Positive electrode active material for lithium secondary battery, positive electrode containing the same and lithium secondary battery

Abstract

The present invention provides a positive electrode active material capable of realizing a battery having excellent capacity characteristics and life characteristics, and a first lithium transition metal oxide and an average particle diameter (D 50 The present invention relates to a positive electrode active material comprising a second lithium transition metal oxide having a small particle strength, the first lithium transition metal oxide having a particle strength of 140 MPa or more, and a difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide having a particle strength of 10 MPa or less, a positive electrode including the positive electrode active material, and a lithium secondary battery including the positive electrode active material.

Description

[Technical field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0058086 filed on May 4, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a positive electrode active material for a lithium secondary battery, and a positive electrode and a lithium secondary battery containing the same. [Background technology]

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

[0004] Lithium transition metal composite oxides are used as the positive electrode active material for lithium secondary batteries. Among them, LiCoO has a high working voltage and excellent capacity characteristics. 2 Lithium-cobalt composite metal oxides such as LiCoO 2 However, the thermal properties are poor due to the instability of the crystal structure caused by the delithiation. 2 However, because of its high cost, there are limitations to its mass use as a power source in fields such as electric vehicles.

[0005] The LiCoO 2 As a replacement for lithium manganese mixed metal oxide (LiMnO 2 or LiMn 2 O 4 etc.), lithium iron phosphate compounds (LiFePO 4 etc.) or lithium nickel composite metal oxide (LiNiO 2Among them, research and development of lithium nickel composite metal oxides, which have a high reversible capacity of about 200 mAh / g and can easily be used to realize large-capacity batteries, has been actively carried out. However, the LiNiO 2 LiCoO 2 In comparison with LiNiO, the thermal stability is inferior, and if an internal short circuit occurs due to external pressure during charging, the positive electrode active material itself decomposes, causing the battery to crack and catch fire. 2 As a method for improving the low thermal stability while maintaining the excellent reversible capacity of lithium transition metal oxides, lithium transition metal oxides in which a part of Ni is substituted with Co, Mn or Al have been developed.

[0006] In the case of a lithium-ion battery using such a lithium transition metal oxide, particularly a lithium transition metal oxide containing a high content of nickel (Ni-rich), as a positive electrode active material, the battery capacity, whether it can generate high power, and whether it generates gas at high temperatures are affected not only by chemical properties such as the composition of the positive electrode active material, the content of impurities, and the content of lithium by-products present on the surface, but also by physical properties such as the size, surface area, density, and shape of the positive electrode active material particles.

[0007] In general, in order to maximize the volumetric energy density of a battery, a method is used in which a positive electrode active material having a large particle size and a positive electrode active material having a small particle size are mixed together to fill gaps between the particles of the positive electrode active material having a large particle size, thereby improving the volumetric energy density of the battery, and a method is used in which the positive electrode active material layer is rolled using a roll press to manufacture a positive electrode active material layer having a denser structure. Here, due to the difference in particle strength between the positive electrode active material having a large particle size and the positive electrode active material having a small particle size, excessive cracks are generated in the particles having relatively weak particle strength during rolling, causing the particles to lose their original shape and also causing the contact area with the electrolyte to become excessively large, resulting in a problem of reduced life characteristics when this is applied to a battery.

[0008] Therefore, there is a need for development of a positive electrode active material that can improve the energy volume density, suppress particle cracking during rolling for manufacturing the positive electrode, and improve life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a positive electrode active material that can suppress particle cracking when the positive electrode active material contains two types of lithium transition metal oxides having different average particle sizes and is rolled. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides a positive electrode active material, a positive electrode, and a lithium secondary battery.

[0011] (1) The present invention relates to a first lithium transition metal oxide and a method for producing the first lithium transition metal oxide having an average particle size (D 50 ) a second lithium transition metal oxide having a small particle strength, the first lithium transition metal oxide having a particle strength of 140 MPa or more, and a difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide having a particle strength of 10 MPa or less.

[0012] (2) The present invention provides a positive electrode active material according to the above (1), in which the second lithium transition metal oxide has a particle strength equal to or smaller than that of the first lithium transition metal oxide.

[0013] (3) The present invention provides the positive electrode active material according to (1) or (2) above, wherein the second lithium transition metal oxide has a particle strength of 130 MPa or more and 190 MPa or less.

[0014] (4) The present invention provides a positive electrode active material in any one of the above (1) to (3), wherein the positive electrode active material has a bimodal particle size distribution.

[0015] (5) In any one of the above (1) to (4), the present invention is characterized in that the average particle size (D 50 ) provides a positive electrode active material having a particle size of 6 μm to 25 μm.

