Boosting Li-Ion Conductivity of Fluoride Solid Electrolyte by Low-Temperature Molten Salt Ablation and Particle Boundary Doping.

Abstract

Halide solid electrolytes (SEs) are attracting great attention, owing to their high ionic conductivity and excellent high-voltage compatibility. However, severe moisture sensitivity, poor thermal stability, and instability at the lithium metal anode interface with chloride and bromide SEs retard their applications in solid-state lithium metal batteries. Fluoride SEs are expected to solve these problems, but they are now plagued by inadequate room-temperature (RT) ionic conductivity. Herein, a low-temperature molten salt (LiCl+1.33AlCl₃) ablation method is proposed to enhance the ionic conductivity of monoclinic Li₃GaF₆ by particle boundary doping. The RT ionic conductivity of Li₃GaF₆ is correspondingly increased by 2 orders of magnitude, and the conductivity reaches 10^-4 S cm^-1 at 60 °C. The improved ionic conductivity benefits from the enhancement of interfacial ion transport, with the formation of more conductive chlorine-doped Li₃GaF_6-<i>_x</i>Cl_<i>x</i> and in situ binder LiAlCl₄ to cement surrounding nanoparticles. The as-synthesized Li₃GaF₆ demonstrates outstanding humidity tolerance without conductivity degradation after exposure to a relative humidity of up to 35%. It also exhibits the widest electrochemical stability window experimentally (close to 6 V) compared with other state-of-the-art SEs. The solid-state Li/Li₃GaF₆/LiFePO₄ cell with a stable Li⁺-conductive polymer interface is successfully driven for at least 200 cycles at 0.5C. Our study provides a solution to various chemical and electrochemical stability issues encountered by the halide SE family.