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 including the same.SOLUTION: The positive electrode for a lithium secondary battery sequentially includes a first composite material layer and a second composite material layer on a positive electrode current collector, and uses different kinds of fluorine-based binders having different miscibilities and molecular weights in the first composite material layer and the second composite material layer, and thus, when a metal body such as a nail penetrates the electrode from the outside, penetration resistance may be increased to secure safety of the battery, and interfacial adhesion of each layer constituting the positive electrode and wettability of the composite material layer with respect to an electrolyte solution may be increased, and as a result, there is an advantage in that life characteristics of the battery are excellent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery, and more particularly to a positive electrode for a lithium secondary battery and a lithium secondary battery with improved safety. This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0039316, filed March 26, 2021, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference. [Background technology]

[0002] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density, working potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0003] Recently, lithium secondary batteries have been used as power sources for medium- to large-sized devices such as electric vehicles, and there is a growing demand for higher capacity, higher energy density, and lower cost lithium secondary batteries. As a result, active research is being conducted into the use of inexpensive elements such as Ni, Mn, and Fe instead of expensive Co.

[0004] One of the main research topics for lithium secondary batteries is to develop high-capacity, high-power electrode active materials while improving the safety of batteries using them. Current lithium secondary batteries are designed for use within a specific voltage range (typically 4.4 V or less) to ensure durability and safety. However, the cell potential can unintentionally rise above this range (up to 12 V). If this state of overcharging exceeds the allowable current or voltage, the lithium secondary battery may explode or catch fire, causing serious safety issues. In particular, lithium secondary batteries used in medium- to large-sized battery packs as power sources for electric vehicles and hybrid vehicles require long life, and ensuring safety is even more important due to the large number of closely packed battery cells.

[0005] In this regard, Patent Document 1 discloses a technology in which an overcharge prevention layer is interposed between a positive electrode current collector and a positive electrode active material layer to increase resistance during overcharge, thereby blocking the charging current and ensuring battery safety. However, as described above, an electrode equipped with an overcharge prevention layer improves safety. However, differences in composition between the overcharge prevention layer and the positive electrode active material layer can cause interlayer cracking, which can result in poor battery life characteristics. Furthermore, the electrode has a low penetration resistance, which reduces safety when penetrated by needle-shaped objects.

[0006] Therefore, there is a need for technological development of a positive electrode for a secondary battery that improves safety without reducing lifespan characteristics and increases penetration resistance when an external metal object such as a nail penetrates the electrode. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 2019-0047203 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide a positive electrode for a secondary battery, which can ensure the safety of the battery by increasing the penetration resistance when an external metal object such as a nail penetrates the electrode, and can improve the life characteristics of the battery, and a lithium secondary battery including the same. [Means for solving the problem]

[0009] To solve the above problem, In one embodiment, the present invention provides A current collector, a first composite layer, and a second composite layer are laminated in this order; and the first mixture layer includes a first binder containing a fluorine-based homopolymer, the second composite layer includes a second binder containing a fluorine-based copolymer having a miscible functional group; The miscible functional group is derived from one or more monomers selected from the group consisting of (meth)acrylic acid, C1-10 alkyl (meth)acrylate, C1-10 alkyl (meth)acrylonitrile, and C1-10 alkyl (meth)acrylamide, to provide a positive electrode for a lithium secondary battery.

[0010] In this case, the first binder and the second binder may satisfy the following formula 1:

[0011] [Formula 1] 1.0≦Mw 2nd / Mw 1st ≦2.0

[0012] In formula 1, Mw 1st represents the weight average molecular weight of the first binder, and Mw 2nd indicates the weight average molecular weight of the second binder.

[0013] The first binder has a weight average molecular weight (Mw) of 10,000 g / mol to 1,000,000 g / mol. 1st ).

[0014] The content of the miscible functional group may be 0.1 mol % to 10 mol % based on the fluorine-based copolymer.

[0015] The first mixture layer may contain 75 to 98 parts by weight of the first active material, 2 to 20 parts by weight of the first binder, and 5 parts by weight or less of the first conductive material, with respect to a total of 100 parts by weight.

[0016] Here, the first active material may include lithium iron phosphate represented by the following Chemical Formula 1:

[0017] [Chemical formula 1] Li 1+x Fe 1-y M 1 y (PO 4-z )X z

[0018] In the above chemical formula 1, M 1 is one or more elements selected from Al, Mg, and Ti, and X is one or more elements selected from F, S, and N, -0.5≦x≦+0.5, 0≦y≦0.5, 0≦z≦0.1.

[0019] Furthermore, the first composite layer may satisfy one or more of the following formulas 2 and 3:

[0020] [Formula 2] 4≦a / b≦20

[0021] [Formula 3] 15

[0022] In the above formula 2 and formula 3, a represents the content of the first active material, b represents the content of the first binder, c represents the content of the first conductive material.

[0023] ​The second mixture layer may contain, for a total of 100 parts by weight, 80 to 98 parts by weight of the second active material, 1 to 10 parts by weight of the second binder, and 10 parts by weight or less of the second conductive material.

