Lithium secondary battery and method for manufacturing the same

Abstract

The present invention relates to a lithium secondary battery including a positive electrode, a separator, and a negative electrode, wherein the positive electrode includes a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co), the negative electrode includes a silicon-based active material and a carbon-based active material, and the efficiency constants of the silicon-based active material and the carbon-based active material and the efficiency constant of the lithium composite transition metal compound satisfy a specific formula.

Description

[Technical field]

[0001] The present invention relates to a lithium secondary battery and a method for producing the same.

[0002] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0131766, filed with the Korean Intellectual Property Office on September 30, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

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

[0004] Such secondary batteries have the primary advantage of dramatically reducing the use of fossil fuels, as well as the advantage of not producing any by-products associated with energy use, and are therefore attracting attention as a new energy source that is environmentally friendly and can improve energy efficiency.

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

[0006] As the use of secondary batteries increases, various battery performances are required. In order to improve battery performance, appropriate attempts have been made to determine the composition of the electrode active material layer, and although some battery performances may be improved by selecting or combining materials, other performances may be deteriorated. Therefore, it is necessary to research the selection or combination of materials appropriate for the performance of the secondary battery to be improved. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a lithium secondary battery having a high energy density and excellent life characteristics.

Means for Solving the Problems

[0008] One embodiment of the present invention is a lithium secondary battery including a positive electrode, a separator, and a negative electrode, wherein the positive electrode includes a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co), the negative electrode includes a silicon-based active material and a carbon-based active material, when an efficiency constant of the silicon-based active material and an efficiency constant of the carbon-based active material are a and b, respectively, a weight part of the silicon-based active material is a* based on 100 weight parts of a total amount of the silicon-based active material and the carbon-based active material, and an efficiency constant of the lithium composite transition metal compound is c, a, a*, b, and c satisfy the following formula 1, and a lithium secondary battery is provided: [Formula 1] [(a × a*) + {b × (100 - a*)}] / c > 103.5

[0009] Another embodiment of the present invention is forming a positive electrode including a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co); forming a negative electrode including a silicon-based active material and a carbon-based active material; and forming a lithium secondary battery including the positive electrode and the negative electrode, when an efficiency constant of the silicon-based active material and an efficiency constant of the carbon-based active material are a and b, respectively, a weight part of the silicon-based active material is a* based on 100 weight parts of a total amount of the silicon-based active material and the carbon-based active material, and an efficiency constant of the lithium composite transition metal compound is c, including adjusting so that a, a*, b, and c satisfy the following formula 1, and a method for manufacturing a lithium secondary battery according to the above-described embodiment is provided: [Formula 1] [(a × a*) + {b × (100 - a*)}] / c > 103.5

Advantages of the Invention

[0010] According to the embodiments described in this specification, materials excellent in charging and discharging efficiency can be selected as the active materials of the positive electrode and the negative electrode to achieve a high energy density, and the efficiency constants and contents due to the types of active materials contained in the positive electrode and the negative electrode are adjusted so as to satisfy a specific mathematical formula, thereby matching the efficiency balance between the positive electrode and the negative electrode, and thus, the battery life and rapid charging performance can be improved.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, the present invention will be described in more detail to facilitate understanding of the present invention. The present invention may be realized in various different forms and is not limited to the embodiments described herein. At this time, the terms and words used in this specification and the claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors, in accordance with the principle that they can appropriately define the concepts of the terms in order to explain their invention in the best way, are to be construed as having meanings and concepts that conform to the technical idea of the present invention.

[0012] In this specification, terms such as "including", "providing", or "having" are intended to specify that the implemented features, numbers, steps, components, or combinations thereof exist, and it should be understood that they do not preclude the existence or addition possibility of one or more other features, numbers, steps, components, or combinations thereof in advance.

[0013] Also, when it is stated that a certain part such as a layer exists "on" or "above" another part, this includes not only the case where it exists "directly above" the other part but also the case where there are other parts in between. Conversely, when it is stated that a certain part exists "directly above" another part, it means that there are no other parts in between. Also, existing "on" or "above" a reference part does not necessarily mean being located "above" or "higher" in the opposite direction of gravity, but means being located above or below the reference part.

