Lithium secondary battery

Abstract

The lithium secondary battery according to the present invention includes a battery case, and an electrode assembly and an electrolyte housed in the battery case, in which the electrode assembly includes a positive electrode, a loading amount of a positive electrode active material layer included in the positive electrode is 9.4 mg / cm2 or less, and the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, and the second positive electrode active material has a smaller average particle diameter D50 than the first positive electrode active material.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the priority benefits of Korean Patent Application No. 10-2022-0140775, filed on October 27, 2022, and Korean Patent Application No. 10-2023-0143339, filed on October 24, 2023, the disclosures of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a lithium secondary battery. Background Art

[0004] With the recent increase in the development of technologies and demands for power tools, electric vehicles, and energy storage systems (ESS), the demand for batteries as an energy source has increased significantly. Therefore, various studies have been conducted on batteries that can meet various demands. As the market demand for high-capacity lithium secondary batteries as a power source for such devices has increased, research has been actively conducted on increasing the energy density of batteries. In addition, as a battery applicable to devices that require high output (such as power tools), the demand for batteries with excellent capacity characteristics and excellent rate characteristics has been increasing.

[0005] However, when a secondary battery embedded in a power tool discharges at a high output, the system operation of the power tool terminates due to a voltage drop of the secondary battery. That is, since the potential value of the secondary battery decreases significantly during high-rate discharge, even if the battery capacity remains, the system determines that the battery capacity is insufficient, resulting in the power supply of the power tool being turned off.

[0006] In addition, cylindrical batteries are generally used as batteries for power tools. In the case of cylindrical batteries, since the weight of the battery can is heavy, when using a cylindrical battery, the total weight of the power tool increases, and thus there is a problem of reduced work convenience. Summary of the Invention

[0007] Technical problem

[0008] One aspect of the present invention provides a lithium secondary battery that can prevent a power-off phenomenon of a power tool due to a low degree of voltage drop of the battery during high-rate discharge.

[0009] Another aspect of the present invention provides a pouch-type lithium secondary battery that has a low degree of voltage drop during high-rate discharge, has a relatively light weight for excellent work convenience, and has low resistance and excellent output characteristics.

[0010] Technical solution

[0011] According to one aspect of the present invention, a lithium secondary battery is provided, the lithium secondary battery comprising:

[0012] a battery case; and

[0013] an electrode assembly and an electrolyte accommodated in the battery case,

[0014] wherein the electrode assembly includes a positive electrode,

[0015] the loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm 2 hereinafter,

[0016] the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, and

[0017] the second positive electrode active material has an average particle diameter D smaller than that of the first positive electrode active material 50 .

[0018] According to the present invention, the first positive electrode active material may include a compound represented by the following Chemical Formula 1:

[0019] [Chemical Formula 1]

[0020] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y

[0021] In the above Chemical Formula 1,

[0022] 0.8 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.2, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.1, a + b + c + d = 1, 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.2,

[0023] M1 is Mn, Al or a combination thereof,

[0024] M2 is at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and

[0025] X1 is at least one element selected from F, Cl, Br, I, At, P and S.

[0026] According to the present invention, the second positive electrode active material may include a compound represented by the following Chemical Formula 2:

[0027] [Chemical Formula 2]​

[0028] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w

[0029] In the above chemical formula 2,

[0030] 0.85 ≤ g ≤ 0.95, 0 ≤ h ≤ 0.15, 0 ≤ i ≤ 0.15, 0 ≤ j ≤ 0.1, g + h + i + j = 1, 0 ≤ z ≤ 0.3, 0 ≤ w ≤ 0.2,

[0031] M3 is Mn, Al, or a combination thereof,

[0032] M4 is at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and

[0033] X2 is at least one element selected from F, Cl, Br, I, At, P, and S.

[0034] According to the present invention, the first positive electrode active material may include a lithium nickel-based oxide containing at least 80 mol%, specifically 80 - 85 mol%, of nickel relative to the total molar amount of transition metals other than lithium.

[0035] According to the present invention, the second positive electrode active material may include a lithium nickel-based oxide containing at least 85 mol% of nickel relative to the total molar amount of transition metals other than lithium.

[0036] According to the present invention, the first positive electrode active material may be secondary particles in which at least 50 primary particles are aggregated with each other.

[0037] According to the present invention, the second positive electrode active material may be single particles composed of one primary particle, quasi-single particles that are aggregates of 2 to 30 primary particles, or a combination thereof.

[0038] According to the present invention, the first positive electrode active material may have an average particle diameter D of 8 μm to 12 μm 50 .

[0039] According to the present invention, the second positive electrode active material may have an average particle diameter D of 1 μm to 7 μm 50 .

[0040] According to the present invention, the weight ratio of the first positive electrode active material to the second positive electrode active material may be 7:3 to 9:1.​

[0041] According to the present invention, the electrode assembly may include a negative electrode, and the negative electrode may include artificial graphite.

[0042] According to the present invention, the voltage retention rate V during 24C discharge represented by the following Mathematical Formula 1 d-24 is at least 83.8%, specifically 83.8% to 93.5%:

[0043] [Mathematical Formula 1]

[0044] V d-24 (%) = (V f-24 / V i-24 ) × 100

[0045] In the above Mathematical Formula 1,

[0046] V f-24 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 24C, and

[0047] V i-24 is the voltage of the fully charged lithium secondary battery before applying a discharge pulse at a rate of 24C.

[0048] According to the present invention, the voltage retention rate V' during continuous discharge represented by the following Mathematical Formula 2 d is at least 82.6%, specifically 83% to 92%:

[0049] [Mathematical Formula 2]

[0050] V' d (%) = (V' f / V' i ) × 100

[0051] In the above Mathematical Formula 2,

[0052] V' f is the voltage of the lithium secondary battery after applying two discharge pulses while increasing the rate by 4C, and

[0053] V' i is the initial voltage of the fully charged lithium secondary battery before applying a discharge pulse.

[0054] According to the present invention, the battery case may be a pouch-type battery case.

