Transport Properties of Flexible Composite Electrolytes Composed of Li_1.5Al_0.5Ti_1.5(PO₄)₃and a Poly(vinylidene fluoride- co-hexafluoropropylene) Gel Containing a Highly Concentrated Li[N(SO₂CF₃)₂]/Sulfolane Electrolyte

Flexible solid-state electrolyte membranes are beneficial for feasible construction of solid-state batteries. In this study, a flexible composite electrolyte was prepared by combining a Li+-ion-conducting solid electrolyte Li1.5Al0.5Ti1.5(PO4)3 (LATP) and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) gel containing a highly concentrated electrolyte of Li[N(SO2CF3)2] (LiTFSA)/sulfolane using a solution casting method. We successfully demonstrated the operation of Li/LiCoO2 cells with the composite electrolyte; however, the rate capability of the cell degraded with increasing LATP content. We investigated the Li-ion transport properties of the composite electrolyte and found that the gel formed a continuous phase in the composite electrolyte and Li-ion conduction mainly occurred in the gel phase. Solid-state 6Li magic-angle spinning NMR measurements for LATP treated with the 6LiTFSA/sulfolane electrolyte suggested that the Li+-ion exchange occurred at the interface between LATP and 6LiTFSA/sulfolane. However, the kinetics of Li+ transfer at the interface between LATP and the PVDF-HFP gel was relatively slow. The interfacial resistance of LATP/gel was evaluated to be 67 ω·cm2 at 30 °C, and the activation energy for interfacial Li+ transfer was 39 kJ mol-1. The large interfacial resistance caused the less contribution of LATP particles to the Li-ion conduction in the composite electrolyte.