Lithium-ion battery

Abstract

To provide a lithium secondary battery with high energy density and excellent lifespan characteristics. [Solution] The present invention relates to a lithium secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the positive electrode comprises a lithium composite transition metal compound comprising nickel (Ni) and cobalt (Co), the negative electrode comprises a mixed negative electrode active material of a silicon-based active material and a carbon-based active material, the electrolyte comprises fluoroethylene carbonate, and the efficiency constants of the mixed negative electrode active material and the lithium composite transition metal compound and the weight of the fluoroethylene carbonate satisfy a specific formula.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0131734, filed with the Korean Intellectual Property Office on October 13, 2022, and all of its contents are incorporated herein by reference.

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

Background Art

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

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

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

[0006] As the utilization of secondary batteries increases, various battery performances are required. Appropriate attempts have been made to adjust the composition of the electrode active material layer to improve battery performance. Although some battery performances can be improved by material selection or combination, some performances may deteriorate instead. Therefore, research on the selection or combination of suitable materials according to the performance of the secondary battery to be improved is necessary.

Summary of the Invention

Means for Solving the Problems

[0008] One embodiment of the present invention is a lithium secondary battery including a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the positive electrode includes a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co), the negative electrode includes a mixed negative electrode active material of a silicon-based active material and a carbon-based active material, the electrolyte includes fluoroethylene carbonate, when the efficiency constants of the mixed negative electrode active material and the lithium composite transition metal compound are a and b, respectively, and the weight part of the fluoroethylene carbonate is c based on 100 weight parts of the electrolyte, a, b, and c satisfy the following formula 1 to provide a lithium secondary battery: [Formula 1] [(b / a)×100 - c]≦95

Advantages of the Invention

[0009] According to the embodiments described herein, 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 balance between the positive electrode and the negative electrode can be adjusted by adjusting the efficiency constant according to the types and mixing ratios of the active materials contained in the positive electrode and the negative electrode and the content of a specific electrolyte additive so as to satisfy a specific mathematical formula. As a result, the life and rapid charging performance of the battery can be improved. Specifically, fluoroethylene carbonate helps to form a film that protects the silicon-based compound, and the appropriate ratio of fluoroethylene carbonate when the deterioration of the negative electrode is severe effectively acts due to the efficiency balance between the positive / negative electrodes. The larger the remaining amount of fluoroethylene carbonate, the more gas generation and cell deterioration due to side reactions occur, so an appropriate amount based on the efficiency balance between the positive / negative electrodes is required.

Modes for Carrying Out the Invention

[0010] The present invention will be described in more detail below to aid in understanding the invention. The present invention may be realized in various different forms and is not limited to the embodiments described herein. In this regard, the terms and words used herein and in the claims shall not be interpreted to be limited to their usual or dictionary meanings, but rather to be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0011] In this specification, terms such as “include,” “provide,” or “have” are intended to indicate the presence of implemented features, figures, steps, components, or combinations thereof, and should not preemptively exclude the existence or possibility of adding one or more other features, figures, steps, components, or combinations thereof.

[0012] Furthermore, when we say that a layer or other part exists "on top of" or "above" another part, this includes not only when it is "directly above" the other part, but also when there are other parts in between. Conversely, when we say that a part exists "directly above" another part, it means that there are no other parts in between. Also, to exist "on top of" or "above" a reference part means to be located above or below the reference part, and does not necessarily mean to be located "above" or "above" in the opposite direction of gravity.

[0013] In this specification, "primary particle" means a particle that, when observed with a scanning electron microscope at a field of view of 5,000x to 20,000x, does not appear to have a grain boundary.

[0014] In this specification, "secondary particles" refer to particles formed by the aggregation of the primary particles.

[0015] In this specification, "single particle" is a term used to distinguish it from conventionally used secondary particulate cathode active material particles formed by the aggregation of tens to hundreds of primary particles, and is a concept that includes single particles consisting of one primary particle and aggregate particles of 10 or fewer primary particles.

[0016] In this specification, the term "particle" may include one or all of the following: single particles, secondary particles, and primary particles.

