Positive electrode for lithium secondary battery and lithium secondary battery

Abstract

To provide a positive electrode for a lithium secondary battery having increased penetration resistance when a metal body such as a nail penetrates the electrode from the outside while having high capacity, high output performance, excellent cycle characteristics and thermal stability.SOLUTION: And one or more second positive electrode material mixture layers disposed on the first positive electrode material mixture layer, wherein the first positive electrode material mixture layer includes a first positive electrode active material and a first binder, the second positive electrode material mixture layer includes a second positive electrode active material and a second binder, the first positive electrode active material has an average particle diameter (D50) of 3 μm or less in a smaller range than an average particle diameter (D50) of the second positive electrode active material, and the positive electrode has an elongation of 0.5% to 2.0%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0153024, filed on November 16, 2020, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery, and more particularly to a positive electrode for a lithium secondary battery and a lithium secondary battery with improved safety. [Background technology]

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

[0004] Recently, as lithium secondary batteries are increasingly used as power sources for medium- to large-sized devices such as electric vehicles, there is a growing demand for higher capacity, higher energy density, and lower cost lithium secondary batteries. As a result, active research is being conducted into the use of inexpensive elements such as Ni, Mn, and Fe instead of expensive Co.

[0005] One of the main research topics for lithium secondary batteries is to develop electrode active materials with high capacity and high power output while improving the safety of the batteries. Lithium transition metal composite oxides are used as the positive electrode active material for lithium secondary batteries, and among them, the lithium cobalt composite metal oxide LiCoO2 is the most commonly used, due to its high operating voltage and excellent capacity characteristics. However, LiCoO2 has very poor thermal properties due to the instability of its crystal structure caused by delithiation, and is expensive, limiting its mass use as a power source in fields such as electric vehicles.

[0006] Materials that have been developed to replace LiCoO2 include lithium manganese composite metal oxides (such as LiMnO2 and LiMn2O4), lithium iron phosphate compounds (such as LiFePO4), and lithium nickel composite metal oxides (such as LiNiO2). Among these, research and development has been particularly active on lithium nickel composite metal oxides, which have a high reversible capacity of approximately 200 mAh / g and make it easy to create high-capacity batteries. However, LiNiO2 has poorer thermal stability than LiCoO2, and if an internal short circuit occurs due to external pressure while charging, the positive electrode active material itself decomposes, resulting in the battery exploding and catching fire.

[0007] As a result, a method of substituting part of nickel (Ni) with cobalt (Co) or manganese (Mn) was proposed as a way to improve the low thermal stability of LiNiO2 while maintaining the excellent reversible capacity. 1-α Co α O2 (α = 0.1 to 0.3) exhibits excellent charge / discharge characteristics and lifespan characteristics, but suffers from the problem of low thermal stability. Nickel-manganese-based lithium composite metal oxides, in which part of the Ni is replaced with Mn, which has excellent thermal stability, and nickel-cobalt-manganese-based lithium composite metal oxides, in which Mn and Co are replaced (hereinafter simply referred to as "NCM-based lithium oxides"), have the advantage of relatively excellent cycle characteristics and thermal stability, but because their low penetration resistance prevents internal short circuits when a metal object such as a nail penetrates, they can pose serious safety issues, such as fires or explosions due to momentary overcurrent.

[0008] Patent Document 1 discloses a technology in which an overcharge prevention layer is interposed between a positive electrode current collector and a positive electrode active material layer to increase resistance during overcharge and cut off the charging current, thereby ensuring battery safety. However, in this patent document, the penetration resistance of the overcharge prevention layer is low, which can cause safety problems when needle-shaped structures penetrate.

[0009] Therefore, there is a need for technological development of a positive electrode for a secondary battery that has increased penetration resistance when an external metal object such as a nail penetrates the electrode. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Korean Patent Publication No. 2019-0047203 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to provide a positive electrode for a secondary battery that has high capacity, high output performance, excellent cycle characteristics, and thermal stability, while also increasing penetration resistance when an external metal object such as a nail penetrates the electrode, and a lithium secondary battery including the same. [Means for solving the problem]

[0012] The positive electrode for a lithium secondary battery according to the present invention includes a first positive electrode mixture layer in contact with a positive electrode current collector, and one or more second positive electrode mixture layers disposed on the first positive electrode mixture layer, the first positive electrode mixture layer including a first positive electrode active material and a first binder, the second positive electrode mixture layer including a second positive electrode active material and a second binder, and the first positive electrode active material having an average particle size (D 50 ) is the average particle size (D 50 ) and its specific surface area is 3 μm or less, which is smaller than (BET)3m 2 / g or more.

