Positive electrode for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode for a lithium secondary battery, which includes a positive electrode active material layer containing: a first positive electrode active material represented by Formula 1 and having a grain size of 150 nm or more, a conductive agent containing single-walled carbon nanotubes (SWCNTs), and a binder, and to a lithium secondary battery including the positive electrode.

Description

Technical Field

[0001] This application claims the benefit of Korean Patent Application No. 10-2020-0182745, filed on December 24, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0002] This invention relates to a positive electrode for lithium secondary batteries and a lithium secondary battery comprising the same, and more particularly to a positive electrode having excellent lifespan and resistance characteristics and a lithium secondary battery comprising the same. Background Technology

[0003] With the development and increasing demand for mobile device technology, the demand for secondary batteries as an energy source has increased significantly. Among these secondary batteries, lithium secondary batteries, which have high energy density, high voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Recently, with the development of technologies such as electric vehicles, the demand for high-capacity rechargeable batteries has been increasing. Therefore, active research has been conducted on high-Ni cathode active materials with excellent capacity characteristics. High-Ni cathode active materials are cathode active materials with a nickel content of 80 atm% or more, and can achieve high capacity. However, the limitation is that as the nickel content increases, the structural stability of the cathode active material decreases, which in turn reduces its high-temperature life characteristics.

[0005] In addition, the limitation is that high-Ni cathode active materials have high resistance and large volume changes during charging and discharging. Therefore, the conductive path is broken during repeated charging and discharging, resulting in an increased resistance rate.

[0006] To address the aforementioned limitations, reducing the grain size of the positive electrode active material can be considered to decrease the volume change during charging and discharging. However, this method has the limitation that while reducing the grain size decreases the volume change, it increases the initial resistance. Specifically, in the case of high-Ni positive electrode active materials with high nickel content, the limitation lies in the fact that the resistivity characteristics deteriorate at low state of charge (SOC) due to the presence of a large amount of NiO rock salt phase on the surface of the high-Ni positive electrode active material, thus degrading the output characteristics. Furthermore, this limitation becomes more severe as the initial resistance increases. In particular, for use as a battery in electric vehicles, the battery should be able to generate sufficient output to enable the vehicle to operate even at low states of charge.

[0007] Therefore, it is necessary to develop lithium secondary batteries using high-Ni cathode active materials to achieve high capacity characteristics and low resistance increase rate, thereby generating sufficient output to have excellent high-temperature life characteristics. Summary of the Invention

[0008] Technical issues

[0009] One aspect of the present invention provides a high-capacity positive electrode with improved resistance characteristics and high-temperature life characteristics, and a lithium secondary battery comprising the positive electrode.

[0010] Technical solution

[0011] According to one aspect of the present invention, a positive electrode for a lithium secondary battery is provided, comprising a positive electrode active material layer, the positive electrode active material layer comprising: a first positive electrode active material represented by Formula 1 and having a grain size of 150 nm or more; a conductive agent comprising single-walled carbon nanotubes (SWCNTs); and a binder.

[0012] [Formula 1]

[0013] Li x1 [Ni a1 Co b1 M 1 c1 M 2 d1 O2

[0014] In Equation 1 above,

[0015] M 1 It is Mn, Al, or a combination thereof.

[0016] M 2 It is selected from at least one of the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and

[0017] 0.9≤x1≤1.1, 0.8≤a1<1, 0 <b1<0.2,0<c1<0.2,0≤d1≤0.1。

[0018] According to another aspect of the present invention, a lithium secondary battery comprising the positive electrode for the lithium secondary battery is provided.

[0019] Beneficial effects

[0020] The cathode of this invention employs a high-Ni cathode active material with a specific grain size and single-walled carbon nanotubes, thereby achieving high capacity characteristics and reducing the rate of resistance increase, thus improving output characteristics. Specifically, this invention achieves low initial resistance and excellent capacity characteristics by using a cathode active material with a grain size of 150 nm or larger, and by using single-walled carbon nanotubes, the loss of conductive pathways is minimized even during repeated charge-discharge cycles. Therefore, lithium secondary batteries using the cathode of this invention exhibit excellent capacity characteristics, resistance characteristics, and high-temperature lifetime characteristics.

[0021] In contrast, when the grain size of the positive electrode active material deviates from the scope of this invention, it is difficult to improve the resistance even when using single-walled carbon nanotubes. When single-walled carbon nanotubes are not used, even when using a positive electrode active material with a grain size that meets the scope of this invention, the resistance improvement effect is not obvious, and the high-temperature life characteristics deteriorate. Attached Figure Description

[0022] Figure 1 This is a graph showing the relationship between the resistance characteristics and SOC of the lithium secondary batteries prepared according to Comparative Example 1 and Examples 1 to 3.

