Theoretical Calculation Of LiMPO₄（M=Fe,Mn） As Cathode Material For Metal Ion Doped Lithium Ion Batteries

LiMPO4(M=Fe,Mn)is one of the most widely used cathode materials for power batteries.With the increasing demand for electric vehicles,the electrochemical performance requirements of batteries are becoming higher and higher,and a new generation of high-potential electrode materials is urgently needed.Studies have confirmed that the M-position substitution of transition elements can improve the conductivity of materials,but its mechanism of action is still unclear.Rare earth materials are expensive,and there are few reports on their doping modification.The theoretical calculation method avoids the blindness caused by the existence of a large number of possible materials,which saves costs and shortens the cycle.In our work,we first use the density functional theory to calculate the lattice constants and electronic structure of the undoped system.Through the analysis of the energy band structure,the composite band gap of LiFePO4 is 0.21 eV for a typical semiconductor,and the band gap of LiMnPO4 is 2.24eV close to the insulator.The experimental results of other research groups are in agreement.It fully proves the accuracy and rationality of the calculation parameters used in the calculation of this paper and lays a foundation for the doping modification and related properties calculation of the subsequent materials.After the different transition elements are doped with LiFePO4.By analyzing its defect formation energy,it has good thermodynamic stability.For the analysis of the band structure,the impurity level will appear near the Fermi level after doping with rare earth elements,which reduces the energy barrier of the electron transition.When the doping element is Nb,the minority-spin band overlap the Fermi level,indicating that it’s a half mental.The elastic properties of the transition group elements after doping are analyzed,and the characteristics of shear deformation of the material are improved after doping.The shear modulus after Co doping is 65.6GPa,which is the most obvious improvement.The calculation of anisotropy also shows that the doping of Co minimizes the anisotropy of shear modulus of the material and the risk of microcrack generation.The band gaps of different concentrations of La doping were investigated.When the doping concentration was 1/8,the material had a minimum band gap of 0.14eV.The elastic properties after doping are calculated.When the doping concentration is 1/4,which is too large and the material tends to be unstable.The effects of different concentrations of La doping on the conductivity of LiFePO4 were investigated from the lattice distortion caused by doping ions and the scattering mechanism of defects on Li ion transport.The diffusion energy barrier was the smallest when the doping concentration was 1/16.The radius of the rare earth element is large.By the calculation of the defect formation can determine its thermodynamic stability.It is found that the band gap is the smallest after Nd doping from the band structure,which is most conducive to the transition of electrons.In addition to Nd doping,the rare earth elements do not improve the shear deformation of the material.The doping of LiMnPO4 with different concentrations of La has a minimum band gap when the doping amount is 1/4.With the decrease of doping concentration,the band gap is closer to the undoped system.The diffusion coefficient of the system is not significantly improved after doping,even lower than that of the undoped system.The smaller the doping concentration is,the larger the diffusion coefficient is,which indicates that the doping of La is not conducive to the diffusion of Li ions in LiMnPO4.After doping with different rare earth elements,the electronic structure of LiMnPO4 changes significantly,and there are differences between the doping of different metal ions,but the doping reduces the band gap of the material,which is beneficial to the improvement of electronic conductivity.