A composition of general formula (1): Nax[MnaNibCrc]O2+y (1), wherein: 0.6≤x≤0.8; −0.1≤y≤0.1; 0.55≤a≤0.7; 0.25≤b≤0.3; c≤0.05; and a+b+c≤1.0, an intermediate product for preparing a composition of general formula (1) and a process of synthesis, wherein the mixed sodium-transition metal oxide of general formula (1) may generally show an essentially or solely P2 structure, and may be used as a positive electrode material for a sodium ion secondary battery.