Positive electrode for lithium secondary battery and lithium secondary battery comprising 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 the distribution of the positive electrode active material particles, the ratio of the total area of the contact surfaces between the positive electrode active material particles and the total number of the contact surfaces between the positive electrode active material particles satisfies a predetermined range. The positive electrode active material particles may include lithium metal oxide particles containing nickel and having a molar fraction of cobalt of 0.02 or less among all elements other than lithium and oxygen.

Description

Cross-references to related applications

[0001] This application is a divisional application of Chinese Patent Application No. 202310949821.9, filed on July 31, 2023, entitled "Positive Electrode for Lithium Secondary Batteries and Lithium Secondary Batteries Including the Positive Electrode". This application claims priority to KR10-2022-0116418 (September 15, 2022). Technical Field

[0002] This invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode. More specifically, this invention relates to a positive electrode for a lithium secondary battery comprising a nickel-based positive electrode active material and a lithium secondary battery including the positive electrode. Background Technology

[0003] Rechargeable batteries are batteries that can be repeatedly charged and discharged, and they are widely used as power sources for portable electronic devices such as mobile phones and laptops.

[0004] Lithium-ion batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design, so they are being actively developed and applied.

[0005] For example, a lithium secondary battery may include: an electrode assembly comprising a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode; and an electrolyte impregnating the electrode assembly.

[0006] The positive electrode may 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 may contain lithium metal oxide as the positive electrode active material.

[0007] For example, 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 as the positive electrode active material.

[0008] Furthermore, cobalt is relatively expensive compared to other metals such as nickel and manganese. Therefore, the extensive use of lithium metal oxide particles containing high concentrations of cobalt in large machinery such as electric vehicles is costly.

[0009] However, when lithium metal oxides do not contain cobalt, the crystal structure may be unstable, which may reduce the operational reliability and electrochemical stability of lithium secondary batteries.

[0010] For example, Korean Patent Publication No. 10-2020-0085679 discloses a lithium secondary battery that improves lifespan characteristics by using lithium metal oxide particles containing only trace amounts of cobalt on the surface. Summary of the Invention

[0011] Technical problems to be solved One technical problem of the present invention is to provide a positive electrode for lithium secondary batteries with improved operational reliability and electrochemical stability.

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

[0013] Technical solution A positive electrode for a lithium secondary battery according to an exemplary embodiment may 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 comprising positive electrode active material particles and satisfying Formula 1.

[0014] The positive electrode active material particles may include lithium metal oxide particles, wherein the lithium metal oxide particles contain nickel and the molar fraction of cobalt in all elements other than lithium and oxygen is less than 0.02.

[0015] [Formula 1] Ca / Cn≤6.44μm 2 In Equation 1, Cn is the total number of contact surfaces formed between the positive active material particles in a three-dimensional model representing the distribution of the positive active material particles in the positive active material layer, and Ca is the total area of ​​the contact surfaces.

[0016] In one implementation, Ca / Cn can be 4.5-6.44 μm. 2 .

[0017] In one embodiment, the lithium metal oxide particles may contain nickel and manganese, but may not contain cobalt.

[0018] In one embodiment, the molar fraction of nickel in the lithium metal oxide particles, excluding lithium and oxygen, can be from 0.7 to 0.85.

[0019] In one embodiment, the lithium metal oxide particles may comprise: a first lithium metal oxide particle having a secondary particle form consisting of a plurality of primary particles aggregated together; and a second lithium metal oxide particle having a single particle form.

[0020] In one embodiment, 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 can be from 1 / 3 to 3.

[0021] In one embodiment, the lithium metal oxide particles may comprise first lithium metal oxide particles and a particle size (D). 50 The second lithium metal oxide particle is smaller than the first lithium metal oxide particle.

[0022] In one embodiment, the particle size (D) of the first lithium metal oxide particles 50 The particle size (D) of the second lithium metal oxide particles can be 9-18 μm. 50 The size can range from 2 to 7 μm.