[0014] In this specification, the "primary particle" means a particle that does not have grain boundaries in appearance when observed at a magnification of 5000 to 20000 times using a scanning electron microscope.

[0015] In this specification, the "secondary particle" is a particle formed by aggregation of the primary particles.

[0016] In this specification, the single particle is a term used to distinguish from the secondary particle-like positive electrode active material particles formed by aggregation of several tens to several hundreds of primary particles that have been generally used in the past, and is a concept including a single particle composed of one primary particle and an aggregate particle of 10 or fewer primary particles.

[0017] In this specification, when "particle" is described, it may mean any one or all of a single particle, a secondary particle, and a primary particle.

[0018] A lithium secondary battery according to an embodiment of this specification includes a positive electrode, a separator, and a negative electrode. The positive electrode includes a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co). The negative electrode includes a silicon-based active material and a carbon-based active material. The efficiency constants of the silicon-based active material and the carbon-based active material are a and b, respectively. When the weight part of the silicon-based active material is a* based on 100 weight parts of the total amount of the silicon-based active material and the carbon-based active material, and the efficiency constant of the lithium composite transition metal compound is c, a, a*, b, and c satisfy the following formula 1.

[0019] [Formula 1] [(a × a*) + {b × (100 - a*)}] / c > 103.5

[0020] The value of the formula 1 can be calculated as a value up to one decimal place.

[0021] In order to achieve a high energy density of the battery, the inventors recognized that it is necessary to design high-efficiency positive and negative electrode materials with good charging / discharging efficiency. However, when using a silicon-based active material with a large capacity per unit weight as the negative electrode material, the deterioration of the negative electrode is caused by low efficiency, and thus there is a problem that the long-term life characteristics deteriorate. Therefore, the inventors used a high-efficiency silicon-based active material as the negative electrode active material and a lithium nickel cobalt-based compound as the positive electrode active material. It was clarified that when specific efficiency parameters are satisfied, the efficiency balance between the positive electrode and the negative electrode can be adjusted, which can affect the long life, and thus the present invention was completed. Specifically, when the above-described components contained in the positive electrode and the negative electrode satisfy the above formula 1, a battery having a high energy density can be realized, the efficiency balance between the positive electrode and the negative electrode can be improved, the initial deterioration of the negative electrode can be prevented, and the life characteristics can be significantly improved.

[0022] According to one embodiment, the silicon-based active material may be a silicon carbon composite or silicon oxide, and the carbon-based active material may be graphite. The lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) may be a single particle. According to one embodiment, the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) may contain 80 mol% or more, for example, 80 mol% or more and less than 100 mol% of nickel among the metals excluding lithium. When the content of nickel is high in this way, the efficiency of the positive electrode can be increased, and a high energy density of the battery can be realized.

[0023] According to one embodiment, the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) may further contain at least one of manganese and aluminum. Specifically, the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) may be represented by the following chemical formula 1.

[0024] [Chemical formula 1] Li a Ni (1-x-y) Co x M1 y M2w O2

[0025] In Chemical Formula 1, 1.0 ≤ a ≤ 1.5, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, 0 ≤ w ≤ 0.1, and 0 ≤ x + y ≤ 0.2, M1 is at least one metal selected from Mn and Al, M2 is one or more metal elements selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo.

[0026] The average particle size (D50) of the lithium composite transition metal compound may be 12 μm to 30 μm, for example, 13 μm to 28 μm, 15 μm to 25 μm, or 17 μm to 23 μm. The BET specific surface area of the lithium composite transition metal compound is 0.5 m 2 / g to 1.1 m 2 / g, for example, 0.6 m 2 / g to 1 m 2 / g, or 0.7 m 2 / g to 0.9 m 2 / g. The composition ratio, average particle size (D50), BET specific surface area, etc. of the elements constituting the lithium composite transition metal compound affect the efficiency constant c of the lithium composite transition metal compound.

[0027] According to one embodiment, based on 100 parts by weight of the cathode active material, the lithium composite transition metal compound containing the nickel (Ni) and cobalt (Co) may be contained in an amount of 90 parts by weight to 100 parts by weight, for example, more than 92 parts by weight, more than 94 parts by weight, more than 96 parts by weight, more than 98 parts by weight, or 98 parts by weight to 100 parts by weight.