[0013] In one embodiment of the present invention, the first positive electrode active material has an average particle size (D 50 ) is 0.1 μm to 2 μm, and its specific surface area is (BET)5m 2 / g~25m 2 / g.

[0014] In one embodiment of the present invention, the adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer is greater than the adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer.

[0015] In one embodiment of the present invention, the adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer is 100 N / m to 500 N / m.

[0016] In one embodiment of the present invention, the adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer is 10 N / m to 40 N / m.

[0017] In one embodiment of the present invention, the first binder and the second binder are binders of the same nature.

[0018] In one embodiment of the present invention, the weight ratio of the first binder based on the total weight of the first positive electrode mixture layer is greater than the weight ratio of the second binder based on the total weight of the second positive electrode mixture layer.

[0019] In one embodiment of the present invention, the weight ratio of the first binder based on the total weight of the first positive electrode mixture layer is 0.01 to 0.3.

[0020] The positive electrode for a lithium secondary battery according to one embodiment of the present invention has an elongation ratio of 0.5% to 2.0%.

[0021] In one embodiment of the present invention, when the thickness of the first positive electrode mixture layer is defined as A and the thickness of the second positive electrode mixture layer is defined as B, A / B≦0.3.

[0022] In one embodiment of the present invention, the first positive electrode mixture layer has a thickness of 1 μm to 20 μm.

[0023] In one embodiment of the present invention, at least one of the first positive electrode active material and the second positive electrode active material includes a lithium transition metal oxide represented by the following Chemical Formula 1:

[0024] [Chemical formula 1] Li a Ni1-x-y Co x Mn y M z O2

[0025] In the above Chemical Formula 1, M is one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and Cr, and 0.9≦a≦1.5, 0≦x≦1, 0≦y≦0.5, 0≦z≦0.1, 0≦x+y≦1.

[0026] In one embodiment of the present invention, the first positive electrode active material includes a lithium iron phosphate compound having an olivine structure represented by Chemical Formula 2 below.

[0027] [Chemical formula 2] Li 1+a Fe 1-x M x (PO 4-b )X b

[0028] In the above chemical formula 2, M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

[0029] In one embodiment of the present invention, at least one of the first positive electrode mixture layer and the second positive electrode mixture layer further includes a conductive material.

[0030] The lithium secondary battery of the present invention includes the above-described positive electrode, separator, and negative electrode.

[0031] In a lithium secondary battery according to one embodiment of the present invention, the adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer, the adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer, and the adhesive strength c between the second positive electrode mixture layer and the separator satisfy the relationship a>b>c.

[0032] In a lithium secondary battery according to one embodiment of the present invention, the adhesive strength c between the second positive electrode mixture layer and the separator is 5 N / m to 30 N / m. [Effects of the Invention]

[0033] In the positive electrode for a lithium secondary battery and the lithium secondary battery including the same according to the present invention, the first positive electrode active material contained in the first positive electrode mixture layer is composed of small particles, and the elongation rate of the positive electrode current collector is reduced due to the influence of the small particles of the first positive electrode active material, which is more advantageous in preventing electrode breakage when penetrated by a needle-shaped conductor.In addition, the first positive electrode mixture layer reduces the exposed area of ​​the current collector, thereby improving safety against penetration.

[0034] In addition, the positive electrode for a secondary battery and the secondary battery including the same have the advantage that, during overcharge, the resistance of the first positive electrode mixture layer increases, reducing the current flowing through the electrode and thereby terminating charging, thereby improving overcharge safety. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 2 is a cross-sectional view of a positive electrode according to one embodiment of the present invention. [Figure 2] FIG. 2 is a conceptual diagram for explaining the penetration resistance of a positive electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will be described in detail below. Before that, the terms and words used in the specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his or her invention.

[0037] In this application, terms such as "comprise" and "have" are intended to specify the presence of a feature, numeral, step, operation, component, part, or combination thereof described in the specification, and should be understood as not excluding the possible presence or addition of one or more other features, numerals, steps, operations, components, parts, or combinations thereof. Furthermore, when a layer, film, region, plate, or other part is described as being "on" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part between them. Conversely, when a layer, film, region, plate, or other part is described as being "under" another part, this includes not only the case where it is "directly under" the other part, but also the case where there is another part between them. Furthermore, in this application, "over" can include not only the case where it is "on top" but also the case where it is "under" the other part.

