By combining first principles calculations and experimental XPS measurements, we investigate the electronic structure of potential Li-ion battery cathode materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that determine small hole
polaron formation and migration. We show that small hole polaron formation depends on features in the electronic structure near the valence-band maximum and that, calculationally, these features depend on the methodology chosen for dealing with the correlated nature of the transition-metal d-derived states in these systems. Comparison with experiment reveals that a hybrid functional approach is superior to GGA+U in correctly reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot support small hole polarons, but that the other three compounds can. The migration barrier is determined mainly by the strong or weak bonding nature of the states at the top of the valence band, resulting in a substantially higher barrier for LiMnPO4 than for LiCoPO4 or LiFePO4.
One year after their initial discovery, two schools of thought have crystallized regarding the electronic structure and magnetic properties of ferropnictide systems. One postulates that these are itinerant weakly correlated metallic systems that beco
me magnetic by virtue of spin-Peierls type transition due to near-nesting between the hole and the electron Fermi surface pockets. The other argues these materials are strongly or at least moderately correlated, the electrons are considerably localized and close to a Mott-Hubbard transition, with the local magnetic moments interacting via short-range superexchange. In this paper we argue that neither picture is fully correct. The systems are moderately correlated, but with correlations driven by Hunds rule coupling rather than by the on-site Hubbard repulsion. The iron moments are largely local, driven by Hunds intra-atomic exchange. Superexchange is not operative and the interactions between the Fe moments are considerably long-range and driven mostly by one-electron energies of all occupied states.
