Negative electrode and secondary battery

Abstract

To provide a negative electrode for a secondary battery and a secondary battery including the same.SOLUTION: A negative electrode for a secondary battery includes a current collector, a first negative electrode active material layer disposed on the current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, and at least one of the first negative electrode active material layer and the second negative electrode active material layer contains lithium-substituted carboxymethyl cellulose, the first negative electrode active material layer contains a carbon-based active material, and only the second negative electrode active material layer contains a silicon-based active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a secondary battery and a secondary battery including the same.

[0002] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0125384, filed with the Korean Intellectual Property Office on September 30, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

[0003] Secondary batteries are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs), which are powered by electrical sources.

[0004] Such secondary batteries have the primary advantage of dramatically reducing the use of fossil fuels, as well as the advantage of not producing any by-products from energy use, making them environmentally friendly and drawing attention as a new energy source for improving energy efficiency.

[0005] In general, a secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. In addition, the electrodes such as the positive electrode and the negative electrode may have an electrode active material layer formed on a current collector.

[0006] As the use of secondary batteries increases, various battery performances are being demanded. Attempts have been made to improve battery performance by adding additives to the active material layer, but depending on the type of additive, some battery performance may be improved, while other performance may be deteriorated. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a negative electrode for a secondary battery that can provide a secondary battery with improved charging performance and life, and a secondary battery including the same. [Means for solving the problem]

[0008] One embodiment of the present invention comprises: Current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer; Including, at least one of the first negative electrode active material layer and the second negative electrode active material layer contains lithium-substituted carboxymethyl cellulose; The first negative electrode active material layer contains a carbon-based active material, and only the second negative electrode active material layer contains a silicon-based active material.

[0009] Another embodiment of the present invention provides a secondary battery comprising the above-described negative electrode for a secondary battery, a positive electrode, and a separator. [Effects of the Invention]

[0010] The synergistic effect of the two-layer structure of the negative electrode active material layer containing silicon-based active materials and the addition of lithium-substituted carboxymethylcellulose improves the battery's charging performance and ensures battery life. Furthermore, silicon-based active materials tend to have lower electrical conductivity than carbon-based active materials. Therefore, if the silicon-based active material is uniformly distributed across the thickness of the negative electrode active material layer (T in Figure 1), this can lead to non-uniform electrical conductivity throughout the electrode, which can be detrimental to charging performance (fast charging). However, when the silicon-based active material is included only in the second negative electrode active material layer (202 in Figure 1) of the two-layer structure, electrical conductivity is increased, thereby reducing contact resistance. Furthermore, the addition of lithium-substituted carboxymethylcellulose, which has high Li-ion mobility, can maximize this effect. The addition of lithium-substituted carboxymethylcellulose increases Li-ion mobility, thereby ensuring the battery's lifespan, which is slightly inferior when the silicon-based active material is included only in the second negative electrode active material layer.

[0011] In particular, when charging at a high current during fast charging, the lithium ion flux is relatively greater than that during normal charging (normal cycle). If the anode active material can rapidly accept lithium ions when this large lithium ion flux is generated, the fast charging performance of the cell can be improved. Silicon-based active materials accept lithium ions through alloying and begin charging at a lower potential, making them more advantageous for improving fast charging performance than graphite-based anode active materials, which accept lithium ions through intercalation. Furthermore, by arranging the silicon-based active material close to the cathode, which is the first to receive the lithium ion flux, i.e., in the upper layer (second anode active material layer), the silicon-based active material can react rapidly with lithium ions, significantly improving fast charging performance. [Brief explanation of the drawings]

[0012] [Figure 1] 1 illustrates an example of a negative electrode according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in more detail below to facilitate understanding of the present invention. The present invention can be embodied in various different forms and is not limited to the embodiments described herein. In this regard, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his or her invention.

[0014] In this specification, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and are understood not to preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0015] Furthermore, when a part such as a layer is said to be "above" another part, this does not only mean that it is "directly above" that part, but also includes cases where there is another part between them. Conversely, when a part is said to be "directly above" another part, it means that there is no other part between them. Note that being "above" a reference part refers to being located above or below the reference part, and does not necessarily mean being located "above" in the opposite direction of gravity.

[0016] In this specification, the description simply referring to the "negative electrode active material layer" without the terms "first" and "second" can apply to both the first negative electrode active material layer and the second negative electrode active material layer.

