Multilayer asymmetric solid polymer electrolyte with modified interface for high-voltage solid-state Li metal batteries

Solid polymer electrolytes (SPEs) are promising for achieving safe solid-state Li metal batteries (SSLMBs). However, unstable electrode/electrolyte interface contact of SPEs limits their application at high voltage. To address this issue, we designed a multi-layer asymmetric SPE with a sandwich structure based on the hydroxyapatite (HAP) enhanced PVDF-HFP matrix. Two different interfacial modification layers were introduced on the anode and cathode sides. Methyl (2,2,2-trifluoromethyl) carbonate (FEMC) and tetramethylene sulfone (TMS) were selected as plasticizers in the layers, respectively, contacting the anode and cathode to reduce the reactivity of the anode interface and enhance the high-voltage compatibility of the cathode interface. Unlike usual designs, each layer of the asymmetric SPE has essentially the same composition except the plasticizers, effectively eliminating interface resistance and promoting Li⁺ fast migration. Consequently, the asymmetric SPE exhibits excellent ionic conductivity at room temperature (8.8 × 10⁻⁴ S cm⁻¹), superior interfacial stability, and Li dendrite inhibition ability. In addition, LiFePO₄||SPE||Li cell demonstrates a stable retention rate of over 92 % after 500 cycles at 1 C. These findings provide a new approach for implementing SSLMBs under high voltage.