Cathode additive, preparation method thereof, and cathode and lithium secondary battery comprising the same

Abstract

The present disclosure relates to a cathode additive, a method for preparing the same, and a cathode and a lithium secondary battery including the same. More specifically, one embodiment of the present disclosure provides a cathode additive that can offset an irreversible capacity imbalance, and increase the initial charge capacity of a cathode.

Claims

1. A cathode mixture comprising: a cathode active material including one or more composite oxides of: a metal of cobalt, manganese, nickel, and lithium, wherein the composite oxide includes cobalt, manganese, nickel, and lithium in a composite oxide; and a cathode additive composition represented by the following Chemical Formula 1:
{.sub.x(Li.sub.2+aNi.sub.bM.sub.1-bO.sub.2+c)}.{.sub.y(NiO)}.{.sub.z(Li.sub.2O)}  [Chemical Formula 1] wherein, in Chemical Formula 1, M is one or more metal atoms forming a divalent cation or a trivalent cation, −0.2≤a≤0.2, 0.5≤b≤1.0, −0.2≤c≤0.2, 0.7≤x≤0.93, 0<y≤0.15, 0.07≤z≤0.15, and x+y+z=1.

2. The cathode mixture according to claim 1, wherein y=z.

3. The cathode mixture according to claim 1, wherein, for the cathode additive, a peak by lithium oxide (Li.sub.2O) is detected in at least one of a range which 2θ is 30 to 35°, a range in which 2θ is 35 to 40°, or a range in which 2θ is 55 to 60°, by XRD (X-Ray Diffraction) measurement by Fe Kα X-ray (X-rα).

4. The cathode mixture according to claim 1, wherein the content of lithium oxide (Li.sub.2O) in the total amount of the cathode additive (100 wt %) is greater than 7 wt % to 15 wt %.

5. The cathode mixture according to claim 1, wherein for the cathode additive, a peak by nickel oxide (NiO) is detected in at least one of a range in which 2θ is 35 to 40°, a range in which 2θ is 40 to 45°, or a range in which 2θ is 50 to 55°, by XRD (X-Ray Diffraction) measurement by Fe Kα X-ray (X-rα).

6. The cathode mixture according to claim 1, wherein the content of nickel oxide (NiO) in the total amount of the cathode additive (100 wt %) is greater than 0 wt % and equal to or less than 15 wt %.

7. The cathode mixture according to claim 1, wherein the cathode additive is included in the content of 1 to 30 wt %, based on the total weight of the mixture (100 wt %).

8. The cathode mixture according to claim 1, further comprising a conductive material, a binder, or a mixture thereof.

9. The cathode mixture according to claim 1, wherein 0.7≤x≤0.86, 0.07≤y≤0.15, 0.07≤z≤0.15, and x+y+z=1.

10. A lithium secondary battery comprising: a cathode comprising the cathode mixture of claim 1; an electrolyte; and an anode.

11. The lithium secondary battery according to claim 10, wherein the anode includes one or more anode active materials selected from the group consisting of a carbon-based anode active material, a lithium metal, a lithium alloy, Si, SiO.sub.x (0<x<2), a Si—C composite, a Si-Q alloy (wherein Q is an alkali metal, an alkali earth metal, atoms of Groups 13 to 16, a transition metal, a rare earth atom, or a combination thereof, provided that it is not Si), Sn, SnO.sub.2, a Sn—C composite, and a Sn-R (R is an alkali metal, an alkali earth metal, atoms of Groups 13 to 16, a transition metal, a rare earth metal, or a combination thereof, provided that it is not Sn).

Description

BRIEF DESCRIPTION OF DRAWING

(1) FIG. 1 is a graph showing the initial charge/discharge property of each lithium secondary battery of Examples 1 to 3 and Comparative Example 1.

(2) FIG. 2 is graph showing the long-term charge/discharge property (capacity retention) of each battery of Comparative Example 2 and Examples 4 and 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) Hereinafter, the actions and the effects of the present disclosure will be explained in more detail through specific examples of the present disclosure. However, these examples are presented only as illustrations of the present disclosure, and the scope of the right of the present disclosure is not limited thereby.

I. Confirmation of the Structure and Advantages of a Cathode Additive Including Lithium Nickel Oxide, Nickel Oxide (NiO), and Lithium Oxide (Li.SUB.2.O)

Example 1: {.SUB.x.(Li.SUB.2.NiO.SUB.2.)}.{y(NiO)}.{z(Li.SUB.2.O)}, x=0.86, y=0.07, z=0.07

(4) (1) Preparation of Cathode Additive

(5) A nickel hydroxide precursor, Ni(OH).sub.2, was heat treated under an inert atmosphere of 600° C. for 10 hours to obtain a nickel-based oxide NiO.