[0017] A secondary battery negative electrode according to one embodiment of the present specification includes a current collector; a first negative electrode active material layer disposed on the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein at least one of the first negative electrode active material layer and the second negative electrode active material layer contains lithium-substituted carboxymethyl cellulose, the first negative electrode active material layer contains a carbon-based active material, and only the second negative electrode active material layer contains a silicon-based active material. In other words, the secondary battery negative electrode is characterized by containing lithium-substituted carboxymethyl cellulose together with two negative electrode active material layers. The distribution of the silicon-based active material can be confirmed through a scanning electron microscope (SEM) image of a cross-section of the electrode. Therefore, the region containing the silicon-based active material can be defined as the second negative electrode active material layer.

[0018] FIG. 1 illustrates an example of a negative electrode including a first negative electrode active material layer 201, a second negative electrode active material layer 202, and a current collector 101.

[0019] The present inventors have discovered that the lithium-substituted carboxymethyl cellulose is advantageous in improving the charging performance and ensuring the lifespan of a battery compared to carboxymethyl cellulose partially substituted with sodium, and that when the lithium-substituted carboxymethyl cellulose is used together with a multilayer structure of an anode active material layer containing a silicon-based active material, a synergistic effect can be achieved to further maximize the charging performance and lifespan of the battery, thereby completing the present invention.

[0020] The lithium-substituted carboxymethylcellulose may be contained in both the first and second negative electrode active material layers, or in one of the first and second negative electrode active material layers. For example, the lithium-substituted carboxymethylcellulose may be contained in a larger amount in the second negative electrode active material layer than in the first negative electrode active material layer, or may be contained only in the second negative electrode active material layer. According to one embodiment, one or both of the first and second negative electrode active material layers may contain lithium-substituted carboxymethylcellulose and no sodium-substituted carboxymethylcellulose. However, in other embodiments, sodium-substituted carboxymethylcellulose may be further contained. For example, one of the first and second negative electrode active material layers may contain lithium-substituted carboxymethylcellulose, and the other may contain sodium-substituted carboxymethylcellulose. According to one embodiment, the lithium-substituted carboxymethylcellulose and sodium-substituted carboxymethylcellulose are the only carboxymethylcellulose salts contained in the electrode.

[0021] In one embodiment of the present specification, the silicon-based active material is SiO x (0≦x<2), SiM y (M is a metal, 1≦y≦4) and Si / C. Only one type of silicon-based active material may be contained, or two or more types may be contained together.

[0022] In one embodiment of the present specification, the second negative electrode active material layer containing the silicon-based active material may further contain a carbon-based active material. In this case, the silicon-based active material may be contained in an amount of 1 to 40 parts by weight, for example, 2 to 35 parts by weight, 3 to 30 parts by weight, 5 to 25 parts by weight, or 7 to 15 parts by weight, based on 100 parts by weight of the total active materials contained in the second negative electrode active material layer containing the silicon-based active material.

[0023] In one embodiment of the present specification, the first negative electrode active material layer contains a carbon-based active material, and only the second negative electrode active material layer contains a silicon-based active material. In this case, the first negative electrode active material layer does not contain a silicon-based active material.

[0024] As the silicon-based active material, SiO x (0 ≦ x < 2) The active material containing may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0025] In this specification, a composite particle or a composite means that two or more materials or substances are physically aggregated without a chemical bond.

[0026] The SiO x (0 < x < 2) corresponds to a matrix in the silicon-based composite particles. The SiO x (0 < x < 2) may be in a form containing Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2). When the silicon-based composite particles contain the SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0027] The silicon-based composite particles may further contain at least one of a Mg compound and a Li compound. The Mg compound and the Li compound can correspond to a matrix in the silicon-based composite particles.

[0028] The Mg compound and / or the Li compound may be present inside and / or on the surface of the SiO x (0 < x < 2). The initial efficiency of the battery can be improved by the Mg compound and / or the Li compound.

[0029] The Mg compound may contain at least one selected from the group consisting of Mg silicate, Mg silicide, and Mg oxide. The Mg silicate may contain at least one of Mg2SiO4 and MgSiO3. The Mg silicide may contain Mg2Si. The Mg oxide may contain MgO.

