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Silyl-Functionalized Electrolyte Additives and Their Reactivity toward Lewis Bases in Li-Ion Cells
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Cite this: Chem. Mater. 2022, 34, 8, 3831–3838
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https://doi.org/10.1021/acs.chemmater.2c00345
Published April 12, 2022

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Abstract

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Silyl groups are included in a wide range of electrolyte additives to enhance the performance of state-of-the-art Li-ion batteries. A recognized representative thereof is tris-(trimethylsilyl)phosphate (TMSPa) which, along with the similarly structured phosphite, has been at the center of numerous electrolyte studies. Even though the silyl group has already been widely reported to be specifically reactive towards fluorides, herein, a reactivity towards several Lewis bases typically found in Li-ion cells is postulated and investigated with the aim to establish a more simplified and generally applicable reaction mechanism thereof. Both gaseous and electrolyte soluble reactants and products are monitored by combining nuclear magnetic resonance and injection cell-coupled mass spectrometry. Experimental observations are supported by computational models. The results clearly demonstrate that the silyl groups react with water, hydroxide, and methoxide and thereby detach in a stepwise fashion from the central phosphate in TMSPa. Intermolecular interaction between TMSPa and the reactants likely facilitates dissolution and lowers the free energy of reaction. Lewis bases are well known to trigger side reactions involving both the Li-ion electrode and electrolyte. By effectively scavenging these, the silyl group can be explained to lower cell impedance and prolong the lifetime of modern Li-ion batteries.

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Organic carbonate-based Li-ion battery electrolytes are state of the art because of their superior combination of performance metrics, such as high ionic conductivity, low viscosity, and thermal stability over a wide temperature range. (1) These electrolytes are also known for their ability to form electrode-passivating layers on the negative Li-ion graphite electrode, commonly termed solid electrolyte interphases (SEIs), which are necessary to stabilize the electrode/electrolyte interfaces and prevent cell failure. When the graphite electrode potential is pushed sufficiently negative (≲1 V vs Li+/Li), the organic carbonate solvent reductively decomposes, forming products of which Lewis bases (LBs) (e.g., RO–, C2O42–, RCO32–, etc.) are the most notable. Depending on their respective solubility, these Lewis bases may combine with Li+ and precipitate on the electrode in form of an electrically insulating yet Li+-conductive SEI. (2,3) Ethylene carbonate (EC) is the archetype Li-ion electrolyte solvent and is known to decompose into both organic and inorganic compounds like lithium alkyl carbonates, alkoxides, oxalates, and various other lithium salts. (1) Lithium hexafluorophosphate (LiPF6), similarly being the archetype electrolyte salt, normally decomposes to form fluorides (e.g., lithium or hydrogen fluoride (LiF/HF)) and phosphorus-containing compounds. Regardless of the species formed, only a few of the products contribute to the effective SEI while the rest simply lead to loss of active lithium and higher cell impedance. (4) Possibly worse, some of the formed products could even further propagate unwanted side reactions and accelerate the failure of the cell. (5)
Electrolyte additives are typically functional molecules dissolved at low concentrations (<5 wt %) in the electrolyte to optimize their physicochemical properties. Examples are SEI formers (e.g., vinylene carbonate), flame retardants (e.g., hexamethoxycyclophosphazene (HMPN) and trimethyl phosphate (TMP)), cathode surface layer formers (e.g., n-butylferrocene), etc. (1) A large group of performance-enhancing additives contains organosilicon silyl groups (R3Si), which when included are known to react with HF, a harmful molecule often encountered in operating Li-ion cells. Particularly, the silyl-functionalized phosphites (P(OR)3) and phosphates (O═P(OR)3) have been researched for years. (6−10) One well-known representative is tris-(trimethylsilyl)phosphate (TMSPa), which, apart from scavenging HF, has also been claimed to be “multifunctional”, e.g., in the sense of being an electrode layer former (both on the positive and negative electrodes). (11,12) Indeed, the group of Jeff Dahn and others showed that LiNi1/3Mn1/3Co1/3O2(NMC)/graphite-based Li-ion cells with TMSPa added display longer cycle and calendar life and exhibit lower impedance. (13,14) Bolli et al. showed that TMSPa used as an additive in Li-rich NMC/graphite cells does not necessarily form a SEI or CEI by being electrochemically reduced/oxidized, but by also scavenging other fluorides, such as LiF residues, lowers the cell impedance. (11) Removing the highly resistive LiF from the SEI while replacing it with side products from TMSPa degradation, such as P- and O-rich compounds, would increase Li+ conductivity while at the same time suppress adverse side reactions within the cell. (15)
Previous studies from our group and others also indicated that TMSPa may not only remove fluorides but could also react with oxide-based impurities, (14,16,17) such as hydroxides. Experimental evidence was, however, merely based on the observation of reduced extent of CO2 evolution (presumed to derive from the OH–-catalyzed EC ring opening described in reactions 1 and 2) within Li-ion cells containing TMSPa, and no further support for a specific reaction mechanism was provided. (14) In principle, all Lewis bases could initiate a nucleophilic attack on the carbonyl carbon of EC and thereby trigger a ring-opening reaction and release of CO2
Degradation of EC at elevated temperatures, initiated by Lewis bases, leads to the formation of poly(ethylene glycol) (PEG)-like polymers, (18) which either deposit on the active materials or dissolve in the electrolyte with increased cell impedance as a result. (5) Moreover, this process is accompanied by an evolution of CO2, which, in turn, could be scavenged by remaining Lewis bases to form a range of carbonates, (19,20) formates, (21) etc.
Basic OH– reacting with CO2 forming inorganic carbonates would be the most prominent example thereof. These thermally activated processes are competing and highly complex with the involvement of several different Lewis bases, as reflected by the richness of various species typically found in the SEI. If these Lewis bases remain active after the essential SEI is formed, the SEI will continue to grow, increase impedance, and shorten the lifetime of the cell.
Three of the most commonly reported Lewis bases formed in the Li-ion cell are F–, OH–, and –OCH3. F– is formed as a result of decomposition of fluorinated salts (like LiPF6) and additives (e.g., fluoroethylene carbonate (FEC)). LiF is practically insoluble in the solvents and hence precipitates on the electrode surfaces whenever formed. OH–, on the other hand, is typically formed from the reduction of trace water in the electrolyte. (22) Hydroxides may also result from the reaction of Li2O with water. (23) LiOH is predicted to be more soluble than LiF in conventionally used solvents. –OCH3 may form as a result of electrochemical reduction of semicarbonates like diethylene carbonate (DEC). The solubility of LiOCH3 is predicted to lie in between LiF and LiOH. (24)
The aim of this study is to investigate the reactivity of the silyl group, as included in TMSPa, towards Lewis bases typically formed in Li-ion cells, such as F–, OH–, and –OCH3. We hypothesize that a significant beneficial action of silyl-functionalized electrolyte additives derives from their general ability to scavenge Lewis bases and thereby suppress electrolyte decomposition and cell impedance growth associated with the above reactions 1–3. An elucidation of the underlying reaction mechanisms of silyl-group-functionalized electrolyte additives promises to unlock a deeper understanding of Li-ion chemistry and lead the way to further optimization of the degradation products formed within the battery cells.

Experimental Section

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Battery-grade EC (Sigma-Aldrich) and DEC (Sigma-Aldrich) were introduced into the glovebox as received from the suppliers. DEC was dried with molecular sieves and then mixed with EC (which was heated up to 50 °C to melt). The mixture of EC/DEC used in this study is in a 1:1 volume ratio, unless otherwise stated. The solvent mixture was further dried with molecular sieves. The water content is around 10 ppm as tested by the Karl Fisher method. LiF (Alfa Aesar, 99.85%), LiOH (Alfa Aesar, 99.85%), and LiOCH3 (Sigma-Aldrich, 98%) were dried at 120 °C in a Büchi oven overnight under vacuum inside the glovebox. Saturated Lewis base solutions were prepared by adding 100 mg of Lewis bases in 2 mL of EC/DEC mixture in separate vials. The vials were sealed and allowed to stir in a magnetic stirrer at least overnight before performing any test. This procedure was performed to make sure that a saturated Lewis base solution is taken as a starting sample. TMSPa, hydroxy-trimethylsilane (TMSOH), and methoxy-trimethylsilane (TMSOCH3) from Sigma-Aldrich (≥98, 97.5, and 99% respectively) were used as received from suppliers.

Nuclear Magnetic Resonance

For NMR, a JEOL (1H, 500 MHz; 31P, 162 MHz; 19F, 376 MHz) spectrometer was used, and chemical shifts were recorded in parts per million. Anhydrous dimethylsulfoxide (DMSO)-d6 (99.9%) from VWR was further dried with molecular sieves and was used as a deuterated solvent. Plastic falcon tubes for electrolyte storage and fluorinated ethylene propylene (FEP) NMR tube liners (outer diameter: 5 mm, Wilmad-LabGlass) were used. They were used as a precautionary measure to avoid any reaction between glass and fluorides. The mouth of the FEP tube was closed with poly(tetrafluoroethylene) (PTFE) plug. For each NMR sample test, 400 μL of DMSO-d6 was added to 100 μL of sample solution. Then, each FEP NMR tube was placed inside a glass NMR tube (inner diameter: 5 mm), which was again closed with a PTFE stopper. The solution storage and sample preparation were done in the glovebox and the H2O content inside it was kept below 1 ppm. All spectra were processed and analyzed by MestReNova 6.0.2-5475.

Mass Spectrometry

The gas analysis setup consists of a custom-made injection cell, which is a blue-capped bottle of 10 mL volume (Duran). The lid of the bottle has three connections: (1) an outlet to periodically sample the evolved gases to the mass spectrometer (Pfeiffer PrismaPlus QME220), (2) an inlet to refill the cell with fresh Ar gas, and (3) a syringe for injection. The temperature and pressure in the cell are monitored using a high precision transducer (Keller-Druck). The sampling system and the cell are integrated inside a temperature-controlled incubator chamber (Tritec, Germany). The temperature is set to 30 °C for all of the experiments. (25) LiF, LiOH, and LiOCH3 were grinded in a mortar and pestle for at least 5 min before making the sample solution. To monitor gas evolution, two different sets of samples were prepared with different protocols: (1) 50 mg of Lewis bases and 800 μL of DEC were mixed in the bottom of the vial and 400 μL of EC/DEC was taken in the syringe, and this mixture is referred to as saturated or unfiltered and (2) 50 mg of Lewis base was mixed with 800 μL of DEC. Then, this mixture was filtered into a reaction vial and 400 μL of EC/DEC was taken in the syringe. This mixture is referred to as filtered. For the experiments with the additive, 5 vol % TMSPa was taken together with 400 μL of EC/DEC in the syringe. The time between each sampling point was set to 180 s, and an initial baseline was performed until there was a stable background signal. Once a stable background is achieved, reactants from the syringe were mixed with the reactants in the vial and periodic sampling was done as stated previously. The solution was magnetically stirred to ensure a homogeneous mixing. The partial pressure of gas species in Ar flow is documented as normalized ion current, with respect to Ar and surface area of the Lewis base powder. Each gas species is recorded at a relevant mass-to-charge ratio (m/z): H2-2, O2-32, Ar-36, CO2-44, TMSPa-73, TMSOH-75, TMSF-77, TMSOCH3-89, and DEC-91. The gas profiles for each experiment were compared with its blank, i.e., EC/DEC mixture only. For the experiments with additives, the gas evolved was compared with additive-added blank samples.

Computational Details

Screening studies using the GFN2-xTB model (26) to compute the free energy of reaction at 298 K have been performed to sample the manifold of structural conformations possible for the reactants and the products at every reaction step in the decomposition of TMSPa. The GFN2-xTB model is generally good at describing geometries and intermolecular bond energies for molecular systems. However, for intramolecular bond energies, semiempirical tight-binding methods are generally not as reliable. Thus, before the screening study, we performed density functional theory (DFT) calculations for the gas-phase reactions using the molecular structures obtained from the xTB geometry optimizations as input to ensure that the semiempirical xTB model accurately captures the correct trends in computed reaction energies (breaking and formation of intramolecular bonds). For this purpose, we used Gaussian09 (27) and the B3LYP functional (28−31) with a Dunning correlation consistent basis set (aug-cc-pVDZ). Weak nonlocal interactions (vdW-forces) were accounted for using the Grimme D3 aposteriori vdW correction scheme. All structures were geometry optimized until the forces on each atom were less than 0.00077 eV/Å. While absolute values can be off by a maximum of 0.4 eV for the computed reaction energies, the trends, i.e., signs and magnitudes of the computed reaction energies, were found to be in line with the DFT data; see results in the Supporting Information. In the screening study, we always computed the reaction energies considering all reactants and products in each simulation to also account for the important intermolecular interactions. For example, the TMSPa/H2O reaction was simulated, allowing TMSPa and H2O to interact first, as compared to the comparison above for the intramolecular reaction energies, where each molecule was treated separately in the calculation. To obtain reliable reaction energies, we further tested the effect the environment has on the reaction energies by considering three different cases: (i) gas phase, (ii) using an implicit solvation model, namely, the polarizable continuum model (PCM), and (iii) the inclusion of effects from explicit solvation by adding EC to satisfy the possibilities to form hydrogen bonds. After a thorough testing procedure, we found that accounting for the possible formation of molecular complexes with reactants and products has the largest effect and is necessary to obtain reaction energies that support the experimental observations. Therefore, all reaction energies reported were modeled keeping all reaction products along the reaction chain. For a complete discussion of the validity of the theoretical calculations and for figures showing the structural models used, we refer to the Supporting Information.