[0023] In one embodiment, the positive electrode active material layer may further comprise a conductive material, and the content of the lithium metal oxide particles in the total weight of the positive electrode active material layer may be 80-99% by weight.

[0024] In one embodiment, the conductive material may comprise: a dot-type conductive material having an aspect ratio of 0.5 to 1.5; and a linear conductive material having an aspect ratio of 2 or higher.

[0025] In one embodiment, the particle size (D50) of the dot-type conductive material can be 10-60 nm, and the length of the linear conductive material can be 15-65 μm.

[0026] In one embodiment, the ratio of the content of the point-type conductive material to the content of the linear conductive material in the total weight of the positive electrode active material layer can be from 0.6 to 0.8.

[0027] In one embodiment, the ratio of the content of the point-type conductive material to the content of the linear conductive material in the total weight of the positive electrode active material layer can be from 1.8 to 3.5.

[0028] In one embodiment, the density of the positive electrode active material layer can be 3.4-3.7 g / cm³ (g / cc).

[0029] A lithium secondary battery according to an exemplary embodiment may include a positive electrode for the lithium secondary battery and a negative electrode disposed opposite to the positive electrode.

[0030] Beneficial effects According to an exemplary embodiment of the present invention, a positive electrode for a lithium secondary battery can be provided that uses lithium metal oxide particles containing a low amount of cobalt and / or cobalt-free lithium metal oxide particles as the positive electrode active material. Even when using the lithium metal oxide particles, the positive electrode can exhibit a low bulk resistance by satisfying the following formula 1.

[0031] The lithium secondary battery according to an exemplary embodiment of the present invention includes the positive electrode, and therefore can exhibit improved resistance and power characteristics. Attached Figure Description

[0032] Figure 1 This is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0033] Figure 2 and Figure 3 These are, respectively, a plan view and a cross-sectional view of a lithium secondary battery according to an exemplary embodiment. Detailed Implementation

[0034] According to an exemplary embodiment of the present invention, a positive electrode for a lithium secondary battery comprising lithium metal oxide particles can be provided.

[0035] According to an exemplary embodiment of the present invention, a lithium secondary battery including the positive electrode can be provided.

[0036] Hereinafter, with reference to the accompanying drawings, a positive electrode for a lithium secondary battery and a lithium secondary battery according to exemplary embodiments of the present invention will be described in more detail. However, the drawings and embodiments are merely exemplary, and the present invention is not limited to the drawings and embodiments.

[0037] Positive electrode for lithium secondary batteries Figure 1 This is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0038] Reference Figure 1 The positive electrode 100 for a lithium secondary battery may include a positive electrode current collector 105 and a positive electrode active material layer 110 formed on the positive electrode current collector 105.

[0039] For example, the positive electrode active material layer 110 can be formed on one or both sides of the positive electrode current collector 105.

[0040] For example, the positive current collector 105 may contain stainless steel, nickel, aluminum, titanium, copper, or alloys thereof.

[0041] The positive electrode active material layer 110 may contain positive electrode active material particles that enable reversible insertion and extraction of lithium ions. The positive electrode active material layer 110 may contain multiple positive electrode active material particles.

[0042] In an exemplary embodiment, the positive electrode active material particles may comprise lithium metal oxide particles in which the molar fraction of cobalt (Co) in all elements other than lithium (Li) and oxygen (O) is less than 0.02.

[0043] In one embodiment, the molar fraction of Co in the lithium metal oxide particles may be less than 0.015, preferably less than 0.01.

[0044] In one embodiment, the lithium metal oxide particles may be substantially free of Co. For example, Co may not be detectable when the composition of the lithium metal oxide particles is analyzed using ICP.

[0045] In an exemplary embodiment, the lithium metal oxide particles may contain nickel (Ni). In one embodiment, the lithium metal oxide particles may contain both Ni and manganese (Mn).