[0055] Advantageous effects

[0056] The lithium secondary battery according to the present invention is characterized in that by using one having 9.4 mg / cm 2The following positive electrode active material layer loadings and bimodal positive electrode active materials having different average particle diameters. As a result, even when the lithium secondary battery of the present invention is discharged at a high rate, the degree of voltage drop of the battery is not large. Therefore, when the battery according to the present invention is used as a power source for an electric tool, the phenomenon of power interruption of the electric tool due to high-rate discharge can be prevented.

[0057] In addition, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, the lithium secondary battery has a relatively light weight and thus has excellent work convenience.

[0058] In addition, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, there are a plurality of electrode tabs. Therefore, compared with a cylindrical battery, the lithium secondary battery has a low resistance and excellent output characteristics and is thus suitable as a power source for an electric tool. Detailed Description

[0059] The advantages and features of the present invention and methods for realizing them will be clarified by the following embodiments to be described. However, the present invention can be embodied in different forms and should not be construed as limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In addition, the present invention is defined only by the scope of the claims.

[0060] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be intended to have the meanings understood by those skilled in the art. In addition, terms defined in a general dictionary should not be construed abnormally or exaggeratedly unless specifically defined otherwise explicitly.

[0061] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to limit the present invention. In the specification, unless otherwise indicated to the contrary, the singular forms of the terms may include the plural forms. It should also be understood that when used in this specification, the terms "comprises" and / or "comprising" specify the presence of the stated elements, but do not preclude the presence or addition of one or more other elements.

[0062] In this specification, unless otherwise specified, when a part is referred to as "comprising" an element, the part does not exclude other elements, but may further include other elements.

[0063] In this specification, "A and / or B" means A or B, or A and B.

[0064] In this specification, unless otherwise clearly specified, the expression "%" means weight %.

[0065] The expression "D" in this specification 50" refers to the particle size at which the cumulative volume in the particle size distribution curve of the particles is 50%. For example, D can be measured by using the laser diffraction method. 50 The laser diffraction method can generally measure particle sizes from the submicron level to several millimeters and can obtain highly reproducible and high-resolution results.

[0066] In this specification, the expression "specific surface area" is measured by the Brunauer-Emmett-Teller (BET) method. Specifically, the specific surface area can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II from Bell Japan Co., Ltd.

[0067] Hereinafter, the present invention will be described in more detail.

[0068] Lithium secondary battery

[0069] The lithium secondary battery according to the present invention includes:

[0070] a battery case; and

[0071] an electrode assembly and an electrolyte accommodated in the battery case,

[0072] The electrode assembly includes a positive electrode,

[0073] The loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm 2 Hereinafter,

[0074] The positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, and

[0075] The second positive electrode active material has a smaller average particle size D than the first positive electrode active material 50 .

[0076] Generally, when a lithium secondary battery discharges at a high rate, the voltage of the battery decreases significantly. When the battery is used as a power source for power tools, even if the battery capacity remains, the power tool system may determine that the battery capacity is insufficient and the power source of the power tool may be turned off.

[0077] As a result of a large amount of repeated research for solving such a problem, the present inventors have found that when the loading amount of the positive electrode active material layer is 9.4 mg / cm 2 Hereinafter, but when using a bimodal positive electrode active material having different average particle sizes, even when the battery discharges at a high rate, the degree of voltage drop is not large. Therefore, when the battery is used as a power source for power tools, power-off of the power tools can be prevented, and thus the present invention has been completed.

[0078] Hereinafter, each component of the lithium secondary battery according to the present invention will be described in more detail.

[0079] <Electrode assembly>

[0080] The electrode assembly according to the present invention may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0081] Hereinafter, each component of the electrode assembly will be described in more detail.

[0082] (1) Positive electrode

[0083] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector.

[0084] There is no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause adverse chemical changes in the battery. As the current collector, for example, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0085] The thickness of the positive electrode current collector may be 8 μm to 500 μm, specifically 8 μm to 300 μm, and more specifically 10 μm to 50 μm. In order to improve the output characteristics of the battery, when the loading amount of the positive electrode active material layer is reduced to reduce the resistance of the positive electrode, the positive electrode active material is embedded in the positive electrode current collector, and thus the positive electrode current collector may be disconnected. However, when the thickness of the positive electrode current collector according to the present invention satisfies the above numerical range, the thickness of the positive electrode current collector is thicker than that of the conventional positive electrode current collector, and thus disconnection of the current collector can be prevented.

[0086] The positive electrode active material layer may include a first positive electrode active material and a second positive electrode active material, and may further include a conductive material, a binder, etc. as needed.

[0087] The first positive electrode active material and the second positive electrode active material are compounds capable of reversibly inserting and extracting lithium, and the first positive electrode active material and the second positive electrode active material may include a lithium composite metal oxide, and the lithium composite metal oxide includes lithium and at least one metal, such as cobalt, manganese, nickel or aluminum. Specifically, the first positive electrode active material and the second positive electrode active material may each include a lithium nickel-based oxide, and thus it is easy to realize a large-capacity battery.

[0088] The first positive electrode active material and the second positive electrode active material may have different compositions.

[0089] For example, the first positive electrode active material according to the present invention may contain a compound represented by the following Chemical Formula 1:

[0090] [Chemical Formula 1]

[0091] Li1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y

[0092] In the above Chemical Formula 1, 0.8 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.2, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.1, a + b + c + d = 1, 0 ≤ x ≤ 0.3, and 0 ≤ y ≤ 0.2 can be satisfied.

[0093] In Chemical Formula 1, M1 can be Mn, Al, or a combination thereof.

[0094] In the above Chemical Formula 1, M2 can be at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0095] In the above Chemical Formula 1, X1 can be at least one element selected from F, Cl, Br, I, At, P, and S.

[0096] The lithium nickel-based oxide included in the first positive electrode active material can contain at least 80 mol%, specifically 80 mol% to 95 mol%, and more specifically 80 mol% to 85 mol% of nickel relative to the total number of moles of transition metals other than lithium. When the nickel content in the lithium nickel-based oxide included in the first positive electrode active material satisfies the above numerical range, the positive electrode energy density increases to ensure sufficient battery capacity. Therefore, the battery of the present invention is suitable for use as a battery for power tools that require a large capacity.

