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 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 include: a first positive electrode active material layer formed on the positive electrode current collector and including first lithium metal oxide particles having a secondary particle form; a second positive electrode active material layer formed on the first positive electrode active material layer and including second lithium metal oxide particles having a single particle form; and a third positive electrode active material layer formed on the second positive electrode active material layer and including third lithium metal oxide particles having a secondary particle form.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode. More specifically, the present invention relates to a positive electrode for a lithium secondary battery including a positive electrode active material layer of a multi-layer structure and a lithium secondary battery including the positive electrode. Background Art

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged and are widely used as a power source for portable electronic devices such as mobile phones and laptops.

[0003] Lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous in terms of charging speed and lightness, and therefore are being actively developed and applied.

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

[0005] For example, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. For example, the positive electrode active material layer may include lithium metal oxide particles as a positive electrode active material.

[0006] Generally, the lithium metal oxide particles morphologically have a secondary particle form in which a plurality of primary particles are aggregated, and the primary particles crystallographically have a polycrystalline structure.

[0007] However, the lithium metal oxide particles having a secondary particle form and a polycrystalline structure may generate cracks in the particles during rolling in the positive electrode manufacturing process. In addition, during repeated charge and discharge of the lithium secondary battery, cracks may be generated in the particles due to the insertion and extraction of lithium.

[0008] Therefore, there may be problems such as generation of gas due to a side reaction between the lithium metal oxide particles and the electrolyte, reduction in life characteristics of the lithium secondary battery, etc. In addition, the above problems may be more serious in a high temperature environment. Summary of the invention

[0009] 1. Technical issues to be resolved

[0010] A technical problem of the present invention is to provide a positive electrode active material for a lithium secondary battery having improved electrochemical performance and electrochemical stability.

[0011] A technical problem of the present invention is to provide a lithium secondary battery with improved electrochemical performance and electrochemical stability.

[0012] (II) Technical solution

[0013] A positive electrode for a lithium secondary battery according to an exemplary embodiment of the present invention may include a positive electrode collector and a positive electrode active material layer formed on the positive electrode collector.

[0014] The positive electrode active material layer may include: a first positive electrode active material layer, which is formed on the positive electrode current collector and includes first lithium metal oxide particles in the form of secondary particles formed by aggregation of multiple primary particles; a second positive electrode active material layer, which is formed on the first positive electrode active material layer and includes second lithium metal oxide particles in the form of single particles; and a third positive electrode active material layer, which is formed on the second positive electrode active material layer and includes third lithium metal oxide particles in the form of secondary particles formed by aggregation of multiple primary particles.

[0015] In one embodiment, the positive electrode may satisfy Formula 1.

[0016] [Formula 1]

[0017] 0.2≤T2 / (T1+T2+T3)≤0.7

[0018] In one embodiment, the positive electrode may satisfy Formula 2.

[0019] [Formula 2]

[0020] 0.15≤T1 / (T2+T3)≤0.85

[0021] In one embodiment, the positive electrode may satisfy Formula 3.

[0022] [Formula 3]

[0023] 0.15≤T3 / (T1+T2)≤0.85

[0024] In Formulas 1 to 3, T1 is the thickness of the first positive electrode active material layer measured in a scanning electron microscope (SEM) image of a cross section of the positive electrode, T2 is the thickness of the second positive electrode active material layer in the scanning electron microscope image, and T3 is the thickness of the third positive electrode active material layer in the scanning electron microscope image.

[0025] In one embodiment, the sum of the thickness of the first positive electrode active material layer, the thickness of the second positive electrode active material layer, and the thickness of the third positive electrode active material layer may be 40 μm to 160 μm.

[0026] In one embodiment, the content of the first lithium metal oxide particles may be 90 wt % or more in the total weight of the first positive active material layer.

[0027] In one embodiment, the content of the second lithium metal oxide particles may be 90 wt % or more in the total weight of the second positive active material layer.

[0028] In one embodiment, the content of the third lithium metal oxide particles may be 90 wt % or more in the total weight of the third positive electrode active material layer.

[0029] In one embodiment, the first lithium metal oxide particles and the second lithium metal oxide particles may contain nickel (Ni), respectively. The concentration of nickel in the first lithium metal oxide particles may be greater than the concentration of nickel in the second lithium metal oxide particles.

[0030] In one embodiment, the second lithium metal oxide particles and the third lithium metal oxide particles may contain nickel (Ni), respectively. The concentration of nickel in the third lithium metal oxide particles may be greater than the concentration of nickel in the second lithium metal oxide particles.

[0031] In one embodiment, the particle size (D 50 ) may be larger than the particle size (D 50 ).

[0032] In one embodiment, the particle size (D 50 ) may be smaller than the particle size (D 50 ).

[0033] In one embodiment, the particle size (D 50 ) and the particle size of the second lithium metal oxide particle (D 50 ) can be greater than 0.75 and less than 1.4.

[0034] In one embodiment, the particle size (D 50 ) may be 5 μm to 15 μm, and the particle size (D 50 ) can be from 1 μm to 5 μm.

[0035] A lithium secondary battery according to an exemplary embodiment of the present invention may include: the positive electrode for the lithium secondary battery; and a negative electrode disposed opposite to the positive electrode.

[0036] (III) Beneficial effects

[0037] According to an exemplary embodiment of the present invention, a positive electrode for a lithium secondary battery having improved power characteristics, life characteristics, high temperature storage characteristics, and resistance characteristics may be provided.

