Cathode material, cathode containing same, and lithium secondary battery

Abstract

The present invention relates to a positive electrode material for lithium secondary batteries comprising a large particle size positive electrode active material and a small particle size positive electrode active material, the large particle size positive electrode active material and the small particle size positive electrode active material each independently containing a lithium nickel-based oxide, in which, when a cation mixing ratio of the large particle size positive electrode active material is Ml and a cation mixing ratio of the small particle size positive electrode active material is Ms, Ml / Ms is 0.6 to 4.0, and when an average particle size of primary particles of the large particle size positive electrode active material is Pl and an average particle size of primary particles of the small particle size positive electrode active material is Ps, Pl / Ps is 0.1 to 2.0; and a lithium secondary battery including the positive electrode material.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0039234 filed on March 29, 2022 and Korean Patent Application No. 10-2023-0040836 filed on March 28, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a cathode material, a cathode and a lithium secondary battery including the same, and more particularly to a cathode material having improved durability and high-temperature life by adjusting the ratio of cation mixing rates and the ratio of primary particle sizes between two cathode active materials having different particle sizes, and a cathode and a lithium secondary battery including the same. [Background technology]

[0003] A lithium secondary battery generally comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and the negative electrode contain active materials capable of intercalating and deintercalating lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2, LiMnO4, etc.), and lithium iron phosphate (LiFePO4) have been used as positive electrode active materials in lithium secondary batteries. Among these, lithium cobalt oxide boasts high operating voltage and excellent capacity characteristics. However, the high cost and unstable supply of cobalt, its raw material, make it difficult to commercialize in large-capacity batteries. Meanwhile, lithium nickel oxide suffers from poor structural stability, making it difficult to achieve sufficient lifespan. Lithium manganese oxide offers excellent stability but poor capacity characteristics. To address these issues, lithium composite transition metal oxides containing two or more transition metals have been developed, with lithium nickel cobalt manganese oxide being the most widely used.

[0005] Generally, lithium nickel cobalt manganese oxide is formed into spherical secondary particles formed by the aggregation of tens to hundreds of primary particles. This means that during the rolling process used to manufacture the cathode, the primary particles are prone to falling off, resulting in particle cracking. Furthermore, repeated charge / discharge cycles can cause cracks along the interfaces between the primary particles as the unit lattice experiences volume changes due to the insertion and extraction of lithium ions. When particle cracks or fractures occur in the cathode active material, the area of ​​contact with the electrolyte increases, which can lead to increased gas generation and degradation of the active material due to side reactions with the electrolyte, resulting in reduced lifespan.

[0006] Furthermore, in order to realize a high energy density, recent attempts have been actively made to increase the nickel content in lithium nickel cobalt manganese oxide. However, in this case, the high reactivity and anisotropic volume change of nickel increase the possibility of intraparticle cracks occurring during charge and discharge, which may cause structural collapse and degrade battery performance.

[0007] Therefore, there is a need to develop a high-nickel cathode material that has excellent structural stability and high energy density. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 10-2021-0031325 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to solve the above problems by providing a bimodal cathode material for lithium secondary batteries that has reduced cracking and improved surface durability by optimizing the ratio of cation mixing rates between large particles and small particles and the ratio of primary particle sizes.

[0010] The present invention also aims to provide a positive electrode and a lithium secondary battery with improved high-temperature life by applying the above-mentioned positive electrode material. [Means for solving the problem]

[0011] According to one embodiment, the present invention includes a large particle size positive electrode active material and a small particle size positive electrode active material, where Ml is a cation mixing ratio of the large particle size positive electrode active material and Ms is a cation mixing ratio of the small particle size positive electrode active material, Ml / Ms is 0.6 to 4.0, The present invention provides a positive electrode material for lithium secondary batteries in which Pl / Ps is 0.1 to 2.0, where Pl is the average particle size of the primary particles of the large particle size positive electrode active material and Ps is the average particle size of the primary particles of the small particle size positive electrode active material.

[0012] Also provided are a positive electrode containing the positive electrode material for lithium secondary batteries and a lithium secondary battery containing the positive electrode. [Effects of the Invention]

[0013] The cathode material for lithium secondary batteries according to the present invention exhibits excellent durability by controlling the ratio of the cation mixing rate between the large particle size cathode active material and the small particle size cathode active material and the ratio of the average particle size of the primary particles within an optimal range. Therefore, lithium secondary batteries using the cathode material of the present invention have excellent life characteristics at high temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in more detail below.