[0017] A lithium secondary battery according to one embodiment of this specification comprises a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the positive electrode comprises a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co), the negative electrode comprises a mixed negative electrode active material of a silicon-based active material and a carbon-based active material, and the electrolyte comprises fluoroethylene carbonate. In the above-described embodiment, when the efficiency constants of the mixed negative electrode active material and the lithium composite transition metal compound are a and b, respectively, and the weight of the fluoroethylene carbonate is c based on 100 parts by weight of the electrolyte, then a, b, and c satisfy the following formula 1.

[0018] [Formula 1] [(b / a)×100-c]≦95

[0019] The value of formula 1 can be calculated to one decimal place.

[0020] The charging capacity and discharging capacity of the active materials of the negative electrode and the positive electrode can be measured as follows.

[0021] First, the active material and Super-C® conductive material: carboxymethylcellulose (CMC) thickener: styrene-butadiene rubber (SBR) binder polymer are added to water in a weight ratio of 95:1:1:3 to produce a slurry. This slurry is then coated onto copper foil, and the mixture is heated to a depth of 1.4875 cm². 2An electrode is manufactured by punching out a shape with a certain surface area, rolling it, and then drying it. Along with the electrode, a lithium metal is used as the counter electrode, and an electrode assembly is manufactured by interposing a polypropylene separator. Ethylene carbonate and ethyl methyl carbonate are mixed in a volume ratio of 3:7, vinylene carbonate is added to an organic solvent to a concentration of 1% by weight, and 1M LiPF6 is added to this to manufacture a non-aqueous electrolyte. This is then injected into the electrode assembly to manufacture a coin-type half cell (CHC).

[0022] The coin-type half-cell (CHC) manufactured as described above can be charged by applying a constant current down to 0.005V at a speed of 0.2C using the CC-CV (constant current constant voltage) method, and then controlling the current with a constant voltage at 0.005V. During discharge, it can be cut off at 1.5V using the CC (constant current) method at a speed of 0.2C, and the discharge efficiency can be measured.

[0023] The percentage ratio of discharge capacity to charge capacity measured using a mixed negative electrode active material as the active material, i.e., [discharge capacity (負極) / charging capacity (負極) Let the efficiency constant be [×100], and use a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as the active material to measure the percentage ratio of discharge capacity to charge capacity, i.e., [discharge capacity (正極) / charging capacity (正極) The efficiency constant b can be defined as [×100].

[0024] The efficiency constants of the negative and positive electrode active materials may vary depending on the type of active material and / or the mixing ratio.

[0025] The inventors of this invention recognized that in order to achieve high energy density in batteries, it is necessary to design highly efficient positive and negative electrode materials with good charge / discharge efficiency. However, when silicon-based active materials with a large capacity per unit weight are used as the negative electrode material, low efficiency causes degradation of the negative electrode, which leads to a problem of degraded long-term life characteristics. Therefore, the inventors of this invention have demonstrated that by using silicon-based active materials and lithium nickel cobalt-based compounds in single particle form as the negative electrode active material and positive electrode active material, respectively, and using fluoroethylene carbonate as an electrolyte additive, it is possible to balance the efficiency of the positive and negative electrodes and influence the long lifespan when certain efficiency parameters are met, thus completing the present invention. Specifically, by having the aforementioned components contained in the positive electrode, negative electrode, and electrolyte satisfy the above formula 1, it is possible to realize a battery with high energy density, improve the efficiency balance of the positive and negative electrodes, and significantly improve the life characteristics.

[0026] According to one embodiment, the lithium-compound transition metal compound containing nickel (Ni) and cobalt (Co) may contain 80 mol% or more of nickel among the metals other than lithium, for example, 80 mol% or more but less than 100 mol%. When the nickel content is high in this way, the efficiency of the positive electrode can be increased, and a high energy density of the battery can be achieved.

[0027] According to one embodiment, the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) is a single particle.

[0028] According to one embodiment, the lithium-compound transition metal compound containing nickel (Ni) and cobalt (Co) may further contain at least one of manganese and aluminum. The lithium-compound transition metal compound may contain 80 mol% or more of nickel among the metals other than lithium, for example, 80 mol% or more and less than 100 mol%. Specifically, the lithium-compound transition metal compound containing nickel (Ni) and cobalt (Co) may be represented by the following chemical formula 1.

