Stable Li|LAGP Interface Enabled by Confining Solvate Ionic Liquid in a Hyperbranched Polyanionic Copolymer for NASICON-Based Solid-State Batteries

The NASICON-type Li_1.5Al_0.5Ge_1.5P₃O₁₂ (LAGP) ceramic electrolyte has the advantages of relatively high ionic conductivity, a wide electrochemical potential window, and air stability. However, side reactions and poor thermal stability between lithium metal and LAGP are enormous challenges. Here, we report a gel polymer electrolyte (GPE) interlayer composed of a solvate ionic liquid and a hyperbranched polyanionic copolymer to stabilize the Li|LAGP interface. The GPE with epoxy groups ensures excellent compatibility and protection between the LAGP and Li metal anode to suppress unfavorable reactions. Moreover, introducing a solvate ionic liquid with non-inflammability can effectively avoid the hidden danger of thermal runaway between lithium metal and LAGP at high temperatures (300 °C). The Li|GPE|LAGP|LiFePO₄ full cell with the gel interface layer delivers a high reversible capacity of 139.5 mA h g^-1 at 0.3 C and can stably cycle 300 times with a retention of 93.4%. This work provides an enlightening strategy for unstable electrolyte interfaces in promising, safe, and outstanding solid-state batteries.