Revealing the doping mechanism and effect of cobalt on the HTB-type iron fluoride: A first-principle study

Abstract

FeF3 with a hexagonal-tungsten-bronze structure (HTB-FeF3) has been proposed as a promising cathode material with high theoretical specific capacity and voltage. However, it suffers from the drawbacks of poor Li kinetics and cycling life due to poor electrical conductivity. Moreover, its fundamental information, such as its geometrical structure, elastic properties and electronic structure, can rarely be found. Hence, first-principle calculations have been carried out to systematically investigate the effects of Co doping on the FeF3. We focus on CoxFe1-xF3 systems, in which x is equal to x = 0.08, 0.17 and 0.25, respectively. Firstly, the formation energies of CoxFe1-xF3 are obtained according to the elemental chemical potentials determined by the phase stability conditions. The calculated formation energies indicate that CoxFe1-xF3 can be synthesized under the F-rich growth conditions. Moreover, difficulty of Co doping FeF3 decreases with the increase of Co doping concentration. By analysis of the electronic structure of CoxFe1-xF3, it is found that the band gap of CoxFe1-xF3decreases remarkably and Co0.17Fe0.83F3 has the lowest band gap (0.993 eV), This phenomenon can be attributed to the Co-3d impurity energy levels occurring between valence band and conduction band. In order to further clarify the doping mechanism of Co on the FeF3, the elastic constants, bulk modulus and Debye temperature of FeF3 and Co0.17Fe0.83F3 are calculated. The results suggest that Co doping can help avoid particle fracture of FeF3 during charge/discharge cycling in the Li-ion batteries. Besides, it also can help us to further understand thermodynamic properties of FeF3 and Co0.17Fe0.83F3.