Anode and secondary battery

Abstract

The present invention relates to a negative electrode for a secondary battery, comprising: a 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, 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, and a secondary battery including the same.

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, Electric Vehicles) and hybrid electric vehicles (HEVs, Hybrid Electric Vehicles) that 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 the use of energy, and are therefore attracting attention as a new energy source that is environmentally friendly and can improve 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, etc. In addition, the electrodes such as the positive electrode and the negative electrode may have an electrode active material layer provided on a current collector.

[0006] As the use of secondary batteries increases, various battery performances are required. Attempts have been made to add additives to the active material layer in order to improve battery performance, but while some battery performances may be improved depending on the type of additive, other battery performances 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, which can provide a secondary battery having 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-mentioned negative electrode for secondary batteries, a positive electrode, and a separator. Effect of the Invention

[0010] The two-layer structure of the negative electrode active material layer containing the silicon-based active material and the addition of lithium-substituted carboxymethylcellulose produce a synergistic effect, thereby improving the charging performance of the battery and ensuring the battery life performance. In addition, since the silicon-based active material may have a lower electrical conductivity than the carbon-based active material, if the silicon-based active material is uniformly distributed in the thickness direction of the negative electrode active material layer (T in FIG. 1), it may cause unevenness in the entire electrode in terms of electrical conductivity, which may be disadvantageous in charging performance (fast charging). However, if the silicon-based active material is included only in the second negative electrode active material layer (202 in FIG. 1) in the two-layer structure, the electrical conductivity can be increased, thereby reducing the contact resistance between the two layers, and the effect can be maximized by applying lithium-substituted carboxymethylcellulose, which has high Li electric / ion mobility. When lithium-substituted carboxymethylcellulose is applied, the mobility of Li ions increases, so that the life durability, which is slightly inferior when the silicon-based active material is included only in the second negative electrode active material layer, can be ensured.

[0011] In particular, when charging at a high current during fast charging, the flux of lithium ions is relatively greater than that during normal charging (normal cycle). If the negative electrode active material can rapidly receive lithium ions when the lithium ion flux is large, the fast charging performance of the cell can be improved. Silicon-based active materials receive lithium ions through alloying and start charging at a lower potential, so they are more advantageous in improving fast charging performance than graphite-based negative electrode active materials that receive lithium ions through intercalation. In addition, by arranging the silicon-based active material close to the positive electrode, which is the direction that receives the lithium ion flux first, i.e., by arranging it in the upper layer (second negative electrode active material layer), the silicon-based active material can rapidly react with lithium ions to greatly improve fast charging performance. [Brief description of the drawings]

[0012] [Figure 1] 1 illustrates an anode according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention will be described in more detail below to help the 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 or words used in the present specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, but are to be construed as having meanings and concepts that correspond to the technical ideas of the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his / her invention.

[0014] As used herein, terms such as "comprise", "comprise", or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and are understood as not precluding 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" the other 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, a description referring only to a "negative electrode active material layer" without the expressions "first" and "second" can be applied to both the first negative electrode active material layer and the second negative electrode active material layer.

[0017] The negative electrode for secondary batteries according to one embodiment of the present specification includes a 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, wherein at least one of the first negative electrode active material layer and the second negative electrode active material layer includes lithium-substituted carboxymethylcellulose, the first negative electrode active material layer includes a carbon-based active material, and only the second negative electrode active material layer includes a silicon-based active material. In other words, the negative electrode for secondary batteries includes lithium-substituted carboxymethylcellulose together with two negative electrode active material layers. The distribution of the silicon-based active material can be confirmed through a SEM (scanning electron microscope) image of a cross-section of the electrode. Therefore, the region where the silicon-based active material exists 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 found that the lithium-substituted carboxymethylcellulose has an advantage in improving the charging performance and ensuring the life performance of a battery compared to carboxymethylcellulose partially substituted with sodium, and in particular, when the lithium-substituted carboxymethylcellulose is used together with a multi-layer structure of a negative electrode active material layer containing a silicon-based active material, the charging performance and life of the battery can be further maximized due to a synergistic effect, thereby completing the present invention.

