Nano-scale mechanism of crack nucleation/propagation and lithium penetration in solid electrolyte

This work presents a direct observation and quantification of the fundamental mechanism of lithium penetration in Li6PS5Cl (LPS) solid electrolyte. Lithium plating in a nano-sized defect on the LPS surface led to the nucleation and propagation of a crack without external force. It also revealed a reduction in the mechanical strength of LPS when altering the electrochemical charge/discharge bias condition. A first principles atomistic simulation was performed to confirm that the disorder in the crystal structure of the LPS, both in lithium deficient and excess states, contributes to the reduced mechanical strength, and a decrease in the modulus is observed when lithium concentration is decreased from the stoichiometric amount. The results of this study suggest the importance of minimizing defects at the surface and grain boundaries to improve the stability of the solid-state electrolyte (SSE). Interfaces and boundaries can be bottlenecks for lithium diffusion, creating the concentration gradient. This can reduce the mechanical stabilities of the SSE, accelerating lithium penetration and degradation in all-solid-state lithium batteries. The insights obtained in this study provide useful information towards understanding the mechanism and designing the materials/structures to solve this issue.