C–LiFePO4 composites were synthesized by mechano-chemical activation using iron and lithium phosphates, and also cellulose as carbon precursor; this mixture was treated with 2 thermal treatments (“slow” or “fast” treatment) and 2 temperatures (575 °C or 800 °C). The four synthesized samples were pure olivine-type materials (XRD), but chemical analyses and Mössbauer spectroscopy show Li/Fe and P/Fe ratios larger than 1 and the presence of 5–6 at.% of Fe3+. Three hypotheses were considered to explain the presence of Fe3+ ions: (i) a chemical formula Li1 + xFe1 − xPO4 for the olivine-type phase, (ii) a mixture of olivine-type LiFePO4 and Fe3+ containing impuritie(s) or (iii) LiFePO4 with Fe3+ surface defects. These hypotheses were checked using different characterization methods such as X-ray diffraction, neutron diffraction and magnetic measurements, hypothesis (i) was shown not to be valid but none of hypotheses (ii) and (iii) was definitely confirmed. One can assume either the presence of a Fe3+-rich phase (amorphous or nanocrystalline) or the occurrence of surface defects which can play a significant role as far as nanomaterials are concerned.