[0016] (6) In any one of the above (1) to (5), the present invention is characterized in that the average particle size (D 50 ) provides a positive electrode active material having a particle size of 1 μm to 12 μm.

[0017] (7) The present invention provides a positive electrode active material according to any one of (1) to (6) above, wherein the first lithium transition metal oxide and the second lithium transition metal oxide are each independently a lithium transition metal oxide in which a molar ratio of nickel among the transition metals is 60% or more.

[0018] (8) The present invention provides a positive electrode active material in any one of the above (1) to (7), wherein the first lithium transition metal oxide and the second lithium transition metal oxide each independently have a composition represented by the following chemical formula 1: [Chemical formula 1] Li 1+a Ni x Co y M 1 z M 2 w O 2 In the above Chemical Formula 1, 0≦a≦0.30, 0.60≦x<1.0, 0 <y<0.40、0<z<0.40、0≦w≦0.10であり、 M 1 is one or more selected from Mn and Al, M 2 is one or more selected from W, Mo, Cr, Zr, Ti, Mg, Ta and Nb.

[0019] (9) The present invention provides a positive electrode active material according to any one of the above (1) to (8), wherein the first lithium transition metal oxide and the second lithium transition metal oxide are contained in a weight ratio of 60:40 to 90:10.

[0020] (10) The present invention provides a positive electrode, comprising a positive electrode active material layer including the positive electrode active material according to any one of (1) to (9) above, wherein a first lithium transition metal oxide contained in the positive electrode active material layer satisfies the following formula 1: [Formula 1]

number

[0021] (11) The present invention provides the positive electrode according to the above (10), wherein the positive electrode active material layer has a porosity of 15 vol % to 30 vol %.

[0022] (12) The present invention provides a lithium secondary battery comprising the positive electrode according to (10) or (11) above. Effect of the Invention

[0023] In the positive electrode active material according to the present invention, the particle strength of the large particle size lithium transition metal oxide is equal to or greater than a specific value, and the difference in particle strength between the large particle size lithium transition metal oxide and the small particle size lithium transition metal oxide is small, so that the cracking phenomenon of the lithium transition metal oxide particles during rolling for manufacturing the positive electrode can be effectively suppressed. Therefore, when using the positive electrode active material according to the present invention, the occurrence of side reactions with the electrolyte caused by the cracking of the lithium transition metal oxide particles can be minimized, and excellent capacity characteristics and life characteristics can be achieved. [Brief description of the drawings]

[0024] [Figure 1] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Example 1. [Diagram 2] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Example 2. [Diagram 3]1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Example 3. [Figure 4] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Comparative Example 1. [Diagram 5] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Comparative Example 2. [Figure 6] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Comparative Example 3. [Figure 7] 1 is a SEM image showing a cross section of a positive electrode prepared using the positive electrode active material prepared in Comparative Example 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention will now be described in more detail.

[0026] The terms and words used in this specification and the claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a manner that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best describe their invention.

[0027] In this specification, the term "average particle size (D 50 The average particle size (D) can be defined as the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using a laser diffraction method. The laser diffraction method generally makes it possible to measure particle sizes in the range of several nm to several mm, and can obtain results with high reproducibility and high resolution.

[0028] In this specification, particle strength (MPa) is a value obtained by taking a sample of lithium transition metal oxide particles, placing the sample on glass, and applying a certain pressure (100 mN) to the sample with a tip, and measuring the force until the particle breaks and the tip comes into contact with the glass on which the sample is placed.

[0029] positive electrode active material The inventors have discovered that by mixing two types of lithium transition metal oxides having different average particle sizes and optimizing the difference in particle strength between a first lithium transition metal oxide having a large particle size with a relatively large average particle size and a second lithium transition metal oxide having a small particle size with a relatively small average particle size, and using a first lithium transition metal oxide having a large particle size with a particle strength of 140 MPa or more, it is possible to have a high energy volume density and suppress particle cracking, thereby ensuring excellent capacity characteristics and long life characteristics when applied to a battery, and have completed the present invention.

[0030] The positive electrode active material according to the present invention will be specifically described below.

[0031] The positive electrode active material according to the present invention has a first lithium transition metal oxide and an average particle size (D 50 ) is small, the first lithium transition metal oxide has a particle strength of 140 MPa or more, and the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is 10 MPa or less.

[0032] The positive electrode active material according to the present invention has a first lithium transition metal oxide having a different average particle size and a second lithium transition metal oxide having a different average particle size (D 50 ) contains a small secondary lithium transition metal oxide and has a bimodal particle size distribution.

[0033] For example, the first lithium transition metal oxide may be particles having a relatively large average particle size, and the second lithium transition metal oxide may be particles having a relatively small average particle size.

