Positive electrode active material, method of preparing the same, and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode active material, wherein the positive electrode active material is a lithium transition metal oxide including a first doping element (A) and a second doping element (B), wherein the first doping element is one or more selected from the group consisting of Zr, La, Ce, Nb, Gd, Y, Sc, Ge, Ba, Sn, Sr, Cr, Mg, Sb, Bi, Zn, and Yb, the second doping element is one or more selected from the group consisting of Al, Ta, Mn, Se, Be, As, Mo, V, W, Si, and Co, and a weight ratio (A/B ratio) of the first doping element to the second doping element is 0.5 to 5.

Claims

1. A positive electrode active material, wherein the positive electrode active material is a lithium transition metal oxide comprising a first doping element (A) and a second doping element (B), wherein the first doping element is Zr, the second doping element is one or more selected from the group consisting of Al, Ta, and Co, and a weight ratio (A/B ratio) of the first doping element to the second doping element is 0.5 to 2, wherein the lithium transition metal oxide is composed of a lithium layer and a transition metal layer, the first doping element is included in the lithium layer, and the second doping element is included in the transition metal layer, and the positive electrode active material is represented by Formula 1 or Formula 2 below:
Li.sub.1-aZr.sub.a[(Ni.sub.xMn.sub.yCo.sub.z).sub.1-bM.sup.2.sub.b]O.sub.2  [Formula 1]
Li.sub.1-c-dZr.sub.cM.sup.2.sub.d[(Ni.sub.xMn.sub.yCo.sub.z).sub.1-e-fZr.sub.eM.sup.2.sub.f]O.sub.2  [Formula 2] wherein 0<a<0.1, 0<b<0.1, 0<c<0.1, 0<d<0.1, 0<e<0.1, 0<f<0.1, x+y+z=1, M.sup.2 is one or more selected from the group consisting of Al, Ta, and Co.

2. The positive electrode active material according to claim 1, wherein the first doping element is included in the lithium layer and the transition metal layer in a weight ratio of 80:20 to 100:0.

3. The positive electrode active material according to claim 1, wherein the second doping element is included in the transition metal layer and the lithium layer present in a weight ratio of 80:20 to 100:0.

4. The positive electrode active material according to claim 1, wherein the first doping element is included in an amount of 500 ppm to 10,000 ppm based on a total weight of the positive electrode active material.

5. The positive electrode active material according to claim 1, wherein the second doping element is included in an amount of 100 ppm to 10,000 ppm based on a total weight of the positive electrode active material.

6. A positive electrode for lithium secondary batteries comprising the positive electrode active material according to claim 1.

7. A lithium secondary battery, comprising the positive electrode according to claim 6.

8. A method of preparing the positive electrode active material according to claim 1, the method comprising: a step of mixing a lithium compound, a transition metal compound, a compound comprising the first doping element (A), and a compound comprising the second doping element (B); and a step of sintering the mixture obtained by the aforementioned step.

9. The method according to claim 8, wherein the lithium transition metal oxide composed of a transition metal layer and a lithium layer is formed by the sintering.

10. The method according to claim 9, wherein, during the sintering, doping the lithium layer with the first doping element (A), and doping the transition metal layer with the second doping element (B).

11. The method according to claim 10, wherein doping positions of the first doping element (A) and the second doping element (B) are determined by a diffusion rate difference between the elements and effective ion radii thereof during the sintering.

Description

MODE FOR CARRYING OUT THE INVENTION

(1) Hereinafter, exemplary embodiments of the present invention are described in detail for those of ordinary skill in the art to easily implement. However, the present invention may be implemented in various different forms and is not limited to these embodiments.

EXAMPLE 1

(2) A transition metal precursor (Ni.sub.0.6Co.sub.0.2Mn.sub.0.2(OH).sub.2) and a lithium salt (Li.sub.2CO.sub.3) were dry-mixed in a stoichiometric ratio of 1:1.06. The lithium used in a large amount was added in an amount of 10% or less in consideration of the uniformity of mixing with the precursor, the purity of the lithium salt itself, loss due to volatilization of the lithium salt at high temperature, and the like. The transition metal precursor and the lithium salt were mixed such that the amount of a positive electrode active material obtained after sintering was 200 g, using a Henschel mixer capable of applying shear force, due to different specific gravities and average particle sizes thereof. An addition amount of each doping element was determined by being converted into a ratio of the doping element to a total of doping compounds. For example, (Zr(OH).sub.4, 100 ppm) represented that the content of Zr, not Zr(OH).sub.4, was 100 ppm. Zirconium hydroxide (Zr(OH).sub.4, 2,000 ppm) and tantalum oxide (Ta.sub.2O.sub.5, 1,000 ppm) were added to the mixture of the transition metal precursor and the lithium salt compound, followed by sintering in a 800° C. furnace with a temperature control function for 12 hours. As a result, a positive electrode active material was prepared.

