First-principles Studies On The Physical And Electrochemical Properties Of Fe/Mn-polyanion Of Sodium-ion Batteries

With the advent of Internet,Internet of Things,artificial intelligence,smart cities,and 5G in the 21st century,the energy storage industry has ushered in a new round of spring.The sodium-ion batteries(SIBs)have attracted great attentions due to its many advantages,such as abundant resources,lower prices,especially suitable for large-scale energy storage,and similar chemical properties to lithium-ion batteries(LIBs).At present,high-performance cathode materials for LIBs such as lithium iron phosphate have been widely used in various energy fields as a substitute for the traditional lithium cobalt oxide.With a similar structural framework to lithium iron phosphate,sodiumion phosphate polyanion compounds have also attracted great interests in recent years.The general formula for sodium-ion phosphate polyanion compounds is NaMPO4(M=Fe,Mn,Co,Ni,and other transition metal elements).In this paper,first-principles calculations based on density functional theory have been performed on the sodium-ion cathode material NaMPO4(M=Fe,Mn)to further understand the physical and electrochemical properties of the material.The main research results include:In the first part,based on the first-principles calculations and the analysis of the calculated Bader charges,the redox reactions of the NaFePO4 during the charge and discharge processes were analyzed.Anionic redox could provide additional theoretical capacity,so the study of anionic redox reactions is of great significance.Unlike LiFePO4,NaFePO4 has two phases,the triphylite and the maricite.The results show that there are obviously visible anion redox reactions in NaFePO4 in both the two phases.The calculated electronic partial density of states(PDOS),volume changes,deformation charge densities and magnetization of the two materials during the desodiation processes are also analyzed,which should help the understanding of the processes of redox reactions.In the second part,first-principles calculations were performed on sodium-ion cathode material NaMnPO4 with the triphylite structure.The triphylite-NaMnPO4 is also important,because it is cost-effective,safe,environmentally-friendly,and most importantly,has a one-dimensional sodium ion transmission channel.First,the crystal structure of NaMnPO4 is analyzed in detail,and then the spin-polarized band structures,the total and partial density of states as well as the changes in the deformation charge densities and magnetic moments of the material are calculated.In addition,compared with the full-desodiation phase MnPO4,magnetic moments decreased from NaMnPO4 to MnPO4.Finally,the distortion on MnO6 octahedron during the redox process was explained by using the Jahn-Teller effect.These thoretical studies may further help the experimentist to understand the physical and electrochemical properties of triphyliteNaMnPO4 material.