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

Abstract

The present invention 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 invention provides a cathode additive that can offset an irreversible capacity imbalance, increase the initial charge capacity of a cathode, and simultaneously inhibit the generation of gas in a battery.

Claims

1. 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)}.{.sub.wLi.sub.5MO.sub.4}  [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,
0.6≤x≤1.0, 0<y≤0.15, 0<z≤0.15, 0≤w≤0.1, x+y+z+w=1, provided that when b=1.0, 0<w≤0.1, and when w=0, 0.5≤b<1.0, wherein a structure of the cathode additive composition includes a core including the (Li.sub.2+aNi.sub.bM.sub.1−bO.sub.2+c), (NiO) and (Li.sub.2O), and a coating layer including the Li.sub.5MO.sub.4.

2. The cathode additive according to claim 1, wherein M includes Al.

3. The cathode additive according to claim 2, wherein a peak by Li.sub.5AlO.sub.4 is detected in at least one of a range in which 2θ is 33 to 36°, or a range in which 2θ is 42 to 45°, by XRD (X-Ray Diffraction) measurement by Fe Kα X-ray (X-rα).

4. The cathode additive according to claim 2, wherein the content of Li.sub.5AlO.sub.4 in the total amount of the cathode additive (100 wt %) is greater than 0 wt % and equal to or less than 15 wt %.

5. The cathode additive according to claim 1, wherein y=z.

6. The cathode additive according to claim 1, wherein a peak by lithium oxide (Li.sub.2O) is detected in at least one of a range in 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).

7. The cathode additive 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 0 wt % and equal to or less than 15 wt %.

8. The cathode additive according to claim 1, wherein a peak by nickel oxide (N.sub.iO) 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).

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

10. A cathode mixture comprising: the cathode additive of claim 1; and a cathode active material.

11. The cathode mixture according to claim 10, wherein the cathode additive is included in the content in an amount of 1 to 30 wt %, based on the total weight of the mixture (100 wt %).

12. The cathode mixture according to claim 10, wherein the cathode active material includes one or more composite oxides of: a metal of cobalt, manganese, nickel, or a combination thereof; and lithium.

13. The cathode mixture according to claim 10, further comprising a conductive material, a binder, or a mixture thereof.

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

15. The lithium secondary battery according to claim 14, 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, an atom 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 Sn—R (R is an alkali metal, an alkali earth metal, an atom of Groups 13 to 16, a transition metal, a rare earth metal, or a combination thereof, provided that it is not Sn).

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the results of XRD (X-Ray Diffraction) analysis by Fe Kα X-ray (X-rα), for the cathode additives of Example 1 and Comparative Example 1.

(2) FIG. 2 shows the ex-situ XRD analysis results for the cathode additives of Example 1 and Comparative Example 1. Specifically, FIG. 2a relates to Example 1, and FIG. 2b relates to Comparative Example 1.

(3) FIG. 3 shows the results of real time analysis of gas pressure during charging of the batteries of Example 1 and Comparative Example 1.

(4) FIG. 4 shows the results of evaluating the initial charge capacities of the batteries of Examples 2 and 3 and Comparative Example 3.

(5) FIG. 5 shows the results of evaluating the cycle life characteristics of the batteries of Examples 2 and 3 and Comparative Example 3.

ADVANTAGEOUS EFFECTS

(6) According to the lithium secondary battery applying the cathode additive of one embodiment for a cathode, the initial irreversible capacity of an anode may be decreased, the initial capacity and efficiency of a cathode may be efficiently increased, and a decrease in energy density during the operation may be inhibited, such that an excellent cycle life characteristic can be exhibited.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

(7) Hereinafter, the actions and 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 the illustrations of the present disclosure, and the scope of the right of the present disclosure is not limited thereby.

Example 1

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

(9) The nickel-based oxide NiO was mixed with lithium oxide (Li.sub.2O) at a mole ratio (NiO:Li.sub.2O) of 1:1.02, aluminum oxide (Al.sub.2O.sub.3) was mixed in the content of 2000 ppm in the total weight of the raw materials, and the mixture was 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.