[0028] In this specification, the silicon-carbon composite is a composite of Si and C, which may be represented by an Si / C-based active material and is distinguished from silicon carbide (Silicon carbide) represented by SiC. The silicon-carbon composite may be a composite of silicon and graphite or the like, and may form a structure surrounded by graphene or amorphous carbon or the like around a core in which silicon and graphite or the like are composite. The silicon dispersed in the silicon-carbon composite may be nanosilicon.

[0029] The silicon oxide may be represented by SiO x (0 ≦ x < 2), and may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0030] The SiO x (0 < x < 2) corresponds to the matrix in the silicon-based composite particles. The SiO x (0 < x < 2) may be in a form containing Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2).

[0031] The average particle size (D50) of the silicon-based active material may be 2 μm to 15 μm, specifically 3 μm to 13 μm, and more specifically 4 μm to 12 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. Further, the size of the Si crystal grains contained in the silicon-based active material may be 10 nm to 30 nm, for example, 15 nm to 25 nm.

[0032] The composition ratio of the elements constituting the silicon-based active material, the presence or absence and type of the coating layer on the particle surface, the average particle size (D50), the size of the Si crystal grains, etc. affect the efficiency constant a of the silicon-based active material.

[0033] In this specification, the average particle diameter (D50) can be defined as the particle diameter corresponding to 50% of the volume cumulative amount in the particle diameter distribution curve of the particles. The average particle diameter (D50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle diameters in the range from the submicron region to about several millimeters, and can obtain highly reproducible and highly resolved results.

[0034] In this specification, the "crystallite size" can be quantitatively analyzed using X-ray diffraction analysis (XRD) with Cu Kα X-rays.

[0035] In this specification, the "specific surface area" is measured by the BET method. Specifically, for the measurement object, degassing is performed at 130°C for 2 hours using a BET measuring device (BEL-SORP-mini, Nippon Bell), and N2 adsorption / desorption is performed at 77K for measurement.

[0036] According to one embodiment, the silicon-based active material has a discharge efficiency of 85% to 95%.

[0037] The charge capacity and discharge capacity of the active materials of the negative electrode and the positive electrode can be measured as follows.

[0038] First, the active material: Super-C (registered trademark) conductive material: carboxymethyl cellulose (CMC) thickener: styrene-butadiene rubber (SBR) binder polymer are put into water at a weight ratio of 95:1:1:3 to produce a slurry. The slurry is coated on a copper foil, 1.4875 cm 2It is punched out to have the area, rolled, dried, and then an electrode is manufactured. Together with the said electrode, lithium metal is used as a counter electrode, and a polypropylene separator is interposed to manufacture an electrode assembly. Ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 3:7, vinylene carbonate is added to make the concentration 1 wt%, 1 M LiPF6 is added to the organic solvent thus obtained to manufacture a non-aqueous electrolyte, and then it is injected into the said electrode assembly to manufacture a coin half cell (CHC).

[0039] The coin half cell (CHC) manufactured above, during charging, a constant current is applied up to 0.005 V in a CC-CV (constant current constant voltage) manner at a rate of 0.2C, and then the current is controlled at a constant voltage of 0.005 V to measure the charge capacity. During discharging, it is cut off at 1.5 V in a CC (constant current) manner at a rate of 0.2C to measure the discharge capacity.

[0040] The percentage of the discharge capacity with respect to the charge capacity measured using a silicon-based active material as the said active material, that is, [discharge capacity (シリコン系) / charge capacity (シリコン系) ×100] is defined as the efficiency constant a, and the percentage of the discharge capacity with respect to the charge capacity measured using a carbon-based active material as the said active material, that is, [discharge capacity (炭素系) / charge capacity (炭素系) ×100] is defined as the efficiency constant b, and the percentage of the discharge capacity with respect to the charge capacity measured using a positive electrode active material as the said active material, that is, [discharge capacity (ニッケルコバルト系) / charge capacity (ニッケルコバルト系) ×100] can be defined as the efficiency constant c.

[0041] According to one example, a may be 70 to 85, for example 75 to 80, b may be 85 to 98, for example 90 to 95, and c may be 85 to 90, for example 86 to 89.

