Positive electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery containing 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] This invention relates to a positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery containing the same. [Background technology]

[0002] Lithium-ion batteries are used as power sources for portable electronic devices such as video cameras, mobile phones, and laptop computers. Compared to existing lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, and nickel-zinc batteries, rechargeable lithium-ion batteries have an energy density more than three times higher per unit weight and can be charged quickly.

[0003] The lithium secondary battery produces electrical energy through oxidation and reduction reactions that occur when lithium ions are absorbed and released into the positive and negative electrodes, with an organic electrolyte or polymer electrolyte filled between the positive and negative electrodes, which contain an active material capable of intercalating and releasing lithium ions.

[0004] In particular, research on thick film electrodes is actively progressing in order to realize high-capacity secondary batteries. These thick film electrodes have advantages such as enabling high-capacity batteries by reducing the thickness of the substrate / separator, and saving battery costs. However, there is a problem of reduced battery performance due to the increased distance electrons or lithium traveled as a result of the thicker electrode film.

[0005] In particular, the performance degradation of such thick-film electrodes is noticeable in the non-uniform charge-discharge characteristics in the thickness direction of the electrode. Specifically, increasing the film thickness of the electrode due to thickening causes an increase in polarization during charging and discharging. Such polarization is caused by a potential difference in the thickness direction of the electrode, and such polarization induces a difference in the depth of charge and discharge of the active material during charging and discharging, with some parts having a high potential and others having a low potential. In the end, the deeper the polarization phenomenon, the deeper the degradation of the parts that maintain a high potential, leading to an overall deterioration of battery performance.

[0006] As a solution to this problem, a method was proposed to increase the amount of conductive material and raise the porosity of the electrode plate, but this method had the problem of limiting the ability to increase the capacitance.

[0007] Therefore, there is a need for a method that minimizes the degradation of electrode performance while using thick film electrodes.

[0008] In addition, lithium-containing metal oxides are commonly used as the positive electrode active material in the positive electrode of the lithium secondary battery. For example, transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium nickel oxide (LiNiO2), or composite oxides in which some of these transition metals are substituted with other transition metals, are used as the positive electrode active material in the lithium secondary battery.

[0009] Recently, research is actively underway to use LiCoO2 (hereinafter referred to as "LCO"), which is widely used in the IT field, in combination with NCM-based cathode active materials in which some of the nickel in lithium nickel oxide is replaced with manganese or cobalt.

[0010] Mixing LCO with NCM-based positive electrode active material has the advantage of increasing battery capacity and reducing costs compared to using LCO alone. However, NCM-based positive electrode active material has the problem of being less stable at high voltages compared to LCO. In addition, when the two materials are used together, problems such as an increase in positive electrode potential occur due to the voltage difference generated by polarization during charging and discharging.

[0011] Therefore, when using a mixture of NCM-based positive electrode active material and LCO, there is a need for a method to improve degradation in high-voltage environments and enhance stability. [Overview of the project] [Problems that the invention aims to solve]

[0012] One aspect of the present invention is to provide a positive electrode for a secondary battery with a novel configuration that includes multiple layers of different positive electrode active materials arranged in a specific order on a current collector.

[0013] Another aspect of the present invention is to provide a method for manufacturing the positive electrode for the secondary battery. Another aspect of the present invention is to provide a lithium secondary battery employing the aforementioned positive electrode for secondary batteries. [Means for solving the problem]

[0014] In one aspect of the present invention, a positive electrode current collector and Distributed on at least one surface of the positive electrode current collector, a first layer containing a first positive electrode active material, A second layer disposed on the first layer and containing a second positive electrode active material, The aforementioned first positive electrode active material is represented by the following chemical formula 1, The above-mentioned second positive electrode active material is represented by the following chemical formula 1 or chemical formula 2, 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: [C1] Li x Ni y M 1-y O2 [Case 2] Li α Co β M' 1-β O2 In the aforementioned 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, independently of each other, one or more metal or transition metal elements with an oxidation state of +2 or +3.

[0015] In other aspects, the process involves applying a first composition containing a first positive electrode active material to at least one surface of a positive electrode current collector to form a first layer. The process includes the step of applying a second composition containing a second positive electrode active material onto the first layer to form a second layer, 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 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 improve its life characteristics by adopting a positive electrode including a novel layered structure. [Brief Description of the Drawings]

[0018] [Figure 1] It is a schematic diagram of a lithium secondary battery according to an exemplary embodiment. [Figure 2] It is a schematic diagram of a positive electrode for a secondary battery according to an exemplary embodiment. [Figure 3] It is a schematic diagram of a positive electrode for a secondary battery according to another exemplary embodiment. [Figure 4] It is a schematic diagram showing a method for manufacturing a positive electrode for a secondary battery according to an exemplary embodiment. [Figure 5] This is a schematic diagram of the positive electrode manufactured in Example 1. [Figure 6] This is a schematic diagram of the positive electrode manufactured in Comparative Example 1. [Figure 7] This graph shows the initial charge profile results of lithium secondary batteries using the positive electrodes manufactured in Example 1 and Comparative Example 1. [Figure 8] This graph shows the cycle life of lithium secondary batteries using the positive electrodes manufactured in Example 1 and Comparative Example 1. [Figure 9] This graph shows the discharge profiles of lithium secondary batteries using the positive electrodes manufactured in Example 4 and Comparative Example 4. [Figure 10] This graph shows the cycle life of lithium secondary batteries using the positive electrodes manufactured in Example 4 and Comparative Example 4. [Modes for carrying out the invention]

[0019] The present invention will be described in more detail below. Hereinafter, with reference to Figure 2, a positive electrode for a secondary battery according to one embodiment of the present invention will be described. Figure 2 is a schematic diagram of a positive electrode for a secondary battery according to an exemplary embodiment. Referring to Figure 2, the positive electrode 10 for a secondary battery, as seen from one side, includes a positive electrode current collector 11, a first layer 12 disposed on at least one side 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 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: [C1] Li x Ni y M 1-y O2 In the aforementioned chemical formula 1, 0.9≦x≦1.2 and 0.1≦y≦0.98, M is one or more metal or transition metal elements with an oxidation state of +2 or +3.