[0097] On the other hand, the second positive electrode active material according to the present invention can include a compound represented by the following Chemical Formula 2:

[0098] [Chemical Formula 2]

[0099] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w

[0100] In the above Chemical Formula 2, 0.85 ≤ g ≤ 0.95, 0 ≤ h ≤ 0.15, 0 ≤ i ≤ 0.15, 0 ≤ j ≤ 0.1, g + h + i + j = 1, 0 ≤ z ≤ 0.3, and 0 ≤ w ≤ 0.2 can be satisfied.

[0101] ​​In Chemical Formula 2, M3 can be Mn, Al, or a combination thereof.

[0102] In Chemical Formula 2 above, M4 can be at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0103] In Chemical Formula 2 above, X2 can be at least one element selected from F, Cl, Br, I, At, P, and S.

[0104] The lithium nickel-based oxide included in the second positive electrode active material can contain at least 85 mol%, specifically 85 mol% to 95 mol%, and more specifically 85 mol% to 90 mol% of nickel relative to the total number of moles of transition metals other than lithium. When the nickel content in the lithium nickel-based oxide included in the second positive electrode active material satisfies the above numerical range, compared with the case of using only the first positive electrode active material, the average nickel content in the positive electrode active material increases, so the positive electrode energy density further increases, and thus it is easy to achieve a large-capacity battery.

[0105] On the other hand, the first positive electrode active material and the second positive electrode active material according to the present invention have different average particle sizes D 50 . Specifically, the second positive electrode active material has a smaller average particle size D than the first positive electrode active material 50 . As a result, compared with the case of using only the first positive electrode active material, the total specific surface area of the positive electrode active material relatively increases, so the number of reaction sites for lithium ions can be increased, and thus the output characteristics of the battery can be improved.

[0106] The first positive electrode active material can be secondary particles in which primary particles are aggregated with each other. For example, the secondary particles can be particles in which at least 50, specifically 50 to 80, and more specifically 50 to 70 primary particles are aggregated with each other.

[0107] The first positive electrode active material can have an average particle size D of 8 μm to 12 μm, specifically 8 μm to 10 μm, and more specifically 8 μm to 9 μm 50 . When the average particle size D of the first positive electrode active material 50 satisfies the above numerical range, the charging capacity of the battery can be ensured at an appropriate level.

[0108] The BET specific surface area of the first positive electrode active material can be 0.38 m 2 / g to 0.58 m 2 / g, specifically 0.42 m 2 / g to 0.53 m 2 / g, and more specifically 0.45 m 2 / g to 0.51 m 2 / g. When the BET specific surface area of the first positive electrode active material satisfies the above numerical range, even when the battery is discharged at a high rate, there is an effect of maintaining the output level.

[0109] The first positive electrode active material can be formed by washing a lithium transition metal oxide with water, followed by drying and heat treatment. Through the water washing treatment, boron (B) coating, zirconium (Zr) doping, and / or strontium (Sr) doping can be carried out on the surface of the particles, thereby improving the capacity and stability of the battery.

[0110] The second positive electrode active material is single particles and / or quasi-single particles. The single particles can be composed of one primary particle. The quasi-single particles can be particles in which at least 2 to 30, specifically 2 to 20, more specifically 2 to 10 primary particles are aggregated.

[0111] The second positive electrode active material can have an average particle diameter D smaller than that of the first positive electrode active material 50 . More particularly, the second positive electrode active material can have an average particle diameter D of 1 μm to 7 μm, specifically 2 μm to 6 μm, more specifically 4 μm to 5 μm 50 . When the average particle diameter D of the second positive electrode active material 50 satisfies the above numerical range, since the second positive electrode active material has a high specific surface area, the total specific surface area of the positive electrode active material can be increased.

[0112] The BET specific surface area of the second positive electrode active material can be 0.54 m 2 / g to 0.74 m 2 / g, specifically 0.59 m 2 / g to 0.69 m 2 / g, more specifically 0.61 m 2 / g to 0.67 m 2 / g. When the BET specific surface area of the second positive electrode active material satisfies the above numerical range, the total specific surface area of the positive electrode active material increases, so that the number of reaction sites for lithium ions can be increased, thereby improving the output characteristics of the battery.

[0113] The second positive electrode active material can be obtained without washing the lithium transition metal oxide with water. When manufacturing the second positive electrode active material, there is an effect of reducing the initial resistance by omitting the water washing treatment.

[0114] Relative to the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 60% by weight to 99% by weight, preferably 70% by weight to 99% by weight, more preferably 80% by weight to 98% by weight. When the content of the positive electrode material satisfies the above range, the battery capacity can be improved by ensuring sufficient positive electrode energy density.

[0115] In this case, the weight ratio of the first positive electrode active material to the second positive electrode active material can be from 70:30 to 90:10, specifically from 75:25 to 85:15, and more specifically from 78:22 to 82:18. When the first positive electrode active material and the second positive electrode active material are mixed and used in the above weight ratio, the total specific surface area of the positive electrode active material is controlled to an appropriate level, thereby minimizing the reduction of the capacity and resistance characteristics of the battery and improving the output characteristics.

[0116] The positive electrode conductive material is a component for further improving the conductivity of the positive electrode active material, and there is no particular limitation on the conductive material as long as it has conductivity and does not cause adverse chemical changes in the battery.

[0117] For example, as the positive electrode conductive material, conductive materials such as carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal cracking carbon black; graphite powders such as natural graphite or artificial graphite having a very developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powders, aluminum powders and nickel powders; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxides; or polyphenylene derivatives can be used.

[0118] The positive electrode conductive material according to the present invention may include a dot-shaped conductive material and a linear conductive material. Specifically, the conductive material may include the dot-shaped conductive material and the linear conductive material in a weight ratio of 10:1 to 50:1, preferably 10:1 to 40:1, and more preferably 20:1 to 40:1. When the weight ratio of the dot-shaped conductive material to the linear conductive material is less than 10:1, as the specific surface area of the conductive material increases, side reactions increase, so there is a problem of excessive gas generation in the hot box test. When the weight ratio of the dot-shaped conductive material to the linear conductive material is greater than 50:1, there is a problem that the conductive network in the positive electrode is not sufficiently ensured, so the positive electrode resistance value increases.

