First-principles Study The Mg²⁺-doping Effects On Li₃V₂（PO₄）₃

The polyanion phosphate cathode materials with the advantages of abundant raw materials,good safety performance,structural stability,low price and good electrochemical performance and etc,so it is considered to be the candidate of the most likely to commericalized as cathode materials for lithium ion batteries.Monoclinic lithium vanadium phosphate cathode materials with the advantages of relative high discharge voltage,high theoretical capacity density,structure stability and good safety performance which have drawn great attention to researchers.However,Li₃V₂（PO₄）₃ meets the problems of low electronic conductivity and diffusion coefficient of Li⁺which have seriously affected the electrochemical performance and future impeded its commercial development.Researchers usually employ the methods of surface coating,particle size reducing and aliovalent ions doping to improve its electrochemical performance,but there are few study on the crystal and electronic structure of Li₃V₂（PO₄）₃ which result in the lack of corresponding theoretical basis.It is useful on the materials design at the level of atomic and molecular,shorten the test cycle,improve the efficiency and save the cost of tests by using the first principles method.In this paper,the first principles within ultrasoft pseudo-potential combined with the generalized gradient approximation method based on the density functional theory（DFT）is used to study the crystal and electronic structures of the lithium ion battery cathode material Li₃V₂（PO₄）₃.The unit cell parameters,atomic population analysis,density of states,electron density difference contours and band structure were calculated.Theoretical lattice constants of Li₃V₂（PO₄）₃ at ground state in this work is obtained.The method of GGS+PBE we used can be verified through comparison with the theoretical and experimental results.At the same time,three kinds of doping models of Mg²⁺substitute at different sites of Li₃V₂（PO₄）₃(Li_2.5Mg_0.25V₂（PO₄）₃,Li₃V_1.75Mg_0.25（PO₄）₃,Li_2.75Mg_0.5V_1.75（PO₄）₃)were established by using the Castep module in Materials Studio software.The atomic positions and lattice constants were optimized by using the generalized gradient approximation（GGA）with PBE algorithm.The formation energies,lattice constants,total and partial density of states,band structures and electron density difference were calculated by using the method of ultra-soft pseudo-potential plane wave based on density functional theory.The results shows:firstly,Mg-doping at V-sites in Li₃V₂（PO₄）₃ have negative defects formation energies and smaller cell parameters change which indicate Mg²⁺is more likely to substitute the V-sites to form stable compounds rather than intermediate or metastable phase.Secondly,it can be seen from the band structure and density of state of the doped Li₃V₂（PO₄）₃ that Mg²⁺doping can significantly reduce the band gap of Li₃V₂（PO₄）₃,where doped at V-sites have the lowest band gaps,show that the Mg²⁺doping can improve the electron conductivity of Li₃V₂（PO₄）₃ and benefits to the improvement the electrochemical performance of the lithium ion batteries,especially the rate capability.Thirdly,the atomic population analysis showed that,the lithiums in the structure of Li₃V₂（PO₄）₃ are in the ionic state and relatively freedom,and the oxygen bonding with vanadium and phosphate to form strong covalent bonds.The net charge of lithiums can be reduced by Mg²⁺doping and hence the ionicity of lithium is enhanced.Lithium ion can insertion and extraction more easily in the process of charge and discharge,which will improve the cycle performance.Lastly,it can be found that,by investigating the charge density difference of Mg²⁺doped Li₃V₂（PO₄）₃,for the Mg²⁺doped at V-sites Li₃V₂（PO₄）₃,the positive charge accumulation in the vicinity of Mg²⁺was smaller,indicating that the diffusion channel can be broadened,diffusion activation energy of lithium ion can be decreased and hence the migrating rate of lithium ion is increased.