Method for manufacturing lithium secondary battery and lithium secondary battery manufactured by the same

Abstract

To provide a lithium secondary battery capable of ultimately reducing only a lithium consumption amount without reducing resistance and life properties by introducing a positive electrode material to mitigate the issues caused by a negative electrode material having a large irreversible capacity loss.SOLUTION: A method for manufacturing a lithium secondary battery comprises the following steps (1)-(5). (1) a step of mixing and heating a small particle lithium composite transition metal oxide having an average particle size (D50) of less than 7 μm with a boron-containing material to manufacture a first positive electrode active material; (2) a step of mixing and heating a large particle lithium composite transition metal oxide having an average particle size (D50) of 8 μm or more with a cobalt-containing material and a boron-containing material to manufacture a second positive electrode active material; (3) a step of mixing the first positive electrode active material with the second positive electrode active material to manufacture a positive electrode material having a bimodal particle size distribution; (4) a step of coating the positive electrode material onto a positive electrode current collector to manufacture a positive electrode; and (5) a step of assembling the positive electrode, a negative electrode containing a silicon-based negative electrode active material and a separation membrane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2020-0162322, filed with the Korean Intellectual Property Office on November 27, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a lithium secondary battery and a lithium secondary battery manufactured thereby. [Background technology]

[0003] In recent years, with the rapid spread of battery-powered electronic devices such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small, lightweight, and have relatively high capacity has been rapidly increasing. In particular, lithium secondary batteries, which are lightweight and have high energy density, have been attracting attention as a driving power source for portable devices. As a result, research and development efforts to improve the performance of lithium secondary batteries have been actively conducted.

[0004] Carbonaceous materials such as graphite are commonly used as negative electrode materials for lithium secondary batteries, but carbonaceous materials have a drawback in that their low capacity per unit mass makes it difficult to increase the capacity of lithium secondary batteries. For this reason, non-carbonaceous negative electrode materials that form intermetallic compounds with lithium, such as silicon, tin, and their oxides, have been developed and used to exhibit higher capacity than carbonaceous materials. However, these negative electrode materials suffer from a large irreversible capacity loss during initial charge and discharge.

[0005] To overcome this problem, methods have been proposed to overcome the irreversible capacity loss of the negative electrode by using a positive electrode material that can provide a lithium ion source or reservoir and remains electrochemically active after the first cycle without degrading the overall battery performance. Specifically, lithium nickel-based oxides such as Li2NiO2 are used in the positive electrode as sacrificial positive electrode materials or overdischarge inhibitors.

[0006] However, most of the lithium nickel-based oxides are expensive and have problems such as generating a large amount of lithium by-products, resulting in a large amount of gas generation. Therefore, alternative methods are being sought. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a lithium secondary battery that can ultimately reduce the amount of lithium consumed without degrading resistance and life characteristics by introducing a cathode material that compensates for the problem of anode materials with large irreversible capacity loss. [Means for solving the problem]

[0008] The present invention provides (1) Average particle size (D 50 ) is less than 7 μm, and a boron-containing raw material, and heat-treating the mixture to prepare a first positive electrode active material; (2) Average particle size (D 50 a step of mixing the large particle lithium composite transition metal oxide having a particle size of 8 μm or more with a cobalt-containing raw material and a boron-containing raw material, followed by heat treatment to prepare a second positive electrode active material; (3) mixing the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material having a bimodal particle size distribution; (4) coating the cathode material onto a cathode current collector to produce a cathode; (5) assembling the positive electrode, the negative electrode including a silicon-based negative electrode active material, and the separator; The present invention provides a method for producing a lithium secondary battery, comprising:

[0009] The present invention also provides a lithium secondary battery including a positive electrode including a positive electrode material having a bimodal particle size distribution, a negative electrode including a silicon-based negative electrode active material, and a separator, the positive electrode material includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material has an average particle size (D 50) a small particle lithium composite transition metal oxide having a particle size of less than 7 μm, and a boron-containing coating layer formed on the small particle lithium composite transition metal oxide, The second positive electrode active material has an average particle size (D 50 and a coating layer containing cobalt and boron formed on the large-particle lithium composite transition metal oxide. [Effects of the Invention]

[0010] The method for manufacturing a lithium secondary battery according to the present invention makes it possible to manufacture a lithium secondary battery with improved output characteristics and high-temperature life by using a high-capacity silicon-based negative electrode active material while compensating for the problem of irreversible capacity loss by using a bimodal positive electrode material. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a photograph of a cross section of a positive electrode prepared in Example 1, which was analyzed by EPMA and then photographed by SEM. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will now be described in more detail to aid in its understanding.