[0030] In one embodiment of the present specification, the Mg element may be contained at 0.1% to 20% by weight, or may be contained at 0.1% to 10% by weight, based on 100% by weight of the total silicon-based active material. Specifically, the Mg element may be contained at 0.5% to 8% by weight or 0.8% to 4% by weight. When the above range is satisfied, the Mg compound can be contained in an appropriate content in the silicon-based active material, so that the volume change of the silicon-based active material can be easily suppressed during charging and discharging of the battery, and the discharge capacity and initial efficiency of the battery can be improved.

[0031] The Li compound may contain at least one selected from the group consisting of Li silicate, Li silicide, and Li oxide. The Li silicate may contain at least one of Li2SiO3, Li4SiO4, and Li2Si2O5. The Li silicide may contain Li7Si2. The Li oxide may contain Li2O.

[0032] In one embodiment of the present invention, the Li compound may contain lithium silicate in the form. The lithium silicate is Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and can be classified into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate can exist in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5 in the silicon-based composite particles, and the amorphous lithium silicate is Li a Si b O cIt may be in the form of (2≦a≦4, 0<b≦2, 2≦c≦5), and is not limited to the above form.

[0033] In one embodiment of the present specification, the Li element may be contained in an amount of 0.1% to 20% by weight, or may be contained in an amount of 0.1% to 10% by weight based on 100% by weight of the total silicon-based active material. Specifically, the Li element may be contained in an amount of 0.5% to 8% by weight, and more specifically, may be contained in an amount of 0.5% to 4% by weight. When the above range is satisfied, the Li compound can be contained in an appropriate content in the silicon-based active material, so that the change in the volume of the negative electrode active material during charging and discharging of the battery can be easily suppressed, and the discharge capacity and initial efficiency of the battery can be improved.

[0034] The content of the Mg element or Li element can be confirmed by ICP (Inductively Coupled Plasma) analysis. After accurately sampling a certain amount (about 0.01 g) of the negative electrode active material for the ICP analysis, it is transferred to a platinum crucible, and nitric acid, hydrofluoric acid, and sulfuric acid are added and completely decomposed on a hot plate. Then, using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICPAES, Perkin-Elmer 7300), the intensity of a standard solution (5 mg / kg) prepared using a standard solution at the specific wavelength of the Mg element or Li element is measured to create a calibration curve. Then, the pretreated sample solution and the background sample are introduced into the instrument, the intensity of each is measured to calculate the actual intensity, and after calculating the concentration of each component by comparing with the calibration curve created, the Mg element or Li element content of the manufactured silicon-based active material is analyzed by converting so that the total sum becomes the theoretical value. <000017​​​​​ In one embodiment of the present specification, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0037] The average particle size (D 50 ) may be 2 μm to 15 μm, specifically 3 μm to 12 μm, and more specifically 4 μm to 10 μm. When the above range is satisfied, side reactions between the silicon-based composite particles and the electrolyte solution are controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0038] In this specification, the average particle size (D 50 The average particle size (D) 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. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0039] The silicon-based active material containing Si / C is a composite of Si and C, and is distinguished from silicon carbide, which is also abbreviated as SiC. The silicon-carbon composite may be a composite of silicon and graphite, or may have a structure in which a core of silicon and graphite is surrounded by graphene or amorphous carbon. The silicon dispersed in the silicon-carbon composite may be nanosilicon. The average particle size (D 50 ) may be 2 μm to 15 μm, specifically 3 μm to 12 μm, and more specifically 4 μm to 10 μm. A carbon layer may be provided on the surface of the active material containing Si / C.

[0040] In one embodiment of the present specification, the carbon-based active material may be graphite, and the graphite may be natural graphite, artificial graphite, or a mixture thereof. Based on 100 parts by weight of the entire first negative electrode active material layer, the first negative electrode active material layer may contain 80 parts by weight to 99.8 parts by weight, for example, 90 parts by weight to 99 parts by weight, or 93 parts by weight to 97 parts by weight of the carbon-based negative electrode active material. Based on 100 parts by weight of the active material contained in the second negative electrode active material layer, the second negative electrode active material layer may contain 60 parts by weight to 99 parts by weight, for example, 75 parts by weight to 98 parts by weight, or 85 parts by weight to 95 parts by weight of the carbon-based negative electrode active material.

[0041] In one embodiment of the present specification, the negative electrode active material may be contained in an amount of 80 parts by weight or more and 99.8 parts by weight or less, preferably 90 parts by weight or more and 99.5 parts by weight or less, and more preferably 95 parts by weight or more and 99 parts by weight or less, in 100 parts by weight of the negative electrode active material layer.