Results and Discussion

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NMR spectra for the reaction medium (EC/DEC), the individual reactants (TMSPa and Lewis bases) dissolved in EC/DEC, and anticipated products were first acquired, as shown in Figure 1, before the reactivity of the silyl group toward the LBs was investigated. Most relevant peaks are observed in the 3–4 δ (ppm) and −0.1–0.3 δ (ppm) range (see the Supporting Information for full spectral ranges recorded (Figure S1)). For the EC/DEC solution, well-known satellite peaks assigned to DEC are visible for ∼3.9 ppm and are included as a reference here. H2O trace impurities are still present in the mixture and identified as a singlet at 3.28 ppm in 1H NMR. Indeed, Karl Fischer titration showed that the solvent mixture contains a few tens of ppm of water as impurities, thus indicating that the protonated species observed at a chemical shift in the range 3–4 ppm could be detected with high sensitivity. TMSPa displays a main set of peaks at ∼0.18 ppm assigned to its methyl −CH3 groups, whereas minor yet detectable peaks at 0.03 ppm assigned to an unknown impurity in the TMSPa additive solution are identified (Figure 1a, orange). Interestingly, when TMSPa is added to the EC/DEC solution, the peak assigned to water vanishes (Figure 1b, black), while a new peak at −0.02 ppm appears. The latter peak is assigned to TMSOH, a proposed product of the reaction
(where LB is the OH–) confirmed by the reference TMSOH spectrum (Figure 1b, pink). BMSPa stands for bis-(trimethylsilyl)phosphate, represents TMSPa that lost 1 trimethylsilyl (TMS) unit, and is also believed to be observed as a minor peak at −15 ppm by 31P NMR (Figure 2a, black). Interestingly, in our initial simulations of hydrolysis of TMSPa according to reaction 4, the energy of reaction ΔE > 0. Thus, in the gas phase, the reaction is found to be endothermic (cf. Table S1 in the Supporting Information). Allowing for intermolecular interactions for both reactants and products, together with the addition of explicit solvent molecules (EC), lowers ΔE and makes the reaction exothermic and spontaneous. In this case, a molecular complex between BMSPa and the additional EC is formed, cf. reaction routes illustrated in Figure S2 in the Supporting Information. These results suggest that the ability of the reaction product to form energetically favorable intermolecular interactions (primarily hydrogen bonding) is important for an efficient breakage of the TMSPa’s O–Si bond. The resulting lowest energies of reaction from the GFN2-xTB calculations of the first step of TMSPa/H2O reaction (Figure 3) show that the hydrolysis of TMSPa is slightly exothermic. Results presented thus far evidence that TMSPa does react with neither EC nor DEC but rather with H2O, which is desired from a practical perspective, to remove residual water from a Li-ion battery electrolyte. After establishing this baseline, the reactivity of three lithiated Lewis bases typically found in the SEI, namely, LiOH, LiOCH3, and LiF toward TMSPa was investigated.

Figure 1

Figure 1. 1H liquid NMR spectra of (a) EC/DEC, orange: pure TMSPa, (b) TMSPa in EC/DEC, pink: TMSOH in EC/DEC, (c) LiOH mixed in EC/DEC, (d) LiOH mixed in EC/DEC with TMSPa, (e) LiOCH3 mixed in EC/DEC, olive green: pure TMSOCH3, (f) LiOCH3 mixed in EC/DEC with TMSPa, and (g) LiF mixed in EC/DEC, blue: LiF mixed in EC/DEC with TMSPa.

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Figure 2

Figure 2. 31P liquid NMR spectra showing the formation of the intermediates upon the reaction of LiOH and TMSPa in the presence of solvents. The broadening of the 31P spectral lines is associated with scalar hydrogen coupling. (36)

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Figure 3

Figure 3. Lowest energy of reaction vs reaction coordinate found in GFN2-xTB screening for the TMSPa/H2O, TMSPa/LiOH, and TMSPa/OCH3 reactions. For the TMSPa/H2O reaction, only the first step is shown. In the modeling, we explicitly account for the intermolecular interactions between reactants and reaction products. In the case of H2O, an explicit EC molecule is added to ensure that all hydrogen bonds are accounted for (cf. Supporting Information).

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LiOH

Adding LiOH to EC/DEC results in the formation of new compound(s) as evidenced from the appearance of new peaks at 3.36, 3.53, and 3.61 ppm (Figure 1c). The peak at 3.36 ppm is assigned to a −[CH2–CH2–O]– fragment of ethylene glycol (EG–), which is likely derived from LB-triggered ring opening of EC by OH– along with the release of CO2 according to Reaction 1, as also previously reported. (32) EG– thus formed may further polymerize EC forming poly(ethylene glycol) (PEG–) along with additional CO2 (Reaction 2). Indeed, our group has previously observed increased electrolyte viscosity upon the formation of OH– during the first few cycles of the Li-ion cell. (33) Further support for such a scenario was found in the form of a continuous evolution of CO2 gas when EC/DEC contacted LiOH compared to the blank experiment in Figure 4a. In the blank case, only the EC/DEC solution is injected into the reaction vial containing DEC and only CO2 already dissolved in these solvents escapes to the continuously flushed Ar headspace. When LiOH is present, a part of CO2 will be scavenged by OH– forming lithium bicarbonate and eventually Li2CO3 (Reaction 3), hence explaining the initially lower CO2 concentrations in the headspace (Figure 4a). (34) Interestingly, when TMSPa was added to LiOH in a EC/DEC mixture significantly, more CO2 evolved (Figure 4a) along with the formation of much more PEG, other solvent degradation products, and TMSOH (Figure 1d). Since neither TMSPa nor TMSOH decomposes EC/DEC to any greater extent (Figure 1b, pink), the enhanced EC decomposition rate is related to an increased reactivity of OH– when TMSPa is present. We hypothesize that TMSPa acts as an anion receptor (35) increasing the dissolution of LiOH, thus dissolving more OH–, which accelerates the ring opening of EC and the evolution of CO2. To confirm, the CO2 evolution was monitored for a filtered LiOH/EC/DEC solution, i.e., thus removing any eventual solid LiOH in EC/DEC when it comes in contact with TMSPa. Upon filtration, there will thus be a limited OH– concentration and no additional LiOH to be dissolved in the reaction cell due to the presence of TMSPa. A filtered solution clearly shows lower CO2 evolution, which supports the notion that TMSPa is able to produce free OH– by solubilizing more LiOH in the unfiltered case, where there is an excess of solid LiOH powder sitting at the bottom of the reaction vial.

Figure 4

Figure 4. (a) CO2 gas evolution profile of blank solution and Lewis bases with solvents with and without TMSPa. Gray area represents the region of background baseline. (b) TMS-LB evolution profiles, as recorded from the mass spectrometer.

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In any case, OH– eventually breaks the O–Si bond in TMSPa to form TMSOH, as evidenced by the relatively strong peak at −0.02 ppm in Figure 1d, and the continuous evolution of gaseous TMSOH (Figure 4b) when TMSPa comes in contact with LiOH. More importantly, TMSPa was largely consumed in the LiOH-saturated EC/DEC solution as evidenced by the nearly complete disappearance of the peak at 0.18 ppm (Figure 1d).
The influence of the TMSPa concentration was therefore more closely examined by mixing saturated LiOH in EC/DEC with TMSPa in 9:1, 7:3, 1:1, and 3:7 volume ratios. Indeed, 31P NMR spectra reveal that the signal from the nearly completely consumed TMSPa (Figure 2b) reappears at higher concentrations (Figure 2c–e). Moreover, two peaks assigned to the intermediate products BMSPa and mono(trimethylsilyl)phosphate (MMSPa, loss of 2 TMS units from TMSPa), as illustrated in Figure 3, are distinguished. The TMS units attached to the central phosphate group of TMSPa would thus be stepwise consumed until only the final product, Li3PO4, without any TMS units attached remains. The GFN2-xTB calculations show that the reaction between LiOH and TMSPa is exothermic, and we expect that the reaction proceeds until Li3PO4 once the reactants meet (Figure 3). These data are in line with the experimental observations of largely consumed MMSPa when LiOH is in excess (Figure 2b). Lithium phosphate does have a low solubility in EC/DEC (hence not detected by our 31P NMR spectrometer, Figure 2b).

LiOCH3

Another LB typically found in Li-ion cells is methoxide (OCH3–), e.g., resulting from the reduction of the linear carbonate solvents. (37) The 1H NMR of LiOCH3 mixed in EC/DEC (Figure 1e) shows new signals at 3.37 and 3.54 ppm. The peak at 3.37 ppm was first tentatively assigned to 2-methoxyethanolate formed as a result of the OCH3–-induced EC ring-opening reaction. However, the CO2 gas profile from MS (Figure 4a) showed otherwise. CO2 present in the EC/DEC solvent mixture is rather observed to be readily consumed when in contact with LiOCH3 likely to form lithium methyl carbonate (LMC)
which is an often reported component of the SEI, (38) though with a poor Li+ conductivity. (39) Based on these results, the new peak at 3.37 ppm appearing in the 1H NMR spectrum is assigned to LMC instead of methoxyethanolate. LiOCH3 is thus not anticipated to induce EC ring opening, but rather scavenges CO2, judging from the nearly complete disappearance of CO2 evolution from the solvents (Figure 4b).
To investigate the reaction between OCH3– and TMSPa, 1H and 31P NMR were performed on a EC/DEC solution in which LiOCH3 is dissolved and to which TMSPa is added (Figures 1f and 2f, respectively). The 31P NMR shows a single and slightly broadened peak at −15.05 ppm assigned to BMSPa. The 1H NMR also shows peaks belonging to BMSPa along with new peaks at 3.13 and 3.28 ppm assigned to TMSOCH3. Interestingly, the signal from TMSPa disappears, again demonstrating that the additive is completely consumed. Addition of TMSPa also removed the species appearing at 3.37 ppm, tentatively assigned to LMC. Either TMSPa reacts directly with LMC or indirectly, e.g., favoring the backward direction of reaction 5 by scavenging LiOCH3 according to Le Chatelier’s principle. Indeed, the addition of TMSPa to LiOCH3 solution increases the evolution of CO2, but only to a level comparable to if all CO2 originally dissolved in the EC/DEC solution would be released (Figure 4a). Evidently, LiOCH3 reacts with TMSPa breaking the molecule at the O–Si bond with TMSOCH3 and BMSPa as reaction products. Upon a direct comparison with the 1H NMR spectrum of pure TMSOCH3 (Figure 1e), the chemical shifts and integrals of the peaks at 0.02 and 3.28 ppm were found to fit very well with the −OCH3 and −TMS groups of pure TMSOCH3, respectively. Moreover, the reaction product TMSOCH3 was clearly detected by MS when TMSPa came in contact with LiOCH3 and continued to evolve throughout the experiment (Figure 4b). Interestingly, only the peak at −15.05 ppm in 31P NMR and 0.07 ppm in 1H NMR attributed to the BMSPa moiety is identified. The reaction between TMSPa and LiOCH3 apparently stops at BMSPa and does not proceed to MMSPa and hence likely not to Li3PO4 either. Indeed, in the GFN2-xTB calculations for the TMSPa/LiOCH3 reaction, a minimum in the reaction energy with respect to the reaction coordinate is found for BMSPa, cf. Figure 3. The LiBMSPa product side is found at a higher energy than anticipated from the calculated gas-phase reactions due to the formation of a molecular complex with the reaction product. Hence, accounting for explicit interactions is important given that the gas-phase reactions with and without the addition of an implicit solvent description are strongly exothermic (see Table S1 in the Supporting Information). In a continuous reaction chain, these stabilizations are even larger, making LiBMSPa/LiOCH3 and Li2MMSPa/LiOCH3 endothermic. The most stable configurations found with the GFN2-xTB method are shown in Figure S5 in the Supporting Information.

LiF

The reactivity of silyl groups towards fluorides has already been investigated extensively, also as included in TMSPa. (14,40) LiF is practically insoluble in EC/DEC, and we confirm that adding LiF to EC/DEC resulted in no signs of new products, neither in 1H NMR (Figure 1g) nor 19F NMR within the timescale of the experiment. When TMSPa was added to LiF in EC/DEC mixture, no new species appeared in the 1H NMR spectrum (Figure 1g), but in the 19F NMR spectrum, a characteristic peak at −154.71 ppm was observed, which was attributed to TMSF (see Figure S1 in the Supporting Information). Indeed, the gas analysis also confirmed previous observations of TMSPa reacting spontaneously with LiF to yield TMSF analogously to Reaction 4 in which F– breaks the Si–O bond of TMSPa and combines with the TMS moiety. The dissolved TMSF is volatile as observed by the MS headspace analysis (Figure 4b). (12,41) Since these observations merely confirm both experimental and theoretical observations and the conclusion made previously, we do not proceed with further analysis. We do however highlight that F– displays the same expected reaction as for OH– and –OCH3 toward TMSPa, thus demonstrating that the reactivity of the silyl groups is not specific toward fluorides but may scavenge several Lewis bases generally found in Li-ion cells.

