Secondary battery

Abstract

To provide a secondary battery.SOLUTION: The present specification provides a secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes a negative electrode active material, the negative electrode active material includes a Si-based active material and a carbon-based active material, the positive electrode includes a Ni-based active material, and a cobalt oxide layer is provided on a part of a particle surface of the Ni-based active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0112181, filed with the Korean Intellectual Property Office on September 5, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a secondary battery. [Background technology]

[0003] Secondary batteries, which are highly adaptable to various product groups and have electrical properties such as high energy density, are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs), which are powered by electrical sources.

[0004] These secondary batteries not only have the primary advantage of dramatically reducing the use of fossil fuels, but also have the advantage of not producing any by-products from energy use, making them environmentally friendly and 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. The positive electrode and the negative electrode may each have an active material layer formed on a current collector, the active material layer including a positive electrode active material and a negative electrode active material. Generally, the positive electrode uses a lithium-containing metal oxide such as LiCoO2 or LiMn2O4 as the positive electrode active material, while the negative electrode uses a carbon-based compound, a silicon-based compound, or a mixture thereof as the negative electrode active material.

[0006] In recent years, in order to develop batteries capable of rapid charging, a mixture of carbon-based compounds such as graphite and silicon-based compounds has been used in the negative electrode. However, when a silicon-based compound is included, the negative electrode resistance rises sharply at the end of discharge, increasing the difference between the negative electrode resistance and the positive electrode resistance, resulting in problems such as a shortened battery life and a deterioration in room temperature cycle characteristics. Therefore, there is a need to develop a battery that solves these problems. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2012-0037409 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a secondary battery that contains a silicon-based compound in the negative electrode, but reduces the difference between the negative electrode resistance and the positive electrode resistance at the discharge end, thereby improving the battery life and room temperature cycle characteristics.

[0009] However, the technical problems that the present invention aims to solve are not limited to the above-mentioned problems, and other problems not mentioned will be clearly understood by those skilled in the art from the description of the invention described below. [Means for solving the problem]

[0010] One embodiment of the present invention provides a secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes a negative electrode active material, the negative electrode active material including a silicon (Si)-based active material and a carbon-based active material, and the positive electrode includes a nickel (Ni)-based active material, the nickel (Ni)-based active material having a cobalt oxide layer on a portion of a particle surface thereof.

[0011] Another embodiment of the present invention provides a secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) is 2.06 or more. [Effects of the Invention]

[0012] The secondary battery of the present invention contains a nickel (Ni)-based active material in the positive electrode, which has a cobalt oxide layer formed on a portion of the particle surface, thereby reducing the sudden decrease in positive electrode resistance at the end of discharge. This reduces the difference between the positive electrode resistance and the negative electrode resistance, resulting in a secondary battery with improved life and room-temperature cycle characteristics.

[0013] Specifically, when a cobalt oxide layer is provided on the entire surface of nickel (Ni)-based active material particles to improve the positive electrode resistance, the resistance at the discharge end is significantly reduced. This results in an excessive increase in the use of silicon (Si)-based active material contained in the negative electrode (negative electrode) (increased depth of use) to enable rapid charging, resulting in a decrease in battery life. The secondary battery of the present invention has a cobalt oxide layer provided on a portion of the surface of nickel (Ni)-based active material particles, enabling rapid charging and improving the positive electrode resistance. This reduces the difference in resistance between the positive and negative electrodes and the depth of use of the silicon (Si)-based active material, thereby resulting in a secondary battery with improved room-temperature life. [Brief explanation of the drawings]

[0014] [Figure 1] This shows an SEM image of a Ni-based active material with a cobalt oxide layer on part of the particle surface. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below. The following description is provided to aid in understanding the present invention, and is not intended to define or limit the scope of the invention.

[0016] In this specification, when a part is said to "comprise" a certain component, this means that it may further include other components, rather than excluding other components, unless specifically stated to the contrary.

[0017] In this specification, when a member is said to be located "on" another member, this includes not only when the member is in contact with the other member, but also when there is another member between the two members.

