Method of Washing Positive Electrode Active Material, and Positive Electrode Active Material Prepared Thereby

Abstract

A method of washing a positive electrode active material includes (1) preparing a lithium composite transition metal oxide which contains Ni, Co and Mn, and has the Ni content of 60 mol % or more; (2) putting the lithium composite transition metal oxide into water; and (3) adding a weak acid to water to which the lithium composite transition metal oxide is added to adjust the pH to 7 to 10, wherein the acid is a weak acid.

Claims

1. A method of washing a positive electrode active material, comprising: (1) preparing a lithium composite transition metal oxide which comprises Ni, Co, and Mn, and an Ni content is 60 mol % or more; (2) putting the lithium composite transition metal oxide into water; and (3) adding a weak acid to the water to adjust the pH to 7 to 10.

2. The method of claim 1, wherein the Ni content is 80 mol % or more.

3. The method of claim 1, wherein the lithium composite transition metal oxide is represented by Formula 1 below
LiNi.sub.aCo.sub.bMn.sub.cM.sub.dO.sub.2   [Formula 1] wherein a≥0.6, 0<b<0.25, 0<c<0.25, 0<d<0.2, a+b+c+d=1, and M is one or more doping elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

4. The method of claim 1, wherein in step (2), an amount of the water is 30 parts by weigh to 150 parts by weight with respect to 100 parts by weight of the lithium composite transition metal oxide.

5. The method of claim 1, wherein the weak acid is one or more selected from the group consisting of phosphorous pentoxide (P.sub.2O.sub.5), phosphoric acid, acetic acid, oxalic acid, citric acid and boric acid.

6. The method of claim 1, wherein the adding a weak acid is performed by adding phosphorous pentoxide (P.sub.2O.sub.5) powder, citric acid powder or a mixture thereof to the water containing the lithium composite transition metal oxide.

7. A method of preparing a positive electrode active material, comprising the method of washing the positive electrode active material according to claim 1.

8. The method of claim 7, further comprising forming a coating layer on a surface of the lithium composite transition metal oxide.

9. A positive electrode active material, comprising: a lithium composite transition metal oxide and a lithium by-product present on the surface of the lithium composite transition metal oxide, wherein the lithium by-product is included at 0.3 mol % to 0.41 mol % with respect to a total mole number of the lithium composite transition metal oxide, and Li.sub.2CO.sub.3 and LiOH are included at a molar ratio of 1:1.8 to 1:3.

10. The positive electrode active material of claim 9, wherein the lithium composite transition metal oxide includes Li.sub.2CO.sub.3 on its surface at 0.05 mol % to 0.14 mol % with respect to the total mole number of the lithium composite transition metal oxide.

11. The positive electrode material of claim 9, wherein the lithium composite transition metal oxide includes LiOH on its surface at 0.15 mol % to 0.27 mol % with respect to the total mole number of the lithium composite transition metal oxide.

12. The positive electrode material of claim 9, further comprising a coating layer including one or more metal or metalloid elements on the surface of the lithium composite transition metal oxide.

13. A positive electrode for a lithium secondary battery, comprising the positive electrode active material of claim 9.

14. A lithium secondary battery, comprising the positive electrode for a lithium secondary battery of claim 13.

Description

MODE FOR CARRYING OUT THE INVENTION

[0108] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily carry out the present invention. However, the present invention may be implemented in a variety of different forms, and is not limited to the embodiments described herein.

EXAMPLE 1

[0109] 50 g of a LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2 (NMC(811)) positive electrode active material was put into 50 g of water and stirred for 2 minutes, and then 1.25 g of a 10% (w/w) P.sub.2O.sub.5 aqueous solution was added until the pH became 7. The resulting solution was further stirred for 2 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

EXAMPLE 2

[0110] The washing of a positive electrode active material was completed by the method described in Example 1, except that 25 g of water was used.

EXAMPLE 3

[0111] The washing of a positive electrode active material was completed by the method described in Example 1, except that 0.75 g of a 10% (w) P.sub.2O.sub.5 aqueous solution was added until the pH became 9.

