Positive electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery including the same

Abstract

To provide a positive electrode for a secondary battery of a novel construction.SOLUTION: The present invention provides a positive electrode for a secondary battery, a manufacturing method of them, and a lithium secondary battery containing them. The positive electrode for a secondary battery, contains: a positive electrode collector; a first layer that is arranged onto at least one surface of the positive electrode collector, and contains a first positive electrode active material; and a second layer that is arranged onto the first layer, and contains a second positive electrode active material. The first positive electrode active material is expressed by the following chemical formula 1, the second positive electrode active material is expressed by the following chemical formula 1 or the following chemical formula 2, and the first positive electrode active material and the second positive electrode active material are different from each other: [Chemical 1] LixNiyM1-yO2; and [Chemical 2] LiαCoβM'1-βO2. In the chemical formula 1 and the chemical formula 2, a definition according to x, y, α, β, M, and M' are referred to a detailed explanation of the invention.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

Background Art

[0002] Lithium secondary batteries are used as driving power sources for portable electronic devices such as video cameras, mobile phones, and notebook computers. Rechargeable lithium secondary batteries have an energy density per unit weight that is more than three times higher and enable fast charging compared to existing lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, etc.

[0003] The lithium secondary battery produces electrical energy through oxidation and reduction reactions when lithium ions are intercalated / deintercalated into the positive and negative electrodes in a state where an organic electrolyte or a polymer electrolyte is filled between a positive electrode and a negative electrode containing an active material capable of intercalating and deintercalating lithium ions.

[0004] In particular, for the realization of high-capacity secondary batteries, research related to thick film electrode plates has been actively promoted. The thick film electrode plate can realize a high-capacity battery through thickness reduction of a base material / separator, etc., and has advantages such as cost reduction of the battery, but there is a problem of deterioration of battery performance due to an extended migration distance of electrons or lithium due to thickening of the electrode plate.

[0005] In particular, such performance degradation of the thick film electrode plate is prominent in the non-uniform charge and discharge characteristics in the thickness direction of the electrode plate. Specifically, an increase in the film thickness of the electrode plate due to thickening causes an increase in the polarization phenomenon during charge and discharge, and such a polarization phenomenon is caused by a potential difference in the electrode plate thickness direction. Such a polarization phenomenon causes a charge and discharge depth difference of the active material during charge and discharge by having a high potential in a part and a low potential in another part. Eventually, as the polarization phenomenon deepens, the part maintaining the high potential deteriorates further, causing deterioration of the overall battery performance.

[0006] As a solution to solve this problem, a solution of increasing the conductive substance and increasing the porosity of the electrode plate has been proposed. However, in that case, there is a problem that there is a limit to increasing the capacity.

[0007] Therefore, while using a thick film electrode plate, there is a practical need for a solution to minimize the performance degradation of the electrode.

[0008] Note that as the positive electrode active material contained in the positive electrode of the lithium secondary battery, a lithium-containing metal oxide is generally used. For example, as the positive electrode active material of the lithium secondary battery, transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium nickel oxide (LiNiO2), and composite oxides in which a part of these transition metals is substituted with other transition metals are used.

[0009] Recently, research on using a NCM-based positive electrode active material in which a part of nickel in lithium nickel oxide is substituted with manganese or cobalt is actively being carried out in combination with LiCoO2 (hereinafter referred to as "LCO") which is frequently used in the IT field.

[0010] When mixing a NCM-based positive electrode active material with LCO, there is an advantage that the capacity of the battery can be increased and the cost can be reduced compared to the case of using LCO alone. However, the NCM-based positive electrode active material has a problem that it is vulnerable to high voltage stability compared to LCO. Moreover, when using the two substances in combination, problems such as an increase in the positive electrode potential occur due to the generation of a voltage difference caused by the polarization phenomenon during charge and discharge.

[0011] Therefore, when using a mixture of a NCM-based positive electrode active material and LCO, there is a practical need for a solution to improve the degradation in a high voltage environment and improve the stability.

Summary of the Invention

Problems to be Solved by the Invention

[0012] One aspect of the present invention is to provide a positive electrode for a secondary battery having a novel structure including a plurality of different positive electrode active material layers arranged in a certain order on a current collector.

[0013] Another aspect of the present invention is to provide a method for manufacturing the positive electrode for a secondary battery. Still another aspect of the present invention is to provide a lithium secondary battery employing the positive electrode for a secondary battery.

Means for Solving the Problems

[0014] In one aspect of the present invention, a positive electrode current collector, a first layer disposed on at least one surface of the positive electrode current collector and containing a first positive electrode active material, a second layer disposed on the first layer and containing a second positive electrode active material, are included, the first positive electrode active material is represented by the following Chemical Formula 1, the second positive electrode active material is represented by the following Chemical Formula 1 or the following Chemical Formula 2, the first positive electrode active material and the second positive electrode active material are different from each other, and a positive electrode for a secondary battery is provided: [Chemical Formula 1] Li x Ni y M 1-y O2 [Chemical Formula 2] Li α Co β M’ 1-β O2 In the Chemical Formulas 1 and 2, 0.9 ≦ x ≦ 1.2, 0.1 ≦ y ≦ 0.98, 0.9 ≦ α ≦ 1.2, 0 ≦ β ≦ 1.0, M and M’ are each independently one or more metals or transition metal elements having an oxidation number of +2 or +3.

[0015] In another aspect, applying a first composition containing a first positive electrode active material on at least one surface of a positive electrode current collector to form a first layer, applying a second composition containing a second positive electrode active material on the first layer to form a second layer, are included, The first positive electrode active material is represented by the following Chemical Formula 1: The second positive electrode active material is represented by the following Chemical Formula 1 or the following Chemical Formula 2: A method for manufacturing a positive electrode for a secondary battery is provided, wherein the first positive electrode active material and the second positive electrode active material are different from each other. [Chemical Formula 1] Li x Ni y M 1-y O2 [Chemical Formula 2] Li α Co β M’ 1-β O2 In the Chemical Formulas 1 and 2, 0.9 ≦ x ≦ 1.2 and 0.1 ≦ y ≦ 0.98, 0.9 ≦ α ≦ 1.2 and 0 ≦ β ≦ 1.0, M and M’ are each independently one or more metals or transition metal elements having an oxidation number of +2 or +3.

[0016] In still another aspect, a lithium secondary battery is provided, which includes the positive electrode for a secondary battery, a negative electrode disposed opposite to the positive electrode, and an electrolyte disposed between the positive electrode and the negative electrode.

Advantages of the Invention

[0017] According to an embodiment, a lithium secondary battery can have improved life characteristics by adopting a positive electrode including a novel layered structure.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mode for Carrying Out the Invention

[0019] Hereinafter, the present invention will be described more specifically. Hereinafter, with reference to FIG. 2, a positive electrode for a secondary battery according to an embodiment of the present invention will be described. FIG. 2 is a schematic diagram of a positive electrode for a secondary battery according to an exemplary embodiment. Referring to FIG. 2, the positive electrode 10 for a secondary battery according to one aspect includes a positive electrode current collector 11, a first layer 12 disposed on at least one surface of the positive electrode current collector 11 and containing a first positive electrode active material, and a second layer 13 disposed on the first layer 12 and containing a second positive electrode active material.

[0020] The first positive electrode active material and the second positive electrode active material are each independently represented by the following Chemical Formula 1, and the first positive electrode active material and the second positive electrode active material are different from each other: [Chemical Formula 1] Li x Ni y M 1-y O2 In Chemical Formula 1, 0.9 ≦ x ≦ 1.2, 0.1 ≦ y ≦ 0.98, M is one or more metals or transition metal elements having an oxidation number of +2 or +3.

[0021] As can be understood from the above, the positive electrode 10 for a secondary battery according to the present invention forms a positive electrode containing a Ni-based substance, and introduces a multi-layer structure containing a plurality of positive electrode active materials having different transition metal compositions. While realizing high capacity by thickening, it is possible to suppress deterioration of battery performance and improve life characteristics. However, the M is an element other than Ni.

