Positive electrode and lithium secondary battery containing the positive electrode

Abstract

This invention discloses a positive electrode and a lithium secondary battery comprising the positive electrode. The positive electrode comprises: a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer has a lower region facing the positive electrode current collector comprising a first positive electrode active material and a first binder polymer, and an upper region facing the lower region comprising a second positive electrode active material and a second binder polymer, and the Ni content of the second positive electrode active material in the upper region is greater than the Ni content of the first positive electrode active material in the lower region.

Description

Technical Field

[0001] This disclosure relates to a positive electrode and a lithium secondary battery comprising the positive electrode. In particular, this disclosure relates to a positive electrode exhibiting high initial efficiency and excellent fast-charging performance, and a lithium secondary battery comprising the positive electrode.

[0002] This application claims priority to Korean Patent Application No. 10-2021-0002853, filed in Korea on January 8, 2021, the disclosure of which is incorporated herein by reference. Background Technology

[0003] With technological advancements and increasing demand for mobile devices, the need for miniaturized and high-capacity rechargeable batteries is growing. Furthermore, among such rechargeable batteries, lithium-ion batteries with high energy density and operating voltage have been commercialized and are widely used.

[0004] The structure of a lithium secondary battery includes: an electrode assembly having a positive electrode and a negative electrode and a porous membrane disposed between the two electrodes, wherein each electrode contains an active material coated on an electrode current collector; and an electrolyte containing a lithium salt injected into the electrode assembly. The electrode is obtained by applying a slurry containing an active material, a binder, and a conductive material dispersed in a solvent onto the current collector, followed by drying and pressing.

[0005] Lithium-ion batteries are secondary batteries in which lithium ions participate in the conduction between electrodes during charging and discharging. Compared with other secondary batteries such as nickel-metal hydride or nickel-cadmium batteries, they exhibit higher energy density and lower memory effect. Therefore, the applications of these lithium-ion batteries have expanded from small power sources in portable electronic devices and household appliances to medium and large power sources for power storage devices, UPS equipment, power leveling equipment, etc., or power sources for driving ships, railway vehicles, hybrid vehicles, electric vehicles, etc., and there is a need to improve the performance of these batteries. For example, in automotive applications such as hybrid or electric vehicles, high-energy-density secondary batteries are needed to achieve long-distance driving, or high-output secondary batteries are needed to improve acceleration response.

[0006] To achieve electric vehicle secondary batteries with high output and high energy density, electrodes with high nickel (Ni) content have been used to obtain electrodes.

[0007] However, when using this Ni-rich cathode, compared to cathodes with low Ni content, the electrode's resistance changes depending on the battery's state of charge (SOC), and most secondary batteries exhibit increased resistance. To address these issues, further research is needed to control the electrode or battery resistance through cathode structure or composition design. Summary of the Invention

[0008] [Technical Issues]

[0009] The present disclosure aims to solve the problems in the related art. Therefore, the present disclosure provides a positive electrode and a secondary battery including the same, wherein the positive electrode can prevent an increase in resistance at a low SOC during discharge.

[0010] [Technical Solution]

[0011] In one aspect of the present disclosure, a secondary battery according to any one of the following embodiments is provided.

[0012] According to a first embodiment, there is provided a positive electrode, comprising:

[0013] a positive electrode current collector; and

[0014] a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer has a lower region facing the positive electrode current collector that comprises a first positive electrode active material and a first binder polymer, and an upper region facing the lower region that comprises a second positive electrode active material and a second binder polymer,

[0015] wherein the first positive electrode active material in the lower region is represented by Chemical Formula 1 below, the second positive electrode active material in the upper region is represented by Chemical Formula 2 below, and a Ni content of the second positive electrode active material is greater than a Ni content of the first positive electrode active material:

[0016] [Chemical Formula 1]

[0017] Li 1+a [Ni x Mn y Co z M1 t O2

[0018] wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1 and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0019] [Chemical Formula 2]

[0020] Li 1+b [Ni u Mn v Co w M2 s O2

[0021] wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1, u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0022] According to a second embodiment, there is provided the positive electrode according to the first embodiment, wherein based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 40 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 80 mol% to 90 mol%.

[0023] According to a third embodiment, there is provided the positive electrode according to the first or second embodiment, wherein based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 50 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 81 mol% to 90 mol%.

[0024] According to a fourth embodiment, there is provided the positive electrode according to any one of the first to third embodiments, wherein based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 70 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 86 mol% to 90 mol%.