(10) After the heat treatment was finished, {.sub.x(Li.sub.2NibAl.sub.1−bO.sub.2)}.Math.{y(NiO)}.Math.{z(Li.sub.2O)}.Math.{.sub.wLi.sub.5MO.sub.4}, where x=0.83, y=0.07, z=0.07, w=0.03, and b=0.97, was finally obtained, which was designated as a cathode additive of Preparation Example 1.

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

Example 2

(12) In order to evaluate the correlation between the cathode additive of Example 1 and the initial properties of a battery (Experimental Example 3), a cathode was prepared using the cathode additive of Example 1, without using the cathode active material, and a lithium secondary battery including the prepared cathode was manufactured.

(13) Specifically, the cathode additive of Example 1 {.sub.0.83(Li.sub.2Ni.sub.0.97Al.sub.0.03O.sub.2)}.Math.{.sub.0.07(NiO)}.Math.{.sub.0.07(Li.sub.2O)}.Math.{.sub.0.03Li.sub.5MO.sub.4}, 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.

(14) 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.

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

Examples 3 and 4

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

(17) Specifically, the cathode additive of Example 1 ({.sub.0.83(Li.sub.2Ni.sub.0.97Al.sub.0.03O.sub.2)}.Math.{.sub.0.07(NiO)}.Math.{.sub.0.07(Li.sub.2O)}.Math.{.sub.0.03Li.sub.5MO.sub.4}), an NCM-based cathode active material (LiNi.sub.0.83Co.sub.0.11Mn.sub.0.06O.sub.2), a 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 Examples 3 and 4.

(18) In Examples 3 and 4, the weight ratios of the cathode additive of Example 1: cathode active material:conductive material:binder were 4.25:80.75:10:5 (Example 3) and 8.5:76.5:10:5 (Example 4), respectively.

(19) Each 2032 coin half-cell was manufactured by the same method as Example 2, using each cathode of Examples 3 and 4 instead of the cathode of Example 2

Comparative Example 1

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

(21) The nickel-based oxide (NiO) was mixed with lithium oxide (Li.sub.2O) at a mole ratio of 1:1.02 (NiO:Li.sub.2O), and the mixture was heat treated under an inert atmosphere at 680° C. for 18 hours. At this time, the speed of heating and cooling was fixed to 5° C. per minute.

(22) After the heat treatment was finished, {.sub.x(Li.sub.2NibAl.sub.1−bO.sub.2)}.Math.{y(NiO)}.Math.{z(Li.sub.2O)}.Math.{wLi.sub.5MO.sub.4}, where x=0.87, y=0.07, z=0.07, w=0, and b=0 was finally obtained, which was determined as a cathode additive of Comparative Example 1.

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

Comparative Example 2

(24) In order to evaluate the correlation between the cathode additive of Comparative Example 1 and the initial properties of a battery (Experimental Example 3), a cathode was prepared using the cathode additive of Comparative Example 1, without using a cathode active material, and a lithium secondary battery including the prepared cathode was manufactured.

(25) The preparation methods of the cathode and the lithium secondary battery of Comparative Example 2 were identical to those of Example 2, except that the cathode additive of Comparative Example 1 was used instead of the cathode additive of Example 1.

Comparative Example 3

(26) A cathode was prepared by the same method as Example 2, except that no cathode additive was used, and instead of the cathode additive of Example 1, the same amount of cathode active material was used, and a lithium secondary battery including the prepared cathode was manufactured.

(27) Experimental Example 1: XRD Analysis

(28) For each cathode additive of Example 1 and Comparative Example 1, XRD (X-Ray Diffraction) analysis by Fe Kα X ray (X-rα) was conducted, and the results were show in the following Table 1 and FIG. 1.

