Positive electrode and lithium secondary battery for 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 document of said Korean patent application are incorporated as a part of the present specification.

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

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

[0004] Recently, as lithium secondary batteries have come to be used as power sources for medium and large-sized devices such as electric vehicles, higher capacity, higher energy density, and lower cost of lithium secondary batteries are further required. Accordingly, active research is being conducted to replace expensive Co with low-cost Ni, Mn, Fe, and the like.

[0005] One of the main research subjects for such lithium secondary batteries is to realize a high-capacity, high-output electrode active material while improving the safety of a battery using the same. Lithium transition metal composite oxides are used as positive electrode active materials for lithium secondary batteries, and among them, LiCoO2 lithium cobalt composite metal oxide, which has a high operating voltage and excellent capacity characteristics, is mainly used. However, LiCoO2 has very poor thermal properties due to destabilization of the crystal structure caused by delithiation, and is expensive, so there is a limit to its large-scale use as a power source in fields such as electric vehicles.

[0006] As alternatives to LiCoO2, materials such as lithium manganese composite metal oxides (LiMnO2 and LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), and lithium nickel composite metal oxides (LiNiO2, etc.) have been developed. Among these, research and development of lithium nickel composite metal oxides, which have a high reversible capacity of approximately 200 mAh / g and are easy to realize as high-capacity batteries, are being actively pursued. However, LiNiO2 has poorer thermal stability compared to LiCoO2, and if an internal short circuit occurs due to external pressure while charged, the positive electrode active material itself decomposes, leading to battery rupture and ignition.

[0007] Therefore, methods have been proposed to improve the low thermal stability of LiNiO2 while maintaining its excellent reversible capacity, by substituting some of the nickel (Ni) with cobalt (Co) or manganese (Mn). However, LiNiO2 with some of the nickel substituted with cobalt... 1-α Co α In the case of O2 (α=0.1~0.3), it exhibits excellent charge / discharge characteristics and life characteristics, but has the problem of low thermal stability. Furthermore, nickel-manganese lithium composite metal oxides in which part of the Ni is replaced with Mn, which has excellent thermal stability, and nickel-cobalt-manganese lithium composite metal oxides in which Mn and Co are replaced (hereinafter simply referred to as "NCM lithium oxides") have the advantage of relatively superior cycle characteristics and thermal stability, but because of their low penetration resistance, internal short circuits do not occur when a metal object such as a nail penetrates them, which can cause serious safety problems such as ignition or explosion due to instantaneous overcurrent.

[0008] Patent Document 1 discloses a technology that ensures battery safety by interposing an overcharge prevention layer between the positive electrode current collector and the positive electrode active material layer to increase resistance during overcharging and interrupt the charging current. However, the above-mentioned patent document has a low penetration resistance of the overcharge prevention layer, which may cause problems in terms of safety when needle-like objects penetrate it.

[0009] Therefore, there is a need for technological development of positive electrodes for secondary batteries that increase the penetration resistance when a metal object, such as a nail, penetrates the electrode from the outside. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Korean Published Patent No. 2019-0047203 [Overview of the project] [Problems that the invention aims to solve]

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

[0012] The positive electrode for a lithium secondary battery according to the present invention comprises 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, wherein the first positive electrode mixture layer comprises a first positive electrode active material and a first binder, and the second positive electrode mixture layer comprises a second positive electrode active material and a second binder, and the first positive electrode active material has an average particle size (D 50 ) is the average particle size (D) of the second positive electrode active material. 50 ) is smaller than 3 μm and has a specific surface area of ​​(BET) 3 m 2 It is 1 / g or more.

[0013] In one embodiment of the present invention, the first positive electrode active material has an average particle size (D 50 The surface area is 0.1 μm to 2 μm, and its specific surface area is (BET) 5 m². 2 / g~25m 2 It could be / g

[0014] In one embodiment of the present invention, the adhesive force a between the positive electrode current collector and the first positive electrode mixture layer is greater than the adhesive force 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 force 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 force 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 having the same properties.

[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] A 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 holds.

[0022] In one embodiment of the present invention, the thickness of the first positive electrode mixture layer is 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 comprises 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 the following inequalities apply: 0.9 ≤ a ≤ 1.5, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and 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 shown in the following chemical formula 2.