[0022] In one embodiment, the M is also one or more elements independently selected from the group consisting of Al, Mg, Mn, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi.

[0023] In one embodiment, the first positive electrode active material is represented by the following Chemical Formula 1A, and the second positive electrode active material is also represented by the following Chemical Formula 1B: [Chemical Formula 1A] Li x1 Ni y1 M1 1-y1 O2 In the Chemical Formula 1A, [Chemical Formula 1B] Li x2 Ni y2 M 1-y2 O2 In the Chemical Formulas 1A and 1B, the definitions related to M1, x1, y1, M2, x2, and y2 refer to the definitions of M, x, and y in this specification. However, y1 > y2.

[0024] That is, in order to solve the problems of deepening of the polarization phenomenon and life deterioration, which are the problems of the thick-film electrode as described above, the positive electrode 10 for a secondary battery according to the present invention has a multi-layer structure including a plurality of positive electrode active materials having different compositions, for example, different ratios of transition metals. In addition, in the first layer 12 close to the positive electrode current collector 11, a lithium transition metal oxide having a high Ni content is arranged as the positive electrode active material so as to be advantageous for high capacity, and in the second layer 13 close to the separator (not shown), a lithium transition metal oxide having a low Ni content is arranged as the positive electrode active material, and deterioration suppression due to polarization and life improvement in the thickened positive electrode can be realized.

[0025] On the other hand, even if Ni-based positive electrode active materials having different compositions are included in a multi-layer structure, unlike the definition described above, when y1 < y2 and the positive electrode active material having a further higher Ni content is arranged in the second layer 13, deterioration due to polarization cannot be suppressed by the electrode plate structure of the positive electrode, and there may arise a problem that the life is rather deteriorated.

[0026] In one embodiment, the first positive electrode active material and the second positive electrode active material are independently represented by the following Chemical Formula 1-1 or the following Chemical Formula 1-2, and the first positive electrode active material and the second positive electrode active material may be different from each other: [Chemical Formula 1-1] Li x’ Ni y’ Co 1-y’-z’ Al z’ O2 [Chemical Formula 1-2] Li x’ Ni y’ Co 1-y’-z’ Mn z’ O2 In Chemical Formula 1-1 and Chemical Formula 1-2, 0.9 ≦ x' ≦ 1.2, 0.1 ≦ y' ≦ 0.98, 0 < z' < 0.5, and 0 < 1 - y' - z' < 0.5.

[0027] For example, in the chemical formula 1-1 and the chemical formula 1-2, y’ represents the content of Ni in the lithium transition metal oxide, and 0.5 ≦ y’ ≦ 0.98. For example, in the chemical formula 1-1 and the chemical formula 1-2, 0.6 ≦ y ≦ 0.98. For example, in the chemical formula 1-1 and the chemical formula 1-2, 0.7 ≦ y ≦ 0.98. For example, in the chemical formula 1-1 and the chemical formula 1-2, 0.8 ≦ y ≦ 0.98.

[0028] In one embodiment, the content of Ni in the first positive electrode active material is also 0.6 mol or more based on the total number of moles of transition metals.

[0029] In one embodiment, the content of Ni in the second positive electrode active material is also 0.6 mol or less based on the total number of moles of transition metals.

[0030] As described above, in the case of using, as the positive electrode active material, a lithium transition metal oxide in which the molar fraction of Ni is 0.6 or more and the content of Ni is large among transition metals, although there is an advantage that a high-capacity battery can be realized, there are disadvantages that the life characteristics, the high-temperature stability, and the high-temperature storage characteristics are significantly deteriorated, and due to such disadvantages, commercialization is difficult. Therefore, as a configuration for solving this, the lithium secondary battery forms, on the lithium transition metal oxide layer in which the content of Ni is large and the molar fraction of Ni is 0.6 or less as described above, a lithium transition metal oxide in which the content of Ni is small, thereby suppressing deterioration due to polarization and improving the life characteristics.

[0031] For example, the first positive electrode active material is LiNi 0.6 Co 0.2 Mn 0.2 O2 or LiNi 08 Co 0.1 Mn 012 O2.

[0032] For example, the second positive electrode active material is LiNi 0.6 Co 0.2 Mn 0.2 O2 or LiNi 033 Co0.33 Mn 0.33 is also O2.

[0033] In one embodiment, the weight ratio of the first positive electrode active material and the second positive electrode active material is also 3:7 to 7:3. For example, the weight ratio of the first positive electrode active material and the second positive electrode active material is also 3:7 to 5:5. If it is outside the above range and the weight ratio of the first positive electrode active material and the second positive electrode active material is less than 3:7, there is a problem that the content of the high-content Ni-based material is excessively reduced and it is not easy to realize a high battery capacity. On the other hand, if the weight ratio of the first positive electrode active material and the second positive electrode active material exceeds 7:3, there is a problem that it is not easy to control the deterioration phenomenon of the positive electrode.

[0034] In one embodiment, the current density ratio of the first layer 12 to the second layer 13 is also 2:8 to 8:2. For example, the current density ratio of the first layer 12 to the second layer 13 is also 3:7 to 7:3.

[0035] In one embodiment, the thickness ratio of the first layer 12 to the second layer 13 is also 2:8 to 8:2. In one embodiment, the thickness of the first layer 12 is also 10 μm to 70 μm. In another embodiment, the thickness of the second layer 13 is also 10 μm to 70 μm. If it is outside the above range, the thickness ratio of the first layer 12 to the second layer 13 is less than 2:8, the thickness of the first layer 12 is less than 10 μm, or the thickness of the second layer 13 exceeds 70 μm, and the second layer 13 is excessively thick compared to the first layer 12, there is a problem that the capacity of the lithium secondary battery is excessively small in terms of volume ratio. On the other hand, if the thickness ratio of the first layer 12 to the second layer 13 exceeds 8:2, the thickness of the first layer 12 exceeds 70 μm, or the thickness of the second layer 13 is less than 10 μm, and the first layer 12 is excessively thick compared to the second layer 13, although there is an effect that the capacity of the lithium secondary battery can be increased, the side reaction occurring on the positive electrode surface cannot be sufficiently prevented, resulting in deterioration of the life characteristics, and in particular, the high-temperature characteristics such as the high-temperature life characteristics and the high-temperature stability are reduced.

[0036] For example, the current density of the positive electrode 10 is 3 to 6 mAh / cm2 It is also so. In one embodiment, the thickness of the positive electrode active material layer including the first layer 11 and the second layer 12 is also 40 μm or more. For example, the thickness of the positive electrode active material layer is also 40 μm to 110 μm.

[0037] That is, the positive electrode 10 according to the present invention has a thickness in the range as described above, embodies a thick film structure, and through which a high-capacity battery can be embodied.

[0038] The positive electrode 10 for a secondary battery according to another aspect includes a positive electrode current collector 11, a first layer 12 disposed on at least one surface of the positive electrode current collector 11 and containing a first positive electrode active material, and a second layer 13 disposed on the first layer 12 and containing a second positive electrode active material.

[0039] The first positive electrode active material is represented by the following Chemical Formula 1, and the second positive electrode active material is represented by the following Chemical Formula 2: [Chemical Formula 1] Li x Ni y M 1-y O2 [Chemical Formula 2] Li α Co β M’ 1-β O2 In the Chemical Formulas 1 and 2, 0.9 ≦ x ≦ 1.2, 0.1 ≦ y ≦ 0.98, 0.9 ≦ α ≦ 1.2, 0 ≦ β ≦ 1.0, M and M’ are each independently one or more metals or transition metal elements having an oxidation number of +2 or +3.

[0040] As can be seen from the above, the positive electrode 10 for a secondary battery according to the present invention introduces a multi-layer structure including the separate first layer 12 and second layer 13 as described above in order to solve the problem of deterioration of the Ni-based material due to the polarization phenomenon that occurs when the LCO-based material and the Ni-based material are simply mixed and coated together.

[0041] That is, the positive electrode 10 for a secondary battery according to the present invention has a multilayer structure containing a plurality of positive electrode active materials having different compositions, and arranges the first positive electrode active material represented by Chemical Formula 1 in a layer closer to the positive electrode current collector 11 and the second positive electrode active material represented by Chemical Formula 2 in a layer closer to the negative electrode.

