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

摘要

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 (Sn-Bi(0), Sn-Bi(1-), Sn-Bi(1+), V-Bi1(2-) V-Bi2(2-) and V-F(0)) 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, Sn1/2Bi30/32F3 with V-Bi1(2-) (Bi vacancy defect) induced by Sn4+ ion doping has the most stable structure under the rich-F growth condition. Here, Delta mu(Sn), Delta mu(Bi) and Delta mu(F) 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 V-Bi1(2-) were further investigated. It is found that the crystal volume of Sn1/32Bi30/32F3 with V-Bi1(2-) is larger than that of pure BiF3 because the length of Bi-F bond around V-Bi1(2-) 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 V-Bi1(2-) is 2.70 eV, which is smaller than that of pure BiF3. Furthermore, Sn1/32Bi30/32F3 with V-Bi1(2-) has better theoretical voltage and theoretical capacity than pure BiF3. (C) 2015 Elsevier B.V. All rights reserved.