Summary and Conclusions

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Silyl groups are important functional components in a wide range of molecular electrolyte additives developed to enhance the performance of state-of-the-art Li-ion batteries. Focus herein is on a well-known representative thereof, trimethylsilylphosphate, which connects three silyl groups to a central phosphate. Our hypothesis that the silyl groups not only specifically react with fluorides, as already widely reported, but also generally react with Lewis bases is confirmed. Water (H2O), hydroxide (OH–), and methoxide (−OCH3) are here brought forward as candidates well representing three types of Lewis bases found in Li-ion cells. 1H, 19F, and 31P-nuclear magnetic resonance combined with headspace gas analysis are applied to monitor TMSPa, the Lewis base, as well as both soluble and volatile reaction products. Theoretical simulations have been performed, which provide support for the experimental observations. On the basis of our findings, we conclude that silyl-group-functionalized molecular additives are beneficial because of their ability to scavenge a range of potentially harmful Lewis bases in the Li-ion cell. Lewis bases are here confirmed to ring-open the electrolyte solvent ethylene carbonate (EC) to release CO2 and ethylene glycols (Reaction 1). The latter degradation product further polymerizes and increases ionic resistance of the electrolyte (Reaction 2). (5) CO2 may be scavenged to form resistive electrode deposits (Reaction 3), such as LMC (Reaction 5). Including silyl-group-based additives in the electrolyte may hence effectively eliminate such adverse effects immediately upon cell assembly by removing Lewis bases and break the autocatalytic electrolyte decomposition cycle of the solvent (combining Reactions 1 and 2). Later on, upon cycling, silyl groups can dissolve and replace highly resistive deposits of the graphite SEI, be it LMC or LiF, with the more ionically conducting Li3PO4. (42) In addition, removal of Lewis bases, such as H2O, is also known to have a beneficial influence on the stability of the commonly employed LiPF6 electrolyte salt. (43) These findings explain why Li-ion cells with silyl-group-based additives display lower impedance and longer cycle life. Although the fate of the resulting TMS-LB species is not further investigated here, these species are soluble in the electrolyte and may not necessarily further react to form resistive deposits and/or induce active lithium loss in the cell. Intermolecular interaction between TMSPa, Lewis bases, and the products is found to be necessary to explain the reaction mechanisms. Gas-phase reaction energies, treating only reactants and products explicitly, are found not sufficient to account for the hypothesized and experimentally proven behavior. This is particularly seen in the cases where there are products offering the formation of new hydrogen bonds, such as the TMSPa/H2O and TMSPa/LiOH reactions. Intermolecular physicochemical effects, such as anion reception, ionic complex formation, and hydrogen bonding, are found more important than expected but remain a subject of future studies. Model studies combining experiments and simulations, as presented herein, are essential to provide the building blocks necessary to establish a deeper fundamental understanding of the Li-ion cell chemistry and the highly complex reaction mechanisms found therein.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.2c00345.

  • 1H, 19F NMR spectra, and computational details etc (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Erik Bowall - Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden
    • Robin Lundström - Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden;  Orcidhttps://orcid.org/0000-0001-9070-9264
    • Nataliia Mozhzhukhina - Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden;  Present Address: Department of Physics, Chalmers University of Technology, SE 412 96 Göteborg, Sweden;  Orcidhttps://orcid.org/0000-0001-6798-9704
    • Guiomar Hernández - Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden;  Orcidhttps://orcid.org/0000-0002-2004-5869
    • Peter Broqvist - Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-751 21 Uppsala, Sweden;  Orcidhttps://orcid.org/0000-0002-9842-4332
  • Funding

    The authors acknowledge Knut and Alice Wallenberg (KAW) Foundation (Grant 2017.0204), Swedish Research Council (2016-04069), and Stiftelsen för Strategisk Forskning (SSF, FFL18-0269) for financial support and StandUp for Energy and the national strategic e-Science program eSSENCE for base funding. The simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at HPC2N and NSC.

  • Notes
    The authors declare no competing financial interest.

References

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This article references 43 other publications.

