Electrolyte Engineering with Carboranes for Next-Generation Mg Batteries

To realize an energy storage transition beyond Li-ion competitive technologies, earth-abundant elements, such as Mg, are needed. Carborane anions are particularly well-suited to realizing magnesium-ion batteries (MIBs), as their inert and weakly coordinating properties beget excellent electrolyte performance. However, utilizing these materials in actual electrochemical cells has been hampered by the reliance on the Mg2+ salts of the commercially available [HCB11H11]− anion, which is not soluble in more weakly binding solvents apart from the higher glymes. Herein, we demonstrate it is possible to iteratively engineer the [HCB11H11]− anion surface synthetically to address previous solubility issues and yield a highly conductive (up to 7.33 mS cm–1) and electrochemically stable (up to +4.2 V vs Mg2+/0) magnesium electrolyte that surpasses the state of the art. This novel non-nucleophilic electrolyte exhibits highly dissociative behavior regardless of concentration and is tolerant of prolonged periods of cycling in symmetric cells at high current densities (up to 2.0 mA cm–2, 400 h). The hydrocarbon functionalized carborane electrolyte presented here demonstrates >96% Coulombic efficiency when paired with a Mo6S8 cathode. This approach realizes a needed candidate to discover next-generation cathode materials that can enable the design of practical and commercially viable Mg batteries.


General Considerations
All manipulations were carried out using standard Schlenk or glovebox techniques under a dinitrogen or UHP (99.995%) argon atmosphere (glovebox, Schlenk line) unless otherwise stated.Dry THF and DME was obtained via distillation under argon from potassium using benzophenone ketyl radical as an indicator.Unless specifically stated, reagents were purchased from commercial vendors and used without further purification.Nuclear magnetic resonance (NMR) spectroscopy was carried out using: Bruker Avance 600 MHz and Bruker NEO 400 MHz (Prodigy LN2 cryoprobe), Varian Inova 500 MHz and Bruker NEO 600 (CP-MAS).NMR chemical shifts are reported in parts per million (ppm) with 1 H and 13 C chemical shifts referenced to the residual non-deutero solvent.High-resolution mass spectrometry (HRMS) was collected on an Agilent Technologies 6210 (TOF LC/MS) featuring a direct injection with multimode electrospray ionization/atmospheric-pressure chemical ionization (ESI/APCI).

Synthesis of Anion Series 2
Cs + [HCB 11 H 11 ] -was prepared following known literature procedures from decaborane (B 10 H 14 ) which was sublimed prior to use. 1 [HNMe 3 ] + [HCB 11 H 11 ] -was prepared by dissolution of Cs + [HCB 11 H 11 ] -in warm water followed by the addition of NMe 3 •HCl, and the solid was collected via vacuum filtration.[HNMe 3 ] + [HCB 11 H 11 ] -was dried for 12 hours in vacuo at 170°C.Alkylated carborane anions were prepared from a modified literature procedure 2 : Working in a glovebox, [HNMe 3 ] + [HCB 11 H 11 ] -was dissolved in THF and 2.5 eq. of n-BuLi (2.5 M solution in hexane) was added dropwise following concentration under vacuum.Once the effervescence of NMe 3 and butane gas seized, the reaction was allowed to stir for 1 hour before confirming complete conversion to dianionic species [CB11H11] 2-[Li] + 2 by 11 B{ 1 H} NMR.The THF solution containing dianionic species [CB11H11] 2- [Li] + 2 was added dropwise to a stirring solution of pentane or hexane approximately 2/1 v/v the volume of the crude reaction mixture during which a white precipitate formed.The precipitate was allowed to settle and the pentane/hexane were carefully decanted.Remaining pentane/hexane was removed under vacuum.To the solid, 1.1 mol eq. of alkyl bromide were added followed by a minimum of THF.The reaction was typically complete after 3 hours, but was often left to stir overnight.The crude reaction mixture was precipitated in hexanes followed by dissolution in a minimum of water yielding a stirring biphasic solution of [R-CB 11 H 11 ] -[Li] + .CsCl was added and the solution was allowed to stir for 15 minutes as [R-CB 11 H 11 ] -[Cs] + gradually precipitated.The slurry was filtered and allowed to dry yielding crude [R-CB 11 H 11 ] -[Cs] + as a white solid.[R-CB 11 H 11 ] -[Cs] + was crystallized from water to yield a white, pearlescent material.

