Positive electrode active material and preparation method thereof, positive electrode plate, secondary battery and electric device

The positive active material comprises a first positive active material A and a second positive active material B. The chemical formula of the first positive active material A is Lia1 [Nix1Coy1Mnz1M1b1M3c1] O2, 0 < x1 < 1, 0 < = y1 < 0.5, 0 < = z1 < 0.5, 0.9 < a1 < 1.2, 0 < b1 < 0.2, 0 < c1 < 0.2, and x1 + y1 + z1 + b1 + c1 = 1, M1 is selected from at least one of W, Nb and Mo, and M3 is selected from at least one of Zr, Ti, Mg, Al, Sb, B and Co; the chemical formula of the second positive active material B is Lia2 [Nix2Coy2Mnz2M2b2M4c2] O2, 0 < x2 < 1, 0 < = y2 < 0.5, 0 < = z2 < 0.5, 0.9 < a2 < 1.2, 0 < b2 < 0.2, 0 < c2 < 0.2, x2 + y2 + z2 + b2 + c2 = 1, M2 is selected from at least one of Sr, Y and V, and M4 is selected from at least one of Zr, Ti, Mg, Al, Sb, B and Co.