A method for the synthesis of compounds of the formula C--Li.sub.xM.sub.1-yM'.sub.y(XO.sub.4).sub.n, where C represents carbon cross-linked with the compound Li.sub.xM.sub.1-yM'.sub.y(XO.sub.4).sub.n, in which x, y and n are numbers such as 0.ltoreq.x.ltoreq.2, 0.ltoreq.y.ltoreq.0.6, and 1.ltoreq.n.ltoreq.1.5, M is a transition metal or a mixture of transition metals from the first period of the periodic table, M' is an element with fixed valency selected among Mg.sup.2+, Ca.sup.2+, Al.sup.3+, Zn.sup.2+ or a combination of these same elements and X is chosen among S, P and Si, by bringing into equilibrium, in the required proportions, the mixture of precursors, with a gaseous atmosphere, the synthesis taking place by reaction and bringing into equilibrium, in the required proportions, the mixture of the precursors, the procedure comprising at least one pyrolysis step of the carbon source compound in such a way as to obtain a compound in which the electronic conductivity measured on a sample of powder compressed at a pressure of 3750 Kgcm.sup.-2 is greater than 10.sup.-8 Scm.sup.-1. The materials obtained have excellent electrical conductivity, as well a very improved chemical activity.