Study On The Mild Expanded Graphite Anode Materials For Lithium Ion Batteries

Lithium-ion battery is widely used as the power sources of portable electricalappliances and electric vehicles, due to its highlights such as high voltage, high energydensity, good cycling performance, small self-discharge, no memory effect andenvironmental friendliness. Electrode material is one of the critical materials in lithium-ionbatteries （LIBs）, while carbon and graphite materials are widely used as anode materials forcommercial LIBs.Natural flake graphite is a rich mineral resource, and heat temperature treatment is notneeded because of its highly graphitization, so when used as anode materials, it has pricepreve. However, the interlayer bonding energy of natural graphite is only16.7kJ/mol,graphene layer of graphite is easy to exfoliate, so its cycling performance isn’t good. Thusnatural graphite has other defects such as low specific capacity, bad compatibility withelectrolyte and not ideal rate capacity. So natural flake graphite needs to be modified, beforeused as anode materials for LIBs.In this paper, a method about mild expansion treatment of graphite was advanced, theinterlayer distance of graphite is mild expanded to improve its electrochemical performance.As the raw material, the natural graphite was mild oxidized before intercalation of aninorangic acid or orangic acid, and graphite was converted into graphite intercalationcompounds （GICs）, then the GICs decomposed under high temperature heat treatment, as aresult, the graphene layer of graphite was mild expanded.Correlation between composition, process, structure and electrochemical behavior ofmild expanded graphite was investigated. The structure and morphology of the sampleswere characterized by XRD, Raman spectra and SEM analysis, respectively. The BETsurface area and DFT pore distribution of graphite particles were determined by thenitrogen adsorption method. Tap density and average particle size of samples were tested bythe related equipments. The electrochemical behavior of the samples was studied usinggalvanostatic charge-discharge test, powder cavity microelectrode technique,electrochemical impedance spectra （EIS） and cyclic voltammogram （CV）.When inorganic acid was used as the intercalate, average interlayer distance d002ofgraphite was mild increased, and the surface of graphite particles was shaped at a certainextent. As for spherical flake graphite, the shape of particles was destroyed, and somegraphene sheets show flexural and sculptural margin after modification. Electrochemicalmeasures show that reversible capacity of modified graphite increases from345.5to381.4 mAh g^-1, and cycling performance is improved. Meanwhile, the lithium-intercalationpotential of modified graphite shifted to high voltage orientation by8-9mV, and duringdeintercalation process of the modified graphite, stage transformation from stage1Li-GICs（lithium-graphite intercalation compounds） to stage2Li-GICs begins to occur at lowervoltage, compared with that of the raw graphite, this indicates that Li+can deintercalatefrom graphite more easily. Thus, the rate discharge capacity of modified graphite isimproved, when when charged at0.1C and discharged at3C, its discharge capacity is360mAh g^-1, so the capacity retention is94%.When orangic acid was used as the intercalate, average interlayer distance of graphitewas increased from0.3355nm to0.3366nm, the average in-plane crystallite domain size Laand Lcdecrease, while graphite particles remain spherical shape. After modification, thesurface of particles become smoother, and the BET specific surface area decreases a bit,compared with that the raw graphite. The initial columbic efficiency of graphite anodeincreases from91.6%to92.7%, and reversible capacity of the30thcycle increases from345mAh g^-1to379.8mAh g^-1, meanwhile both the cycling performance and rate dischargecapacity are improved. The capacity of modified graphite at the60thcycle is370mAh g^-1,the corresponding retention is98.04%. The discharge capacity of modified graphite is354mAh g^-1when charged at0.1C and discharged at3C, so the capacity retention is93%.After modification, the interlayer distance d002of graphite is mild expanded, itselectrochemical performance, including specific capacity, cycling performance and ratecapacity, are efficiently improved. Compared with the raw graphite, the lithium-intercalatepotential of graphite anode material rises a bit, the electrochemical polarization decreases,and the reversibility of lithium intercalation/deintercalation reaction increases, so Li+candeintercalate from the graphite more easily.