[0038] According to an exemplary embodiment of the present invention, a lithium secondary battery having improved power characteristics, life characteristics, high temperature storage characteristics, and resistance characteristics by including the positive electrode may be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0040] Figure 2 and Figure 3 1 and 2 are respectively a plan view and a cross-sectional view of a lithium secondary battery according to an exemplary embodiment. DETAILED DESCRIPTION

[0041] According to an exemplary embodiment of the present invention, a positive electrode for a lithium secondary battery is provided, the positive electrode for a lithium secondary battery including a positive electrode active material layer including lithium metal oxide particles having a predetermined morphology and having a multilayer structure. In addition, a lithium secondary battery including the positive electrode is provided.

[0042] Hereinafter, a positive electrode for a lithium secondary battery and a lithium secondary battery according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the drawings and embodiments are merely exemplary, and the present invention is not limited thereto.

[0043] Positive electrode for lithium secondary battery

[0044] Figure 1 : is a cross-sectional view schematically showing a positive electrode for a lithium secondary battery (hereinafter, simply referred to as a positive electrode) according to one embodiment of the present invention.

[0045] Reference Figure 1 The positive electrode 100 may include a positive electrode collector 105 and a positive electrode active material layer 110 formed on the positive electrode collector 105. The positive electrode active material layer 110 may be formed on one side or both sides of the positive electrode collector 105.

[0046] The positive active material layer 110 may include a first positive active material layer 112 , a second positive active material layer 114 , and a third positive active material layer 116 sequentially formed on the positive current collector 105 .

[0047] In one embodiment, the first positive active material layer 112 may be directly formed on the positive current collector 105 , the second positive active material layer 114 may be directly formed on the first positive active material layer 112 , and the third positive active material layer 116 may be directly formed on the second positive active material layer 114 .

[0048] The positive electrode active material layer 110 may include a positive electrode active material that can intercalate and deintercalate lithium. For example, the positive electrode active material may include lithium metal oxide particles.

[0049] In one embodiment, the lithium metal oxide particles may include lithium cobalt-based oxide particles (LCO), lithium manganese-based oxide particles (LMO), lithium nickel-based oxide particles (LNO), lithium nickel-manganese-based oxide particles (LNMO), lithium nickel-cobalt-manganese-based oxide particles (NCM), lithium nickel-cobalt-aluminum-based oxide particles (NCA), lithium iron phosphate-based oxide particles (LFP), over-lithiated oxide particles (OLO), and the like.

[0050] In an exemplary embodiment of the present invention, the first positive active material layer 112 may include first lithium metal oxide particles in the form of secondary particles in which a plurality of primary particles are aggregated.

[0051] The second positive active material layer 114 may include second lithium metal oxide particles in the form of single particles.

[0052] The third positive active material layer 116 may include third lithium metal oxide particles in the form of secondary particles in which a plurality of primary particles are aggregated.

[0053] For example, the combination of the first positive electrode active material layer 112 , the second positive electrode active material layer 114 , and the third positive electrode active material layer 116 can improve the resistance characteristics of the positive electrode, the power characteristics, the life characteristics, and the high-temperature storage characteristics of the lithium secondary battery.

[0054] The secondary particles and the single particles may be distinguished from each other morphologically. For example, based on a cross-sectional image of the particles measured by a scanning electron microscope (SEM), the secondary particles and the single particles may be distinguished from each other.

[0055] For example, the secondary particles may include a plurality of primary particles, and the boundaries of the primary particles may be observed in a SEM cross-sectional image. For example, more than 10, more than 30, more than 50, or more than 100 primary particles may be aggregated in the secondary particles.

[0056] For example, the single particle may represent a monolith rather than an aggregate, and unlike the secondary particle, the boundary of the primary particle may not be observed in a SEM cross-sectional image.

[0057] In addition, fine particles (for example, particles having a volume of 1 / 100 or less relative to the volume of the single particle) may be attached to the surface of the single particle, and this form is not excluded from the concept of the single particle.

[0058] For example, the single particles may be present in contact with each other. For example, 2 to 10, 2 to 5, or 2 or 3 single particles may be present in contact with each other.

[0059] In one embodiment, the positive electrode 100 may satisfy the following Formula 1. Therefore, the lifespan characteristics and high-temperature storage characteristics of the lithium secondary battery may be further improved.

[0060] [Formula 1]

[0061] 0.2≤T2 / (T1+T2+T3)≤0.7

[0062] In some embodiments, T2 / (T1+T2+T3) may be 0.2 to 0.5 or 0.25 to 0.4.

[0063] In one embodiment, the positive electrode 100 may satisfy the following Formula 2. In this case, the resistance characteristics of the positive electrode may be further improved.

[0064] [Formula 2]

[0065] 0.15≤T1 / (T2+T3)≤0.85

[0066] In some embodiments, T1 / (T2+T3) may be 0.15 to 0.7, 0.2 to 0.65, 0.3 to 0.6, or 0.4 to 0.6.

[0067] In one embodiment, the positive electrode 100 may satisfy the following Formula 3. In this case, the power characteristics of the lithium secondary battery may be further improved.

[0068] [Formula 3]

[0069] 0.15≤T3 / (T1+T2)≤0.85

[0070] In some embodiments, T3 / (T1+T2) may be 0.15 to 0.7, 0.2 to 0.65, 0.3 to 0.6, or 0.4 to 0.6.

