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

Abstract

A positive electrode for a lithium secondary battery according to an exemplary embodiment includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer contains positive electrode active material particles. In a three-dimensional model representing a distribution of the positive electrode active material particles, a ratio of a total area of contact surfaces between the positive electrode active material particles to a total number of the contact surfaces between the positive electrode active material particles satisfies a prescribed range. The positive electrode active material particles can contain lithium metal oxide particles containing nickel and having a molar fraction of cobalt among all elements other than lithium and oxygen of 0.02 or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. In more detail, the present application relates to a positive electrode for a lithium secondary battery including a nickel-based positive electrode active material and a lithium secondary battery including the same. BACKGROUND

[0002] A secondary battery is a battery that can be repeatedly charged and discharged, and is widely used as a power source for portable electronic devices such as mobile phones, notebook computers, etc.

[0003] A lithium secondary battery has a high operating voltage and energy density per unit weight, and is advantageous in terms of charging speed and light weight, and thus is actively developed and applied.

[0004] For example, a lithium secondary battery can include an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode; and an electrolyte impregnating the electrode assembly.

[0005] The positive electrode can include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode active material layer can include a lithium metal oxide as a positive electrode active material.

[0006] For example, as the positive electrode active material, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2, LiMn2O4, etc.), lithium iron phosphate compound (LiFePO4), NCM-based lithium metal oxide containing nickel, cobalt, and manganese, NCA-based lithium metal oxide containing nickel, cobalt, and aluminum, etc. are used.

[0007] In addition, the price of cobalt is relatively high compared to other metals such as nickel, manganese, etc. Therefore, it is not advantageous in terms of cost to use lithium metal oxide particles containing a high concentration of cobalt in large machines such as electric vehicles.

[0008] However, when the lithium metal oxide does not contain cobalt, the crystal structure can not be stable, and thus the operating reliability and electrochemical stability of the lithium secondary battery can be reduced.

[0009] For example, Korean Patent Laid-Open No. 10-2020-0085679 discloses a lithium secondary battery that improves life characteristics, etc. by using lithium metal oxide particles containing only a trace amount of cobalt in a surface portion. SUMMARY

[0010] Technical problem to be solved

[0011] One technical problem of the present application is to provide a positive electrode for a lithium secondary battery having improved operating reliability and electrochemical stability.

[0012] One technical problem of the present application is to provide a lithium secondary battery having improved operational reliability and electrochemical stability.

[0013] Technical Solution

[0014] A positive electrode for a lithium secondary battery according to an exemplary embodiment can include: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer contains positive electrode active material particles and satisfies Formula 1.

[0015] The positive electrode active material particles can contain lithium metal oxide particles having a molar fraction of cobalt among all elements other than lithium and oxygen in the lithium metal oxide particles is 0.02 or less.

[0016] [Formula 1]

[0017] Ca / Cn≤6.44 μm 2

[0018] In Formula 1, Cn is a total number of contact surfaces formed by contact between the positive electrode active material particles in a three-dimensional model representing a distribution of the positive electrode active material particles in the positive electrode active material layer, and Ca is a total area of the contact surfaces.

[0019] In one embodiment, Ca / Cn can be 4.5-6.44 μm 2 .

[0020] In one embodiment, the lithium metal oxide particles can contain nickel and manganese, and can not contain cobalt.

[0021] In one embodiment, among all elements other than lithium and oxygen in the lithium metal oxide particles, a molar fraction of nickel can be 0.7 to 0.85.

[0022] In one embodiment, the lithium metal oxide particles can include: first lithium metal oxide particles in a form of secondary particles in which a plurality of primary particles are aggregated; and second lithium metal oxide particles in a form of single particles.

[0023] In one embodiment, a ratio of a content of the second lithium metal oxide particles to a content of the first lithium metal oxide particles in a total weight of the positive electrode active material layer can be 1 / 3 to 3.

[0024] In one embodiment, the lithium metal oxide particles can include first lithium metal oxide particles and second lithium metal oxide particles having a particle diameter (D 50 ) smaller than the first lithium metal oxide particles.

[0025] In one embodiment, the particle size (D 50 ) of the first lithium metal oxide particles can be 9 to 18 μm, and the particle size (D 50 ) of the second lithium metal oxide particles can be 2 to 7 μm.

[0026] In one embodiment, the positive electrode active material layer can further include a conductive material, and the content of the lithium metal oxide particles can be 80 to 99 wt% in the total weight of the positive electrode active material layer.

