CN118661281A - Negative electrode and secondary battery - Google Patents

Abstract

The present invention relates to a negative electrode for a secondary battery and a secondary battery comprising the negative electrode, wherein the negative electrode comprises: a current collector; a first negative electrode active material layer, which is arranged on the current collector and contains a silicon-carbon composite; and a second negative electrode active material layer, which is arranged on the first negative electrode active material layer and contains silicon oxide.

Description

Negative electrode and secondary battery

[ Technical field ]

The present application claims priority and benefit from korean patent application No. 10-2022-013514 filed on 10 month 14 of 2022 and korean patent application No. 10-2023-0135580 filed on 10 month 12 of 2023, which are incorporated herein by reference in their entireties.

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

[ Background Art ]

The secondary battery is generally applied not only to a portable device but also to an Electric Vehicle (EV) or a Hybrid Electric Vehicle (HEV) driven by an electric drive source.

Since the secondary battery has a major advantage in that the use of fossil fuel can be greatly reduced and an advantage in that no by-product is generated by using energy, the secondary battery has been attracting attention as a novel energy source for improving the eco-friendliness and energy efficiency.

In general, a secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, an electrolyte, and the like. The positive electrode, the negative electrode, and the like may be provided with an electrode active material layer on the current collector.

As the utilization of secondary batteries increases, various battery performances are required. In order to improve battery performance, attempts have been made to appropriately constitute the electrode active material layer. Some of the performance of the battery may be improved according to the selection or combination of materials, but some of the performance may be deteriorated instead. Therefore, it is necessary to study how to select or combine appropriate materials according to the performance of the secondary battery to be improved.

[ Summary of the invention ]

[ Technical problem ]

The present invention is directed to providing a negative electrode capable of providing a secondary battery having a rapid charge performance and an improved lifetime, and a secondary battery including the same.

Technical scheme

An exemplary embodiment of the present invention provides a negative electrode for a secondary battery, including: a current collector; a first anode active material layer disposed on the current collector and containing a silicon-carbon composite; and a second anode active material layer that is provided on the first anode active material layer and contains silicon oxide, wherein a ratio (D _50,2nd/D_50,1st) of an average particle diameter (D _50,2nd) of the second anode active material layer to an average particle diameter (D _50,1st) of the first anode active material layer is 0.75 to 1.5.

Another exemplary embodiment of the present invention provides a secondary battery including the above-described negative electrode, positive electrode, and separator for a secondary battery.

[ Advantageous effects ]

According to the exemplary embodiments described in the present specification, disposing the silicon-carbon composite and the silicon oxide having different efficiencies and expansion characteristics in the silicon-based active material at specific positions in the two anode active material layers may provide a secondary battery having excellent rapid charge performance and improved lifetime.

Detailed description of the preferred embodiments

The present invention will be described in more detail below to aid in understanding the present invention. The present invention may be embodied in a variety of different forms and is not limited to the exemplary embodiments described herein. The terms or words used throughout the specification and claims should not be construed as limited to common meanings or dictionary meanings, but interpreted as having meanings and concepts consistent with technical ideas of the present invention based on the principle that the inventor can properly define terms or concepts of the terms to best explain the present invention.

It will be further understood that the terms "comprises," "comprising," "includes" and/or "having," when used in this specification, specify the presence of stated features, integers, steps, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components, and/or groups thereof.

Furthermore, it will be understood that when an element such as a layer is referred to as being "on" another element, it can be "directly on" the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present. Further, when an element is referred to as being "on" a reference portion, the element may be located above or below the reference portion, and does not necessarily mean that the element is located "above" in a direction opposite to gravitational force.

In this specification, a description called only "active material layer" without first and second expressions may be applied to both the first active material layer and the second active material layer.

