Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode, comprising: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer contains a silicon-based active material and a negative electrode binder, and the negative electrode binder comprises a water-based binder and a rubber-based binder in a weight ratio of 82:18 to 88:12, and the water-based binder contains at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylonitrile, and polyacrylamide.

Description

Technical Field

[0001] Cross-reference to Related Applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2019-0028273, filed with the Korean Intellectual Property Office on Mar. 12, 2019, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a negative electrode and a secondary battery including the same. Background Art

[0005] Recently, due to the rapid popularization of electronic devices using batteries, such as mobile phones, laptop computers, electric vehicles, etc., the demand for small and lightweight secondary batteries with relatively high capacity has increased rapidly. In particular, lithium secondary batteries are lightweight and have a high energy density, and thus have attracted attention as a driving power source for portable devices. Therefore, research and development efforts to improve the performance of lithium secondary batteries have been actively carried out.

[0006] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. In addition, the positive electrode and the negative electrode may have an active material layer containing a positive electrode active material or a negative electrode active material on a current collector. In the positive electrode, a lithium-containing metal oxide such as LiCoO 2 or LiMn 2 O 4 is used as the positive electrode active material. Correspondingly, in the negative electrode, a carbon-based active material or a silicon-based active material without lithium is used as the negative electrode active material.

[0007] In particular, among negative electrode active materials, silicon-based active materials have attracted attention because they have a capacity about 10 times higher than that of carbon-based active materials, and due to their high capacity, silicon-based active materials have the advantage that high energy density can be achieved even with a thin electrode. However, due to the problem of volume expansion during charge / discharge and the resulting deterioration of life performance, silicon-based active materials are not commonly used.

[0008] Therefore, there is a need to develop a secondary battery that can improve life performance while achieving the high capacity and high energy density of silicon-based active materials.

[0009] Korean Patent Publication No. 10-2017-0074030 relates to a negative electrode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the same, and discloses a negative electrode active material including a porous silicon-carbon composite material. However, there are limitations in solving the above problems.

[0010] [Prior Art Documents]

[0011] [Patent Document]

[0012] Korean Patent Publication No. 10-2017-0074030 Summary of the Invention

[0013] Technical Problem

[0014] One aspect of the present invention provides a negative electrode capable of achieving improved life performance.

[0015] Another aspect of the present invention provides a negative electrode capable of being implemented as a thin film.

[0016] Still another aspect of the present invention provides a secondary battery including the above negative electrode.

[0017] Technical Solution

[0018] According to one aspect of the present invention, there is provided a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer includes a silicon-based active material and a negative electrode binder, and the negative electrode binder includes an aqueous binder and a rubber-based binder in a weight ratio of 82:18 to 88:12, and the aqueous binder includes at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylonitrile, and polyacrylamide.

[0019] According to another aspect of the present invention, there is provided a secondary battery including the above negative electrode, a positive electrode opposite to the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0020] Advantageous Effects

[0021] The negative electrode of the present invention uses a negative electrode binder including a specific aqueous binder and a rubber-based binder in a specific weight ratio, thereby minimizing the volume expansion of the silicon-based active material to improve life performance and enabling a thin film electrode that is both thin and satisfies the required energy density. Brief Description of the Drawings

[0022] Figure 1 It is a graph showing the evaluation of the capacity retention rate of the secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 5, respectively. Detailed Description

[0023] It should be understood that the words or terms used in the specification and claims of the present invention should not be construed as being limited to the meanings defined in a common dictionary. It should be further understood that, based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, the words or terms should be construed as having meanings consistent with their meanings in the context of the relevant field and technical concept of the present invention.

[0024] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, singular terms may include plural forms.

[0025] In this specification, it should be understood that the terms "comprising", "including" or "having" are intended to indicate the presence of the stated features, numbers, steps, elements or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements or combinations thereof.

[0026] In this specification, the average particle diameter (D 50 ) can be defined as the particle diameter corresponding to 50% volume measurement in the particle size distribution curve of the particles. The average particle diameter (D 50 ) can be measured by, for example, laser diffraction method. The laser diffraction method can generally measure particle diameters from the submicron region to several millimeters, and thus high reproducibility and high resolution results can be obtained.

[0027] Hereinafter, the present invention will be described in more detail.

