Lithium secondary battery

Abstract

The lithium secondary battery according to the present invention includes a battery case, an electrode assembly housed in the battery case, and an electrolyte. The electrode assembly includes a positive electrode, and the loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm. 2 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 an average particle size D 50 is small.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on 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, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

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

[0003] As technological development and demand for power tools, electric vehicles, and energy storage systems (ESS) increases, the demand for batteries as energy sources is growing rapidly, and various research projects are being conducted on batteries that can meet various needs. As the market demand for high-capacity lithium secondary batteries as power sources for such devices grows, research is being actively conducted to increase cell energy density. In addition, there is an increasing demand for batteries that have excellent not only capacity characteristics but also rate characteristics, as batteries that can be used in devices that require high output, such as power tools.

[0004] However, when a secondary battery installed in a power tool is discharged at high output, the voltage drop of the secondary battery can cause the system to shut down. That is, during high-rate discharge, the potential of the secondary battery drops significantly, causing the system to determine that the battery has insufficient capacity, even though there is still capacity remaining in the battery, resulting in the power being turned off.

[0005] In addition, cylindrical batteries are currently commonly used as batteries for power tools. However, in the case of cylindrical batteries, the battery can is heavy, so when such batteries are used, the total weight of the power tool increases, resulting in a problem of reduced ease of use. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above problems, and aims to provide a lithium secondary battery that reduces the degree of voltage drop of the battery during high-rate discharge and can prevent the power-off phenomenon of power tools.

[0007] Another object of the present invention is to provide a pouch-type lithium secondary battery that exhibits a low voltage drop during high-rate discharge, is relatively light in weight and easy to work with, has low resistance, and has excellent output characteristics. [Means for solving the problem]

[0008] According to the present invention, the battery includes a battery case, an electrode assembly housed in the battery case, and an electrolyte. The electrode assembly includes a positive electrode, and the loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm. 2 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 an average particle size D 50 A lithium secondary battery with a small capacity is provided.

[0009] According to the present invention, the first positive electrode active material may include a compound represented by the following Formula 1: [Chemical formula 1] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y In 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; M1 is Mn, Al, or a combination thereof; M2 is one metal element selected from the group consisting of 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 X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0010] According to the present invention, the second positive electrode active material may include a compound represented by the following Formula 2: [Chemical formula 2] Li 1+z [Ni g Co h M3 i M4 j ] 1-z O 2-w X2 w In 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; M3 is Mn, Al, or a combination thereof; M4 is one metal element selected from the group consisting of 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 one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0011] According to the present invention, the first positive electrode active material may include a lithium nickel-based oxide containing 80 mol % or more, specifically 80 mol % to 85 mol %, of nickel relative to the total number of moles of transition metals other than lithium.

[0012] According to the present invention, the second positive electrode active material may include a lithium nickel-based oxide containing 85 mol % or more of nickel based on the total number of moles of transition metals other than lithium.

[0013] According to the present invention, the first positive electrode active material may be secondary particles formed by agglomeration of 50 or more primary particles.

[0014] According to the present invention, the second positive electrode active material may be a single particle consisting of one primary particle, a quasi-single particle which is an aggregate of 2 to 30 of the primary particles, or a combination thereof.

[0015] According to the present invention, the average particle size D of the first positive electrode active material 50 The thickness can be 8 μm to 12 μm.

[0016] According to the present invention, the average particle size D of the second positive electrode active material 50 The thickness can be 1 μm to 7 μm.

[0017] 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.

[0018] According to the present invention, the electrode assembly may include a negative electrode, and the negative electrode may include artificial graphite.

[0019] According to the present invention, the voltage maintenance rate V during 24 C discharge, which is expressed by the following [Equation 1], d-24 can be 83.8% or more, specifically 83.8% to 93.5%. [Formula 1] V d-24 (%)=(V f-24 / V i-24 ) x 100 In the 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 a fully charged lithium secondary battery before applying a discharge pulse at a 24C rate.

[0020] According to the present invention, the voltage maintenance ratio V' during continuous discharge, which is expressed by the following [Equation 2], d can be 82.6% or more, specifically 83% to 92%. [Formula 2] V' d (%)=(V' f / V' i ) x 100 In the formula 2, V' f is the voltage of the lithium secondary battery after applying two discharge pulses at a rate increasing by 4C, and V' i is the initial voltage of a fully charged lithium secondary battery before applying a discharge pulse.

[0021] According to the present invention, the battery case may be a pouch-type battery case. [Effects of the Invention]

[0022] The lithium secondary battery according to the present invention has a positive electrode active material layer loading of 9.4 mg / cm 2 The lithium secondary battery of the present invention is characterized by the use of a bimodal cathode active material having different average particle diameters, as shown below. As a result, even when the lithium secondary battery of the present invention is subjected to high-rate discharge, the voltage drop of the battery is not large, and therefore, when the battery of the present invention is used as a power source for a power tool, the power-off phenomenon of the power tool due to high-rate discharge can be prevented.