[0071] In Formulas 1 to 3, T1 is the thickness of the first positive electrode active material layer 112 in a scanning electron microscope (SEM) image of a cross section of the positive electrode, T2 is the thickness of the second positive electrode active material layer 114 in the scanning electron microscope image, and T3 is the thickness of the third positive electrode active material layer 116 in the scanning electron microscope image.

[0072] In some embodiments, T2 / (T1+T2+T3) may be 0.25 to 0.4, T1 / (T2+T3) may be 0.4 to 0.6, and T3 / (T1+T2) may be 0.4 to 0.6. Within the above range, the power characteristics, life characteristics, high temperature storage characteristics, and resistance characteristics of the lithium secondary battery may be further improved.

[0073] In one embodiment, the thickness of the positive electrode active material layer 110 (for example, the sum of T1, T2 and T3) can be 20 μm to 500 μm, 30 μm to 450 μm, 35 μm to 300 μm, 40 μm to 250 μm, 40 μm to 200 μm, 40 μm to 160 μm, 50 μm to 160 μm, 60 μm to 160 μm or 80 μm to 160 μm. Within the above range, the power characteristics, life characteristics, high temperature storage characteristics and resistance characteristics of the lithium secondary battery can be further improved. In addition, the energy density of the lithium secondary battery can be further improved.

[0074] In one embodiment, the positive active material layer 110 may further include a conductive material and a binder.

[0075] In some embodiments, the adhesive may include an organic adhesive such as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, polymethyl methacrylate (PMMA), or a water-based adhesive such as styrene-butadiene rubber (SBR). For example, the adhesive may also be used together with a thickener such as carboxymethyl cellulose (CMC).

[0076] In some embodiments, the conductive material may include carbon-based conductive materials such as graphite, graphene, carbon black, acetylene black, carbon nanotubes (CNTs), etc.; metal-based conductive materials including tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3 and other perovskite materials, etc.

[0077] In one embodiment, the content of the first lithium metal oxide particles in the total weight of the first positive electrode active material layer 112 may be 70 wt % or more, in one embodiment, the content of the first lithium metal oxide particles may be 80 wt % or more, in one embodiment, the content of the first lithium metal oxide particles may be 90 % or more.

[0078] In one embodiment, the content of the second lithium metal oxide particles in the total weight of the second positive electrode active material layer 114 may be 70 wt % or more, in one embodiment, the content of the second lithium metal oxide particles may be 80 wt % or more, in one embodiment, the content of the second lithium metal oxide particles may be 90 % or more.

[0079] In one embodiment, the content of the third lithium metal oxide particles in the total weight of the third positive electrode active material layer 112 may be 70 wt % or more. In one embodiment, the content of the third lithium metal oxide particles may be 80 wt % or more. In one embodiment, the content of the third lithium metal oxide particles may be 90 wt % or more.

[0080] In one embodiment, the particle size (D 50 ) may be larger than the particle size (D 50 ). Therefore, the resistance characteristics of the positive electrode can be further improved.

[0081] In one embodiment, the particle size (D 50 ) may be smaller than the particle size (D 50 ).

[0082] In one embodiment, the particle size (D 50 ) may be 3 μm to 20 μm. In one embodiment, the particle size (D 50 ) may be 4 μm to 18 μm. In one embodiment, the particle size (D 50 ) can be 5μm to 15μm.

[0083] In one embodiment, the particle size (D 50 ) may be 1 μm to 9 μm. In one embodiment, the particle size (D 50 ) may be 1 μm to 7 μm. In one embodiment, the particle size (D 50 ) can be from 1 μm to 5 μm.

[0084] In one embodiment, the particle size (D 50 ) may be 1 μm to 9 μm. In one embodiment, the particle size (D 50) may be 1 μm to 7 μm. In one embodiment, the particle size (D 50 ) may be 1 μm to 5 μm. Therefore, the power characteristics of the lithium secondary battery can be further improved.

[0085] For example, the particle size (D 50 ) is a particle size based on 50% of the volume cumulative particle size distribution and can be measured by a laser diffraction method.

[0086] In one embodiment, the particle size (D 50 ) and the particle size of the second lithium metal oxide particle (D 50 ) can be 0.75 or more and less than 1.4. Within the above range, the resistance characteristics of the positive electrode, the power characteristics and the fast charging performance of the lithium secondary battery can be further improved.

[0087] In some embodiments, the particle size (D 50 ) and the particle size of the second lithium metal oxide particle (D 50 ) can be in the range of 0.8 to 1.35, 0.85 to 1.3, 0.9 to 1.25, 0.9 to 1.2 or 0.9 to 1.1.

[0088] In one embodiment, the first lithium metal oxide particle, the second lithium metal oxide particle, and the third lithium metal oxide particle may contain nickel, respectively.

[0089] In some embodiments, the first lithium metal oxide particle, the second lithium metal oxide particle, and the third lithium metal oxide particle may further contain cobalt and manganese, respectively.

[0090] In some embodiments, the first lithium metal oxide particle, the second lithium metal oxide particle, and the third lithium metal oxide particle may be represented by the following Chemical Formula 1, respectively.

