Positive electrode for lithium secondary battery and lithium secondary battery

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery. Specifically, the positive electrode includes: a first positive electrode mixture layer in contact with a positive electrode current collector; and at least one second positive electrode mixture layer provided on the first positive electrode mixture layer. The first positive electrode mixture layer contains a first positive electrode active material and a first binder, and the second positive electrode mixture layer contains a second positive electrode active material and a second binder. The average particle diameter (D 50 ) of the first positive electrode active material is smaller than the average particle diameter (D 50 ) of the second positive electrode active material and is 3 μm or less. The specific surface area (BET) of the first positive electrode active material is 3 m 2 / g or more.

Description

Technical Field

[0001] This application claims the benefit of priority to Korean Patent Application No. 10-2020-0153024, filed on November 16, 2020, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to a positive electrode for lithium secondary batteries and lithium secondary batteries, and more specifically, to a positive electrode for lithium secondary batteries and lithium secondary batteries with improved safety. Background Technology

[0003] With technological advancements and increasing demands for mobile devices, the need for secondary batteries as energy sources is also rapidly growing. Among these secondary batteries, lithium-ion batteries, characterized by high energy density and operating potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Recently, as lithium-ion batteries are used as power sources for medium to large devices such as electric vehicles, there is a demand for high capacity, high energy density, and low cost. Therefore, research has been actively conducted to replace expensive cobalt with low-cost alternatives such as nickel, manganese, and iron.

[0005] One of the key research tasks related to lithium-ion batteries is to develop high-capacity and high-output electrode active materials and improve the safety of batteries using these materials. Lithium transition metal composite oxides are commonly used as positive electrode active materials, with lithium-cobalt composite metal oxides being the primary choice due to their excellent capacity characteristics and high operating voltage. However, LiCoO2 suffers from unstable crystal structure due to delithiation, very poor thermal properties, and is expensive, making its widespread use in electric vehicles and other applications difficult.

[0006] Lithium-manganese composite metal oxides (LiMnO2 or LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.), and lithium-nickel composite metal oxides (LiNiO2, etc.) have been developed. Among them, lithium-nickel composite metal oxides have a large reversible capacity of about 200 mAh / g, which can easily realize high-capacity batteries, and are currently under active research and development. However, compared with LiCoO2, LiNiO2 has poor thermal stability. In the charging state, when external pressure causes an internal short circuit, the positive electrode active material itself decomposes, leading to battery rupture and fire.

[0007] Therefore, as a method to improve the low thermal stability while maintaining the excellent reversible capacity of LiNiO2, a method of replacing part of the nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. However, in LiNiO2 obtained by replacing part of the nickel with cobalt... 1-α Co αIn the case of O2 (α = 0.1~0.3), the charge-discharge characteristics and lifetime characteristics are excellent, but the thermal stability is low. In addition, in the case of nickel-manganese lithium composite metal oxides obtained by replacing part of Ni with Mn, which has excellent thermal stability, and in the case of nickel-cobalt-manganese lithium composite metal oxides obtained by replacing part of Ni with Mn and Co (hereinafter referred to as "NCM lithium oxides"), the cycle characteristics and thermal stability are excellent, but due to the low penetration resistance, when a metal body such as a nail penetrates, an internal short circuit will not occur, and therefore it may catch fire or explode due to overcurrent.

[0008] Korean Patent Publication No. 2019-0047203 discloses a technology that increases the resistance during overcharging by inserting an overcharge prevention layer between the positive electrode current collector and the positive electrode active material, thereby blocking the charging current and ensuring battery safety. However, in the aforementioned prior art, the overcharge prevention layer has low penetration resistance. Therefore, from a safety perspective, there may be problems when it is pierced by a needle.

[0009] Therefore, there is a need for a technology for the positive electrode of a secondary battery that improves penetration resistance when a metal object, such as a nail, penetrates the electrode from the outside. Summary of the Invention

[0010] Technical issues

[0011] The purpose of this invention is to provide a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode. The positive electrode can improve penetration resistance when metal objects such as nails penetrate the electrode from the outside, while having high capacity and high output performance, excellent cycle characteristics and thermal stability.

[0012] Technical solution

[0013] The positive electrode for a lithium secondary battery of the present invention comprises: a first positive electrode agent layer in contact with a positive electrode current collector; at least one second positive electrode agent layer disposed on the first positive electrode agent layer; wherein the first positive electrode agent layer comprises a first positive electrode active material and a first binder, wherein the second positive electrode agent layer comprises a second positive electrode active material and a second binder, wherein the average particle size (D) of the first positive electrode active material is [not specified in the original text]. 50 The particle size is smaller than the average particle size (D) of the second positive electrode active material. 50 And the surface area is equal to or less than 3 μm, and the specific surface area (BET) of the first positive electrode active material is equal to or greater than 3 m². 2 / g.

[0014] In one embodiment of the present invention, the average particle size (D) of the first positive electrode active material is... 50 The surface area can be in the range of 0.1 μm to 2 μm, and the specific surface area of ​​the first positive electrode active material can be 5 m². 2 / g~25m2 Within the range of / g.

