Tetrafluoroterephthalonitrile: A Novel Electrolyte Additive for High-Voltage Lithium Cobalt Oxide/Graphite Battery

A novel electrolyte additive, tetrafluoroterephthalonitrile (TFTPN), is proposed to improve the cyclic stability of lithium cobalt oxide (LiCoO₂)/graphite lithium-ion full cells up to 4.4 V. Electrochemical measurements indicate that TFTPN can be reduced on graphite electrode and oxidized on LiCoO₂ electrode preferentially compared to the baseline electrolyte, 1.0 M LiPF₆ in EC/DEC/EMC (1/1/1, in weight), and thus improves the cyclic stability of graphite/Li and LiCoO₂/Li half cells, respectively. Further charge/discharge tests demonstrate that the cyclic stability of LiCoO₂/graphite full cell can be significantly improved by TFTPN. A high capacity retention of 91% is achieved for the full cell using 0.5% TFTPN-containing electrolyte after cycling at 0.5C between 3.0 and 4.4 V for 300 cycles, compared to the 79% for that using the baseline electrolyte. This effect is attributed to the simultaneously formed protective interphase films on graphite and LiCoO₂ by TFTPN due to its preferential reduction or oxidation. The resulting interphase films are verified by physical characterizations and theoretical calculations.