Synthesis And Characterization Of Li₂ FeSiO₄/C Nanocomposite Cathode Material For Lithium-Ion Batteries

Because of its potentially high theoretical capacity, excellent safety performance, low-cost sources of raw materials and other prominent environmental advantages, silicate polyanionic material Li₂MSiO₄ (Fe, Mn, etc.) is considered to be a new competitive class of cathode materials for development of lithium-ion batteries. One of the most representative Li₂FeSiO₄, which is similar to LiFePO₄, has the advantage of electrochemical cycling stability, outstanding rate performance, thermal stability, and so on. But because the material's electronic conductivity and lithium ion migration rate are low, the structure of the second lithium is difficult to emerge leading high theoretical capacity can not be achieved. These disadvantages limit its application in high-rate and high-capacity electrode materials preparation. In this paper, Li₂FeSiO₄/C nanocomposites are prepared by citric acid assisted sol-gel method and mechanical milling high temperature solid-state method. We studied the impact of different preparation methods and conditions of to material morphology and composition. The Li₂FeSiO₄/C nanocomposite prepared under different sintering temperature and carbon content in the control of different preparation methods, were in-depth analysis and study by physical, chemical and electrochemical methods.The lithium-ion battery cathode materials Li₂FeSiO₄ were prepared by a sol-gel method used stoichiometric amount of analytical reagents, CH₃COOLi·2H₂O, C₆H₅FeO₇·5H₂O, Si(C₂H₅O)₄, and citric acid. The Li₂FeSiO₄ prepared under different sintering temperature were analyzed by X-ray diffraction, scanning electron microscopy. The results showed that, most of the particle size distribution range of 2⁵μm.In the 1.5 4.7 V voltage range, 1/16C rate constant current charge and discharge test, the first charge capacity of Li₂FeSiO₄ cathode material is 149.3 mAh g^-1, the first discharge capacity is 118.0 mAh g^-1 and after 35 cycles the rest discharge capacity is 79 mAh g^-1 with good electrochemical performance. Li₂FeSiO₄/C materials are prepared by sol-gel method with sucrose as a carbon source. The prepared material with sintering temperature of 600℃, have a more flat charge and discharge platform between 2.6 2.8 V. The first charge capacity is 86mAh g^-1 and discharge 65 mAh g^-1. After 12 cycles, the reversible capacity decline to 45 mAh g^-1 with a capacity retention rate of 69%.The lithium-ion battery cathode materials Li₂FeSiO₄/C were prepared by solid-state reaction method used stoichiometric amount of Li₂CO₃, FeC₂O₄·2H₂O, nano-SiO₂ as starting materials and sucrose as the carbon source under different sintering temperature 600℃,650℃,700℃,750℃. The Li₂FeSiO₄/C was analyzed by XRD, SEM and TEM. SEM results show that the Li₂FeSiO₄/C consist of partially agglomerated nanoparticles with an average particle size of about 100 nm.The TEM and the HRTEM show that the primary particles of Li₂FeSiO₄/C 650℃and 700℃are composed of many Li₂FeSiO₄ nano-crystals measured 8¹0 nm which are surrounded by amorphous carbon which can provide a conductive network to facilitate electron transfer and make good connections of the active material particles. In the 1.5 4.75 V voltage range, different rate constant current charge and discharge test, Li₂FeSiO₄/C prepared under 700℃has the best performence of cycle stability and the first discharge capacity under 0.1C is 138 mAh g^-1. The first discharge capacity is about 100mAh g^-1 under 0.5C. After 50 cycles, the discharge capacity remained at 90 mAh g^-1 or so. The initial discharge under 1C is 60 mAh g^-1. After 20 cycles increased to 80 mAh g^-1. After 50 cycles the discharge capacity remained at 70 mAh g^-1 or more. Li₂FeSiO₄/C composite cathode materials were prepared by solid-phase methode with sucrose as carbon source. The results of optimizing carbon content is that the discharge capacity of the Li₂FeSiO₄/C material added 7.5% carbon is up to 141 mAh g^-1 under 0.1C. The first discharge capacity is about 103.6 mAh g^-1 under 0.5C. After 50 cycles, the discharge capacity remained at 97.3 mAh g^-1. The initial discharge under 1C is 88.6 mAh g^-1. After 50 cycles the discharge capacity remained at 78 mAh g^-1.