Study On Preparation And Modification Of Li₂FeSiO₄/C Cathode Materials

Lithium-ion batteries?LIBs?have been widely used in portable electronics,electric vehicles and large-scale energy storage application due to their high energy density,long cycle life,low self-discharge,nonmemory effect and high operating voltage.As a novel polyanion compound,lithium iron silicate?Li2FeSi O₄?is regarded as one of the most promising candidates for Li FePO₄,because of its low cost,environmental benignancy,high theoretical capacity and excellent chemical stability.However,Li₂FeSiO₄ cathode material suffers from its inherent disadvantages such as the slow lithium ion diffusion rate?approximately 1×10-14cm2/s?and poor electronic conductivity?approximately 6×10-14 s/cm?,which hinders the attainment of high electrochemical capacity and cycling performance,and then limits its large scale applications in LIBs.To overcome above severe issues,in this paper,carbon coating,tailor the particle size to nanoscale,morphology and size optimization and metal cation doping were adopted to improve the electronic conductivity and lithium ion diffusion velocity,thus enhance the electrochemistry performances of Li₂FeSiO₄ cathode material.And the primary study tasks are as follow:?1?The Li₂FeSiO₄/C composite cathode material is synthesized by sol-gel method,and the influences of sintering temperatures on the electrochemical properties of Li₂FeSiO₄/C are studied in detail.The results reveal that the sample sintered at650 °C exhibits best electrochemical properties among all the samples due to its high purity,uniform particle size and shorter Li+diffusion pathway,which delivers a higher reversible capacity of 159.1 mAh/g at a rate of 0.1 C in a potential range of 1.5-4.8V with an excellent capacity retention of 92.1% after 50 cycles.?2?The morphology and size controlled Li₂FeSiO₄/C cathode material is successfully synthesized by a sol-gel method,in which nano-sized,spherical and cubic Fe₂O₃ are respectively used as iron sources.By adjusting the Fe₂O₃ precursors' morphologies,the morphology and grain size of the Li₂FeSiO₄/C materials can be purposefully controlled.The results show that the as-synthesized Li₂FeSiO₄/C samples exhibit well-defined morphologies due to the homogeneous and well-crystallized Fe₂O₃ precursors.Comparing with Li₂FeSiO₄/C synthesized by commercial Fe₂O₃ precursor,the Li₂FeSiO₄/C materials prepared by spherical or cubic Fe₂O₃ precursors exhibit eminent electrochemical properties,e.g.,high initialreversible capacities of 214.1 m Ah/g for LFS/C-S and 206.5 mAh/g for LFS/C-C at0.1 C and good cyclic stability?93.1% for LFS/C-S and 91.5% for LFS/C-C?.In particular,the spherical LFS/C-S material with better fluidity and less agglomeration displays excellent rate capability of 105.4 mAh/g at 10 C.?3?In order to promote the insertion/extraction of more than one lithium ion per formula unit during charge/discharge processes,the Mn doped spherical Li₂FeSiO₄/C cathode materials are successfully synthesized through a sol-gel method.The effects of alien Mn with various amounts on the structure,morphology and electrochemical performance were studied systematically.The result shows that the introduced Mn not only doesn't affect the structure and morphology of the host material,but also improve the structure stability of Li₂FeSiO₄ cathode material as well as facilitate the transfer of lithium ion during long cycling process.Besides,the electrochemical measurements reveal that the sample doped with 5% Mn delivers the highest initial discharge capacity of 248.5 mAh/g at 0.1 C under ambient temperature in a voltage range of1.5-4.8 V,the retention of the capacity is as high as 91.9% after 200 cycles.Additionally,the sample also exhibits a good rate capability with a discharge capacity of 105.6 mAh/g even at a high rate of 10 C.