[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 elements selected from Al, Mg, and Ti, and X is one or more elements selected from F, S, and N, with -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 positive electrode, separator membrane, and negative electrode described above.

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

[0032] In one embodiment of the present invention, the lithium secondary battery has an adhesive force c between the second positive electrode mixture layer and the separation membrane of 5 N / m to 30 N / m. [Effects of the Invention]

[0033] The positive electrode for a lithium secondary battery according to the present invention and the lithium secondary battery containing the same are further advantageous in preventing electrode breakage when a needle-shaped conductor penetrates the positive electrode current collector because the first positive electrode active material contained in the first positive electrode mixture layer is composed of small particles, which reduces the elongation rate of the positive electrode current collector due to the influence of the small particles of the first positive electrode active material. In addition, the first positive electrode mixture layer has the effect of improving penetration safety by reducing the exposed area of ​​the current collector.

[0034] Furthermore, the positive electrode for secondary batteries and secondary batteries containing it have the added benefit of improving overcharge safety by increasing the resistance of the first positive electrode mixture layer during overcharging, thereby reducing the current flowing through the electrode and terminating the charging process. [Brief explanation of the drawing]

[0035] [Figure 1] This is a cross-sectional view of a positive electrode according to one embodiment of the present invention. [Figure 2] This is a conceptual diagram illustrating the through-resistance of the positive electrode according to an embodiment of the present invention. [Modes for carrying out the invention]

[0036] The present invention will now be described in detail. Before that, however, the terms and words used in this specification and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather should be interpreted as meanings and concepts consistent with the technical idea of ​​the present invention, based on the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0037] In this application, terms such as "includes" and "have" are intended to specify the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood not to preemptively exclude the presence or possibility of adding one or more other features, numbers, steps, actions, components, parts, or combinations thereof. Furthermore, when a part such as a layer, film, region, or plate is said to be "on top" of another part, this includes not only when it is "directly on top" of the other part, but also when there is another part in between. Conversely, when a part such as a layer, film, region, or plate is said to be "below" another part, this includes not only when it is "directly below" the other part, but also when there is another part in between. Also, in this application, "located on top" may include being located at the bottom as well as at the top.

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

[0039] Figure 1 is a cross-sectional view of a positive electrode according to one embodiment of the present invention. Referring to Figure 1, the positive electrode 100 for a lithium secondary battery of 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) of the second positive electrode active material. 50 ) is smaller than 3 μm and has a specific surface area of ​​(BET) 3 m 2 It is 1 / g or more.

[0040] Figure 2 is a conceptual diagram illustrating the through-resistance of a positive electrode according to an embodiment of the present invention. Referring to Figure 2, the positive electrode of the present invention has a first positive electrode active material that constitutes the first positive electrode mixture layer in contact with the positive electrode current collector, with the particle size of the first positive electrode mixture layer being small to reduce the elongation of the first positive electrode mixture layer. As a result, the adjacent positive electrode current collector is more likely to break the positive electrode due to the reduced elongation of the first positive electrode mixture layer.

[0041] Therefore, if a metal object such as a nail penetrates the positive electrode, the positive electrode current collector will break without stretching along the metal object, thus reducing the contact area between the positive electrode current collector and the metal object. Furthermore, if the positive electrode current collector stretches along the metal object, it may come into contact with the negative electrode current collector, which has the opposite polarity. However, the positive electrode of the present invention can suppress contact between the positive electrode current collector and the negative electrode current collector, which reduces the stretching rate.

[0042] On the other hand, Figure 1 illustrates an embodiment in which the second positive electrode mixture layer is composed of a single layer. However, the embodiments of the present invention are not limited to this, and the second positive electrode mixture layer can also be composed of a multilayer structure of two or more layers in order to improve energy density and conductivity.

[0043] The average particle size (D) of the first positive electrode active material mentioned above. 50 ) is the average particle size (D) of the second positive electrode active material. 50 ) could be between 5% and 80%.