[0029] [Chemical formula 1] Li a Ni (1-x-y) Co x M1 y M2 w O2 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 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.

[0030] According to one embodiment, based on 100 parts by weight of the positive electrode active material, the lithium composite transition metal compound containing the nickel (Ni) and cobalt (Co) may be contained in an amount of 90 to 100 parts by weight.

[0031] In one embodiment of the present specification, the silicon-based active material is SiO x (0 ≤ x < 2), SiM y (M is a metal, 1 ≤ y ≤ 4), and at least one of Si / C. The silicon-based active material may contain only one kind, or two or more kinds may be contained together. When both of the two negative electrode active material layers contain the silicon-based active material, the same kind of silicon-based active material may be used for the two active material layers, or different types or different combinations of silicon-based active materials may be used. The silicon-based active material may be contained in an amount of 1 part by weight or more and less than 20 parts by weight, for example, 1 to 10 parts by weight, based on 100 parts by weight of the total of the above-described mixed negative electrode active material.

[0032] The active material containing SiO x (0 ≤ x < 2) may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0033] The SiO x (0 < x < 2) corresponds to a matrix in the silicon-based composite particles. The SiOx (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). When the silicon-based composite particles contain the SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0034] The silicon-based composite particles may further contain at least one of a magnesium (Mg) compound and a lithium (Li) compound. The magnesium (Mg) compound and the lithium (Li) compound may correspond to a matrix within the silicon-based composite particles.

[0035] The magnesium (Mg) compound and / or the lithium (Li) compound may be present inside and / or on the surface of the SiO x (0 < x < 2). The initial efficiency of the battery can be improved by the Mg compound and / or the Li compound.

[0036] The magnesium (Mg) compound may contain at least any one selected from the group consisting of magnesium (Mg) silicate, magnesium (Mg) silicide, and magnesium (Mg) oxide. The magnesium (Mg) silicate may contain at least any one of Mg2SiO4 and MgSiO3. The magnesium (Mg) silicide may contain Mg2Si. The Mg oxide may contain MgO.

[0037] In one embodiment of the present specification, the magnesium (Mg) element may be contained in an amount of 0.1% to 20% by weight, or 0.1% to 10% by weight, based on 100% by weight of the total silicon-based active material. Specifically, the magnesium (Mg) element may be contained in an amount of 0.5% to 8% by weight, or 0.8% to 4% by weight. When the above range is satisfied, the magnesium (Mg) compound can be contained in an appropriate content in the silicon-based active material, so that the volume change of the silicon-based active material during charging and discharging of the battery can be easily suppressed, and the discharge capacity and initial efficiency of the battery can be improved.

[0038] The lithium (Li) compound may contain at least one selected from the group consisting of lithium (Li) silicate, lithium (Li) silicide, and lithium (Li) oxide. The lithium (Li) silicate may contain at least one of Li2SiO3, Li4SiO4, and Li2Si2O5. The lithium (Li) silicide may contain Li7Si2. The lithium (Li) oxide may contain Li2O.

[0039] In one embodiment of the present invention, the lithium (Li) compound may include a form of lithium silicate. The lithium silicate is represented by Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and can be classified into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate may exist in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5 in the silicon-based composite particles, and the amorphous lithium silicate may be in the form of Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and is not limited to the above form.

[0040] In one embodiment of this specification, the lithium (Li) element may be present in an amount of 0.1% to 20% by weight, or 0.1% to 10% by weight, based on 100% by weight of the total silicon-based active material. Specifically, the lithium (Li) element may be present in an amount of 0.5% to 8% by weight, or more specifically, 0.5% to 4% by weight. When the above range is met, the lithium (Li) compound can be included in the silicon-based active material in an appropriate amount, so that volume changes of the negative electrode active material during charging and discharging of the battery can be easily suppressed, and the discharge capacity and initial efficiency of the battery can be improved.