[0091] [Chemical formula 1]

[0092] Li x Ni (1-a-b) M1 a M2 b O y

[0093] In Chemical Formula 1, M1 and M2 may be independently at least one of Co, Mn, Al, Zr, Ti, Cr, B, Mn, Ba, Si, Y, W, and Sr, and may be 0.9≤x≤1.2, 1.9≤y≤2.1, and 0≤a+b≤0.5.

[0094] In some embodiments, 0.9 ≤ x ≤ 1.1.

[0095] In some embodiments, 0 < a + b ≤ 0.4, 0 < a + b ≤ 0.3, or 0 < a + b ≤ 0.2.

[0096] In one embodiment, the concentration of nickel in the first lithium metal oxide particles may be greater than the concentration of nickel in the second lithium metal oxide particles. The concentration of nickel may refer to the molar ratio of nickel in the lithium metal oxide particles to the total number of moles of all elements other than lithium and oxygen. In this case, the capacity characteristics, life characteristics, and high-temperature storage characteristics of the lithium secondary battery can be further improved.

[0097] In some embodiments, the difference in the concentration of nickel between the first lithium metal oxide particles and the second lithium metal oxide particles may be 0.05 or more, 0.1 or more, 0.2 or more, 0.3 or more. Additionally, the difference in the concentration of nickel may be 0.5 or less.

[0098] In one embodiment, the concentration of nickel in the third lithium metal oxide particles may be greater than the concentration of nickel in the second lithium metal oxide particles.

[0099] In some embodiments, the difference in the concentration of nickel between the second lithium metal oxide particles and the third lithium metal oxide particles may be 0.05 or more, 0.1 or more, 0.2 or more, 0.3 or more. Additionally, the difference in the concentration of nickel may be 0.5 or less.

[0100] In one embodiment, the first lithium metal oxide particles, the second lithium metal oxide particles, and the third 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.

[0101] In one embodiment, a coating may be formed on the surfaces of the first lithium metal oxide particles, the second lithium metal oxide particles, and the third lithium metal oxide particles. For example, the coating may contain Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, etc.

[0102] Lithium secondary battery

[0103] Figure 2 is a plan view schematically showing a lithium secondary battery according to an embodiment of the present invention, Figure 3 is a cross-sectional view taken along the I-I' line of Figure 2

[0104] Refer to​ Figure 2 and Figure 3 In the case of a lithium secondary battery, it may include a positive electrode 100 and a negative electrode 130 disposed opposite to the positive electrode 100.

[0105] The positive electrode 100 may be the positive electrode for the lithium secondary battery described above.

[0106] 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 surfaces of the negative electrode current collector 125.

[0107] For example, the negative electrode active material layer 120 may contain a negative electrode active material. In one embodiment, the negative electrode active material layer 120 may further contain an adhesive, a conductive material, etc.

[0108] For example, the negative electrode current collector 125 may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0109] For example, the negative electrode active material may be a material that allows lithium ions to be intercalated and deintercalated. For example, the negative electrode active material may include a lithium alloy, a carbon-based active material, a silicon-based active material, etc., and these may be used alone or in combination of two or more.

[0110] For example, the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc.

[0111] For example, the carbon-based active material may include crystalline carbon, amorphous carbon, a carbon composite, carbon fiber, etc. For example, the amorphous carbon may include hard carbon, coke, mesocarbon microbeads, mesophase pitch-based carbon fiber, etc. For example, the crystalline carbon may include natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbeads (MCMB), graphitized mesophase pitch-based carbon fiber (MPCF), etc.

[0112] 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, etc. In this case, a lithium secondary battery with a high capacity can be achieved.

[0113] 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.

[0114] For example, the positive electrode 100 and the negative electrode 130 may be alternately and repeatedly arranged to form an electrode assembly 150.

[0115] In one embodiment, the separator 140 may be disposed 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.

[0116] In one embodiment, the separator 140 may include a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer, ethylene-methacrylate copolymer, etc. In addition, the separator 140 may include a non-woven fabric formed of high melting point glass fiber, polyethylene terephthalate fiber, etc.

[0117] For example, the electrode assembly 150 may be housed together with an electrolyte in the case 160 to form a lithium secondary battery.

[0118] For example, the lithium secondary battery may be manufactured in a cylindrical shape, a prismatic shape, a pouch shape, a coin shape, or the like.

[0119] For example, the electrolyte may include a lithium salt and an organic solvent.

[0120] In one embodiment, the lithium salt may be Li + X - For example, the X - 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 - 、(CF3CF2SO2)2N - wait.

[0121] In some embodiments, the lithium salt may include LiBF4, LiPF6, and the like.

[0122] In one embodiment, the organic solvent may include a carbonate-based solvent, an ester-based (carboxylate-based) solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, an aprotic solvent, and the like.

[0123] For example, the carbonate-based solvent may include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), and the like.

[0124] For example, the ester-based solvent may include methyl propionate, ethyl propionate, propyl acetate, butyl acetate, ethyl acetate, butyrolactone, caprolactone, valerolactone, and the like.

[0125] For example, the ether-based solvent may include dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, and the like.

[0126] For example, the ketone-based solvent may include cyclohexanone or the like.

[0127] For example, the alcohol-based solvent may include ethyl alcohol, isopropyl alcohol, and the like.

[0128] For example, the aprotic solvent may include a nitrile-based solvent, an amide-based solvent (eg, dimethylformamide), a dioxolane-based solvent (eg, 1,3-dioxolane), a sulfolane-based solvent, and the like.

