Positive electrode optimized for improved high-temperature life characteristics and secondary battery including the same

Abstract

To provide a positive electrode in which a positive electrode mixture containing a positive electrode active material is formed on a positive electrode current collector, and a secondary battery comprising the same.SOLUTION: A positive electrode active material includes a lithium transition metal oxide powder represented by chemical formula 1, LiaNixCoyMzO2-wAw (Formula 1), M being at least one selected from a group consisting of Mn, Ti, Mg, Al, Zr, Mn and Ni, A being an oxygen-substituted halogen, and 1.00≤a≤1.05, 0.1≤x≤0.8, 0.1≤y≤0.8, 0.01≤z≤0.4, and 0≤w≤0.001. The lithium transition metal oxide powder has large particles which are secondary particles having an average particle diameter (D50) of 7 μm to 17 μm, and small particles which are single particles having average particle diameter (D50) of 2 μm to 7 μm, weight ratio of large particles to small particles is 5:5 to 9:1, and the positive electrode mixture has a porosity of 22% to 35%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0170943, filed December 19, 2019, and Korean Patent Application No. 10-2020-0131094, filed October 12, 2020, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a positive electrode optimized for improving high-temperature life characteristics and a secondary battery including the same. [Background technology]

[0003] The rapid increase in fossil fuel use has led to an increasing demand for alternative and clean energy, and one of the most actively researched fields in this area is electrochemical power generation and storage.

[0004] Currently, secondary batteries are a typical example of electrochemical elements that use electrochemical energy, and the range of their use is tending to expand more and more.

[0005] Recently, with the technological development and increasing demand for portable devices such as portable computers, portable phones, and cameras, the demand for secondary batteries as energy sources has increased dramatically. Among such secondary batteries, much research has been conducted on lithium secondary batteries, which exhibit high energy density and working potential, long cycle life, and low self-discharge rate, and they have also been commercialized and are widely used.

[0006] Furthermore, with growing concern about environmental issues, much research is being conducted into electric vehicles and hybrid electric vehicles that can replace vehicles that use fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution. Nickel-metal hydride secondary batteries are mainly used as the power source for such electric vehicles and hybrid electric vehicles, but research into using lithium secondary batteries, which have high energy density and discharge voltage, is also actively underway, and some of them are already at the commercialization stage.

[0007] Currently, lithium-containing cobalt oxide (LiCoO2) is mainly used as the positive electrode material for lithium secondary batteries, but other lithium-containing manganese oxides such as LiMnO2 with a layered crystal structure and LiMn2O4 with a spinel crystal structure, as well as lithium-containing nickel oxide (LiNiO2) are also being considered.

[0008] Among the cathode active materials, LiCoO2 is the most widely used due to its excellent lifespan characteristics and charge / discharge efficiency. However, it has the disadvantages of reduced structural stability and being expensive due to limited cobalt resources used as a raw material, limiting its price competitiveness. This limits its mass use as a power source in fields such as electric vehicles.

[0009] LiNiO2-based positive electrode active materials are relatively inexpensive and exhibit high discharge capacity battery characteristics, but they have the problem of a sudden phase transition in the crystal structure due to volume changes that occur with charge / discharge cycles, and a sudden decrease in safety when exposed to air and moisture.

[0010] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of being excellent in thermal safety and inexpensive, but have problems such as small capacity, poor cycle characteristics, and poor high-temperature characteristics.

[0011] Therefore, recently, ternary or quaternary lithium transition metal oxides containing three or more transition metals have been developed by substituting a portion of the LiNiO2 with Co, Mn, Al, or the like.

[0012] Such ternary or quaternary lithium transition metal oxides are generally used by converting primary particles into secondary particles.

[0013] However, such secondary particles of lithium transition metal oxides are prone to particle cracking during the electrode rolling process, which increases the specific surface area of ​​the active material and severely reduces storage and life performance at high temperatures. Such particle cracking of secondary particles is more prevalent in small particles with an average particle size (D50) of approximately 5 μm.

