Cathode material, a cathode containing the same, and a lithium secondary battery

Abstract

The present invention relates to a bimodal cathode material including a large particle size cathode active material and a small particle size cathode active material, in which the large particle size cathode active material is a single particle consisting of one nodule, and the small particle size cathode active material is a pseudo-single particle that is an aggregate of 2 to 30 nodules.

Description

[Technical Field]

[0001] This application claims priority under Korean Patent Application No. 10-2021-0146643 dated October 29, 2021, and all content disclosed in the said Korean Patent Application is incorporated herein by reference.

[0002] The present invention relates to a positive electrode material, a positive electrode containing the same, and a lithium secondary battery, and more particularly to a bimodal positive electrode material that can achieve excellent lifespan and resistance characteristics by applying large-particle and small-particle-sized particles of a specific shape, and a positive electrode containing the same and a lithium secondary battery. [Background technology]

[0003] A lithium secondary battery generally consists of a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and negative electrode contain an active material that allows for the insertion and deintercalation of lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMnO4, etc.), and lithium iron phosphate compounds (LiFePO4) have been used as positive electrode active materials for lithium secondary batteries. Of these, lithium cobalt oxide has the advantage of a high operating voltage and excellent capacity characteristics, but the price of cobalt, which is the raw material, is high and the supply is unstable, making commercial application to high-capacity batteries difficult. Lithium nickel oxide has poor structural stability, making it difficult to achieve sufficient lifespan characteristics. On the other hand, lithium manganese oxide has excellent stability but has the problem of poor capacity characteristics. Therefore, in order to complement the problems of lithium transition metal oxides containing Ni, Co, or Mn alone, lithium composite transition metal oxides containing two or more transition metals have been developed, and among them, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn is widely used.

[0005] Conventional lithium nickel cobalt manganese oxide generally has a spherical secondary particle morphology in which dozens to hundreds of primary particles are aggregated. However, in the case of lithium nickel cobalt manganese oxide in the form of secondary particles with such a large number of aggregated primary particles, particle cracking in which primary particles are detached occurs easily during the rolling process during the production of the positive electrode, and there is a problem that cracks occur inside the particles during the charge-discharge process. When particle cracking and cracks occur in the positive electrode active material, the contact area with the electrolyte increases, resulting in an increase in gas generation and deterioration of the active material due to side reactions with the electrolyte, and thus there is a problem that the life characteristics deteriorate.

[0006] In order to solve such problems, single-particle-form lithium nickel cobalt manganese oxide has been developed. Single-particle-form lithium nickel cobalt manganese oxide consists of one particle and can be produced by a method of over-firing at a high firing temperature after mixing a precursor for a positive electrode active material and a lithium source. Such a single-particle-form positive electrode active material has higher particle strength than a positive electrode active material in the form of secondary particles, so that particle cracking can be prevented during electrode rolling. However, since the single-particle positive electrode active material has significantly inferior lithium mobility compared to the positive electrode active material in the form of secondary particles, when the single-particle positive electrode active material is used alone, there is a problem that the resistance is too high to ensure sufficient capacity and output characteristics.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention is for solving the above problems, and aims to provide a positive electrode material with less particle cracking during electrode rolling and minimized increase in resistance during cycle charge-discharge by including a large-particle-size positive electrode active material and a small-particle-size positive electrode active material with controlled particle shapes.

[0008] Also, the present invention aims to provide a positive electrode and a lithium secondary battery with improved life characteristics and resistance characteristics by applying the above positive electrode material.

Means for Solving the Problems

[0009] According to one embodiment, the present invention provides a bimodal positive electrode material including a large particle size positive electrode active material and a small particle size positive electrode active material, wherein the large particle size positive electrode active material is a single particle composed of one nodule, and the small particle size positive electrode active material is a pseudo-single particle which is an aggregate of 2 to 30 nodules.

[0010] According to another embodiment, the present invention provides a positive electrode including the positive electrode material according to the present invention and a lithium secondary battery including the positive electrode.

Effect of the Invention

[0011] In the positive electrode material according to the present invention, as the large particle size positive electrode active material, a single particle composed of one nodule is used, and as the small particle size positive electrode active material, a pseudo-single particle which is an aggregate of 2 to 30 nodules is used, so that there are few particle cracks during rolling of the electrode, and an increase in resistance during charge and discharge is suppressed. Thereby, when the positive electrode material of the present invention is applied, a secondary battery excellent in life characteristics and resistance characteristics can be manufactured.

