Effect of fluoroesters on the low temperature electrochemical characteristics of graphite electrode

The electrochemical behavior of graphite electrode has been investigated in 1 M LiClO4-ethylene carbonate (EC)/diethyl carbonate (DEC) and fluoroester-mixed (4.8 vol %) EC/DEC solutions at low temperatures to evaluate the effect of addition of fluoroesters on the electrochemical characteristics. The cyclic voltammetry study revealed that the fluoroesters used were electrochemically reduced at more positive potentials between 0.87 V and 1.41 V vs. Li/Li+, mainly at 0.99-1.10 V, than EC reduced at 0.60 V, and with decreasing reduction potentials of the fluoroesters, the larger reduction and oxidation currents reversibly flowed in the fluoroester-mixed solvents than in EC/DEC at 0 degrees C. The charge-discharge cycling accompanying the lithium ion deintercalation and intercalation reactions has been also studied by a constant current method at 25 degrees C, 0 degrees C and -4 degrees C. A low molecular weight fluoroester, CHF2COOCH3-mixed EC/DEC demonstrated the much larger charge capacities than EC/DEC itself and other fluoroester-mixed solvents at 0 degrees C and -4 degrees C, though its capacity was comparable to that in EC/DEC at 25 degrees C. The charge capacities obtained in the solvents mixed with other fluoroesters having larger molecular weights were not largely different from that in EC/DEC at 0 degrees C. (C) 1998 Elsevier Science S.A.