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

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

(31) TABLE-US-00001 TABLE 1 Structural analysis cell parameter (Å) Quantitative analysis a c crystallite size NiO Li.sub.2O Li.sub.5AlO.sub.4 axis axis volume (Å.sup.3) (nm) (wt %) (wt %) (wt %) Example 1 2.779 9.025 93.98 182 7 7 3 Comparative 2.779 9.026 94.01 182 7 7 — Example 1

(32) It is already known that Comparative Example 1 has an orthorhombic crystal structure with a point group of 1 mmm. Further, from the results of structural analysis of Table 1, it can be seen that Comparative Example 1 and Example 1 have identical crystal structures. Thus, it can be seen that Example 1 also includes a compound represented by Li.sub.2+aNi.sub.bAl.sub.1−bO.sub.2+c.

(33) Meanwhile, from the analysis results of Table 1, it can be confirmed that Li.sub.5AlO.sub.4 was not detected in Comparative Example 1, but Li.sub.5AlO.sub.4 was detected in Example 1.

(34) In the lithium nickel oxide having many Li by-products on the surface, particularly LiOH by-products exist in a large quantity. It can be seen that when aluminum oxide (Al.sub.2O.sub.3) is doped, it reacts with LiOH on the surface of the cathode additive of Example 1 to form a coating layer of Li.sub.5AlO.sub.4 on the surface, and the remaining components are positioned in the core at the lower part of the coating layer.

(35) Experimental Example 2: Ex-Situ XRD Analysis

(36) For each battery of Example 2 and Comparative Example 2, a 0.1C charge was conducted by each voltage to conduct ex-situ XRD analysis, and the results are reported in FIG. 2a (Example 2) and FIG. 2b (Comparative Example 2).

(37) Referring to FIG. 2a (Example 2) and FIG. 2b (Comparative Example 2), it can be confirmed that Comparative Example 2 maintains a Li.sub.2NiO.sub.2 structure till 3.9 V on the basis of a 0.1C charge of a coin half-cell, while Example 2 maintains a Li.sub.2NiO.sub.2 structure to 4.1V.

(38) Experimental Example 3: Evaluation of the Correlation Between a Cathode Additive and the Initial Properties of a Battery (Evaluation of the Initial Capacity of a Battery and Gas Generation Amount)

(39) For each battery of Example 2 and Comparative Example 2, the 1.sup.st charge/discharge was progressed under the following conditions. Further, a gas pressure during the charge of each battery was analyzed in real time using Differential electrochemical mass spectrometer (DEMS), and the amount of gas generation of each battery is recorded in FIG. 3 and the following Table 2.

(40) Charge: 0.1C, CC/CV, 4.25 V, 0.005C cut-off

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

(42) TABLE-US-00002 TABLE 2 Anode 0.1 C 0.1 C Cathode Charge Discharge Efficiency Gas 1.sup.st Cycle (mAh/g) (mAh/g) (%) (μl, 45° C.) Example 2 385 143.6 37.3 258.2 Comparative 383 142.5 37.2 376.1 Example 2

(43) According to FIG. 3 and Table 2, it was confirmed that in the battery of Example 2, the initial performance was improved, and gas generation was inhibited, compared to Comparative Example 2. This is considered to result from the application of Al doping and coating in the cathode additive of Example 1.

(44) Meanwhile, in Example 2, the initial performance of a battery and the degree of gas generation may vary according to the content of Li.sub.2O in the cathode additive, Al doping amount, and coating amount. As the content of Li.sub.2O in the cathode additive increases, the initial performance of a battery is improved, which is the effect resulting from the provision of extra Li by Li.sub.2O. Further, as the Al doping amount and coating amount increases in the cathode additive, gas generation of a battery may be inhibited, which is the effect resulting from the stabilization of the crystal structure of the core (particularly, {.sub.x(Li.sub.2Ni.sub.bAl.sub.1−bO.sub.2)}) by Al doping, and the inhibition of direct contact of the core and electrolyte by Al coating.