[0038] The present invention will be described in detail below.

[0039] FIG. 1 is a cross-sectional view of a positive electrode according to one embodiment of the present invention. Referring to FIG. 1, a positive electrode 100 for a lithium secondary battery according to the present invention includes a first positive electrode mixture layer 120 in contact with a positive electrode current collector 110, and one or more second positive electrode mixture layers 130 disposed on the first positive electrode mixture layer 120. The first positive electrode mixture layer includes a first positive electrode active material and a binder, and the second positive electrode mixture layer includes a second positive electrode active material and a binder. The first positive electrode active material has an average particle size (D 50 ) is the average particle size (D 50 ) and its specific surface area is 3 μm or less, which is smaller than (BET)3m 2 / g or more.

[0040] 2 is a conceptual diagram illustrating the penetration resistance of a positive electrode according to an embodiment of the present invention. Referring to FIG. 2, the positive electrode of the present invention has a first positive electrode active material constituting a first positive electrode mixture layer in contact with a positive electrode current collector, and the particle size of the first positive electrode active material is small, thereby reducing the elongation rate of the first positive electrode mixture layer. As a result, the adjacent positive electrode current collector is more susceptible to fracture of the positive electrode due to the reduced elongation rate of the first positive electrode mixture layer.

[0041] Therefore, when a metal object such as a nail penetrates the positive electrode, the positive electrode current collector is broken without being stretched along the metal object, thereby reducing the contact area between the positive electrode current collector and the metal object. Furthermore, when the positive electrode current collector is stretched along the metal object, it may come into contact with the negative electrode current collector of the opposite polarity, but the positive electrode of the present invention can suppress contact between the positive electrode current collector and the negative electrode current collector, which would reduce the stretch rate.

[0042] Meanwhile, FIG. 1 illustrates an embodiment in which the second positive electrode mixture layer is configured as a single layer, but the embodiment of the present invention is not limited thereto, and the second positive electrode mixture layer may be configured as a multilayer structure of two or more layers in order to improve energy density and conductivity.

[0043] The average particle size (D 50 ) is the average particle size (D 50 ) can be 5 to 80%.

[0044] That is, according to one embodiment of the present invention, the average particle size (D 50 The first positive electrode active material having a small average particle size (D 50 The second positive electrode active material having a large pore size can be coated on the upper layer of the positive electrode, thereby reducing the elongation rate of the first positive electrode mixture layer adjacent to the positive electrode current collector.

[0045] In the present invention, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using a laser diffraction method. 50 The measurement method for the average particle size (D) is as follows: After dispersing the particles of the positive electrode active material in a dispersion medium, the particles are introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT3000) and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. Then, the average particle size (D) corresponding to 50% of the cumulative volume in the measuring device is measured. 50) can be calculated.

[0046] Specifically, the average particle size (D 50 The average particle size (D ) of the first positive electrode active material may be 3 μm or less, more preferably 0.1 μm to 2 μm, and even more preferably 0.1 to 1.5 μm. 50 If the particle size is less than 0.1 μm, side reactions may occur or dispersibility may be poor during the electrode manufacturing process. If the particle size exceeds 3 μm, the adhesive strength with the positive electrode current collector may decrease, resulting in little improvement in safety.

[0047] The specific surface area of ​​the first positive electrode active material is 3 m 2 / g or more, preferably 5m 2 / g~25m 2 / g, and more preferably 7m 2 / g~20m 2 / g. The specific surface area is 3m 2 If it is less than 1 / g, the elongation ratio of the first positive electrode mixture layer may increase, which is not preferable.

[0048] In the present invention, the specific surface area is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mino II manufactured by BEL Japan.

[0049] The second positive electrode active material has an average particle size (D 50 ) may be relatively large particles.

[0050] Specifically, the average particle size (D 50 The average particle size (D ) of the second positive electrode active material may be 3 μm or more, specifically 3 to 50 μm, and preferably 3 to 30 μm. 50 If the thickness is less than 3 μm, there may be process difficulties in the rolling step during the electrode manufacturing process.

[0051] The specific surface area of ​​the second positive electrode active material is 2 m 2 / g or less, preferably 0.1m 2 / g~1.5m 2 / g, more preferably 0.2m 2 / g~1.2m 2 / g.