[0041] The content of magnesium (Mg) or lithium (Li) elements can be confirmed by ICP analysis. For the ICP analysis, a fixed amount (approximately 0.01 g) of the negative electrode active material is accurately separated, transferred to a platinum crucible, and completely decomposed on a hot plate with the addition of nitric acid, hydrofluoric acid, and sulfuric acid. Subsequently, a reference calibration curve is created by measuring the intensity of a standard solution prepared using a standard solution (5 mg / kg) at wavelengths specific to magnesium (Mg) or lithium (Li) elements using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300). Then, the pre-treated sample solution and a blank sample are introduced into the instrument, their respective intensities are measured to calculate the actual intensities, and the concentrations of each component are calculated against the created calibration curve. The total is then converted to a theoretical value, and the content of magnesium (Mg) or lithium (Li) elements in the manufactured silicon-based active material can be analyzed.

[0042] In one embodiment of this specification, a carbon layer may be provided on the surface and / or inside the pores of the silicon-based composite particles. The carbon layer imparts conductivity to the silicon-based composite particles, thereby improving the initial efficiency, life characteristics, and capacity characteristics of a secondary battery containing a negative electrode active material including the silicon-based composite particles. The total weight of the carbon layer may be 5% to 40% by weight based on 100% by weight of the total silicon-based composite particles.

[0043] In one embodiment of this specification, the carbon layer may contain at least one of amorphous carbon and crystalline carbon.

[0044] The average particle size (D50) of the silicon-based active material may be 2 μm to 15 μm, more specifically 3 μm to 12 μm, and more specifically 4 μm to 10 μm. When the above range is satisfied, the side reactions between the silicon-based composite particles and the electrolyte are controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0045] In this specification, the average particle size (D50) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size (D50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters, and can obtain highly reproducible and high-resolution results.

[0046] In this specification, the silicon-carbon composite is a composite of Si and C, and may also be represented as a Si / C-based active material, and is distinguished from silicon carbide, represented as SiC. The silicon-carbon composite may be a composite of silicon and graphite, etc., and may form a structure in which a core of silicon and graphite, etc. is surrounded by graphene or amorphous carbon, etc. The silicon in the silicon-carbon composite may be nanosilicon.

[0047] In one embodiment of this specification, the carbon-based active material may be graphite, and the graphite may be natural graphite, artificial graphite, or a mixture thereof. Based on 100 parts by weight of the mixed negative electrode active material, the carbon-based active material may be included in an amount of 60 parts by weight or more and 99 parts by weight or less.

[0048] According to one embodiment, the negative electrode may include a current collector and a negative electrode active material layer provided on the current collector.

[0049] The negative electrode active material may be present in an amount of 80 parts by weight or more and 99.9 parts by weight or less per 100 parts by weight of the negative electrode active material layer, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0050] According to one embodiment of this specification, the negative electrode active material layer may further include a negative electrode binder in addition to a mixed negative electrode active material comprising a silicon-based active material and a carbon-based active material.

[0051] The negative electrode binder can improve adhesion between negative electrode active material particles and adhesion between negative electrode active material particles and negative electrode current collector. The negative electrode binder can be any substance known in the art, and non-limiting examples include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, and may also include various copolymers thereof.

[0052] The negative electrode binder may be included in an amount of 0.1 parts 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 parts by weight or more and 20 parts by weight or less, and more preferably 0.5 parts by weight or more and 10 parts by weight or less.

[0053] The negative electrode active material layer does not necessarily have to contain a conductive material, but may further contain a conductive material as needed. 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 is conductive, and may be, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives. The content of the conductive material in the negative electrode active material layer may be 0.01 to 20 parts by weight, preferably 0.03 to 18 parts by weight, per 100 parts by weight of the negative electrode active material layer.

[0054] In one embodiment of this specification, the thickness of the negative electrode active material layer may be 5 μm or more and 500 μm or less.

[0055] In one embodiment of this specification, the negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Specifically, transition metals that readily adsorb carbon, 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 is not limited thereto.

[0056] In one embodiment of this specification, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, which comprises 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.

[0057] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 1 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0058] In one embodiment, the positive 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 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less, with respect to 100 parts by weight of the positive electrode active material layer.

[0059] According to further embodiments of this specification, the positive electrode active material layer may further include a positive electrode binder and a conductive material.