[0028] <Negative electrode>

[0029] The present invention relates to a negative electrode, and more particularly, to a negative electrode for a lithium secondary battery.

[0030] The negative electrode of the present invention includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer includes a silicon-based active material and a negative electrode binder, wherein the negative electrode binder includes an aqueous binder and a rubber-based binder in a weight ratio of 82:18 to 88:12, and the aqueous binder includes at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylonitrile, and polyacrylamide.

[0031] Generally, it is known that silicon-based active materials have a capacity about 10 times higher than that of carbon-based active materials. Therefore, when silicon-based active materials are applied to the negative electrode, it is expected that a thin film electrode with a high level of energy density can be achieved. However, silicon-based active materials have a problem of volume expansion / contraction due to lithium insertion / extraction during charge and discharge, and thus it is not easy to commonly use silicon-based active materials.

[0032] When a silicon-based active material is used for the negative electrode of the present invention, the negative electrode active material layer contains a negative electrode binder including a specific aqueous binder and a rubber-based binder having a specific weight ratio. The aqueous binder has strong stress and can suppress the volume expansion of the silicon-based active material during charging and discharging, while the rubber-based binder can reduce the strong stress of the aqueous binder to a specific level and prevent the bending problem of the negative electrode active material layer caused by the use of the aqueous binder. Therefore, the negative electrode of the present invention can solve the volume expansion problem of the silicon-based active material, thereby improving the life performance of the battery and achieving high capacity. At the same time, a thin-film negative electrode with high energy density can be realized.

[0033] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum cadmium alloy, etc. can be used as the negative electrode current collector.

[0034] To achieve a thin negative electrode, the thickness of the negative electrode current collector can generally be 3 μm to 100 μm, preferably 4 μm to 40 μm.

[0035] The negative electrode current collector may have fine irregularities formed on its surface to improve the adhesion of the negative electrode active material. For example, negative electrode current collectors in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric bodies can be used.

[0036] The negative electrode active material layer is formed on the negative electrode current collector.

[0037] The negative electrode active material layer contains a silicon-based active material.

[0038] The silicon-based active material may contain a compound represented by SiO x (0 ≤ x < 2). Since SiO 2 does not react with lithium ions and thus cannot store lithium, it is preferred that x is within the above range.

[0039] Specifically, the silicon-based active material may contain Si. Generally, the advantage of Si is that its capacity is about 2.5 to 3 times higher than that of silicon oxides (such as SiO x (0 < x < 2)).

[0040] However, the volume expansion / contraction of Si due to charging and discharging is much larger than that of silicon oxides, making it not easy to commercialize Si. However, in the present invention, the negative electrode binder described later is used, so that the problem of deterioration of life performance caused by the volume expansion of the silicon-based active material can be effectively solved, and the advantages of the silicon-based active material such as high capacity and energy density can be more preferably realized.

[0041] In terms of ensuring the structural stability of the active material during charging and discharging, more smoothly forming a conductive network to maintain conductivity, or making it easier for the negative electrode binder used to bond the active material and the current collector to access, the average particle size (D 50 ) of the silicon-based active material can be 1 μm to 10 μm, preferably 2 μm to 6 μm.

[0042] In terms of fully realizing the high capacity of the silicon-based active material in the secondary battery, the amount of the silicon-based active material contained in the negative electrode active material layer can be 60% by weight to 90% by weight, preferably 65% by weight to 75% by weight.

[0043] The negative electrode active material layer contains a negative electrode binder. The negative electrode binder contains an aqueous binder and a rubber-based binder.

[0044] The aqueous binder can be dissolved in an aqueous solvent such as water and contains at least one selected from the group consisting of polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacrylamide (PAM).

[0045] The aqueous binder has hydrophilic properties and is generally insoluble in the electrolytes or electrolytic solutions commonly used in secondary batteries. When applied to the negative electrode or the secondary battery, this property can endow the aqueous binder with strong stress or strong tensile strength. Therefore, the volume expansion / contraction problem of the silicon-based active material during charging and discharging can be effectively suppressed.