[0023] Furthermore, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it is relatively light in weight and easy to work with.

[0024] Furthermore, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it has a large number of electrode tabs, which means that it has lower resistance and exhibits superior output characteristics compared to cylindrical batteries, making it suitable for use as a power source for power tools. DETAILED DESCRIPTION OF THE INVENTION

[0025] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following detailed examples. However, the present invention is not limited to the following examples, and can be realized in various different forms. However, the following examples are provided to fully disclose the present invention and to fully convey the scope of the invention to those skilled in the art. The present invention is defined by the claims.

[0026] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense that can be commonly understood by a person having ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless they are clearly and specifically defined.

[0027] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular includes the plural unless otherwise stated in the text. As used in this specification, the words "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements in addition to the elements mentioned.

[0028] In this specification, when a part is said to include a certain component, this does not mean that it may exclude other components, but that it may further include other components, unless otherwise specified to the contrary.

[0029] In this specification, the phrase "A and / or B" means A, or B, or A and B.

[0030] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0031] In this specification, D 50 means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve.50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0032] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0033] The present invention will now be described in further detail.

[0034] Lithium secondary battery The lithium secondary battery according to the present invention includes a battery case, an electrode assembly housed in the battery case, and an electrolyte. The electrode assembly includes a positive electrode, and the loading amount of the positive electrode active material layer included in the positive electrode is 9.4 mg / cm. 2 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 an average particle size D 50 is small.

[0035] Generally, when a lithium secondary battery is subjected to high-rate discharge, the battery voltage drops significantly. If such a battery is used as a power source for a power tool, the power tool system may determine that the battery has insufficient capacity, even though there is actually remaining capacity in the battery, and the power source of the power tool may be terminated.

[0036] The present inventors have conducted extensive research to solve this problem, and as a result, have found that the loading amount of the positive electrode active material layer is 9.4 mg / cm 2The present inventors have found that when a bimodal cathode active material having different average particle diameters is used, the voltage drop is not large even when the battery is discharged at a high rate, and therefore, when such a battery is used as a power source for a power tool, the power-off phenomenon of the power tool can be prevented, leading to the completion of the present invention.

[0037] Each component of the lithium secondary battery according to the present invention will be described in more detail below.

[0038] <Electrode assembly> 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.

[0039] Hereinafter, each component of the electrode assembly will be described in more detail.

[0040] (1) Positive electrode The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0041] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0042] 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. When the loading amount of the positive electrode active material layer is reduced to lower the positive electrode resistance in order to improve the battery output characteristics, the positive electrode active material may be embedded in the positive electrode current collector, resulting in the positive electrode current collector being disconnected. However, when the thickness of the positive electrode current collector according to the present invention satisfies the above numerical range, the positive electrode current collector is thicker than conventional ones, preventing the current collector from being disconnected.

[0043] The positive electrode active material layer may contain a positive electrode active material, and may further contain a conductive material, a binder, and the like, as needed.

[0044] The first and second positive electrode active materials are compounds capable of reversible intercalation and deintercalation of lithium, and may include lithium metal oxides containing lithium and one or more metals, such as cobalt, manganese, nickel, or aluminum. Specifically, the first and second positive electrode active materials may each include a lithium-nickel-based oxide to facilitate the realization of a large-capacity battery.

[0045] The first positive electrode active material and the second positive electrode active material may have different compositions.

[0046] For example, the first positive electrode active material according to the present invention may include a compound represented by the following Formula 1:

[0047] [Chemical formula 1] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y

[0048] In 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.

[0049] In the above Chemical Formula 1, M1 can be Mn, Al, or a combination thereof.

[0050] In Chemical Formula 1, M2 may be one metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0051] In the above Chemical Formula 1, X may be one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0052] The lithium nickel-based oxide contained in the first positive electrode active material may contain 80 mol% or more, specifically 80 mol% to 95 mol%, and more specifically 80 mol% to 85 mol% of nickel relative to the total moles of transition metals other than lithium. When the nickel content in the lithium nickel-based oxide contained in the first positive electrode active material satisfies the above numerical range, the positive electrode energy density increases and sufficient battery capacity can be ensured, making the battery of the present invention suitable for use as a battery for power tools that require large capacity.

[0053] Meanwhile, the second positive electrode active material according to the present invention may include a compound represented by the following Formula 2:

[0054] [Chemical formula 2] Li 1+z [Ni g Co h M3 i M4 j ] 1-z O 2-w X2 w

[0055] In 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.

[0056] In the above Chemical Formula 2, M3 can be Mn, Al, or a combination thereof.

