An electrochemically active material resulting from substituting a portion of the nickel in a single-phase composite oxide of nickel and lithium of the LiNiO.sub.2 type, characterized in that the active material satisfies the formula: Li(M.sup.1.sub.(1-a-b-c)Li.sub.aM.sup.2.sub.bM.sup.3.sub.c)O.sub.2 in which: 0.020; and in that said active material, regardless of its state of charge, satisfies the relationship: 0.40