[0025] According to a fifth embodiment, there is provided the positive electrode according to any one of the first to fourth embodiments, wherein the weight ratio of the lower region to the upper region of the positive electrode active material layer is 20:80 to 80:20.

[0026] According to a sixth embodiment, there is provided a method for manufacturing a positive electrode, comprising the following steps:

[0027] preparing a slurry for a lower layer containing a first positive electrode active material represented by Chemical Formula 1 below, a first binder polymer and a first dispersion medium, and a slurry for an upper layer containing a second positive electrode active material represented by Chemical Formula 2 below, a second binder polymer and a second dispersion medium; wherein the Ni content of the second positive electrode active material is higher than the Ni content of the first positive electrode active material;

[0028] coating the slurry for the lower layer on one surface of a positive electrode current collector, and coating the slurry for the upper layer on the slurry for the lower layer; and

[0029] simultaneously drying the coated slurry for the lower layer and the coated slurry for the upper layer to form a positive electrode active material layer:

[0030] [Chemical Formula 1]

[0031] Li 1+a [Ni x Mn y Co z M1 t O2

[0032] wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1 and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0033] [Chemical Formula 2]

[0034] Li 1+b [Ni u Mn v Co w M2 s O2

[0035] wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1 and u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0036] According to a seventh embodiment, there is provided the method for producing the positive electrode according to the sixth embodiment, wherein based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 40 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 80 mol% to 90 mol%.

[0037] According to an eighth embodiment, there is provided the method for producing the positive electrode according to the sixth or seventh embodiment, wherein based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 50 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 81 mol% to 90 mol%.

[0038] According to the ninth embodiment, a method for manufacturing a positive electrode according to any one of the sixth to eighth embodiments is provided, wherein, based on the total transition metal of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 70 mol%, and based on the total transition metal of the second positive electrode active material, the Ni content of the second positive electrode active material is 86 mol% to 90 mol%.

[0039] According to the tenth embodiment, a lithium secondary battery is provided, which includes a positive electrode as described in any one of the first to fifth embodiments.

[0040] [Beneficial Effects]

[0041] The positive electrode of this disclosure includes a positive electrode active material layer having a lower region facing the positive electrode current collector, comprising a first positive electrode active material and a first binder polymer, and an upper region disposed on the lower region. The Ni content of a second positive electrode active material in the upper region is controlled to be greater than the Ni content in the first positive electrode active material in the lower region. In this way, an increase in resistance at low SOC (which could lead to rapid output degradation) can be prevented, and a positive electrode with improved high-temperature cycling characteristics and a secondary battery including this positive electrode can be provided.

[0042] Furthermore, according to embodiments of this disclosure, the above-mentioned effects can be achieved more significantly in a high-load cathode with a high coating amount of positive electrode active material. As a result, batteries for electric vehicles (EVs) with high capacity and high energy density can be realized. Attached Figure Description

[0043] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, serve to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure should not be construed as being limited to the drawings.

[0044] Figure 1 The graph shows the relationship between the high-temperature capacity retention rate and the number of cycles for the secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2, respectively. Detailed Implementation

[0045] Preferred embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Before description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its general and dictionary meaning, but should be interpreted according to the meaning and concept corresponding to the technical aspects of the present disclosure, based on the principle that the inventor is allowed to appropriately define the terminology for the best interpretation.

[0046] In one aspect of this disclosure, a positive electrode is provided, comprising:

[0047] Positive current collector; and

[0048] A positive electrode active material layer disposed on at least one surface of a positive electrode current collector, comprising a lower layer region facing the positive electrode current collector that contains a first positive electrode active material and a first binder polymer, and an upper layer region facing the lower layer region that contains a second positive electrode active material and a second binder polymer,

[0049] wherein the first positive electrode active material in the lower layer region is represented by the following Chemical Formula 1, the second positive electrode active material in the upper layer region is represented by the following Chemical Formula 2, and the Ni content of the second positive electrode active material is higher than the Ni content of the first positive electrode active material:

[0050] [Chemical Formula 1]

[0051] Li 1+a [Ni x Mn y Co z M1 t O2

[0052] wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1, and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0053] [Chemical Formula 2]

[0054] Li 1+b [Ni u Mn v Co w M2 s O2

[0055] wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1, and u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0056] The present invention provides a method for manufacturing a positive electrode, wherein through a double-layer (DLD) coating technology for a positive electrode, two different types of positive electrode active materials with different Ni contents are disposed in an upper layer and a lower layer of the positive electrode, wherein the Ni content of the positive electrode active material in the upper layer is controlled to be higher than the Ni content of the positive electrode active material in the lower layer. The present disclosure aims to control the resistance in a low SOC region that may affect the output of a secondary battery by using the above positive electrode.