EXAMPLE 4

[0112] 50 g of a NMC(811) positive electrode active material was put into 50 g of water and stirred for 2 minutes, and then a 20% (w/w) P.sub.2O.sub.5 aqueous solution was added until the pH became 7. The resulting solution was further stirred for 5 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

EXAMPLE 5

[0113] The washing of a positive electrode active material was completed by the method described in Example 1, except that a 0.1 mol % citric acid (C.sub.6H.sub.8O.sub.7) aqueous solution, instead of the 10% (w/w) P.sub.2O.sub.5 aqueous solution used in Example 1, was added until the pH became 7.

COMPARATIVE EXAMPLE 1

[0114] 50 g of a NMC(811) positive electrode active material was put into 50 g of water and stirred for 5 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

COMPARATIVE EXAMPLE 2

[0115] 50 g of a NMC(811) positive electrode active material was put into 25 g of water and stirred for 5 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

COMPARATIVE EXAMPLE 3

[0116] 50 g of a NMC(811) positive electrode active material was put into 50 g of water and stirred for 2 minutes, and a 20%(w/v) HCl aqueous solution was added until the pH became 7. The resulting solution was further stirred for 5 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

COMPARATIVE EXAMPLE 4

[0117] 50 g of a NMC(811) positive electrode active material was put into 50 g of water and stirred for 2 minutes, and a 3% (w/v) HCl aqueous solution was added until the pH became 7. The resulting solution was further stirred for 5 minutes, water was removed using a reduced pressure filter for 2 minutes, and then the resulting product was dried in a vacuum oven, thereby completing the washing of the positive electrode active material.

EXPERIMENTAL EXAMPLE 1

[0118] The content of a lithium by-product was measured by a Warder titration method of titrating amounts of an OH.sup.− ion and a CO.sub.3.sup.2− ion in the washed positive electrode active material in each of Examples 1 to 5 and Comparative Examples 1 to 4 using an 888 Titrando instrument (Mettler Toledo) and the result is shown in Table 1 below.

TABLE-US-00001 TABLE 1 Lithium by-product (mol %) Li.sub.2CO.sub.3 LiOH Total Li.sub.2CO.sub.3/LiOH Example 1 0.102 0.249 0.351 0.40 Example 2 0.133 0.268 0.401 0.49 Example 3 0.133 0.254 0.387 0.52 Example 4 0.095 0.238 0.323 0.35 Example 5 0.112 0.251 0.363 0.45 Comparative 0.155 0.266 0.421 0.58 Example 1 Comparative 0.186 0.306 0.492 0.61 Example 2 Comparative 0.068 0.192 0.260 0.35 Example 3 Comparative 0.083 0.218 0.301 0.38 Example 4

[0119] Referring to Table 1, compared to the washed positive electrode active materials in Comparative Examples 1 and 2, the washed positive electrode active materials in Examples 1 to 5 have a low amount of a lithium by-product, and have a higher amount of removed Li.sub.2CO.sub.3 than that of removed LiOH, confirming that the ratio of Li.sub.2CO.sub.3/LiOH is relatively small.

[0120] In Comparative Example 3, the amount of a residual lithium by-product was merely 0.260 mol %, which cannot satisfy an amount of residual lithium needed when a coating layer is formed on the positive electrode active material, and leads to degradation of capacity characteristics and lifetime characteristics due to corrosion (release of internal lithium to outside) of the surface of the positive electrode active material.

EXPERIMENTAL EXAMPLE 2

[0121] A positive electrode slurry was prepared by adding a mixture of the washed positive electrode active material according to each of Examples 1 to 4 and Comparative Examples to 3 with carbon black as a conductive material and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 97.5:1:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent.

[0122] Each of the prepared positive electrode slurries was applied on an aluminum (Al) thin film as a positive electrode current collector to a thickness of approximately 20 μm and dried, and then subjected to roll pressing, thereby manufacturing a positive electrode.

