Modifications Of Natural Graphite Used For Lithium Ion Batteries

In the present study, natural graphite, which has good lithium storage performanceand bad compatibility with electrolyte, was deep investigated for preparing excellentperformance and low-cost anode materials for Lithium ion battery (LIB). Naturalgraphite was modified by gas-phase oxidation and liquid-phase oxidation which wereattempted to improve coulombic efficiency. The methods of chemical coating andphysical coating which were used to improve tap density were introduced to solve thedeficiencyof small tap densityof the natural graphite. Two natural graphite compositematerials with large tap density were prepared, and their electrochemicalperformances were studied primarily. X-ray diffraction (XRD), Scanning electron microscope (SEM), X-rayphotoelectron spectroscopy (XPS), cyclic voltammograms and constantcharge-discharge were used to analyze the structure and electrochemical performanceof many kinds of typical natural graphite in China, especially to studycharge/discharge behavior of the sample HT10 in EC/EMC and 3EC/3EMC/4PCsolvent. Exfoliation product from partial graphite flake was observed by SEM. Theformation of solid electrolyte interface (SEI) film in the first cycle was analyzed bycyclic voltammograms and constant charge-discharge tests. It was proved that SEI caninhibit the decompositionof solvent further and avoid the exfoliation of graphite flakein the electrolyte containing 40% PC solvent which can make the anode performstable charge/discharge cycling. Therefore, natural graphite HT10 containingrhombohedral phase can exhibit excellent performance in two kinds of electrolyte. Oxidation treatment in which natural graphite was modified by air and H2O2 wasinvestigated. It was demonstrated that lithium storage sites increased and the particlesize became more uniform by air oxidation because these changes make the insertioncapacity increase and is benefit to form stable and uniform SEI film which can inhabitthe cointercalation of solvent in the cycling process. It was proved that the oxygencontent on the surface of the graphite decreased with the change of surface functionalgroup by oxidation treatment. At the same time, the change of surface structure wasbeneficial to decrease the consumption of lithium ion in the process of the formationof SEI film. Furthermore, this change inhibited the decomposition of solvent andelectrolyte and improved the coulombic efficiency in the first cycle. The firstirreversible capacity of HONG which was modified by H2O2 was decreased from52.9mAh/g to 38.8mAh/g, the reversible capacity kept above 340mAh/g, and thecoulombic efficiency reached 89.8% in the first cycle. Two methods of chemical coating and physical coating can also improve thesphericityand tap density of natural graphite,and the electrochemical performance ofcomposite materials were enhanced to a certain extent. Especially the tap density ofthe graphite CPNG11 which was modified by chemical coating was increased from0.61g/cm3 to 0.87g/cm3, the discharge capacity reached 338mAh/g and the coulombicefficiencyattained 86.4% in the first cycle.