[0057] Furthermore, this disclosure exhibits even greater effectiveness in high-load electrodes with high coating amounts of positive electrode active material, and can be applied to the manufacture of batteries for electric vehicles (EVs) with high capacity and high energy density. Generally, high-Ni positive electrode active material, i.e., positive electrode active material with high Ni content, refers to a positive electrode active material containing 80 mol% or more (based on total transition metals) of Ni.

[0058] Such high-Ni cathode active materials undergo greater structural changes during charging and discharging, resulting in lower structural stability. Consequently, they exhibit a rapid increase in resistance within a lower SOC range, leading to a decrease in output.

[0059] According to this disclosure, a cathode active material with low Ni content is provided in the lower region of the bilayer cathode active material layer, thereby improving structural stability and mitigating the problem of increased resistance.

[0060] According to embodiments of this disclosure, based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material can be 40 mol% to 75 mol%, 50 mol% to 75 mol%, 55 mol% to 75 mol%, 60 mol% to 75 mol%, 65 mol% to 75 mol%, or 65 mol% to 70 mol%.

[0061] Furthermore, based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material can be 80 mol%–90 mol%, 81 mol%–90 mol%, 82 mol%–90 mol%, 84 mol%–90 mol%, or 86 mol%–90 mol%.

[0062] The first and second positive electrode active materials can each be primary particles, or secondary particles formed by aggregating primary particles through a granulation process.

[0063] In the particle size distribution, the average particle size (D50) of the first positive electrode active material can be 3μm to 15μm, especially 5μm to 12μm, and even more especially 6μm to 10μm.

[0064] In addition, the D50 of the second positive electrode active material can be 5μm to 20μm, especially 7μm to 18μm, and even more especially 10μm to 15μm.

[0065] As used herein, “particle size, Dn” refers to the particle size corresponding to n% of the cumulative particle number distribution. Thus, “D50” refers to the particle size corresponding to 50% of the cumulative particle number distribution based on particle size, “D90” refers to the particle size corresponding to 90% of the cumulative particle number distribution based on particle size, and “D10” refers to the particle size corresponding to 10% of the cumulative particle number distribution based on particle size.

[0066] Furthermore, Dn can be determined using laser diffraction. Specifically, the powder to be analyzed is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The differences in the diffraction pattern (based on particle size) as the particles pass through the laser beam are then determined, and the particle size distribution is calculated. The particle size is then calculated at each point according to the cumulative particle number distribution at 10%, 50%, and 90% of the particle size to determine D10, D50, and D90, respectively.

[0067] According to one embodiment of this disclosure, the first positive electrode active material contained in the lower region of the positive electrode and the second positive electrode active material contained in the upper region are different types of nickel-cobalt-manganese-lithium oxide active materials with different Ni contents. Therefore, there may be an intermixed region where these different types of active materials are mixed at the contact between the lower and upper regions. This is because when a lower slurry containing the first positive electrode active material and an upper slurry containing the second positive electrode active material are simultaneously coated or continuously coated at very short time intervals and then dried simultaneously to form an active material layer, a certain intermixed region will be generated at the interface where the lower and upper slurries contact before drying. Then, when the lower and upper slurries are subsequently dried, this intermixed region is formed into the shape of an intermixed region layer.

[0068] According to one embodiment of this disclosure, the weight ratio (load per unit area ratio) of the lower region to the upper region of the positive electrode active material layer can be 20:80 to 80:20, particularly 30:70 to 70:30. When the above weight ratio is met, the adhesion between the current collector and the positive electrode active material layer is enhanced, and excellent fast charging performance can be achieved.

[0069] According to one embodiment of this disclosure, the thickness ratio of the lower layer to the upper layer can be 20:80 to 80:20, particularly 30:70 to 70:30. When the above thickness ratio is met, the adhesion between the current collector and the positive electrode active material layer is enhanced, and excellent fast charging performance can be achieved.

[0070] According to one embodiment of this disclosure, the total thickness of the positive electrode active material layer is not particularly limited. For example, the total thickness of the positive electrode active material layer can be 40 μm to 200 μm. Furthermore, the thickness of the lower region of the positive electrode active material layer can be 20 μm to 150 μm or 30 μm to 100 μm, and the thickness of the upper region can be 20 μm to 150 μm or 30 μm to 100 μm.

[0071] When the thicknesses of the lower and upper regions meet the above range, the adhesion between the current collector and the positive electrode active material layer is enhanced, and excellent fast charging performance can be achieved.

