Study On Controllable Synthesis And Modification Of Cathode Material ?LiFePO₄,LiCoO₂? For Lithium Ion Batteries

Cathode materials play an important role on the electrochemical performance of lithium-ion secondary batteries.Lithium iron phosphate and lithium cobaltate are commonly used as cathode materials for commercial lithium-ion batteries.Lithium iron phosphate possesses high structural stability,but its poor electrical conductivity and low lithium ions diffusivity limit its high-rate capability.Lithium cobaltate has a high theoretical capacity,however about half of the lithium ions in Li Co O₂ can be reversibly inserted and extracted within the voltage range of 3.0-4.2 V.In order to improve the performance of lithium ion battery cathode materials,modification methods such as morphology control,composite metal were chosen to improve the electrochemical performance of lithium iron phosphate and lithium cobaltate.The morphology of lithium iron phosphate is controlled by solvothermal method.Effect of the ratio of ethylene glycol to water in the solvent on the morphology and properties of the as-synthesized lithium iron phosphate was studied.The particle size of lithium iron phosphate decreases and their morphology shrinks in the axial direction with increasing of the ratio of ethylene glycol in solvent.Lithium iron phosphate nanorods were synthesized with ethylene glycol as the solvent,which have the most excellent electrochemical performance,and the capacity is above 125 m Ah/g at 20 C current density.Lithium iron phosphate nanoplates with exposed?010?crystal planes were performed by solvothermal method with AOT as surfactant.Li Fe PO₄nanoplates have been further modified with graphene.The Li Fe PO₄/GO composite exhibits enhanced electrochemical performance,which attributes to the synergistically increase of electronic conductivity and lithium ions diffusivity.The Li Fe PO₄/Ag composites were synthesized with varying contents of Ag as an addition in order to invest the effect of Ag additives of cathode materials.The metal Ag would be irreversible oxidized during the first charge process.Oxidized Ag dissolves into electrolyte and deposited on anode or separators.A certain amount of Ag addition can effectively enhance the electrochemical performances of Li Fe PO₄,but too much Ag adding lower the capacity and safety.The basic cobalt carbonate with a three-dimensional layered morphology were used as cobalt source.The lithium cobalt oxide retained the nanosheet morphology from the cobalt source,which is beneficial to the improvement of electrochemical performance.The Li Co O₂/Ag composite material was prepared by in-situ composite Ag,Ag suppresses the phase change of lithium cobaltate at high voltage and improves the cycle stability of lithium cobaltate at 4.5 V...