Synthesis of Magnesium Salts
[R-CB 11 H 11 ] -[Cs] + was stirred on equal v/v solution of water/CHCl 3 to obtain a slurry.1.5 mol eq.NMe 3 HCl was added to the slurry and the mixture was stirred vigorously for 15 minutes.The biphasic solution was allowed to separate into two layers, and the CHCl 3 containing [R-CB 11 H 11 ] -[HNMe 3 ] + was separated and dried in-vacuo to yield [R-CB 11 H 11 ] -[HNMe 3 ] + as a white powder.This powder was stirred in a minimum of cold water for 15 minutes, filtered (2x), and allowed to dry.The solid was collected and dried in-vacuo at 80 °C for a minimum of 24 hours.Working in a glovebox, the dry [R-CB 11 H 11 ] -[HNMe 3 ] + was dissolved in a minimum of THF and 0.65 mol eq.n-Bu sec-Bu magnesium was added dropwise to the stirring solution upon which ([R-CB 11 H 11 ] -) 2 Mg 2+ immediately precipitated.The reaction was allowed to stir for 1 hour upon which 40 mL of hexane was added, and the solid ([R-CB 11 H 11 ] -) 2 [Mg(THF) 6 ] 2+ was collected via vacuum filtration.The ([R-CB 11 H 11 ] -) 2 [Mg(THF) 6 ] 2+ species were recrystallized by THF/Hexane vapor diffusion from a minimum of THF in which the species exhibit limited solubility.The crystalline, white solid was dissolved in 10 mL of dimethoxyethane (DME), stirred for 30 minutes, and dried invacuo to yield

Electrolyte Formulation
0.8 M solutions of Mg2g/DME were obtained by dissolution of ([R-CB 11 H 11 ] -) 2 [Mg(DME) 3 ] 2+ (800 mg, 1.1 mmol) in dimethoxyethane (0.72 mL).The solution was allowed to stir for 1 hour or until complete dissolution of the salt was achieved.The resulting electrolyte was filtered through a glass microfiber filter (Whatman, F Grade) and stored in a 10 mL Teflon sealed Schlenk flask and used without further purification.H 2 O content of the electrolytes used in this study were verified to be <10 ppm by Karl Fisher titration prior to use.

Electrode Fabrication
Chevrel phase Cu 2 Mo 6 S 8 was first synthesized via solid state reaction by heating stoichiometric mixture of elemental Cu, Mo and S powders.Stoichiometric amounts of Cu powder (Alfa Aesar, 10 micron, 99.9%), Mo powder (Alfa Aesar, 2-4 micron, 99.9%) and S powder (Sigma Aldrich, 99.5-100%) were thoroughly mixed and sealed in an evacuated quartz tube.The quartz tube was subsequently heated in a muffle furnace.The temperature ramp program was set as follows: The temperature was ramped up to 450 °C (1 °C min -1 ) and held for 24 h.The temperature was then ramped up to 700 °C (1 °C min -1 ) and held for another 24 h.The temperature was thereafter ramped up again to 1050 °C (1 °C min -1 ) and held for 48 h.Finally, the furnace was allowed to cool down to room temperature naturally.The synthesized Chevrel phase Cu 2 Mo 6 S 8 then underwent a chemical leaching process to yield the final product of Mo 6 S 8 as follows: Cu 2 Mo 6 S 8 was added into 20 ml 6M HCl solution.Oxygen was bubbled into the solution for 8 hours while stirring.Following the reaction, the obtained Mo 6 S 8 was centrifuged, washed with adequate amount of deionized water, and dried in vacuum oven at 50 °C overnight.The resulting product was characterized by SEM/EDX and powder XRD to verify purity.The electrode slurry was made by mixing 80 wt.% Mo 6 S 8 , 10 wt.% carbon black, and 10 wt.% PVDF in N-Methyl-2-pyrrolidone (NMP) solution via a mechanical mixer for 15 min.The slurry was coated on thin Al foil with Mo 6 S 8 loading of 1.5 mg cm -2 .