[0057] In Chemical Formula 2, M4 may be one metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0058] In the above Chemical Formula 2, X2 may be one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0059] The lithium nickel-based oxide contained in the second positive electrode active material may contain 85 mol% or more, specifically 85 mol% to 95 mol%, and more specifically 85 mol% to 90 mol% of nickel relative to the total moles of transition metals other than lithium. When the nickel content in the lithium nickel-based oxide contained in the second positive electrode active material satisfies this numerical range, the average nickel content in the positive electrode active material increases compared to when only the first positive electrode active material is used, thereby further increasing the positive electrode energy density and making it easier to realize a large-capacity battery.

[0060] On the other hand, the first and second positive electrode active materials according to the present invention have an average particle size D 50 Specifically, the second positive electrode active material has an average particle size D 50 As a result, compared to when only the first positive electrode active material is used, the total specific surface area of ​​the positive electrode active material is relatively increased, which increases the number of reaction sites for lithium ions and improves the output characteristics of the battery.

[0061] The first positive electrode active material may be a secondary particle formed by aggregation of primary particles. For example, the secondary particle may be a particle formed by aggregation of 50 or more, specifically 50 to 80, more specifically 50 to 70 primary particles.

[0062] The average particle size D of the first positive electrode active material 50 The average particle size D of the first positive electrode active material may be 8 μm to 12 μm, specifically 8 μm to 10 μm, and more specifically 8 μm to 9 μm. 50 When the value of the charge capacity satisfies the above range, the charge capacity of the battery can be maintained at an appropriate level.

[0063] The BET specific surface area of ​​the first positive electrode active material is 0.38 m 2 / g~0.58m 2 / g, specifically 0.42m 2 / g~0.53m 2 / g, more specifically 0.45m 2 / g~0.51m 2 When the BET specific surface area of ​​the first positive electrode active material is in this range, the output level can be maintained even when the battery is discharged at a high rate.

[0064] The first positive electrode active material may be formed by washing a lithium transition metal oxide with water, followed by drying and heat treatment. The washing treatment enables boron (B) coating, zirconium (Zr) doping, and / or strontium (Sr) doping on the particle surface, thereby improving the capacity and stability of the battery.

[0065] The second positive electrode active material is a single particle and / or a quasi-single particle. The single particle may consist of one primary particle. The quasi-single particle may be an aggregate of 2 to 30, specifically 2 to 20, more specifically 2 to 10 primary particles.

[0066] The second positive electrode active material has an average particle size D 50 More specifically, the average particle diameter D of the second positive electrode active material can be small. 50 The average particle size D of the second positive electrode active material can be 1 μm to 7 μm, specifically 2 μm to 6 μm, and more specifically 4 μm to 5 μm. 50 When satisfies the above numerical range, the second positive electrode active material has a high specific surface area, and therefore the total specific surface area of ​​the positive electrode active material can be increased.

[0067] The BET specific surface area of ​​the second positive electrode active material is 0.54 m 2 / g~0.74m 2 / g, specifically 0.59m 2 / g~0.69m 2 / g, more specifically 0.61m 2 / g~0.67m 2When the BET specific surface area of ​​the second positive electrode active material satisfies this range, the total specific surface area of ​​the positive electrode active material increases, thereby increasing the number of reaction sites for lithium ions and improving the output characteristics of the battery.

[0068] The second positive electrode active material may be a lithium transition metal oxide that has not been subjected to a water washing treatment. By omitting the water washing treatment during the preparation of the second positive electrode active material, an initial resistance can be reduced.

[0069] The positive electrode active material may be contained in an amount of 60 to 99 wt %, preferably 70 to 99 wt %, and more preferably 80 to 98 wt %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode active material satisfies the above range, sufficient positive electrode energy density can be ensured, thereby improving the capacity of the battery.

[0070] In this case, the weight ratio of the first positive electrode active material to the second positive electrode active material may be 70:30 to 90:10, specifically 75:25 to 85:15, and more specifically 78:22 to 82:18. When the first positive electrode active material and the second positive electrode active material are mixed and used in this weight ratio, the total specific surface area of ​​the positive electrode active material can be controlled to an appropriate level, thereby minimizing the decrease in capacity and resistance characteristics of the battery and improving output characteristics.

[0071] The positive electrode conductive material is a component for further improving the conductivity of the positive electrode active material, and such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity.

[0072] For example, the positive electrode conductive material may be carbon powder such as carbon black, 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 crystal structure; conductive fibers such as carbon fiber or metal fiber; carbon fluoride powder; conductive powder such as 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.

[0073] The positive electrode conductive material according to the present invention may contain both dot-like conductive materials and linear conductive materials. Specifically, the conductive material may contain dot-like conductive materials and linear conductive materials in a weight ratio of 10:1 to 50:1, preferably 10:1 to 40:1, and more preferably 20:1 to 40:1. If the weight ratio of dot-like conductive materials to linear conductive materials is less than 10:1, the specific surface area of ​​the conductive material increases, resulting in an increase in side reactions and excessive gas generation in hot box tests. If the weight ratio of dot-like conductive materials to linear conductive materials exceeds 50:1, the conductive network within the positive electrode is insufficient, resulting in high positive electrode resistance.