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    Tris(trimethylsilyl) phosphite (TMSPi) has emerged as an useful electrolyte additive for Li ion cells. This work examines the use of TMSPi and a structurally analogous compd., tri-Et phosphite (TEPi), in LiNi0.5Mn0.3Co0.2O2-graphite full cells, contg. a (baseline) electrolyte with 1.2 M LiPF6 in EC:EMC (3:7 wt./wt.) and operating between 3.0-4.4 V. Galvanostatic cycling data reveal a measurable difference in capacity fade between the TMSPi and TEPi cells. Furthermore, lower impedance rise is obsd. for the TMSPi cells, because of the formation of a P- and O-rich surface film on the pos. electrode that was revealed by XPS data. Elemental anal. on neg. electrodes harvested from cycled cells show lower contents of transition metal (TM) elements for the TMSPi cells than for the baseline and TEPi cells. Our findings indicate that removal of TMS groups from the central P-O core of the TMSPi additive enables formation of the oxide surface film. This film is able to block the generation of reactive TM-O radical species, suppress H abstraction from the electrolyte solvent, and minimize oxidn. reactions at the pos. electrode-electrolyte interface. In contrast, oxidn. of TEPi does not yield a protective pos. electrode film, which results in inferior electrochem. performance.
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    ACS Applied Energy Materials (2020), 3 (1), 290-299CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
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    Kim, Jongjung; Adiraju, Venkata A. K.; Rodrigo, Nuwanthi; Hoffmann, Jennifer; Payne, Martin; Lucht, Brett L.
    ACS Applied Materials & Interfaces (2021), 13 (19), 22351-22360CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The beneficial role of lithium bis(trimethylsilyl) phosphate (LiTMSP), which may act as a novel bifunctional additive for high-voltage LiNi1.5Mn0.5O4 (LNMO)/graphite cells, has been investigated. LiTMSP is synthesized by heating tris(trimethylsilyl) phosphate with lithium tert-butoxide. The cycle performance of LNMO/graphite cells at 45°C significantly improved upon incorporation of LiTMSP (0.5 wt %). NMR anal. suggests that the trimethylsilyl (TMS) group in LiTMSP can react with hydrogen fluoride (HF), which is generated through the hydrolysis of lithium hexafluorophosphate (LiPF6) by residual water in an electrolyte soln. or water generated via oxidative electrolyte decompn. reactions to form TMS fluoride. Inhibition of HF leads to a decrease in the concn. of transition-metal ion-dissoln. (Ni and Mn) from the LNMO electrode, as detd. by inductively coupled plasma mass spectrometry. In addn., the generation of the superior passivating surface film derived by LiTMSP on the graphite electrode, suppressing further electrolyte reductive decompn. as well as deterioration/reformation caused by migrated transition metal ions, is supported by a combination of chronoamperometry, XPS, and field-emission SEM. Furthermore, a LiTMSP-derived surface film has better lithium ion cond. with a decrease in resistance of the graphite electrode, as confirmed by electrochem. impedance spectroscopy, leading to improvement in the rate performance of LNMO/graphite cells. The HF-scavenging and film-forming effects of LiTMPS are responsible for the less polarization of LNMO/graphite cells enabling improved cycle performance at 45°C.
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  16. 16
    Kim, D. Y.; Park, H.; Choi, W. I.; Roy, B.; Seo, J.; Park, I.; Kim, J. H.; Park, J. H.; Kang, Y. S.; Koh, M. Ab Initio Study of the Operating Mechanisms of Tris(Trimethylsilyl) Phosphite as a Multifunctional Additive for Li-Ion Batteries. J. Power Sources 2017, 355, 154– 163,  DOI: 10.1016/j.jpowsour.2017.04.062
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    16
    Ab initio study of the operating mechanisms of tris(trimethylsilyl) phosphite as a multifunctional additive for Li-ion batteries
    Kim, Dong Young; Park, Hosang; Choi, Woon Ih; Roy, Basab; Seo, Jinah; Park, Insun; Kim, Jin Hae; Park, Jong Hwan; Kang, Yoon-Sok; Koh, Meiten
    Journal of Power Sources (2017), 355 (), 154-163CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Tris(trimethylsilyl) phosphite (P(OSi(CH3)3)3) is a multifunctional electrolyte additive for scavenging HF and forming a cathode electrolyte interphase (CEI). Systematic anal. of the HF reaction pathways and redox potentials of P(OSi(CH3)3)3, OP(OSi(CH3)3)3, P(OSiF3)3, and OP(OSiF3)3, and their reaction products, using ab initio calcns. allowed us to elucidate the operating mechanism of P(OSi(CH3)3)3 and verify the rules that det. its HF reaction pathways and electrochem. stability. While O-Si cleavage is the predominant HF scavenging pathway for P(OSi(CH3)3)3, O-P cleavage is stabilized by replacing CH3 with an electron-withdrawing group. Thus, P(OSiF3)3 scavenges HF mainly through O-P cleavage to produce PF3, which has high oxidn. stability. However, the O-Si cleavage pathway produces P(OSi(CH3)3)2OH, P(OSi(CH3)3) (OH)2, and P(OH)3 sequentially, along with Si(CH3)3F. These PO3 systems, which are oxidized earlier than carbonate solns. and form tightly bonded units following oxidn., act as seed units for compact CEI growth. Moreover, the HF scavenging ability of PO3 systems is maintained during oxidn. until all O-Si bonds are broken. As a strategy for developing additives with enhanced functionality, modifying P(OSi(CH3)3)3 by replacing CH3 with an electron-donating group to exclusively utilize the O-Si cleavage pathway for HF scavenging is recommended.
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  17. 17
    Mai, S.; Xu, M.; Liao, X.; Xing, L.; Li, W. Improving Cyclic Stability of Lithium Nickel Manganese Oxide Cathode at Elevated Temperature by Using Dimethyl Phenylphosphonite as Electrolyte Additive. J. Power Sources 2015, 273, 816– 822,  DOI: 10.1016/j.jpowsour.2014.09.171
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    17
    Improving cyclic stability of lithium nickel manganese oxide cathode at elevated temperature by using dimethyl phenylphosphonite as electrolyte additive
    Mai, Shaowei; Xu, Mengqing; Liao, Xiaolin; Xing, Lidan; Li, Weishan
    Journal of Power Sources (2015), 273 (), 816-822CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    A novel electrolyte additive, di-Me phenylphosphonite (DMPP), is reported in this paper to be able to improve significantly the cyclic stability of LiNi0.5Mn1.5O4 cathode of high-voltage lithium-ion battery at elevated temp. When subjecting the cathode to charge/discharge cycling at 50° at 1C (1C = 146.7 mA-h/g) rate in a std. electrolyte (1.0M LiPF6 in ethylene carbonate/dimethyl carbonate of 1:2 vol. ratio), LiNi0.5Mn1.5O4 suffers serious discharge capacity decay, with a capacity retention of 42% after 100 cycles. By adding 0.5% DMPP into the std. electrolyte, the capacity retention is increased to 91%. This improvement can be ascribed to the preferential oxidn. of DMPP to the std. electrolyte and the subsequent formation of a protective film on LiNi0.5Mn1.5O4, which suppresses the electrolyte decompn. and protects LiNi0.5Mn1.5O4 from destruction. Theor. calcns. together with voltammetric analyses demonstrate the preferential oxidn. of DMPP and the consequent suppression of electrolyte decompn., while the observations from SEM, XPS and FTIR spectroscopy confirm the protection that DMPP provides for LiNi0.5Mn1.5O4.
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    Metzger, M.; Strehle, B.; Solchenbach, S.; Gasteiger, H. A. Hydrolysis of Ethylene Carbonate with Water and Hydroxide under Battery Operating Conditions. J. Electrochem. Soc. 2016, 163, A1219– A1225,  DOI: 10.1149/2.0411607jes
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    Hydrolysis of Ethylene Carbonate with Water and Hydroxide under Battery Operating Conditions
    Metzger, Michael; Strehle, Benjamin; Solchenbach, Sophie; Gasteiger, Hubert A.
    Journal of the Electrochemical Society (2016), 163 (7), A1219-A1225CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    This study deals with the decompn. of ethylene carbonate (EC) by H2O in the absence and presence of catalytically active hydroxide ions (OH-) at reaction conditions close to lithium-ion battery operation. We use Online Electrochem. Mass Spectrometry (OEMS) to quantify the CO2 evolved by these reactions, referred to as H2O-driven and OH--driven EC hydrolysis. By examg. both reactions at various temps. (10 - 80°C) and water concns. (<20 ppm or 200, 1000, and 5000 ppm H2O) with or without catalytically active OH- ions in EC with 1.5 M LiClO4, we det. an Arrhenius relationship between the CO2 evolution rate and the cell temp. While the apparent activation energy for the base electrolyte (<20 ppm H2O) is very large (app. Ea ≈153 kJ/mol), substantially lower values are obtained in the presence of H2O (app. Ea ≈99 ± 3 kJ/mol), which are even further decreased in the presence of catalytically active OH- (app. Ea ≈43 ± 5 kJ/mol). Our data show that OH--driven EC hydrolysis is relevant already at room temp., whereas H2O-driven EC hydrolysis (i.e., without catalytically active OH-) is only relevant at elevated temp. (≥40°C), as is the case for the base electrolyte. Thus, catalytic quantities of OH-, e.g., from hydroxide contaminants on the surface of transition metal oxide based active materials, would be expected to lead to considerable CO2 gassing in lithium-ion cells.
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    Peng, Z.; Merz, K. M., Jr. Theoretical Investigation of the CO2 + OH- ─ > HCO3- Reaction in the Gas and Aqueous Phases. 1993, 115, 9640 9647.  DOI: 10.1021/ja00074a032 .
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    Zhuang, G. V.; Yang, H.; Ross, P. N.; Xu, K.; Jow, T. R. Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate. Electrochem. Solid-State Lett. 2006, 9, A64– A68,  DOI: 10.1149/1.2142157
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    Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate
    Zhuang, Guorong V.; Yang, Hui; Ross, Philip N., Jr.; Xu, Kang; Jow, T. Richard
    Electrochemical and Solid-State Letters (2006), 9 (2), A64-A68CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    To improve the understanding of passive film formation on metallic Li in org. electrolyte, the authors synthesized and characterized Li Me carbonate (LiOCO2Me), a prototypical component of the film. The structure of this compd. was characterized with NMR and FTIR spectroscopy, and its thermal stability and decompn. pathway was studied by TGA. The FTIR spectrum of the synthesized compd. enables resoln. of multiple products in the passive film on Li in a di-Me carbonate soln. Li Me carbonate is only one of the components, the others being Li oxalate and Li methoxide.
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    McMahon, T. B.; Northcott, C. J. The Fluoroformate Ion, FCO 2 −. An Ion Cyclotron Resonance Study of the Gas Phase Lewis Acidity of Carbon Dioxide and Related Isoelectronic Species. Can. J. Chem. 1978, 56, 1069– 1074,  DOI: 10.1139/v78-181
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    The fluoroformate ion, FCO2-. An ion cyclotron resonance study of the gas phase Lewis acidity of carbon dioxide and related isoelectronic species
    McMahon, Terrance Brian; Northcott, Colleen Joan
    Canadian Journal of Chemistry (1978), 56 (8), 1069-74CODEN: CJCHAG; ISSN:0008-4042.
    The gas phase ion mol. reactions of a no. of potential fluoride donors with CO2 and carbonyl fluoride have been studied. The fluoride affinities of CO2 and carbonyl fluoride are 33 ± 3 kcal/mol and 35 ± 3 kcal/mol, resp. In addn., from gas phase acidity studies of acetyl fluoride and 2-fluoropropene, the fluoride affinities of ketene and allene have been calcd. to be 38 ± 2 kcal/mol and 15 ± 2 kcal/mol, resp.
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    Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. Identification of Surface Films Formed on Lithium in Propylene Carbonate Solutions. J. Electrochem. Soc. 1987, 134, 1611– 1620,  DOI: 10.1149/1.2100722
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    Identification of surface films formed on lithium in propylene carbonate solutions
    Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E.
    Journal of the Electrochemical Society (1987), 134 (7), 1611-20CODEN: JESOAN; ISSN:0013-4651.
    FTIR, IR, and XPS were used to study the films formed on Li in propylene carbonate solns. of LiClO4, LiAsF6, and LiSO3CF3. Over a range of conditions, the main components detected in the initial surface films were Li alkyl carbonates (RCO3Li, R = alkyl). Another alkyl carbonate solvent, di-Et carbonate, was found to react with Li to form Li Et carbonate. In addn. to solvent redn., XPS measurements gave indication of salt redn. reactions. LiClO4, LiAsF6, and LiSO3CF3 were reduced by Li to form halide ions, which were detected on the Li surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species such as OH-, while the other involves one-electron redn. of propylene carbonate by Li metal, followed by free radical termination reactions. When high concns. of H2O were present, Li2CO3 was formed by further reaction of the alkyl carbonates with H2O. On Li surfaces without a mech. stable surface film, such as those of Li amalgams, the redn. reaction is believed to proceed by an overall 2-electron process, and the primary product is Li carbonate.
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    Zhuang, G. V.; Ross, P. N. Analysis of the Chemical Composition of the Passive Film on Li-Ion Battery Anodes Using Attentuated Total Reflection Infrared Spectroscopy. Electrochem. Solid-State Lett. 2003, 6, A136  DOI: 10.1149/1.1575594
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    Analysis of the Chemical Composition of the Passive Film on Li-Ion Battery Anodes Using Attenuated Total Reflection Infrared Spectroscopy
    Zhuang, Guorong V.; Ross, Philip N., Jr.
    Electrochemical and Solid-State Letters (2003), 6 (7), A136-A139CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    FTIR spectroscopy with attenuated total reflection geometry was used to study the surface of graphite anodes obtained from Li-ion batteries. The batteries were of the 18650-type and subjected to calender aging (60% state of charge) at 55°. The compn. of the film on an anode from a control cell (not aged) consisted of Li2C2O4, RCOOLi, and LiOMe. After aging, there was also LiOH and MeOH, and in some cases LiHCO3, probably due to the reaction of H2O with the methoxide and oxalate. There is substantial variation in the relative amts. of the 5 compds. over the surfaces of the electrodes. Alkyl carbonates may form early on, but they decomp. to more inorg. compds. with aging. The multicomponent compn. reflects the complex chem. of passive film formation in real Li-ion cells.
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    Tasaki, K.; Harris, S. J. Computational Study on the Solubility of Lithium Salts Formed on Lithium Ion Battery Negative Electrode in Organic Solvents. J. Phys. Chem. C 2010, 114, 8076– 8083,  DOI: 10.1021/jp100013h
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    Computational Study on the Solubility of Lithium Salts Formed on Lithium Ion Battery Negative Electrode in Organic Solvents
    Tasaki, Ken; Harris, Stephen J.
    Journal of Physical Chemistry C (2010), 114 (17), 8076-8083CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The soly. of lithium salts, found in solid-electrolyte interface (SEI) films on the anode surface in lithium-ion batteries, has been examd. in org. solvents by atomistic computer simulations. The salts included lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium oxalate ([LiCO2]2), lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methoxide (LiOCH3), lithium Me carbonate (LiOCO2CH3), lithium Et carbonate (LiOCO2C2H5), and dilithium ethylene glycol dicarbonate ([CH2OCO2Li]2). The org. solvents were di-Me carbonate (DMC) and ethylene carbonate. The at. charges in the force field have been fitted to the electrostatic potential obtained from d. functional theory calcns. for each salt. The heat of dissoln. in DMC for the salts calcd. from computer simulations ranged from exothermic heats for the org. salts in general to endothermic heats for the inorg. salts in the order of [CH2OCO2Li]2 < LiOCO2CH3 < LiOH < LiOCO2C2H5 < LiOCH3 < LiF < [LiCO2]2 < Li2CO3 < Li2O where the value of the heat went from more neg. in the left to more pos. in the right. In ethylene carbonate, the order was more or less the same, but the salts were found to dissolve more than in DMC in general. The anal. from simulations was performed to rationalize the soly. of each salt in DMC and also the soly. difference between in DMC and ethylene carbonate. The latter was found to be due not only to the difference in polarity between the two solvents, but we also suspect that it may be due to the mol. shapes of the solvents. We also found that the conformation of [CH2OCO2Li]2 changed in going from DMC to ethylene carbonate, which contributed to the difference in the soly.
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    Lundström, R.; Berg, E. J. Design and Validation of an Online Partial and Total Pressure Measurement System for Li-Ion Cells. J. Power Sources 2021, 485, 229347  DOI: 10.1016/j.jpowsour.2020.229347
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    Bannwarth, C.; Ehlert, S.; Grimme, S. GFN2-XTB - An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions. J. Chem. Theory Comput. 2019, 15, 1652– 1671,  DOI: 10.1021/acs.jctc.8b01176
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    GFN2-xTB-An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions
    Bannwarth, Christoph; Ehlert, Sebastian; Grimme, Stefan
    Journal of Chemical Theory and Computation (2019), 15 (3), 1652-1671CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    An extended semiempirical tight-binding model is presented, which is primarily designed for the fast calcn. of structures and noncovalent interactions energies for mol. systems with roughly 1000 atoms. The essential novelty in this so-called GFN2-xTB method is the inclusion of anisotropic second order d. fluctuation effects via short-range damped interactions of cumulative at. multipole moments. Without noticeable increase in the computational demands, this results in a less empirical and overall more phys. sound method, which does not require any classical halogen or hydrogen bonding corrections and which relies solely on global and element-specific parameters (available up to radon, Z = 86). Moreover, the at. partial charge dependent D4 London dispersion model is incorporated self-consistently, which can be naturally obtained in a tight-binding picture from second order d. fluctuations. Fully anal. and numerically precise gradients (nuclear forces) are implemented. The accuracy of the method is benchmarked for a wide variety of systems and compared with other semiempirical methods. Along with excellent performance for the "target" properties, we also find lower errors for "off-target" properties such as barrier heights and mol. dipole moments. High computational efficiency along with the improved physics compared to it precursor GFN-xTB makes this method well-suited to explore the conformational space of mol. systems. Significant improvements are furthermore obsd. for various benchmark sets, which are prototypical for biomol. systems in aq. soln.
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    Density-functional thermochemistry. III. The role of exact exchange
    Becke, Axel D.
    Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
    Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
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    Lee, C.; Yang, W.; Parr, R. G. Development of the Colic-Salvetti Correlation-Energy into a Functional of the Electron Density Formula. Phys. Rev. B 1988, 37, 785– 789,  DOI: 10.1103/physrevb.37.785
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    Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density
    Lee, Chengteh; Yang, Weitao; Parr, Robert G.
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    A correlation-energy formula due to R. Colle and D. Salvetti (1975), in which the correlation energy d. is expressed in terms of the electron d. and a Laplacian of the 2nd-order Hartree-Fock d. matrix, is restated as a formula involving the d. and local kinetic-energy d. On insertion of gradient expansions for the local kinetic-energy d., d.-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calcns. on a no. of atoms, pos. ions, and mols., of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
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    Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: A Critical Analysis. Can. J. Phys. 1980, 58, 1200– 1211,  DOI: 10.1139/p80-159
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    Various approx. forms for the correlation energy per particle of the spin-polarized homogeneous electron gas that have frequently been used in applications of the local spin d. approxn. to the exchange-correlation energy functional are assessed. By accurately recalcg. the RPA correlation energy as a function of electron d. and spin polarization, the inadequacies of the usual approxn. for interpolating between the para- and ferro-magnetic states are demonstrated and an accurate new interpolation formula is presented. A Pade approximant technique was used to accurately interpolate the recent Monte Carlo results. These results can be combined with the RPA spin-dependence so as to produce a correlation energy for a spin-polarized homogeneous electron gas with an estd. max. error of 1 mRy and thus should reliably det. the magnitude of non-local corrections to the local spin d. approxn. in real systems.
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    Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. A 1994, 98, 11623– 11627,  DOI: 10.1021/j100096a001
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    Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields
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    Journal of Physical Chemistry (1994), 98 (45), 11623-7CODEN: JPCHAX; ISSN:0022-3654.
    The unpolarized absorption and CD spectra of the fundamental vibrational transitions of the chiral mol. 4-methyl-2-oxetanone are calcd. ab initio. Harmonic force fields are obtained using d. functional theory (DFT), MP2 and SCF methodologies, and a [5s4p2d/3s2p] (TZ2P) basis set. DFT calcns. use the LSDA, BLYP, and Becke3LYP (B3LYP) d. functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with expt. The MP2 force field yields spectra in slightly worse agreement with expt. than the B3LYP force field. The SCF force field yields spectra in poor agreement with expt. The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreement with expt.
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    Aurbach, D. Nonaqueous Electrochemistry; Marcel Dekker, Inc., 1999.
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    Kitz, P. G.; Lacey, M. J.; Novák, P.; Berg, E. J. Operando Investigation of the Solid Electrolyte Interphase Mechanical and Transport Properties Formed from Vinylene Carbonate and Fluoroethylene Carbonate. J. Power Sources 2020, 477, 228567  DOI: 10.1016/j.jpowsour.2020.228567
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    Operando investigation of the solid electrolyte interphase mechanical and transport properties formed from vinylene carbonate and fluoroethylene carbonate
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  • Abstract

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    Figure 1

    Figure 1. 1H liquid NMR spectra of (a) EC/DEC, orange: pure TMSPa, (b) TMSPa in EC/DEC, pink: TMSOH in EC/DEC, (c) LiOH mixed in EC/DEC, (d) LiOH mixed in EC/DEC with TMSPa, (e) LiOCH3 mixed in EC/DEC, olive green: pure TMSOCH3, (f) LiOCH3 mixed in EC/DEC with TMSPa, and (g) LiF mixed in EC/DEC, blue: LiF mixed in EC/DEC with TMSPa.

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    Figure 2

    Figure 2. 31P liquid NMR spectra showing the formation of the intermediates upon the reaction of LiOH and TMSPa in the presence of solvents. The broadening of the 31P spectral lines is associated with scalar hydrogen coupling. (36)

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    Figure 3

    Figure 3. Lowest energy of reaction vs reaction coordinate found in GFN2-xTB screening for the TMSPa/H2O, TMSPa/LiOH, and TMSPa/OCH3 reactions. For the TMSPa/H2O reaction, only the first step is shown. In the modeling, we explicitly account for the intermolecular interactions between reactants and reaction products. In the case of H2O, an explicit EC molecule is added to ensure that all hydrogen bonds are accounted for (cf. Supporting Information).

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    Figure 4

    Figure 4. (a) CO2 gas evolution profile of blank solution and Lewis bases with solvents with and without TMSPa. Gray area represents the region of background baseline. (b) TMS-LB evolution profiles, as recorded from the mass spectrometer.

