Synthesis And Electrochemical Performance Of Modification Positive Materials Of Lithium Iron Silicate & Lithium Iron Phosphate

Lithium-ion batteries have been widely used in various portable electronic products, and will be used in electric vehicles and hybrid electric vehicle. The cathode material determines the performance-price ratio of the lithium-ion batteries. The polyanion-type cathode materials, Li2FeSiO4 and LiFePO4, are the most promising cathode materials for rechargeable lithium-ion batteries due to their cheap, environmentally benign, cycling performance and good thermal stability.In this dissertation, series of doped Li2FeSiO4 and LiFePO4 materials were synthesized by different sintering method. Among them, different kinds of the Li2FeSiO4/C were prepared by doping Zr4+ to replace partly Fe2+, or F- to O2-, or co-doping F- and Zr4+ to replace partly O2- and Fe2+ in the structure. The different kinds of the LiFePO4/C were prepared by doping Nb5 +or Sc3+ to replace partly Fe2+ in the structure. Many advanced techniques, such as, X-ray diffraction, charge-discharge cycling, cyclic voltammetry experiment, electrochemical impedance, Fourier transform infrared spectroscopy, ultraviolet diffuse reflection spectroscopy and scanning electron microscopy were employed to investigate the relationship among synthesis conditions, structures and electrochemical performances. The main results are given as follows:(1) Monoclinic (Space group:P21/n) Li2.05FeSiO4Fx/C (x=0,0.01,0.02,0.03,0.04) composites were prepared by using a hydrolysis method. The composite with the theoretical composition of Li2.05FeSiO4F0.02/C exhibits a capacity of 91.9 mAh·g-1 in the 1st cycle and 87.4 mAh·g-1 in the 30th cycle at 0.3 C rate and 30℃, and it exhibits a 1st cycle capacity of 116.7 mAh·g-1 at 0.3 C rate,106.5 mAh·g-1 at 1 C rate,99.2 mAh·g-1 at 2 C rate, respectively, at 55℃.(2)Monoclinic (Space group:P21/n) Li2.05Fe1-2ZrySiO4F0.02/C (y=0.025,0.0375, 0.050) composites were prepared by using a hydrolysis method, which associated with impurity phase of SiO2. Among them, the micron Li2.05Fe0.95Zr0.025SiO4F0.02/C composite prepared by sintered at 400℃and 600℃exhibits the best electrochemical performance in the F-Zr co-doped composites. The composite exhibits a capacity of 160.3 mAh·g-1 at 0.3 C rate,111.5 mAh·g-1 at 1 C rate,99.2 mAh·g-1 at 2 C rate,86.5 mAh·g-1 at 5C rate at 55℃, respectively. It is the first report that the energy gap of the lithium iron silicate decreases with doping of fluoride and zirconium ion.(3) Olivine Nb doped lithium iron phosphate/C composites with the micro size were prepared by doping Nb ions to replace partial lithium ions or iron ions in the structure. Ampng them, Li1.05Fe0.98Nb0.02PO4/C composite exhibits a 1st discharge capacity of 120.5 mAh·g-1 at 1 C rate,116.5 mAh·g-1 at 2 C rate and 103.3 mAh·g-1 at 5 C rate, respectively.(4) Olivine Sc doped lithium iron phosphate/C composites with the microsize were prepared by doping Sc3+ ions to replace partial lithium ions or iron ions in the structure. Li1.05Fe0.99Sc0.01PO4/C composite exhibits a 1st discharge capacity of 120.5 mAh·g-1 at 1 C rate,116.2 mAh·g-1 at 2 C rate and 107.4 mAh·g-1 at 5 C rate,85.8 mAh·g-1 at 10C rate and 69.1 mAh·g-1 at 20 C rate, espectively. Compared to the energy gap of the undoped composite (3.07 eV), the energy gap of the Sc doped composite decreased to 2.89 eV.