[0014] Therefore, there is a strong need to develop a positive electrode that can minimize particle cracking during rolling of the positive electrode containing the lithium transition metal oxide as an active material, thereby improving the life characteristics at high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0015] The present invention aims to improve the high-temperature life characteristics of a secondary battery including a positive electrode active material by optimizing the composition, mixing conditions, and rolling conditions of the positive electrode active material to minimize particle cracking of the positive electrode active material. [Means for solving the problem]

[0016] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0017] Hereinafter, a cathode active material according to an embodiment of the present invention, a method for manufacturing the same, and a secondary battery including the same will be described.

[0018] According to one embodiment of the present invention, there is provided a positive electrode in which a positive electrode mixture containing a positive electrode active material is formed on a positive electrode current collector, The positive electrode active material includes a lithium transition metal oxide powder represented by the following chemical formula 1: Li a Ni x Co y M z O 2-w A w (chemical formula 1) M is at least one selected from the group consisting of Mn, Ti, Mg, Al, Zr, Mn, and Ni; A is an oxygen-substituted halogen; 1.00≦a≦1.05, 0.1≦x≦0.8, 0.1≦y≦0.8, 0.01≦z≦0.4, and 0≦w≦0.001; The lithium transition metal oxide powder is The primary particles are aggregated into secondary particles, and the average particle size (D50) of the secondary particles is large particles of 7 μm to 17 μm. It is composed of small particles with an average particle size (D50) of 2 μm to 7 μm. The mixing ratio of the large particles to the small particles is 5:5 to 9:1 by weight, The positive electrode mixture has a porosity of 22% to 35%.

[0019] Specifically, the large particles may have an average particle size (D50) of 9 μm to 11 μm, and the small particles may have an average particle size (D50) of 4 μm to 6 μm.

[0020] More specifically, the mixing ratio of the large particles to the small particles may be 6:4 to 8:2 by weight.

[0021] More specifically, the porosity of the positive electrode mixture may be 24% to 30%.

[0022] Furthermore, the electrode density of the positive electrode may be 3.0 g / cc to 3.6 g / cc, and more specifically, 3.2 g / cc to 3.45 g / cc.

[0023] Meanwhile, in the lithium transition metal oxide powder of Formula 1 contained as the positive electrode active material, M is Mn b Al c where 0≦b≦1 and 0≦c≦1.

[0024] The positive electrode mixture may further include a binder and a conductive material in addition to the positive electrode active material. In this case, the binder is included in an amount of 1 to 5 wt % based on the total weight of the positive electrode mixture, and the conductive material is included in an amount of 0.5 to 5 wt % based on the total weight of the positive electrode mixture.

[0025] According to another embodiment of the present invention, the positive electrode; negative electrode; and The secondary battery includes an electrode assembly including a separator interposed between the positive electrode and the negative electrode, and the electrode assembly is impregnated with an electrolyte and housed in a battery case. [Effects of the Invention]

[0026] The positive electrode according to the present invention includes a positive electrode active material in which small particles of a lithium transition metal oxide powder are broken down into single particles, and by configuring the porosity of the positive electrode mixture within an optimal range by adjusting the mixing ratio of large particles to small particles and the rolling conditions, it is possible to improve the high-temperature life characteristics of a secondary battery containing the positive electrode. DETAILED DESCRIPTION OF THE INVENTION

[0027] The positive electrode and secondary battery according to the present invention will be described in more detail below.

[0028] According to one embodiment of the present invention, A positive electrode in which a positive electrode mixture containing a positive electrode active material is formed on a positive electrode current collector, The positive electrode active material includes a lithium transition metal oxide powder represented by the following chemical formula 1: Li a Ni x Co y M z O 2-w A w (chemical formula 1) M is at least one selected from the group consisting of Mn, Ti, Mg, Al, Zr, Mn, and Ni; A is an oxygen-substituted halogen; 1.00≦a≦1.05, 0.1≦x≦0.8, 0.1≦y≦0.8, 0.01≦z≦0.4, and 0≦w≦0.001; The lithium transition metal oxide powder is The primary particles are aggregated into secondary particles, and the average particle size (D50) of the secondary particles is large particles of 7 μm to 17 μm. It is composed of small particles with an average particle size (D50) of 2 μm to 7 μm. The mixing ratio of the large particles to the small particles is 5:5 to 9:1 by weight, The positive electrode mixture has a porosity of 22% to 35%.