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  • References

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      Shi, Feifei; Zhao, Hui; Liu, Gao; Ross, Philip N.; Somorjai, Gabor A.; Komvopoulos, Kyriakos
      Journal of Physical Chemistry C (2014), 118 (27), 14732-14738CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The formation of passive films on electrodes due to electrolyte decompn. significantly affects the reversibility of Li-ion batteries (LIBs); however, understanding of the electrolyte decompn. process is still lacking. The decompn. products of ethylene carbonate (EC)-based electrolytes on Sn and Ni electrodes were studied in this study by FTIR spectroscopy. The ref. compds., di-Et 2,5-dioxahexane dicarboxylate (DEDOHC) and polyethylene carbonate (poly-EC), were synthesized, and their chem. structures were characterized by FTIR spectroscopy and NMR. Assignment of the vibration frequencies of these compds. was assisted by quantum chem. (Hartree-Fock) calcns. The effect of Li-ion solvation on the FTIR spectra was studied by introducing the synthesized ref. compds. into the electrolyte. EC decompn. products formed on Sn and Ni electrodes were identified as DEDOHC and poly-EC by matching the features of surface species formed on the electrodes with ref. spectra. The results of this study demonstrate the importance of accounting for the solvation effect in FTIR anal. of the decompn. products forming on LIB electrodes.
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    4. 4
      Heiskanen, S. K.; Kim, J.; Lucht, B. L. Generation and Evolution of the Solid Electrolyte Interphase of Lithium-Ion Batteries. Joule 2019, 3, 2322– 2333,  DOI: 10.1016/j.joule.2019.08.018
      4
      Generation and Evolution of the Solid Electrolyte Interphase of Lithium-Ion Batteries
      Heiskanen, Satu Kristiina; Kim, Jongjung; Lucht, Brett L.
      Joule (2019), 3 (10), 2322-2333CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      A review. A solid electrolyte interphase (SEI) is generated on the anode of lithium-ion batteries during the first few charging cycles. The SEI provides a passivation layer on the anode surface, which inhibits further electrolyte decompn. and affords the long calendar life required for many applications. However, the SEI remains poorly understood. Recent investigations of the structure of the initial SEI, along with changes which occur to the SEI upon aging, have been conducted. The investigations provide significant new insight into the structure and evolution of the anode SEI. The initial redn. products of ethylene carbonate (EC) are lithium ethylene dicarbonate (LEDC) and ethylene. However, the instability of LEDC generates an intricate mixt. of compds., which greatly complicates the compn. of the SEI. Mechanisms for the generation of the complicated mixt. of products are presented along with the differences in the SEI structure in the presence of electrolyte additives.
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    5. 5
      Kitz, P. G.; Novák, P.; Berg, E. J. Influence of Water Contamination on the SEI Formation in Li-Ion Cells: An Operando EQCM-D Study. ACS Appl. Mater. Interfaces 2020, 12, 15934– 15942,  DOI: 10.1021/acsami.0c01642
      5
      Influence of Water Contamination on the SEI Formation in Li-Ion Cells: An Operando EQCM-D Study
      Kitz, Paul G.; Novak, Petr; Berg, Erik J.
      ACS Applied Materials & Interfaces (2020), 12 (13), 15934-15942CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The interphase formation on carbon (C) anodes in LiPF6/EC + DEC Li-ion battery electrolyte is analyzed by combining operando electrochem. quartz crystal microbalance with dissipation monitoring (EQCM-D) with in situ online electrochem. mass spectrometry (OEMS). EQCM-D enables unique insights into the anode solid electrolyte interphase (SEI) mass/thickness, its viscoelastic properties, and changes of electrolyte viscosity during the initial formation cycles. The interphase in pure electrolyte is relatively soft (G'SEI ≈ 0.2 MPa, ηSEI ≈ 10 mPa s) and changes its viscoelastic properties dynamically as a function of electrode potential. With increasing electrolyte water content, the SEI becomes thicker and much more rigid. Doubly labeled D218O is added to the electrolyte in order to precisely track the reaction pathway of water at the anode by OEMS. In the first cycle between 2.6 - 1.7 V vs. Li+/Li water is reduced and hydroxide ions initiate an autocatalytic hydrolysis of EC. With large amts. of water initially present in the electrolyte, most of the formed CO2 gas is scavenged by reactions with hydroxide and alkoxide ions forming a thick, rigid, and Li2CO3 rich early interphase on the C anode. This layer alleviates the following electrolyte decompn. processes and slows the redn. of EC < 1 V vs. Li+/Li.
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    6. 6
      Tarnopolsky, V. Electrolyte for rechargeable lithium battery, and rechargeable lithium battery including same. U.S. Patent US7494746B22009.
      There is no corresponding record for this reference.
    7. 7
      Mai, S.; Xu, M.; Liao, X.; Hu, J.; Lin, H.; Xing, L.; Liao, Y.; Li, X.; Li, W. Tris(Trimethylsilyl)Phosphite as Electrolyte Additive for High Voltage Layered Lithium Nickel Cobalt Manganese Oxide Cathode of Lithium Ion Battery. Electrochim. Acta 2014, 147, 565– 571,  DOI: 10.1016/j.electacta.2014.09.157
      7
      Tris(trimethylsilyl) phosphite as electrolyte additive for high voltage layered lithium nickel cobalt manganese oxide cathode of lithium ion battery
      Mai, Shaowei; Xu, Mengqing; Liao, Xiaolin; Hu, Jiana; Lin, Haibin; Xing, Lidan; Liao, Youhao; Li, Xiaoping; Li, Weishan
      Electrochimica Acta (2014), 147 (), 565-571CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Tris(trimethylsilyl) phosphite (TMSPi) is reported as an effective electrolyte additive for high-voltage layered lithium nickel cobalt manganese oxide (LiNi1/3Co1/3Mn1/3O2) cathode of lithium-ion battery. Charge/discharge tests demonstrate that the cyclic stability and rate capability of LiNi1/3Co1/3Mn1/3O2 can be improved significantly by adding 0.5 wt.% TMSPi into a std. electrolyte, 1.0M LiPF6 in ethylene carbonate/dimethyl carbonate (1:2 vol. ratio). The capacity retention of LiNi1/3Co1/3Mn1/3O2 is improved from 75.2% to 91.2% after 100 cycles at 0.5C rate (1C = 160 mA/g), while its discharge capacity at 5C is enhanced from 122.4 mA-h/g to 134.4 mA-h/g. This improvement can be ascribed to the suppression of electrolyte decompn. and transition metal ion dissoln. by TMSPi, due to the preferential oxidn. of TMSPi to electrolyte and the formation of a protective solid electrolyte interphase on LiNi1/3Co1/3Mn1/3O2, which are confirmed by electrochem. measurements and surface characterizations.
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    8. 8
      Song, Y. M.; Han, J. G.; Park, S.; Lee, K. T.; Choi, N. S. A Multifunctional Phosphite-Containing Electrolyte for 5 V-Class LiNi 0.5Mn1.5O4 Cathodes with Superior Electrochemical Performance. J. Mater. Chem. A 2014, 2, 9506– 9513,  DOI: 10.1039/c4ta01129e
      8
      A multifunctional phosphite-containing electrolyte for 5 V-class LiNi0.5Mn1.5O4 cathodes with superior electrochemical performance
      Song, Young-Min; Han, Jung-Gu; Park, Soojin; Lee, Kyu Tae; Choi, Nam-Soon
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (25), 9506-9513CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      We report a highly promising organophosphorus compd. with an org. substituent, tris(trimethylsilyl)phosphite (TMSP), to improve the electrochem. performance of 5 V-class LiNi0.5Mn1.5O4 cathode materials. Our investigation reveals that TMSP alleviates the decompn. of LiPF6 by hydrolysis, effectively eliminates HF promoting Mn/Ni dissoln. from the cathode, and forms a protective layer on the cathode surface against severe electrolyte decompn. at high voltages. Remarkable improvements in the cycling stability and rate capability of high voltage cathodes were achieved in the TMSP-contg. electrolyte. After 100 cycles at 60 °C, the discharge capacity retention was 73% in the baseline electrolyte, whereas the TMSP-added electrolyte maintained 90% of its initial discharge capacity. In addn., the LiNi0.5Mn1.5O4 cathode with TMSP delivers a superior discharge capacity of 105 mA h g-1 at a high rate of 3 C and an excellent capacity retention of 81% with a high coulombic efficiency of over 99.6% is exhibited for a graphite/LiNi0.5Mn1.5O4 full cell after 100 cycles at 30 °C.
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    9. 9
      Wang, D. Y.; Dahn, J. R. A High Precision Study of Electrolyte Additive Combinations Containing Vinylene Carbonate, Ethylene Sulfate, Tris(Trimethylsilyl) Phosphate and Tris(Trimethylsilyl) Phosphite in Li[Ni 1/3 Mn 1/3 Co 1/3]O 2 /Graphite Pouch Cells. J. Electrochem. Soc. 2014, 161, A1890– A1897,  DOI: 10.1149/2.0841412jes
      9
      A high precision study of electrolyte additive combinations containing vinylene carbonate, ethylene sulfate, tris(trimethylsilyl) phosphate and tris(trimethylsilyl) phosphite in Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells
      Wang, David Yaohui; Dahn, J. R.
      Journal of the Electrochemical Society (2014), 161 (12), A1890-A1897, 8 pp.CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The effects of electrolyte additive combinations contg. vinylene carbonate (VC), ethylene sulfate (DTD), tris(trimethylsilyl) phosphate (TTSP) and tris(trimethylsilyl) phosphite (TTSPi) in Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells have been studied using the ultra high precision charger (UHPC) at Dalhousie University, electrochem. impedance spectroscopy (EIS), gas evolution measurements and long term cycling. Combinations of electrolyte additives that act synergistically can be more effective than a single electrolyte additive (2%VC). These additive mixts.: reduce parasitic reactions at the pos. electrode >4.1 V compared to VC alone; improve coulombic efficiency, reduce charge end-point capacity slippage and also reduce impedance of the cells. A particularly useful additive combination is 2% VC + 1% DTD + 0.5% TTSP + 0.5% TTSPi.
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    10. 10
      Peebles, C.; Sahore, R.; Gilbert, J. A.; Garcia, J. C.; Tornheim, A.; Bareño, J.; Iddir, H.; Liao, C.; Abraham, D. P. Tris(Trimethylsilyl) Phosphite (TMSPi) and Triethyl Phosphite (TEPi) as Electrolyte Additives for Lithium Ion Batteries: Mechanistic Insights into Differences during LiNi 0.5 Mn 0.3 Co 0.2 O 2 -Graphite Full Cell Cycling. J. Electrochem. Soc. 2017, 164, A1579– A1586,  DOI: 10.1149/2.1101707jes
      10
      Tris(trimethylsilyl) Phosphite (TMSPi) and Triethyl Phosphite (TEPi) as Electrolyte Additives for Lithium Ion Batteries: Mechanistic Insights into Differences during LiNi0.5Mn0.3Co0.2O2-Graphite Full Cell Cycling
      Peebles, Cameron; Sahore, Ritu; Gilbert, James A.; Garcia, Juan C.; Tornheim, Adam; Bareno, Javier; Iddir, Hakim; Liao, Chen; Abraham, Daniel P.
      Journal of the Electrochemical Society (2017), 164 (7), A1579-A1586CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Tris(trimethylsilyl) phosphite (TMSPi) has emerged as an useful electrolyte additive for Li ion cells. This work examines the use of TMSPi and a structurally analogous compd., tri-Et phosphite (TEPi), in LiNi0.5Mn0.3Co0.2O2-graphite full cells, contg. a (baseline) electrolyte with 1.2 M LiPF6 in EC:EMC (3:7 wt./wt.) and operating between 3.0-4.4 V. Galvanostatic cycling data reveal a measurable difference in capacity fade between the TMSPi and TEPi cells. Furthermore, lower impedance rise is obsd. for the TMSPi cells, because of the formation of a P- and O-rich surface film on the pos. electrode that was revealed by XPS data. Elemental anal. on neg. electrodes harvested from cycled cells show lower contents of transition metal (TM) elements for the TMSPi cells than for the baseline and TEPi cells. Our findings indicate that removal of TMS groups from the central P-O core of the TMSPi additive enables formation of the oxide surface film. This film is able to block the generation of reactive TM-O radical species, suppress H abstraction from the electrolyte solvent, and minimize oxidn. reactions at the pos. electrode-electrolyte interface. In contrast, oxidn. of TEPi does not yield a protective pos. electrode film, which results in inferior electrochem. performance.
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    11. 11
      Bolli, C.; Guéguen, A.; Mendez, M. A.; Berg, E. J. Operando Monitoring of F - Formation in Lithium Ion Batteries. Chem. Mater. 2019, 31, 1258– 1267,  DOI: 10.1021/acs.chemmater.8b03810
      11
      Operando Monitoring of F- Formation in Lithium Ion Batteries
      Bolli, Christoph; Gueguen, Aurelie; Mendez, Manuel A.; Berg, Erik J.
      Chemistry of Materials (2019), 31 (4), 1258-1267CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Online electrochem. mass spectrometry (OEMS) was applied to study the influence of tris(trimethylsilyl)phosphate (TMSPa) as an additive in 1 M LiPF6 (fluoroethylene carbonate/diethylene carbonate (DEC)) electrolyte on the gas evolution in Li-rich/NCM full cells during cycling. The results indicate that TMSPa neither influences the solid electrolyte interphase (SEI) formation on the anode nor the surface reconstruction on the cathode but acts as a chem. scavenger for HF and LiF. TMSPa thus lowers the electrolyte acidity and suppresses further LiPF6 decompn., resulting in lower impedance and higher lithium ion battery (LIB) performance. Furthermore, the selective reactivity of TMSPa toward fluorides leads to the formation of Me3SiF enabling the additive to act as a chem. probe and to study HF/LiF formation operando by OEMS. By this methodol., we were able to identify contributions from SEI formation, proton and reactive oxygen formation >4.2 V, cross-talk between the anode and cathode, and the polyvinylidene fluoride binder to the fluoride formation in LIBs.
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    12. 12
      Qi, X.; Tao, L.; Hahn, H.; Schultz, C.; Gallus, D. R.; Cao, X.; Nowak, S.; Röser, S.; Li, J.; Cekic-Laskovic, I.; Rad, B. R.; Winter, M. Lifetime Limit of Tris(Trimethylsilyl) Phosphite as Electrolyte Additive for High Voltage Lithium Ion Batteries. RSC Adv. 2016, 6, 38342– 38349,  DOI: 10.1039/c6ra06555d