[0074] In this case, the content of the linear conductive material can be 0.5% by weight or less, specifically 0.05% to 0.3% by weight, and more specifically 0.05% to 0.2% by weight, relative to the total weight of the positive electrode active material layer.

[0075] The positive electrode conductive material may be contained in an amount of 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %, 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, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0076] The positive electrode binder is a component that serves to bind the active material to the conductive material and the like and to bind the active material to the current collector.

[0077] Examples of such positive electrode binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers, and one or more of these may be used alone or in combination.

[0078] The positive electrode binder may be contained in an amount of 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 %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode binder satisfies this range, the contact area between the positive electrode binder and the positive electrode active material is increased, ensuring excellent positive electrode adhesive strength.

[0079] On the other hand, the loading amount of the positive electrode active material layer was 9.4 mg / cm 2 Specifically, 8.0 mg / cm 2 ~9.4mg / cm 2 , more specifically 8.20 mg / cm 2 ~9.35mg / cm 2 The loading amount of the positive electrode active material layer can be 9.4 mg / cm 2 In the case of a positive electrode resistance exceeding 1000 kJ / cm2, the battery resistance increases as the positive electrode resistance increases, resulting in a problem of a decrease in output characteristics.

[0080] The porosity of the positive electrode active material layer may be greater than 30%, specifically 32% to 38%, more specifically 32% to 37%. When the porosity of the positive electrode active material layer satisfies this range, the interface resistance of the positive electrode is reduced, and the output characteristics of the battery can be improved.

[0081] The packing density of the positive electrode active material layer may be 2.8 g / cc or more, specifically 2.8 g / cc to 3.5 g / cc, more specifically 2.9 g / cc to 3.2 g / cc, thereby allowing the porosity of the positive electrode active material layer to fall within the above-mentioned appropriate numerical range.

[0082] Meanwhile, the positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except for using the above-described positive electrode active material. Specifically, the positive electrode can be manufactured by preparing a positive electrode slurry composition containing the above-described positive electrode active material, a positive electrode conductive material, and / or a positive electrode binder, applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling the composition.

[0083] Alternatively, the positive electrode can be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0084] (2) Negative electrode The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, or may be a graphite electrode made of carbon (C), or may be a metal itself.

[0085] The 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, sintered carbon, copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like can be used.

[0086] The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to strengthen the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0087] The negative electrode active material layer may contain a negative electrode active material, and may further contain a conductive material, a binder, and the like, as needed.

[0088] The negative electrode active material is a material that can reversibly intercalate / deintercalate lithium metal and lithium ions.

[0089] The negative electrode active material may include at least one selected from the group consisting of a carbon-based material, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0090] The carbon-based active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon (low-temperature sintered carbon), hard carbon, mesophase pitch carbide, and sintered coke.

[0091] The metal or the alloy of these metals with lithium may be a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or an alloy of these metals with lithium.

[0092] Examples of the metal composite oxides include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, and Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1) and Sn x Me 1-x Me' y O z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) can be selected from the group consisting of those used.

[0093] As substances that can dope and de-dope lithium, Si, SiO x (0 < x ≤ 2), Si-Y alloy (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, SnO2, 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), etc. can be mentioned, and also, at least one of these can be mixed with SiO2 and used. As the element Y, it can be selected from the group consisting of 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.

[0094] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0095] Specifically, the negative electrode active material according to the present invention can be artificial graphite, and more specifically, it can be artificial graphite surface-coated with hard carbon. When using artificial graphite as the negative electrode active material, it has the effect of improving the cycle life characteristics of the battery by reducing the resistance of the negative electrode to improve the capacity characteristics of the battery and reducing the overvoltage during charging to prevent lithium precipitation due to side reactions. [[ID=十五]] [[ID=十六]]

[0096] [[ID=十七]] The negative electrode active material may be contained in an amount of 60 wt% to 99 wt%, preferably 70 wt% to 99 wt%, and more preferably 80 wt% to 99 wt%, based on the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies this range, the contents of the negative electrode conductive material and the negative electrode binder can be maintained at preferred levels, and sufficient negative electrode energy density can be secured, thereby improving the battery capacity.

[0097] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. Examples of such a conductive material include carbon powders such as carbon black, 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 fiber and metal fiber; carbon fluoride powder; conductive powders such as 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.

[0098] The negative electrode conductive material may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies the above range, a negative electrode conductive network is ensured, thereby improving the electrical conductivity of the negative electrode.

