Trimethylsilyl Compounds for the Interfacial Stabilization of Thiophosphate‐Based Solid Electrolytes in All‐Solid‐State Batteries

Abstract Argyrodite‐type Li6PS5Cl (LPSCl) has attracted much attention as a solid electrolyte for all‐solid‐state batteries (ASSBs) because of its high ionic conductivity and good mechanical flexibility. LPSCl, however, has challenges of translating research into practical applications, such as irreversible electrochemical degradation at the interface between LPSCl and cathode materials. Even for Li‐ion batteries (LIBs), liquid electrolytes have the same issue as electrolyte decomposition due to interfacial instability. Nonetheless, current LIBs are successfully commercialized because functional electrolyte additives give rise to the formation of stable cathode‐electrolyte interphase (CEI) and solid‐electrolyte interphase (SEI) layers, leading to supplementing the interfacial stability between electrolyte and electrode. Herein, inspired by the role of electrolyte additives for LIBs, trimethylsilyl compounds are introduced as solid electrolyte additives for improving the interfacial stability between sulfide‐based solid electrolytes and cathode materials. 2‐(Trimethylsilyl)ethanethiol (TMS‐SH), a solid electrolyte additive, is oxidatively decomposed during charge, forming a stable CEI layer. As a result, the CEI layer derived from TMS‐SH suppresses the interfacial degradation between LPSCl and LiCoO2, thereby leading to the excellent electrochemical performance of Li | LPSCl | LiCoO2, such as superior cycle life over 2000 cycles (85.0% of capacity retention after 2000 cycles).

.       Table S1.Quantitative molar ratio of Li, P, and Si elements for LPSCl powders adsorbed with TMS-SH.The corresponding weight ratio of LPSCl and TMS-SH in LPSCl powders adsorbed with TMS-SH was calculated from the molar ratio of Li, P, and Si.Elemental analysis was performed using ICP-AES.

Figure S2 .
Figure S2.a-d) EDS spectra for bare LPSCl powders (a) and LPSCl powders adsorbed with solid electrolyte additives, such as TMS-SH (b), TMS-OH (c), and Bis-TMS (d).Horizontal and vertical axes in the EDS spectra represent energy (keV) and counts per seconds per electronvolt (cps eV -1 ), respectively.The insets show the EDS spectra expanded for clarity.

Figure S3 .
Figure S3.TEM and EDS mapping images of LPSCl powders adsorbed with TMS-OH.Red, green, violet, and blue colors represent silicon, sulfur, phosphorus, and chlorine elements, respectively.

Figure S4 .
Figure S4.a, b) TEM-EDS spectra of LPSCl adsorbed with TMS-SH (a) and TMS-OH (b).Horizontal and vertical axes in the spectra represent energy (keV) and counts, respectively.

Figure S5 .
Figure S5.a) Nyquist plots and b) chronoamperometry profiles of bare LPSCl and LPSCl adsorbed with TMS-OH and TMS-SH under 0.25 V bias.Symbols and solid lines in (a) represent raw data and linear fit lines, respectively.The inset in (a) shows the equivalent circuit model for fitting the Nyquist plots (SS: stainless steel, R: resistance, and CPE: constant phase element).The inset in (b) shows the chronoamperometry profiles on an expanded y-axis scale in the selected time period for clarity.Bare, TMS-OH, and TMS-SH in the figure legends represent bare LPSCl, LPSCl adsorbed with TMS-OH, and LPSCl adsorbed with TMS-SH, respectively.

Figure S7 .
Figure S7.Schematic image of the home-made bulk-type cell.

Figure S9 .
Figure S9.a, b) Coulombic efficiency of various electrochemical cells shown in Figure 4c (a) and Figure 5a (b).The inset shows their Coulombic efficiencies at the first cycle.LiCoO2 and LiNbO3 were denoted as LCO and LNO, respectively.

Figure S10 .
Figure S10.a) Cycle performances and b) Coulombic efficiencies of Li | LPSCl | LiCoO2 cells at a charge current density of 0.66 mA cm -2 of bare LPSCl and TMS-LPSCl for the composite cathode pellets.The cells were discharged at the same current density of 1.1 mA cm -2 using a CC mode in the voltage range of 2.5 -4.3 V (vs.Li/Li + ) at 30 °C.

Figure S11 .
Figure S11.a-d) Voltage profiles of Li | LPSCl | LiCoO2 cells at a various charge current densities, such as 0.44 mA cm -2 (a, b) and 0.66 mA cm -2 (c, d), for bare LPSCl (a, c) and TMS-LPSCl (b, d) in the composite cathode pellets.The cells were discharged at the same current density of 1.1 mA cm -2 using a CC mode in the voltage range of 2.5 -4.3 V (vs.Li/Li + ) at 30 °C.To clearly display the voltage fluctuation in the end of charge due to micro-short circuit, the voltage profiles of the selected voltage range of 4.1 − 4.3 V in (i) were displayed in (ii).