[0024] Furthermore, the first composite layer and the second composite layer can satisfy one or more of the conditions of the following formulas 4 to 6:

[0025] [Formula 4] a <a’

[0026] [Formula 5] b>b'

[0027] [Formula 6] c <c’

[0028] In the above formulas 4 to 6, a, b, and c are the same as defined in Formula 2 and Formula 3 above; a' represents the content of the second active material, b' represents the content of the second binder, c' represents the content of the second conductive material. The first composite layer may have an average thickness of 0.1 μm to 300 μm, and specifically may have an average thickness of 0.1 μm to 10 μm.

[0029] Furthermore, in one embodiment, the present invention provides a lithium secondary battery comprising the positive electrode according to the present invention, a negative electrode, and a separator located between the positive electrode and the negative electrode. [Effects of the Invention]

[0030] The positive electrode for a lithium secondary battery according to the present invention includes a first composite layer and a second composite layer sequentially disposed on a positive electrode current collector, and the first composite layer and the second composite layer use different types of fluorine-based binders having different miscibility and molecular weights. This not only increases the penetration resistance when an external metal object such as a nail penetrates the electrode, thereby ensuring the safety of the battery, but also simultaneously increases the interfacial adhesion of each layer constituting the positive electrode and the wettability of the composite layer to the electrolyte, resulting in excellent battery life characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention is susceptible to various modifications and embodiments, and therefore, specific embodiments will be described in detail in the detailed description.

[0032] However, it should be understood that this is not intended to limit the invention to any particular embodiment, but rather to include all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0033] In the present invention, the terms "comprise" or "have" and the like are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the presence or possibility of addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0034] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly below" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0035] The present invention will now be described in more detail.

[0036] <Positive electrode for lithium secondary batteries> In one embodiment, the present invention provides A current collector, a first composite layer, and a second composite layer are laminated in this order; and the first mixture layer includes a first binder containing a fluorine-based homopolymer, the second composite layer includes a second binder containing a fluorine-based copolymer having a miscible functional group; The miscible functional group is derived from one or more monomers selected from the group consisting of (meth)acrylic acid, C1-10 alkyl (meth)acrylate, C1-10 alkyl (meth)acrylonitrile, and C1-10 alkyl (meth)acrylamide, to provide a positive electrode for a lithium secondary battery.

[0037] The positive electrode for a lithium secondary battery according to the present invention has a structure in which a first mixture layer and a second mixture layer are sequentially laminated on a current collector.

[0038] In this case, the first mixture layer may include a first active material, a first binder, and a first conductive material, and the second mixture layer may include a second active material, a second binder, and a second conductive material.

[0039] Specifically, the first and second mixture layers may contain a first and second active material, respectively, which are positive electrode active materials capable of reversible intercalation and deintercalation.

[0040] For example, the first active material included in the first composite layer may include one or more lithium iron phosphates represented by the following Chemical Formula 1:

[0041] [Chemical formula 1] Li 1+x Fe 1-y M 1 y (PO 4-z )X z

[0042] In the above chemical formula 1, M 1 is one or more elements selected from Al, Mg, and Ti, X is one or more elements selected from F, S, and N, and -0.5≦x≦+0.5, 0≦y≦0.5, and 0≦z≦0.1.

[0043] The first active material is lithium iron phosphate represented by Chemical Formula 1 and may include one or more compounds selected from the group consisting of LiFePO4, Li(Fe,Al)PO4, Li(Fe,Mg)PO4, and Li(Fe,Ti)PO4, and specifically, LiFePO4 can be used.

[0044] The lithium iron phosphate represented by Chemical Formula 1 may have an olivine structure. At an overcharge voltage of about 4.5 V or higher, lithium iron phosphate with an olivine structure loses its lithium and its volume shrinks. This quickly interrupts the conductive path of the first composite layer, allowing the first composite layer to function as an insulating layer. This also increases the resistance of the first composite layer, blocking the charging current and allowing the overcharge termination voltage to be reached. To this end, the first active material may have a large specific surface area and a small particle size to quickly convert the first composite layer into an insulating layer under overcharge voltage conditions. Specifically, the first active material may have an average particle size of less than 10 μm, more specifically, an average particle size of 9 μm or less, 8 μm or less, 7 μm or less, 0.01 to 8 μm, 0.01 to 6 μm, 0.01 to 5 μm, 0.01 to 3 μm, or 0.1 to 1 μm.

[0045] As another example, the second active material included in the second composite layer may include one or more lithium nickel oxides represented by the following Chemical Formula 2, and may be used in combination with the lithium iron phosphate represented by Chemical Formula 1 included in the first active material:

[0046] [Chemical formula 2] Li p Ni 1-q-r-s Co q Mn r M 2 s O2

[0047] In the above chemical formula 2, M 2 is one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and Cr, and 0.9≦p≦1.5, 0≦q≦1, 0≦r≦0.5, 0≦s≦0.1, 0≦q+r+s≦1.