[0015] In the present invention, a "single particle" refers to a particle consisting of one single primary particle. In the present invention, a "primary particle" refers to a particle unit that may be a single crystal lacking a crystalline grain boundary, or a polycrystalline particle that appears to have no grain boundaries when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope (SEM). In the present invention, a "quasi-single particle" refers to a particle that is a complex formed of approximately 30 or less primary particles.

[0016] In the present invention, the term "primary particle" refers to a particle unit that does not appear to have grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000 times, and the term "secondary particle" refers to a particle formed by agglomeration of several tens to several hundreds of primary particles. Specifically, the secondary particle may be an agglomeration of more than about 30 primary particles.

[0017] In the present invention, the "average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of at least 20 primary particles observed in a scanning electron microscope image. Here, the particle size refers to the diameter of the longest axis of the primary particles.

[0018] In the present invention, the "D50 of a positive electrode active material" refers to the particle size corresponding to 50% cumulative volume in the volume cumulative particle size distribution of a positive electrode active material powder, and may be measured using a laser diffraction method. For example, the positive electrode active material powder may be dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac's S-3500), and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W to obtain a graph of the volume cumulative particle size distribution. The particle size at the point where the volume cumulative amount is 50% on the obtained volume cumulative particle size distribution graph may be measured.

[0019] The "specific surface area" in the present invention is measured by the BET method, and specifically, may be calculated from a nitrogen adsorption isotherm obtained under a liquid nitrogen atmosphere at 77 K using a BELSORP-MAX (MicrotracBEL corp.).

[0020] In the present invention, the "cation mixing ratio" refers to the ratio of nickel ions (Ni ions) to the total amount of lithium sites (Li sites) in the lithium layer of the layered lithium nickel oxide. 2+ ) is the percentage of the mixed material, and is measured by X-ray diffraction analysis (XRD).

[0021] Specifically, the X-ray diffraction analysis was performed as follows. Using a Bruker D8 Endeavor (Cu Kα, λ = 1.54 Å) equipped with a LynxEye XE-T position sensitive detector, the diffraction pattern of the sample was measured at an FDS of 0.5°, from 15° to 90° 2θ (theta), with a step size of 0.02° and a total scan time of approximately 20 minutes. Rietveld refinement was performed on the measured data, taking into account the charge at each site (+3 for metals at transition metal sites, +2 for Ni at Li sites) and cation mixing. Instrumental line broadening during analysis was considered using the Fundamental Parameter Approach (FPA) provided by the Bruker TOPAS program, and all peaks in the measurement range were used during fitting. Peak shapes are fitted using only the Lorentzian contribution as a first principle (FP) type available in TOPAS, without considering strain. The Ni excess value determined after Rietveld refinement is defined as the cation mixing ratio.

[0022] Cathode material The cathode material according to the present invention includes a large particle size cathode active material having a relatively large particle size and a small particle size cathode active material having a relatively small particle size, specifically, a bimodal particle size distribution. In this case, the small particle size cathode active material fills the voids of the large particle size cathode active material during electrode rolling, thereby increasing the electrode density and enabling the realization of a high energy density.

[0023] However, when a large particle size positive electrode active material and a small particle size positive electrode active material are mixed to form an electrode, the small particle size active material located between the large particle size particles acts to reduce direct contact between the large particle size particles, which can result in the small particle size positive electrode active material being subjected to relatively greater physical stress, leading to deeper particle cracking. Furthermore, a high-nickel content positive electrode active material exhibits greater anisotropic contraction and expansion during charge and discharge than a low-nickel content positive electrode active material, which increases the likelihood of intra-particle cracking. Surface cracks can induce electrolyte penetration, resulting in structural degradation of the positive electrode material and shortening the battery life.

[0024] To solve this problem, the inventors adjusted the ratio of the cation mixing ratio (Ml) of the large particle size positive active material (Ml) to the cation mixing ratio (Ms) of the small particle size positive active material (Ms) from 0.6 to 4.0, thereby suppressing the acceleration of overall cathode degradation by lowering the cation mixing ratio of the small particle size positive active material, which is prone to severe particle cracking. Specifically, cation mixing in the active material is likely to occur primarily on the surface, which has relatively high interfacial energy and is therefore unstable, rather than within the primary particles. Therefore, during charge and discharge, chain ion exchange reactions occur around the sites where cation mixing occurs, tending to propagate structural collapse, which affects the generation and propagation of microcracks.

