The electronic structure and band gap of LiFePO4 and LiMnPO4

TL;DR

This study uses ab initio GGA+U calculations to analyze the electronic structure and band gap of LiFePO4 and LiMnPO4, revealing good agreement with experimental optical data and highlighting the importance of electron correlation in these materials.

Contribution

It applies the GGA+U method to accurately determine the electronic properties of olivine LiMPO4 materials, emphasizing the role of electron correlation and polaronic conduction.

Findings

GGA+U results agree well with experimental optical gaps

Standard GGA makes large errors in electronic structure prediction

Olivines are likely polaronic conductors with extrinsic carriers

Abstract

Materials with the olivine LixMPO4 structure form an important new class of materials for rechargeable Li batteries. There is significant interest in their electronic properties because of the importance of electronic conductivity in batteries for high rate applications. The density of states of LixMPO4 (x = 0, 1 and M = Fe, Mn) has been determined with the ab initio GGA+U method, appropriate for these correlated electron systems. Computed results are compared with the optical gap of LiFePO4, as measured using UV-Vis-NIR diffuse reflectance spectroscopy. The results obtained from experiment (3.8-4.0 eV) and GGA+U computations (3.7 eV) are in very good agreement. However, standard GGA, without the same level of treatment of electron correlation, is shown to make large errors in predicting the electronic structure. It is argued that olivines are likely to be polaronic conductors with extrinsically determined carrier levels and that their electronic conductivity is therefore not simply related to the band gap.