[0029] As described above, the lithium transition metal oxide represented by Chemical Formula 1 is generally used as a positive electrode active material in the form of secondary particles.

[0030] In addition, large particles with an average particle size (D50) of approximately 11 μm and small particles with an average particle size (D50) of approximately 5 μm are mixed together, but there was a problem that the secondary particles of the lithium transition metal oxide powder would crack during the electrode rolling process.

[0031] In particular, most of the particle cracks occurred in small particles, while some of the large particles cracked.

[0032] To solve this problem, the inventors of the present application have confirmed that particle cracking can be reduced by monolithicizing small particles in the lithium transition metal oxide powder. However, they have also found that mixing secondary large particles with monolithic small particles can increase cracking in the secondary large particles and result in a greater reduction in lifespan than when existing secondary large particles and small particles are mixed, depending on the mixing conditions and rolling conditions of the large and small particles. They have found an optimized mixing ratio of large particles to small particles, a porosity due to rolling, and ultimately an optimized electrode density that do not increase cracking in the secondary large particles compared to existing configurations, and have completed the present invention.

[0033] Therefore, in the positive electrode according to the present invention, the lithium transition metal oxide powder of Formula 1 contained as a positive electrode active material is composed of a mixture of large secondary particles and small monoparticulated particles.

[0034] Generally, the monolithic lithium transition metal oxide has an average particle size (D50) of about 5 μm. While it is easy to prepare small monolithic particles, it is not easy to prepare large monolithic particles because the heat treatment temperature and time are excessively increased. As the particle size of the monolithic particles increases, the charge / discharge efficiency and C-rate characteristics decrease rapidly. Therefore, it is preferable to use large secondary particles as the large particles.

[0035] In the present invention, the term "secondary particles" refers to a state in which tens to hundreds of primary particles are aggregated, and the term "single particles" refers to a state in which primary particles exist individually or are aggregated in less than 10 particles.

[0036] In this case, the diameter of the major axis of the primary particles constituting the secondary large particles may be, for example, 100 nm to 1000 nm, and the diameter of the major axis of the primary particles constituting the monoparticulated small particles may be, for example, 500 nm to 7000 nm.

[0037] The average particle size (D50) of the large secondary particles formed by agglomeration of tens to hundreds of such primary particles may be 7 μm to 17 μm, as defined above, and more specifically, 9 μm to 11 μm.

[0038] Furthermore, the average particle size (D50) of the small particles formed by aggregation of less than 10 primary particles or existing individually as the primary particles may be 2 μm to 7 μm, as defined above, and more specifically, 4 μm to 6 μm.

[0039] If the average particle size (D50) of the large particles and the average particle size (D50) of the small particles are outside the above range, i.e., if the average particle size of the large particles is too large, the low efficiency and low C-rate characteristics of the monolithic small particles may be exacerbated, resulting in reduced capacity and output. If the average particle size of the large particles is too small, the electrode rolling properties may be deteriorated, and in particular, the monolithic small particles may cause severe cracking of the large particles during rolling, resulting in reduced high-temperature life. On the other hand, if the average particle size of the monolithic small particles is too small, manufacturing becomes difficult due to reduced processability, and if it is too large, charge / discharge efficiency and C-rate characteristics may be reduced, which is undesirable.

[0040] Here, the average particle size (D50) refers to the diameter at 50% of the cumulative particle volume distribution by particle size. The average particle size (D50) can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in the diffraction pattern depending on the particle size is measured to calculate the particle size distribution. D50 can be measured by calculating the particle diameter at 50% of the cumulative particle volume distribution by particle size in the analyzer.

[0041] On the other hand, according to the present invention, as described above, when small particles that have been divided into single particles and large particles that have been divided into secondary particles are mixed, particle cracking of the large particles that have been divided into secondary particles may increase depending on the mixing conditions, the porosity due to rolling, and the electrode density. Therefore, it is extremely important to adjust the mixing ratio of the large particles to the small particles and the porosity due to rolling.

[0042] First, the large particles and the small particles can be prepared separately and then mixed together. In this case, the mixing ratio for achieving the effects of the present invention may be specifically 5:5 to 9:1, more specifically 6:4 to 8:2, and even more specifically 7:3.