[0099] The negative electrode binder is a component that helps bind the negative electrode conductive material, negative electrode active material, and negative electrode current collector together. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

[0100] The negative electrode binder may be included in an amount of 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer. When the content of the negative electrode binder satisfies this range, the negative electrode active material particles are smoothly bound together, minimizing the problem of volume expansion of the negative electrode active material and allowing the negative electrode active material to be well attached to the negative electrode current collector.

[0101] On the other hand, when a metal is used without forming a negative electrode active material layer on the negative electrode, the negative electrode can be manufactured by physically bonding, rolling, or depositing a metal thin film on the metal thin film itself or on the negative electrode current collector. The deposition method can be an electrolytic deposition method or a chemical vapor deposition method.

[0102] For example, the metal to be bonded / rolled / deposited onto the metal thin film itself or the negative electrode current collector may include one metal or an alloy of two metals selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0103] (3) Separator The separator can be any separator that is normally used as a separator in a lithium secondary battery, and is particularly preferred to have low resistance to ion migration of the electrolyte and excellent ability to retain moisture in the electrolyte solution.

[0104] For example, the separator may be a porous polymer film containing a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Also, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used as the separator.

[0105] The thickness of the separator can be 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 6 μm to 13 μm. When the thickness of the separator satisfies the above range, short-circuiting between the positive electrode and the negative electrode can be prevented and the cell resistance can be minimized. As a result, the life and output characteristics of the lithium secondary battery can be improved.

[0106] <Electrolyte> Meanwhile, the lithium secondary battery according to the present invention may include an electrolyte.

[0107] The electrolyte may include an organic solvent and a lithium salt commonly used in the art, and is not particularly limited.

[0108] The organic solvent may be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester-based solvents such as methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC), which may be used alone or in combination.

[0109] Preferably, the electrolyte can use a mixed solution of a carbonate-based solvent and an ester-based solvent as the organic solvent, and specifically, a mixed solution of a cyclic carbonate-based solvent and an ester-based solvent, or a mixed solution of a cyclic carbonate-based solvent, a linear carbonate-based solvent and an ester-based solvent can be used.

[0110] The cyclic carbonate solvent may be at least one of ethylene carbonate and propylene carbonate, and the linear carbonate solvent may be at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0111] The ester-based solvent may be ethyl propionate (EP). Ethyl propionate has a lower viscosity than conventional electrolyte components, and when it is included, the ionic conductivity of the electrolyte can be increased.

[0112] Ethyl propionate may be contained in an amount of 80 wt % or less, specifically 5 wt % to 80 wt %, more specifically 40 wt % to 70 wt %, based on the total weight of the electrolyte. When the content of ethyl propionate satisfies the above range, the viscosity of the electrolyte can be optimized to achieve excellent ionic conductivity of the electrolyte.

[0113] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiN(SO2F)2, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt is preferably contained in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0114] The electrolyte according to the present invention may further include an additive to further improve the physical properties of the secondary battery.

[0115] Examples of such additives include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, nitrile-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0116] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinyl ethylene carbonate (VEC).

[0117] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0118] The nitrile compound may be, for example, succinonitrile, adiponitrile, hexanetricyanide, 1,4-dicyano-2-butene, or the like.

[0119] The sultone compound may be, for example, 1,3-propane sultone, 1,3-propene sultone, or the like.

[0120] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0121] The phosphate-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0122] The borate-based compound may be, for example, tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), or the like.

[0123] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0124] The lithium salt-based compound is a compound different from the lithium salt contained in the bis-electrolyte solution, and may be one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bis(oxalatoborate) (LiB(C2O4)2)), and LiBF4.

[0125] Meanwhile, the additives may be used alone or in combination of two or more.

[0126] The total amount of the additives may be 1 wt % to 25 wt %, preferably 1 wt % to 20 wt %, and more preferably 5 wt % to 20 wt %, based on the total weight of the electrolyte. When the additives are contained within this range, a stable coating is formed on the electrode, which can suppress ignition during overcharge, and can prevent side reactions from occurring during the initial activation process of the secondary battery, or the additives from remaining or being precipitated.

[0127] <Battery case> The lithium secondary battery according to the present invention may include a cylindrical, prismatic, pouch, or coin-type battery case, and preferably includes a pouch-type battery case.

[0128] The pouch-type battery case includes a barrier layer, a substrate layer formed on one side of the barrier layer, and a sealant layer formed on the other side of the barrier layer, and includes at least one cup portion recessed in one direction.

[0129] Specifically, the pouch-type battery case can be manufactured by a method in which a flexible pouch film laminate having a base layer, a barrier layer, and a sealant layer laminated in that order is inserted into a press molding device, and pressure is applied to a portion of the pouch film laminate with a punch to stretch it, thereby forming a cup portion having a recessed shape in one direction.

[0130] The base layer is disposed on the outermost layer of the pouch to protect the electrode assembly from external impact and to provide electrical insulation.

[0131] The substrate layer may be made of a polymer material, for example, one or more polymer materials selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark).

