MULTI-LAYER NEGATIVE ELECTRODE COMPRISING NATURAL GRAPHITE AND ARTIFICIAL GRAPHITE AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

Abstract

The present disclosure relates to a multilayer negative electrode comprising a negative electrode current collector configured to transfer electrons between an outer lead and a negative electrode active material, a first negative electrode mixture layer formed on one surface or both surfaces of the current collector and containing natural graphite as a negative electrode active material and a second negative electrode mixture layer formed on the first negative electrode mixture layer and containing artificial graphite as a negative electrode active material, and a lithium secondary battery including the same.

Claims

1. A multilayer negative electrode comprising: a negative electrode current collector configured to transfer electrons between an outer lead and a negative electrode active material; a first negative electrode mixture layer formed on one surface or both surfaces of the current collector and comprising natural graphite and artificial graphite as a negative electrode active material; and a second negative electrode mixture layer formed on the first negative electrode mixture layer and comprising artificial graphite as a negative electrode active material, wherein a loading amount of the first negative electrode mixture layer is smaller than a loading amount of the second negative electrode mixture layer.

2. The multilayer negative electrode of claim 1, wherein an amount of the natural graphite present in the first negative electrode mixture layer is from 5% by weight to 79% by weight based on a total weight of the first negative electrode mixture layer.

3. The multilayer negative electrode of claim 1, wherein the second negative electrode mixture layer further comprises natural graphite, and an amount of the natural graphite present in the second negative electrode mixture layer is from 0.1% by weight to 10% by weight based on the total weight of the second negative electrode mixture layer.

4. The multilayer negative electrode of claim 1, wherein a weight ratio of the first negative electrode mixture layer to the second negative electrode mixture layer is 1:9 to 2:1.

5. The multilayer negative electrode of claim 6, wherein the weight ratio of the first negative electrode mixture layer to the total weight of the first negative electrode mixture layer and the second negative electrode mixture layer is decreased as the content of the natural graphite of the first negative electrode mixture layer is increased.

6. The multilayer negative electrode of claim 1, wherein the natural graphite has a specific surface area (BET) of 2 m.sup.2/g to 8 m.sup.2/g.

7. The multilayer negative electrode of claim 1, wherein the natural graphite is flake graphite, vein graphite, or amorphous graphite.

8. The multilayer negative electrode of claim 1, wherein the natural graphite has a tap density of 0.9 g/cc to 1.3 g/cc.

9. The multilayer negative electrode of claim 1, wherein a ratio of I.sub.110 to I.sub.003 of particles of the natural graphite at XRD diffraction is 20 to 40.

10. The multilayer negative electrode of claim 1, wherein the natural graphite has an average particle diameter (D50) of 5 μm to 30 μm.

11. The multilayer negative electrode of claim 1, wherein the artificial graphite is 0.5 m.sup.2/g to 5 m.sup.2/g in a range in which a specific surface area (BET) of the artificial graphite is smaller than that of the natural graphite.

12. The multilayer negative electrode of claim 1, wherein the artificial graphite is in a form of a powder, a flake, a block, a plate, or a rod.

13. The multilayer negative electrode of claim 1, wherein a ratio of I.sub.110 to I.sub.003 of particles of the artificial graphite at XRD diffraction is 5 to 20.

14. The multilayer negative electrode of claim 1, wherein at least one of the natural graphite and the artificial graphite is pitch-coated.

15. The multilayer negative electrode of claim 1, wherein some of solid contents of the first negative electrode mixture layer and the second negative electrode mixture layer is mixed therebetween so as not to form a boundary surface.

16. A lithium secondary battery comprising the multilayer negative electrode of claim 1.

17. The multilayer negative electrode of claim 1, wherein a weight ratio of natural graphite to artificial graphite in the first negative electrode mixture layer is from 4:6 to less than 1:1.

18. The multilayer negative electrode of claim 1, wherein the loading amount of the first negative electrode mixture layer ranges from 6 mg/cm.sup.2 to 8 mg/cm.sup.2.

19. The multilayer negative electrode of claim 1, wherein the loading amount of the second negative electrode mixture layer ranges from 8 mg/cm.sup.2 to 10 mg/cm.sup.2.

20. The multilayer negative electrode of claim 1, wherein the second negative electrode mixture layer is applied on the first negative electrode mixture layer before drying the first negative electrode mixture layer.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0089] Hereinafter, the present invention will be described in detail with reference to the following examples. However, these are provided as preferable examples of the present invention, and do not limit the scope of the present invention in any respect.

Example 1

[0090] 1-1. Preparing of First Negative Electrode Slurry

[0091] Natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm used as a first negative electrode active material, SBR used as a binder, CMC used as a thickener, and carbon black used as a conductive material were weighed to have a weight ratio of 94:2.5:2:1.5, and then placed in distilled water and mixed to prepare a first negative electrode slurry.

