External pressure's influence on lithium-ion transport within solid-state electrolytes

High-performance solid-state electrolytes (SSEs) are central to enabling next-generation all-solid-state batteries (ASSBs), where fast Li + transport remains a critical bottleneck for practical deployment. However, the mechanism by which external pressure modulates ionic conductivity in representative SSEs—halide-based Li 3 InCl 6 (LIC) and sulfide-based Li 6 PS 5 X (LPSX, X = Cl, Br, I)—remains poorly understood. To address this challenge, we fabricated pellet sandwiched between two stainless steel rods with 10 mm diameter with both SSEs and performed in situ investigations of their pressure-dependent conductivity. The results reveal that battery assembly methods are pivotal: when an initial compaction pressure of 400 MPa was applied and subsequently released, ionic conductivity decreased steadily with increasing external pressure again. In contrast, without prior compaction, conductivity initially increased with pressure before declining beyond a critical threshold. The conductivity of LPSC decreased to 70% of its maximum value once the pressure exceeded 1000 MPa, and further dropped to 25% at 1657 MPa (relative to 414 MPa). LIC exhibited a much earlier deterioration, retaining only ∼25% of its initial conductivity at 625 MPa and just 10% above 1000 MPa. These findings highlight the decisive role of pressure in governing Li + transport within SSEs and provide essential guidance for optimizing assembly pressures in practical ASSBs.