Study on Preparation and Properties of the LiFePo4 Cathode Material for Lithium-Ion Battery

Author:Chen Ming Hua

Supervisor:kou kai chang

Database:Doctor

Degree Year:2018

Download:11

Pages:127

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As one of the most promising energy-storage devices,lithium-ion secondary batteries have gained greatly commercial success in the fields of portable electronic devices,hybrid electric vehicles(HEVs)and electric vehicles(EVs)due to their outstanding properties such as high energy density and environmental friendliness.Olivine-structured LiFePO4 is regarded as a potential cathode material for lithium-ion batteries owing to its low cost,high safety,high specific capacity and excellent cycle performance.However,the pure LiFePO4has the drawbacks of poor electronic conductivity and low lithium-ion diffusion that hinder it from commercial use.Therefore,the optimization of preparation process and the study of modificationare of great significance to improve the electronic conductivity and lithium-ion diffusion of LiFePO4.The main contents of this paper are as followings:(1)Starting from one-dimensional nanometer size design,low cost iron nitrate was used as iron source and polyvinylpyrrolidone was used as spinning medium.Lithium-ion batteries LiFePO4/C nanocomposites were synthesized via the electrospinning technique.Through the crystal structure,surface morphology and microstructure analysis and electrochemical performance measurement,it was observed that there existed a little impurity phase in LiFePO4/C with initial specific discharge capacity of 122.5 mAh?g-1 at 0.1 C.The prepared LiFePO4/C nanocmposites exhibited not only favorable rate cycle performance due to one-dimensional nano-structure but also relatively low specific discharge capacity due to excessive carbon content.(2)Starting from zero-dimensional nanometer size design,low cost iron nitrate was used as iron source and citric acid was used as carbon source.LiFePO4@C nanocomposites were prepared via the sol-gel method.The effects of synthesis temperature and carbon content on the electrochemical performance of electrode materials were systematically investigated.The electrode materials with initial specific discharge capacity of 153.1 mAh?g-1 at 0.1 C and better rate cycle performance were obtained.The electrochemical performance of electrode materials which were prepared via two methods were contrasted and the sol-gel method was identified as the best method for synthesis of LiFePO4 electrode materials.(3)Starting from two-dimensional nanometer size design,graphene-LiFePO4@C electrode composites were prepared via the sol-gel method.The in-situ of LiFePO4@C and graphene was achieved.The effect of graphene on the structure,morphology and lithium storage performance of LiFePO4@C was investigated.The electrochemical performance at high rate was selective analyzed and the effect of graphene content on the electrochemical performance of LiFePO4@C was investigated.It was observed that the crystal structure of LiFePO4 did not change and LiFePO4@C particles with average size of 90 nm were uniformly distributed on the surface of graphene.Benefiting from the ultrahigh electron conductivity and significant chemical stability,when the graphene content is 2 wt%,electrode materials exhibited a initial specific discharge capacity of 158.6 mAh?g-1 at 0.1 C and 128.6 mAh?g-1 at10 C.The specific capacity retention rate was 96.7%after 300 cycles,exhibiting the best lithium storage performance.(4)Starting from improving the intrinsic conductivity of LiFePO4,V2O5 was used as the dopant.Vanadium ion doped LiFePO4@C composites were prepared via the sol-gel method.The effect of different doping contents on the on the structure,morphology and electrochemistry performance was discussed.It was observed that the doped specimens maintained the integrated olivine-structured crystal structure.The vanadium doping can further improve the electrochemistry performance of electrode materials.When the vanadium doping is 5 mol%,specimens exhibited a initial specific discharge capacity of 160.5 mAh?g-1at 0.1 C and favorable rate capability and cycling stability.(5)Starting from improving the conductivity of carbon film,N-doped carbon was applied to modify LiFePO4.The N-doped carbon-coated LiFePO4 composite were prepared through the sol-gel method using C2H4N4 as the nitrogen dopant.Meanwhile,the effect of nitrogen doping contents on the structure and performance of LiFePO4@C was investigated.The obtained results showed that the citric acid and C2H4N4 in-situ decomposed at high temperature and a layer of N-doped carbon film was coated on the surface of LiFePO4particles,which effectively enhanced the electronic contact between them and thus improved the electrochemical performance of LiFePO4 electrode materials.When the nitrogen doping content was 0.35 wt%,the corresponding electrode materials exhibited the best electrochemical performance.The initial discharge capacity of LiFePO4@N0.35%C reached157.2 mAh?g-1 at 0.1 C between 2.5 and 4.2 V,and this capacity almost preserved after30 cycles.This paper investigates how to enhance the electrochemical performances of LiFePO4.Starting from the synthetic methods and chemical modifications,some useful researches have been made on the aspects of size control,carbon,nitrogen and grapheme coating,etc.The research contents mentioned above are aspected to provide some references on the application of LiFePO4 in battery industry.