[0060] The positive electrode binder can improve adhesion between positive electrode active material particles and adhesion between positive electrode active material particles and positive electrode current collector. Any positive electrode binder known in the art can be used, and non-limiting examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0061] The positive electrode binder may be included in an amount of 0.1 parts by weight or more and 50 parts by weight or less based on 100 parts by weight of the positive electrode active material layer, for example, preferably in an amount of 0.3 parts by weight or more and 35 parts by weight or less, and more preferably in an amount of 0.5 parts by weight or more and 20 parts by weight or less.

[0062] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity without causing chemical changes within the battery. Specific examples include graphite such as natural graphite or 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. One of these alone or a mixture of two or more may be used.

[0063] Specifically, the conductive material may include one or more single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The conductive material may be included in an amount of 0.1 parts by weight or more and 2 parts by weight or less based on 100 parts by weight of the positive electrode active material layer composition, for example, preferably 0.3 parts by weight or more and 1.5 parts by weight or less, and more preferably 0.5 parts by weight or more and 1.2 parts by weight or less.

[0064] The positive electrode and the negative electrode can be manufactured by conventional methods for manufacturing positive and negative electrodes, except that the positive and negative electrode active materials described above are used. Specifically, they can be manufactured by coating a composition for forming an active material layer, which includes the active material described above and selectively contains a binder and a conductive material, onto a current collector, followed by drying and rolling. In this case, the types and contents of the positive and negative electrode active materials, binder, and conductive material are as described above. The solvent can be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of solvent used should be sufficient to dissolve or disperse the active material, conductive material, and binder, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to have a viscosity that allows for excellent thickness uniformity when coating for the manufacture of the positive and negative electrodes. Alternatively, the positive and negative electrodes can also be manufactured by casting the active material layer-forming composition onto another support, peeling it off the support, and then laminating the resulting film onto the current collector.

[0065] The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is the type of separator normally used in secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymeric substances may be used, and they may be selectively used as single-layer or multi-layer structures.

[0066] 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.

[0067] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0068] 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 derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0069] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents because they have high dielectric constants and dissociate lithium salts well. Furthermore, when such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be produced, and therefore they can be used even more preferably.

[0070] As the metal salt, a lithium salt may be used, and 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 - , and (CF3CF2SO2)2N - You may use one or more selected from the group consisting of the following:

[0071] The aforementioned fluoroethylene carbonate is added to the electrolyte. The fluoroethylene carbonate may be present in the electrolyte, or it may be present in the form of a coating on the positive electrode surface or the negative electrode surface. The amount of fluoroethylene carbonate may be 1 to 10 parts by weight based on 100 parts by weight of the electrolyte.

[0072] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0073] A lithium secondary battery according to one embodiment of the present invention includes an assembly comprising a positive electrode, a negative electrode, a separator, and an electrolyte.

[0074] Further embodiments of the present invention provide a battery module and a battery pack containing the aforementioned lithium secondary battery as a unit cell. Because the battery module and battery pack contain the secondary battery having high capacity, high rate characteristics and cycle characteristics, they 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.

[0075] The lithium secondary battery according to the embodiment of the present invention exhibits excellent discharge capacity, output characteristics, and cycle performance stably, and can therefore be used as a power source for not only portable devices such as mobile phones, laptop computers, and digital cameras, but also 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. For example, the battery module or battery pack can be used as a power source for one or more medium- and large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0076] The following are preferred embodiments to aid in understanding the present invention. These embodiments are merely illustrative examples of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present description and the technical concept, and that such variations and modifications fall within the scope of the appended claims.

[0077] Example 1 A negative electrode was manufactured by coating a 10 μm thick copper foil with a negative electrode active material composition containing SiO and graphite (3 parts by weight of SiO based on 100 parts by weight of negative electrode active material), 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, and then drying it to achieve a dry thickness of 140 μm.

[0078] Li 1.0 Ni 0.86 Co 0.06 Mn 0.08 A positive electrode was manufactured by coating a 15 μm thick aluminum foil with a composition for forming a positive electrode active material layer, containing O2 lithium nickel oxide, a conductive material (CNT), and a binder (PVDF) in a weight ratio of 97:1:2, to a dry thickness of 130 μm, and then drying it.