[0023] Figure 2 The graph shows the relationship between the resistance characteristics and the state of charge (SOC) of the lithium secondary batteries prepared according to Comparative Examples 2 and 3.

[0024] Figure 3 This is a graph showing the relationship between the resistance characteristics and SOC of the lithium secondary batteries prepared according to Example 4 and Comparative Example 4.

[0025] Figure 4 This is a graph showing the high-temperature life characteristics of the lithium secondary batteries prepared according to Comparative Example 1 and Examples 1 to 3.

[0026] Figure 5 This is a graph showing the high-temperature life characteristics of the lithium secondary batteries prepared according to Example 4 and Comparative Example 4. Detailed Implementation

[0027] In this invention, the term "grain" refers to a single-crystal particle unit having a regular atomic arrangement. Grain size can be determined by analyzing XRD data using the Rietveld refinement method, the XRD data being obtained through X-ray diffraction analysis of the positive electrode active material powder. Here, a Bruker D8 Endeavor (Cu-Kα) sensor equipped with a LynxEye XE-T position-sensitive detector is used. The sample was placed in the groove of a general powder support, and a glass slide was used to flatten the sample surface and fill the sample so that the sample height corresponded to the edge of the support. X-ray diffraction analysis was then performed under the following conditions: a step size of 0.02° in the FDS 0.5° region, 2θ = 15°–90°, and a total scan time of approximately 20 minutes. Rietveld refinement was applied to the measured data, taking into account the charge at each site (+3 at transition metal sites for metals, +2 at Li sites for Ni) and cation mixing. Specifically, in the analysis of grain size, the instrument topology was considered using the fundamental parameter method (FPA) embedded in the Bruker TOPAS program, and the total peak within the measurement range was used during fitting. Among the available peak shapes in TOPAS, only the Lorenzian contribution was used as a first-principles (FP) method to fit the peak shapes, without considering strain.

[0028] In this invention, the term "primary particle" refers to the smallest particle unit that is distinguished as a single block when the cross-section of the positive electrode active material is observed by scanning electron microscopy (SEM), and it may consist of a single grain or multiple grains. In this invention, the average particle size of the primary particles can be determined by measuring the size of each particle distinguished from the cross-sectional SEM image of the positive electrode active material particles and calculating their arithmetic mean.

[0029] In this invention, the term "secondary particle" refers to a secondary structure formed by the aggregation of multiple primary particles. The average particle size of the secondary particles can be measured using a particle size analyzer; in this invention, the Microtrac S3500 is used as the particle size analyzer.

[0030] In this invention, the term "particle size Dn" for positive electrode active materials refers to the particle size at n% of the volumetric cumulative distribution of particle size. In other words, D50 is the particle size at 50% of the volumetric cumulative distribution of particle size, D90 is the particle size at 90% of the volumetric cumulative distribution of particle size, and D10 is the particle size at 10% of the volumetric cumulative distribution of particle size. Dn can be measured using laser diffraction. Specifically, after dispersing the target powder in a dispersion medium (distilled water), the dispersion medium is introduced into a commercially available laser diffraction particle size measurement device (e.g., Microtrac S3500), and the particle size distribution is calculated by measuring the differences in the diffraction pattern caused by particle size as the particles pass through the laser beam. D10, D50, and D90 can be measured by calculating the particle size at 10%, 50%, and 90% of the volumetric cumulative distribution of particle size in the measurement device.

[0031] In this specification, the term "specific surface area" is measured by the Brunauer-Emmett-Teller (BET) method, specifically, the specific surface area can be calculated from the amount of nitrogen adsorbed at liquid nitrogen temperature (77K) using the BELSORP-mino II manufactured by Bell Japan Inc.

[0032] The invention will be described in more detail below.

[0033] The inventors have conducted research to improve the resistivity characteristics of lithium-ion secondary batteries using high-Ni cathode active materials. As a result, the inventors discovered that the resistivity characteristics of high-capacity cathodes can be improved by using single-walled carbon nanotubes and high-Ni cathode active materials with specific grain sizes, and thus completed this invention.