      12
      Lifetime limit of tris(trimethylsilyl) phosphite as electrolyte additive for high voltage lithium ion batteries
      Qi, Xin; Tao, Liang; Hahn, Hendrik; Schultz, Carola; Gallus, Dennis Roman; Cao, Xia; Nowak, Sascha; Roser, Stephan; Li, Jie; Cekic-Laskovic, Isidora; Rad, Babak Rezaei; Winter, Martin
      RSC Advances (2016), 6 (44), 38342-38349CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The tris(trimethylsilyl) phosphite (TMSPi) is considered as an ideal electrolyte additive for lithium ion batteries. In this work, its pos. effect as well as its failure mechanism in a LiPF6 contg. electrolyte was studied by means of selected electrochem., structural and anal. techniques. The LiNi0.5Co0.2Mn0.3O2/graphite cells with TMSPi as electrolyte additive were cycled between 2.8 and 4.6 V. Thanks to the compact cathode electrolyte interphase formed by the oxidative decompn. of TMSPi in a freshly prepd. TMSPi contg. electrolyte, both the discharge capacity and the cycling stability of cells were enhanced. However, our results also show that TMSPi actually reacts with LiPF6 at room temp. TMSPi is consumed by this spontaneous reaction after aging for certain time. In addn., a part of the fluorophosphates, generated from the hydrolysis of LiPF6, is bonded to one or two TMS groups, causing a decrease in the fluorophosphate content in the CEI film. Consequently, the cycling stability of the lithium ion cells with aged TMSPi contg. electrolyte deteriorates. The obtained results offer important insights into the practical application of TMSPi, which means that TMSPi can only be used as an effective additive in a freshly prepd. LiPF6 contg. electrolyte.
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    13. 13
      Sinha, N. N.; Burns, J. C.; Dahn, J. R. Comparative Study of Tris(Trimethylsilyl) Phosphate and Tris(Trimethylsilyl) Phosphite as Electrolyte Additives for Li-Ion Cells. J. Electrochem. Soc. 2014, 161, A1084– A1089,  DOI: 10.1149/2.087406jes
      13
      Comparative Study of Tris(trimethylsilyl) Phosphate and Tris(trimethylsilyl) Phosphite as Electrolyte Additives for Li-Ion Cells
      Sinha, Nupur Nikkan; Burns, J. C.; Dahn, J. R.
      Journal of the Electrochemical Society (2014), 161 (6), A1084-A1089CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The effects of tris(trimethlysilyl) phosphate (TTSP) and tris(trimethlysilyl) phosphite electrolyte (TTSPi) additives with and without vinylene carbonate (VC) in Li[Ni1/3Mn1/3Co1/3]/graphite (NMC/graphite) pouch cells were studied using the ultra high precision charger at Dalhousie University and electrochem. impedance spectroscopy. The coulombic efficiency of NMC/graphite cells was improved when TTSPi was added to the electrolyte. TTSP and TTSPi both reduce the cell impedance significantly. The addn. of VC to NMC/graphite cells contg. TTSP or TTSPi improves the coulombic efficiency and decreases the charge end point capacity slippage. However, when VC was added the impedance of cells contg. either TTSP or TTSPi increased. In the presence of VC, the TTSPi additive decreased the rate of parasitic reactions, reduced the charge end point capacity slippage and also the reduced impedance of NMC/graphite cells compared to cells with TTSP and VC. These results demonstrate that TTSPi is a better additive than TTSP when added to control electrolyte and to electrolyte contg. VC for NMC/graphite cells.
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    14. 14
      Guéguen, A.; Bolli, C.; Mendez, M. A.; Berg, E. J. Elucidating the Reactivity of Tris(Trimethylsilyl)Phosphite and Tris(Trimethylsilyl)Phosphate Additives in Carbonate Electrolytes - A Comparative Online Electrochemical Mass Spectrometry Study. ACS Appl. Energy Mater. 2020, 3, 290– 299,  DOI: 10.1021/acsaem.9b01551
      14
      Elucidating the Reactivity of Tris(trimethylsilyl)phosphite and Tris(trimethylsilyl)phosphate Additives in Carbonate Electrolytes-A Comparative Online Electrochemical Mass Spectrometry Study
      Gueguen, Aurelie; Bolli, Christoph; Mendez, Manuel A.; Berg, Erik J.
      ACS Applied Energy Materials (2020), 3 (1), 290-299CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      The electrolyte additives tris(trimethylsilyl)phosphite (TMPSi) and tris(trimethylsilyl)phosphate (TMSPa) have shown their potential to improve lifetime and decrease impedance build-up in lithium ion batteries (LIBs) in several studies. Online electrochem. mass spectrometry (OEMS) was applied to study the effect of TMSPi and TMSPa on gas releasing parasitic reaction during cycling of HE-NCM/graphite cells in different electrolytes to elucidate their working mechanisms, for which so far contradictive claims can be found in literature. The results underline the chem. instability of the two additives in soln. and show that they have, amongst for one exception, little or no effect on the release of O2 and CO2 which would be expected in case of a passivating surface layer suppressing electrolyte decompn. The decreased POF3 evolution and the formation of Me3SiF indicates that the main role of both additives is that of a chem. scavenger, e.g. for HF, which lead to decreased acidification of the electrolyte and thus longer lifetime of LIBs contg. these electrolytes.
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    15. 15
      Kim, J.; Adiraju, V. A. K.; Rodrigo, N.; Hoffmann, J.; Payne, M.; Lucht, B. L. Lithium Bis(Trimethylsilyl) Phosphate as a Novel Bifunctional Additive for High-Voltage LiNi1.5Mn0.5O4/Graphite Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2021, 13, 22351– 22360,  DOI: 10.1021/acsami.1c02572
      15
      Lithium Bis(trimethylsilyl) Phosphate as a Novel Bifunctional Additive for High-Voltage LiNi1.5Mn0.5O4/Graphite Lithium-Ion Batteries
      Kim, Jongjung; Adiraju, Venkata A. K.; Rodrigo, Nuwanthi; Hoffmann, Jennifer; Payne, Martin; Lucht, Brett L.
      ACS Applied Materials & Interfaces (2021), 13 (19), 22351-22360CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The beneficial role of lithium bis(trimethylsilyl) phosphate (LiTMSP), which may act as a novel bifunctional additive for high-voltage LiNi1.5Mn0.5O4 (LNMO)/graphite cells, has been investigated. LiTMSP is synthesized by heating tris(trimethylsilyl) phosphate with lithium tert-butoxide. The cycle performance of LNMO/graphite cells at 45°C significantly improved upon incorporation of LiTMSP (0.5 wt %). NMR anal. suggests that the trimethylsilyl (TMS) group in LiTMSP can react with hydrogen fluoride (HF), which is generated through the hydrolysis of lithium hexafluorophosphate (LiPF6) by residual water in an electrolyte soln. or water generated via oxidative electrolyte decompn. reactions to form TMS fluoride. Inhibition of HF leads to a decrease in the concn. of transition-metal ion-dissoln. (Ni and Mn) from the LNMO electrode, as detd. by inductively coupled plasma mass spectrometry. In addn., the generation of the superior passivating surface film derived by LiTMSP on the graphite electrode, suppressing further electrolyte reductive decompn. as well as deterioration/reformation caused by migrated transition metal ions, is supported by a combination of chronoamperometry, XPS, and field-emission SEM. Furthermore, a LiTMSP-derived surface film has better lithium ion cond. with a decrease in resistance of the graphite electrode, as confirmed by electrochem. impedance spectroscopy, leading to improvement in the rate performance of LNMO/graphite cells. The HF-scavenging and film-forming effects of LiTMPS are responsible for the less polarization of LNMO/graphite cells enabling improved cycle performance at 45°C.
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    16. 16
      Kim, D. Y.; Park, H.; Choi, W. I.; Roy, B.; Seo, J.; Park, I.; Kim, J. H.; Park, J. H.; Kang, Y. S.; Koh, M. Ab Initio Study of the Operating Mechanisms of Tris(Trimethylsilyl) Phosphite as a Multifunctional Additive for Li-Ion Batteries. J. Power Sources 2017, 355, 154– 163,  DOI: 10.1016/j.jpowsour.2017.04.062
      16
      Ab initio study of the operating mechanisms of tris(trimethylsilyl) phosphite as a multifunctional additive for Li-ion batteries
      Kim, Dong Young; Park, Hosang; Choi, Woon Ih; Roy, Basab; Seo, Jinah; Park, Insun; Kim, Jin Hae; Park, Jong Hwan; Kang, Yoon-Sok; Koh, Meiten
      Journal of Power Sources (2017), 355 (), 154-163CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Tris(trimethylsilyl) phosphite (P(OSi(CH3)3)3) is a multifunctional electrolyte additive for scavenging HF and forming a cathode electrolyte interphase (CEI). Systematic anal. of the HF reaction pathways and redox potentials of P(OSi(CH3)3)3, OP(OSi(CH3)3)3, P(OSiF3)3, and OP(OSiF3)3, and their reaction products, using ab initio calcns. allowed us to elucidate the operating mechanism of P(OSi(CH3)3)3 and verify the rules that det. its HF reaction pathways and electrochem. stability. While O-Si cleavage is the predominant HF scavenging pathway for P(OSi(CH3)3)3, O-P cleavage is stabilized by replacing CH3 with an electron-withdrawing group. Thus, P(OSiF3)3 scavenges HF mainly through O-P cleavage to produce PF3, which has high oxidn. stability. However, the O-Si cleavage pathway produces P(OSi(CH3)3)2OH, P(OSi(CH3)3) (OH)2, and P(OH)3 sequentially, along with Si(CH3)3F. These PO3 systems, which are oxidized earlier than carbonate solns. and form tightly bonded units following oxidn., act as seed units for compact CEI growth. Moreover, the HF scavenging ability of PO3 systems is maintained during oxidn. until all O-Si bonds are broken. As a strategy for developing additives with enhanced functionality, modifying P(OSi(CH3)3)3 by replacing CH3 with an electron-donating group to exclusively utilize the O-Si cleavage pathway for HF scavenging is recommended.
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    17. 17
      Mai, S.; Xu, M.; Liao, X.; Xing, L.; Li, W. Improving Cyclic Stability of Lithium Nickel Manganese Oxide Cathode at Elevated Temperature by Using Dimethyl Phenylphosphonite as Electrolyte Additive. J. Power Sources 2015, 273, 816– 822,  DOI: 10.1016/j.jpowsour.2014.09.171
      17
      Improving cyclic stability of lithium nickel manganese oxide cathode at elevated temperature by using dimethyl phenylphosphonite as electrolyte additive
      Mai, Shaowei; Xu, Mengqing; Liao, Xiaolin; Xing, Lidan; Li, Weishan
      Journal of Power Sources (2015), 273 (), 816-822CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      A novel electrolyte additive, di-Me phenylphosphonite (DMPP), is reported in this paper to be able to improve significantly the cyclic stability of LiNi0.5Mn1.5O4 cathode of high-voltage lithium-ion battery at elevated temp. When subjecting the cathode to charge/discharge cycling at 50° at 1C (1C = 146.7 mA-h/g) rate in a std. electrolyte (1.0M LiPF6 in ethylene carbonate/dimethyl carbonate of 1:2 vol. ratio), LiNi0.5Mn1.5O4 suffers serious discharge capacity decay, with a capacity retention of 42% after 100 cycles. By adding 0.5% DMPP into the std. electrolyte, the capacity retention is increased to 91%. This improvement can be ascribed to the preferential oxidn. of DMPP to the std. electrolyte and the subsequent formation of a protective film on LiNi0.5Mn1.5O4, which suppresses the electrolyte decompn. and protects LiNi0.5Mn1.5O4 from destruction. Theor. calcns. together with voltammetric analyses demonstrate the preferential oxidn. of DMPP and the consequent suppression of electrolyte decompn., while the observations from SEM, XPS and FTIR spectroscopy confirm the protection that DMPP provides for LiNi0.5Mn1.5O4.
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    18. 18
      Metzger, M.; Strehle, B.; Solchenbach, S.; Gasteiger, H. A. Hydrolysis of Ethylene Carbonate with Water and Hydroxide under Battery Operating Conditions. J. Electrochem. Soc. 2016, 163, A1219– A1225,  DOI: 10.1149/2.0411607jes
      18
      Hydrolysis of Ethylene Carbonate with Water and Hydroxide under Battery Operating Conditions
      Metzger, Michael; Strehle, Benjamin; Solchenbach, Sophie; Gasteiger, Hubert A.
      Journal of the Electrochemical Society (2016), 163 (7), A1219-A1225CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      This study deals with the decompn. of ethylene carbonate (EC) by H2O in the absence and presence of catalytically active hydroxide ions (OH-) at reaction conditions close to lithium-ion battery operation. We use Online Electrochem. Mass Spectrometry (OEMS) to quantify the CO2 evolved by these reactions, referred to as H2O-driven and OH--driven EC hydrolysis. By examg. both reactions at various temps. (10 - 80°C) and water concns. (<20 ppm or 200, 1000, and 5000 ppm H2O) with or without catalytically active OH- ions in EC with 1.5 M LiClO4, we det. an Arrhenius relationship between the CO2 evolution rate and the cell temp. While the apparent activation energy for the base electrolyte (<20 ppm H2O) is very large (app. Ea ≈153 kJ/mol), substantially lower values are obtained in the presence of H2O (app. Ea ≈99 ± 3 kJ/mol), which are even further decreased in the presence of catalytically active OH- (app. Ea ≈43 ± 5 kJ/mol). Our data show that OH--driven EC hydrolysis is relevant already at room temp., whereas H2O-driven EC hydrolysis (i.e., without catalytically active OH-) is only relevant at elevated temp. (≥40°C), as is the case for the base electrolyte. Thus, catalytic quantities of OH-, e.g., from hydroxide contaminants on the surface of transition metal oxide based active materials, would be expected to lead to considerable CO2 gassing in lithium-ion cells.
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    19. 19
      Peng, Z.; Merz, K. M., Jr. Theoretical Investigation of the CO2 + OH- ─ > HCO3- Reaction in the Gas and Aqueous Phases. 1993, 115, 9640 9647.  DOI: 10.1021/ja00074a032 .
      There is no corresponding record for this reference.
    20. 20
      Zhuang, G. V.; Yang, H.; Ross, P. N.; Xu, K.; Jow, T. R. Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate. Electrochem. Solid-State Lett. 2006, 9, A64– A68,  DOI: 10.1149/1.2142157
      20
      Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate
      Zhuang, Guorong V.; Yang, Hui; Ross, Philip N., Jr.; Xu, Kang; Jow, T. Richard
