Negative electrode sheet and method for preparing the same, secondary battery, battery module, battery pack, and device

1. A negative electrode sheet, comprising:

a current collector; and

a first active material layer and a second active material layer that are sequentially provided on at least one surface of the current collector;

wherein:

the first active material layer includes a first negative electrode active material, particle sizes of the first negative electrode active material satisfying formula (I): 0.02≤A 1 =(Dn10) 1 /(Dv50) 1 ≤0.2, formula (I);

the second active material layer includes a second negative electrode active material, particle sizes of the second negative electrode active material satisfying formula (II): 0.02≤A 2 =(Dn10) 2 /(Dv50) 2 ≤0.3, formula (II); and

in the formula (I) and the formula (II), A 1 and A 2 satisfy 1<A 2 /A 1 <2.5.

2. The negative electrode sheet according to claim 1 , wherein:

(D v 50) 1 of the first negative electrode active material is 12 μm to 20 μm.

3. The negative electrode sheet according to claim 1 , wherein:

(D v 50) 2 of the second negative electrode active material is 8 μm to 17 μm.

4. The negative electrode sheet according to claim 1 , wherein:

(D n 10) 1 of the first negative electrode active material is 0.5 μm to 2 m; and

(D n 10) 2 of the second negative electrode active material is 0.5 μm to 2 μm.

5. The negative electrode sheet according to claim 1 , wherein:

(D v 50) 2 of the second negative electrode active material is smaller than (D v 50) 1 of the first negative electrode active material, and a difference between (D v 50) 1 and (D v 50) 2 is not less than 3 μm.

6. The negative electrode sheet according to claim 1 , wherein:

a thickness T 1 of the first active material layer is 20 μm to 60 μm.

7. The negative electrode sheet according to claim 1 , wherein:

a thickness T 2 of the second active material layer is 25 μm to 70 μm.

8. The negative electrode sheet according to claim 7 , wherein:

the thickness T 2 of the second active material layer is not less than a thickness T 1 of the first active material layer.

9. The negative electrode sheet according to claim 7 , wherein:

a ratio of the thickness T 2 of the second active material layer to a thickness T 1 of the first active material layer is 1.1 to 2.4.

10. The negative electrode sheet according to claim 9 , wherein:

a product of T 2 /T 1 and A 2 /A 1 satisfies: 1.1≤(T 2 /T 1 )×(A 2 /A 1 )≤2.9.

11. The negative electrode sheet according to claim 1 , wherein:

the first negative electrode active material includes graphite; and/or

the second negative electrode active material includes graphite and silicon.

12. The negative electrode sheet according to claim 11 , wherein:

in the second negative electrode active material, in terms of mass percentage, a content of silicon converted into silicon monoxide accounts for 1-20% of the second negative electrode active material.

13. The negative electrode sheet according to claim 11 , wherein:

graphite in the first negative electrode active material and the second negative electrode active material is selected from artificial graphite, and the artificial graphite is primary particle and/or secondary particle.

14. A method for preparing a negative electrode sheet, comprising:

Step S1: providing a first negative electrode active material, particle sizes of the first negative electrode active material satisfying formula (I): 0.02≤A 1 =(Dn10) 1 /(Dv50) 1 ≤0.2, formula (I);

Step S2: preparing a slurry A containing the first negative electrode active material;

Step S3: providing a second negative electrode active material, particle sizes of the second negative electrode active material satisfying formula (II): 0.02≤A 2 =(Dn10) 2 /(Dv50) 2 ≤0.3, formula (II);

Step S4: preparing a slurry B containing the second negative electrode active material;

Step S5: coating the slurry A on at least one surface of a current collector to form a first active material layer;

coating the slurry B on a surface of the first active material layer to form a second active material layer; and

conducting oven drying, cold pressing, slitting, and die cutting to obtain the negative electrode sheet;

wherein A 1 and A 2 satisfy 1<A 2 /A 1 <2.5.

15. The method according to claim 14 , wherein:

in the step S1, the first negative electrode active material is prepared by processes of raw material selection, shaping and classification, graphitization, and demagnetization and sieving sequentially to obtain the first negative electrode active material with an average particle size (D v 50) 1 from 12 μm to 20 μm and (D n 10) 1 from 0.5 μm to 2 μm.

16. The method according to claim 14 , wherein:

in the step S3, the second negative electrode active material includes graphite and silicon;

the graphite is prepared by processes of raw material selection, shaping and classification, granulation, graphitization, carbonization, and demagnetization and sieving sequentially, and preparation of the silicon comprises a process of carbon cladding; and

the graphite and the silicon are mixed to obtain the second negative electrode active material with an average particle size (D v 50) 2 from 8 μm to 17 μm and (D n 10) 1 from 0.5 μm to 2 μm.

17. The method according to claim 14 , wherein:

in the step S2 and the step S4, a conductive agent comprises one or more of conductive carbon black, graphene, vapor grown carbon fiber VGCF, and carbon nanotube.

18. The method according to claim 14 , wherein:

in the step S4, the second negative electrode active material includes silicon and graphite; and

a conductive agent includes graphene or carbon nanotube.

19. A secondary battery, comprising a negative electrode sheet including:

a current collector; and

a first active material layer and a second active material layer that are sequentially provided on at least one surface of the current collector;

wherein:

the first active material layer includes a first negative electrode active material, particle sizes of the first negative electrode active material satisfying formula (I): 0.02≤A 1 =(Dn10) 1 /(Dv50) 1 ≤0.2, formula (I);

the second active material layer includes a second negative electrode active material, particle sizes of the second negative electrode active material satisfying formula (II): 0.02≤A 2 =(Dn10) 2 /(Dv50) 2 ≤0.3, formula (II); and

in the formula (I) and the formula (II), A 1 and A 2 satisfy 1<A 2 /A 1 <2.5.