[0042] As a result, in the positive electrode 10 for a secondary battery according to the present invention, since the second positive electrode active material, which is an LCO-based material relatively close to a high voltage, is located near the negative electrode, it is possible to control the deterioration of the positive electrode due to the polarization phenomenon, and the first positive electrode active material, which is a Ni-based material as a high-capacity material, can be sufficiently applied, and the effect of enabling high capacity implementation can be exhibited. On the other hand, even when the LCO-based material and the Ni-based material are included in a multilayer structure in the same manner as in the present invention, if the LCO-based material is arranged closer to the current collector side and the Ni-based material is arranged closer to the negative electrode, the Ni-based material will be placed in a high-voltage situation due to polarization, and there may arise a problem that the life is further deteriorated.

[0043] In one embodiment, the M and the M' are each independently also one or more elements selected from the group consisting of Al, Mg, Mn, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi.

[0044] For example, the M is also one or more selected from among Al, Mn, and Co. For example, the M is also Al and Co, and is also Mn and Co.

[0045] In one embodiment, the first positive electrode active material contained in the first layer 12 is also represented by the following Chemical Formula 1-1 or the following Chemical Formula 1-2: [Chemical Formula 1-1] Li x’ Ni y’ Co 1-y’-z’ Al z’ O2 [Chemical Formula 1-2] Li x’ Ni y’ Co 1-y’-z’ Mn z’ O2 In the aforementioned Chemical Formula 1-1 and Chemical Formula 1-2, 0.9 ≦ x’ ≦ 1.2, 0.1 ≦ y’ ≦ 0.98, 0 < z’ < 0.5, and 0 < 1 - y’ - z’ < 0.5.

[0046] For example, in the aforementioned Chemical Formula 1-1 and Chemical Formula 1-2, y’ represents the content of Ni in the lithium transition metal oxide, and 0.5 ≦ y’ ≦ 0.98, for example, 0.5 < y’ ≦ 0.98. For example, in the aforementioned Chemical Formula 1-1 and Chemical Formula 1-2, 0.6 ≦ y ≦ 0.98. For example, in the aforementioned Chemical Formula 1-1 and Chemical Formula 1-2, 0.7 ≦ y ≦ 0.98. For example, in the aforementioned Chemical Formula 1-1 and Chemical Formula 1-2, 0.8 ≦ y ≦ 0.98.

[0047] As described above, in the case of using a lithium transition metal oxide with a high content of Ni, where the molar fraction of Ni in the transition metal is 0.5 or more, as the positive electrode active material, despite the advantage of being able to realize a high-capacity battery, there are disadvantages such as significant degradation in life characteristics, high-temperature stability, and high-temperature storage characteristics. Due to such disadvantages, commercialization is difficult. Therefore, as a configuration for solving this problem, the lithium secondary battery forms the aforementioned LCO-based material on the first layer containing the Ni-based material, and by means of a mechanism such as preventing side reactions occurring on the positive electrode surface, it can exhibit excellent life characteristics and high-temperature stability.

[0048] For example, the positive electrode for the secondary battery is LiNi 0.8 Co 0.15 Mn 0.05 O2, LiNi 0.85 Co 0.1 Mn 0.05 O2, LiNi 0.88 Co 0.08 Mn 0.04 O2, LiNi 0.88 Co 0.08 Al 0.04 O2, Li 1.02 Ni 0.80 Co 0.15 Mn 0.05 O2, Li 1.02 Ni 0.85 Co 0.10Mn 0.05 O2, Li 1.02 Ni 0.88 Co 0.08 Mn 0.04 O2, Li 1.02 Ni 0.88 Co 0.08 Al 0.04 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.88 Co 0.1 Al 0.02 O2, LiNi 0.88 Co 0.12 Mn 0.04 O2, LiNi 0.85 Co 0.1 Al 0.05 O2, LiNi 0.88 Co 0.1 Mn 0.02 It can contain O2 and one or more lithium transition metal oxides as the positive electrode active material.

[0049] For example, in Chemical Formula 2, 0 < β ≦ 1.0 also holds. In one embodiment, the second positive electrode active material contained in the second layer 13 is also LiCoO2.

[0050] In one embodiment, the weight ratio of the first positive electrode active material to the second positive electrode active material is also 3:7 to 7:3. For example, the weight ratio of the first positive electrode active material to the second positive electrode active material is also 3:7 to 5:5. If the weight ratio of the first positive electrode active material and the second positive electrode active material is less than 3:7 outside the above range, the content of the Ni-based material decreases excessively, and there is a problem that it is not easy to realize a high battery capacity. On the other hand, if the weight ratio of the first positive electrode active material and the second positive electrode active material exceeds 7:3, there is a problem that it is not easy to control the deterioration phenomenon of the positive electrode.

[0051] In one embodiment, the thickness ratio of the first layer 12 to the second layer 13 is also 3:7 to 7:3. In one embodiment, the thickness of the first layer 12 is also 3 μm to 50 μm. In another embodiment, the thickness of the second layer 13 is also 3 μm to 50 μm. If outside the above range, that is, the thickness ratio of the first layer 12 to the second layer 13 is less than 3:7, the thickness of the first layer 12 is less than 3 μm, or the thickness of the second layer 13 exceeds 50 μm, and the second layer 13 is excessively thick compared to the first layer 12, there is a problem that the capacity of the lithium secondary battery is excessively small in terms of volume ratio. On the other hand, if the thickness ratio of the first layer 12 to the second layer 13 exceeds 7:3, the thickness of the first layer 12 exceeds 50 μm, or the thickness of the second layer 13 is less than 3 μm, and the first layer 12 is excessively thick compared to the second layer 13, although there is an effect that the capacity of the lithium secondary battery can be increased, the side reactions occurring on the positive electrode surface cannot be sufficiently prevented, resulting in deterioration of the life characteristics. In particular, high-temperature characteristics such as high-temperature life characteristics and high-temperature stability deteriorate.

[0052] For example, the current density of the positive electrode 10 is also 2 to 10 mAh / cm 2 as well. In one embodiment, the active material loading amount ratio of the first layer 12 to the second layer 13 is also 3:7 to 7:3. In one embodiment, the loading amount of the first layer 12 is also 3 to 40 mg / cm 2 as well. In another embodiment, the loading amount of the second layer 13 is also 40 to 3 mg / cm 2 as well.

[0053] In one embodiment, the positive electrode for the secondary battery may further contain one or more of a binder and a conductive material. For the description related to the binder and the conductive material, refer to the following description.

[0054] In one embodiment, based on the total weight of the positive electrode for the secondary battery, the sum of the contents of the first positive electrode active material and the second positive electrode active material is also 80 to 98% by weight.

[0055] Figure 3 is a schematic diagram of a positive electrode for a secondary battery according to another exemplary embodiment. Referring to Figure 3, the positive electrode 20 for a secondary battery includes a positive electrode current collector 21, a first layer 22 disposed on at least one surface of the positive electrode current collector 21 and containing the first positive electrode active material, and a second layer 23 disposed on the first layer 22 and containing the second positive electrode active material. In addition, it further includes a third layer 24 disposed on the second layer 23 and containing a third positive electrode active material. The third positive electrode active material is represented by the following Chemical Formula 1, and the third positive electrode active material has a composition different from that of the first positive electrode active material and the second positive electrode active material: [Chemical Formula 1] Li x Ni y M 1-y O2 In Chemical Formula 1, 0.9 ≦ x ≦ 1.2, 0.1 ≦ y ≦ 0.98, M is one or more metal or transition metal elements with an oxidation number of +2 or +3.

[0056] That is, the positive electrode 20 for a secondary battery according to the present invention may include, in addition to the aforementioned first layer 22 and second layer 23, a separate positive electrode active material layer containing a lithium transition metal oxide with a different composition as the third layer 24. Although not separately described, it may further include an additional layer containing a lithium transition metal oxide with a different composition. At this time, the number of layers of the added layer is not particularly limited.