[0018] The terms or words used in this specification should not be construed as being limited to their ordinary or dictionary meanings, but should be construed in a meaning and concept that is consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0019] As used herein, the singular expression of a term includes the plural expression unless the context clearly dictates otherwise.

[0020] In this specification, the crystallinity of the structure contained in the positive or negative electrode active material can be confirmed by X-ray diffraction analysis. X-ray diffraction analysis can be performed using an X-ray diffraction (XRD) analysis device (product name: D4-Endavor, manufacturer: Bruker), and in addition to the above device, any device used in the art can be appropriately adopted.

[0021] In this specification, the presence or absence of elements and the content of elements in the active material of the positive or negative electrode can be confirmed by ICP (inductively coupled plasma) analysis, and the ICP analysis can be performed using an inductively coupled plasma optical emission spectrometer (ICPAES, Perkin-Elmer 7300).

[0022] The present invention relates to a secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode contains a negative electrode active material, the negative electrode active material includes a silicon (Si)-based active material and a carbon-based active material, the positive electrode contains a nickel (Ni)-based active material, and the nickel (Ni)-based active material has a cobalt oxide layer provided on a part of the particle surface.

[0023] In this specification, when the negative electrode active material includes a silicon (Si)-based active material, the negative electrode resistance rapidly increases at the end of discharge, and the positive electrode resistance using a positive electrode material having low resistance characteristics rapidly decreases. Therefore, the difference between the negative electrode resistance and the positive electrode resistance becomes very large. As a result, there is a problem that the deterioration of the negative electrode progresses rapidly, the life of the battery becomes short, and the normal temperature cycle characteristics deteriorate. In order to solve this problem, a cobalt oxide layer is provided on a part of the surface of the Ni-based active material particles, which are the positive electrode material, to reduce the rapid decrease in the positive electrode resistance, and a battery with improved life and normal temperature cycle characteristics can be obtained.

[0024] In this specification, the end of discharge means a region where the state of charge (SOC) of the battery (Full cell) is 10% or less.

[0025] <Ni-based active material> The positive electrode of the present invention contains a nickel (Ni)-based active material, and a cobalt oxide layer is provided on a part of the particle surface of the nickel (Ni)-based active material. In a part of the particle surface, "a part" means not the entire surface of the nickel (Ni)-based active material particles. Specifically, providing a cobalt oxide layer on a part of the particle surface of the nickel (Ni)-based active material means excluding the case where a cobalt oxide layer is formed on the entire particle surface of the nickel (Ni)-based active material.

[0026] In the present invention, the "cobalt oxide layer" is a layer containing cobalt oxide. The cobalt oxide layer may consist only of cobalt oxide, may contain cobalt oxide as a main component, and may further contain a trace amount of additional components. For example, the cobalt oxide layer may contain cobalt oxide at 90 wt% or more and 100 wt% or less.

[0027] According to one embodiment of the present invention, the cobalt oxide layer may be formed on 1% to 99%, 5% to 99%, or 10% to 80% of the surface area of ​​the nickel (Ni)-based active material particles.

[0028] In one embodiment of the present invention, the cobalt oxide layer may be formed on a portion of the surface of a nickel (Ni)-based active material particle, and may be island-shaped. The term "island-shaped" refers to a form in which the entire periphery is not continuously connected. This term is used to distinguish it from a form in which the entire or a portion of the surface of the nickel (Ni)-based active material particle is continuously surrounded, and the term itself does not limit the form, size, or area. As can be seen in FIG. 1, the cobalt oxide layer refers to island-shaped particles formed on the surface of the nickel (Ni)-based active material particle, and the form is not limited.

[0029] In one embodiment of the present invention, the nickel (Ni)-based active material comprises two or more, five or more, ten or more, or twenty or more cobalt oxide layers on the particle surface, and the cobalt oxide layers are in the form of islands. The islands may be in the form of spheres, distorted spheres, or agglomerates bonded together, formed on the surface of the nickel (Ni)-based active material particles, but the form is not limited thereto.

[0030] The cobalt oxide layer may be disposed at intervals on the surface of a particle of the nickel (Ni)-based active material, or may be distributed discontinuously on a portion of the surface of the particle of the Ni-based active material.

