| Artificial graphite is the main anode material of lithium ion batteries in China. It has stable performances and high capacity, but its cost is high.The reserve of natural graphite is large in China and the price is low. However, natural graphite can not be used as anode material directly, because it has bad compatibility with electrolyte and this causes lower coulombic efficiency and poor cycle life.In order to improve relative performances of natural graphite as anode of lithium ion battery, the methods of air oxidation and coating with phenolic resin are used respectively.First,Several natural graphite samples were analyzed preparatorily in structures and properties through X-ray diffifraction(XRD), Brunauer-Emmer-Teller(BET),Scan electronic microscope(SEM), Laser Particle diameter and electrochemical test. It was found that the electrochemical performance was affected by the factors such as crystal structure, impurity content, particle diameter and BET surface area. The special natural graphite with appropriate parameters (D50 16μm, BET 5～6 m2/g,TD 1.0g/cm3,first charge capacity 330mAh/g,first coulombic efficiency 85%) was selected as modification material.Results of air oxidation for spherical natural graphite have demonstrated that some active parts is removed, lithium storage sites increased and the particle size became more uniform. These changes make the insertion capacity increase and are of benefit to form stable and uniform SEI film which can prevent the cointercalation of solvent in the cycling process. At the same time, the change of surface structure is beneficial to decrease the consumption of lithium ion in the process of the formation of SEI film. Furthermore, this change prenvents the decomposition of solvent and electrolyte and improved the coulombic efficiency in the first cycle. The first irreversible capacity of graphite sample oxidized at 600℃for 2 hours decreased from 52.9mAh/g to 38.8mAh/g, the reversible capacity and first coulombic efficiency attained 345mAh/g and 88.2% respectably.This paper established a model for surface structure of coated graphite composite material.The function and mechanism of carbon-coating layer is analyzed. And the insertion behavior of Li+ is described. Furthermore, the effection of coating thickness to improve the compatibility with electrolyte is analyzed.The two process dipping- filtration and solvent evaporation are used to coat a layer of phenolic resin on spherical natural graphite according to different mix rates. The test results of SEM, XRD and Raman indicate that the core-shell composite of disorder carbon coating graphite has been prepared after pyrogenation.Disorder carbon materials are prepared by pyrolysis of phenolic resin from 700℃to 1000℃. The structure and micro-morphology of pyrolytic carbons are tested through XRD, SEM and so on. Pyrogenation process is at 1000℃for 2 hours in inert atmosphere because of the coated graphite has better performance.The constant charge-discharge and cyclic voltammograms are used to analyze the influence of coating amount to electrochemical performance such as reversible capacity and coulombic efficiency. The results show that the disorder carbon-coated graphite material with stable the core-shell structure can be prepared by coating 5% of the percentage of phenolic resin on spherical natural graphite after pyrolysis. Test results show that: the two important performances reversible capacity charge-discharge efficiency increased obviously after modified. The reversible capacity of modified graphite samples increased from 330 mAh / g to 363mAh / g, the first time coulombic efficiency increased from 85% to 90.2%, which was attained basic requirements of lithium-ion battery production. The achievements laid an important foundation for industrialization of natural graphite as the anode material of lithium-ion battery.
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