[0043] If the content of small particles is too high outside the above range, the small particles have a higher particle strength, which may cause severe cracking of the large particles during electrode rolling, resulting in a shortened high-temperature life. If the content is too low, the filling rate of the small particles is low, resulting in a reduced electrode density, which is undesirable.

[0044] Second, in order to exert the effects of the present invention, the porosity of the positive electrode mixture may be adjusted by adjusting the rolling conditions to be 22% to 35%, specifically, 24% to 30%, and more specifically, 25% to 28%.

[0045] Here, the porosity is calculated as follows using the ratio of the electrode density to the true density of the entire solid content of the positive electrode mixture. (true density - electrode density) / true density x 100

[0046] If the porosity is outside the above range and is too small, this means that the rolling strength is high, which increases the problem of particle cracking of large secondary particles. If the porosity is too large, the rolling strength is low, which reduces the electrode density, which is undesirable in terms of energy density, and there is a problem of reduced output due to loss of interparticle conductive paths.

[0047] The most preferable range of electrode density for achieving the effects of the present invention is determined by optimizing the porosity by the mixing ratio of the large particles and the small particles and the rolling conditions, and the preferable range of electrode density for the positive electrode according to the present invention may be 3.0 g / cc to 3.6 g / cc, specifically 3.3 g / cc to 3.45 g / cc.

[0048] The electrode density can be calculated as the weight per unit volume of the electrode excluding the current collector.

[0049] If the electrode density is too low outside the above range, the energy per volume ratio decreases, which is undesirable. If the electrode density is too high, the particle cracking of the secondary large particles increases, which may result in a decrease in high-temperature life characteristics.

[0050] Meanwhile, the positive electrode active material may include a lithium transition metal oxide powder represented by the above Chemical Formula 1, and more specifically, in the above Chemical Formula 1, M is Mn b Al c where 0≦b≦1 and 0≦c≦1 may be satisfied. In other words, the lithium transition metal oxide powder contained as the positive electrode active material specifically contains one or more transition metals selected from the group consisting of Li-Ni-Co-Mn oxide containing Ni, Co, and Mn, Li-Ni-Co-Al oxide containing Ni, Co, and Al, and Li-Ni-Co-Mn-Al oxide containing Ni, Co, Mn, and Al.

[0051] The term "comprising a lithium transition metal oxide powder represented by Chemical Formula 1" may mean that the lithium transition metal oxide powder having one composition selected from the range of Chemical Formula 1 is contained alone, or that lithium transition metal oxide powders having various compositions within the range of Chemical Formula 1 are mixed.

[0052] Furthermore, the positive electrode active material may be LiNiO2, LiMnO2, LiMn2O2, LiCoO2, or the like in addition to the lithium transition metal oxide represented by Chemical Formula 1. 1-d Mn d O2, LiNi 1-dMn d O2 (0.2 < d < 1), Li(Ni a Co b Mn c )O4 (0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), LiMn 2-e Ni e O4, LiMn 2-e Co e O4 (0 < e < 2), LiCoPO4, or LiFePO4, etc., which are compounds conventionally known as cathode active materials, can be further included, and it is of course possible that any one or a mixture of two or more of these may be further included in a small amount.

[0053] However, the lithium transition metal oxide represented by Chemical Formula 1 is contained at least 60% by weight or more based on the total weight of the cathode active material.

[0054] In addition to such a cathode active material, the cathode binder can further include a binder and a conductive material, and optionally, a filler may be further included.

[0055] The conductive material is used to impart conductivity to the electrode, and in the battery being constructed, it can be used without special restrictions as long as it has electron conductivity without undergoing a chemical change. Specific examples include carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT); graphite such as natural graphite and artificial graphite; metal powders or metal fibers such as copper, nickel, aluminum, silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc. One kind alone or a mixture of two or more of these can be used. The conductive material is contained at 0.1% to 30% by weight, specifically 0.3% to 10% by weight, and more specifically 0.5% to 5% by weight based on the total weight of the cathode binder.

[0056] The binder improves adhesion between positive electrode active material particles and 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, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder is included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 10 wt%, and more specifically 1 wt% to 5 wt% of the total weight of the positive electrode mixture.