[0042] According to one embodiment, the silicon-based active material may be included in an amount of 1 part by weight to 20 parts by weight based on 100 parts by weight of the negative electrode active material. Also, the silicon-based active material may be included in an amount of 1 part by weight to 20 parts by weight, for example, 2 parts by weight to 18 parts by weight, 3 parts by weight to 15 parts by weight, or 3 parts by weight to 10 parts by weight based on 100 parts by weight of the total amount of the silicon-based active material and the carbon-based active material. When the content of the silicon-based active material is 1% by weight or more, the advantage of using the material can be achieved by the capacity gain being a certain level or more, and when it is 20% by weight or less, it is advantageous for the lifespan and battery characteristics by preventing excessive swelling, and also has price competitiveness from the perspective of cost.

[0043] According to one embodiment, the graphite included in the negative electrode may be natural graphite, artificial graphite, or a mixture thereof. The graphite may be included in an amount of 80 parts by weight or more and 99 parts by weight or less, for example, 85 parts by weight to 98 parts by weight, 87 parts by weight to 97 parts by weight, or 92 parts by weight to 96 parts by weight based on 100 parts by weight of the active material included in the negative electrode. When the graphite contains both artificial graphite and natural graphite, the content ratio thereof may be 5:5 to 9:1, for example, 6:4 to 8:2, or 7:3. The average particle size (D50) of the artificial graphite may be 5 μm to 20 μm, for example, 6 μm to 18 μm, 7 μm to 15 μm, or 8 μm to 12 μm, and the average particle size (D50) of the natural graphite may be 5 μm to 30 μm, for example, 7 μm to 28 μm, 10 μm to 27 μm, 12 μm to 26 μm, or 15 μm to 25 μm.

[0044] According to one embodiment, the negative electrode may include a current collector; and a negative electrode active material layer provided on the current collector.

[0045] With respect to 100 parts by weight of the negative electrode active material layer, the negative electrode active material may be included in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 88 parts by weight or more and 99 parts by weight or less, 90 parts by weight or more and 98 parts by weight or less, and more preferably 95 parts by weight or more and 99.9 parts by weight or less.

[0046] According to one embodiment of the present specification, the negative electrode active material layer may further include a negative electrode binder in addition to the silicon-based active material and the carbon-based active material.

[0047] The negative electrode binder can play a role in improving the adhesion between negative electrode active material particles and the adhesive force between the negative electrode active material particles and the negative electrode current collector. As the negative electrode binder, any material well-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 at least any one selected from the group consisting of substances in which hydrogen thereof is substituted with Li, Na, Ca, etc., and various copolymers thereof may also be included.

[0048] The negative electrode binder may be included 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 0.3 part by weight or more and 20 parts by weight or less, more preferably 0.5 part by weight or more and 10 parts by weight or less.

[0049] The negative electrode active material layer may not contain a conductive material, but may further contain a conductive material if necessary. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it does not cause a chemical change 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; metal powders such as fluorocarbons, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives may be used. The content of the conductive material in the negative electrode active material layer may be 0.01 parts by weight to 20 parts by weight, preferably 0.03 parts by weight to 18 parts by weight, based on 100 parts by weight of the negative electrode active material layer.

[0050] For the purpose of the present invention, when the carbon-based active material is graphite such as natural graphite or artificial graphite, the parts by weight of the graphite used as the carbon-based active material are not considered when defining the total parts by weight of the conductive material. Similarly, when the conductive material selected according to the negative electrode is graphite, the parts by weight of the described conductive material are not included when defining the total parts by weight of the carbon-based negative electrode active material. Therefore, when graphite is selected as both the carbon-based negative electrode active material and the negative electrode conductive material, the total parts by weight of graphite correspond to the sum of the parts by weight of the graphite used as the carbon-based negative electrode active material and the parts by weight of the graphite used as the negative electrode conductive material.

[0051] According to an example, as the conductive material contained in the negative electrode active material layer, 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 may be used.

[0052] In one embodiment of the present specification, the thickness of the negative electrode active material layer may be 5 μm or more and 500 μm or less.