[0046] On the other hand, because the aqueous binder has strong stress, when the aqueous binder is used alone, there is a risk of negative electrode bending, cracks caused by bending, and deterioration of life performance. The rubber-based binder can be well dissolved in the electrolytes or electrolytic solutions commonly used in secondary batteries. Thus, when used together with the aqueous binder, the stress of the aqueous binder can be reduced to a specific level. Therefore, by using the negative electrode binder containing the aqueous binder and the rubber-based binder in a specific weight ratio, the negative electrode of the present invention can improve the life performance by effectively solving the volume expansion / contraction problem of the silicon-based active material, and can achieve a thin but high-energy-density negative electrode by solving the bending problem that occurs during the manufacture of the thin-film negative electrode.

[0047] The negative electrode binder contains an aqueous binder and a rubber-based binder in a weight ratio of 82:18 to 88:12.

[0048] When the content of the aqueous binder in the negative electrode binder is less than 82% by weight and the content of the rubber-based binder is greater than 18% by weight, the silicon-based active material cannot be firmly adhered, so that the volume expansion / contraction problem of the active material accompanying charge and discharge may not be effectively controlled. When the content of the aqueous binder in the negative electrode binder is greater than 88% by weight and the content of the rubber-based binder is less than 12% by weight, the content of the aqueous binder is too high, thus exacerbating the bending problem during the manufacture of the thin-film negative electrode, so there is a risk of product defects and deterioration of life performance, and due to the property that the aqueous binder cannot be well dissolved in the electrolyte, the negative electrode resistance may be increased, which is not preferred.

[0049] The negative electrode binder may preferably contain the aqueous binder and the rubber-based binder in a weight ratio of 82:18 to 88:12, more preferably 83.5:16.5 to 86.5:13.5, and when within the above range, an improvement in life performance and the realization of a thin-film negative electrode can be more preferably obtained when applying the silicon-based active material to the negative electrode.

[0050] The aqueous binder can be dissolved in an aqueous solvent such as water and may contain at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyacrylonitrile, and polyacrylamide. From the aspect of having excellent tolerance to the volume expansion / contraction of the silicon-based active material, it is preferably at least one selected from the group consisting of polyvinyl alcohol and polyacrylic acid, more preferably polyvinyl alcohol and polyacrylic acid. When the aqueous binder contains polyvinyl alcohol and polyacrylic acid, the above advantages of the aqueous binder can be achieved in a better way. Specifically, the aqueous binder may contain polyvinyl alcohol and polyacrylic acid in a weight ratio of 50:50 to 90:10, preferably 55:45 to 80:20.

[0051] From the aspect of promoting dispersion in an aqueous solvent such as water when preparing a slurry for forming a negative electrode active material layer and improving the adhesion force by more smoothly coating the active material, the aqueous binder may include those aqueous binders in which hydrogen in the aqueous binder is replaced by Li, Na, or Ca.

[0052] The weight-average molecular weight of the aqueous binder may be 250,000 g / mol to 500,000 g / mol, preferably 350,000 g / mol to 400,000 g / mol, and when the weight-average molecular weight is within the above range, the aqueous binder has a suitable viscosity level and is well dispersed in a solvent, etc., which is thus preferred.

[0053] The rubber-based binder is a material different from the water-based binder and can be defined as a material that does not dissolve well in water-based solvents such as water but can be smoothly dispersed in water-based solvents. Specifically, the rubber-based binder may include at least one selected from the group consisting of styrene-butadiene rubber (SBR), hydrogenated nitrile rubber (HNBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, and fluororubber. In terms of easy dispersion and excellent phase stability, at least one selected from the group consisting of styrene-butadiene rubber and hydrogenated nitrile rubber is preferred, and styrene-butadiene rubber is more preferred.

[0054] Preferably, the water-based binder may include at least one selected from the group consisting of polyvinyl alcohol and polyacrylic acid, and the rubber-based binder may include at least one selected from the group consisting of styrene-butadiene rubber and hydrogenated nitrile rubber. Preferably, the water-based binder may include polyvinyl alcohol and polyacrylic acid, and the rubber-based binder may include styrene-butadiene rubber. The negative electrode binder containing the water-based binder and the rubber-based binder can sufficiently control the volume expansion / shrinkage of the silicon-based active material and impart excellent flexibility, and thus is preferred.

[0055] The amount of the negative electrode binder contained in the negative electrode active material layer may be 10% by weight to 30% by weight, preferably 15% by weight to 25% by weight. When within the above range, the volume expansion problem of the active material can be minimized by better bonding the silicon-based active material. At the same time, the dispersion of the binder can be promoted when preparing the slurry for forming the negative electrode active material layer, and the coating performance and phase stability of the slurry can be improved.