[0042] In one embodiment herein, one or both of the first negative electrode active material layer and the second negative electrode active material layer contains lithium-substituted carboxymethyl cellulose.

[0043] The lithium-substituted carboxymethyl cellulose may have a degree of substitution of hydroxy (-OH) groups with lithium carboxymethyl groups (-CHCOOLi) of 0.1 or more, for example, 0.5 or more, specifically 0.7 to 1.3, or 0.8 to 1.2, and more specifically 0.8 to 1.0.

[0044] The lithium-substituted carboxymethyl cellulose may have a molecular weight (Mn) of 300,000 to 1,000,000, for example, 350,000 to 900,000, and in particular 500,000 to 900,000.

[0045] The lithium-substituted carboxymethyl cellulose may be contained in a negative electrode active material layer containing a silicon-based active material, or in a negative electrode active material layer containing only a carbon-based active material without a silicon-based active material, or in a second negative electrode active material layer containing a silicon-based active material and a carbon-based active material.

[0046] The lithium-substituted carboxymethyl cellulose may be contained only in the first negative electrode active material layer, only in the second negative electrode active material layer, or in both the first negative electrode active material layer and the second negative electrode active material layer.

[0047] The lithium-substituted carboxymethyl cellulose may be included in an amount of 0.1 to 5 parts by weight, for example, 0.1 to 3 parts by weight, 0.2 to 3 parts by weight, 0.3 to 2 parts by weight, or 0.5 to 1 part by weight, based on 100 parts by weight of the total weight of the first and second negative electrode active material layers. The lithium-substituted carboxymethyl cellulose may be included in an amount of 0.1 to 5 parts by weight, for example, 0.1 to 3 parts by weight, 0.2 to 3 parts by weight, 0.3 to 2 parts by weight, or 0.5 to 1 part by weight, based on 100 parts by weight of the first or second negative electrode active material layer containing the lithium-substituted carboxymethyl cellulose.

[0048] According to a further embodiment of the present specification, the negative electrode active material layer may further contain a negative electrode binder in addition to the silicon-based active material and the carbon-based active material.

[0049] The negative electrode binder may improve adhesion between negative electrode active material particles and between the negative electrode active material particles and the negative electrode current collector. The negative electrode binder may be any binder known in the art, and non-limiting examples thereof may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof.

[0050] The negative electrode binder may be included in an amount of 0.1 parts by weight to 20 parts by weight, for example, preferably 0.3 parts by weight to 20 parts by weight, and more preferably 0.5 parts by weight to 10 parts by weight, based on 100 parts by weight of the negative electrode active material layer.

[0051] The negative electrode active material layer may not contain a conductive material, but may further contain a conductive material if necessary. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material 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 fiber and metal fiber; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, 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 content of the conductive material in the negative electrode active material layer may be 0.01 to 20 parts by weight, preferably 0.03 to 18 parts by weight, per 100 parts by weight of the negative electrode active material layer.

[0052] For purposes of the present invention, when the carbon-based active material is graphite, such as natural graphite or artificial graphite, the weight parts of the graphite used as the carbon-based active material are not taken into account when defining the total weight parts of the conductive material. Similarly, when the conductive material selected for the negative electrode is graphite, the weight parts of the conductive material described when defining the total weight parts of the carbon-based negative electrode active material are not included. Therefore, when graphite is selected as both the carbon-based negative electrode active material and the negative electrode conductive material, the total weight parts of graphite correspond to the sum of the weight parts of the graphite used as the carbon-based negative electrode active material and the weight parts of the graphite used as the negative electrode conductive material.

[0053] According to an example, the conductive material contained in the negative electrode active material layer may be carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fiber such as carbon fiber or metal fiber; or conductive tube such as carbon nanotube.

[0054] In one embodiment of the present specification, the thickness of the first negative electrode active material layer and the thickness of the second negative electrode active material layer may each be 30 μm or more and 100 μm or less, for example, 45 μm or more and 75 μm or less, and the sum of the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer may be 90 μm or more and 150 μm or less.

[0055] In one embodiment of the present specification, the negative electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that effectively adsorbs carbon, such as copper or nickel, can be used as the current collector. The thickness of the current collector can be 1 μm to 500 μm, but is not limited thereto.

[0056] A further embodiment herein provides a secondary battery comprising an anode, a cathode and a separator according to the previous embodiment.

[0057] In one embodiment of the present specification, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material. The positive electrode active material layer may have a thickness of 20 μm to 500 μm.