[0057] The filler is selectively used as a component to suppress expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material that does not induce chemical changes in the battery, and examples of such fibrous materials include olefin polymers such as polyethylene and polypropylene, glass fiber, carbon fiber, etc. In this case, the filler is added in an amount of 0.1 to 3 wt % based on the total weight of the positive electrode mixture.

[0058] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. 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. The positive electrode current collector can have a thickness of 3 μm to 500 μm, and the surface of the current collector can be formed with fine irregularities to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0059] According to another embodiment of the present invention, the positive electrode; negative electrode; and The secondary battery includes an electrode assembly including a separator interposed between the positive electrode and the negative electrode, and the electrode assembly is impregnated with an electrolyte and housed in a battery case.

[0060] In particular, the secondary battery may be a lithium secondary battery.

[0061] Like the positive electrode, the negative electrode can have a structure in which a negative electrode mixture containing a negative electrode active material is formed on a negative electrode current collector, and the negative electrode mixture can also contain, in addition to the negative electrode active material, the above-mentioned conductive material and binder, and, if necessary, a filler.

[0062] The negative electrode active material may be a compound capable of reversibly inserting and extracting lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. x(0 < x < 2), metal oxides capable of doping and undoping lithium such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compound and the carbonaceous material such as Si-C composites or Sn-C composites, etc. may be mentioned, and any one or a mixture of two or more of these can be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Note that all carbon materials such as low-crystalline carbon and high-crystalline carbon can be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0063] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used. Further, the negative electrode current collector can usually have a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the binding force of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven bodies, etc.

[0064] 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 with low resistance to electrolyte ion movement and excellent electrolyte humidification capacity is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin 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 can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, a separator coated with a ceramic component or a polymer material to ensure heat resistance or mechanical strength can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0065] Furthermore, examples of the electrolytic solution 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.

[0066] Specifically, the electrolyte solution may include an organic solvent and a lithium salt.

[0067] The organic solvent may be any solvent that functions 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; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that 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 mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred, as they have high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries. In this case, mixing the cyclic carbonate and the linear carbonate at a volume ratio of about 1:1 to about 1:9 can result in superior electrolyte performance.

[0068] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt concentration is preferably in the range of 0.1M to 2.0M. When the lithium salt concentration falls within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0069] In addition to the above components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds (e.g., 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, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives are included in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0070] As described above, the secondary battery according to the present invention can be used as a device power source in portable devices such as mobile phones, notebook computers, and digital cameras, and in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0071] Although the present invention may be embodied in various different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention is not limited to the embodiments set forth herein.

[0072] <Production Example 1> (Secondary large particles) Nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a 70:10:20 molar ratio in a 5-L batch reactor set at 60°C to prepare a 2M metal salt solution. The containers containing the metal salts were connected to the reactor, and a 4M NaOH solution and a 7% NH4OH solution were prepared and connected to the reactor, respectively. After adding 3 L of deionized water to a 5-L coprecipitation reactor, nitrogen gas was purged into the reactor at a rate of 2 L / min to remove dissolved oxygen in the water and create a non-oxidizing atmosphere inside the reactor. After adding 100 mL of 4M NaOH, the reactor was maintained at a pH of 12.0 at 60°C with a stirring speed of 1200 rpm. The metal salt solution, NaOH solution, and NH4OH solution were added at rates of 180 mL / hr, 180 mL / hr, and 10 mL / hr, respectively, for 10 hours to conduct the coprecipitation reaction. Ni 0.7 Co 0.1 Mn 0.2 A nickel-cobalt-manganese composite oxyhydroxide (OH)2 with an average particle size (D50): 11 μm was synthesized.

[0073] The resulting nickel-cobalt-manganese composite metal hydroxide particles were mixed with lithium hydroxide as a lithium source material in a molar ratio of 1:1.07, and then heat-treated in air at 860°C for 15 hours to form large-particle LiNi 0.7 Co 0.1 Mn 0.2 O2 powder (average particle size (D50): 11 μm) was obtained.

[0074] In SEM observation, the large particle positive electrode powder was found to be composed of several tens of primary particles agglomerated to form secondary particles, which were spherical or elliptical in shape.

