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

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, YF₃-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 YF₃, weakening Li⁺-polymer coordination and thus promoting ionic conductivity, while YF₃ 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 YF₃/PCL design validates a practical route and offers a unique dual-interface stabilization strategy for high-voltage ASSLMBs.