Cathode active material for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to an anode active material for lithium secondary battery and a lithium secondary battery including the same, and more specifically it relates to an anode active material for lithium secondary battery in which the a lithium ion diffusion path in the primary particles is formed to exhibit specific directivity, and a lithium secondary battery including the same. The cathode active material for lithium secondary battery of the present invention has a lithium ion diffusion path exhibiting specific directivity in the primary particles and the secondary particles, thus not only the conduction velocity of the lithium ion is fast and the lithium ion conductivity is high but also the cycle characteristics are improved as the crystal structure hardly collapses despite repeated charging and discharging.

Claims

1. A cathode active material for lithium secondary battery, which is a cathode active material having a layered structure containing a transition metal, and comprises secondary particles formed by aggregation of primary particles, wherein an aspect ratio of the primary particles is 1 or more, and an a-axis direction that is a lithium ion diffusion path in the primary particles is formed to be parallel to a longer side of the primary particles, wherein the a-axis direction exhibits directivity in a center direction of the secondary particles, and wherein the lithium ion diffusion path in the primary particles is formed in a direction towards a center of the entirety of the particles, such that a lithium ion diffusion path from a surface to a center of the secondary particles has a one-dimensional or two-dimensional tunnel structure.

2. The cathode active material for lithium secondary battery according to claim 1, wherein the a-axis direction forms an angle of ±40° with a connecting line connecting a surface of the secondary particles with a center of the secondary particles.

3. The cathode active material for lithium secondary battery according to claim 1, wherein an area occupied by the primary particles having the lithium ion diffusion path, which forms an angle of ±40° with a connecting line connecting a surface of the secondary particles with a center of the secondary particles, is 10% or more of an area of the secondary particles.

4. The cathode active material for lithium secondary battery according to claim 1, wherein an area occupied by the primary particles having the aspect ratio of 1 or more and the lithium ion diffusion path in the particles formed to be parallel to a major axis of the particles among the primary particles is 40% or more of an area of the secondary particles.

5. The cathode active material for lithium secondary battery according to claim 1, wherein the secondary particles are represented by the following Formula 1 and a concentration of transition metal is constant in the entire particles:
Li.sub.xNi.sub.1−a−b−cCo.sub.aMn.sub.bMe.sub.cO.sub.2−yX.sub.y  <Formula 1> (in Formula 1, 0.9≦x≦1.15, 0≦a≦0.5, 0≦b≦0.65, 0≦c≦0.15, 0≦y≦0.1, Me is at least one or more elements selected from the group consisting of Al, Mg, B, P, Ti, Si, Zr, Ba, and any combination thereof; and X is at least one or more elements or molecules selected from anions of F, BO.sub.3, and PO.sub.4).

6. A lithium secondary battery comprising the cathode active material for lithium secondary battery according to claim 1.

7. A lithium secondary battery comprising the cathode active material for lithium secondary battery according to claim 2.

8. A lithium secondary battery comprising the cathode active material for lithium secondary battery according to claim 3.

9. A lithium secondary battery comprising the cathode active material for lithium secondary battery according to claim 4.

10. A lithium secondary battery comprising the cathode active material for lithium secondary battery according to claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a rhombohedral crystal structure.

(2) FIG. 2 illustrates SEM images of the cathode active material particles produced in Examples 1 to 6 and Comparative Examples 1 and 2 of the present invention.

(3) FIG. 3 illustrates the results of the fracture surfaces of the cathode active material particles produced in Examples 1 to 6 and Comparative Examples 1 and 2 of the present invention taken as SEM images.

(4) FIG. 4 illustrates the results of the shape and structure of the primary particles in the particles of Example 4 of the present invention determined by TEM.

(5) FIG. 5 illustrates the measurement results of the shape and structure of the primary particles in the particles of Comparative Example 1 of the present invention determined by TEM.

(6) FIG. 6 illustrates a schematic diagram of the particle cross-section, primary particles, and lithium ion diffusion path of the cathode active material according to the present invention.

MODE FOR CARRYING OUT THE INVENTION

(7) Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples.

Example 1

(8) Into a co-precipitation reactor (volume of 100 L, output of rotary motor: 80 W or more), 20 L of distilled water and 1000 g of ammonia as a chelating agent were introduced and stirred at 350 rpm using the motor while maintaining the internal temperature of the reactor at 48° C. Nitrogen gas was continuously supplied to the reactor at a flow rate of 3 L/min.