[0048] The second active material is not necessarily limited to lithium nickel oxide represented by chemical formula 2, but may be a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; 1+x1 Mn 2-x1 O4 (where x1 is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x2 M a x2 O2 (where M a = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x2 = 0.01 to 0.3), Ni-site lithium nickel oxide, chemical formula LiMn 2-x3 M b x3 O2 (where M b = Co, Ni, Fe, Cr, Zn or Ta, and x3 = 0.01 to 0.1) or Li2Mn3M c O8 (where M c Lithium manganese composite oxide represented by the formula (wherein LiNi is Fe, Co, Ni, Cu or Zn); x4 Mn 2-x4 Examples of such compounds include lithium manganese composite oxides with a spinel structure represented by LiMn2O4 (where x4 = 0.01 to 1); LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; disulfide compounds; and Fe2(MoO4)3.

[0049] Specifically, the lithium nickel oxide shown in the above chemical formula 2 is LiCoO2, LiCo 0.5 Zn 0.5 O2, LiCo 0.7 Zn 0.3 O2, LiNiO2, LiNi 0.5 Co 0.5 O2, LiNi 0.6 Co 0.4 O2, LiNi 1 / 3 Co 1 / 3 Al1 / 3 O2, LiMnO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 and more specifically, LiCoO2, LiCo 0.7 Zn 0.3 O2, LiNi 0.5 Co 0.5 O2 or LiNi 0.8 Co 0.1 Mn 0.1 O2 may be used alone or in combination.

[0050] The first mixture layer and the second mixture layer also contain a positive electrode additive containing lithium cobalt oxide represented by the following Chemical Formula 3:

[0051] [Chemical formula 3] Li a Co 1-b M 3 b O4

[0052] In the above chemical formula 3, M 3 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and a and b are 5≦a≦7, 0≦b≦0.3, respectively.

[0053] The positive electrode additive contains a large amount of lithium, and therefore can provide lithium to replace lithium consumed due to irreversible chemical and physical reactions at the negative electrode during initial charging, thereby increasing the charge capacity of the battery and reducing the irreversible capacity, thereby improving the battery life characteristics.

[0054] As such a positive electrode additive, the present invention includes a lithium cobalt oxide represented by Chemical Formula 3. Here, the lithium cobalt oxide represented by Chemical Formula 3 is Li6CoO4, Li6Co 0.5 Zn 0.5 O4, Li6Co 0.7 Zn 0.3 It may include O4 etc.

[0055] The content of the positive electrode additive may be 0.1 to 5 parts by weight, specifically 0.1 to 3 parts by weight, or 1 to 3 parts by weight, based on 100 parts by weight of the first composite layer or the second composite layer. By adjusting the content of the positive electrode additive as described above, the present invention can maximize the initial charge capacity of the battery while reducing the amount of gas generated during subsequent charge and discharge.

[0056] In addition, the first composite layer and the second composite layer contain a first binder and a second binder, respectively, to bind the active material and conductive material contained in each layer and at the same time impart adhesive strength between the current collector and the first composite layer and / or between the first composite layer and the second composite layer.

[0057] In this case, the first binder contained in the first composite layer may be a homopolymer derived from one type of fluorine-based monomer. Specifically, the first binder may be derived from a fluorine-based monomer such as vinylidene fluoride (VDF or VF2), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinyl fluoride (VF), hexafluoroisobutylene (HFIB), perfluorobutylethylene (PFBE), pentafluoropropene, 3,3,3-trifluoro-1-propene, or 2-trifluoromethyl-3,3,3-trifluoropropene.

[0058] For example, the first binder may include polyvinylidene fluoride (PVDF) homopolymer. The terms "vinylidene fluoride homopolymer" or "PVDF" used herein may refer to a polymer containing 90 mol% or more, 95 mol% or more, or 98 mol% or more vinylidene fluoride. The present invention uses a fluorine-based homopolymer derived from a fluorine-based monomer composed solely of fluorinated alkyl chains without any additional functional groups as the first binder. This minimizes side reactions with active materials having a large specific surface area of ​​less than 10 μm while maintaining low swelling in the electrolyte solution within a certain range, thereby preventing a decrease in battery capacity and lifespan and a decrease in the bonding strength between the first composite layer and the current collector.

[0059] The weight average molecular weight (Mw 1st ) may be 10,000 g / mol to 1,000,000 g / mol, specifically 50,000 g / mol to 1,000,000 g / mol; 100,000 g / mol to 1,000,000 g / mol; 300,000 g / mol to 1,000,000 g / mol; 500,000 g / mol to 1,000,000 g / mol; 750,000 g / mol to 1,000,000 g / mol; 850,000 g / mol to 1,000,000 g / mol; 200,000 g / mol to 500,000 g / mol; 300,000 g / mol to 700,000 g / mol; 400,000 g / mol to 850,000 g / mol; or 100,000 g / mol to 250,000 g / mol.

[0060] The present invention is directed to the weight average molecular weight (Mw 1st ) within the above range, it is possible to prevent the first active material and the first conductive material contained in the first composite layer from agglomerating due to a low molecular weight, while it is possible to prevent a sudden increase in viscosity of the composition for forming the first composite layer due to a high molecular weight.

