Anode and secondary battery including said anode

Abstract

The present invention relates to a negative electrode and a secondary battery, the negative electrode active material layer including a negative electrode active material layer including a negative electrode active material and a binder, the binder including: a) an acrylic polymer that is a polymer of a monomer mixture including a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; and b) cellulose nanofibers.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0187721, filed December 24, 2021, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to an anode including an anode active material layer, the anode active material layer including an anode active material and a binder, the binder including: a) an acrylic polymer that is a polymer of a monomer mixture including a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; and b) cellulose nanofibers; and a secondary battery including the anode. [Background technology]

[0003] The rapid increase in fossil fuel use has led to an increasing demand for alternative and clean energy, and one of the most actively researched fields in this area is the field of power generation and storage using electrochemical reactions.

[0004] Currently, secondary batteries are a typical example of electrochemical devices that use electrochemical energy, and their range of use is expanding. Recently, with the development of technologies and increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as energy sources has rapidly increased. Among these secondary batteries, much research has been conducted on high-energy density, i.e., high-capacity lithium secondary batteries, which have been commercialized and widely used.

[0005] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material that inserts and extracts lithium ions from the positive electrode. Silicon-based particles with a large discharge capacity can be used as the negative electrode active material.

[0006] However, silicon-based particles have low initial efficiency and undergo excessive volume change during charging and discharging. As a result, the silicon-based particles may detach from the anode during battery operation or the conductive paths within the anode may be reduced, resulting in a shortened battery life. In particular, the above-mentioned problems become more pronounced when so-called pure silicon silicon particles (particles made of silicon) are used.

[0007] Attempts have been made to improve the binder to prevent the silicon-based particles from detaching and maintain the conductive path even when the silicon-based particles undergo large volume changes, such as the use of an acrylic binder instead of carboxymethyl cellulose and styrene-butadiene rubber.

[0008] In order to prevent the silicon particles from detaching and maintain the conductive path while using an acrylic binder, thereby improving the battery life, the acrylic binder has been adjusted to have a high modulus. For example, the weight average molecular weight of the acrylic binder is as high as 400,000 or more, and the T g However, if the weight average molecular weight of the acrylic binder is high, it is difficult to disperse the negative electrode slurry, which reduces the conductivity within the negative electrode. g If the value is too high, the brittleness of the negative electrode active material layer increases excessively, and the adhesive strength of the negative electrode decreases. g There is a need to move away from the traditional method of adjusting values. Therefore, new technologies that can improve the life characteristics of batteries are required. Summary of the Invention [Problem to be solved by the invention]

[0009] One problem to be solved by the present invention is to provide a negative electrode with improved battery life characteristics. Another problem to be solved by the present invention is to provide a secondary battery including the negative electrode. [Means for solving the problem]

[0010] According to one embodiment of the present invention, there is provided a negative electrode including a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a binder, the binder including: a) an acrylic polymer which is a polymer of a monomer mixture including a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; and b) cellulose nanofibers. According to another embodiment of the present invention, there is provided a secondary battery including the negative electrode. [Effects of the Invention]

[0011] According to the present invention, the acrylic polymer can be uniformly dispersed in the negative electrode slurry, and the cellulose nanofibers can act as a filler in the negative electrode active material layer, thereby improving the durability of the negative electrode.

[0012] In addition, the carboxyl groups on the surface of the cellulose nanofibers bond with the hydroxyl groups of the acrylic polymer, providing the binder with high tensile strength. This allows the negative electrode structure to be maintained even when the negative electrode active material, particularly silicon particles, undergoes rapid volumetric changes, preventing detachment of the negative electrode active material and effectively maintaining a conductive path within the negative electrode active material layer. This improves the battery's lifespan.

[0013] Furthermore, the cellulose nanofibers are longer than conventionally used carboxymethyl cellulose and can therefore be easily adsorbed onto the negative electrode active material. Unlike styrene butadiene rubber, the cellulose nanofibers can be surface-adhered to surrounding components, further improving the adhesive strength of the negative electrode and preventing detachment of the negative electrode active material during battery operation, thereby further improving the battery life. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a graph showing the capacity retention rates of secondary batteries using the negative electrodes of Examples 1 to 6 and Comparative Examples 1 and 2 at room temperature. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will now be described in more detail to aid in understanding the invention. The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0016] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless otherwise clearly indicated in the context.

