Metal-doped positive electrode active material for high voltage

Abstract

Provided are a metal element-doped positive electrode active material for a high voltage and a preparation method thereof. The positive electrode active material may include a lithium cobalt oxide having a layered crystal structure; and a metal element (M) incorporated into the lithium cobalt oxide in an amount of 0.2 parts by weight to 1 part by weight with respect to 100 parts by weight of the lithium cobalt oxide, wherein the metal element (M) does not form a chemical bond with the elements of the lithium cobalt oxide, and wherein the layered crystal structure in maintained at a positive electrode potential of more than 4.5 V (based on Li potential) when fully charged.

Claims

1. A positive electrode active material, comprising: M-doped lithium cobalt oxide particles, the particles comprising: a lithium cobalt oxide represented by the following Formula 1, the lithium cobalt oxide has a layered crystal structure; and a metal element (M) which is incorporated into the lithium cobalt oxide and does not form a chemical bond with the elements of the lithium cobalt oxide, wherein M is present in an amount of 0.3 parts by weight to 0.9 part by weight with respect to 100 parts by weight of the lithium cobalt oxide, wherein M is Mg, wherein the layered crystal structure is maintained at a positive electrode potential of more than 4.5 V based on Li potential:
Li.sub.1+xCo.sub.1-xO.sub.2  [Formula 1] wherein x satisfies 0≤x≤0.2.

2. The positive electrode active material of claim 1, wherein the layered crystal structure is maintained when the positive electrode potential ranges from more than 4.5 V to 4.8 V or less.

3. The positive electrode active material of claim 1, wherein a peak intensity of an X-ray diffraction (XRD) peak corresponding to the (003) plane of the layered crystal structure when measured at a positive electrode potential of 4.55 V is 30% or more of a peak intensity of the XRD peak corresponding to the (003) plane when measured at a positive electrode potential of 4.50 V.

4. The positive electrode active material of claim 1, wherein a peak intensity of an X-ray diffraction (XRD) peak corresponding to the (003) plane of the layered crystal structure when measured at a positive electrode potential of 4.55 V is 40% or more of a peak intensity of the XRD peak corresponding to the (003) plane of the layered crystal structure when measured at a positive electrode potential of 4.50 V.

5. The positive electrode active material of claim 1, further comprising: a coating layer which is formed on the M-doped lithium cobalt oxide particles, wherein the coating layer includes one or more metal oxides selected from the group consisting of Al.sub.2O.sub.3, MgO, ZrO, Li.sub.2ZrO.sub.3, and TiO.sub.2.

6. A lithium secondary battery comprising a positive electrode including the positive electrode active material of claim 1; a negative electrode; and an electrolyte.

7. The lithium secondary battery of claim 6, wherein at a positive electrode potential of 4.55 V (based on Li potential) when fully charged, a capacity retention ratio after 50 cycles of charge/discharge is 85% or more of an initial capacity.

8. A method of preparing the positive electrode active material of claim 1, the method comprising: dry-mixing a cobalt acid salt, a lithium precursor, and a doping precursor; and sintering the mixture at a temperature of 900° C. or higher to form the M-doped lithium oxide particles.

9. The method of claim 8, further comprising: forming a metal oxide coating layer on the M-doped lithium cobalt oxide particles.

10. The method of claim 8, wherein the sintering temperature ranges from 900° C. to 1200° C.

11. The method of claim 8, where the sintering is performed for a time period ranging from 4 hours to 12 hours.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 is a graph showing a capacity retention ratio of a coin-type half cell including a lithium cobalt oxide doped with Mg of 1,000 ppm or 3,000 ppm, when charged at an upper voltage limit of 4.5 V;

(3) FIG. 2 is a graph showing a capacity retention ratio of the coin-type half cell including the lithium cobalt oxide doped with Mg of 1,000 ppm or 3,000 ppm, when charged at an upper voltage limit of 4.55 V;

(4) FIG. 3 is a graph showing a capacity retention ratio of a coin-type half cell including a lithium cobalt oxide doped with Al of 1,000 ppm or 3,000 ppm, when charged at an upper voltage limit of 4.5 V;