[0013] In this specification, the term "average particle size (D 50 The average particle size (D) is defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using a laser diffraction method. More specifically, lithium composite transition metal oxide particles are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. Then, the average particle size (D 50 ) can be calculated.

[0014] The method for producing a lithium secondary battery of the present invention includes the following steps (1) to (5). (1) Average particle size (D 50 mixing the small particle lithium composite transition metal oxide having a particle size of less than 7 μm with a boron-containing raw material, followed by heat treatment to prepare a first positive electrode active material; (2) Average particle size (D 50 a step of mixing the large particle lithium composite transition metal oxide having a particle size of 8 μm or more with a cobalt-containing raw material and a boron-containing raw material, followed by heat treatment to prepare a second positive electrode active material; (3) mixing the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material having a bimodal particle size distribution; (4) coating the cathode material onto a cathode current collector to produce a cathode; and (5) Assembling the positive electrode, the negative electrode containing a silicon-based negative electrode active material, and the separator.

[0015] To develop high-capacity cells, it is essential to use silicon-based anode active materials with high capacities. However, silicon-based anode active materials have a low initial charge / discharge efficiency of less than 85%, which poses a problem of high lithium ion loss due to irreversible reactions.

[0016] Furthermore, when a positive electrode active material with high initial charge / discharge efficiency is used, the charge capacity of the cell decreases, which is disadvantageous in achieving high energy.

[0017] Therefore, by using a low-efficiency positive electrode active material that can provide lithium to the silicon-based negative electrode active material during initial charge / discharge, the loss of lithium ions can be reduced and energy can be increased.

[0018] The method for manufacturing a lithium secondary battery according to the present invention provides a cathode material with improved output and reduced consumption of lithium ions due to low initial charge / discharge efficiency by applying a boron (B) and cobalt (Co) composite coating.

[0019] Boron coating forms a lithium boron oxide (LBO) phase on the surface of the positive electrode active material. This LBO phase has high ionic conductivity, which increases the battery capacity and reduces resistance, and its low electrical conductivity prevents side reactions between the positive electrode surface and the electrolyte.

[0020] Cobalt coating forms cobalt oxide (Co3O4) on the surface of the positive electrode active material during low-temperature heat treatment, which can reduce discharge efficiency and increase initial resistance.

[0021] In the present invention, the composite coating of boron and cobalt can reduce the efficiency to match the silicon-based negative electrode active material without reducing the resistance, and can improve the output and lifespan compared to a coating of boron alone.

[0022] In particular, in a bimodal cathode active material consisting of small and large particles, cobalt coating is not applied to the small particles, which have a large specific surface area and thus have a significant impact on output performance, and cobalt coating is applied only to the large particles, which have a relatively small specific surface area, thereby minimizing the increase in initial resistance and output reduction caused by cobalt oxide.

[0023] Each step will be described in detail below.

[0024] <Cathode material manufacturing> In the step of producing the first positive electrode active material of the present invention, the average particle size (D 50 The small particle lithium transition metal composite oxide having a particle size of less than 7 μm is mixed with a boron-containing raw material and heat-treated.

[0025] The average particle size (D 50 ) may be 2 μm or more and less than 7 μm, preferably 3 μm to 6 μm.

[0026] The boron-containing raw materials include H3BO3, B2O3, B4C, BF3, (C3H7O)3B, (C6H5O)3B, [CH3(CH2)3O]3B, C 13 H 19 The boron-containing raw material may be one or more selected from O3, C6H5B(OH)2, and B2F4, preferably one or more selected from H3BO3 and B2O3, and more preferably H3BO3. H3BO3 has a lower melting point and is more reactive with lithium ions than other boron-containing raw materials, and therefore serves to lower the ambient reaction temperature, specifically, the reaction temperature of a plasticity additive that promotes grain growth or a raw material with a high melting point. The same explanation applies to the boron-containing raw material used in steps (1) and (2) of the present invention.