[0021] As can be seen from the above, the positive electrode 10 for secondary batteries according to the present invention incorporates a multilayer structure in which multiple positive electrode active materials with different transition metal compositions are each included, while constituting a positive electrode containing a Ni-based material. This allows for high capacity through thick film construction, while suppressing degradation of battery performance and improving lifespan characteristics. However, M is an element other than Ni.

[0022] In one embodiment, 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: [Case 1A] Li x1 Ni y1 M1 1-y1 O2 In the aforementioned chemical formula 1A, [C1B] Li x2 Ni y2 M 1-y2 O2 In the aforementioned chemical formulas 1A and 1B, the definitions relating to M1, x1, y1, M2, x2, and y2 shall be as defined herein for M, x, and y. However, y1 > y2.

[0024] That is, in order to solve the problems of polarization phenomenon deepening 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 with 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 with a high Ni content is arranged as the positive electrode active material so as to be advantageous for high capacity. In the second layer 13 close to the separator (not shown), a lithium transition metal oxide with 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 with different compositions are included in a multi-layer structure, different from the definition described above, when y1 < y2 and the positive electrode active material with a higher Ni content is arranged in the second layer 13, the 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 chemical formulas 1-1 and 1-2, y' represents the Ni content in the lithium transition metal oxide, and 0.5 ≤ y' ≤ 0.98. For example, in chemical formulas 1-1 and 1-2, 0.6 ≤ y ≤ 0.98. For example, in chemical formulas 1-1 and 1-2, 0.7 ≤ y ≤ 0.98. For example, in chemical formulas 1-1 and 1-2, 0.8 ≤ y ≤ 0.98.

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

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

[0030] As mentioned above, when using lithium transition metal oxides with a high Ni content, where the mole fraction of Ni is 0.6 or more, as the positive electrode active material, despite the advantage of being able to realize high-capacity batteries, there is a significant disadvantage of reduced lifespan, high-temperature stability, and high-temperature storage characteristics, which makes commercialization difficult. Therefore, as a configuration to solve this problem, the lithium secondary battery can suppress degradation due to polarization and improve lifespan by forming lithium transition metal oxides with a low Ni content, where the mole fraction of Ni is 0.6 or less, on top of the lithium transition metal oxide layer with a high Ni content.

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

[0032] For example, the second cathode active material is LiNi 0.6 Co 0.2 Mn 0.2 O2 or LiLi 033 Co0.33 Mn 0.33 It 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 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 3:7 to 5:5. If the weight ratio of the first positive electrode active material and the second positive electrode active material falls outside this range and is less than 3:7, there is a problem that the content of high-content Ni-based material is excessively reduced, making it difficult to achieve 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 difficult to control the degradation phenomenon of the positive electrode.

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

[0035] In one embodiment, the thickness ratio of the first layer 12 to the second layer 13 is 2:8 to 8:2. In another embodiment, the thickness of the first layer 12 is 10 μm to 70 μm. In yet another embodiment, the thickness of the second layer 13 is 10 μm to 70 μm. If the thickness ratio of the first layer 12 to the second layer 13 falls outside this range, or if the thickness of the first layer 12 is less than 2:8, or if the thickness of the second layer 12 is less than 10 μm, or if the thickness of the second layer 13 exceeds 70 μm, and the second layer 13 is excessively thicker than the first layer 12, there is a problem in that the capacity of the lithium secondary battery is excessively small relative to its volume. On the other hand, if the thickness ratio of the first layer 12 to the second layer 13 exceeds 8:2, or if the thickness of the first layer 12 exceeds 70 μm, or if the thickness of the second layer 13 is less than 10 μm, and the first layer 12 is excessively thicker than the second layer 13, it has the effect of increasing the capacity of the lithium secondary battery, but it does not adequately prevent side reactions occurring on the positive electrode surface, leading to a deterioration of the lifespan characteristics, and in particular, a decrease in high-temperature characteristics such as high-temperature lifespan characteristics and high-temperature stability.

[0036] For example, the current density of the positive electrode 10 is 3 to 6 mAh / cm².2 But so. In one embodiment, the thickness of the positive electrode active material layer, including the first layer 11 and the second layer 12, is 40 μm or more. For example, the thickness of the positive electrode active material layer can be 40 μm to 110 μm.

[0037] In other words, the positive electrode 10 according to the present invention has a thickness within the range described above, embodies a thick film structure, and through it, can realize a high-capacity battery.

[0038] The positive electrode 10 for the secondary battery, according to other aspects, 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: [C1] Li x Ni y M 1-y O2 [Case 2] Li α Co β M' 1-β O2 In the aforementioned 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, independently of each other, one or more metal or transition metal elements with an oxidation state of +2 or +3.

