Method of Preparing Positive Electrode Active Material

Abstract

A method of preparing a positive electrode active material that includes introducing a reaction mixture including a lithium source material and a nickel-manganese-cobalt precursor into a continuous firing furnace and subjecting the same to primary heat treatment, thereby preparing a fired mixture; subjecting the fired mixture to pulverization or size classification; and introducing the fired mixture having been pulverized or size-classified into a rotary kiln and subjecting the same to secondary heat treatment, thereby forming a lithium nickel manganese cobalt-based positive electrode active material.

Claims

1. A method of preparing a positive electrode active material comprising: introducing a reaction mixture including a lithium source material and a nickel-manganese-cobalt precursor into a continuous firing furnace and performing primary heat treatment, thereby preparing a fired mixture; subjecting the fired mixture to pulverization or size classification; and introducing the fired mixture having been pulverized or size-classified into a rotary kiln and performing secondary heat treatment, thereby forming a lithium nickel manganese cobalt-based positive electrode active material.

2. The method of claim 1, wherein the continuous firing furnace is a roller hearth kiln.

3. The method of claim 1, wherein the rotary kiln includes a rotary cylindrical tube having an inlet portion and an outlet portion, wherein the rotary cylindrical tube is disposed such that it is inclined at an angle of 2 to 8 and the outlet portion is not level with the inlet portion.

4. The method of claim 3, wherein the rotary cylindrical tube is rotated at a rate of 0.5 rpm to 8 rpm during the secondary heat treatment.

5. The method of claim 3, wherein the rotary cylindrical tube has the length-to-diameter ratio of 2 to 20.

6. The method of claim 1, wherein the primary heat treatment is performed in a temperature range of 300 C. to 900 C.

7. The method of claim 1, wherein the primary heat treatment is performed under an oxygen atmosphere or an air atmosphere.

8. The method of claim 1, wherein the secondary heat treatment is performed in a temperature range of 600 C. to 1,000 C.

9. The method of claim 1, wherein the secondary heat treatment is performed under an atmosphere in which oxygen partial pressure is no more than 20%.

10. The method of claim 1, wherein the secondary heat treatment is performed under a nitrogen atmosphere or a vacuum atmosphere.

11. The method of claim 1, wherein the primary heat treatment is performed for 3 hours to 15 hours.

12. The method of claim 1, wherein the secondary heat treatment is performed for 1 hour to 7 hours.

13. The method of claim 1, wherein the lithium nickel manganese cobalt-based positive electrode active material is represented by the following Chemical Formula 1:
Li.sub.1+x[Ni.sub.aMn.sub.bCo.sub.cM.sup.1.sub.1abc].sub.1xO.sub.2[Chemical Formula 1] wherein, in Chemical Formula 1, 0.2x0.2, 0<a<1, 0<b<1, 0<c<1, and M.sup.1 is one or more 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.

Description

EXAMPLE 1

[0076] LiOH and Ni.sub.0.8Co.sub.0.1Mn.sub.0.1(OH).sub.2 were mixed in a mixer such that the lithium-to-transition metal atomic ratio was 1.01:1, thereby preparing a reaction mixture. 7,500 g (volume: 4,000 ml) of the reaction mixture was introduced into a roller hearth kiln (with a closeable entrance and exit, internal volume: 1000 m.sup.3, roller speed: 9 mm/min, oxygen feed rate: 650 m.sup.3/min) and subjected to primary heat treatment for 10 hours at 650 C. under an oxygen atmosphere, thereby preparing a fired mixture, which was then cooled. The prepared fired mixture had a volume of 1,800 ml and a weight of about 5,000 g.

[0077] The fired mixture weighing 5,000 g (volume: 1,800 ml) was subjected to crushing and sieving, and the crushed fired mixture was introduced into a rotary kiln (quartz tube, diameter: 0.3 m, length: 3 m, rotational speed: 2 rpm, raw material feed rate: 2,000 g/h, inclination: 5) and subjected to secondary heat treatment for three hours at 760 C. under a nitrogen atmosphere, thereby preparing the positive electrode active material of Example 1 (weight: 5,000 g, volume: 1,800 ml).