[0027] In step (1), the boron-containing raw material may be mixed in an amount of 0.03 wt% to 0.25 wt%, preferably 0.05 wt% to 0.15 wt%, based on the total content of the small-particle lithium composite transition metal oxide. When the content of the boron-containing raw material is 0.03 wt% or more based on the total content of the small-particle lithium composite transition metal oxide, the LBO phase, a coating layer formed by boron interacting with lithium by-products such as lithium hydroxide and lithium carbonate present on the surface of the positive electrode active material, is sufficiently formed, thereby achieving the effects of increased capacity and reduced resistance and preventing side reactions between the surface of the positive electrode active material and the electrolyte. When the content of the boron-containing raw material is 0.25 wt% or less, the formation of boron oxide, BO, which would increase resistance, is prevented. Specifically, when the boron content exceeds the above range, the amount of boron exceeds the amount of lithium by-products capable of reacting to form the LBO phase, and the boron oxide, BO, is formed in addition to LBO and acts as a resistor, which can undesirably increase resistance.

[0028] The heat treatment in step (1) may be carried out at 250°C to 400°C, preferably 280°C to 350°C. A heat treatment temperature of 250°C or higher in step (1) is preferable because it is a sufficient temperature for boron to react with lithium by-products on the surface of the positive electrode active material, and therefore no unreacted boron source remains. A heat treatment temperature of 400°C or lower produces sufficient LBO phase, contributing to improved capacity. Heat treatment at a temperature higher than this is undesirable because it is higher than the optimum temperature for LBO phase production and may actually result in a decrease in capacity.

[0029] The heat treatment in step (1) may be carried out for 50 to 500 minutes.

[0030] In the step of producing the second positive electrode active material of the present invention, the average particle size (D 50 The large particle lithium transition metal composite oxide having a particle size of 8 μm or more is mixed with a cobalt-containing raw material and a boron-containing raw material, and the mixture is heat-treated.

[0031] The average particle size (D 50 ) may be 8 μm or more and 20 μm or less, preferably 9 μm to 16 μm.

[0032] Cobalt coating is not applied to the small particle lithium composite transition metal oxide, which has a relatively small average particle size and therefore a large specific surface area, and therefore has a significant impact on output performance. However, cobalt coating is applied to the large particle lithium composite transition metal oxide, which has a relatively small specific surface area. This reduces the initial charge / discharge efficiency of the positive electrode active material, but minimizes the effects of increased resistance and reduced output.

[0033] The mixing in steps (1) and (2) may be dry mixing in which mixing is performed without a solvent.

[0034] The cobalt-containing raw material may be one or more selected from Co3O4, Co(OH)2, Co2O3, Co3(PO4)2, CoF3, Co(OCOCH3)2·4H2O, Co(NO3)·6H2O, Co(SO4)2·7H2O, and CoC2O4, preferably one or more selected from Co3O4 and Co(OH)2, and more preferably Co(OH)2. Co(OH)2 has a lower melting point than other cobalt-containing raw materials, and therefore has the advantage that a sufficient cobalt coating effect can be achieved even if the two are heat-treated together at a temperature suitable for the boron-containing raw material, which has a lower reaction temperature than the cobalt-containing raw material.

[0035] In step (2) of the present invention, the boron-containing raw material and the cobalt-containing raw material are H3BO3 and Co(OH)2, respectively. This combination has the advantage of lowering the reaction temperature of the surrounding raw materials of H3BO3, and simultaneously achieving the coating effect of boron and cobalt at a low temperature due to the relatively low melting point of Co(OH)2.

[0036] In one embodiment of the present invention, the coating raw material components of the first and second positive electrode active materials may be different. More specifically, in the step of preparing the first positive electrode active material, no other coating raw material may be mixed in addition to the boron-containing raw material, and in the step of preparing the second positive electrode active material, no other coating raw material may be mixed in addition to the cobalt-containing raw material and the boron-containing raw material.

[0037] The heat treatment in step (2) may be carried out at 250°C to 400°C, preferably 280°C to 350°C. If the heat treatment temperature in step (2) is 250°C or higher, it is sufficient to allow the boron and cobalt sources to react, and is therefore preferred in that no unreacted boron or cobalt remains. If the heat treatment temperature is 400°C or lower, the LBO phase is sufficiently generated, contributing to an improvement in capacity. If the heat treatment is carried out at a temperature higher than this, it is not preferred in that it is higher than the optimum temperature for LBO phase generation and may result in a decrease in capacity.