[0031] According to one embodiment of the present invention, the cobalt oxide layer has a thickness of 5 nm to 10 nm, where "thickness" refers to the length of the longest straight line that crosses the cobalt oxide layer.

[0032] In one embodiment of the present invention, the cobalt oxide layers formed on the particle surfaces of the nickel (Ni)-based active material may have the same shape and thickness or may be different from each other.

[0033] According to an embodiment of the present invention, the cobalt oxide layer is contained in an amount of 0.5 to 3 parts by weight with respect to 100 parts by weight of the nickel (Ni)-based active material.

[0034] The Ni-based active material of the present invention may include a lithium composite metal oxide containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, a lithium-nickel-manganese-cobalt-based oxide (for example, Li(Ni , Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), a lithium-nickel-cobalt-transition metal (M) oxide (for example, Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of independent elements, 0 < p2 < 1,​​​​​​​​​​​​​According to one embodiment of the present invention, the carbon-based active material may be artificial graphite, natural graphite, carbon black, or the like.

[0038] Conventionally, graphite-based compounds have been used exclusively as negative electrode active materials. However, in recent years, as demand for high-capacity batteries has increased, attempts to mix silicon-based active materials in order to increase capacity have been increasing.

[0039] The inventors have devised the present invention to solve the problem of room temperature cycle life, which does not occur when only a carbon-based compound is included as the negative electrode active material, whereas room temperature cycle life problems occur when a silicon-based active material is included.

[0040] In one embodiment of the present invention, the silicon (Si)-based active material is SiO β (0<β<2) or a Si-C composite.

[0041] According to one embodiment of the present invention, the silicon (Si)-based active material is included in an amount of 1 to 10 parts by weight, preferably 5 to 10 parts by weight, based on 100 parts by weight of the negative electrode active material. When the content of the silicon (Si)-based active material satisfies this range, improved effects are achieved in terms of energy density and cell resistance, and volume expansion during charge / discharge is reduced, resulting in excellent effects in terms of lifespan.

[0042] The negative electrode active material layer may be formed by applying a negative electrode slurry containing a negative electrode active material, a binder, and / or a conductive material to at least one surface of a negative electrode current collector, drying, and rolling.

[0043] The negative electrode slurry contains the negative electrode active material, a binder, and / or a conductive material.

[0044] According to one embodiment of the present specification, the negative electrode slurry may further contain an additional negative electrode active material in addition to the silicon (Si)-based active material and the carbon-based active material described above.

[0045] The additional negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; SiO β (0<β<2), SnO 2、 Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as vanadium oxide, lithium titanium oxide, and lithium vanadium oxide, and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and SnC composites. One or a mixture of two or more of these may also be used. Furthermore, a metallic lithium thin film may also be used as the negative electrode active material.

[0046] In one embodiment of the present invention, the weight ratio of the negative electrode active material to the additional negative electrode active material contained in the negative electrode slurry may be 10:90 to 90:10, specifically 10:90 to 50:50.

[0047] The negative electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel 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 6 μm to 20 μm, but is not limited thereto.

[0048] The binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, 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 may include various copolymers thereof.

[0049] The conductive material is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, 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; fluorocarbon powder; metal powder such as 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 may be used.

[0050] The negative electrode slurry may further contain a thickener such as Na-CMC (sodium carboxymethyl cellulose), Li-CMC (carboxymethyl cellulose lithium), or CNF (cellulose nanofiber).

[0051] The negative electrode slurry according to an embodiment of the present invention may further include a solvent for forming the negative electrode slurry. Specifically, the solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, specifically distilled water, in order to facilitate dispersion of components.

[0052] In one embodiment of the present invention, the solid content weight of the negative electrode slurry may be 20 parts by weight to 75 parts by weight, specifically 30 parts by weight to 70 parts by weight, based on 100 parts by weight of the negative electrode slurry in total.