The negative electrode for a secondary battery according to an exemplary embodiment of the present specification includes: a current collector; a first anode active material layer disposed on the current collector and containing a silicon-carbon composite; and a second anode active material layer that is provided on the first anode active material layer and contains silicon oxide, wherein a ratio (D _50,2nd/D_50,1st) of an average particle diameter (D _50,2nd) of the second anode active material layer to an average particle diameter (D _50,1st) of the first anode active material layer is 0.75 to 1.5.

Recently, in order to improve rapid charging, which is the greatest requirement of an automotive battery, attempts have been made to design the battery by disposing a silicon-based active material in the negative electrode. However, the present inventors found that when a silicon-based active material having a large expansion is disposed on the anode upper layer, the use of the upper layer is dense during charge and discharge, resulting in significant deterioration of the silicon-based active material of the upper layer and poor life characteristics. Accordingly, the present inventors have found that when two silicon-based active materials having different efficiency and expansion characteristics are used and disposed at specific positions in two anode active material layers, a battery having excellent rapid charge and improved life characteristics can be provided, and completed the present invention.

Specifically, the anode for a secondary battery of the above-described exemplary embodiment includes a silicon-carbon composite among silicon-based active materials in a first anode active material layer disposed adjacent to a current collector, and includes silicon oxide in a second anode active material layer disposed away from the current collector. Although silicon oxide is relatively inefficient, it has longer lifetime characteristics, while silicon carbon composites have higher efficiency characteristics. Therefore, when the silicon oxide and the silicon carbon composite are disposed on the upper and lower layers of the two anode active material layers, respectively, excellent quick charge performance and improved life characteristics can be obtained.

Further, the ratio (D _50,2nd/D_50,1st) of the average particle diameter (D _50,2nd) of the second anode active material layer to the average particle diameter (D _50,1st) of the first anode active material layer may be 0.75 to 1.5, 0.9 to 1.1, or 0.9 to 1.

If the average particle diameter of the first anode active material layer and the average particle diameter of the second anode active material layer deviate from the above ranges, the pore structure is not uniform, thereby impairing the mobility of lithium ions, and the reactivity of the first anode active material layer and the second anode active material layer is different, adversely affecting the battery life.

According to one exemplary embodiment, the discharge efficiency of the silicon carbon composite contained in the first anode active material layer is 85% to 95%. Further, the discharge efficiency of the silicon oxide contained in the second anode active material layer is 75% to 85%.

The discharge efficiency of the anode active material can be measured in the following manner.

First, a negative electrode active material, super-C (registered trademark) as a conductive material, carboxymethyl cellulose (CMC) as a thickener, and Styrene Butadiene Rubber (SBR) binder polymer were added to water at a weight ratio of 95:1:1:3 to prepare a negative electrode slurry. Copper foil was coated with the negative electrode slurry, punched and rolled to an area of 1.4875cm ², and then dried to prepare a negative electrode. A negative electrode and lithium metal as a counter electrode were used to prepare an electrode assembly, and a polypropylene separator was interposed between the two electrodes. The nonaqueous electrolytic solution was prepared by adding 1M LiPF ₆ to an organic solvent in which ethylene carbonate and methylethyl carbonate were mixed in a volume ratio of 3:7 and ethylene carbonate was added to a concentration of 1 wt%. Then, a nonaqueous electrolytic solution was injected into the electrode assembly to prepare a coin-type secondary half cell (coin half cell; CHC).

When the negative electrode coin-type secondary half cell (CHC) prepared as described above was charged, a constant current was applied to 0.005V at a rate of 0.2C by a constant current constant voltage (CC-CV) method, and then a current was controlled at a constant voltage of 0.005V. During discharge, the discharge efficiency can be measured by a Constant Current (CC) method at a rate of 0.2C cut off at 1.5V. The discharge efficiency can be calculated as discharge capacity/charge capacity×100.

