Research On High Performance Li₂FeSiO₄ Cathode Material For Lithium Batteries

Lithium iron silicate（Li₂FeSiO₄） is a promising polyanionic cathode material for lithium-ion batteries. Similar to lithium iron phosphate（LiFePO4）, Li₂FeSiO₄ shows merits of also abundant raw materials, low cost, non-toxicity and environmental friendliness. In addition, it can afford a theoretical capacity ³31m Ah/g through two-electron reaction, and an energy density as high as 1200Wh/kg. This energy is twice that of most other cathodes, rendering Li₂FeSiO₄ an ideal choice for next generation lithium batteries.In this thesis, we report our recent works on the synthesis and modification of Li₂FeSiO₄, focusing on the structure, morphology, modification and electrochemical properties. The main conclusions can be summarized as follows:Firstly, we synthesize three-dimensional（3D） porous Li₂FeSiO₄/C nanocomposites based on hydrothermal reaction and subsequent annealing. The obtained 3D porous materials were further carbon coated to improve the electronic conductivity and electrochemical properties as well. The modified material provides a reversible capacity of 170mAh/g at the voltage window of 1.5–4.5V. Compared to the previous papers, it can be proved that: reduction of the size, formation of 3D porous structure and coating of amorphous carbon all can ameliorate the electrochemical performance by increasing the ion transmission rate and electronic conductivity of Li₂FeSiO₄.Secondly, we designed a 3D framework coaxial structure supported by carbon nanotubes to improve the conductivity and reduce the contact resistance of the material. The CNT@ Li₂FeSiO₄ was prepared through a sol-gel process. After exploration of experimental conditions, the designed structure composite was carried out, with dense Li₂FeSiO₄ coated on the carbon nanotubes. Moreover, the coaxial nanotubes interweaved, forming network structure which accelerating the ion transmission speed. The synthesized products were confirmed as orthorhombic Li₂FeSiO₄ by XRD test. The specific surface area was much larger than normal Li₂FeSiO₄, owing to the 3D coaxial network structure. For further electrochemical tests, the CNT@ Li₂FeSiO₄ can afford ²00m Ah/g at the voltage window of 1.5–4.6V.Lastly, Li₂FeSiO₄ nanofibers（LFS NFs） were successfully prepared by electrospinning method. After exploring the configurations of the solution for electrospinning and the annealing conditions, we successfully synthesized Li₂FeSiO₄ nanofibers. Characterized by structure and morphologies, the product we synthesized was confirmed to be Li₂FeSiO₄ of orthorhombic phase. We believe that the Li₂FeSiO₄ nanofibers can perform excellent electrochemical capacities due to the hollow porous and interwoven mesh structures.