A sacrificial positive active material for a lithium-ion electrochemical element which is a compound of formula (Li.sub.2O).sub.x (MnO.sub.2).sub.y(MnO).sub.z(MO.sub.a).sub.t in which: x+y+z+t=1; 1−x−y≥0; 0.97≥x≥0.6; y≤0.45; x −0.17; y≥0; y+z>0; t≥0; 1≤a<3. M is selected from the group consisting of Fe, Co, Ni, B, Al, Ti, Si, V, Mo, Zr and a mixture thereof.

The present application discloses a positive electrode active material and its preparation method, a sodium ion battery and an apparatus containing the sodium ion battery. The positive electrode active material satisfies a chemical formula Na.sub.2+xCu.sub.hMn.sub.kM.sub.lO.sub.7−y, wherein M is one or more selected from Li, B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba and W, 0≤x≤0.5, 0.1<h≤2, 1≤k≤3, 0≤l≤0.5, and 0≤y≤1, 2≤h+k+l≤3.5, and 0.57≤(2+x)/(h+k+l)≤0.9.

A sacrificial positive active material for a lithium-ion electrochemical element which is a compound of formula (Li.sub.2O).sub.x (MnO.sub.2).sub.y(MnO).sub.z(MO.sub.a).sub.t in which: x+y+z+t=1; 1xy0; 0.97x0.6; y0.45; x 0.17; y0; y+z>0; t0; 1a<3. M is selected from the group consisting of Fe, Co, Ni, B, Al, Ti, Si, V, Mo, Zr and a mixture thereof.

The present application discloses a positive electrode active material and its preparation method, a sodium ion battery and an apparatus containing the sodium ion battery. The positive electrode active material satisfies a chemical formula Na.sub.2+xCu.sub.hMn.sub.kM.sub.lO.sub.7-y, wherein M is one or more selected from Li, B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba and W, 0x0.5, 0.1<h2, 1k3, 0l0.5, and 0y1, 2h+k+l3.5, and 0.57(2+x)/(h+k+l)0.9.