[0034] positive electrode

[0035] The positive electrode of the present invention includes a positive electrode active material layer comprising (1) a positive electrode active material, (2) a conductive agent and (3) a binder. In this case, the positive electrode active material comprises a first positive electrode active material represented by Formula 1 below and having a grain size of 150 nm or more, and the conductive agent comprises single-walled carbon nanotubes (SWCNTs).

[0036] [Formula 1]

[0037] Li x1 [Ni a1 Co b1 M 1 c1 M 2 d1 O2

[0038] In Equation 1 above,

[0039] M 1 It is Mn, Al, or a combination thereof.

[0040] M 2 It is selected from at least one of the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and

[0041] 0.9≤x1≤1.1, 0.8≤a1<1, 0 <b1<0.2,0<c1<0.2,0≤d1≤0.1。

[0042] The components of the positive electrode of the present invention will be described in detail below.

[0043] (1) Positive electrode active material

[0044] The positive electrode of the present invention comprises a lithium composite transition metal oxide represented by the following [Formula 1] as the first positive electrode active material.

[0045] [Formula 1]

[0046] Li x1 Ni a1 Co b1 M 1 c1 M 2 d1 O2

[0047] In Equation 1 above, the above M 1 It can be Mn, Al, or a combination thereof, preferably Mn or a combination of Mn and Al.

[0048] M 2It may be at least one selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S. In terms of improving the structural stability of the lithium composite transition metal oxide, M 2 More preferably, it includes Zr.

[0049] The above x1 represents the molar ratio of lithium in the lithium composite transition metal oxide, where x1 may satisfy 0.9 ≤ x1 ≤ 1.1, preferably 0.95 ≤ x1 ≤ 1.08, and more preferably 1 ≤ x1 ≤ 1.08.

[0050] The above a1 represents the atomic ratio of nickel in the metals other than lithium in the lithium composite transition metal oxide, where a1 may satisfy 0.80 ≤ a1 ≤ 1, 0.80 ≤ a1 ≤ 0.99, or 0.85 ≤ x1 ≤ 0.95. When the nickel content satisfies the above range, high-capacity characteristics can be achieved.

[0051] The above b1 represents the atomic ratio of cobalt in the metals other than lithium in the lithium composite transition metal oxide, where b1 may satisfy 0 < b1 < 0.2, 0 < b1 ≤ 0.15, or 0.01 ≤ b1 ≤ 0.10.

[0052] The above c1 represents the atomic ratio of M in the metals other than lithium in the lithium composite transition metal oxide 1 where c1 may satisfy 0 < c1 < 0.2, 0 < c1 ≤ 0.15, or 0.01 ≤ c1 ≤ 0.10.

[0053] The above d1 represents the atomic ratio of M in the metals other than lithium in the lithium composite transition metal oxide 2 where d1 may satisfy 0 ≤ d1 ≤ 0.1 or 0 ≤ d1 ≤ 0.05.

[0054] Meanwhile, the grain size of the first positive electrode active material is 150 nm or more, preferably 150 nm to 200 nm, and more preferably 160 nm to 190 nm. When using a high-Ni positive electrode active material with a grain size less than 150 nm, the initial resistance is high, and the charge and discharge capacity are insufficient, so it is difficult to achieve high capacity and high output characteristics.

[0055] Meanwhile, the average particle size D of the first positive electrode active material 50 may be 10 μm to 20 μm, preferably 10 μm to 18 μm, and more preferably 10 μm to 16 μm. When the average particle size of the first positive electrode active material satisfies the above range, excellent life characteristics can be achieved.

[0056] Meanwhile, in addition to the first positive electrode active material, the positive electrode of the present invention further includes a second positive electrode active material as the positive electrode active material. The second positive electrode active material may be a positive electrode active material having a different average particle size from the first positive electrode active material.

[0057] When at least two types of positive electrode active materials with different average particle sizes are used, the relatively smaller positive electrode active material fills the spaces between the relatively larger positive electrode active materials, thus achieving the effect of improving energy density and preventing the active materials from being damaged by the gap pressure during the rolling process.

[0058] Meanwhile, the average particle size D of the second positive electrode active material 50 The particle size can be from 1 μm to 8 μm, preferably from 1 μm to 7 μm, and more preferably from 2 μm to 7 μm. When the average particle size D of the second positive electrode active material... 50 When the above range is met, the electrode filling degree increases, thereby improving the energy density and output characteristics.

[0059] Meanwhile, the composition of the second positive electrode active material can be the same as or different from that of the first positive electrode active material. For example, in this invention, the nickel content of the first positive electrode active material can be higher than that of the second positive electrode active material. When the nickel content of the first positive electrode active material with a larger particle size is higher than that of the second positive electrode active material with a smaller particle size, the capacity characteristics can be excellent.