(5) FIG. 4 is a graph showing a capacity retention ratio of the coin-type half cell including the lithium cobalt oxide doped with Al of 1,000 ppm or 3,000 ppm, when charged at an upper voltage limit of 4.55 V;

(6) FIG. 5 shows the result of TOF-SIMS (Time of flight secondary ion mass spectrometry) analysis of a positive electrode active material of Example 1; and

(7) FIG. 6 is an XRD graph showing a peak intensity of a coin-type half cell including the positive electrode active material of Example 1 or Comparative Example 1, as measured while increasing an upper limit voltage from 4.5 V to 4.54 V at 0.01 V intervals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(8) Hereinafter, embodiments of the present invention will be described with reference to drawings. However, the embodiments are provided only for better understanding, but the scope of the present invention is not limited thereby.

(9) Preparation of Positive Electrode Active Material

Example 1

(10) 0.294 g of MgO, 80.27 g of Co.sub.3O.sub.4, and 36.94 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Mg was included in an amount of 3,000 ppm, based on the total weight of a positive electrode active material. Then, the mixture was sintered in a furnace at 1,050° C. for 10 hours to prepare a Mg-doped lithium cobalt oxide.

Example 2

(11) 0.147 g of Al.sub.2O.sub.3, 80.27 g of Co.sub.3O.sub.4, and 36.94 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Al was included in an amount of 3,000 ppm, based on the total weight of a positive electrode active material. Then, the mixture was sintered in a furnace at 1,050° C. for 10 hours to prepare an Al-doped lithium cobalt oxide.

Example 3

(12) 0.245 g of MgO, 80.27 g of Co.sub.3O.sub.4, and 36.94 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Mg was included in an amount of 5,000 ppm, based on the total weight of a positive electrode active material. Then, the mixture was sintered in a furnace at 1,050° C. for 10 hours to prepare an Mg-doped lithium cobalt oxide.

Example 4

(13) 0.343 g of MgO, 80.27 g of Co.sub.3O.sub.4, and 36.94 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Mg was included in an amount of 7,000 ppm, based on the total weight of a positive electrode active material. Then, the mixture was sintered in a furnace at 1,050° C. for 10 hours to prepare an Mg-doped lithium cobalt oxide.

Example 5

(14) 0.441 g of MgO, 80.27 g of Co.sub.3O.sub.4, and 36.94 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Mg was included in an amount of 9,000 ppm, based on the total weight of a positive electrode active material. Then, the mixture was sintered in a furnace at 1,050° C. for 10 hours to prepare an Mg-doped lithium cobalt oxide.

Example 6

(15) To form a coating layer on the Mg-doped lithium cobalt oxide prepared in Example 1, 500 ppm of Al.sub.2O.sub.3 was dry-mixed with lithium cobalt oxide particles to coat the lithium cobalt oxide particles, and then sintered in a furnace at 700° C. for 5 hours to prepare a positive electrode active material on which the coating layer was formed.

Comparative Example 1

(16) A Mg-doped lithium cobalt oxide was prepared in the same manner as in Example 1, except that Mg was included in an amount of 1,000 ppm, based on the total weight of the positive electrode active material.

Comparative Example 2

(17) An Al-doped lithium cobalt oxide was prepared in the same manner as in Example 2, except that Al was included in an amount of 1,000 ppm, based on the total weight of the positive electrode active material.

Comparative Example 3

(18) A Mg-doped lithium cobalt oxide was prepared in the same manner as in Example 1, except that Mg was included in an amount of 10,000 ppm, based on the total weight of the positive electrode active material.

(19) Manufacture of Secondary Battery

(20) Each of the metal-doped positive electrode active materials prepared in Examples 1 to 6, and Comparative Examples 1 to 3, a PVdF binder, and a natural graphite conductive material were mixed well at a weight ratio of 96:2:2 (positive electrode active material: binder: conductive material) in NMP, and then applied to an Al foil having a thickness of 20 μm, and dried at 130° C. to manufacture each positive electrode. As a negative electrode, a lithium foil was used, an electrolyte containing 1M LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 was used to manufacture each coin-type half cell.

