Surface structure change and electrode characteristics of carbon materials by fluorination reaction

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(1)Surface structure change of petroleum cokes by fluorinationPetroleum coke and those heat-treated at 1860℃,2100℃,2300℃,2600℃ and 2800℃ were fluorinated by elemental fluorine at 150℃-300℃ for 2 min and by plasma treatment at 90℃ using CF_4. The fluorine contents obtained by elemental analysis were less than 1 at% and surface fluorine concentrations obtained by XPS were in the range of 5-20 at%. BET surface areas were not changed or slightly reduced by surface fluorination. However, surface disorder was increased. Transmission electron microscopic observation revealed that the edge plane of petroleum coke was closed by carbon-carbon bond formation during the graphitization process at high temperatures and surface fluorination opened the closed edge surface with increase in the surface disorder.(2)Electrode characteristics of surface-fluorinated petroleum cokes in organic solventsElectrode characteristics of surface-fluorinated petroleum coke samples were examined by cyclic voltammetry and galvanostatic charge/discharge cycling in 1 M LiClO_4 ethylene carbonate (EC)/diethyl carbonate (DEC) at 25℃. Cyclic voltammetry study showed that electrochemical decomposition of EC observed at 0.6 V vs Li/Li^+ was not changed for PC,PC1860 and PC2100,but significantly reduced for PC2300,PC2600 and PC2800. Charge/discharge experiments at current densities of 60 and 150 mA/g indicated that first coulombic efficiencies of PC2300,PC2600 and PC2800 were highly increased. This would be because the closed edge of graphitized petroleum cokes was opened by fluorination and the opening of the closed edge facilitated the surface film formation and Li^+ ion insertion.