[0052] The stretch ratio of the positive electrode mixture layer (a laminate of a first positive electrode mixture layer and a second positive electrode mixture layer) according to one embodiment of the present invention is 0.5% to 2%, preferably 0.5% to 1.8%, and more preferably 0.6% to 1.5%. The stretch ratio of the positive electrode mixture layer in the present invention is a value measured using a UTM device, and is measured by measuring the change in length of the positive electrode mixture layer from the original length until it is maximally stretched when the positive electrode mixture layer is stretched at a speed of about 5 mm / min after installation. When the stretch ratio of the positive electrode mixture layer satisfies the above numerical range, it is possible to increase the penetration resistance while improving cell performance such as life characteristics.

[0053] In one embodiment of the present invention, the positive electrode for a lithium secondary battery of the present invention satisfies a>b>c, where a is the adhesive strength between the positive electrode current collector and the first positive electrode mixture layer, b is the adhesive strength between the first positive electrode mixture layer and the second positive electrode mixture layer, and c is the adhesive strength between the second positive electrode mixture layer and the separator.

[0054] This is to minimize the contact area between the metal object and the current collector when an external metal object, such as a nail, penetrates the positive electrode. That is, when the metal object penetrates the positive electrode, an external force is applied to the positive electrode. This external force can create gaps between the positive electrode current collector and the first positive electrode mixture layer, between the first positive electrode mixture layer and the second positive electrode mixture layer, and between the second positive electrode mixture layer and the separator. In this case, if adhesive strength a is relatively greater than adhesive strengths b and c, even if the first positive electrode mixture layer is detached from the second positive electrode mixture layer, the first positive electrode mixture layer remains attached to the positive electrode current collector, making it difficult for the metal object to come into direct contact with the positive electrode current collector. Furthermore, when the adhesive strength b is relatively greater than the adhesive strength c, even if the second positive electrode mixture layer is detached from the separation membrane, the second positive electrode mixture layer adheres to the first positive electrode mixture layer and can protect the first positive electrode mixture layer, thereby suppressing the tendency of the first positive electrode mixture layer to be detached from the positive electrode current collector due to the external force of the metal body.

[0055] As described above, the positive electrode of the present invention has excellent adhesive strength between the first positive electrode mixture layer and the positive electrode current collector. Therefore, when a metal object such as a nail penetrates the positive electrode from the outside, the first positive electrode mixture layer reduces the exposed area of ​​the positive electrode current collector. As a result, the short-circuit current of the positive electrode of the present invention is reduced, thereby improving safety.

[0056] In this case, a is 5 to 12 times, and preferably 6 to 10 times, the value of b. When the relationship between a and b satisfies the above numerical range, the effect of penetration safety can be more effectively exhibited.

[0057] The adhesive strength a between the current collector and the first positive electrode mixture layer may be 100 N / m to 500 N / m, preferably 150 N / m to 300 N / m, and more preferably 200 N / m to 300 N / m.

[0058] The adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer may be 10 N / m to 40 N / m, preferably 15 N / m to 35 N / m, and more preferably 20 N / m to 35 N / m.

[0059] The adhesive strength c between the second positive electrode mixture layer and the separator is in the range of 5 N / m to 30 N / m, preferably 7 N / m to 25 N / m, and more preferably 10 N / m to 20 N / m, which is smaller than the adhesive strength b.

[0060] The first and second positive electrode mixture layers of the present invention contain a binder. The binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The binder may be present in an amount of 1 wt% to 30 wt% based on the total weight of the positive electrode mixture layer.

[0061] In one embodiment of the present invention, the first binder contained in the first positive electrode mixture layer and the second binder contained in the second positive electrode mixture layer may be binders of the same nature. For example, if the first binder is a hydrophilic binder, the second binder may also be a hydrophilic binder, and if the second binder is a lipophilic binder, the second binder may also be a lipophilic binder. The concept of "having the same nature" also encompasses embodiments in which the first binder and the second binder are the same type.

[0062] In one specific example, the weight ratio of the first binder based on the total weight of the first positive electrode mixture layer may be greater than the weight ratio of the second binder based on the total weight of the second positive electrode mixture layer. In the present invention, the particle size and specific surface area of ​​the first positive electrode active material contained in the first positive electrode mixture layer and the porosity of the first positive electrode mixture layer can be controlled to predetermined conditions to control the adhesive force a between the first positive electrode mixture layer and the positive electrode current collector so that it is greater than the adhesive forces b and c without adjusting the binder content. Alternatively, the adhesive force a can be further increased by increasing the content of the first binder in the first positive electrode mixture layer greater than the content of the second binder in the second positive electrode mixture layer. Here, the binder content refers to the weight ratio of the first binder to the total weight of the first positive electrode mixture layer and the weight ratio of the second binder to the total weight of the second positive electrode mixture layer.

