Synthesis and characterization of polymer electrolytes based on cross-linked phenoxy-containing polyphosphazenes

A new method to prepare the polymer electrolytes for lithium-ion batteries is proposed. The polymer electrolytes were prepared by reacting poly(phosphazene)s (MEEPP) having 2-(2-methoxyethoxy)ethoxy and 2-(phenoxy)ethoxy units with 2,4,6-tris[bis(methoxymethyl)amino]-1,3,5-triazine (CYMEL) as a cross-linking agent. This method is simple and reliable for controlling the cross-linking extent, thereby providing a straightforward way to produce a flexible polymer electrolyte membrane. The 6 mol % cross-linked polymer electrolyte (ethylene oxide unit (EO)/Li = 24:1) exhibited a maximum ionic conductivity of 5.36 × 10^-5 S cm^-1 at 100 °C. The ⁷Li linewidths of solid-state static NMR showed that the ionic conductivity was strongly related to polymer segment motion. Moreover, the electrochemical stability of the MEEPP polymer electrolytes increased with an increasing extent of cross-linking, the highest oxidation voltage of which reached as high as 7.0 V. Moreover, phenoxy-containing polyphosphazenes are very useful model polymers to study the relationship between the polymer flexibility; that is, the cross-linking extent and the mobility of metal ions.