Modified Natural Graphite As Lithium-ion Battery Anode Material

Natural graphite is one of the dominant minerals in china, which owns the advantages of wide source, low price and mature purification/separation technologies. It is considered to be an ideal anode material for lithium ion battery because of its high theoretical lithium intercalating capacity and flat and low charge/discharge plateau at low potential. Nevertheless, its disadvantages, such as bad compatibility with electrolytes, poor cycle performance, can't bear high current charge-discharge, seriously limits its application in anode materials for lithium ion battery.In this thesis, in order to overcome these disadvantages, natural graphite was modified by oxidation and coating through pyrolysis of phenolic resin. The electrochemical properties of natural graphite before and after oxidation and coating were investigated, as well as the paticle size effect on the electrochemicall performance The results showed that the carbon coated natural graphite with small particle size has a batter performance. This paper serves the purpose of developing a anode materials which has good performance, low price, simple synthesis technique and feasible for industrial production. The main contents and results are as follows:(1) By oxidation and carbon coating through pyrolysis of phenolic resin, modified graphite with good performance was prepared. The spherical natural graphite was oxidized by 98% sulfuric acid at about 200℃for 10 h. The oxidation natural graphite was mixed with alcohol solution which contains phenolic resin at different concentrations. After the alcohol evaporated, the graphite was heat-treated at high temperature to make the phenolic resin coated on graphite pyrolyze to form an amorphous carbon film. SEM, XRD, TEM, Raman and galvanostatic charge-discharge measurement showed that the surface defects and impurities of the graphite were eliminated by oxidation treatment and carbon coating. The as-obtained natural graphite sample presented good electrochemical performance, when the covering amount of phenolic resin is 9%, its reversible capacity remains 361.6 mAh/g after 40 cycles, while Natural graphite only 332.1 mAh/g.(2) By comparing the electrochemical properties of three kinds of spherical natural graphites with different particle size (12μm,16μm,20μm),we found that the graphite with the particle size of 12μm showed the most excellent electrochemical performance at large charge/discharge current. Its reversible capacity remains 270 rnAh/g when current density reaches to 270 mA/g. Coating graphite by phenolic resin pyrolytic carbon, its performance at haig rate was improved. Its reversible capacity remains 325 rnAh/g when charging and discharging current density is 210 mA/g. The good rate performance of carbon coated small-sized natural graphite for lithium ion battery makes it a promising candidate as anode materials for electric vehicle dynamic lithium-ion batteries.