LITHIUM SECONDARY BATTERY CATHODE ACTIVE MATERIAL, MANUFACTURING METHOD THEREFOR, AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

The present invention comprises: an overlithiated layered oxide represented by chemical formula 1 below; and an ion-conductive coating layer on the overlithiated layered oxide represented by chemical formula 1: [chemical formula 1] .sub.rLi.sub.2MnO.sub.3.Math.(1-r)Li.sub.aNi.sub.xCo.sub.yMn.sub.zM1.sub.1−(x+y+z)O.sub.2 (in chemical formula 1, 0<r≤0.6, 0<a≤1, 0≤x≤1, 0≤y<1, 0≤z<1, and 0<x+y+z<1, and M1 is at least one selected from among Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Cu, In, S, B, Ge, Si, and Bi).

Claims

1. A cathode active material for a lithium secondary battery comprising: overlithiated layered oxide represented by Formula 1 below; and an ion-conductive coating layer formed on a surface of the overlithiated layered oxide represented by Formula 1, [Formula 1] rLi.sub.2MnO.sub.3.Math.(1−r)Li.sub.aNi.sub.xCo.sub.yMn.sub.zM1.sub.1−(x+y+z)O.sub.2 wherein r, a, x, y, and z satisfy 0<r<0.6, 0<a<1, 0<x<1, 0<y<1, 0<z<1, and 0<x+y+z<1, and M1 comprises at least one selected from Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Cu, In, S, B, Ge, Si, and Bi.

2. The cathode active material according to claim 1, wherein the ion-conductive coating layer comprises at least one selected from Ti, Al, and Zr.

3. The cathode active material according to claim 1, wherein the ion-conductive coating layer comprises a material represented by Formula 2 below: [Formula 2] Li.sub.aM2.sub.bO.sub.c wherein a, b, and c satisfy 0<a<4, 0<b<5, and 0<c<12, and M2 comprises at least one selected from Ti, Al, and Zr.

4. The cathode active material according to claim 1, wherein the ion-conductive coating layer is present in an amount of 0.05 to 5 mol % based on an amount of the overlithiated layered oxide.

5. The cathode active material according to claim 1, wherein the ion-conductive coating layer has a thickness of 1 to 100 nm.

6. The cathode active material according to claim 1, wherein primary particles aggregate to form secondary particles, and primary particles having a size of 300 nm to 10 μm are present in an amount of 50 to 100 vol % based on a total amount of the primary particles constituting the secondary particles.

7. The cathode active material according to claim 1, wherein M1 in Formula 1 acts as a flux growing the primary particles.

8. The cathode active material according to claim 1, wherein M1 in Formula 1 comprises at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W.

9. The cathode active material according to claim 1, wherein M1 in Formula 1 is present in an amount of 0.001 to 10 mol % based on a total number of moles of metals of the overlithiated layered oxide.

10. The cathode active material according to claim 1, wherein a ratio (Li/Ni+Co+Mn) of a number of moles of lithium to a total number of moles of all metals contained among Ni, Co, or Mn in the overlithiated layered oxide represented by Formula 1 is 1.1 to 1.6.

11. The cathode active material according to claim 1, wherein a ratio (Mn/Ni) of a number of moles of Mn to a total number of moles of Ni in the overlithiated layered oxide represented by Formula 1 is 1 to 4.5.

12. A method of preparing the cathode active material for a secondary battery according to claim 1, the method comprising: a first step of preparing a cathode active material precursor; a second step of mixing the cathode active material precursor with a lithium compound and calcining the resulting mixture to form a lithium composite oxide; and a third step of mixing the lithium composite oxide formed in the second step with a coating precursor to form an ion-conductive coating layer.

13. The method according to claim 12, wherein, in the second step, a compound containing M1 of Formula 1 is further mixed and calcined.

14. The method according to claim 12, wherein the precursor comprises at least one selected from TiO.sub.2, Al.sub.2O.sub.3, Al(OH).sub.3, ZrO.sub.2, and Zr(OH).sub.4.

15. A secondary battery comprising the cathode active material according to claim 1.

Description

DESCRIPTION OF DRAWINGS

[0080] FIG. 1 shows the results of EDS performed on an embodiment of the present invention.

[0081] FIG. 2 shows the result of EDS performed on an embodiment of the present invention.

[0082] FIG. 3 shows the results of SEM performed on Examples and Comparative Examples of the present invention.

[0083] FIG. 4 shows the results of XRD analysis performed on Examples and Comparative Examples of the present invention.

