Formation of small hole polarons in olivine phosphate cathode materials

摘要

We investigate small hole polaron formation in olivine phosphate materials, LiMnPO_4, LiFePO_4, LiCoPO_4 and LiNiPO_4, 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.