Interfacial Behavior of a Thio-LISICON Solid-State Electrolyte under External Pressure

External pressure can greatly affect the cycling performance of solid-state batteries, although pressure-driven changes in electrochemical processes are still not fully understood. For instance, Li10GeP2S12 (LGPS) is known to strongly react with lithium metal. However, under external pressure, with the use of additional Li6PS5Cl (LPSCl) outer layers forming a sandwich structure, stable cycling over 1800 cycles can be achieved (Ye and Li. Nature, 2021, 593, 218-222). This finding motivated us to investigate the interfacial behavior of Li/LGPS under external pressure (i.e., 7.5 MPa) to shed light on the improved electrochemical performance. Our results indicate that the growth rate of the interfacial resistance is substantially decreased while the cycle life before open-circuit failure is substantially extended under the MPa-scale external pressure. The reactivity between lithium metal and LGPS is extremely severe, forming island features upon cycling. Using selected area Raman spectroscopy, these "islands" were confirmed to be interfacial decomposition products, in line with the expected volume expansion of LGPS when reacting with lithium metal. Under 7.5 MPa external pressure, the interface products were formed in a densely packed and homogeneously distributed manner after 10 cycles. While the application of external pressure did not alter the types of interface components, the MPa-level pressure functioned in improving physical contact of the interface and homogenizing the interface reactions. In addition, a lithium depletion layer was formed in the interface region, which can contribute to extra interfacial resistance. Our work marks an important step to understanding the interfacial electrochemical behavior of the Li/LGPS interface under an MPa-scale external pressure.