Electrochemical behavior of elemental alloy anodes in solid-state batteries

Lithium alloy anodes in the form of dense foils offer significant potential advantages over lithium metal and particulate alloy anodes for solid-state batteries (SSBs). However, the reaction and degradation mechanisms of dense alloy anodes remain largely unexplored. Here, we investigate the electrochemical lithiation/delithiation behavior of 12 elemental alloy anodes in SSBs with Li6PS5Cl solid-state electrolyte (SSE), enabling direct behavioral comparisons. The materials show highly divergent first-cycle Coulombic efficiency, ranging from 99.3% for indium to ∼20% for antimony. Through microstructural imaging and electrochemical testing, we identify lithium trapping within the foil during delithiation as the principal reason for low Coulombic efficiency in most materials. The exceptional Coulombic efficiency of indium is found to be due to unique delithiation reaction front morphology evolution in which the high-diffusivity LiIn phase remains at the SSE interface. This study links composition to reaction behavior for alloy anodes and thus provides guidance toward better SSBs.

Li0.5In counter electrodes were prepared with lithium (MSE Supplies) and indium foils with an atomic ratio of 1:2 (lithium:indium).These prepared lithium and indium foils were stacked in an indium/lithium/indium configuration and mixed via an accumulative roll bonding process in an Ar-filled glove box.The prepared Li0.5In counter electrodes were punched into disks of 15 mm diameter before electrochemical testing.
Cell assembly: Solid-state half cells were assembled using elemental working electrodes, LPSC solid-state electrolyte (SSE), and a lithium counter electrode.Elemental electrodes, with a diameter of 10 mm, were prepared through punching or polishing.90 mg of the ultrafine LPSC powder (particle size ~1 μm) was poured into a polyether ether ketone (PEEK) die (inner diameter: 10 mm) and uniaxially pressed to a pressure of ~250 MPa for 5 min using titanium plungers.Subsequently, the alloy working electrode was added and subjected to further pressing up to ~375 MPa.The lithium counter electrode was attached to a titanium plunger and inserted to contact the densified LPSC pellet.Finally, the alloy anode/LPSC/lithium stack was then uniaxially pressed to ~60 MPa to form interfacial contact between the lithium counter electrode and the LPSC pellet.
Liquid-electrolyte pouch cells were assembled using elemental working electrodes, liquid electrolyte, separators, and Li0.5In counter electrodes.The elemental electrodes were punched or polished into discs with a diameter of 15 mm.The liquid electrolyte used was 1.0 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DEC, 1:1 by volume, Sigma Aldrich) with 10 vol% fluoroethylene carbonate (FEC, Sigma Aldrich).
Glass fiber membranes (Cytiva Whatman TM ) were used as separators.The separator was soaked in electrolyte before assembly and an additional 50 μL of electrolyte was added to each cell before vacuum sealing.
The lithium symmetric cell was assembled by uniaxially pressing 90 mg of ultrafine LPSC powder at ~375 MPa for 5 min using titanium plungers.Subsequently, the lithium electrodes were attached to titanium plungers and inserted to contact both sides of the densified LPSC pellet.The mass of the lithium electrodes was controlled to be the same.Finally, the lithium/LPSC/lithium stack was uniaxially pressed to 60 MPa to form interfacial contact between the lithium counter electrode and LPSC pellet.The Li0.5In symmetric cell was assembled with Li0.5In electrodes, liquid electrolyte composed of 1.0 M LiPF6 in EC/DEC with 10 vol% FEC, and glass fiber membranes as separators.The mass of Li0.5In at each electrode was controlled to be the same.
Solid-state full cells were assembled by first uniaxially pressing 90 mg of ultrafine LPSC powder at ~125 MPa in the PEEK die.Then, 30 mg of cathode composite powder, equivalent to ~5.0 mAh cm -2 , was added on top of LPSC pellet and pressed to ~250 MPa.Finally, the alloy electrode was added on the other side of the LPSC pellet and pressed to ~375 MPa for 5 min.All cells were assembled inside the Ar-filled glove box.
Electrochemical measurements: Before electrochemical testing, all cells (solid-state half cells, liquid-electrolyte pouch cells, and solid-state full cells) were sandwiched between steel stack plates, and a constant stack pressure of 8 MPa was applied by tightening nuts at each corner using a digital torque wrench.S2 Springs at each corner were used to ensure uniform stack pressure distribution across the cell.S3 The stack pressure was calibrated with a pressure sensor prior to the tests.Solid-state half cell tests were performed with Arbin and Landt Instruments battery cyclers, with a current density of 50 µA cm −2 and a voltage range of 0 to 1.5 V vs. Li/Li + .For high-temperature half cell testing at 60 °C, heat tape was wrapped around the PEEK die during the test.Due to the insulating properties of PEEK, the outer diameter of the die was reduced to ~12 mm to improve heat transfer to the alloy anode/LPSC/lithium stack.The liquid-electrolyte pouch cell was tested with a Neware battery testing system, with a current density of 50 µA cm −2 and a voltage range of -0.63 to 0.87 V vs. Li0.5In/Li+ .The symmetric cells were cycled for one cycle at a current density of 50 µA cm −2 with capacity cutoff of 5 mAh cm -2 for each half-cycle.For the solid-state full cell cycling test, a current density of 0.2 mA cm −2 was used for the first two cycles and 0.5 mA cm −2 for the subsequent cycles, with a voltage range of 2.0 to 4.1 V.The areal charge capacity was limited to 5 mAh cm -2 for every cycle.The cyclic voltammetry (CV) tests of solid-state half cells were performed with a Bio-Logic SP-200 potentiostat.The sweep rate was 0.01 mV s -1 with a voltage range of 0 to 1.0 V.For the rate performance tests of solid-state full cells, a current density of 0.2 mA cm −2 was used for the first two cycles followed by increasing the current density in steps of 0.5, 1.0, and 2.0 mA cm −2 , with three cycles for each current density.The current density was then reduced back to 0.2 mA cm −2 for the rest of the cycles.All the solid-state cells were tested in Ar-filled glove box.Except for the elevated temperature solid-state half cell tests, all cells were tested at room temperature (25 °C).