[0057] For example, the first positive electrode active material is represented by the following Chemical Formula 1A, the second positive electrode active material is represented by the following Chemical Formula 1B, and the third positive electrode active material is also represented by the following Chemical Formula 1C: [Chemical Formula 1A] Li x1 Ni y1 M1 1-y1 O2 In Chemical Formula 1A, [Chemical Formula 1B] Li x2 Ni y2 M2 1-y2 O2 [Chemical Formula 1C] Li x3 Ni y3 M3 1-y3O2 In the chemical formulas 1A, 1B, and 1C, the definitions related to M1, x1, y1, M2, x2, y2, M3, and x3 are referred to the places where M, x, and y are defined in this specification, where y1 > y2 > y3.

[0058] That is, when including a positive electrode active material layer of more than two layers, the positive electrode active material layers are arranged in a direction where the Ni content decreases from the layer close to the positive electrode current collector 21 (the first layer 22) to the layer far from it (the third layer 24). Through this, the high-capacity characteristics and the suppression of battery performance deterioration embodied in the present invention can be exhibited.

[0059] For example, the current density of the third layer is less than or equal to the current density of the second layer, and the current density of the second layer is also less than or equal to the current density of the first layer.

[0060] Hereinafter, with reference to FIG. 4, a method for manufacturing a positive electrode for a secondary battery according to an embodiment of the present invention will be described. FIG. 4 is a schematic diagram showing a method for manufacturing a positive electrode for a secondary battery according to an exemplary embodiment. Referring to FIG. 4, a method for manufacturing a positive electrode 10 for a secondary battery according to another aspect of the present invention includes applying a first composition containing a first positive electrode active material on at least one surface of a positive electrode current collector 11 to form a first layer 12, and applying a second composition containing a second positive electrode active material on the first layer 12 to form a second layer 13. The first positive electrode active material is represented by the following chemical formula 1, the second positive electrode active material is represented by the following chemical formula 1 or the following chemical formula 2, and the first positive electrode active material and the second positive electrode active material are different from each other: [Chemical formula 1] Li x Ni y M 1-y O2 [Chemical formula 2] Li α Co β M’ 1-β O2 In the chemical formulas 1 and 2, 0.9 ≤ x ≤ 1.2, 0.1 ≤ y ≤ 0.98, 0.9 ≤ α ≤ 1.2, 0 ≤ β ≤ 1.0, M and M’ are, independently of each other, one or more metals or transition metal elements with an oxidation number of +2 or +3.

[0061] For the detailed descriptions of x, y, α, β, M and M’ in Chemical Formulas 1 and 2, refer to the foregoing.

[0062] Also, although not separately described, when further including a third layer containing an additional third positive electrode active material, a method similar to the step of forming the second layer 12 on the first layer 11 may be used.

[0063] In one specific example, the first composition or the second composition may further include a binder and a conductive material.

[0064] The binder is a component that aids in the binding of a lithium transition metal oxide, i.e., a cathode active material or an inorganic substance, to a conductive material and in the binding of the cathode active material to a current collector, and is also included between the cathode current collector and the cathode active material layer, within the cathode active material layer, between the cathode active material layer and the inorganic substance layer, or within the inorganic substance layer, and is added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the cathode active material or the inorganic substance. For example, the binder can be added in the range of 1 to 30 parts by weight, 1 to 20 parts by weight, or 1 to 15 parts by weight based on 100 parts by weight of the cathode active material or the inorganic substance. For example, the binder is one or more selected from the group consisting of polyvinylidene fluoride, polyvinyl chloride, polybenzimidazole, polyimide, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polystyrene, polymethyl methacrylate, polyaniline, acrylonitrile butadiene styrene, phenol resin, epoxy resin, polyethylene terephthalate, polytetrafluoroethylene, polyphenylene sulfide, polyamideimide, polyetherimide, polyethersulfone, polyamide, polyacetal, polyphenylene oxide, polybutylene terephthalate, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), and fluororubber. For example, when there are two or more binders in the above examples, various copolymers obtained by polymerizing the two or more binders are also binders.

[0065] The first composition or the second composition may further selectively contain a conductive material in order to provide a conductive path to the aforementioned positive electrode active material or inorganic substance and further improve electrical conductivity. As the conductive material, any material can be used as long as it is generally used in a lithium secondary battery. Examples thereof include carbon-based substances such as carbon black, acetylene black, ketjen black, and carbon fiber (e.g., vapor-grown carbon fiber); metal-based substances such as metal powders or metal fibers of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or a conductive material containing a mixture thereof. The content of the conductive material can be appropriately adjusted and used. For example, the weight ratio of the positive electrode active material or inorganic substance and the conductive material is also added in the range of 99:1 to 90:10.

[0066] The positive electrode current collector has a thickness of 3 μm to 500 μm and is not particularly limited as long as it does not induce a chemical change in the battery and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a surface-treated product with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. The current collector can also form fine irregularities on its surface to enhance the adhesion of the positive electrode active material, and various forms such as a film, sheet, foil, net, porous body, foam, and non-woven fabric body are possible.

[0067] The prepared first composition can be directly applied onto a positive electrode current collector and dried to manufacture a positive electrode plate. As an alternative, after casting the first composition onto a separate support, the film obtained by peeling it from the support can be laminated onto the positive electrode current collector to manufacture a positive electrode plate.

[0068] On the other hand, the first composition or the second composition may further contain a solvent. As the solvent, N-methylpyrrolidone, acetone, water, etc. can be used, but it is not limited thereto, and any can be used as long as it can be used in the technical field.

[0069] In one specific example, based on the total weight of the first composition, the content of the first positive electrode active material is also 80 to 98% by weight. For example, based on the total weight of the first composition, the content of the first positive electrode active material is also 85 to 98% by weight, but it is not limited thereto.

[0070] In one specific example, based on the total weight of the second composition, the content of the second positive electrode active material is also 80 to 98% by weight. For example, based on the total weight of the second composition, the content of the second positive electrode active material is also 85 to 98% by weight, but it is not limited thereto.

[0071] For example, when drying after applying the first composition or the second composition, the drying is also performed by primary drying at a temperature range of 80 to 130 °C for about 5 to 30 minutes.

[0072] In another aspect of the present invention, the lithium secondary battery includes a positive electrode for a secondary battery as described above, a negative electrode disposed opposite to the positive electrode, and an electrolyte disposed between the positive electrode and the negative electrode.

[0073] In one specific example, the operating voltage of the lithium secondary battery is also 2.5 to 4.5 V. For example, the operating voltage of the lithium secondary battery is also 3.0 to 4.4 V.

[0074] The positive electrode for a secondary battery essentially includes the positive electrode current collector, the first layer, and the second layer described above, and the related description refers to the above. Further, the first layer and the second layer each include the first positive electrode active material and the second positive electrode active material described above as essential components, and in addition to the essential components, a positive electrode active material material generally used in lithium secondary batteries may be additionally included.

[0075] For example, Li a A’ 1-b B b D2 (in the chemical formula, 0.90 ≦ a ≦ 1 and 0 ≦ b ≦ 0.5); Li a E 1-b B b O 2-c D c (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05); LiE 2-b B b O 4-c D c (in the chemical formula, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05); Li a Ni 1-b-c Co b B c D α (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 < α ≦ 2); Li a Ni 1-b-c Co b B c O 2-α F α (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 < α < 2); Li a Ni 1-b-c Co b B c O 2-α F2 (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 < α < 2); Li a Ni 1-b-c Mn b B c D α (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 < α ≦ 2); Li a Ni 1-b-c Mn b B c O 2-α F α (in the chemical formula, 0.90 ≦ a ≦ 1, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 < α < 2); Li a Ni 1-b-c Mn b B c O 2-αF2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α < 2); Li a Ni b E c G d O2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1); Li a Ni b Co c Mn d GeO2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1); Li a NiG b O2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (in the chemical formula, 0.90 ≤ a ≤ 1, 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (in the chemical formula, 0.90 ≤ a ≤ 1, 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiIO2; LiNiVO4; Li 3-f J2(PO4)3 (0 ≤ f ≤ 2); Li 3-f A compound represented by any one of the chemical formulas of Fe2(PO4)3 (0 ≤ f ≤ 2); LiFePO4 may be further included:

[0076] In the above chemical formula, A’ is Ni, Co, Mn, or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; F is F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q is Ti, Mo, Mn, or a combination thereof; I is Cr, V, Fe, Sc, Y, or a combination thereof; J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof. For example, LiCoO2, LiMn x’’ O 2x’’ (x’’ = 1, 2), LiNi 1-x’’ Mn x’’ O 2x’’ (0 < x’’ < 1), LiNi 1-x’’-y’’ Co x’’ Mn y’’ O2(0 ≤ x’’ ≤ 0.5, 0 ≤ y’’ ≤ 0.5), FePO4, etc.

