Homogeneous Fluorine Doping toward Highly Conductive and Stable Li₁₀GeP₂S₁₂ Solid Electrolyte for All-Solid-State Lithium Batteries.

Abstract

The unique structure and exceptionally high lithium ion conductivity over 10 mS cm^-1 of Li₁₀GeP₂S₁₂ have gained extensive attention in all-solid-state lithium batteries. However, its poor resistivity to moisture and chemical/electrochemical incompatibility with lithium metal severely impede its practical application. Herein, a fluorine functionalized Li₁₀GeP₂S₁₂ is synthesized by stannous fluoride doping and employed as a monolayer solid electrolyte to realize stable all-solid-state lithium batteries. The atomic-scale mechanism underlying the impact of fluorine doping on both moisture and electrochemical stability of Li₁₀GeP₂S₁₂ is revealed by density functional theory calculations. Fluorine surface doping significantly reduces surface hydrophilicity by electronic regulation, thereby retarding the hydrolysis reaction of Li₁₀GeP₂S₁₂. After exposed to a relative humidity of 35%-40% for 20 min, the ionic conductivity of Li_9.98Ge_0.99Sn_0.01P₂S_11.98F_0.02 maintains as high as 2.21 mS cm^-1, nearly one order of magnitude higher than that of Li₁₀GeP₂S₁₂ with 0.31 mS cm^-1. Meanwhile, bulk doping of highly electronegative fluorine promotes the formation of lithium vacancies in the Li₁₀GeP₂S₁₂ system, thus allowing stable lithium plating/stripping in Li | Li symmetric batteries, boosting a critical current density reaching 2.1 mA cm^-2. The LiCoO₂ | lithium all-solid-state batteries display improved cycling stability and rate capability, showing 80.1% retention after 600 cycles at 1C.