Brief Description of the Drawings

[0012] [Figure 1] It is a scanning electron microscope image showing the particle shape of the large particle size positive electrode active material A manufactured according to Production Example 1. [Figure 2] It is a scanning electron microscope image showing the particle shape of the large particle size positive electrode active material B manufactured according to Production Example 2. [Figure 3] It is a scanning electron microscope image showing the particle shape of the large particle size positive electrode active material C manufactured according to Production Example 3. [Figure 4] It is a scanning electron microscope image showing the particle shape of the small particle size positive electrode active material D manufactured according to Production Example 4. [Figure 5] It is a scanning electron microscope image showing the particle shape of the small particle size positive electrode active material E manufactured according to Production Example 5. [Figure 6]This figure shows the press density of the cathode material powders of Examples 1-2 and Comparative Examples 1-4. [Figure 7] This figure shows the high-temperature life characteristics of lithium secondary batteries using the cathode materials of Examples 1-2 and Comparative Examples 1-4. [Modes for carrying out the invention]

[0013] The present invention will be described in more detail below.

[0014] The terms and words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may define the concepts of terms as appropriate to best describe their invention.

[0015] In this invention, "single particle" refers to a particle consisting of one single nodule. In this invention, "pseudo-single particle" refers to a particle that is a composite formed of 30 or fewer nodules.

[0016] In the present invention, "nodule" means a particle unit body that constitutes a single particle or a pseudo-single particle. The nodule can be a single crystal lacking crystalline grain boundaries, or a polycrystalline material that appears to have no grain boundaries when observed at a field of view of 5,000 to 20,000 times using a scanning electron microscope (SEM) or electron backscatter diffraction (EBSD). The average grain size of the nodule refers to the arithmetic mean calculated after measuring the grain size of the nodule observed via SEM or EBSD.

[0017] In this invention, "secondary particle" refers to a particle formed by the aggregation of several tens to hundreds of primary particles. More specifically, a secondary particle is an aggregate of 40 or more primary particles.

[0018] As used in this invention, the term "particle" may include any one or all of the following: single particles, pseudo-single particles, primary particles, nodules, and secondary particles. In this invention, "average particle size D 50 This refers to the particle size at the 50% reference point of the volume-cumulative particle size distribution of the positive electrode active material powder, and can be measured using the laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, it can be introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiated with ultrasound at approximately 28 kHz with an output of 60 W, and then measured by obtaining a volume-cumulative particle size distribution graph and determining the particle size corresponding to 50% of the volume-cumulative amount.

[0019] In this invention, "particle strength" is determined by placing the positive electrode material powder into a particle strength measuring device (model: MCT-510), confirming the shape of the positive electrode active material particles to be measured with a microscope, and then applying a force to the positive electrode active material particles with a tip to measure the force at which the particles break. Considering the inter-particle deviation, the values ​​measured 30 or more times are mapped, and the average value is used as the particle strength value.

[0020] In this invention, "press density" was measured using HPRM-1000. Specifically, 5g of positive electrode material powder was placed into a cylindrical mold with a cross-sectional diameter of 1.2cm, and then the mold containing the positive electrode material was pressurized with a force of 2000kgf. Next, the height of the pressurized mold was measured with a vernier caliper, and the press density was determined.

[0021] In this invention, the average number of nodules aggregated in the particle cross-section of the positive electrode active material can be measured by the following method. A positive electrode manufactured using the positive electrode material is cut by ion milling, and a cross-sectional image in the thickness direction of the cut positive electrode is obtained using a scanning electron microscope (SEM). Then, at least 30 particles are selected from the image for both large-particle positive electrode active material particles and small-particle positive electrode active material particles, and the number of nodules in the cross-section of each positive electrode active material particle is measured by SEM image analysis. Subsequently, the arithmetic mean of the measured number of nodules is calculated, and this value is evaluated as the average number of nodules aggregated in the particle cross-section.

[0022] Cathode material The positive electrode material according to the present invention has an average particle size (D 50 This is a bimodal cathode material containing two types of cathode active materials that differ in their properties, namely, a large-particle cathode active material with relatively large particle sizes and a small-particle cathode active material with relatively small particle sizes.