[0077] On the other hand, the negative electrode is also manufactured by the following method. For example, a negative electrode active material, a conductive material, a binder, and a solvent are mixed to prepare a negative electrode active material composition. The negative electrode active material composition is directly coated on a metal current collector and dried to manufacture a negative electrode plate. As an alternative, after the negative electrode active material composition is cast on a separate support, a film peeled off from the support is laminated on a metal current collector to manufacture a negative electrode plate.

[0078] Any negative electrode active material is possible as long as it can be used as a negative electrode active material of a lithium battery in the relevant technical field. For example, it may contain one or more selected from the group consisting of lithium metal, a metal alloyable with lithium, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0079] For example, the metals capable of alloying with lithium include Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y alloys (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination of these elements and is not Si), Sn-Y alloys (where Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, or a combination of these elements and is not Sn), and the like. Examples of the element Y include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, or Te. For example, the transition metal oxides also include lithium titanate, vanadium oxide, lithium vanadate, and the like. For example, the non-transition metal oxides include SnO2, SiO x’’’ (0 < x''' < 2), and the like.

[0080] The carbon-based material may also be crystalline carbon, amorphous carbon, or a mixture thereof. The crystalline carbon may also be graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, and the amorphous carbon may also be soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0081] As the negative electrode active material composition, a conductive material, a binder, and a solvent that are the same as those in the case of the positive electrode active material composition (i.e., the first composition) can be used.

[0082] The contents of the negative electrode active material, the conductive material, the binder, and the solvent are at levels generally used in lithium batteries. Depending on the use and configuration of the lithium battery, one or more of the conductive material, the binder, and the solvent may be omitted. Next, a separator inserted between the positive electrode and the negative electrode is prepared.

[0083] If the separator is one generally used in a lithium battery, any of them can be used. A separator having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention ability can be used. For example, it can be selected from among glass fiber, polyester, Teflon (registered trademark), polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a non-woven fabric or a woven fabric. For example, a rollable separator such as polyethylene or polypropylene is used for a lithium-ion battery, and a separator having excellent impregnation ability for an organic electrolyte can be used for a lithium-ion polymer battery. For example, the separator can also be manufactured by the following method.

[0084] A polymer resin, a filler, and a solvent are mixed to prepare a separator composition. The separator composition can be directly coated on the upper part of the electrode and dried to form a separator. Or, after the separator composition is cast on a support and dried, the separator film peeled off from the support is laminated on the upper part of the electrode to form a separator.

[0085] The polymer resin used for manufacturing the separator is not particularly limited, and any can be used as long as it is a substance used as a binder for an electrode plate. For example, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or a mixture thereof can be used.

[0086] Next, an electrolyte is prepared. For example, the electrolyte is also an organic electrolyte solution. Also, the electrolyte is a solid. For example, it is also boric oxide, lithium oxynitride, etc., but is not limited thereto, and any can be used as long as it can be used as a solid electrolyte in the technical field. The solid electrolyte is also formed on the negative electrode by a method such as sputtering. For example, the organic electrolyte is also produced by dissolving a lithium salt in an organic solvent.

[0087] Any of the organic solvents can be used as long as they can be used as organic solvents in the relevant technical field. For example, propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, dibutyl carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, diethylene glycol, dimethyl ether, or a mixture thereof.

[0088] Any of the lithium salts can be used as long as they can be used as lithium salts in the relevant technical field. For example, LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiAlO2, LiAlCl4, LiN(C x’’’’ F 2x’’’’+1 SO2)(Cy ’’’’ F 2y’’’’+1 SO2)(where x’’’’, y’’’’ are natural numbers), LiCl, LiI, or a mixture thereof.

[0089] As can be seen from FIG. 1, the lithium secondary battery 1 includes a positive electrode 3, a negative electrode 2, and a separator 4. The aforementioned positive electrode 3, negative electrode 2, and separator 4 are wound or folded and housed in a battery case 5. Next, an organic electrolyte is injected into the battery case 5, sealed with a cap assembly 6, and the lithium secondary battery 1 is completed. The battery case 5 can also be cylindrical, rectangular, thin-film type, etc. For example, the lithium secondary battery 1 is also a thin-film battery. The lithium secondary battery 1 is also a lithium-ion battery.

[0090] A separator is disposed between the positive electrode and the negative electrode, and a battery structure can be formed. After the battery structure is laminated in a bicell structure, impregnated with an organic electrolyte, and the resulting product is housed and sealed in a pouch, a lithium-ion polymer battery is completed.

[0091] The lithium secondary battery is used not only as a power source for small devices but also as a unit battery of a medium- to large-sized device battery module including a large number of batteries.

[0092] Examples of the medium- to large-sized devices include power tools; xEVs including electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV); electric two-wheel vehicles including E-bikes and E-scooters; electric golf carts; electric trucks; electric commercial vehicles; or power storage systems; etc., but are not limited thereto. Also, the lithium secondary battery is used in all other applications that require high output, high voltage, and high-temperature driving.

[0093] Exemplary embodiments will be described in more detail through the following examples and comparative examples. However, these examples are for illustrating the technical idea and the scope of the present invention is not limited only by them.

[0094] (Manufacture of Lithium Secondary Battery) Example 1 (Manufacture of Positive Electrode) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 O2 96 wt%, as the conductive material, super-p 2 wt%, and as the binder, polyvinylidene fluoride 2 wt% were mixed to produce a first composition. The first composition was applied to an aluminum (Al) thin film, which is a positive electrode current collector with a thickness of about 20 μm, dried at about 80°C for 20 minutes, and then subjected to roll press to obtain a positive electrode with the first layer applied.

[0095] On the first layer, as the second positive electrode active material, LiNi 0.6 Mn 0.2 Co 0.2 O2 96 wt%, as the conductive material, super-p 2 wt%, and as the binder, polyvinylidene fluoride 2 wt% were mixed to form a second composition. Then, it was dried at about 80°C for 20 minutes to produce a double-coated positive electrode.

[0096] On the second layer, as the third positive electrode active material, LiNi 0.33 Mn 0.33 Co 0.33 O2 96 wt%, as the conductive material, super-p 2 wt%, and as the binder, polyvinylidene fluoride 2 wt% were mixed to form a third composition. Then, it was dried at about 80°C for 20 minutes to produce a triple-coated positive electrode.

[0097] At this time, the current density of the first layer is about 2.24 mAh / cm 2 and the current density of the second layer is about 1.68 mAh / cm 2 and the current density of the third layer is about 1.68 mAh / cm 2and the current density ratio of the first layer, the second layer, and the third layer was about 4:3:3. On the other hand, in the positive electrode, the content ratio of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material was 3.6:3:3.4.

[0098] At this time, the capacity ratio of the first layer, the second layer, and the third layer was 4:3:3, the thickness of the positive electrode active material layer including the first layer to the third layer was about 80 μm, and the overall current density was about 5.6 mAh / cm 2 It was.

[0099] At this time, the loading level of the positive electrode was 28 mg / cm 2 It was. A schematic diagram showing the structure of the positive electrode is illustrated in FIG. 5.

[0100] (Manufacture of Electrolyte) As a lithium salt, 1.15 M LiPF6 was included, and 7 wt% of fluoroethylene carbonate (FEC) was added to a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propylene propionate (PP) in a volume ratio of 2:1:2:5 to produce an electrolyte for a lithium secondary battery.