[0060] Specifically, the second positive electrode active material may comprise a lithium composite transition metal oxide represented by Formula 2:

[0061] [Equation 2]

[0062] Li x2 Ni a2 Co b2 M 3 c2 M 4 d2 O2

[0063] In Equation 2 above, the above M 3 It can be Mn, Al, or a combination thereof, preferably Mn or a combination of Mn and Al.

[0064] M 4 M can be selected from at least one of the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and in terms of improving the structural stability of lithium complex transition metal oxides, 4 More preferably, Zr is included.

[0065] The above x2 represents the molar ratio of lithium in the lithium composite transition metal oxide, wherein x2 can satisfy 0.9≤x2≤1.1, preferably 0.95≤x2≤1.08, and more preferably 1≤x2≤1.08.

[0066] The above a2 represents the atomic ratio of nickel in the metals other than lithium in the lithium composite transition metal oxide, where a2 can satisfy 0.60 ≤ a2 ≤ 0.90, 0.65 ≤ a2 ≤ 0.85, or 0.80 ≤ a2 ≤ 0.85. When the nickel content satisfies the above range, high-capacity characteristics can be achieved.

[0067] The above b2 represents the atomic ratio of cobalt in the metals other than lithium in the lithium composite transition metal oxide, where b2 can satisfy 0 < b2 < 0.40, 0 < b2 ≤ 0.35, or 0.01 ≤ b2 ≤ 0.20.

[0068] The above c2 represents the atomic ratio of M in the metals other than lithium in the lithium composite transition metal oxide 3 where c2 can satisfy 0 < c2 < 0.40, 0 < c2 ≤ 0.35, or 0.01 ≤ c2 ≤ 0.20.

[0069] The above d2 represents the atomic ratio of M in the metals other than lithium in the lithium composite transition metal oxide 4 where d2 can satisfy 0 ≤ d2 ≤ 0.1 or 0 ≤ d2 ≤ 0.05.

[0070] Meanwhile, the grain size of the second positive electrode active material is 90 nm to 160 nm, preferably 100 nm to 160 nm, more preferably 100 nm to 150 nm. When the positive electrode active material with a grain size satisfying the above range is used as the second positive electrode active material, the phenomenon of particle breakage of the positive electrode active material during the rolling process can be effectively suppressed. When the grain size of the second positive electrode active material is too large or too small, it is difficult to control particle breakage.

[0071] Meanwhile, the weight ratio of the first positive electrode active material and the second positive electrode active material included in the positive electrode of the present invention can be 50:50 to 95:5, preferably 60:40 to 90:10, more preferably 70:30 to 90:10. When the mixing ratio of the first positive electrode active material and the second positive electrode active material satisfies the above range, the packing density of the positive electrode active material can be increased, thereby having an excellent effect of improving the battery capacity.

[0072] Meanwhile, based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 70% by weight to 99% by weight, preferably 80% by weight to 99% by weight, more preferably 90% by weight to 99% by weight. When the content of the positive electrode active material satisfies the above range, a positive electrode with low electrode resistance and high energy density can be manufactured.

[0073] In this case, the content of the positive electrode active material is the total weight of the first positive electrode active material and the second positive electrode active material.

[0074] (2) Conductive agent

[0075] The positive electrode of this invention incorporates single-walled carbon nanotubes as a conductive agent. The single-walled carbon nanotubes are used to improve the conductivity of the positive electrode by providing a conductive pathway between the positive electrode active materials.

[0076] Since single-walled carbon nanotubes have a higher specific surface area than other conductive agents and can form three-dimensional conductive pathways, when single-walled carbon nanotubes are applied together with high-Ni cathode active materials, the loss of conductive pathways caused by the expansion and contraction of cathode active materials during charging and discharging can be minimized.

[0077] The BET specific surface area of ​​single-walled carbon nanotubes can reach 700 m². 2 / g to 1,500m 2 / g, preferably 800m 2 / g to 1,400m 2 / g, more preferably 900m 2 / g to 1,300m 2 / g. Furthermore, the average diameter of the single-walled carbon nanotubes can be from 0.5 nm to 3 nm, preferably from 0.7 nm to 2 nm. When the specific surface area and average diameter of the single-walled carbon nanotubes meet the above ranges, they exhibit excellent dispersibility, thus enabling the formation of uniform conductive pathways within the positive electrode active material layer.