[0056] In the present invention, when measuring the bending height by the following method, the bending height of the binder film prepared with the negative electrode binder may be 1 cm or less:

[0057] The step of preparing a solution for forming a binder film by adding 2 g of the negative electrode binder to distilled water so that the solid content is 15% by weight;

[0058] The step of applying the solution for forming a binder film to a square Teflon substrate having a size of 10 cm × 10 cm to an area of 10 cm × 10 cm;

[0059] The step of drying the applied solution for forming a binder film at 60 °C for 48 hours to prepare a binder film, the binder film including a vertex spaced apart from the Teflon substrate;

[0060] A step of defining the maximum vertical distance between the adhesive film spaced apart from the Teflon substrate and the Teflon substrate as the bending height when vertically pressing the vertex spaced apart from the Teflon substrate against the Teflon substrate with a force of 3 kgf.

[0061] When the bending height of the adhesive film is 1 cm or less, volume expansion / contraction of the adhesive accompanying charging and discharging of the silicon-based active material can be suppressed, and bending problems occurring during manufacturing of the thin film negative electrode can be effectively solved. Preferably, the bending height can be 0.6 cm or less, and from the aspect of smoothly controlling volume expansion / contraction of the silicon-based active material by using a negative electrode adhesive in the active material layer, it is more preferably 0.35 cm to 0.6 cm, and even more preferably 0.46 cm to 0.55 cm.

[0062] The Teflon substrate has low adhesiveness to the negative electrode adhesive. When drying the solution for forming the adhesive film applied on the Teflon substrate, at least two vertices, specifically four vertices, of the adhesive film can be spaced apart from the vertices of the Teflon substrate due to the stress of the negative electrode adhesive. Therefore, when vertically pressing one vertex of the adhesive film spaced apart from the Teflon substrate against the Teflon substrate, the maximum vertical distance between the adhesive film spaced apart from the Teflon substrate and the Teflon substrate is measured, and the measured distance is defined as the bending height. For example, when vertically pressing one vertex of the adhesive film spaced apart from the Teflon substrate against the Teflon substrate, another vertex of the adhesive film, specifically another vertex in the diagonal direction of the one vertex, moves vertically upward from the Teflon substrate. At this time, the minimum distance between the another vertex and the Teflon substrate can be defined as the maximum vertical distance between the adhesive film and the Teflon substrate.

[0063] The minimum distance can be defined as the distance between another vertex of the adhesive film and the vertex of the Teflon substrate opposite to the another vertex.

[0064] In addition to containing the above-mentioned silicon-based active material and negative electrode adhesive, the negative electrode active material layer may further contain a negative electrode conductive material.

[0065] The negative electrode conductive material can be used to assist and improve the conductivity in a secondary battery, and there is no particular limitation as long as it has conductivity without causing chemical changes. Specifically, the negative electrode conductive material may include graphite, such as natural graphite or artificial graphite; carbon materials, 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; carbon fluorides; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; and polyphenylene derivatives. From the aspect of achieving high conductivity, carbon black is preferably included.

[0066] From the aspect of promoting the dispersion of the negative electrode conductive material and further improving the conductivity when preparing the slurry for forming the negative electrode active material layer, the specific surface area of the negative electrode conductive material can be 80 m 2 / g to 200 m 2 / g, preferably 100 m 2 / g to 150 m 2 / g.

[0067] The amount of the negative electrode conductive material contained in the negative electrode active material layer can be 5% by weight to 20% by weight, preferably 7% by weight to 15% by weight. When within the above range, it is preferable from the aspect of being able to form an excellent conductive network while reducing the increase in resistance caused by the negative electrode binder.

[0068] The negative electrode active material layer has excellent adhesion to the silicon-based active material due to the above-mentioned negative electrode binder, and a thin-film negative electrode with a high energy density can be achieved. Specifically, the thickness of the negative electrode active material layer can be 10 μm to 40 μm, preferably 20 μm to 30 μm.

[0069] The porosity of the negative electrode can be 37% to 45%, preferably 38% to 41%. When within the above range, the conductivity can be improved by appropriately accommodating the volume expansion / contraction of the silicon-based active material while maintaining the contact degree between the active materials at an appropriate level, which is preferable.

