A Solid‐State Electrolyte Based on Li0.95Na0.05FePO4 for Lithium Metal Batteries

Solid‐state electrolytes (SSEs) play a crucial role in developing lithium metal batteries (LMBs) with high safety and energy density. Exploring SSEs with excellent comprehensive performance is the key to achieving the practical application of LMBs. In this work, the great potential of Li0.95Na0.05FePO4 (LNFP) as an ideal SSE due to its enhanced ionic conductivity and reliable stability in contact with lithium metal anode is demonstrated. Moreover, LNFP‐based composite solid electrolytes (CSEs) are prepared to further improve electronic insulation and interface stability. The CSE containing 50 wt% of LNFP (LNFP50) shows high ionic conductivity (3.58 × 10−4 S cm−1 at 25 °C) and good compatibility with Li metal anode and cathodes. Surprisingly, the LMB of Li|LNFP50|LiFePO4 cell at 0.5 C current density shows good cycling stability (151.5 mAh g−1 for 500 cycles, 96.5% capacity retention, and 99.3% Coulombic efficiency), and high‐energy LMB of Li|LNFP50|Li[Ni0.8Co0.1Mn0.1]O2 cell maintains 80% capacity retention after 170 cycles, which are better than that with traditional liquid electrolytes (LEs). This investigation offers a new approach to commercializing SSEs with excellent comprehensive performance for high‐performance LMBs. A flexible composite solid electrolyte based on Li0.95Na0.05FePO4 with excellent interface compatibility by a simple preparation method. Due to its excellent interfacial stability with lithium metal and cathodes, the lithium metal battery of Li||LFP at 0.5 C current density shows excellent cycling stability (151.5 mAh g−1 for 500 cycles, 96.5% capacity retention, and 99.3%).