The Effects Of Surface Modifications On The Performance Of A Natural Graphite As Anode Material Of Lithium Ion Batteries

The principle and the development of lithium ion batteries have been described in this dissertation. The physicochemical properties, electrochemical performance and some modification methods of graphite anode have been introduced with emphasis. Basde on this, a natural graphite was modified by chemical oxidation, coating SnO₂ and Sn, and the structure and electrochemical performance of modified graphite were investigated.In this third chapter, a natural graphite was modified bu the solution oxidation with K₂FeO₄, and the effects of various factors on the structure and electrochemical performance of natural graphite were investicated by different physical techniques and electrochemical methods. The results of SEM, EDX and Raman spectra showed that after the oxidation treatment with K₂FeO₄, some mesoscopic pores/channels and thin oxide layer are introduced on the surface of graphite. The galvanostatic charge-discharge experiments and EIS indicated that the oxidated natural graphite displays much larger reversible capacity, better polarizations than pristine natural graphite. It was found that the graphite sample oxidated at pH=7 and 80 ℃ displays better electrochemical performance, and its largest reversible capacity reaches 363mAh/g.SnO₂-graphite composites were prepared by adding natural graphite powders into the mixture solution of SnCl₂ and Na₂CO₃. SnO₂ was coated on the surface of the natural graphite powders in the composite material. The electrochemical performance of the composite samples are greatly improved and its largest electrochemical reversible capacity can reach 421mAh/g. Tin oxide has higher capacitiy (about 500mAh/g), but its capacity fades quickly due to larger volume change during the lithiation and de-lithiation. The SnCVgraphite composite material displays the advantages of the two kinds of materials, that is, better electrochemical-cycling stability and much larger capacity.The Sn-coated graphite was prepared by electroless plating. The results of charge-discharge experiments showed that the natural graphite with Sn surface layer has excellent electrochemical-cycling stability. The tin surface layer has may prevent the exfloliation of the graphene planes caused by the co-intercalation of electrolyte molecules. This might be mainly responsible for the great improvement on the cycling stability of the Sn-coated graphite.