First-principles Investigation Of Electrochemical And Elastic Properties Of Metal Doped LiFeSO₄F And NaFePO₄

Li ion batteries as promising power sources are widely used in cell phones, laptops and electric vehicles due to their high energy density. Na is located below the Li in the periodic table, and they have similar chemical properties. Na ion batteries have received wide attention due to the aboundant Na resources, low cost and environmental friendliness. The cathode material is a key component that governs the electrochemistry of the batteries. In the present study, the electrochemical performance and elastic properties of AFeSO4F (A=Li, Na) and NaFePO4 as cathode materials for LIB and SIB are investigated using first-principles calculation.The structure stability, electrochemical performance and elastic properties of AFe1-xMxSO4F (A=Li, Na; M=Co, Ni, Mg; x=0,0.5,1) were investigated using first-principles calculation. Ni-doped AFeSO4F undergo negligible volume changes upon the extraction of Li/Na ions, indicating that Ni doping is beneficial to cycle stability of the battery. The computational results reveal that the substitutions of Fe by Ni, Co and Mg enhance the open circuit voltage of the materials. The band gaps of AFe1-xMxSO4F show transition metal-doped AFeSO4F, especially for Ni doping case have better electronic conductivity in comparison with the pure phase and Mg-doped AFeSOF. By the calculation of elastic property, LiFeSO4F is found to be ductile, while NaFeSOF shows brittle property. Further calculation shows NaFe0.5Ni0.5SO4F have a higher value of BIG than NaFe0.5Ni0.5SO4F, indicating NaFe0.5Ni0.5SO4F can improve the ductility. Therfore, Ni doping is one of the effective methods to improve the electrochemical properties of AFeSO4F. The fundamental study of AFeSO4F may broaden the understanding of fluorosulfate compounds and provide certain theoretical guidance for experimenter.The crystal stability, electrochemical and elastic properties of various metals (Co, Mn, Mg) doped NaFePO4 are performed by first-principles calculation. The calculated change of volume about the NaFe1-xMxPO4 upon the extraction of Na ions demonstrates that the change of cell volume decreases with the increase of Mg adulteration. We predicted Mg-doped NaFePO4 is beneficial to cycle stability of the battery. Furthermore, the calculated voltage increases with the increasing of doping mental content, and follow the series Mg>Co>Mn. We also find the band gaps of NaFe0.75Co0.25PO4 and NaFe0.25Co0.75PO4 are lower than NaFePO4, showing a better electronic conductivity. Analysis of the elastic property reveals that the value of BIG decreases with the increase of Co, Mn and Mg, indicating they can not improve the ductility of NaFePO4. Therfore, to a certain extent, various metal doping can improve the electrochemical properties of NaFePO4. The fundamental study of NaFePO4 may provide certain theoretical guidance for experimenter and optimize them for a better performing.