Preparation And Modification Of Li₂FeSiO₄/C Cathode Material For Lithium Ion Batteries

Polyanion-type Li₂FeSiO₄ cathode material has attracted great attention because of its high theory specific capacity,low cost and environmental friendly properties.However,Li₂FeSiO₄ is plagued by the low electronic conductivity and lithium-ion diffusion efficiency,which greatly limit its development.Carbon coating is regarded as a common way to enhance the conductivity of electrode materials.However,the traditional carbon coating has gradually been unable to meet the higher pursuit of Li₂FeSiO₄ performance.Therefore,on the basis of carbon coating,we introduced a nitrogen source and highly conductive carbon nanotubes to improve the electrochemical performance of Li₂FeSiO₄ material.The research contents are concluded as following:(1)N-doped carbon coating Li₂FeSiO₄/C composites were synthesized by sol-gel method using biuret as nitrogen source.The effect of nitrogen source amount on the electrochemical performance of materials has been studied.The results showed that the electrochemical performance of all N-doped samples is significantly higher than that of the undoped sample.In particular,the sample with a molar ratio of carbon source and nitrogen source of 6:1 showed the best electrochemical performance,with a reversible capacity of 210.8 m Ah/g at 0.2C that correspond to 1.27 reversible Li⁺extraction/insertion.And it delivers the discharge capacity of 232.1,177.6,161.2,and144.2 m Ah/g at rates of 0.2,0.5,1,and 2C,respectively.(2)N-doped carbon coating and carbon nanotubes synergistically modified Li₂FeSiO₄ composites were prepared by adding carbon nanotubes in the sol-gel process on the basis of the optimal amount of nitrogen doping.By adjusting the content of carbon nanotubes,it is found that the material can form a continuous 3D conducting network when the addition amount is 6 wt%,which can accelerate the electron transport and stabilize the structure.Therefore,the sample exhibits excellent electrochemical performance with the discharge capacitiy of 270.6 m Ah/g at 0.2C(about 1.63 Li⁺per molecular).After 100 cycles,the capacity is still maintained at 256.7 m Ah/g.And the sample shows 267.9,194.7,178.5,and 173.6 m Ah/g at 0.2,0.5,1,and 2C,respectively.