Cathode active material of lithium secondary battery

Abstract

The present invention relates to a cathode active material for a lithium secondary battery, and more particularly, to a cathode active material for a lithium secondary battery, which includes a core portion and a shell portion surrounding the core portion, in which a total content of cobalt in the core portion and the shell portion is 5 to 12 mol %, and the content of cobalt in the core portion and the shell portion is adjusted to be within a predetermined range. In the cathode active material precursor and the cathode active material for a secondary battery prepared using the same according to the present invention, optimal capacity of a lithium secondary battery may be increased by adjusting the cobalt content in the particles of the cathode active material, and life characteristics may be enhanced by improving stability.

Claims

1. A cathode active material for a lithium secondary battery, the cathode active material comprising: a core portion and a shell portion surrounding the core portion, that together form a particle, wherein a total content of cobalt in the core portion and the shell portion is 5 to 12 mol %, wherein the content of cobalt in the entire particle is W, the content of cobalt in the shell portion is 0.2 W to 1.0 W, wherein the particle comprises cobalt and the content of cobalt in the shell portion is more than 20 mol %, wherein when the particle comprises manganese, the content of manganese in the shell portion is less than 20 mol %, wherein a diameter of the entire particle D is 1 μm to 25 μm, and wherein a thickness of the shell portion is 0.01 D to 0.3 D.

2. The cathode active material of claim 1, wherein the cathode active material for a lithium secondary battery is represented by Chemical Formula 1 below:
Li.sub.aNi.sub.xCo.sub.yMn.sub.1−x−y−zO.sub.2 (where 0.9≤a≤1.3, 0.7≤x<1.0, 0.05≤y≤0.12, 0.0≤z≤0.3, and 0.0≤1−x−y−z≤0.3, wherein M is one or more elements selected from among B, Ba, Ce, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, and Cu).

3. The cathode active material of claim 2, wherein a concentration of cobalt in the core portion and a concentration of cobalt in the shell portion are not equal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, 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) FIGS. 1 and 2 show results of measuring a size and an internal metal concentration of a cathode active material prepared according to an embodiment of the present invention.

(3) FIGS. 3 to 6 show results of measuring characteristics of batteries including cathode active materials of Inventive Examples of the present invention and Comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(4) Hereinafter, the present invention will be described in more detail with reference to examples. These embodiments are only for illustrating the present invention and the scope of the present invention is not construed as being limited by these embodiments.

Inventive Example: Preparation of Precursor

(5) In order to prepare a cathode active material, nickel sulfate, cobalt sulfate, and manganese sulfate were prepared to first prepare precursors 1 to 3 including a core portion and a shell portion by a co-precipitation reaction. Here, the precursors were prepared such that compositions of the entire core and shell portions were 5 mol %, 9 mol %, and 12 mol % (Inventive Examples 1 to 3), respectively.

(6) LiOH was added as a lithium compound and subjected to a first heat—treatment in the presence of N.sub.2 and O.sub.2/(1˜100 LPM) at a heating rate of 1° C./min˜20° C./min for 4 to 20 hours (with respect to a maintained range). After the first heat treatment, 0 to 10 mol % of a compound containing Al was added to the resultant mixture and subjected to a secondary heat-treatment to prepare a cathode active material for a lithium secondary battery.

(7) Next, distilled water was prepared and maintained at a constant temperature of 5 to 40° C. Thereafter, the prepared cathode active material for a lithium secondary battery was put into the distilled water and rinsed for 0.1 to 10 hours, while the temperature was maintained.

(8) The rinsed cathode active material was filter-pressed and then dried at 50 to 300° C. for 3 to 24 hours under the atmosphere of oxygen.

Comparative Example: Preparation of Precursor

(9) A cathode active material was prepared in the same manner as that of Inventive Example except that the cobalt content in the entire core and shell portions was 3 mol %.