[0015] In one embodiment of the present invention, the adhesive force (a) between the positive current collector and the first positive electrode compound layer is greater than the adhesive force (b) between the first positive electrode compound layer and the second positive electrode compound layer.

[0016] In one embodiment of the present invention, the adhesive force (a) between the positive current collector and the first positive electrode binder layer is in the range of 100 N / m to 500 N / m.

[0017] In one embodiment of the present invention, the adhesive force (b) between the first positive electrode adhesive layer and the second positive electrode adhesive layer is in the range of 10 N / m to 40 N / m.

[0018] In one embodiment of the present invention, the first adhesive and the second adhesive are adhesives having the same physical properties.

[0019] In one embodiment of the present invention, the weight ratio of the first adhesive based on the total weight of the first positive electrode adhesive layer is greater than the weight ratio of the second adhesive based on the total weight of the second positive electrode adhesive layer.

[0020] In one embodiment of the present invention, the weight ratio of the first adhesive to the total weight of the first positive electrode adhesive layer is in the range of 0.01 to 0.3.

[0021] In one embodiment of the present invention, the elongation of the positive electrode for lithium secondary batteries is in the range of 0.5% to 2.0%.

[0022] In one embodiment of the present invention, A / B ≤ 0.3. Here, A represents the thickness of the first positive electrode compound layer, and B represents the thickness of the second positive electrode compound layer.

[0023] In one embodiment of the present invention, the thickness of the first positive electrode compound layer is in the range of 1 μm to 20 μm.

[0024] 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:

[0025] [Chemical Formula 1]

[0026] Li a Ni 1-x-y Co x Mn y M z O2

[0027] Here, M is selected from at least one of the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, and wherein 0.9≤a≤1.5, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.1, and 0≤x+y≤1.

[0028] In one embodiment of the present invention, the first positive electrode active material comprises lithium iron phosphate having an olivine structure, represented by the following chemical formula 2:

[0029] [Chemical Formula 2]

[0030] Li 1+a Fe 1-x M x (PO 4-b )X b

[0031] Here, M is selected from at least one of the groups consisting of Al, Mg and Ti, X is selected from at least one of the groups consisting of F, S and N, and -0.5≤a≤+0.5, 0≤x≤0.5, 0≤b≤0.1.

[0032] In one embodiment of the present invention, at least one of the first positive electrode compound layer and the second positive electrode compound layer further contains a conductive material.

[0033] The lithium secondary battery of the present invention includes the above-mentioned positive electrode, separator, and negative electrode.

[0034] In a 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 compound layer, the adhesive force b between the first positive electrode compound layer and the second positive electrode compound layer, and the adhesive force c between the second positive electrode compound layer and the separator satisfy the condition a > b > c.

[0035] In a lithium secondary battery according to one embodiment of the present invention, the adhesive force c between the second positive electrode binder layer and the separator is in the range of 5 N / m to 30 N / m.

[0036] Beneficial effects

[0037] In the positive electrode for lithium secondary batteries and the lithium secondary battery including the positive electrode of the present invention, since the first positive electrode active material contained in the first positive electrode compound layer is composed of small particles, the elongation of the positive electrode current collector is reduced, and therefore it is advantageous to disconnect the electrode in the event of penetration by a needle-like conductor, and since the first positive electrode compound layer reduces the exposed area of ​​the current collector, penetration safety is improved.

[0038] Furthermore, according to the positive electrode for secondary batteries and the secondary battery including the positive electrode, during overcharging, the resistance of the first positive electrode flux layer increases, the current flowing in the electrode decreases, thereby terminating charging and improving overcharging safety. Attached Figure Description

[0039] Figure 1 This is a cross-sectional view of the positive electrode according to one embodiment of the present invention.

[0040] Figure 2 This is a conceptual diagram illustrating the penetration resistance of the positive electrode in one embodiment of the present invention. Detailed Implementation

[0041] The invention will be described in detail below with reference to the accompanying drawings. The terms and words used in this specification and claims should not be construed as limited to common or dictionary terms; the inventors may appropriately define the concepts of the terms to best describe their invention. The terms and words should be interpreted as having meanings and concepts consistent with the technical concept of the invention.

[0042] In this application, it should be understood that terms such as "comprising" or "having" are intended to indicate the presence of the features, quantities, steps, operations, components, parts, or combinations thereof described in the specification, and they do not preclude the possibility of the presence or addition of one or more other features or quantities, steps, operations, components, parts, or combinations thereof. Furthermore, when a portion such as a layer, membrane, region, plate, etc., is referred to as being "on" another portion, this includes not only the case where the portion is "directly" on the other portion, but also the case where other portions are inserted between them. On the other hand, when a portion such as a layer, membrane, region, plate, etc., is referred to as being "below" another portion, this includes not only the case where the portion is "directly" below the other portion, but also the case where other portions are inserted between them. Additionally, the "on" of a setting in this application can include both bottom and top settings.

[0043] The invention will now be described in detail with reference to the accompanying drawings.

