Ammonium-assisted wet chemistry was reported to synthesize various superionic halide electrolytes with a submicrometer size.

There are three main situations in which the uniformity and microstructure of the slurry change with the solid content.

Halide solid electrolytes (SEs) have garnered significant attention due to their decent ionic conductivity and exceptional high-voltage stability. However, their structure–property relationship remains enigmatic. In this work, we synthesize a range of Li_3−xLu_1−xZr_xCl₆ (LLZC, 0 < x < 1) solid solutions to investigate the influence of lithium ion and vacant site contents on ionic conductivity. Contrary to our previous understanding, it is found that achieving a balance in lithium ion and vacant site content, rather than aliovalent substitution-induced structural changes that introduce stacking faults, plays a crucial role in optimizing ion transport in a hexagonal close packing (hcp) anion framework. This balance culminates in the highest ionic conductivity (1.5 mS cm⁻¹) and lowest activation energy (0.285 eV) of LLZC. In addition, aliovalent substitution strengthens the oxidative stability of halide SSEs but reduces their reduction stability. Using LLZC as SEs and LiMn₂O₄ as cathodes, cobalt-free all-solid-state batteries undergo 1000 cycles of stable cycling at 0.3 C with negligible capacity decay.

All‐solid‐state lithium metal batteries (ASSLMBs) featuring sulfide solid electrolytes (SEs) are recognized as the most promising next‐generation energy storage technology because of their exceptional safety and much‐improved energy density. However, lithium dendrite growth in sulfide SEs and their poor air stability have posed significant obstacles to the advancement of sulfide‐based ASSLMBs. Here, a thin layer (approximately 5 nm) of g‐C₃N₄ is coated on the surface of a sulfide SE (Li₆PS₅Cl), which not only lowers the electronic conductivity of Li₆PS₅Cl but also achieves remarkable interface stability by facilitating the in situ formation of ion‐conductive Li₃N at the Li/Li₆PS₅Cl interface. Additionally, the g‐C₃N₄ coating on the surface can substantially reduce the formation of H₂S when Li₆PS₅Cl is exposed to humid air. As a result, Li–Li symmetrical cells using g‐C₃N₄‐coated Li₆PS₅Cl stably cycle for 1000 h with a current density of 0.2 mA cm⁻². ASSLMBs paired with LiNbO₃‐coated LiNi_0.6Mn_0.2Co_0.2O₂ exhibit a capacity of 132.8 mAh g⁻¹ at 0.1 C and a high‐capacity retention of 99.1% after 200 cycles. Furthermore, g‐C₃N₄‐coated Li₆PS₅Cl effectively mitigates the self‐discharge behavior observed in ASSLMBs. This surface‐coating approach for sulfide solid electrolytes opens the door to the practical implementation of sulfide‐based ASSLMBs.