[0079] The positive electrode and the negative electrode were stacked with a separator in between, and an electrolyte was injected to fabricate a battery. As the separator, a 12 μm thick separator was used, which had a triple-layered base film of PE / PP / PE with a coating layer containing Al2O3 and PVDF binder. As the electrolyte, 1M LiPF6, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio 3 / 7), and vinylene carbonate (VC) / propane sultone (PS) / fluoroethylene carbonate (FEC) (contained in amounts of 3 parts by weight, 1.5 parts by weight, and 3 parts by weight, respectively, based on 100 parts by weight of the electrolyte) were used.

[0080] Table 1 shows the values ​​of Equation 1 mentioned above for the fabricated batteries. Table 2 below shows the capacity retention rate when the fabricated batteries are subjected to 200 charge-discharge cycles at a specific temperature.

[0081] Comparative Example 1 The procedure was carried out in the same manner as in Example 1, except that 10 parts by weight of SiO was included per 100 parts by weight of the negative electrode active material.

[0082] Comparative Example 2 The procedure was carried out in the same manner as in Comparative Example 1, except that fluoroethylene carbonate was not added to the electrolyte.

[0083] Table 1 below shows the efficiency constants of the electrode active materials used in the batteries manufactured in the examples and comparative examples, as well as the content of the additive (fluoroethylene carbonate) and the value of Equation 1.

[0084] [Table 1]

[0085] The batteries manufactured in the examples and comparative examples were subjected to constant current / constant voltage (CC / CV) charging at 0.33C up to 4.2V at room temperature (25°C) (0.05C cutoff), followed by constant current (CC) discharge at 0.5C (2.5V cutoff). The number of cycles required to reach 80% of the initial capacity is shown in Table 2 below.

[0086] [Table 2]

[0087] As shown in Table 2 above, it was confirmed that the battery of the example had a significantly larger number of cycles to reach 80% capacity compared to the comparative example.

[0088] Furthermore, the present invention may also preferably include the following examples. [Section 1] A lithium secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, The positive electrode comprises a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co). The aforementioned negative electrode includes a mixed negative electrode active material of a silicon-based active material and a carbon-based active material. The electrolyte comprises fluoroethylene carbonate, When the efficiency constants of the mixed negative electrode active material and the lithium composite transition metal compound are a and b, respectively, and the amount of fluoroethylene carbonate is c relative to 100 parts by weight of the electrolyte, then a, b, and c satisfy the following formula 1, [Formula 1] [(b / a)×100-c]≦95 This is a lithium-ion secondary battery. [Section 2] The lithium-ion secondary battery according to item 1, wherein the lithium-ion composite transition metal compound containing nickel (Ni) and cobalt (Co) contains 80 mol% or more of nickel among the metals other than lithium. [Section 3] The lithium secondary battery according to item 1, wherein the lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) is a single particle. [Section 4] The lithium-complex 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 O2 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 from among Mn or Al. The lithium secondary battery described in item 1, wherein M2 is one or more metallic elements selected from the group consisting of Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo. [Section 5] The lithium secondary battery according to item 1, wherein the positive electrode contains 90 to 100 parts by weight of a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) based on 100 parts by weight of the positive electrode active material. [Section 6] The silicon-based active material is SiO x (0≦x<2), SiM yA lithium secondary battery as described in item 1, comprising (M is a metal, 1 ≤ y ≤ 4) and at least one of Si / C. [Section 7] The lithium secondary battery according to item 1, wherein the silicon-based active material is contained in an amount of 1 part by weight or more and less than 20 parts by weight based on 100 parts by weight of the mixed negative electrode active material. [Section 8] The lithium secondary battery according to item 1, wherein the fluoroethylene carbonate is contained in an amount of 1 to 10 parts by weight based on 100 parts by weight of the electrolyte.

Claims

[Claim 1] A lithium secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, The positive electrode comprises a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co). The aforementioned negative electrode includes a mixed negative electrode active material of a silicon-based active material and a carbon-based active material. The electrolyte comprises fluoroethylene carbonate, When the efficiency constants of the mixed negative electrode active material and the lithium composite transition metal compound are a and b, respectively, and the weight of the fluoroethylene carbonate is c relative to 100 parts by weight of the electrolyte, then a, b, and c satisfy the following formula 1, [Formula 1] [(b / a)×100-c]≦95 This is a lithium-ion secondary battery.

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