[0020] The lithium-substituted carboxymethylcellulose may be included in both the first and second negative electrode active material layers, or may be included in one layer of the first or second negative electrode active material layers. For example, the lithium-substituted carboxymethylcellulose may be included in the second negative electrode active material layer more than in the first negative electrode active material layer, or may be included 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 may not contain sodium-substituted carboxymethylcellulose at all, but in other embodiments, they may further contain additional sodium-substituted carboxymethylcellulose. 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 the sodium-substituted carboxymethylcellulose are the only carboxymethylcellulose salts included 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. The silicon-based active material may be contained alone or in combination of two or more kinds.

[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, based on a total of 100 parts by weight of the active materials contained in the second negative electrode active material layer containing the silicon-based active material, the silicon-based active material may be contained in an amount of 1 part by weight to 40 parts by weight, for example, 2 parts by weight to 35 parts by weight, 3 parts by weight to 30 parts by weight, 5 parts by weight to 25 parts by weight, or 7 parts by weight to 15 parts by weight.

[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 chemical bonding.

[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 SiO 2 , 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 include at least one selected from the group consisting of Mg silicate, Mg silicide, and Mg oxide. 2 SiO 4 and MgSiO 3 The Mg silicide may include at least one of Mg 2 The Mg oxide may include MgO.

[0030] In one embodiment of the present specification, the Mg element may be included in an amount of 0.1 wt% to 20 wt%, or 0.1 wt% to 10 wt%, based on the total weight of the silicon-based active material being 100%. Specifically, the Mg element may be included in an amount of 0.5 wt% to 8 wt% or 0.8 wt% to 4 wt%. If the above range is satisfied, the Mg compound can be included in the silicon-based active material in an appropriate content, so that the volume change of the silicon-based 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.

[0031] The Li compound may include at least one selected from the group consisting of Li silicate, Li silicide, and Li oxide. 2 SiO 3 , Li 4 SiO 4 , and Li 2 S 2 O 5 The Li silicide may include at least one of Li 7 S 2 The Li oxide may include Li 2 It may contain O.

[0032] In one embodiment of the present invention, the Li compound may include a form of lithium silicate. The lithium silicate is Li a S 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 Li 2 SiO 3 , Li 4 SiO 4 and Li 2 Si 2 O 5 in the silicon-based composite particles. The amorphous lithium silicate may be in the form of Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), but is not limited to this 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 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, 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 volume change of the negative electrode 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.

[0034] The content of the Mg element or Li element can be confirmed by ICP (inductively coupled plasma) analysis. For the ICP analysis, a certain amount (about 0.01 g) of the negative active material is accurately taken and then 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 optical emission spectroscope (ICPAES, Perkin-Elmer 7300), the intensity of a standard solution prepared using a standard solution (5 mg / kg) at a wavelength specific to Mg element or Li element is measured to create a reference calibration curve. Then, the pretreated sample solution and the base sample are introduced into the instrument, and the intensity of each is measured to calculate the actual intensity, and the concentration of each component is calculated relative to the created calibration curve, and the Mg element or Li element content of the silicon-based active material manufactured can be analyzed by converting the total sum to a theoretical value.

[0035] In one embodiment of the present specification, a carbon layer may be provided on the surface and / or inside the pores of the silicon-based composite particles. The carbon layer imparts electrical conductivity to the silicon-based composite particles, and the initial efficiency, life characteristics, and battery capacity characteristics of a secondary battery including a negative electrode active material including the silicon-based composite particles may be improved. The total weight of the carbon layer may be 5% by weight to 40% by weight based on 100% by weight of the silicon-based composite particles in total.

[0036] 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 of the silicon-based composite particles (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 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 50The average particle size (D) can be defined as the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by the laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to about 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 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 a composite 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 of the carbon-based negative electrode active material, for example, 90 parts by weight to 99 parts by weight, or 93 parts by weight to 97 parts by weight. 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 or more and 99 parts by weight or less, for example, 75 to 98 parts by weight, or 85 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 in 100 parts by weight of the negative electrode active material layer 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.