[0044] Figure 1 This is a cross-sectional view of the positive electrode according to one embodiment of the present invention. (Refer to...) Figure 1 The positive electrode 100 for a lithium secondary battery of the present invention includes: a first positive electrode binder layer 120 in contact with a positive electrode current collector 110; and at least one second positive electrode binder layer 130 disposed on the first positive electrode binder layer 120. Here, the first positive electrode binder layer comprises a first positive electrode active material and a first binder, and the second positive electrode binder layer comprises a second positive electrode active material and a second binder. Furthermore, the average particle size (D) of the first positive electrode active material is... 50 The particle size is smaller than the average particle size (D) of the second positive electrode active material. 50 And the surface area is equal to or less than 3 μm, and the specific surface area (BET) of the first positive electrode active material is equal to or greater than 3 m². 2 / g.

[0045] Figure 2 This is a conceptual diagram illustrating the penetration resistance of the positive electrode in one embodiment of the present invention. (Refer to...) Figure 2 In the positive electrode of the present invention, since the first positive electrode active material constituting the first positive electrode compound layer in contact with the positive electrode current collector is composed of small particles, the elongation of the first positive electrode compound layer is reduced. As a result, due to the reduced elongation of the first positive electrode compound layer, the positive electrode current collector adjacent to it becomes more favorable for positive electrode disconnection.

[0046] Therefore, when a metal object such as a nail penetrates the positive electrode, the positive current collector does not elongate along the metal object but instead breaks the circuit. Consequently, the contact area between the positive current collector and the metal object decreases. Furthermore, as the positive current collector elongates along the metal object, it may come into contact with the negative current collector, which has the opposite polarity. Here, the positive electrode of the present invention can suppress contact between the negative current collector and the positive current collector, whose elongation has decreased.

[0047] also, Figure 1 The embodiment shown is formed by a single layer of the second positive electrode compound layer, but the embodiments of the present invention are not limited thereto. The second positive electrode compound layer may also be formed by two or more layers to improve energy density or conductivity.

[0048] The average particle size (D) of the first positive electrode active material 50 This can correspond to the average particle size (D) of the second positive electrode active material. 50 5% to 80% of ).

[0049] That is, according to one embodiment of the present invention, the first positive electrode active material can be coated on the lower part of the positive electrode to be adjacent to the positive electrode current collector, and has a large particle size (D). 50 The second positive electrode active material can be coated on the upper part of the positive electrode. As a result, the elongation of the first positive electrode binder layer adjacent to the positive electrode current collector is reduced.

[0050] In this invention, the average particle size D 50 The average particle size D can be defined as the particle size corresponding to 50% of the volumetric accumulation in the particle size distribution curve. For example, laser diffraction can be used to measure the average particle size D. 50 For example, based on the average particle size (D) of the positive electrode active material... 50 The measurement method involves dispersing particles of the positive electrode active material in a dispersion medium, then irradiating them with approximately 28 kHz ultrasound at a 60 W output using a commercially available laser diffraction particle size measurement device (e.g., Microtrac MT 3000). The average particle size (D) corresponding to 50% of the volumetric accumulation in the measurement device can then be calculated. 50 ).

[0051] Specifically, the average particle size of the first positive electrode active material can be equal to or less than 3 μm. More preferably, the average particle size (D) of the first positive electrode active material is... 50 The particle size can be in the range of 0.1 μm to 2 μm, preferably in the range of 0.1 μm to 1.5 μm. When the average particle size (D) of the first positive electrode active material... 50 When the particle size is less than 0.1 μm, electrode side reactions may occur or the dispersibility may decrease during electrode manufacturing; when the average particle size (D) of the first positive electrode active material is less than 0.1 μm, electrode side reactions may occur or the dispersibility may decrease; 50 When the diameter is greater than 3μm, the adhesion to the positive current collector may decrease, and the effect of improving safety may be reduced.

[0052] Furthermore, the specific surface area of ​​the first positive electrode active material is equal to or greater than 3 m². 2 / g, preferably at 5m 2 / g~25m 2 Within the range of / g, more preferably within 7m 2 / g~20m 2 Within the range of / g. If the specific surface area is less than 3m² 2 If the elongation of the first positive electrode compound layer is increased by / g, it may be undesirable.

[0053] In this invention, the specific surface area is measured using the BET method. Specifically, it can be calculated by using the amount of nitrogen adsorbed at liquid nitrogen temperature (77K) using the Belserp-mino II from BEL Japan.

[0054] The second positive electrode active material can be an average particle size (D) 50 The relative particle size is larger than the average particle size (D) of the first positive electrode active material. 50 ) particles.

[0055] Specifically, the average particle size (D) of the second positive electrode active material 50 The particle size can be equal to or greater than 3 μm, preferably in the range of 3 μm to 50 μm, and more preferably in the range of 3 μm to 30 μm. When the average particle size (D) of the second positive electrode active material... 50 When the diameter is less than 3μm, there may be difficulties in the rolling process when manufacturing the electrode.

[0056] In addition, the specific surface area of ​​the second positive electrode active material is equal to or greater than 2m². 2 / g, preferably at 0.1m 2 / g~1.5m 2 Within the range of / g, more preferably within 0.2m 2 / g~1.2m 2 Within the range of / g.

