The electronic structure of LiFePO4 and delithiated FePO4 is revisited in the light of the previous calculations taking into account the coulomb correlation potential for d-electrons. The nature of the optical transitions across the energy gap is investigated. In LiFePO4, these are intra-atomic Fe2+−Fe3+ transitions suffering a strong Franck−Condon effect due to the local distortion of the lattice in FePO4, which is indirect evidence of the formation of a small polaron. This situation contrasts with that met in the much more covalent delithiated phase, where the optical transition across the energy gap is associated with a transfer of an electron from the p-states of the oxygen to the d-states of iron ions. The small polarons in LiFePO4 are associated with the presence of Fe3+ ions introduced by native defects in relative concentration [Fe3+]/[Fe2++Fe3+] = 3 × 10-3 in the samples known to be optimized with respect to their electrochemical properties. The nearest iron neighbors around the central polaron s...