[0034] The first lithium transition metal oxide has an average particle size (D 50 ) can be 6 μm to 25 μm, specifically 6 μm to 22 μm, and more specifically 8 μm to 20 μm, and the second lithium transition metal oxide can have an average particle size (D 50 ) can be from 1 μm to 12 μm, specifically from 2 μm to 10 μm, and more specifically from 2 μm to 8 μm.

[0035] The first lithium transition metal oxide and the second lithium transition metal oxide have an average particle size (D 50 ) satisfies the above range, the particles of the second lithium transition metal oxide are filled between the particles of the first lithium transition metal oxide, thereby improving the tap density of the positive electrode active material including the same. The higher the tap density, the higher the packing density of the electrode. When an electrode is manufactured using the same, a slurry including a positive electrode active material having the above tap density can be thinly coated on the surface of a positive electrode current collector, and the thickness of the electrode is improved after coating. In the process of rolling the same, the pressure required to reach the electrode thickness for matching the rolling density is small, and cracks of the positive electrode active material due to rolling can be improved. Furthermore, the volumetric energy density is improved, thereby further improving the capacity characteristics.

[0036] Meanwhile, the particle strength of the first lithium transition metal oxide is 140 MPa or more, and the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is 10 MPa or less. In this case, since the particle strengths of the first lithium transition metal oxide and the second lithium transition metal oxide are adjusted to be the same, physical force is not concentrated on either one during rolling for manufacturing the positive electrode but is dispersed, and the first lithium transition metal oxide particles and the second lithium transition metal oxide particles can each maintain their original shape, and particle cracking can be suppressed.

[0037] On the other hand, if the particle strength of the first lithium transition metal oxide is less than 140 MPa, the large-sized particles of the first lithium transition metal oxide that occupy a large part of the electrode are easily broken, and the specific surface area available for reaction with the electrolyte becomes larger. Also, if the difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is more than 10 MPa, the small-sized particles of the second lithium transition metal oxide may be completely broken between the large-sized particles of the first lithium transition metal oxide, and the problem of the large specific surface area available for reaction with the electrolyte may become even worse.

[0038] The particle strength of the first lithium transition metal oxide may be, specifically, 140 MPa to 200 MPa, more specifically, 140 MPa to 160 MPa. The second lithium transition metal oxide may have a particle strength smaller than or equal to that of the first lithium transition metal oxide, specifically, 130 MPa or more and less than 190 MPa, more specifically, 130 MPa to 150 MPa. When the particle strengths of the first lithium transition metal oxide and the second lithium transition metal oxide satisfy the above range, cracking of the particles during rolling for manufacturing the positive electrode can be reduced. As a result, excellent rolling density can be achieved, and the high temperature life characteristics of the battery can be optimized.

[0039] Meanwhile, the particle strength of lithium transition metal oxides varies depending on the composition and characteristics of the raw material, that is, the precursor for the positive electrode active material (transition metal hydroxide, transition metal oxyhydroxide, etc.) and the firing conditions. Therefore, by appropriately adjusting the firing conditions (temperature and time) depending on the composition and / or characteristics (surface area, density, shape, etc.) of the precursor for the positive electrode active material, it is possible to produce lithium transition metal oxides having the desired particle strength.

[0040] Meanwhile, the first lithium transition metal oxide and the second lithium transition metal oxide may each independently be a lithium transition metal oxide having a nickel molar ratio of 80% or more among the transition metals. The high-nickel-containing lithium transition metal oxide has a large capacity per unit volume, and therefore can achieve excellent capacity characteristics when applied to a battery.

[0041] Specifically, the first lithium transition metal oxide and the second lithium transition metal oxide may each independently have a composition represented by the following Chemical Formula 1.

[0042] [Chemical formula 1] Li 1+a Ni x Co y M 1 z M 2 w O 2

[0043] In the above Chemical Formula 1, M 1 is one or more selected from Mn and Al, and specifically may contain Mn and Al simultaneously.

[0044] Said M 2 can be one or more selected from W, Mo, Cr, Zr, Ti, Mg, Ta and Nb.

[0045] The 1+a indicates the molar ratio of lithium in the lithium transition metal oxide, and can be 0≦a≦0.30, preferably 0≦a≦0.20.

[0046] Wherein x represents the molar ratio of nickel among the total transition metals, and 0.60 ≦ x < 1.0, 0.60 ≦ x ≦ 0.99, 0.60 ≦ x ≦ 0.95, 0.60 ≦ x ≦ 0.90, or 0.60 ≦ x ≦ 0.85. When the nickel content satisfies the above range, excellent capacity characteristics can be realized.

[0047] Wherein y represents the molar ratio of cobalt among the total transition metals, and 0 < y < 0.40, 0 < y < 0.30, or 0.01 ≦ y ≦ 0.20.

