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 ([Py1,4]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 Li2CO3 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 LiFePO4 (LFP) cathode delivers a specific capacity of ≈120 mAh g−1 with a Coulombic efficiency greater than 96% at 10 mA g(LFP) −1.