[0053] <Secondary battery> A secondary battery according to an embodiment of the present invention may include the anode according to the embodiment described above. Specifically, the secondary battery may include an anode, a cathode, a separator and an electrolyte interposed between the cathode and the anode, and the anode is the same as the anode described above. Since the anode has been described above, detailed description thereof will be omitted.

[0054] According to one embodiment of the present invention, the density of the electrode can be increased by a pressing step during the electrode fabrication process.

[0055] A secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) is 2.06 or more, this value being rounded to two decimal places.

[0056] In another embodiment, the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) may be 2.06 or more, or 2.06 or more to 2.47 or less, or 2.06 or more to 2.30 or less. According to another embodiment, the electrode density of the positive electrode is 3.5 g / cc or more.

[0057] In still another embodiment, the electrode density of the positive electrode is 3.5 g / cc to 3.73 g / cc.

[0058] According to another embodiment, the electrode density of the positive electrode is 3.5 g / cc to 3.55 g / cc.

[0059] According to another embodiment, the electrode density of the negative electrode is 1.7 g / cc or less.

[0060] In another embodiment, the electrode density of the negative electrode is 1.51 g / cc to 1.7 g / cc.

[0061] In another embodiment, the electrode density of the negative electrode is 1.51 g / cc to 1.62 g / cc.

[0062] According to another embodiment, there is provided a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprising a silicon (Si)-based active material and a carbon-based active material, the positive electrode comprises a nickel (Ni)-based active material, and the nickel (Ni)-based active material has a cobalt oxide layer provided on a portion of the particle surface thereof, and the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) is 2.06 or more.

[0063] In another embodiment, the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) may be 2.06 or more, or may be 2.06 or more and 2.47 or less, or may be 2.06 or more and 2.30 or less.

[0064] According to another embodiment, the electrode density of the positive electrode is 3.5 g / cc or more.

[0065] In still another embodiment, the electrode density of the positive electrode is 3.5 g / cc to 3.73 g / cc.

[0066] According to another embodiment, the electrode density of the positive electrode is 3.5 g / cc to 3.55 g / cc.

[0067] According to another embodiment, the electrode density of the negative electrode is 1.7 g / cc or less.

[0068] In another embodiment, the electrode density of the negative electrode is 1.51 g / cc to 1.7 g / cc.

[0069] In another embodiment, the electrode density of the negative electrode is 1.51 g / cc to 1.62 g / cc.

[0070] The electrode density can be measured by the following method.

[0071] Electrode density = {electrode weight (excluding foil weight)} / {(electrode thickness) x (electrode area)}

[0072] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and including the positive electrode active material.

[0073] The positive electrode current collector in the positive electrode is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the collector surface to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0074] The positive electrode active material may be the nickel (Ni)-based active material described above.

[0075] According to one embodiment of the present invention, an additional positive electrode active material may be included, such as layered compounds, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium iron oxides, such as LiFe3O4; 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.3 is satisfied); 2-c3 M c3 Examples of suitable lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≦c3≦0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4, in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be Li-metal.

[0076] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-described positive electrode active material.

[0077] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be any material that has electronic conductivity without undergoing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more.

[0078] The positive electrode binder serves to improve adhesion between particles of the positive electrode active material and between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-coHFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0079] The cathode slurry according to an embodiment of the present invention may further include a solvent for forming the cathode slurry. Specifically, the solvent for forming the cathode slurry may include methylpyrrolidone (NMP) to facilitate dispersion of components.

[0080] In one embodiment of the present invention, the solid content weight of the positive electrode slurry may be 20 parts by weight to 85 parts by weight, specifically 30 parts by weight to 80 parts by weight, based on a total of 100 parts by weight of the positive electrode slurry.

[0081] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without limitation. It is particularly preferable that the separator has low resistance to electrolyte ion movement and excellent electrolyte humidification capability. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and it can be used in a single-layer or multi-layer structure.

[0082] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries.

[0083] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0084] Examples of the non-aqueous organic solvent that may be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0085] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, and therefore, these cyclic carbonates are more preferably used.

[0086] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3- , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0087] 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, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0088] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, excellent life characteristics, and excellent cycle characteristics, and can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0089] In the following, preferred embodiments are presented to aid in understanding the present invention, but these embodiments are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present description and technical concept. Such changes and modifications are naturally intended to fall within the scope of the appended claims.