[0025] If the Ml / Ms ratio is less than 0.6, there is a problem of accelerated deterioration due to the surface condition of small particles as described above, and if it exceeds 4.0, it is undesirable because deterioration is accelerated when microcracks occur in large particles. Specifically, the Ml / Ms ratio may be 0.8 to 3.5, and more specifically, 1.0 to 3.0.

[0026] Meanwhile, the cation mixing ratio (Ml) of the large particle size positive electrode active material may be 1.0 at% to 2.0 at%, more preferably 1.2 at% to 1.5 at%. When the cation mixing ratio (Ml) of the large particle size positive electrode active material satisfies this range, it is preferable in that it can minimize the formation of microcracks in the large particle size positive electrode active material during battery operation, improve durability, and thereby extend the lifespan. Here, at% means atomic %.

[0027] The small particle size positive electrode active material may have a cation mixing ratio (Ms) of 0.5 at% to 1.5 at%, more preferably 0.8 at% to 1.3 at%. When the cation mixing ratio (Ms) of the small particle size positive electrode active material satisfies the above range, it is preferable in that deterioration due to particle cracking of the small particle size positive electrode active material can be suppressed.

[0028] The inventors also adjusted the Pl / Ps ratio, which is the ratio of the average particle size (Pl) of the primary particles of the large-particle size positive active material to the average particle size (Ps) of the primary particles of the small-particle size positive active material, to 0.1 to 2.0. A Pl / Ps ratio less than 0.1 or greater than 2.0 indicates poor particle strength of the large or small particles. This increases the amount of fine powder generated as the small-particle cracking increases during the electrode rolling process, thereby increasing the area of ​​side reactions, which accelerates battery degradation and reduces high-temperature life and output characteristics. Specifically, the Pl / Ps ratio may be 0.1 to 1.5, and more specifically, 0.3 to 1.0.

[0029] In other words, adjusting Ml / Ms and Pl / Ps within the above ranges suppresses particle cracking of large particle size positive electrode active materials and suppresses surface deterioration of small particle size positive electrode active materials, thereby improving the durability of the positive electrode material and ultimately improving the lifespan of the battery.

[0030] In one embodiment of the present invention, the D50 of the large particle size positive electrode active material may be 8 μm to 18 μm, preferably 10 μm to 18 μm, and more preferably 10 μm to 15 μm. The D50 of the small particle size positive electrode active material may be 2 μm to 7 μm, preferably 3 μm to 6 μm, and more preferably 4 μm to 6 μm. When the D50 of the large particle size positive electrode active material and the D50 of the small particle size positive electrode active material satisfy the above ranges, it is preferable because electrode rolling characteristics can be maximized.

[0031] Meanwhile, the large particle size positive electrode active material and the small particle size positive electrode active material may each independently be composed of a single particle, a similar-single particle, a secondary particle formed by agglomeration of a number of primary particles, or a combination thereof. Specifically, the large particle size positive electrode active material may be composed of secondary particles, and the small particle size positive electrode active material may be composed of a single particle and / or a similar-single particle. In this case, cracking and deterioration of the small particles may be suppressed, and the rolling characteristics of the electrode may be improved.

[0032] The large particle size positive electrode active material may have an average primary particle size (Pl) of 0.2 μm to 1.0 μm, more preferably 0.2 μm to 0.8 μm. When the average primary particle size (Pl) of the large particle size positive electrode active material satisfies this range, particle cracking is minimized, which is preferable.

[0033] The average particle size (Ps) of the primary particles of the small-particle-size positive electrode active material may be 0.5 μm to 2.5 μm, preferably 0.5 μm to 2.0 μm, and more preferably 0.8 μm to 2.0 μm. When the average particle size (Ps) of the primary particles of the small-particle-size positive electrode active material satisfies the above range, it is preferable in terms of suppressing cracking of the small particles and suppressing side reactions.

[0034] In one embodiment of the present invention, where Bl is the specific surface area of ​​the large particle size positive active material and Bs is the specific surface area of ​​the small particle size positive active material, Bl / Bs may be 0.15 to 1.6, preferably 0.3 to 1.6, and more preferably 0.5 to 1.5. A larger specific surface area of ​​the positive active material increases the surface area that can participate in charge / discharge reactions, thereby improving output characteristics. However, this also means that the surface area that is susceptible to deterioration increases, so it is important to adjust the specific surface area appropriately. Considering that small particles have a larger surface area per weight than large particles, it is preferable to adjust Bl / Bs within the above range for the positive electrode material according to the present invention.

