Positive electrode active material particle including core including lithium cobalt oxide and shell including lithium cobalt phosphate and preparation method thereof

Abstract

Provided is a positive electrode active material particle including a core that includes lithium cobalt oxide represented by the following Chemical Formula 1; and a shell that is located on the surface of the core and includes lithium cobalt phosphate represented by the following Chemical Formula 2, wherein the shell has a tetrahedral phase:
Li.sub.aCo.sub.(1-x)M.sub.xO.sub.2-yA.sub.y(1) wherein M is at least one of Ti, Mg, Zn, Si, Al, Zr, V, Mn, Nb, or Ni, A is oxygen-substitutional halogen, and 0.95a1.05, 0x0.2, 0y0.2, and 0x+y0.2,
Li.sub.bCoPO.sub.4(2) wherein 0b1.

Claims

1. A positive electrode active material particle comprising a core that includes lithium cobalt oxide represented by the following Chemical Formula 1; and a tetrahedral phase shell that is located on a surface of the core and includes lithium cobalt phosphate represented by the following Chemical Formula 2:
Li.sub.aCo.sub.(1-x)M.sub.xO.sub.2-yA.sub.y(1) wherein M is at least one of Ti, Mg, Zn, Si, Al, Zr, V, Mn, Nb, or Ni, A is oxygen-substitutional halogen, and 0.95a1.05, 0x0.2, 0y0.2, and 0x+y0.2,
Li.sub.bCoPO.sub.4(2) wherein 0b1.

2. The positive electrode active material particle of claim 1, wherein a weight of the tetrahedral phase shell relative to a weight of the core is 0.1% by weight to 3.0% by weight.

3. The positive electrode active material particle of claim 1, wherein an average particle diameter (D50) of the lithium cobalt oxide is 5 micrometer to 25 micrometer.

4. The positive electrode active material particle of claim 1, wherein a thickness of the tetrahedral phase shell is 1 to 1 m.

5. The positive electrode active material particle of claim 1, wherein the tetrahedral phase shell is formed on the area of 50% to 100% with respect to the surface area of the core.

6. The positive electrode active material particle of claim 1, wherein the lithium cobalt phosphate of the tetrahedral phase shell undergoes a phase transfer to an olivine phase at a temperature of 220 C. or higher.

7. The positive electrode active material particle of claim 6, wherein a tetrahedral phase of the lithium cobalt phosphate has higher ionic conductivity than the olivine phase.

8. The positive electrode active material particle of claim 1, wherein due to an electrochemical reaction by Co.sup.2+/Co.sup.3+ redox coupling at a potential of 4.9 V or higher, the lithium cobalt phosphate of the tetrahedral phase shell has a property of inhibiting an oxygen release phenomenon which is caused by lithium deintercalation in the lithium cobalt oxide of the core during overcharging.

9. The positive electrode active material particle of claim 1, wherein the surface of the tetrahedral phase shell is coated with Al.sub.2O.sub.3.

10. The positive electrode active material particle of claim 9, wherein a thickness of the Al.sub.2O.sub.3 coating is 5 nm to 100 nm.

11. A method of preparing the positive electrode active material particle of claim 1, the method comprising: preparing a mixed solution in which a cobalt source, a phosphorus source, and a lithium source are mixed; adding lithium cobalt oxide in a particle state to the mixed solution, followed by mixing to form a solution; and performing a hydrothermal reaction of the solution.

12. The method of claim 11, wherein the cobalt source is cobalt oxide or cobalt nitride.

13. The method of claim 11, wherein the phosphorus source is phosphoric acid or a salt thereof.

14. The method of claim 11, wherein the lithium source is lithium hydroxide or lithium carbonate.

15. The method of claim 11, wherein the lithium cobalt oxide is mixed so that a weight ratio of the lithium cobalt phosphate to the lithium cobalt oxide in the positive electrode active material particle prepared by the hydrothermal reaction becomes 0.1% by weight to 3.0% by weight.