[0061] In addition, the first binder may satisfy the following formula 1:

[0062] [Formula 1] 1.0≦Mw 2nd / Mw 1st ≦2.0

[0063] In formula 1, Mw 1st represents the weight average molecular weight of the first binder, and Mw 2nd indicates the weight average molecular weight of the second binder.

[0064] Equation 1 above indicates the condition for the ratio of the weight-average molecular weight of the first binder contained in the first composite layer to the weight-average molecular weight of the second binder contained in the second composite layer, meaning that the second binder used in the present invention can have a weight-average molecular weight that is equal to or up to twice as large as that of the first binder. Specifically, the present invention can satisfy Equation 1 with 1 or 2, more specifically, 1.05 to 1.95; 1.05 to 1.5; 1.05 to 1.2; 1.2 to 1.5; 1.6 to 2; 1.8 to 2; or 1.3 to 1.95. By satisfying the condition of Equation 1 above, the present invention can prevent degradation of the interfacial performance between the first composite layer and the second composite layer due to a difference in weight-average molecular weight between the first binder and the second binder, while also preventing deterioration of battery life.

[0065] The second binder may be a fluorine-based copolymer obtained by using two or more fluorine-based monomers. Specifically, the fluorine-based copolymer may be a copolymer of two or more fluorine-based monomers selected from the group consisting of vinylidene fluoride (VDF or VF2), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinyl fluoride (VF), hexafluoroisobutylene (HFIB), perfluorobutylethylene (PFBE), pentafluoropropene, 3,3,3-trifluoro-1-propene, and 2-trifluoromethyl-3,3,3-trifluoropropene.

[0066] The fluorine-based copolymer may also contain a miscible functional group. Here, the "miscible functional group" may refer to a functional group that can improve the miscibility with the solid-phase material contained in the second composite layer, such as the second active material and the second conductive material, and improve the affinity between the positive electrode composite layer and the electrolyte. Such a miscible functional group may include, for example, a functional group derived from one or more monomers selected from the group consisting of (meth)acrylic acid, C1-10 alkyl(meth)acrylate, C1-10 alkyl(meth)acrylonitrile, and C1-10 alkyl(meth)acrylamide.

[0067] In addition, the content of the miscible functional group may be 0.1 to 10 mol% based on 100 mol% of the entire fluorine-based copolymer serving as the second binder, specifically 1 to 5 mol%, 3 to 8 mol%, 5 to 10 mol%, or 1 to 3 mol%. By controlling the content of the miscible functional group contained in the fluorine-based copolymer within the above range, the present invention can prevent a situation in which the miscibility between the second active material and the second conductive material is insufficient due to a low content of the miscible functional group, while preventing a decrease in the adhesive strength between the first composite layer and the second composite layer due to a miscible functional group content of 10 mol% or more.

[0068] For example, the fluorine-based copolymer may be a copolymer produced by copolymerizing 70 to 99 mol% of vinylidene fluoride (VdF) and 1 to 30 mol% of hexafluoropropene (HFP), which are fluorine-based monomers, and may contain 3 to 5 mol% of miscible functional groups using (meth)acrylic acid, based on 100 mol% of the entire copolymer.

[0069] The first and second conductive materials contained in the first and second composite layers may each comprise at least one material selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber. For example, the first and second conductive materials may comprise acetylene black.

[0070] Furthermore, the first composite layer may contain the first active material, first binder, and first conductive material in specific amounts and / or content ratios. Specifically, the first composite layer may contain 75 to 98 parts by weight of the first active material, 2 to 20 parts by weight of the first binder, and 5 parts by weight or less of the first conductive material, per 100 parts by weight of the total. More specifically, the first composite layer may contain 80 to 98 parts by weight, 85 to 98 parts by weight, 90 to 98 parts by weight, 75 to 85 parts by weight, 85 to 95 parts by weight, 85 to 90 parts by weight, or 90 to 95 parts by weight of the first active material, per 100 parts by weight of the total. Furthermore, the first composite layer may contain, relative to a total of 100 parts by weight, 4 to 20 parts by weight, 4 to 15 parts by weight, 4 to 12 parts by weight, 4 to 8 parts by weight, 8 to 20 parts by weight, 8 to 15 parts by weight, or 15 to 25 parts by weight of the first binder, and may contain, relative to a total of 100 parts by weight, 5 parts by weight or less, 3 parts by weight or less, 0.5 to 3 parts by weight, or 0.5 to 2.5 parts by weight of the first conductive material.

[0071] Furthermore, the first composite layer may satisfy one or more of the following formulas 2 and 3:

[0072] [Formula 2] 4≦a / b≦20

[0073] [Formula 3] 15

[0074] In the above formulas 2 and 3, a represents the content of the first active material, b represents the content of the first binder, and c represents the content of the first conductive material.