[0046] In one embodiment, the molar fraction of Ni in the lithium metal oxide particles, excluding Li and O, can be 0.6 to 0.9, 0.65 to 0.85, or 0.7 to 0.85.

[0047] In one embodiment, the molar fraction of Mn in the lithium metal oxide particles, excluding Li and O, can be 0.05 to 0.3, 0.1 to 0.3, or 0.15 to 0.3.

[0048] In one embodiment, the lithium metal oxide particles may comprise a crystal structure or chemical structure represented by the following chemical formula 1.

[0049] [Chemical Formula 1] Li a Ni x Mn y Co z M 1-x-y-z O2 In chemical formula 1, M may contain at least one of Mg, V, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V and Bi, and may be 0.9≤a≤1.2, 0.6≤x≤0.9, 0≤z≤0.02, 0.8≤x+y+z≤1.

[0050] In some implementations, the value can be 0.9 ≤ a < 1.1.

[0051] In some implementations, the value can be 0.65≤x≤0.85 or 0.7≤x≤0.85.

[0052] In some implementations, z can be ≤0.015 or z≤0.01.

[0053] In some implementations, z can be greater than 0 or greater than 0.001.

[0054] In some implementations, z can be 0.

[0055] The lithium metal oxide particles contain low levels of Co or no Co at all, thereby reducing manufacturing costs. However, the electrical conductivity and crystal structure stability of the lithium metal oxide particles may be reduced. Consequently, the resistance of the cathode and secondary battery using these lithium metal oxide particles will increase, potentially leading to a decrease in power output.

[0056] However, the positive electrode active material layer 110 according to an exemplary embodiment of the present invention can satisfy the following formula 1 and can improve the above-mentioned problems caused by the reduction of Co content.

[0057] [Formula 1] Ca / Cn≤6.44μm 2 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 average contact area of ​​each contact surface.

[0058] According to an exemplary implementation, a three-dimensional model of the positive electrode can be obtained 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). Under the conditions of Formula 1, even when using the lithium metal oxide particles, the lithium secondary battery can have improved resistance characteristics.

[0059] If Equation 1 is not satisfied, the resistance of the positive electrode may increase. Furthermore, when the lithium secondary battery operates at high temperatures, its internal resistance (DCIR) may increase significantly.

[0060] When using lithium metal oxide particles (e.g., NCM 811) with a Co molar fraction exceeding 0.02 (based on all elements except Li and O), the difference in effect may be negligible depending on the conditions satisfying Equation 1.

[0061] In one implementation, Ca / Cn can be 3μm. 2 Above, 4μm 2 Above, 4.5μm 2 Above, 5μm 2Above or 5.2μm 2 In this case, the power characteristics and fast charging characteristics of lithium secondary batteries can be improved.

[0062] For example, the Ca / Cn value can be adjusted by various factors such as: the particle size 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 the multiple positive electrode active material particles when the positive electrode active material particles are a mixture of multiple positive electrode active material particles with different physical properties; the morphology and content of the conductive material; the mixing ratio of the multiple conductive materials when the conductive material is a mixture of multiple conductive materials with different physical properties; and the density of the positive electrode active material layer 110.

[0063] In one embodiment, the lithium metal oxide particles may further contain doping elements. For example, the doping elements may include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, etc.

[0064] In one embodiment, the positive electrode active material may further comprise a coating formed on at least a portion of the surface of the lithium metal oxide particles. For example, the coating may contain Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, etc.

[0065] In one embodiment, the lithium metal oxide particles may comprise first lithium metal oxide particles and a particle size (D). 50 The second lithium metal oxide particle is smaller than the first lithium metal oxide particle.

[0066] The particle size (D) 50 The particle size (D) can be the particle size at 50% of the volumetric particle size distribution. For example, the particle size (D) 50 The laser diffraction method can be used for measurement.