(6) The nickel-based oxide NiO was mixed with lithium oxide (Li.sub.2O) at a mole ratio (NiO:Li.sub.2) of 1:1.1, and heat treated at 680° C. (inert atmosphere) for 18 hours. At this time, the heating and cooling rates were fixed to 5° C. per minute.

(7) After the heat treatment was finished, {.sub.x(Li.sub.2NiO.sub.2)}.{y(NiO)}.{z(Li.sub.2O)}, x=0.86, y=0.07, z=0.07, was finally obtained, which was designated as a cathode additive of Example 1.

(8) The above chemical formula was calculated from Experimental Example 1 described below.

(9) (2) Manufacture of a Cathode and a Lithium Secondary Battery (Coin Half Cell)

(10) A cathode was prepared using the cathode additive of Example 1, and a lithium secondary battery including the prepared cathode was manufactured.

(11) Specifically, the cathode additive of Example 1, {.sub.0.86(Li.sub.2NiO.sub.2)}.{.sub.0.07NiO}.{.sub.0.07Li.sub.2O}, a conductive material (Super-P, Denka black), and a binder (PVdF) were mixed in an organic solvent (NMP) at a weight ratio of 85:10:5 (cathode additive:conductive material:binder), to prepare a cathode mixture in the form of a slurry, and then the cathode mixture was coated on an aluminum current collector and dried in a vacuum oven at 120° C. for 30 minutes to prepare a cathode.

(12) As a counter electrode, Li-metal was used, and as an electrolyte, a solution of 2 wt % of VC dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:2 was used.

(13) A 2032 half coin cell was manufactured using the above-described constructional elements, according to a common manufacturing method.

Example 2: {.SUB.x.(Li.SUB.2.NiO.SUB.2.)}.{.SUB.y.(NiO)}.{.SUB.z.(Li.SUB.2.O)}, x=0.80, y=0.10, z=0.10

(14) (1) Preparation of Cathode Additive

(15) {.sub.x(Li.sub.2NiO.sub.2)}.{.sub.y(NiO)}.{.sub.z(Li.sub.2O)}, x=0.80, y=0.10, z=0.10, was obtained by the same method as Example 1, except that the nickel-based oxide NiO was mixed with lithium oxide (Li.sub.2O) at a mole ratio of 1:1.2, and it was designated as a cathode additive of Example 2.

(16) The above chemical formula was calculated from Experimental Example 1 described below.

(17) (2) Manufacture of a Cathode and a Lithium Secondary Battery (Coin Half Cell)

(18) The cathode and lithium secondary battery of Example 2 were manufactured by the same method as Example 1, except that the cathode additive of Example 2 was used instead of the cathode additive of Example 1.

Example 3: {.SUB.x.(Li.SUB.2.NiO.SUB.2.)}.{.SUB.y.(NiO)}.{.SUB.z.(Li.SUB.2.O)}, x=0.76, y=0.12, z=0.12

(19) (1) Preparation of Cathode Additive

(20) {.sub.x(Li.sub.2NiO.sub.2)}.{.sub.y(NiO)}.{.sub.z(Li.sub.2O)}, x=0.76, y=0.12, z=0.12, was obtained by the same method as Example 1, except that the nickel-based oxide NiO was mixed with lithium oxide (Li.sub.2O) at a mole ratio of 1:1.3, and the above Chemical Formula was calculated from Experimental Example 1 described below.

(21) (2) Manufacture of a Cathode and a Lithium Secondary Battery (Coin Half Cell)

(22) The cathode and lithium secondary battery of Example 3 were manufactured by the same method as Example 1, except that the cathode additive of Example 3 was used instead of the cathode additive of Example 1.

Comparative Example 1: .SUB.x.(Li.SUB.2.NiO.SUB.2.), x=0.86

(23) (1) Preparation of Cathode Additive

(24) A cathode additive was prepared by the same method as Example 1, and then non-reacted nickel-based oxide NiO and lithium oxide (Li.sub.2O) were sieved through a 400 mesh sieve to finally obtain .sub.x(Li.sub.2NiO.sub.2), x=0.86, having an orthorhombic crystal structure with a point group of Immm, which was designated as the cathode additive of Comparative Example 1.

(25) (2) Manufacture of a Cathode and a Lithium Secondary Battery (Coin Half Cell)

(26) The cathode and lithium secondary battery of Comparative Example 1 were manufactured by the same method as Example 1, except that the cathode additive of Comparative Example 1 was used instead of the cathode additive of Example 1.

Experimental Example 1: XRD Analysis

(27) For each cathode additive of Examples 1 to 3 and Comparative Example 1, XRD (X-Ray Diffraction) analysis by Cu Kα X ray (X-rα) was conducted, and the results are recorded in the following Table 1.