[0017] It should be understood that in this specification, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0018] In this specification, D 50 can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, the laser diffraction method, which generally allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0019] In this specification, the term "weight average molecular weight (Mw)" refers to a value measured by gel permeation chromatography (GPC) and converted to standard polystyrene. Specifically, the weight average molecular weight is a value measured by GPC under the following conditions and converted, and standard polystyrene from an Agilent system was used to create a calibration curve.

[0020] <Measurement conditions> Measuring device: Ultrahydrogel Linear×2 Solvent: 0.1M NaNO3, pH 7 phosphate buffer (0.45μm filtered) Flow rate: 1.0ml / min Sample concentration: 1.0 mg / ml Injection volume: 100μl Column temperature: 40℃ Detector: Waters RI detector Standard sample: Polyacrylic acid (corrected by a cubic function) Data processing: Empower 3

[0021] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0022] In this specification, the modulus can be measured by the following method. The polymer solution was dried at room temperature to prepare a film, which was then punched out into a 10mm x 20mm sample. The modulus was confirmed by measuring the strain-stress at a strain rate of 5mm / min using a text analyzer.

[0023] <Negative electrode> A negative electrode according to one embodiment of the present invention includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material and a binder. The binder may include: a) an acrylic polymer that is a polymer of a monomer mixture including a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; and b) cellulose nanofibers.

[0024] The negative electrode may include a negative electrode active material layer, and the negative electrode active material layer may be disposed on a negative electrode current collector. The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that easily adsorbs carbon, such as copper or nickel, may be used as the negative electrode current collector. The thickness of the negative electrode current collector may be 6 μm to 20 μm, but the thickness of the negative electrode current collector is not limited thereto. The negative electrode active material layer may be disposed on one or both surfaces of the current collector. The negative electrode active material layer may include a negative electrode active material and a binder.

[0025] 1.Negative electrode active material The negative electrode active material may include a silicon-based active material, which enables the negative electrode to have a high capacity.

[0026] In particular, the negative electrode active material may contain silicon particles. The silicon particles may be so-called pure silicon silicon particles (particles made of silicon). The silicon particles can effectively improve the capacity of the negative electrode. The silicon particles undergo rapid volume changes during the charge and discharge process of the battery, which can reduce the life characteristics of the negative electrode. However, in the present invention, the binder contains an acrylic polymer containing acrylamide-derived units and cellulose nanofibers, which prevents the silicon particles from detaching even when the volume of the silicon particles changes rapidly, and maintains the conductive paths in the negative electrode active material layer, thereby improving the life characteristics of the negative electrode.

[0027] The average particle size (D 50 ) may be 1 μm to 10 μm, specifically 3 μm to 5 μm. The silicon particles may be contained in the negative electrode active material layer in an amount of 75 to 85 wt %, and when this range is satisfied, the capacity of the negative electrode can be effectively improved.

[0028] 2. Binder The binder may include: a) an acrylic polymer that is a polymer of a monomer mixture containing a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; and b) cellulose nanofibers.

[0029] (1) Acrylic polymer The acrylic polymer may serve as a binder and a thickener in the negative electrode active material layer.

[0030] The acrylic polymer may be a polymer of a monomer mixture containing (a1) a (meth)acrylamide group-containing monomer, (a2) an unsaturated carboxylic acid monomer, and (a3) ​​a monomer having a solubility in water of 100 g / L or less.

[0031] The (meth)acrylamide group-containing monomer may be, for example, one or more monomers selected from the group consisting of acrylamide, N-methylolacrylamide, N-butoxymethylacrylamide, N-methylolmethacrylamide, N-butoxymethylmethacrylamide, N,N-diethylacrylamide, N,N-dimethylacrylamide, N,N-diethylmethacrylamide, N-ethylacrylamide, N-propylacrylamide, and N-tert-butylacrylamide, used alone or in combination, but is not limited thereto. Specifically, the (meth)acrylamide group-containing monomer may be acrylamide.

[0032] The (meth)acrylamide group-containing monomer may be included in an amount of 60 to 90 parts by weight, specifically 70 to 85 parts by weight, based on 100 parts by weight of the monomer mixture. When the content of the (meth)acrylamide group-containing monomer satisfies this range, the adhesive strength of the negative electrode may be improved, detachment of the negative electrode active material may be suppressed, and the durability of the negative electrode may be improved.