[0067] In some embodiments, the particle size (D) of the first lithium metal oxide particles 50 The particle size (D) can be 9-18 μm or 10-18 μm. In some embodiments, the particle size (D) of the second lithium metal oxide particles is... 50 The size can be 1-9μm or 2-7μm.

[0068] 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, preferably 1 / 3 to 3.

[0069] In one embodiment, the lithium metal oxide particles may comprise: a first lithium metal oxide particle having a secondary particle form of a plurality of primary particles aggregated together; and a second lithium metal oxide particle having a single particle form.

[0070] 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).

[0071] For example, the secondary particle can refer to a particle in which multiple primary particles are aggregated and considered as a single particle or observed as a single particle. For example, in the case of the secondary particle, the boundary of the primary particle can be observed in the SEM cross-sectional image.

[0072] For example, the secondary particles may contain more than 10, 30, 50, or 100 primary particles.

[0073] For example, the single particle may refer to a monolith rather than an aggregate. For example, in the case of the single particle, unlike the secondary particles, the boundary of the primary particle may not be observable in the SEM cross-sectional image.

[0074] In one embodiment, the surface of the single particle may be covered with fine particles (e.g., particles having a volume of less than 1 / 100 relative to the volume of the single particle), and this form is not excluded from the concept of the single particle.

[0075] For example, the individual particles may also be in contact with each other. For example, 2-10, 2-5, or 2-3 individual particles may be in contact with each other.

[0076] In one embodiment, the density of the positive electrode active material layer 110 can be 2.5-3.7 g / cm³, 2.8-3.7 g / cm³, 3.0-3.7 g / cm³, 3.2-3.7 g / cm³, or 3.4-3.7 g / cm³.

[0077] In one embodiment, the positive electrode active material layer 110 may further comprise a conductive material. In some embodiments, the positive electrode active material layer 110 may further comprise an adhesive.

[0078] In one embodiment, the content of the positive electrode active material particles in the total weight of the positive electrode active material layer 110 can be 80-99% by weight, 85-98% by weight, or 90-95% by weight.

[0079] In one embodiment, the content of the lithium metal oxide particles in the total weight of the positive electrode active material layer 110 can be 80-99% by weight, 85-98% by weight, or 90-95% by weight.

[0080] In some embodiments, the content of the conductive material in the total weight of the positive electrode active material layer 110 can be 1-10% by weight, 1-5% by weight, or 1-3% by weight.

[0081] In some embodiments, the content of the binder in the total weight of the positive electrode active material layer 110 may be 1-10% by weight, 1-5% by weight, or 1-3% by weight.

[0082] In one embodiment, the conductive material may comprise a linear conductive material. For example, the linear conductive material may comprise rod-shaped conductive materials and fibrous conductive materials, etc.

[0083] In some embodiments, the aspect ratio (length / diameter) of the linear conductive material can be 2 to 10,000, 10 to 5,000, 50 to 3,000, or 100 to 1,000.

[0084] In some embodiments, the linear conductive material may comprise carbon nanotubes (CNTs).

[0085] For example, the carbon nanotubes (CNTs) may include single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), multi-walled carbon nanotubes (MWCNTs), rope carbon nanotubes, etc.

[0086] In some embodiments, the length of the linear conductive material can be 15-65 μm.

[0087] In some embodiments, the conductive material may further comprise 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. Therefore, a decrease in conductivity due to the use of the lithium metal oxide particles can be prevented.

[0088] When the point-type conductive material is combined with lithium metal oxide particles (e.g., NCM 811) having a Co molar fraction of more than 0.02 (based on all elements except Li and O) to satisfy Formula 1, the fast-charging characteristics of the lithium secondary battery may actually be reduced.

[0089] In some embodiments, the aspect ratio (length / diameter) of the point-type conductive material can be from 0.5 to 1.5.

[0090] In some embodiments, the sphericity of the point-type conductive material may be 0.7 to 1, 0.8 to 1, or 0.9 to 1.