In the present specification, the silicon-carbon composite is a composite of Si and C, and may be represented as a Si/C-based active material, unlike silicon carbide represented as SiC. The silicon-carbon composite may be a composite of silicon and graphite or the like, and may form a structure in which a core composite of silicon and graphite or the like is surrounded by graphene or amorphous carbon or the like. In the silicon-carbon composite, the silicon may be nano-silicon.

In the present specification, silicon oxide may be represented as SiO _x (0.ltoreq.x < 2). The silicon oxide may be a silicon-based composite particle containing SiO _x (0 < x < 2) and voids.

SiO _x (0 < x < 2) corresponds to the matrix in the silicon-based composite particles. SiO _x (0 < x < 2) may be in a form comprising Si and SiO ₂, si may form a phase. That is, x corresponds to the ratio of the number of O to Si contained in SiO _x (0 < x < 2). When the silicon-based composite particles include SiO _x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

The silicon-based composite particles may further include at least one of Mg compound and Li compound. The Mg compound and the Li compound may correspond to the matrix in the silicon-based composite particle.

Mg compounds and/or Li compounds may be present inside SiO _x (0 < x < 2) and/or on the surface of SiO _x (0 < x < 2). Mg compounds and/or Li compounds may improve the initial efficiency of the battery.

The Mg compound may include at least one selected from the group consisting of magnesium silicate, magnesium silicide, and magnesium oxide. The magnesium silicate may include at least one of Mg ₂SiO₄ and MgSiO ₃. The magnesium silicide may include Mg ₂ Si. The magnesium oxide may include MgO.

In one exemplary embodiment of the present specification, the Mg element may be contained in an amount of 0.1 to 20wt%, or 0.1 to 10 wt%, based on 100 wt% of the total amount of the silicon-based active material. Specifically, the content of Mg element may be 0.5 to 8wt%, or 0.8 to 4 wt%. When the above range is satisfied, an appropriate amount of Mg compound may be contained in the silicon-based active material, and thus the volume change of the silicon-based active material during charge and discharge of the battery may be easily suppressed, and the discharge capacity and initial efficiency of the battery may be improved.

The Li compound may include at least one selected from the group consisting of lithium silicate, lithium silicide, and lithium oxide. The lithium silicate may include at least one of Li ₂SiO₃、Li₄SiO₄ and Li ₂Si₂O₅. The lithium silicide may include Li ₇Si₂. The lithium oxide may include Li ₂ O.

In an exemplary embodiment of the present invention, the Li compound may include a form of lithium silicate. Lithium silicate is represented by Li _aSi_bO_c (a is more than or equal to 2 and less than or equal to 4, b is more than or equal to 0 and less than or equal to 2 and c is more than or equal to 5), and can be classified into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate may be present in the silicon-based composite particles in the form of at least one lithium silicate selected from the group consisting of Li ₂SiO₃、Li₄SiO₄ and Li ₂Si₂O₅, and the amorphous lithium silicate may be in the form of Li _aSi_bO_c (2.ltoreq.a.ltoreq.4, 0.ltoreq.b.ltoreq.2, 2.ltoreq.c.ltoreq.5). However, the present invention is not limited thereto.

In one exemplary embodiment of the present specification, the content of Li element may be 0.1 to 20 wt%, or 0.1 to 10 wt%, based on 100 wt% of the total silicon-based active material. Specifically, the content of the Li element may be 0.5 to 8 wt%, more specifically, 0.5 to 4 wt%. When the above range is satisfied, a suitable content of Li compound may be contained in the silicon-based active material, and thus the volume change of the anode active material during charge and discharge of the battery may be easily suppressed, and the discharge capacity and initial efficiency of the battery may also be improved.