      Electrochemical and Solid-State Letters (2006), 9 (2), A64-A68CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      To improve the understanding of passive film formation on metallic Li in org. electrolyte, the authors synthesized and characterized Li Me carbonate (LiOCO2Me), a prototypical component of the film. The structure of this compd. was characterized with NMR and FTIR spectroscopy, and its thermal stability and decompn. pathway was studied by TGA. The FTIR spectrum of the synthesized compd. enables resoln. of multiple products in the passive film on Li in a di-Me carbonate soln. Li Me carbonate is only one of the components, the others being Li oxalate and Li methoxide.
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    21. 21
      McMahon, T. B.; Northcott, C. J. The Fluoroformate Ion, FCO 2 −. An Ion Cyclotron Resonance Study of the Gas Phase Lewis Acidity of Carbon Dioxide and Related Isoelectronic Species. Can. J. Chem. 1978, 56, 1069– 1074,  DOI: 10.1139/v78-181
      21
      The fluoroformate ion, FCO2-. An ion cyclotron resonance study of the gas phase Lewis acidity of carbon dioxide and related isoelectronic species
      McMahon, Terrance Brian; Northcott, Colleen Joan
      Canadian Journal of Chemistry (1978), 56 (8), 1069-74CODEN: CJCHAG; ISSN:0008-4042.
      The gas phase ion mol. reactions of a no. of potential fluoride donors with CO2 and carbonyl fluoride have been studied. The fluoride affinities of CO2 and carbonyl fluoride are 33 ± 3 kcal/mol and 35 ± 3 kcal/mol, resp. In addn., from gas phase acidity studies of acetyl fluoride and 2-fluoropropene, the fluoride affinities of ketene and allene have been calcd. to be 38 ± 2 kcal/mol and 15 ± 2 kcal/mol, resp.
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    22. 22
      Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. Identification of Surface Films Formed on Lithium in Propylene Carbonate Solutions. J. Electrochem. Soc. 1987, 134, 1611– 1620,  DOI: 10.1149/1.2100722
      22
      Identification of surface films formed on lithium in propylene carbonate solutions
      Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E.
      Journal of the Electrochemical Society (1987), 134 (7), 1611-20CODEN: JESOAN; ISSN:0013-4651.
      FTIR, IR, and XPS were used to study the films formed on Li in propylene carbonate solns. of LiClO4, LiAsF6, and LiSO3CF3. Over a range of conditions, the main components detected in the initial surface films were Li alkyl carbonates (RCO3Li, R = alkyl). Another alkyl carbonate solvent, di-Et carbonate, was found to react with Li to form Li Et carbonate. In addn. to solvent redn., XPS measurements gave indication of salt redn. reactions. LiClO4, LiAsF6, and LiSO3CF3 were reduced by Li to form halide ions, which were detected on the Li surface. Two possible mechanisms for the formation of alkyl carbonates are discussed. One is the nucleophilic reaction of propylene carbonate with basic species such as OH-, while the other involves one-electron redn. of propylene carbonate by Li metal, followed by free radical termination reactions. When high concns. of H2O were present, Li2CO3 was formed by further reaction of the alkyl carbonates with H2O. On Li surfaces without a mech. stable surface film, such as those of Li amalgams, the redn. reaction is believed to proceed by an overall 2-electron process, and the primary product is Li carbonate.
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    23. 23
      Zhuang, G. V.; Ross, P. N. Analysis of the Chemical Composition of the Passive Film on Li-Ion Battery Anodes Using Attentuated Total Reflection Infrared Spectroscopy. Electrochem. Solid-State Lett. 2003, 6, A136  DOI: 10.1149/1.1575594
      23
      Analysis of the Chemical Composition of the Passive Film on Li-Ion Battery Anodes Using Attenuated Total Reflection Infrared Spectroscopy
      Zhuang, Guorong V.; Ross, Philip N., Jr.
      Electrochemical and Solid-State Letters (2003), 6 (7), A136-A139CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      FTIR spectroscopy with attenuated total reflection geometry was used to study the surface of graphite anodes obtained from Li-ion batteries. The batteries were of the 18650-type and subjected to calender aging (60% state of charge) at 55°. The compn. of the film on an anode from a control cell (not aged) consisted of Li2C2O4, RCOOLi, and LiOMe. After aging, there was also LiOH and MeOH, and in some cases LiHCO3, probably due to the reaction of H2O with the methoxide and oxalate. There is substantial variation in the relative amts. of the 5 compds. over the surfaces of the electrodes. Alkyl carbonates may form early on, but they decomp. to more inorg. compds. with aging. The multicomponent compn. reflects the complex chem. of passive film formation in real Li-ion cells.
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    24. 24
      Tasaki, K.; Harris, S. J. Computational Study on the Solubility of Lithium Salts Formed on Lithium Ion Battery Negative Electrode in Organic Solvents. J. Phys. Chem. C 2010, 114, 8076– 8083,  DOI: 10.1021/jp100013h
      24
      Computational Study on the Solubility of Lithium Salts Formed on Lithium Ion Battery Negative Electrode in Organic Solvents
      Tasaki, Ken; Harris, Stephen J.
      Journal of Physical Chemistry C (2010), 114 (17), 8076-8083CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The soly. of lithium salts, found in solid-electrolyte interface (SEI) films on the anode surface in lithium-ion batteries, has been examd. in org. solvents by atomistic computer simulations. The salts included lithium oxide (Li2O), lithium carbonate (Li2CO3), lithium oxalate ([LiCO2]2), lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methoxide (LiOCH3), lithium Me carbonate (LiOCO2CH3), lithium Et carbonate (LiOCO2C2H5), and dilithium ethylene glycol dicarbonate ([CH2OCO2Li]2). The org. solvents were di-Me carbonate (DMC) and ethylene carbonate. The at. charges in the force field have been fitted to the electrostatic potential obtained from d. functional theory calcns. for each salt. The heat of dissoln. in DMC for the salts calcd. from computer simulations ranged from exothermic heats for the org. salts in general to endothermic heats for the inorg. salts in the order of [CH2OCO2Li]2 < LiOCO2CH3 < LiOH < LiOCO2C2H5 < LiOCH3 < LiF < [LiCO2]2 < Li2CO3 < Li2O where the value of the heat went from more neg. in the left to more pos. in the right. In ethylene carbonate, the order was more or less the same, but the salts were found to dissolve more than in DMC in general. The anal. from simulations was performed to rationalize the soly. of each salt in DMC and also the soly. difference between in DMC and ethylene carbonate. The latter was found to be due not only to the difference in polarity between the two solvents, but we also suspect that it may be due to the mol. shapes of the solvents. We also found that the conformation of [CH2OCO2Li]2 changed in going from DMC to ethylene carbonate, which contributed to the difference in the soly.
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    25. 25
      Lundström, R.; Berg, E. J. Design and Validation of an Online Partial and Total Pressure Measurement System for Li-Ion Cells. J. Power Sources 2021, 485, 229347  DOI: 10.1016/j.jpowsour.2020.229347
      There is no corresponding record for this reference.
    26. 26
      Bannwarth, C.; Ehlert, S.; Grimme, S. GFN2-XTB - An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions. J. Chem. Theory Comput. 2019, 15, 1652– 1671,  DOI: 10.1021/acs.jctc.8b01176
      26
      GFN2-xTB-An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quantum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions
      Bannwarth, Christoph; Ehlert, Sebastian; Grimme, Stefan
      Journal of Chemical Theory and Computation (2019), 15 (3), 1652-1671CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      An extended semiempirical tight-binding model is presented, which is primarily designed for the fast calcn. of structures and noncovalent interactions energies for mol. systems with roughly 1000 atoms. The essential novelty in this so-called GFN2-xTB method is the inclusion of anisotropic second order d. fluctuation effects via short-range damped interactions of cumulative at. multipole moments. Without noticeable increase in the computational demands, this results in a less empirical and overall more phys. sound method, which does not require any classical halogen or hydrogen bonding corrections and which relies solely on global and element-specific parameters (available up to radon, Z = 86). Moreover, the at. partial charge dependent D4 London dispersion model is incorporated self-consistently, which can be naturally obtained in a tight-binding picture from second order d. fluctuations. Fully anal. and numerically precise gradients (nuclear forces) are implemented. The accuracy of the method is benchmarked for a wide variety of systems and compared with other semiempirical methods. Along with excellent performance for the "target" properties, we also find lower errors for "off-target" properties such as barrier heights and mol. dipole moments. High computational efficiency along with the improved physics compared to it precursor GFN-xTB makes this method well-suited to explore the conformational space of mol. systems. Significant improvements are furthermore obsd. for various benchmark sets, which are prototypical for biomol. systems in aq. soln.
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    27. 27
      Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Had, M.; Fox, D. J. Gaussian 09 Citation;Gaussian, Inc.: Wallingford CT, 2016.
      There is no corresponding record for this reference.
    28. 28
      Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648– 5652,  DOI: 10.1063/1.464913
      28
      Density-functional thermochemistry. III. The role of exact exchange
      Becke, Axel D.
      Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
      Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
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    29. 29
      Lee, C.; Yang, W.; Parr, R. G. Development of the Colic-Salvetti Correlation-Energy into a Functional of the Electron Density Formula. Phys. Rev. B 1988, 37, 785– 789,  DOI: 10.1103/physrevb.37.785
      29
      Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density
      Lee, Chengteh; Yang, Weitao; Parr, Robert G.
      Physical Review B: Condensed Matter and Materials Physics (1988), 37 (2), 785-9CODEN: PRBMDO; ISSN:0163-1829.
      A correlation-energy formula due to R. Colle and D. Salvetti (1975), in which the correlation energy d. is expressed in terms of the electron d. and a Laplacian of the 2nd-order Hartree-Fock d. matrix, is restated as a formula involving the d. and local kinetic-energy d. On insertion of gradient expansions for the local kinetic-energy d., d.-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calcns. on a no. of atoms, pos. ions, and mols., of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
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    30. 30
      Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: A Critical Analysis. Can. J. Phys. 1980, 58, 1200– 1211,  DOI: 10.1139/p80-159
      30
      Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis
      Vosko, S. H.; Wilk, L.; Nusair, M.
      Canadian Journal of Physics (1980), 58 (8), 1200-11CODEN: CJPHAD; ISSN:0008-4204.
      Various approx. forms for the correlation energy per particle of the spin-polarized homogeneous electron gas that have frequently been used in applications of the local spin d. approxn. to the exchange-correlation energy functional are assessed. By accurately recalcg. the RPA correlation energy as a function of electron d. and spin polarization, the inadequacies of the usual approxn. for interpolating between the para- and ferro-magnetic states are demonstrated and an accurate new interpolation formula is presented. A Pade approximant technique was used to accurately interpolate the recent Monte Carlo results. These results can be combined with the RPA spin-dependence so as to produce a correlation energy for a spin-polarized homogeneous electron gas with an estd. max. error of 1 mRy and thus should reliably det. the magnitude of non-local corrections to the local spin d. approxn. in real systems.
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    31. 31
      Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. A 1994, 98, 11623– 11627,  DOI: 10.1021/j100096a001
      31
      Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields
      Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
      Journal of Physical Chemistry (1994), 98 (45), 11623-7CODEN: JPCHAX; ISSN:0022-3654.
      The unpolarized absorption and CD spectra of the fundamental vibrational transitions of the chiral mol. 4-methyl-2-oxetanone are calcd. ab initio. Harmonic force fields are obtained using d. functional theory (DFT), MP2 and SCF methodologies, and a [5s4p2d/3s2p] (TZ2P) basis set. DFT calcns. use the LSDA, BLYP, and Becke3LYP (B3LYP) d. functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with expt. The MP2 force field yields spectra in slightly worse agreement with expt. than the B3LYP force field. The SCF force field yields spectra in poor agreement with expt. The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreement with expt.
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    32. 32
      Aurbach, D. Nonaqueous Electrochemistry; Marcel Dekker, Inc., 1999.
      There is no corresponding record for this reference.
    33. 33
      Kitz, P. G.; Lacey, M. J.; Novák, P.; Berg, E. J. Operando Investigation of the Solid Electrolyte Interphase Mechanical and Transport Properties Formed from Vinylene Carbonate and Fluoroethylene Carbonate. J. Power Sources 2020, 477, 228567  DOI: 10.1016/j.jpowsour.2020.228567
      33
      Operando investigation of the solid electrolyte interphase mechanical and transport properties formed from vinylene carbonate and fluoroethylene carbonate
      Kitz, Paul G.; Lacey, Matthew J.; Novak, Petr; Berg, Erik J.