[0092] 1-2. Preparing of Second Negative Electrode Slurry

[0093] Artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm used as a second negative electrode active material, SBR used as a binder, CMC used as a thickener, and carbon black used as a conductive material were weighed to have a weight ratio 94:2.5:2:1.5, and then placed in distilled water and mixed to prepare a second negative electrode slurry.

[0094] 1-3. Preparing of a Negative Electrode

[0095] The first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried, and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Example 2

[0096] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.5 m.sup.2/g, a tap density: 1.00 g/cc, a degree of orientation: 28) having an average particle diameter D50 of 15 μm was used as a first negative electrode active material.

Example 3

[0097] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a first negative electrode active material.

Example 4

[0098] A negative electrode was prepared in the same manner as in Example 3, except that the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount) and dried, and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried, and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Example 5

[0099] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 2.2 m.sup.2/g, a tap density: 0.94 g/cc, a degree of orientation: 22) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a first negative electrode active material.

Example 6

[0100] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 6.0 m.sup.2/g, a tap density: 0.85 g/cc, a degree of orientation: 20) having an average particle diameter D50 of 4 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a first negative electrode active material.

Example 7

[0101] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 1.8:8.2 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 10 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 6 mg/cm.sup.2 and dried.

Example 8

[0102] A negative electrode was prepared in the same manner as in Example 3, except that the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 10 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 6 mg/cm.sup.2 and dried, and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Example 9

[0103] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 1:9 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 10

[0104] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 4:6 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 11

[0105] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 1:1 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 12

[0106] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 6:4 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 13

[0107] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 7:3 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 14

[0108] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 8:2 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 6 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 10 mg/cm.sup.2 and dried.

Example 15

[0109] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 8 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 8 mg/cm.sup.2 and dried.

Example 16

[0110] A negative electrode was prepared in the same manner as in Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a first negative electrode active material, and the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 8 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 8 mg/cm.sup.2 and dried.

Comparative Example 1

[0111] A negative electrode was prepared in the same manner as in Example 1, except that artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm was used as a first negative electrode active material, and amorphous graphite (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 3:7 as a second negative electrode active material.

Comparative Example 2

[0112] A negative electrode was prepared in the same manner as in Comparative Example 1, except that the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 10 mg/cm.sup.2 (based on a post-drying amount), and the second negative electrode slurry was applied to the first negative electrode slurry at a loading amount of 6 mg/cm.sup.2 and dried, and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Comparative Example 3

[0113] A negative electrode was prepared in the same manner as in Comparative Example 1, except that natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm were mixed at a weight ratio of 5:95 as a first negative electrode active material.

Comparative Example 4

[0114] Using a mixture obtained by mixing natural graphite in an amorphous form (a specific surface area: 3.0 m.sup.2/g, a tap density: 0.95 g/cc, a degree of orientation: 25) having an average particle diameter D50 of 11 μm and artificial graphite in a flake form (a specific surface area: 1.5 m.sup.2/g, a tap density: 0.9 g cc, a degree of orientation: 12) having an average particle diameter D50 of 20.8 μm at a weight ratio of 11:89 as a negative electrode active material, SBR as a binder, CMC as a thickener and carbon black as a conductive material, negative electrode active material:binder:thickener:conductive material were mixed with distilled water at a ratio of 94:2.5:2:1.5 to prepare a negative electrode slurry.

[0115] The negative electrode slurry was applied to a copper foil current collector at a loading amount of 16 mg/cm.sup.2 (based on a post-drying amount) and dried, and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Comparative Example 5

[0116] A negative electrode was prepared in the same manner as in Example 1, except that only the second negative electrode slurry was applied to a copper foil current collector at a loading amount of 16 mg/cm.sup.2 (based on a post-drying amount), and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Comparative Example 6

[0117] A negative electrode was prepared in the same manner as in Example 1, except that only the first negative electrode slurry was applied to a copper foil current collector at a loading amount of 16 mg/cm.sup.2 (based on a post-drying amount), and then they were rolled so that a density of an electrode was 1.6 g/cc to prepare a negative electrode.

Experimental Example 1

[0118] The negative electrode plates prepared in Examples 1 to 9 and Comparative Examples 1 to 5 were cut to have a width of 15 mm and fixed on a slide glass, and then the current collector was peeled off at a rate of 300 mm/min to measure a 180 degree peel strength, the results of which are shown in Table 1 below.

Experimental Example 2

[0119] Li (Ni.sub.1/3Co.sub.1/3Mn.sub.1/3)O.sub.2 used as a positive electrode active material was placed in distilled water with carbon black and PVDF at a ratio of 96:2:2 and mixed to prepare a positive electrode slurry. The prepared positive electrode slurry was applied to an aluminum foil current collector at a loading amount of 29.2 mg/cm.sup.2 (based on a post-drying amount) and dried, and then rolled at an electrode density of 3.4 g/cc to prepare a positive electrode.

