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

石油焦经1860℃、2100℃、2300℃、2600℃和2800℃热处理后，分别在150℃~300℃和90℃条件下用CF4等离子体处理2min，元素氟含量小于1at%，X射线光电子能谱分析得到的表面氟含量在5~20at%之间。表面氟化对BET比表面积没有影响或略有减小。然而，表面无序性增加了。透射电子显微镜观察表明，在高温石墨化过程中，石墨化过程中碳-碳键的形成封闭了石油焦的边缘平面，表面氟化打开了封闭的边缘表面，表面无序度增加。(2)表面氟化石油焦在有机溶剂中的电极特性通过循环伏安和恒流充放电循环研究了表面氟化石油焦在1M LiClO_4/碳酸二乙酯(EC)/碳酸二乙酯(DEC)中25℃的循环伏安和恒流充放电循环。循环伏安研究表明，PC、PC1860和PC2100在0.6 V VS Li/Li^+下的电化学分解没有改变，而PC2300、PC2600和PC2800的电化学分解明显减少。在60 mA/g和150 mA/g电流密度下的充放电实验表明，PC2300、PC2600和PC2800的一次库仑效率显著提高。这可能是因为石墨化石油焦的封闭边是通过氟化打开的，而封闭边的打开促进了表面膜的形成和锂离子的插入。

(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.