| Under enormous pressure of high-carbon energy in general use in the world today, all countries have chosen the development of electric vehicle industry as a transition to green economy. In all technical scheme, lithium-ion battery has become the first selection of power battery used in electric vehicles because of the obvious advantages of high energy density and long cycle life. Li Fe PO4 cathode material is considered to be the best choice for battery power due to its advantages, such as outstanding security and reliability, excellent cycle and stable performance, low cost, environmentally friendly and so on. However, the low electronic conductivity and lithium ion diffusion coefficient determined by crystal structure of Li Fe PO4 make this cathode material has a poor rate electrochemical performance, especially the high rate cycle performance is not ideal. These disadvantages limit large-scale application of Li Fe PO4 in power lithium-ion battery area.In this dissertation, the purpose is to improve ratio electrochemical performance of Li Fe PO4 cathode materials. For structure modification of Li Fe PO4, the doping modification, rate electrochemical performance and possible theoretical position of different valence metal ions Mn2+ and Mo6+ have been studied. Besides, effects and mechanism explanation of Fe vacancies defect in Li Fe PO4 lattice have been discussed. For surface modification of Li Fe PO4, coating with different conductive materials including graphene, modified graphene and nano-Cu particles generated by chemical method has been investigated. Detailed research works are as follows:Through the combination of experimental synthesis and theoretical calculation, Mn2+ doped Li Fe1-x Mnx PO4 and Mo6+ doped Li Fe1-3x Mox PO4/C samples were synthesized by carbothermal reduction. It was found that a small amount of metal ions Mn2+ and Mo6+ were present in Li Fe PO4 lattice in the form of solid solution. In the two doped samples, the valence of Mn was +2, and the valence of Mo was mainly +6 and a small amount of mixed valence state. Both of two samples doped metal ion showed more excellent electrochemical performance. Li Fe0.97Mn0.03PO4 has a discharge capacity of 96.3 m Ah·g-1 at 5C and its capacity retention can reach 98% after 60 times different rate cycles. Li Fe0.85Mo0.05PO4/C has a discharge capacity of 105.1 m Ah·g-1 at 5C and its capacity retention can reach 91% after 90 times different rate cycles. Through the first-principles theoretical calculations, it is evidenced that metal ions Mn2+ and Mo6+ doping can significantly expand lithium ion channels of Li Fe PO4 and improve its electronic conductivity, and Mo is easier to occupy Fe site in the lattice from formation energy calculations.Li Fe PO4/Graphene composites were synthesized by solid-state reaction from new carbon source graphene. Graphene dispersed between Li Fe PO4 particles to form a conductive network, and then greatly improved the composites discharge specific capacity at high rate. The discharge capacity of Li Fe PO4/Graphene composites were 128.1, 116.3, 95.9 and 69.3 m Ah·g-1 at 5C, 10 C, 20 C and 50 C, respectively, and hardly decreased after 60 cycles at 0.5C. In addition, graphene(R-Graphene) was prepared by reducing graphene oxide from Fe2+ in alkaline conditions, and achieving an efficient, environmentally friendly and low cost reduction of graphene oxide. On the basis of them, Li Fe PO4/R-Graphene composites were prepared by solid-state method, and R-Graphene dispersed to form a better composite than amorphous carbon among Li Fe PO4 particles. So Li Fe PO4/R-Graphene composites had more excellent electrochemical performance with capacity of 112.1 and 94.7 m Ah·g-1 at 5 and 10 C. Moreover, Li Fe0.85Mo0.05PO4/C/R-Graphene composites were synthesized by high temperature annealing, and had an improved electrochemical performance that the discharge capacity can achieve 114.2 m Ah·g-1 at 5C.Li Fe0.92PO4 cathode material was synthesized via carbothermal reduction method, and Fe vacancy defects were generated to change the electrochemical properties. The valence of Fe in Li Fe0.92PO4 with complete Li Fe PO4 crystal structure was mainly +2 and a little +3. The capacity of Li Fe0.92PO4 had no advantage at small rate, but Li Fe0.92PO4 exhibited more excellent electrochemical rate characteristics and cycle stability than Li Fe PO4 at more than 0.5 C. In particular, the discharge capacities of Li Fe0.92PO4 were 116.6, 105.3, 92.1, and 69.0 m Ah·g-1 at 1C, 2C, 5C, and 10 C, respectively, and its capacity retention rate can reach 96% after 100 cycles. On the basis of them, Li Fe0.92PO4/C/Graphene composites were prepared and its discharge capacities were 131.5, 124.1, 118.2, 108.2, and 90.5 m Ah·g-1 at 0.5C, 1C, 2C, 5C, and 10 C, respectively. Capacity retention rate of composites can reach 96% after 100 cycles, showing a good electrochemical application performance. Through the first-principles theoretical calculations, Li Fe11/12PO4 model with Fe vacancy defect was analyzed, and it is confirmed that this model had wider lithium ion migration channels in theory. Moreover, the strong interaction between Fe atom beside Fe vacancies and O atom played an important role in increase of electronic conductivity.Cu nanoparticles prepared by liquid phase chemical reduction were used to modify Li Fe PO4. Through the single factor experiment method, generation rate and reaction incubation time of Cu nanoparticles in reduction reaction were comparatively studied, and then optimum parameters of reaction was determined: concentration of HCHO was 16 m L/L, p H value of solution was 11.5, reaction temperature was 40 ℃, and reaction time was 12 min. Cu nanoparticles obtained by the above parameters distributed between Li Fe PO4 particles to improve the conductivity of the composites and had little effect on the tap density, wherein the content of Cu nanoparticles was 6.18% of composites. The discharge capacities of Li Fe PO4/Cu composites were 141.5, 128.5, 112.1, and 88.3 m Ah·g-1 at 0.5C, 1C, 2C, and 5C, respectively, and its capacity retention rate was 82.6% at various rates after 90 cycles. |
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