[0075] ​Equation 2 above is a condition indicating that the content ratio of the first active material to the first binder contained in the first mixture layer is 4 to 20 (i.e., 4≦a / b≦20), and the positive electrode of the present invention may satisfy Equation 2 as 4 to 20; 4 to 10; 4 to 8; 5 to 10; 8 to 19; 8 to 11; or 15 to 20. By satisfying the condition of Equation 2 within the above range, the present invention can prevent a decrease in energy density and deterioration in lifespan caused by a content ratio of the first active material to the first binder being less than 4, while also preventing a decrease in adhesive strength between the current collector and the first mixture layer caused by a content ratio exceeding 20, which can reduce charge-discharge cycle lifespan and stability.

[0076] In addition, the above formula 3 is expressed as follows: the ratio of the content of the first active material to the content of the first conductive material contained in the first mixture layer is greater than 15 (i.e., 15

[0077] Similarly to the first mixture layer, the second mixture layer may contain the second active material, the second binder, and the second conductive material in specific amounts and / or content ratios.

[0078] ​Specifically, the second composite layer may contain, per 100 parts by weight, 80 to 98 parts by weight of the second active material, 1 to 10 parts by weight of the second binder, and 10 parts by weight or less of the second conductive material. More specifically, the second composite layer may contain, per 100 parts by weight, 85 to 98 parts by weight, 75 to 98 parts by weight, 75 to 85 parts by weight, 85 to 95 parts by weight, 85 to 90 parts by weight, 90 to 95 parts by weight, 90 to 98 parts by weight, or 93 to 97 parts by weight of the second active material. Furthermore, the first composite layer may contain, per 100 parts by weight, 1 to 10 parts by weight, 1 to 5 parts by weight, or 1 to 3 parts by weight of the first binder, and 10 parts by weight or less, 7.5 parts by weight or less, 5 parts by weight or less, 4 parts by weight or less, 1 to 4.5 parts by weight, or 2 to 4 parts by weight of the first conductive material.

[0079] Furthermore, the second mixture layer can satisfy one or more of the conditions of the following formulas 4 to 6:

[0080] [Formula 4] a <a’

[0081] [Formula 5] b>b'

[0082] [Formula 6] c <c’

[0083] In the above formulas 4 to 6, a, b, and c are the same as defined in the above formulas 2 and 3, a' represents the content of the second active material, b' represents the content of the second binder, and c' represents the content of the second conductive material.

[0084] The above formulas 4 to 6 show the content ratios of each active material, binder, and conductive material contained in the first composite layer and the second composite layer. Specifically, the first active material and first conductive material contained in the first composite layer have a lower content than the second active material and second conductive material contained in the second composite layer, respectively, and the first binder contained in the first composite layer has a higher content than the second binder contained in the second composite layer. By controlling the content of each component contained in the first composite layer and the second composite layer to satisfy one or more of the above formulas 4 to 6, the present invention can prevent desorption at the interface between each layer and increase the short-circuit resistance of the first composite layer and the second composite layer under non-ideal conditions.

[0085] The first and second composite layers may each have an average thickness of 500 μm or less. For example, the first and second composite layers may each have an average thickness of 0.1 μm to 300 μm, specifically 0.1 μm to 200 μm, 0.1 μm to 100 μm, 0.1 μm to 50 μm, 0.1 μm to 20 μm, 0.1 μm to 10 μm, 0.1 μm to 5 μm, 1 μm to 3 μm, 10 μm to 50 μm, 50 μm to 100 μm, 100 μm to 300 μm, or 100 μm to 200 μm. For example, the first composite layer may have an average thickness of 3±0.5 μm, and the second composite layer may have an average thickness of 150±10 μm. The first composite layer may have an average thickness of 8±0.5 μm, and the second composite layer may have an average thickness of 60±5 μm.

[0086] Furthermore, the positive electrode for a lithium secondary battery according to the present invention can use a positive electrode current collector that has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. can be used. In the case of aluminum or stainless steel, it is also possible to use a material that has been surface-treated with carbon, nickel, titanium, silver, etc. Furthermore, the positive electrode current collector can be formed with fine irregularities on its surface to enhance the adhesive strength of the positive electrode active material, and can be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. Furthermore, the average thickness of the positive electrode current collector is preferably 3 to 500 μm, taking into account the conductivity and total thickness of the positive electrode to be manufactured.

[0087] The positive electrode for a lithium secondary battery according to the present invention comprises, as described above, a first composite layer and a second composite layer sequentially disposed on a positive electrode current collector, and the first composite layer and the second composite layer use different types of fluorine-based binders having different miscibility and molecular weights. This not only increases the penetration resistance when an external metal object such as a nail penetrates the electrode, thereby ensuring the safety of the battery, but also increases the interfacial adhesion of each layer constituting the positive electrode and the wettability of the composite layer to the electrolyte, thereby providing the advantage of excellent battery life characteristics.

[0088] <Lithium secondary battery> In one embodiment, the present invention provides a lithium secondary battery including the above-described positive electrode of the present invention, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0089] The lithium secondary battery according to the present invention may include an electrode assembly in which a separator is disposed between the cathode and anode of the present invention, and may have a structure in which the electrode assembly is immersed in an electrolyte solution containing a lithium salt so that the cathode and anode are wetted by the electrolyte solution.

[0090] Here, the negative electrode is prepared by applying a negative electrode active material to a negative electrode current collector, followed by drying and pressing, and may optionally further contain the above-mentioned conductive material, organic binder polymer, filler, etc., as needed.