(3) The obtained positive electrode active material was sintered according to melting of the lithium salt, and thus, a portion of the positive electrode active material was present in an agglomerated state. Accordingly, to prepare the positive electrode active material having particle distribution similar to that of the precursor, an average particle size of the positive electrode active material was adjusted by pulverization and classification processes.

(4) The pulverization process was performed by means of a Henschel mixer. After the pulverization process, a classification process was performed using a mesh scale net equipped with an ultrasonic device. Finally, a positive electrode active material (Ni:Co:Mn=6:2:2 in a mole ratio) having an average particle size (D.sub.50) of 11.5 μm was prepared.

EXAMPLE 2

(5) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that the contents of zirconium hydroxide and tantalum oxide were 2,000 ppm and 2,000 ppm, respectively.

EXAMPLE 3

(6) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that the contents of zirconium hydroxide and tantalum oxide were 2,000 ppm and 4,000 ppm, respectively.

EXAMPLE 4

(7) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that zirconium oxide (ZrO.sub.2, 2,000 ppm) and tantalum oxide (1,000 ppm) were used instead of zirconium hydroxide and tantalum oxide.

EXAMPLE 5

(8) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that zirconium hydroxide (Zr(OH).sub.4, 2,000 ppm) and cobalt hydroxide (Co(OH).sub.2, 2,000 ppm) were used instead of zirconium hydroxide and tantalum oxide.

EXAMPLE 6

(9) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that zirconium hydroxide (Zr(OH).sub.4, 2,000 ppm) and aluminum hydroxide (Al(OH).sub.3, 2,000 ppm) were used instead of zirconium hydroxide and tantalum oxide.

Comparative Example 1

(10) A non-doped positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) was obtained in the same manner as in Example 1, except that zirconium hydroxide and tantalum oxide were not added.

Comparative Example 2

(11) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with only one element was obtained in the same manner as in Example 1, except that zirconium hydroxide (2,000 ppm) was only used instead of zirconium hydroxide and tantalum oxide.

Comparative Example 3

(12) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with only one element was obtained in the same manner as in Example 1, except that only aluminum hydroxide (Al(OH).sub.3 2,000 ppm) was used instead of zirconium hydroxide and tantalum oxide.

Comparative Example 4

(13) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that zirconium hydroxide and tantalum oxide were respectively used in amounts of 2,000 ppm and 350 ppm.

Comparative Example 5

(14) A positive electrode active material powder (Ni:Co:Mn=6:2:2 in a mole ratio) doped with two elements was obtained in the same manner as in Example 1, except that zirconium hydroxide and tantalum oxide were respectively used in amounts of 2,000 ppm and 4,500 ppm.

Experimental Example 1: Analysis of Components of Positive Electrode Active Material

(15) The positive electrode active materials prepared according to Examples 1 to 6 and Comparative Example 4 and 5 were subjected to rietveld refinement by XRD to investigate the content of a dopant distributed in the lithium layer and the transition metal layer of each thereof. Results are summarized in Table 1 below.

(16) TABLE-US-00001 TABLE 1 Dopant type and content Transition Lithium layer metal layer (content, % (content, % Weight ratio of by weight) by weight) first doping First Second First Second element to doping doping doping doping second doping element element element element element Example 1 99.5 1.2 0.5 98.8 2 Example 2 99.4 1.4 0.6 98.6 1 Example 3 99.5 1.6 0.5 98.4 0.5 Example 4 99.3 1.3 0.7 98.7 2 Example 5 99.1 0.5 0.9 99.5 1 Example 6 99.2 0.5 0.8 99.5 1 Comparative 99.2 0.9 0.8 99.1 5.7 Example 4 Comparative 99.2 1.3 0.8 98.7 0.44 Example 5

Experimental Example 2: Cycle Characteristic Evaluation Experiments

(17) The positive electrode active material prepared according to each of Examples 1 to 6 and Comparative Examples 1 to 5, acetylene black, and polyvinylidene fluoride (PVdF) in a weight ratio of 97:2:1 were added to N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a positive electrode slurry. An aluminum (Al) thin film was coated with the positive electrode slurry in an amount of 400 mg/25 cm.sup.2 and then dried, thereby manufacturing a positive electrode. The positive electrode was roll-pressed.

