Negative electrode and secondary battery

Abstract

The present invention relates to a negative electrode for a secondary battery and a secondary battery including the same, the negative electrode including 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 the porosity of the second negative electrode active material layer is 5% to 20% greater than the porosity of the first negative electrode active material layer.

Description

Technical Field

[0001] This specification claims the benefit of the filing date of Korean Patent Application No. 10-2022-0131727, filed with the Korean Intellectual Property Office on October 13, 2022, and all contents disclosed in the document of the Korean patent application are included herein.

[0002] The present invention relates to a negative electrode for a secondary battery and a secondary battery including the same.

Background Art

[0003] Secondary batteries are widely applied not only to portable devices but also to electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., which are driven by an electric drive source.

[0004] Such secondary batteries have not only the primary advantage of being able to significantly reduce the use of fossil fuels but also the advantage of generating no by-products due to the use of energy, so they are environmentally friendly and are attracting attention as a new energy source for improving energy efficiency.

[0005] Generally, a secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Further, an electrode such as a positive electrode or a negative electrode may have an electrode active material layer provided on a current collector.

[0006] As the utilization of secondary batteries increases, various battery performances are required. In order to improve battery performance, attempts have been made to control the composition of the active material or additive in the positive electrode active material layer or the negative electrode active material layer, but an incorrect combination of materials may rather have an adverse effect on the performance of the final battery. For this reason, research on improving battery performance by the combination of materials in the positive electrode and the negative electrode is needed.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention aims to provide a negative electrode for a secondary battery and a secondary battery including the same, which can provide a secondary battery with improved rapid charging characteristics.

Means for Solving the Problems

[0008] One embodiment of the present invention is 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 including and providing a negative electrode for a secondary battery, wherein the porosity of the second negative electrode active material layer is 5% to 20% larger than the porosity of the first negative electrode active material layer.

[0009] Another embodiment of the present invention provides a secondary battery including the negative electrode for a secondary battery, a positive electrode, and a separator.

Advantages of the Invention

[0010] According to the embodiments described in this specification, by adjusting the porosity of the two layers in the two-layer structure of the negative electrode active material layer to be different, the rapid charging performance can be improved. Specifically, the higher the porosity, the more advantageous it is for rapid charging. However, if the porosity of the upper layer (the second negative electrode active material layer) is higher than that of the lower layer (the first negative electrode active material layer), although the overall porosity is the same, the porosity of the portion closer to the electrolyte will be high, showing an improved rapid charging effect.

Brief Description of the Drawings

[0011]

Figure 1

Modes for Carrying Out the Invention

[0012] Hereinafter, to assist in understanding the present invention, the present invention will be described in more detail. The present invention can be embodied in various different forms and is not limited to the embodiments described herein. Here, terms or words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor, in accordance with the principle that the concept of terms can be appropriately defined in order to best explain his invention, is construed in a meaning and concept that conforms to the technical idea of the present invention.

[0013] In this specification, terms such as "comprising", "including", or "having" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and are understood not to preclude the presence or addition of one or other other features, numbers, steps, components, or combinations thereof in advance.

[0014] Also, when a certain part such as a layer is "on" another part, this includes not only the case where it is "directly on" another part, but also the case where there is another part in between. Conversely, when a certain part is "directly on" another part, it means that there is no other part in between. Note that being "on" the reference part refers to being located above or below the reference part, and does not necessarily mean being located "upward" in the opposite direction of gravity.

[0015] In this specification, an explanation that only mentions the "negative electrode active material layer" without the first and second expressions can be applied to both the first negative electrode active material layer and the second negative electrode active material layer.

[0016] The negative electrode for a secondary battery according to an embodiment of this 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, and is characterized in that the porosity of the second negative electrode active material layer is 5% to 20% higher than that of the first negative electrode active material layer.

[0017] If the porosity of the second negative electrode active material layer is higher than that of the first negative electrode active material layer, although the overall porosity is the same, it will have the effect of a higher porosity in the part closer to the electrolyte, and an improved rapid filling effect can be achieved.