[0048] Wherein z represents the molar ratio of M 1 among the total transition metals, and 0 < z < 0.40, 0 < z < 0.30, or 0.01 < z < 0.20.

[0049] Wherein w represents the molar ratio of M 2 among the total transition metals, and 0 ≦ w ≦ 0.10, or 0 ≦ w ≦ 0.05.

[0050] More specifically, the first lithium transition metal oxide and the second lithium transition metal oxide can each independently have a composition represented by the following Chemical Formula 2.

[0051] [Chemical Formula 2] Li 1+a Ni x Co y Mn z1 Al z2 M 2 w O 2

[0052] In Chemical Formula 2, a, x, y, w, and M 2 are as defined in Chemical Formula 1.

[0053] That is, M 2 can be one or more selected from the group consisting of W, Mo, Cr, Zr, Ti, Mg, Ta, and Nb.

[0054] The 1+a indicates the molar ratio of lithium in the lithium transition metal oxide, and can be 0≦a≦0.30, preferably 0≦a≦0.20.

[0055] The x represents a molar ratio of nickel to all transition metals, and may be 0.60≦x<1.0, 0.60≦x≦0.99, 0.60≦x≦0.95, 0.60≦x≦0.90, or 0.60≦x≦0.85. When the nickel content satisfies the above range, excellent capacity characteristics can be achieved.

[0056] The y represents the molar ratio of cobalt to the total transition metals, and is 0 <y<0.40、0<y<0.30、または0.01≦y≦0.20であることができる。

[0057] The w is M among all transition metals. 2 and may be 0≦w≦0.10, or 0≦w≦0.05.

[0058] On the other hand, z1 represents the molar ratio of Mn among the transition metals, and is 0 <z1<0.40、0<z1<0.30、または0<z1<0.20であることができる。

[0059] The z2 represents the molar ratio of Al among the transition metals, and is 0 <z2<0.20、0<z2<0.15、または0<z2<0.10であることができる。

[0060] The first lithium transition metal oxide and the second lithium transition metal oxide may have the same or different compositions. For example, the first lithium transition metal oxide may be a lithium transition metal oxide having a molar ratio of nickel among the transition metals of 85 mol % or more, and the second lithium transition metal oxide may be a lithium transition metal oxide having a molar ratio of nickel among the transition metals of 80 mol % to 85 mol %.

[0061] Meanwhile, the first lithium transition metal oxide and the second lithium transition metal oxide may further include, if necessary, a coating layer on the surface of the lithium transition metal oxide, the coating layer including one or more elements (hereinafter referred to as "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, Mo, Sr, Sb, Bi, Si, and S. When the coating layer is included, the contact between the lithium transition metal oxide and the electrolyte is blocked, and gas generation and elution of the transition metal due to side reactions with the electrolyte can be effectively suppressed.

[0062] The coating layer may be formed by mixing a lithium transition metal oxide and a raw material containing the coating element, and then heat treating the mixture at a temperature of 200°C to 500°C.

[0063] In the positive electrode active material according to the present invention, the first lithium transition metal oxide and the second lithium transition metal oxide may be contained in a weight ratio of 60:40 to 90:10, specifically, a weight ratio of 60:40 to 90:10, more specifically, a weight ratio of 70:30 to 90:10. In this case, an effect of improving the tap density of the positive electrode active material can be obtained.

[0064] positive electrode The present invention also provides a positive electrode including a positive electrode active material layer containing the positive electrode active material. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector and containing the positive electrode active material. The positive electrode active material layer may have a porosity of 15 volume % to 30 volume %, specifically 18 volume % to 30 volume %, and more specifically 20 volume % to 30 volume %.

[0065] Here, the first lithium transition metal oxide contained in the positive electrode active material layer satisfies the following formula 1. That is, when a positive electrode is manufactured using the positive electrode active material according to the present invention, the first lithium transition metal oxide particles are less likely to crack, and the form factor value can be 0.4 or more, specifically 0.4 to 1. In contrast, when a positive electrode is manufactured using a positive electrode active material that does not satisfy the present invention, the first lithium transition metal oxide particles are cracked during rolling for manufacturing the positive electrode, and as a result, the perimeter of the first lithium transition metal oxide particles, i.e., the boundary length, increases, and the form factor value does not satisfy 0.4 or more, and becomes low.

[0066] [Formula 1]

number

[0067] In the above formula 1, A and P are the cross-sectional area and perimeter, respectively, of a particle of the first lithium transition metal oxide.

[0068] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector can usually have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. For example, it can be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0069] The positive electrode active material layer may contain a conductive material and a binder in addition to the above-described positive electrode material according to the present invention.