[0063] In this case, the weight ratio of the first binder based on the total weight of the first positive electrode material mixture layer may be 0.01 to 0.3, and preferably 0.05 to 0.2. The thickness of the first positive electrode material mixture layer is specifically 1 μm to 20 μm, and preferably 1 μm to 10 μm.

[0064] The first positive electrode active material and / or the second positive electrode active material of the present invention may contain a lithium transition metal oxide represented by Chemical Formula 1 below.

[0065] [Chemical formula 1] Li a Ni 1-x-y Co x Mn y M z O2

[0066] In the above chemical formula 1, M is one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and Cr, and 0.9≦a≦1.5, 0≦x≦1, 0≦y≦0.5, 0≦z≦0.1, 0≦x+y≦1.

[0067] However, the first positive electrode active material and / or the second positive electrode active material are not necessarily limited to the lithium transition metal oxide represented by Chemical Formula 1, and the first positive electrode active material and / or the second positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals, or a compound represented by Chemical Formula Li 1+x1 Mn 2-x1 O4 (where x1 is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2, lithium copper oxide (Li2CuO2), vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7, with the chemical formula LiNi 1-x2 M 1 x2 O2 (where M 1 is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x2 is 0.01 to 0.3), Ni-site lithium nickel oxide, chemical formula LiMn 2-x3 M 2 x3 O2 (where M 2 = Co, Ni, Fe, Cr, Zn or Ta, and x3 = 0.01 to 0.1) or Li2Mn3M 3 O8 (where M 3 Lithium manganese composite oxide, LiNi x4 Mn 2-x4 Examples of lithium manganese composite oxides include those with a spinel structure represented by LiMn2O4 (where x4 = 0.01 to 1), LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion, disulfide compounds, and Fe2(MoO4)3.

[0068] The first and second positive electrode active materials may contain lithium transition metal oxides of the same composition or different compositions. In a preferred embodiment of the present invention, the first positive electrode active material preferably contains a lithium iron phosphate compound having an olivine structure represented by the following Chemical Formula 2:

[0069] [Chemical formula 2] Li 1+a Fe 1-x M x (PO 4-b )X b

[0070] In the above chemical formula 2, M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

[0071] The olivine-structured positive electrode active material loses lithium from the first positive electrode active material and contracts in volume at overcharge voltages of about 4.5 V or higher, rapidly interrupting the conductive path of the first positive electrode mixture layer, causing the first positive electrode mixture layer to act as an insulating layer and increasing resistance, thereby cutting off the charging current and allowing the overcharge termination voltage to be reached. Therefore, in the present invention, selecting the olivine-structured positive electrode active material as the first positive electrode active material contained in the first positive electrode mixture layer provides a synergistic effect in terms of improving safety.

[0072] In this way, the positive electrode of the present invention selects a lithium iron phosphate compound with an olivine structure represented by Chemical Formula 2 as the first positive electrode active material contained in the first positive electrode mixture layer, thereby increasing the electrical resistance of the first positive electrode mixture layer and enabling the first positive electrode mixture layer to function as a resistance layer at high voltages. As a result, the resistance of the positive electrode increases significantly during overcharge, causing a decrease in charging current and terminating charging, thereby ensuring safety. In this case, the first positive electrode mixture layer not only functions as a safety layer (SFL) to prevent overcharge, but also allows the positive electrode active material to develop capacity under normal operating conditions of the battery.

[0073] Meanwhile, in the positive electrode of the present invention, the first positive electrode mixture layer and the second positive electrode mixture layer may have different active materials. For example, the first positive electrode mixture layer may function as an overcharge prevention layer, and the first positive electrode active material may be a lithium iron phosphate compound with an olivine structure represented by Chemical Formula 2 above. The second positive electrode mixture layer may then have the lithium transition metal oxide represented by Chemical Formula 1 above as the second positive electrode active material. In this case, the high capacity / high energy density characteristics of the second positive electrode active material may provide a secondary battery with excellent capacity characteristics.

[0074] In a preferred embodiment of the present invention, when the thickness of the first positive electrode mixture layer is defined as A and the thickness of the second positive electrode mixture layer is defined as B, the thickness ratio A / B between the first positive electrode mixture layer and the second positive electrode mixture layer may be 0.3 or less, and preferably 0.1 or less. The first positive electrode mixture layer of the present invention is a layer provided for ensuring safety, and does not need to be thick because it is sufficient as long as it is thick enough to increase penetration resistance when penetrated by a conductor such as a metal body.