[0027] In one embodiment, the conductive material can include a point-type conductive material having an aspect ratio of 0.5 to 1.5, and a line-type conductive material having an aspect ratio of 2 or more.

[0028] In one embodiment, the particle size (D 50 ) of the point-type conductive material can be 10 to 60 nm, and the length of the line-type conductive material can be 15 to 65 μm.

[0029] In one embodiment, the ratio of the content of the point-type conductive material to the content of the line-type conductive material can be 0.6 to 0.8 in the total weight of the positive electrode active material layer.

[0030] In one embodiment, the ratio of the content of the point-type conductive material to the content of the line-type conductive material can be 1.8 to 3.5 in the total weight of the positive electrode active material layer.

[0031] In one embodiment, the density of the positive electrode active material layer can be 3.4 to 3.7 g / cm3.

[0032] A lithium secondary battery according to an exemplary embodiment can include the positive electrode for a lithium secondary battery and a negative electrode disposed opposite the positive electrode.

[0033] Advantageous Effects

[0034] According to an exemplary embodiment of the present application, a positive electrode for a lithium secondary battery using lithium metal oxide particles containing a low content of cobalt and / or lithium metal oxide particles containing no cobalt as a positive electrode active material can be provided. The positive electrode can exhibit a low bulk resistance by satisfying Equation 1 even if the lithium metal oxide particles are used.

[0035] A lithium secondary battery according to an exemplary embodiment of the present application includes the positive electrode, and thus can exhibit improved resistance and power characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0037] FIG. 2 and FIG. 3 are a plan view and a cross-sectional view of a lithium secondary battery according to an exemplary embodiment, respectively. DETAILED DESCRIPTION

[0038] According to an exemplary embodiment of the present application, a positive electrode for a lithium secondary battery including lithium metal oxide particles can be provided.

[0039] According to an exemplary embodiment of the present application, a lithium secondary battery including the positive electrode can be provided.

[0040] Hereinafter, a positive electrode for a lithium secondary battery and a lithium secondary battery according to an exemplary embodiment of the present application will be described in greater detail with reference to the accompanying drawings. However, the accompanying drawings and the embodiments are merely illustrative, and the present application is not limited to the accompanying drawings and the embodiments.

[0041] Positive electrode for lithium secondary battery

[0042] FIG. 1 is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0043] Referring to FIG. 1 , the positive electrode for a lithium secondary battery 100 can include a positive electrode current collector 105 and a positive electrode active material layer 110 formed on the positive electrode current collector 105.

[0044] For example, the positive electrode active material layer 110 can be formed on one side or both sides of the positive electrode current collector 105.

[0045] For example, the positive electrode current collector 105 can include stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0046] The positive electrode active material layer 110 can include positive electrode active material particles in which lithium ions are reversibly intercalated and deintercalated. The positive electrode active material layer 110 can include a plurality of positive electrode active material particles.

[0047] In an exemplary embodiment, the positive electrode active material particles can include lithium metal oxide particles in which the mole fraction of cobalt (Co) among all elements except lithium (Li) and oxygen (O) is 0.02 or less.

[0048] In one embodiment, the mole fraction of Co in the lithium metal oxide particles can be 0.015 or less, and preferably can be 0.01 or less.

[0049] In one embodiment, the lithium metal oxide particles can be substantially free of Co. For example, Co can not be detectable when the composition of the lithium metal oxide particles is analyzed using ICP.

[0050] In an exemplary embodiment, the lithium metal oxide particles can contain nickel (Ni). In one embodiment, the lithium metal oxide particles can contain Ni and manganese (Mn).

[0051] In one embodiment, the mole fraction of Ni among all elements other than Li and O in the lithium metal oxide particles can be 0.6 to 0.9, 0.65 to 0.85, or 0.7 to 0.85.

[0052] In one embodiment, the mole fraction of Mn among all elements other than Li and O in the lithium metal oxide particles can be 0.05 to 0.3, 0.1 to 0.3, or 0.15 to 0.3.

[0053] In one embodiment, the lithium metal oxide particles can include a crystal structure or a chemical structure represented by the following Chemical Formula 1.

[0054] [Chemical Formula 1]

[0055] Li a Ni x Mn y Co z M 1-x-y-z O2

[0056] In Chemical Formula 1, M can include at least one of Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and can be 0.9≤a≤1.2, 0.6≤x≤0.9, 0≤z≤0.02, 0.8≤x+y+z≤1.