[0033] The unsaturated carboxylic acid monomer may be, for example, one or more selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, maleic anhydride, fumaric acid, and itaconic acid, used alone or in combination, but is not limited thereto. Specifically, the unsaturated carboxylic acid monomer may be acrylic acid.

[0034] The unsaturated carboxylic acid monomer may be included in an amount of 8 parts by weight to 30 parts by weight, specifically 10 parts by weight to 25 parts by weight, based on 100 parts by weight of the monomer mixture. When the content of the unsaturated carboxylic acid monomer satisfies this range, the negative electrode active material and the conductive material can be effectively dispersed, and sufficient adhesion between the negative electrode current collector and the negative electrode active material can be ensured.

[0035] The monomer having a solubility in water of 100 g / L or less means a monomer that dissolves in an amount of 100 g or less when added to 1 L of water at 25° C. When the acrylic polymer contains a monomer having such low solubility in water, it is possible to obtain an effect of reducing the water content in the negative electrode.

[0036] The monomer having a solubility in water of 100 g / L or less may be, for example, one or more monomers selected from the group consisting of alkyl (meth)acrylates having 1 to 10 carbon atoms, (meth)acrylonitrile, and styrene, but is not limited thereto.

[0037] Specific examples of the alkyl (meth)acrylate having 1 to 10 carbon atoms include, but are not limited to, methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-ethylhexyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethylhexyl methacrylate, 2-ethylhexyl methacrylate, etc. Specific examples of the (meth)acrylonitrile include acrylonitrile and methacrylonitrile.

[0038] The monomer having a solubility in water of 100 g / L or less may be included in an amount of 2 parts by weight to 10 parts by weight, specifically 4 parts by weight to 8 parts by weight, based on 100 parts by weight of the monomer mixture. When the content of the monomer having a solubility in water of 100 g / L or less satisfies this range, an acrylic polymer that is easily soluble in water can be obtained while reducing the water content in the negative electrode.

[0039] The glass transition temperature (T g) may be 100° C. to 150° C., specifically 110° C. to 140° C., more specifically 120° C. to 130° C. When the temperature is within this range, the processability during the production of the negative electrode may be improved, and the adhesive strength of the negative electrode may be improved.

[0040] The acrylic polymer may be contained in the negative electrode active material layer in an amount of 1 wt % to 30 wt %, specifically 3 wt % to 20 wt %, more specifically 5 wt % to 15 wt %, for example, 1 wt % to 9.5 wt %. When the amount is within this range, detachment of the negative electrode active material is effectively suppressed, and the battery life can be further improved.

[0041] The acrylic polymer can be produced by mixing the monomer components to form a monomer mixture and then polymerizing the mixture. The polymerization can be carried out using a conventional polymerization method known in the art, such as solution polymerization or emulsion polymerization. The polymerization temperature and polymerization time can be determined appropriately depending on the polymerization method, the type of polymerization initiator, and other factors. For example, the polymerization temperature can be 50°C to 100°C, and the polymerization time can be 1 to 10 hours. Furthermore, during the polymerization, a polymerization initiator, a molecular weight modifier, and the like can be further added as needed. The polymerization initiator can be an inorganic or organic peroxide, and water-soluble initiators such as potassium persulfate, sodium persulfate, and ammonium persulfate can be used. The molecular weight modifier can be, for example, mercaptans, terpenes such as terpinolene, dipentene, and t-terpinene, or halogenated hydrocarbons such as chloroform and carbon tetrachloride.

[0042] (2) Cellulose nanofiber The cellulose nanofibers, when used in combination with the acrylic binder, improve the toughness of the binder, thereby preventing the negative electrode active material (especially silicon particles) from being detached from the negative electrode active material layer even when excessive volume change occurs during the charge and discharge process of the battery, and maintaining the conductive path of the negative electrode active material layer, thereby improving the lifespan of the battery using the negative electrode.

[0043] In particular, the cellulose nanofibers are highly effective when used in combination with the acrylic polymer. When the negative electrode active material layer uses only an acrylic polymer as a binder, the acrylic polymer is a linear or partially branched polymer, making it difficult to prevent structural collapse of the negative electrode during excessive volume change of the negative electrode active material, resulting in low adhesive strength of the negative electrode.

[0044] In contrast, when the negative electrode active material layer contains an acrylic polymer and cellulose nanofibers as a binder, the acrylic polymer can be uniformly dispersed in the negative electrode active material layer, and the cellulose nanofibers can act as a filler in the negative electrode active material layer, thereby improving the durability of the negative electrode.