16. The method of claim 11, wherein a shell of lithium cobalt phosphate having a tetrahedral phase is formed on a surface of the lithium cobalt oxide by the hydrothermal reaction.

17. The method of claim 11, wherein the hydrothermal reaction is performed at a temperature of 200 C. to 400 C. and a pressure of 1 bar to 10 bar for 5 minutes to 20 minutes.

18. A secondary battery comprising a positive electrode including the positive electrode active material particle of claim 1, a negative electrode, and an electrolyte.

19. The secondary battery of claim 18, wherein the electrolyte includes electrolyte additives, wherein the electrolyte additives include at least one of ethylene carbonate, vinyl acetate, vinyl ethylene carbonate, thiophene, 1,3-propane sultone, succinic anhydride, or dinitrile additive, and wherein the dinitrile additive is at least one of malononitrile, succinonitrile, glutaronitrile, adiponitrile, or phthalonitrile.

20. The secondary battery of claim 19, wherein the dinitrile additive is included in an amount of 5% by weight or less based on the total weight of the electrolyte.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates a comparison of crystal structures of lithium cobalt phosphate; and

(2) FIG. 2 shows ARC test results according to Experimental Example 2 of the present invention; and

(3) FIG. 3 shows safety test results according to Experimental Example 3 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereinafter, the present invention will be described in more detail with reference to Examples of the present invention. However, the scope of the present invention is not limited thereby.

Example 1

(5) LiCoO.sub.2, of which particle diameter was distributed in the range of 10 micrometer to 20 micrometer, was prepared in a particle state. 50 g of LiCoO.sub.2 in a particle state was mixed and dispersed in 50 ml of a solution in which 0.26 g of CH.sub.3COOLi.2H.sub.2O, 0.96 g of (CH.sub.3COO).sub.3Co.4H.sub.2O, and 0.41 g of (NH.sub.4).sub.2HPO.sub.4 were dissolved in DI water at a weight ratio of 1(source):30(DI water), and the temperature was raised to 220 C. for 10 minutes and maintained at a pressure of 5 bar for 5 minutes to carry out heat-treatment, thereby preparing positive electrode active material particles wherein a core and a shell were formed at a weight ratio of core:shell of 100:1 and lithium cobalt phosphate with a tetrahedral phase was coated on the surface of lithium cobalt oxide at a thickness of 10 nanometer to 100 nanometer.

Example 2

(6) Positive electrode active material particles were prepared in the same manner as in Example 2, except that a core and a shell were formed at a weight ratio of core:shell of 100:3.

Example 3

(7) Positive electrode active material particles were prepared in the same manner as in Example 2, except that a core and a shell were formed at a weight ratio of core:shell of 100:5.

Comparative Example 1

(8) Positive electrode active material particles were prepared in the same manner as in Example 1, except that a shell including Al.sub.2O.sub.3 was formed on the surface of lithium cobalt oxide using an Al(OH).sub.3 aqueous solution.

Comparative Example 2

(9) LiCoO.sub.2, of which particle diameter was distributed in the range of 10 micrometer to 20 micrometer, was prepared in a particle state. A solution was prepared by dissolving 1.26 g of Co(NO.sub.3).sub.3.9H.sub.2O, 0.41 g of (NH.sub.4).sub.2HPO.sub.4, and 0.07 g of LiOH in DI water. In 500 ml of this solution, 50 g of LiCoO.sub.2 in a particle state was mixed, dispersed, filtered, and dried under vacuum to recover powder. The recovered powder was heat-treated at 900 C. for 5 hours, thereby preparing positive electrode active material particles wherein a core and a shell were formed at a weight ratio of core:shell of 100:1 and lithium cobalt phosphate with an olivine phase was coated on the surface of lithium cobalt oxide at a thickness of 10 nanometer to 100 nanometer.