[0129] In one embodiment, the lithium secondary battery may include a positive electrode lead 107 connected to the positive electrode 100 and protruding outside the case 160 , and a negative electrode lead 127 connected to the negative electrode 130 and protruding outside the case 160 .

[0130] For example, the positive electrode lead 107 may be electrically connected to the positive electrode collector 105. In addition, the negative electrode lead 127 may be electrically connected to the negative electrode collector 125.

[0131] The positive electrode collector 105 may include a protruding positive electrode tab 106 on one side. The positive electrode active material layer 110 may not be formed on the positive electrode tab 106. The positive electrode tab 106 may be formed integrally with the positive electrode collector 105, or may be connected by welding or the like. The positive electrode collector 105 and the positive electrode lead 107 may be electrically connected through the positive electrode tab 106.

[0132] The negative electrode current collector 125 may include a protruding negative electrode tab 126 on one side. The negative electrode active material layer 120 may not be formed on the negative electrode tab 126. The negative electrode tab 126 may be formed integrally with the negative electrode current collector 125, or may be 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.

[0133] Hereinafter, exemplary embodiments of the present invention will be described, but the following embodiment is merely one embodiment of the present invention, and the present invention is not limited to the following embodiment.

[0134] Preparation Example 1-1: NCM 523 (D 50 =10μm)

[0135] NiSO4, CoSO4 and MnSO4 were added to distilled water from which internal dissolved oxygen was removed at a molar ratio of 5:2:3 to prepare a mixed solution.

[0136] The mixed solution, NaOH (precipitant) and NH4OH (chelating agent) were added into a reactor and coprecipitated for 30 hours to obtain metal hydroxide particles (Ni 0.5 Co 0.2 Mn 0.3 (OH)2). The metal hydroxide particles were dried at 80°C for 12 hours and then dried again at 110°C for 12 hours.

[0137] The metal hydroxide particles and lithium hydroxide were added into a dry high-speed mixer so that the molar ratio of the metal hydroxide particles to the lithium hydroxide was 1:1.03, thereby preparing a mixture.

[0138] The mixture was placed in a calcining furnace, and the temperature was raised to 700° C. at a rate of 2° C. / min, and maintained at 700° C. for 10 hours. During the calcining process, oxygen was continuously introduced into the calcining furnace at a flow rate of 10 mL / min.

[0139] After the calcination was completed, the calcined product was cooled to room temperature, and then crushed and classified to obtain lithium metal oxide particles (NCM 523; LiNi 0.5 Co 0.2 Mn 0.3 O2).

[0140] Measure the particle size (D 50 ). Particle size (D 50 ) is the particle size based on 50% of the volume cumulative particle size distribution and is measured by laser diffraction method (Microtrac MT 3000). The measured particle size (D 50 ) is 10μm.

[0141] Preparation Example 1-2: NCM 523 (D 50 =5μm)

[0142] The same method as in Preparation Example 1-1 was used, except that the coprecipitation reaction time was changed to obtain lithium metal oxide particles in the form of secondary particles (D 50 =5μm).

[0143] Preparation Example 1-3: NCM 523 (D 50 =7μm)

[0144] The same method as in Preparation Example 1-1 was used, except that the coprecipitation reaction time was changed to obtain lithium metal oxide particles in the form of secondary particles (D 50 =7μm).

[0145] Preparation Example 1-4: NCM 811 (D 50 =10μm)

[0146] The lithium metal oxide particles (NCM 811; LiNi 200,000; NCM ... 0.8 Co 0.1 Mn 0.1 O2,D 50 =10μm).

[0147] Preparation Example 1-5: NCM 811 (D 50 =5μm)

[0148] The same method as in Preparation Example 1-4 was used, except that the coprecipitation reaction time was changed to obtain lithium metal oxide particles in the form of secondary particles (D 50 =5μm).

[0149] Preparation Example 2-1: NCM 523 (D 50 =5μm)

[0150] NiSO4, CoSO4 and MnSO4 were added to distilled water from which internal dissolved oxygen was removed at a molar ratio of 5:2:3 to prepare a mixed solution.

[0151] The mixed solution, NaOH (precipitant) and NH4OH (chelating agent) were added to a reactor and coprecipitated for 72 hours to obtain metal hydroxide particles (Ni 0.5 Co 0.2 Mn 0.3 (OH)2). The metal hydroxide particles were dried at 100°C for 12 hours and then dried again at 120°C for 12 hours.

[0152] The metal hydroxide particles and lithium hydroxide were added into a dry high-speed mixer so that the molar ratio of the metal hydroxide particles to the lithium hydroxide was 1:1.03, thereby preparing a mixture.

[0153] The mixture was placed in a calcining furnace, and the temperature was raised to 980° C. at a rate of 2° C. / min, and maintained at 980° C. for 12 hours. During the calcining process, oxygen was continuously introduced into the calcining furnace at a flow rate of 10 mL / min.

[0154] After the calcination is completed, the calcined product is cooled to room temperature, crushed and classified to obtain lithium metal oxide particles (LiNi 0.5 Co 0.2 Mn 0.3 O2).

[0155] Measure the particle size (D 50 ). Particle size (D 50 ) is the particle size based on the 50% volume cumulative particle size distribution and is measured using the laser diffraction method (Microtrac MT 3000). The measured particle size (D 50 ) is 5μm.