[0040] As can be seen from the above, the positive electrode 10 for secondary batteries according to the present invention introduces a multilayer structure including the aforementioned separate first layer 12 and second layer 13 in order to solve the problem of preventing degradation of the Ni-based material due to polarization phenomena that occur when LCO-based material and Ni-based material are simply mixed and coated together.

[0041] In other words, the positive electrode 10 for a secondary battery according to the present invention has a multilayer structure containing multiple positive electrode active materials of different compositions, wherein the first positive electrode active material represented by chemical formula 1 is placed in the layer closest to the positive electrode current collector 11, and the second positive electrode active material represented by chemical formula 2 is placed in the layer closest to the negative electrode.

[0042] As a result, the positive electrode 10 for secondary batteries according to the present invention can control the deterioration of the positive electrode due to polarization by positioning the second positive electrode active material, which is an LCO-based material with a relatively high voltage, near the negative electrode, and can fully utilize the first positive electrode active material, which is a Ni-based material that is a high-capacity material, thereby exhibiting the effect of realizing high capacity. On the other hand, even if an LCO-based material and a Ni-based material are included in a multilayer structure similar to the present invention, if the LCO-based material is placed near the current collector and the Ni-based material is placed near the negative electrode, polarization may cause the Ni-based material to be subjected to a higher voltage condition, which can lead to a problem of further deterioration of its lifespan.

[0043] In one embodiment, M and M' are 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.

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

[0045] In one embodiment, the first positive electrode active material contained in the first layer 12 can also be represented by the following chemical formula 1-1 or chemical formula 1-2: [C1-1] Li x’ Ni y’ Co 1-y’-z’ Al z’ O2 [Formation 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 embody a high-capacity battery, there are drawbacks such as significant deterioration in life characteristics, high-temperature stability, and high-temperature storage characteristics. Due to such drawbacks, commercialization is difficult. Therefore, as a configuration for solving this, 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, can exhibit excellent life characteristics, high-temperature stability, etc.

[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 [[ID=4K]]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 to the second positive electrode active material is less than 3:7 outside the above range, there is a problem that the content of the 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 to 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 3:7 to 7:3. In another embodiment, the thickness of the first layer 12 is 3 μm to 50 μm. In yet another embodiment, the thickness of the second layer 13 is 3 μm to 50 μm. If the thickness ratio of the first layer 12 to the second layer 13 falls outside this range, or if the thickness of the first layer 12 is less than 3:7, or if the thickness of the second layer 13 is less than 3 μm, or if the thickness of the second layer 13 exceeds 50 μm, and the second layer 13 is excessively thicker than the first layer 12, there is a problem in that the capacity of the lithium secondary battery is excessively low relative to its volume. On the other hand, if the thickness ratio of the first layer 12 to the second layer 13 exceeds 7:3, or if the thickness of the first layer 12 exceeds 50 μm, or if the thickness of the second layer 13 is less than 3 μm, and the first layer 12 is excessively thicker than the second layer 13, it has the effect of increasing the capacity of the lithium secondary battery, but it does not adequately prevent side reactions occurring on the positive electrode surface, leading to a deterioration of the lifespan characteristics, and in particular, a decrease in high-temperature characteristics such as high-temperature lifespan characteristics and high-temperature stability.

[0052] For example, the current density of the positive electrode 10 is 2 to 10 mAh / cm². 2 But so. In one embodiment, the ratio of the active material loading amounts of the first layer 12 to the second layer 13 is 3:7 to 7:3. In one embodiment, the loading amount of the first layer 12 is 3 to 40 mg / cm³. 2 In other embodiment, the loading amount of the second layer 13 is 40 to 3 mg / cm³. 2 But so.

[0053] In one embodiment, the positive electrode for the secondary battery may further include one or more of a binder and a conductive material. A description of the binder and conductive material will be provided below.

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

[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, a second layer 23 disposed on the first layer 22 and containing the second positive electrode active material, and 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 being represented by the following chemical formula 1, and the third positive electrode active material having a different composition from the first positive electrode active material and the second positive electrode active material: [C1] Li x Ni y M 1-y O2 In the aforementioned chemical formula 1, 0.9≦x≦1.2 and 0.1≦y≦0.98, M is one or more metal or transition metal elements with an oxidation state of +2 or +3.

[0056] In other words, the positive electrode 20 for a secondary battery according to the present invention may include, in addition to the first layer 22 and second layer 23 described above, a separate positive electrode active material layer containing a lithium transition metal oxide of a different composition as a third layer 24, and may further include additional layers containing lithium transition metal oxides having different compositions, although not described separately. In this case, the number of additional layers 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: [Case 1A] Li x1 Ni y1 M1 1-y1 O2 In the aforementioned chemical formula 1A, [C1B] Li x2 Ni y2 M2 1-y2 O2 [C1C] Li x3 Ni y3 M3 1-y3O2 In the aforementioned chemical formulas 1A, 1B, and 1C, the definitions relating to M1, x1, y1, M2, x2, y2, M3, x3, and y3 shall be as defined herein for M, x, and y. However, y1 > y2 > y3.

[0058] In other words, when the positive electrode active material layer contains more than two layers, the Ni content decreases as you move from the layer closest to the positive electrode current collector 21 (first layer 22) to the layer furthest away (third layer 24). Through this arrangement, the high capacity characteristics and suppression of battery performance degradation embodied in the present invention can be achieved.