[0078] Meanwhile, based on the total weight of the positive electrode active material layer, the content of single-walled carbon nanotubes can be from 0.01% to 2% by weight, preferably from 0.05% to 1.5% by weight, and more preferably from 0.1% to 1.2% by weight. When the content of single-walled carbon nanotubes in the positive electrode active material layer meets the above range, sufficient conductive pathways can be formed.

[0079] Furthermore, when necessary, the positive electrode of the present invention may also contain different types of conductive agents along with the single-walled carbon nanotubes as conductive agents. For example, the conductive agent may also include at least one selected from the group consisting of: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; metal powders, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; and conductive metal oxides, such as titanium oxide, and preferably carbon black. When conductive agents other than single-walled carbon nanotubes are also included as described above, the improvement in conductivity is even more excellent.

[0080] Meanwhile, when a conductive agent other than single-walled carbon nanotubes is also included, the content of the conductive agent other than single-walled carbon nanotubes is 0.1% to 10% by weight, preferably 0.1% to 5% by weight, and more preferably 0.5% to 5% by weight, based on the total weight of the positive electrode active material layer. When the content of the conductive agent other than single-walled carbon nanotubes meets the above range, the capacity reduction caused by the reduction in the active material content can be minimized, and the conductivity can be improved.

[0081] (3) Adhesive

[0082] The positive electrode of the present invention includes an adhesive. The adhesive is used to improve the adhesion between the positive electrode active materials and the adhesion between the positive electrode active materials and the current collector, and adhesives known in the art can be used.

[0083] For example, as an adhesive, polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof can be used, and any one of them alone or a mixture of two or more of them can be used.

[0084] Meanwhile, based on the total weight of the positive electrode active material layer, the binder content can be from 0.1% to 10% by weight, preferably from 0.15% to 7% by weight, and more preferably from 0.2% to 5% by weight. When the binder content meets the above range, the deintercalation of the positive electrode active material can be suppressed, while minimizing the increase in resistance.

[0085] The positive electrode of the present invention can be prepared by the following method: mixing positive electrode active material, conductive agent and binder in solvent to prepare positive electrode slurry composition, coating the positive electrode slurry composition on positive electrode current collector or separate support, and then drying and rolling the coated positive electrode current collector or coated support to form a positive electrode active material layer.

[0086] In this case, commonly used solvents in the art for preparing cathode slurry compositions can be used as solvents, such as N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropanol, acetone, or water, either alone or in combination of two or more. Considering the coating thickness and preparation yield of the slurry, the amount of solvent used may be sufficient if it can dissolve or disperse the cathode active material, conductive agent, and binder, and if its viscosity can exhibit excellent thickness uniformity during subsequent coating for cathode preparation.

[0087] There are no particular limitations on the positive electrode current collector, as long as it is conductive and does not cause adverse chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and microscopic irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0088] Lithium secondary batteries

[0089] Next, the lithium secondary battery of the present invention will be described.

[0090] The lithium secondary battery may include the above-described positive electrode of the present invention, and in particular includes the positive electrode, a negative electrode disposed facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0091] In addition, lithium secondary batteries may optionally include a battery container that houses electrode assemblies of a positive electrode, a negative electrode, and a separator, as well as a sealing member that seals the battery container.

[0092] Since the positive electrode is the same as described above, its detailed description will be omitted, and only the remaining structure will be described in detail below.

[0093] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a layer of negative electrode active material placed on top.

[0094] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and does not cause adverse chemical changes in the battery. For example, it can be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys. Furthermore, the thickness of the negative electrode current collector can typically range from 3 μm to 500 μm, and, similar to the positive electrode current collector, microscopic irregularities can be formed on its surface to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0095] In addition to the negative electrode active material, the negative electrode active material layer may optionally include a binder and a conductive agent.

[0096] As anode active materials, compounds capable of reversibly inserting and deintercalating lithium can be used. Specific examples of anode active materials can be: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metallic materials that can be alloyed with lithium, such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; and metal oxides that can be de-doped and doped with lithium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or composites containing metallic substances and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of them can be used. Furthermore, lithium metal films can be used as negative electrode active materials. In addition, both low-crystallinity carbon and high-crystallinity carbon can be used as carbon materials. Typical examples of low-crystallinity carbon can be soft carbon and hard carbon, and typical examples of high-crystallinity carbon can be irregular, plate-like, sheet-like, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch and high-temperature sintered carbon, such as coke derived from petroleum or coal tar pitch.