[0053] In one embodiment of the present specification, the negative electrode current collector may be any material that does not cause a chemical change in the battery and has conductivity, 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 of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Specifically, transition metals that adsorb carbon well, such as copper and nickel, may be used as the current collector. The thickness of the current collector may be 1 μm to 500 μm, but the thickness of the current collector is not limited thereto.

[0054] 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 a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co). The thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

[0055] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Further, the positive electrode current collector may usually have a thickness of 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, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven bodies, etc.

[0056] In one embodiment, based on 100 parts by weight of the positive electrode active material layer, the positive electrode active material may be contained in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less.

[0057] According to a further embodiment of the present specification, the positive electrode active material layer may further include a positive electrode binder and a conductive material.

[0058] 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, any material well-known in the technical field 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.

[0059] The positive electrode binder may be contained in an amount of 0.1 part 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, preferably, it may be contained in an amount of 0.3 part by weight or more and 35 parts by weight or less, and more preferably 0.5 part by weight or more and 20 parts by weight or less.

[0060] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode, and any material having electron conductivity without causing a chemical change in the battery can be used without particular limitation. 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 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.

[0061] Specifically, 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 included in an amount of 0.1 part 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 may preferably be included in an amount of 0.3 part by weight or more and 1.5 parts by weight or less, and more preferably 0.5 part by weight or more and 1.2 parts by weight or less.

[0062] The positive electrode and the negative electrode can be manufactured by the usual manufacturing methods for positive and negative electrodes, except that the above-described positive and negative electrode active materials are used. Specifically, after applying a composition for forming an active material layer, which includes the above-described active material and optionally a binder and a conductive material, onto a current collector, it can be manufactured by drying and rolling. At this time, the types and contents of the positive and negative electrode active materials, the binder, and the conductive material are as described above. As the solvent, any solvent generally used in the art may be used, 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 thereof may be used. The amount of the solvent used is such that, considering the coating thickness of the slurry and the manufacturing yield, the active material, the conductive material, and the binder are dissolved or dispersed, and then it has a viscosity that can exhibit excellent thickness uniformity during coating for the manufacture of the positive and negative electrodes. Also, as another method, the positive and negative electrodes can also be manufactured by casting the composition for forming the active material layer onto another support, and then laminating the film obtained by peeling it from the support onto the current collector.

[0063] The separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Usually, any separator that can be used as a separator in a secondary battery can be used without particular limitation, and it is particularly preferably low in resistance to the ion migration of the electrolyte and excellent in the ability to hold the electrolyte solution. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. Also, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, 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 may be used, and it may be selectively used as 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 in the manufacture of lithium secondary batteries.

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

[0066] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-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 may be used.

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

[0068] As the metal salt, a lithium salt may be used. The lithium salt is a substance that is easily soluble 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 - 、および(CF3CF2SO2)2N - One or more selected from the group consisting of may also be used.

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

[0070] A lithium secondary battery according to an 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 lithium 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 lithium 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- to 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 any one or more of medium- to large-sized devices including power tools; electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); or power storage systems.

[0073] One embodiment of the present invention provides a method for manufacturing a lithium secondary battery, which includes: forming a positive electrode including a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co); forming a negative electrode including a silicon-based active material and a carbon-based active material; and forming a lithium secondary battery including the positive electrode and the negative electrode, wherein the efficiency constants of the silicon-based active material and the carbon-based active material are a and b, respectively, the weight part of the silicon-based active material is a* based on 100 weight parts of the total amount of the silicon-based active material and the carbon-based active material, and the efficiency constant of the lithium composite transition metal compound is c, and the method includes adjusting a, a*, b, and c so as to satisfy the following formula (1): [Formula 1] [(a × a*) + {b × (100 - a*)}] / c > 103.5

[0074] The step of adjusting a, a*, b, and c so as to satisfy formula (1) includes: forming a positive electrode including a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co); forming a negative electrode including a silicon-based active material and a carbon-based active material; Forming a half-cell including the positive electrode or the negative electrode, a reference electrode, and an electrolyte; Measuring the charge capacity and discharge capacity of the half-cell; and It may include calculating an efficiency constant using the following formula 2: [Formula 2] Efficiency constant = [discharge capacity / charge capacity] Regarding the positive electrode, negative electrode, and efficiency constant of the method for manufacturing a lithium secondary battery according to the embodiment, the descriptions regarding the lithium secondary battery described above can be applied.