[0091] In addition, the negative electrode active material may be, for example, graphite having a completely formed layered crystal structure like natural graphite, soft carbon having a low-crystalline layered crystal structure (graphene structure; a structure in which planes of a hexagonal honeycomb pattern of carbon are arranged in layers), hard carbon in which such a structure is mixed with an amorphous portion, artificial graphite, expanded graphite, carbon fiber, carbon that is difficult to graphitize, carbon black, carbon nanotubes, fullerenes, activated carbon, and other carbon and graphite materials; Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1), Sn x Me 1-x Me’ y O z (Me is Mn, Fe, Pb, Ge, Me’ is Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), etc. metal composite oxides; lithium metal; lithium alloys; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; titanium oxides; lithium titanium oxides, etc. can be used.

[0092] As an example, the negative electrode active material may contain both graphite and silicon (Si)-containing particles. The graphite may include any one or more of natural graphite having a layered crystal structure and artificial graphite having an isotropic structure. The silicon (Si)-containing particles may be particles having silicon (Si) as a main component as a metal component, and may include silicon (Si) particles, silicon oxide (SiO2) particles, or a mixture of the silicon (Si) particles and the silicon oxide (SiO2) particles.

[0093] In this case, the negative electrode active material may contain 80 to 95 parts by weight of graphite and 1 to 20 parts by weight of silicon (Si)-containing particles, based on 100 parts by weight of the total. By adjusting the contents of graphite and silicon (Si)-containing particles in the negative electrode active material within the above ranges, the present invention can reduce lithium consumption and irreversible capacity loss during initial charge and discharge of the battery, while improving charge capacity per unit mass.

[0094] The negative electrode mixture layer may have an average thickness of 100 μm to 200 μm, specifically 100 μm to 180 μm, 100 μm to 150 μm, 120 μm to 200 μm, 140 μm to 200 μm, or 140 μm to 160 μm.

[0095] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc. can be used. In the case of copper or stainless steel, it is also possible to use those that have been surface-treated with carbon, nickel, titanium, silver, etc. In addition, like the positive electrode current collector, the negative electrode current collector can have fine irregularities on its surface to strengthen the bonding force with the negative electrode active material, and can be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. In addition, the average thickness of the negative electrode current collector can be suitably 3 to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0096] The separator may be an insulating thin film having high ion permeability and mechanical strength. Specifically, the separator may be any material commonly used in the art. For example, a sheet or nonwoven fabric made of one or more chemically resistant and hydrophobic materials selected from the group consisting of polypropylene, polyethylene, polyethylene-polypropylene copolymer, and glass fiber may be used. In some cases, a composite separator may be used in which inorganic particles / organic particles are coated on a porous polymer substrate such as a sheet or nonwoven fabric with an organic binder polymer.

[0097] The separation membrane may have an average pore diameter of 0.01 to 10 μm and an average thickness of 5 to 300 μm. For example, the separation membrane may have an average pore diameter of 0.01 to 1 μm and an average thickness of 10 to 30 μm.

[0098] The separator may have an air permeability of 150 to 350 sec / 100 ml, specifically 160 to 300 sec / 100 ml or 200 to 280 sec / 100 ml. By controlling the air permeability of the separator within the above range, the present invention can prevent heat generation and / or fire in the battery when penetrated by an external needle-shaped conductor, while maintaining high wettability with the electrolyte and high electrical performance of the battery.

[0099] Meanwhile, the electrode assembly may be wound in a jelly roll shape and housed in a cylindrical battery, a prismatic battery, or a pouch-type battery, or may be housed in a pouch-type battery in a folded or stacked and folded form, but is not limited thereto.

[0100] In addition, the lithium salt-containing electrolyte solution according to the present invention may be composed of an electrolyte solution and a lithium salt, and the electrolyte solution may be a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like.

[0101] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0102] Examples of the organic solid electrolyte that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociative groups.

[0103] Examples of the inorganic solid electrolyte that can be used include nitrides, halides, and sulfates of Li, such as LiN, LiI, LiNi, LiN-LiI-LiOH, LiSiO, LiSiO-LiI-LiOH, LiSiS, LiSiO, LiSiO-LiI-LiOH, and LiPO-LiS-SiS.

[0104] The lithium salt is a substance that is easily dissolved in a non-aqueous electrolyte, such as LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylates, lithium 4-phenylboronate, imides, and the like may be used.

[0105] In addition, to improve charge / discharge characteristics, flame retardancy, etc., the electrolyte may contain additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride. In some cases, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene may be added to impart non-flammability, and carbon dioxide gas may be added to improve high-temperature storage properties. Fluoro-ethylene carbonate (FEC), propene sultone (PRS), etc. may also be added.

[0106] <Battery module> Furthermore, in one embodiment, the present invention provides a battery module including the above-described secondary battery as a unit battery, and provides a battery pack including the battery module.

