The Research On Modified Graphite As Anodes Materials For Lithium-ion Batteries

Modified graphite was synthesized by three ways,i.e.,mildly-expanded modification,mechanical ball-milling and carbon-coating,as anode materials for lithium-ion batteries with superior electrochemical performances.The influence of four parameters on the composites microcrystalline structures and electrical performances,which are mixture mass ratio,ball-milling time,carbon-coating temperature and time was studied by a series of characterization methods,such as:Scanning electron microscopy?SEM?,Transmission electronmicroscope?TEM?,X-raydiffraction?XRD?,Ramanspectrum,Brunauer-Emmett-Teller gas adsorption?BET?,galvanostatic charge/discharge test,cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?.The research focused on three parts as follows:?1?The composite of expanded-graphite?EG?was prepared by a intercalation reaction in which H₂SO₄ acts as solvent,H₂O₂ as oxidizing agent,?NH₄?₂S₂O₈ as intercalating agent.It was found that EG delivered an initial charge capacity of 662 mAh?g^-1 at 0.1 C,which was increased by 46%in comparison with natural graphite.EG showed a reversible capacity of 265 mAh?g^-1 at the rate of 1 C increased by 70%,respectively.The composite shows good electrochemical performance due to not only defects and active sites brought by oxidant,but also fluffy structure caused by decomposition of intercalating agent,which is beneficial to Li⁺transport.?2?Ball-milling expanded graphite?BMEG?with the superior capacity of 1008mAh?g^-1 and initial coulomb efficiency?ICE?of 89.3%at 0.1 C was synthesized when mixture mass ratio was 0.75g,ball-milling time was 30 h.It is indicated that graphite are exfoliated by the shear force from the collision of steel ball,which is effective in reducing the particle size of composite,keeping balance of defect disorder and graphite crystallization,as well as microporous and mesoporous coexistence of hierarchical pore structure.?3?Chemical vapor deposition method was used to modify the BMEG via acetylene pyrolysis.Carbon coating on ball-milling expanded graphite?C@BMEG?prepared at400°C for 0.5 h exhibits higher ICE of 93%.Furthermore,after 100 cycles,the C@BMEG had a higher capacity retention of 68.4%than the BMEG with 22.2%.Such excellent stability of C@BMEG attributes to amorphous carbon layer coated on the composite,which should lead to smooth surface and small specific surface area as well as alleviating the volume expansion caused by solvated lithium-ion insertion into graphitic layer during cycling.