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

[0043] The lithium-substituted carboxymethyl cellulose is a carboxymethyl cellulose having a lithium carboxymethyl group (-CH 2 The degree of substitution of hydroxy (-OH) groups with COOLi) may be 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 carboxymethylcellulose 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 carboxymethylcellulose may be included in a negative electrode active material layer including a silicon-based active material, or may be included in a negative electrode active material layer including only a carbon-based active material without including a silicon-based active material. The lithium-substituted carboxymethylcellulose may be included in a second negative electrode active material layer including 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 parts by weight, based on 100 parts by weight of the total weight of the first negative electrode active material layer and the second negative electrode active material layer. 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 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 the lithium-substituted carboxymethyl cellulose.

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

[0049] The negative electrode binder may play a role in improving the adhesion between the negative electrode active material particles and the adhesive strength between the negative electrode active material particles and the negative electrode current collector. The negative electrode binder may be any one known in the art, and non-limiting examples thereof 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, Ca, or the like, 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 include a conductive material, but may further include a conductive material if necessary. The conductive material included in the negative electrode active material layer is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity, 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 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 parts by weight to 20 parts by weight, preferably 0.03 parts by weight to 18 parts by weight, based on 100 parts by weight of the negative electrode active material layer.

[0052] For the purposes of the present invention, when the carbon-based active material is graphite, such as natural graphite or artificial graphite, the weight parts of 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 according to 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. Thus, 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 graphite used as the carbon-based negative electrode active material and the weight parts of 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. The sum of the thickness of the first negative electrode active material layer and the thickness of 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, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. Specifically, transition metals that adsorb carbon well, such as copper and nickel, can be used as the current collector. The thickness of the current collector can be 1 μm to 500 μm, but the thickness of the current collector 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 including the positive electrode active material. The positive electrode active material layer may have a thickness of 20 μm or more and 500 μm or less.

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

[0059] In one embodiment of the present specification, the positive electrode may include a lithium composite transition metal compound including 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 nickel in an amount of 80 mol % or more, for example, 80 mol % or more and less than 100 mol %, among metals other than lithium.

[0060] In one embodiment, the positive electrode active material in 100 parts by weight of the positive electrode active material layer 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.

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

[0062] The positive electrode binder may play a role in improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material particles and the positive electrode current collector. As the positive electrode binder, those known in the art may be used, and non-limiting examples 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, or various copolymers thereof, and one or more of these 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, and can be used without any particular limitation as long as it does not undergo chemical changes in the battery 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, carbon fiber, and carbon nanotubes; 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; or conductive polymers such as polyphenylene derivatives, and the like. One of these may be used alone or a mixture of two or more of them may be used. Specifically, in one embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNT); and multi-walled carbon nanotubes (MWCNT).

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

[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 Here, the weight is based on the weight (solid content) after drying excluding the solvent. This range is advantageous for high energy density and rapid charging characteristics.

[0067] The positive electrode and the negative electrode may be manufactured according to a typical method for manufacturing a positive electrode and a negative electrode, except for using the positive electrode active material and the negative electrode active material. Specifically, the active material layer forming composition containing the active material and, optionally, a binder and a conductive material may be applied to a current collector, followed by drying and rolling. In this case, the types and contents of the positive electrode and negative electrode active materials, the binder, and the 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 one or more of these may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the active material, the conductive material, and the binder in consideration of the coating thickness of the slurry and the manufacturing yield, and to have a viscosity that can show excellent thickness uniformity when applied to manufacture the positive electrode and the negative electrode. As another method, 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 off 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 passage for lithium ions to move. Any separator that is generally used in secondary batteries can be used without any particular limitation. In particular, it is preferable that the separator has low resistance to the ion movement of the electrolyte and excellent electrolyte humidification ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymeric material may be used to ensure heat resistance or mechanical strength, and may be selectively used in a single layer or multilayer structure.

[0069] Examples of the electrolyte include 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 the manufacture of lithium secondary batteries, but are not limited to these.

