POSITIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY, PREPARATION METHOD THEREFOR, AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

The present invention relates to a positive electrode active material comprising an overlithiated layered oxide (OLO) and, more specifically, to a positive electrode active material comprising: an OLO represented by chemical formula 1 below; and an amorphous free oxide coating layer of an amorphous free oxide on the surface of the OLO represented by chemical formula 1. [Chemical formula 1] Li.sub.2MnO.sub.3.(1-r)Li.sub.aNi.sub.xCo.sub.yMn.sub.zM1.sub.1-(x+y+z)O.sub.2 (wherein, 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 any 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).

Claims

1. A cathode active material for a lithium secondary battery comprising: overlithiated layered oxide (OLO) represented by Formula 1 below; and an amorphous glassy oxide coating layer formed on a surface of the overlithiated layered oxide represented by Formula 1,
rLi.sub.2MnO.sub.3.(1-r)Li.sub.aNi.sub.xCo.sub.yMn.sub.zM1.sub.1-(x+y+z)O.sub.2  [Formula 1] 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 amorphous glassy oxide coating layer comprises at least one selected from Si, B, P, and Ge.

3. The cathode active material according to claim 1, wherein the amorphous glassy oxide coating layer comprises a material represented by Formula 2 below:
xLi.sub.2O*(1-x)M2.sub.aO.sub.b  [Formula 2] wherein x, a, and b satisfy 0<x≤8, 0<a≤2, and 0<b≤5, and M2 comprises at least one selected from Si, B, P, and Ge.

4. The cathode active material according to claim 1, wherein the amorphous glassy oxide 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 amorphous glassy oxide 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 in 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 material formed in the second step with a coating precursor to form an amorphous glassy oxide 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 third step further comprises, after mixing the material formed in the second step with the coating precursor, allowing the mixture to stand at 250 to 700° C. for 7 to 12 hours and then subjecting the mixture to furnace cooling.

15. The method according to claim 14, wherein the coating precursor comprises at least one selected from B.sub.2O.sub.3, P.sub.2O.sub.5, H.sub.3BO.sub.3, NH.sub.4HPO.sub.4, NH.sub.4H.sub.2PO.sub.4, (NH.sub.4).sub.2HPO.sub.4, and H.sub.3PO.sub.4.

16. The method according to claim 12, further comprising, after the first step and before the second step, roasting the prepared precursor at 300 to 600° C.

17. The method according to claim 12, further comprising, after the first step and before the second step, washing the calcined material with water.

18. The method according to claim 12, further comprising, after the second step and before the third step, washing the calcined material with water.

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

Description

DESCRIPTION OF DRAWINGS

[0076] FIG. 1 shows the results of SEM performed on cathode active materials according to Examples and Comparative Examples of the present invention.

[0077] FIG. 2 shows the result of EDS performed on cathode active materials according to an embodiment of the present invention.

[0078] FIG. 3 shows the results of XRD analysis of cathode active materials according to Examples and Comparative Examples of the present invention.

[0079] FIG. 4 shows the results of EDS performed on cathode active materials according to Examples of the present invention.

[0080] FIG. 5 shows the results of EDS performed on cathode active materials of Examples according to the present invention.

[0081] FIG. 6 is a charge/discharge graph of Examples and Comparative Examples of the present invention.

[0082] FIG. 7 is an overvoltage curve of Examples and Comparative Examples of the present invention.

[0083] FIG. 8 is a rate characteristic curve according to Examples and Comparative Examples of the present invention.

[0084] FIG. 9 shows the life characteristics according to Examples and Comparative Examples of the present invention.

[0085] FIG. 10 shows the voltage retention ratio according to Examples and Comparative Examples of the present invention.

BEST MODE

[0086] 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.

[0087] As used herein, terms such as “comprising” should be understood as open-ended terms that do not preclude the inclusion of other elements.

[0088] 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

[0089] Synthesis

[0090] 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.4.6H.sub.2O, CoSO.sub.4.7H.sub.2O, and MnSO.sub.4H.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 8.0 to 11.0, 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.

[0091] 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.0.17Co.sub.0.106Mn.sub.0.719CO.sub.3 precursor having a size of 4 μm to 20 μm.

[0092] Roasting

[0093] 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.

[0094] Calcination

[0095] 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).

[0096] 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 a cathode active material.