[0156] Preparation Example 2-2: NCM 523 (D 50 =7μm)

[0157] The same method as in Preparation Example 2-1 was used, except that the coprecipitation reaction time was changed to prepare lithium metal oxide particles in the form of single particles (D 50 =7μm).

[0158] Examples and Comparative Examples

[0159] Example 1

[0160] (1) Production of positive electrode (including positive electrode active material layer with three-layer structure)

[0161] The lithium metal oxide particles of Preparation Example 1-1, carbon nanotubes (CNTs), and polyvinylidene fluoride (PVDF) were dispersed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 98:1:1 to prepare a first positive electrode slurry.

[0162] The lithium metal oxide particles of Preparation Example 2-1, carbon nanotubes (CNTs), and PVDF were dispersed in NMP at a weight ratio of 98:1:1 to prepare a second positive electrode slurry.

[0163] The lithium metal oxide particles of Preparation Example 1-2, carbon nanotubes (CNTs), and PVDF were dispersed in NMP at a weight ratio of 98:1:1 to prepare a third positive electrode slurry.

[0164] The first positive electrode slurry, the second positive electrode slurry and the third positive electrode slurry are sequentially coated on an area other than the protrusion (positive electrode tab) of an aluminum foil having a protrusion on one side, and dried and rolled to obtain a positive electrode including a first positive electrode active material layer, a second positive electrode active material layer and a third positive electrode active material layer.

[0165] The density (i.e., compaction density) of the positive electrode active material layer was adjusted to 3.5 g / square centimeter (cc). The total thickness of the positive electrode active material layer was adjusted to 150 μm. The coating amount of the first positive electrode slurry to the third positive electrode slurry was adjusted so that the first positive electrode active material layer to the third positive electrode active material layer satisfied the thickness relationship recorded in Table 1.

[0166] (2) Manufacturing of lithium secondary batteries

[0167] Artificial graphite, carbon nanotubes (CNTs), styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were dispersed in distilled water at a weight ratio of 95:3:1:1 to prepare a negative electrode slurry.

[0168] The negative electrode slurry was applied to a copper foil having a protrusion (negative electrode tab) on one side except for the protrusion, and dried and rolled to prepare a negative electrode.

[0169] A polyethylene separator (thickness of 20 μm) was placed between the positive electrode and the negative electrode to form an electrode assembly. A positive electrode lead and a negative electrode lead were connected to the positive electrode tab and the negative electrode tab, respectively, by welding.

[0170] A 1M LiPF6 solution (a mixed solvent of EC and EMC with a volume ratio (v / v) of 30:70) was prepared. Based on the total weight of the electrolyte, 1 wt% of fluoroethylene carbonate (FEC), 1 wt% of lithium difluorophosphate (LiPO2F2), 0.5 wt% of 1,3-propane sultone (PS) and 0.5 wt% of 1-propylene-1,3-sultone (PRS) were added to prepare an electrolyte.

[0171] The electrode assembly is housed in a soft pack such that a portion of the positive electrode lead and the negative electrode lead are exposed to the outside, and three surfaces except for an electrolyte injection portion surface are sealed.

[0172] The electrolyte is injected into the interior of the soft package, and the electrolyte injection portion is sealed to manufacture a lithium secondary battery.

[0173] Example 2 to Example 7

[0174] A positive electrode and a lithium secondary battery are manufactured by the same method as in Example 1, except that the thicknesses of the first to third positive electrode active material layers are adjusted to different thicknesses by adjusting the coating amounts of the first to third positive electrode slurries to different coating amounts.

[0175] The total thickness of the positive electrode active material layer remains the same as that of Example 1.

[0176] Example 8

[0177] A positive electrode and a lithium secondary battery were manufactured by the same method as in Example 1, except that the lithium metal oxide particles of Preparation Example 1-3 were used when preparing the third positive electrode slurry.

[0178] Example 9

[0179] A positive electrode and a lithium secondary battery were manufactured by the same method as in Example 1, except that the lithium metal oxide particles of Preparation Example 2-2 were used when preparing the second positive electrode slurry.

[0180] Example 10

[0181] A positive electrode and a lithium secondary battery were manufactured by the same method as in Example 1, except that the lithium metal oxide particles of Preparation Examples 1-4 were used in preparing the first positive electrode slurry, and the lithium metal oxide particles of Preparation Examples 1-5 were used in preparing the third positive electrode slurry.

[0182] Comparative Example 1

[0183] The first positive electrode slurry and the second positive electrode slurry are sequentially coated on an aluminum foil and dried and rolled to prepare a positive electrode including a first positive electrode active material layer and a second positive electrode active material layer.

[0184] The thickness of the first positive electrode active material layer and the thickness of the second positive electrode active material layer were adjusted to 1:1. The total thickness of the positive electrode active material layer was maintained at the same thickness as that of Example 1.

[0185] A lithium secondary battery was manufactured by the same method as in Example 1, except that the positive electrode was used.

[0186] Comparative Example 2

[0187] The second positive electrode slurry and the first positive electrode slurry are sequentially coated on an aluminum foil and dried and rolled to prepare a positive electrode including a first positive electrode active material layer and a second positive electrode active material layer.

[0188] The thickness of the first positive electrode active material layer and the thickness of the second positive electrode active material layer were adjusted to 1:1. The total thickness of the positive electrode active material layer was maintained at the same thickness as that of Example 1.