      Journal of Power Sources (2020), 477 (), 228567CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode. In this study combined simultaneous electrochem. impedance spectroscopy (EIS) and operando electrochem. quartz crystal microbalance with dissipation monitoring (EQCM-D) are employed together with in situ gas anal. (OEMS) to study the influence of VC and FEC on the passivation process and the interphase properties at carbon-based anodes. In small quantities both additives reduce the initial interphase mass loading by 30-50%, but only VC also effectively prevents continuous side reactions and improves anode passivation significantly. VC and FEC are both reduced at potentials above 1 V vs. Li+/Li in the first cycle and change the SEI compn. which causes an increase of the SEI shear storage modulus by over one order of magnitude in both cases. As a consequence, the ion diffusion coeff. and cond. in the interphase is also significantly affected. While small quantities of VC in the initial electrolyte increase the SEI cond., FEC decompn. products hinder charge transport through the SEI and thus increase overall anode impedance significantly.
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    34. 34
      Mozhzhukhina, N.; Flores, E.; Lundström, R.; Nystrom, V.; Kitz, P. G.; Edström, K.; Berg, E. J. Direct Operando Observation of Double Layer Charging and Early SEI Formation in Li-Ion Battery Electrolytes. J. Phys. Chem. Lett. 2020, 11, 4119– 4123,  DOI: 10.1021/acs.jpclett.0c01089
      34
      Direct Operando Observation of Double Layer Charging and Early Solid Electrolyte Interphase Formation in Li-Ion Battery Electrolytes
      Mozhzhukhina, Nataliia; Flores, Eibar; Lundstroem, Robin; Nystroem, Ville; Kitz, Paul G.; Edstroem, Kristina; Berg, Erik J.
      Journal of Physical Chemistry Letters (2020), 11 (10), 4119-4123CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The solid electrolyte interphase (SEI) is the most crit. yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF6 and org. carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, online electrochem. mass spectrometry, and electrochem. quartz crystal microbalance. The elec. double layer is directly obsd. to charge as Li+ solvated by ethylene carbonate (EC) progressively accumulates at the neg. charged electrode surface. Further neg. polarization triggers SEI formation, as evidenced by H2 evolution and electrode mass deposition. Electrolyte impurities, HF and H2O, are reduced early and contribute in a multistep (electro)chem. process to an inorg. SEI layer rich in LiF and Li2CO3. This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.
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    35. 35
      Qin, Y.; Chen, Z.; Lee, H. S.; Yang, X. Q.; Amine, K. Effect of Anion Receptor Additives on Electrochemical Performance of Lithium-Ion Batteries. J. Phys. Chem. C 2010, 114, 15202– 15206,  DOI: 10.1021/jp104341t
      35
      Effect of anion receptor additives on electrochemical performance of lithium-ion batteries
      Qin, Yan; Chen, Zonghai; Lee, H. S.; Yang, X.-Q.; Amine, K.
      Journal of Physical Chemistry C (2010), 114 (35), 15202-15206CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Four boron-based anion receptors were investigated as electrolyte additives for lithium-ion batteries. The electrochem. performance of lithium-ion cells was found to strongly depend on the structure of the anion receptor added to the electrolyte. The capacity retention of the lithium-ion cell was slightly improved by adding 0.07 M bis(1,1,1,3,3,3-hexafluoroisopropyl)pentafluorophenylboronate additive, whereas the addn. of 2,5-bis(trifluoromethylphenyl)tetrafluoro-1,3,2-benzodioxaborole dramatically deteriorated the electrochem. performance. The addn. of a certain type of anion receptor can promote the electrochem. decompn. of the electrolyte, resulting in high interfacial impedance and accelerated capacity fading of lithium-ion cells. Ab initio calcns. showed that the electrochem. performance of anion receptors had good correlation to the degree of localization of the LUMO at the boron center of anion receptors, which can potentially be used in the search for new anion receptors for lithium-ion batteries.
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    36. 36
      Lindon, J. C.; Tranter, G. E.; Holmes, J. L. Encyclopedia of Spectroscopy and Spectrometry; Elsevier, 2000.
      There is no corresponding record for this reference.
    37. 37
      Xu, K.; Zhuang, G. V.; Allen, J. L.; Lee, U.; Zhang, S. S.; Ross, P. N.; Jow, T. R. Syntheses and Characterization of Lithium Alkyl Mono- and Bicarbonates as Components of Surface Films in Li-Ion Batteries. J. Phys. Chem. B 2006, 110, 7708– 7719,  DOI: 10.1021/jp0601522
      37
      Syntheses and Characterization of Lithium Alkyl Mono- and Di-carbonates as Components of Surface Films in Li-Ion Batteries
      Xu, Kang; Zhuang, Guorong V.; Allen, Jan L.; Lee, Unchul; Zhang, Sheng S.; Ross, Philip N., Jr.; Jow, T. Richard
      Journal of Physical Chemistry B (2006), 110 (15), 7708-7719CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      A homologous series of Li alkyl mono- and di-carbonate salts was synthesized as refs. compds. for the proposed components formed at the electrolyte/electrode interface in Li-ion batteries. The physicochem. characterization of these ref. compds. in the bulk state using thermal analyses and XPS, NMR, and FTIR provides a reliable database for comparison in studies of the surface chem. of electrodes from Li-ion batteries.
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    38. 38
      Parimalam, B. S.; MacIntosh, A. D.; Kadam, R.; Lucht, B. L. Decomposition Reactions of Anode Solid Electrolyte Interphase (SEI) Components with LiPF6. J. Phys. Chem. C 2017, 121, 22733– 22738,  DOI: 10.1021/acs.jpcc.7b08433
      38
      Decomposition Reactions of Anode Solid Electrolyte Interphase (SEI) Components with LiPF6
      Parimalam, Bharathy S.; MacIntosh, Alex D.; Kadam, Rahul; Lucht, Brett L.
      Journal of Physical Chemistry C (2017), 121 (41), 22733-22738CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The anode solid electrolyte interface (SEI) on the anode of lithium ion batteries contains lithium carbonate (Li2CO3), lithium Me carbonate (LMC), and lithium ethylene dicarbonate (LEDC). The development of a strong phys. understanding of the properties of the SEI requires a strong understanding of the evolution of the SEI compn. over extended timeframes. The thermal stability of Li2CO3, LMC, and LEDC in the presence of LiPF6 in di-Me carbonate (DMC), a common salt and solvent, resp., in lithium ion battery electrolytes, has been investigated to afford a better understanding of the evolution of the SEI. The residual solids from the reaction mixts. have been characterized by a combination of XPS and IR spectroscopy with attenuated total reflectance (IR-ATR), while the soln. and evolved gases have been investigated by NMR (NMR) spectroscopy and gas chromatog. with mass selective detection (GC-MS). The thermal decompn. of Li2CO3 and LiPF6 in DMC yields CO2, LiF, and F2PO2Li. The thermal decompn. of LMC and LEDC with LiPF6 in DMC results in the generation of a complicated mixt. including CO2, LiF, ethers, phosphates, and fluorophosphates.
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    39. 39
      Zhuang, G. V.; Yang, H.; Ross, P. N.; Xu, K.; Jow, T. R. Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate. Electrochem. Solid-State Lett. 2006, 9, A64– A68,  DOI: 10.1149/1.2142157
      39
      Lithium Methyl Carbonate as a Reaction Product of Metallic Lithium and Dimethyl Carbonate
      Zhuang, Guorong V.; Yang, Hui; Ross, Philip N., Jr.; Xu, Kang; Jow, T. Richard
      Electrochemical and Solid-State Letters (2006), 9 (2), A64-A68CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      To improve the understanding of passive film formation on metallic Li in org. electrolyte, the authors synthesized and characterized Li Me carbonate (LiOCO2Me), a prototypical component of the film. The structure of this compd. was characterized with NMR and FTIR spectroscopy, and its thermal stability and decompn. pathway was studied by TGA. The FTIR spectrum of the synthesized compd. enables resoln. of multiple products in the passive film on Li in a di-Me carbonate soln. Li Me carbonate is only one of the components, the others being Li oxalate and Li methoxide.
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    40. 40
      Bolli, C.; Guéguen, A.; Mendez, M. A.; Berg, E. J. Operando Monitoring of F - Formation in Lithium Ion Batteries. Chem. Mater. 2019, 31, 1258– 1267,  DOI: 10.1021/acs.chemmater.8b03810
      40
      Operando Monitoring of F- Formation in Lithium Ion Batteries
      Bolli, Christoph; Gueguen, Aurelie; Mendez, Manuel A.; Berg, Erik J.
      Chemistry of Materials (2019), 31 (4), 1258-1267CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Online electrochem. mass spectrometry (OEMS) was applied to study the influence of tris(trimethylsilyl)phosphate (TMSPa) as an additive in 1 M LiPF6 (fluoroethylene carbonate/diethylene carbonate (DEC)) electrolyte on the gas evolution in Li-rich/NCM full cells during cycling. The results indicate that TMSPa neither influences the solid electrolyte interphase (SEI) formation on the anode nor the surface reconstruction on the cathode but acts as a chem. scavenger for HF and LiF. TMSPa thus lowers the electrolyte acidity and suppresses further LiPF6 decompn., resulting in lower impedance and higher lithium ion battery (LIB) performance. Furthermore, the selective reactivity of TMSPa toward fluorides leads to the formation of Me3SiF enabling the additive to act as a chem. probe and to study HF/LiF formation operando by OEMS. By this methodol., we were able to identify contributions from SEI formation, proton and reactive oxygen formation >4.2 V, cross-talk between the anode and cathode, and the polyvinylidene fluoride binder to the fluoride formation in LIBs.
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    41. 41
      Myers, E. L.; Butts, C. P.; Aggarwal, V. K. BF3·OEt2 and TMSOTf: A Synergistic Combination of Lewis Acids. Chem. Commun. 2006, 4434– 4436,  DOI: 10.1039/b611333h
      41
      BF3·OEt2 and TMSOTf: A synergistic combination of Lewis acids
      Myers, Eddie L.; Butts, Craig P.; Aggarwal, Varinder K.
      Chemical Communications (Cambridge, United Kingdom) (2006), (42), 4434-4436CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The combination of BF3·OEt2 and TMSOTf gives BF2OTf·OEt2, which is a more powerful Lewis acid than its components and esp. effective in CH3CN solvent; the complex formed was characterized by 1H, 19F, 11B and 31P (using Et3PO as an additive) NMR spectroscopy.
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    42. 42
      Banerjee, A.; Wang, X.; Fang, C.; Wu, E. A.; Meng, Y. S. Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes. Chem. Rev. 2020, 120, 6878– 6933,  DOI: 10.1021/acs.chemrev.0c00101
      42
      Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes
      Banerjee, Abhik; Wang, Xuefeng; Fang, Chengcheng; Wu, Erik A.; Meng, Ying Shirley
      Chemical Reviews (Washington, DC, United States) (2020), 120 (14), 6878-6933CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A Review. All-solid-state batteries (ASSBs) have attracted enormous attention as one of the crit. future technologies for safe and high energy batteries. With the emergence of several highly conductive solid electrolytes in recent years, the bottleneck is no longer Li-ion diffusion within the electrolyte. Instead, many ASSBs are limited by their low Coulombic efficiency, poor power performance, and short cycling life due to the high resistance at the interfaces within ASSBs. Because of the diverse chem./phys./mech. properties of various solid components in ASSBs as well as the nature of solid-solid contact, many types of interfaces are present in ASSBs. These include loose phys. contact, grain boundaries, and chem. and electrochem. reactions to name a few. All of these contribute to increasing resistance at the interface. Here, we present the distinctive features of the typical interfaces and interphases in ASSBs and summarize the recent work on identifying, probing, understanding, And engineering them. We highlight the complicated, but important, characteristics of interphases, namely the compn., distribution, and electronic and ionic properties of the cathode-electrolyte and electrolyte-anode interfaces; understanding these properties is the key to designing a stable interface. Thorough and in-depth understanding on the complex interfaces and interphases is essential to make a practical high-energy ASSB.
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    43. 43
      Xu, K.; Zhang, S.; Jow, T. R.; Xu, W.; Angell, C. A. LiBOB as Salt for Lithium-Ion Batteries:A Possible Solution for High Temperature Operation. Electrochem. Solid-State Lett. 2002, 5, A26  DOI: 10.1149/1.1426042
      43
      Li[bis(oxalato)borate] as salt for lithium-ion batteries a possible solution for high temperature operation
      Xu, Kang; Zhang, Shengshui; Jow, T. Richard; Xu, Wu; Angell, C. Austen
      Electrochemical and Solid-State Letters (2002), 5 (1), A26-A29CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      A new lithium salt based on a chelated borate anion [bis(oxalato)borate] is evaluated as the electrolyte solute for lithium-ion cells by both electrochem. means and cell testing. Controlled potential coulometry study reveals that the anion can stabilize aluminum substrate to more pos. potentials than the popular hexafluorophosphate (PF6-) anion does, while slow scan cyclic voltammograms show good compatibility of the salt with graphitizable carbonaceous anode as well as satisfactory stability against charged cathode surface. The lithium-ion cells contg. this salt as electrolyte solute exhibit excellent capacity utilization, capacity retention as well as rate capability at room temp. Probably due to the fact that the new anion contains no labile fluorine and is thermally stable, electrolyte solns. based on it demonstrate stable performance in cells even at 60°C, where LiPF6-based electrolytes would usually fail. The preliminary results reported herein provide a possible soln. to the instability of the Li-ion cell performance at the elevated temps. anticipated for heavy duty applications such as elec. or hybrid elec. vehicle missions.
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  • Supporting Information

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