[0023] Bimodal cathode materials have the advantage that, during electrode rolling, small-particle cathode active material fills the voids in large-particle cathode active material, increasing electrode density and thereby achieving high energy density. However, in the case of conventional bimodal cathode materials using cathode active material in secondary particle form, there was a problem that the particle strengths of the large-particle cathode active material and the small-particle cathode active material differed from each other, resulting in severe particle cracking of the large-particle cathode active material. To solve the above-mentioned problems, the present invention applies single particles and pseudo-single particles with high particle strength to the large-particle cathode active material and the small-particle cathode active material. Specifically, the present invention applies single particles as the large-particle cathode active material and pseudo-single particles as the small-particle cathode active material in the bimodal cathode material. Here, a single particle means a particle consisting of one nodule, and the pseudo-single particle means an aggregate of 2 to 30 nodules, preferably 2 to 25, more preferably 2 to 20.

[0024] Single-particle or pseudo-single-particle positive electrode active materials have higher particle strength compared to existing secondary-particle positive electrode active materials, which consist of tens to hundreds of primary particles aggregated together. As a result, there is less particle cracking during rolling, less change due to volume expansion and contraction during charging and discharging, and a significant reduction in the occurrence of cracks inside the particles. Therefore, when single particles are used as large-particles and pseudo-single particles as small-particles, as in the present invention, deterioration of battery performance due to gas generation and cracks inside the positive electrode active material can be minimized, and lithium secondary batteries with excellent lifespan characteristics, especially high-temperature lifespan characteristics, can be manufactured.

[0025] However, when using single particles or pseudo-single particles as the positive electrode active material, lithium mobility is inferior compared to conventional secondary particle positive electrode active materials, leading to increased resistance and a decrease in capacitance and power characteristics. The inventors diligently conducted research to develop a positive electrode material with excellent lifetime characteristics and minimal resistance increase. As a result, they discovered that resistance increase can be suppressed when the particle morphology of the large-particle positive electrode active material and the small-particle positive electrode active material satisfies specific conditions, namely, when the large-particle positive electrode active material is in single-particle form and the small-particle positive electrode active material is in pseudo-single-particle form, thus completing the present invention.

[0026] During the rolling of electrodes, cracks may occur along the interfaces between the subparticles (i.e., primary particles and nodules) that make up the positive electrode active material particles. If cracks occur inside the active material, the electrolyte penetrates into the cracked area, increasing resistance. On the other hand, when cracks occur in large-particle positive electrode active material, the area in contact with the electrolyte due to the cracks is larger than that of small-particle positive electrode active material, resulting in a more severe increase in resistance. Therefore, in this invention, by using a single particle consisting of one nodule as the large-particle positive electrode active material, the occurrence of cracks in the large-particle positive electrode active material is minimized, thereby suppressing the increase in resistance.

[0027] On the other hand, the number of nodules constituting the small particle size positive electrode active material can be 2 to 30, preferably 2 to 25, more preferably 2 to 20, and even more preferably 2 to 10.

[0028] If the number of nodules constituting the small-particle positive electrode active material exceeds 30, particle cracking may occur during electrode rolling, potentially reducing its lifespan. Furthermore, if the small-particle positive electrode active material is in single-particle form, its electrode rolling properties may decrease, leading to a lower electrode density and potentially degrading its capacity characteristics.

[0029] Preferably, when r' is the average number of nodules aggregated in the particle cross-section of the small-particle-sized positive electrode active material, 1 / r' can be 0.5 or less, preferably 0.1 to 0.5. According to the inventors' research, when 1 / r' satisfies the above range, a better improvement in lifetime characteristics and resistance characteristics is observed.

[0030] On the other hand, the number of nodules constituting the large-particle-sized positive electrode active material and the small-particle-sized positive electrode active material can be adjusted by controlling the firing conditions. Generally, positive electrode active material is manufactured by mixing a precursor, which is a transition metal hydroxide, with a lithium source and then firing it. However, the higher the firing temperature and the longer the firing time, the fewer nodules constituting the positive electrode active material particles there are. Therefore, the number of nodules constituting the positive electrode active material particles can be adjusted by adjusting the firing temperature and / or firing time during the manufacturing of the positive electrode active material. For example, if the composition of the large-particle-sized positive electrode active material and the small-particle-sized positive electrode active material is the same, the firing temperature of the large-particle-sized positive electrode active material can be increased by 20°C or more, preferably 20°C to 80°C, compared to the firing temperature of the small-particle-sized positive electrode active material, so that 1 / r' becomes 0.5 or less.

[0031] On the other hand, in the present invention, the large particle size positive electrode active material has an average particle size D 50 The particle size can be 10 μm to 20 μm, preferably 10 μm to 18 μm, and more preferably 10 μm to 15 μm. Furthermore, the average particle size D of the small particle size positive electrode active material is 50 The average particle size D of the large-particle-size positive electrode active material and the small-particle-size positive electrode active material can be 1 μm to 8 μm, preferably 3 μm to 8 μm, and more preferably 3 μm to 7 μm. 50When the above range is satisfied, the press density of the positive electrode material increases, whereby, during the production of the electrode, the electrode density can be improved and excellent energy density can be achieved.

