Lithium secondary battery

Abstract

The lithium secondary battery of the present invention comprises: a battery case; and an electrode assembly and an electrolyte accommodated in the battery case; the electrode assembly includes a positive electrode including a first positive electrode active material, a second positive electrode active material, and a conductive agent, where the second positive electrode active material is a single particle composed of one primary particle, a quasi-single particle as an aggregate of 2 to 30 primary particles, or a combination thereof; the weight ratio of the first positive electrode active material to the second positive electrode active material is 7.5: 2.5 to 9.5: 0.5, and the conductive agent includes a dot-shaped conductive agent and a linear conductive agent in a weight ratio of 10: 1 to 50: 1.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application Nos. 10-2022-0140776, filed on October 27, 2022, and 10-2023-0143340, filed on October 24, 2023, the disclosures of which are incorporated herein by reference.

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

[0004] With recent technological developments and the increasing demand 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 requirements. Due to the increasing demand for the market of lithium secondary batteries with high capacity as a power source for such devices, research on increasing the energy density of batteries has been actively carried out. 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 the voltage drop of the secondary battery. That is, since the potential value of the secondary battery decreases significantly during high-rate discharge, even if there is remaining battery capacity, the system determines that the battery capacity is insufficient, resulting in the power cut-off of the power tool.

[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 is heavy, when using a cylindrical battery, the total weight of the power tool increases, so there is a limitation in reducing work convenience. Summary of the Invention

[0007] Technical problem

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

[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, is relatively light in weight and has excellent work convenience, and has low resistance and excellent output characteristics.

[0010] Technical solution

[0011] According to one aspect of the present invention, there is provided a lithium secondary battery comprising: a battery case, and an electrode assembly and an electrolyte accommodated in the battery case, wherein the electrode assembly includes a positive electrode, the positive electrode includes a first positive electrode active material, a second positive electrode active material, and a conductive agent, the second positive electrode active material is a single particle composed of one primary particle, a quasi-single particle as an aggregate of 2 to 30 primary particles, or a combination thereof, the weight ratio of the first positive electrode active material to the second positive electrode active material is 7.5:2.5 to 9.5:0.5, and the conductive agent includes a dot-shaped conductive agent and a linear conductive agent having a weight ratio of 10:1 to 50:1.

[0012] According to the present invention, the first positive electrode active material may include a compound represented by Formula 1:

[0013] [Formula 1]

[0014] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y

[0015] In 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 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 X1 is at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0016] According to the present invention, the second positive electrode active material may include a compound represented by Formula 2:

[0017] [Formula 2]

[0018] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w

[0019] ​​In Formula 2 above, 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 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 at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0020] According to the present invention, the first positive electrode active material may include a lithium nickel-based oxide, and the lithium nickel-based oxide contains at least 80 mol%, specifically 80 to 85 mol%, of nickel relative to the total molar amount of transition metals other than lithium.

[0021] According to the present invention, the second positive electrode active material may include a lithium nickel-based oxide, and the lithium nickel-based oxide contains at least 85 mol% of nickel relative to the total molar amount of transition metals other than lithium.

[0022] According to the present invention, the first positive electrode active material may be secondary particles formed by agglomeration of at least 50 primary particles.

[0023] According to the present invention, the average particle diameter D of the second positive electrode active material 50 may be smaller than that of the first positive electrode active material.

[0024] According to the present invention, the average particle diameter D of the first positive electrode active material 50 may be 8 μm to 12 μm.

[0025] According to the present invention, the average particle diameter D of the second positive electrode active material 50 may be 1 μm to 7 μm.

[0026] According to the present invention, the dot-shaped conductive agent may be carbon black, and the linear conductive agent may be multi-walled carbon nanotubes.

[0027] According to the present invention, the battery resistance (DC-IR) at SOC 100 may be 7 mΩ or less, specifically 1 mΩ to 7 mΩ.

[0028] According to the present invention, the battery case may be a pouch-type battery case.

[0029] Advantageous effects

[0030] The lithium secondary battery of the present invention is characterized in that it uses a bimodal positive electrode active material containing large-sized secondary particles (first positive electrode active material) with a weight ratio of 7.5:2.5 to 9.5:0.5, small-sized single particles and / or quasi-single particles (second positive electrode active material), and also uses a positive electrode conductive agent containing a dot-shaped conductive agent and a linear conductive agent with a weight ratio of 10:1 to 50:1. As a result, even when the lithium secondary battery of the present invention is discharged at a high rate, the voltage drop of the battery is not significant. Therefore, when the battery of the present invention is used as a power source for power tools, power outages of the power tools due to high-rate discharge can be prevented.

[0031] In addition, when the lithium secondary battery of the present invention is a pouch-type lithium secondary battery, the weight of the lithium secondary battery is relatively light, so it has excellent working convenience.

[0032] In addition, when the lithium secondary battery of the present invention is a pouch-type lithium secondary battery, there are a plurality of tabs. Therefore, compared with cylindrical batteries, the lithium secondary battery has low resistance and excellent output characteristics, and is thus suitable for use as a power source for power tools. Detailed Embodiments

[0033] Through the embodiments described below, the advantages and features of the present invention and the methods for realizing them will be clarified. However, the present invention can be implemented in different forms and should not be construed as being limited to the embodiments described herein. On the contrary, these embodiments are provided so that the present invention is thorough and complete, and fully conveys the scope of the present invention to those skilled in the art. In addition, the present invention is only defined by the scope of the claims.

