Electrochemical properties of the solid polymer electrolyte PEO20–LiSO3CF3–Urea1.5

In this work, a new type of solid polymer electrolyte, formulated as PEO20–LiSO3CF3–Ureax, is prepared by solution-casting technique. Among the different urea-containing samples, PEO20–LiTf–Urea1.5 presents an optimum ambient temperature ionic conductivity of 5.1 × 10− 6 S/cm, enhanced by nearly 100 times over the PEO20-LiTf system. The mechanism for the increase in conductivity is discussed by the joint analysis of DSC and ATR-IR measurements. Urea molecules coordinate with lithium ions via carbonyl groups, which efficiently decrease the concentration of triple cations [Li2Tf]+. The addition of urea results in augmented cation transference and increased interface impedance. Analysis of the salt diffusion coefficient confirms that urea enhances the transport of lithium salts. The prepared SPE is tested by galvanostatic charge-discharge cycling in lithium secondary cells. Data obtained for PEO20–LiSO3CF3–Urea1.5 reveals that the upper voltage limit in the LiCoO2-based cells should be about 4.0 V versus Li/Li+.

► Correlation between decreased concentration of triple cation [Li2Tf]+ and increase ionic conductivity for PEO20-LiTf.
► Illustration of microstructure for polymer electrolytes based on a reasonable assumption.
► Addition of urea in PEO20-LiTf results in augmented cation transference number and increased interface impedance.
► Urea substantially enhances the salt diffusion rate.
► The prepared SPE is anodically stable below 4.0 V for LiCoO2.