Phosphorus flame retardant in situ fixed on a gel polymer electrolyte for lithium metal batteries with enhanced safety and superior electrochemical performance†

Abstract

Non-flammable polymer electrolytes are attractive due to their inherent potential to eliminate the fire hazards of conventional liquid electrolytes. However, the most widely used flame-retardant additives can facilely react with Li anodes and are unfavorable for the conduction of lithium ions, restricting the improvement of electrochemical performance. Herein, a flame-retardant gel polymer electrolyte (FGPE) is prepared by in situ copolymerization of a flame-retardant additive on a polymer backbone. The phosphorus additive 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) presents excellent flame-retardant properties, while the abundant CO/C–O groups on polyethylene glycol methyl ether methacrylate (PEGMEMA) and polyethylene glycol diacrylate (PEGDA) promote the rapid transfer of lithium ions. After the reaction between the P–H bond in the former and the unsaturated double bond in the latter, the adverse reactions between DOPO and the Li anode are suppressed, and an excellent polymer electrolyte with high safety and outstanding electrochemical performance is obtained. Given the feasibility of our strategy, a record of sustained firing for 190 s without ignition is presented. Moreover, a high ionic conductivity (1.13 mS cm⁻¹ at 25 °C) is achieved after specifically regulating the content of DOPO; the assembled LiFePO₄/FGPE/Li battery shows excellent cycling performance after 300 cycles with a capacity of 165.7 mA h g⁻¹ and capacity retention of 99.7%, realizing a balance between high safety and high performance. This electrolyte design philosophy provides a promising path for high-safety and high-energy-density lithium metal batteries.