[0070] In this specification, the porosity of the negative electrode can be calculated by the following formula 1.

[0071] [Formula 1]

[0072] Porosity (%) of the negative electrode = {1 - (true density of the negative electrode / electrode density of the negative electrode)} × 100 In the above formula 1, the true density of the negative electrode is the density of the negative electrode active material layer measured by collecting a negative electrode of a specific size and then pressing the collected negative electrode with a pressing device until the thickness of the negative electrode remains unchanged. The electrode density of the negative electrode refers to the density of the negative electrode active material layer measured by collecting a negative electrode of a specific size.

[0073] The negative electrode can be manufactured by coating a negative electrode slurry containing a negative electrode active material, a negative electrode binder, and optionally a negative electrode conductive material and / or a solvent for the negative electrode slurry on a negative electrode current collector, and then drying and roll-pressing.

[0074] In terms of promoting the dispersion of the negative electrode active material, the negative electrode binder, and / or the negative electrode conductive material, the solvent for forming the negative electrode slurry may include, for example, at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, and preferably distilled water.

[0075] Considering the viscosity, coating performance, and dispersibility of the negative electrode slurry, the amount of the solvent for forming the negative electrode slurry contained in the negative electrode slurry may be such that the concentration of the solid containing the negative electrode active material, the negative electrode binder, and optionally the negative electrode conductive material is 15 wt% to 45 wt%, preferably 20 wt% to 30 wt%, and more preferably 24 wt% to 27 wt%.

[0076] <Secondary battery>

[0077] The present invention provides a secondary battery including the above negative electrode, specifically a lithium secondary battery.

[0078] Specifically, the secondary battery according to the present invention includes the above negative electrode, a positive electrode opposite to the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0079] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0080] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause a chemical change in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., or an aluminum cadmium alloy, etc. may be used as the positive electrode current collector.

[0081] The positive electrode current collector generally may have a thickness of 3 μm to 500 μm.

[0082] The positive electrode current collector may have fine irregularities formed on its surface to improve the adhesion of the positive electrode active material. For example, positive electrode current collectors in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric bodies may be used.

[0083] The positive electrode active material layer may contain a positive electrode active material.

[0084] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium. Specifically, it may contain a lithium transition metal composite oxide containing at least one transition metal selected from the group consisting of nickel, cobalt, manganese, and aluminum and lithium; preferably, it contains a lithium transition metal composite oxide containing a transition metal containing nickel, cobalt, or manganese and lithium.

[0085] More specifically, the lithium transition metal composite oxide may be a lithium manganese-based oxide (e.g., LiMnO 2 , LiMn 2 O 4 etc.), a lithium cobalt-based oxide (e.g., LiCoO 2 etc.), a lithium nickel-based oxide (e.g., LiNiO 2 etc.), a lithium nickel manganese-based oxide (e.g., LiNi 1-Y Mn Y O 2 (where 0 < Y < 1), LiMn 2-z Ni z O 4 (where 0 < Z < 2) etc.), a lithium nickel cobalt-based oxide (e.g., LiNi 1- Y1 Co Y1 O 2 (where 0 < Y1 < 1) etc.), a lithium manganese cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O 2 (where 0 < Y2 < 1), LiMn 2- z1 Co z1 O 4 (where 0 < Z1 < 2) etc.), a lithium nickel manganese cobalt-based oxide (e.g., Li(Ni p Co q Mn r1 )O 2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O 4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2) etc.), or a lithium nickel cobalt transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 A S2 )O 2(where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, p2, q2, r3, and s2 are each atomic fractions of independent elements, and 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1), etc., and may include any one or two or more of its compounds. Among these, from the aspect of being able to increase the capacity and stability of the battery, the lithium transition metal composite oxide may be LiCoO 2 , LiMnO 2 , LiNiO 2 , lithium nickel manganese cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 , Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 , Li(Ni 0.7 Mn 0.15 Co 0.15 )O 2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 etc.), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 etc.). When considering the significant improvement effects obtained by controlling the type and content ratio of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide may be Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 , Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 , Li(Ni 0.7 Mn 0.15 Co 0.15 )O 2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 etc., and any one or two or more of its mixtures may be used.