(28) Specifically, lithium nickel oxide and the nickel oxide (NiO) can be detected as crystalloids, through XRD (X-Ray Diffraction) by Cu Kα X ray (X-rα).

(29) Particularly, quantitative analysis results were obtained through the calculation of intensity after XRD (X-Ray Diffraction) analysis.

(30) TABLE-US-00001 TABLE 1 Structural analysis Quantitative Cell parameter analysis (Å) Volume Crystallite NiO Li.sub.2O a axis c axis (Å.sup.3) size (nm) (wt %) (wt %) Comparative 2.779 9.025 93.98 182 0 0 Example 1 Example 1 2.779 9.026 94.01 182 7 7 Example 2 2.779 9.028 93.96 205 10 10 Example 3 2.780 9.028 93.96 210 12 12

(31) It is already known that Comparative Example 1 has an orthorhombic crystal structure with a point group of Immm. Further, from the results of structural analysis of Table 1, it can be seen that Comparative Example 1 and Examples 1 to 3 have identical crystal structures. Thus, it can be seen that Examples 1 to 3 also include a compound represented by Li.sub.2+aNi.sub.bM.sub.1-bO.sub.2+c.

(32) From the results of quantitative analysis of Table 1, it can be confirmed that Li.sub.2O was not detected in Comparative Example 1. However, it can be confirmed that in Examples 1 to 3, based on the total amount (100 wt %), 7 wt % (Example 1), 10 wt % (Example 2), and 12 wt % (Example 3) of Li.sub.2O were respectively detected.

Experimental Example 2: Evaluation of the Initial Charge/Discharge Property of a Battery Applying Additive

(33) For each battery of Examples 1 to 3 and Comparative Example 1, the initial charge/discharge property was evaluated under the following conditions. The results of evaluation are recorded in FIG. 1 and Table 2.

(34) Charge: 0.1 C, CC/CV, 4.25 V, 0.05 C cut-off

(35) Discharge: 0.1 C, CC, 2.5 V, cut-off

(36) According to FIG. 1 and Table 2, it can be confirmed that in Examples 1 to 3, the initial irreversible capacity of an anode decreased and the initial efficiency of a cathode increased, compared to Comparative Example 1.

(37) TABLE-US-00002 TABLE 2 Anode 0.1 C 0.1 C Cathode Charge Discharge Efficiency 1.sup.st Cycle (mAh/g) (mAh/g) (%) Capacity Comparative 329.8 95.2 28.9 (mAh/g) Example 1 Example 1 382 142.3 37.3 Example 2 394.2 142.8 36.2 Example 3 402.2 143.6 35.7

(38) In Examples 1 to 3, in order to confirm the effect of improvement in the initial performance of a battery by the cathode additive of one embodiment, a cathode mixture was prepared using each cathode additive in the same amount as the common cathode active material, and a cathode and a lithium secondary battery were manufactured.

(39) As explained above, the cathode additive of one embodiment can irreversibly discharge lithium ions and oxygen at the initial charge voltage, for example, 2.5 to 4.25 V (vs. Li/Li.sup.+), and then can be converted into a composition capable of reversible intercalation and deintercalation of lithium ions. Thus, as in Examples 1 to 3, the cathode additive of one embodiment may be utilized as an additive for compensating the initial irreversible capacity of an anode, and also as an active material enabling reversible intercalation and deintercalation of lithium.

(40) However, since it may have small reversible capacity compared to a common cathode active material due to the Li content and the structural limitations, in case the initial performance of a battery is to be simultaneously improved while securing the long-time life cycle characteristics, the cathode additive of one embodiment may be combined with the cathode active material at an appropriate mixing ratio according to the desired properties of a battery.

III. Examples of Practical Application Forms of a Cathode Additive Including Lithium Nickel Oxide, Nickel Oxide (NiO), and Lithium Oxide (Li.SUB.2.O)

Examples 4 and 5: Application of the Cathode Additive of Example 1 in Combination with the Cathode Active Material

(41) For the practical application form of the cathode additive of Example 1, the cathode additive of Example 1 was used in combination with a cathode active material prepare a cathode, and a lithium secondary battery including the prepared cathode was manufactured.

(42) Specifically, the cathode additive of Example 1 ({.sub.x(Li.sub.2NiO.sub.2)}.{.sub.y(NiO)}.{.sub.z(Li.sub.2O)}, x=0.86, y=0.07, z=0.07), the cathode active material of NCM (LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2), the conductive material (Super-P, Denka Black), and a binder (PVdF) were mixed in an organic solvent (NMP) to prepare a cathode mixture in the form of a slurry, and then the cathode mixture was coated on an aluminum current collector and dried in a vacuum oven at 120° C. for 30 minutes to manufacture each cathode of Example 4 and 5.