[0018] According to one embodiment, the porosity of the second negative electrode active material layer may be 5% to 20%, for example, 10% to 20% larger than the porosity of the first negative electrode active material layer. The larger the porosity difference, the better the rapid charging performance may be theoretically. However, if the porosity difference is too large, it may have an adverse effect on the cell life and durability.

[0019] According to one embodiment, the porosity of the first negative electrode active material layer and the porosity of the second negative electrode active material layer may each be 15% to 40%.

[0020] According to one embodiment, the first negative electrode active material layer and the second negative electrode active material layer may include one or more of a dot-shaped conductive material, a linear conductive material, and a planar conductive material as a conductive material. The dot-shaped conductive material, linear conductive material, and / or planar conductive material included in the first negative electrode active material layer and the second negative electrode active material layer may use the same type or different types from each other.

[0021] As the dot-shaped conductive material, for example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc. can be used.

[0022] As the linear conductive material, conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); metal powders such as fluorocarbons, aluminum, and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

[0023] Examples of the planar conductive material include graphene and the like.

[0024] According to one embodiment, the negative electrode conductive material may be in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of each of the first negative electrode active material layer and the second negative electrode active material layer.

[0025] The first negative electrode active material layer and the second negative electrode active material layer contain a negative electrode active material, and the negative electrode active material may include a silicon-based active material and a carbon-based active material. The carbon-based active material may include artificial graphite and / or natural graphite.

[0026] 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 at least one of Si / C. The silicon-based active material may include only one type, or two or more types may be included together. When both of the two-layer negative electrode active material layers contain a silicon-based active material, the same type of silicon-based active material may be used for the two-layer active material layers, or different types or different combinations of silicon-based active materials may be used.

[0027] In one embodiment of the present specification, the first negative electrode active material layer and the second negative electrode active material layer may contain the silicon-based active material in an amount of 1 to 40 parts by weight, for example, 1 to 20 parts by weight, based on 100 parts by weight in total of the negative electrode active material.

[0028] The active material containing SiO x (0 ≦ x < 2) may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0029] The SiO x (0 < x < 2) corresponds to a matrix within 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 within (0 < x < 2). When the silicon-based composite particles contain x (0 < x < 2), the discharge capacity of the secondary battery can be improved. x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0030] 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 within the silicon-based composite particles.

[0031] The Mg compound and / or the Li compound may be present inside and / or on the surface of x (0 < x < 2). The initial efficiency of the battery can be improved by the Mg compound and / or the Li compound.

[0032] 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.

[0033] In one embodiment of the present specification, the Mg element may be contained in an amount of 0.1 wt% to 20 wt%, or 0.1 wt% to 10 wt% based on 100 wt% of the total of the silicon-based active material. Specifically, the Mg element may be contained in an amount of 0.5 wt% to 8 wt% or 0.8 wt% to 4 wt%. When the above range is satisfied, the Mg compound can be contained in an appropriate content within the silicon-based active material, 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 the initial efficiency of the battery can be improved.

[0034] 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.

[0035] In one embodiment of the present invention, the Li compound may contain the form of lithium silicate. The lithium silicate is represented by 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 within the silicon-based composite particles, and 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), and is not limited to this form.

[0036] In one embodiment of this specification, the Li element may be contained in an amount of 0.1 wt% to 20 wt%, or may be contained in an amount of 0.1 wt% to 10 wt% based on 100 wt% of the total silicon-based active material. Specifically, the Li element may be contained in an amount of 0.5 wt% to 8 wt%, and more specifically, may be contained in an amount of 0.5 wt% to 4 wt%. When the above range is satisfied, the Li compound can be contained in an appropriate content within 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.

[0037] The content of the Mg element or Li element can be confirmed by ICP (Inductively Coupled Plasma) analysis. For the ICP analysis, after accurately weighing a certain amount (about 0.01 g) of the negative electrode active material, 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 optical 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. After that, the pretreated sample solution and the substrate 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 above, the content of the Mg element or Li element in the silicon-based active material manufactured by conversion so that the total sum becomes the theoretical value can be analyzed.

