Ionic Liquid and Polymer Coated Garnet Solid Electrolytes for High‐Energy Solid‐State Lithium Metal Batteries

Garnet solid‐state electrolytes are promising for lithium metal batteries in terms of safety and stability. However, their rigid and brittle nature under humid air increase the interfacial resistance at the electrode/electrolyte interfaces. Herein, the ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py 1,4 ]TFSI) is employed to dissolve poly(ethylene oxide) (PEO) polymer microstructures that facilitate ion transport through an amorphous rather than a crystalline polymer matrix. The PEO‐IL coatings on Ga doped garnet (LLZO) solid‐state electrolyte can effectively suppress the formation of Li 2 CO 3 passivation layers on the garnet surface. Electrochemical impedance spectroscopy results show that the interfacial resistance of PEO‐IL coated Ga‐LLZO is much lower than that of uncoated Ga‐LLZO. In addition, the surface of PEO‐IL@Ga‐LLZO is flatter and it has a better contact with the Li anode than uncoated Ga‐LLZO. A symmetric cell of Li/PEO‐IL coated solid electrolyte/Li exhibits flat voltage profiles with overpotentials of less than 15 mV and shows a stable lithium plating/stripping process. A solid‐state battery based on PEO‐IL@Ga‐LLZO solid electrolyte combined with lithium metal anode and LiFePO 4 (LFP) cathode delivers a specific capacity of ≈120 mAh g −1 with a Coulombic efficiency greater than 96% at 10 mA g (LFP) −1 .