Lithium secondary battery comprising spherical graphite as anode active material

Abstract

Disclosed is an anode active material for lithium secondary batteries that includes natural graphite particles consisting of spherical particles of agglomerated graphite sheets, outer surfaces of which are not coated with a carbon-based material, wherein the surfaces of the particles have a degree of amorphization of at least 0.3 within a range within which an R value [R=I.sub.1350/I.sub.1580] (I.sub.1350 is the intensity of Raman around 1350 cm.sup.−1 and I.sub.1580 is the intensity of Raman around 1580 cm.sup.−1) of a Raman spectrum is in the range of 0.30 to 1.0.

Claims

1. An anode mixture for secondary batteries, comprising: an anode active material for lithium secondary batteries; an aqueous binder; and an aqueous thickening agent, wherein the aqueous thickening agent is carboxy methyl cellulose (CMC), and the aqueous binder is styrene-butadiene rubber (SBR), wherein the anode active material comprises natural graphite particles comprising spherical particles of agglomerated graphite sheets, outer surfaces of which are not coated with a carbon-based material, and wherein the surfaces of the particles have a degree of amorphization ranging from 0.3 to 0.5 to control viscosity and stability of the mixture and have an R value of a Raman spectrum ranging from 0.3 to 1.0, wherein the R value is represented by the following formula:
R=I.sub.1350/I.sub.1580 wherein I.sub.1350 is a Raman intensity of about 1350 cm.sup.−1 and I.sub.1580 is a Raman intensity of about 1580 cm.sup.−1.

2. The anode mixture according to claim 1, wherein the R value is 0.30 to 0.50.

3. The anode mixture according to claim 1, wherein the natural graphite particles have a hydrophilic substituent combined to at least some of the carbon atoms at surfaces thereof.

4. The anode mixture according to claim 1, wherein the natural graphite particles have an average particle diameter of 21 μm to 25 μm.

5. The anode mixture according to claim 1, wherein the natural graphite particles have a specific surface area of 4.5 m.sup.2/g to 5.5 m.sup.2/g.

6. The anode mixture according to claim 1, wherein an amount of the natural graphite particles is 90 wt % based on a total weight of the anode active material.

7. The anode mixture according to claim 1, wherein an amount of the CMC is 1.0 wt % to 2.0 wt % based on a total weight of the anode mixture.

8. The anode mixture according to claim 1, wherein an amount of the SBR is 1.0 wt % to 2.0 wt % based on a total weight of the anode mixture.

9. An anode manufactured by coating an anode current collector with the anode mixture according to claim 8 and drying the coated anode current collector.

10. A lithium secondary battery comprising the anode according to claim 9, a separator, and a cathode.

11. A battery module comprising the lithium secondary battery according to claim 10 as a unit battery.

12. The anode mixture according to claim 1, wherein the R value is great than 0.50 to 1.0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a graph showing evaluation results of measurement of the viscosity of a slurry in Experimental Example 1; and

(3) FIG. 2 is a graph showing evaluation results of lifespan characteristics of coin half-cells in Experimental Example 2.

BEST MODE

(4) Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only for illustration of the present invention and should not be construed as limiting the scope and spirit of the present invention.

Example 1

(5) Graphite sheets were agglomerated by milling to prepare spherized natural graphite particles having a diameter of 21 μm, surfaces of which had a degree of amorphization of 0.30, and the natural graphite particles, CMC, and SBR were mixed with water in a weight ratio of 98:1:1 (spherized natural graphite particles:CMC:SBR) to prepare a slurry. The slurry was coated onto a Cu foil and dried and rolled, thereby completing fabrication of an anode.

Example 2

(6) An anode was manufactured in the same manner as in Example 1, except that spherized natural graphite particles having a diameter of 23 μm, surfaces of which had a degree of amorphization of 0.40, were used.

Example 3

(7) An anode was manufactured in the same manner as in Example 1, except that spherized natural graphite particles having a diameter of 21 μm, surfaces of which had a degree of amorphization of 0.45, were used.

Comparative Example 1

(8) An anode was manufactured in the same manner as in Example 1, except that spherized natural graphite particles having a diameter of 22 μm, surfaces of which had a degree of amorphization of 0.23, were used.

(9) Specific surface areas and tap densities of the natural graphite particles prepared according to Examples 1 to 3 and Comparative Example 1 are shown in Table 1 below.

(10) TABLE-US-00001 TABLE 1 Particle Specific diameter Degree of surface Tap density (μm) amorphization area (m.sup.2/g) (g/cm.sup.3) Example 1 21 0.30 5.3 0.96 Example 2 23 0.40 5.4 1.03 Example 3 21 0.45 5.4 1.10 Comparative 22 0.23 5.1 0.94 Example 1

[Experimental Example 1] Viscosity Measurement Results

(11) Viscosity and stability of the slurries used in Examples 1 to 3 and Comparative Example 1 were measured and measurement results are shown in FIG. 1.

(12) Referring to FIG. 1, the slurries of Examples 1 to 3 have lower viscosity than the slurry of Comparative Example 1, and the slurry of Example 3 does not show a viscosity increase section according to an increase in shear rate, from which it can be confirmed that there is no agglomeration of particles of the anode material. From the results, it can be confirmed that, as a degree of amorphization of the particle surfaces increases, the anode active material and the aqueous binder are smoothly mixed in the slurry using the aqueous binder and, accordingly, viscosity and slurry stability are enhanced.

[Experimental Example 2] Evaluation of Lifespan Characteristics of Coin Half-Cell

(13) Each of the anodes manufactured according to Examples 1 to 3 and Comparative Example 1 was assembled with a cathode including LiCoO.sub.2 as a cathode active material, PVdF as a binder, and natural graphite as a conductive material to manufacture a coin half-cell. Lifespan characteristics of the coin half-cells were measured by performing charging and discharging at a voltage of 3.0 V to 4.2 V. Measurement results are illustrated in FIG. 2.

(14) Referring to FIG. 2, the coin half-cells including the anodes of Examples 1 to 3 have superior lifespan characteristics to the coin half-cell including the anode of Comparative Example 1. This is assumed because, as shown in Table 1 above, the anode active materials used in Examples 1 to 3 have higher density than the anode active material used in Comparative Example 1, which indicates that, as the density of an anode active material increases, an electrode is smoothly impregnated with an electrolyte and thus lithium ions vigorously migrate into the electrode, and the lithium ions more rapidly migrate at a surface of an anode active material having a high degree of surface amorphization and thus resistance to migration of lithium ions at the surface of the anode active material is low.

(15) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

(16) As described above, an anode active material according to the present invention includes natural graphite particles having specific exterior appearance and surface characteristics and thus, when fabricating an electrode using the same, slurry processability may be enhanced, excellent battery lifespan characteristics may be obtained, and manufacturing costs of a battery may be reduced.