Electrochemical characterization of two types of PEO-based polymer electrolytes with room-temperature ionic liquids

Two types of gel polymer electrolytes containing ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), are prepared by the solution casting method. Fourier-transform infrared spectroscopy indicates that incorporating both EMI-TFSI and PP13-TFSI can change the phase structure of poly(ethylene oxide) (PEO) and cause a more amorphous phase, can weaken the association between Li(+) and TFSI(-), and cause more free Li(+). The TFSI(-) anion from these two kinds of ionic liquids plays a main plasticizing role in the P(EO)(20)LiTFSI systems. From cyclic voltammetry results, the good reversibility of the Li deposition and dissolution, as well as a stable interfacial passive film, can be expected by incorporating PP13-TFSI and a relatively small quantity of EMI-TFSI. The addition of EMI-TFSI and PP13-TFSI to P(EO)(20)LiTFSI polymer electrolyte results in an increase of the ionic conductivity, a wider electrochemical stability window, a decrease of the glass transition temperature (T(g)), and a better interfacial stability. The tests performed on Li metal/LiFePO(4) batteries containing these two types of polymer electrolytes have shown a good capacity performance at a moderate temperature. (C) 2008 The Electrochemical Society.