[0045] In addition, the carboxyl groups on the surface of the cellulose nanofibers bond with the hydroxyl groups of the acrylic polymer, providing the binder with high tensile strength. This allows the negative electrode structure to be maintained even when the negative electrode active material, particularly silicon particles, undergoes rapid volumetric changes, preventing detachment of the negative electrode active material and effectively maintaining a conductive path within the negative electrode active material layer. This improves the battery's lifespan.

[0046] Furthermore, the cellulose nanofibers are longer than conventionally used carboxymethyl cellulose, and therefore can be easily adsorbed onto the negative electrode active material. Unlike styrene-butadiene rubber, the cellulose nanofibers can be surface-adhered to surrounding components, further improving the adhesive strength of the negative electrode.

[0047] The cellulose nanofibers may be made from polysaccharides having a β-1,4-glucan structure. Examples of cellulose nanofibers include cellulose nanofibers derived from higher plants (e.g., natural cellulose fibers (pulp fibers) such as wood fibers (e.g., wood pulp from conifers, broad-leaved trees, etc.), bamboo fibers, sugarcane fibers, seed hair fibers (e.g., cotton linters, bombax cotton, kapok, etc.), bast fibers (e.g., hemp, paper mulberry, Mitsumata, etc.), and leaf fibers (e.g., Manila hemp, New Zealand hemp, etc.)), animal-derived cellulose fibers (e.g., sea squirt cellulose), bacterial-derived cellulose fibers (e.g., the cellulose contained in nata de coco), and chemically synthesized cellulose fibers (e.g., rayon, cellulose esters (e.g., cellulose acetate), hydroxyalkyl celluloses (e.g., hydroxyethyl cellulose (HEC), hydroxypropyl cellulose), and cellulose ether derivatives such as alkyl celluloses (e.g., methyl cellulose, ethyl cellulose, etc.)). These cellulose nanofibers may be used alone or in combination of two or more kinds.

[0048] Among the above-mentioned cellulose nanofibers, cellulose fibers derived from higher plants, for example, cellulose fibers derived from pulp such as wood fibers (wood pulp from conifers, broad-leaved trees, etc.) and seed hair fibers (cotton linter pulp, etc.), are preferred because they are easy to produce cellulose nanofibers with an appropriate aspect ratio.

[0049] The average diameter of the cellulose nanofibers may be 2 nm to 50 nm, specifically 2.5 nm to 25 nm, and more specifically 3 nm to 5 nm. When this range is satisfied, the cellulose nanofibers can be uniformly dispersed in the negative electrode active material layer, thereby further improving the conductivity of the negative electrode, the adhesive strength of the negative electrode, and the battery life characteristics. The average diameter refers to the average value of the diameters of the top 10 cellulose nanofibers and the bottom 10 cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed under tens of thousands of magnifications (e.g., 10,000 times) using a TEM.

[0050] The average length of the cellulose nanofibers may be 0.3 μm to 20 μm, specifically 0.5 μm to 10 μm, and more specifically 1 μm to 5 μm. When this range is satisfied, the cellulose nanofibers can be uniformly dispersed in the negative electrode active material layer, thereby further improving the conductivity of the negative electrode, the adhesive strength of the negative electrode, and the battery life characteristics. The average length refers to the average length of the top 10 longest and bottom 10 longest cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed using a TEM at a magnification of several tens of thousands (e.g., 10,000 times).

[0051] The average aspect ratio of the cellulose nanofibers may be 30 to 2,000, specifically 50 to 1,000, and more specifically 300 to 500. When this range is satisfied, the cellulose nanofibers can be uniformly dispersed in the negative electrode active material layer, thereby further improving the conductivity of the negative electrode, the adhesive strength of the negative electrode, and the battery life characteristics. Here, the aspect ratio refers to the length relative to the diameter of the cellulose nanofibers. The average aspect ratio refers to the average of the aspect ratios of the top 10 cellulose nanofibers and the bottom 10 cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed using a TEM at a magnification of several tens of thousands (e.g., 10,000 times).

[0052] The cellulose nanofibers may be contained in the negative electrode active material layer in an amount of 0.1 wt % to 3 wt %, specifically 0.2 wt % to 1.5 wt %, more specifically 0.4 wt % to 1.0 wt %, for example, 0.5 wt % to 1.0 wt %. When the amount is within this range, the tensile strength of the binder is effectively improved, and the battery life characteristics can be further improved.