[0123] A coin-type half cell was manufactured by interposing a polyethylene porous membrane between the manufactured positive electrode and a lithium metal as a negative electrode, and injecting an electrolyte in which 1M LiPF.sub.6 was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 30:70.

[0124] The half cell manufactured as described above was charged once at 0.1 C. Afterward, the half cell was allowed to stand for 20 minutes and discharged at a constant current (CC) of 0.1 C to measure a charge capacity and a discharge capacity and initial efficiency, and then the result is shown in Table 2 below.

TABLE-US-00002 TABLE 2 Measurement of initial efficiency (0.1 C) Charge capacity Discharge Efficiency (mA) capacity (mAh/g) (%) Example 1 228.6 206.6 90.4 Example 2 227.0 205.5 90.5 Example 3 228.4 206.2 90.3 Example 4 226.6 205.4 90.7 Example 5 228.6 206.4 90.3 Comparative 227.3 206.0 90.6 Example 1 Comparative 223.5 203.7 91.1 Example 2 Comparative 222.6 200.1 89.9 Example 3 Comparative 223.7 202.8 90.6 Example 4

EXPERIMENTAL EXAMPLE 3

[0125] A coin-type half cell manufactured using each of the washed positive electrode active materials according to Example 4 and Comparative Example 3, which were manufactured in Experimental Example 2, was charged/discharged at 45° C. and 0.33 C, thereby measuring a discharge capacity, and also a voltage drop for 60 seconds of discharging was measured to measure cell resistance. The above-described process was repeatedly performed for 1 to 50 cycles, and the result is shown in Table 3 below.

TABLE-US-00003 TABLE 3 Evaluation of lifetime Evaluation of resistance characteristic (capacity) increase 30 cycles 50 cycles 30 cycles 50 cycles Example 1 97.8% 95.8%  45% 101% Example 2 96.4% 94.1%  57% 160% Example 3 97.2% 95.1%  51% 142% Example 4 97.0% 94.4%  60% 178% Example 5 97.5% 95.2%  49% 112% Comparative 94.3% 90.7% 152% 323% Example 1 Comparative 91.5% 84.1% 195% 423% Example 2 Comparative 93.8% 89.4% 134% 289% Example 3 Comparative 94.1% 91.2% 105% 223% Example 4

[0126] Referring to Table 2, the batteries manufactured using the washed positive electrode active materials according to Examples 1 to 5 and Comparative Examples 1 to 4 had no significant difference in initial efficiency.

[0127] However, referring to Table 3, compared to the batteries manufactured using the positive electrode active materials of Comparative Examples 1 and 2 to which an acid was not added and the batteries manufactured using positive electrode active materials of Comparative Examples 3 and 4 in which hydrochloric acid was used as an acid, it can be confirmed that the batteries manufactured using the positive electrode active materials of Examples 1 to 4 to which phosphoric acid was added in the process of pH adjustment and Example 5 in which citric acid was added have an excellent lifetime characteristics, and a low resistance increase rate according to an increased cycle number.

[0128] Meanwhile, when a strong acid, HCL, was used at a low concentration as in Comparative Example 4, as confirmed in Table 1, the total amount of a residual lithium by-product was as low as 0.301 mol %, the Li.sub.2CO.sub.3/LiOH ratio was relatively low, and this case was more effective than Comparative Examples 1 and 2 using water and Comparative Example 3 using a relatively high concentration of HCl. However, as confirmed in Table 3, when a strong acid was used at a low concentration, compared to Examples 1 to 5 using a weak acid, this is less effective in terms of lifetime characteristics and an increase in resistance according to an increased cycle number.

[0129] This is because, when a weak acid such as phosphoric acid or citric acid is used as an acid as in Examples 1 to 5, compared to the case using a strong acid, the lithium composite transition metal oxide was less corroded by an acid in the washing process.

[0130] In addition, it can be confirmed that, when a weak acid was used for pH adjustment as in Examples and Comparative Examples, compared to the case using a strong acid, it was easy to adjust pH, and thus the removal of the lithium by-product was more effectively and easily performed.