[0038] 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 can impart conductivity to the silicon-based composite particles, and the initial efficiency, life characteristics, and battery capacity characteristics of a secondary battery including the negative electrode active material containing the silicon-based composite particles can be improved. The total weight of the carbon layer may be included at 5 wt% to 40 wt% based on 100 wt% in total of the silicon-based composite particles.

[0039] In one embodiment of the present specification, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0040] The average particle size (D 50 ) of the silicon-based active material is 2 μm to 15 μm, specifically 3 μm to 12 μm, and more specifically may be 4 μm to 10 μm. When the above range is satisfied, the side reaction between the silicon-based composite particles and the electrolyte can be controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0041] In the present specification, the average particle size (D 50) can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The average particle size (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes ranging from the submicron region to about several millimeters, and can obtain highly reproducible and highly resolved results.

[0042] The active material containing Si / C as the silicon-based active material is a composite of Si and C, and is distinguished from silicon carbide denoted as SiC. The silicon-carbon composite may be one in which silicon and graphite are compounded, or may form a structure in which a core in which silicon and graphite are compounded is surrounded by graphene or amorphous carbon. In the silicon-carbon composite, the silicon may be nanosilicon.

[0043] In one embodiment of the present specification, the artificial graphite and the natural graphite may be contained in an amount of 60 parts by weight or more and 99 parts by weight or less based on 100 parts by weight of the negative electrode active material. The artificial graphite and the natural graphite may be contained in a weight ratio of 1:9 to 9:1, for example, 2:8 to 8:2.

[0044] In one embodiment of the present specification, the negative electrode active material in each 100 parts by weight of the first negative electrode active material layer and the second negative 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 80 parts by weight or more and 90 parts by weight or less.

[0045] 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 negative electrode active material.

[0046] As the negative electrode binder, it can play a role in improving the adhesion between negative electrode active material particles and the adhesive force between negative electrode active material particles and the negative electrode current collector. As the negative electrode binder, those known in the art can be used. Non-limiting examples include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, poly acrylic acid, and substances in which their hydrogens are substituted with Li, Na, or Ca, etc. It may contain at least one selected from the group consisting of them, and may also contain various copolymers thereof.

[0047] The negative electrode binder may be contained in an amount of 0.1 part by weight or more and 20 parts by weight or less based on 100 parts by weight of the negative electrode active material layer. For example, preferably it is contained in an amount of 0.3 part by weight or more and 20 parts by weight or less, and more preferably 0.5 part by weight or more and 10 parts by weight or less.

[0048] In one embodiment of this specification, the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer may each be 5 μm or more and 100 μm or less, for example, 10 μm or more and 70 μm or less.

[0049] In one embodiment of the present specification, the negative electrode current collector may be any material as long as it has conductivity without inducing a chemical change in the battery, and is not particularly limited. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. Specifically, a transition metal that can effectively adsorb 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 the thickness of the current collector is not limited thereto.

[0050] A further embodiment of the present specification provides a secondary battery including a negative electrode, a positive electrode, and a separator according to the foregoing embodiments.

[0051] 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 thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

[0052] The positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. Further, the positive electrode current collector may usually have a thickness of 1 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0053] 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 active materials. The lithium composite transition metal compound may further include at least one of manganese and aluminum. The lithium composite transition metal compound may contain 80 mol% or more, for example, 80 mol% or more and less than 100 mol% of nickel among the metals excluding lithium.

[0054] 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, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and still more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0055] According to a further embodiment of the present specification, the positive electrode active material layer according to the foregoing embodiment may further include a positive electrode binder and a conductive material.

[0056] The positive electrode binder can play a role in improving the adhesion between positive electrode active material particles and the adhesive force between the positive electrode active material particles and the positive electrode current collector. As the positive electrode binder, those known in the art can be used. 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. Among these, one kind alone or a mixture of two or more kinds may be used.