[0097] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be from 80% to 99% by weight.

[0098] Adhesives are components that facilitate adhesion between conductive materials, active materials, and current collectors. The amount of adhesive added is typically from 0.1% to 10% by weight, based on the total weight of the negative electrode active material layer. Examples of adhesives include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0099] A conductive agent is a component used to further improve the conductivity of the negative electrode active material. Based on the total weight of the negative electrode active material layer, the amount of conductive agent added can be less than 10% by weight, preferably less than 5% by weight. There are no particular limitations on the conductive agent, as long as it is conductive and does not cause adverse chemical changes in the battery. Examples of conductive agents that can be used include graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; or conductive materials such as polyphenylene derivatives; and so on.

[0100] For example, the negative electrode active material layer can be prepared by dissolving or dispersing the negative electrode active material, along with optional binders and conductive agents, in a solvent to prepare a negative electrode slurry composition, coating the composition onto a negative electrode current collector, and drying the coated negative electrode current collector; or it can be prepared by casting the negative electrode slurry composition onto a separate support, and then stacking the film layer separated from the support onto the negative electrode current collector.

[0101] In lithium-ion secondary batteries, the separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion secondary batteries. Specifically, separators with high electrolyte retention capacity and low resistance to the movement of electrolyte ions can be used. In particular, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin-based polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, or ethylene / methacrylate copolymers), or laminated structures having two or more layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. In addition, coated separators including ceramic or polymer components can be used to ensure heat resistance or mechanical strength, and can optionally be used in single-layer or multi-layer structures.

[0102] In addition, the electrolyte used in this invention may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes or molten inorganic electrolytes that can be used to manufacture lithium secondary batteries.

[0103] Specifically, electrolytes can include organic solvents and lithium salts.

[0104] Any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, organic solvents that can be used include: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic solvents, such as benzene or fluorobenzene; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); alcohol solvents, such as ethanol or isopropanol; nitriles, such as R-CN (where R is a straight-chain, branched, or cyclic C2-C20 hydrocarbon group, and may include double-bonded aromatic rings or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane.

[0105] Any compound can be used as a lithium salt without particular limitation, as long as it can provide lithium ions in a lithium secondary battery. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used. Lithium salts can be used in concentration ranges from 0.1M to 2.0M. When the concentration of the lithium salt is within this range, the electrolyte exhibits suitable conductivity and viscosity, thus demonstrating excellent performance, and lithium ions can move efficiently.

[0106] Lithium secondary batteries containing the positive electrode active material of the present invention as described above stably exhibit excellent discharge capacity, output characteristics and lifespan characteristics, and therefore can be used in fields such as portable devices (e.g., mobile phones, laptops or digital cameras) or electric vehicles (e.g., hybrid electric vehicles (HEVs)).

[0107] Therefore, according to another embodiment of the present invention, a battery module comprising the above-mentioned lithium secondary battery as a unit cell and a battery pack comprising the battery module are provided.

[0108] The battery module or battery pack can be used as a power source for at least one medium to large-sized device: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0109] The shape of the lithium secondary battery of the present invention is not particularly limited, and it can be cylindrical, prismatic, pouch-shaped or coin-shaped.

[0110] The lithium secondary battery of the present invention can be used not only as a battery cell for use as a power source for small devices, but also as a unit cell in medium and large battery modules that include multiple battery cells.

[0111] Example

[0112] The invention will now be described in more detail with reference to specific embodiments. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments described herein. Rather, these exemplary embodiments are provided to make this specification thorough and complete, and to fully convey the scope of the invention to those skilled in the art.

[0113] Example 1

[0114] Li[Ni] with a grain size of 180 nm and an average grain size D50 of 14 μm was used. 0.9 Co0.05 Mn 0.05 O2, Single-walled carbon nanotubes (OSCiAl, TUBALL, BET) specific surface area = 1160 m² 2 The positive electrode slurry composition was prepared by mixing N-methylpyrrolidone with carbon black and PVDF binder in a weight ratio of 96:0.5:1:2.5. The positive electrode slurry composition was coated on one surface of an aluminum current collector, dried at 130°C, and then rolled to prepare the positive electrode.

[0115] In addition, artificial graphite, carbon black (SUPER C-65), and acrylic binder (BM-L302, available from Zeon Corporation) were mixed in a weight ratio of 95:1:4 and added to water as a solvent to prepare a negative electrode slurry composition. The negative electrode slurry composition was coated onto a copper current collector, dried, and then rolled to prepare the negative electrode.

