Enhancing the Mechanical Properties of Solid Electrolytes for Crack‐Resistant Cathodes in All‐Solid‐State Batteries

The electrochemical performance of all‐solid‐state batteries (ASSBs) is strongly influenced by the physicochemical characteristics of solid electrolytes; however, the impact of their mechanical properties on cell performance remains comparatively underexplored. Here, SiO 2 incorporation during Li 6 PS 5 Cl synthesis induces Si/O co‐substitution, forming Li 6.1 P 0.9 Si 0.1 S 4.8 O 0.2 Cl with modified mechanical properties. Combined theoretical and experimental investigations show an increase in Young's modulus from 16 to 21 GPa upon Si/O co‐substitution. AFM nanoindentation analysis further reveals higher elastic stiffness and hardness for Li 6.1 P 0.9 Si 0.1 S 4.8 O 0.2 Cl. Cells employing Li 6.1 P 0.9 Si 0.1 S 4.8 O 0.2 Cl‐based composite cathodes exhibit markedly improved cycling performance, achieving a capacity retention of 90.8% after 500 cycles, surpassing that of cells based on conventional Li 6 PS 5 Cl. Overall, this study underscores the importance of structural engineering for tailoring the mechanical properties of solid electrolytes and provides a design framework for developing mechanically robust composite cathodes for next‐generation ASSBs.