[0057] In one embodiment of the present invention, the elongation of the positive electrode compound layer (a laminate of a first positive electrode compound layer and a second positive electrode compound layer) is in the range of 0.5% to 2%, preferably in the range of 0.5% to 1.8%, and more preferably in the range of 0.6% to 1.5%. In this invention, the elongation of the positive electrode compound layer is a value measured using a UTM device, and the elongation is determined by the change in length relative to the original length of the positive electrode compound layer when it elongates at a rate of approximately 5 mm / min after installation. When the elongation of the positive electrode compound layer is within the above-mentioned range, battery performance, such as penetration resistance and battery life characteristics, can be improved.

[0058] In a positive electrode for a 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 compound layer, the adhesive force b between the first positive electrode compound layer and the second positive electrode compound layer, and the adhesive force c between the second positive electrode compound layer and the separator satisfy the condition a > b > c.

[0059] This is to minimize the contact area between the current collector and a metal object such as a nail when the metal object penetrates the positive electrode. Specifically, when the metal object penetrates the positive electrode, applying external force to the positive electrode may create gaps between the positive electrode current collector and the first positive electrode adhesive layer, between the first and second positive electrode adhesive layers, and between the second positive electrode adhesive layer and the separator. Here, when the adhesive force a is relatively greater than adhesive forces b and c, even if the first positive electrode adhesive layer is peeled from the second positive electrode adhesive layer, the first positive electrode adhesive layer will still adhere to the positive electrode current collector. Therefore, it is difficult for the metal object to directly contact the positive electrode current collector. Furthermore, when the adhesive force b is relatively greater than adhesive force c, even if the second positive electrode adhesive layer is peeled from the separator, the second positive electrode adhesive layer can adhere to the first positive electrode adhesive layer and protect it. This suppresses the tendency for the first positive electrode adhesive layer to peel off from the positive electrode current collector due to the external force of the metal object.

[0060] Similarly, in the positive electrode of the present invention, the adhesion between the first positive electrode adhesive layer and the positive electrode current collector is excellent. Thus, when a metal object such as a nail penetrates the positive electrode, the first positive electrode adhesive layer reduces the exposed area of ​​the positive electrode current collector. Therefore, in the positive electrode of the present invention, safety is improved due to the reduction in short-circuit current.

[0061] At this point, a is preferably 5 to 12 times that of b, and more preferably 6 to 10 times that of b. When a and b are within the above-mentioned value range, excellent penetration safety performance can be achieved.

[0062] The adhesive force α between the current collector and the first positive electrode adhesive layer can be in the range of 100 N / m to 500 N / m, preferably in the range of 150 N / m to 300 N / m, and more preferably in the range of 200 N / m to 300 N / m.

[0063] The adhesive force b between the first positive electrode layer and the second positive electrode layer can be in the range of 10 N / m to 40 N / m, preferably in the range of 15 N / m to 35 N / m, and more preferably in the range of 20 N / m to 35 N / m.

[0064] The adhesive force c between the second positive electrode binder layer and the separator can be less than the adhesive force b and is in the range of 5 N / m to 30 N / m, preferably in the range of 7 N / m to 25 N / m, and more preferably in the range of 10 N / m to 20 N / m.

[0065] The first and second positive electrode binder layers of this invention contain an adhesive that improves the adhesion between positive electrode active material particles and enhances the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), PVDF-co-HFP copolymer, 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, or various copolymers thereof, and one or more mixtures thereof may be used. Based on the total weight of the positive electrode binder layers, the adhesive content can be from 1% to 30% by weight.

[0066] In one embodiment of the present invention, the first adhesive contained in the first positive electrode layer and the second adhesive contained in the second positive electrode layer may be adhesives having the same physical properties. For example, if the first adhesive is a hydrophilic adhesive, then the second adhesive may also be a hydrophilic adhesive; if the first adhesive is an oleophilic adhesive, then the second adhesive may also be an oleophilic adhesive. "Same physical properties" refers to the concept of embodiments in which the types of the first adhesive and the second adhesive are the same.

[0067] In one specific example, the weight ratio of the first adhesive based on the total weight of the first positive electrode agent layer can be greater than the weight ratio of the second adhesive based on the total weight of the second positive electrode agent layer. In this invention, by controlling the porosity of the first positive electrode agent layer and ensuring that the particle size and specific surface area of ​​the first positive electrode active material contained in the first positive electrode agent layer meet predetermined conditions, the adhesive force 'a' between the first positive electrode agent layer and the positive electrode current collector can be controlled to be greater than both adhesive forces 'b' and 'c'. However, the adhesive force 'a' can also be further improved by making the content of the first adhesive in the first positive electrode agent layer greater than the content of the second adhesive in the second positive electrode agent layer. Here, the adhesive content refers to the weight ratio of the first adhesive in the total weight of the first positive electrode agent layer and the weight ratio of the second adhesive in the total weight of the second positive electrode agent layer.

[0068] At this point, the weight ratio of the first adhesive to the total weight of the first positive electrode binder layer can be in the range of 0.01 to 0.3, preferably in the range of 0.05 to 0.2. The thickness of the first positive electrode binder layer can be in the range of 1 μm to 20 μm, preferably in the range of 1 μm to 10 μm.