[0058] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector may typically have a thickness of 1 to 500 μm, and fine irregularities can be formed on the collector surface to enhance the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0059] In one embodiment of the present specification, the positive electrode may include a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as an active material. The lithium composite transition metal compound may further include at least one of manganese and aluminum. The lithium composite transition metal compound may include 80 mol % or more, for example, 80 mol % or more but less than 100 mol %, of nickel among metals excluding lithium.

[0060] In one embodiment, the positive electrode active material may be contained in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, and more preferably 95 parts by weight or more and 99.8 parts by weight or less, in 100 parts by weight of the positive electrode active material layer.

[0061] According to a further embodiment of the present specification, the positive electrode active material layer according to the aforementioned embodiment may further include a positive electrode binder and a conductive material.

[0062] The positive electrode binder may serve to improve adhesion between positive electrode active material particles and between the positive electrode active material particles and the positive electrode current collector. The positive electrode binder may be any binder known in the art, and non-limiting 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, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

[0063] The positive electrode binder may be included in an amount of 0.1 parts by weight to 20 parts by weight, for example, preferably 0.3 parts by weight to 18 parts by weight, and more preferably 0.5 parts by weight to 15 parts by weight, based on 100 parts by weight of the positive electrode active material layer.

[0064] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode. Any conductive material can be used without particular limitations as long as it does not undergo chemical changes within the battery and has electronic conductivity. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. Specifically, in one embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

[0065] The conductive material may be included in an amount of 0.1 parts by weight to 10 parts by weight, for example, preferably 0.2 parts by weight to 7 parts by weight, and more preferably 0.3 parts by weight to 5 parts by weight, based on 100 parts by weight of the composition for a positive electrode active material layer.

[0066] According to one embodiment, the sum of the weight of the first negative electrode active material layer and the weight of the second negative electrode active material layer of the negative electrode is 170 to 280 mg / 25 cm 2 The weight here is based on the weight (solid content) after drying excluding the solvent. This range is advantageous for achieving high energy density and rapid charging characteristics.

[0067] The positive and negative electrodes can be fabricated according to conventional methods for fabricating positive and negative electrodes, except for using the positive and negative active materials. Specifically, they can be fabricated by coating a composition for forming an active material layer containing the active material and, optionally, a binder and a conductive material, on a current collector, followed by drying and rolling. The types and contents of the positive and negative active materials, binder, and conductive material are as described above. The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used should be sufficient to dissolve or disperse the active material, conductive material, and binder, taking into account the coating thickness and manufacturing yield of the slurry, and to provide a viscosity that allows excellent thickness uniformity when applied to fabricate positive and negative electrodes. Alternatively, the positive electrode and the negative electrode may be produced by casting the active material layer-forming composition on a separate support, peeling the composition from the support, and laminating the resulting film on a current collector.

[0068] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitation. It is particularly preferable for the separator to have low resistance to electrolyte ion movement and excellent electrolyte humidification capability. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and it can be used in a single-layer or multi-layer structure.

[0069] 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.

[0070] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0071] Examples of the non-aqueous organic solvent that may be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-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.

[0072] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, and therefore, these cyclic carbonates are more preferably used.

[0073] 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:

[0074] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0075] A secondary battery according to one embodiment of the present invention includes an assembly including a positive electrode, a negative electrode, a separator, and an electrolyte, and may be a lithium secondary battery.

[0076] A further embodiment of the present invention provides a battery module including the aforementioned secondary battery as a unit cell, and a battery pack including the same. The battery module and battery pack include the secondary battery having high capacity, excellent rate-limiting characteristics, and excellent cycle characteristics, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0077] The secondary battery according to the embodiment of the present invention stably exhibits excellent discharge capacity, output characteristics, and cycle performance, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, as well as portable devices such as mobile phones, laptops, and digital cameras. For example, the battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0078] In the following, preferred embodiments are presented to aid in understanding the present invention, but these embodiments are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present description and technical concept. Such changes and modifications are naturally intended to fall within the scope of the appended claims.

[0079] Examples 1 to 3 and Comparative Examples 1 to 6 <Cel production> Example 1 Positive electrode production A lithium composite transition metal compound containing nickel (Ni), cobalt (Co), and manganese (Mn) in an atomic ratio of 84:8:8 as the positive electrode active material and doped with aluminum (Al), a conductive material (CNT), and a binder (PVDF) were added to a methylpyrrolidone (NMP) solvent in a weight ratio of 97:1:2 to prepare a positive electrode slurry (the solid content of the positive electrode slurry was 70 parts by weight of the total positive electrode slurry).