Experimental Example: Measurement of Particle Size

(10) A particle size of Inventive Example 1 was measured and results thereof are shown in FIG. 1. As shown in FIG. 1, it can be seen that the particle size prepared according to Inventive Example of the present invention is 10 to 25 μm.

Experimental Example: Measurement of Thickness of Shell Portion

(11) A thickness of the shell was measured on the basis of metal concentrations from a surface to the inside of the particle prepared in Inventive Example 1 and results thereof are shown in FIG. 2.

(12) As shown in FIG. 2, it can be seen that a thickness of the shell of the particle prepared according to Inventive Example is 1.6 um.

Manufacturing Example: Manufacturing of Half-Cell

(13) 94 wt % of the cathode active materials prepared according to Inventive Examples 1 to 3 and Comparative Example, 3 wt % of a conductive material (super-P), and 3 wt % of a binder (PVDF) were mixed in the ratio of 4.7 g:0.15 g:0.15 g, respectively, mixed at 1,900 rpm/10 min. by a stirrer, applied to an AI foil by a micro-film applicator, and subsequently dried in a dry oven at 135° C. for four hours to manufacture a positive plate.

(14) A coin cell was manufactured using a lithium metal foil as a negative plate, polypropylene of W-Scope-20 μm as a separator, and 1.15 M LiPF having a composition of EC/EMC=7/3 as an electrolyte.

Experimental Example: Measurement of Charging/Discharging Characteristics

(15) The charging/discharging characteristics of the particles of Inventive Examples 1 to 3 and the particles of Comparative Example were measured, and results thereof are shown in FIG. 3 and Table 1.

(16) As shown in FIG. 3 and Table 1, it can be seen that, when a mole fraction of Co in the entire core and shell portions was 9%, the charging/discharging characteristics were significantly improved, as compared with Comparative Example.

(17) TABLE-US-00001 TABLE 1 Charge Discharge C-rate Cycle Capacity Capacity Efficiency Retention (%) EIS Retention (%) Remark Co (mAh/g) (mAh/g) (%) (5 C/0.1 C) (Ohm) (50 Cyc) Composition Comparative 241.1 218.1 90.5% 77.8 44.7 91.1 3% Example Inventive 238.3 220.8 92.7% 80.4 28.4 95.1 5% Example 1 Inventive 244.5 229.8 94.8% 82.1 16.7 94.6 9% Example 2 Inventive 242.0 227.6 94.0% 82.0 18.4 95.0 12%  Example 3

Experimental Example 2: Measurement of Output Characteristics

(18) Output characteristics of the particles of Inventive Examples 1 to 3 and the particles of Comparative Example were measured and results thereof are shown in FIG. 4 and Table 1.

(19) As shown in FIG. 4 and Table 1, it can be seen that, when a mole fraction of Co in the entire core and the shell portions was 9%, the output characteristics were significantly improved, as compared with Comparative Example.

(20) Also, in FIG. 4 and Table 1, it can be seen that the secondary battery including the cathode active material according to the present invention has especially improved high-rate characteristics.

Experimental Example 3: Measurement of Electrochemical Impedance Spectroscopy (EIS) Characteristics

(21) EIS resistance characteristics of the particles of Inventive Examples 1 to 3 and the particles of Comparative Example were measured, and results thereof are shown in FIG. 5 and Table 1.

(22) As shown in FIG. 5 and Table 1, it can be seen that, when a mole fraction of Co in the entire core and shell portions was 9%, the EIS resistance characteristics were significantly improved, as compared with the Comparative Example.

Experimental Example 4: Measurement of Life Characteristics

(23) Life characteristics of the particles of Inventive Examples 1 to 3 and the particles of Comparative Example were measured, and results thereof are shown in FIG. 6 and Table 1.

(24) As shown in FIG. 6 and Table 1, it can be seen that, when a mole fraction of Co in the entire core and shell portions was 12%, the life characteristics were significantly improved, as compared with Comparative Example.