[0132] The substrate layer may have a single layer structure or a multi-layer structure in which different polymer films are laminated. When the substrate layer has a multi-layer structure, an adhesive layer may be interposed between the polymer films.

[0133] Meanwhile, the substrate layer may have a total thickness of 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 multilayer structure, the thickness includes the thickness of the adhesive layer. When the substrate layer satisfies the above range, it exhibits excellent durability, insulating properties, and moldability. If the thickness of the substrate layer is too thin, durability may be poor and the substrate layer may be damaged during the molding process. If the thickness of the substrate layer is too thick, moldability may be impaired, the overall thickness of the pouch may increase, the battery storage space may be reduced, and the energy density may be reduced.

[0134] The barrier layer is intended to ensure the mechanical strength of the pouch-type battery case, to block the entry and exit of gases or moisture outside the secondary battery, and to prevent leakage of the electrolyte.

[0135] The barrier layer may have a thickness of 40 μm to 100 μm, 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, moldability is improved, the molding depth of the cup portion is increased, and even during two-cup molding, cracks and / or pinholes are reduced, and resistance to external stress after molding is improved.

[0136] On the other hand, the barrier layer may be made of a metal material, specifically, an aluminum alloy thin film.

[0137] The aluminum alloy thin film may contain aluminum and one or more metal elements other than aluminum, such as iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0138] The sealant layer is bonded by heat and pressure to seal the pouch, and is located as the innermost layer of the pouch film laminate.

[0139] The sealant layer is the surface that comes into contact with the electrolyte and the electrode assembly after the pouch is formed, and therefore must have insulating and corrosion resistance. It must also have high sealing properties because it must completely seal the interior and prevent material transfer between the inside and outside.

[0140] The sealant layer may be made of a polymer material, such as one or more selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark). Among these, it is particularly preferable that the sealant layer contains polypropylene (PP), which has excellent mechanical properties such as tensile strength, rigidity, surface hardness, abrasion resistance, and heat resistance, and excellent chemical properties such as corrosion resistance.

[0141] The sealant layer may have a single layer structure or a multi-layer structure including two or more layers made of different polymer materials.

[0142] The sealant layer may have a total thickness of 60 μm to 100 μm, preferably 60 μm to 90 μm, and more preferably 70 μm to 90 μm. If the sealant layer is too thin, the seal durability and insulating properties may decrease, while if it is too thick, the flexibility may be poor, the total thickness of the pouch film laminate may increase, and the energy density per volume may decrease.

[0143] Meanwhile, the pouch film laminate may be manufactured by a method known in the art, for example, by attaching a substrate layer to the upper surface of a barrier layer via an adhesive, and forming a sealant layer on the lower surface of the barrier layer by coextrusion or an adhesive, but the method is not limited thereto.

[0144] The pouch-type battery case may be sealed with the electrode assembly housed therein such that a portion of the electrode lead, i.e., a terminal portion, is exposed. Specifically, the electrode lead may be connected to the electrode tab of the electrode assembly, an insulating portion may be formed on a portion of the electrode lead, the electrode assembly may be housed in a housing space provided in the cup portion, an electrolyte may be injected, and then the pouch-type battery case may be sealed.

[0145] The thickness of the electrode lead may be 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 is in this range, which is thicker than conventional thicknesses, the resistance of the battery is reduced, and heat generation during an external short circuit is reduced, resulting in improved heat resistance.

[0146] When the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it is lighter in weight than a cylindrical secondary battery, and the presence of many tabs reduces resistance, resulting in excellent output characteristics, making it suitable for use as a battery for power tools.

[0147] On the other hand, in the lithium secondary battery according to the present invention, the voltage retention rate V at 24 C discharge represented by the following formula 1 is d-24 can be 83.8% or more, specifically 83.8% to 95.0%, and more specifically 83.8% to 93.5%.

[0148] [Formula 1] V d-24 (%)=(V f-24 / V i-24 ) x 100

[0149] In the 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 a fully charged lithium secondary battery before applying a discharge pulse at a 24C rate. d-24 When the value of ρ satisfies the above range, the voltage drop is not large even when the battery is subjected to high-rate discharge, and therefore the battery of the present invention is suitable for use as a power source for power tools.

[0150] On the other hand, in the lithium secondary battery according to the present invention, the voltage maintenance rate V' during continuous discharge, which is represented by the following formula 2, d can be 82.6% or more, specifically 82.6% to 92.5%, and more specifically 83% to 92%.

[0151] [Formula 2] V' d (%)=(V' f / V' i ) x 100

[0152] In the formula 2, V' f is the voltage of the lithium secondary battery after applying two discharge pulses at a rate increasing by 4C, and V' i is the initial voltage of a lithium secondary battery that was fully charged before applying a discharge pulse, the first discharge pulse was applied at a 24C rate, and the second discharge pulse was applied at a 28C rate.