[0044] In other words, according to one embodiment of the present invention, the relative average particle size (D 50 A first positive electrode active material with a small particle size (D) is coated on the lower layer of the positive electrode adjacent to the positive electrode current collector, and the average particle size (D) is relatively small. 50 A second positive electrode active material with a large ) can be coated onto the upper layer of the positive electrode. This reduces 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 ) can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve. The above average particle size (D 50 ) can be measured, for example, using the laser diffraction method. For example, the average particle size (D) of the positive electrode active material. 50 The measurement method involves dispersing the positive electrode active material particles in a dispersion medium, then introducing them into a commercially available laser diffraction particle size analyzer (e.g., microtrac MT3000), irradiating them with ultrasound at approximately 28 kHz with an output of 60 W, and then measuring the average particle size (D) corresponding to 50% of the cumulative volume in the measuring device. 50It is possible to calculate ).

[0046] Specifically, the average particle size (D) of the first positive electrode active material. 50 The average particle size (D) of the first positive electrode active material may be 3 μm or less. More preferably it may be 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, electrode side reactions may occur, or there may be problems with dispersibility during the electrode manufacturing process. Furthermore, if it exceeds 3 μm, the adhesion to the positive electrode current collector decreases, and the effect of improving safety may be minimal.

[0047] Furthermore, the specific surface area of ​​the first positive electrode active material is 3 m². 2 / g or more, preferably 5m 2 / g~25m 2 / g, and more preferably 7m 2 / g~20m 2 It is / g. The specific surface area is 3m². 2 If the value is less than / g, the elongation rate of the first cathode mixture layer may increase, which is undesirable.

[0048] In this 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-minoII from BEL Japan.

[0049] The above-mentioned second positive electrode active material has a larger average particle size (D) than the above-mentioned first positive electrode active material. 50 ) can be relatively large particle size particles.

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

[0051] Furthermore, the specific surface area of ​​the second positive electrode active material is 2 m². 2It is less than or equal to / g, preferably 0.1m 2 / g~1.5m 2 / g, more preferably 0.2m 2 / g~1.2m 2 It is / g.

[0052] The elongation rate 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 elongation rate of the positive electrode mixture layer in the present invention is a value measured using a UTM device. When the positive electrode mixture layer is stretched at a speed of approximately 5 mm / min after installation, the elongation rate was measured by the change in length from the existing length of the positive electrode mixture layer until it is maximally stretched. When the elongation rate of the positive electrode mixture layer satisfies the above numerical range, it is possible to improve cell performance such as life characteristics while increasing penetration resistance.

[0053] In one embodiment of the present invention, the positive electrode for a lithium secondary battery satisfies a>b>c when the adhesive force between the positive electrode current collector and the first positive electrode mixture layer is defined as a, the adhesive force between the first positive electrode mixture layer and the second positive electrode mixture layer is defined as b, and the adhesive force between the two positive electrode mixture layers and the separation membrane is defined as c.

[0054] This is intended to minimize the contact area between the metal object and the current collector when a metal object, such as a nail, penetrates the positive electrode from the outside. In other words, when a metal object penetrates the positive electrode, an external force is applied to the positive electrode, and 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 separation film. At this time, if adhesive force a is relatively greater than adhesive forces 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, if adhesive strength b is relatively greater than adhesive strength c, even if the second positive electrode mixture layer detaches from the separation film, the second positive electrode mixture layer can adhere to the first positive electrode mixture layer and protect it. This suppresses the tendency for the first positive electrode mixture layer to detach from the positive electrode current collector due to external forces from the metal body.

[0055] Thus, the positive electrode of the present invention has excellent adhesion between the first positive electrode mixture layer and the positive electrode current collector. 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 positive electrode of the present invention experiences reduced short-circuit current and improved safety.

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

[0057] Furthermore, the adhesive force 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 force 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 force c between the second positive electrode mixture layer and the separation film is in a range smaller than the adhesive force b, from 5 N / m to 30 N / m, preferably from 7 N / m to 25 N / m, and more preferably from 10 N / m to 20 N / m.

[0060] The first positive electrode mixture layer and the second positive electrode mixture layer of the present invention include a binder. The binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The binder may be included in an amount of 1% to 30% by weight relative to the total weight of the positive electrode mixture layer.