[0086] Considering the full capacity of the positive electrode active material, the amount of the positive electrode active material contained in the positive electrode active material layer may be 80% to 99% by weight, preferably 92% to 98.5% by weight.

[0087] In addition to containing the above positive electrode active material, the positive electrode active material layer may further contain a positive electrode binder and / or a positive electrode conductive material.

[0088] The positive electrode binder is a component for assisting the adhesion of the active material, conductive material, etc. and the adhesion to the current collector. Specifically, it may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0089] From the aspect of ensuring sufficient adhesion between components such as the positive electrode active material, the amount of the positive electrode binder contained in the positive electrode active material layer may be 1% to 20% by weight, preferably 1.2% to 10% by weight.

[0090] The positive electrode conductive material can be used to assist and improve the conductivity in the secondary battery, and there is no particular limitation as long as it has conductivity without causing chemical changes. Specifically, the positive electrode conductive material may include graphite, such as natural graphite or artificial graphite; carbon materials, 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 carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives, and from the aspect of improving conductivity, carbon black may be preferably included.

[0091] From the aspect of promoting the dispersion of the positive electrode conductive material and further improving the conductivity when preparing the slurry for forming the positive electrode active material layer, the specific surface area of the positive electrode conductive material may be 80 m 2 / g to 200 m 2 / g, preferably 100 m 2 / g to 150 m 2 / g.

[0092] From the aspect of ensuring sufficient conductivity, the amount of the positive electrode conductive material contained in the positive electrode active material layer may be 1% to 20% by weight, preferably 1.2% to 10% by weight.

[0093] The thickness of the positive electrode active material layer may be from 30 μm to 400 μm, preferably from 50 μm to 110 μm.

[0094] The positive electrode can be manufactured by coating a positive electrode slurry containing a positive electrode active material, and optionally a positive electrode binder, a positive electrode conductive material, and a solvent for forming the positive electrode slurry on a positive electrode current collector, followed by drying and rolling.

[0095] The solvent for forming the positive electrode slurry may contain an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and the amount used may be such that a preferred viscosity is obtained when the positive electrode active material and optionally the positive electrode binder, the positive electrode conductive material, etc. are included. For example, the amount of the solvent for forming the positive electrode slurry contained in the positive electrode slurry may be such that the concentration of the solids including the positive electrode active material and optionally the positive electrode binder and the positive electrode conductive material is 50% by weight to 95% by weight, preferably 70% by weight to 90% by weight.

[0096] The separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used without particular limitation as long as it is a separator commonly used in secondary batteries. In particular, a separator having excellent electrolyte wettability and low resistance to ion movement in the electrolyte is preferred. Specifically, a porous polymer membrane can be used, for example, a porous polymer membrane made of a polyolefin polymer such as a homopolymer of ethylene, a homopolymer of propylene, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof. Also, a typical porous non-woven fabric can be used, for example, a non-woven fabric formed of glass fibers having a high melting point, polyethylene terephthalate fibers, etc. In addition, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and it can be selectively used in a single-layer or multi-layer structure.

[0097] In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of secondary batteries, but is not limited thereto.

[0098] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0099] Any organic solvent can be used without particular limitation as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain a double bond, an aromatic ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among the above solvents, carbonate solvents are preferred, and a mixture of a cyclic carbonate having high ionic conductivity and high dielectric constant (such as ethylene carbonate or propylene carbonate) and a low-viscosity linear carbonate compound (such as methyl ethyl carbonate, dimethyl carbonate, or diethyl carbonate) that can improve the charge / discharge performance of the battery is more preferred. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

[0100] Any compound can be used as the lithium salt without particular limitation as long as it can provide lithium ions used in the lithium secondary battery. Specifically, LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF 6 、LiAlO 4 、LiAlCl 4 、LiCF 3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5 SO 3 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiCl、LiI、LiB(C 2 O 4 ) 2The like can be used as the lithium salt. The concentration range of the lithium salt used can be 0.1 - 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby showing excellent performance, and lithium ions can move effectively.

[0101] The secondary battery can be manufactured according to a typical method for manufacturing a secondary battery by inserting a separator between the above-mentioned negative electrode and positive electrode and then injecting an electrolyte solution thereto.

