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, including 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.