[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] The following describes a method for manufacturing a positive electrode for a secondary battery according to one embodiment of the present invention, with reference to Figure 4. Figure 4 is a schematic diagram showing a method for manufacturing a positive electrode for a secondary battery according to an exemplary embodiment. Referring to Figure 4, the method for manufacturing a positive electrode 10 for a secondary battery according to another aspect of the present invention includes the steps of: applying a first composition containing a first positive electrode active material to 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, wherein 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: [C1] Li x Ni y M 1-y O2 [Case 2] Li α Co β M' 1-β O2 In the aforementioned 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, independently of each other, one or more metal or transition metal elements with an oxidation state of +2 or +3.

[0061] For a detailed explanation of x, y, α, β, M, and M' in the aforementioned chemical formulas 1 and 2, please refer to the section above.

[0062] Furthermore, although not otherwise described, if a third layer containing an additional third positive electrode active material is further included, 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 helps to bond the lithium transition metal oxide, i.e., the positive electrode active material or inorganic material, to the conductive material, and to the positive electrode active material and the current collector. It is also included between the positive electrode current collector and the positive electrode active material layer, within the positive electrode active material layer, between the positive electrode active material layer and the inorganic material layer, or within the inorganic material layer, and is added in amounts of 1 to 50 parts by weight based on 100 parts by weight of the positive electrode active material or inorganic material. For example, the binder can be added in amounts 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 positive electrode active material or inorganic material. For example, the binder is one or more selected from the group consisting of polyvinylidene fluoride, polyvinylidene chloride, polybenzimidazole, polyimide, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polystyrene, polymethyl methacrylate, polyaniline, acrylonitrile butadiene styrene, phenolic resin, epoxy resin, polyethylene terephthalate, polytetrafluoroethylene, polyphenylene sulfide, polyamide-imide, polyetherimide, polyethersulfone, polyamide, polyacetal, polyphenylene oxide, polybutylene terephthalate, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber. For example, if there are two or more binders in the above example, then a variety of copolymers obtained by polymerizing the two or more binders are also binders.

[0065] The first or second composition may optionally further contain a conductive material to provide conductive passages to the aforementioned positive electrode active material or inorganic material and to further improve electrical conductivity. The conductive material can be any material commonly used in lithium secondary batteries, and examples include carbon-based materials such as carbon black, acetylene black, Ketjen black, and carbon fiber (e.g., vapor-grown carbon fiber); metallic materials such as metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof. The content of the conductive material can be appropriately adjusted. For example, the weight ratio of the positive electrode active material or inorganic material to the conductive material may be in the range of 99:1 to 90:10.

[0066] The positive electrode current collector is not particularly limited as long as it has a thickness of 3 μm to 500 μm, does not induce chemical changes in the battery, and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatment with carbon, nickel, titanium, silver, etc., can be used. The current collector can also have fine irregularities formed on its surface to enhance the adhesion of the positive electrode active material, and can take various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0067] The prepared first composition can be directly applied to a positive electrode current collector and dried to produce a positive electrode plate. Alternatively, the first composition can be cast onto a separate support, and the resulting film, obtained by peeling it off the support, can be laminated onto the positive electrode current collector to produce a positive electrode plate.

[0068] On the other hand, the first composition or the second composition may further contain a solvent. The solvent may be N-methylpyrrolidone, acetone, or water, but is not limited to these; any solvent that is usable in the art may be used.

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

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

[0071] For example, when drying after applying the first or second composition, the drying may be carried out 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 2.5 to 4.5V. For example, the operating voltage of the lithium secondary battery is 3.0 to 4.4V.

[0074] The positive electrode for the secondary battery includes the positive electrode current collector, the first layer, and the second layer as essential components, and a description relating thereto should be seen above. Furthermore, the first layer and the second layer each have the first positive electrode active material and the second positive electrode active material as essential components, and may also further include positive electrode active material materials commonly used in lithium secondary batteries in addition to these essential components.

[0075] For example, Li a A' 1-b B b D2 (In the above 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 above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE 2-b B b O 4-c D c (In the above chemical formula, 0 ≤ b ≤ 0.5 and 0 ≤ c ≤ 0.05); Li a Ni 1-b-c Co b B c D α (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2); Li a Ni 1-b-c Co b B c O 2-α F α (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Co b B c O 2-α F2 (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b B c D α (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2); Li a Ni 1-b-c Mn b B c O 2-α F α (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni 1-b-c Mn b B c O 2-αF2 (In the above chemical formula, 0.90 ≤ a ≤ 1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2); Li a Ni b E c G d O2 (where the above chemical formula has the following parameters: 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 (where the above chemical formula has the following parameters: 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 (where the above chemical formula is such that 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (where the above chemical formula is such that 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (where the above chemical formula is such that 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (in the above chemical formula, 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiIO2; LiNiVO4; Li 3-f J2(PO4)3(0≦f≦2);Li 3-f It may also contain additional compounds represented by any one of the chemical formulas Fe2(PO4)3(0≦f≦2);LiFePO4:

[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, rare earth elements, 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; and 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’’ Examples include O2 (0 ≤ x'' ≤ 0.5, 0 ≤ y'' ≤ 0.5) and FePO4.

[0077] On the other hand, the negative electrode can also be 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 onto a metal current collector and dried to produce a negative electrode plate. Alternatively, the negative electrode plate can also be produced by casting the negative electrode active material composition onto a separate support, and then laminating the film, which is peeled off the support, onto a metal current collector.