[0075] At least one of the first positive electrode mixture layer and the second positive electrode mixture layer of the present invention further contains a conductive material. The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of suitable conductive materials include graphite (e.g., natural graphite or artificial graphite); carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or fibers (e.g., copper, nickel, aluminum, and silver); conductive whiskers (e.g., zinc oxide and potassium titanate); conductive metal oxides (e.g., titanium oxide); and conductive polymers (e.g., polyphenylene derivatives). The conductive material may be present in an amount of 1 wt % to 30 wt % based on the total weight of the positive electrode mixture layer.

[0076] In the present invention, the positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance the adhesive strength of the positive electrode active material. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0077] The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, graphite such as natural graphite or artificial graphite, carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black, conductive fibers such as carbon fibers or metal fibers, conductive tubes such as carbon nanotubes, fluorocarbon, metal powders such as aluminum or nickel powder, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives may be used.

[0078] The present invention also provides an electrochemical device comprising the above-described positive electrode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0079] The lithium secondary battery specifically includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and may optionally further include a battery case that accommodates the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery case.

[0080] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer located on the negative electrode current collector.

[0081] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector typically has a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the negative electrode current collector may have a surface with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0082] The negative electrode mixture layer includes a negative electrode active material, a binder, and a conductive material. For example, the negative electrode mixture layer may be manufactured by applying a composition for forming the negative electrode mixture layer, which includes the negative electrode active material and, optionally, the binder and the conductive material, onto a negative electrode current collector and drying the composition, or by casting the composition for forming the negative electrode mixture layer onto a separate support, peeling it off from the support, and laminating the resulting film onto the negative electrode current collector.

[0083] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbon such as petroleum or coal tar pitch-derived cokes.

[0084] The binder and conductive material may be the same as those used in the positive electrode.

[0085] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without limitation. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent humidification ability for the electrolyte is preferred. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and can be used in a single-layer or multi-layer structure.

[0086] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0087] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0088] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carnonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; and R-CN (R is C2 to C6). 20 Examples of suitable solvents include nitriles such as ethylene carbonate (a hydrocarbon group having a linear, branched, or cyclic structure, which may contain a double bond, an aromatic ring, or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, mixing the cyclic carbonate and the linear carbonate in a volume ratio of approximately 1:1 to 9 can provide excellent electrolyte performance.

[0089] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(CF, SO), LiN(CF, SO), LiN(CF, SO), LiCl, LiI, or LiB(CO) . The lithium salt concentration is preferably within the range of 0.1 M to 2.0 M. When the lithium salt concentration falls within the above range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0090] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as a haloalkylene carbonate compound (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0091] As described above, the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of mobile devices such as mobile phones, laptops, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0092] According to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0093] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools, electric vehicles (EVs), hybrid electric vehicles, and electric vehicles including plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0094] DETAILED DESCRIPTION OF THE INVENTION The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0095] Example 1 Average particle size (D 50 ) is 1 μm, and the BET specific surface area is 15 m 2 93 parts by weight of LiFePO4 positive electrode active material having a solubility of 1 / g, 2 parts by weight of carbon black as a conductive material, and 5 parts by weight of PVDF as a binder were mixed in an N-methylpyrrolidone (NMP) solvent to prepare a first positive electrode active material slurry.

[0096] Average particle size (D 50 ) is 4 μm, and the BET specific surface area is 0.7 m 2 / g LiNi 0.8 Co 0.1 A second positive electrode active material slurry was prepared by mixing 96 parts by weight of MnO2 positive electrode active material, 2 parts by weight of carbon black as a conductive material, and 2 parts by weight of PVDF as a binder in an N-methylpyrrolidone (NMP) solvent.

[0097] The first and second positive electrode active material slurries were applied to aluminum foil, dried, and rolled to produce a positive electrode having an aluminum foil / first positive electrode mixture layer / second positive electrode mixture layer structure. The first positive electrode mixture layer had a thickness of 10 μm, and the second positive electrode mixture layer had a thickness of 80 μm.

[0098] Examples 2 and 3 A positive electrode was fabricated in the same manner as in Example 1, except that the composition of the first positive electrode active material slurry was changed as shown in Table 1 below.

