A method for synthesizing particles of a carbon-coated complex oxide, said complex oxide having substantially an olivine structure and conforming to formula Li x M 1-y M' y (PO 4) n, wherein: x, y and n are numbers such that 0 ≤ x ≤ 2, 0 ≤ y ≤ 0.6 and 1 ≤ n ≤ 1.5; M is a transition metal or a mixture of transition metals from the first row of the periodic table; M' is a fixed-valence element selected from Mg 2+, Ca 2+, Al 3+, ZN 2+ or a combination of the same and P is optionally partially replaced by at least one element selected from Si and S, said method being characterized in that: - the synthesis is made by a thermodynamic or kinetic reaction for balancing a mixture, in required proportions, of the following source compounds: a) a source of M; b) a source of M' when y > 0; c) a source compound of lithium; d) a source compound of P and optionally a source compound of a Si or S element; e) a source of conductive carbon; with a reducing gaseous atmosphere; - the synthesis is performed by controlling the composition of said gaseous atmosphere, the temperature of the synthesis reaction and the proportion of the source compound c) with respect to other source compounds a), b) and d), to impose the oxydation state of the transition metal to the valence degree desired for constituting the compound of the LiMPO 4 type; - the temperature of the synthesis is higher than the temperature at which the reactive atmosphere is capable of reducing the transition element(s) to their required oxidation states in the LiMPO 4 compound, the temperature and duration of the synthesis are lower than, respectively, the temperature and duration leading to a reduction of the transition element(s) to the metallic state or an oxidation of the carbon resulting from the pyrolysis of the organic substance; - the method includes at least a step of pyrolyzing the source compound e). - the synthesis is made by a thermodynamic or kinetic reaction for balancing a mixture, in required proportions, of the following source compounds: a) a source of M; b) a source of M' when y > 0; c) a source compound of lithium; d) a source compound of P and optionally a source compound of a Si or S element; e) a source of conductive carbon; with a reducing gaseous atmosphere; a) a source of M; b) a source of M' when y > 0; c) a source compound of lithium; d) a source compound of P and optionally a source compound of a Si or S element; e) a source of conductive carbon; with a reducing gaseous atmosphere; - the synthesis is performed by controlling the composition of said gaseous atmosphere, the temperature of the synthesis reaction and the proportion of the source compound c) with respect to other source compounds a), b) and d), to impose the oxydation state of the transition metal to the valence degree desired for constituting the compound of the LiMPO 4 type; - the temperature of the synthesis is higher than the temperature at which the reactive atmosphere is capable of reducing the transition element(s) to their required oxidation states in the LiMPO 4 compound, the temperature and duration of the synthesis are lower than, respectively, the temperature and duration leading to a reduction of the transition element(s) to the metallic state or an oxidation of the carbon resulting from the pyrolysis of the organic substance; - the method includes at least a step of pyrolyzing the source compound e).