The electrochemical behavior of organo-fluorine compounds with antioxidation ability has been investigated. Oxidation currents of fluorine-compound-containing ethylene carbonate (EC)/diethyl carbonate (DEC) solutions were much smaller than those of EC/DEC and EC/DEC/propylene carbonate (PC) at potentials higher than 6 V vs
Li/Li+
. Electrochemical reduction of fluorine compounds started at ca. 2 V vs
Li/Li+
, higher than those for EC, DEC, and PC. However the first coulombic efficiencies for natural graphite electrodes in fluorine-compound-containing EC/DEC mixtures were nearly the same as those in EC/DEC without an increase in irreversible capacities. Furthermore the first coulombic efficiencies in fluorine-compound-containing EC/DEC/PC mixtures were much larger than those in EC/DEC/PC itself. The results show that the fluorine compounds used in the present study can be used as nonflammable solvents for lithium ion batteries.
Differential scanning calorimetry study demonstrated that mixing of fluoro-ethers and fluoro-carbonates improved the thermal stability of 0.67 mol/L
LiClO4
–ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1 by volume). The oxidation currents were smaller in the fluorine compound-mixed electrolyte solutions than in 0.67 mol/L
LiClO4
–EC/DEC/PC, which also shows a high stability of the fluorine compound-mixed electrolyte solutions against electrochemical oxidation. Electrochemical reduction of fluorine compounds took place at the higher potentials than EC, DEC, and PC, as suggested by the highest occupied molecular orbital and lowest unoccupied p-molecular orbital energies of the fluorine compounds. However, charge/discharge experiments using natural graphite (NG) electrodes showed that the fluorine compounds increased first coulombic efficiencies due to the quick formation of the solid electrolyte interphase on NG in PC-containing solvents.
Surface fluorination of natural-graphite samples with average particle sizes of 5, 10, and 15 m ͑abbreviated to NG5 m, NG10 m, and NG15 m͒ was performed by F 2 ͑3 ϫ 10 4 Pa͒ at 200 and 300°C for 2 min and electrochemical properties of surface-fluorinated natural-graphite samples were investigated in 1 mol/dm 3 LiClO 4 -ethylene carbonate ͑EC͒/diethyl carbonate ͑DEC͒/propylene carbonate ͑PC͒ ͑1:1:1 in volume͒. Surface fluorine concentrations were in the range of 11.1-15.3 and 17.7-20.5 atom % for natural-graphite samples fluorinated at 200 and 300°C, respectively, for three natural-graphite samples. The surface areas of NG10 m and NG15 m were only slightly increased by fluorination, while the increase in surface area was large in the case of NG5 m. However, the increase in surface disorder by fluorination was larger for NG10 m and NG15 m than for NG5 m. Electrochemical reduction of PC on NG10 m and NG15 m was highly reduced by surface fluorination, which led to a large increase in first coulombic efficiencies. The increase in first coulombic efficiencies for surface-fluorinated NG10 m and NG15 m is attributed to the increase in their surface disorder and probably actual electrode area by fluorination.
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