[0099] Comparative Example 1 Average particle size (D 50 ) is 4 μm, and the BET specific surface area is 0.7 m 2 / g LiNi 0.8 Co 0.1 Mn 0.1 A first positive electrode active material slurry was prepared by mixing 96 parts by weight of O2 positive electrode active material, 2 parts by weight of carbon black as a conductive material, and 2 parts by weight of PVDF as a binder in an N-methylpyrrolidone (NMP) solvent.

[0100] Average particle size (D 50 ) is 1 μm, and the BET specific surface area is 15 m 2 93 parts by weight of LiFePO4 positive electrode active material having a solubility of 1 / g, 2 parts by weight of carbon black as a conductive material, and 5 parts by weight of PVDF as a binder were mixed in an N-methylpyrrolidone (NMP) solvent to prepare a second positive electrode active material slurry.

[0101] The first and second positive electrode active material slurries were applied to aluminum foil, dried, and rolled to produce a positive electrode having an aluminum foil / first positive electrode mixture layer / second positive electrode mixture layer structure. The first positive electrode mixture layer had a thickness of 10 μm, and the second positive electrode mixture layer had a thickness of 80 μm.

[0102] Comparative Example 2 In Example 1, the LiFePO4 contained in the first positive electrode active material slurry was mixed with the average particle size (D 50A positive electrode was prepared in the same manner as in Example 1, except that LiFePO4 having a particle size of 4 μm and a specific surface area of ​​2.8 m / g was used, and the composition of the first positive electrode active material slurry was changed as shown in Table 1 below.

[0103] [Table 1]

[0104] Experimental Example 1: Measurement of elongation ratio The positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were prepared as specimens. The specimens were then loaded into a UTM device and stretched at a rate of approximately 5 mm / min. The stretch rate was measured by measuring the change in length from the original positive electrode to the maximum stretched length. The results are shown in Table 2.

[0105] Experimental Example 2: Adhesion Strength Measurement The positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were cut to a width and length of 25 mm and 70 mm, respectively, and then a separator was laminated thereon, followed by lamination using a press under conditions of 70°C and 4 MPa to prepare test specimens.

[0106] The prepared specimen was attached to a glass plate using double-sided tape, with the positive electrode facing the glass plate. The separator portion of the specimen was peeled off at a 90° angle at 25°C, a speed of 100 mm / min, using a tensile tester. The peel force was measured in real time, and the average value was defined as the interfacial adhesion strength (c) between the second positive electrode mixture layer and the separator. The results are shown in Table 2.

[0107] The interfacial adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer and the interfacial adhesive strength a between the first positive electrode mixture layer and the positive electrode current collector were also measured in the same manner, and the results are shown in Table 2.

[0108] Experimental example 3: Evaluation of penetration safety The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 and 2 were used to produce lithium secondary batteries, respectively.

[0109] First, natural graphite as an anode active material, carbon black conductive material, and PVDF binder were mixed in a weight ratio of 85:10:5 in N-methylpyrrolidone solvent to prepare a slurry for forming an anode, which was then coated on copper foil to prepare an anode.

[0110] A porous polyethylene separator was interposed between the negative electrode and each of the positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 and 2 to prepare an electrode assembly. Each electrode assembly was then placed inside a case, and an electrolyte solution was injected into the case to prepare a lithium secondary battery. The electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent consisting of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC in a volume ratio of 3 / 4 / 3).

[0111] The lithium secondary batteries manufactured using the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 and 2 were evaluated for the presence or absence of ignition when a metal object having a diameter of 3 mm was lowered at a speed of 80 mm / sec to penetrate the cell, in the same manner as the PV8450 certification conditions. The results are shown in Table 2 below.

[0112] [Table 2]

[0113] Referring to Table 2, secondary batteries including positive electrodes according to the examples of the present invention exhibit improved penetration safety compared to secondary batteries including positive electrodes according to the comparative examples. The positive electrodes of Comparative Examples 1 and 2 satisfy the relationship a>b>c, but the elongation ratio of the positive electrode exceeds 2.0%. Therefore, in the present invention, in order to improve penetration safety, it is preferable that the elongation ratio of the positive electrode be 2.0% or less.

