Implications of the DFT plus U method on polaron properties in energy materials

To model polaronic behavior in strongly correlated transition-metal oxideswith ab initio methods, one typically requires a level of theory beyond that of local density or general gradient density functional theory (DFT) approximations to account for the strongly correlated d-shell interactions of transition-metal oxides. In the present work, we utilize density functional theory with additional on-site Hubbard corrections (DFT+ U) to calculate polaronic properties in two lithium ion battery cathode materials, Li x FePO4 and Li x Mn2O4, and two photocatalytic materials, TiO2 and Fe2O3. We investigate the effects of the + U on-site projection on polaronic properties. Through systematic comparison with hybrid functional calculations, it is shown that + U projection in these model materials can impact upon the band gap, polaronic hopping barrier, and polaronic eigenstate offset from the band edges in a nontrivial manner. These properties are shown to have varying degrees of coupling and dependence on the + U projection in each example material studied, which has important implications for arriving at systematic material predictions of polaronic properties in transition-metal oxides.