Li⁺ Conduction in a Polymer/Li_1.5Al_0.5Ge_1.5(PO₄)₃ Solid Electrolyte and Li-Metal/Electrolyte Interface

The solid oxide electrolyte Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) with a NASICON structure has a high bulk ionic conductivity of 10^-4 S cm^-1 at room temperature and good stability in the air because of the strong P⁵⁺-O^2- covalence bonding. However, the Ge⁴⁺ ions in LAGP are quickly reduced to Ge³⁺ on contact with the metallic lithium anode, and the LAGP ceramic has insufficient physical contact with the electrodes in all-solid-state batteries, which limits the large-scale application of the LAGP electrolyte in all-solid-state Li-metal batteries. Here, we prepared flexible PEO/LiTFSI/LAGP composite electrolytes, and the introduction of LAGP as a ceramic filler in polymer electrolytes increases the total ionic conductivity and the electrochemical stability of the composite electrolyte. Moreover, the flexible polymer shows good contact with the electrodes, resulting in a small interfacial resistance and stable cycling of all-solid-state Li-metal batteries. The influence of the external pressure and temperature on Li⁺ transfer across the Li/electrolyte interface is also investigated.