[0116] Lithium-ion batteries are prepared by the following process: an electrode assembly is prepared by placing a polyethylene separator between the positive and negative electrodes prepared above, the electrode assembly is placed in a battery case, and then an electrolyte solution is injected into the case.

[0117] In this case, the electrolyte solution described above is used in which LiPF6 is dissolved in a mixed organic solvent at 1M, wherein the mixed organic solvent is an organic solvent in which ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) are mixed in a volume ratio of 3:4:3.

[0118] Example 2

[0119] The cathode and lithium secondary battery were prepared in the same manner as in Example 1, except that Li[Ni] was used with a grain size of 156 nm and an average grain size D50 of 14 μm. 0.9 Co 0.05 Mn 0.05 O2 is used as the positive electrode active material.

[0120] Example 3

[0121] The cathode and lithium secondary battery were prepared in the same manner as in Example 1, except that Li[Ni] was used with a weight ratio of 75:25, a grain size of 180 nm, and an average grain size D50 of 14 μm. 0.9 Co 0.05 Mn 0.05 O2 and Li[Ni] with a grain size of 139 nm and an average grain size D50 of 4 μm 0.8 Co 0.1 Mn 0.1 A mixture of O2 is used as the positive electrode active material.

[0122] Example 4

[0123] The cathode and lithium secondary battery were prepared in the same manner as in Example 1, except that Li[Ni] was used with a grain size of 159 nm and an average grain size D50 of 14 μm. 0.8 Co 0.1 Mn 0.1 O2 is used as the positive electrode active material.

[0124] Comparative Example 1

[0125] The cathode and lithium secondary battery were prepared in the same manner as in Example 1, except that single-walled carbon nanotubes were not used.

[0126] Comparative Example 2

[0127] The cathode and lithium secondary battery were prepared in the same manner as in Example 1, except that Li[Ni] was used with a grain size of 124 nm and an average grain size D50 of 14 μm. 0.9 Co 0.05 Mn 0.05 O2 is used as the positive electrode active material.

[0128] Comparative Example 3

[0129] The cathode and lithium secondary battery were prepared in the same manner as in Comparative Example 2, except that single-walled carbon nanotubes were not used.

[0130] Comparative Example 4

[0131] The cathode and lithium secondary battery were prepared in the same manner as in Example 4, except that single-walled carbon nanotubes were not used.

[0132] Experimental Example 1 - Evaluation of Resistance Characteristics

[0133] While charging the lithium secondary batteries prepared according to Comparative Examples 1 to 4 and Examples 1 to 4, the relationship between resistance and state of charge (SOC) was measured by hybrid pulse power characterization (HPPC) testing. Specifically, the lithium secondary batteries prepared according to Comparative Examples 1 to 4 and Examples 1 to 4 were charged at a C rate of 0.3C in CCCV mode to 4.2V at room temperature (25°C), and then discharged to 2.5V. Then, the lithium secondary batteries were charged at a C rate of 0.3C in CCCV mode to 4.2V and then discharged. When discharging at a C rate of 2C, the 60-second voltage drop resistance of each SOC was measured.

[0134] The measurement results are presented in Figures 1 to 3 middle. Figure 1 This is a graph showing the relationship between the resistance characteristics and the state of charge (SOC) of the lithium secondary batteries prepared according to Comparative Example 1 and Examples 1 to 3. Figure 2The graph shows the relationship between the resistance characteristics and the state of charge (SOC) of the lithium secondary batteries prepared according to Comparative Examples 2 and 3. Figure 3 This is a graph showing the relationship between the resistance characteristics and SOC of the lithium secondary batteries prepared according to Example 4 and Comparative Example 4.

[0135] See Figure 1 It can be confirmed that the lithium secondary batteries of Examples 1 to 3 exhibit lower resistance than the lithium secondary battery of Comparative Example 1 across the entire SOC range.

[0136] Additionally, see Figure 2 It can be confirmed that the initial resistance of Comparative Example 3, which uses a positive electrode active material with a small grain size of less than 150 nm and does not use single-walled carbon nanotubes, is significantly increased compared to Comparative Example 1. At the same time, it can be confirmed that the initial resistance of Comparative Example 2, which uses a single-walled carbon nanotube with a small grain size of less than 150 nm, is reduced compared to Comparative Example 3, but has a slight effect on reducing the initial resistance compared to Comparative Example 1, which uses a positive electrode active material with a grain size of 180 nm. Furthermore, as the SOC increases due to charging, Comparative Example 2 exhibits resistance characteristics comparable to Comparative Example 3 by increasing the resistance.