[0070] The positive electrode material may be included in an amount of 80 to 99 wt %, more specifically 85 to 98 wt %, based on the total weight of the positive electrode active material layer. When included in the above range, excellent capacity characteristics can be exhibited.

[0071] The conductive material is used to give conductivity to the electrode, and can be used without any particular limitation as long as it does not cause a chemical change in the battery and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, and among these, one type alone or a mixture of two or more types can be used. The conductive material can be contained in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0072] The binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene 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, and one or more of these may be used alone or in combination. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0073] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material according to the present invention. Specifically, the positive electrode may be manufactured by applying a composition for forming a positive electrode active material layer, which is prepared by dissolving or dispersing the positive electrode active material and, optionally, a binder and a conductive active material in a solvent, onto a positive electrode current collector, followed by drying and rolling. In this case, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0074] The solvent may be a solvent commonly used in the art, and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc., and one or more of these may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to provide a viscosity that allows excellent thickness uniformity during subsequent coating for the production of a positive electrode, taking into consideration the coating thickness of the slurry and the manufacturing yield.

[0075] As another method, the positive electrode can also be produced by casting the composition for forming a positive electrode active material layer on another support, peeling the composition from the support, and laminating the resulting film on a positive electrode current collector.

[0076] Lithium secondary battery In addition, the present invention can provide an electrochemical device including the positive electrode. The electrochemical device can be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0077] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Since the positive electrode is as described above, a detailed description thereof will be omitted, and only the remaining components will be described in detail below.

[0078] Also, the lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0079] 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.

[0080] The negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity, 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 aluminum-cadmium alloy. The negative electrode current collector may have a thickness of usually 3 μm to 500 μm, and like the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0081] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0082] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples of the negative electrode active material 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; SiO β (0<β<2), SnO 2Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as vanadium oxide and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites, and any one or a mixture of two or more of these may be used. Also, a thin film of metallic lithium may be used as the negative electrode active material. Also, the carbonaceous material may be either low crystalline carbon or high crystalline carbon. Representative examples of low crystalline carbon include soft carbon and hard carbon, and representative examples of high crystalline carbon include amorphous, plate-like, flake-like, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, carbon microbeads, mesophase pitches, and high-temperature fired carbon such as petroleum or coal tar pitch derived cokes.

[0083] The negative electrode active material may be included in an amount of 80 parts by weight to 99 parts by weight with respect to 100 parts by weight of the total weight of the negative electrode active material layer.

[0084] The binder is a component that facilitates bonding between the conductive material, the active material, and the current collector, and may be added in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of 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.

[0085] 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 parts by weight or less, specifically 5 parts by weight or less, based on 100 parts by weight of the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples of such conductive materials 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 fibers and metal fibers; metal powders such as carbon fluoride, 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.

[0086] For example, the negative electrode active material layer can be produced by applying a negative electrode mixture, which is prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, onto a negative electrode current collector and drying the applied material; alternatively, the negative electrode mixture can be cast onto another support, and then peeled off from the support to obtain a film, which can be laminated onto the negative electrode current collector.

[0087] 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 that is generally used as a separator in a lithium secondary battery can be used without any particular limitation. In particular, a separator that has low resistance to ion movement of the electrolyte and has excellent electrolyte moisture-absorbing ability is preferable. Specifically, a porous polymer film, for example, 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, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, etc. can be used. In addition, a coated separator containing a ceramic component or a polymeric material can be used to ensure heat resistance or mechanical strength, and can be selectively used in a single layer or multilayer structure.

[0088] In addition, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel 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.

[0089] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0090] As the organic solvent, any solvent can be used without particular limitation as long as it serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, 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; carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double bond, aromatic ring, or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can enhance the charge-discharge performance of the battery, and a linear carbonate compound with low viscosity (e.g., ethylmethylcarbonate, dimethylcarbonate, or diethylcarbonate) is more preferred. In this case, the cyclic carbonate and the linear carbonate are preferably mixed and used at a volume ratio of about 1:1 to about 1:9, so that the electrolyte can exhibit excellent performance.

[0091] The lithium salt can be used without any particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiSbF 6 , LiAlO 4 , LiAlCl 4 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN(C 2 F 5 SO 3 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , LiCl, LiI, or LiB(C 2 O 4 ) 2 etc. can be used. The lithium salt is preferably used in a concentration range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, and therefore can exhibit excellent electrolyte performance, allowing lithium ions to migrate effectively.

[0092] 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, triethyl alcohol amine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc. Here, the additives may be contained in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the total weight of the electrolyte.

[0093] As described above, the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and life characteristics, and is therefore useful in portable devices such as mobile phones, notebook computers and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0094] 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.

[0095] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, such as a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0096] The external 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.