[0078] The negative electrode active material can be any material that can be used as a negative electrode active material for a lithium battery in the art. For example, it may include one or more materials selected from the group consisting of lithium metal, metals alloyable with lithium, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0079] For example, metals that can be alloyed 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 thereof, and not Si), and 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 thereof, and not Sn). The element Y may be 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, Ti, Ge, P, As, Sb, Bi, S, Se, or Te. For example, the transition metal oxide may be lithium titanium oxide, vanadium oxide, lithium vanadium oxide, etc. For example, the non-transition metal oxide may be SnO2, SiO2 x’’’ (0 <x’’’<2)などでもある。

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

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

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

[0083] Any separator commonly used in lithium batteries can be used. A separator that exhibits low resistance to electrolyte ion movement while having excellent electrolyte moisture absorption capacity may be used. For example, it may be selected from glass fiber, polyester, Teflon®, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or combinations thereof, and may be in the form of a nonwoven or woven fabric. For example, a rollable separator such as polyethylene or polypropylene may be used in lithium-ion batteries, and a separator with excellent organic electrolyte impregnation capacity may be used in lithium-ion polymer batteries. For example, the separator may also be manufactured by the following method.

[0084] A separator composition is prepared by mixing a polymer resin, a filler, and a solvent. The separator composition can be directly coated onto the top of an electrode and dried to form a separator. Alternatively, the separator composition can be cast onto a support and dried, and then the separator film, peeled off the support, can be laminated onto the top of an electrode to form a separator.

[0085] The polymer resin used in the manufacture of the separator is not particularly limited, and any substance that can be used as a binder for the electrode plate may be used. For example, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or mixtures thereof may be used.

[0086] Next, electrolytes are prepared. For example, the electrolyte may also be an organic electrolyte. Furthermore, the electrolyte may also be a solid. For example, it may be boron oxide, lithium oxynitride, etc., but is not limited to these; any material that can be used as a solid electrolyte in the relevant art is acceptable. The solid electrolyte may also be formed on the negative electrode by a method such as sputtering. For example, the organic electrolyte can also be produced by dissolving a lithium salt in an organic solvent.

[0087] The aforementioned organic solvent can be any organic solvent that can be used in the art. 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 mixtures thereof.

[0088] Any of the aforementioned lithium salts can be used if they are usable as lithium salts in the relevant art. 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 mixtures thereof, etc.

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

[0090] A separator can be placed between the positive electrode and the negative electrode to form a battery structure. After the battery structure is stacked in a bicell structure, it is impregnated with an organic electrolyte, and the resulting product is housed in a pouch and sealed to complete a lithium-ion polymer battery.

[0091] The aforementioned lithium secondary battery is used not only as a power source for small devices, but also as a unit battery in medium- and large-sized device battery modules containing numerous batteries.

[0092] Examples of the aforementioned medium- and large-sized devices include, but are not limited to, power tools; xEVs, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric motorcycles, including e-bikes and e-scooters; electric golf carts; electric trucks; electric commercial vehicles; or power storage systems. Furthermore, the lithium secondary batteries are also used in all other applications requiring high power, high voltage, and high-temperature operation.

[0093] The following examples and comparative examples illustrate exemplary embodiments of the invention. However, these examples are for illustrative purposes only and do not limit the scope of the invention in themselves.

[0094] (Manufacturing of lithium-ion batteries) Example 1 (Manufacturing of positive electrodes) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 A first composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The first composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, and dried at about 80°C for 20 minutes. After that, a roll press was performed to obtain a positive electrode with the first layer coated.

[0095] On the first layer, LiNi is used as the second positive electrode active material. 0.6 Mn 0.2 Co 0.2 A second composition, consisting of 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder, was applied to form a second layer. Next, it was dried at approximately 80°C for 20 minutes to produce a double-coated positive electrode.

[0096] On the aforementioned second layer, LiNi is used as the third positive electrode active material. 0.33 Mn 0.33 Co 0.33 A third composition was applied to form a third layer, which consisted of 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The mixture was then dried at approximately 80°C for 20 minutes to produce a triple-coated positive electrode.

[0097] At this time, the current density of the first layer is approximately 2.24 mAh / cm³. 2 The current density of the second layer is approximately 1.68 mAh / cm³. 2 The current density of the third layer is approximately 1.68 mAh / cm². 2The current density ratio of the first, second, and third layers was approximately 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] In this case, the capacitance ratio of the first, second, and third layers is 4:3:3, the thickness of the positive electrode active material layer including the first to third layers is approximately 80 μm, and the overall current density is approximately 5.6 mAh / cm². 2 That was the case.

[0099] At this time, the loading level of the positive electrode is 28 mg / cm³. 2 That was the case. A schematic diagram showing the structure of the positive electrode is shown in Figure 5.

[0100] (Electrolyte manufacturing) An electrolyte for lithium secondary batteries was prepared by adding 7% by weight of fluoroethylene carbonate (FEC) to a mixed solvent containing 1.15 M LiPF6 as a lithium salt, in a 2:1:2:5 volume ratio, consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propylene propionate (PP).

[0101] (Assembly of lithium-ion batteries) A half-cell lithium secondary battery was manufactured using the aforementioned positive electrode, lithium metal negative electrode, 18 μm thick polyethylene separator coated with ceramics, and the aforementioned electrolyte. 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 is approximately 1.2 mAh / cm². 2 Therefore, the current density of the second layer is approximately 0.9 mAh / cm². 2 The current density of the third layer is approximately 0.9 mAh / cm². 2A lithium secondary battery was manufactured in the same manner as in Example 1, except that the current density ratio of the first, second, and third layers was adjusted to approximately 4:3:3.