EXAMPLE 2

[0078] Li.sub.2CO.sub.3 and Ni.sub.0.6Co.sub.0.2Mn.sub.0.2(OH).sub.2 were mixed in a mixer such that the lithium-to-transition metal atomic ratio was 1.07:1, thereby preparing a reaction mixture. 7,500 g (volume: 4,000 ml) of the reaction mixture was introduced into the same roller hearth kiln as in Example 1 and subjected to primary heat treatment for 10 hours at 750 C. under an oxygen atmosphere, thereby preparing a fired mixture, which was then cooled. The prepared fired mixture had a volume of 1,800 ml and a weight of about 5,000 g.

[0079] The fired mixture weighing 5,000 g (volume: 1,800 ml) was subjected to crushing and sieving, and the crushed fired mixture was introduced into the same rotary kiln as in Example 1 and subjected to secondary heat treatment for three hours at 820 C. under a nitrogen atmosphere, thereby preparing the positive electrode active material of Example 2 (weight: 5,000 g, volume: 1,800 ml).

Comparative Example 1

[0080] LiOH and Ni.sub.0.8Co.sub.0.1Mn.sub.0.1(OH).sub.2 were mixed in a mixer such that the lithium-to-transition metal atomic ratio was 1.01:1, thereby preparing a reaction mixture. 7,500 g (volume: 4,000 ml) of the reaction mixture was introduced into the same roller hearth kiln as in Example 1 and subjected to a 13-hour heat treatment at 650 C. under an oxygen atmosphere, cooling, crushing, and sieving, thereby preparing the positive electrode active material of Comparative Example 1 (weight: 5,000 g, volume: 1,700 ml).

Comparative Example 2

[0081] LiOH and Ni.sub.0.8Co.sub.0.1Mn.sub.0.1(OH).sub.2 were mixed in a mixer such that the lithium-to-transition metal atomic ratio was 1.01:1, thereby preparing a reaction mixture. 7,500 g (volume: 4,000 ml) of the reaction mixture was introduced into the same rotary kiln as in Example 1 and subjected to a 13-hour heat treatment at 650 C. under an oxygen atmosphere, cooling, crushing, and sieving, thereby preparing the positive electrode active material of Comparative Example 2. The positive electrode active material prepared by the above-describe process had a weight of 500 g and a volume of 180 ml. In Comparative Example 2, the weight of the prepared positive electrode active material was significantly lower than the weight of the introduced reaction mixture, which is considered to be due to the clogging of the inlet due to the agglomeration of the reaction mixture caused by the moisture contained in LiOH and the like.

Experimental Example 1: Determination of Crystal Size and Cation Mixing in Positive Electrode Active Material

[0082] The crystal size and cation mixing of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were determined by XRD refinement analysis, results of which are shown in the following Table 1. Cation mixing is a phenomenon in which some Ni.sup.2+ ions switch positions with some Li.sup.+ ions due to the similarity in ionic sizes thereof, resulting in the presence of Ni atoms in the Li layer rather than in the transition metal layer where they normally belong. A lower cation mixing ratio can be considered to indicate more successful synthesis of a positive electrode active material.

TABLE-US-00001 TABLE 1 Cation mixing Crystal size (nm) ratio (%) Example 1 135.2 1.12 Example 2 120.4 1.03 Comparative Example 1 110.2 2.68 Comparative Example 2 84.3 15.63

[0083] Referring to Table 1, the positive electrode active materials of Examples 1 and 2, which were prepared by performing primary and secondary heat-treatment processes in series in a continuous firing furnace and a rotary kiln, were evaluated to have a larger crystal size and a lower cation mixing ratio than those of the comparative examples.

[0084] On the other hand, in the case of Comparative Example 1, where only a roller hearth kiln was used, a higher cation mixing ratio, and accordingly, large quality inconsistency were exhibited despite the same duration of heat treatment as in the examples. Likewise, in the case of Comparative Example 2, where only a rotary kiln was used, it was evaluated that the cation mixing ratio was very high because the occurrence of agglomeration hindered sufficient firing of the mixture.

