Hydrothermal Modification Of Natural Graphite And Electrochemical Performance

Artificial graphite is a widely used anode material for lithium-ion batteries,but its preparation requires calcining petroleum coke at high temperature,resulting in huge energy waste and pollution to the atmosphere,while natural graphite(NG)is cheap and has a higher reversible capacity than artificial graphite,its production process is simple and low cost.However,the NG anode has obstacles such as poor rate performance,poor cycle stability,electrolyte molecules easily embed and easy to produce lithium dendrite during the charge and discharge process,which restrict its application in the field of high-performance lithium-ion batteries.Based on this,this paper uses inorganic salt modifiers to modify natural graphite by hydrothermal method,hydrothermal modification boosts the interaction of modifier with NG anode,which improves the rate performance and cycle stability of natural graphite anode materials.The main contents are as follows:(1)Mg(NO3)2 was used as an modifier to optimize the electrochemical performance of NG.After the hydrothermal modification with 0.4 wt%Mg(NO3)2 at 200℃ for 12 h,the modified NG exhibits high capacity retention at high current rates(about 88%at 0.5 C relative to the capacity at 0.1 C)and excellent cyclability(attaining a reversible capacity of 446.7 mAh g-1 after 350 cycles at 0.3 C).Detailed characterizations demonstrate,the hydrothermal modification with Mg(NO3)2 brings about the high graphitization degree of NG favoring for the enhanced structural stability and prolonged cycle life,additional active sites for storing more Li-ions.During the first lithiation process,NO3-interacts with lithium ions to form Li3N,LiNxOy and Li2O solid electrolytes,which is conducive to the rapid migration of lithium ions.and in situ created solid electrolytes of Li3N,LiNxOy and Li2O for fast Li-ion migration.Moreover,the modification alleviates electrolyte decomposition,Li dendrite formation and polarization during cycling.(2)MgCl2 was adopted to modify NG by taking advantage of the characteristics that halogen elements are easy to form acceptor-type intercalation compounds with graphite.NG was hydrothermally treated with MgCl2 at 150℃ for 12 h,and the modified NG showed an initial discharge capacity of 546.1 mAh g-1,and a capacity of 477.4 mAh g-1 maintained after 800 cycles,denoting significantly improved cycle stability of the NG anodes.Systematic characterizations show that Mg2+can improve the graphitization degree of NG in the hydrothermal process,and Cl-enters the interlayer of NG to form intercalation compounds,which can improve the initial capacity of NG.During the electrochemical cycling,Mg2+ can reversibly provide more lithium storage sites,the composite of the two further improve the lithium storage performance and specific capacity of the NG anode.Partial Cl-deposited on the surface of NG to form LiCl,favoring to improve the diffusion rate of lithium ions.(3)Iodized salt was used to modified NG anode by the hydrothermal method.The initial discharge capacity of the modified NG is 480.2 mAh g-1,and the capacity retention at 0.5 C is about 74%(relative to the capacity at 0.1 C).The characterizations show that Cl-can enter into the interlayer of NG to generate intercalation compound Cn[Cl]in the hydrothermal process,favorable to improve the lithium storage capacity of the NG anode.The IO3-in the iodized salt can react with lithium to form LiI and Li2O,conducive to improving lithium ion diffusion rate and inhibiting the formation of lithium dendrites.Na+ plays an role in optimizing the performance of SEI films,reducing interfacial polarization.Partial Cl-is deposited on the surface of NG to form LiCl,contributing to good chemical stability and corrosion resistance of NG,and thus greatly improves the stability of NG during charging and discharging.