[0035] Specifically, the specific surface area of ​​the large particle size positive electrode active material is 0.2 m 2 / g to 1.0m 2 / g, preferably 0.4m 2 / g to 0.8m 2 / g, more preferably 0.6m 2 / g to 0.8m 2 / g, and the specific surface area of ​​the small particle size positive electrode active material may be 0.5 m 2 / g to 1.2m 2 / g, preferably 0.5m 2 / g to 1.0m 2 / g, more preferably 0.6m 2 / g to 1.0m 2 / g.

[0036] Specifically, the large particle size positive electrode active material and the small particle size positive electrode active material may each include a lithium nickel-based oxide containing nickel, cobalt, and manganese.

[0037] In one embodiment of the present invention, the large particle size positive electrode active material and the small particle size positive electrode active material each may contain a lithium nickel-based oxide in which the molar ratio of nickel among the transition metals is 70 mol % or more, more preferably 80 mol % or more.

[0038] On the one hand, the large-particle-size positive electrode active material and the small-particle-size positive electrode active material can each contain a lithium nickel-based oxide represented by the following Chemical Formula 1, and the lithium nickel-based oxide contained in the large-particle-size positive electrode active material and the lithium nickel-based oxide contained in the small-particle-size positive electrode active material may be the same as or different from each other.

[0039] [Chemical Formula 1] Li 1+x (Ni a Co b Mn c M d )O 2-y In the Chemical Formula 1, M is one or more selected from the group consisting of Al, Mg, V, Ti, Zr, Nb, and W, x, a, b, c, and d are respectively -0.10 ≤ x ≤ 0.20, 0.50 ≤ a < 1.0, 0 < b ≤ 0.40, 0 < c ≤ 0.30, 0 ≤ d ≤ 0.10, a + b + c + d = 1, and 0 ≤ y ≤ 0.05.

[0040] The 1 + x indicates the molar ratio of lithium in the lithium nickel-based oxide, and may be -0.1 ≤ x ≤ 0.2 or 0 ≤ x ≤ 0.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium nickel-based oxide can be stably formed.

[0041] The a indicates the molar ratio of nickel among all the metals excluding lithium in the lithium nickel-based oxide, and may be 0.60 ≤ a < 1, 0.70 ≤ a < 1, or 0.85 ≤ a < 1. When the molar ratio of nickel satisfies the above range, a high energy density is shown, so that a high capacity can be realized.

[0042] The b indicates the molar ratio of cobalt among all the metals excluding lithium in the lithium nickel-based oxide, and may be 0 < b ≤ 0.30, 0 < b ≤ 0.20, or 0 < b ≤ 0.10. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0043] <00002The above-mentioned c represents the molar ratio of manganese among the total metals excluding lithium in the lithium nickel-based oxide, and it may be 0 < c ≤ 0.25, 0 < c ≤ 0.20, or 0 < c ≤ 0.10. When the molar ratio of manganese satisfies the above range, the structural stability of the positive electrode active material is excellently exhibited.

[0044] In one embodiment of the present invention, the large particle size positive electrode active material can contain one or more doping elements selected from the group consisting of Al, Mg, V, Ti, Zr, Nb, and W. In this case, there is an effect of suppressing structural deterioration and improving high-temperature durability. Preferably, the large particle size positive electrode active material can contain Al as a doping element. That is, the above-mentioned d representing the molar ratio of the doping element among the total metals excluding lithium in the lithium nickel-based oxide may be 0 < d ≤ 0.08, 0 < d ≤ 0.06, or 0 < d ≤ 0.05.

[0045] In one embodiment of the present invention, the molar ratio of nickel among the transition metals in the lithium nickel-based oxide may be 70 mol% or more, and more preferably 80 mol% or more.

[0046] On the other hand, the lithium nickel-based oxide contained in the large particle size positive electrode active material may have a higher molar ratio of nickel among the transition metals than the lithium nickel-based oxide contained in the small particle size positive electrode active material. In this case, there is an advantage that the Ni composition of small particles with a large reaction area and relatively severe particle cracking can be set relatively low, and deterioration of the positive electrode scale can be suppressed.

[0047] Meanwhile, the large particle size positive electrode active material and the small particle size positive electrode active material according to the present invention may further include a coating layer on the surface of the lithium nickel-based oxide particles, if necessary. In this case, the coating layer may include 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-based oxide, the coating layer reduces contact between the electrolyte and the lithium nickel-based oxide, thereby reducing the elution of transition metals and the generation of gas 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 the coating layer includes Co, it can suppress side reactions with the electrolyte, as well as improve output and reduce resistance.

