Study On The Pitch Carbon Coated Natural Graphite Anode Materials For Lithium Ion Batteries

Graphite anodes dominate the lithium-ion battery anode market because of hightheoretical capacity of up to372mAh/g and good cycling performance, low voltageplateau and low cost, outperforming other competitive anode materials such as spinellithium titanate with low capacity and high voltage plateau, and alloy or transitionalmetal oxide anodes with poor cycling performance and high cost when prepared asnano-scale particles. Untreated natural graphite has poor compatibility with organicelectrolyte and has low initial coulombic efficiency caused by "3D" SEI film formingon and within the particle. carbon coating through liquid phase method is not only afacile way to prepare “core-shell” structured carbon-coated graphite, but also cheaperthan chemical vapor deposition, and more uniform than solid phase coating. Vacuumimpregnation not only can uniformly coat the surface of the graphite particles but alsofill the internal pores of the graphite particles, then result in improved performance.This study applied the vacuum impregnation liquid coating method on two kinds ofgraphite.Self-designed experimental apparatus was used to prepare coated graphitesamples. effects of the amount of coating, carbonization temperature, heating rate andheating duration on the structure and electrochemical properties of coated graphite aresystematically studied. XRD, Raman spectra, SEM and other methods are used tostudy the crystal structure, surface structure and surface morphology of the asprepared coated graphite. N2absorption-desorption and laser particle size distributionanalyzer are applied to determine BET specific surface area and particle sizedistribution. Galvanostatic charge-discharge, cyclic scan voltammetry andelectrochemical impedance is engaged to study the electrochemical properties of thecoated graphite.The results show that: vacuum impregnation liquid coating methods can prepared"core-shell" structured carbon-coated graphite easily. Coating amount of asphaltcarbon and carbonization temperature have a great influence on the structure andelectrochemical performance of the coated graphite while heating rate and heatingduration have little. Excessive coating amount lead to the graphite particles bondedtogether, result in poor processing and electrochemical performance. The optimumcoating amount of microcrystalline graphite and flake graphite are6%and3%,respectively. The higher the carbonization temperature, the better the electrochemical performance of the coated graphite is achieved. Taking cost into consideration, theoptimum carbonization temperature of1100℃is chosen. Effect of heating rate andheating duration on the electrochemical properties of the coated graphite are smaller,but0.5℃/min and2h are the best choice, respectively. When applied all the optimumparameters for coating, considerable improvement on electrochemical performance ofmicrocrystalline graphite and flake graphite was gained, e.g. initial coulombicefficiencies increased from80.2%and90.8%to90.7%and92.6%respectively,cycling performance of microcrystalline graphite considerably enhanced.A new phenomenon in the first charge and discharge curves of coatedmicrocrystalline and flake graphite was found, voltage rebound which induced bystructure relaxation of coated graphites resulted from expansion caused by lithiumintercalation. For the coated microcrystalline graphite case, voltage rebound happenedand the extents increased gradually when coating amount increased from3%to9%.Voltage rebound of untreated microcrystalline graphite was severe, while that ofuntreated flake graphite does not appear. Due to no structural relaxation, there is novoltage rebound in microcrystalline and flake graphite with coating amount of12%and15%. It can be predicted that strictly control experimental conditions and furtherquantitatively study the relationship between voltage rebound phenomenon andelectrochemical properties will be an auxiliary judge of the coating efficience.