[0038] The heat treatment in step (2) may be carried out for 50 to 500 minutes.

[0039] In step (2), the cobalt-containing raw material may be mixed in an amount of 0.1 wt % to 1.5 wt %, preferably 0.15 wt % to 1.3 wt %, based on the total content of the large-particle lithium composite transition metal oxide. When the amount of the cobalt-containing raw material is 0.1 wt % or more based on the total content of the large-particle lithium composite transition metal oxide, sufficient cobalt oxide is formed, which effectively prevents the efficiency of the positive electrode from being reduced. When the amount of the cobalt-containing raw material is 1.5 wt % or less, an appropriate amount of cobalt oxide is formed on the surface of the positive electrode active material. Excessive cobalt oxide formed on the surface of the positive electrode active material can result in a decrease in capacity and an increase in resistance.

[0040] In step (2), the boron-containing raw material may be mixed in an amount of 0.03 wt% to 0.25 wt%, preferably 0.05 wt% to 0.15 wt%, based on the total content of the large-particle lithium composite transition metal oxide. When the content of the boron-containing raw material is 0.03 wt% or more based on the total content of the large-particle lithium composite transition metal oxide, the LBO phase, a coating layer formed by boron interacting with lithium by-products such as lithium hydroxide and lithium carbonate present on the surface of the positive electrode active material, is sufficiently formed, thereby achieving the effects of increased capacity and reduced resistance and preventing side reactions between the surface of the positive electrode active material and the electrolyte. When the content of the boron-containing raw material is 0.25 wt% or less, the formation of boron oxide, BO, which would increase resistance, is prevented. Specifically, when the boron content exceeds the above range, the amount of boron exceeds the amount of lithium by-products capable of reacting to form the LBO phase. This results in the formation of boron oxide, BO, which acts as a resistor in addition to LBO, and may actually increase resistance, which is undesirable.

[0041] In the present invention, the lithium transition metal composite oxide is represented by the following chemical formula 1.

[0042] [Chemical Formula 1] Li 1+x (Ni a Co b Mn c M d )O2 In Chemical Formula 1 above, M is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, x, a, b, c, and d are respectively 0 ≦ x ≦ 0.2, 0.70 ≦ a < 1, 0 < b ≦ 0.25, 0 < c ≦ 0.25, and 0 ≦ d ≦ 0.1.

[0043] Preferably, in Chemical Formula 1 above, M may be Al.

[0044] Also, a, b, c, and d may respectively preferably be 0.70 ≦ a ≦ 0.90, 0.05 ≦ b ≦ 0.25, 0.05 ≦ c ≦ 0.25, and 0 ≦ d ≦ 0.05, and more preferably 0.80 ≦ a ≦ 0.90, 0.05 ≦ b ≦ 0.15, 0.05 ≦ c ≦ 0.15, and 0 ≦ d ≦ 0.05.

[0045] That is, in the lithium composite transition metal oxide, the content of nickel (Ni) in the total content of transition metals may be 70 mol% or more, preferably 80 mol% or more.

[0046] The method for manufacturing a lithium secondary battery of the present invention includes step (3) of mixing the first positive electrode active material and the second positive electrode active material. In step (3), the first positive electrode active material and the second positive electrode active material may be mixed at a weight ratio of 10:90 to 40:60, preferably 15:85 to 30:70, and may be present within the same content range in the manufactured positive electrode material.

[0047] <Manufacture of Positive Electrode> In the step of manufacturing the positive electrode of the present invention, the positive electrode material is coated on a positive electrode current collector. This may be performed by a conventional positive electrode manufacturing method, except for using the above-described positive electrode material. Specifically, the positive electrode may be manufactured by applying a positive electrode slurry containing the above-described positive electrode material and, optionally, a binder and a conductive material, onto a positive electrode current collector, followed by drying and rolling.

[0048] 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; baked carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like can be used.

[0049] The solvent for the positive electrode slurry may be an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a preferred viscosity when the positive electrode material and, optionally, a binder and a conductive material are contained. For example, the positive electrode slurry may be contained so that the solids concentration is 10% by weight to 90% by weight, preferably 40% by weight to 85% by weight.

