Studies On The Preparation And Modification Of LiMnPO₄/C Composite As Cathode Material For Lithium Ion Batteries

Driven by environmental benignity,high voltage platform,low processing cost,etc.,LiMnPO₄ has been regarded as a very appealing and potential cathode candidate for lithium ion baterries?LIBs?.However,it is still a large obstacle for its large-scale practical applications,due to its sluggish electrochemical kinetics of electron,low lithium ion transport rate and Jahn-Teller effect during the Li ion insertion-extraction processes,which will cause the problem of poor cycling stability,low rate capability and will restrict its application.Accordingly,this thesis is aimed to improve the cycling stability and rate property of LiMnPO₄,through the method of fabricating nanostructures,doping with metal cationic ions and in situ compositing graphene,so as to achieve highly desirable electrochemical properties.The mesoporous LiMnPO₄ nanocrystal composite has been successfully designed and synthesized via a glycine assisted solvothermal and the later carbon thermal reduction.Then anocrystalline and the inset mesoporous structure of the mesoporous LiMnPO₄/C nanocrystalline composites can provide Li+fast transmission channel,so as to improve the rate performance of the composite.Meanwhile,coating with carbon layer can improve the cycle stability of the composite by improving the electronic conductivity,and preventing the erosion on the electrode materials by electrolyte.The results indicate that the composite shows an obvious capacitance behavior,which is conducive to cushion the impact of the big electricity to the electrode material,so as to improve the high rate performance of the material.The discharge capacity value?167.1 mAh g-1?for mesoporous Li MnPO₄ is close to the theoretical capacity of LiMnPO₄ at 0.1 C,even in the high rate of 5 C,its discharge capacity is still high about 128.7 mAh g-1.In order to improve the electroconductibility of Li MnPO₄,3D porous LiMnPO₄/nitrogen doped graphene aerogels was synthesised through the static adsorption-hydrothermal self-assembly method.Li MnPO₄ nanoparticles was coated by the aerogel structure which is constructed by crosslinking and nitrating graphene.Graphene conductive frame forms a double consecutive channel for the Lithium ions and electrons,greatly improving the electronic conductivity and the lithium ion diffusion rate of the composite,improving the cycle stability of the composite.The structure can enlarge the specific surface area of material and increase the active site of the electrochemical reaction process.Graphene doping with nitrogen can highlyincrease the conductivity of the composite and promote the infiltration of the organic electrolyte.When the composite was test at 1 C,its discharge capacity is 135.2 mAh g-1,and maintain 83.2% after 100 cycles.LiMn_0.8Fe_0.2PO₄/C nanocomposite is synthesised in situ on the surface of graphene by solvothermal method with the further high temperature calcination to reduct the oxide graphene and making use of the rich functional oxygen on oxide graphene surface?such as hydroxyl,carboxyl,etc.?to combine with Mn2+ and Li+.The results indicate that LiMn_0.8Fe_0.2PO₄ particles load on the graphene layer uniformly,and particles can connect with other particles through the graphene,forming a porous ion/electrical conductive network.Meanwhile,the porous structure constructs is beneficial to the penetration of the electrolyte and the buffer of volume inflation for the electrode composite in the charging and discharging process,therefore,the composite can achieve an excellent electrochemical performance.When the composite were discharged at 0.1,0.5,1,2,5 and 10 C,its specific capacity were163.5,149.2,136.0,120.6,100.1 and 84.5 mAh g-1 respectively.