The correlation between the cycling efficiency, surface chemistry and morphology of Li electrodes in electrolyte solutions based on methyl formate

The correlation between Li-cycling efficiency, morphology and surface chemistry in methyl formate (MF) solutions have been investigated. The salt included LiAsF6, LiClO4, LiBF4, and their mixtures. Propylene ethylene, dimethyl and diethyl carbonates, tetrahydrofuran and 1,3-dioxolane were used as co-solvents. The impact of CO2 as an additive was also explored. Ex situ surface sensitive Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy were used in conjunction with standard electrochemical techniques. Li-cycling efficiency was measured in half-cell testing of electrodes prepared by Li deposition on copper. The Li-cycling efficiency in all MF solutions is poor due to the instability of the surface films formed on Li in this highly polar solvent. The use of CO2 enabled high Li-cycling efficiency to be obtained due to an efficient passivation of Li2CO3 formed on Li when CO2 was present. The performance of Li electrodes in these systems could be further optimized by the use of the appropriate co-solvents. The factors which determine the anode utility in these systems are discussed.