[0050] The binder in the positive electrode slurry is a component that aids in bonding between the positive electrode material and the conductive material and to the current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene termonomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0051] The conductive material in the positive electrode slurry is a substance that does not cause a chemical change in the battery and imparts conductivity, and may be added in an amount of 0.5 wt % to 20 wt % relative to the total weight of the solid content in the positive electrode slurry.

[0052] Examples of conductive materials that can be used include carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with highly developed crystal structures; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0053] The cathode material may be contained in an amount of 80% by weight to 99% by weight, specifically 90% by weight to 99% by weight, based on the total weight of the solid content in the cathode slurry. If the content of the cathode material is 80% by weight or less, the energy density may be reduced, and the capacity may be reduced.

[0054] <Production of negative electrodes> The negative electrode according to the present invention includes a silicon-based negative electrode active material and may be prepared by coating a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, a solvent, etc., followed by drying and rolling.

[0055] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those with the surface of copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. Also, similar to the positive electrode current collector, fine irregularities can be formed on the surface to strengthen the binding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0056] In the present invention, the silicon-based negative electrode active material is Si, S i O x (0 < x < 2) and Si-Y alloy (where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), and is one or more selected therefrom, preferably Si or SiO.

[0057] Since the silicon-based negative electrode active material has a capacity nearly about 10 times higher than that of graphite, the mass loading (mg·cm -2 ) can be lowered to improve the rapid charging performance of the battery. However, there is a problem that the loss rate of lithium ions due to the irreversible reaction is high, but by applying the above-described positive electrode material, such a problem can be solved.

[0058] The anode of the present invention may further include a carbon-based anode active material in addition to the silicon-based anode active material. Any carbon-based anode active material commonly used in lithium-ion secondary batteries may be used without particular limitation. Representative examples of such a material include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

[0059] In the present invention, the silicon-based negative electrode active material may be contained in an amount of 1 wt % to 100 wt %, preferably 3 wt % to 10 wt %, based on the total weight of the negative electrode active material.

[0060] The negative electrode active material may be contained in an amount of 80% by weight to 99% by weight based on the total weight of the solid content in the negative electrode slurry.

[0061] The binder is a component that helps to bond between the conductive material, negative electrode active material, and current collector, and may be added in a content of 1 wt% to 30 wt% based on the total weight of the solid content in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0062] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 1 wt % to 20 wt % based on the total weight of the solid content in the negative electrode slurry. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fiber or metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, 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.

[0063] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a preferred viscosity when the negative electrode active material, binder, conductive material, etc. are contained. For example, the solvent may be contained so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 50 wt % to 75 wt %, preferably 50 wt % to 65 wt %.

[0064] <Lithium secondary battery manufacturing> The method for producing a lithium secondary battery of the present invention includes a step (5) of assembling the positive electrode, a negative electrode containing a silicon-based negative electrode active material, and a separator.

[0065] Specifically, in step (5), the positive electrode and the negative electrode are sequentially stacked with a separator interposed therebetween and dried to prepare a battery assembly, which is then inserted into a case, and an electrolyte is injected and sealed to prepare a lithium secondary battery.

[0066] The separator separates the positive and negative electrodes and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. It is particularly preferable that the separator exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification. Specifically, porous polymer films, such as those made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, to ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can be used, and it can be selectively used as a single-layer or multi-layer structure.

[0067] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries.

[0068] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0069] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Specifically, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether solvent such as dibutyl ether or tetrahydrofuran; a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene or fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene carbonate). Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constants, which enhance the charge / discharge performance of batteries, and low-viscosity chain carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, excellent electrolyte performance can be achieved when the cyclic carbonate and chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0070] The lithium salt can be any compound capable of providing lithium ions for use in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or a combination thereof. The lithium salt concentration is preferably within the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0071] 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 purposes of improving battery life characteristics, suppressing battery capacity reduction, improving battery discharge capacity, etc. Here, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0072] <Lithium secondary battery> The lithium secondary battery according to the present invention comprises: A lithium secondary battery comprising a positive electrode including a positive electrode material having a bimodal particle size distribution, a negative electrode including a silicon-based negative electrode active material, and a separator, the positive electrode material includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material has an average particle size (D 50) a small particle lithium composite transition metal oxide having a particle size of less than 7 μm, and a boron-containing coating layer formed on the small particle lithium composite transition metal oxide, The second positive electrode active material has an average particle size (D 50 ) is 8 μm or more, and a cobalt- and boron-containing coating layer formed on the large-particle lithium composite transition metal oxide.