[0053] In the negative electrode active material layer, the weight ratio of the acrylic polymer to the cellulose nanofibers may be 6:4 to 9.8:0.2, specifically 7:3 to 9.8:0.2, more specifically 8:2 to 9.5:0.5, for example, 8:2 to 9:1. When this range is satisfied, the tensile strength (toughness) of the entire binder can be sufficient, thereby improving the prevention of detachment of the negative electrode active material and the establishment of conductive paths, thereby improving the battery life. Furthermore, the viscosity of the negative electrode slurry can be maintained at an appropriate level, thereby improving the phase stability of the negative electrode slurry.

[0054] The negative electrode active material layer may further include a conductive material. The conductive material is not particularly limited as long as it does not induce chemical changes in the battery and is conductive, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes; fluorocarbons; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The conductive material may be contained in the negative electrode active material layer in an amount of 5% by weight to 10% by weight, specifically 5% by weight to 15% by weight.

[0055] <Secondary battery> A secondary battery according to another embodiment of the present invention may include the negative electrode of the above-described embodiment. Specifically, the secondary battery may include the negative electrode of the above-described embodiment, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, detailed description thereof will be omitted.

[0056] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material. The positive electrode current collector in the positive electrode is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector may typically have a thickness of 3 to 500 μm, and the surface of the current collector may be provided with fine irregularities to enhance adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0057] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 1+a M b O 2+c may include:

[0058] More specifically, the positive electrode active material is Li 1+a M b O 2+ccomprising, and M may be at least any one element selected from the group consisting of Ni, Co, Mn, Fe, P, Al, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo, and V, and -0.2 ≦ a ≦ 0.2, 0 < b ≦ 2, 0 ≦ c ≦ 2 may hold. The said a preferably satisfies -0.1 ≦ a ≦ 0.1, more preferably 0 ≦ a ≦ 0.1. Specifically, the said Li 1+a M b O 2+c is Li 1+a [Ni p Co q M 1 r M 2 s O2 or may be the said Li 1+a [Ni p Co q M 1 r M 2 s O2. In the said Li 1+a [Ni p Co q M 1 r M 2 s O2, the said M 1 may be at least any one element of Al and Mn, and M 2 may be at least any one element selected from the group consisting of Fe, P, Mg, Ca, Zr, Zn, Ti, Ru, Nb, W, B, Si, Na, K, Mo, and V. The said p satisfies 0 < p < 1, preferably 0.3 < p < 1, more preferably 0.5 < p < 1. The said q satisfies 0 < q < 1, preferably 0 < q < 0.7, more preferably 0 < q < 0.5. The said r satisfies 0 < r < 1, preferably 0 < r < 0.7, more preferably 0 < r < 0.5. The said s satisfies 0 ≦ s ≦ 0.2, preferably 0 ≦ s ≦ 0.1. The said Li 1+a M b O 2+c is LiCoO2, LiNiO2, LiMnO2, Li[Ni 0.5 Co 0.3 Mn 0.2 O2、Li[Ni0.6 Co 0.2 Mn 0.2 ]O2, Li[Ni 0.7 Co 0.1 Mn 0.2 ]O2, Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2, Li[Ni 0.9 Co 0.05 Mn 0.05 ]O2, LiMn2O4, LiFePO4, 0.5Li2MnO3·0.5Li[Mn 0.4 Ni 0.3 Co 0.3 ]O2. Preferably, the Li 1+a M b O 2+c is the Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2, Li[Ni 0.7 Co 0.1 Mn 0.2 ]O2, Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2, Li[Ni 0.9 Co 0.05 Mn 0.05 ]O2. The positive electrode active material may be contained in the positive electrode active material layer in an amount of 95% by weight to 99% by weight. When the positive electrode is used in combination with the negative electrode of the above-described embodiment, it is possible to prevent overpotential from occurring in the positive electrode.

[0059] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-described positive electrode active material. In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it does not cause chemical changes in the battery that is constructed and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, and the like, and one or more of these may be used alone or in combination.

[0060] In addition, the positive electrode binder serves to improve adhesion between particles of the positive electrode active material and the adhesive strength between the positive electrode active material and the positive electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, Poly Examples include tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, and one of these may be used alone or two or more may be used in combination.