[0061] In one embodiment of the present invention, the first binder contained in the first cathode mixture layer and the second binder contained in the second cathode mixture layer may be binders of the same properties. 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 meaning of having the same properties includes embodiments in which the first binder and the second binder are of the same type.

[0062] In one specific example, the weight ratio of the first binder relative to the total weight of the first positive electrode mixture layer may be greater than the weight ratio of the second binder relative to the total weight of the second positive electrode mixture layer. The present invention can control the adhesive force a between the first positive electrode mixture layer and the positive electrode current collector to be greater than adhesive forces b and c by controlling 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 to predetermined conditions, without adjusting the binder content. However, the adhesive force a can be further increased by making 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 weight of the first binder relative to the total weight of the first positive electrode mixture layer, and the weight ratio of the weight of the second binder relative to the total weight of the second positive electrode mixture layer.

[0063] In this case, the weight ratio of the first binder to the total weight of the first positive electrode mixture layer may be 0.01 to 0.3, and preferably 0.05 to 0.2. Specifically, the thickness of the first positive electrode mixture layer is 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 include a lithium transition metal oxide represented by the following chemical formula 1.

[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 the following inequalities apply: 0.9 ≤ a ≤ 1.5, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and 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 lithium transition metal oxides represented by chemical formula 1, and the first positive electrode active material and / or the second positive electrode active material may be layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals, or compounds with chemical formula Li 1+x1 Mn 2-x1 Lithium manganese oxides such as O4 (where x1 is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc., lithium copper oxide (Li2CuO2), vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7, etc., chemical formula LiNi 1-x2 M 1 x2 O2(here, M 1 Lithium nickel oxide of the Ni site type, represented by the chemical formula LiMn (where x is Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x² is 0.01-0.3) 2-x3 M 2 x3 O2(here, M 2 (=Co, Ni, Fe, Cr, Zn or Ta, and x3 = 0.01~0.1) or Li2Mn3M 3 O8 (Here, M 3 Lithium manganese composite oxide, LiNi (=Fe, Co, Ni, Cu or Zn) x4 Mn 2-x4 This can include lithium manganese composite oxides with a spinel structure represented as O4 (where x4 = 0.01 to 1), LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions, disulfide compounds, Fe2(MoO4)3, and others.

[0068] On the other hand, the first positive electrode active material and the second positive electrode active material may contain lithium transition metal oxides of the same composition or lithium transition metal oxides of different compositions. In one 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, and X is one or more selected from F, S, and N, with -0.5 ≤ a ≤ +0.5, 0 ≤ x ≤ 0.5, and 0 ≤ b ≤ 0.1.

[0071] The above-described olivine-structured positive electrode active material, at an overcharge voltage of approximately 4.5V or higher, causes lithium to escape from the first positive electrode active material, resulting in a volume contraction. This quickly blocks the conductive path of the first positive electrode mixture layer, causing the first positive electrode mixture layer to act as an insulating layer, increasing resistance, and interrupting the charging current, thus reaching the overcharge termination voltage. Therefore, in this invention, selecting the above-described 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] Thus, the positive electrode of the present invention is configured such that the first positive electrode active material contained in the first positive electrode mixture layer is a lithium iron phosphate compound with an olivine structure represented by chemical formula 2, thereby increasing the electrical resistance of the first positive electrode mixture layer and enabling it to act as a resistive layer at high voltages. As a result, the resistance of the positive electrode increases significantly during overcharging, causing a decrease in charging current and terminating the charge, thus ensuring safety. In such a case, the first positive electrode mixture layer functions as a safety layer (SFL) to prevent overcharging, and under normal battery operating conditions, the positive electrode active material also plays a role in expressing its capacity.

[0073] On the other hand, in the positive electrode of the present invention, the types of active materials in the first positive electrode mixture layer and the second positive electrode mixture layer may differ from each other. For example, the first positive electrode mixture layer may be configured to function as an overcharge prevention layer, and the first positive electrode active material may be selected to be a lithium iron phosphate compound with an olivine structure represented by chemical formula 2. The second positive electrode mixture layer may be configured to be configured to be a lithium transition metal oxide represented by chemical formula 1. In such a case, the high capacity / high energy density characteristics of the second positive electrode active material make it possible to 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 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 safety and only needs to be thick enough to increase penetration resistance when penetrated by a conductor such as a metal body; therefore, it does not need to be thick.