[0057] In some embodiments, it can be 0.9≤a<1.1.

[0058] In some embodiments, it can be 0.65≤x≤0.85 or 0.7≤x≤0.85.

[0059] In some embodiments, it can be z≤0.015 or z≤0.01.

[0060] In some embodiments, it can be z>0 or z≥0.001.

[0061] In some embodiments, it can be z=0.

[0062] The lithium metal oxide particles contain a low content of Co or do not contain Co, so that manufacturing costs can be reduced. However, the electrical conductivity and the crystal structure stability of the lithium metal oxide particles can be reduced. Therefore, the resistance of the positive electrode and the secondary battery using the lithium metal oxide particles can increase, so that the power can be reduced.

[0063] However, the positive electrode active material layer 110 according to the exemplary embodiment of the present application can satisfy Equation 1 below, and can improve the above-mentioned problems due to the reduction of the Co content.

[0064] [Equation 1]

[0065] Ca / Cn≤6.44 μm 2

[0066] In Equation 1, Cn is the total number of contact surfaces between the positive electrode active material particles in a three-dimensional model representing the distribution of the positive electrode active material particles in the positive electrode active material layer, and Ca is the total area of the contact surfaces. Ca / Cn can refer to the size of the average contact area of each of the contact surfaces.

[0067] According to the exemplary embodiment, a three-dimensional model of a positive electrode can be obtained by using XRM. By analyzing the three-dimensional model using Geodict S / W, the values of Cn and Ca can be automatically calculated (see Evaluation Example 1)

[0068] Under the condition of Equation 1, even if the lithium metal oxide particles are used, a lithium secondary battery can have improved resistance characteristics.

[0069] Under the condition that Equation 1 is not satisfied, the resistance of the positive electrode can increase. In addition, in the case where the lithium secondary battery operates at a high temperature, the internal resistance (DCIR) of the lithium secondary battery can be greatly increased.

[0070] When using lithium metal oxide particles having a Co mole fraction (based on all elements except Li and O) exceeding 0.02 (for example, NCM 811), the difference in effect according to the condition that Equation 1 is satisfied can be insignificant.

[0071] In one embodiment, Ca / Cn can be 3 μm 2 4 μm 2 4.5 μm 2 5 μm 2 5.2 μm 2 or more. In this case, the power characteristics and the rapid charging characteristics of the lithium secondary battery can be improved.

[0072] For example, the Ca / Cn value can be adjusted by various factors such as the particle diameter and content of the positive electrode active material particles in the positive electrode active material layer 110; the morphology of the positive electrode active material particles; the mixing ratio between a plurality of positive electrode active material particles having different physical properties from each other when the positive electrode active material particles are a mixture of a plurality of positive electrode active material particles having different physical properties from each other; the morphology and content of the conductive material; the mixing ratio of a plurality of conductive materials when the conductive material is a mixture of a plurality of conductive materials having different physical properties from each other; the density of the positive electrode active material layer 110; and the like.

[0073] In one embodiment, the lithium metal oxide particles can further contain a doping element. For example, the doping element can include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, and the like.

[0074] In one embodiment, the positive electrode active material can further include a coating layer formed on at least a portion of the surface of the lithium metal oxide particles. For example, the coating layer can contain Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, and the like.

[0075] In one embodiment, the lithium metal oxide particles can include first lithium metal oxide particles and second lithium metal oxide particles having a particle diameter (D 50 ) smaller than the first lithium metal oxide particles.

[0076] The particle diameter (D 50 ) can be a particle diameter at 50% of a volume particle diameter distribution. For example, the particle diameter (D 50 ) can be measured using a laser diffraction method.

[0077] In some embodiments, the particle diameter (D 50 ) of the first lithium metal oxide particles can be 9 to 18 μm or 10 to 18 μm. In some embodiments, the particle diameter (D 50 ) of the second lithium metal oxide particles can be 1 to 9 μm or 2 to 7 μm.

[0078] In some embodiments, the ratio of the content of the second lithium metal oxide particles to the content of the first lithium metal oxide particles in the total weight of the positive electrode active material layer 110 can be 1 / 4 to 4, and preferably can be 1 / 3 to 3.

[0079] In one embodiment, the lithium metal oxide particles can include: first lithium metal oxide particles in the form of secondary particles in which a plurality of primary particles are aggregated; and second lithium metal oxide particles in the form of single particles.

