Preparation Of Nitrogen-doped Carbon-coated Li₂FeSiO₄ Nanocomposites And Research On Lithium Storage Performance

The polyaniontype compound Li2FeSiO4(LFS)is considered to be a promising lithium ion positive electrode material because of its favourable properties such as high theoretical capacity-and good cycle stability.However,Li2FeSiO4 inherently possess two major issues linking to low electron conductivity and slow Li+diffusion,which obviously restrict its development.Carbon coating can improve the performance of the electrode material.N-doping carbon coating can not only significantly improve the apparent conductivity of the carbon layer,but also destroy the carbon network structure and generate numerous defects and vacancies.Lithium ion diffusion channel is formed,which increases the migration rate of Li+.In this thesis,N-doping carbon-coated Li2FeSiO4 electrode materials were synthesized by different nitrogen sources and carbon source,and the electrochemical performance of Li2FeSiO4 cathode materials was improved The research results are as follows:1.N-doping carbon-coated Li2FeSiO4@N-C nanocomposites were prepared by sol-gel method using tartaric acid and urea as carbon and nitrogen sources,respectively.According to sol-gel theory,the addition amount of urea is adjusted to optimize the pH value of the reaction system,and effectively reducing the particle size of Li2FeSiO4.The N-doped carbon coating significantly increases the electrical conductivity and the lithium ion migration rate of the material and improves the electrochemical performance of Li2FeSiO4@N-C.The sample Li2FeSiO4@N-C1 with the smallest particle size exhibits higher capacity,good cycle stability and rate performance.The Li2FeSiO4@N-C1 delivers a high capacity of 238.5 mAh g-1 at 0.2 C that corresponds to 1.44 reversible Li+ extraction/insertion.After charging and discharging at 0.5 C for 100 cycles,the capacity is maintained at about 220 mAh/g without significant attenuation.2.N-doping carbon-coated modified Li2FeSiO4-N nanocomposites were prepared by sol-gel method using tartaric acid and dicyandiamide as carbon and nitrogen sources.After N-doping in the carbon coating layer,the electrochemical performance of Li2FeSiO4-N nanocomposite is significantly improved,especially rate performance and cycle stability.At a magnification of 0.5 C,the discharge specific capacity of LFS-N2 is 252.2 mAhg-1 at the first circle,and the discharge specific capacity after 100 cycles is still maintained at 243.9 mAhg-1,and the capacity retention rate is as high as 96.7%.