Study On Structure And Electrochemical Performance Of Lithium Iron Phosphate/Graphene Composite

Lithium iron phosphate?LiFePO₄?with olivine-type structure is considered to be a promising cathode material for lithium-ion batteries due to its advantages of high discharge voltage,long cycle life,good safety,and low cost.However,its performance during high current charge and discharge reduced for its low lithium ion diffusion rate and low electronic conductivity,which limits its practical application as a power battery.Ion doped can cause lattice distortion in lithium iron phosphate to improve the conductivity of lithium iron phosphate.Surface coating can improve the conductivity of the material and prevent the oxidation of Fe²⁺.In this paper,manganese and nickel ion-doped lithium iron phosphate composite materials,graphene-based composites and lithium iron phosphate/graphene-based composites were synthesized.The influence of doping ion and graphene coating on the structure,morphology and electrochemical performance of the composite materials was investigated.The main contents are as following:?1?LiFePO₄/C composites were prepared by hydrothermal method.The effects of different hydrothermal reaction temperature and reaction time on the performance of LiFePO₄/C composites were investigated and the optimal condition for preparing parameters LiFePO₄/C were determined.Li Fe_1-xMn_xPO₄/C and Li Fe_1-xNi_xPO₄/C composites were prepared by hydrothermal method,and the effects of doped manganese and nickel ions on the structures and electrochemical properties of lithium iron phosphate were studied.The results show that proper doping of manganese and nickel ions did not change the crystal structures of lithium iron phosphate,however change the lattice parameters and cell volume,reduce the particle size,shorten the diffusion path of lithium ions and improve the electrochemical performances of lithium iron phosphate.The results show that the first discharge specific capacity of Li Fe_0.98Mn_0.02PO₄/C reaches 156 m Ah·g^-1at 0.1 C and 110 m Ah·g^-1at 5 C rate.the first discharge specific capacity of Li Fe_0.97Ni_0.03PO₄/C is 167.8 m Ah·g^-1at 0.1 C and the capacity retention rate is still 93.9%after 200 cycles at 10 C rate.?2?Graphene oxide?GO?was prepared by Hummer method and graphene oxide was reduced to graphene?RGO?by hydrothermal reduction method.Nitrogen-doped graphene was prepared by using melamine as the nitrogen source and sodium sulfide Sulfur as sulfur-doped graphene composite.The obtained materials were characterized and analyzed by XRD,SEM,Raman,FTIR and XPS.The results show that doping nitrogen and sulfur element can increase the defects and disorder of graphene.?3?Carbon-coating lithium iron phosphate/graphene composites were prepared by hydrothermal method and the effects of the amount of GO added on the performance of the composite were discussed.The results show that when the added amount of GO is 5%of theoretical weight of LiFePO₄,the composite exhibits the most excellent electrochemical performance and the specific discharge capacity reaches 160.8 m Ah·g^-1at 0.1 C rate,which is 94.6%of the theoretical specific capacity(170 m Ah·g^-1).Carbon-coating lithium iron phosphate/nitrogen-doped graphene and carbon-coated lithium iron phosphate/sulfur-doped graphene composites were prepared,and the structures and electrochemical performances were studied.The results show that nitrogen and sulfur-doped graphene-coated lithium iron phosphate have no effects on the LiFePO₄crystal structures.Appropriate amounts of nitrogen and sulfur doped can reduce the grain size of the composites and form a more conductive three-dimensional network structures.