[0119] In this case, the content of the linear conductive material may be 0.5% by weight or less, specifically 0.05% by weight to 0.3% by weight, and more specifically 0.05% by weight to 0.2% by weight, based on the total weight of the positive electrode active material layer.

[0120] The content of the positive electrode conductive material may be 1% by weight to 20% by weight, preferably 1% by weight to 15% by weight, and more preferably 1% by weight to 10% by weight, based on the total weight of the positive electrode active material layer. When the content of the positive electrode conductive material in the positive electrode active material layer satisfies the above range, the conductivity of the positive electrode can be improved by ensuring the positive electrode conductive network.

[0121] The positive electrode binder is a component that helps the binding between the active material and the conductive material and the binding with the current collector.

[0122] Examples of the positive electrode binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene (PE), polypropylene, ethylene propylene diene monomer rubber, sulfonated ethylene propylene diene monomer rubber, styrene-butadiene rubber, fluororubber, various copolymers, etc., and any one of them alone or a mixture of two or more thereof may be used.

[0123] Relative to the total weight of the positive electrode active material layer, the content of the positive electrode binder may be 0.5 wt% to 5.0 wt%, specifically 1.0 wt% to 4.0 wt%, and more specifically 1.0 wt% to 3.5 wt%. When the content of the positive electrode binder satisfies the above range, the contact area between the positive electrode binder and the positive electrode active material increases, thereby ensuring excellent positive electrode adhesion.

[0124] On the other hand, the loading of the positive electrode active material layer may be 9.4 mg / cm 2 Hereinafter, specifically 8.0 mg / cm 2 to 9.4 mg / cm 2 , and more specifically 8.20 mg / cm 2 to 9.35 mg / cm 2 . When the loading of the positive electrode active material layer is greater than 9.4 mg / cm 2 , there is a problem that the resistance of the battery increases as the positive electrode resistance value increases, resulting in deterioration of the output characteristics.

[0125] The porosity of the positive electrode active material layer may be greater than 30%, specifically 32% to 38%, and more specifically 32% to 37%. When the porosity of the positive electrode active material layer satisfies the above range, the interfacial resistance of the positive electrode decreases, thereby improving the output characteristics of the battery.

[0126] The packing density of the positive electrode active material layer may be at least 2.8 g / cc, specifically 2.8 g / cc to 3.5 g / cc, and more specifically 2.9 g / cc to 3.2 g / cc. Therefore, the porosity of the positive electrode active material layer may have the above appropriate numerical range.

[0127] On the other hand, the positive electrode can be prepared according to a typical method for preparing a positive electrode, except that the above positive electrode active material is used. Specifically, the positive electrode can be prepared by preparing a positive electrode paste composition containing the above positive electrode active material, positive electrode conductive material and / or positive electrode binder, coating the positive electrode paste composition on the positive electrode current collector, and drying and calendering the positive electrode paste composition.

[0128] Alternatively, as another method, the positive electrode can be prepared by casting a positive electrode paste composition on a separate carrier and then laminating the film separated from the carrier on the positive electrode current collector.

[0129] (2) Negative electrode

[0130] The negative electrode may include a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. Alternatively, the negative electrode may be a graphite electrode made of carbon (C). Alternatively, the negative electrode may be the metal itself.

[0131] There is no particular limitation on the negative electrode current collector as long as it has high conductivity and does not cause adverse chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used.

[0132] The negative electrode current collector generally may have a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to improve the binding force of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as a film, sheet, foil, net, porous body, foam body, and non-woven fabric body.

[0133] The negative electrode active material layer may include a negative electrode active material and may further include a conductive material, a binder, etc. as needed.

[0134] The negative electrode active material is lithium metal and / or a material capable of reversibly inserting / extracting lithium ions.

[0135] The negative electrode active material may include at least one selected from the following: carbonaceous materials, metals or alloys of lithium and metals, metal composite oxides, materials capable of doping and de-doping lithium, and transition metal oxides.

[0136] The carbonaceous active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon may include graphite, such as irregular, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, and examples of amorphous carbon may include soft carbon (low-temperature sintered carbon) or hard carbon, mesophase pitch carbide, sintered coke, etc.

[0137] As the metal or alloy of lithium and metal, metals selected from Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or alloys of lithium and metals can be used.

[0138] As the metal composite oxide, those selected from PbO, PbO 2 , Pb 2 O 3 , Pb3 O 4 、 Sb 2 O 3 、 Sb 2 O 4 、 Sb 2 O 5 、 GeO, GeO 2 、 Bi 2 O 3 、 Bi 2 O 4 、 Bi 2 O 5 、 Li x Fe 2 O 3 (0 ≤ x ≤ 1), Li x WO 2 (0 ≤ x ≤ 1) and Sn x Me 1-x Me' y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements of groups I, II and III of the periodic table, or halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8).

[0139] Materials capable of doping and de-doping lithium may include Si, SiO x (0 < x ≤ 2), Si-Y alloys (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO 2 and Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), and mixtures of SiO 2 and at least one of them may also be used. Element Y may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0140] Transition metal oxides may include lithium-containing titanium composite oxides (LTO), vanadium oxides, and lithium vanadium oxides.

[0141] Specifically, the negative electrode active material according to the present invention may be artificial graphite, and more specifically, may be artificial graphite coated with hard carbon on its surface. When artificial graphite is used as the negative electrode active material, the capacity characteristics of the battery are improved by reducing the resistance of the negative electrode, and the cycle life characteristics of the battery are improved by reducing the overvoltage during charging to prevent lithium precipitation caused by side reactions.

[0142] Relative to the total weight of the negative electrode active material layer, the content of the negative electrode active material may be 60% by weight to 99% by weight, preferably 70% by weight to 99% by weight, and more preferably 80% by weight to 99% by weight. When the content of the negative electrode active material satisfies the above range, the contents of the negative electrode conductive material and the negative electrode binder can be maintained at desired levels, and sufficient negative electrode energy density can be ensured, thereby improving the battery capacity.

[0143] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material, and there is no particular limitation on the conductive material as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, conductive materials such as carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal cracking carbon black; graphite powders such as natural graphite or artificial graphite conductive fibers with a very developed crystal structure such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powders, aluminum powders and nickel powders; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxides; or polyphenylene derivatives can be used.