Figure S12 .
Figure S12.a) 2D molecular structure of DDT-SH.b) Raman and c) IR spectra of THF solutions containing DDT-SH before and after adsorption of DDT-SH on the LPSCl surface.The regions of the Raman (2500 -3100 cm -1 ) and IR (3200 − 2400 cm -1 ) spectra are expanded for clarity.

Figure S14 .
Figure S14.Nyquist plots of bare LPSCl (black square), THF-LPSCl (blue triangle), and TMS-LPSCl (red circle).Symbols and solid lines represent raw data and linear fit lines, respectively.The inset shows the equivalent circuit model for the Nyquist plots (SS: stainless steel, R: resistance, and CPE: constant phase element).

Figure S16 .
Figure S16.Comparison of the cycle performances of LiCoO2 cathode materials with sulfidebased solid electrolytes reported in recent literatures.Symbols and colors represent anode type and operating temperature, respectively: circle (bare Li metal), square (protected Li metal such as Li-In and coated Li), blue (25 ~ 35 ℃ including room temperature), orange (35 ~ 45 ℃), red (45 ~ 60 ℃), and green (N/A).The detailed cell conditions are listed in TableS5.

Figure S17 .
Figure S17.a, b) Ionic conductivities (a) and the corresponding Nyquist plots (b) of LPSCl adsorbed with TMS-SH for various concentrations of TMS-SH in THF.The weight percent of TMS-SH to LPSCl was measured using ICP-AES analysis.Symbols and solid lines represent raw data and linear fit lines, respectively.The inset shows the equivalent circuit model for the Nyquist plots (SS: stainless steel, R: resistance, and CPE: constant phase element).

Figure S18 .
Figure S18.a) Voltage profiles of LiCoO2 with TMS-LPSCl for various concentrations of TMS-SH, such as 0.05 M (blue line), 0.1 M (red line), and 0.2 M (green line), in the voltage range of 2.5 -4.3 V (vs.Li/Li +) at 30 °C.The cells were examined at the two current protocols for charge and discharge: (i) at a 0.1C rate (0.11 mA cm -2 , solid line) for charge and discharge, and (ii) at a 0.2 C rate (0.22 mA cm -2 ) for charge and a 0.5 C rate (0.54 mA cm -2 ) for discharge (dashed line).b) Cycle performance of LiCoO2 with TMS-LPSCl for various concentrations of TMS-SH, such as 0.05 M (blue square), 0.1 M (red circle), and 0.2 M (green triangle), in the voltage range of 2.5 -4.3 V (vs.Li/Li + ) at 30 °C.The cells were charged at a 0.2 C rate and discharged at a 0.5 C rate.

Figure S19 .
Figure S19.Normalized TOF-SIMS spectra of bare LPSCl and TMS-LPSCl for Si(CH3)3 + fragments.Black and grey spectra represent TMS-LPSCl and bare LPSCl, respectively.The scale bars of normalized intensity are inserted in the figures.

Figure S21 .
Figure S21.Schematic illustration for the home-made reactor to observe the chemical degradation of LPSCl powders in a dry oxygen atmosphere at room temperature.

Figure S22 .
Figure S22.a) Voltage profiles and b) cycle performance of bare LPSCl and TMS-LPSCl, which were retrieved after storage in oxygen, at a current density of 0.1 C (0.11 mA cm -2).The cells were examined in the voltage range of 2.5 -4.3 V (vs.Li/Li + ) at 30 °C.Both powders were stored in a home-made container under a dry oxygen atmosphere for six days at room temperature.

Table S2 .
The ionic and electronic conductivities of bare LPSCl and LPSCl adsorbed with TMS-SH and TMS-OH at room temperature.

Table S3 .
Fitting parameters for the EIS data shown in FigureS5a.

Table S4 .
Fitting parameters for the EIS data shown in FigureS14.

Table S5 .
Comparison of the electrochemical performances and cell parameters of LiCoO2 cathode materials with sulfide-based solid electrolytes reported in recent literatures.

Table S6 .
Fitting parameters for the EIS data shown in FigureS17b.

Table S7 .
Quantitative molar ratio of Li, P, and Si elements for LPSCl powders adsorbed with TMS-SH for various concentrations of TMS-SH in THF (0.05 M and 0.2 M).The corresponding weight ratio of LPSCl and TMS-SH in LPSCl powders adsorbed with TMS-SH was calculated from the molar ratio of Li, P, and Si.Elemental analysis was performed using ICP-AES.

Table S8 .
Fitting parameters for the EIS data shown in Figure9b and c