Positive electrode active material, sodium-ion secondary battery comprising the same and electrical apparatus

1. A positive electrode active material which is in a particulate form and comprises a compound represented by Formula 1:

Na x A y M1[M2(CN) 6 ] δ ·z H 2 O  Formula 1

wherein, A is selected from at least one of an alkali metal element and an alkaline earth metal element, and an ionic radius of A is greater than an ionic radius of sodium; M1 and M2 are each independently selected from at least one of a transition metal element, (<y≤0.2, 0<x+y≤2, 0<δ≤1, and 0≤z≤10; and particles of the positive electrode active material have a gradient layer in which a content of an A element decreases from a particle surface to a particle interior.

2. The positive electrode active material according to claim 1 , wherein 0.04≤y≤0.2.

3. The positive electrode active material according to claim 1 ,

wherein in the particles of the positive electrode active material, a thickness of the gradient layer with respect to an A element content is in a range of 10-100 nm, wherein the thickness of the gradient layer with respect to the A element content refers to a distance from a layer with a content ratio of Na to A element reaching Na:A-50At %: 50At % as measured when element contents of the particles of the positive electrode active material is quantitatively measured by a transmission electron microscopy-energy spectrum line scanning characterization method to the particle surface.

4. The positive electrode active material according to claim 1 , wherein the A is selected from at least one of Li, K, Rb, Cs, Be, Mg, Ca, Sr and Ba.

5. The positive electrode active material according to claim 1 , wherein the M1 and M2 are each independently selected from at least one of Fe, Mn, Ni, Co, Cu and Zn.

6. The positive electrode active material according to claim 1 , wherein a volume median particle diameter Dv50 of the positive electrode active material is in a range of 1-5 μm.

7. The positive electrode active material according to claim 1 , wherein a specific surface area of the positive electrode active material is in a range of 1-10 m 2 /g.

8. The positive electrode active material according to claim 1 , wherein a water content of the positive electrode active material is ≤2 wt %.

9. A method for preparing a positive electrode active material, the method comprising the steps of:

1) Adding a positive electrode active material precursor into a solvent and dispersing to obtain a suspension, wherein the positive electrode active material precursor is a compound represented by Formula 2:

Na n M1[M2(CN) 6 ] δ   Formula 2

wherein the M1 and M2 are each independently selected from at least one of transition metal elements, 0<n≤2, and 0<δ≤1;

2) Cooling the suspension obtained in step 1), then adding a salt of A into the suspension in the cooled state, and stirring and aging, wherein A is selected from at least one of an alkali metal element and an alkaline earth metal element, and an ionic radius of A is greater than an ionic radius of sodium; and

3) Filtering the suspension obtained in step 2), washing and drying the filtered precipitate to obtain a positive electrode active material;

the positive electrode active material is in particulate form, and comprises a compound represented by Formula 1:

Na x A y M1[M2(CN) 6 ]δ·z H 2 O  Formula 1

wherein, A, M1, M2 and δ are each as defined above, 0<y≤0.2, 0<x+y≤2, and 0≤z=10; and a particle of the positive electrode active material has a gradient layer in which a content of A element decreases from a surface of the particle to an interior of the particle.

10. The method according to claim 9 , wherein the solvent in step 1) is a polar solvent, solubility of the positive electrode active material precursor in the solvent is not higher than 0.02 mol/L, and solubility of the salt of A in the solvent is not lower than 0.05 mol/L.

11. The method according to claim 9 , wherein the solvent is selected from one or more of acetonitrile, adiponitrile, methanol, ethanol, water, formamide and dimethyl sulfoxide.

12. The method according to claim 9 , wherein in the step 2), the suspension obtained in step 1) is cooled to below 10° C.

13. The method according to claim 9 , wherein a molar ratio of A in the salt of A to sodium in the positive electrode active material precursor is 1:5 or less.

14. The method according to claim 9 , wherein,

the positive electrode active material precursor in the step 1) is prepared by a method comprising the steps of:

i) dissolving a soluble salt of the transition metal element M1 and optionally a Na-containing slow-releasing agent in water to make a solution a;

ii) dissolving a soluble transition metal cyano complex of the transition metal element M2 in water to make a solution b;

iii) adding the solution a dropwise to the solution b under stirring, and then stirring and aging after completion of dropwise addition of the solution a to the solution b; and

iv) filtering the suspension obtained in step iii), and washing and drying the filtered precipitate.

15. The method according to claim 14 , wherein a molar ratio of M1 in the soluble salt of the transition metal element M1 to M2 in the soluble transition metal cyano complex of the transition metal element M2 is in a range of 1:0.8 to 0.8:1.

16. The method according to claim 14 , wherein in the step iii), the solution is maintained in a temperature range of 20° C.-120° C.

17. The method according to claim 9 , wherein the stirring and aging in step 2) is performed at a stirring rate of 50 to 1800 rpm for 1 to 30 minutes.

18. A secondary battery comprising the positive electrode active material according to claim 1 .

19. An electrical apparatus comprising the secondary battery according to claim 18 .