[0069] The first positive electrode active material and / or the second positive electrode active material of the present invention may comprise a lithium transition metal oxide represented by the following chemical formula 1:

[0070] [Chemical Formula 1]

[0071] Li a Ni 1-x-y Co x Mn y M z O2

[0072] Here, M is selected from at least one of the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, and wherein 0.9≤a≤1.5, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.1, and 0≤x+y≤1.

[0073] However, the first and / or second positive electrode active materials are not necessarily limited to lithium transition metal oxides represented by Formula 1, and the first and / or second positive electrode active materials can be: layered compounds, such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium manganese oxides, such as Li... 1+x Mn 2-x O4 (here, x is between 0 and 33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, LiFe3O4, V2O5 and Cu2V2O7; and LiNi 1-x2 M 1 x2 O2 (here, M) 1 Lithium nickel oxides represented by (Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x2 = 0.01 to 0.3); Li2Mn3M 3 O8 (where M) 3 Lithium-manganese composite oxides (represented by Fe, Co, Ni, Cu, or Zn); LiNi x4 Mn 2-x4 O4 (here, x4 = 0.01~1); LiMn2O4, in which part of the Li is replaced by alkaline earth metal ions; disulfides; Fe2(MoO4)3, etc.

[0074] Furthermore, the first and second positive electrode active materials may contain lithium transition metal oxides of the same composition or lithium transition metal oxides of different compositions.

[0075] In one embodiment of the present invention, the first positive electrode active material preferably comprises lithium iron phosphate having an olivine structure, represented by the following chemical formula 2:

[0076] [Chemical Formula 2]

[0077] Li 1+a Fe 1-x M x (PO 4-b )X b

[0078] Here, M is selected from at least one of the groups consisting of Al, Mg and Ti, X is selected from at least one of the groups consisting of F, S and N, and -0.5≤a≤+0.5, 0≤x≤0.5, 0≤b≤0.1.

[0079] The volume of the positive electrode active material with an olivine structure decreases as lithium in the first positive electrode active material discharges at an overcharge voltage of approximately 4.5V or higher. Therefore, the conductive path of the first positive electrode binder layer is rapidly blocked. In this way, the first positive electrode binder layer acts as an insulating layer, increasing its resistance and blocking the charging current, thereby reaching the overcharge termination voltage. Therefore, in this invention, by selecting a positive electrode active material with an olivine structure as the first positive electrode active material contained in the first positive electrode binder layer, a synergistic effect can be achieved in improving safety.

[0080] Similarly, the positive electrode of the present invention improves the resistance of the first positive electrode compound layer by selecting lithium iron phosphate with an olivine structure, represented by Chemical Formula 2, as the first positive electrode active material contained in the first positive electrode compound layer. This allows the first positive electrode compound layer to act as a resistive layer under high voltage, thus significantly increasing the resistance of the positive electrode during overcharging, leading to a decrease in charging current and termination of charging, thereby ensuring safety. In this case, the first positive electrode compound layer serves as a safety layer (SFL) to prevent overcharging, and the positive electrode active material achieves capacity under normal battery operating conditions.

[0081] Furthermore, in the positive electrode of the present invention, the type of active material of the first positive electrode compound layer can be different from the type of active material of the second positive electrode compound layer. For example, the first positive electrode compound layer can be selected as lithium iron phosphate with an olivine structure represented by Chemical Formula 2 as the first positive electrode active material, and the second positive electrode compound layer can be selected as lithium transition metal oxide represented by Chemical Formula 1 as the second positive electrode active material. In this case, due to the high capacity / high energy density characteristics of the second positive electrode active material, a secondary battery with excellent capacity characteristics can be provided.

[0082] In a preferred embodiment of the present invention, assuming the thickness of the first positive electrode compound layer is A and the thickness of the second positive electrode compound layer is B, then A / B can be equal to or less than 0.3, preferably equal to or less than 0.1. The first positive electrode compound layer of the present invention is a layer prepared to ensure safety; it does not need to be thick, as long as it can improve the penetration resistance when penetrated by conductors such as metal bodies.

[0083] At least one of the first and second positive electrode additive layers of the present invention further contains a conductive material. Such a conductive material is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. Examples include: graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon fiber; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Based on the total weight of the positive electrode active additive layers, the content of the conductive material can be from 1% to 30% by weight.

[0084] In this invention, the positive electrode current collector is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. Examples of positive electrode current collectors can include: stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. Furthermore, the thickness of the positive electrode current collector is typically from 3 μm to 500 μm. By forming minute irregularities on the surface of the positive electrode current collector, the adhesion of the positive electrode active material can be improved. It can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0085] There are no particular restrictions on conductive materials, as long as they are conductive and do not cause chemical changes in the battery. Examples include: natural graphite, artificial graphite, etc.; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorinated carbon, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0086] Furthermore, the present invention provides an electrochemical device including the aforementioned positive electrode. Specifically, this electrochemical device can be a battery or a capacitor, and more specifically, it can be a lithium secondary battery.

