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/Ni^{2 + 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 LiNiTi_0.5V_0.5O₄ from Li₂NiTi_0.5V_0.5O₄, by activating the redox couple V⁺³/V⁺⁴ in Li₂NiTi_0.5V_0.5O₄. 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.