(9) A 2.5 M aqueous precursor solution prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate at a molar ratio of 1:1:1 and a 28% aqueous ammonia solution were continuously introduced into the reactor at a rate of 3.25 L/hr and 0.15 L/hr, respectively. In addition, in order to adjust the pH, a 25% aqueous solution of sodium hydroxide was continuously supplied to the liquid surface in the reactor at a rate of 0.835 L/hr so as to have a pH of 11.5. The temperature of the reaction solution was maintained at from 48 to 50° C. and the 25% aqueous solution of sodium hydroxide was added to the reaction solution so as to maintain the pH at 11.5, thereby forming metal hydroxide particles. The precipitate of spherical nickel-manganese-cobalt composite hydroxide was collected from the reactor after the reaction was terminated.

(10) The composite metal hydroxide precipitated was filtered, washed with water, dried in a hot air dryer at 100° C. for 12 hours, thereby obtaining a precursor powder in the form of a composite metal hydroxide having a composition of (Ni.sub.0.33Co.sub.0.33Mn.sub.0.33)(OH).sub.2.

(11) The composite metal hydroxide and lithium carbonate (Li.sub.2CO.sub.3) were mixed together at a molar ratio of 1:1.00 to 1.10, then heated at a temperature rising rate of 2° C./min, and then fired at from 750 to 1000° C. for from 10 to 20 hours, thereby obtaining the powder of an cathode active material of Li(Ni.sub.0.33Co.sub.0.33Mn.sub.0.33)O.sub.2.

Example 2

(12) The powder of a cathode active material was produced by the same method as in Example 1 except that the molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate introduced was adjusted so that the cathode active material had a composition of Li(Ni.sub.0.5Co.sub.0.2Mn.sub.0.3)O.sub.2.

Example 3

(13) The powder of a cathode active material was produced by the same method as in Example 1 except that the molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate introduced was adjusted so that the cathode active material had a composition of Li(Ni.sub.0.7Co.sub.0.2Mn.sub.0.1)O.sub.2.

Example 4

(14) The powder of a cathode active material was produced by the same method as in Example 1 except that the molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate introduced was adjusted so that the cathode active material had a composition of Li(Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)O.sub.2.

Example 5

(15) The powder of a cathode active material was produced by the same method as in Example 4 except that the molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate introduced was adjusted so that the cathode active material had a composition of Li(Ni.sub.0.820Co.sub.0.145Mn.sub.0.035)O.sub.2.

Example 6

(16) The powder of an cathode active material was produced by the same method as in Example 4 except that the molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate introduced was adjusted so that the cathode active material had a composition of Li(Ni.sub.0.90Co.sub.0.07Mn.sub.0.03)O.sub.2.

Comparative Example 1

(17) Into a co-precipitation reactor having a volume of 100 L, 80 L of distilled water and 1000 g of ammonia as a chelating agent were introduced and stirred at 5000 rpm using the motor while maintaining the internal temperature of the reactor at 50±2° C. In addition, nitrogen gas was continuously supplied to the reactor at a flow rate of 3 L/min. Next, a 1 M aqueous precursor solution prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate at a molar ratio of 8:1:1 and a 28% aqueous ammonia solution were continuously introduced into the reactor at a rate of 6.5 L/hr and 0.6 L/hr, respectively. In addition, in order to adjust the pH, a 25% aqueous solution of sodium hydroxide was continuously supplied to the liquid surface in the reactor at a rate of from 1.5 to 2.0 L/hr so as to have a pH of from 11 to 12. The temperature of the reaction solution was maintained at 50±2° C. and the 25% aqueous solution of sodium hydroxide was added to the reaction solution so as to maintain the pH at from 11 to 12, thereby forming metal hydroxide particles. In 30 hours after the inside of the reactor reached a steady-state, the hydroxide particles discharged from the overflow pipe were continuously collected, washed with water, dried in a hot air dryer at 100° C. for 12 hours, thereby obtaining a precursor powder in the form of a composite metal hydroxide having a composition of (Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)(OH).sub.2.

(18) The composite metal hydroxide and lithium hydroxide (LiOH.H.sub.2O) were mixed together at a molar ratio of 1:1.00 to 1.10, then heated at a temperature rising rate of 2° C./min, then subjected to the heat treatment at 550° C. for 10 hours, and then fired at 750° C. for 20 hours, thereby obtaining the powder of an cathode active material.

Comparative Example 2

(19) The powder of an cathode active material was synthesized by the same method as in Comparative Example 1 except that a 1 M aqueous precursor solution prepared by mixing nickel sulfate, cobalt sulfate, and aluminum nitrate at a molar ratio of 81.5:15:3.5 was used.

Taking of SEM Image

(20) SEM images of the particles and fracture surfaces of the cathode active materials produced in Examples 1 to 6 and Comparative Examples 1 and 2 were taken and the results are illustrated in FIG. 2 and FIG. 3.

(21) From FIG. 2, it can be seen that the cathode active material particles produced in Examples 1 to 6 and Comparative Examples 1 and 2 are spherical secondary particles formed by aggregation of the primary particles.