Experimental Example 1

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

(11) The coin-type half cells thus manufactured were used to examine output properties (rate properties) at 2.0 C/0.2 C, and the results are shown in Table 1 below. C-rate was measured by setting 1 C as 40 mA. Charge-discharge performance was conducted between 2.5 V and 4.5 V under the CC-CV (charge) and CC (discharge) mode.

(12) TABLE-US-00001 TABLE 1 Com- Com- Output parative parative property Example 1 Example 2 Example 3 Example 1 Example 2 2.0 C/0.2 C 94.5 94.2 94.0 93.8 89.0 discharge capacity (%)

(13) Referring to Table 1, the battery according to Example 1, wherein the lithium cobalt oxide coated with LiCoPO.sub.4 having the tetrahedral phase was used as the positive electrode active material, showed the most excellent output properties. Moreover, the battery according to Comparative Example 2, wherein the lithium cobalt oxide coated with LiCoPO.sub.4 having the olivine phase was used as the positive electrode active material, showed poorer output properties than the battery including lithium cobalt oxide coated with Al.sub.2O.sub.3.

(14) These results suggest that the tetrahedral phase shows excellent ionic conductivity, because it has a more open structure than the olivine phase.

Experimental Example 2

(15) Each of the positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 2 was used as a positive electrode active material, PVdF was used as a binder, and natural graphite was used as a conductive material. The positive electrode active material:binder:conductive material at a weight ratio of 96:2:2 were mixed well in NMP, and this mixture was applied to Al foil with a thickness of 20 m, and dried at 130 C. to manufacture a positive electrode.

(16) Artificial graphite, PVd, and carbon black at a weight ratio of 96:2:2 were mixed well in NMP, and this mixture was applied to Cu foil with a thickness of 20 m, and dried at 130 C. to manufacture a negative electrode.

(17) A separator (Celgard) was interposed between the positive electrode and the negative electrode to construct an electrode assembly, which was mounted in a pouch-type battery case, and an electrolyte containing 1 M LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 was used to manufacture each battery cell.

(18) An ARC test was performed by simultaneously measuring temperature change, voltage, and current change of the battery cells thus manufactured using an accelerating rate calorimeter under the charging condition of 4.4 V, and the results of measuring the onset time of the thermal runaway temperature, at which the temperature of the battery cell uncontrollably increased, are shown in the following Table 2 and FIG. 2.

(19) TABLE-US-00002 TABLE 2 Comparative ARC test Example 1 Comparative Example 1 Example 2 Onset time of 888 725 755 thermal runaway (min)

(20) Referring to Table 2 and FIG. 2, it was confirmed that the battery of Example 1 using the positive electrode active material particles according to the present invention showed slow thermal runaway, indicating more excellent high-temperature stability.

(21) It is because Al.sub.2O.sub.3 coating (Comparative Example 1) is sparsely formed in a particle shape on the surface of lithium cobalt oxide, lithium ions relatively freely move to the core-shell electrolyte, and thus there is little effect of reducing ionic conductivity in the shell, and as a result, thermal runaway occurs most rapidly, whereas LiCoPO.sub.4 coating, regardless of olivine phase or tetrahedral phase, basically has the effect of reducing lithium ionic conductivity in the shell, and thus the onset of thermal runaway is delayed. However, like the Al.sub.2O.sub.3 coating, the LiCoPO.sub.4 coating of the olivine phase (Comparative Example 2) is sparsely formed in a particle shape on the surface of lithium cobalt oxide, and thus the effect of reducing ionic conductivity in the shell is less than that of Example 1, whereas the LiCoPO.sub.4 coating of the tetrahedral phase (Example 1) forms a coating layer by hydrothermal reaction to form a much more compact coating layer than the wet coating method used in Comparative Example 1 and Comparative Example 2, and as a result, lithium ionic conductivity in the shell is greatly reduced and small leakage current flows in the internal short circuit.

