Enhancement of electronic conductivity ofLiFePO4by 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 ${\mathrm{Li}}_{1\ensuremath{-}3x}{\mathrm{Cr}}_{x}{\mathrm{FePO}}_{4}$ with $x=0.01$ 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.