(43) In Examples 4 and 5, the weight ratios of the cathode additive of Example 1:cathode active material:conductive material:binder were 4.825:91.675:1.5:2 (Example 4) and 9.65:86.85:1.5:2.0 (Example 5), respectively.

(44) Each 2032 half coin cell was manufactured by the same method as Example 1, using each cathode of Examples 4 and 5 instead of the cathode of Example 1

Comparative Example 2: Application of Cathode Active Material Alone

(45) A cathode was prepared by the same method as Example 1, except that no cathode additive was used, and instead of the cathode additive of Example 1, the same amount of cathode active material (LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2) was used, and a lithium secondary battery including the prepared cathode was manufactured.

Experimental Example 3: Evaluation of the Practical Application Form of a Cathode Additive (Evaluation of the Properties of the Initial Capacity and Life Cycle Characteristics of a Battery)

(46) Specifically, charge/discharge of each battery of Comparative Example 2 and Examples 4 and 5 was progressed at a room temperature of 25° C. under the following conditions. The results are shown in FIG. 2 and Table 3.

(47) Charge: 0.2 C, CC/CV, 4.25 V, 0.005 C cut-off

(48) Discharge: 0.2 C, CC, 2.5 V, cut-off

(49) According to FIG. 2 and Table 3, it is confirmed that, compared to the case of using the cathode active material only (Comparative Example 2), in the case where the cathode additive of Example 1 was used in combination with the cathode active material (Examples 3 and 4), both the initial charge capacity and the life cycle characteristics of a battery are improved.

(50) TABLE-US-00003 TABLE 3 Composition of additive and weight ratio Initial operation Long term operation property of battery of additive and active material property of battery Capacity Capacity Capacity Additive:Active 0.2 C 0.2 C Retention Retention Retention material Charge Discharge (%, @ 30.sup.th (%, @ 100.sup.th (%, @ 200.sup.th ({.sub.x(Li.sub.2NiO.sub.2)}•{.sub.y(NiO)}•{.sub.z(Li.sub.2O)}) (weight ratio) (mAh/g) (mAh/g) cycle) cycle) cycle) Comparative — Active material 226.1 206.2 95.1 92.8 89.5 Example 2 100% (Ref.) Example 3 x = 0.86, y = 0.07, z = 0.07  5:95 235.7 204.5 96.5 94.2 91.8 Example 4 x = 0.86, y = 0.07, z = 0.07 10:90 243.0 199.2 96.6 95.1 92.9

(51) Putting the above results and Experimental Examples 1 to 2 together, it can be confirmed that the cathode additive including the lithium nickel oxide, the nickel oxide (NiO), and the lithium oxide (Li.sub.2O) irreversibly discharges lithium ions and oxygen preferentially over the cathode active material at the initial charge voltage of a battery, thereby compensating the initial irreversible capacity of an anode and increasing the initial charge capacity of a cathode.

(52) Moreover, according to FIG. 2 and Table 3, it can be confirmed that when the cycle number of a battery is identical, the capacity retention rates of Examples 3 and 4 are remarkably high, compared to the capacity retention rates of Comparative Example 2.

(53) Such a difference in the capacity retention rates becomes severe as the cycle number of a battery increases, and particularly, it is confirmed that in Comparative Example 2, after the operation of 100 cycles, 92.8% of the initial capacity is maintained, and after the operation of 200 cycles, 89.5% is maintained. On the other hand, it is confirmed that in the case of Examples 3 and 4, after the operation of 100 cycles, 94.2% or more of the initial capacity is maintained, and even after the operation of 200 cycles, 91.8% or more of the initial capacity is maintained.

(54) This means that in cases where battery cycles are progressed to a greater number of cycles, the cathode additive of these embodiments provide for an increase in the initial capacity, with the added benefit of a lower capacity loss over the life cycle of the battery. It also means that after the cathode additive of these embodiments irreversibly discharges lithium ions and oxygen at the initial charge voltage of a battery, it is converted into a composition capable of reversible intercalation and deintercalation of lithium ions, which may at least partly contribute to the capacity being maintained even through the progression of battery cycles.

(55) Meanwhile, among Examples 3 and 4, in Example 4 wherein a cathode mixture including a higher content of a cathode additive of one embodiment is used, the initial charge capacity and the cycle life characteristic of a battery are further improved. This means that as a cathode mixture including a high content of a cathode additive of one embodiment is used, the initial charge capacity of a battery is further improved, and thus the cycle life characteristic can be more effectively improved.

(56) Thus, as explained above, in case the initial performance of a battery is to be improved while simultaneously securing the long-term life cycle characteristic, the cathode additive of one embodiment may be used in combination with the cathode active material at an appropriate mixing ratio, according to the desired battery properties.