[0090] <Cel production> Example 1 Fabrication of the positive electrode A nickel (Ni)-based positive electrode active material with a cobalt oxide layer on part of the particle surface, a conductive material (carbon nanotubes (CNTs)), and a binder (PVDF) were added to a methylpyrrolidone (NMP) solvent in a weight ratio of 97.0:1.2:1.8 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).

[0091] The nickel (Ni)-based positive electrode active material contains nickel (Ni), cobalt (Co), and manganese (Mn) in an atomic ratio of 86:8:6.

[0092] The positive electrode slurry prepared above was applied onto an aluminum (Al) current collector, dried at a high temperature, and then rolled at room temperature to prepare a positive electrode.

[0093] The electrode density of the positive electrode prepared above is 3.64 g / cc.

[0094] Fabrication of the negative electrode Anode active materials including an SiO-based active material and a carbon-based active material (artificial graphite, natural graphite, and SiO-based active material were included in amounts of 75 parts by weight, 20 parts by weight, and 5 parts by weight, respectively, based on 100 parts by weight of the total anode active materials), a conductive material (carbon black), a binder (SBR), and a thickener (CMC) were added to a distilled water solvent in a weight ratio of 95.6:1.0:2.3:1.1 to prepare anode slurry (the solid content of the anode slurry was 50 parts by weight of the total anode slurry).

[0095] The negative electrode slurry prepared above was applied onto a copper (Cu) current collector, dried at a high temperature, and then rolled at room temperature to prepare a negative electrode.

[0096] The electrode density of the negative electrode prepared above is 1.62 g / cc.

[0097] 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.

[0098] -Electrolyte composition: 1M LiPF6, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio 3 / 7), vinylene carbonate (VC) / propane sultone (PS) (contained in the electrolyte at 3 parts by weight and 1.5 parts by weight, respectively) - Activation: 0.1C, 3 hours after charging, high temperature / normal temperature aging, degas

[0099] The electrode density of the positive electrode / the electrode density of the negative electrode of the cell manufactured as above is 2.25.

[0100] Example 2. A cell was fabricated in the same manner as in Example 1, except that the negative electrode contained 10 parts by weight of an SiO-based active material relative to the total negative electrode active material.

[0101] Example 3 A cell was fabricated in the same manner as in Example 1, except that the negative electrode contained 5 parts by weight of an SiC-based active material instead of an SiO-based active material, based on the total negative electrode active material.

[0102] Comparative Example 1 A cell was fabricated in the same manner as in Example 1, except that the positive electrode was coated with cobalt hydroxide instead of a cobalt oxide layer.

[0103] Comparative Example 2 A cell was fabricated in the same manner as in Example 1, except that a nickel (Ni)-based active material without a cobalt oxide layer was used for the positive electrode.

[0104] Reference example 1. A cell was fabricated in the same manner as in Example 1, except that only a carbon-based active material was used in the negative electrode instead of a silicon-based active material.

[0105] Reference example 2. A cell was fabricated in the same manner as in Example 1, except that a nickel (Ni)-based active material with a cobalt oxide layer formed on the entire surface was used for the positive electrode, and only a carbon-based active material was used for the negative electrode instead of a silicon-based active material.

[0106] <Test Results> Experimental Example 1. Cell resistance performance The fabricated cell was charged at 0.33 C with constant current / constant voltage (CC / CV) to 4.2 V (0.05 C cutoff) and discharged at 0.33 C with constant current (CC) (2.5 V cutoff) three times, and the third discharge capacity was measured (initial capacity). After the same charge, the cell was discharged at 0.33 C to set the SOC at 50%, and then pulse discharged at 2.5 C for 10 seconds to measure the resistance (initial resistance). The results are shown in Table 1 below.