[0114] Experimental Example 4: Evaluation of overcharge safety Lithium secondary batteries were fabricated using the same negative electrode and separator material and method as in Experimental Example 3 using each of the positive electrodes of Example 2 and Comparative Example 1. Each fabricated lithium secondary battery was CCCV charged at 0.33 C to 4.2 V to prepare a 100% SOC cell. The 100% SOC cells were then CC charged at 1 C-rate to 10% and 20% of the corresponding cell capacity to prepare 110% and 120% SOC cells, respectively. The resistances of each cell at 100%, 110%, and 120% SOC were measured using electrochemical impedance spectroscopy.

[0115] The resistance of the overcharged battery is shown in Table 3 below.

[0116] [Table 3]

[0117] As shown in Table 3, the positive electrodes according to the examples of the present invention exhibited resistance at a similar level to that of the positive electrodes according to the comparative examples at operable charge states (SOC 100%, 110%), but the resistance increased significantly during overcharge (SOC 120%) compared to the positive electrodes according to the comparative examples. Therefore, the positive electrodes of the present invention are expected to increase resistance during overcharge, thereby causing charge termination and ensuring safety.

[0118] The above description is merely illustrative of the technical concept of the present invention, and various modifications and variations are possible within the scope of the essential characteristics of the present invention, if one skilled in the art to which the present invention pertains. Therefore, the drawings disclosed in the present invention are for the purpose of explaining, rather than limiting, the technical concept of the present invention, and the scope of the technical concept of the present invention is not limited by such drawings. The scope of protection of the present invention should be interpreted by the following claims, and all technical concepts within the scope equivalent thereto should be interpreted as being within the scope of the present invention.

Claims

1. a first positive electrode mixture layer in contact with a positive electrode current collector, and one or more second positive electrode mixture layers disposed on the first positive electrode mixture layer; the first positive electrode mixture layer includes a first positive electrode active material and a first binder, the second positive electrode mixture layer includes a second positive electrode active material and a second binder, The first positive electrode active material has an average particle size (D ５０ ) is the average particle size (D ５０ ) and its specific surface area is 3 μm or less, which is smaller than (BET) 3 m ２ / g or more.

2. The first positive electrode active material has an average particle size (D ５０ ) is 0.1 μm to 2 μm, and its specific surface area is (BET) 5 m ２ / g~25m ２ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode has a Cr content of 0.15 or more.

3. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein an adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer is greater than an adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer.

4. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein an adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer is 100 N / m to 500 N / m.

5. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein an adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer is 10 N / m to 40 N / m.

6. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first binder and the second binder are binders of the same nature.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein a weight ratio of the first binder based on a total weight of the first positive electrode mixture layer is greater than a weight ratio of the second binder based on a total weight of the second positive electrode mixture layer.

8. 8. The positive electrode for a lithium secondary battery according to claim 7, wherein a weight ratio of the first binder based on the total weight of the first positive electrode mixture layer is 0.01 to 0.

3.

9. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode for a lithium secondary battery has an elongation rate of 0.5% to 2.0%.

10. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein, when the thickness of the first positive electrode mixture layer is defined as A and the thickness of the second positive electrode mixture layer is defined as B, A / B≦0.

3.

11. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first positive electrode mixture layer has a thickness of 1 μm to 20 μm.

12. At least one of the first positive electrode active material and the second positive electrode active material includes a lithium transition metal oxide represented by the following Chemical Formula 1: [Chemical formula 1] Li ａ Ni １－ｘ－ｙ Co ｘ Mn ｙ M ｚ O ２ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein in Chemical Formula 1, M is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and Cr, and 0.9≦a≦1.5, 0≦x≦1, 0≦y≦0.5, 0≦z≦0.1, and 0≦x+y≦1.

13. The first positive electrode active material includes a lithium iron phosphate compound having an olivine structure represented by the following Chemical Formula 2: [Chemical formula 2] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein, in Chemical Formula 2, M is at least one selected from Al, Mg, and Ti; X is at least one selected from F, S, and N; and -0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.

1.

14. The positive electrode for a lithium secondary battery according to claim 1 , wherein at least one of the first positive electrode mixture layer and the second positive electrode mixture layer further contains a conductive material.

15. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 1, a separator, and a negative electrode.

16. 16. The lithium secondary battery according to claim 15, wherein an adhesive strength a between the positive electrode current collector and the first positive electrode mixture layer, an adhesive strength b between the first positive electrode mixture layer and the second positive electrode mixture layer, and an adhesive strength c between the second positive electrode mixture layer and the separator satisfy a>b>c.

17. 17. The lithium secondary battery according to claim 16, wherein the adhesive strength c between the second positive electrode mixture layer and the separator is 5 N / m to 30 N / m.

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