[0061] The separator separates the negative electrode and positive electrode and provides a path for lithium ion migration. Any separator typically used in secondary batteries can be used without particular limitations. It is particularly preferable that the separator exhibits low resistance to electrolyte ion migration and has excellent electrolyte humidifying ability. Specifically, porous polymer films, such as those made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material may be used, and it may be selectively used as a single-layer or multi-layer structure.

[0062] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries.

[0063] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt. Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0064] In particular, among the carbonate organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents, because they have a high dielectric constant and dissociate lithium salts well. When such cyclic carbonates are mixed in an appropriate ratio with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, an electrolyte having high electrical conductivity can be prepared, and therefore such cyclic carbonates can be used more preferably.

[0065] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3- , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0066] In addition to the constituent components of the electrolyte, the electrolyte may further include one or more additives, such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, in order to improve the life characteristics of the battery, suppress a decrease in battery capacity, and improve the discharge capacity of the battery.

[0067] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and high cycle characteristics, and can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0068] Below, preferred examples are presented to help understand the present invention. However, the following examples are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope and technical spirit of the present description, and it goes without saying that such changes and modifications fall within the scope of the appended claims.

[0069] Examples and Comparative Examples An acrylic polymer and cellulose nanofibers were prepared as follows.

[0070] (A) Acrylic polymer (A-1) An acrylic polymer (T) produced using 60 parts by weight of acrylamide, 30 parts by weight of acrylic acid, and 10 parts by weight of acrylonitrile g : 120℃, Elastic modulus: 7.99GPa) (A-2) Acrylic polymer (T) produced using 80 parts by weight of acrylamide, 15 parts by weight of acrylic acid, and 5 parts by weight of acrylonitrile g : 110℃, Elastic modulus: 3.12GPa)

[0071] (B) Cellulose nanofiber (B-1) Single-filament cellulose nanofiber (B-2) Molecular fibrous cellulose nanofiber

[0072] Example 1: Preparation of negative electrode Average particle size (D 50 A negative electrode slurry was produced containing: silicon particles having a size of 5 μm; a conductive material containing carbon black and single-walled carbon nanotubes; an acrylic polymer (A-1); cellulose nanofibers (B-1); and water.

[0073] 150 mg / 25 cm on both sides of a copper (Cu) metal thin film, which is a 20 μm thick negative electrode current collector 2The negative electrode slurry was coated and dried using a loading of 1000 kJ / cm2 and the circulating air temperature was 70° C. The negative electrode current collector coated with the slurry and dried was then rolled and dried in a vacuum oven at 130° C. for 8 hours to prepare a negative electrode including a negative electrode active material layer. The weight ratio of the silicon particles, conductive material, acrylic polymer (A-1), and cellulose nanofibers (B-1) was 80:10:9.8:0.2.

[0074] Examples 2 to 6 and Comparative Example 1: Production of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the conditions were changed as shown in Table 1 below.

[0075] [Table 1]

[0076] Comparative Example 2: Production of negative electrode A negative electrode was produced in the same manner as in Example 1, except that styrene butadiene rubber and carboxymethyl cellulose were used instead of the acrylic polymer (A-1) and the cellulose nanofibers (B-1). The weight ratio of the silicon particles, conductive material, styrene butadiene rubber, and carboxymethyl cellulose was 80:10:3:7.

[0077] [Table 2]

[0078] The average diameter refers to the average value of the diameters of the top 10 cellulose nanofibers and the bottom 10 cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed under 10,000x magnification by TEM. The average length refers to the average value of the lengths of the top 10 cellulose nanofibers and the bottom 10 cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed under 10,000x magnification by TEM.

[0079] The average aspect ratio refers to the average value of the aspect ratios of the top 10 cellulose nanofibers and the bottom 10 cellulose nanofibers when the negative electrode active material layer of the manufactured negative electrode is observed at 10,000 magnifications using a TEM.

[0080] Experimental example 1: Evaluation of life characteristics (capacity retention rate) Secondary batteries were produced using the negative electrodes of Examples 1 to 6 and Comparative Examples 1 and 2, respectively.

[0081] As the positive electrode active material, Li[Ni 0.9 Mn 0.05 Co 0.05 The positive electrode active material, multi-walled carbon nanotubes (MWCNT) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 96:2:2 with N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode slurry.