[0073] The lithium secondary battery can be manufactured by the above-mentioned method for manufacturing a lithium secondary battery, and the explanation of the above-mentioned method for manufacturing a lithium secondary battery can be cited for each component.

[0074] In the lithium secondary battery of the present invention, the coating layers included in the first and second positive electrode active materials can be confirmed using an electron probe micro analyzer (EPMA).

[0075] In one embodiment of the present invention, the coating layer of the first positive electrode active material and the coating layer of the second positive electrode active material are different in constituent components. More specifically, the coating layer of the first positive electrode active material is made of boron, and the coating layer of the second positive electrode active material is made of cobalt and boron.

[0076] The lithium secondary battery according to the present invention can be applied to portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0077] According to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0078] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0079] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. [Example]

[0080] [Examples and Comparative Examples: Production of Positive Electrode] Example 1 Li(Ni) 0.8 Co 0.1 Mn 0.1 )O2(D 50 A lithium composite transition metal oxide represented by the formula (I = 4 μm) and H3BO3 were dry-mixed in a weight ratio of 1:0.05. The mixture was heat-treated in an air atmosphere at 290°C for 200 minutes to prepare a first positive electrode active material having a boron-containing coating layer.

[0081] Separately, Li(Ni 0.8 Co 0.1 Mn 0.1 )O2(D 50 A lithium composite transition metal oxide represented by the formula (I = 13 μm), Co(OH)2, and H3BO3 were dry-mixed in a weight ratio of 1:0.4:0.05. The mixture was heat-treated in an air atmosphere at 290°C for 200 minutes to prepare a second positive electrode active material having a cobalt- and boron-containing coating layer.

[0082] The first and second positive electrode active materials were mixed in a weight ratio of 15:85 to prepare a bimodal positive electrode material.

[0083] The cathode material, conductive material (carbon black), and binder (polyvinylidene fluoride (PVdF)) were mixed in a weight ratio of 97.5:1.0:1.5 in N-methyl-2-pyrrolidone (NMP) solvent to prepare a cathode slurry (solid content: 50 wt%). The cathode slurry was applied to one side of an aluminum current collector, dried at 130°C, and rolled to prepare a cathode.

[0084] Figure 1 shows a cross section of the positive electrode analyzed with an electron probe microanalyzer (EPMA) and photographed with a scanning electron microscope (SEM). In Figure 1, the green area is the Co coating layer, and the peak shown directly above the photograph indicates the Co concentration. Figure 1 shows that the Co coating layer is formed only on large particles.

[0085] Example 2. A positive electrode was manufactured in the same manner as in Example 1, except that in manufacturing the second positive electrode active material, the content of Co(OH) was increased so that the weight ratio of the lithium composite transition metal oxide, Co(OH) and HBO was 1:1.1:0.05.

[0086] Example 3 A positive electrode was manufactured in the same manner as in Example 1, except that, in manufacturing the first positive electrode active material, the content of H3BO3 was increased so that the weight ratio of the lithium composite transition metal oxide to H3BO3 was 1:0.13, and in manufacturing the second positive electrode active material, the content of H3BO3 was increased so that the weight ratio of the lithium composite transition metal oxide, Co(OH)2, and H3BO3 was 1:0.4:0.13.

[0087] Comparative Example 1 A positive electrode was manufactured in the same manner as in Example 1, except that no coating layer was formed on either the first or second positive electrode active material.

[0088] Comparative Example 2 A positive electrode was manufactured in the same manner as in Example 1, except that when forming the coating layer of the second positive electrode active material, cobalt was not used and only boron coating was applied.

[0089] Comparative Example 3. A positive electrode was manufactured in the same manner as in Example 1, except that a coating layer was not formed on the first positive electrode active material, and only a cobalt coating was applied without boron when forming a coating layer on the second positive electrode active material.

[0090] Comparative Example 4. A positive electrode was manufactured in the same manner as in Example 1, except that when forming the coating layer of the first positive electrode active material, Co(OH)2 was added so that the weight ratio of the lithium composite transition metal oxide, Co(OH)2, and H3BO3 was 1:0.4:0.05, thereby forming a cobalt- and boron-containing coating layer.