[0084] FIG. 5 shows the results of XRD analysis performed on Examples and Comparative Examples of the present invention.

[0085] FIG. 6 is a graph showing charge/discharge characteristics of Examples and Comparative Examples of the present invention.

[0086] FIG. 7 is an overvoltage curve of Examples and Comparative Examples of the present invention.

[0087] FIG. 8 is a rate characteristic curve according to Examples and Comparative Examples of the present invention.

[0088] FIG. 9 shows the life characteristics according to Examples and Comparative Examples of the present invention.

[0089] FIG. 10 shows the life characteristics according to Examples and Comparative Examples of the present invention.

[0090] FIG. 11 is a voltage graph according to Examples and Comparative Examples of the present invention.

[0091] FIG. 12 is a voltage graph according to Examples and Comparative Examples of the present invention.

BEST MODE

[0092] Hereinafter, the present invention will be described in more detail with reference to Examples. However, these examples should not be construed as limiting the present invention.

[0093] As used herein, terms such as “comprising” should be understood as open-ended terms that do not preclude the inclusion of other elements.

[0094] As used herein, the terms “preferred” and “preferably” refer to embodiments of the present invention that may provide certain advantages in specific environments, and are not intended to exclude other embodiments from the scope of the invention.

<Example 1>Preparation Of Cathode Active Material

Synthesis

[0095] A spherical Ni.sub.0.2Co.sub.0.1Mn.sub.0.7CO.sub.3 precursor was synthesized using a co-precipitation method. 25 wt % of NaCO.sub.3 and 28 wt % of NH.sub.4OH were added to a 2.5M composite transition metal sulfate solution prepared by mixing NiSO.sub.4.Math.6H.sub.2O, CoSO.sub.4.Math.7H.sub.2O, and MnSO.sub.4.Math.H.sub.2O at a molar ratio of 20:10:70 in a 90 L reactor. At this time, the pH in the reactor was maintained at 10.0 to 12.0, and the temperature thereof was maintained at 45 to 50° C. In addition, N.sub.2, which is an inert gas, was injected into the reactor to prevent oxidation of the prepared precursor.

[0096] After completion of synthesis, washing and dehydration were performed using a filter press (F/P). Finally, the dehydrated product was dried at 120° C. for 2 days and filtered through a 75 μm (200 mesh) sieve to obtain a Ni.sub.0.2O.sub.0.1Mn.sub.0.7CO.sub.3 precursor having a size of 18 μm and 4 μm.

Roasting

[0097] The prepared precursor was maintained in an O.sub.2 or air (50 L/min) atmosphere in a box furnace, and the temperature was elevated at a rate of 2° C/min and maintained at 550° C. for 1 to 6 hours, followed by furnace cooling.

Calcination

[0098] LiOH or Li.sub.2CO.sub.3 was weighed such that the roasted precursor had a Li/(Ni+Co+Mn) ratio of 1.45, and 0.6 mol % of Nb.sub.2O.sub.5 was weighed as a flux dopant, followed by mixing using a manual mixer (MM).

[0099] The mixture was maintained under an O.sub.2 or air (50 L/min) atmosphere in a box furnace, heated at 2° C/min, and maintained at a calcination temperature of 900° C. for 7 to 12 hours, followed by furnace cooling to prepare lithium composite oxide particles.

Coating

[0100] 1.0 mol % of TiO.sub.2 was weighed as a surface treatment dopant and mixed using a mixer (manual mixer, MM).

[0101] The mixture was maintained under an O.sub.2 or air (50 L/min) atmosphere in a box furnace, heated at 4.4° C/minute, and maintained at a sintering temperature of 700° C. for 7 to 12 hours, followed by furnace cooling to prepare a cathode active material.

<Example 2>Preparation Of Cathode Active Material

[0102] A cathode active material was prepared in the same manner as in Example 1, except that 1.0 mol % of Al.sub.2O.sub.3 was mixed as a surface treatment dopant in the coating step of Example 1.

<Example 3>Preparation of cathode active material

[0103] A cathode active material was prepared in the same manner as in Example 1, except that 1.0 mol % of n-ZrO.sub.2 was mixed as a surface treatment dopant in the coating step of Example 1.

[0104] <Example 4>Preparation Of Cathode Active Material

[0105] A cathode active material was prepared in the same manner as in Example 1, except that 0.19 mol % of TiO.sub.2, 0.77 mol % of Al.sub.2O.sub.3, and 0.04 mol % of n-ZrO.sub.2 were mixed as a surface treatment dopant in the coating step of Example 1.