[0032] On the other hand, in the present invention, the large particle size positive electrode active material and the small particle size positive electrode active material can each independently contain a lithium nickel-based oxide.

[0033] Specifically, the large particle size positive electrode active material and the small particle size positive electrode active material can each independently contain a lithium nickel-based oxide represented by the following [Chemical Formula 1].

[0034] [Chemical Formula 1] Li a Ni b Co c M 1 d M 2 e O2

[0035] In the above Chemical Formula 1, M 1 can be Mn, Al or a combination thereof, preferably Mn or Mn and Al.

[0036] The above M 2 is one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably one or more selected from the group consisting of Zr, Y, Mg, and Ti, and more preferably Zr, Y or a combination thereof. The M 2 element is not necessarily included, but when included in an appropriate amount, it can play a role in promoting grain growth during firing or improving crystal structure stability.

[0037] The a represents the molar ratio of lithium in the lithium nickel-based oxide, and can be 0.8 ≦ a ≦ 1.3, 0.9 ≦ a ≦ 1.3, or 1.0 ≦ a ≦ 1.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium nickel-based oxide can be stably formed.

[0038] The b represents the molar ratio of nickel among all the metals other than lithium in the lithium nickel-based oxide, and can be 0.8 ≦ b < 1, 0.82 ≦ b < 1, 0.83 ≦ b < 1, 0.85 ≦ b < 1 or 0.87 ≦ b < 1. When the molar ratio of nickel satisfies the above range, it shows a high energy density and high capacity can be realized.

[0039] The c represents the molar ratio of cobalt among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 < c < 0.2, 0 < c < 0.18, or 0.01 ≦ c ≦ 0.17. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0040] The d represents the molar ratio of the M 1 element among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 < d < 0.2, 0 < d < 0.18, or 0.01 ≦ d ≦ 0.17. When the molar ratio of the M 1 element satisfies the above range, the structure stability of the positive electrode active material is excellent.

[0041] The e represents the molar ratio of the M 2 element among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 ≦ e ≦ 0.1, or 0 ≦ e ≦ 0.05.

[0042] On the other hand, the large-particle-grain positive electrode active material and the small-particle-grain positive electrode active material according to the present invention may further include a coating layer on the surface of the lithium nickel oxide particles as needed. Here, the coating layer may include, for example, one or more coating elements selected from the group consisting of Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mn, Sr, Sb, Bi, Si, and S. When a coating layer is present on the surface of the lithium nickel oxide, the coating layer reduces contact between the electrolyte and the lithium nickel oxide, thereby reducing the elution of transition metals and the generation of gases due to side reactions with the electrolyte. Preferably, the coating layer may include B, Co, Al, or a combination thereof, and more preferably Co. When Co is included in the coating layer, an improvement in output and a reduction in resistance can be obtained along with the effect of suppressing side reactions with the electrolyte.

[0043] The large-particle-sized positive electrode active material and the small-particle-sized positive electrode active material may have the same or different compositions. For example, the Ni molar ratio of the large-particle-sized positive electrode active material may be higher than that of the small-particle-sized positive electrode active material. Because the large-particle-sized positive electrode active material is in single-particle form, it has superior structural stability compared to the small-particle-sized positive electrode active material, which is in a pseudo-single-particle form. As a result, even with a high Ni content, there is less structural degradation due to heat and repeated charge-discharge cycles. Therefore, when a lithium nickel-based oxide with a relatively higher Ni content is used as the large-particle-sized positive electrode active material, a secondary battery with high capacity and excellent lifespan characteristics can be realized. Specifically, the large-particle-sized positive electrode active material may contain a lithium nickel-based oxide containing 85 mol% or more, preferably 87 mol% or more, of nickel, and the small-particle-sized positive electrode active material may contain a lithium nickel-based oxide containing 80 mol% or more and less than 87 mol%, preferably 80 mol% to 86 mol%, and more preferably 83 mol% to 86 mol%, of nickel.

