High‐Voltage All‐Solid‐State Lithium Metal Batteries Mediated by the YF ₃ Strengthened Polycaprolactone Electrolytes

ABSTRACT Integrating lithium‑rich/nickel‐rich high‑voltage cathodes with polycaprolactone‑based all‑solid‑state polymer electrolytes markedly boosts the safety and specific energy of all‐solid‑state batteries. However, interfacial incompatibility at high voltages poses a significant challenge for polycaprolactone (PCL)‐based all‐solid‐state polymer electrolytes (SPEs), therefore limiting their pairing in high‐voltage cathodes. Herein, YF ₃ ‐improved PCL SPEs are designed for 4.5 V‐, and 4.8 V‐class all‐solid‐state lithium metal batteries (ASSLMBs). In modified SPEs, PCL carbonyl groups donate electron density to YF ₃ , weakening Li ⁺ ‐polymer coordination and thus promoting ionic conductivity, while YF ₃ with Lewis‐acid properties simultaneously improves anti‐oxidation capability. The optimized membrane delivers 5.1 V oxidative stability, high thermal robustness, and forms stable cathodic and anodic interphases. Remarkably, Ni‐rich (NCM9055) cells with the optimized electrolyte achieve 500 cycles at 4.5 V with 69.8% capacity retention, while Li‐rich cells sustain 200 cycles at 4.8 V – performance that was previously difficult to achieve. This YF ₃ /PCL design validates a practical route and offers a unique dual‐interface stabilization strategy for high‐voltage ASSLMBs.