[0144] Relative to the total weight of the negative electrode active material layer, the content of the negative electrode conductive material may be 1% by weight to 20% by weight, preferably 1% by weight to 15% by weight, and more preferably 1% by weight to 10% by weight. When the content of the negative electrode active material satisfies the above range, the conductivity of the negative electrode can be improved by ensuring the negative electrode conductive network.

[0145] The negative electrode binder is a component that helps the binding between the negative electrode conductive material, the negative electrode active material and the negative electrode current collector. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene propylene diene monomer rubber, sulfonated ethylene propylene diene monomer rubber, styrene-butadiene rubber, fluororubber, various copolymers thereof, etc., and any one of them alone or a mixture of two or more of them can be used.

[0146] Based on the total weight of the negative electrode active material layer, the content of the negative electrode binder can be 1 wt% to 20 wt%, preferably 1 wt% to 15 wt%, more preferably 1 wt% to 10 wt%. When the content of the negative electrode binder meets the above range, the negative electrode active material particles can be smoothly combined to minimize the volume expansion problem of the negative electrode active material, and the negative electrode active material can be well attached to the negative electrode current collector.

[0147] On the other hand, when using the metal itself without forming a negative electrode active material layer on the negative electrode, the negative electrode can be prepared by physically combining, rolling, or depositing the metal on the metal thin film itself or the negative electrode current collector. The deposition method can use the electroplating method or chemical deposition method of the metal.

[0148] For example, the metal combined / rolled / deposited on the metal thin film itself or the negative electrode current collector can include one metal selected from lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In), or an alloy of two of these metals.

[0149] (3) Separator

[0150] The separator can be used without particular limitation as long as it is generally used as a separator in lithium secondary batteries. In particular, a separator having a high moisture retention capacity for the electrolyte and a low resistance to the transfer of electrolyte ions is preferred.

[0151] As the separator, for example, a porous polymer including polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a stacked structure having two or more layers thereof can be used. In addition, typical porous non-woven fabrics, such as non-woven fabrics formed of high melting point glass fibers or polyethylene terephthalate fibers, can be used as the separator.

[0152] The thickness of the separator can be 5 μm to 20 μm, preferably 5 μm to 15 μm, more preferably 6 μm to 13 μm. When the thickness of the separator meets the above range, the battery resistance value can be minimized while preventing a short circuit between the positive electrode and the negative electrode. As a result, the life characteristics and output characteristics of the lithium secondary battery can be improved.

[0153] <Electrolyte>

[0154] On the other hand, the lithium secondary battery according to the present invention can include an electrolyte.

[0155] The electrolyte can include commonly used organic solvents and lithium salts in the art and is not particularly limited.

[0156] Any organic solvent can be used as the organic solvent without particular limitation, as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, γ-butyrolactone or ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene or fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC) can be used alone or in combination.

[0157] Preferably, in the electrolyte, a mixed solution of a carbonate solvent and an ester solvent can be used as the organic solvent. Specifically, a mixed solution of a cyclic carbonate solvent and an ester solvent, or a mixed solution of a cyclic carbonate solvent, a linear carbonate solvent and an ester solvent can be used.

[0158] The cyclic carbonate solvent can be at least one of ethylene carbonate and propylene carbonate. The linear carbonate solvent can be at least one selected from dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0159] The ester solvent can be ethyl propionate (EP). Since ethyl propionate has a lower viscosity than conventional electrolyte components, when ethyl propionate is included, the effect of increasing the ionic conductivity of the electrolyte can be obtained.

[0160] Relative to the total weight of the electrolyte, the content of ethyl propionate can be 80% by weight or less, specifically 5% to 80% by weight, more specifically 40% to 70% by weight. When the content of ethyl propionate satisfies the above numerical range, the viscosity of the electrolyte can be optimized to achieve excellent ionic conductivity of the electrolyte.

[0161] A lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, LiN(SO 2 F) 2 、LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF 6 、LiAlO 4 、LiAlCl 4 、LiCF 3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5SO 3 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiCl、LiI、LiB(C 2 O 4 ) 2 etc.) can be used as lithium salts. The lithium salts are preferably included in the electrolyte at a concentration of about 0.6 mol% to about 2 mol%.

[0162] The electrolyte according to the present invention may further include additives to further improve the physical properties of the secondary battery.

[0163] Examples of the additives may include at least one selected from cyclic carbonate compounds, halogen-substituted carbonate compounds, nitrile compounds, sultone compounds, sulfate compounds, phosphate compounds, borate compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0164] The cyclic carbonate compounds may be, for example, vinylene carbonate (VC) or ethylene vinylene carbonate (VEC).

[0165] The halogen-substituted carbonate compounds may be, for example, fluoroethylene carbonate (FEC), etc.

[0166] The nitrile compounds may be, for example, succinonitrile, adiponitrile, hexantricyanide, 1,4-dicyano-2-butene, etc.

[0167] The sultone compounds may be, for example, 1,3-propane sultone, 1,3-propene sultone, etc.

[0168] The sulfate compounds may be, for example, ethylene sulfite (Esa), trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), etc.

[0169] The phosphate compounds may include, for example, at least one compound selected from the following: lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyl trimethylsilyl phosphate, trimethylsilyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphite.

[0170] The borate compounds may be, for example, tetraphenyl borate, lithium oxalyl difluoroborate (LiODFB), etc.

[0171] Benzene compounds may include, for example, fluorobenzene, amine compounds may include triethanolamine or ethylenediamine, and silane compounds may include tetravinylsilane.

[0172] The lithium salt compound is a compound different from the lithium salts included in the non-aqueous electrolyte, and the lithium salt compound may include a compound selected from LiPO 2 F 2 , LiODFB, LiBOB (lithium bis(oxalato)borate (LiB(C 2 O 4 )) 2 ), and LiBF 4 , and at least one compound selected therefrom.

[0173] On the other hand, the additive may be used alone or in combination of two or more thereof.

[0174] Relative to the total weight of the electrolyte, the total amount of the additive may be 1 wt% to 25 wt%, preferably 1 wt% to 20 wt%, more preferably 5 wt% to 20 wt%. When the additive is included within the above range, a film can be stably formed on the electrode, and the ignition phenomenon during overcharging of the battery can be suppressed, and side reactions during the initial activation process of the secondary battery can be prevented, or the residue or precipitation of the additive can be prevented.

