Effects And Mechanism Of Fluoroethylene Carbonate On Low Temperature Performance Of Li-ion Batteries

In this work, the methods of evaluating electrolyte's cell performance for Li-ion battery in laboratory were established; then the compatibility of graphite electrode with electrolyte at low temperature, and the influence and mechanism of FEC additive on the low temperature performance of Li-ion cells were thoroughly studied. The main results obtained in this work are listed as follows.(1) The testing temperature, the cell's model and the oxidation film formed on the lithium plate greatly impact the battery performance, while the types of graphites and the ratio of graphite, conductive agent and binder have little effect on it.(2) Among the graphite electrodes which were studied, natural graphite AGP-8 delivers the highest discharge capacity and the lowest charge voltage, and shows the best compatibility with the electrolyte selected at low temperature. Using natural graphite AGP-8 as electrode could improve the reliability of the electrolyte evaluation.(3) FEC can improve the cycle performance of Li-ion battery at 20℃and reduce the volatility of the Li-ion's capacity at non-constant temperatures. Also, FEC additive can increase the first discharge capacity, improve the cycle performance and rate performance, decrease the delithiation voltage of the Li-ion battery at-20℃.(4) The FEC additive could reduce the resistance of the SEI film and the impedance of charge transfer on the graphite electrode. The SEI film contacted with the FEC additive was demonstrated to have improved morphology and richer LiF. Therefore, the improved low temperature performance of Li-ion cells with FEC additive could be ascribed to a richer LiF contained SEI film formed on the graphite electrode.