[0075] <Production Example 2> (Small particles made into single particles) Ni was prepared in the same manner as in Production Example 1, except that the coprecipitation reaction time was shortened to 3 hours. 0.7 Co 0.1 Mn 0.2 A nickel-cobalt-manganese composite oxyhydroxide (OH)2 with an average particle size (D50): 4.5 μm was synthesized.

[0076] The resulting nickel-cobalt-manganese composite metal hydroxide particles were mixed with lithium hydroxide as a lithium source material in a 1:1 molar ratio, and then heat-treated in air at 950°C for 15 hours to form small particles of LiNi 0.7 Co 0.1 Mn 0.2 O2 powder (average particle size (D50): 5 μm) was obtained.

[0077] In SEM observation, the small particle cathode powder was found to have primary particles that existed individually or aggregated together in groups of less than 10, and was spherical or elliptical in shape.

[0078] <Production Example 3> (Secondary small particles) Ni was prepared in the same manner as in Production Example 1, except that the coprecipitation reaction time was shortened to 3 hours. 0.7 Co 0.1 Mn 0.2 A nickel-cobalt-manganese composite oxyhydroxide (OH)2 with an average particle size (D50): 4.5 μm was synthesized.

[0079] The resulting nickel-cobalt-manganese composite metal hydroxide particles were mixed with lithium hydroxide as a lithium source material in a molar ratio of 1:1.7, and then heat-treated in air at 840°C for 15 hours to form large-particle LiNi 0.7 Co 0.1 Mn 0.2 O2 powder (average particle size (D50): 4.5 μm) was obtained.

[0080] In SEM observation, the small particle positive electrode powder was found to be composed of several tens of primary particles agglomerated to form secondary particles, which were spherical or elliptical in shape.

[0081] Example 1 The large particle positive electrode powder prepared in Preparation Example 1 and the small particle positive electrode powder prepared in Preparation Example 2 were mixed in a weight ratio of 7:3 to form a positive electrode active material, with PVdF as the binder and carbon black as the conductive material. The positive electrode active material, binder, and conductive material were thoroughly mixed in NMP at a weight ratio of 96:2:2 to form a slurry (viscosity: 5000 mPa s). This slurry was applied to a 20 μm thick Al current collector, dried at 130°C, and rolled at a pressure of 1.7 ton / cm to form a 74.5 μm thick positive electrode.

[0082] <Example 2> A positive electrode was manufactured in the same manner as in Example 1, except that the large particle positive electrode powder manufactured in Preparation Example 1 and the small particle positive electrode powder manufactured in Preparation Example 2 were mixed in a weight ratio of 5:5 to use as a positive electrode active material.

[0083] Example 3 A positive electrode was prepared in the same manner as in Example 1, except that the large particle positive electrode powder prepared in Preparation Example 1 and the small particle positive electrode powder prepared in Preparation Example 2 were mixed in a weight ratio of 8:2 to form a positive electrode active material.

[0084] Example 4 A positive electrode was produced in the same manner as in Example 1, except that the pressure during rolling was 2.1 ton / cm and the thickness of the electrode was 73.1 μm.

[0085] <Example 5> A positive electrode was produced in the same manner as in Example 1, except that the pressure during rolling was 1.1 ton / cm and the thickness of the electrode was 76.8 μm.

[0086] Example 6 A positive electrode was manufactured in the same manner as in Example 1, except that the pressure during rolling was 0.7 ton / cm to a thickness of 78.4 μm.

[0087] <Comparative Example 1> A positive electrode was manufactured in the same manner as in Example 1, except that the large particle positive electrode powder manufactured in Preparation Example 1 and the small particle positive electrode powder manufactured in Preparation Example 3 were mixed in a weight ratio of 7:3.

[0088] <Comparative Example 2> A positive electrode was manufactured in the same manner as in Example 1, except that the positive electrode powder of large particles manufactured in Preparation Example 1 and the positive electrode powder of small particles manufactured in Preparation Example 2 were mixed in a weight ratio of 4:6 to use as a positive electrode active material.

[0089] <Comparative Example 3> A positive electrode was manufactured in the same manner as in Example 1, except that the pressure during rolling was 2.5 ton / cm to a thickness of 71.8 μm.