[0097] The composition of the cathode active material prepared in Example 1 was Li:Ni:Co:Mn:Nb=15.3:15.1:9.3:59.8:0.4 (wt %).

[0098] Coating

[0099] 1.5 mol % of H.sub.3BO.sub.3 was weighed as a surface treatment dopant and mixed using a mixer (manual mixer, MM).

[0100] 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 300° C. for 7 to 12 hours, followed by furnace cooling to prepare a cathode active material.

<Example 2> Preparation of Cathode Active Material

[0101] A cathode active material was prepared in the same manner as in Example 1, except that 0.5 mol % of NH.sub.4HPO.sub.4 was mixed as a surface treatment dopant in the coating step of Example 1 and the calcination temperature in the coating step was 600° C.

[0102] The composition of the cathode active material prepared in Example 2 was Li:Ni:Co:Mn:Nb=15.0:14.8:9.3:60.0:0.8 (wt %).

<Comparative Example 1> Preparation of Cathode Active Material

[0103] 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

[0104] 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.

<Comparative Example 3> Preparation of Cathode Active Material

[0105] A cathode active material was prepared in the same manner as in Example 1, except that a flux dopant was not mixed in Example 1.

<Production Example> Production of Lithium Secondary Battery

[0106] 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.

[0107] 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.

[0108] 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

[0109] As can be seen from FIG. 1, when comparing Comparative Example 2 with Examples 1 and 2, there was no change in the size or shape of the primary particles due to the B and P coatings. When comparing Comparative Example 1 with Comparative Example 3, it can be seen that there was no particle growth due to the amorphous glassy oxide coating layer.

[0110] As can be seen from FIG. 2, not only Ni, Co, and Mn elements, but also Nb, which is a flux dopant inducing particle growth, are homogeneously distributed in the particles.

[0111] The XRD analysis of FIG. 3 was performed at a wavelength of CuK α radiation=1.5406 Å. As can be seen from FIG. 3, when Nb is added as a flux dopant, the (003) peak shifts, which supports the notion that the overlithiated layered oxide lattice is doped with Nb, a flux dopant.

[0112] As can be seen from FIG. 4, the coating layer containing B is homogeneously distributed on the surface of the cathode active material according to Example 1.

[0113] As can be seen from FIG. 5, the coating layer containing P is homogeneously distributed on the surface of the cathode active material according to Example 2.

[0114] As can be seen from FIG. 6, compared to Comparative Example 1, in Comparative Example 2, the capacity decreased slightly as the primary particle size increased. This is because the kinetics decreases as the lithium ion diffusion distance increases. However, in Example, in which the amorphous glassy oxide coating layer is formed, the ion conductivity increases and the kinetics of the lithium ions increases, so the capacity increases.

[0115] 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 amorphous glassy oxide coating layer is formed, the overvoltage can be reduced due to the increased kinetics of lithium ions.

[0116] As can be seen from FIG. 8, the rate characteristics of Examples are improved by about 10% or more compared to Comparative Examples. This is because the resistance is reduced due to the amorphous glassy oxide coating layer.

[0117] As can be seen from FIG. 9, the lifetime characteristics of Comparative Example 3, in which the amorphous glassy oxide coating layer is formed, are improved by about 10% or more compared to Comparative Example 1. In addition, Examples 1 and 2, in which the particles are grown and the amorphous glassy oxide coating layer is formed, exhibit a capacity retention rate of 80% or more during cycling. This is because resistance is reduced by the amorphous glassy oxide coating layer, Mn elution is suppressed, the phase change from the spinel phase to the rock salt phase starting from the surface during cycling is suppressed, and the lifetime is improved.

[0118] As can be seen from FIG. 10, Examples exhibited a voltage retention rate of 97%. This is because the phase change of the overlithiated layered oxide, occurring during cycling, was suppressed due to the amorphous glassy oxide coating layer.

[0119] The experimental results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Comparative Comparative Comparative ITEM Example 1 Example 2 Example 1 Example 2 Example 3 Initial stage CH. mAh/g 294 286 276 286 291 (@25° C.) DCH. 261 254 245 241 257 0.1 C Eff. % 89 89 89 84 88 2.0-4.6 V High rate Rate 53 54 36 43 42 5 C/0.1 C Lifetime Cycle Life 90 79 60 64 68 (@25° C.) (50cycle) 1 C/1 C Voltage Decay 97.4 96.3 95 95 96 2.0-4.6 V (50cycle)