[0057] The positive electrode binder may be contained in an amount of 0.1 parts by weight or more and 50 parts by weight or less based on 100 parts by weight of the positive electrode active material layer. For example, it is preferably contained in an amount of 0.3 parts by weight or more and 35 parts by weight or less, and more preferably 0.5 parts by weight or more and 20 parts by weight or less.

[0058] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode, and can be used without particular limitation as long as it has electron conductivity without causing a chemical change in the battery. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers 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. Among these, one kind alone or a mixture of two or more kinds may be used.

[0059] Specifically, in one embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The conductive material may be contained in an amount of 0.1 parts by weight or more and 2 parts by weight or less based on 100 parts by weight of the composition for the positive electrode active material layer. For example, it is preferably contained in an amount of 0.3 parts by weight or more and 1.5 parts by weight or less, and more preferably 0.5 parts by weight or more and 1.2 parts by weight or less.

[0060] The positive electrode and the negative electrode can be manufactured according to the normal manufacturing methods of positive and negative electrodes, except for using the above-mentioned positive electrode active material and negative electrode active material. Specifically, it can be manufactured by applying a composition for forming an active material layer containing the above-mentioned active material and optionally a binder and a conductive material onto a current collector, and then drying and rolling. At this time, the types and contents of the positive electrode and negative electrode active materials, binder, and conductive material are as described above. The solvent may be a solvent generally used in the technical field, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of the solvent used is such that it dissolves or disperses the active material, conductive material, and binder in consideration of the coating thickness of the slurry and the manufacturing yield, and then has a viscosity that can exhibit excellent thickness uniformity during coating for manufacturing the positive and negative electrodes. Also, as another method, the positive electrode and the negative electrode may be manufactured by casting the composition for forming the active material layer on another support, and then laminating the film obtained by peeling from the support onto the current collector.

[0061] A further embodiment of this specification provides a method for manufacturing a negative electrode for a secondary battery according to the above-described embodiment. The manufacturing method is as follows: Coating a first composition containing a negative electrode active material and a conductive material on a current collector, and rolling to form a first negative electrode active material layer; and Coating a second composition containing a negative electrode active material and a conductive material on the first negative electrode active material layer, rolling, and forming a second negative electrode active material layer having a porosity 5% to 20% larger than the porosity of the first negative electrode active material layer.

[0062] Together with the specific types of active materials and conductive materials in each of the aforementioned negative electrode active material layers, by performing separate rolling during the formation of each negative electrode active material layer by the manufacturing method, the porosity of the first negative electrode active material layer can be made different from that of the second negative electrode active material layer, specifically, smaller. For example, after coating the first negative electrode active material layer, rolling is carried out to match the target porosity, and then, after coating the second negative electrode active material layer, rolling is secondarily carried out to match the target porosity, and then drying can be advanced in a batch manner.

[0063] As the separator, it separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Usually, it can be used without particular limitation as long as it is used as a separator in a secondary battery. In particular, it is preferably low in resistance to the ion migration of the electrolyte while being excellent in the ability to hold the electrolyte solution. Specifically, a porous polymer film, for example, a porous polymer film made from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance can also be used, and it may be selectively used in a single-layer or multi-layer structure.

[0064] 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 during the manufacture of lithium secondary batteries.

[0065] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0066] 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, triester phosphate, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, etc. may be used.

[0067] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well, so they are preferably used. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate at an appropriate ratio and used, an electrolyte having high electrical conductivity can be produced and can be more preferably used.

[0068] As the metal salt, a lithium salt can be used. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte. For example, as the anion of the lithium salt, 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 can be used.

[0069] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc.

[0070] The 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 this may be a lithium secondary battery.

[0071] 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. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics, they can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0072] The secondary battery according to an embodiment of the present invention can be used as a power source for not only portable devices such as mobile phones, notebook computers, and digital cameras, but also medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, in order to stably exhibit excellent discharge capacity, output characteristics, and cycle performance. For example, the battery module or battery pack can be used as a power source for one or more medium and large-sized devices among power tools; electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); or power storage systems.