[0107] The battery pack can also be used as a power source for medium- to large-sized devices that require high-temperature stability, long cycle characteristics, and high rate characteristics. Specific examples of such medium- to large-sized devices include power tools powered by battery-type motors; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles, including electric bicycles (E-bikes) and electric scooters; electric golf carts; and power storage systems, more specifically, hybrid electric vehicles (HEVs) are examples, but are not limited to these.

[0108] The present invention will be described in more detail below based on examples and experimental examples.

[0109] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0110] <Examples 1 to 5 and Comparative Examples 1 to 10, Production of Positive Electrodes for Lithium Secondary Batteries> As the first active material, LiFePO4 (average particle size (D 50 ):1μm, BET specific surface area: 15m 2 / g), a PVDF homopolymer as a first binder, and carbon black as a first conductive material were prepared and mixed in an N-methylpyrrolidone (NMP) solvent to prepare a slurry for the first composite layer.

[0111] Separately, LiNi was used as the second active material. 0.8 Co 0.1 Mn 0.1 The second binder was a PVDF-HFP copolymer (PVDF:HFP = 75-85 mol%: 15-25 mol%), and the second conductive material was carbon black. These materials were mixed in N-methylpyrrolidone (NMP) to prepare a slurry for the second composite layer. The second binder was a PVDF-HFP copolymer containing a methyl acrylate group as a miscible functional group.

[0112] In addition, the weight average molecular weight (Mw 1st ), the ratio of the weight average molecular weight of the first binder to that of the second binder (Mw 2nd / Mw 1st The content of the miscible functional group contained in the second binder is shown in Table 1 below. The active material, binder, and conductive material contained in each slurry were weighed and mixed as shown in Table 2 below.

[0113] The first and second composite layer slurries prepared above were sequentially applied to aluminum foil, dried, and rolled to produce a positive electrode for a lithium secondary battery in which the first and second composite layers were formed on the aluminum foil. At this time, the average thicknesses of the first and second composite layers were 8 μm and 120 μm, respectively.

[0114] [Table 1]

[0115] [Table 2]

[0116] <Experimental Example> In order to evaluate the performance of the positive electrode for a lithium secondary battery according to the present invention, the following experiment was carried out.

[0117] a) Evaluation of electrolyte wettability The surface contact angle was measured using a Drop Shape Analysis System (DSA100) (KRUSS, Germany). Specifically, a drop of deionized water was dropped onto the surface of the positive electrode for the lithium secondary battery manufactured in the Examples and Comparative Examples using a syringe needle. Then, a photograph of the deionized water drop on the surface of the positive electrode was captured, and the captured photograph was analyzed to measure the contact angle. This process was repeated 20 times, and the average value was obtained. The results are shown in Table 3 below.

[0118] b) Evaluation of interlayer adhesive strength The positive electrodes prepared in the Examples and Comparative Examples were cut to 25 mm and 70 mm in length and width, respectively, and laminated using a press at 70°C and 4 MPa to prepare test specimens. The prepared test specimens were attached to a glass plate using double-sided tape, with the current collector facing the glass plate. Using a tensile tester, the second composite layer of the test specimen was peeled at a 90° angle at 25°C and a speed of 100 mm / min. The peel force was measured in real time, and the average value was defined as the interfacial adhesion strength "B" between the first composite layer and the second composite layer. The interfacial adhesion strength "A" between the current collector and the first composite layer was also measured using the same method, and the results are shown in Table 3 below.

[0119] c) Nail penetration test Lithium secondary batteries were fabricated using the positive electrodes fabricated in the Examples and Comparative Examples. Specifically, negative electrode active materials, natural graphite, a carbon black conductive material, and a PVDF binder were mixed in an N-methylpyrrolidone solvent at a weight ratio of 85:10:5 to prepare a negative electrode slurry, which was then coated on copper foil to fabricate a negative electrode.

[0120] The fabricated negative electrode was stacked on a prepared positive electrode with a separator between them to fabricate an electrode assembly. The fabricated electrode assembly was then placed inside a battery case, and an electrolyte solution was injected into the case to fabricate a lithium secondary battery. A porous polyethylene (PE) film (thickness: approximately 16 μm, pore diameter: 0.9 μm, air permeability: 200-280 sec / 100 ml) was used as the separator. The electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent consisting of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC in a volume ratio of 3 / 4 / 3).

[0121] The manufactured lithium secondary batteries were evaluated for the presence or absence of ignition when a metal object with a diameter of 3 mm was dropped at a speed of 80 mm / sec to penetrate the cell, in the same manner as the PV8450 certification conditions, and the results are shown in Table 3 below.

[0122] d) Evaluation of cycle life performance Lithium secondary batteries were fabricated using the positive electrodes fabricated in the Examples and Comparative Examples using the same method as in the nail penetration test. The fabricated lithium secondary batteries were subjected to 200 charge / discharge cycles (n=200) at 25°C under the conditions of a charge cut-off voltage of 4.25V, a discharge cut-off voltage of 2.5V, and a current of 0.33C / 0.33C, and the capacity retention (Capacity Retention [%]) was measured. The capacity retention was calculated using the following formula 7, and the results are shown in Table 3.