[0097] 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.

[0098] Hereinafter, the present invention will be described in detail with reference to examples in order to specifically explain the present invention. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0099] Examples and Comparative Examples Example 1 -Preparation of the first lithium transition metal oxide NiSO 4 , CoSO 4 , MnSO 4 The above was dissolved in water in an amount such that the molar ratio of nickel:cobalt:manganese became 88:5:7, to prepare a transition metal-containing solution having a concentration of 2M.

[0100] The container filled with the transition metal-containing solution and the container further containing a 25% by weight NaOH aqueous solution and a 15% by weight NH 4 The aqueous OH solutions were each connected to a 200 L batch reactor set at 55°C.

[0101] Next, deionized water was added to the batch reactor, and nitrogen gas was purged to remove dissolved oxygen in the water and create a non-oxidizing atmosphere in the reactor. NaOH was then added and stirred at a stirring speed of 250 rpm to maintain the pH in the coprecipitation reactor at 11.7.

[0102] Next, the transition metal-containing solution was added to the coprecipitation reactor at a rate of 250 mL / hr, and NH 4 The OH aqueous solution was added at a rate of 40 mL / hr, and the NaOH aqueous solution was added at a rate that maintained the pH of the reaction solution at 11.7. After 6 hours of reaction, stirring was stopped and the supernatant was removed to concentrate the mixture. This process was repeated 4 to 5 times to obtain the average particle size (D 50 The particles were grown until the diameter of the particles was approximately 10 μm.

[0103] The particles thus produced were filtered using a filter press, then dried at 130°C for 24 hours, and Ni 0.88 Co 0.05 Mn 0.07 (OH) 2 A first positive electrode active material precursor having the composition was obtained.

[0104] Next, the first positive electrode active material precursor was mixed with LiOH.H 2 Add 1.06 equivalents of O and Al(OH) 3 The above was mixed and calcined in an oxygen atmosphere at 775° C. for 13.5 hours to produce a lithium transition metal oxide having a nickel:cobalt:manganese:aluminum molar ratio of 86:5:7:2.

[0105] The lithium transition metal oxide was stirred with distilled water in a weight ratio of 1:1.1 and washed with water.

[0106] After washing with water, the lithium transition metal oxide was 3 BO 3 were mixed and heat treated at 295° C. for 5 hours to produce a B-coated first lithium transition metal oxide.

[0107] -Preparation of secondary lithium transition metal oxides NiSO 4 , CoSO 4 , MnSO 4 The transition metal-containing solution was dissolved in water in an amount such that the molar ratio of nickel:cobalt:manganese was 83:5:12, and the concentration was 2M. The concentration process was repeated 2-3 times during the preparation of the precursor, and the average particle size (D 50A second positive electrode active material precursor was prepared in the same manner as the first positive electrode active material precursor, except that the particles were grown until the diameter of the first positive electrode active material precursor reached about 5 μm.

[0108] The average particle size (D 50 ) is about 5μm and has a composition of Ni 0.83 Co 0.05 Mn 0.12 (OH) 2 It was.

[0109] Thereafter, the second positive electrode active material precursor was treated with LiOH. 2 Add 1.06 equivalents of O and Al(OH) 3 The above was mixed and calcined in an oxygen atmosphere at 790° C. for 13.5 hours to produce a lithium transition metal oxide having a nickel:cobalt:manganese:aluminum molar ratio of 81:5:12:2.

[0110] The lithium transition metal oxide was stirred with distilled water in a weight ratio of 1:1.1 and washed with water.

[0111] After washing with water, the lithium transition metal oxide was 3 BO 3 were mixed and heat treated at 295° C. for 5 hours to produce a B-coated secondary lithium transition metal oxide.

[0112] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0113] Example 2 -Preparation of first lithium transition metal oxide The first lithium transition metal oxide was produced in the same manner as in Example 1.

[0114] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 795°C instead of 790°C.

[0115] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0116] Example 3 -Preparation of first lithium transition metal oxide A first lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 780°C instead of 775°C.

[0117] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 805°C instead of 790°C.

[0118] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0119] Comparative Example 1 -Preparation of first lithium transition metal oxide A first lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 785°C instead of 775°C.

[0120] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1.

[0121] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0122] Comparative Example 2 -Preparation of first lithium transition metal oxide NiSO 4 , CoSO 4 , MnSO 4was dissolved in water in an amount such that the molar ratio of nickel:cobalt:manganese was 88:5:7, to prepare a transition metal-containing solution with a concentration of 2M.

[0123] The container filled with the transition metal-containing solution and the container further containing a 25% by weight NaOH aqueous solution and a 15% by weight NH 4 The aqueous OH solutions were each connected to a 200 L batch reactor set at 55°C.