The content of Mg element or Li element can be confirmed by ICP analysis. For ICP analysis, a predetermined amount (about 0.01 g) of the anode active material was precisely separated, transferred to a platinum crucible, and completely decomposed on a hot plate by adding nitric acid, hydrofluoric acid, and sulfuric acid thereto. Then, a reference calibration curve was obtained by measuring the intensity of a standard solution prepared using a standard solution (5 Mg/kg) at the intrinsic wavelength of Mg element or Li element by using an inductively coupled plasma atomic emission spectrometer (ICP-AES, perkin-Elmer 7300). Subsequently, the pretreated sample solution and the blank sample are introduced into a spectrometer, the actual intensity is calculated by measuring the intensity of each component, the concentration of each component is calculated based on the obtained calibration curve, and then conversion is performed so that the sum of the calculated concentrations of the components is equal to the theoretical value, so that the content of Mg element or Li element in the prepared silicon-based active material can be analyzed.

In one exemplary embodiment of the present description, the carbon layer may be disposed on the surface and/or inside the pores of the silicon-based composite particles. The carbon layer imparts conductivity to the silicon-based composite particles, so that the initial efficiency, life characteristics, and battery capacity characteristics of a secondary battery including a negative electrode active material containing the silicon-based composite particles can be improved. The total amount of the carbon layer contained may be 5 to 40% by weight based on 100% by weight of the total amount of the silicon-based composite particles.

In an exemplary embodiment of the present specification, the carbon layer may include at least one of amorphous carbon or crystalline carbon.

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

In the present specification, the average particle diameter (D ₅₀) may be defined as a particle diameter corresponding to 50% of the cumulative volume in the particle diameter distribution curve of the particles. The average particle diameter (D ₅₀) can be measured using, for example, a laser diffraction method. In the laser diffraction method, the particle diameter ranging from submicron to several millimeters can be measured in general, and the result of high reproducibility and high resolution can be obtained.

According to one exemplary embodiment, the content of the silicon carbon composite in the first anode active material layer may be 1 to 20 wt% based on 100 wt% of the first anode active material layer, and the content of the silicon oxide in the second anode active material layer may be 1 to 20 wt% based on 100 wt% of the second anode active material layer. When the content of the silicon-carbon composite or the silicon oxide in the first anode active material layer or the second anode active material layer is 1 wt% or more, the capacity gain can be made to be a level or more, and thus the advantage of using these materials can be exhibited; when the content is 20 wt% or less, excessive swelling can be prevented, which is advantageous in terms of life and battery characteristics, and also has price competitiveness from the viewpoint of cost.

According to one exemplary embodiment, the first and second anode active material layers further include a carbon-based active material. The carbon-based active material may be graphite, which may be natural graphite, artificial graphite, or a mixture thereof. The content of the carbon-based active material may be 80 parts by weight or more and 99 parts by weight or less based on 100 parts by weight of the total amount of the anode active material contained in each anode active material layer.

In one exemplary embodiment of the present specification, the content of the anode active material may be 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 most preferably 98 parts by weight or more and 99.9 parts by weight or less, based on 100 parts by weight of each anode active material layer.

According to another exemplary embodiment of the present specification, the anode active material layer may include an anode binder in addition to the silicon-based active material and the carbon-based active material.

The anode binder may be used to improve adhesion between anode active material particles and an anode current collector. As the anode binder, those known in the art can be used. Non-limiting examples thereof may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and the above materials in which hydrogen is substituted with Li, na, ca, etc., and may further include various copolymers thereof.

The content of the anode binder may be 0.1 parts by weight or more and 20 parts by weight or less, for example, preferably 0.3 parts by weight or more and 20 parts by weight or less, more preferably 0.5 parts by weight or more and 10 parts by weight or less, based on 100 parts by weight of the anode active material layer.

The anode active material layer may not contain a conductive material, but may also contain a conductive material if necessary. The conductive material contained in the anode active material layer is not particularly limited as long as the conductive material has conductivity without causing chemical changes in the battery, and for example, it is possible to use: graphite, such as natural graphite or artificial graphite; carbon blacks 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 powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives and the like. The content of the conductive material in the anode active material layer may be 0.01 to 20 parts by weight, preferably 0.03 to 18 parts by weight, based on 100 parts by weight of the anode active material layer.