Experimental Example 3

(22) Each of the positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 2 was used as a positive electrode active material, PVdF was used as a binder, and natural graphite was used as a conductive material. The positive electrode active material:binder:conductive material at a weight ratio of 96:2:2 were mixed well in NMP, and this mixture was applied to Al foil with a thickness of 20 m, and dried at 130 C. to manufacture a positive electrode.

(23) Artificial graphite, PVdF, and carbon black at a weight ratio of 96:2:2 were mixed well in NMP, and this mixture was applied to Cu foil with a thickness of 20 m, and dried at 130 C. to manufacture a negative electrode.

(24) A separator (Celgard) was interposed between the positive electrode and the negative electrode to construct an electrode assembly, which was mounted in a pouch-type battery case, and an electrolyte containing 1 M LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 was used to manufacture each battery cell.

(25) The battery cells thus manufactured was overcharged in CC/CV charging mode of an upper voltage limit of 4.55 V for 24 hours, and temperature changes of the cells were compared, and the results are shown in FIG. 3.

(26) Referring to FIG. 3, the cell temperature of the battery using the active material particles of Example 1 according to the present invention was maintained low, as compared with those of the batteries using the active material particles of Comparative Examples 1 and 2, indicating higher high-voltage stability.

(27) It is because the cobalt ions in the lithium cobalt phosphate with the tetrahedral phase has low coulombic efficiency in the oxidation-reduction reaction under high-voltage environment, as compared with Al.sub.2O.sub.3-coated LiCoO.sub.2 (Comparative Example 1) and lithium cobalt phosphate with the olivine phase (Comparative Example 2), and therefore, the battery operation is stopped by rapid depletion of the electrolyte before occurrence of an oxygen release phenomenon caused by excessive deintercalation of Li. Particularly, the Al.sub.2O.sub.3 coating of Comparative Example 1 does not participate in the oxidation-reduction reaction under high-voltage environment not to greatly contribute to prevention of overcharging.

(28) Although the present invention has been described in connection with the exemplary embodiments of the present invention, it will be apparent to those skilled in the art that various modifications and changes may be made thereto without departing from the scope and spirit of the invention.

(29) [ Industrial Availability]

(30) As described above, the positive electrode active material particles according to the present invention may include a core that includes lithium cobalt oxide; and a shell that is coated on the surface of the core and includes lithium cobalt phosphate with a tetrahedral phase. As compared with existing positive electrode active material particles including lithium cobalt oxide, the oxidation number of Co ion of the shell is maintained at +3 or less, and therefore, reactions with an electrolyte are remarkably reduced to prevent a problem of stability reduction such as a swelling phenomenon due to gas generation, release of Co ions is reduced due to a strong PO bond, and changes of the surface structure are inhibited under a high voltage due to a high operating voltage of lithium cobalt phosphate included in the shell, and therefore, structural stability of the positive electrode active material particles may be improved and lifespan property of a secondary battery may be also improved, and movement of lithium ions through the shell may be allowed to effectively prevent deterioration of a rate property of a secondary battery which is caused by formation of a coating layer.

(31) Further, since the shell of the positive electrode active material particles according to the present invention has a tetrahedral phase, it may have excellent ionic conductivity due to a more open structure with many voids, as compared with the lithium cobalt phosphate having an olivine phase. Further, since the shell may undergo a phase transfer to the olivine phase under high temperature environment, the ionic conductivity of the shell may be decreased, and thus smaller leakage current flows in the internal short circuit to improve the high-temperature stability. Furthermore, since the shell has low coulombic efficiency in the oxidation-reduction reaction of cobalt ions under high-voltage environment, as compared with lithium cobalt phosphate having the olivine phase, the battery operation is stopped by rapid depletion of the electrolyte before occurrence of an oxygen release phenomenon caused by excessive deintercalation of Li. There is also an effect of improving safety.