[0124] Next, deionized water was added to the batch reactor, and nitrogen gas was purged to remove dissolved oxygen in the water, creating a non-oxidizing atmosphere in the reactor. Next, NaOH was added and stirred at a stirring speed of 250 rpm to maintain the pH in the coprecipitation reactor at 11.7.

[0125] Next, the transition metal-containing solution was added to the coprecipitation reactor at a rate of 250 mL / hr, and NH 4 The OH aqueous solution was added at a rate of 40 mL / hr, and the NaOH aqueous solution was added at a rate that maintained the pH of the reaction solution at 11.7. After 6 hours of reaction, stirring was stopped and the supernatant was removed to concentrate the mixture. This process was repeated 4 to 5 times to obtain the average particle size (D 50 The particles were grown until the diameter of the particles was approximately 10 μm.

[0126] The particles thus produced were filtered using a filter press and then dried at 130°C for 24 hours to obtain Ni 0.88 Co 0.05 Mn 0.07 (OH) 2 A first positive electrode active material precursor having the composition was obtained.

[0127] Next, the first positive electrode active material precursor was mixed with LiOH.H 2 Add 1.06 equivalents of O and Al(OH) 3 The above was mixed and calcined in an oxygen atmosphere at 795° C. for 13.5 hours to produce a lithium transition metal oxide having a nickel:cobalt:manganese:aluminum molar ratio of 86:5:7:2.

[0128] The lithium transition metal oxide was stirred with distilled water in a weight ratio of 1:1.1 and washed with water.

[0129] After washing with water, the lithium transition metal oxide was 3 BO 3 were mixed and heat treated at 295° C. for 5 hours to produce a B-coated first lithium transition metal oxide.

[0130] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 780°C instead of 790°C.

[0131] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0132] Comparative Example 3 -Preparation of the first lithium transition metal oxide A first lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 770°C instead of 775°C.

[0133] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 780°C instead of 790°C.

[0134] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0135] Comparative Example 4 -Preparation of the first lithium transition metal oxide A first lithium transition metal oxide was produced in the same manner as in Example 1, except that the calcination was carried out at 780°C instead of 775°C.

[0136] -Preparation of secondary lithium transition metal oxides A second lithium transition metal oxide was produced in the same manner as in Example 1.

[0137] -Production of cathode active materials The first lithium transition metal oxide and the second lithium transition metal oxide prepared above were mixed in a weight ratio of 8:2 to prepare a positive electrode active material.

[0138] [Table 1]

[0139] Experimental example 1: Confirmation of form factor (F) Positive electrodes were manufactured using the positive electrode active materials manufactured in Examples 1 to 3 and Comparative Examples 1 to 4. Specifically, the positive electrode active materials manufactured in Examples 1 to 3 and Comparative Examples 1 to 4, a conductive material (FX35), and a binder (KF9700 and BM73OH mixed in a weight ratio of 1.35:0.15) were mixed in a weight ratio of 97.5:1:1.5 in N-methyl-2-pyrrolidone (NMP) solvent to manufacture a positive electrode slurry. The positive electrode slurry was applied to one side of an aluminum current collector, dried at 130°C, and then rolled to a porosity of 24% by volume to manufacture a positive electrode.

[0140] The positive electrodes produced using the positive electrode active materials produced in Examples 1 to 3 and Comparative Examples 1 to 4 were cut with an Ar beam using a Cross-Section Polisher (HITACHI IM5000; accelerating voltage: 7 kV), and cross-sectional SEM images of each positive electrode are shown in FIGS. 1 to 7.

[0141] The SEM images of FIGS. 1 to 7 were subjected to image processing (using Mountains Software) to detect the first lithium transition metal oxide and the second lithium transition metal oxide particles contained in the positive electrode, and the form factor value of the first lithium transition metal oxide particles was extracted. The average form factor value for each positive electrode is shown in Table 2 below.

[0142] [Table 2]

[0143] As shown in Table 2, when the positive electrode active materials prepared in Examples 1 to 3 were used, the average form factor was relatively large compared to the comparative example, which indicates that the positive electrode active material particles were less likely to crack when a positive electrode was prepared by applying pressure.

[0144] Experimental Example 2 Lithium secondary batteries were produced using positive electrodes containing the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 4 produced in Experimental Example 1, and the life characteristics of each lithium secondary battery were evaluated.