EXPERIMENTAL EXAMPLE 1

Analysis of Capacity Retention Ratio

(21) Each of the coin-type half cells manufactured above was charged at 0.5 C to an upper voltage limit of 4.5 V or 4.55 V at 25° C., and then discharged at 1.0 C to a lower voltage limit of 3 V. This procedure was regarded as 1 cycle, and capacity retention ratios after 50 cycles were measured. The results are shown in the following Table 1 and FIGS. 1 to 4. Table 2 shows initial capacities when charged at 4.5 V and 4.55 V, respectively.

(22) TABLE-US-00001 TABLE 1 Capacity retention ratio(%) after 50 cycles Upper voltage limit 4.5 V 4.55 V Example 1 72 89 (Mg, 3,000 ppm) Example 2 93 89 (Al, 3,000 ppm) Example 3 70 89 (Mg, 5,000 ppm) Example 4 72 90 (Mg, 7,000 ppm) Example 5 72 91 (Mg. 9,000 ppm) Example 6 80 92 (Example 1 + formation of coating layer) Comparative Example 1 92 73 (Mg, 1,000 ppm) Comparative Example 2 96 73 (Al, 1,000 ppm) Comparative Example 3 69 85 (Mg, 10,000 ppm)

(23) TABLE-US-00002 TABLE 2 4.5 V 4.55 V Initial Initial charge charge capacity Initial capacity Initial Upper voltage limit (mAh/g) efficiency (mAh/g) efficiency Example 1 195 95.3 215 95.2 (Mg, 3,000 ppm) Example 2 193 94.7 212 94.5 (Al, 3,000 ppm) Example 3 193 94.5 213 94.4 (Mg, 5,000 ppm) Example 4 194 94.0 212 93.8 (Mg, 7,000 ppm) Example 5 194 93.2 208 93.3 (Mg. 9,000 ppm) Example 6 195 97.0 215 96.7 (formation of coating layer) Comparative Example 1 196 97.3 220 97.2 (Mg, 1,000 ppm) Comparative Example 2 196 97.2 218 97.3 (Al, 1,000 ppm) Comparative Example 3 190 92.5 205 92.0 (Mg, 10,000 ppm)

(24) Referring to Table 1, FIGS. 1 to 4, and Table 2, when 3,000 ppm of Mg or Al was doped as in Examples 1 or 2, the capacity retention ratio was lower than that of Comparative Example 1 or 2 doped with 1,000 ppm of Mg or Al, after 50 cycles of charge/discharge at 4.5 V. FIG. 1 shows that when the positive electrode active material of Example 1 was used, the capacity retention ratio was higher than that of Comparative Example 1 up to about 30 cycles, but the capacity retention ratio of Example 1 was rapidly decreased over 30 cycles.

(25) However, after 50 cycles of charge/discharge at 4.55 V, Examples 1 and 2 having the higher doping contents showed remarkably higher capacity retention ratios.

(26) Example 2 having the Al doping content of 3,000 ppm showed a lower capacity retention ratio at 4.5 V than 4.55 V. However, its capacity retention ratio was decreased at 4.5 V whereas its capacity retention ratio was increased at 4.55 V, as compared with Comparative Example 1 having the Al doping content of 1,000 ppm.

(27) Meanwhile, Comparative Example 2 having the Mg doping content of 10,000 ppm showed a low capacity retention ratio, as compared with Examples.

(28) This is because the positive electrode active material undergoes a reversible phase transition during charge/discharge processes, and the reversibility of the phase transition is reduced with increasing charge potential, leading to reduction in the capacity. However, when lithium cobalt oxide is doped with a metal within a particular range in order to prepare a high potential lithium cobalt oxide with high capacity, an irreversible phase transition may be minimized to prevent the decrease of the capacity retention ratio. However, when the doping metal content is too small as in Comparative Examples 1 and 2, it is difficult to obtain the above effect, and thus the capacity retention ratio is rapidly decreased during charge/discharge processes at 4.55 V.

(29) Accordingly, when lithium cobalt oxide is doped with Mg or Al within a particular content range, the phase transition at a high potential may be minimized to exhibit the high capacity retention ratio.

