Effect of addition of fluoroethers to organic solvents for lithium ion secondary batteries

Electrochemical characteristics of graphite have been investigated in fluoroether-mixed ethylene carbonate (EC)/diethyl carbonate (DEC) solutions at 0 degreesC and in fluoroether- or fluoroester-mixed EC/DEC/propylene carbonate (PC) solutions at 0, -5 and -10 degreesC. Cyclic voltammetry studies indicate that the reduction potentials of the five fluoroethers used are in the range of 0.57-0.81 V versus Li/Li+ in EC/DEC. The charge capacities of graphite electrodes higher than that in EC/DEC are observed in both EC/DEC and EC/DEC/PC solutions containing fluoroethers or fluoroesters with slightly higher reduction potentials than EC. The reduction potentials and irreversible capacities are decreased with increasing LUMO energies of fluoroethers and fluoroesters. The improvement of low temperature characteristics of the graphite electrode is discussed on the basis of the formation of a surface film with C-F groups on graphite. (C) 2001 Elsevier Science B.V. All rights reserved.