[0153] Voltage maintenance rate V' during continuous discharge d When satisfies the above numerical range, the battery voltage does not become excessively low even when the battery is continuously discharged, and therefore the battery of the present invention is suitable for use as a power source for power tools that require continuous discharge.

[0154] The lithium secondary battery according to the present invention can be used as a battery cell used as a power source for small devices such as power tools, and can also be used as a unit battery in a medium- to large-sized battery module including a plurality of battery cells. Preferred examples of the medium- to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems (ESS).

[0155] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely illustrative for facilitating understanding of the present invention and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present description, and it goes without saying that such changes and modifications are within the scope of the appended claims.

[0156] Examples and Comparative Examples Example 1 (Production of a lithium secondary battery) Hard carbon coated artificial graphite (average particle size D50 =19μm, BET specific surface area=0.8m 2 The negative electrode slurry was prepared by mixing 100% cellulose acetate (CMC) and 100% styrene-butadiene rubber (SBR) in a weight ratio of 94.3:2.0:1.2:2.5, followed by the addition of distilled water. The solid content of the negative electrode slurry was 44 wt%.

[0157] The negative electrode slurry was applied to one surface of a 10 μm thick copper (Cu) metal thin film at a rate of 6.96 mg / cm. 2 The slurry was applied in a loading amount of 1000 kJ / cm2 and then dried in a vacuum. The dried negative electrode slurry was then rolled and dried in a vacuum oven at 130°C for 12 hours, and then punched out to prepare a negative electrode.

[0158] The first positive electrode active material has an average particle size D 50 is 10 μm, and the BET specific surface area is 0.57 m 2 / g, and the secondary particles Li[Ni 0.83 Co 0.12 Mn 0.05 ]O2 was used as the second positive electrode active material, and 50 is 4 μm, and the BET specific surface area is 0.65 m 2 / g and single-particle and / or quasi-single-particle Li[Ni 0.86 Co 0.08 Mn 0.06 ]O2 was used.

[0159] A cathode active material (a mixture of the first and second cathode active materials in a weight ratio of 8:2), a cathode conductive material (a mixture of carbon black and carbon nanotubes (CNTs) in a weight ratio of 30:1), polyvinylidene fluoride (PVDF), and hydrogenated nitrile butadiene rubber (H-NBR) were added to an N-methylpyrrolidone (NMP) solvent in a weight ratio of 95.28:3.1:1.3:0.32 and stirred to prepare a cathode slurry. The solid content of the cathode slurry was 70 wt%.

[0160] The positive electrode slurry was applied to one surface of a 15 μm thick aluminum thin film at a rate of 9.24 mg / cm. 2 The slurry was applied in a loading amount of 10 ...

[0161] The negative and positive electrodes prepared as described above and a porous polyethylene separator having a thickness of 10 μm were stacked to prepare an electrode assembly.

[0162] An electrolyte was prepared by dissolving LiPF6 and LiFSi to a concentration of 0.7M in a solvent in which ethylene carbonate (EC) and ethyl propionate (EP) were mixed in a weight ratio of 35:65.

[0163] The electrode assembly was placed in a pouch-type battery case, the electrolyte was injected, and the case was sealed to manufacture a lithium secondary battery.

[0164] Example 2 (Production of Lithium Secondary Battery) Loading amount 9.3mg / cm 2 A positive electrode was produced in the same manner as in Example 1, except that the following was used.

[0165] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0166] Example 3 (Production of Lithium Secondary Battery) A positive electrode was manufactured in the same manner as in Example 1, except that a positive electrode active material in which the first positive electrode active material and the second positive electrode active material were mixed in a weight ratio of 7:3 was used.

[0167] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0168] Example 4 (Production of Lithium Secondary Battery) A positive electrode was prepared in the same manner as in Example 1, except that a positive electrode active material obtained by mixing the first positive electrode active material and the second positive electrode active material in a weight ratio of 9:1 was used.

[0169] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0170] Comparative Example 1 (Production of Lithium Secondary Battery) A positive electrode was manufactured in the same manner as in Example 1, except that the second positive electrode active material was not used and only the first positive electrode active material was used.

[0171] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0172] Comparative Example 2 (Production of Lithium Secondary Battery) Loading amount 11.0mg / cm 2 A positive electrode was produced in the same manner as in Example 1, except that the following was used.

[0173] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0174] Comparative Example 3 (Production of Lithium Secondary Battery) Loading amount 9.5mg / cm 2 A positive electrode was produced in the same manner as in Example 1, except that the following was used.

[0175] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0176] In Examples 1 to 4 and Comparative Examples 1 to 3, the compositions of the positive electrode active materials and the loading amounts of the positive electrode active material layers are as shown in Table 1 below.