(45) In Example 1, in order to confirm the effects of improvement in the initial performance of a battery and inhibition of gas generation by the cathode additive of one embodiment, a cathode mixture was prepared using each cathode additive in the same amount as a common cathode active material, without combining the cathode active material, and a cathode and a lithium secondary battery were manufactured.

(46) 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+), and then can be converted into a composition capable of reversible intercalation and deintercalation of lithium ions. Thus, as in Example 1, the core may be utilized as an additive for compensating the initial irreversible capacity of an anode, and also as active material enabling reversible intercalation and deintercalation of lithium.

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

(48) Hereinafter, Examples 3 and 4 wherein the active cathode material is combined with the cathode additive of one embodiment are presented, and the battery properties will be evaluated.

(49) Experimental Example 4: Evaluation of the Practical Application form of Cathode Additive (Evaluation of the Initial Capacity and Cycle Life Characteristic of a Battery)

(50) Specifically, in comparison with the case wherein only the cathode active material is applied for a cathode (Comparative Example 3), the cathode additive of Example 1 and the cathode active material were applied for a cathode at a weight ratio of 5:95 (Example 3) and 10:90 (Example 4), respectively, and the initial capacity and life cycle characteristics of the batteries were evaluated, and the results were shown in FIG. 3, FIG. 4, and the following Table 3.

(51) TABLE-US-00003 TABLE 3 Comparison of the initial charge capacity/retention for Comparative Example 3, Examples 3 and 4 Comparative Example 3 Example 4 Example 3 [additive: [additive: [NCM 100% NCM = 5:95 NCM = 10:90 (Ref.)] (w; w)] (w; w)] Charge (0.2 C) mAh/g 230.0 240.5 248.8 Discharge mAh/g 216.3 213.8 210.8 (0.2 C) Efficiency % 94.0 88.9 84.7 Capacity % 92.0 93.1 94.2 Retention (@ 30.sup.th cycle) Capacity % 87.0 91.0 92.5 Retention (@ 100.sup.th cycle) Capacity % 81.4 88.0 90.1 Retention (@ 200.sup.th cycle)

(52) According to FIG. 3, FIG. 4, and Table 3, it is confirmed that in case the cathode additive of one embodiment and the cathode active material are used in combination (Examples 3 and 4), compared to the case of using only the cathode active material for a cathode (Comparative Example 3), both the initial charge capacity and the life cycle characteristic of a battery are improved.

(53) Specifically, according to FIG. 3 and Table 3, it can be confirmed that although the initial charge capacity of Comparative Example 3 is just 230.0 mAh/g, the initial charge capacities of Examples 3 and 4 increased by 10 mAh/g or more. Thus, it can be confirmed that the cathode additive of one embodiment can irreversibly discharge lithium ions and oxygen 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.

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

(55) 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 3, after operation of 100 cycles, 87.0% of the initial capacity is maintained, and after operation of 200 cycles, 81.4% is maintained. On the other hand, it is confirmed that in the case of Examples 3 and 4, after operation of 100 cycles, 91.0% or more of the initial capacity is maintained, and even after operation of 200 cycles, 88.0% or more of the initial capacity is maintained.

(56) This means that in case battery cycles are progressed while the initial capacity of a cathode increases by the cathode additive of one embodiment, the capacity loss decreases. This also means that after the cathode additive of one embodiment 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, thus partly contributing to the capacity even during the progression of battery cycles.

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

(58) However, even if the cathode additive of one embodiment irreversibly discharges lithium ions and oxygen at the initial charge voltage of a battery and then is converted into a composition capable of reversible intercalation and deintercalation of lithium ions, it exhibits low reversible (discharge) capacity due to the Li content and structural limitation, and thus the initial efficiency of Example 4 becomes lower than that of Example 3.

(59) Therefore, as explained above, in case the initial performance of a battery is to be improved simultaneously while 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 aimed battery properties.