EFFECT OF FLUOROESTERS ON THE LOW-TEMPERATURE ELECTROCHEMICAL CHARACTERISTICS OF GRAPHITE ELECTRODE

The electrochemical behavior of graphite electrode has been investigat ed in 1 M LiClO4-ethylene carbonate (EC)/diethyl carbonate (DEC) and f luoroester-mixed (4.8 vol %) EC/DEC solutions at low temperatures to e valuate the effect of addition of fluoroesters on the electrochemical characteristics. The cyclic voltammetry study revealed that the fluoro esters 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 flu oroesters, the larger reduction and oxidation currents reversibly flow ed in the fluoroester-mixed solvents than in EC/DEC at 0 degrees C. Th e charge-discharge cycling accompanying the lithium ion deintercalatio n and intercalation reactions has been also studied by a constant curr ent method at 25 degrees C, 0 degrees C and -4 degrees C. A low molecu lar weight fluoroester, CHF2COOCH3-mixed EC/DEC demonstrated the much larger charge capacities than EC/DEC itself and other fluoroester-mixe d solvents at 0 degrees C and -4 degrees C, though its capacity was co mparable to that in EC/DEC at 25 degrees C. The charge capacities obta ined in the solvents mixed with other fluoroesters having larger molec ular weights were not largely different from that in EC/DEC at 0 degre es C. (C) 1998 Elsevier Science S.A.