First-principles study on doping effect of Sn in BiF3 as cathode materials for Li-ion battery

Abstract

First-principles calculations were carried out to investigate the structural relaxation, formation energy, electronic structure and electrochemical properties of Sn-doped BiF3. When Sn was doped into BiF3, two common oxidation states of Sn, +2 and +4, were considered. In addition, some typical neutral and charge defects (SnBi0,SnBi1−,SnBi1+,VBi12−VBi22− and VF0) were discussed in detail. Calculated formation energies indicate that Sn4+ ion is much easier to dope into BiF3 than Sn2+ ion. When Fermi level lies at the bottom of conduction band, Sn1/32Bi30/32F3 with VBi12− (Bi vacancy defect) induced by Sn4+ ion doping has the most stable structure under the rich-F growth condition. Here, ΔμSn, ΔμBi and ΔμF are −13.18 eV,−9.71 eV and 0, respectively. What's more, the crystal structure, electronic structure and electrochemical properties of Sn1/32Bi30/32F3 with VBi12− were further investigated. It is found that the crystal volume of Sn1/32Bi30/32F3 with VBi12− is larger than that of pure BiF3 because the length of Bi–F bond around VBi12− in the Sn1/32Bi30/32F3 becomes much longer relative to the length of Bi–F bond in the pure BiF3. Besides, the calculated band gap of Sn1/32Bi30/32F3 with VBi12− is 2.70 eV, which is smaller than that of pure BiF3. Furthermore, Sn1/32Bi30/32F3 with VBi12− has better theoretical voltage and theoretical capacity than pure BiF3.