[0087] Specifically, a lithium secondary battery includes a positive electrode, a negative electrode opposite to the positive electrode, a separator between the positive and negative electrodes, and an electrolyte, wherein the positive electrode is as described above. Additionally, a lithium secondary battery may optionally include a battery case housing an electrode assembly that accommodates the positive electrode, negative electrode, and separator, and a sealing component for sealing the battery case.

[0088] In a lithium secondary battery, the negative electrode includes a negative current collector and a negative electrode flux layer disposed on the negative current collector.

[0089] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and will not cause chemical changes in the battery. Examples include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or copper or stainless steel, aluminum-cadmium alloys surface-treated with carbon, nickel, titanium, or silver. Furthermore, the thickness of the negative electrode current collector can typically range from 3 μm to 500 μm. By forming tiny irregularities on the surface of the current collector, similar to the positive electrode current collector, the bonding force of the negative electrode active material can be enhanced. It can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0090] The negative electrode binder layer comprises a negative electrode active material, a binder, and a conductive material. For example, the negative electrode binder layer can be prepared by applying a negative electrode binder layer forming composition comprising a negative electrode active material and optionally a binder and a conductive material onto a negative electrode current collector and drying it, or by casting the negative electrode binder layer forming composition onto a separate support and then laminating a film obtained by peeling the slurry from the support onto the negative electrode current collector.

[0091] Compounds capable of reversibly inserting and de-intercalating lithium can be used as anode active materials. Specific examples include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metallic materials 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 metal oxides capable of doping and de-doping lithium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above-mentioned metallic materials and carbonaceous materials, such as Si-C composites or Sn-C composites, may be used, or any one or a mixture of two or more of them. Additionally, lithium metal films may be used as the negative electrode active material. Both low-crystallinity carbon and high-crystallinity carbon may be used as carbon materials. Examples of low-crystallinity carbon include soft carbon and hard carbon. Examples of high-crystallinity carbon include amorphous, flake-like, spherical, or fibrous natural or artificial graphite, condensate (Kish) graphite, pyrolytic carbon, mesophase pitch-like carbon fibers, mesophase carbon microspheres, mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0092] In addition, the adhesive and conductive material can be the same as those described in the previous positive electrode.

[0093] Meanwhile, in lithium-ion secondary batteries, the separator is used to separate the negative electrode from the positive electrode and provide a path for lithium ions to move. Any separator commonly used in lithium-ion secondary batteries can be used without any particular limitations. In particular, separators with high electrolyte wetting ability and low resistance to ion movement in the electrolyte are preferred. Specifically, porous polymer membranes can be used, for example, porous polymer membranes made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexane copolymer, and ethylene / methacrylate copolymer. Furthermore, nonwoven fabrics made of conventional porous nonwoven fabrics, such as high-melting-point glass fiber or polyethylene terephthalate fiber, can be used. To ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymer materials can be used, and can optionally be used as single-layer or multi-layer structures.

[0094] Examples of electrolytes used in this invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries, but the invention is not limited to these examples.

[0095] Specifically, electrolytes may include organic solvents and lithium salts.

[0096] Organic solvents can be any organic solvent capable of serving as a medium through which ions participating in the electrochemical reactions of the battery can move. Specifically, examples of such organic solvents include: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and γ-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic 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 carbonate (PC); alcohol solvents, such as ethanol and isopropanol; nitriles, such as R-CN (where R is a straight-chain, branched, or cyclic C2–C20 hydrocarbon group, which may contain double-bonded aromatic rings or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Preferably, a carbonate solvent is used; more preferably, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, and linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) with low viscosity, which can improve the charge / discharge performance of the battery, is preferred. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0097] Lithium salts can be used without any particular restrictions, as long as they are compounds capable of providing lithium ions for lithium secondary batteries. Specifically, lithium salts can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, or LiN(CF3SO2)2. LiCl, LiI, or LiB(C2O4)2 can also be used. The concentration of the lithium salt is preferably in the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0098] In addition to the electrolyte component, to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, the electrolyte may contain one or more of the following: halogenated alkyl carbonate compounds (such as ethylene difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. Here, based on the total weight of the electrolyte, the content of the additives can be from 0.1% to 5% by weight.

[0099] Since lithium secondary batteries containing the positive electrode active material of the present invention stably exhibit excellent discharge capacity, output characteristics and capacity retention, they can be used in portable devices (such as mobile phones, laptops and digital cameras) and electric vehicles (such as hybrid vehicles).

[0100] Therefore, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0101] Battery modules or battery packs can be used as a power source for one or more medium and large devices, including: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0102] The embodiments of the present invention will be described in detail below to enable those skilled in the art to readily implement the invention. However, the present invention can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0103] Example 1

[0104] The average particle size (D) of 93% by weight50 The surface area is 1 μm and the BET specific surface area is 15 m². 2 The first positive electrode slurry is prepared by mixing 2% by weight of LiFePO4 positive electrode active material, 2% by weight of carbon black as a conductive material, and 5% by weight of PVdF as a binder in N-methylpyrrolidone (NMP) as a solvent.

[0105] The average particle size (D) of 96% by weight 50 The micrometer diameter (4μm) and the specific surface area (BET) are 0.7m². 2 / g of LiNi 0.8 Co 0.1 Mn 0.1 O2 positive electrode active material, 2% by weight carbon black as a conductive material, and 2% by weight PVdF as a binder are mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a second positive electrode slurry.