[0137] Also see Figure 3 It can be confirmed that the lithium secondary battery of Example 4 exhibits lower resistance than the lithium secondary battery of Comparative Example 4 (whose positive electrode active material has the same composition and grain size as that of Example 4) across the entire SOC range.

[0138] Experimental Example 2 - Evaluation of Lifetime Characteristics

[0139] While performing 100 charge-discharge cycles at 45°C, 0.33°C, and 2.5V to 4.2V, the capacity retention rate and resistance increase rate of the lithium secondary batteries manufactured according to Examples 1 to 4 and Comparative Examples 1 and 4 were measured.

[0140] For capacity retention, the capacity fraction of each cycle is measured based on the second discharge capacity. For resistance increase rate, the resistance as a voltage drop is measured during the first 60 seconds of discharge, and the increase rate is measured based on the resistance after the second discharge.

[0141] The measurement results are presented in Figure 4 and 5 middle. Figure 4 This is a graph showing the lifetime characteristics of the lithium secondary batteries prepared according to Comparative Example 1 and Examples 1 to 3. Figure 5 This is a graph showing the lifespan characteristics of the lithium secondary batteries prepared according to Example 4 and Comparative Example 4.

[0142] See Figure 4It can be confirmed that the lithium secondary batteries of Examples 1 to 3 exhibit significantly superior lifespan characteristics compared to the lithium secondary battery of Comparative Example 1. Additionally, see [link to related documentation]. Figure 5 It can be confirmed that the lithium secondary battery of Example 4 exhibits significantly superior life characteristics, especially resistance characteristics, compared to the lithium secondary battery of Comparative Example 4.

Claims

1. A positive electrode for a lithium secondary battery, comprising a positive electrode active material layer, the positive electrode active material layer comprising: A first positive electrode active material represented by Formula 1 and having a grain size of 150 nm or more; A conductive agent containing single-walled carbon nanotubes; and A binder: [Formula 1] Li x1 [Ni a1 Co b1 M 1 c1 M 2 d1 ]O2 in, In Formula 1 above, M 1 It is Mn, Al, or a combination thereof. M 2 It is selected from at least one of the following groups: Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S. 0.9 ≤ x1 ≤ 1.1, 0.8 ≤ a1 < 1, 0 < b1 < 0.2, 0 < c1 < 0.2, 0 ≤ d1 ≤ 0.

1.

2. The positive electrode for a lithium secondary battery as described in claim 1, wherein, The grain size of the first positive electrode active material is 150 nm to 200 nm.

3. The positive electrode for a lithium secondary battery as described in claim 1, wherein, The average particle size D of the first positive electrode active material 50 The size ranges from 10 μm to 20 μm.

4. The positive electrode for a lithium secondary battery as described in claim 1, wherein, The positive electrode active material layer further includes a second positive electrode active material, wherein the average particle size D of the second positive electrode active material is... 50 The range is from 1μm to 8μm.

5. The positive electrode for a lithium secondary battery as described in claim 4, wherein, The second positive electrode active material is represented by Formula 2 and has a grain size of 90 nm to 160 nm: [Formula 2] Li x2 [Ni a2 Co b2 M 3 c2 M 4 d2 ]O2 Wherein, in Formula 2 above, M 3 It is Mn, Al, or a combination thereof. M 4 It is selected from at least one of the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and 0.9 ≤ x2 ≤ 1.1, 0.6 ≤ a2 ≤ 0.90, 0 < b2 < 0.40, 0 < c2 < 0.40 and 0 ≤ d2 ≤ 0.

1.

6. The positive electrode for a lithium secondary battery as described in claim 4, wherein, The weight ratio of the first positive electrode active material and the second positive electrode active material comprised is 50:50 to 95:

5.

7. The positive electrode for a lithium secondary battery as described in claim 1, wherein, The specific surface area of ​​the single-walled carbon nanotubes is 700 m². 2 / g to 1,500m 2 / g.

8. The positive electrode for a lithium secondary battery as described in claim 1, wherein, The conductive agent further comprises at least one selected from the group consisting of graphite, carbon black, conductive fibers, metal powders, conductive whiskers, and conductive metal oxides.

9. The positive electrode for a lithium secondary battery as described in claim 1, wherein, Based on the total weight of the positive electrode active material layer, the content of the single-walled carbon nanotubes is 0.01 wt% to 2 wt%.

10. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to any one of claims 1 to 9.

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