[0072] The weight percentage (wt%) of the first adhesive polymer in the lower region can be greater than the weight percentage (wt%) of the second adhesive polymer in the upper region.

[0073] Specifically, the ratio (a / b) of the weight percentage (wt%) of the first adhesive polymer in the solid component of the lower layer slurry (a) to the weight percentage (wt%) of the second adhesive polymer in the solid component of the upper layer slurry (b) can be 1-5, 1.1-5, 1-4, 1.2-4, 1-3, 1.5-3 or 2.1-3.

[0074] Here, when the ratio of the weight percentage (wt%) of the first adhesive polymer in the lower region to the weight percentage (wt%) of the second adhesive polymer in the upper region meets the above range, it can achieve excellent adhesion between the current collector and the positive electrode active material layer and excellent fast charging performance.

[0075] According to one embodiment of this disclosure, the weight percentage (wt%) of the first binder polymer in the lower region of the positive electrode active material layer can be 1 wt% to 3 wt%, 1.5 wt% to 2.5 wt%, or 1.5 wt% to 2.4 wt%, and the weight percentage (wt%) of the second binder polymer in the upper region of the positive electrode active material layer can be 0.5 wt% to 3 wt%, 1 wt% to 2.5 wt%, or 1 wt% to 2.4 wt%.

[0076] According to one embodiment of this disclosure, the ratio (by weight) of the total weight of the first binder polymer and the second binder polymer to the total weight of the positive electrode active material layer can be 1% to 3% by weight, 1% to 2% by weight, 2% to 3% by weight, 1% to 2.4% by weight, or 2.4% to 3% by weight.

[0077] In another aspect of this disclosure, a method for manufacturing a positive electrode is provided, comprising the following steps:

[0078] A lower layer slurry containing a first positive electrode active material, a first binder polymer, and a first dispersion medium, represented by the following chemical formula 1, and an upper layer slurry containing a second positive electrode active material, a second binder polymer, and a second dispersion medium, represented by the following chemical formula 2, wherein the Ni content of the second positive electrode active material is greater than the Ni content of the first positive electrode active material;

[0079] The lower layer slurry is applied to one surface of the positive electrode current collector, and the upper layer slurry is applied onto the lower layer slurry; and

[0080] The lower and upper slurries are dried simultaneously after coating to form the positive electrode active material layer:

[0081] [Chemical Formula 1]

[0082] Li 1+a [Ni x Mn y Co z M1 t O2

[0083] wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1 and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0084] [Chemical Formula 2]

[0085] Li 1+b [Ni u Mn v Co w M2 s O2

[0086] wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1 and u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd.

[0087] The positive electrode current collector is not particularly limited as long as it has electrical conductivity and does not cause any chemical change in the corresponding battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver and the like, and aluminum-cadmium alloy can be used. Although the thickness of the positive electrode current collector is not particularly limited, it may have a thickness of 3 μm to 500 μm that is conventionally used.

[0088] Based on the total weight of the lower layer slurry and the upper layer slurry, the amounts of the first positive electrode active material and the second positive electrode active material may each be 80 wt% to 99 wt%.

[0089] The positive electrode active material layer may further comprise a conductive material, and thus each of the lower layer slurry and the upper layer slurry may further comprise a conductive material.

[0090] There are no particular restrictions on the conductive material, as long as it does not cause a chemical change in the corresponding battery and is conductive. Specific examples of conductive materials include: carbon black-based carbonaceous materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; metal powders, such as fluorocarbons, aluminum powder, or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials such as polyphenylene derivatives; and so on. Based on the total weight of the positive electrode slurry composition, the amount of conductive material added can be from 0.1% to 20% by weight.

[0091] The first and second adhesive polymers are each components that contribute to the adhesion of conductive materials, active materials, or positive electrode current collectors, and their addition amounts can range from 0.1% to 20% by weight based on the total weight of the positive electrode slurry composition. Specific examples of the first and second adhesive polymers independently include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber (SBR), lithium polyacrylate (Li-PAA), etc.

[0092] The first and second dispersion media may each independently include water or an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and their amounts may be such that the upper and lower slurries containing the first positive electrode active material / second positive electrode active material, the first binder polymer / second binder polymer, conductive materials, etc., can have desired viscosity levels.

[0093] Furthermore, there are no particular restrictions on the methods for applying the lower and upper coating slurries, as long as they are methods conventionally used in the field. For example, coating methods using a stencil, Mayer rod coating, gravure coating, dip coating, spray coating, etc., can be used.

