Sn vs. Ge: Effects of Elastic and Plastic Deformation of LGPS-type Solid Electrolytes on Charge-discharge Properties of Composite Cathodes for All-solid-state Batteries

Abstract

The charge-discharge properties of all-solid-state batteries are affected by both chemical and physical factors. Physical issues mainly arise from the microstructure of the composites and the mechanical properties of the solid electrolytes themselves. However, physical issues have been investigated by focusing on the microstructures of the composites rather than the mechanical properties of the solid electrolytes themselves. In this study, composite cathodes with similar microstructures were fabricated using LiCoO₂ as the active material and either Li_9.81Sn_0.81P_2.19S₁₂ or Li₁₀GeP₂S₁₂ as the solid electrolyte. The composite with Li_9.81Sn_0.81P_2.19S₁₂ exhibited higher capacity retention and coulombic efficiency with increasing C-rates at 1.9–3.6 V vs. In-Li than that with Li₁₀GeP₂S₁₂. Moreover, during charging–discharging at 1.9–3.8 V, the expansion and shrinkage of LiCoO₂ were greater those at 1.9–3.6 V for the composite with Li_9.81Sn_0.81P_2.19S₁₂, leading to a higher capacity, capacity retention, and coulombic efficiency than those of the composite with Li₁₀GeP₂S₁₂. These results are attributed to the high elastic modulus, high yield stress, and volumetrically-large elastic-deformability, which enable Li_9.81Sn_0.81P_2.19S₁₂ to reversibly deform while maintaining contact with LiCoO₂, unlike Li₁₀GeP₂S₁₂. These results demonstrate that solid electrolytes with low elastic moduli are not absolutely suitable for all-solid-state batteries, and that a high yield stress and volumetrically-large elastic-deformability are especially significant for reversible deformation. These findings provide new insights for the development of composite electrodes for all-solid-state batteries.