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

[0104] Furthermore, the capacitance ratio of the first, second, and third layers is 4:3:3, the thickness of the positive electrode active material layer including the first to third layers is approximately 45 μm, and the overall current density is approximately 3 mAh / cm². 2 That was the case. At this time, the loading level of the positive electrode is 15 mg / cm³. 2 That was the case.

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

[0106] (Manufacturing of positive electrodes) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 A first composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The first composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, and dried at about 80°C for 20 minutes. After that, a roll press was performed to obtain a positive electrode with the first layer coated.

[0107] On the first layer, LiNi is used as the second positive electrode active material. 0.33 Mn 0.33 Co 0.33 A second composition, consisting of 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder, was applied to form a second layer. Next, it was dried at approximately 80°C for 20 minutes to produce a double-coated positive electrode.

[0108] At this time, the current density of the first layer is approximately 3.08 mAh / cm³. 2 The current density of the second layer is approximately 2.52 mAh / cm². 2 The current density ratio between the first and second layers was approximately 11:9. On the other hand, in the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 4.9:5.1.

[0109] In this case, the capacitance ratio of the first layer to the second layer is 11:9, the thickness of the positive electrode active material layer including the first layer and the second layer is approximately 80 μm, and the overall current density is approximately 5.6 mAh / cm². 2 That was the case. At this time, the loading level of the positive electrode is 28 mg / cm³. 2 That was the case.

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

[0111] (Manufacturing of positive electrodes) As the positive electrode active material, LiNi 0.6 Mn 0.2 Co 0.2 A positive electrode composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The positive electrode composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of approximately 20 μm, and dried at approximately 80°C for 20 minutes. After drying, the positive electrode was roll-pressed to obtain the positive electrode. At this time, the thickness of the positive electrode active material layer in the positive electrode was approximately 80 μm, and the current density of the positive electrode was 5.6 mAh / cm². 2 The loading level is 28 mg / cm³. 2 That was the case. A schematic diagram showing the structure of the positive electrode is shown in Figure 6.

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

[0113] (Manufacturing of positive electrodes) As the positive electrode active material, LiNi 0.6 Mn 0.2 Co 0.2 A positive electrode composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The positive electrode composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of approximately 20 μm, and dried at approximately 80°C for 20 minutes. After drying, the positive electrode was roll-pressed to obtain the positive electrode. At this time, the thickness of the positive electrode active material layer in the positive electrode was approximately 43 μm, and the current density of the positive electrode was 3 mAh / cm². 2 The loading level is 15 mg / cm³. 2 That was the case.

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

[0115] (Manufacturing of positive electrodes) As the first positive electrode active material, LiNi 0.33 Mn 0.33 Co 0.33 A first composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The first composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, and dried at about 80°C for 20 minutes. After that, a roll press was performed to obtain a positive electrode with the first layer coated.

[0116] On the first layer, LiNi is used as the second positive electrode active material. 0.6 Mn 0.2 Co 0.2A second composition, consisting of 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder, was applied to form a second layer. Next, it was dried at approximately 80°C for 20 minutes to produce a double-coated positive electrode.

[0117] On the aforementioned second layer, LiNi is used as the third positive electrode active material. 0.8 Mn 0.1 Co 0.1 A third composition was applied to form a third layer, which consisted of 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The mixture was then dried at approximately 80°C for 20 minutes to produce a triple-coated positive electrode.

[0118] At this time, the current density of the first layer is approximately 1.68 mAh / cm³. 2 The current density of the second layer is approximately 1.68 mAh / cm³. 2 The current density of the third layer is approximately 2.24 mAh / cm³. 2 The current density ratio of the first, second, and third layers was approximately 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 3.4:3:0.3.6.

[0119] In this case, the capacitance ratio of the first, second, and third layers is 3:3:4, the thickness of the positive electrode active material layer including the first to third layers is approximately 80 μm, and the overall current density is approximately 5.6 mAh / cm². 2 That was the case. At this time, the loading level of the positive electrode is 28 mg / cm³. 2 That was the case.

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

[0121] The evaluation results are shown in Figure 7. Referring to Figure 7, the lithium secondary battery according to Example 1 differs from the lithium secondary battery according to Comparative Example 1 in that it contains lithium transition metal oxide layers of various compositions. However, by appropriately adjusting the ratio of each layer, the Ni metal content can be matched to 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 the lithium secondary battery in Comparative Example 1 can be realized.

[0123] Evaluation Example 2: Lifetime Characteristics Evaluation The lithium secondary batteries manufactured in Example 1 and Comparative Example 1 were charged and discharged under charging conditions (constant current / constant voltage of 0.7C / 4.35V, cutoff current of 0.025C, 10-minute rest) and discharge conditions (constant voltage of 1.0C, cutoff voltage of 3.0V, 10-minute rest), and the capacity over 40 cycles was measured and is shown in Figure 8.

[0124] Referring to Figure 8, it can be confirmed that the lithium secondary battery according to Example 1 maintains its capacity characteristics even better as the number of cycles increases compared to the lithium secondary battery according to Comparative Example 1. In other words, even when introducing a thick film cathode with the same loading level, it can be confirmed that the lithium secondary battery according to Example 1, which introduces a multi-layer cathode active material layer, exhibits superior life characteristics compared to the lithium secondary battery according to Comparative Example 1, which has a single-layer structure.