[0091] In some embodiments, the particle size (D) of the point-type conductive material 50 The wavelength can be 10-60nm.

[0092] For example, the dot-type conductive material may include carbon-based conductive materials such as graphite, carbon black, and graphene; and metal-based conductive materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3.

[0093] In some embodiments, the ratio of the content of the point-type conductive material to the content of the linear conductive material in the total weight of the positive electrode active material layer 110 can be 0.6 to 0.9 or 0.6 to 0.8.

[0094] In some embodiments, the ratio of the content of the point-type conductive material to the content of the linear conductive material in the total weight of the positive electrode active material layer 110 can be 1.5 to 4, 1.8 to 3.5, or 2 to 3.

[0095] In one embodiment, the adhesive may comprise a fluoropolymer adhesive. In some embodiments, the fluoropolymer adhesive may comprise polyvinylidene fluoride (PVDF), PVDF-co-HFP, etc.

[0096] Lithium secondary batteries Figure 2 and Figure 3 These are, respectively, a plan view and a cross-sectional view of a lithium secondary battery according to an exemplary embodiment.

[0097] The negative electrode 130 may 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 may be formed on one or both sides of the negative electrode current collector 125.

[0098] The negative electrode active material layer 120 may contain a negative electrode active material that enables reversible insertion and extraction of lithium ions.

[0099] In one embodiment, the negative electrode active material layer 120 may further include an adhesive, a conductive material, and the like.

[0100] For example, the negative electrode current collector 125 may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0101] In one embodiment, the negative electrode active material may include a lithium alloy, a carbon-based active material, a silicon-based active material, and the like.

[0102] For example, the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, and the like.

[0103] For example, the carbon-based active material may include crystalline carbon, amorphous carbon, carbon composites, carbon fibers, and the like.

[0104] For example, the amorphous carbon may include hard carbon, coke, mesocarbon microbeads, mesophase pitch-based carbon fibers, and the like.

[0105] For example, the crystalline carbon may include natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbeads (MCMB), graphitized mesophase pitch-based carbon fibers (MPCF), and the like.

[0106] In one embodiment, the negative electrode active material may include a silicon-based active material. For example, the silicon-based active material may include Si, SiO x (0 < x < 2), Si / C, SiO / C, Si metal (Metal), etc. In this case, a lithium secondary battery with a high capacity can be achieved.

[0107] In some embodiments, the area of the negative electrode 130 may be larger than the area of the positive electrode 100. Therefore, lithium ions generated from the positive electrode 100 can migrate smoothly to the negative electrode 130 without precipitation in the middle.

[0108] For example, the electrode assembly 150 may be formed by alternately repeating the arrangement of the positive electrode 100 and the negative electrode 130.

[0109] In one embodiment, a separator 140 may be inserted between the positive electrode 100 and the negative electrode 130. For example, the electrode assembly 150 may be formed by winding, stacking, z-folding, etc. of the separator 140.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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 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.

[0115] For example, electrode assembly 150 and electrolyte can be housed in housing 160 to form a lithium secondary battery.

[0116] For example, the lithium secondary battery can be made into cylindrical, triangular, pouch, or coin shapes.

[0117] In one embodiment, the electrolyte may contain lithium salt and organic solvent.

[0118] 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 - Any one of them.

[0119] For example, the organic solvent may include carbonate-based solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC); ester-based solvents such as methyl propionate, ethyl propionate, ethyl acetate, propyl acetate, butyl acetate, butyrolactone, caprolactone, and valproic acid; 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; and aprotic solvents such as amide-based solvents (e.g., dimethylformamide), dioxolane-based solvents (e.g., 1,3-dioxolane), sulfolane-based solvents, and nitrile-based solvents.