[0080] For example, the single particles and the secondary particles can be distinguished based on cross-sectional images of the particles measured by a Scanning Electron Microscope (SEM).

[0081] For example, the secondary particles can refer to particles in which a plurality of primary particles are aggregated to be regarded as one particle or observed as one particle. For example, in the case of the secondary particles, the boundaries of the primary particles can be observed in SEM cross-sectional images.

[0082] For example, more than 10, more than 30, more than 50, or more than 100 primary particles can be aggregated in the secondary particles.

[0083] For example, the single particles can refer to monoliths rather than aggregates. For example, in the case of the single particles, unlike the secondary particles, the boundaries of the primary particles can not be observed in SEM cross-sectional images.

[0084] In one embodiment, fine particles (for example, particles having a volume of 1 / 100 or less with respect to the volume of the single particles) can be attached to the surface of the single particles, and this form does not exclude the concept of the single particles.

[0085] For example, the single particles can also exist in contact with each other. For example, 2 to 10, 2 to 5, or 2 to 3 single particles can exist in contact with each other.

[0086] In one embodiment, the density of the positive active material layer 110 can be 2.5 to 3.7 g / cm3, 2.8 to 3.7 g / cm3, 3.0 to 3.7 g / cm3, 3.2 to 3.7 g / cm3, or 3.4 to 3.7 g / cm3.

[0087] In one embodiment, the positive active material layer 110 can further include a conductive material. In some embodiments, the positive active material layer 110 can further include a binder.

[0088] In one embodiment, the content of the lithium metal oxide particles can be 80 to 99% by weight, 85 to 98% by weight, or 90 to 95% by weight in the total weight of the positive electrode active material layer 110.

[0089] In one embodiment, the content of the lithium metal oxide particles can be 80 to 99% by weight, 85 to 98% by weight, or 90 to 95% by weight in the total weight of the positive electrode active material layer 110.

[0090] In some embodiments, the content of the conductive material can be 1 to 10% by weight, 1 to 5% by weight, or 1 to 3% by weight in the total weight of the positive electrode active material layer 110.

[0091] In some embodiments, the content of the conductive material can be 1 to 10% by weight, 1 to 5% by weight, or 1 to 3% by weight in the total weight of the positive electrode active material layer 110.

[0092] In one embodiment, the conductive material can include a linear conductive material. For example, the linear conductive material can include a rod-shaped conductive material, a fiber-shaped conductive material, and the like.

[0093] In some embodiments, the linear conductive material can have an aspect ratio (length / diameter) of 2 to 10,000, 10 to 5,000, 50 to 3,000, or 100 to 1,000.

[0094] In some embodiments, the linear conductive material can include a carbon nanotube (CNT).

[0095] For example, the carbon nanotube (CNT) can include a single-walled CNT (SWCNT), a double-walled CNT (DWCNT), a multi-walled CNT (MWCNT), a rope CNT, and the like.

[0096] In some embodiments, the linear conductive material can have a length of 15 to 65 µm.

[0097] In some embodiments, the conductive material can further include a point-type conductive material. In this case, the linear conductive material can form a large-scale conductive network, and the point-type conductive material can further form a small-scale conductive network. Accordingly, a decrease in electrical conductivity due to the use of the lithium metal oxide particles can be prevented.

[0098] In the case where the point-type conductive material is combined with a lithium metal oxide particle having a Co molar fraction (based on all elements except Li and O) exceeding 0.02 (for example, NCM 811) and used to satisfy the formula 1, the rapid charging characteristics of the lithium secondary battery can rather decrease.

[0099] In some embodiments, the aspect ratio (length / diameter) of the point-type conductive material can be 0.5 to 1.5.

[0100] In some embodiments, the sphericity of the point-type conductive material can be 0.7 to 1, 0.8 to 1, or 0.9 to 1.

[0101] In some embodiments, the particle size (D 50 ) of the point-type conductive material can be 10-60 nm.

[0102] For example, the point-type conductive material can include a carbon-based conductive material such as graphite, carbon black, graphene, etc.; a metal-based conductive material such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.; or the like.

[0103] In some embodiments, the ratio of the content of the point-type conductive material to the content of the linear-type conductive material can be 0.6 to 0.9 or 0.6 to 0.8 in the total weight of the positive electrode active material layer 110.