[0044] On the other hand, it is preferable that the particle strength of the large-particle-sized positive electrode active material is higher than that of the small-particle-sized positive electrode active material. Specifically, the particle strength of the large-particle-sized positive electrode active material can be twice or more, preferably 2 to 4 times, and more preferably 2 to 3 times, that of the small-particle-sized positive electrode active material. When the particle strength of the large-particle-sized positive electrode active material is twice or more that of the small-particle-sized positive electrode active material, particle cracking of the large-particle-sized positive electrode active material is minimized, and an excellent effect of suppressing resistance increase can be obtained.

[0045] Specifically, the particle strength of the large-particle-sized positive electrode active material can be 150 MPa to 300 MPa, preferably 180 MPa to 220 MPa, and the particle strength of the small-particle-sized positive electrode active material can be 75 MPa to 150 MPa, preferably 90 MPa to 110 MPa. When the particle strengths of the large-particle-sized positive electrode active material and the small-particle-sized positive electrode active material satisfy the above ranges, particle cracking during electrode rolling is minimized, and the porosity and electrode density of the positive electrode active material layer can be appropriately formed.

[0046] On the other hand, the positive electrode material can contain large-particle positive electrode active material and small-particle positive electrode active material in a weight ratio of 60:40 to 90:10, preferably 70:30 to 80:20. When the weight ratio of large-particle positive electrode active material to small-particle positive electrode active material satisfies the above range, excellent capacity characteristics and life characteristics can be achieved.

[0047] On the other hand, the positive electrode material may have a press density of 3.0 g / cc to 4.0 g / cc, preferably 3.0 g / cc to 3.8 g / cc, and more preferably 3.0 g / cc to 3.6 g / cc, measured by pressing the positive electrode material powder at 2000 kgf. When the press density of the positive electrode material satisfies the above range, a secondary battery with excellent energy density can be manufactured.

[0048] positive electrode Next, the positive electrode according to the present invention will be described.

[0049] The positive electrode according to the present invention includes the positive electrode material of the present invention described above. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode material according to the present invention.

[0050] Since the cathode material has been described above, a detailed explanation will be omitted, and only the remaining components will be described in detail below.

[0051] The positive electrode current collector is not particularly limited as long as it contains a highly conductive metal, allows for easy adhesion of the positive electrode active material layer, and is unreactive within the battery voltage range. Examples of materials that can be used for the positive electrode current collector include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments such as carbon, nickel, titanium, or silver. Furthermore, the positive electrode current collector can typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on its surface to enhance adhesion to the positive electrode active material. It can be used in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0052] The positive electrode active material layer may, as necessary, selectively include a conductive material and a binder together with the positive electrode material.

[0053] Here, the positive electrode material can be included in an amount of 80% to 99% by weight, more specifically 85% to 98.5% by weight, relative to the total weight of the positive electrode active material layer. When included within the above content range, it can exhibit excellent capacitance characteristics.

[0054] The conductive material is used to impart conductivity to the electrodes and can be used without particular limitations in the battery it is configured in, as long as it does not cause chemical changes and has electronic conductivity. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive tubes such as carbon nanotubes; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Of these, one or more can be used. The conductive material may be included in an amount of 0.1% to 10% by weight relative to the total weight of the positive electrode active material layer.

[0055] The binder plays a role in improving the adhesion between positive electrode material particles and the adhesion between the positive electrode material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers in which the hydrogens of these are substituted with Li, Na, or Ca, or various copolymers thereof. Of these, one or more can be used. The binder can be present in an amount of 0.1% to 15% by weight relative to the total weight of the positive electrode active material layer.

[0056] The positive electrode can be manufactured by a conventional positive electrode manufacturing method, except for using the positive electrode material described above. Specifically, it can be manufactured by coating a positive electrode composite material, prepared by dissolving or dispersing the positive electrode material, and optionally a binder, conductive material, and dispersant, in a solvent, onto a positive electrode current collector, followed by drying and rolling; or by casting the positive electrode composite material onto another support, peeling it off the support, and laminating the resulting film onto the positive electrode current collector.

[0057] The solvent can be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone, or water, and one or more of these can be used individually or in mixtures of two or more. The amount of solvent used should be sufficient to dissolve or disperse the cathode active material, conductive material, binder, and dispersant, taking into account the slurry coating thickness and manufacturing yield, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the cathode.

[0058] Lithium-ion rechargeable battery Next, the lithium secondary battery according to the present invention will be described.

[0059] The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte. As the positive electrode is as described above, a detailed explanation will be omitted, and only the remaining components will be described in detail below.

[0060] Furthermore, the lithium secondary battery may selectively further include a battery container for housing the electrode assembly comprising the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0061] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0062] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatments such as carbon, nickel, titanium, or silver, and aluminum-cadmium alloys can be used. The negative electrode current collector can usually have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as film, sheet, foil, mesh, porous material, foam, and nonwoven fabric.