[0094] According to one embodiment of this disclosure, when the steps of applying a lower layer slurry to one surface of the positive current collector and applying an upper layer slurry on the lower layer slurry are performed simultaneously or at very short time intervals, a device such as a dual-groove die head can be used.

[0095] The step of simultaneously drying the coated lower slurry and the coated upper slurry to form an active material layer may include: simultaneously drying the coated lower slurry and upper slurry to remove the dispersion medium in each slurry; pressing; and vacuum drying to form an active material layer.

[0096] Here, pressing can be performed using methods conventional in the art, such as rolling, for example, at a temperature of 15°C to 30°C and a pressure of 1 MPa to 20 MPa. Furthermore, the pressing conditions can result in a porosity of 20% to 40%, 25% to 35%, 20% to 30%, or 30% to 40% for the pressed electrode (active material layer).

[0097] The drying step of the coated slurry can be carried out at 70℃~110℃, 75℃~100℃, or 80℃~90℃ for 10 minutes~30 minutes, 15 minutes~25 minutes, or 20 minutes~30 minutes, respectively. The drying temperature and time can be appropriately controlled according to the type and content of the dispersion medium.

[0098] In addition, after the dried slurry is pressed, it can be vacuum dried at a temperature of 100℃~170℃, 120℃~150℃ or 130℃~150℃ for about 3 hours to 10 hours or 5 hours to 8 hours. However, the drying temperature and time can be appropriately controlled according to the type and content of the dispersion medium.

[0099] The ratio (a / b) of the weight percentage (wt%) of the first binder polymer in the solid component of the lower layer slurry to the weight percentage (wt%) of the second binder polymer in the solid component of the upper layer slurry can be 1-5, 1.1-5, 1-4, 1.2-4, 1-3, 1.5-3 or 2.1-3.

[0100] Here, when the ratio of the weight percentage (wt%) of the first adhesive polymer in the lower layer slurry after coating to the weight percentage (wt%) of the second adhesive polymer in the upper layer slurry after coating meets the above range, excellent adhesion and fast charging performance can be achieved.

[0101] The weight percentage (wt%) of the first binder polymer in the solid component of the lower layer slurry can be 1 wt% to 3 wt%, 1.5 wt% to 2.5 wt%, or 1.5 wt% to 2.4 wt%, and the weight percentage (wt%) of the second binder polymer in the solid component of the upper layer slurry can be 0.5 wt% to 3 wt%, 1 wt% to 2.5 wt%, or 1 wt% to 2.4 wt%.

[0102] Based on the total solids composition of the lower and upper slurries, the total weight ratio (wt%) of the first and second adhesive polymers can be 1 wt% to 3 wt%, 1 wt% to 2 wt%, 2 wt% to 3 wt%, 1 wt% to 2.4 wt%, or 2.4 wt% to 3 wt%.

[0103] As described above, the positive electrode active material layer has a double-layer structure and includes: a lower region containing a first positive electrode active material and a first binder polymer facing the positive electrode current collector, and an upper region containing a second positive electrode active material and a second binder polymer facing the lower region.

[0104] In another aspect of this disclosure, a lithium secondary battery having the aforementioned positive electrode is provided. Specifically, the lithium secondary battery can be obtained by injecting a lithium salt-containing electrolyte into an electrode assembly comprising the positive electrode, a negative electrode, and a separator disposed between the two electrodes.

[0105] The negative electrode can be obtained by combining a negative electrode active material with a negative electrode current collector using methods known in the art. Non-limiting examples of negative electrode active materials include conventional negative electrode active materials that can be used as negative electrodes in conventional electrochemical devices. In particular, lithium-intercalating materials, such as lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials, are preferred. Non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, copper alloys, or combinations thereof.

[0106] A slurry is prepared by mixing a negative electrode active material, a conductive material, a binder, and a solvent. This slurry can then be directly coated onto a negative electrode current collector. In a variation, the slurry can be cast onto another support, and a negative electrode can be obtained by laminating a negative electrode active material film, obtained by peeling it off from the support, onto the negative electrode current collector.

[0107] The diaphragm can be a conventional porous polymer membrane commonly used as a diaphragm. For example, it can be a porous polymer membrane made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer. Such a porous polymer membrane can be used alone or in the form of a laminate. Furthermore, an insulating film with high ion permeability and mechanical strength can be used. The diaphragm may include a safety-reinforced diaphragm (SRS), which comprises a ceramic material coated on the diaphragm surface in a thin layer. Alternatively, conventional porous nonwoven webs, such as nonwoven webs made of high-melting-point glass fibers or polyethylene terephthalate fibers, can be used, but the scope of this disclosure is not limited thereto.

