| As a new class of polyoxyanion cathodes based on the orthosilicates containing the tetrahedral structures, Li2FeSiO4compounds are the promising candidates for the lithium-ion battery cathode materials, due to its environmental benignity, low cost, abundance of raw materials, simple synthesis and high theoretical capacity. But Li2FeSiO4have many defects like poor electronic conductivity and Li-iondiffusion coefficients, which have resulted its poor electrochemical properties. In this work, the nano-Li2FeSiO4/C composites were prepared by a traditional solid-state reaction method, and were mainly studied from three aspects--synthesis process, surface carbon-coating and metal ion doping to improve the electrochemical properties. The as-prepared materials were characterized by elementary analyzer, XRD, SEM, TEM, FT-IR spectrum, BET and electrochemical test. The main contents are as follows:(1) Nano-Li2FeSiO4/C composites were prepared from three kinds of nano-SiO2(the particle sizes of nano-SiO2are15±5nm,30±5nm and50±5nm, respectively) by a traditional solid-state reaction method. Using ammonium citrate as chelant and carbon source, FeC2O4@2H2O as iron source, Li2CO3as lithium source and SiO2as silicon sourse. The effect of SiO2particle sizes on the structure and electrochemical performance of nano-LiFeSiO4/C composites was investigated. The as-prepared materials were characterized by XRD, BET, SEM, elementary analyzer andΩ electrochemical test. The results showed that the composites(defined as LFS30) fabricated from nano-SiO2(30±5nm) has a tetrahedral structure with best crystallization and less impurities, the average particle size of LFS30is29.1nm, the specific surface areas of composite is35.27m2/g, the carbon content of material is9.9%. The initial discharge capacity of LFS30composite is152.6mA@h/g at0.05C, after60cycles at1C, the discharge capacitys attenuation value is15%. The EIS showed that LFS30has the best AC impedance parameters, which the Rs is5.44Ω, Rct is85Ω, the Li-iondiffusion coefficients Do in oxidation and reduction process is3.24xlO-12cm2·s-1:(2) Nano-Li2FeSiO4/C was prepared via a solid reaction, Li2CO3, FeC2O4·2H2O, nano-SiO2(30±5nm) and ammonium citrate were used as the starting materials. The effect of calcined atmosphere and grinded medium on electrochemical performance of Li2FeSiO4/C composites were studied. The results indicated that the material has the best electrochemical properties under the conditions of argon as calcined atmosphere and acetone as grinded medium. The initial discharge capacity of composite is199.1mA-h/g at0.1C, which means that1.2Li+can be deintercalate/intercalated from one Li2FeSiO4molecule, after60cycles at1C, the discharge capacity’s attenuation value is9.4%;(3) Under the optimized contidion, the effect of different carbon sources on the electrochemical properties of Li2FeSiO4/C material was researched. The carbon sources included sucrose, ammonium citrate, PEG+ammonium citrate and sucrose+PEG+ammonium citrate. The results declared that the sample which ammonium citrate as carbon source had the best electrochemical performance and AC impedance parameters. After20cycles at1C, the discharge capacity of composite reduced from106.6mA-h/g to104.5mA-h/g, the discharge capacitys attenuation value is1.96%. The EIS showed that composite has the best AC impedance parameters, which the Rs is19.4Ω, Rct is195Ω, the Li-iondiffusion coefficients Do in oxidation and reduction process is3.24xlO-12cm2·s-1:(4) The LFS/C/rGO composites were prepared by a facile solid-state reaction method, using ammonium citrate and graphene oxide as carbon source. The effect of graphene oxide amount on electrochemical performance of LFS/C/rGO composites were studied. When the molar ratio of two carbon sources is1:4, the composites have the best electrical conductivity, AC impedance parameters and electrochemical properties, which the electrical conductivity is19.6×10-7s cm-1, Rs is7.79Ω, Ret is225f2, the Li-iondiffusion coefficients Do in oxidation and reduction process is6.3x1O-13cm2·s-1. The initial discharge capacity is191.6mA·h/g at0.1C, after60cycles at1C, the discharge capacity attenuation value is8%. The LFS/C/rGO and LFS/C materials were characterized by XRD, TEM, SEM, FT-IR spectrum and electrochemical test. XRD results reveal that rGO can promote the crystalline grain growing. SEM and TEM results indicate a more valid conductive network is formed in LFS/C/rGO. FT-IR spectrum indicates that1638cm-1peak of rGO’s C=C bonds are observed in LFS/C/rGO and all typical characteristic absorption peaks of LFS/C/rGO exhibit blue shifts compared to the FT-IR spectrum of LFS/C. The electrochemical measurements illustrated that LFS/C/rGO has much better electrochemical performance than that of LFS/C;(5) Mn-doping Li2FexMn1-xSiO4/C was prepared by solid-state reaction. The influence of x value on electrochemical performance of Li2FexMn1_xSiO4/C was researched by electrochemical test. With x value is decreasing, the electrochemical properties of Li2FexMn1-xSiO4/C are decrease. Li2Fe0.9Mn0.1SiO4/C has the highest initial charge/discharge capacity and the value is200.8mA·h/g,168.1mA·h/g at0.1C, respectively. After60cycles at1C, the discharge capacitys attenuation is36%. While the discharge capacitys attenuation value of Li2FeSiO4/C is9.1%and has the best AC impedance parameters. In a word, the Mn-doping improved the charge-discharge performance, but weakened the cycling properties of Li2FexMn1-xSiO4/C composites. |
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