In one exemplary embodiment of the present specification, each of the first and second anode active material layers may have a thickness of 10 μm or more and 150 μm or less. The first and second anode active material layers may each have a weight loading of 50 to 300mg/25cm ². The weight loading is based on the weight of the solids after drying.

In one exemplary embodiment of the present specification, the anode current collector is not particularly limited as long as the anode current collector has conductivity without causing chemical changes in the battery. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like may be used. Specifically, transition metals (e.g., copper and nickel) having good carbon adsorption can be used as the current collector. The thickness of the current collector may be 1 μm to 500 μm. However, the thickness of the current collector is not limited thereto.

Another exemplary embodiment of the present specification provides a secondary battery including the anode, the cathode, and the separator of the above exemplary embodiment.

In one exemplary embodiment of the present specification, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing a positive electrode active material. The thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

The positive electrode current collector is not particularly restricted so long as it has conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like can be used. In addition, the thickness of the positive electrode current collector may be generally 1 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric body, and the like.

In one exemplary embodiment of the present specification, the positive electrode may include a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as an active material. The lithium composite transition metal compound may further comprise at least one of manganese and aluminum. Among metals other than lithium, the lithium composite transition metal compound may contain nickel in an amount of 80mol% or more (for example, 80mol% or more and less than 100 mol%).

In one exemplary embodiment, the content of the positive electrode active material in 100 parts by weight of the positive electrode active material layer may be 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 most preferably 98 parts by weight or more and 99.9 parts by weight or less.

According to another exemplary embodiment of the present specification, the positive electrode active material layer of the above-described exemplary embodiment may further include a positive electrode binder and a conductive material.

The positive electrode binder may be used to improve adhesion between positive electrode active material particles and a positive electrode current collector. For the positive electrode binder, those known in the art may be used. Non-limiting examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, various copolymers thereof, and the like, and a mixture of any one or two or more thereof may be used.

The content of the positive electrode binder may be 0.1 parts by weight or more and 50 parts by weight or less, for example, preferably 0.3 parts by weight or more and 35 parts by weight or less, 0.5 parts by weight or more and 20 parts by weight or less, based on 100 parts by weight of the positive electrode active material layer.

The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode, and may be used without particular limitation as long as the conductive material has electron conductivity without causing chemical changes in the battery. Specific examples may include: graphite, such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal 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 a conductive polymer such as a polyphenylene derivative or the like, any one or a mixture of two or more thereof may be used.

Specifically, in one exemplary embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The content of the conductive material may be 0.1 parts by weight or more and 2 parts by weight or less, for example, preferably 0.3 parts by weight or more and 1.5 parts by weight or less, more preferably 0.5 parts by weight or more and 1.2 parts by weight or less, based on 100 parts by weight of the composition for the positive electrode active material layer.

The positive electrode and the negative electrode may be manufactured according to a conventional method of manufacturing the positive electrode and the negative electrode, except that the above-described positive electrode active material and negative electrode active material are used. Specifically, the electrode may be manufactured by applying an active material layer forming composition containing the above active material and optionally a binder and a conductive material to a current collector, followed by drying and calendaring. In this case, the types and contents of the positive 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 any one or a mixture of two or more thereof may be used. In view of the coating thickness of the slurry and the manufacturing yield, if the solvent can dissolve or disperse the active material, the conductive material, and the binder and then has a viscosity capable of exhibiting excellent thickness uniformity when coated to manufacture the positive electrode and the negative electrode, the amount of the solvent is sufficient. Or the positive electrode and the negative electrode may be manufactured by casting the active material layer forming composition onto a separate support and laminating a film obtained by peeling from the support on a current collector.