[0075] At least one of the first positive electrode mixture layer and the second positive electrode mixture layer of the present invention further comprises a conductive material. The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite, carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber, metal powders or metal fibers such as copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, or conductive polymers such as polyphenylene derivatives can be used. The conductive material may be included in an amount of 1% to 30% by weight relative to 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 without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the positive electrode current collector can usually have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in a variety of forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0077] The conductive material described above is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, it may be used to make 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, metal powders such as fluorocarbon, aluminum, or nickel powder, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, or conductive materials such as polyphenylene derivatives.

[0078] Furthermore, the present invention provides an electrochemical element including the above-mentioned positive electrode. Specifically, the electrochemical element may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

[0079] The lithium secondary battery described above specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, a separator membrane interposed between the positive and negative electrodes, and an electrolyte, wherein the positive electrode is as described above. The lithium secondary battery may also selectively further include a battery case housing the electrode assembly of the positive electrode, negative electrode, and separator membrane, and a sealing member for sealing the battery case.

[0080] In the lithium secondary battery described above, 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 described above is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the negative electrode current collector can usually have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

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

[0083] As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium can be used. 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, or Al alloys; and SiO2. βExamples include lithium-doped and dedoped metal oxides such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites. One or more of these can be used as a mixture of two or more. A metallic lithium thin film may also be used as the negative electrode active material. Furthermore, all types of carbon materials, including low-crystalline carbon and high-crystalline carbon, can be used. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0084] Furthermore, the binder and conductive material described above may be the same as those used in the positive electrode described above.

[0085] On the other hand, in the lithium secondary battery described above, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Generally, any membrane commonly used as a separation membrane in lithium secondary batteries can be used without particular limitations, and those with low resistance to ion movement of the electrolyte while exhibiting excellent moisture-absorbing capacity for the electrolyte are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, can be used. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. Furthermore, coated separators containing ceramic components or polymeric substances can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0086] Furthermore, examples of electrolytes used in the present invention include, but are not limited to, 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 manufacture of lithium secondary batteries.

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

[0088] The above-mentioned organic solvent can be used without particular limitations, as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carnonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; and R-CN (where R is C2-C2). 20 Nitriles such as linear, branched, or cyclic hydrocarbon groups (which may include double-bonded aromatic rings or ether bonds), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes can be used. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to 9 can produce an electrolyte with excellent performance.

[0089] The lithium salt described above can be used without particular limitation as long as it is a compound that can provide lithium ions for use in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably used in the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0090] In addition to the electrolyte components described above, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additives may be present in an amount of 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0091] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and capacity retention rate stably, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0092] The present invention provides a battery module and a battery pack containing the above-mentioned lithium secondary battery as a unit cell, according to another embodiment of the present invention.

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

[0094] Hereinafter, embodiments of the present invention will be described in detail so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be embodied in a variety of different forms and is not limited to the embodiments described herein.

[0095] Example 1 Average particle size (D 50 The surface area is 1 μm and the BET specific surface area is 15 m². 2 A first cathode active material slurry was prepared by mixing 93 parts by weight of LiFePO4 cathode active material (at a concentration of / g), 2 parts by weight of carbon black as a conductive material, and 5 parts by weight of PVDF as a binder in an N-methylpyrrolidone (NMP) solvent.

[0096] Average particle size (D 50 The surface area is 4 μm, and the BET specific surface area is 0.7 m². 2 LiSa 0.8 Co 0.1 A slurry of the second positive electrode active material 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 positive electrode active material slurry and the second positive electrode active material slurry were applied to aluminum foil, dried, and rolled to produce a positive electrode having the structure of aluminum foil / first positive electrode mixture layer / second positive electrode mixture layer. The thickness of the first positive electrode mixture layer was 10 μm, and the thickness of the second positive electrode mixture layer was 80 μm.

