Positive electrode active material, sodium ion secondary battery comprising same and power consuming device

A method for preparing a positive electrode active material, comprising the following steps:

• 1) Dissolving a soluble salt of transition metal element M1 together with an optional Na-containing slow release agent in a solvent to prepare a solution a;
• 2) Dissolving a soluble transition metal cyano complex of transition metal element M2 in a solvent to prepare a solution b;
• 3) Under a stirring state, dropwise adding the solution a to the solution b, followed by stirring and aging after the dropwise adding is finished;
• 4) adding a salt of A to a suspension obtained in step 3), followed by stirring and aging, wherein the A is selected from at least one of alkali metal elements and has an ionic radius larger than that of sodium;
• 5) Filtering a suspension obtained in step 4) and washing a filtered precipitate;
• 6) Mixing the washed precipitate obtained in step 5) and the soluble salt of the transition metal element M1 together in water, followed by stirring and aging to form a suspension d, wherein the amount of the soluble salt of the transition metal element M1 added is selected to obtain a pH value of the suspension d in a range of 8.1 to 9.0;
• and
• 7) Filtering the suspension d and washing a filtered precipitate to obtain the positive electrode active material, wherein
• the positive electrode active material is granular and comprises a compound represented by formula 1,
• (NaxAy)a□bM1[M2(CN)6]δ Formula 1
• wherein 0<y≤0.2, 0<x+y≤2, 0<δ≤1, a+b=2, 0.85≤a≤0.98, □ represents a vacancy, b represents the number of vacancies, and the A, M1, and M2 are defined as above;
• and
• when the positive electrode active material is dissolved in water, an aqueous solution having a concentration of 5 g/100 g water is obtained, a pH value of the aqueous solution at a temperature of 20° C. is 7.6 to 8.5,
• wherein a particle of the positive electrode active material comprises:
• a gradient layer, in which a content of the A element gradually decreases in a direction from an external surface of the particle toward an interior portion of the particle, the gradient layer having a thickness in a range of 10 nm to 100 nm;
• an outermost layer, defined as a region from the external surface of the particle to a position within the particle where the content ratio of Na to A reaches 10 atom %:90 atom %, as measured by transmission electron microscopy-energy spectrum (TEM-EDS) line scan characterization, the outermost layer having a thickness in a range of 10 nm to 50 nm;
• and
• a vacancy layer, defined as a region from the external surface of the particle to a position within the particle where a total content by atom % of Na and A becomes less than a total content by atom % of M1 and M2, as measured by TEM-EDS line scan characterization, the vacancy layer having a thickness d2 in a range of 9 nm to 35 nm.