[0075] Hereinafter, preferred examples are presented to assist in the understanding of the present invention. However, it should be obvious to those skilled in the art that the examples are merely illustrative of the description and various changes and modifications are possible within the scope of the description and the scope of the technical idea. Needless to say, such variations and modifications belong to the scope of the appended claims.

[0076] Example 1 A composition for forming a negative electrode active material layer containing a negative electrode active material containing SiO and graphite (the weight ratio of SiO and graphite is a* and (100 - a*), respectively), a conductive material (carbon black, CNT), a binder (SBR), and a thickener (CMC) in a weight ratio of 95.6:1.0:2.3:1.1 was coated on a copper foil with a thickness of 10 μm so that the dry thickness became 140 μm and then dried to manufacture a negative electrode. Here, SiO had a particle size (D50) of 10 μm and the size of Si crystal grains was 20 nm. The graphite contained artificial graphite and natural graphite in a weight ratio of 7:3, and the average particle sizes (D50) of the artificial graphite and natural graphite were 10 μm and 20 μm, respectively.

[0077] Li 1.0 Ni 0.86 Co 0.06 Mn 0.08A composition for forming a positive electrode active material layer containing lithium nickel-based oxide, binder (PVDF), and conductive material (CNT) in a weight ratio of 97:1:2 was coated on an aluminum foil with a thickness of 15 μm so that the dry thickness became 130 μm, and then dried to produce a positive electrode. Here, the particle size (D50) of the lithium nickel-based oxide was 20 μm, and the BET was 0.8 m 2 / g.

[0078] The positive electrode and the negative electrode were laminated with a separator interposed therebetween, and an electrolytic solution was injected to fabricate a battery. A separator with a thickness of 12 μm provided with a coating layer containing Al2O3 and a PVDF binder was used on a base film having a triple structure of PE / PP / PE. As the electrolytic solution, 1M LiPF6, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio 3 / 7), and vinylene carbonate (VC) / propane sultone (PS, propane sultone) (3 parts by weight and 1.5 parts by weight respectively based on 100 parts by weight of the electrolyte) were used.

[0079] The values of Formula 1 described above for the fabricated battery are shown in Table 1. In order to measure the normal temperature life and high temperature life of the fabricated battery, the capacity retention rate when performing charge and discharge cycles 200 times under the following conditions is shown in Table 2 below.

[0080] * Normal temperature life: Charging was performed at a constant current / constant voltage (CC / CV) up to 4.2V at 0.33C at normal temperature (25°C) (0.05C cut-off), and discharging was performed at a constant current (CC) of 0.5C (2.5V cut-off) to perform cycles.

[0081] * High temperature life: Charging was performed at a constant current / constant voltage (CC / CV) up to 4.2V at 0.33C at high temperature (45°C) (0.05C cut-off), and discharging was performed at a constant current (CC) of 0.33C (2.5V cut-off) to perform cycles.

[0082] * Fast charging life: Charging was performed from 0% to 80% of the state of charge (SOC) for 25 minutes, and discharging was performed at a constant current (CC) of 0.33C (2.5V cut-off) to perform cycles.

[0083] Example 2 It was carried out in the same manner as in Example 1, except that a silicon-carbon composite (Si / C) was used instead of SiO in the negative electrode active material. Here, the silicon-carbon composite (Si / C) had a particle size (D50) of 10 μm and the size of Si crystal grains was 20 nm.

[0084] Comparative Example 1 Li 1.0 Ni 0.86 Co 0.06 Mn 0.08 0.08 Instead of the LiNiO2 lithium nickel-based oxide, Li 1.0 Ni 0.84 Co 0.08 Mn 0.08 0.08 It was carried out in the same manner as in Example 1, except that the LiNiO2 lithium nickel-based oxide was used. Here, the particle size (D50) of the lithium nickel-based oxide was 10 μm and the BET was 1.2 m 2 / g.

