First-principles investigation of the structural stability and electronic properties of LiV1–xMxPO4F (M = Mn, Fe, Co, and Ni)

Abstract

Density functional theory calculations corrected by on-site Coulomb interactions have been conducted to study the crystal structure and electronic property of the LiV1–xMxPO4F (M = Mn, Fe, Co, and Ni) electrode systems. The calculation results show that the substitution of V atoms in LiVPO4F by Fe-dopants resulted in the lowest formation energy, and the crystal structure is, therefore, most thermodynamically stable, while Co and Ni doping seems to be difficult due to their relatively higher formation energy than that of doping with Fe and Mn. Furthermore, Fe substitution can effectively suppress the volume change of the LiVPO4F materials during electrochemical delithiation, leading to improvement of the cycle performance of LiVPO4F electrode. The results of charge analysis predict that Fe substitution is able to donate extra electrons for charge compensation during the Li+ reversible extraction/insertion process. The calculation of energy band structure and density of states indicate that the Mn, Fe, Co, and Ni doping reduce the band gaps through variations of V-3d spin orbitals. However, only Fe doping introduced impurity level located at 0.75 eV above valence band in the forbidden band of LiVPO4F, providing a platform for electronic transition, which is helpful to enhance electronic conductivity of the LiVPO4F electrode.