[0080] The positive electrode slurry prepared above was applied onto an Al current collector, dried, and then rolled at room temperature to prepare a positive electrode.

[0081] Negative electrode production For the first negative electrode active material layer, a carbon-based active material (artificial graphite and natural graphite in a weight ratio of 8:2), a conductive material (carbon black), a binder (SBR), and a thickener (Li-CMC) were added to a distilled water solvent in a weight ratio of 96:1:2:1 to prepare a negative electrode slurry (the solid content of the negative electrode slurry was 50 parts by weight of the total negative electrode slurry).

[0082] For the second negative electrode active material layer, negative electrode active materials including Mg-doped SiO active material and carbon-based active material (including artificial graphite and natural graphite in an 8:2 weight ratio) (the Mg-doped SiO active material was 5 parts by weight based on 100 parts by weight of the total negative electrode active materials (10 parts by weight based on 100 parts by weight of the negative electrode active materials in the second negative electrode active material layer)), a conductive material (carbon black), a binder (SBR), and a thickener (Li-CMC) were added to a distilled water solvent in a weight ratio of 96:1:2:1 to prepare a negative electrode slurry (the solid content of the negative electrode slurry was 50 parts by weight of the total negative electrode slurry).

[0083] The first negative electrode active material layer slurry prepared above was applied onto a Cu current collector, and then the second negative electrode active material layer slurry was sequentially applied onto the first negative electrode active material layer. The first negative electrode active material layer and the second negative electrode active material layer were simultaneously dried and then rolled at room temperature to prepare a negative electrode.

[0084] Cell preparation A separator was interposed between the positive electrode and negative electrode prepared above, and the electrodes were assembled, and an electrolyte was poured into the electrodes, followed by activation to prepare a cell.

[0085] - Electrolyte composition: 1M LiPF6, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio 3 / 7), vinylene carbonate (VC) / propane sultone (PS, propane sultone) (contained in the electrolyte at 3 parts by weight and 1.5 parts by weight, respectively) - Activation: 0.1C, after 3 hours of charging, high temperature / room temperature aging, degas

[0086] Example 2 An electrode and a cell were produced in the same manner as in Example 1, except that the slurry thickener for the first negative electrode active material layer was made of Na-CMC instead of Li-CMC.

[0087] Example 3 An electrode and a cell were produced in the same manner as in Example 1, except that the slurry thickener for the second negative electrode active material layer was made of Na-CMC instead of Li-CMC.

[0088] Comparative Example 1 An electrode and a cell were produced in the same manner as in Example 1, except that the slurry thickener for the first negative electrode active material layer and the second negative electrode active material layer was made of Na-CMC instead of Li-CMC.

[0089] Comparative Example 2 An electrode and a cell were fabricated in the same manner as in Comparative Example 1, except that the negative electrode was fabricated using only a carbon-based active material as the negative electrode active material in both the first negative electrode active material layer slurry and the second negative electrode active material layer slurry.

[0090] Comparative Example 3 An electrode and a cell were produced in the same manner as in Comparative Example 6, except that the first negative electrode active material layer slurry of Comparative Example 6 was applied to a Cu current collector in a single layer (the thickness of the single layer was the same as the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer of Comparative Example 6), and the second negative electrode active material layer was not formed.

[0091] Comparative Example 4 An electrode and a cell were prepared in the same manner as in Comparative Example 6, except that the first negative electrode active material layer slurry in Comparative Example 6 contained Na-CMC instead of Li-CMC as a thickener, was applied to a Cu current collector in a single layer, and no second negative electrode active material layer was formed.

[0092] Comparative Example 5 An electrode and a cell were produced in the same manner as in Example 1, except that the slurry for the second negative electrode active material layer in Example 1 was first applied to a Cu current collector using the slurry for the first negative electrode active material layer, and the slurry for the first negative electrode active material layer in Example 1 was then applied to the first negative electrode active material layer using the slurry for the second negative electrode active material layer.

