Polyvinyl formal based gel polymer electrolyte prepared using initiator free in-situ thermal polymerization method

Novel polyvinyl formal (PVFM) based gel polymer electrolytes (GPEs) are developed using an initiator free thermal polymerization method. The polymerization mechanism during the cross-linking process is investigated by means of Fourier transform infrared (FTIR) spectroscopy measurements. With the prepared GPEs (containing 2 to 5 wt % PVFM), Li polymer batteries with LiFePO₄ as the cathode are assembled, and the electrochemical properties such as interfacial impedance, electrochemical stability window and cycling performance are evaluated. The resulting PVFM based GPEs present a better thermal stability compared with the corresponding conventional liquid electrolyte and an acceptable conductivity of ∼10^-3 S cm ^-1 at ambient temperatures. Cyclic voltammetric (CV) curves reveal that the electrochemical stability window of PVFM based GPE is 1.5-5V vs. Li/Li⁺ and wider than that for the corresponding liquid electrolyte which is 1.8-4.4 V. The discharge capacity of the polymer Li/LiFePO₄ battery is 145 mAh g^-1 over a voltage range of 2.5-4.25 V at 1/10 C rate after 80 cycles with a small capacity fade.