[0101] (Assembly of Lithium Secondary Battery) Using the positive electrode, a lithium metal negative electrode, a 18-μm-thick polyethylene separator coated with ceramics, and the electrolyte, a lithium secondary battery in a half-cell form was manufactured. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V.

[0102] Example 2 The current density of the first layer was about 1.2 mAh / cm 2 and the current density of the second layer was about 0.9 mAh / cm 2 and the current density of the third layer was about 0.9 mAh / cm 2A lithium secondary battery was manufactured in the same manner as in Example 1, except that the current density ratio of the first layer, the second layer, and the third layer was adjusted to about 4:3:3.

[0103] At this time, in the positive electrode, the content ratio of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material was 3.6:3.0:3.4.

[0104] Also, the capacity ratio of the first layer, the second layer, and the third layer was 4:3:3, the thickness of the positive electrode active material layer including the first layer to the third layer was about 45 μm, and the overall current density was about 3 mAh / cm 2 It was. At this time, the loading level of the positive electrode was 15 mg / cm 2 It was.

[0105] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a positive electrode manufactured by the following method was used.

[0106] (Manufacture of positive electrode) As the first positive electrode active material, 96% by weight of LiNi 0.8 Mn 0.1 Co 0.1 O2, 2% by weight of super-p as a conductive material, and 2% by weight of polyvinylidene fluoride as a binder were mixed to produce a first composition. The first composition was applied to an aluminum (Al) thin film, which is a positive electrode current collector having a thickness of about 20 μm, dried at about 80°C for 20 minutes, and then roll-pressed to obtain a positive electrode with the first layer applied.

[0107] On the first layer, as the second positive electrode active material, 96% by weight of LiNi 0.33 Mn 0.33 Co 0.33 O2, a second composition in which 2% by weight of super-p as a conductive material and 2% by weight of polyvinylidene fluoride as a binder were mixed was applied to form a second layer. Next, it was dried at about 80°C for 20 minutes to manufacture a double-coated positive electrode.

[0108] At this time, the current density of the first layer was about 3.08 mAh / cm 2 and the current density of the second layer was about 2.52 mAh / cm 2 The current density ratio of the first layer and the second layer was about 11:9. On the other hand, in the positive electrode, the content ratio of the first positive electrode active material and the second positive electrode active material was 4.9:5.1.

[0109] At this time, the capacity ratio of the first layer and the second layer was 11:9, the thickness of the positive electrode active material layer including the first layer and the second layer was about 80 μm, and the overall current density was about 5.6 mAh / cm 2 At this time, the loading level of the positive electrode was 28 mg / cm 2

[0110] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a positive electrode manufactured by the following method was used.

[0111] (Manufacture of positive electrode) As the positive electrode active material, 96% by weight of LiNi 0.6 Mn 0.2 Co 0.2 O2, 2% by weight of super-p as the conductive material, and 2% by weight of polyvinylidene fluoride as the binder were mixed to produce a positive electrode composition. The positive electrode composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, dried at about 80 °C for 20 minutes, and then roll-pressed to obtain a positive electrode. At this time, in the positive electrode, the thickness of the positive electrode active material layer was about 80 μm, the current density of the positive electrode was 5.6 mAh / cm 2 and the loading level was 28 mg / cm 2 A schematic diagram showing the structure of the positive electrode is illustrated in FIG. 6.

[0112] Comparative Example 2 ​​​A lithium secondary battery was manufactured in the same manner as in Example 1, except that a positive electrode manufactured by the following method was used.

[0113] (Manufacture of Positive Electrode) As a positive electrode active material, 296% by weight of LiNi 0.6 Mn 0.2 Co 0.2 O2, 2% by weight of super-p as a conductive material, and 2% by weight of polyvinylidene fluoride as a binder were mixed to produce a positive electrode composition. The positive electrode composition was applied to an aluminum (Al) thin film, which is a positive electrode current collector with a thickness of about 20 μm, dried at about 80 °C for 20 minutes, and then roll-pressed to obtain a positive electrode. At this time, in the positive electrode, the thickness of the positive electrode active material layer was about 43 μm, the current density of the positive electrode was 3 mAh / cm 2 and the loading level was 15 mg / cm 2 .

[0114] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a positive electrode manufactured by the following method was used.

[0115] (Manufacture of Positive Electrode) As a first positive electrode active material, 296% by weight of LiNi 0.33 Mn 0.33 Co 0.33 O2, 2% by weight of super-p as a conductive material, and 2% by weight of polyvinylidene fluoride as a binder were mixed to produce a first composition. The first composition was applied to an aluminum (Al) thin film, which is a positive electrode current collector with a thickness of about 20 μm, dried at about 80 °C for 20 minutes, and then roll-pressed to obtain a positive electrode coated with a first layer.

[0116] On the first layer, as a second positive electrode active material, LiNi 0.6 Mn 0.2 Co 0.2A second composition in which 96% by weight of O₂, 2% by weight of super-p as a conductive material, and 2% by weight of polyvinylidene fluoride as a binder were mixed was applied to form a second layer. Next, it was dried at about 80°C for 20 minutes to produce a double-coated positive electrode.

[0117] On the second layer, as a third positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 A third composition in which 96% by weight of O₂, 2% by weight of super-p as a conductive material, and 2% by weight of polyvinylidene fluoride as a binder were mixed was applied to form a third layer. Next, it was dried at about 80°C for 20 minutes to produce a triple-coated positive electrode.

[0118] At this time, the current density of the first layer was about 1.68 mAh / cm 2 and the current density of the second layer was about 1.68 mAh / cm 2 and the current density of the third layer was about 2.24 mAh / cm 2 and the current density ratio of the first layer, the second layer, and the third layer was about 3:3:4. On the other hand, in the positive electrode, the content ratio of the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material was 3.4:3:0.36.

[0119] At this time, the capacity ratio of the first layer, the second layer, and the third layer was 3:3:4, the thickness of the positive electrode active material layer including the first layer to the third layer was about 80 μm, and the overall current density was about 5.6 mAh / cm 2 It was. At this time, the loading level of the positive electrode was 28 mg / cm 2 It was.

[0120] Evaluation Example 1: Initial Charge Profile Characteristic Evaluation Regarding the lithium secondary batteries manufactured in Example 1 and Comparative Example 1, within a voltage range of 2.8 to 4.3 V, they were charged at a constant current of 0.2C for 1 cycle, and the charge profile characteristics showing the capacity change due to the voltage change were evaluated.

[0121] The evaluation results are shown in Fig. 7. Referring to Fig. 7, the lithium secondary battery according to Example 1, unlike the lithium secondary battery according to Comparative Example 1, includes lithium transition metal oxide layers with various compositions. However, the ratio of each layer can be appropriately adjusted, and the content of Ni metal can be adjusted to match the Ni metal content level in the lithium secondary battery according to Comparative Example 1.

[0122] Through this, it can be confirmed that a high-capacity battery at a level similar to that of the lithium secondary battery according to Comparative Example 1 is realized.

[0123] Evaluation Example 2: Life Characteristic Evaluation Regarding the lithium secondary batteries manufactured in Example 1 and Comparative Example 1, charging and discharging were performed under charging conditions (constant current / constant voltage of 0.7C / 4.35V, 0.025C cut-off current, 10-minute rest) and discharging conditions (constant voltage of 1.0C, 3.0V cut-off voltage, 10-minute rest), and the capacity during 40 cycles was measured and shown in Fig. 8.

[0124] Referring to Fig. 8, it can be confirmed that the lithium secondary battery according to Example 1 maintains better capacity characteristics as the number of cycles increases compared to the lithium secondary battery according to Comparative Example 1. That is, even when a thick-film positive electrode having the same loading level is introduced, the lithium secondary battery according to Example 1 in which a positive electrode active material layer with a multilayer structure is introduced exhibits excellent life characteristics compared to the lithium secondary battery according to Comparative Example 1 having a single-layer structure.

[0125] On the other hand, under the same conditions, not only in Example 1 and Comparative Example 1, but also for Example 2 and 3, and Comparative Example 2 and 3, the capacity retention rate (life) after 40 cycles was measured and shown in Table 1 below.