[0093] Comparative Example 6 An electrode and a cell were fabricated in the same manner as in Example 1, except that a negative electrode active material including an Mg-doped SiO active material and a carbon-based active material (including artificial graphite and natural graphite in a weight ratio of 8:2) was used instead of the carbon-based active material of the first negative electrode active material layer slurry, and the Mg-doped SiO active material contained in both the first negative electrode active material layer and the second negative electrode active material layer was included in an amount of 5 parts by weight based on 100 parts by weight of the total negative electrode active material.

[0094] Experimental Example 1. Cell resistance performance The fabricated cell was charged at 0.33C with constant current / constant voltage (CC / CV) up to 4.2V (0.05C cutoff) and discharged at 0.33C with constant current (CC) (2.5V cutoff) three times, and the third discharge capacity was measured. After the same charge, the cell was discharged at 0.33C to set the SOC at 50%, and then pulse discharged at 2.5C for 10 seconds to measure the resistance (initial resistance). The results are shown in Table 2 below.

[0095] Experimental Example 2: Cell Charging C-Rate Performance The fabricated cells were charged at constant current / constant voltage (CC / CV) up to 4.2 V at different C-rates (0.1 C / 2.0 C) (0.05 C cut) and discharged at constant current (CC) at 0.33 C (2.5 V cut) to measure the charge capacity at the C-rate. The cell charge C-rate performance (2.0 C charge capacity / 0.1 C charge capacity x 100) was calculated based on the measured charge capacity and is shown in Table 2.

[0096] Experimental example 3: High temperature cycle life performance of the cell The cells were cycled at a high temperature (45°C) by constant current / constant voltage (CC / CV) charging at 0.33 C up to 4.2 V (0.05 C cut), followed by constant current (CC) discharging at 0.33 C (2.5 V cut). After 200 cycles, the capacity was measured in the same manner as in Experimental Example 1, and the capacity retention rate (capacity after 400 cycles / initial capacity x 100%) was measured. The high-temperature cycle life performance of the cells of Examples 1 to 3 was measured to be 96.0%, 95.5%, and 94.9%, respectively.

[0097] [Table 1]

[0098] [Table 2]

[0099] As shown in Table 2, the cell resistance and C-rate performance of the example were superior to those of the comparative example. Furthermore, the cell manufactured in the example also maintained excellent lifespan.

Claims

1. A secondary battery comprising a negative electrode and a positive electrode, The aforementioned negative electrode is Current collector, A first negative electrode active material layer provided on the current collector, and A second negative electrode active layer provided on the first negative electrode active layer Includes, At least one of the first negative electrode active material layer and the second negative electrode active material layer comprises lithium-substituted carboxymethylcellulose. The first negative electrode active material layer contains a carbon-based active material, and only the second negative electrode active material layer contains a silicon-based active material. The lithium-substituted carboxymethylcellulose is included in an amount of 0.5 to 1.3 parts by weight based on 100 parts by weight of the first negative electrode active material layer or the second negative electrode active material layer containing it. The positive electrode comprises a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as active materials in a secondary battery.

2. The aforementioned silicon-based active material is SiO ｘ (0≦x<2), SiM ｙ The secondary battery according to claim 1, comprising (M is a metal, 1 ≤ y ≤ 4), and at least one of Si / C.

3. The secondary battery according to claim 1, wherein the carbon-based active material comprises at least one of artificial graphite and natural graphite.

4. The secondary battery according to claim 1, wherein the second negative electrode active material layer further comprises a carbon-based active material.

5. The secondary battery according to claim 4, wherein the carbon-based active material of the second negative electrode active material layer comprises at least one of artificial graphite and natural graphite.

6. The secondary battery according to claim 4, wherein the silicon-based active material is contained in an amount of 1 to 40 parts by weight, based on a total of 100 parts by weight of the active material contained in the second negative electrode active material layer containing the silicon-based active material.

7. The secondary battery according to claim 1, wherein the silicon-based active material includes a magnesium compound.

8. The first negative electrode active material layer and the second negative electrode active material layer each contain a conductive material, the conductive material each containing carbon black, and the secondary battery according to claim 1.

9. The sum of the weight of the first negative electrode active material layer and the weight of the second negative electrode active material layer is 170 to 280 mg / 25 cm ２ The secondary battery according to claim 1.

10. The secondary battery according to claim 1, wherein the lithium composite transition metal compound further comprises at least one of manganese and aluminum.

11. The secondary battery according to claim 1, wherein the cell resistance performance of the secondary battery is 1.85 Ω or less.

12. The secondary battery according to claim 1, wherein the cell charge C-rate performance of the secondary battery is 97% or more.

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