[0085] Comparative Example 2 Li 1.0 Ni 0.86 Co 0.06 Mn 0.08 0.08 Instead of the LiNiO2 lithium nickel-based oxide, Li 1.0 Ni 0.84 Co 0.08 0.08 It was carried out in the same manner as in Example 1, except that the LiNi0.08MnO2 lithium nickel-based oxide was used and the silicon-carbon composite (SiC) was used instead of SiO in the negative electrode active material. Here, the particle size (D50) of the lithium nickel-based oxide was 10 μm and the BET was 1.2 m 2 / g.

[0086] Comparative Example 3 Li 1.0 Ni 0.86 Co 0.06 Mn 0.08 0.08 Instead of the LiNiO2 lithium nickel-based oxide, Li 1.0 Ni 0.84 Co 0.08 Mn 0.08Except for using O2 lithium nickel-based oxide and having the weight ratio a* of SiO in the negative electrode active material being 3, it was carried out in the same manner as in Example 1. Here, the particle size (D50) of the lithium nickel-based oxide was 10 μm, and the BET was 1.2 m 2 / g.

[0087]

Table 1

[0088]

Table 2

[0089] As shown in Table 2 above, it was confirmed that the capacity retention rates after 200 cycles at normal temperature and high temperature and the capacity retention rate after 50 cycles of rapid charging were superior in Example 1 and Example 2 compared with the comparative examples.

Claims

1. A lithium secondary battery including a positive electrode, a separator, and a negative electrode, wherein the positive electrode includes a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co), the negative electrode includes a silicon-based active material and a carbon-based active material, the efficiency constant of the silicon-based active material is a, the efficiency constant of the carbon-based active material is b, the weight part of the silicon-based active material is a* based on 100 weight parts of the total amount of the silicon-based active material and the carbon-based active material, and when the efficiency constant of the lithium composite transition metal compound is c, a, a*, b, and c satisfy the following formula 1, 【Formula 1】 [(a × a*) + {b × (100 - a*)}] / c > 103.5 A lithium secondary battery.

2. The lithium secondary battery according to claim 1, wherein the silicon-based active material is a silicon carbon composite or silicon oxide, and the carbon-based active material is graphite.

3. The lithium secondary battery according to claim 1, wherein the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) contains 80 mol% or more of nickel among the metals excluding lithium.

4. The lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) is represented by the following chemical formula 1, 【Chemical Formula 1】 Li ａ Ni （１－ｘ－ｙ） Co ｘ M1 ｙ M2 ｗ O ２ In Chemical Formula 1, 1.0 ≤ a ≤ 1.5, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, 0 ≤ w ≤ 0.1, 0 ≤ x + y ≤ 0.2, M1 is at least one metal among Mn or Al, M2 is one or more metal elements selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo. The lithium secondary battery according to claim 1.

5. The lithium secondary battery according to claim 2, wherein the silicon carbon composite has a discharge efficiency of 85% to 95%.

6. The lithium secondary battery according to claim 2, wherein the positive electrode contains 90 to 100 weight parts of the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) based on 100 weight parts of the positive electrode active material.

7. A method for manufacturing the lithium secondary battery according to any one of claims 1 to 6, comprising: forming a positive electrode including a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co); forming a negative electrode including a silicon-based active material and a carbon-based active material; forming a lithium secondary battery including the positive electrode and the negative electrode. The efficiency constant of the silicon-based active material is a, the efficiency constant of the carbon-based active material is b, the weight part of the silicon-based active material is a* based on 100 weight parts of the total amount of the silicon-based active material and the carbon-based active material, and when the efficiency constant of the lithium composite transition metal compound is c, it includes the step of adjusting a, a*, b, and c so as to satisfy the following formula 1: [Formula 1] [(a × a*) + {b × (100 - a*)}] / c > 103.5 A method for manufacturing a lithium secondary battery, which is as follows.

8. The step of adjusting a, a*, b, and c so as to satisfy Formula 1 includes: forming a positive electrode including a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co); forming a negative electrode including a silicon-based active material and a carbon-based active material; forming a half cell including the positive electrode or the negative electrode, a reference electrode, and an electrolyte; measuring the charge capacity and the discharge capacity of the half cell; calculating an efficiency constant using the following formula 2, and includes: [Formula 2] Efficiency constant = [Discharge capacity / Charge capacity] The method for manufacturing a lithium secondary battery according to claim 7, which is as follows.

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