[0189] A lithium secondary battery was manufactured by the same method as in Example 1, except that the positive electrode was used.

[0190] Comparative Example 3

[0191] The second positive electrode slurry, the first positive electrode slurry and the second positive electrode slurry are sequentially coated on an aluminum foil and dried and rolled to prepare a positive electrode including a first positive electrode active material layer, a second positive electrode active material layer and a third positive electrode active material layer.

[0192] A lithium secondary battery was manufactured by the same method as in Example 1, except that the positive electrode was used.

[0193] Experimental Example 1: Measurement of Thickness of the First to Third Positive Electrode Active Material Layers

[0194] The positive electrodes of Example and Comparative Example 3 were cut in the width direction (ie, the direction in which the minor axis of the positive electrode extends), and the cross sections were measured using a scanning electron microscope (SEM).

[0195] In the SEM cross-sectional image, thicknesses T1 , T2 , and T3 of each of the first positive electrode active material layer, the second positive electrode active material layer, and the third positive electrode active material layer were measured.

[0196] Among them, during rolling, a small amount of lithium metal oxide particles in the form of single particles may be mixed between lithium metal oxide particles in the form of secondary particles. Therefore, a mixed region may be formed between the first positive electrode active material layer and the second positive electrode active material layer. The thickness of the mixed region is considered to be the thickness of the layer (e.g., the first positive electrode active material layer) in which the lithium metal oxide particles in the form of single particles are mixed.

[0197] The values ​​of the following Formulas 1 to 3 are calculated using the measured T1 to T3.

[0198] [Formula 1]

[0199] T2 / (T1+T2+T3)

[0200] [Formula 2]

[0201] T1 / (T2+T3)

[0202] [Formula 3]

[0203] T3 / (T1+T2)

[0204] Experimental Example 2: Measurement of bulk resistance and interface resistance of positive electrode

[0205] The interface resistance (resistance between the positive electrode active material layer and the positive electrode current collector) and the bulk resistance (resistance of the positive electrode active material layer) of the positive electrodes of the examples and comparative examples were measured.

[0206] The interface resistance and the bulk resistance were measured using a resistivity meter (RESISTIVITY METER) manufactured by HIOKI Corporation.

[0207] A current is passed through the surface of the positive electrode active material layer, and the potential generated on the surface is measured at multiple points. The positive electrode is modeled as a three-dimensional resistance matrix, and the potential is calculated by passing a current through the three-dimensional resistance matrix in a computer simulation, and the resistance value when the actual measured surface potential distribution is consistent with the potential distribution in the simulation is used as the body resistance.

[0208] Experimental Example 3

[0209] (1) Measurement of high temperature storage capacity retention

[0210] The lithium secondary batteries of the examples and comparative examples were subjected to repeated CC / CV charging (0.5C 4.2V, 0.05C CUT-OFF) and CC discharging (0.5C 2.7V CUT-OFF) 3 times at 25°C, and the third discharge capacity H1 was measured.

[0211] The lithium secondary battery was again subjected to CC / CV charging (0.5C 4.2V 0.05C cutoff).

[0212] The charged lithium secondary battery was stored at 60° C. for 12 weeks, and then further left at room temperature for 30 minutes, and subjected to CC discharge (0.5 C 2.75 V cutoff), and the discharge capacity H2 was measured.

[0213] The capacity retention ratio was calculated as follows.

[0214] Capacity retention rate (%) = H2 / H1×100 (%)

[0215] (2) Measurement of DCIR increase rate during high temperature storage

[0216] The lithium secondary batteries of the examples and comparative examples were charged at 25° C. at 0.5 C CC / CV (4.2 V 0.05 C cutoff) and then discharged at 0.5 C CC to SOC 60%.

[0217] At SOC 60%, the C-rate was changed to 0.2C, 0.5C, 1C, 1.5C, 2C, 2.5C and 3.0C, and discharge and supplementary charge were performed for 10 seconds respectively, and DCIR R1 was measured.

[0218] When the above-mentioned discharge and supplementary charge are performed, the end point of the voltage is taken as the straight line equation, and its slope is taken as DCIR.

[0219] The charged lithium secondary battery was left for 12 weeks under the condition of being exposed to the atmosphere at 60° C. and then further left at normal temperature for 30 minutes, and DCIR R2 was measured by the same method as described above.

[0220] The internal resistance increase rate was calculated as follows.

[0221] Internal resistance increase rate (%) = (R2-R1) / R1×100 (%)

[0222] (3) Confirmation of vent timing

[0223] The lithium secondary batteries of the examples and comparative examples were charged (CC / CV 0.33C, 4.3V, 0.05C cutoff) to SOC 100%.

[0224] The charged lithium secondary battery was left to stand under atmospheric conditions at 60°C.

[0225] Confirm the time when the sealing part of the soft bag is degassed.

[0226] When the seal portion swelled or individual vent holes appeared, it was evaluated that venting occurred.

[0227] Experimental Example 4

[0228] (1) Measurement of fast charge capacity retention rate

[0229] The lithium secondary batteries of the embodiment and the comparative example were charged to SOC 8% at 0.33C at 25°C, and step-charged at 2.5C-2.25C-2C-1.75C-1.5C-1.0C in the SOC 8-80% range, and charged again at 0.33C (4.3V, 0.05C cut-off) in the SOC 80-100% range, for a total of 25 minutes. The time was adjusted according to the same benchmark, and then CC discharged to 2.5V at 0.33C.

