Process for the preparation of an insertion compound of an alkali metal in which the following successive stages are carried out: a) an organic complex of a transition metal or of a mixture of transition metals M in an oxidation state of greater than 2 is brought into contact with an alkali metal A in the ionic form and with an entity of formula Hb(XO4), where X is chosen from Si, S, Al, P, Ge, As or Mo and b has a value from 0 to 5, in a liquid medium in a closed chamber; the chamber is brought to a temperature T which makes possible the decomposition of the organic complex in the said liquid medium; the temperature and the pressure in the chamber are brought back to ambient temperature and atmospheric pressure and the insertion compound for an alkali metal of formula AMXO4, in which M is in the +2 oxidation state, is recovered.