Study On Preparation And Modification Of LiFePO₄ Used As Cathode Material For Li-ion Battery

With the growing popularity of a variety of portable electronic products, lithium ion batteries have developed rapidly since 1990 s. In addition, energy crisis and the rapid development of modern science and technology also contributed to the higher demands on the performance of lithium ion battery to some extent. Among them, olivine-typed LiFePO4 cathode material is the widely investigated one for its high theoretical capacity and good cycle performance, rich raw material sources, low cost, good safety performance and environmental friendly characteristics. However, the feature of LiFePO4 crystal structure resulted in its low electronic conductivity and lithium ion diffusion coefficient, seriously affecting the discharge rate property and limiting its commercial application. Therefore, it is of far-reaching significance to improve the electronic conductivity and lithium ion diffusion coefficient of Li FePO4 by synthetic process optimization and modification research. Till now, the common ways to improve the electrochemical performance of Li FePO4 are minimizing the material particle size, adding conductive agent and doping metal ions.In this paper, olivine-typed LiFePO4 was chosen as the cathode material. LiFePO4 and LiFePO4/G cathode materials were synthesized by the high temperature solid-state reaction without using inert gas; Li0.99Nb0.01Fe0.97Ti0.03PO4/G was synthesized by the sol-gel method. The effects of different graphene coating amount and ion-doping on the materials properties are investigated by XRD, EDS, SEM, charge-discharge tests, CV and EIS, respectively.In this research, graphene coating was adopted to modify LiFePO4. The coating can not only increase the conductivity of LiFePO4 particles and inhibit the grain from growing up, but also effectively prevent the oxidation of Fe2+. In this way, can the as prepared LiFePO4/G particles have a more regular, smaller and distributed uniformly morphology. What is more, the influence of graphene coating amount on the properties of Li FePO4 was alos discussed in this thesis. When the graphene content was 8 wt.%, the obtained sample could present the best electrochemical properties with the first discharge capacity of 164 mAh/g at 0.1 C rate.Thus, on the basis of graphene coating, Nb, Ti-codoped Li FePO4/G samples were synthesized through the sol-gel method. The XRD analysis indicated the perfect olivine structure of the sample and the high valent ion doping modification could not affect the structure at all. In a word, through graphene coating and doping modification, the electrochemical performance of Li0.99Nb0.01Fe0.97Ti0.03PO4/G could be greatly enhanced, not only the high specific capacity and coulomb efficiency, but also the improved cycle performance.