[0145] Here, a lithium metal disk was used as the negative electrode, and a separator was interposed between the positive and negative electrodes to prepare an electrode assembly, which was then placed inside a battery case, and an electrolyte was injected into the case to prepare a lithium secondary battery. Here, the electrolyte was 1M LiPF in an organic solvent in which ethylene carbonate: ethyl methyl carbonate: diethyl carbonate were mixed in a volume ratio of 3:3:4. 6 The electrolyte used was a solution of

[0146] Next, each of the manufactured lithium secondary batteries was charged in CC / CV mode at a constant current of 0.1 C at 25° C. up to 4.25 V (CV 0.05 C), and then discharged in CC mode down to 3.0 V, and the first initial charge capacity and discharge capacity were measured.

[0147] In addition, 30 charge / discharge cycles were performed at 45°C, in the range of 3.0 to 4.25 V, and at a constant current of 0.33 C, and the capacity retention rate (discharge capacity at 30 cycles / initial discharge capacity x 100) and resistance increase rate (resistance at 30 cycles / initial resistance x 100) of each lithium secondary battery at 30 cycles were measured. The measurement results are shown in Table 3 below.

[0148] [Table 3]

[0149] Referring to Table 3, when the positive electrode active materials prepared in Examples 1 to 3 are applied to a battery, not only is the initial charge / discharge capacity excellent, but the capacity retention rate of the battery is high and the resistance increase rate is low, so that it is possible to confirm that the battery also has excellent life characteristics.

[0150] On the other hand, referring to Figures 4 to 7 and Table 3, in the cases of Comparative Examples 1, 2, and 4, in which the difference in particle strength between the large particle size lithium transition metal oxide and the small particle size lithium transition metal oxide exceeds 10 MPa, a problem occurs in which the small particle size lithium transition metal oxide particles crack, and in the cases of Comparative Examples 2 and 3, in which the particle strength of the large particle size lithium transition metal oxide is less than 140 MPa, a problem occurs in which the large particle size lithium transition metal oxide particles crack, and it can be seen that the life characteristics are reduced due to side reactions.

[0151] As a result, it has been confirmed that when two types of lithium transition metal oxides having different average particle sizes are mixed and used as in the present invention, in which the particle strength of the large particle size lithium transition metal oxide is 140 MPa or more and the difference in particle strength between the large particle size lithium transition metal oxide and the small particle size lithium transition metal oxide is 10 MPa or less, a battery having particularly high energy volume density and suppressing particle cracks can be realized, minimizing the occurrence of side reactions caused by particle cracks of the lithium transition metal oxide, and thus realizing a battery having excellent capacity characteristics and life characteristics.

Claims

1. A first lithium transition metal oxide and an average particle size (D ５０ ) a second lithium transition metal oxide having a small The particle strength of the first lithium transition metal oxide is 140 MPa or more; a difference in particle strength between the first lithium transition metal oxide and the second lithium transition metal oxide is 10 MPa or less; the first lithium transition metal oxide and the second lithium transition metal oxide are each independently a lithium transition metal oxide in which a molar ratio of nickel among transition metals is 60% or more.

2. The positive electrode active material of claim 1 , wherein the second lithium transition metal oxide has a particle strength equal to or smaller than that of the first lithium transition metal oxide.

3. The positive electrode active material according to claim 1 , wherein the second lithium transition metal oxide has a particle strength of 130 MPa or more and 190 MPa or less.

4. 10. The cathode active material of claim 1, wherein the cathode active material has a bimodal particle size distribution.

5. The average particle size (D ５０ 2. The positive electrode active material according to claim 1, wherein the thickness of the first electrode is 6 μm to 25 μm.

6. The average particle size (D ５０ 2. The positive electrode active material according to claim 1, wherein the thickness of the first electrode is 1 μm to 12 μm.

7. The positive electrode active material according to claim 1 , wherein the first lithium transition metal oxide and the second lithium transition metal oxide each independently have a composition represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ Ni ｘ Co ｙ M １ ｚ M ２ ｗ O ２ In the above Chemical Formula 1, 0≦a≦0.30, 0.60≦x<1.0, 0<y<0.40, 0<z<0.40, and 0≦w≦0.10, M １ is one or more selected from Mn and Al, M ２ is one or more selected from W, Mo, Cr, Zr, Ti, Mg, Ta and Nb.

8. 2. The positive electrode active material of claim 1, wherein the first lithium transition metal oxide and the second lithium transition metal oxide are contained in a weight ratio of 60:40 to 90:

10.

9. A positive electrode active material layer comprising the positive electrode active material according to claim 1, A positive electrode, wherein the first lithium transition metal oxide contained in the positive electrode active material layer satisfies the following formula 1: [Formula 1] [0010] In the above formula 1, A and P are the cross-sectional area and perimeter, respectively, of a particle of the first lithium transition metal oxide.

10. The positive electrode according to claim 9, wherein the positive electrode active material layer has a porosity of 15% by volume to 30% by volume.

11. A lithium secondary battery comprising the positive electrode according to claim 9.

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