[0126]

Table 1

[0127] Referring to Table 1 above, when comparing Comparative Examples 1 and 3 and Examples 1 and 3 in which thick-film structured cathodes of the same thickness were introduced, in the case of the lithium secondary batteries of Examples 1 and 3 that included a multi-layer structured cathode active material layer but arranged different cathode active materials in a certain order, it can be confirmed that, under the same conditions, they exhibit excellent life characteristics not only compared to the lithium secondary battery of Comparative Example 1 with a single-layer structure, but also compared to the lithium secondary battery of Comparative Example 3 with the opposite stacking order. In particular, in the present invention, in the cathode current collector defined with the desired stacking order, it can be confirmed that the lithium secondary battery of Comparative Example 3 stacked in the direction opposite to the direction in which the Ni content decreases as going in the far direction has rather deteriorated life characteristics compared to the lithium secondary battery of Comparative Example 1 with a single-layer structure. This is presumably because the cathode active material with a low Ni content is advantageous in terms of life characteristics, and the cathode active material with a high Ni content is advantageous in terms of capacity characteristics, but due to the different stacking orders, the effects that should be realized cannot be fully realized.

[0128] In addition, it can be confirmed that the lithium secondary battery of Example 1 with a triple-layer structure having one more layer added compared to the lithium secondary battery of Example 3 configured in a double layer exhibits an even more excellent effect of improving life characteristics.

[0129] On the other hand, the lithium secondary batteries of Examples 2 and Comparative Example 2 introduced a thin-film cathode structure instead of the thick-film cathode structure, respectively, and it can be confirmed that the effect of improving the life by introducing the multi-layer cathode structure of the present invention is shown not only in the thick-film structure but also in the thin-film structure. However, such an improvement effect is smaller compared to the case of the thick-film structure, which is presumably because the problem of life reduction is not significant in the cathode of the thin-film structure.

[0130] Example 4 (Manufacture of Cathode) As the first cathode active material, LiNi 0.8 Mn 0.1 Co 0.12% by weight of O296, 2% by weight of super-p as the conductive material, and 2% by weight of polyvinylidene fluoride as the binder were mixed to produce the first composition. The first composition was applied to an aluminum (Al) thin film, which is a positive electrode current collector with a thickness of about 20 μm, dried at about 80°C for 20 minutes, and then roll-pressed to obtain a positive electrode with the first layer applied.

[0131] On the first layer, a second composition obtained by mixing 95% by weight of LiCoO2 as the second positive electrode active material and 5% by weight of polyvinylidene fluoride as the binder was applied to form a second layer. Next, it was dried at about 80°C for 20 minutes to produce a double-coated positive electrode. At this time, the loading amount of the first layer was about 5.25 mg / cm 2 and the loading amount of the second layer was about 12.25 mg / cm 2 and the loading amount ratio of the first layer to the second layer was 3:7. On the other hand, the current density of the positive electrode including the first layer and the second layer was 3.0 mAh / cm 2 . In the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 7:3.

[0132] (Manufacture of negative electrode) As the negative electrode active material, 98% by weight of graphite and 2% by weight of the binder were mixed, put into distilled water, and then dispersed for 60 minutes using a mechanical stirrer to produce a negative electrode active material composition. The negative electrode active material composition was applied to a copper current collector with a thickness of 10 μm to a thickness of about 60 μm using a doctor blade, dried with a hot air dryer at 100°C for 0.5 hour, and then further dried and rolled once under vacuum at 120°C for 4 hours to produce a negative electrode with a negative electrode active material layer formed on the current collector.

[0133] (Manufacture of electrolyte) As the lithium salt, 1.15 M LiPF6 was included, and 7% by weight of fluoroethylene carbonate (FEC) was added to a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propylene propionate (PP) in a volume ratio of 2:1:2:5 to produce an electrolyte for a lithium secondary battery.

[0134] (Assembly of Lithium Secondary Battery) Using the positive electrode, the negative electrode, a 18-μm-thick polyethylene separator coated with ceramics, and the electrolyte, a lithium secondary battery was manufactured. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V.

[0135] Example 4-1 The current density of the positive electrode is 2.6 mAh / cm 2 A lithium secondary battery was manufactured in the same manner as in Example 4, except that the contents of the first positive electrode active material and the second positive electrode active material were adjusted so as to be. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V. In the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 7:3.

[0136] Example 4-2 The current density of the positive electrode is 3.4 mAh / cm 2 A lithium secondary battery was manufactured in the same manner as in Example 4, except that the contents of the first positive electrode active material and the second positive electrode active material were adjusted so as to be. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V. In the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 7:3.

[0137] Example 4-3 A lithium secondary battery was manufactured in the same manner as in Example 4, except that the operating voltage of the lithium secondary battery was adjusted to 3.0 to 4.4 V. In the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 7:3.

[0138] Example 5 The loading amount of the first layer is about 8.75 mg / cm 2 and the loading amount of the second layer is about 8.75 mg / cm 2A lithium secondary battery was manufactured in the same manner as in Example 4, except that the loading amount ratio between the first layer and the second layer was adjusted to be about 5:5. In the positive electrode, the content ratio between the first positive electrode active material and the second positive electrode active material was 5:5.

[0139] Comparative Example 4 (Manufacture of Positive Electrode) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 O2 28.8 wt%, as the second positive electrode active material, LiCoO2 67.2 wt%, as the conductive material, super-p 2 wt%, and as the binder, polyvinylidene fluoride 2 wt% were mixed to produce a positive electrode composition. The positive electrode composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, dried at about 80 °C for 20 minutes, and then roll-pressed to produce a positive electrode. At this time, the thickness of the positive electrode active material layer was about 40 μm. Also, the current density of the positive electrode was 3.0 mAh / cm 2 It was.

[0140] (Manufacture of Negative Electrode) The negative electrode used in Example 4 was used.

[0141] (Manufacture of Electrolyte) The electrolyte used in Example 4 was used.

[0142] (Assembly of Lithium Secondary Battery) Using the positive electrode, the negative electrode, a 18-μm-thick polyethylene separator coated with ceramics, and the electrolyte solution, a lithium secondary battery was manufactured. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V.

[0143] Comparative Example 4-1 The current density of the positive electrode was 2.6 mAh / cm 2A lithium secondary battery was manufactured in the same manner as in Comparative Example 4, except that the contents of the first positive electrode active material and the second positive electrode active material were adjusted so as to be [specific conditions]. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V.

[0144] Comparative Example 4-2 The current density of the positive electrode is 3.4 mAh / cm 2 A lithium secondary battery was manufactured in the same manner as in Comparative Example 4, except that the contents of the first positive electrode active material and the second positive electrode active material were adjusted so as to be [specific conditions]. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35 V.

[0145] Comparative Example 4-3 A lithium secondary battery was manufactured in the same manner as in Comparative Example 4, except that the operating voltage of the lithium secondary battery was adjusted to 3.0 to 4.4 V.

[0146] Comparative Example 5 As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 A lithium secondary battery was manufactured in the same manner as in Comparative Example 4, except that 48% by weight of LiNiMnCoO2 was used as the first positive electrode active material, 48% by weight of LiCoO2 was used as the second positive electrode active material, 2% by weight of super-p was used as the conductive material, and 2% by weight of polyvinylidene fluoride was used as the binder to produce a positive electrode composition.

[0147] Evaluation Example 3: Discharge Profile Evaluation With respect to the lithium secondary batteries manufactured in Example 4 and Comparative Example 4, charging and discharging were performed under charging conditions (constant current / constant voltage of 0.2C / 4.35V, 0.025C cut-off current, 10-minute rest) and discharging conditions (constant voltage of 0.2C, 3.0V cut-off voltage, 10-minute rest), and the discharge profile was measured and shown in FIG. 9. Referring to FIG. 9, it can be confirmed that the lithium secondary battery according to Example 4, even though it contains LCO with relatively inferior capacity characteristics while applying a multi-layer cathode active material, exhibits a discharge capacity at a level not significantly inferior compared to the lithium secondary battery according to Comparative Example 4 that applies an NCM-based cathode active material.