The separator serves to separate the anode and the cathode and provide a movement path of lithium ions, wherein any separator may be used as the separator without particular limitation as long as it is commonly used for secondary batteries, and in particular, a separator having a high moisture retention ability for an electrolyte and a low resistance to movement of electrolyte ions is preferably used. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer; or a laminated structure having two or more layers. A conventional porous nonwoven fabric, for example, a nonwoven fabric formed of high-melting glass fibers, polyethylene terephthalate fibers, or the like may also be used. In addition, a coated separator including a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure may be selectively used.

Examples of the electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, or the like, which may be used to manufacture a lithium secondary battery, but are not limited thereto.

In particular, the electrolyte may include a nonaqueous organic solvent and a metal salt.

As the nonaqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, 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, phosphotriester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfane, 1, 3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate or ethyl propionate can be used.

In particular, among carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents, and since they have a high dielectric constant, lithium salts can be dissociated well, and thus can be preferably used. When the cyclic carbonate is mixed with a linear carbonate having a low viscosity and a low dielectric constant (e.g., dimethyl carbonate and diethyl carbonate) in an appropriate ratio and used, an electrolyte having high conductivity may be prepared, and thus may be more preferably used.

A lithium salt may be used as the metal salt, and the lithium salt is a material that is easily soluble in a nonaqueous electrolytic solution, wherein, for example, one or more anions ：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 (CF ₃CF₂SO₂)₂N^-) selected from the group consisting of the following may be used as the lithium salt.

In addition to the above electrolyte components, one or more additives such as halogenated alkylene carbonate compounds such as ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, N-glycol ether, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride may be included in the electrolyte for the purpose of improving the life characteristics of the battery, suppressing the decline of the battery capacity, improving the discharge capacity of the battery, and the like.

The secondary battery of an exemplary embodiment of the present invention includes an assembly including a positive electrode, a negative electrode, a separator, and an electrolyte, and may be a lithium secondary battery.

Another exemplary embodiment of the present invention provides a battery module including the above secondary battery as a unit cell and a battery pack including the same. Since the battery module and the battery include the secondary battery having high capacity, high rate performance, and high cycle characteristics, the battery module and the battery pack may 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 electric power storage systems.

Since the secondary battery of the exemplary embodiment of the present invention stably exhibits excellent discharge capacity, output characteristics, and cycle performance, the secondary battery may be used as a power source for portable devices such as mobile phones, notebook computers, and digital cameras, and 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. For example, the battery module or battery pack may be used as a power source for medium-to-large devices of any one or more of the following: an electric tool; electric vehicles, including Electric Vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or a power storage system.

Examples

Hereinafter, preferred embodiments will be provided to better understand the present invention. It will be apparent to those skilled in the art that the embodiments are provided only for illustrating the present invention, and that various modifications and changes may be made within the scope and technical spirit of the present invention. Such modifications and variations naturally also fall within the scope of the claims contained herein.

Example 1

A first negative electrode active material layer-forming composition containing a silicon-carbon composite (discharge efficiency: 90%, D ₅₀: 10 μm) and a carbon-based active material (content of silicon-carbon composite: 5 parts by weight based on 100 parts by weight of negative electrode active material), a conductive material (containing carbon black, CNT), a binder (SBR), and a thickener (CMC) in a weight ratio of 95.9:1.0:2.0:1.1 was coated on a copper foil (thickness 10 μm) until the thickness in a dry state was 70 μm, and then dried, thereby forming a first negative electrode active material layer. Then, a second anode active material layer-forming composition containing SiO (discharge efficiency: 80%, D ₅₀: 10 μm) and a carbon-based active material (content of SiO: 5 parts by weight based on 100 parts by weight of the anode active material), a conductive material (containing carbon black, CNT), a binder (SBR), and a thickener (CMC) in a weight ratio of 94.9:1.0:3.0:1.1 was coated on the first anode active material layer until the thickness in a dry state was 70 μm, and dried, thereby forming a second anode active material layer, whereby an anode was prepared.