Raman Spectroscopy
Raman spectra were acquired on a confocal Raman microspectrometer (LabRam Evolution, Horiba, Japan) following excitation at 532 nm.An 1800 groove/mm grating was selected to provide < 1cm -1 spectral resolution.A 50x long-working distance objective (LMPlanFL, Olympus, Japan, wd: 10.6 mm, na: 0.5) was used to image the sample (spot size: <1.3 mm).Samples were prepared in a glovebox under nitrogen atmosphere and sealed in borosilicate glass NMR tubes.

Electrochemistry and Device Fabrication
Three-electrode cyclic voltammetry was carried out in a 200 mL glass Gamry 5 neck cell with a platinum (Gamry, 99.999%) working electrode (3 mm disc) and two Mg plates (0.25 mm thickness, 99.9%, Alfa Aesar) as the reference electrode and counter electrode, respectively.The Pt working electrode was polished with alumina particle (0.05 μm) water dispersion on a polishing pad, sonicated in ethanol, and dried at least 12 hours under vacuum prior to each experiment.Overpotential for 3electrode experiments was determined by the potential difference between onset of reduction and 0 V vs. Mg 2+/0 .Twoelectrode symmetric (Mg||Mg) and half cell (Mg||Cu and Mg||Mo 6 S 8 ) experiments in both 2016 and 2032 stainless steel casing were employed for coin cell fabrication and glass microfiber was used as a separator (Whatman, F grade).Voltage hysteresis for constant current experiments undertaken with Mg||Cu was determined by the potential difference between the reduction/oxidation plateau.Magnesium metal (Sigma Aldrich, 99.9 %) was obtained as a foil, polished with 2000 grain sandpaper (3M), and punched into disks 15.75 mm in diameter.Mg anodes submerged in hexane were sonicated for 1 hour, washed with 5 mL of hexane (3x), and dried in-vacuo for 2 hours prior to use.Identical Mg metal electrodes were used as the working and counter electrode.Chevrel Phase molybdenum sulfide (Mo 6 S 8 ) wasprepared following previously reported procedures and characterized via SEM/EDX and powder XRD following synthesis. 3Cells were assembled in an argon-filled glove box (>0.5 ppm H2O, >1 ppm O2).Galvanostatic charge-discharge cycling tests were performed by a battery cycler system (MACCOR).Electrochemical Impedance Spectroscopy (EIS) of Mg2g/DME was conducted in a 2-electrode T-cell apparatus consisting of electrodes of known surface area and separation distance over a 25°C-60°C temperature range.
Copper rods of known surface area were separated at a fixed path length and used to measure electrolyte resistance.Two copper rods were placed on two sides of the T cell and wrapped with Teflon tape to prevent leaking.The cell was filled with electrolyte Mg2g/DME until the electrodes were fully submerged.Finally, the cell was sealed at the top with a third copper rod.The resistance of the electrolyte was measured by EIS with frequency from 30 kHz to 10 Hz under voltage of 10 mV rms.The resistance was determined as the first data point intercepted with the x axis (Figure S37).Ionic conductivity was calculated from ohm's law where  is conductivity,  is resistivity, L is the length between the electrodes across the electrolyte, A is the surface area of electrodes, and R is the resistance of the electrolyte:

Surface Characterization
Samples were prepared via electrodeposition of magnesium metal in a Mg||Cu 2032 coin cell configuration followed by disassembly in an argon-filled glovebox.Copper electrodes were washed liberally with anhydrous DME and allowed to dry under vacuum for 12 hours.The surface morphology and elemental composition of the samples were characterized with scanning electron microscopy (SEM, Nova Nano S450) and energy-dispersive X-ray (EDX) spectroscopy.

