Study On Preparation And Electrochemical Performance Of Nano Li₂FeSiO₄/C Composite As Cathode Material

The theoretical capacity of Li₂FeSiO₄ is 332 m Ah g^-1,which is much higher than that of other cathode materials.However,the poor diffusivity ability of Li⁺and low conductivity limit the application of Li₂FeSiO₄.In this paper,the specific surface areas of nano Li₂FeSiO₄ and nano Li₂FeSiO₄/C cathode materials were controlled by two kinds of nano fumed silicas with different specific surface areas,in order to investigate the effects of specific surface area on the structure,morphology and electrochemical properties of nano Li₂FeSiO₄ and nano Li₂FeSiO₄/C.In addition,the properties of Li₂FeSiO₄ are improved by carbon materials coating and Nd³⁺doping.The main contents are as follows:(1)Two nano Li₂FeSiO₄,named as LFS-325 and LFS-221,were synthesized by solid phase reaction from two kinds of nano fumed silicas with specific surface area of 325m² g^-1 and 221 m² g^-1,respectively.The specific surface areas of LFS-325 and LFS-221determined by N₂ adsorption and desorption isotherm are 23.6 m² g^-1 and 18.3 m² g^-1.In the presence of carbon source,two kinds of nano Li₂FeSiO₄/C composites,named as LFS/C-325 and LFS/C-221,were prepared as same as experimental conditions of Li₂FeSiO₄.The specific surface areas of LFS/C-325 and LFS/C-221 determined by N₂adsorption and desorption isotherm are 53.9 m² g^-1 and 51.5 m² g^-1,respectively.Therefore,it can be concluded that the specific surface areas of nano Li₂FeSiO₄ and nano Li₂FeSiO₄/C composites can be well controlled by nano fumed silicas with different specific surface areas.XRD proves that the crystals of LFS/C-325 and LFS/C-221 are the monoclinic system of P2₁ space group.FTIR detects the absorption peaks of Li-O bond and Si-O bond.SEM shows that LFS-325 and LFS-221 have similar spherical morphology,and the main particle size distribution range is 60-120 nm.LFS/C-325 and LFS/C-221 also have similar spherical morphology,and the main particle size distribution range is 15-30 nm.Therefore,a layer of carbon material coated on the surface of Li₂FeSiO₄ can effectively inhibit the growth of its particles.LFS/C-325 with carbon materials coating has larger specific surface area and smaller particle size,so it possesses the best electrochemical performance.Under the test conditions of0.1 C and 1.5V-4.8 V,the first discharge specific capacity of LFS/C-325 is 185.2 m Ah g^-1,and the coulomb efficiency is 98.3%.The specific capacity of the 100th discharge at 1 C is 125.4 m Ah g^-1,and the capacity retention is 90.6%.(2)Two nano Li₂FeSiO₄/C composites,named as LFS/C-PVP and LFS/C-CA,were synthesized by polyvinylpyrrolidone(PVP)and citric acid(CA).XRD indicates that the carbon layer obtained from pyrolysis of PVP and CA is coated on the surface of Li₂FeSiO₄ crystal in an amorphous form,and the carbon layer does not change its crystal.SEM shows that LFS/C-CA has irregular spherical morphology,and the particle size is mainly distributed in the range of 20-50 nm.The morphology of LFS/C-PVP is similar to LFS/C-CA.The particle size of LFS/C-PVP is mainly distributed in 15-30 nm.The electrical resistivity test indicates that the electrical conductivity of LFS/C-PVP(0.00126 S mm^-1)is higher than that of LFS/C-CA(0.00116 S mm^-1).Elemental analysis shows that there is a trace amount of N element in PVP,which can increase the conductivity of LFS/C-PVP.A trace amount of N element in PVP is beneficial to increase the capacity of the LFS/C-PVP.So,LFS/C-PVP possesses excellent electrochemical performance.(3)A certain amount of Nd(NO₃)₃·6H₂O was substituted for the corresponding molar amount of FeC₂O₄·2H₂O,and then neodymium ion(Nd³⁺)doped Li₂FeSiO₄/C composite(LFNS)was prepared by solid-phase method.XRD shows that the structures of LFNS and pure Li₂FeSiO₄(LFS)are monoclinic with the P2₁ space group.So Nd³⁺doping does not change the crystal system of Li₂FeSiO₄.SEM shows that the particle size of LFNS with spherical morphology is mainly distributed in 15-45 nm.The first discharge specific capacity of LFNS and LFS at 0.1 C is 221.7 m Ah g^-1 and 184 m Ah g^-1 respectively.The specific capacities of LFNS and LFS after 100 cycles at 1 C are 122m Ah g^-1 and 113.9 m Ah g^-1,and the capacity retention rates are 94%and 93.4%,respectively.The diffusion coefficients of lithium ion in LFNS and LFS are 5.42×10^-15cm² s^-1 and 2.36×10^-15cm² s^-1,respectively.