[0120] The prepared positive electrode was punched to a size of 3×4 cm, and the negative electrodes prepared in Examples 1 to 9 and Comparative Examples 1 to 5 were punched to a size of 3.2×4.2 cm. Afterward, a PE separator was interposed between the positive electrode and the negative electrode, and pouch cells were prepared by sealing an electrolyte containing 1 M of LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 with an aluminum pouch.

[0121] The cells were charged and discharged (3.0 V) for 50 cycles in a 1 C CC/CV mode at an ambient temperature of 25° C. and an upper limit voltage of 4.25 V, and a capacity retention ratio was measured. The results are shown in Table 1 below.

[0122] Discharge resistance was calculated from a voltage of the cell after charging the cell in a 1 C CC/CV mode at 4.55 V and applying a current corresponding to 2.5 C for 30 seconds at SOC 50, and the discharge resistances are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Capacity Discharge Adhesion Retention Resistance (gf/15 mm) Ratio (%) @ SOC 50% Example 1 62 91.1 1.371 Example 2 64 90.8 1.382 Example 3 54 99.1 1.330 Example 4 36 91 1.341 Example 5 47 99.2 1.352 Example 6 27 97.5 1.324 Example 7 18 93.1 1.329 Example 8 47 91.3 1.366 Example 9 17.5 99.3 1.320 Example 10 54.5 98.4 1.354 Example 11 56 97.9 1.371 Example 12 57 96.2 1.384 Example 13 59 94.3 1.398 Example 14 61 90.1 1.421 Example 15 49 98.9 1.361 Example 16 66 92.4 1.382 Comparative Example 1 9 88.2 1.411 Comparative Example 2 11 89.3 1.432 Comparative Example 3 12 99.4 1.318 Comparative Example 4 15 99.1 1.393 Comparative Example 5 7 96.3 1.311 Comparative Example 6 48 84.2 1.486

[0123] Hereinafter, Table 1 and Experimental Examples 1 and 2 will be described together.

[0124] First, referring to Examples 3 to 7 and Comparative Examples 2 and 4, or Examples 8 and 11 and Comparative Example 1 in which an overall negative electrode contains similar amounts of artificial graphite and natural graphite, it can be seen that when an active material in a form of a mixture of artificial graphite and natural graphite is coated with a two-layer structure to be positioned close to a current collector and a first negative electrode mixture layer contains the active material (Examples 3 to 7 and Example 8, Example 11) as in the present invention, adhesion, a capacity retention rate and discharge resistance characteristics (output characteristics) are superior to those of Comparative Example 1 or 2 in which only the artificial graphite was contained in the first negative electrode mixture layer or Comparative Example 4 in which a content of natural graphite located near the current collector is relatively small because the content of the natural graphite contained in the overall electrode is great and widely spread as the active material is coated with a single layer even when the mixed form of the active material is used.

[0125] Comparing Example 3 and Example 8 having the same configurations but differing amounts of loading of the first negative mixture layer and of the second negative mixture layer, it can be seen that superior adhesion, capacity retention ratio, and output characteristics are exhibited when the first negative electrode mixture layer is applied in a smaller amount than the second negative electrode mixture layer (Example 3).

[0126] Comparing Example 3 and Examples 9 to 14 in which contents of the natural graphite contained in the first negative mixture layer are different, it can be seen that the capacity retention ratio and discharge resistance are similar and the adhesion is remarkably excellent in a case in which natural graphite and artificial graphite are mixed at a weight ratio of 3:7 or more in the first negative electrode mixture layer (Example 3, Examples 10 to 14) in comparison to a case in which natural graphite and artificial graphite were mixed at 1:9 (Example 9).

[0127] Comparing Examples 3, 5, and 6 in which types of natural graphite were different, it can be seen that the adhesion and capacity retention rates of Examples 3 and 5 satisfying the particle diameter and tap density of the present invention are better than those of Example 6 in which the particle diameter and tap density of the present invention are different.

[0128] Comparing Example 3 and Example 4 in which the coating methods of the first negative electrode slurry and the second negative electrode slurry were different, it can be seen that the coating of the second negative electrode slurry before drying after the coating of the first negative electrode slurry is superior in all the properties.

[0129] In conclusion, when the above results are examined as a whole, it can be seen that the most excellent performance of a battery in terms of adhesive force, capacity retention ratio, and output characteristics is obtained in a case in which natural graphite is contained in a first negative electrode slurry at a weight ratio of 3:7 or more in comparison to artificial graphite, a loading amount of the first negative electrode slurry is made smaller than a loading amount of a second negative electrode slurry, and a wetting electrode coating is performed when preparing a negative electrode in a multilayer structure.

[0130] Comparative Example 5 using only artificial graphite and Comparative Example 6 using only natural graphite have a problem in that adhesion is very low (Comparative Example 5) and capacity retention and output characteristics are very poor (Comparative Example 6).

[0131] It should be understood by those skilled in the art that various changes may be made without departing from the spirit and scope of the present disclosure.