[0177] [Table 1]

[0178] Experimental example 1 - Continuous discharge test evaluation At a temperature of 23°C, each of the lithium secondary batteries of the Example and Comparative Examples was first fully charged to 4.2 V at 0.6 C. Then, using the PNE Cycle program (CTSMonPro, PNE SOLUTION CO., LTD.), discharge pulses were applied under the conditions shown in Table 2 below, with a rest period between each pulse, to perform a continuous discharge test on the lithium secondary batteries.

[0179] In the continuous discharge test, the voltage of the battery was measured before and after the first to fifth pulses were applied, and the results are shown in Table 3 below. The voltage of the battery before each pulse application is V i and the battery voltage after each pulse is V f and the battery voltage V before each pulse application. i The battery voltage V after each pulse application f The ratio of V d The voltage maintenance rate V during 24C discharge, expressed as the following formula 1, is calculated from the measured voltage value. d-24 and the voltage maintenance rate V' during continuous discharge, which is expressed by the following formula 2: d were calculated and are shown in Table 4 below.

[0180] [Formula 1] V d-24 (%)=(V f-24 / V i-24 ) x 100

[0181] In the 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 a fully charged lithium secondary battery before applying a discharge pulse at a 24C rate.

[0182] [Formula 2] V' d (%)=(V' f / V' i ) x 100

[0183] In the formula 2, V' fis the voltage of the lithium secondary battery after applying two consecutive discharge pulses, and V' i is the voltage of a fully charged lithium secondary battery before applying a discharge pulse. During the continuous discharge, the discharge pulse was applied at a rate increasing by 4C, with the first discharge pulse rate being 24C and the second discharge pulse rate being 28C.

[0184] [Table 2]

[0185] [Table 3]

[0186] [Table 4]

[0187] As shown in Tables 3 and 4, in Examples 1 to 4, the voltage retention rate V of the lithium secondary battery at 24 C discharge d-24 It can be seen that the voltage retention rate V' during continuous discharge of the lithium secondary batteries in Examples 1 to 4 is 84.00% or more, which is higher than those in Comparative Examples 1 to 3. d It can be seen that the voltage drop of the battery is 83.40% or more, which is higher than that of Comparative Examples 1 to 3. This means that the voltage drop of the battery is smaller in Examples 1 to 4 during high-rate discharge and continuous discharge than in Comparative Examples 1 to 3.

Claims

1. A lithium secondary battery, A battery case and an electrode assembly and an electrolyte housed in the battery case, The electrode assembly includes a positive electrode, The loading amount of the positive electrode active material layer included in the positive electrode was 9.4 mg / cm ２ is as follows: the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, The second positive electrode active material has an average particle size D ５０ It is a small lithium secondary battery.

2. The first positive electrode active material includes 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 X ｙ 2. The lithium secondary battery of claim 1, wherein, in 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; M1 is Mn, Al, or a combination thereof; M2 is one metal element selected from the group consisting of 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 X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

3. The second positive electrode active material includes a compound represented by the following Chemical Formula 2: [Chemical formula 2] Li １＋ｚ −8) ｇ Co ｈ 73 ｉ 74 ｊ ) １－ｚ 9 ２－ｗ 82 ｗ 2. The lithium secondary battery of claim 1, wherein, in 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; M3 is Mn, Al, or a combination thereof; M4 is one metal element selected from the group consisting of 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 one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

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

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

6. 2. The lithium secondary battery according to claim 1, wherein the second positive electrode active material comprises a lithium nickel-based oxide containing 85 mol% or more 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 a secondary particle formed by agglomeration of 50 or more primary particles.

8. 2. The lithium secondary battery according to claim 1, wherein the second positive electrode active material is a single particle consisting of one primary particle, a pseudo-single particle which is an aggregate of 2 to 30 of the primary particles, or a combination thereof.

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

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

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

1.

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

13. The voltage maintenance rate V during 24 C discharge is expressed by the following [Equation 1] ｄ－２４ is 83.8% or more, [Formula 1] V ｄ－２４ (%)=(V ｆ－２４ / V ｉ－２４ )×100 In the 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 2. The lithium secondary battery according to claim 1, wherein V is the voltage of the lithium secondary battery fully charged before applying a discharge pulse at a 24C rate.

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

15. The voltage maintenance ratio V' during continuous discharge is expressed by the following [Equation 2] ｄ is 82.6% or more, [Formula 2] V' ｄ (%)=(V' ｆ / V' ｉ )×100 In the formula 2, V' ｆ is the voltage of the lithium secondary battery after applying two discharge pulses at a rate increasing by 4C, and V' ｉ 3. The lithium secondary battery according to claim 1, wherein V is the initial voltage of a fully charged lithium secondary battery before applying a discharge pulse.

16. The voltage maintenance ratio V' during continuous discharge ｄ The lithium secondary battery according to claim 15, wherein the .lambda. / .lambda. ratio is 83% to 92%.

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

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