[0082] The prepared positive electrode slurry was applied to a 15 μm thick aluminum metal thin film, which was a positive electrode current collector, and dried in an air circulating temperature of 110° C. The coating was then rolled and dried in a vacuum oven at 130° C. for 2 hours to form a positive electrode active material layer.

[0083] The negative electrode, the positive electrode, and a porous polyethylene separator were assembled using a winding method. An electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio)) and lithium hexafluorophosphate (LiPF6 1 mol) were injected into the assembled battery to prepare a lithium secondary battery.

[0084] The secondary battery was charged and discharged, and the life characteristics (capacity retention rate) were evaluated. The first and second cycles were charged and discharged at 0.1 C, and the third and fourth cycles were 2 Charge and discharge were performed at 0.5C up to 00 cycles.

[0085] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off) Discharge condition: CC (constant current) condition 1.5V The capacity retention rates were calculated as follows. Capacity retention rate (%) = (200th discharge capacity / 1st discharge capacity) x 100

[0086] [Table 3]

[0087] 1, the capacity retention rates of the secondary batteries using the negative electrodes of Examples 1 to 6, which used a combination of an acrylic polymer and cellulose nanofibers, were higher than that of the secondary battery using the negative electrode of Comparative Example 1, which used only the acrylic polymer, and were also higher than that of the secondary battery using the negative electrode of Comparative Example 2, which used styrene-butadiene rubber and carboxymethyl cellulose. Furthermore, comparing Example 4 and Example 5, it was found that Example 5, which used an acrylic polymer in which acrylamide was used in an amount of 80 parts by weight based on 100 parts by weight of the monomer mixture, had a higher capacity retention rate than Example 4, which used an acrylic polymer in which acrylamide was used in an amount of 60 parts by weight based on 100 parts by weight of the monomer mixture.

[0088] Furthermore, it can be seen that Example 5, in which cellulose nanofibers with an aspect ratio of 400 are present in the negative electrode active material layer, has a higher capacity retention rate than Example 6, in which the aspect ratio is 85.

Claims

1. a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material and a binder, The binder is a) an acrylic polymer which is a polymer of a monomer mixture containing a (meth)acrylamide group-containing monomer, an unsaturated carboxylic acid monomer, and a monomer having a solubility in water of 100 g / L or less; b) cellulose nanofibers; Including, In the negative electrode active material layer, the weight ratio of the acrylic polymer to the cellulose nanofibers is 6:4 to 9.5:0.5; the acrylic polymer is contained in the negative electrode active material layer in an amount of 5% by weight to 20% by weight, The monomer having a solubility in water of 100 g / L or less is a monomer that dissolves in an amount of 100 g or less when added to 1 L of water at 25°C. Negative electrode.

2. 2. The negative electrode according to claim 1, wherein the (meth)acrylamide group-containing monomer is contained in an amount of 60 to 90 parts by weight based on 100 parts by weight of the polymer of the monomer mixture.

3. 2. The negative electrode according to claim 1, wherein the (meth)acrylamide group-containing monomer is acrylamide.

4. 2. The negative electrode according to claim 1, wherein the unsaturated carboxylic acid monomer is contained in an amount of 8 to 30 parts by weight based on 100 parts by weight of the polymer of the monomer mixture.

5. The negative electrode according to claim 1 , wherein the unsaturated carboxylic acid monomer is acrylic acid.

6. 2. The negative electrode according to claim 1, wherein the monomer having a solubility in water of 100 g / L or less is contained in an amount of 2 to 10 parts by weight based on 100 parts by weight of the polymer of the monomer mixture.

7. 2. The negative electrode according to claim 1, wherein the monomer having a solubility in water of 100 g / L or less is acrylonitrile.

8. The glass transition temperature (T ｇ 2. The negative electrode according to claim 1, wherein the temperature is 100°C to 150°C.

9. The negative electrode according to claim 1, wherein the cellulose nanofibers have an average diameter of 2 nm to 50 nm.

10. The negative electrode according to claim 1, wherein the cellulose nanofibers have an average length of 0.3 μm to 20 μm.

11. The negative electrode according to claim 1, wherein the cellulose nanofibers have an average aspect ratio of 30 to 2,000.

12. 2. The negative electrode according to claim 1, wherein the cellulose nanofibers are contained in the negative electrode active material layer in an amount of 0.1% by weight to 3% by weight.

13. The anode of claim 1 , wherein the anode active material comprises silicon particles.

14. A secondary battery comprising the negative electrode according to claim 1.

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