Experimental Example 2: Evaluation of Lifetime Characteristics and Direct Current Resistance Increase Rate

[0085] Each one of the positive electrode active materials prepared according to Examples 1 and 2 and Comparative Examples 1 and 2 was dispersed, along with a PVdF binder and carbon black at a weight ratio of 97.5:1.5:1.0, in an NMP solution to prepare a slurry. The slurry was then applied on an Al current collector and later rolled by a roll press, thereby producing a positive electrode.

[0086] Meanwhile, a natural graphite negative electrode active material, a carbon black conductive material, and a PVdF binder, at a weight ratio of 95.6:1.0:3.4, were mixed in an N-methylpyrrolidone solvent to prepare a composition for forming a negative electrode, which was then applied on a Cu current collector, thereby producing a negative electrode.

[0087] A porous polyethylene separator was interposed between the positive electrode and the negative electrode produced as described above, thereby producing an electrode assembly. After the electrode assembly was placed in a case, a liquid electrolyte was injected into the case, thereby manufacturing a coin cell lithium secondary battery. Here, the liquid electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF.sub.6) at a concentration of 0.7 M in an organic solvent composed of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC-to-EMC volumetric mixing ratio=3:7).

[0088] The coin cell prepared as described above was subjected to two cycles of charging and discharging at room temperature under the conditions of an end-of-charge voltage of 4.25 V, an end-of-discharge voltage of 2.5 V, and a rate of 0.1 C/0.1 C, and then the initial charge-discharge capacity and initial resistance were determined. Afterwards, the capacity retention rate (in %) and direct current resistance (DCR) increase rate (in %) after 50 cycles were measured while subjecting the coin cell to charging and discharging at 45 C. under the conditions of an end-of-charge voltage of 4.25 V, an end-of-discharge voltage of 2.5 V, and a rate of 0.3 C/0.3 C. The measurement results are shown in Table 2.

TABLE-US-00002 TABLE 2 Initial Capacity retention DCR increase charge/discharge Initial rate after 50 cycles rate after 50 Classification capacity (mA/g) resistance () (%) cycles (%) Example 1 227.6/206.3 13.6 95.6 158 Example 2 195.7/180.5 14.4 97.7 146 Comparative 227.6/206.3 16.2 93.4 236 Example 1 Comparative 185.2/143.5 31.5 42.5 573 Example 2

[0089] Referring to Table 2, the positive electrode active materials of Examples 1 and 2 prepared by performing primary and secondary heat-treatment processes in series in a continuous firing furnace and a rotary kiln were evaluated to exhibit excellent capacity retention, low resistance, and a low DCR increase rate compared to those of Comparative Example 1 where only a roller hearth kiln was used and Comparative Example 2 where only a rotary kiln was used.

Experimental Example 3: Measurement of Amount of Residual Lithium in Prepared Positive Electrode Active Material

[0090] The amount of residual lithium in the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 was measured using an 888 Titrando instrument manufactured by Metrohm AG, by the Warder titration method in which the amount of OH.sup. ions and CO.sub.3.sup.2 ions is determined by titration. The ratio (in mol %) of the number of moles of unreacted residual lithium to the total number of moles of lithium introduced in preparation of the positive electrode active materials determined by the above-described method is shown in the following Table 3.

TABLE-US-00003 TABLE 3 Li.sub.2CO.sub.3 LiOH Total Classification (mol %) (mol %) (mol %) Example 1 0.343 0.481 0.824 Example 2 0.629 0.356 0.985 Comparative 0.493 0.605 1.098 Example 1 Comparative 1.158 1.323 2.481 Example 2

[0091] Referring to Table 3, it can be seen that the amount of residual lithium was significantly low in the positive electrode active materials of Examples 1 and 2 prepared by performing primary and secondary heat-treatment processes in series in a continuous firing furnace and a rotary kiln as compared with the comparative examples.

[0092] On the other hand, in the case of Comparative Example 1 where only a roller hearth kiln was used and Comparative Example 2 where only a rotary kiln was used, it was evaluated that the amount of unreacted residual lithium was large because sufficient firing could not be achieved despite the same duration of heat treatment as in the examples.