[0102] The secondary battery according to the present invention can be used in portable devices such as mobile phones, laptop computers, and digital cameras, and in the field of electric vehicles such as hybrid electric vehicles (HEVs). In particular, it can preferably be used as a constituent battery of a medium and large-sized battery module. Therefore, the present invention also provides a medium and large-sized battery module including the above-mentioned secondary battery as a unit cell.

[0103] The medium and large-sized battery module as described above can preferably be applied to power sources requiring high output and large capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices.

[0104] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be embodied in many different forms and is not limited to the embodiments described herein.

[0105] Embodiment

[0106] Example 1: Manufacturing a negative electrode

[0107] The silicon-based active material Si (average particle diameter (D 50 ) : 3.5 μm), which is a negative electrode active material, carbon black (product name: Super C65, manufacturer: Timcal) as a negative electrode conductive material, and a negative electrode binder are added to distilled water as a solvent for forming a negative electrode slurry at a weight ratio of 70:10:20 to prepare a negative electrode slurry (solid content concentration: 25 wt%). The negative electrode binder is prepared by mixing a mixture of polyvinyl alcohol (PVA) and polyacrylic acid (PAA) (weight average molecular weight: about 360,000 g / mol) mixed at a weight ratio of 66:34 as an aqueous binder and styrene-butadiene rubber (SBR) as a rubber-based binder at a weight ratio of 85:15.

[0108] The negative electrode slurry is applied at 2.91 mg / cm 2 (7.38 mAh / cm 2) was coated on one surface of a copper current collector (thickness: 8 μm) serving as a negative electrode current collector, roll-pressed, and then dried in a vacuum oven at 130 °C for 10 hours to form a negative electrode active material layer (thickness: 24 μm), which was prepared into a negative electrode according to Example 1 (negative electrode thickness: 32 μm, porosity 40.5%).

[0109] Examples 2 to 3 and Comparative Examples 1 to 5

[0110] The negative electrodes of Examples 2 and 3 and Comparative Examples 1 to 5 were each manufactured in the same manner as in Example 1, except that the contents of the silicon-based active material, the negative electrode conductive material, and the negative electrode binder used in Example 1 were adjusted according to Table 1 below.

[0111] [Table 1]

[0112]

[0113] In Table 1 above, the bending height and the porosity were measured by the following methods.

[0114] 1) Bending height

[0115] A solution for forming an adhesive film was prepared by adding 2 g of the negative electrode binder used in each of Examples 1 to 3 and Comparative Examples 1 to 5 to distilled water so that the solid content was 15% by weight. The solution for forming an adhesive film was applied to a square Teflon substrate having a size of 10 cm × 10 cm over an area of 10 cm × 10 cm. The applied solution for forming an adhesive film was dried at 60 °C for 48 hours to prepare an adhesive film, which included a vertex spaced apart from the Teflon substrate. When a vertex spaced apart from the Teflon substrate was vertically pressed downward against the Teflon substrate with a force of 3 kgf, the maximum vertical distance between the adhesive film spaced apart from the Teflon substrate and the Teflon substrate was measured and defined as the bending height.

[0116] 2) Porosity

[0117] The porosity of the negative electrode was measured by Equation 1 below.

[0118] [Equation 1]

[0119] Porosity of negative electrode (%) = {1 - (true density of negative electrode / electrode density of negative electrode)} × 100 In Equation 1 above, the true density of the negative electrode was the density of the negative electrode active material layer measured by collecting a negative electrode of a specific size and then pressing the collected negative electrode with a pressing device until the thickness of the negative electrode became constant, and the electrode density of the negative electrode referred to the density of the negative electrode active material layer measured by collecting a negative electrode of a specific size.

[0120] Experimental Example

[0121] Experimental Example 1: Evaluation of Capacity Retention Rate

[0122] <Manufacture of secondary battery>

[0123] Use lithium metal as the positive electrode.

[0124] Insert a polyethylene separator between the negative electrode and the positive electrode of each of Examples 1 to 3 and Comparative Examples 1 to 5 manufactured above, and inject an electrolyte therein to manufacture a half-cell type secondary battery. The electrolyte is prepared by adding vinylene carbonate in an amount of 3 wt% based on the total weight of the electrolyte and LiPF with a concentration of 1 M as a lithium salt to an organic solvent in which fluoroethylene carbonate (FEC) and diethyl carbonate (DMC) are mixed at a volume ratio of 30:70. 6 and prepared.