[0120] Hereinafter, preferred embodiments and comparative examples of the present invention will be described. However, the following embodiments are merely preferred embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0121] Preparation Example 1 (Preparation of secondary particles with 0 mol% Co) A mixed solution was prepared by adding NiSO4 and MnSO4 in a molar ratio of 75:25 to distilled water after removing dissolved oxygen.

[0122] The mixed solution, NaOH, and NH4OH were added to the reactor, and a co-precipitation reaction was carried out for 55 hours to prepare metal hydroxide particles (Ni). 0.75 Mn 0.25 (OH)2).

[0123] The metal hydroxide particles and lithium hydroxide are added to a dry high-speed mixer to prepare a mixture with a molar ratio of 1:1.03.

[0124] The mixture was placed in a calcining furnace, and the temperature of the calcining furnace was increased to 780°C at a rate of 2°C / min, and maintained at 780°C for 13 hours. During calcination, oxygen was passed through the calcining furnace at a flow rate of 10 mL / min.

[0125] The calcined material was then naturally cooled to room temperature and pulverized and graded to obtain lithium metal oxide particles (LiNi). 0.75 Mn 0.25 O 2) .

[0126] The cross-section of the lithium metal oxide particles was observed using a scanning electron microscope (SEM), confirming that they have a secondary particle form formed by primary particle aggregation.

[0127] The particle size (D) of the lithium metal oxide particles was measured using laser diffraction. 50 The particle size (D) of the lithium metal oxide particles. 50 The value is 13μm.

[0128] Preparation Example 2 (Preparation of single particles of 0 mol% Co) A mixed solution was prepared by adding NiSO4 and MnSO4 in a molar ratio of 75:25 to distilled water after removing dissolved oxygen.

[0129] The mixed solution, NaOH, and NH4OH were added to the reactor, and a co-precipitation reaction was carried out for 60 hours to prepare Ni metal hydroxide particles. 0.75 Mn 0.25 (OH)2.

[0130] The metal hydroxide particles and lithium hydroxide are added to a dry high-speed mixer to prepare a mixture with a molar ratio of 1:1.03.

[0131] The mixture was placed in a calcining furnace, and the temperature of the calcining furnace was increased to 950°C at a rate of 2°C / min, and calcined at 950°C for 10 hours. During calcination, oxygen was passed through the calcining furnace at a flow rate of 10 mL / min.

[0132] The calcined material was then naturally cooled to room temperature and pulverized and graded to obtain lithium metal oxide particles (LiNi). 0.75 Mn 0.25 O2).

[0133] The cross-section of the lithium metal oxide particles was observed using a scanning electron microscope (SEM), confirming that they were in single-particle form.

[0134] The particle size (D) of the lithium metal oxide particles was measured using laser diffraction. 50 The particle size (D) of the lithium metal oxide particles. 50 The value is 3.5 μm.

[0135] Preparation Example 3 (Preparation of secondary particles with 2 mol% Co) A mixed solution was prepared by adding NiSO4, CoSO4 and MnSO4 in a molar ratio of 75:2:23 to distilled water after removing dissolved oxygen.

[0136] In addition to using the aforementioned mixed solution, lithium metal oxide particles (LiNi) were prepared by the same method as in Preparation Example 1. 0.75 Co 0.02 Mn 0.23 O2).

[0137] The cross-section of the lithium metal oxide particles was observed using a scanning electron microscope (SEM), confirming that they have a secondary particle form formed by primary particle aggregation.

[0138] The particle size (D) of the lithium metal oxide particles was measured using laser diffraction. 50 The particle size (D) of the lithium metal oxide particles. 50 The value is 13μm.

[0139] Preparation Example 4 (Preparation of single particles with 2 mol% Co) A mixed solution was prepared by adding NiSO4, CoSO4 and MnSO4 in a molar ratio of 75:2:23 to distilled water after removing dissolved oxygen.

[0140] In addition to using the aforementioned mixed solution, lithium metal oxide particles (LiNi) were prepared using the same method as in Preparation Example 2. 0.75 Co 0.02 Mn 0.23 O2).