[0063] The negative electrode active material layer selectively includes a binder and a conductive material together with the negative electrode active material.

[0064] As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. β Examples include metallic oxides that can be doped and dedoped with lithium, such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used. A metallic lithium thin film can also be used as the negative electrode active material. The negative electrode active material can be present in an amount of 80% to 99% by weight relative to the total weight of the negative electrode active material layer.

[0065] The binder is typically added in an amount of 0.1% to 10% by weight relative to the total weight of the negative electrode active material layer, as a component that facilitates bonding between the conductive material, active material, and current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0066] The conductive material can be added in an amount of 10% by weight or less, preferably 5% by weight or less, relative to the total weight of the negative electrode active material layer, as a component to further improve the conductivity of the negative electrode active material. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used.

[0067] The negative electrode active material layer can be manufactured by coating a negative electrode composite material, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and then drying it, or by casting the negative electrode composite material onto another support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0068] On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Generally, any separator used in lithium secondary batteries can be used without particular limitations, but those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof can be used. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, coated separators containing ceramic components or polymeric substances can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0069] Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0070] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0071] The organic solvent can be used without particular limitations as long as it serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC); alcoholic solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and can include a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0072] The lithium salt can be used without particular limitations as long as it is a compound that can provide lithium ions for use in lithium secondary batteries. Specifically, the anion of the lithium salt is F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 - CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - The lithium salt can be selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 0.1M to 4.0M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0073] In addition to the electrolyte components, the electrolyte may also contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethyl alcoholamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethyl alcohol, or aluminum trichloride. Here, the additive may be present in an amount of 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0074] As described above, lithium secondary batteries containing the bimodal cathode material according to the present invention exhibit excellent discharge capacity, output characteristics, and life characteristics in a stable manner, making them useful in portable devices such as mobile phones, notebook computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0075] Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

[0076] The aforementioned battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0077] The external shape of the lithium secondary battery of the present invention is not particularly limited, but it can be cylindrical, rectangular, pouch-type, or coin-type, using a can.

[0078] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also preferably as a unit battery in medium- and large-sized battery modules containing a large number of battery cells.

[0079] Examples of the aforementioned medium- and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0080] The present invention will be described in more detail below with reference to specific examples.

[0081] Manufacturing Example 1 - Manufacturing of large-particle cathode active material A (single particle) Average particle size D 50 Ni is 12 μm 0.88 Co 0.08 Mn 0.04 (OH)2 and LiOH are mixed so that the molar ratio of transition metal (Ni+Co+Mn):Li is 1:1.03, and the mixture is fired at 850°C for 12 hours to produce a large particle size positive electrode active material A (LiNi 0.88 Co 0.08 Mn 0.04 O2) was manufactured. The powder of the manufactured positive electrode active material A was observed with a scanning electron microscope (SEM) to confirm the particle morphology. Figure 1 shows an SEM image of the positive electrode active material A manufactured by the above method. As shown in Figure 1, the positive electrode active material A was in the form of a single particle consisting of one nodule.

[0082] Manufacturing Example 2 - Manufacturing of large-particle cathode active material B (secondary particles) Except for the fact that the firing was carried out at 800°C for 10 hours, the large particle size positive electrode active material B (LiNi) was manufactured using the same method as in Manufacturing Example 1. 0.88 Co 0.08 Mn 0.04O2) was manufactured. The powder of the manufactured positive electrode active material B was observed with a scanning electron microscope to confirm the particle morphology. Figure 2 shows an SEM image of the positive electrode active material B manufactured by the above method. As shown in Figure 2, the positive electrode active material B had a secondary particle morphology in which one or more primary particles were aggregated.

[0083] Manufacturing Example 3 - Manufacturing of large-particle cathode active material C (pseudo-single particle) Except for firing at 830°C for 10 hours, the large particle size cathode active material C(LiNi) was manufactured using the same method as in Manufacturing Example 1. 0.88 Co 0.08 Mn 0.04 O2) was manufactured. The powder of the manufactured positive electrode active material C was observed with a scanning electron microscope to confirm its particle morphology. Figure 3 shows a SEM image of the positive electrode active material C manufactured by the above method. As shown in Figure 3, the positive electrode active material C was in a pseudo-single particle morphology, consisting of aggregates of 10 to 20 nodules.

