First-principles Study On Doping Modification Of Sodium Vanadium Phosphate Cathode Materials

Sodium vanadium phosphate（Na₃V₂（P O₄）₃,NVP）with the structure of Na-superionic conductor（NASC ION）,it not only has a three-dimensional sodium ion transport channel,a theoretical reversible specific capacity of up to 117 m Ah g^-1,and the abilit y of two sodium ions to be deem bedded at a voltage of 3.4 V（vs.Na⁺/Na）,but also a stable structure and good cycling performance.Therefore,it is widely used in various secondary batteries.In this paper,titanium sodium phosphate（Na Ti ₂（PO₄）₃,NTP）and Na₃V₂（PO₄）₃material were calculated by VASP software using DFT+U method in first Principles.The research contents and calculation results are as follows:First,the mechanism of sodium removal of Na₃V₂（P O₄）₃ was calculated.The results show that Na₃V₂（P O₄）₃ has good structural stability and can support charge and discharge for a long time.And t he debonding mechanism of Na₃V₂（P O₄）₃ structure is in agreement with the experimental results.The density of states of Na_xV₂（PO₄）₃（x=0,1,2,3）is mainly composed of V-3d states and O-2p states,and the band gap decreases with the removal of Sodium,indicating that the electron conductivity of Na_xV₂（PO₄）₃（x=0,1,2,3）increases with the removal of Na ions,which contributes to the enhancement of the rate performance of the electrode material.the migration energy barrier of Na⁺in Na₃V₂（P O₄）₃ is 0.490 e V and 0.546 e V,respectively,corresponding to the migration path of Na2-Na2 and Na2-Na1（Na2:Na⁺in 18e sites,Na1:Na⁺in 6b sites）.The calculated results show that the transport of Na⁺tends to the migration mode of Na2-Na2.Secondly,the el ectronic structure and the migration energy barrier of Sodium ion were calculated and analyzed by Ti and Fe doping with different concentration of Na₃V₂（PO₄）₃.The results show that the band gap of Na₃V₂（P O₄）₃ decreases with the doping of Ti and Fe,which the doping obviously improved the electronic conductivity.the Na₃V_1.5Ti_0.5（PO₄）₃ and Na₃V_1.5Fe_0.5（PO₄）₃ doping systems have the smallest band gap value of 0.39e V and 0.65 e V respectively,that is,the optimal doping concentration is0.5.In addition,after doping,the diffusion distance of Na⁺decreases and the local structure changes lead to the enlargement of diffusion channels,the decrease of migration energy barrier,and the increase of Na⁺diffusion rate.Thus,For Na₃V_1.5Ti_0.5（PO₄）₃,the calculated migration energy barrier corresponding to Na2-Na2 and Na2-Na1 migration paths is 0.329 e V and0.436 e V,respectively.However,for Na₃V_1.5Fe_0.5（PO₄）₃,The migration energy barrier corresponding to Na2-Na2 and Na2-Na1 migration paths is0.350 e V and 0.493 e V,respectively.Finally,calculate the NVP||NTP of the model electronic structure and sodium ion migration energy barrier.The results show that the improved conductive ability,band gap reduced from 2.38 e V to 1.89 e V.Na2-Na2 and Na2-Na1 migration path corresponding migration energy barrier 0.269 e V and 0.357 e V,respectively,showed that compared with excessive metal doping,NVP||NTP model of sodium ion migration rate faster.In conclusion,by studying and analyzing the evolution of electronic structure and microstructure of Na₃V₂（P O₄）₃ in the process of sodium removal,Na₃V₂（P O₄）₃ has excellent structural stability and clear sodium ion transport mechanism,and can effectively change the electronic conductivity of Na₃V₂（PO₄）₃ and reduce the sodium ion migration energy barrier by means of doping.It provides an effective theoretical basis for the doping element and doping ratio of Na₃V₂（PO₄）₃ experiment field.