[0048] Meanwhile, the cathode material may contain a large particle size cathode active material and a small particle size cathode active material in a weight ratio of 50:50 to 90:10, preferably 50:50 to 80:20. When the weight ratio of the large particle size cathode active material and the small particle size cathode active material is within this range, the small particles may be arranged between the large particles while minimizing space loss. This optimizes the packing density and is advantageous for distributing stress applied between the active materials during rolling, which is advantageous for improving the rolling characteristics of the electrode and realizing high energy.

[0049] positive electrode Next, the positive electrode according to the present invention will be described.

[0050] The positive electrode according to the present invention includes the positive electrode material according to the present invention. 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.

[0051] Since the cathode material has been described above, a detailed description thereof will be omitted, and only the remaining components will be described in detail below.

[0052] The positive electrode current collector may include a highly conductive metal, and is not particularly limited as long as it is easily adhered to the positive electrode active material layer and is non-reactive within the voltage range of the battery. The positive electrode current collector may be made of, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The positive electrode current collector may typically have a thickness of 3 to 500 μm, and the surface of the current collector may be micro-irregularized to enhance adhesion of the positive electrode active material. The positive electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0053] The positive electrode active material layer may optionally contain a conductive material and a binder in addition to the positive electrode material, as required.

[0054] The positive electrode material may be included in an amount of 80 to 99 wt %, more specifically, 85 to 98.5 wt %, based on the total weight of the positive electrode active material layer, and excellent capacity characteristics can be exhibited when included in this range.

[0055] The conductive material is used to impart conductivity to the electrode. Any material that exhibits electronic conductivity without causing chemical changes in the resulting battery can be used without any particular limitations. Specific examples include 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; carbon-based materials, such as carbon fiber; metal powders or 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; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be present in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0056] The binder serves to improve adhesion between particles of the positive electrode material and between the positive electrode material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, polymers in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof. These may be used alone or in combination. The binder may be contained in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0057] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode material. Specifically, the positive electrode may be manufactured by coating a positive electrode mixture, which is prepared by dissolving or dispersing the positive electrode material and, optionally, a binder, a conductive material, and a dispersant in a solvent, on a positive electrode current collector, followed by drying and rolling, or by casting the positive electrode mixture on a separate support, peeling it from the support, and laminating the resulting film on the positive electrode current collector.

[0058] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethyl formamide (DMF), acetone, or water, and may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the cathode active material, conductive material, binder, and dispersant, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to provide a viscosity that allows excellent thickness uniformity when the slurry is subsequently applied to manufacture a cathode.

[0059] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0060] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Since the positive electrode is the same as described above, detailed description thereof will be omitted, and only the remaining components will be described in detail below.

[0061] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0062] 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.

[0063] The negative electrode current collector may be any material that has high conductivity without inducing chemical changes in the battery, and may be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0064] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0065] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples thereof 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, and Al alloys; and SiO. β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites. A mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. The negative electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0066] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 0.1 to 10 wt % based on the total weight of the negative electrode active material layer. Examples of such binders 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, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0067] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powder 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.

[0068] The negative electrode active material layer may be prepared by coating a negative electrode mixture prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating. Alternatively, the negative electrode mixture may be cast on a separate support, peeled from the support, and then laminated onto the negative electrode current collector as a film.

[0069] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular limitations. In particular, a separator having low resistance to ion movement in the electrolyte and excellent humidifying ability for the electrolyte is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be used in a single-layer or multi-layer structure.

[0070] Furthermore, examples of the electrolyte used in the present invention include 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 production of lithium secondary batteries, but are not limited to these.

[0071] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0072] The organic solvent may be any solvent capable of acting as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Specific examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (R is a hydrocarbon group having a linear, branched, or cyclic structure and having 2 to 20 carbon atoms, which may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / 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.

[0073] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without any particular limitation. Specifically, the anion of the lithium salt may be 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 may be at least one selected from the group consisting of LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(CF, SO), LiN(CF, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The concentration of the lithium salt is preferably in the range of 0.1 to 4.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0074] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, water, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the battery's lifespan, suppressing battery capacity loss, and improving the battery's discharge capacity. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0075] As described above, the lithium secondary battery including the cathode material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, and is therefore useful in the fields of portable devices such as mobile phones, laptops, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0076] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0077] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0078] The shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0079] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a large number of battery cells.

[0080] Examples of the medium- to large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0081] The present invention will be described in more detail below through specific examples.