[0091] [Experimental Example] Experimental example 1: Confirmation of capacity and initial resistance An electrode assembly was fabricated by interposing a 15 μm thick polyethylene separator between each of the positive electrodes and lithium metal negative electrodes fabricated in Examples 1 to 3 and Comparative Examples 1 to 4, and then the assembly was placed inside a battery case. An electrolyte solution was then injected into the case to fabricate a lithium secondary battery. The electrolyte solution was a 1:2 volume ratio organic solvent mixture of ethylene carbonate (EC):ethyl methyl carbonate (EMC) in which 1 M LiPF6 was dissolved. The capacity and resistance of the lithium secondary batteries were measured during 0.1 C charge / discharge.

[0092] Specifically, the lithium secondary batteries employing the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 were charged at 25° C. with a constant current of 0.1 C to 4.25 V with a 0.05 C cutoff. Then, the batteries were discharged at a constant current of 0.1 C to 3.0 V to measure the initial charge / discharge capacity. The initial resistance was measured by dividing the voltage drop during the initial 10 seconds of discharge by the current value, and is shown in Table 1 below.

[0093] [Table 1]

[0094] As can be seen from Table 1 above, the batteries incorporating the positive electrodes of Examples 1 to 3 exhibited low initial resistance and initial charge / discharge efficiency when combined with a lithium metal negative electrode. This indicates that they are advantageous in that they can reduce lithium consumption when used with a silicon-based negative electrode active material, which has a high lithium ion loss rate. In contrast, the positive electrodes of Comparative Example 1, in which neither the small nor large particles were coated, and Comparative Example 2, in which both the small and large particles were coated with boron alone, exhibited very high initial charge / discharge efficiency when combined with a lithium metal negative electrode. Therefore, it is expected that they will lose a large amount of lithium ions when used with a silicon-based negative electrode active material. Meanwhile, the positive electrodes of Comparative Example 3, in which only the large particles were coated with cobalt alone and the small particles were not coated, and Comparative Example 4, in which both the small and large particles were coated with boron and cobalt, exhibited low initial charge / discharge efficiency but had the drawback of high initial resistance.

[0095] Experimental example 2: Evaluation of output characteristics The room temperature output resistance of the lithium secondary batteries including the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 and the silicon-based negative electrodes was measured.

[0096] Specifically, anode active material (3 wt% SiO and 97 wt% artificial graphite), binder (SBR-CMC), and conductive material (carbon black) were mixed in a weight ratio of 95:3.5:1.5 with water as a solvent to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to a 6 μm-thick copper (Cu) thin film as anode current collector, dried, and then roll-pressed to prepare anode.

[0097] In each of Examples 1 to 3 and Comparative Examples 1 to 4, a 15 μm thick polyethylene separator was interposed between the positive electrode and the negative electrode to prepare an electrode assembly, which was then placed inside a battery case. An electrolyte solution was then injected into the case to prepare a lithium secondary battery. The electrolyte solution was prepared by dissolving 1M LiPF6 in a mixed organic solvent of ethylene carbonate (EC):ethyl methyl carbonate (EMC) in a volume ratio of 1:2.

[0098] Each of the lithium secondary batteries was charged at 0.5 C in CCCV mode at 25° C. to 4.2 V, and then discharged at a constant current of 2.0 C for 30 seconds, and the output resistance was measured from the voltage drop over 30 seconds. The results are shown in Table 2 below.

[0099] [Table 2]

[0100] The results in Table 2 above show that the positive electrodes of Examples 1 to 3 have the effect of lowering the output resistance when combined with a negative electrode containing a Si-based negative electrode active material.

[0101] Experimental example 3: Evaluation of high-temperature life characteristics The high-temperature life characteristics of lithium secondary batteries including the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 and silicon-based negative electrodes were measured.

[0102] Specifically, anode active material (3 wt% SiO and 97 wt% artificial graphite), binder (SBR-CMC), and conductive material (carbon black) were mixed in a weight ratio of 95:3.5:1.5 with water as a solvent to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to a 6 μm-thick copper (Cu) thin film as anode current collector, dried, and then roll-pressed to prepare anode.