(30) Further, as the content of the doping element is increased, the initial efficiency is reduced at a voltage of higher than 4.5 V, and thus it is expected to easily balance the positive electrode and the negative electrode at the time of manufacturing a battery cell. It is also expected that the remaining amount of the positive electrode active material is decreased, which will help to reduce the production cost of battery cells. Accordingly, it is expected that materials suitable for operating voltage conditions of battery cells may be designed, performances required for the battery cells may be realized, and the production cost may be greatly reduced.

(31) Meanwhile, at 4.55 V, as the doping amount of Mg was increased, the lifespan was slightly improved, which is presumably attributed to structural stabilization. However, at 4.55 V, as the doping amount of Mg was increased, the initial efficiency was decreased. It is expected that when the doping amount of Mg is increased to 10,000 ppm or more, the resistance increase of the positive electrode active material, rather than structural stability, may be further promoted, capacity may be deteriorated, and the lifespan stability may be reduced.

(32) Meanwhile, after 50 cycles of charge/discharge at 4.55 V, the doped lithium cobalt oxide having the coating layer of Example 6 exhibited a high capacity retention ratio, as compared with Example 1 having no coating layer.

(33) Accordingly, it can be seen that the positive electrode active materials of Examples exhibit improved lifespan characteristics at a potential of higher than 4.5 V.

EXPERIMENTAL EXAMPLE 2

TOF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) Analysis

(34) The result of analyzing the active material obtained in Example 2 by TOF-SIMS (Time of flight secondary ion mass spectrometry) is shown in FIG. 5.

(35) Referring to FIG. 5, it was confirmed that the doped metal element Al was distributed on the lithium cobalt oxide, and in particular, the doped metal element Al was predominantly distributed in the region close to the surface of the lithium cobalt oxide. These analysis results indicate that the metal element Al was doped into the lithium cobalt oxide to form a physical/crystallographic connection without complex formation via a chemical bond with lithium cobalt oxide.

EXPERIMENTAL EXAMPLE 3

XRD Analysis

(36) In order to examine changes in the crystal structures of the lithium cobalt oxides of Example 1 and Comparative Example 1, coin-type half cells including the same were manufactured, and peak intensity was measured while increasing the upper limit voltage from 4.5 V to 4.55 V at 0.01 V intervals. The XRD graphs (2-theta-scale) thus measured are shown in FIG. 6.

(37) Referring to FIG. 6, the positive electrode active material of Example 1 showed a peak corresponding to the (003) plane in the crystal structure of lithium cobalt oxide in the range of 23 to 24 degree when measured at a positive electrode potential of 4.40 V to 4.55 V and a peak in the range of 24 to 25 degree when measured at a positive electrode potential of 4.55 V. When the charge potential of the positive electrode active material is 4.54 V or higher, the crystal structure undergoes a phase transition to another phase, and when the charge potential is 4.55 V, a new peak indicating the crystal structure after phase transition is observed in the range of 24 to 25 degree. However, the peak corresponding to the (003) plane was observed continuously, indicating that the phase transition reversibly occurred. Therefore, even during charge/discharge processes, the capacity retention ratio may be maintained.

(38) Specifically, FIG. 6 shows peaks before/after phase transition at each voltage. A peak intensity of a peak corresponding to the (003) plane when measured at a positive electrode potential of 4.55 V was 30% or more of a peak intensity of the peak corresponding to the (003) plane when measured at 4.50 V. The metal-doped lithium cobalt oxides minimized the phase transition to exhibit high capacity retention ratios, thereby exhibiting remarkably improved lifespan characteristics.

(39) However, the positive electrode active material of Comparative Example 1 showed a remarkably low peak intensity of the peak corresponding to the (003) plane when measured at 4.55 V, and a peak indicating the crystal structure after phase transition was higher than a peak indicating the crystal structure before phase transition, suggesting that reversibility of the phase transition was remarkably decreased. Accordingly, it can be seen that when the doping metal content is low, lifespan characteristics may be greatly deteriorated.

(40) It will be apparent to those skilled in the art to which the present invention pertains that various modifications and changes may be made thereto without departing from the scope of the invention.