[0082] <Manufacturing Example: Manufacturing of Positive Electrode Active Material> (1) Production Example 1 - Production of large particle size active material (production of positive electrode active materials A to E) Ni 0.90 Co 0.05 Mn 0.05 (OH)2, LiOH, and Al(OH)3 were added to a 700L Henschel mixer in a molar ratio of (Ni + Co + Mn):Li:Al of 1:1.05:0.03 and mixed for 20 minutes at 400 rpm at the center. The mixed powder was placed in a 330mm x 330mm alumina crucible and heat-treated (calcined) for 15 hours at 750°C under an oxygen (O2) atmosphere. The heat-treated active material was mixed with deionized water (Di-water) in a 50:50 weight ratio, stirred for 10 minutes, washed with water, and then dried in a drying oven at 120°C for 12 hours. The dried active material was then mixed with 0.1 wt% H3BO3 based on the total weight of the active material and heat-treated at 300°C for 10 hours to produce large-particle size cathode active material A.

[0083] Positive electrode active materials B to E were prepared through the same process as the positive electrode active material A, except that the molar ratio of Li and Al and the firing temperature were changed as shown in Table 1 below.

[0084] [Table 1]

[0085] (2) Production Example 2 - Production of small particle size active material (production of positive electrode active materials F to J) Ni 0.88 Co 0.06 Mn 0.06 (OH)2 and LiOH were added to a 700L Henschel mixer in a molar ratio of (Ni+Co+Mn):Li:Al of 1:1.05:0.03 and mixed for 20 minutes at 400 rpm in the center. The mixed powder was placed in a 330mm x 330mm alumina crucible and heat-treated (calcined) for 15 hours at 790°C under an oxygen (O2) atmosphere. The heat-treated active material was mixed with deionized water (Di-water) in a 50:50 weight ratio, stirred for 10 minutes, washed with water, and then dried in a drying oven at 120°C for 12 hours. The dried active material was then mixed with 0.1 wt% H3BO3 based on the total weight of the active material and heat-treated at 300°C for 10 hours to produce small particle size cathode active material F.

[0086] Positive electrode active materials G to K were prepared through the same process as the positive electrode active material F, except that the molar ratio of Li and Al and the firing temperature were changed as shown in Table 2 below.

[0087] [Table 2]

[0088] (3) Measurement of the physical properties of the positive electrode active material The cation mixing ratio, average particle size of primary particles, D50, and specific surface area of ​​each of the positive electrode active materials prepared in Preparation Examples 1 and 2 were measured by the following methods, and the results are shown in Table 3 below.

[0089] Cation Mixture Ratio Diffraction patterns of the samples were measured using a Bruker D8 Endeavor (Cu Kα, λ = 1.54 Å) equipped with a LynxEye XE-T position-sensitive detector. The FDS was set to 0.5°, and the 2θ (theta) range was set to 15° to 90° with a step size of 0.02°, for a total scan time of approximately 20 min. Rietveld refinement of the measured data was performed, taking into account the charge at each site (+3 for metals at transition metal sites, +2 for Ni at Li sites) and cation mixing. Instrumental line broadening during analysis was accounted for using the Fundamental Parameter Approach (FPA) provided by the Bruker TOPAS program. All peaks in the measurement range were used for fitting. Peak shapes were fitted using the Lorentzian contribution only in the FP (First Principle) method available in TOPAS, without considering strain. The Ni excess value determined after Rietveld refinement was defined as the cation mixing ratio.

[0090] Average diameter of primary particles A cross-sectional image of each positive electrode active material was observed using a scanning electron microscope, and the longest axis diameters of 20 primary particles in the cross section of the positive electrode active material were measured, followed by calculating the arithmetic average value.

[0091] D50 0.05 g of powder of each positive electrode active material was placed in a particle size distribution confirmation device (Microtrac, MTS-3500) and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W to obtain a graph of volume cumulative particle size distribution, and the particle size at the point where the volume cumulative amount reached 50% was measured on the obtained graph of volume cumulative particle size distribution.

[0092] specific surface area 3 g of each positive electrode active material powder was placed in a BELSORP-MAX (MicrotracBEL Corp.) and nitrogen adsorption isotherms were obtained under a liquid nitrogen atmosphere at 77 K, and the BET specific surface area was calculated using the isotherms.

[0093] [Table 3]

[0094] <Examples and Comparative Examples: Production of Cathode Material> Example 1 The positive electrode active material A prepared in Preparation Example 1 and the positive electrode active material F prepared in Preparation Example 2 were mixed in a weight ratio of 80:20 to prepare a positive electrode material.