[0125] <Evaluation of capacity retention rate>

[0126] Use an electrochemical charge / discharge device to evaluate the capacity retention rate of the secondary batteries manufactured in each of Examples 1 to 3 and Comparative Examples 1 to 5.

[0127] The secondary battery is 1) charged (0.1C CC / CV charge 0.005V 0.05C cut-off) and discharged (0.1C CC discharge 1.5V cut-off), which is set as the first cycle, 2) charged (0.1C CC / CV charge 0.005V 0.05C cut-off) and discharged (0.1C CC discharge 1.0V cut-off), which is set as the second cycle, 3) charged (0.5C CC / CV charge 0.005V 0.05C cut-off) and discharged (0.5C CC discharge 1.0V cut-off), which is set as the third cycle, and 4) charged and discharged under the same conditions as the third cycle until the 27th cycle.

[0128] Evaluate the capacity retention rate of the charge / discharge cycles of the secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 5 according to Equation 2 below, and shown in Figure 1 . In Figure 1 , Examples 1 to 3 are denoted as "Operation 1" to "Operation 3" in sequence, while Comparative Examples 1 to 5 are denoted as "Comparison 1" to "Comparison 5" in sequence.

[0129] [Equation 2]

[0130] Capacity retention rate (%) = {(Discharge capacity at the Nth cycle) / (Discharge capacity at the first cycle)} × 100

[0131] (In the above formula, N is an integer between 1 and 27)

[0132] In addition, evaluate the capacity retention rate of the 27th cycle according to Equation 2 above, and shown in Table 2.

[0133] [Table 2]

[0134] Capacity Retention Rate at 27 Cycles (%) Embodiment 1 87.1 Embodiment 2 85.2 Embodiment 3 85.6 Comparative Example 1 80.3 Comparative Example 2 82.5 Comparative Example 3 83.2 Comparative Example 4 82.9 Comparative Example 5 80.0

[0135] Referring to Table 2, in the case of the example, a negative electrode binder containing an aqueous binder and a rubber binder in a preferred ratio was used, thereby preventing the expansion / shrinkage problem that occurs when a silicon-based active material is used. Thus, it can be seen that the capacity retention rate was significantly increased compared to the comparative example.

Claims

1. A negative electrode, comprising: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a silicon-based active material and a negative electrode binder, and the negative electrode binder comprises an aqueous binder and a rubber-based binder in a weight ratio of 82:18 to 88:12, and wherein the aqueous binder contains polyvinyl alcohol and polyacrylic acid in a weight ratio of 55:45 to 80:20, the amount of the silicon-based active material contained in the negative electrode active material layer is 60 wt% to 90 wt%; and the amount of the negative electrode binder contained in the negative electrode active material layer is 10 wt% to 30 wt%, wherein the porosity of the negative electrode is 37% to 45%, and wherein the rubber-based binder is styrene-butadiene rubber.

2. The negative electrode according to claim 1, wherein the silicon-based active material contains Si.

3. The negative electrode according to claim 1, wherein the thickness of the negative electrode active material layer is 10 μm to 40 μm.

4. The negative electrode according to claim 1, wherein when measuring the bending height by the following method, the bending height of the adhesive film prepared with the negative electrode binder is 1 cm or less: preparing a solution for forming an adhesive film by adding 2 g of the negative electrode binder to distilled water so that the solid content is 15 wt%; applying the solution for forming an adhesive film onto a square Teflon substrate having a size of 10 cm × 10 cm to an area of 10 cm × 10 cm; drying the applied solution for forming an adhesive film at 60 °C for 48 hours to prepare an adhesive film, the adhesive film comprising a vertex spaced apart from the Teflon substrate; and when vertically pressing the vertex spaced apart from the Teflon substrate against the Teflon substrate with a force of 3 kgf, defining the maximum vertical distance between the adhesive film spaced apart from the Teflon substrate and the Teflon substrate as the bending height.

5. The negative electrode according to claim 1, wherein the negative electrode active material layer further comprises a negative electrode conductive material.

6. The negative electrode according to claim 5, wherein the amount of the negative electrode conductive material contained in the negative electrode active material layer is 5 wt% to 20 wt%.

7. A secondary battery, comprising: the negative electrode according to claim 1; a positive electrode opposite to the negative electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

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