Cyclotriphosphazene-based flame-retardant polymer electrolytes for high performance sodium metal batteries

A flexible flame-retardant polymer electrolyte (FRPE) based on the cross-linked network containing cyclophosphazene groups in the presence of poly(ethylene glycol)-functionalized cyclotriphosphazene (PEG-CP) is fabricated via thiol-acrylate and radical photopolymerizations under UV irradiation for the application in sodium metal batteries (SMBs). The PEG-CP with star-shaped structure applied as the ionic conductor and flame-retardant reinforcer, improves simultaneously the flame retardancy and ionic conductivity of FRPEs. With the increase of PEG-CP content, the crystallinity and glass-transition temperature of FRPEs decrease significantly, which is beneficial to the improvement of ionic conductivity. More importantly, the introduction of cyclic boronic ester improves the sodium-ion transference number and interfacial stability owing to the interaction between anions of sodium salt and boronic ester. The Na/FRPE-40/Na3V2(PO4)3 cell exhibits high specific capacity of 88.0 mAh/g at 0.1C, and retains 91.4 % of the initial capacity after 100 cycles. This work demonstrates that the FRPEs have the great potential for SMBs with high safety and excellent electrochemical performances.