Enhancement of electronic conductivity of ${\mathrm{LiFePO}}_4$ by Cr doping and its identification by first-principles calculations

We present a first-principles electronic band structure for pure ${\mathrm{LiFePO}}_4,$ delithiated ${\mathrm{FePO}}_4,$ and Cr-doped ${\mathrm{LiFePO}}_4.$ 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 ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+},$ 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, ${\mathrm{Cr}}^{3+}.$ We also confirm experimentally, for ${\mathrm{Li}}_{1-3x}{\mathrm{Cr}}_x{\mathrm{FePO}}_4$ with $x=0.01\mathrm{}$ and 0.03, an enhancement of the electronic conductivity up to eight orders of magnitude comparing with pure ${\mathrm{LiFePO}}_4.$ 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.