[0107] [Table 1]

[0108] Experimental example 2: Cell room temperature cycle life performance The cells were cycled at room temperature (25°C) by constant current / constant voltage (CC / CV) charging at 0.33 C to 4.2 V (0.05 C cutoff), followed by constant current (CC) discharging at 0.5 C (2.5 V cutoff). After 400 cycles, the capacity and resistance were measured in the same manner as in Experimental Example 1, and the capacity retention rate (capacity after 400 cycles / initial capacity x 100%) and resistance increase rate (resistance after 400 cycles / initial resistance x 100-100%) were measured. The results are shown in Table 2 below.

[0109] [Table 2]

[0110] From the experimental results in Table 1, it can be seen that Examples 1 to 3 of the present invention, which have a cobalt oxide layer on the positive electrode, showed an increase in cell resistance at the end of discharge (SOC 5%). This indicates that the increase in cell resistance was due to an increase in the positive electrode resistance. The increase in positive electrode resistance reduces the depth of use of the SiO-based negative electrode active material, preventing negative electrode degradation and providing an excellent effect in terms of lifespan. This can be confirmed by the fact that the electrodes of Examples 1 to 3 in Table 2 showed high capacity retention and low resistance increase rates in room temperature cycle experiments.

[0111] In contrast, Comparative Examples 1 and 2 correspond to cases where cobalt hydroxide is used instead of cobalt oxide or where no cobalt oxide layer is provided, and thus the positive electrode resistance is low at the discharge end, the SiO-based negative electrode active material is used for a longer period of time, and negative electrode degradation progresses rapidly. This can be confirmed by the fact that the electrodes of Comparative Examples 1 and 2 have lower capacity retention rates and larger resistance increase rates than the electrodes of Examples 1 to 3 in the room temperature cycle experiment shown in Table 2.

[0112] Reference Examples 1 and 2 correspond to electrodes using only a carbon-based negative electrode active material as the negative electrode active material, and when only a carbon-based negative electrode active material is included as the negative electrode active material, the room temperature cycle life problem does not occur. This can be confirmed by the fact that there is no significant difference in the capacity retention rate and resistance increase rate between Reference Examples 1 and 2 in Table 2. In other words, the present invention is intended to be applied to electrodes containing a silicon-based active material, rather than electrodes using only a carbon-based negative electrode active material, and to solve the room temperature cycle life problem.

[0113] [1] A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, the negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material and a carbon-based active material, the positive electrode contains a nickel-based active material, A secondary battery in which a cobalt oxide layer is provided on a part of the surface of the particles of the nickel-based active material. [2] The secondary battery according to [1], wherein the cobalt oxide layer is provided on 1% to 99% of the particle surface area of ​​the nickel-based active material. [3] The secondary battery according to [1], wherein the cobalt oxide layer is island-shaped. [4] The secondary battery according to [3], wherein the nickel-based active material contains two or more cobalt oxide layers on the particle surface. [5] The secondary battery according to [1], wherein the cobalt oxide layer has a thickness of 5 nm to 10 nm. [6] The secondary battery according to [1], wherein the cobalt oxide layer is contained in an amount of 0.5 to 3 parts by weight per 100 parts by weight of the nickel-based active material. [7] The secondary battery according to [1], wherein the silicon-based active material is contained in an amount of 1 to 10 parts by weight based on 100 parts by weight of the negative electrode active material. [8] The secondary battery according to any one of [1] to [7], wherein the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) is 2.06 or more. [9] The secondary battery according to [8], wherein the electrode density of the positive electrode is 3.5 g / cc or more.

[10] The secondary battery according to [8], wherein the electrode density of the negative electrode is 1.7 g / cc or less.

[11] A secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, A secondary battery in which the ratio of the electrode density of the positive electrode to the electrode density of the negative electrode (positive electrode density / negative electrode density) is 2.06 or more.

[12] The secondary battery according to

[11] , wherein the electrode density of the positive electrode is 3.5 g / cc or more.

[13] The secondary battery according to

[11] , wherein the electrode density of the negative electrode is 1.7 g / cc or less.

Claims

[Claim 1] A secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, the negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material and a carbon-based active material, the positive electrode contains a nickel-based active material, A secondary battery in which a cobalt oxide layer is provided on a part of the surface of the particles of the nickel-based active material.

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