[0098] Examples 2 and 3 The positive electrode was prepared 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 The surface area is 4 μm, and the BET specific surface area is 0.7 m². 2 LiSa 0.8 Co 0.1 Mn 0.1 A first cathode active material slurry was prepared by mixing 96 parts by weight of O2 cathode 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 The surface area is 1 μm and the BET specific surface area is 15 m². 2 A second cathode active material slurry was prepared by mixing 93 parts by weight of LiFePO4 cathode active material (at a concentration of / g), 2 parts by weight of carbon black as a conductive material, and 5 parts by weight of PVDF as a binder in an N-methylpyrrolidone (NMP) solvent.

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

[0102] Comparative Example 2 In the above Example 1, the LiFePO4 contained in the first positive electrode active material slurry is divided into average particle sizes (D 50A positive electrode was prepared in the same manner as in Example 1, except that LiFePO4 with a 4 μm saturation 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 The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 2 were prepared as test specimens. After mounting these specimens in a UTM device, they were stretched at a speed of approximately 5 mm / min. The stretching rate was measured by the change in length until the positive electrode was maximally stretched compared to the existing positive electrode length. The results are shown in Table 2.

[0105] Experiment Example 2: Measurement of Adhesion The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 2 were cut to have a horizontal and vertical length of 25 mm and 70 mm, respectively. Then, separation membranes were laminated and laminated using a press at 70°C and 4 MPa to prepare test specimens.

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

[0107] The interfacial adhesion force b between the first positive electrode mixture layer and the second positive electrode mixture layer, and the interfacial adhesion force a between the first positive electrode mixture layer and the positive electrode current collector were also measured using the method described above, and the results are shown in Table 2.

[0108] Experimental Example 3: Evaluation of Penetration Safety Lithium secondary batteries were manufactured using the positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 2.

[0109] First, natural graphite, carbon black conductive material, and PVDF binder were mixed in an N-methylpyrrolidone solvent in a weight ratio of 85:10:5 to produce a slurry for forming the negative electrode, which was then applied to copper foil to produce the negative electrode.

[0110] An electrode assembly was manufactured by interposing a porous polyethylene separation membrane between the negative electrode described above and the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 2. After positioning each electrode assembly inside a case, an electrolyte was injected into the case to manufacture a lithium secondary battery. The electrolyte 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 mixed volume ratio of 3 / 4 / 3).

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

[0112] [Table 2]

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

[0114] Experimental Example 4: Evaluation of Overcharge Safety Lithium secondary batteries were manufactured using the same negative electrode, separation membrane material, and method as in Experimental Example 3, using the positive electrodes from Example 2 and Comparative Example 1. Each manufactured lithium secondary battery was charged at 0.33C and 4.2V CCCV to prepare cells with a State of Charge (SOC) of 100%. Then, the cells with a SOC of 100% were CC-charged at 1C-rate to 10% and 20% of their respective cell capacities to produce cells with a SOC of 110% and 120%, respectively. The SOC 100%, 110%, and 120% resistances of each cell were measured by 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 above, the positive electrode according to the embodiment of the present invention exhibits a resistance level similar to that of the positive electrode according to the comparative example in a drivable charge state (SOC 100%, 110%), but its resistance increased significantly during overcharging (SOC 120%) compared to the positive electrode according to the comparative example. Therefore, the positive electrode of the present invention is expected to increase resistance during overcharging, thereby causing the charge to terminate and ensuring safety.

[0118] The above description is merely illustrative of the technical concept of the present invention, and a person with ordinary skill in the art to which the present invention belongs can make various modifications and variations without departing from the essential characteristics of the present invention. Therefore, the drawings disclosed herein are for illustrative purposes only and not to limit 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 an equivalent scope should be interpreted as being included in the scope of rights of the present invention.

Claims

1. It 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 comprises a first positive electrode active material and a first binder. The second positive electrode mixture layer comprises a second positive electrode active material and a second binder. The average particle size (D) of the first positive electrode active material ５０ ) is the average particle size (D) of the second positive electrode active material. ５０ The range is smaller than 0.1 μm to 1.5 μm, A positive electrode for lithium secondary batteries with an elongation ratio of 0.5% to 2.0%.

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

3. The positive electrode for a lithium secondary battery according to claim 1, wherein the adhesive force a between the positive electrode current collector and the first positive electrode mixture layer is 5 to 12 times the adhesive force b between the first positive electrode mixture layer and the second positive electrode mixture layer.

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