[0125] On the other hand, under the same conditions, the capacity retention rate (lifespan) after 40 cycles was measured not only for Example 1 and Comparative Example 1, but also for Examples 2 and 3, and Comparative Examples 2 and 3, and is 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, which introduce thick-film positive electrodes of the same thickness, it can be confirmed that the lithium secondary batteries of Examples 1 and 3, which include a multi-layer positive electrode active material layer but have different positive electrode active materials arranged in a certain order, exhibit superior life characteristics under the same conditions, not only compared to the lithium secondary battery of Comparative Example 1, which has a single-layer structure, but also compared to the lithium secondary battery of Comparative Example 3, which has the stacking order reversed. In particular, in the positive electrode current collector defined in the present invention as having a desirable stacking order, it can be confirmed that the lithium secondary battery of Comparative Example 3, which is stacked in the opposite direction to the direction in which the Ni content decreases as you move away from it, actually has worse life characteristics than the lithium secondary battery of Comparative Example 1, which has a single-layer structure. This is because positive electrode active materials with a low Ni content are advantageous in terms of life characteristics, and positive electrode active materials with a high Ni content are advantageous in terms of capacity characteristics, but the stacking order is different, and the effects that need to be realized cannot be fully realized.

[0128] Furthermore, it can be confirmed that the lithium secondary battery of Example 1, which has a triple-layer structure with an additional layer, exhibits an even better lifespan improvement effect compared to the lithium secondary battery of Example 3, which has a double-layer structure.

[0129] On the other hand, the lithium secondary batteries of Example 2 and Comparative Example 2, by introducing a thin-film cathode structure instead of a thick-film cathode structure, demonstrate that the life-saving effect of introducing the multilayer 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 than in the case of the thick-film structure, which is likely because the life-saving problem is not as significant in the cathode of the thin-film structure.

[0130] Example 4 (Manufacturing of positive electrodes) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1A first composition was prepared by mixing 6 wt% O29, 2 wt% super-p as a conductive material, and 2 wt% polyvinylidene fluoride as a binder. The first composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of about 20 μm, and dried at about 80°C for 20 minutes. After that, a roll press was performed to obtain a positive electrode with the first layer coated.

[0131] 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 a binder, was applied to the first layer to form a second layer. Next, it was dried at approximately 80°C for 20 minutes to produce a double-coated positive electrode. At this time, the loading amount of the first layer was approximately 5.25 mg / cm³. 2 The loading amount for the second layer is approximately 12.25 mg / cm³. 2 The loading ratio of the first layer to the second layer is 3:7. On the other hand, the current density of the positive electrode including the first and second layers is 3.0 mAh / cm². 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] (Manufacturing of negative electrodes) As a negative electrode active material, 98% by weight of graphite and 2% by weight of binder were mixed and added to distilled water, then dispersed using a mechanical stirrer for 60 minutes 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 approximately 60 μm using a doctor blade, dried in a hot air dryer at 100°C for 0.5 hours, then dried once more under vacuum and 120°C conditions for 4 hours, and rolled to produce a negative electrode in which a negative electrode active material layer was formed on the current collector.

[0133] (Electrolyte manufacturing) An electrolyte for lithium secondary batteries was prepared by adding 7% by weight of fluoroethylene carbonate (FEC) to a mixed solvent containing 1.15 M LiPF6 as a lithium salt, in a 2:1:2:5 volume ratio, consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propylene propionate (PP).

[0134] (Assembly of lithium-ion batteries) A lithium secondary battery was manufactured using the positive electrode, the negative electrode, a 18 μm thick polyethylene separator coated with ceramics, and the electrolyte. 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 / cm². 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 to achieve the desired result. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35V. 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 / cm². 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 to achieve the desired result. At this time, the operating voltage of the lithium secondary battery was 3.0 to 4.35V. 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 4.4V. 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 approximately 8.75 mg / cm³. 2 The loading amount of the second layer is approximately 8.75 mg / cm³. 2A lithium secondary battery was manufactured in the same manner as in Example 4, except that the loading ratio between the first layer and the second layer was adjusted to approximately 5:5. In the positive electrode, the content ratio of the first positive electrode active material to the second positive electrode active material was 5:5.

[0139] Comparative Example 4 (Manufacturing of positive electrodes) As the first positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1 A positive electrode composition was prepared by mixing 8.8% by weight of O22, 7.2% by weight of LiCoO26 as the second positive electrode active material, 2% by weight of super-p as the conductive material, and 2% by weight of polyvinylidene fluoride as the binder. The positive electrode composition was applied to an aluminum (Al) thin film, which was a positive electrode current collector with a thickness of approximately 20 μm, and after drying at approximately 80°C for 20 minutes, a roll press was applied to produce the positive electrode. At this time, the thickness of the positive electrode active material layer was approximately 40 μm. The current density of the positive electrode was 3.0 mAh / cm². 2 That was the case.

[0140] (Manufacturing of negative electrodes) The negative electrode used in Example 4 was used.

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

[0142] (Assembly of lithium-ion batteries) A lithium secondary battery was manufactured using the positive electrode, the negative electrode, a 18 μm thick polyethylene separator coated with ceramics, and the electrolyte. 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 is 2.6 mAh / cm². 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 to achieve the desired result. In this case, 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 / cm². 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 to achieve the desired result. In this case, 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 using the same method as in Comparative Example 4, except that the operating voltage of the lithium secondary battery was adjusted to 3.0 to 4.4V.