[0230] The first discharge capacity S1 was measured, the above-mentioned charge and discharge were repeated 100 times, and the 100th discharge capacity S2 was measured.

[0231] The fast charge capacity retention rate was calculated according to the following formula.

[0232] Fast charge capacity retention rate (%) = S2 / S1×100 (%)

[0233] (2) Measurement of cycle capacity retention (life-span)

[0234] The lithium secondary batteries of Examples and Comparative Examples were subjected to CC / CV charging (0.33C 4.3V 0.05C cutoff) and CC discharging (0.33C 2.5V cutoff) at 45°C.

[0235] The above charge and discharge were repeated 500 times, and the discharge capacity C2 at the 500th time was measured.

[0236] The cycle capacity retention rate was calculated according to the following formula.

[0237] Cycle capacity retention rate (%) = C2 / C1×100 (%)

[0238] [Table 1]

[0239]

[0240] [Table 2]

[0241]

[0242] Referring to Table 1 and Table 2, the resistance characteristics, life characteristics, high temperature storage characteristics and / or power characteristics of the positive electrode in the lithium secondary battery of the embodiment are improved compared with the lithium secondary battery of the comparative example.

Claims

1. A positive electrode for a lithium secondary battery, comprising: Positive electrode current collector; a first positive electrode active material layer, the first positive electrode active material layer being formed on the positive electrode current collector and comprising first lithium metal oxide particles in the form of secondary particles formed by aggregation of primary particles; a second positive electrode active material layer formed on the first positive electrode active material layer and including second lithium metal oxide particles in a single particle form; and A third positive electrode active material layer is formed on the second positive electrode active material layer and includes third lithium metal oxide particles in the form of secondary particles in which primary particles are aggregated.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein The positive electrode for the lithium secondary battery satisfies Formula 1, [Formula 1] 0.2≤T2 / (T1+T2+T3)≤0.7 In Formula 1, T1 is the thickness of the first positive electrode active material layer measured in a scanning electron microscope image of a cross section of the positive electrode, T2 is the thickness of the second positive electrode active material layer in the scanning electron microscope image, and T3 is the thickness of the third positive electrode active material layer in the scanning electron microscope image.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein The positive electrode for the lithium secondary battery satisfies Formula 2, [Formula 2] 0.15≤T1 / (T2+T3)≤0.85 In Formula 2, T1 is the thickness of the first positive electrode active material layer measured in a scanning electron microscope image of a cross section of the positive electrode, T2 is the thickness of the second positive electrode active material layer in the scanning electron microscope image, and T3 is the thickness of the third positive electrode active material layer in the scanning electron microscope image.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein The positive electrode for the lithium secondary battery satisfies Formula 3, [Formula 3] 0.15≤T3 / (T1+T2)≤0.85 In Formula 3, T1 is the thickness of the first positive electrode active material layer measured in a scanning electron microscope image of a cross section of the positive electrode, T2 is the thickness of the second positive electrode active material layer in the scanning electron microscope image, and T3 is the thickness of the third positive electrode active material layer in the scanning electron microscope image.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein The sum of the thickness of the first positive electrode active material layer, the thickness of the second positive electrode active material layer, and the thickness of the third positive electrode active material layer is 40 μm to 180 μm.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein The content of the first lithium metal oxide particles is 90 wt % or more in the total weight of the first positive electrode active material layer.

7. The positive electrode for a lithium secondary battery according to claim 1, wherein The content of the second lithium metal oxide particles is 90 wt % or more in the total weight of the second positive electrode active material layer.

8. The positive electrode for a lithium secondary battery according to claim 1, wherein The content of the third lithium metal oxide particles is 90 wt % or more in the total weight of the third positive electrode active material layer.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein The first lithium metal oxide particles and the second lithium metal oxide particles respectively contain nickel (Ni), and a concentration of nickel in the first lithium metal oxide particles is greater than a concentration of nickel in the second lithium metal oxide particles.

10. The positive electrode for a lithium secondary battery according to claim 1, wherein The second lithium metal oxide particles and the third lithium metal oxide particles respectively contain nickel (Ni), and a concentration of nickel in the third lithium metal oxide particles is greater than a concentration of nickel in the second lithium metal oxide particles.

11. The positive electrode for a lithium secondary battery according to claim 1, wherein The particle size D of the first lithium metal oxide particles 50 is larger than the particle size D of the second lithium metal oxide particle 50 .

12. The positive electrode for a lithium secondary battery according to claim 1, wherein The particle size D of the third lithium metal oxide particles 50 smaller than the particle size D of the first lithium metal oxide particle 50 .

13. The positive electrode for a lithium secondary battery according to claim 1, wherein The particle size D of the third lithium metal oxide particles 50 The particle size D of the second lithium metal oxide particle 50 The ratio is greater than or equal to 0.75 and less than 1.

4.

14. The positive electrode for a lithium secondary battery according to claim 1, wherein The particle size D of the first lithium metal oxide particles 50 The particle size D of the second lithium metal oxide particles and the third lithium metal oxide particles is 5 μm to 15 μm. 50 1μm to 5μm.

15. A lithium secondary battery comprising: The positive electrode for a lithium secondary battery according to claim 1; as well as A negative electrode is arranged opposite to the positive electrode.

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