[0104] In some embodiments, the ratio of the content of the point-type conductive material to the content of the linear-type conductive material can be 1.5 to 4, 1.8 to 3.5, or 2 to 3 in the total weight of the positive electrode active material layer 110.

[0105] In one embodiment, the binder can include a fluorine-based polymer binder. In some embodiments, the fluorine-based polymer binder can include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), or the like.

[0106] Lithium secondary battery

[0107] FIG. 2 and FIG. 3 are a plan view and a cross-sectional view of a lithium secondary battery according to exemplary embodiments, respectively.

[0108] The negative electrode 130 can include a negative electrode current collector 125 and a negative electrode active material layer 120 formed on the negative electrode current collector 125. For example, the negative electrode active material layer 120 can be formed on one or both faces of the negative electrode current collector 125.

[0109] The negative electrode active material layer 120 can include a negative electrode active material in which lithium ions can be reversibly intercalated and deintercalated.

[0110] In one embodiment, the negative active material layer 120 can further include a binder and a conductive material, etc.

[0111] For example, the negative current collector 125 can include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0112] In one embodiment, the negative active material can include a lithium alloy, a carbon-based active material, a silicon-based active material, etc.

[0113] For example, the lithium alloy can include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc.

[0114] For example, the carbon-based active material can include crystalline carbon, amorphous carbon, carbon composite, carbon fiber, etc.

[0115] For example, the amorphous carbon can include hard carbon, coke, mesocarbon microbeads, mesophase pitch-based carbon fiber, etc.

[0116] For example, the crystalline carbon can include natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbeads (MCMB), graphitized mesophase pitch-based carbon fiber (MPCF), etc.

[0117] In one embodiment, the negative active material can include a silicon-based active material. For example, the silicon-based active material can include Si, SiO x (0 < x < 2), Si / C, SiO / C, Si metal (Metal), etc. In this case, a lithium secondary battery having high capacity can be implemented.

[0118] In some embodiments, the area of the negative electrode 130 can be greater than the area of the positive electrode 100. Accordingly, lithium ions generated from the positive electrode 100 can smoothly migrate to the negative electrode 130 without being deposited in the middle.

[0119] For example, the electrode assembly 150 can be formed by alternately repeating the arrangement of the positive electrode 100 and the negative electrode 130.

[0120] In one embodiment, a separator 140 can be interposed between the positive electrode 100 and the negative electrode 130. For example, the electrode assembly 150 can be formed by winding, stacking, z-folding, etc. of the separator 140.

[0121] For example, the diaphragm 140 may comprise a porous polymer membrane prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Alternatively, the diaphragm 140 may comprise a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0122] A lithium secondary battery according to an exemplary embodiment may include a positive electrode lead 107 and a negative electrode lead 127, the positive electrode lead 107 being connected to a positive electrode 100 and protruding to the outside of a housing 160, and the negative electrode lead 127 being connected to a negative electrode 130 and protruding to the outside of a housing 160.

[0123] The positive lead 107 can be connected to the positive current collector 105. Additionally, the negative lead 127 can be connected to the negative current collector 125.

[0124] The positive current collector 105 may include a positive electrode tab 106 protruding to one side. A positive active material layer 110 may not be formed on the positive electrode tab 106. The positive electrode tab 106 may be integrally formed with the positive current collector 105 or connected by welding or the like. The positive current collector 105 and the positive lead 107 can be electrically connected through the positive electrode tab 106.

[0125] The negative electrode current collector 125 may include a negative electrode tab 126 protruding to one side. A negative electrode active material layer 120 may not be formed on the negative electrode tab 126. The negative electrode tab 126 may be integrally formed with the negative electrode current collector 125 or connected by means of welding or the like. The negative electrode current collector 125 and the negative electrode lead 127 may be electrically connected through the negative electrode tab 126.

[0126] For example, electrode assembly 150 and electrolyte can be housed in housing 160 to form a lithium secondary battery.

[0127] For example, the lithium secondary battery can be made into cylindrical, triangular, pouch, or coin shapes.

[0128] In one embodiment, the electrolyte may contain lithium salt and organic solvent.

[0129] The lithium salt may contain Li + X - For example, X - It can be F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6- , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - .

[0130] For example, the organic solvent can include carbonate-based solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); ester-based solvents such as methyl propionate, ethyl propionate, ethyl acetate, propyl acetate, butyl acetate, butyrolactone, hexalactone, and valerolactone; ether-based solvents such as dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), and tetrahydrofuran (THF); alcohol-based solvents such as ethanol and isopropanol; ketone-based solvents such as cyclohexanone; aprotic solvents such as amide-based solvents (e.g., dimethylformamide), dioxolane-based solvents (e.g., 1,3-dioxolane), sulfolane-based solvents, nitrile-based solvents, and the like.

[0131] Hereinafter, preferred examples and comparative examples of the present application will be described. However, the following examples are merely one preferred example of the present application, and the present application is not limited to the following examples.

[0132] Preparation Example 1 (0 mol% of Co, preparation of secondary particles)

[0133] NiSO4and MnSO4were added to distilled water from which dissolved oxygen was removed, in a molar ratio of 75:25, thereby preparing a mixed solution.

[0134] The mixed solution, NaOH, and NH4OH were added to a reactor, and a co-precipitation reaction was performed for 55 hours, thereby preparing metal hydroxide particles (Ni 0.75 Mn 0.25(OH)2).

[0135] The metal hydroxide particles and lithium hydroxide were added to a dry high-speed mixer such that the molar ratio of the metal hydroxide particles and lithium hydroxide was 1:1.03, thereby preparing a mixture.

[0136] The mixture was put into a calcination furnace, the temperature of the calcination furnace was increased to 780°C at a rate of 2°C / min, and maintained at 780°C for 13 hours. While calcinating, oxygen was passed through the calcination furnace at a flow rate of 10 mL / min.

[0137] After that, the calcinated product was naturally cooled to room temperature, and pulverized and classified, thereby obtaining lithium metal oxide particles (LiNi 0.75 Mn 0.25 O 2) .

[0138] The cross section of the lithium metal oxide particles was observed with a scanning electron microscope (SEM), and confirmed to have a secondary particle form in which primary particles were aggregated.

[0139] The particle size (D 50 ) of the lithium metal oxide particles was measured using a laser diffraction method. The particle size (D 50 ) of the lithium metal oxide particles was 13 μm.

[0140] Preparation Example 2 (0 mol% of Co, preparation of single particles)

[0141] NiSO4 and MnSO4 were added to distilled water from which dissolved oxygen was removed, in a molar ratio of 75:25, thereby preparing a mixed solution.

[0142] The mixed solution, NaOH, and NH4OH were added to a reactor, and a co-precipitation reaction was performed for 60 hours, thereby preparing metal hydroxide particles Ni 0.75 Mn 0.25 (OH)2.

[0143] The metal hydroxide particles and lithium hydroxide were added to a dry high-speed mixer such that the molar ratio of the metal hydroxide particles and lithium hydroxide was 1:1.03, thereby preparing a mixture.

[0144] The mixture was put into a calcination furnace, the temperature of the calcination furnace was increased to 950°C at a rate of 2°C / min, and calcinated at 950°C for 10 hours. While calcinating, oxygen was passed through the calcination furnace at a flow rate of 10 mL / min.

[0145] After that, the calcinated product was naturally cooled to room temperature, and pulverized and classified, thereby obtaining lithium metal oxide particles (LiNi 0.75 Mn 0.25O2).

[0146] The cross-section of the lithium metal oxide particles was observed with a scanning electron microscope (SEM) to confirm a single particle form.

[0147] The particle size (D 50 ) of the lithium metal oxide particles was measured by a laser diffraction method. The particle size (D 50 ) of the lithium metal oxide particles was 3.5 μm.

[0148] Preparation Example 3 (2 mol% of Co, preparation of secondary particles)

[0149] NiSO4, CoSO4, and MnSO4 were added to distilled water from which dissolved oxygen was removed, in a molar ratio of 75:2:23, thereby preparing a mixed solution.

[0150] A lithium metal oxide particle (LiNi 0.75 Co 0.02 Mn 0.23 O2) was prepared by the same method as in Preparation Example 1, except that the mixed solution was used.

[0151] The cross-section of the lithium metal oxide particles was observed with a scanning electron microscope (SEM) to confirm a secondary particle form in which primary particles were aggregated.

[0152] The particle size (D 50 ) of the lithium metal oxide particles was measured by a laser diffraction method. The particle size (D 50 ) of the lithium metal oxide particles was 13 μm.