[0084] Manufacturing Example 4 - Manufacturing of small-particle cathode active material D (pseudo-single particle) Average particle size D 50 Ni is 3.7 μm 0.86 Co 0.08 Mn 0.06 (OH)2 and LiOH are mixed so that the molar ratio of transition metal (Ni+Co+Mn):Li is 1:1.05, and fired at 800°C for 12 hours to produce a small particle size positive electrode active material D(LiNi 0.86 Co 0.08 Mn 0.06 O2) was manufactured. The powder of the manufactured positive electrode active material D was observed with a scanning electron microscope to confirm the particle morphology. Figure 4 shows an SEM image of the positive electrode active material D manufactured by the above method. As shown in Figure 4, the positive electrode active material D manufactured by the above method had a pseudo-single particle morphology in which 10 to 20 nodules were aggregated.

[0085] Manufacturing Example 5 - Manufacturing of small-particle cathode active material E (secondary particles) Except for firing at 780°C for 10 hours, the elementary particle cathode active material E(LiNi) was manufactured using the same method as in Manufacturing Example 4. 0.86 Co 0.08Mn 0.06 O2) was manufactured. The powder of the manufactured positive electrode active material E was observed with a scanning electron microscope to confirm its particle morphology. Figure 5 shows a SEM image of the positive electrode active material E manufactured by the above method. As shown in Figure 5, the positive electrode active material E had a secondary particle morphology in which dozens or more primary particles were aggregated.

[0086] Example 1 A cathode material was manufactured by mixing the large-particle cathode active material A produced in manufacturing example 1 and the small-particle cathode active material D produced in manufacturing example 4 in a weight ratio of 80:20.

[0087] Example 2 A cathode material was manufactured by mixing the large-particle cathode active material A produced in manufacturing example 1 and the small-particle cathode active material D produced in manufacturing example 4 in a weight ratio of 70:30.

[0088] Comparative Example 1 A cathode material was manufactured by mixing the large-particle cathode active material B produced in manufacturing example 2 and the small-particle cathode active material E produced in manufacturing example 5 in a weight ratio of 80:20.

[0089] Comparative Example 2 A cathode material was manufactured by mixing the large-particle cathode active material B produced in manufacturing example 2 and the small-particle cathode active material D produced in manufacturing example 4 in a weight ratio of 80:20.

[0090] Comparative Example 3 A cathode material was manufactured using the small-particle cathode active material D produced in manufacturing example 4 on its own.

[0091] Comparative Example 4 A cathode material was manufactured by mixing the large-particle cathode active material C produced in manufacturing example 3 and the small-particle cathode active material D produced in manufacturing example 4 in a weight ratio of 80:20.

[0092] Experimental Example 1: Press Density 5 g each of the positive electrode material powders from Examples 1-2 and Comparative Examples 1-4 were placed into a cylindrical mold with a cross-sectional diameter of 1.2 cm. The mold containing the positive electrode material was then pressurized with loads of 400 kgf, 800 kgf, 1200 kgf, 1600 kgf, and 2000 kgf. Next, the height of the pressurized mold was measured using a vernier caliper to determine the press density. The measurement results are shown in Figure 6 below. Referring to Figure 6, it can be confirmed that the positive electrode materials from Examples 1-2 exhibit a higher press density compared to the positive electrode materials from Comparative Examples 1-4.

[0093] Experimental Example 2: Measurement of the number of subparticle aggregates in the particle cross-section The cathode materials, conductive materials (carbon nanotubes, LB-CNTs), and PVdF binders produced in Examples 1-2 and Comparative Examples 1-4 were mixed in N-methylpyrrolidone in a weight ratio of 97.2:1.2:1.6 to produce a cathode slurry. The cathode slurry was applied to one side of an aluminum current collector, dried, and rolled to produce a cathode.

[0094] After obtaining cross-sectional images in the thickness direction by cutting the manufactured positive electrode using the ion milling method, 30 particles were selected from both the large-particle positive electrode active material and the small-particle positive electrode active material by image analysis, and the number of nodules or primary particles in each particle cross-section was measured. The arithmetic mean of the number of nodules or primary particles measured in the particle cross-section of 30 large-particle positive electrode active material particles was defined as the average aggregation number of nodules or primary particles in the particle cross-section of the large-particle positive electrode active material, and the arithmetic mean of the number of nodules or primary particles measured in the particle cross-section of 30 small-particle positive electrode active material particles was defined as the average aggregation number of primary particles in the particle cross-section of the small-particle positive electrode active material, r'. The measurement results are shown in Table 1 below.