[0108] Electrolytes include lithium salts as electrolyte salts and organic solvents used to dissolve lithium salts.

[0109] Any lithium salt commonly used in secondary battery electrolytes can be used, without particular restrictions. For example, the anion of the lithium salt can be selected from F. - Cl - ,Br - I - NO3 - N(CN)2 - BF4- ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - , (SF5)3C-, (CF3SO2)3C-, CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - It is any one or more of the group consisting of SCN-, (CF3CF2SO2)2N-.

[0110] The organic solvent contained in the electrolyte can be any conventionally used organic solvent without particular limitation. Typical examples of organic solvents include at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran.

[0111] In particular, among carbonate organic solvents, ethylene carbonate and propylene carbonate, as cyclic carbonates, are organic solvents with high viscosity and high dielectric constant, and are therefore preferred because they can readily dissociate lithium salts in the electrolyte. More preferably, when such cyclic carbonates are mixed with chain carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate or diethyl carbonate, electrolytes with higher conductivity can be prepared.

[0112] Optionally, the electrolyte used in this disclosure may also include additives found in conventional electrolytes, such as overcharge preventers.

[0113] One embodiment of the lithium secondary battery disclosed herein can be obtained by: providing a separator between a positive electrode and a negative electrode to form an electrode assembly; introducing the electrode assembly into a bag, a cylindrical battery case, or a prismatic battery case; and then injecting an electrolyte therein to complete the secondary battery. In a variation, the lithium secondary battery can be obtained by stacking electrode assemblies, impregnating the stack with an electrolyte, introducing the resulting product into a battery case, and then sealing it.

[0114] According to embodiments of this disclosure, the lithium secondary battery can be a stacked, wound, stacked-folded, or cable-type battery.

[0115] The lithium secondary battery disclosed herein can be used as a battery cell for powering small devices, or preferably as a unit cell in medium or large battery modules comprising multiple battery cells. Specific examples of such medium or large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems, etc. In particular, the lithium secondary battery can be used as a battery for hybrid electric vehicles and as a novel renewable energy storage battery requiring high output.

[0116] Example

[0117] Embodiments will be described more fully below to facilitate a clear understanding of this disclosure. However, the embodiments described below may be implemented in many different forms and should not be construed as limiting oneself to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided to make this disclosure fully complete and to adequately convey the scope of this disclosure to those skilled in the art.

[0118] Example 1

[0119] (1) Manufacturing of the positive electrode

[0120] First, Denka Black, as a conductive material, was introduced into N-methylpyrrolidone (NMP). The resulting mixture was then introduced into a high-speed dispersion system (Spike mill, obtained from Inoue) and dispersed to prepare a pre-dispersion of the conductive material. Next, the positive electrode active material and binder polymer were mixed with the pre-dispersion of the conductive material to prepare a slurry for the lower layer with a viscosity of 10,000 cps.

[0121] Here, the lower layer slurry contains 96.3% by weight of the chemical formula Li[Ni]. 0.65 Mn 0.15 Co 0.20 The positive electrode active material shown in O2 (based on a total transition metal Ni content of 65%), 1.3% by weight Denka Black as a conductive material, and 2.4% by weight polyvinylidene fluoride (PVDF) as a binder polymer.

[0122] Additionally, a top layer slurry with a viscosity of 10000 cps was prepared in the same manner as described above. Here, the top layer slurry contains 96.3% by weight of Li[Ni] 0.86 Mn 0.05 Co 0.07 Al 0.2 The positive electrode active material represented by O2 (based on a total transition metal Ni content of 86%), 1.3 wt% Denka Black as a conductive material, and 2.4 wt% polyvinylidene fluoride (PVDF) as a binder polymer.

[0123] The lower layer slurry is introduced into the lower layer slurry tank of the double-layer coating machine, and the upper layer slurry is introduced into its upper layer slurry tank, thereby applying the lower layer slurry and the upper layer slurry simultaneously onto the aluminum positive electrode current collector with a thickness of 12μm.

[0124] Then, the slurries were dried at 110°C and pressed to obtain a positive electrode with a double-layer coating. Here, the total thickness of the positive electrode active material layer is 175 μm, the thickness of the lower region of the positive electrode active material layer is 87 μm, and the thickness of the upper region of the positive electrode active material layer is 87 μm. The porosity of the positive electrode is 29%. The weight ratio of the lower to upper layers of the positive electrode active material layer is 50:50.

