DFT study of Li2NiTiO4 and vanadium-doped Li2NiTiO4

Abstract

Density functional theory study of Li2NiTiO4 and vanadium-doped Li2NiTiO4 are performed for understanding their structural and electrochemical phenomena, viz., structural geometry like lattice parameters, change in lattice volume with Li+ extraction, Li+ de-intercalation voltage, electrochemical capacity etc. Li2NiTiO4 has cubic symmetry with space group Fm3m (space group number 225). De-intercalated structure of LiNiTiO4 is obtained by removing one Li atom from Li2NiTiO4 unit cell while shifting another Li atom from octahedral site 4b to tetrahedral lattice site 8c. Due to Li+ extraction, the change in unit cell volume is ~ 5.9%. Li+ de-intercalation voltage is calculated by subtracting total energy of the unit cell of LiNiTiO4 and bcc Li from Li2NiTiO4. The voltage comes out to be 4.84 V which is nearly at the threshold for electrochemical stability of used electrolytes. The redox couples in this case are Ni+2/Ni2 + δ and O−2/O−2+γ. The presence of redox couple O−2/O−2+γ leads to the probability of evolution of oxygen during charging. However, partial (50% in this case) vanadium doping at Ti site reduces the redox voltage to 4.64 V considering the de-intercalation reaction forming LiNiTi0.5V0.5O4 from Li2NiTi0.5V0.5O4, by activating the redox couple V+3/V+4 in Li2NiTi0.5V0.5O4. This also reduces the possibility of evolution of oxygen during de-intercalation reaction by shifting the main redox couple from O−2/O−2+γ to V+3/V+4, which leads to greater structural stability of electrode materials during charge-discharge cycles.