Synthesis of Fluorine-Doped Lithium Argyrodite Solid Electrolytes for Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) that utilize novel solid electrolytes (SEs) have garnered much attention because of their potential to yield safe and high-energy-density batteries. Sulfide-based argyrodite-class SEs are an attractive option because of their impressive ionic conductivity. Recent studies have shown that LiF at the interface between Li and SE enhances electrochemical stability. However, the synthesis of F-doped argyrodites has remained challenging because of the high temperatures used in the state-of-the-art solid-state synthesis methods. In this work, for the first time, we report F-doped Li₅₊PS₅F argyrodites with a tunable doping content and dual dopants (F^-/Cl^- and F^-/Br^-) that were synthesized through a solvent-based approach. Among all compositions, Li₆PS₅F_0.5Cl_0.5 exhibits the highest Li-ion conductivity of 3.5 × 10^-4 S cm^-1 at room temperature (RT). Furthermore, Li symmetric cells using Li₆PS₅F_0.5Cl_0.5 show the best cycling performance among the tested cells. X-ray photoelectron spectroscopy and ab initio molecular dynamics simulations revealed that the enhanced interfacial stability of Li₆PS₅F_0.5Cl_0.5 SE against Li metal can be attributed to the formation of a stable solid electrolyte interphase (SEI)-containing conductive species (Li₃P), alongside LiCl and LiF. These findings open new opportunities to develop high-performance SSLMBs using a novel class of F-doped argyrodite electrolytes.