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

Abstract

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 f irst time, we report F-doped Li 5+y PS 5 F y 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, Li 6 PS 5 F 0.5 Cl 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 Li 6 PS 5 F 0.5 Cl 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 Li 6 PS 5 F 0.5 Cl 0.5 SE against Li metal can be attributed to the formation of a stable solid electrolyte interphase (SEI)-containing conductive species (Li 3 P), alongside LiCl and LiF. These findings open new opportunities to develop high-performance SSLMBs using a novel class of F-doped argyrodite electrolytes.

Halogen‐Rich Lithium Argyrodite Solid‐State Electrolytes: A Review

“…Air stability and electrode compatibility are also necessary features. Doping can increase sulfide electrolyte's air stability and electrode compatibility, but it frequently causes the electrolyte‘s ionic conductivity to drop [62–65] …”

“…Doping can increase sulfide electrolyte's air stability and electrode compatibility, but it frequently causes the electrolyte's ionic conductivity to drop. [62][63][64][65] The key to developing all-solid-sate batteries is learning how to modify sulfide electrolytes to equip them with high ionic conductivity, high air stability, and high electrode compatibility. Halogen-rich lithium argyrodite electrolytes with a high enough ionic conductivity allow us a lot of room to improve performance in other areas.…”

Fluorinated Solid‐State Electrolytes for Lithium Batteries: Interface Design and Ion Conduction Mechanisms

“…(Reprinted with permission. [ 128 ] Copyright 2022, ACS). e) A cyclic voltammogram of Li 2 (OH) 0.9 F 0.1 Cl.…”

“…[ 127 ] Not only that, Wang et al also used a solvent method for double doping (F − and Cl − ) to obtain fluorine‐doped lithium argyrodite oxide SSEs Li 6 PS 5 F 0.5 Cl 0.5 . [ 128 ] Due to the enhanced interfacial stability of LiCl and LiF formed by SSEs and Li metal, the assembled full cell exhibits good cycling performance, with a specific discharge capacity exceeding 105 mAh g −1 (Figure 8d). Moreover, the fluorine‐doped antiperovskite lithium‐ion conductor Li 2 (OH)X (X=Cl, Br) is a promising SSE, and the substitution of OH by F enables the crystal phase transition of the material.…”

“…d) Cycling performance of the LFP|Li 6 PS 5 F 0.5 Cl 0.5 |Li cell. (Reprinted with permission [128]. Copyright 2022, ACS).…”