Material characterization: Cryogenic focused-ion beam scanning electron microscopy (cryo-FIB-SEM)
imaging was carried out using a Thermo-Fisher Helios 5CX FIB-SEM equipped with a Ga + source and a Quorum cryogenic stage system.The (de)lithiated electrodes were extracted from the PEEK die inside the Ar-filled glove box and transferred into the SEM chamber, with a few seconds of air exposure.All samples were cooled down to -140 °C prior to FIB milling and imaging to reduce detrimental interactions with the ion beam.S4 An initial cross-sectional milling utilized 100 nA of beam current with 30 kV accelerating voltage.For final polishing of the cross-section, nA of beam current was applied.For imaging the morphology and compositional contrast of (de)lithiated foils, a through-the-lens detector (TLD) was used to simultaneously capture secondary electrons (SE) and backscattered electrons (BSE).Table S3.Alloy anode thickness used in solid-state full cell tests, as well as first-cycle areal charge/discharge capacities, specific charge/discharge capacities, and Coulombic efficiency values from Figure 4.

Figure S1 .
Figure S1.Galvanostatic voltage curves of (a) a lithium symmetric cell with LPSC SSE and (b) a Li0.5In symmetric cell with liquid electrolyte consisting of 1.0 M LiPF6 in EC/DEC with 10 vol% FEC.The symmetric cells were cycled for one cycle at a current density of 50 µA cm −2 with a half-cycle areal capacity of 5 mAh cm -2 .The cells were tested at 25 °C with a uniaxial stack pressure of 8 MPa.Both cells exhibit flat voltage profiles with minimal voltage and no polarization, showing that these electrodes can effectively be used as counter electrodes in half cells under these conditions.

Figure S2 .
Figure S2.First-cycle galvanostatic voltage curves for cryo-FIB-SEM samples in Figure2a-h.The solidstate half cells with indium, aluminum, tin, and silicon electrodes were lithiated to a capacity of 1.5 mAh cm -2 , which is a capacity value that is less than the full theoretical capacity of the electrodes.(a) Indium foil was partially delithiated to a capacity cutoff of 1.0 mAh cm -2 .The inset shows the voltage curve in the range of 0.5 to 0.7 V vs. Li/Li + .(b-d) Aluminum, tin, and silicon electrodes were delithiated to 1.5 V. Red circles on the voltage curves indicate the stages at which ex situ cryo-FIB-SEM imaging was performed.Half cells were tested under a uniaxial stack pressure of 8 MPa and a current density of 50 µA cm −2 at 25 °C.The specific capacities of the lithiated foils were 117.8 mAh g -1 for indium, 314.2 mAh g -1 for aluminum, and 107.6 mAh g -1 for tin.

Figure S3 .
Figure S3.Cryo-FIB-SEM images of indium (a) after lithiation to 0.34 V vs. Li/Li + to completely react the foil to form the LiIn phase and (b) after delithiation to a capacity cutoff of 2.0 mAh cm -2 .

Figure S4 .
Figure S4.First-cycle galvanostatic voltage curve for cryo-FIB-SEM samples in Figure S2.The solid-state half cell with indium electrode was lithiated to 0.34 V vs. Li/Li + and delithiated to a capacity cutoff of 2.0 mAh cm -2 .The inset shows the voltage curve in the range of 0.34 to 0.65 V vs. Li/Li + .Red circles on the voltage curve indicate the stages at which the ex situ cryo-FIB-SEM imaging was performed.The half cell was tested under a uniaxial stack pressure of 8 MPa and a current density of 50 µA cm −2 at 25 °C.

Figure S5 .
Figure S5.First-cycle cyclic voltammetry curves of alloy foil anodes in solid-state half cells utilizing LPSC SSE and lithium metal counter electrodes.(a) Indium, (b) aluminum, (c) tin, and (d) silicon.The sweep rate was 0.01 mV s -1 with a voltage range of 0 to 1.0 V.The half cells were tested at 25 °C with a uniaxial stack pressure of 8 MPa.

Figure S6 .
Figure S6.Rate-dependent galvanostatic testing of alloy foil anodes in solid-state full cells with LPSC SSE and NMC622 cathodes.(a-d) Voltage curves under different current densities from (a) indium (20 µm), (b) aluminum (20 µm), (c) tin (10 µm), and (d) silicon (500 µm) cells.The cathode loading and N:P ratios of these cells were set to be identical to those of the full cells tested in the galvanostatic cycling tests in Figure 4. (e) Discharge capacity values during this rate testing.The areal charge capacity was limited to 5 mAh cm -2 in each cycle.All cells were tested at 25 °C in the voltage range of 2.0 to 4.1 V with a uniaxial stack pressure of 8 MPa.

Table S2 .
Alloy anode thickness used in solid-state half cell tests, as well as areal discharge/charge capacities, specific discharge/charge capacities, and Coulombic efficiencies values from Figure1.