Electrochemically Stable Poly (Vinylidene Fluoride)-Polyurethane Polymer Gel Electrolytes with Polar β-Phase in Lithium Batteries

Abstract

The β-phase in polyvinylidene fluoride (PVDF) exhibits superior dielectric properties as compared to α-phase, with ionic conductivity in liquid electrolytes up to one magnitude higher than that of α-phase, providing excellent ion transportation performance when applied as polymer gel electrolytes (GPEs). However, the strong polarization effect affects the electrochemical stability of GPEs. To improve the electrochemical stability while maintaining high ionic conductivity, PVDF and PU are selected to be co-blended. Porous polymer membranes containing polar β phase are prepared by solution-casting phase separation, and then swollen and activated in the electrolyte to obtain PVDF-PU composite GPEs. The results show that PVDF0.8-PU0.2 GPE exhibits an ionic conductivity of 8.81 × 10-4 S cm-1 at room temperature and 1.88 × 10-3 S cm-1 at 65°C (the polyether segment in PU starts to melt). Li+ transference numbers (~ 0.84), electrochemical oxidation limit (~ 4.3 V) and charge-discharge performance satisfy the commercial requirements. Coulombic efficiency (~ 99%) and capacity retention (~ 96%) indicate that this GPE promises to be commercially available.