[0090] <Comparative Example 4> A positive electrode was manufactured in the same manner as in Example 1, except that the pressure during rolling was 0.2 ton / cm to a thickness of 82.8 μm.

[0091] <Experimental Example 1> The porosity of the positive electrode mixture and the electrode density of the positive electrodes produced in Examples 1 to 6 and Comparative Examples 1 to 4 were determined as follows. The results are shown in Table 1.

[0092] The porosity was calculated as follows using the ratio of the electrode density to the true density of the entire solid content of the positive electrode mixture. (true density - electrode density) / true density x 100

[0093] The electrode density was calculated as the weight per unit volume excluding the current collector of the electrode.

[0094] [Table 1]

[0095] <Experimental Example 2> A half coin cell was fabricated using the positive electrode prepared in Examples 1 to 6 and Comparative Examples 1 to 4 and lithium metal as the negative electrode, and an electrolyte containing 1M LiPF6 in a solvent of EC:DMC:DEC=1:2:1.

[0096] The half coin cell thus prepared was charged at 25°C at a constant current (CC) of 0.1 C to 4.25 V, and then charged at a constant voltage (CV) of 4.25 V for the first charge until the charge current reached 0.05 mAh. After leaving it for 20 minutes, it was discharged at a constant current of 0.1 C to 3.0 V, and the discharge capacity of the first cycle was measured.

[0097] At 45°C, the battery was charged at 0.5C up to 4.25V under constant current / constant voltage (CC / CV) conditions, and then charged to 0.05mAh. After that, the battery was discharged at 1C down to 3.0V under constant current (CC) conditions, which constituted one cycle. This cycle was repeated up to 50 times, and the value calculated as (capacity after 50 cycles / capacity after 1 cycle) x 100 was taken as the high-temperature life retention rate (%), and the results are shown in Table 2.

[0098] [Table 2]

[0099] By referring to Table 2 together with Table 1, it can be seen that when a positive electrode satisfying the conditions of the present invention is used, particle cracking is reduced and high-temperature life characteristics are excellent.

Claims

1. A positive electrode in which a positive electrode mixture containing a positive electrode active material is formed on a positive electrode current collector, The positive electrode active material includes a lithium transition metal oxide powder represented by the following Chemical Formula 1: Li ａ Ni ｘ Co ｙ M ｚ O ２－ｗ A ｗ (Chemical formula 1) M is at least one selected from the group consisting of Mn, Ti, Mg, Al, and Zr; A is an oxygen-substituted halogen; 1.00≦a≦1.05, 0.1≦x≦0.8, 0.1≦y≦0.8, 0.01≦z≦0.4, and 0≦w≦0.001; The lithium transition metal oxide powder is The primary particles are aggregated to form secondary particles, which are large particles with an average particle size (D50) of 7 μm to 17 μm. It is composed of small particles having an average particle size (D50) of 2 μm to 6 μm, The small particles are formed into a single particle, and have a form in which primary particles having a major axis diameter of 500 nm or more exist individually or are aggregated with less than 10 particles, The mixing ratio of the large particles to the small particles is 5:5 to 9:1 by weight, The electrode density of the positive electrode is 3.2 g / cc to 3.45 g / cc.

2. 2. The positive electrode according to claim 1, wherein the large particles have an average particle size (D50) of 9 μm to 11 μm.

3. 2. The positive electrode according to claim 1, wherein the mixing ratio of the large particles to the small particles is 6:4 to 8:2 by weight.

4. The positive electrode according to claim 1, wherein the positive electrode mixture has a porosity of 22% to 30%.

5. The M is Mn ｂ Al ｃ 2. The positive electrode of claim 1, wherein 0≦b≦1 and 0≦c≦1.

6. The positive electrode according to claim 1 , wherein the positive electrode mixture further comprises a binder and a conductive material.

7. The positive electrode according to claim 6, wherein the binder is contained in an amount of 1 to 5 wt % based on the total weight of the positive electrode mixture, and the conductive material is contained in an amount of 0.5 to 5 wt % based on the total weight of the positive electrode mixture.

8. The positive electrode according to claim 1 . a negative electrode, and The secondary battery includes an electrode assembly including a separator interposed between the positive electrode and the negative electrode, the electrode assembly being impregnated with an electrolyte and housed in a battery case.

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