Stabilizing Li4SnS4 Electrolyte from Interface to Bulk Phase with a Gradient Lithium Iodide/Polymer Layer in Lithium Metal Batteries

Sulfide electrolytes promise superior ion conduction in all-solid-state lithium (Li) metal batteries, while suffering harsh hurdles including interior dendrite growth and instability against Li and moist air. A prerequisite for solving such issues is to uncover the nature of the Li/sulfide interface. Herein, air-stable Li4SnS4 (LSS) as a prototypical sulfide electrolyte is selected to visualize the dynamic evolution and failure of the Li/sulfide interface by cryo-electron microscopy. The interfacial parasitic reaction (2Li + 2Li4SnS4 = 5Li2S + Sn2S3) is validated by direct detection of randomly distributed Li2S and Sn2S3 crystals. A bifunctional buffering layer is consequently introduced by self-diffusion of halide into LSS. Both the interface and the grain boundaries in LSS have been stabilized, eliminating the growing path of Li dendrites. The buffering layer enables the durability of Li symmetric cell (1500 h) and high-capacity retention of the LiFePO4 full-cell (95%). This work provides new insights into the hierarchical design of sulfide electrolytes.