[0153] Preparation Example 4 (2 mol% of Co, preparation of single particles)

[0154] NiSO4, CoSO4, and MnSO4 were added to distilled water from which dissolved oxygen was removed, in a molar ratio of 75:2:23, thereby preparing a mixed solution.

[0155] A lithium metal oxide particle (LiNi 0.75 Co 0.02 Mn 0.23 O2) was prepared by the same method as in Preparation Example 2, except that the mixed solution was used.

[0156] The cross-section of the lithium metal oxide particles was observed with a scanning electron microscope (SEM) to confirm a single particle form.

[0157] The particle size (D 50 ) of the lithium metal oxide particles was measured by a laser diffraction method. The particle size (D 50 ) of the lithium metal oxide particles was 3.4 μm.

[0158] Examples and Comparative Examples

[0159] The positive electrode active material described in Table 1 below, PVDF, and the conductive material described in Table 1 below were dispersed in NMP, thereby preparing a positive electrode slurry.

[0160] The content of PVDF was 1.2% by weight, the content of the conductive material was the content described in Table 1 below, and the balance was the positive electrode active material, based on the total of 100% by weight of the positive electrode active material, PVDF, and the conductive material.

[0161] The positive electrode slurry was coated on an aluminum foil (thickness: 15 μm), and drying and pressing were performed, thereby manufacturing a positive electrode in which a positive electrode active material layer was formed. At the time of the pressing, the density of the positive electrode active material layer was adjusted to about 3.5 g / cm3.

[0162] A lithium metal was used as a counter electrode (negative electrode).

[0163] The positive electrode and the negative electrode were cut into a circular shape, respectively, and a circular polyethylene separator (thickness: 13 μm) was interposed between the positive electrode and the negative electrode, thereby manufacturing an electrode assembly.

[0164] The electrode assembly and an electrolyte (1 M LiPF6 solution, solvent: EC / EMC 3:7 v / v) were put into a coin-shaped case, thereby manufacturing a coin-shaped lithium secondary battery.

[0165] Evaluation Example 1: Analysis of contact state of positive electrode active material particles in positive electrode active material layer

[0166] (1) The positive electrode of the examples and the comparative examples was cut into a size of 100 mm x 100 mm, respectively, thereby manufacturing a test sample.

[0167] (2) A three-dimensional model representing the distribution of the positive electrode active material particles in the test sample was prepared (three-dimensional modeling was performed only on the positive electrode active material particles). Specifically, an image was obtained while rotating the positive electrode test sample by 360° using an XRM, and a three-dimensional model was calculated by merging the images.

[0168] (3) The total number of contact surfaces (Cn) formed by the positive electrode active material particles contacting each other and the total area of the contact surfaces (Ca, that is, the sum of the areas of the contact surfaces) were calculated by analyzing the three-dimensional model. Specifically, for the three-dimensional model, the number of contact pixels of the positive electrode active material particles was measured using a three-dimensional image analysis software Geodict S / W, and the area of the contact pixels was calculated.

[0169] (4) The average contact area of each contact surface was calculated by dividing Ca by Cn.

[0170] The three-dimensional modeling and analysis were performed by the following equipment and conditions.

[0171] 1) Measuring equipment: X-ray microscope (XRM, Zeiss company, 620 versa)

[0172] 2) Measuring conditions: Source condition 50 kV / 4.5 W; Voxel size: 300 nm

[0173] 3) Analysis program: Geodict S / W (available / purchasable from Math2Market), Identify Grain

[0174] 4) Analysis conditions: Interface threshold for Grain-fragment reconnection was set to 22%; Set to include Grain at Domain boundary; Grain shape was set to Ellipsoid type.

[0175] Evaluation Example 2: Measurement of bulk resistance of positive electrode

[0176] The bulk resistance of the positive electrode was measured by the following measuring equipment and conditions.

[0177] 1) Measuring equipment: Hioki XF057 Probe unit

[0178] 2) Measuring conditions: Current 1 μA; Voltage range 10 V

[0179] 3) Pin contact number: 500

[0180] Evaluation Example 3: Evaluation of resistance (DCIR) increase rate after cycle at high temperature

[0181] The lithium secondary batteries of the examples and the comparative examples were subjected to CC / CV charging (4.2 V 0.05 C CUT-OFF) at 0.33 C at 45°C and then subjected to CC discharging at 0.33 C to 50% state of charge (SOC).