[0125] (2) Manufacturing of the negative electrode

[0126] First, a negative electrode active material slurry was prepared by dispersing 95.6 wt% of a negative electrode active material (artificial graphite), 1.0 wt% of a conductive material (Super-C), and 3.4 wt% of a binder polymer (styrene-butadiene rubber) in water. This slurry was then applied to an 8 μm thick copper negative electrode current collector, dried at 150 °C, and subsequently pressed to obtain the negative electrode. Here, the total thickness of the negative electrode active material layer was 212 μm, and the porosity was 29.0%.

[0127] (3) Battery manufacturing

[0128] A porous polyethylene membrane, serving as a separator, is placed between the positive and negative electrodes. The resulting structure is then incorporated into a battery casing, and an electrolyte is injected into it, thereby obtaining a secondary battery. Here, the electrolyte is a mixed solution containing ethylene carbonate and ethyl methyl carbonate (volume ratio 3 / 7) and dissolved with 1.1 M LiPF6.

[0129] Example 2

[0130] Except for changing the Ni content in the positive electrode active material as shown in Table 1 below, the positive electrode, negative electrode, and secondary battery were obtained in the same manner as in Example 1.

[0131] Comparative Example 1

[0132] The positive electrode, negative electrode, and secondary battery were obtained in the same manner as in Example 1, except that 96.3% by weight of Li[Ni] was used. 0.86 Mn 0.05 Co 0.07 Al 0.2 The positive electrode active material shown in O2 (based on a Ni content of 86% of total transition metals), 1.3 wt% of Denka Black as a conductive material, and 2.4 wt% of polyvinylidene fluoride (PVDF) as a binder polymer were mixed to prepare a slurry for the lower layer with a viscosity of 10000 cps; and

[0133] 96.3% by weight of the chemical formula Li[Ni 0.65 Mn 0.15 Co 0.20 The positive electrode active material shown in O2 (based on a total transition metal Ni content of 65%), 1.3 wt% Denka Black as a conductive material, and 2.4 wt% polyvinylidene fluoride (PVDF) as a binder polymer were mixed to prepare an upper layer slurry with a viscosity of 10000 cps.

[0134] Comparative Example 2

[0135] First, Denka Black, as a conductive material, was introduced into N-methylpyrrolidone (NMP). The resulting mixture was then introduced into a high-speed dispersion system (Spike mill, obtained from Inoue) and dispersed to prepare a pre-dispersion of the conductive material. Next, the positive electrode active material and binder polymer were mixed with the pre-dispersion of the conductive material to prepare a slurry with a viscosity of 10,000 cps.

[0136] Here, the slurry contains 96.3% by weight of Li[Ni] 0.86 Mn 0.05 Co 0.07 Al 0.2 The positive electrode active material is represented by O2 (based on a total transition metal Ni content of 86%), 1.3% by weight of Denka Black as a conductive material, and 2.4% by weight of polyvinylidene fluoride (PVDF) as a binder polymer.

[0137] The slurry was applied to an aluminum cathode current collector with a thickness of 12 μm using a single-layer coating system.

[0138] The slurry is then dried at 110°C and pressed to obtain a positive electrode with a single-layer coating. Here, the total thickness of the positive electrode active material layer is 175 μm.

[0139] A lithium secondary battery was obtained in the same manner as in Example 1, except that the positive electrode obtained as described above was used.

[0140] The characteristics of each positive electrode and lithium secondary battery in Examples 1 and 2, and Comparative Examples 1 and 2, are evaluated as follows.

[0141] Test Example 1: Evaluation of the increase in resistance in the lower SOC (5%–20%) region compared to the resistance at 50% SOC.

[0142] Using the lithium secondary batteries of Examples 1 and 2, and Comparative Examples 1 and 2, the increase in resistance in the lower SOC (5%–20%) region compared to the resistance at SOC 50% was evaluated under the following conditions using the following methods. The results are shown in Table 1 below.

[0143] For each lithium secondary battery, the state of charge (SOC) was reduced from 100% to 0% at a rate of 5% under an operating voltage in the range of 2.5V to 4.25V at room temperature. At the same time, the resistance was determined when it was discharged to each SOC level at 2.5C, and the results were compared.

[0144] Furthermore, the increase in resistance in the lower SOC (5%–20%) region compared to the resistance at SOC 50% is calculated using the following formula:

[0145] Resistance increase (%) = [(resistance at lower SOC (5%~20%)) / (resistance at SOC 50%)] × 100

[0146] The resistance at lower SOC (5%–20%) is the average of the resistance values ​​determined at SOCs of 5%, 10%, 15%, and 20%.