Crystal Structure Determination
Single crystals suitable for X-ray diffraction were grown by slow diffusion of hexane into dimethoxyethane.A colourless crystal (plate, approximate dimensions 0.49 × 0.16 × 0.12 mm3) was placed onto the tip of a MiTeGen pin and mounted on a Bruker Venture D8 diffractometer equipped with a PhotonIII detector at 180.00 K.The data collection was carried out using Mo Kα radiation (λ = 0.71073 Å, ImS micro-source) with a frame time of 15 seconds and a detector distance of 60 mm.A collection strategy was calculated and complete data to a resolution of 0.84 Å with a redundancy of 5.3 were collected.The frames were integrated with the Bruker SAINT 4 software package using a narrow-frame algorithm to a resolution of 0.84 Å.Data were corrected for absorption effects using the Multi-Scan method (SADABS). 5Please refer to Table S1 for additional crystal and refinement information.
The space group Pna2 1 was determined based on intensity statistics and systematic absences.The structure was solved using the SHELX suite of programs 6,7 and refined using full-matrix least-squares on F 2 within the OLEX2 suite. 8And intrinsic phasing solution was calculated, which provided most non-hydrogen atoms from the E-map.Full-matrix least squares / difference Fourier cycles were performed, which located the remaining non-hydrogen atoms.All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters.The final full matrix least squares refinement converged to R1 = 0.0909 and wR2 = 0.2624 (F 2 , all data).The goodness-of-fit was 1.028.On the basis of the final model, the calculated density was 0.944 g/cm 3 and F(000), 3350 e-.The crystals suffer from solvent loss which leads to large anisotropic displacement parameters and solvent accessible voids of 172 Å 3 in the unit cell.

Fig
Fig S8. 13 C NMR of Mg2b in d 6 -acetone

Figure S38 :
Figure S38: Crystallized Mg2f from a concentrated solution in DME at room temperature (1.4 M).

Figure S39 :
Figure S39: Crystallization behavior of 0.75M Mg1a in DME (left) vs. 0.8M Mg2g in DME (right) allowed to warm from -30 °C at (a) 25 °C over a period of 2 hours and (b) 50 °C over a period of 1 hour.

Figure S40 :
Figure S40: Raman spectroscopy in low frequency region (700 -900 cm -1 ) of concentrated DME solutions of Mg2g.Depression of peak at 820 cm -1 associated with a bending mode of DME is accompanied by gradual increase of peak at 881 cm -1 associated with complexation in [Mg(DME) 3 ] 2+ .

Figure S42 :
Figure S42: Electrochemical Impedance Spectroscopy of 0.8 M Mg2g/DME as a function of temperature.

Figure
FigureS43: A number of previously reported liquid magnesium electrolytes plotted in order of increasing ionic conductivity including Mg2g/DME (green) and previously investigated carboranyl electrolytes (red).

Figure
Figure S44: (left) Zoomed in view of Mg2g 3-electrode cyclic voltammetry with measured overpotential for displayed cycles (related to Fig. 4d) and (right) corresponding coulombic efficiency over entire 50 cycles.

Figure S48 :
Figure S48: SEM images of magnesium metal deposition on copper (a) 50x magnification of surface and (b) 10kx magnification of surface with Mg2g/DME

Figure S52 :
Figure S52: Characterization of synthesized Mo 6 S 8 active material (a) SEM and corresponding elemental mapping of Mo 6 S 8 (b) EDX spectra of Mo 6 S 8 and (c) XRD of Mo 6 S 8 .