[0095] [Table 1]

[0096] Experimental Example 3: Evaluation of Lifetime Characteristics <Manufacturing of secondary batteries> The cathode materials, conductive materials (carbon nanotubes, LB-CNTs), and PVdF binders produced in Examples 1-2 and Comparative Examples 1-4 were mixed in N-methylpyrrolidone in a weight ratio of 97.2:1.2:1.6 to produce a cathode slurry. The cathode slurry was applied to one side of an aluminum current collector, dried, and rolled to produce a cathode.

[0097] A negative electrode slurry was prepared by mixing a negative electrode active material (a mixture of SiO and graphite, with a graphite:Si weight ratio of 94.5:3.3), a mixed conductive material consisting of Super C65 and single-walled carbon nanotubes in a weight ratio of 98:2, and an SBR binder (M37, LG Chem) in water in a weight ratio of 95.57:1:3.43. The negative electrode slurry was applied to one side of a copper current collector, dried, and rolled to produce the negative electrode.

[0098] After placing a separator between the positive and negative electrodes, the battery was inserted into a battery case, and an electrolyte was injected to manufacture a lithium secondary battery.

[0099] For the lithium secondary batteries manufactured as described above, the DCIR increase and capacity retention were measured while performing 50 charge-discharge cycles, with one cycle consisting of charging to 4.25V at 45°C and discharging to 2.5V at 1C. The measurement results are shown in Figure 7 and Table 2 below.

[0100] Here, the resistance increase rate and capacitance retention rate were calculated using the following equations 1 and 2.

[0101] Equation 1: Resistance increase rate (%) = {(Resistance after 50 cycles / Resistance after 1 cycle) × 100} - 100

[0102] Equation 2: Capacity retention rate (%) = (Discharge capacity after 50 cycles / Discharge capacity after 1 cycle) × 100

[0103] [Table 2]

[0104] As shown in Table 2 and Figure 7 above, the lithium secondary batteries using the cathode materials of Examples 1 and 2 showed superior performance in terms of capacity retention rate and resistance increase rate compared to the lithium secondary batteries using the cathode materials of Comparative Examples 1 to 4.

Claims

1. This is a bimodal cathode material containing large-particle cathode active material and small-particle cathode active material. The aforementioned large-particle positive electrode active material is a single particle consisting of one nodule, The aforementioned small-particle positive electrode active material is a pseudo-single particle, which is an aggregate of 2 to 30 nodules. The aforementioned large-particle positive electrode active material has an average particle size D50 of 10 μm to 20 μm. The aforementioned small particle size positive electrode active material has an average particle size D50 of 1 μm to 8 μm. The large-particle-grain positive electrode active material and the small-particle-grain positive electrode active material each independently contain a lithium nickel-based oxide represented by the following [Chemical Formula 1], as a positive electrode material: [Chemical formula 1] Li ａ Ni ｂ Co ｃ M １ ｄ M ２ ｅ O ２ In the chemical formula (1), M １ is Mn, Al or a combination thereof, and M ２ is one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, and 0.8 ≤ a ≤ 1.3, 0.8 ≤ b < 1, 0 < c < 0.2, 0 < d < 0.2, 0 ≤ e ≤ 0.

1.

2. The positive electrode material according to claim 1, wherein when the average number of nodules aggregated in the particle cross-section of the small-particle positive electrode active material is r', 1 / r' is 0.5 or less.

3. The positive electrode material according to claim 2, wherein the aforementioned 1 / r' is 0.1 to 0.

5.

4. The cathode material according to claim 1, wherein the particle strength of the large-particle cathode active material is twice or more the particle strength of the small-particle cathode active material.

5. The particle strength of the aforementioned large-particle positive electrode active material is 150 MPa to 300 MPa. The cathode material according to claim 1, wherein the particle strength of the small-particle cathode active material is 75 MPa to 150 MPa.

6. The positive electrode material according to claim 1, wherein the positive electrode material contains a large-particle positive electrode active material and a small-particle positive electrode active material in a weight ratio of 60:40 to 90:

10.

7. The positive electrode material according to claim 1, wherein the press density measured by pressing the positive electrode material at 2000 kgf is 3.0 g / cc to 4.0 g / cc.

8. A positive electrode comprising the positive electrode material described in any one of claims 1 to 7.

9. A lithium secondary battery comprising the positive electrode described in claim 8.

10. The cathode material according to claim 1, wherein the Ni molar ratio of the large-particle cathode active material is higher than the Ni molar ratio of the small-particle cathode active material.

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