[0141] The cross-section of the lithium metal oxide particles was observed using a scanning electron microscope (SEM), confirming that they were in single-particle form.

[0142] The particle size (D) of the lithium metal oxide particles was measured using laser diffraction. 50 The particle size (D) of the lithium metal oxide particles. 50 The value is 3.4 μm.

[0143] Examples and Comparative Examples The positive electrode active material, PVDF, and conductive material listed in Table 1 below are dispersed in NMP to prepare a positive electrode slurry.

[0144] Based on a total of 100% by weight of the positive electrode active material, PVDF, and the conductive material, the PVDF content is 1.2% by weight, the conductive material content is as shown in Table 1 below, and the remainder is the positive electrode active material.

[0145] The positive electrode slurry is coated onto an aluminum foil (15 μm thick), dried, and pressed to manufacture a positive electrode with a positive electrode active material layer. During the pressing process, the density of the positive electrode active material layer is adjusted to approximately 3.5 g / cm³.

[0146] Lithium metal is used as the counter electrode (negative electrode).

[0147] The positive electrode and the negative electrode are cut into circles, and a circular polyethylene diaphragm (13 μm thick) is inserted between the positive electrode and the negative electrode to manufacture the electrode assembly.

[0148] The electrode assembly and electrolyte (1M LiPF6 solution, solvent: EC / EMC 3:7 v / v) are placed in a coin-shaped housing to manufacture a coin-shaped lithium secondary battery.

[0149] Evaluation Example 1: Analysis of the contact state of positive electrode active material particles in the positive electrode active material layer (1) The positive electrodes of the examples and comparative examples were cut into 100mm×100mm sizes to manufacture samples.

[0150] (2) Create a three-dimensional model representing the distribution of positive electrode active material particles in the sample (three-dimensional modeling is performed only on the positive electrode active material particles). Specifically, the positive electrode sample is rotated 360° using XRM to obtain images, and the three-dimensional model is calculated by merging the images.

[0151] (3) The total number (Cn) of contact surfaces formed by the positive electrode active material particles in contact with each other and the total area of ​​the contact surfaces (Ca, i.e., the sum of the areas of the contact surfaces) are 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 is measured using the three-dimensional image analysis software Geodict S / W, and the area of ​​the contact pixels is calculated.

[0152] (4) The average contact area of ​​each contact surface is calculated by dividing Ca by Cn.

[0153] The 3D modeling and analysis are performed using the following equipment and under the following conditions.

[0154] 1) Measuring equipment: X-ray microscope (XRM, Zeiss, 620 versa) 2) Measurement conditions: Source condition 50kV / 4.5W; Voxel size: 300nm 3) Analysis program: Geodict S / W (available for purchase / use on Math2Market), IdentifyGrain. 4) Analysis conditions: Set the interface threshold for grain-fragment reconnection to 22%; set it to include grains at the domain boundary; set the grain shape to ellipsoid.

[0155] Evaluation Example 2: Measurement of the bulk resistance of the positive electrode The volume resistance of the positive electrode is measured using the following measuring equipment and conditions.

[0156] 1) Measuring equipment: Hiokki XF057 probe unit 2) Measurement conditions: Current 1μA; Voltage range 10V 3) Number of pin contacts: 500 Evaluation Example 3: Evaluating the rate of increase in resistance (DCIR) after cycling at high temperature. At 45°C, the lithium secondary batteries of the examples and comparative examples were CC / CV charged at 0.33C (4.2V 0.05C cut-off) and then CC discharged at 0.33C to 50% state of charge (SOC).

[0157] DCIR R1 was measured by discharging and recharging at a rate of 1C for 10 seconds at 50% state of charge (SOC). Specifically, during the discharge and recharge, the endpoint of the voltage was used as the linear equation, and its slope was used as DCIR.