[0103] In each of Examples 1 to 3 and Comparative Examples 1 to 4, a 15 μm thick polyethylene separator was interposed between the positive electrode and the negative electrode to prepare an electrode assembly, which was then placed inside a battery case. An electrolyte solution was then injected into the case to prepare a lithium secondary battery. The electrolyte solution was prepared by dissolving 1M LiPF6 in a mixed organic solvent of ethylene carbonate (EC):ethyl methyl carbonate (EMC) in a volume ratio of 1:2.

[0104] Each of the lithium secondary batteries was charged at 0.5 C in CCCV mode at 45° C. to 4.2 V and then discharged at a constant current of 0.5 C to 3.0 V, and 200 charge-discharge cycles were performed to measure the capacity retention rate and the resistance increase rate. The results are shown in Table 3.

[0105] [Table 3]

[0106] From the results in Table 3 above, it can be confirmed that the positive electrodes of Examples 1 to 3, when combined with a negative electrode containing a Si-based active material, are more effective in increasing the capacity retention rate of the battery and reducing the resistance increase rate in a high-temperature environment than the positive electrodes of Comparative Examples 1 to 4.

[0107] To summarise the above experimental results, the lithium secondary battery according to one embodiment of the present invention is excellent in output characteristics and high-temperature life.

Claims

1. (1) Average particle size (D ５０ mixing the small particle lithium composite transition metal oxide having a particle size of less than 7 μm with a boron-containing raw material and heat-treating the mixture to produce a first positive electrode active material; (2) Average particle size (D ５０ a cobalt-containing raw material and a boron-containing raw material, and heat-treating the mixture to prepare a second positive electrode active material; (3) mixing the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material having a bimodal particle size distribution.

2. The boron-containing raw material is H ３ BO ３ and B ２ O ３ The method for producing a positive electrode material for a lithium secondary battery according to claim 1, wherein the positive electrode material is one or more selected from the group consisting of:

3. The cobalt-containing raw material is Co ３ O ４ and Co(OH) ２ The method for producing a positive electrode material for a lithium secondary battery according to claim 1 or 2, wherein the positive electrode material is one or more selected from the following:

4. 4. The method for producing a positive electrode material for a lithium secondary battery according to claim 1, wherein in the step (1), the boron-containing raw material is mixed in an amount of 0.05 to 0.13 parts by weight per 1 part by weight of the small particle lithium composite transition metal oxide.

5. 5. The method for producing a positive electrode material for a lithium secondary battery according to claim 1, wherein in the step (2), the cobalt-containing raw material is mixed in an amount of 0.4 parts by weight to 1.1 parts by weight per 1 part by weight of the large-particle lithium composite transition metal oxide.

6. 6. The method for producing a positive electrode material for a lithium secondary battery according to claim 1, wherein in the step (2), the boron-containing raw material is mixed in an amount of 0.05 to 0.13 parts by weight per 1 part by weight of the large-particle lithium composite transition metal oxide.

7. The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 6, wherein the heat treatment in step (1) is carried out at 250 ° C to 400 ° C.

8. The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 7, wherein the heat treatment in step (2) is performed at 250 ° C to 400 ° C.

9. 9. The method for producing a positive electrode material for a lithium secondary battery according to claim 1, wherein in the step (3), the first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 10:90 to 40:

60.

10. The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 9, wherein the small particle and the large particle lithium composite transition metal oxide are each independently represented by the following chemical formula 1: [Chemical formula 1] Li １＋ｘ (N ａ Co ｂ Mn ｃ M ｄ )O ２ In the above chemical formula 1, M is one or more selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, a, b, c, and d are 0≦x≦0.2, 0.70≦a<1, 0<b≦0.25, 0<c≦0.25, and 0≦d≦0.1, respectively.

11. A cathode material having a bimodal particle size distribution, the positive electrode material includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material has an average particle size (D ５０ a small-particle lithium composite transition metal oxide having a particle size of less than 7 μm, and a boron-containing coating layer formed on the small-particle lithium composite transition metal oxide, The second positive electrode active material has an average particle size (D ５０ and a coating layer containing cobalt and boron formed on the large-particle lithium composite transition metal oxide.

12. The positive electrode material for a lithium secondary battery according to claim 11, wherein the first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 10:90 to 40:

60.

13. 13. The positive electrode material for a lithium secondary battery according to claim 11 or 12, wherein the small particle and large particle lithium composite transition metal oxides each independently have a nickel (Ni) content of 70 mol % or more of the total transition metal content.

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