Tuning of Band Gap of Cathode Li2NiPO4F by Replacing P to Nb and Forming Li2NiNbO4F for Application as 5 V Cathode in Lithium Ion Battery: A Density Functional Theory Study

Electrochemical properties of Li2NiPO4F were studied using density functional theory. The obtained voltage, electronic band gap, capacity (∼ for 2 Li+ extraction) and energy density are achieved as 5.33 V, 4.0 eV, 287.3 mAh g−1 and 1531.31 Wh kg−1, respectively. Although, the electrochemical properties of Li2NiPO4F are promising, large electronic band gap would certainly pose a limitation for its commercial application. Nb is a transition metal and its electronegativity is 1.6 which is less than the electronegativity of 2.19 for P. This implies, less operating voltage would be obtained if we replace P in Li2NiPO4F by Nb to form Li2NiNbO4F. However, electronic configuration of Nb is [Kr] 4d45 s1 and the valance state of Nb in Li2NiNbO4F is +5, which in turn specify that, localized Nb d states will reside in conduction band of Li2NiNbO4F and hence the electronic band-gap would be less owing to this localized Nb-d states. Our speculation gets verified by the calculated properties of Li2NiNbO4F obtained through DFT as follows; Voltage, electronic band gap, capacity (∼ for 2 Li+ extraction) and energy density achieved, respectively, are 5.01 V, 3.64 eV (less than LiFePO4), 215.71 mAh g−1, 1080.71 Wh kg−1. Lower electronic band gap of Li2NiNbO4F makes it an alternative to Li2NiPO4F.