[0147] Test Example 2: Evaluation of High Temperature Capacity Retention

[0148] The lithium secondary batteries of Examples 1 and 2, as well as Comparative Examples 1 and 2, were subjected to 170 charge / discharge cycles at 45°C under the following conditions to evaluate high-temperature capacity retention.

[0149] Charging conditions: Constant current (CC) / constant voltage (CV), (4.25V, 0.005C cutoff current)

[0150] Discharge conditions: Constant current (CC) condition, 2.5V

[0151] High-temperature capacity retention was calculated as the ratio of the discharge capacity after 170 cycles to the discharge capacity after the first cycle. The results are shown in Table 1 below. Figure 1 middle.

[0152] [Table 1]

[0153]

[0154] Referring to Table 1, compared with the secondary battery of Comparative Example 1 (using a positive electrode including a bilayer active material layer, wherein the Ni content of the second positive electrode active material and the Ni content of the first positive electrode active material are reversed) and the secondary battery of Comparative Example 2 (using a positive electrode including a monolayer active material layer), each secondary battery of Examples 1 and 2 including a bilayer active material layer (controlled such that the Ni content of the second positive electrode active material in the upper region is greater than the Ni content of the first positive electrode active material in the lower region) showed a lower resistance increase (based on the resistance at SOC 50%) and a higher high-temperature retention rate at a lower SOC (5% to 20%).

Claims

1. A positive electrode, comprising: a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of said positive electrode current collector, and having a lower layer region facing said positive electrode current collector that contains a first positive electrode active material and a first binder polymer, and an upper layer region facing said lower layer region that contains a second positive electrode active material and a second binder polymer, wherein the first positive electrode active material in said lower layer region is represented by the following Chemical Formula 1, the second positive electrode active material in said upper layer region is represented by the following Chemical Formula 2, and the Ni content of the second positive electrode active material is greater than the Ni content of the first positive electrode active material: [Chemical Formula 1] Li 1+a [Ni x Mr y Co z ]M1 t O2 wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1 and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd; [Chemical Formula 2] Li 1+b [Ni u Mr v Co w ]M2 s O2 wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1 and u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd; wherein, based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 80 mol% to 90 mol%.

2. The positive electrode as described in claim 1, wherein, Based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 81 mol% to 90 mol%.

3. The positive electrode as described in claim 1, wherein, Based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 70 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 86 mol% to 90 mol%.

4. The positive electrode as described in claim 1, wherein, The weight ratio of said lower layer region to said upper layer region of said positive electrode active material layer is 20:80 to 80:

20.

5. A method for manufacturing a positive electrode, comprising the following steps: preparing a slurry for a lower layer containing a first positive electrode active material represented by the following Chemical Formula 1, a first binder polymer and a first dispersion medium, and a slurry for an upper layer containing a second positive electrode active material represented by the following Chemical Formula 2, a second binder polymer and a second dispersion medium; wherein the Ni content of the second positive electrode active material is greater than the Ni content of the first positive electrode active material; coating the slurry for the lower layer on one surface of the positive electrode current collector, and coating the slurry for the upper layer on the slurry for the lower layer; and simultaneously drying the coated slurry for the lower layer and the coated slurry for the upper layer to form a positive electrode active material layer: [Chemical Formula 1] Li 1+a [Ni x Mr y Co z ]M1 t O2 wherein 0≤a≤0.2, 0.4≤x≤0.9, 0<y<1, 0<z<1, 0≤t<0.1 and x+y+z+t=1, and M1 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd, [Chemical Formula 2] Li 1+b [Ni u Mr v Co w ]M2 s O2 wherein 0≤b≤0.2, 0.4≤u≤0.9, 0<v<1, 0<w<1, 0≤s<0.1 and u+v+w+s=1, and M2 is at least one element selected from the group consisting of Co, Mn, Ni, Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, W, Sn, Hf, Nd and Gd; wherein, based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 75 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 80 mol% to 90 mol%.

6. The method for manufacturing the positive electrode as described in claim 5, wherein, based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 81 mol% to 90 mol%.

7. The method for manufacturing the positive electrode as described in claim 5, wherein, based on the total transition metals of the first positive electrode active material, the Ni content of the first positive electrode active material is 65 mol% to 70 mol%, and based on the total transition metals of the second positive electrode active material, the Ni content of the second positive electrode active material is 86 mol% to 90 mol%.

8. A lithium secondary battery, comprising the positive electrode according to any one of claims 1 to 4.

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