A compound of formula Li.sub.4+xMnM.sup.1.sub.aM.sup.2.sub.bO.sub.c wherein: M.sup.1 is selected from the group consisting in Ni, Mn, Co, Fe and a mixture thereof; M.sup.2 is selected from the group consisting in Si, Ti, Mo, B, Al and a mixture thereof;
with: −1.2≦x≦3; 0<a≦2.5; 0≦b≦1.5; 4.3≦c≦10; and c=4+a+n.Math.b+x/2
wherein n=2 when M.sup.2 is selected from the group consisting in Si, Ti, Mo or a mixture thereof; and n=1.5 when M.sup.2 is selected from the group consisting in B, Al or a mixture thereof; and n=0 if b=0.