[0123] [Formula 7] Capacity retention rate (%) = (discharge capacity after n charge / discharge cycles / discharge capacity after one charge / discharge cycle) x 100

[0124] [Table 3]

[0125] As shown in Table 3 above, the electrode assemblies of the examples manufactured according to the present invention have the effect of improving safety as well as the life of the battery.

[0126] Specifically, the positive electrodes prepared in the examples were shown to have contact angles of less than 85°, more specifically less than 80°, less than 75°, or 68 to 73°, which means that the positive electrodes have better wettability with respect to the electrolyte than the positive electrodes of Comparative Examples 8 and 9, which do not contain miscible functional groups in the first binder or contain them at significantly lower contents.

[0127] Furthermore, the positive electrode of the example had improved interfacial adhesion between each layer, with the adhesive strength (A) between the current collector and the first composite layer and the adhesive strength (B) between the first composite layer and the second composite layer being high at 380 N / m or more and 26 N / m or more, respectively. It was also confirmed that the short-circuit resistance of the positive electrode increased when a nail penetrated the electrode, suppressing ignition.

[0128] Furthermore, all of the positive electrodes prepared according to the examples were confirmed to have a capacity retention rate of 95% or more even after 200 charge / discharge cycles, which means that the battery life was improved compared to the positive electrodes of the comparative examples.

[0129] Based on these results, the positive electrode for a lithium secondary battery according to the present invention comprises a first composite layer and a second composite layer sequentially on a positive electrode current collector, and the first composite layer and the second composite layer use different types of fluorine-based binders with different miscibility and molecular weights. This not only increases the penetration resistance when an external metal object such as a nail penetrates the electrode, thereby ensuring the safety of the battery, but also increases the interfacial adhesion of each layer constituting the positive electrode and the wettability of the composite layer to the electrolyte, thereby providing the advantage of excellent battery life characteristics.

[0130] Although the present invention has been described above with reference to preferred embodiments, it will be understood that a person skilled in the art or with ordinary knowledge in the art can make various modifications and changes to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims.

[0131] Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification, but should be defined by the claims.

Claims

1. a current collector, a first composite layer, and a second composite layer stacked in this order; the first mixture layer includes a first binder containing a fluorine-based homopolymer, the second composite layer includes a second binder containing a fluorine-based copolymer having a miscible functional group; A positive electrode for a lithium secondary battery, wherein the miscible functional group is derived from one or more monomers selected from the group consisting of (meth)acrylic acid, C1-10 alkyl(meth)acrylate, C1-10 alkyl(meth)acrylonitrile, and C1-10 alkyl(meth)acrylamide.

2. The first binder and the second binder satisfy the following formula 1: [Formula 1] 1.0≦Mw ２ｎｄ / Mw １ｓｔ ≦2.0 In the formula 1, Mw １ｓｔ represents the weight average molecular weight of the first binder, Mw ２ｎｄ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein ≡ represents the weight average molecular weight of the second binder.

3. The first binder has a weight average molecular weight (Mw) of 10,000 g / mol to 1,000,000 g / mol. １ｓｔ 2. The positive electrode for a lithium secondary battery according to claim 1, comprising:

4. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the content of the miscible functional group is 0.1 mol % to 10 mol % based on the fluorine-based copolymer.

5. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first composite layer comprises, with respect to a total of 100 parts by weight of the first composite layer, 75 to 98 parts by weight of a first active material, 2 to 20 parts by weight of the first binder, and 5 parts by weight or less of a first conductive material.

6. The first active material includes lithium iron phosphate represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ｘ Fe １－ｙ M １ ｙ (PO ４－ｚ )X ｚ In the above Chemical Formula 1, M １ represents one or more elements selected from Al, Mg, and Ti, and X represents one or more elements selected from F, S, and N, 6. The positive electrode for a lithium secondary battery according to claim 5, wherein −0.5≦x≦+0.5, 0≦y≦0.5, and 0≦z≦0.

1.

7. The first composite layer satisfies one or more of the following formulas 2 and 3: [Formula 2] 4≦a / b≦20 [Formula 3] 15<a / c In the formulas 2 and 3, a represents the content of the first active material, b represents the content of the first binder, 2. The positive electrode for a lithium secondary battery according to claim 1, wherein c represents the content of the first conductive material.

8. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the second composite layer comprises, with respect to a total of 100 parts by weight of the second composite layer, 80 to 98 parts by weight of a second active material, 1 to 10 parts by weight of a second binder, and 10 parts by weight or less of a second conductive material.

9. The first composite layer and the second composite layer satisfy one or more conditions of the following formulas 4 to 6, [Formula 4] a<a' [Formula 5] b>b' [Formula 6] c<c' In the formulas 4 to 6, a represents the content of the first active material, b represents the content of the first binder, c represents the content of the first conductive material, a' represents the content of the second active material, b' represents the content of the second binder, 2. The positive electrode for a lithium secondary battery according to claim 1, wherein c' represents the content of the second conductive material.

10. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first mixture layer has an average thickness of 0.1 μm to 300 μm.

11. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the first mixture layer has an average thickness of 0.1 μm to 10 μm.

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