Flame-retardant solid polymer electrolyte with a bridged interface towards safe and stable lithium-metal batteries

Abstract

Solid polymer electrolytes (SPEs) have great potential for enabling stable lithium-metal batteries (LMBs), although safety concerns still limit their practical applications. With numerous reports on developing flame-retardant properties in SPEs, the parasitic reaction between the lithium (Li)-metal anode and flame-retardant molecules remains a technical issue. Herein, we propose a flame-retardant solid polymer electrolyte with a bridged interface (FRSPE-BI) that improves the interface stability between the Li-metal anode and flame-retardant molecules via covalent attachment of the electrolyte to the Li-metal anode (Li–O–Si). The FRSPE-BI, synthesized from the copolymerization of 3-(trimethoxysilyl)propyl methacrylate (MPS) and poly(ethylene glycol) methacrylate (PEGMA), is chemically crosslinked using hexamethylene diisocyanate (HDI). With high ambient-temperature ionic conductivity (3.1 × 10⁻⁴ S cm⁻¹) and a decent cationic transport number (0.59), the FRSPE-BI exhibited improved electrochemical performance. Forming a bridged interface with the Li-metal electrode, the Li/FRSPE-BI/Li cells exhibited superior cycling stability with reduced polarization voltage and stable interfacial resistance. The Li/FRSPE-BI/LFP cells demonstrated a high-capacity retention of 76.7% at 1C after 300 cycles under ambient temperature. Moreover, FRSPE-BI can eliminate fire safety risks and enable workable batteries at a high temperature of 237 °C. The design principle of constructing a covalent bond between the electrolyte and Li-metal electrode provides a new pathway toward a solid-state LMB system with high safety and stable cycle performance.