[0073] Hereinafter, preferred examples are presented to assist in understanding the present invention. However, it is obvious to those skilled in the art that the examples are illustrative of the description and various changes and modifications are possible within the scope of the description and the scope of the technical idea. It is natural that such variations and modifications belong to the appended claims.

[0074] Example 1 After coating the negative electrode current collector with the first negative electrode active material layer to a thickness of about 50 μm, cold rolling was performed at room temperature. Next, after coating the second negative electrode active material layer to a thickness of about 50 μm, cold rolling was performed at room temperature. After rolling, batch drying was carried out. The rolling of each layer was carried out so as to have the porosity described in Table 1 below.

[0075] As a composition for manufacturing the first and second negative electrode active material layers, a composition containing a negative electrode active material including artificial graphite, natural graphite, and SiO, a conductive material (including a linear conductive material (CNT) and a planar conductive material (graphene)), a binder (SBR), and a thickener in a weight ratio of 94:1:2:1 was used.

[0076] The porosity of each negative electrode active material layer is shown in Table 1 below. The measurement of the porosity was performed by cross-sectional image analysis and ratio calculation.

[0077] Li on the positive current collector 1.0 Ni 0.86 Co 0.08 Mn 0.06 A composition containing NiCoMnO2, a conductive material (CNT), and a binder (PVDF) in a weight ratio of 97:1:2 was coated, and then a positive electrode was manufactured by drying and rolling.

[0078] The positive electrode and the negative electrode were laminated with a separator in between, and an electrolyte was injected to fabricate a battery. The electrolyte contains 1M LiPF6, EC (ethylene carbonate) / EMC (ethyl methyl carbonate) (3 / 7 volume ratio), 1.5 wt% VC (vinylene carbonate), and 0.5 wt% PS (propane sultone).

[0079] Comparative Example 1 It was carried out in the same manner as in Example 1, except that the first negative electrode active material layer and the second negative electrode active material layer were simultaneously coated with a total thickness of about 100 μm and then roll-pressed and dried together.

[0080]

Table 1

[0081] The depth of chargeable SOC by rate of the negative electrodes of the batteries manufactured in the examples and comparative examples is shown in FIG. 1. According to FIG. 1, it was confirmed that the battery manufactured in Example 1 shows a higher charging depth compared to Comparative Example 1.

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, wherein the porosity of the second negative electrode active material layer is 5% to 20% greater than the porosity of the first negative electrode active material layer, and the negative electrode for a secondary battery.

2. The negative electrode for a secondary battery according to claim 1, wherein the porosity of the second negative electrode active material layer is 10% to 20% greater than the porosity of the first negative electrode active material layer.

3. The negative electrode for a secondary battery according to claim 1, wherein the porosity of the first negative electrode active material layer and the porosity of the second negative electrode active material layer are each 15% to 40%.

4. The first negative electrode active material layer and the second negative electrode active material layer contain a negative electrode active material, the negative electrode active material includes a silicon-based active material, artificial graphite, and natural graphite, and 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.

5. The negative electrode for a secondary battery according to claim 4, wherein the silicon-based active material is contained in an amount of 1 part by weight to 40 parts by weight based on 100 parts by weight in total of the active materials contained in each of the first negative electrode active material layer and the second negative electrode active material layer.

6. The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode active material layer and the second negative electrode active material layer contain a conductive material, and the conductive material contains at least one of a dot-shaped conductive material, a linear conductive material, and a planar conductive material.

7. The negative electrode for a secondary battery according to claim 6, wherein the conductive material is contained in an amount of 0.1 part by weight to 5 parts by weight based on 100 parts by weight in each of the first negative electrode active material layer and the second negative electrode active material layer.

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. A manufacturing method for manufacturing the negative electrode for a secondary battery according to any one of claims 1 to 7, comprising coating a first composition containing a negative electrode active material and a conductive material on a current collector and rolling to form a first negative electrode active material layer, and coating a second composition containing a negative electrode active material and a conductive material on the first negative electrode active material layer, rolling, and forming a second negative electrode active material layer having a porosity 5% to 20% greater than the porosity of the first negative electrode active material layer.

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