Formation of Small Hole Polarons in Olivine Phosphate Cathode Materials

We investigate polaron formation in olivine LiMnPO4, LiFePO4, LiCoPO4 and LiNiPO4 using a combination of first principles calculations and experimental XPS spectra. We determine that details of the electronic structure near the Fermi energy, relevant to localization of a hole polaron, are not accurately captured using the standard GGA+U methodology and require a hybrid functional approach. We show that polaron formation is possible in all but the Ni-based compound and discuss the possibility of an electronic instability caused by electron withdrawal during delithiation caused by oxygen evolution.