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

Abstract

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 4.4V. Electrochemical measurements indicate that TFTPN can be reduced on graphite electrode and oxidized on LiCoO2 electrode preferentially compared to the baseline electrolyte, 1.0M 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 4.4V 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.