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

[0071] Examples of the non-aqueous organic solvent 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, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

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

[0073] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte. For example, the anion of the lithium salt may be F - , Cl - , I - , NO 3 - , N(CN) 2 - , B.F. 4 - , ClO 4 - , P.F. 6 - , (CF 3 ) 2 PF 4 - , (CF 3 )3 PF 3 - , (CF 3 ) 4 PF 2 - , (CF 3 ) 5 PF - , (CF 3 ) 6 P - , C.F. 3 SO 3 - , C.F. 3 CF 2 SO 3 - , (CF 3 SO 2 ) 2 N - , (FSO 2 ) 2 N - , C.F. 3 CF 2 (CF 3 ) 2 CO - , (CF 3 SO 2 ) 2 CH - ,(SCIENCE FICTION 5 ) 3 C - , (CF 3 SO 2 ) 3 C - , C.F. 3 (CF 2 ) 7 SO 3 - , C.F. 3 CO 2 - , C.H. 3 CO 2 - , SCN - and (CF 3 CF 2 SO 2 ) 2 N - 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, and 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, which may be a lithium secondary battery.

[0076] A further embodiment of the present invention provides a battery module including the above-mentioned 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-limiting characteristics and high cycle characteristics, and therefore can be used as a power source for a medium- to large-sized device 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, notebook computers, 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 the understanding of the present invention, but the embodiments are merely illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical ideas of the present invention. Such changes and modifications are naturally 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 a 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 (containing 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 a weight ratio of 8:2) (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 included 50 parts by weight of the total negative electrode slurry).

[0083] The first negative electrode active material layer slurry prepared as above was applied onto a Cu current collector, and the second negative electrode active material layer slurry was then applied onto the first negative electrode active material layer in sequence. 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, an electrolyte was poured into the electrodes, and then the electrodes were activated to prepare a cell.

[0085] - Electrolyte composition: 1M LiPF 6 , 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, 3 hours charging, high temperature / normal 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 produced in the same manner as in Comparative Example 1, except that in both the first negative electrode active material layer slurry and the second negative electrode active material layer slurry, a negative electrode was produced using only a carbon-based active material as the negative electrode active material.

[0090] Comparative Example 3 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 an electrode and a cell were produced in the same manner as in Comparative Example 6, except that 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 in a single layer to a Cu current collector, and a second negative electrode active material layer was not 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 as a slurry for a 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 as a slurry for a 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 a 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 included 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 prepared cell was charged at 0.33C up to 4.2V with constant current / constant voltage (CC / CV) (0.05C cut) and discharged at 0.33C with constant current (CC) (2.5V cut) three times to measure the third discharge capacity. After the above charge, the cell was set to SOC50% with 0.33C discharge, and 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 4.2V at different C rates (0.1C / 2.0C) (0.05C cut) and discharged at constant current (CC) of 0.33C (2.5V cut) to measure the charge capacity according to the C rate. The cell charge C rate performance (2.0C charge capacity / 0.1C 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 cells The cells were cycled at high temperature (45°C) by constant current / constant voltage (CC / CV) charging (0.05C cut) at 0.33C to 4.2V, and constant current (CC) discharging (2.5V cut) at 0.33C. 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 performances of the cells of Examples 1 to 3 were measured to be 96.0%, 95.5%, and 94.9%, respectively.

[0097] [Table 1]

[0098] [Table 2]

[0099] As shown in Table 2, it was confirmed that the cell resistance performance and cell charge C-rate performance of the embodiment are superior to those of the comparative example. Furthermore, the cell manufactured in the embodiment also maintained excellent life performance.

Claims

1. A 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 are included, wherein the first negative electrode active material layer and the second negative electrode active material layer contain 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, a negative electrode for a secondary battery.

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

3. The negative electrode for a secondary battery according to Claim 1, wherein the carbon-based active material contains at least one of artificial graphite and natural graphite.

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

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

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

7. 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 negative electrode for a secondary battery according to claim 1, which is such.

8. A secondary battery including the negative electrode for a secondary battery according to any one of Claims 1 to 7, a positive electrode, and a separator.

9. The secondary battery according to Claim 8, wherein the positive electrode contains a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as active materials.

10. The secondary battery according to Claim 9, wherein the lithium composite transition metal compound further contains at least one of manganese and aluminum.

11. The secondary battery according to Claim 8, wherein the cell resistance performance of the secondary battery is 1.85 Ω or less.

12. The secondary battery according to Claim 8, wherein the cell charge C-rate performance of the secondary battery is 97% or more.

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