First-principles Investigation Of Electrochemical And Dopant Properties Of Li₂FePO₄F And Na₂FeP₂O₇

Since Sony announced the first version of commercialized lithium-ion-battery(LIB) in1991, LIB has rapidly penetrated into our everyday life. LIB is one of post potential candidates as the energy storage device mainly due to its high energy densities with fairly good rate capabilities and a fairly long cycle life. At the same time, sodium-ion batteries(SIB) also been studied. The cathode was indeed the most important one among the three parts(cathode, electrolyte and anode) of the LIB/SIB. One of the most interesting cathode materials is that of poly-anions, chiefly because of the improvements in safety, chemical stability and also environmental friendliness. In this work, we systematically investigated the electrochemical properties and metal-doping of Li2 Fe PO4F and Na2 Fe P2O7 as cathode materials for LIB and SIB, respectively, by using ab initio methods.I、The electrochemical properties of three isotopic Li2 Fe PO4F compounds, as cathode materials under different space groups Pbcn, P1_and Pnma were investigated using first principle calculations. Their structures, average open circuit voltages for step delithiation reactions were explored, the results are in good agreement with the reported experimental data. We estimated the substitution effect of Fe by Co/Mn in Pnma-Li2 Fe PO4F. Our results suggest that Li2Fe1-x Cox PO4F(0<x<1) are stable enough to be a series of new cathode materials, while Li2Fe1-x Mnx PO4 F would be decomposed into Li Mn PO4、Li Fe PO4 and Li F. The substitution of Fe by Co in Li2Fe1-x Cox PO4 F may enhance the discharge potential of the materials, and the rate of its volume change during the redox process is between 0.6% ~ 2.1%, which indicated that Li2Fe1-x Cox PO4 F are stable enough to be new cathode materials. Furthermore, from the projected density of states for Li2Fe0.5Co0.5PO4 F, a strong hybridization for Fe-3d, Co-3d bands near the Fermi level was found, which implied that the Co-doped Li2Fe1-x Cox PO4 F may possess better electronic conductivity than the pure phase. Therefore we believe that the fundamental study presented here may broaden the understanding of fluorophosphates compounds and provide insight into optimizing them for a better performing LIB.II、β-Na2 Fe P2O7 as a safe cathode with high thermal stability has been demonstrated in experimental. The structure and electrochemical properties of β-Na2 Fe P2O7 and β-Na Fe P2O7 as positive materials were investigated using DFT calculations. The calculated voltages and the energy of phase transition from β®α Na Fe P2O7 are in good agreement with the reported experimental data. Four intermediate phase among desodiated process were found(i.e.Na1.9375 Fe P2O7, Na1.5Fe P2O7, Na1.375 Fe P2O7 and Na1.125 Fe P2O7), where the calculated voltage profile agreed well with experiments results. We also estimate the substitution effect of Fe by other transition metals i.e. Na2Fe1-x Mx P2O7(M = V, Mn, Co, Ni; x = 0, 0.25, 0.5, 0.75, 1). In this work, the DFT calculations can help the understanding of pyrophosphate compounds and provide insight to optimize them for a better performing SIB.