[0175] <Battery case>

[0176] The lithium secondary battery according to the present invention may include a cylindrical, prismatic, pouch-type, or coin-type battery case, and preferably may include a pouch-type battery case.

[0177] The pouch-type battery case includes a barrier layer, a substrate layer formed on one surface of the barrier layer, and a sealant layer formed on the other surface of the barrier layer, and includes at least one cup portion recessed in one direction.

[0178] Specifically, the pouch-type battery case is flexible and can be manufactured by a method of inserting a pouch film stack sequentially stacked with a substrate layer, a barrier layer, and a sealant layer into a press-forming device and stretching the pouch film stack by applying pressure to a partial area of the pouch film stack with a punch to form a cup portion recessed in one direction.

[0179] The substrate layer is provided on the outermost layer of the pouch to protect the electrode assembly from external shocks and to electrically insulate it.

[0180] The substrate layer may be made of a polymer material such as those selected from polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobis At least one polymer material among azole, polyarylate, and polytetrafluoroethylene.

[0181] The substrate layer can have a single-layer structure or a multi-layer structure in which different polymer films are stacked. When the substrate layer has a multi-layer structure, an adhesive layer can be inserted between the polymer films.

[0182] On the other hand, the total thickness of the substrate layer can be 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the substrate layer has a multi-layer structure, the thickness includes the thickness of the adhesive layer. When the base layer satisfies the above range, durability, insulation, and formability are excellent. If the substrate layer is too thin, the durability may deteriorate, and damage to the substrate layer may occur during the forming process. If the substrate layer is too thick, the formability may deteriorate, the total thickness of the bag may increase, and the battery accommodation space may decrease, thereby reducing the energy density.

[0183] The barrier layer is used to ensure the mechanical strength of the pouch-type battery case, block the entry of gases, moisture, etc. from outside the secondary battery, and prevent the leakage of the electrolyte.

[0184] The thickness of the barrier layer can be 40 μm to 100 μm, more preferably 50 μm to 80 μm, and more preferably 60 μm to 80 μm. When the thickness of the barrier layer satisfies the above range, the formability is improved to increase the forming depth of the cup portion, or even when two cups are formed, fewer cracks and / or pinholes are generated, thereby improving the resistance to external stress after forming.

[0185] On the other hand, the barrier layer can be made of a metal material, specifically, it can be made of an aluminum alloy thin film.

[0186] The aluminum alloy thin film can include aluminum and metal elements other than aluminum, such as one or more selected from iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0187] The sealant layer is attached by thermal compression to seal the bag and is located on the innermost layer of the bag film stack.

[0188] Since the sealant layer is the surface that contacts the electrolyte and the electrode assembly after the bag is formed, the sealant layer should have insulation and corrosion resistance, and since its interior should be completely sealed to prevent material movement between the inside and the outside, the sealant layer should have high sealing properties.

[0189] The sealant layer may be made of, for example, a polymer material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaben, polyvinyl chloride ... At least one of polytetrafluoroethylene, polyarylate and polytetrafluoroethylene, wherein it is particularly preferred that the sealant layer includes polypropylene (PP) which has excellent mechanical properties such as tensile strength, stiffness, surface hardness, wear resistance and heat resistance, as well as excellent chemical properties such as corrosion resistance.

[0190] The sealant layer may have a single-layer structure, or may have a multi-layer structure including two or more layers made of different polymer materials.

[0191] The sealant layer may have a total thickness of 60 μm to 100 μm, preferably 60 μm to 90 μm, more preferably 70 μm to 90 μm. If the sealant layer is too thin, the sealing durability and insulation may be deteriorated, and if the sealant layer is too thick, the flexibility may be deteriorated, and the total thickness of the bag film stack may increase, resulting in a decrease in energy density relative to volume.

[0192] On the other hand, the bag film stack can be prepared by a method for preparing a bag film stack known in the art. For example, the bag film stack can be prepared by a method for attaching a substrate layer to the upper surface of the barrier layer by an adhesive and forming a sealant layer on the lower surface of the barrier layer by coextrusion or an adhesive, but is not limited thereto.

[0193] The pouch-type battery case may be sealed in a state where the electrode assembly is accommodated therein so that a portion (i.e., an end portion) of the electrode lead is exposed. Specifically, when the electrode lead is connected to the electrode tab of the electrode assembly and the insulating portion is formed on a portion of the electrode lead, the electrode assembly is accommodated in the accommodation space provided in the cup portion, the electrolyte is injected, and then the pouch-type battery case may be sealed.

[0194] The electrode lead may have a thickness of 0.05 mm to 0.5 mm, specifically 0.08 mm to 0.3 mm, and more specifically 0.1 mm to 0.2 mm. When the thickness of the electrode lead satisfies the above numerical range, which is thicker than the existing electrode lead, the following effects are present: the resistance of the battery is reduced, and at the same time, when an external short circuit occurs, heat generation is reduced, thereby improving heat resistance.

[0195] When the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, the lithium secondary battery is lighter than a cylindrical secondary battery and has lower resistance because of the presence of multiple tabs, so that the lithium secondary battery has excellent output characteristics and can be suitably used as a battery for electric tools.

[0196] On the other hand, in the lithium secondary battery according to the present invention, the voltage retention rate V during 24C discharge represented by the following Mathematical Formula 1 d-24 can be at least 83.8%, specifically 83.8% to 95.0%, and more specifically 83.8% to 93.5%.

[0197] [Mathematical Formula 1]

[0198] V d-24 (%) = (V f-24 / V i-24 ) × 100

[0199] In the above Mathematical Formula 1, V f-24 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 24C, and V i-24 is the voltage of the fully charged lithium secondary battery before applying a discharge pulse at a rate of 24C. When the voltage retention rate V d-24 satisfies the above numerical range, even when the battery is discharged at a high rate, the degree of voltage drop is not large. Therefore, the battery according to the present invention can be suitably used as a power source for power tools.

[0200] On the other hand, in the lithium secondary battery according to the present invention, the voltage retention rate V' during continuous discharge represented by the following Mathematical Formula 2 d can be at least 82.6%, specifically 82.6% to 92.5%, and more specifically 83% to 92%.