(22) From FIG. 3 of SEM images of the fracture surfaces of the particles, it can be seen that, in the case of the particles produced in Examples 1 to 6 of the present invention, the aspect ratio of the primary particles is 1 or more, the primary particles grow in the longitudinal direction to exhibit directivity in the center direction of the particles, but in the case of Comparative Examples 1 and 2, the primary particles are close to a circle and the directivity of the primary particles is not observed in the inside of the secondary particles.

Taking of TEM Image

(23) The shape and structure of the primary particles of Example 4 and Comparative Example 1 were determined by TEM, and the results are illustrated in FIG. 4 and FIG. 5, respectively.

(24) From FIG. 4, it can be confirmed that, in the case of the particles produced in Example 4 of the present invention, the a-axis direction that is the lithium ion diffusion path in the primary particles is formed in the longitudinal direction, and at the same time, the a-axis direction that is the lithium ion diffusion path in the primary particles exhibits directivity in the center direction of the secondary particles.

(25) On the other hand, in the case of FIG. 5 illustrating TEM images of the particles produced in Comparative Example 1, it can be confirmed that the a-axis direction that is the lithium ion diffusion path is not only directed toward the center direction of the particles but is also not formed to be parallel to the major axis.

Measurement of Properties of Particles

(26) TABLE-US-00001 TABLE 1 Result of particle Result of ICP (mole %) size (μm) Division Ni Co Mn Al D10 D50 D90 Example 1 33.6 33.7 32.7 — 7.8 10.4 14.1 Example 2 49.6 20.3 30.1 — 7.8 10.6 14.6 Example 3 70.2 20.3 9.6 — 7.5 9.9 12.8 Example 4 79.1 11.3 9.7 — 7.2 9.5 11.9 Example 5 82.1 14.4 — 3.5 4.1 10.7 14.4 Example 6 89.5 7.5 — 3.1 4.5 11.4 15.3 Comparative 80.2 9.9 9.7 — 4.7 9.3 14.0 Example 1 Comparative 81.5 14.8 — 3.4 7.2 11.4 15.3 Example 2

Production of Battery

(27) The cathode active materials produced in Examples 1 to 6 and Comparative Examples 1 and 2, super-P as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed together at a weight ratio of 92:5:3 to prepare a slurry. The slurry was uniformly coated on an aluminum foil having a thickness of 15 μm and vacuum-dried at 135° C. to produce a cathode for lithium secondary battery.

(28) A coin battery was produced according to a usually known manufacturing process using the above cathode, a lithium foil as a counter electrode, a porous polyethylene film (Celgard 2300 manufactured by Celgard, LLC., thickness: 25 μm) as a separator, and a liquid electrolytic solution in which LiPF.sub.6 was dissolved at a concentration of 1.15 M in a solvent prepared by mixing ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7.

Measurement of Properties of Battery

(29) The initial capacity, initial efficiency, rate characteristics, and lifespan characteristics of the batteries produced using the active materials produced in Examples 1 to 6 and Comparative Examples were measured, and the results are presented in the following Table 2.

(30) From Table 2 below, it can be confirmed that the properties of the batteries including the active materials produced in Examples of the present invention are more significantly improved than those of the batteries including the active materials produced in Comparative Examples.

(31) TABLE-US-00002 TABLE 2 Initial capacity Lifespan (mAh/g) Initial Rate performance Li/M BET Charge Discharge efficiency characteristics % at 100.sup.th Division ratio m2/g quantity quantity (%) % (2 C/0.1 C) cycle Example 1 1.07 0.23 180 161 89.4 88.3 96.8 Example 2 1.02 0.25 192 171 89.1 87.6 89.7 Example 3 1.01 0.28 210 199 94.5 87.1 89.5 Example 4 1.01 0.27 224 203 90.6 87.9 90.9 Example 5 1.00 0.32 220 201 91.3 87.2 88.2 Example 6 0.99 0.48 237 210 88.7 86.2 82.8 Comparative 1.02 0.47 227 204 89.8 82.5 81.4 Example 1 Comparative 1.01 0.26 219 201 91.6 83.4 80.3 Example 2

INDUSTRIAL APPLICABILITY

(32) As described above, it can be said that the cathode active material for lithium secondary battery according to the present invention is significantly useful in that the a-axis direction that is the lithium ion diffusion path in the primary particles is formed to be parallel to the longer side of the primary particles and to exhibit directivity in the center direction of the secondary particles, thus the storage of the lithium ion into and release thereof from the primary particles are facilitated in the charging and discharging procedure, and the capacity characteristics and lifespan characteristics of the battery including the cathodeactive material for lithium secondary battery according to the present invention are significantly improved.