The positive electrode was prepared by coating a positive electrode active material layer-forming composition containing lithium nickel-based oxide single particles of Li _1.0Ni_0.84Co_0.08Mn_0.08O₂, a conductive material (CNT), and a binder (PVDF) in a weight ratio of 97:1:2 on an aluminum foil having a thickness of 15 μm until a thickness in a dry state was 130 μm, and then drying it.

A battery was prepared by stacking a positive electrode and a negative electrode with a separator interposed therebetween and injecting an electrolyte. The electrolyte contained 1M LiPF ₆, ethylene Carbonate (EC)/ethylmethyl carbonate (EMC) (volume ratio 3/7) and Vinylene Carbonate (VC)/Propane Sultone (PS) (content of 3 parts by weight and 1.5 parts by weight, respectively, based on 100 parts by weight of the electrolyte).

To confirm the rapid charge life performance of the prepared battery, the battery was cycled at room temperature (25 ℃) to charge to 4.1V at a Constant Current (CC) of 2.0C and discharge (2.5V off) at a Constant Current (CC) of 0.33C. The capacity retention after 100 cycles is shown in table 1 below.

Comparative example 1

The same procedure as in example 1 was conducted except that SiO was contained in the first anode active material layer instead of the silicon-carbon composite.

TABLE 1

	Number of cycles	Capacity retention (%)
			Example 1	100 Cycles	96％
Comparative example 1	100 Cycles	80％

As shown in table 1, it was confirmed that the capacity retention rate of the examples was significantly higher than that of the comparative examples, and thus the quick charge life performance was excellent.

Claims (11)

1. A negative electrode for a secondary battery, comprising: Current collector; a first negative electrode active material layer, the first negative electrode active material layer being disposed on the current collector and containing a silicon-carbon composite; and a second negative electrode active material layer, the second negative electrode active material layer being disposed on the first negative electrode active material layer and containing silicon oxide, The ratio D _50,2nd /D _50,1st of the average particle size D _50,2nd of the second negative electrode active material layer to the average particle size D _50,1st of the first negative electrode active material layer is 0.75 to 1.5. 2 . The negative electrode for a secondary battery according to claim 1 , wherein a discharge efficiency of the silicon-carbon composite contained in the first negative electrode active material layer is 85% to 95%. 3 . The negative electrode for a secondary battery as claimed in claim 1 , wherein a discharge efficiency of the silicon oxide contained in the second negative electrode active material layer is 75% to 85%. 4 . The negative electrode for a secondary battery as claimed in claim 1 , wherein a ratio of an average particle size D _50,2nd of the second negative electrode active material layer to an average particle size D _50,1st of the first negative electrode active material layer is 0.9 to 1. 5 . 5 . The negative electrode for a secondary battery as claimed in claim 1 , wherein each of the first negative electrode active material layer and the second negative electrode active material layer further comprises a carbon-based active material. 6 . The negative electrode for a secondary battery as claimed in claim 5 , wherein the carbon-based active material comprises at least one of artificial graphite and natural graphite. 7. The negative electrode for a secondary battery as claimed in claim 1, wherein the content of the silicon-carbon composite in the first negative electrode active material layer is 1% to 20% by weight based on 100% by weight of the first negative electrode active material layer, and the content of the silicon oxide in the second negative electrode active material layer is 1% to 20% by weight based on 100% by weight of the second negative electrode active material layer. 8 . The negative electrode for a secondary battery as claimed in claim 1 , wherein the first negative electrode active material layer and the second negative electrode active material layer each have a thickness of 10 μm to 150 μm. 9 . A secondary battery comprising the negative electrode according to claim 1 , a positive electrode, and a separator. 10 . The secondary battery according to claim 9 , wherein the positive electrode comprises a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as an active material. 11 . The secondary battery according to claim 10 , wherein the lithium composite transition metal compound further comprises at least one of manganese (Mn) and aluminum.