Enhancement of electronic conductivity of LiFePO4 by Cr doping and its identification by first-principles calculations

We present a first-principles electronic band structure for pure LiFePO4, delithiated FePO4, and Cr-doped LiFePO4. It indicates that not only Fe but also O atoms are oxidized in the delithiation process, while P is little affected. This is in contrast to the usual view of the intercalation reaction that the removal of Li only transforms Fe from Fe2+ to Fe3+, but in agreement with the present x-ray photoemission spectroscopy experiment. Calculation also assumes a significant enhancement of electronic conductivity when lithium ions are replaced by cations with higher valence, Cr3+. We also confirm experimentally, for Li1-3xCrxFePO4 with x=0.01 and 0.03, an enhancement of the electronic conductivity up to eight orders of magnitude comparing with pure LiFePO4. Besides the conventional p-type doping conductivity, another mechanism has been suggested, which involves the electron hopping within a cluster surrounding the doping atom and related vacancies, and electron tunneling between these conducting clusters.