[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 the positive electrode composition was prepared by mixing 8% by weight of O248, 8% by weight of LiCoO248 as the second positive electrode active material, 2% by weight of super-p as the conductive material, and 2% by weight of polyvinylidene fluoride as the binder.

[0147] Evaluation Example 3: Discharge Profile Evaluation The lithium secondary batteries manufactured in Example 4 and Comparative Example 4 were charged and discharged under charging conditions (constant current / constant voltage of 0.2C / 4.35V, cutoff current of 0.025C, 10-minute rest) and discharge conditions (constant voltage of 0.2C, cutoff voltage of 3.0V, 10-minute rest), and the discharge profiles were measured and are shown in Figure 9. Referring to FIG. 9, it can be confirmed that the lithium secondary battery according to Example 4, even when containing LCO with relatively inferior capacity characteristics in the positive electrode active material while applying a multi-layer positive electrode active material, exhibits a discharge capacity at a level not significantly inferior to that of the lithium secondary battery according to Comparative Example 4 applying an NCM-based positive electrode active material.

[0148] Evaluation Example 4: Cycle Characteristics Evaluation Regarding the lithium secondary batteries manufactured in Example 4 and Comparative Example 4, charge-discharge was performed under charging conditions (constant current / constant voltage of 0.7C / 4.35V, 0.025C cut-off current, 10-minute rest) and discharge 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 containing both the LCO-based material and the NCM-based material together, the lithium secondary battery according to Example 4 containing them in a specific order exhibits excellent cycle characteristics compared to the lithium secondary battery according to Comparative Example 4 with a simple mixture.

[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 NCM (first positive electrode active material) and LCO (second positive electrode active material) are applied in the same content ratio, the lithium secondary battery of the example in which NCM and LCO are arranged in a multilayer structure in a specific order exhibits superior life characteristics under the same conditions compared to the comparative lithium secondary battery in which NCM and LCO are simply mixed. Furthermore, it can be confirmed that despite changes in current density, drive voltage, and NCM / LCO ratio, the lithium secondary battery of the example shows little to no change in life characteristics, while the lithium secondary battery of the comparative example with simple mixing shows significant deterioration in life characteristics.

[0153] In the foregoing, preferred embodiments of the present invention have been described with reference to the drawings and embodiments. However, these are merely illustrative examples, and those skilled in the art will understand from them that a variety of modifications and equivalent other embodiments are possible. Accordingly, the scope of protection of the present invention is defined by the claims. [Industrial applicability]

[0154] In one example, a lithium secondary battery may have improved lifespan characteristics by employing a positive electrode that includes a novel layered structure. [Explanation of symbols]

[0155] 1. Lithium-ion rechargeable battery 2 negative electrode 3 Positive electrode 4 Separators 5 Battery case 6 Cap Assembly 10 Positive electrode for secondary batteries 11 Positive electrode current collector 12 1st layer 13 2nd layer 20 Positive electrode for secondary batteries 21 Positive electrode current collector 22 1st layer 23 2nd layer 24 3rd layer

Claims

1. Positive electrode current collector and Distributed on at least one surface of the positive electrode current collector, a first layer containing a first positive electrode active material, A second layer disposed on the first layer and containing a second positive electrode active material, The first positive electrode active material is represented by the following chemical formula 1A, The second positive electrode active material is LiCoO ２ And, The current density ratio of the first layer to the second layer is 2:8 to 8:

2. The ratio of active material loading amounts between the first and second layers is 5:5 to 7:

3. The first positive electrode active material and the second positive electrode active material have different compositions from each other, and are positive electrodes for secondary batteries: [Chemical formula 1A] Li ｘ１ Ni ｙ１ M1 １－ｙ１ O ２ In the aforementioned chemical formula 1A, 0.9 ≤ x1 ≤ 1.2 and 0.7 ≤ y1 ≤ 0.98, M1 is one or more metal or transition metal elements with an oxidation state of +2 or +3.

2. The positive electrode for a secondary battery according to claim 1, wherein M1 is 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.

3. The first positive electrode active material is the positive electrode for a secondary battery according to claim 1, 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’ O ２ [Chemical formula 1-2] Li ｘ’ Ni ｙ’ Co １－ｙ’－ｚ’ Mn ｚ’ O ２ In the aforementioned chemical formulas 1-1 and 1-2, 0.9 ≤ x' ≤ 1.2, 0.7 ≤ y' ≤ 0.98, 0 < z' < 0.3, and 0 < 1 - y' - z' < 0.

3.

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

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

3.

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

3.

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

2.

8. The loading amount for the first layer is 3 mg / cm³. ２ or 40 mg / cm³ ２ The positive electrode for a secondary battery according to claim 1.

9. The loading amount for the second layer is 40 mg / cm³. ２ or 3 mg / cm ２ The positive electrode for a secondary battery according to claim 1.

10. The current density of the positive electrode is 2 to 10 mAh / cm². ２ The positive electrode for a secondary battery according to claim 1.

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

12. The positive electrode for a secondary battery according to claim 11, wherein 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 less than or equal to the current density of the first layer.

13. The positive electrode for a secondary battery according to claim 1, wherein the thickness of the positive electrode active material layer including the first layer and the second layer is 40 μm or more.

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

15. A positive electrode for a secondary battery according to any one of claims 1 to 14, A negative electrode is positioned opposite the positive electrode, A lithium secondary battery comprising an electrolyte disposed between the positive electrode and the negative electrode.

16. The lithium secondary battery according to claim 15, wherein the operating voltage of the lithium secondary battery is 2.5 to 4.5V.

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