[0182] DCIR R1 was measured by discharging at a C-rate of 1 C and supplemental charging for 10 seconds at a state of charge (SOC) of 50%. Specifically, when the discharging and supplemental charging were performed, the end point of the voltage was taken as a linear equation, and the slope thereof was used as DCIR.

[0183] The lithium secondary battery was repeatedly charged and discharged 300 times at 45°C according to the charge and discharge conditions. After the charge and discharge 300 times, the DCIR R2 was measured by the same method as the above.

[0184] The resistance (DCIR) increase rate was calculated by the following equation.

[0185] DCIR increase rate (%) = (R2 - R1) / R1 x 100

[0186] The evaluation results are shown in Table 1 below.

[0187] [Table 1]

[0188]

[0189]

[0190] Referring to Table 1, the bulk resistance and the high temperature resistance increase rate of the positive electrode of the lithium secondary battery of the examples were low compared to the lithium secondary battery of the comparative example.

Claims

1. A positive electrode for a lithium secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector, and containing positive electrode active material particles and satisfying Formula 1, wherein the positive electrode active material particles contain lithium metal oxide particles containing nickel and having a molar fraction of cobalt among all elements other than lithium and oxygen of 0.02 or less, [Formula 1] Cn / Ca < 0.1 Ca / Cn < 6.44 μm 2 In Formula 1, Cn is the total number of contact surfaces between the positive electrode active material particles in a three-dimensional model representing the distribution of the positive electrode active material particles in the positive electrode active material layer, and Ca is the total area of the contact surfaces; wherein the three-dimensional model is calculated by obtaining images while rotating a positive electrode sample 360° using an XRM and merging the images; measuring the number of contact pixels of the positive electrode active material particles and calculating the area of the contact pixels using three-dimensional image analysis software Geodict S / W, the analysis of the three-dimensional model being performed by setting an interface threshold value at which particle fragments are reconnected to 22%, setting to include particles at domain boundaries, and setting the particle shape to an ellipsoidal type.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein Ca / Cn is 4.5-6.44 μm 2 .

3. The positive electrode according to claim 1, wherein The lithium metal oxide particles contain nickel and manganese, and do not contain cobalt.

4. The positive electrode according to claim 1, wherein Among all elements other than lithium and oxygen in the lithium metal oxide particles, the molar fraction of nickel is 0.7 to 0.

85.

5. The positive electrode according to claim 1, wherein The lithium metal oxide particles contain: first lithium metal oxide particles in the form of secondary particles having a plurality of primary particles aggregated; and second lithium metal oxide particles in the form of single particles.

6. The positive electrode according to claim 5, wherein In the positive electrode active material layer, the ratio of the content by weight of the second lithium metal oxide particles to the content by weight of the first lithium metal oxide particles is 1 / 3 to 3.

7. The positive electrode according to claim 1, wherein The lithium metal oxide particles include first lithium metal oxide particles and particles having a particle diameter D 50 second lithium metal oxide particles smaller than the first lithium metal oxide particles.

8. The positive electrode according to claim 7, wherein The particle size D of the first lithium metal oxide particles is 9-18 μm 50 The particle size D of the second lithium metal oxide particles is 2-7 μm 50 The particle size D of the second lithium metal oxide particles is 2-7 μm 9. The positive electrode according to claim 1, wherein The positive electrode active material layer further contains a conductive material, In the total weight of the positive electrode active material layer, the content of the lithium metal oxide particles is 80 to 99% by weight.

10. The positive electrode according to claim 9, wherein The conductive material contains: a point-type conductive material having an aspect ratio of 0.5 to 1.5; and a line-type conductive material having an aspect ratio of 2 or more.

11. The positive electrode according to claim 10, wherein The particle size D of the dot-type conductive material 50 The wavelength range is 10-60 nm, and the length of the linear conductive material is 15-65 μm.

12. The positive electrode according to claim 10, wherein In the positive electrode active material layer, the ratio of the content by weight of the point-type conductive material to the content by weight of the line-type conductive material is 0.6 to 0.

8.

13. The positive electrode according to claim 10, wherein In the positive electrode active material layer, the ratio of the content by weight of the point-type conductive material to the content by weight of the line-type conductive material is 1.8 to 3.

5.

14. The positive electrode according to claim 1, wherein The density of the positive electrode active material layer is 3.4 to 3.7 g / cm3.

15. A lithium secondary battery, comprising: the positive electrode for a lithium secondary battery according to claim 1; and a negative electrode disposed opposite to the positive electrode.

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