[0201] [Mathematical Formula 2]

[0202] V' d (%) = (V' f / V' i ) × 100

[0203] In the above Mathematical Formula 2, V' f is the voltage of the lithium secondary battery after applying two discharge pulses while increasing the rate by 4C, V' i is the initial voltage of the fully charged lithium secondary battery before applying the discharge pulse. The first discharge pulse is applied at a rate of 24C, and the second discharge pulse is applied at a rate of 28C.

[0204] When the voltage retention rate V' during continuous discharge d satisfies the above numerical range, even when the battery is continuously discharged, the battery voltage will not be excessively reduced. Therefore, the battery according to the present invention can be suitably used as a power source for power tools that require continuous discharge.

[0205] The lithium secondary battery according to the present invention can be used as a battery cell for powering small devices such as power tools, and can also be used as a unit cell in medium and large-sized battery modules including a plurality of battery cells. Preferred examples of medium and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems (ESS), etc.

[0206] Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the following embodiments are only for illustrative purposes of the present invention, and the scope of the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications and changes are possible within the scope and spirit of the present invention. Such modifications and changes fall within the scope of the claims included herein.

[0207] Examples and comparative examples

[0208] Example 1 (Preparation of Lithium Secondary Battery)

[0209] Artificial graphite coated with hard carbon (average particle size D 50 = 19 μm, BET specific surface area = 0.8 m 2 / g), carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed at a weight ratio of 94.3:2.0:1.2:2.5, and distilled water was added thereto to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 44% by weight.

[0210] The negative electrode slurry was coated onto one surface of a copper (Cu) metal film having a thickness of 10 μm at a loading amount of 6.96 mAh / cm 2 , and then dried in vacuo. Then, the dried negative electrode slurry was calendered, dried in a vacuum oven at 130 °C for 12 hours, and then punched to prepare a negative electrode.

[0211] Li[Ni 50 having an average particle size D of 10 μm 2 and a BET specific surface area of 0.57 m 0.83 Co 0.12 Mn 0.05 ]O 2 was used as the first positive electrode active material, and Li[Ni 50 having an average particle size D of 4 μm 2 and a BET specific surface area of 0.65 m 0.86 Co 0.08 Mn 0.06 ]O 2 as single particles and / or quasi-single particles was used as the second positive electrode active material.

[0212] The positive electrode active material obtained by mixing a first positive electrode active material and a second positive electrode active material in a weight ratio of 8:2, the positive electrode conductive material obtained by mixing carbon black and carbon nanotubes (CNT) in a weight ratio of 30:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) are introduced into an N-methylpyrrolidone (NMP) solvent in a weight ratio of 95.28:3.1:1.3:0.32, and then stirred to prepare a positive electrode paste. The solid content of the positive electrode paste is 70% by weight.

[0213] The positive electrode paste is coated on one surface of an aluminum thin film with a thickness of 15 μm at a loading amount of 9.24 mg / cm 2 , and then vacuum dried at 130 °C for 10 hours. The dried positive electrode paste is calendered, dried in a vacuum oven at 130 °C for 12 hours, and then stamped to prepare a positive electrode.

[0214] The negative electrode and positive electrode prepared as described above and a porous polyethylene separator with a thickness of 10 μm are assembled in a stacked manner to prepare an electrode assembly.

[0215] An electrolyte is prepared by dissolving LiPF 6 and LiFSi in a solvent mixed with ethylene carbonate (EC) and ethyl propionate (EP) in a weight ratio of 35:65 such that LiPF 6 becomes 0.7 M and LiFSi becomes 0.7 M.

[0216] A lithium secondary battery is prepared by accommodating the electrode assembly in a pouch-type battery case, injecting the electrolyte therein, and sealing the case.

[0217] Example 2 (Preparation of Lithium Secondary Battery)

[0218] A positive electrode is prepared in the same manner as in Example 1 except that a loading amount of 9.3 mg / cm 2 is used.

[0219] A lithium secondary battery is prepared in the same manner as in Example 1 except that the above positive electrode is used.

[0220] Example 3 (Preparation of Lithium Secondary Battery)

[0221] A positive electrode is prepared in the same manner as in Example 1 except that the positive electrode active material obtained by mixing a first positive electrode active material and a second positive electrode active material in a weight ratio of 7:3 is used.

[0222] A lithium secondary battery is prepared in the same manner as in Example 1 except that the above positive electrode is used.

[0223] Example 4 (Preparation of Lithium Secondary Battery)

[0224] The positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material obtained by mixing the first positive electrode active material and the second positive electrode active material at a weight ratio of 9:1 was used.

[0225] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0226] Comparative Example 1 (Preparation of Lithium Secondary Battery)

[0227] The positive electrode was prepared in the same manner as in Example 1, except that only the first positive electrode active material was used without using the second positive electrode active material.

[0228] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0229] Comparative Example 2 (Preparation of Lithium Secondary Battery)

[0230] The positive electrode was prepared in the same manner as in Example 1, except that a loading amount of 11.0 mg / cm 2 was used.

[0231] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0232] Comparative Example 3 (Preparation of Lithium Secondary Battery)

[0233] The positive electrode was prepared in the same manner as in Example 1, except that a loading amount of 9.5 mg / cm 2 was used.

[0234] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0235] In Examples 1 to 4 and Comparative Examples 1 to 3, the composition of the positive electrode active material and the loading amount of the positive electrode active material layer are as shown in Table 1 below.

[0236]

[0237] Experimental example 1 - Evaluation of continuous discharge test

[0238] Each lithium secondary battery in the examples and comparative examples was fully charged to 4.2 V under an initial 0.6 C condition at 23°C. Then, a continuous discharge test of the lithium secondary battery was carried out by using a PNE cycle program (CTSMonPro, PNE SOLUTION CO.LTD), applying a discharge pulse under the conditions shown in Table 2 below and having a rest period between each pulse application.