[0106] A first positive electrode active material slurry and a second positive electrode active material slurry are coated onto an aluminum foil, which is then dried and rolled to manufacture a positive electrode with a structure of aluminum foil / first positive electrode mixture layer / second positive electrode mixture layer. The thickness of the first positive electrode mixture layer is 10 μm, and the thickness of the second positive electrode mixture layer is 80 μm.

[0107] Examples 2-3

[0108] The positive electrode was manufactured using the same method as in Example 1, except that the composition of the first positive electrode active material slurry was changed as shown in Table 1 below.

[0109] Comparative Example 1

[0110] The average particle size (D) of 96% by weight 50 The micrometer diameter (4μm) and the specific surface area (BET) are 0.7m². 2 / g of LiNi 0.8 Co 0.1 Mn 0.1 O2 positive electrode active material, 2% by weight carbon black as a conductive material, and 2% by weight PVdF as a binder are mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a first positive electrode slurry.

[0111] The average particle size (D) of 93% by weight 50 The surface area is 1 μm and the BET specific surface area is 15 m². 2 A second positive electrode slurry is prepared by mixing 1 g of LiFePO4 positive electrode active material, 2 wt% of carbon black as a conductive material, and 5 wt% of PVdF as a binder in N-methylpyrrolidone (NMP) as a solvent.

[0112] A first positive electrode active material slurry and a second positive electrode active material slurry are coated onto an aluminum foil, which is then dried and rolled to manufacture a positive electrode with a structure of aluminum foil / first positive electrode mixture layer / second positive electrode mixture layer. The thickness of the first positive electrode mixture layer is 10 μm, and the thickness of the second positive electrode mixture layer is 80 μm.

[0113] Comparative Example 2

[0114] The positive electrode was manufactured using the same method as in Example 1, except that the average particle size (D) was increased. 50 Its thickness is 4μm and its specific surface area is 2.8m². 2 / g of LiFePO4 is used as the LiFePO4 contained in the first positive electrode active material slurry, and the composition of the first positive electrode active material slurry is changed as shown in Table 1 below.

[0115] [Table 1]

[0116]

[0117] Experimental Example 1: Elongation Measurement

[0118] The positive electrodes prepared in Examples 1-3 and Comparative Examples 1 and 2 were used as samples. These samples were mounted on a UTM device and stretched at a speed of approximately 5 mm / min. The elongation rate of the positive electrode at its maximum length was measured compared to its original length. The results are shown in Table 2.

[0119] Experimental Example 2: Adhesive Strength Measurement

[0120] The positive electrodes manufactured in Examples 1-3 and Comparative Examples 1 and 2 were cut into pieces 25 mm wide and 70 mm long. Subsequently, the separators were stacked and laminated at 70°C and 4 MPa to prepare samples.

[0121] The prepared sample was attached and fixed to a glass plate using double-sided adhesive tape, with the positive electrode facing the glass plate. The diaphragm portion of the sample was peeled off using a tensile testing machine at 25°C, and the peel strength was measured in real time. Here, the average value is defined as the interfacial adhesion force c between the second positive electrode adhesive layer and the diaphragm. The results are shown in Table 2.

[0122] The interfacial adhesion force b between the first positive electrode compound layer and the second positive electrode compound layer, and the interfacial adhesion force a between the first positive electrode compound layer and the positive electrode current collector were also measured in the above scheme, and the results are shown in Table 2.

[0123] Experiment Example 3: Penetration Safety Assessment

[0124] Lithium-ion batteries were manufactured using each of the positive electrodes produced in Examples 1-3 and Comparative Examples 1 and 2.

[0125] First, natural graphite, carbon black conductive material, and PVDF binder are mixed in N-methylpyrrolidone solvent at a weight ratio of 85:10:5 to produce a slurry for forming the negative electrode. This slurry is then coated onto copper foil to produce the negative electrode.

[0126] An electrode assembly was manufactured by inserting a porous polyethylene separator between the negative electrode and each positive electrode manufactured according to Examples 1-3 and Comparative Examples 1-2, and each electrode assembly was placed in a battery case, and an electrolyte was injected into the battery case to manufacture a lithium secondary battery. At this time, a 1.0M concentration of lithium hexafluorophosphate (LiPF6) was dissolved in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC volume ratio = 3 / 4 / 3) to manufacture the lithium secondary battery.

[0127] Lithium-ion batteries were manufactured using each positive electrode produced according to Examples 1-3 and Comparative Examples 1 and 2. A metal body with a diameter of 3 mm was lowered at a speed of 80 mm / s to penetrate the cell of the lithium-ion battery under the same conditions as PV8450. The occurrence of fire was evaluated, and the results are shown in Table 2 below.

[0128] [Table 2]

[0129]

[0130] Referring to Table 2, compared with the secondary battery including the positive electrode of the comparative examples, the secondary battery including the positive electrode of the embodiments of the present invention shows improved penetration safety. The positive electrodes of Comparative Examples 1 and 2 satisfy a > b > c, but the elongation of the positive electrode exceeds 2.0%. Therefore, in the present invention, in order to improve penetration safety, the elongation of the positive electrode is preferably 2.0% or less.

