Outward Charge-Enriched Boundaries Stabilize Halide–Sulfide Solid–Electrolyte Interfaces

Halide-sulfide bilayer electrolytes offer a practical route toward all-solid-state lithium batteries by combining the high-voltage compatibility of halide electrolytes with the favorable interfacial contact and Li+-transport characteristics of sulfide interlayers. However, the intrinsic chemical incompatibility between the halide Li3InCl6 (LIC) and the sulfide Li6PS5Cl (LPSC) drives persistent In-centric interfacial reconstruction, leading to continuous impedance growth and electrochemical decay. Across an investigated Li3In1-xYxCl6 series (nominal x = 0.05-0.20), we show that the nominal x = 0.10 composition generates an outward charge-enriched boundary on the outer Cl- sublattice, markedly improving interfacial compatibility. This localized electronic feature smooths the electrostatic landscape for Li+ migration, increases the room-temperature ionic conductivity to 1.5 mS cm-1, and effectively suppresses the sustained inward propagation of sulfide-associated interfacial reconstruction. As a result, the corresponding halide/sulfide interface develops a much shallower and more spatially confined reconstruction layer. All-solid-state cells based on the optimized chloride electrolyte deliver a specific capacity of 175 mAh g-1 at 1C, with about 88.2% capacity retention after 1000 cycles. These findings establish outward charge-boundary engineering as a route to simultaneously improve bulk ion transport and suppress interfacial degradation in reactive halide-sulfide solid electrolytes.