[0239] In this case, the battery voltage before and after 1 to 5 pulses are applied in a continuous discharge test is measured, and the results are shown in Table 3 below. In this case, the battery voltage before each pulse application is represented by V i and the battery voltage after each pulse application is represented by V f . Also, the ratio of the battery voltage V f after each pulse application to the battery voltage V i before each pulse application is represented by V d (%). In addition, the voltage retention rate V d-24 during 24C discharge represented by the following Mathematical Formula 1 and the voltage retention rate V' d during continuous discharge represented by the following Mathematical Formula 2 are calculated from the measured voltage values, and the results are shown in Table 4 below.

[0240] [Mathematical Formula 1]

[0241] V d-24 (%) = (V f-24 / V i-24 ) × 100

[0242] In the above Mathematical Formula 1, V f-24 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 24C, and V i-24 is the voltage of the fully charged lithium secondary battery before applying a discharge pulse at a rate of 24C.

[0243] [Mathematical Formula 2]

[0244] V' d (%) = (V' f / V' i ) × 100

[0245] In the above Mathematical Formula 2, V'< f is the voltage of the lithium secondary battery after continuously applying two discharge pulses, and V'< i is the voltage of the fully charged lithium secondary battery before applying a discharge pulse. During continuous discharge, a discharge pulse is applied while increasing the rate by 4C, the rate of the first discharge pulse is 24C, and the rate of the second discharge pulse is 28C.

[0246]

[0247]

[0248]

[0249] As shown in Tables 3 to 4, it can be confirmed that the voltage retention rate V of the lithium secondary battery during 24C discharge in Examples 1 to 4d-24 is at least 84.00%, which is higher than the voltage retention rate of the lithium secondary battery during the 24C discharge in Comparative Examples 1 to 3. In addition, it can be confirmed that the voltage retention rate V′ during the continuous discharge of the lithium secondary batteries in Examples 1 to 4 d is at least 83.40%, which is higher than the voltage retention rate of Comparative Examples 1 to 3. This means that the degree of voltage drop of the battery during high-rate discharge and continuous discharge in Examples 1 to 4 is smaller than that of the battery during high-rate discharge and continuous discharge in Comparative Examples 1 to 3.

Claims

1. A lithium secondary battery, the lithium secondary battery comprises: a battery case; and an electrode assembly and an electrolyte accommodated in the battery case, wherein the electrode assembly includes a positive electrode, The loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm 2 Hereinafter, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, and The second positive electrode active material has a smaller average particle size D than the first positive electrode active material 50 .

2. The lithium secondary battery according to claim 1, wherein the first positive electrode active material contains a compound represented by the following Chemical Formula 1: [Chemical Formula 1] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y ​ Wherein, in the above Chemical Formula 1, 0.8 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.2, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.1, a + b + c + d = 1, 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.2, M1 is Mn, Al or a combination thereof, M2 is at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and X1 is at least one element selected from F, Cl, Br, I, At, P and S.

3. The lithium secondary battery according to claim 1, wherein the second positive electrode active material contains a compound represented by the following Chemical Formula 2: [Chemical Formula 2] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w ​ Wherein, in the above Chemical Formula 2, 0.85 ≤ g ≤ 0.95, 0 ≤ h ≤ 0.15, 0 ≤ i ≤ 0.15, 0 ≤ j ≤ 0.1, g + h + i + j = 1, 0 ≤ z ≤ 0.3, 0 ≤ w ≤ 0.2, M3 is Mn, Al or a combination thereof, M4 is at least one metal element selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and X2 is at least one element selected from F, Cl, Br, I, At, P and S.

4. The lithium secondary battery according to claim 1, wherein the first positive electrode active material contains a lithium nickel-based oxide containing at least 80 mol% of nickel relative to the total number of moles of transition metals other than lithium.

5. The lithium secondary battery according to claim 1, wherein the first positive electrode active material contains a lithium nickel-based oxide containing 80 mol% - 85 mol% of nickel relative to the total number of moles of transition metals other than lithium.

6. The lithium secondary battery according to claim 1, wherein the second positive electrode active material contains a lithium nickel-based oxide containing at least 85 mol% of nickel relative to the total number of moles of transition metals other than lithium.

7. The lithium secondary battery according to claim 1, wherein the first positive electrode active material is secondary particles in which at least 50 primary particles are aggregated with each other.

8. The lithium secondary battery according to claim 1, wherein the second positive electrode active material is single particles composed of one primary particle, quasi-single particles that are aggregates of 2 to 30 primary particles, or a combination thereof.

9. The lithium secondary battery according to claim 1, wherein the first positive electrode active material has an average particle diameter D of 8 μm to 12 μm 50 .

10. The lithium secondary battery according to claim 1, wherein the second positive electrode active material has an average particle diameter D of 1 μm to 7 μm 50 .

11. The lithium secondary battery according to claim 1, wherein the weight ratio of the first positive electrode active material to the second positive electrode active material is 7:3 to 9:

1.

12. The lithium secondary battery according to claim 1, wherein the electrode assembly includes a negative electrode, and the negative electrode contains artificial graphite as a negative electrode active material.

13. The lithium secondary battery according to claim 1, wherein the voltage retention rate V during 24C discharge represented by the following Mathematical Formula 1 d-24 is at least 83.8%: [Mathematical formula 1] V d-24 (%)=(V f-24 / V i-24 )×100 Wherein, In the above Mathematical formula 1, V f-24 is the voltage of the lithium secondary battery after applying a discharge pulse at a 24C rate, and V i-24 is the voltage of the lithium secondary battery that is fully charged before applying a discharge pulse at a 24C rate.

14. The lithium secondary battery according to claim 13, wherein the voltage retention rate V during 24C discharge d-24 is 83.8% to 93.5%.

15. The lithium secondary battery according to claim 1, wherein the voltage retention rate V′ during continuous discharge represented by the following mathematical formula 2 d is at least 82.6%: [Mathematical formula 2] V′ d (%)=(V′ f / V′ i )×100 Wherein, In the above Mathematical formula 2, V' f is the voltage of the lithium secondary battery after applying two discharge pulses while increasing the magnification by 4C, and V' i is the initial voltage of the lithium secondary battery that is fully charged before applying a discharge pulse.

16. The lithium secondary battery according to claim 15, wherein a voltage retention rate V' during the continuous discharge d is from 83% to 92%.

17. The lithium secondary battery according to claim 1, wherein the battery case is a pouch-type battery case.

Images