[0158] The lithium secondary battery was repeatedly charged and discharged 300 times at 45°C according to the aforementioned charging and discharging conditions. After 300 charge and discharge cycles, the DCIR R2 was measured using the same method as described above.

[0159] The rate of increase in resistance (DCIR) is calculated using the following formula.

[0160] DCIR increase rate (%) = (R2-R1) / R1×100 The evaluation results are shown in Table 1 below.

[0161] [Table 1]

[0162] Referring to Table 1, compared with the lithium secondary battery of the comparative example, the lithium secondary battery of the embodiment has a lower rate of increase in bulk resistance and high-temperature resistance of the positive electrode.

Claims

1. A positive electrode for a lithium secondary battery, comprising: Positive current collector; as well as A positive electrode active material layer is formed on the positive electrode current collector, and the positive electrode active material layer comprises positive electrode active material particles and conductive material. The positive electrode active material particles comprise lithium metal oxide particles, wherein the lithium metal oxide particles contain nickel and the molar fraction of cobalt among all elements except lithium and oxygen is less than 0.

02. The conductive material includes point-type conductive materials and linear conductive materials. In the total weight of the positive electrode active material layer, the ratio of the content of the point-type conductive material to the content of the linear conductive material is 0.6 to 0.8 or 1.8 to 3.

5.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide particles contain nickel and manganese, but no cobalt.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein, Of all the elements in the lithium metal oxide particles other than lithium and oxygen, the molar fraction of nickel is 0.7 to 0.

85.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide particles comprise: a first lithium metal oxide particle having a secondary particle form consisting of multiple primary particles aggregated together; and a second lithium metal oxide particle having a single particle form.

5. The positive electrode for a lithium secondary battery according to claim 4, wherein, In the total weight of the positive electrode active material layer, the ratio of the content of the second lithium metal oxide particles to the content of the first lithium metal oxide particles is 1 / 3 to 3.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide particles comprise a first lithium metal oxide particle and a particle size D. 50 A second lithium metal oxide particle that is smaller than the first lithium metal oxide particle.

7. The positive electrode for a lithium secondary battery according to claim 6, wherein, The particle size D of the first lithium metal oxide particle 50 The particle size D of the second lithium metal oxide particles is 9-18 μm. 50 It is 2-7μm.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide particles comprise 80-99% of the total weight of the positive electrode active material layer.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein, The conductive material includes: a dot-type conductive material having an aspect ratio of 0.5 to 1.5; and a linear conductive material having an aspect ratio of 2 or more.

10. The positive electrode for a lithium secondary battery according to claim 9, 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.

11. The positive electrode for a lithium secondary battery according to claim 1, wherein, The density of the positive electrode active material layer is 3.4-3.7 g / cm³.

12. The positive electrode for a lithium secondary battery according to claim 1, wherein, The positive electrode active material layer satisfies the following formula 1. [Formula 1] Ca / Cn≤6.44μm 2 In Equation 1, Cn is the total number of contact surfaces formed by the contact of the positive active material particles in a three-dimensional model representing the distribution of the positive active material particles in the positive active material layer, and Ca is the total area of ​​the contact surfaces. The three-dimensional model is calculated by: obtaining an image by rotating the positive electrode sample 360° using XRM and merging the images; measuring the number of contact pixels of the positive active material particles using the three-dimensional image analysis software Geodict S / W and calculating the area of ​​the contact pixels. The analysis of the three-dimensional model is performed by setting the interface threshold for particle fragment reconnection to 22%, setting it to include particles at the domain boundary, and setting the particle shape to ellipsoidal.

13. The positive electrode for a lithium secondary battery according to claim 12, wherein, The Ca / Cn ratio is 4.5-6.44 μm. 2 .

14. A lithium secondary battery, comprising: The positive electrode for a lithium secondary battery as described in claim 1; as well as The negative electrode is positioned opposite to the positive electrode.

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