| Orthosilicates cathode materials Li2FeSiO4 have attracted great attention due to its high theoretical specific capacity(332 mAh·g-1), excellent cycling performance, nontoxic and abundant sources of raw materials. However, since its low electronic conductivity and Li+ diffusion coefficient, the real capacity is less than half of the theoretical one( < 166 mAh·g-1), which highly restrict the commercial production and applications of Li2FeSiO4. Hence, a systematic research is proposed in this paper on the synthesis process and modification methods of Li2FeSiO4 cathode material. The purpose of this study is to improve the electrochemical performance of Li2FeSiO4 by improving its purity, surface morphology and structure then design and manufacture Lithium-Ion batteries with high capacity, high security and low cost.The electrochemical properties of Li2FeSiO4 materials are improved by changing the raw materials, synthesis and surface modification processes methods. Moreover, a variety of analytical methods such as XRD、SEM、TEM、TG、IR、Raman、XPS、EDS、BET、CEA、CV、EIS are used to make a systematic study on the structure, morphology and electrochemical properties of Li2FeSiO4. The main contents include:(1) Li2FeSiO4/C cathode material is synthesized by using the high temperature solid-state method. Considering the effects of different lithium sources and carbon sources on Li2FeSiO4/C material structure, morphology and electrochemical properties, the Li2FeSiO4/C cathode material has been synthesized and characterized by Li2SiO3 and hydroxyethyl cellulose for the first time. The initial discharge capacity of the material is 119 mAh·g-1 and 79.8% of the capacity is maintained after 20 cycles. The hydroxyethyl cellulose formed carbon coating layer on the surface of the particles under anaerobic decomposition conditions which effectively hindered the growth and agglomeration of Li2FeSiO4 grains meanwhile dense carbon coating layer also effectively improved the electronic conductivity of the material surface.(2) The carbothermal reduction method is used for in situ synthesis of Li2FeSiO4/C cathode material. The researcher found that sucrose in the process of the carbon thermal reduction continuous generates highly active carbon cleavage that reacts with Fe2O3, which makes the synthesized Li2FeSiO4 has high purity, less particle agglomeration and carbon-coated surface layer remaining intact. When the sintering temperature is 600 oC, the initial discharge capacity of the material is 140.2 mAh·g-1,and the capacity is not attenuated after 40 cycles. The carbothermal reduction method can simplify the process and reduce production costs.(3) The surface modification solutions of Li2FeSiO4/C materials are investigated. We proposed that it can use chemical vapor deposition method for carbon cladding layer deposited on the Li2FeSiO4 particle surface, by taking ethanol vapor as a carbon source, using Compared to the method of adding the carbon source in the precursor, the feature of vapor deposition method is thin and uniform deposition of carbon. Two-stage sintering method was used to separate the carbon deposition process and the sintering process of Li2FeSiO4, which reduce the amount of carbon-coated with increasing crystallinity of Li2 FeSi O4. The results show that the initial discharge capacity of Li2FeSiO4/C is 155.3 mAh·g-1 and 20 cycles capacity retention rate is 89.8%.(4) In order to further improve the electrochemical properties of Li2FeSiO4/C cathode material, we proposed a method by adding graphene and carbon nanotubes as the conductive frame to further improve the electrochemical properties of Li2FeSiO4 material. We found that the initial discharge capacity of the Li2FeSiO4/Graphene/C material is 153.0 mAh·g-1, with 95% capacity retention rate by 40 cycles, when graphene dosage is 1 wt.%, When the optimum amount of carbon nanotubes is 5 wt.%,the initial discharge capacity of the Li2FeSiO4/CNTs/C is 142.7 mAh·g-1,and 50 cycles capacity retention rate is 97.7%. The graphene and carbon nanotubes serves as the internal conductive network, meanwhile, the restraint of grain growth in the sintering process effectively reduces the grain size of Li2FeSiO4,Hence, the rate and cycle characteristics of Li2 FeSi O4 cathode material were greatly improved.(5) Hydrothermal assisted sol-gel method was first used for the synthesis of porous Li2FeSiO4/C cathode material using Fe(NO3)3·9H2O、CH3COOLi and Si(C2H5O)4 as raw materials, ethylenediamine as a kind of reducing agent and pH regulator. In this synthetic route, ethylenediamine simultaneously plays three roles: adjustment the pH value of sol, reduction of Fe3+ and cross-linked to form a polymer soft template. The average grain diameter of the porous Li2FeSiO4/C material synthesized by this method is only 25 nm; the average pore diameter is 3.6 nm. The initial discharge capacity of the materials is 195.5 mAh·g-1,and shows a very good cyclic performance. |
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