[0095] Example 2. The positive electrode active material B produced in Preparation Example 1 and the positive electrode active material G produced in Preparation Example 2 were mixed in a weight ratio of 80:20 to produce a positive electrode material.

[0096] Example 3. The positive electrode active material A produced in Production Example 1 and the positive electrode active material G produced in Production Example 2 were mixed in a weight ratio of 80:20 to produce a positive electrode material.

[0097] Example 4. The positive electrode active material A produced in Production Example 1 and the positive electrode active material F produced in Production Example 2 were mixed in a weight ratio of 50:50 to produce a positive electrode material.

[0098] Comparative Example 1 The positive electrode active material C prepared in Preparation Example 1 and the positive electrode active material H prepared in Preparation Example 2 were mixed in a weight ratio of 80:20 to prepare a positive electrode material.

[0099] Comparative Example 2 The positive electrode active material D prepared in Preparation Example 1 and the positive electrode active material I prepared in Preparation Example 2 were mixed in a weight ratio of 80:20 to prepare a positive electrode material.

[0100] Comparative Example 3. The positive electrode active material E produced in Production Example 1 and the positive electrode active material J produced in Production Example 2 were mixed in a weight ratio of 80:20 to produce a positive electrode material.

[0101] Comparative Example 4. The positive electrode active material D prepared in Preparation Example 1 and the positive electrode active material G prepared in Preparation Example 2 were mixed in a weight ratio of 80:20 to prepare a positive electrode material.

[0102] Comparative Example 5. The positive electrode active material C prepared in Preparation Example 1 and the positive electrode active material K prepared in Preparation Example 2 were mixed in a weight ratio of 80:20 to prepare a positive electrode material.

[0103] <Experimental Example> Experimental Example 1 - Fine powder generation rate 3 g of each cathode material powder prepared in Examples 1 to 4 and Comparative Examples 1 to 5 was placed in a cylindrical metal mold with a diameter of 2 cm and pressed at a pressure of 6 tons. The cumulative volumetric particle size distribution was then measured to determine the percentage of fine powder less than 1 μm in size. The particle size distribution was measured using a Microtrac S-3500, and the percentage of fine powder less than 1 μm in size was calculated based on the total weight of the cathode material and converted to volume percent. The measurement results are shown in Table 4 below.

[0104] Experimental Example 2: Evaluation of high-temperature battery life characteristics Each of the cathode materials prepared in Examples 1 to 4 and Comparative Examples 1 to 5, a conductive material (acetylene black), and a PVdF binder were mixed in N-methylpyrrolidone at a weight ratio of 97.5:1.0:1.5 to prepare a cathode slurry. The cathode slurry was applied to one side of an aluminum current collector, dried, and rolled to prepare a cathode.

[0105] Specifically, anode active material (a 50:50 mixture of natural graphite and artificial graphite), SBR binder (M37, LG Chem), and conductive material (acetylene black) were mixed with water (solvent) in a weight ratio of 96.0:3.0:1.0 to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to a copper foil (anode current collector), which was then dried and rolled to prepare anode.

[0106] A 15 μm thick polyethylene separator was placed between the positive and negative electrodes to fabricate an electrode assembly, which was then inserted into a battery case and filled with electrolyte to fabricate a lithium secondary battery. The electrolyte used was a solution of 1M LiPF6 dissolved in an organic solvent containing ethylene carbonate (EC): ethyl methyl carbonate (EMC) in a volume ratio of 1:2.

[0107] The lithium secondary battery prepared as described above was charged at 45° C. at 0.3 C up to 4.25 V and discharged at 0.3 C down to 2.5 V, and the DCIR increase and capacity retention were measured after 100 charge-discharge cycles, with one cycle being defined as one cycle. The measurement results are shown in Table 4 below.

[0108] At this time, the resistance increase rate and capacity retention rate were calculated using the following formulas 1 and 2.

[0109] Equation 1: Resistance increase rate (%) = {(resistance after 100 cycles / resistance after 1 cycle) × 100} - 100 Equation 2: Capacity retention rate (%) = (discharge capacity after 100 cycles / discharge capacity after 1 cycle) × 100

[0110] [Table 4]

[0111] The results in Table 4 confirm that the cathode materials of Examples 1 to 4, in which the ratio of the cation mixing ratio of the large particle size cathode active material to the cation mixing ratio of the small particle size cathode active material (Ml / Ms) was between 0.6 and 4.0 and the ratio of the average particle size of the primary particles of the large particle size cathode active material to the average particle size of the primary particles of the small particle size cathode active material (Pl / Ps) was between 0.1 and 2.0, exhibited little particle cracking due to rolling and excellent life at high temperatures. On the other hand, the cathode material of Comparative Example 1, in which the Pl / Ps value was within the range of the present invention but the Ml / Ms value was less than 0.6, exhibited little particle cracking but was inferior to the examples in capacity retention and resistance increase rate at high temperatures. The cathode material of Comparative Example 4, in which the Ml / Ms value exceeded 4.0, exhibited significant particle cracking and poor capacity retention and resistance increase rate at high temperatures.

