Synergistic Regulation of Bulk and Interfacial Properties in PEO Electrolyte by CaF2 for Stable Lithium Metal Batteries

Abstract

The advancement of all-solid-state lithium metal batteries (ASSLMBs) with high energy density is significantly constrained by the inherent drawbacks of solid polymer electrolytes (SPEs), such as low room-temperature ionic conductivity, inadequate mechanical strength to prevent lithium dendrite growth, and unstable interfaces between electrodes and electrolytes.To overcome these limitations, this study introduces a synergistic modification strategy for poly(ethylene oxide) (PEO)-based SPEs by incorporating calcium fluoride (CaF₂), aiming to concurrently improve bulk electrolyte properties and interfacial stability. The introduction of CaF₂ effectively inhibited the crystallization of PEO by steric hindrance and Lewis acid, which made the ionic conductivity of the electrolyte reach 8.49 × 10^-5 S cm^-1 at 40°C (70% higher than that of the original electrolyte) and the lithium-ion migration number increased to 0.34.The composite electrolyte also demonstrates improved mechanical robustness (yield strength of 1.61 MPa) and electrochemical stability window (with decomposition voltage elevated to 5.01 V), enabling a critical current density of 0.8 mA cm^-2 . Theoretical and experimental investigations reveal that CaF₂ promotes the formation of a hybrid solid electrolyte interphase (SEI) enriched with LiF and Ca-Li alloys, which guides uniform lithium deposition. As a result, Li symmetric cells achieve stable cycling over 1600 hours, and LiFePO₄||Li full cells retain 98.1% of their initial capacity after 300 cycles at 0.5 C. This work offers a scalable pathway for designing dendrite-suppressing SPEs through dual optimization of bulk and interfacial characteristics.