[0131] Experiment Example 4: Overcharge Safety Assessment

[0132] Using each positive electrode from Example 2 and Comparative Example 1, lithium secondary batteries were manufactured using the same negative electrode, separator material, and method as in Experimental Example 3. Cells with a state of charge (SOC) of 100% were prepared by charging each manufactured lithium secondary battery at 0.33C and 4.2VCCCV. Furthermore, cells with an SOC of 110% and 120% were manufactured by charging the SOC 100% battery at a 1C rate with 10% and 20% of its capacity, respectively. The resistance of each battery at SOCs of 100%, 110%, and 120% was determined using electrochemical impedance spectroscopy.

[0133] In addition, the resistance of overcharged batteries is shown in Table 3 below.

[0134] [Table 3]

[0135] SOC 100 SOC 110 SOC 120 Example 2 0.31 0.93 5.34 Comparative Example 1 0.31 0.73 1.14

[0136] As shown in Table 3, the positive electrode of the present invention exhibits a similar resistance to the positive electrode of the comparative example under the working charge states (SOC 100%, 110%), but the resistance increases significantly compared to the comparative example during overcharging (SOC 120%). Therefore, it can be expected that the positive electrode of the present invention will terminate charging by increasing the resistance during overcharging, thereby ensuring safety.

[0137] The above description is merely an illustration of the technical concept of the present invention. Those skilled in the art can make various modifications and variations without departing from the essential characteristics of the invention. Therefore, the accompanying drawings disclosed in this invention are not intended to limit the technical concept of the invention, but rather to illustrate it. The scope of the technical concept of the invention is not limited by these drawings. The scope of protection of this invention should be interpreted based on the appended claims, and all technical concepts within the equivalent scope should be interpreted as being included within the scope of this invention.

Claims

1. A positive electrode for a lithium secondary battery, the positive electrode comprising: a first positive electrode mixture layer in contact with a positive electrode current collector; and at least one second positive electrode mixture layer 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, wherein the second positive electrode mixture layer comprises a second positive electrode active material and a second binder, wherein the average particle diameter D 50 of the first positive electrode active material is smaller than the average particle diameter D 50 of the second positive electrode active material, and is equal to or smaller than 3 pm, and the BET specific surface area of the first positive electrode active material is equal to or larger than 3 m 2 / g. wherein the positive electrode for a lithium secondary battery has an elongation in the range of 0.5% to 2.0%, and an adhesion a between the positive electrode current collector and the first positive electrode mixture layer is 6 to 12 times an adhesion b between the first positive electrode mixture layer and the second positive electrode mixture layer.

2. The positive electrode according to claim 1, wherein The average particle diameter D of the first positive electrode active material is preferably in the range of 0.1 μm to 2 μm. 50 The BET specific surface area of the first positive electrode active material is preferably in the range of 5 m 2 / g to 25 m 2 / g.

3. The positive electrode according to claim 1, wherein an adhesion a between the positive electrode current collector and the first positive electrode mixture layer is 6 to 10 times an adhesion b between the first positive electrode mixture layer and the second positive electrode mixture layer.

4. The positive electrode according to claim 1, wherein an adhesion a between the positive electrode current collector and the first positive electrode mixture layer is in the range of 100 N / m to 500 N / m.

5. The positive electrode according to claim 1, wherein an adhesion b between the first positive electrode mixture layer and the second positive electrode mixture layer is in the range of 10 N / m to 40 N / m.

6. The positive electrode according to claim 1, wherein the first binder and the second binder are binders having the same property.

7. The positive electrode 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. The positive electrode according to claim 7, wherein a weight ratio of the first binder to the total weight of the first positive electrode mixture layer is in the range of 0.01 to 0.

3.

9. The positive electrode according to claim 1, wherein the positive electrode for a lithium secondary battery has an elongation in the range of 0.5% to 1.8%.

10. The positive electrode according to claim 1, wherein A / B ≤ 0.3, where A represents a thickness of the first positive electrode mixture layer and B represents a thickness of the second positive electrode mixture layer.

11. The positive electrode according to claim 1, wherein a thickness of the first positive electrode mixture layer is in the range of 1 μm to 20 μm.

12. The positive electrode according to claim 1, wherein 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: [Chemical Formula 1] Li a Ni 1-x-y Co x Mn y M z O2, where M is at least one selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and Cr, and where 0.9 ≤ a ≤ 1.5, 0 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and 0 ≤ x + y ≤ 1.

13. The positive electrode according to claim 1, wherein the first positive electrode active material contains a lithium iron phosphate 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 , where M is at least one selected from the group consisting of Al, Mg, and Ti, X is at least one selected from the group consisting of F, S, and N, and -0.5 ≤ a ≤ +0.5, 0 ≤ x ≤ 0.5, and 0 ≤ b ≤ 0.

1.

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

15. A lithium secondary battery comprising: the positive electrode for a lithium secondary battery of claim 1; a separator; and a negative electrode.

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

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

Images