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 Li 10 GeP 2 S 12 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 Li 10 GeP 2 S 12 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 Li 10 GeP 2 S 12 is revealed by density functional theory calculations. Fluorine surface doping significantly reduces surface hydrophilicity by electronic regulation, thereby retarding the hydrolysis reaction of Li 10 GeP 2 S 12 . After exposed to a relative humidity of 35%–40% for 20 min, the ionic conductivity of Li 9.98 Ge 0.99 Sn 0.01 P 2 S 11.98 F 0.02 maintains as high as 2.21 mS cm −1 , nearly one order of magnitude higher than that of Li 10 GeP 2 S 12 with 0.31 mS cm −1 . Meanwhile, bulk doping of highly electronegative fluorine promotes the formation of lithium vacancies in the Li 10 GeP 2 S 12 system, thus allowing stable lithium plating/stripping in Li | Li symmetric batteries, boosting a critical current density reaching 2.1 mA cm −2 . The LiCoO 2 | lithium all‐solid‐state batteries display improved cycling stability and rate capability, showing 80.1% retention after 600 cycles at 1C.