| Recently, lithium transition-metal silicates Li2FeSiO4have attracted research interest. With the advantages of high safety, low cost and abundant recourses, Li2FeSiO4has been considered as potential cathodes materials for lithium-ion batteries. Because of the (SiO4)4-poly anions with strong Si-O bonds in the crystal structure, Li2FeSiO4has electrochemical and chemical stability. However, Li2FeSiO4has low electronic and ionic conductivity. Many techniques have been employed to improve the electrochemical performance of Li2FeSiO4. These techniques include conductive carbon coating, particle size reducing, super valent cation doping.Cr-doped Li2FeSiO4improved the lithium ion mobility and exhibited higher discharge capacity, lower electrode polarization, and better rate capability. Currently, there is no theoretical study reported of Cr-doped Li2FeSiO4(Li2Fei-xCrxSi04). The electronic structure and electrochemical properties of Cr-doped Li2FeSiO4are investigated by using the first principle calculations in this work.It is widely accepted that Li2FeSiO4has a monoclinic structure with the space group of P21. In this work, the electronic structure of Li2FeSiO4is calculated based on this P2i structure. The lattice structure and density of states of Li2Fe1-xCrxSiO4(x=0,0.25,0.5,0.75) are calculated. The electron distribution near the fermi level are investigated by using XCrySDen software package. In order to study the electrochemical properties in the charge-discharge process, the lattice structure and density of states of LiyFe1-xCrxSiO4(y=2,1.75,1.5,1,0.5) are calculated. The net charges of every atom of the material are calculated by using BADER software package. In the last, the average voltage and the theoretical specific capacity of Li2Fei-xCrxSiO4are calculated. The main research contents are as follows:(1) GGA+U methods are applied to investigate the density of states of Li2FeSiO4. The calculations show that the material is a semiconductor, the electrons near fermi level are mainly consist of Fe-3d electrons. It is consistent with the literature results of experimental and theoretical calculation. The band gap of doped materials is narrower than Li2FeSiO4, so the electronic transition into the conduction band state can arise more easily. In the Cr-doped materials, the electrons near fermi level are mainly consist of Cr-3d electrons and the valence band and conduction band slates are mainly composed of Cr-3d electronic. Cr-doped improves the electronic conductivity of materials.(2) The lattice structure and density of states of LiyFei-xCrxSiO4(y=2,1.75,1.5,1,0.5) are calculated. Compared with Li2FeSiO4, the change of the unit cell shape and volume is not obvious in the process of removing Li+. The Cr-doped material has high cycling stability in the charge-discharge process. The band gap is still determined by Cr-3d electronic, the band gap of the Cr-doped material is narrower than the Li2FeSi04.(3) In the Li2FeSiO4material, the net charge of Fe ions is1.47, and it increases to1.9after removing Li+. In the Cr-doped material, the net charge of Cr ions is1.54, and it will increase to1.96after remove Li+, and then, the net charge of Fe ions will increase. The net charge of the Cr ions will increase to2.3, while doping ratio is0.25and0.5.(4) The average voltage and theoretical specific capacity have little change before and after doping under the condition in which the same number of lithium-ion are removed.1.5lithium ions can be removed in the material of doping with0.25,0.5, and it can improve the average voltage and the theoretical capacity. |
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