[0148] Evaluation Example 4: Cycle Characteristic Evaluation Regarding the lithium secondary batteries manufactured in Example 4 and Comparative Example 4, charging and discharging were performed under charging conditions (constant current / constant voltage of 0.7C / 4.35V, 0.025C cut-off current, 10-minute rest) and discharging conditions (constant voltage of 1.0C, 3.0V cut-off voltage, 10-minute rest), and the capacity during 300 cycles was measured and shown in FIG. 10.

[0149] Referring to FIG. 10, it can be confirmed that the lithium secondary battery according to Example 4 maintains better capacity characteristics as the number of cycles increases compared to the lithium secondary battery according to Comparative Example 4. That is, even when both the LCO-based material and the NCM-based material are contained together, the lithium secondary battery according to Example 4 that contains them in a specific order exhibits excellent cycle characteristics compared to the lithium secondary battery according to Comparative Example 4 that is simply mixed.

[0150] On the other hand, under the same conditions, not only Example 4 and Comparative Example 4, but also Examples 4-1 to 4-3, Example 5, Comparative Examples 4-1 to 4-3, and Comparative Example 5 were measured for the capacity retention rate (life) after 300 cycles and shown in Table 2 below.

[0151]

Table 2

[0152] Referring to Table 2 above, it can be confirmed that even when applying NCM (the first positive electrode active material) and LCO (the second positive electrode active material) with the same content ratio, the lithium secondary battery of the example in which NCM and LCO are arranged in a multilayer structure in a certain order exhibits excellent life characteristics compared to the lithium secondary battery of the comparative example in which NCM and LCO are simply mixed under the same conditions. Not only that, regardless of the changes in the current density, driving voltage, and NCM / LCO ratio, the lithium secondary battery of the example shows little or few changes in life characteristics, while the lithium secondary battery of the comparative example with simple mixing shows significant deterioration in life characteristics.

[0153] In the above, with reference to the drawings and examples, the desirable embodiments according to the present invention have been described, but they are merely exemplary, and those skilled in the art in the relevant technical field will be able to understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the protection scope of the present invention is defined by the scope of the claims.

Industrial Applicability

[0154] The lithium secondary battery according to one embodiment can improve life characteristics by adopting a positive electrode including a layer structure of a novel configuration.

Explanation of Reference Numerals

[0155] 1 Lithium secondary battery 2 Negative electrode 3 Positive electrode 4 Separator 5 Battery case 6 Cap assembly 10 Positive electrode for secondary battery 11 Positive electrode current collector 12 First layer 13 Second layer 20 Positive electrode for secondary battery 21 Positive electrode current collector 22 First layer 23 Second layer 24 Third layer

Claims

1. A positive electrode current collector, a first layer disposed on at least one surface of the positive electrode current collector and containing a first positive electrode active material, a second layer disposed on the first layer and containing a second positive electrode active material, and the first positive electrode active material is represented by the following Chemical Formula 1A, The second positive electrode active material is represented by the following chemical formula 1B or is LiCoO ２ and the current density ratio of the first layer to the second layer is 2:8 to 8:2, the first positive electrode active material and the second positive electrode active material have different compositions from each other, the thickness of the positive electrode active material layer including the first layer and the second layer is 80 μm or more and 110 μm or less, a positive electrode for a secondary battery: [Chemical Formula 1A] Li ｘ１ Ni ｙ１ M1 １－ｙ１ O ２ [Chemical Formula 1B] Li ｘ２ Ni ｙ２ M2 １－ｙ２ O ２ In the Chemical Formulas 1A and 1B, 0.9 ≤ x1 ≤ 1.2, 0.9 ≤ x2 ≤ 1.0, 0.7 ≤ y1 ≤ 0.98, 0.1 ≤ y2 ≤ 0.98, and y1 > y2, M1 and M2 are each independently one or more metal or transition metal elements having an oxidation number of +2 or +3.

2. The M1 and the M2 are each independently one or more elements selected from the group consisting of Al, Mg, Mn, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, and Bi, the positive electrode for a secondary battery according to Claim 1.

3. The first positive electrode active material is represented by the following Chemical Formula 1-1 or the following Chemical Formula 1-2, the second positive electrode active material is represented by the following Chemical Formula 1-1 or the following Chemical Formula 1-2, and the first positive electrode active material and the second positive electrode active material are different from each other, the positive electrode for a secondary battery according to Claim 1: [Chemical Formula 1-1] Li ｘ’ Ni ｙ’ Co １－ｙ’－ｚ’ Al ｚ’ O ２ [Chemical Formula 1-2] Li ｘ’ Ni ｙ’ Co １－ｙ’－ｚ’ Mn ｚ’ O ２ In the Chemical Formulas 1-1 and 1-2, 0.9 ≤ x' ≤ 1.2, 0.1 ≤ y' ≤ 0.98, 0 < z' < 0.5, and 0 < 1 - y' - z' < 0.

5.

4. The content of Ni in the first positive electrode active material is 0.7 mol or more based on the total number of moles of transition metals, the positive electrode for a secondary battery according to Claim 1.

5. When the second positive electrode active material is represented by Chemical Formula 1, the Ni content in the second positive electrode active material is 0.6 mol or less based on the total number of moles of transition metals, the positive electrode for a secondary battery according to Claim 1.

6. The weight ratio of the first positive electrode active material to the second positive electrode active material is 3:7 to 7:3, the positive electrode for a secondary battery according to Claim 1.

7. The active material loading ratio of the first layer to the second layer is 3:7 to 7:3, the positive electrode for a secondary battery according to Claim 1.

8. The current density ratio of the first layer to the second layer is 3:7 to 7:3, the positive electrode for a secondary battery according to Claim 1.

9. The thickness ratio of the first layer to the second layer is 2:8 to 8:2, and the positive electrode for a secondary battery according to claim 1.

10. The loading amount of the first layer is 3 mg / cm ２ to 40 mg / cm ２ The positive electrode for a secondary battery according to claim 1, wherein the loading amount is as described above.

11. The loading amount of the second layer is 40 mg / cm ２ to 3 mg / cm ２ The positive electrode for a secondary battery according to claim 1, which is such.

12. The current density of the positive electrode is 2 to 10 mAh / cm ２ The positive electrode for a secondary battery according to claim 1, which is such that.

13. It further includes a third layer disposed on the second layer and containing a third positive electrode active material, The third positive electrode active material is represented by the following Chemical Formula 1, and the third positive electrode active material has a composition different from that of the first positive electrode active material and the second positive electrode active material. The positive electrode for a secondary battery according to claim 1: [Chemical Formula 1] Li ｘ Ni ｙ M １－ｙ O ２ In the Chemical Formula 1, 0.9 ≤ x ≤ 1.2, 0.1 ≤ y ≤ 0.98, M is one or more metals or transition metal elements with an oxidation number of +2 or +3.

14. The third positive electrode active material is represented by the following Chemical Formula 1C, and the positive electrode for a secondary battery according to claim 13: [Chemical Formula 1C] Li ｘ３ Ni ｙ３ M3 １－ｙ３ O ２ In the Chemical Formula 1C, 0.9 ≤ x3 ≤ 1.2, 0.1 ≤ y3 ≤ 0.98, M3 is one or more metals or transition metal elements with an oxidation number of +2 or +3, However, y1 > y2 > y3.

15. The current density of the third layer is equal to or less than the current density of the second layer, and the current density of the second layer is equal to or less than the current density of the first layer. The positive electrode for a secondary battery according to claim 13.

16. Based on the total weight of the positive electrode for a secondary battery, the sum of the contents of the first positive electrode active material and the second positive electrode active material is 80 to 98% by weight. The positive electrode for a secondary battery according to claim 1.

17. A positive electrode for a secondary battery according to any one of claims 1 to 16, A negative electrode disposed opposite to the positive electrode, An electrolyte disposed between the positive electrode and the negative electrode, and a lithium secondary battery including the same.

18. The operating voltage of the lithium secondary battery is 2.5 to 4.5 V. The lithium secondary battery according to claim 17.

Images