High-Performance Flame-Retardant Polyphosphate ester-based Solid Electrolyte via In-situ Ring-Opening Polymerization for Safe Li Metal Batteries

Abstract

Conventional flammable liquid electrolytes compromise the safety of lithium metal batteries. We address this via the molecular design and in-situ catalyst-free ring-opening polymerization of a trifluoromethyl-functionalized polyphosphoester, yielding a cross-linked solid polymer electrolyte (TFMPE). The deliberate incorporation of phosphate, ether, and carbonate units with -CF3 side groups imparts intrinsic flame retardancy (from P/F) and governs ion transport. The resulting TFMPE membrane exhibits a balanced suite of properties: high ionic conductivity (3.7×10-4 S cm-1), a wide electrochemical window (4.9 V), and a high Li+ transference number (0.51), which promotes uniform Li+ flux and improves interfacial stability. Moreover, in-situ polymerization forms a seamless electrode/electrolyte interface, suppresses Li dendrite growth, and enables stable Li||Li cycling for 2000 h. Full cells using LiFePO4, LiMnFePO4, and LiCoO2 cathodes also show excellent cycling stability.