DFT study of Mg2TiO4 and Ni doped Mg1.5Ni0.5TiO4 as electrode material for Mg ion battery application

Mg2TiO4 is a spinel material, however it is electrochemically inactive. We have simulated Ni doping in its tetrahedral network by partially replacing Mg+2 (0.5 Mg+2) ions with Ni+2 to form Mg1.5Ni0.5TiO4. Mg2TiO4, Mg1.5Ni0.5TiO4 and its de-intercalated end product MgNi0.5TiO4 are studied through analysis of structural parameters and density of states. All the three materials produce almost similar x-ray diffraction pattern ensuring structural stability during charge–discharge. After doping of Ni, Mg1.5Ni0.5TiO4 becomes electrochemically active, as, only Ni states are found to be contributing charges (electrons) to the conduction band of MgNi0.5TiO4 after de-intercalation. This redox activity is also supported from the magnetic moments of Ni indicating change in valance from +2 to +3 on de-intercalation. Calculation of de-intercalation voltage for de-intercalation of tetrahedral Mg from Mg1.5Ni0.5TiO4 indicates a value of 4.22 V. This high voltage with electrochemical capacity 151 mAh g−1 would generate energy density of 637.2 W–h kg−1. Simulation of formation energy (Ef) of Mg1.5Ni0.5TiO4 indicates that with rutile TiO2 the phase formation is probable with slightly negative value of formation energy, however, with anatase TiO2 the phase formation is highly probable with high negative value of formation energy (Ef).