[0112] In addition, it was confirmed that the cathode material of Comparative Example 2, in which the Ml / Ms value satisfies the range of the present invention but the Pl / Ps value exceeds 2.0, exhibited particle cracking and poor high-temperature performance compared to the Examples. It was also found that Comparative Example 5, in which the Pl / Ps value is less than 0.1, exhibited not only particle cracking but also poor high-temperature life characteristics (capacity retention rate) and output characteristics (resistance increase rate) compared to the Examples.

[0113] Furthermore, it can be seen that Comparative Example 3, in which both the Ml / Ms and Pl / Ps values ​​are outside the range of the present invention, exhibits the most deteriorated properties.

Claims

1. comprising a large particle size positive electrode active material and a small particle size positive electrode active material, the large particle size positive electrode active material and the small particle size positive electrode active material each independently contain a lithium nickel-based oxide; where Ml is a cation mixing ratio of the large particle diameter positive electrode active material and Ms is a cation mixing ratio of the small particle diameter positive electrode active material, Ml / Ms is 0.6 to 4.0, where Pl is the average particle size of the primary particles of the large-particle-size positive electrode active material and Ps is the average particle size of the primary particles of the small-particle-size positive electrode active material, Pl / Ps is 0.1 to 2.0, The large particle size positive electrode active material and the small particle size positive electrode active material each contain a lithium nickel-based oxide in which the molar ratio of nickel among the transition metals is 50 mol % or more.

2. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein the large particle size positive electrode active material has a D50 of 8 μm to 18 μm.

3. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein the small particle diameter positive electrode active material has a D50 of 2 μm to 7 μm.

4. 2. The positive electrode material for a lithium secondary battery according to claim 1, wherein the large particle size positive electrode active material and the small particle size positive electrode active material are each independently composed of a single particle composed of a primary particle; a secondary particle composed of an aggregation of a number of primary particles; or a combination thereof.

5. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein the average particle size of the primary particles of the large particle size positive electrode active material is 0.2 μm to 1.0 μm.

6. 2. The positive electrode material for a lithium secondary battery according to claim 1, wherein the average particle size of the primary particles of the small particle size positive electrode active material is 0.5 μm to 2.0 μm.

7. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein Bl / Bs is 0.15 to 1.6, where Bl is the specific surface area of ​​the large particle size positive electrode active material and Bs is the specific surface area of ​​the small particle size positive electrode active material.

8. 2. The positive electrode material for a lithium secondary battery according to claim 1, wherein the large particle size positive electrode active material and the small particle size positive electrode active material each contain a lithium nickel-based oxide represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ｘ (N ａ Co ｂ Mn ｃ M ｄ )O ２－ｙ In the above Chemical Formula 1, M is at least one element selected from the group consisting of Al, Mg, V, Ti, Zr, Nb, and W; x, a, b, c, and d are -0.10≦x≦0.20, 0.50≦a<1.0, 0<b≦0.40, 0<c≦0.30, 0≦d≦0.05, a+b+c+d=1, and 0≦y≦0.05, respectively.

9. 2. The positive electrode material for a lithium secondary battery according to claim 1, wherein the large particle size positive electrode active material and the small particle size positive electrode active material each contain a lithium nickel-based oxide in which the molar ratio of nickel among the transition metals is 70 mol % or more.

10. 2. The positive electrode material for a lithium secondary battery according to claim 1, wherein the large particle size positive electrode active material and the small particle size positive electrode active material are contained in a weight ratio of 50:50 to 90:

10.

11. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein the large particle size positive electrode active material has a cation mixing ratio of 1.0 at % to 2.0 at %.

12. 2. The positive electrode material for lithium secondary batteries according to claim 1, wherein the small particle size positive electrode active material has a cation mixing ratio of 0.5 at % to 1.5 at %.

13. A positive electrode for a lithium secondary battery, comprising the positive electrode material for a lithium secondary battery according to claim 1 .

14. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 13.

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