Dual-Salt-Regulated PEO–LLZTO Composite Polymer Electrolytes for Solid-State Lithium Metal Batteries with Stable Interfaces

Abstract Poly(ethylene oxide) (PEO)-based solid polymer electrolytes are promising for solid-state lithium metal batteries because of their favorable flexibility, processability, and lithium-salt solvation capability. However, their practical application is still limited by insufficient ion transport, a low lithium-ion transference number, and poor lithium-metal interfacial stability. Herein, dual-lithium-salt-regulated composite polymer electrolytes (DSCPEs) were constructed by incorporating a LiTFSI/LiDFOB dual-salt system into a PEO–LLZTO composite polymer electrolyte matrix to achieve a balance between ion transport capability and lithium-metal interfacial stability. The optimized DSCPE-2 (LiTFSI:LiDFOB = 3:2) delivers an ionic conductivity of 6.63 × 10–4 S cm–1, a lithium-ion transference number of 0.69, and an electrochemical stability window of up to 5.1 V. In addition, the Li|DSCPE-2|Li symmetric cell operates stably for over 2000 h at 60 °C under 0.1 mA cm–2, with a critical current density of 1.05 mA cm–2, indicating its enhanced compatibility with lithium metal. When assembled with a LiFePO4 cathode, the Li|DSCPE-2|LFP full cell delivers an initial discharge capacity of 136.54 mAh g–1 and retains 123.41 mAh g–1 after 300 cycles at 0.5 C and 60 °C, corresponding to a capacity retention of 90.38%. Furthermore, the cell maintains a reversible capacity of approximately 115 mAh g–1 even at 5 C. This work provides an effective strategy for regulating ion transport behavior and lithium–metal interfacial stability in PEO-based composite polymer electrolytes.