<Comparative Example 1>Preparation Of Cathode Active Material

[0106] A cathode active material was prepared in the same manner as in Example 1, except that a flux dopant was not mixed and a coating step was not performed.

<Comparative Example 2>Preparation Of Cathode Active Material

[0107] A cathode active material was prepared in the same manner as in Example 1, except that the coating step of Example 1 was not performed.

[0108] <Production Example>Production Of Lithium Secondary Battery

[0109] 90 wt % of each of the cathode active materials according to Examples and Comparative Examples, 5.5 wt % of carbon black, and 4.5 wt % of a PVDF binder were dispersed in 30 g of N-methyl-2 pyrrolidone (NMP) to prepare a cathode slurry. The cathode slurry was applied to a 15 μm-thick aluminum (Al) thin film, which is a cathode current collector, dried, and then roll-pressed to produce a cathode. The loading amount of the cathode was 5.5 mg/cm.sup.2 and the electrode density was 2.3 g/cm.sup.3.

[0110] For the cathode, metallic lithium was used as a counter electrode, and a mixture of 1M LiPF.sub.6 and EC/DMC (1/1, v/v) was used as an electrolyte.

[0111] A separator formed of a porous polyethylene (PE) film was injected between the cathode and the anode to form a battery assembly, and the electrolyte was injected into the battery assembly to produce a lithium secondary battery (coin cell).

Experimental Example

[0112] As can be seen from FIG. 1, the Ti, Al, and Zr coating layers are uniformly distributed on the surface of the cathode active material according to Examples 1 to 3.

[0113] As can be seen from FIG. 2, the Ti, Al and Zr coating layers are uniformly distributed on the surface of the cathode active material according to Example 4.

[0114] As can be seen from FIG. 3, primary particles are grown by the flux dopant Nb, and Ti, Al, and Zr coating layers are formed on the grown primary particles.

[0115] The XRD analysis of FIGS. 4 and 5 were performed using a wavelength of CuKα radiation=1.5406 Å. As can be seen from FIGS. 4 and 5, the dopant for surface treatment is partially doped on the particle surface. In addition, it can be seen that the diffusion of lithium ions is facilitated due to inter-slab expansion caused by partial doping.

[0116] As can be seen from FIG. 6, in Comparative Example 2, the lithium ion diffusion distance increases as the size of primary particles increases, thus causing problems of reduced kinetics and a slight decrease in capacity. On the other hand, in Example, in which an ion conductive coating layer is formed, the capacity increases. This is because the kinetics of lithium ions is increased due to the increased ionic conductivity.

[0117] As can be seen from FIG. 7, as the particle size increases, the diffusion distance of lithium ions increases, which causes a problem in which overpotential is generated due to concentration polarization of lithium ions. However, in Examples, in which the ion-conductive coating layer is formed, the overvoltage is reduced due to the increased kinetics of lithium ions.

[0118] As can be seen from FIG. 8, the rate characteristics of Examples are improved by about 10% or more compared to Comparative Example 2. This is because the resistance is reduced due to the ion-conductive coating layer.

[0119] As can be seen from FIGS. 9 and 10, the lifetime characteristics of Examples are improved, and the capacity retention rate is 80% or more compared to Comparative Example 2. This is because resistance is reduced by the ion-conductive coating layer, Mn elution is suppressed, and the phase change from the spinel phase to the rock salt phase starting from the surface during cycling is suppressed.

[0120] As can be seen from FIGS. 11 and 12, Examples exhibit an improved voltage retention rate compared to Comparative Example 2. This is because the phase change of the overlithiated layered oxide, occurring during cycling, is suppressed due to the ion-conductive coating layer.

[0121] The experimental results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Comparative Comparative Example Example Example Example ITEM Example 1 Example 2 1 2 3 4 Initial CH. mAh/g — 262 265 273 274 270 (@25° C.) DCH. — 218 218 219 222 216 0.1 C. Eff. % — 83.4 82.3 80.2 81.1 79.9 2.0-4.6 V High-rate Rate 36 42.7 47.0 43.7 47.3 45.1 5 C./0.1 C. Lifetime Cycle 60 80.9 85.6 82.2 83.0 82.7 (@25° C.) Life 1 C./1 C. (50 cycle) 2.0-4.6 V Voltage 95 95.7 96.3 96.0 96.1 96.2 Decay (50 cycle)