(18) Natural graphite, carbon black as a conductive material, styrene-butadiene rubber, and carboxymethylcellulose (CMC) were mixed in a weight ratio of 96.3:1:1.5:1.2, thereby preparing a slurry for a negative electrode. A copper foil was coated with the negative electrode slurry in an amount of 350 mg/25 cm.sup.2 and then dried in a 100° C. dry oven for 10 hours, followed by being roll-pressed. As a result, a negative electrode was manufactured.

(19) A polyolefin separator was interposed between the positive electrode and the negative electrode, and then an electrolyte prepared by dissolving 1 M LiPF.sub.6 in a solvent, which was prepared by mixing ethylene carbonate(EC) and diethyl carbonate (DEC) in a volume ratio of 30:70, was injected. As a result, a coin-shaped battery was manufactured.

(20) The following electrochemical evaluation experiment was performed using the coin-shape battery.

(21) In particular, the lithium secondary battery was charged at 0.2 C 4.4 V CC-CV (0.05 C cut-off) and then discharged at 0.2 C 3.0 V CC in a 25° C. chamber to activate (form) the same. Subsequently, charging at 0.7 C CC-CV/discharging at 0.5 C CC was repeatedly performed 100 times (20 minutes left).

(22) Results are summarized in Table 2 below.

(23) TABLE-US-00002 TABLE 2 Capacity retention rate after 100 cycles (%) 25° C. 45° C. Example 1 95.3 94.4 Example 2 95.7 93.8 Example 3 95.4 93.5 Example 4 96.4 94.4 Example 5 95.9 94.1 Example 6 95.1 94.0 Comparative Example 1 92.6 91.1 Comparative Example 2 94.2 92.6 Comparative Example 3 94.0 91.9 Comparative Example 4 93.9 92.1 Comparative Example 5 93.2 91.3

(24) As shown in Table 2, it can be confirmed that the lithium secondary batteries respectively including the positive electrode active materials of Examples 1 to 6 including the lithium transition metal oxide; the first doping element (A); and the second doping element (B), wherein a weight ratio (A/B ratio) of the first doping element to the second doping element is 0.5 to 5, exhibit excellent capacity retention rates after 100 cycles and thus superior long lifespan characteristics at both 25° C. and 45° C. as compared to the lithium secondary batteries respectively including the positive electrode active materials of Comparative Examples 1 to 5.

(25) The positive electrode active material (Comparative Example 1) excluding a doping element, the positive electrode active material (Comparative Examples 2 and 3) including only one doping element, and the positive electrode active materials (Comparative Examples 4 and 5), wherein a weight ratio (A/B ratio) of the first doping element to the second doping element is outside 0.5 to 5, exhibit poor capacity retention rates after 100 cycles as compared to the positive electrode active materials of Examples 1 to 6. This result indicates that, for excellent long lifespan characteristics, both the first doping element and the second doping element should be included and a weight ratio (A/B ratio) of the first doping element to the second doping element should be within a predetermined content ratio.

Experimental Example 3: Thermal Stability Evaluation

(26) Secondary batteries were manufactured using the positive electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 5 and activated according to the method of Experimental Example 1, followed by fully charging once at 0.2 C 4.4 V CC-CV (0.05 C cut-off) to perform differential scanning calorimetry (DSC). Results are summarized in Table 3 below.

(27) In particular, the fully charged coin-shape battery was disassembled in a dry room, and then only the positive electrode was collected. The collected positive electrode was placed in a DSC pan, and 20 μl of an electrolytic solution was injected thereinto, followed by measuring thermal stability while elevating temperature from 35° C. to 600° C. at a temperature elevation rate of 10° C./min using DSC (TGA/DSC 1, manufactured by Mettler toledo).

(28) TABLE-US-00003 TABLE 3 Battery Main peak (° C.) Example 1 255 Example 2 254 Example 3 252 Example 4 254 Example 5 251 Example 6 253 Comparative Example 1 242 Comparative Example 2 246 Comparative Example 3 247 Comparative Example 4 248 Comparative Example 5 250

(29) Referring to Table 3, it can be confirmed that the main peak temperatures of Examples 1 to 6 are higher than those of Comparative Examples 1 to 5. This result indicates that, in the case of the secondary batteries including the positive electrode active material of the present invention, temperature increase due to heat generated when an exothermic reaction occurs in the secondary batteries due to an internal short circuit or an impact is delayed, whereby a series of exothermic reactions can be prevented.

(30) Accordingly, it was confirmed that both the first doping element and the second doping element should be included and a weight ratio (A/B ratio) of the first doping element to the second doping element should be within the predetermined content ratio so as to exhibit excellent long lifespan characteristics and superior thermal stability.