[0034] Unless otherwise defined, all terms used herein (including technical and scientific terms) can be intended to have the meanings understood by those skilled in the art. In addition, unless clearly and obviously defined in the description, terms defined in common dictionaries should not be ideally or overly interpreted as having a formal meaning.

[0035] The terms used herein are for the purpose of describing specific example embodiments only and are not intended to limit the present invention. In this specification, terms in the singular form may include the plural form unless the contrary is mentioned. It will also be understood that when used in this specification, the terms "comprising" and / or "including" specify the presence of the described components, but do not exclude the presence or addition of one or more other components.

[0036] In this specification, when referring to a part as "comprising" a certain element, this part does not exclude other elements, but may also include other elements unless otherwise stated.

[0037] In this specification, "A and / or B" means A, or B, or A and B.

[0038] In this specification, the expression "%" means weight %, unless otherwise clearly specified.

[0039] The expression "D 50 " in this specification represents the particle size at which the cumulative volume is 50% in the particle size distribution curve of the particles. For example, D 50 can be measured by using the laser diffraction method. The laser diffraction method can measure particle sizes in the sub-micron range to the millimeter range, thereby obtaining highly reproducible and high-resolution results.

[0040] 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 using BELSORP-mini II from Bell Japan Co., Ltd. at liquid nitrogen temperature (77K).

[0041] In this specification, the "battery resistance (DC-IR) at SOC 100" can be calculated as follows: Charge the lithium secondary battery to SOC 100%, and then measure the voltage change and current change of the battery respectively according to the pulse of applying 0.1 second and 10 A through the PNE cycle program (CTSMonPro, PNE SOLUTION Co., Ltd.), and divide the voltage change value by the current change value.

[0042] Hereinafter, the present invention will be described in more detail.

[0043] Lithium secondary battery

[0044] The lithium secondary battery of the present invention includes a battery case and an electrode assembly and an electrolyte accommodated in the battery case. The electrode assembly includes a positive electrode, and the positive electrode includes a first positive electrode active material, a second positive electrode active material, and a conductive agent. The second positive electrode active material is a single particle composed of one primary particle, a quasi-single particle that is an aggregate of 2 to 30 primary particles, or a combination thereof. The weight ratio of the first positive electrode active material to the second positive electrode active material is 7.5:2.5 to 9.5:0.5, and the conductive agent includes a dot-shaped conductive agent and a linear conductive agent with a weight ratio of 10:1 to 50:1.

[0045] 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 an electric tool, even if there is remaining battery capacity, the electric tool system may determine that the battery capacity is insufficient, and the power source of the electric tool may be cut off.

[0046] As a result of extensive and repeated research to address this limitation, the present inventors have found that when using a bimodal positive electrode active material containing large-diameter secondary particles (first positive electrode active material) and small-diameter single particles and / or quasi-single particles (second positive electrode active material) in a weight ratio of 7.5:2.5 to 9.5:0.5 and using a dot-shaped conductive agent and a linear conductive agent in a weight ratio of 10:1 to 50:1 together, even when the battery is discharged 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 interruption of the power tools can be prevented, thus completing the present invention.

[0047] Hereinafter, each component of the lithium secondary battery of the present invention will be described in more detail.

[0048] <Electrode assembly>

[0049] The electrode assembly of the present invention may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0050] Hereinafter, each component of the electrode assembly will be described in more detail.

[0051] (1) Positive electrode

[0052] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector.

[0053] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. 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 as the current collector.

[0054] The thickness of the positive electrode current collector may be 10 μm to 500 μm, specifically 10 μm to 300 μm, and more specifically 10 μm to 50 μm. 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 of the present invention satisfies the above numerical range, the thickness of the positive electrode current collector is thicker than that of a conventional positive electrode current collector, and thus disconnection of the current collector can be prevented.

[0055] The positive electrode active material layer may include a first positive electrode active material and a second positive electrode active material, and may also include a conductive agent, a binder, etc. as needed.

[0056] The first positive electrode active material and the second positive electrode active material are compounds capable of reversibly inserting and extracting lithium. Among them, the first positive electrode active material and the second positive electrode active material may include lithium composite metal oxides containing 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 lithium nickel-based oxides, which helps to achieve a large-capacity battery.

[0057] The first positive electrode active material and the second positive electrode active material may have different compositions.

[0058] For example, the first positive electrode active material of the present invention may include a compound represented by the following formula 1:

[0059] [Formula 1]

[0060] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y

[0061] In the above 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.

[0062] In formula 1, M1 may be Mn, Al, or a combination thereof.

[0063] In the above formula 1, M2 may be at least 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.

[0064] In the above formula 1, X1 may be at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0065] Relative to the total molar amount of transition metals other than lithium, the lithium nickel-based oxide contained in the first positive electrode active material may include at least 80 mol%, specifically 80 mol% to 95 mol%, and more particularly 80 mol% to 85 mol% of nickel. 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, thereby ensuring 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.

[0066] Meanwhile, the second positive electrode active material of the present invention may include a compound represented by the following formula 2:​

[0067] [Formula 2]

[0068] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w

[0069] In Formula 2 above, 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.

[0070] In Formula 2, M3 can be Mn, Al, or a combination thereof.

[0071] In Formula 2 above, M4 can be at least 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.

[0072] In Formula 2 above, X2 can be at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0073] Relative to the total molar amount of transition metals other than lithium, the lithium nickel-based oxide contained 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. When the nickel content in the lithium nickel-based oxide contained 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, and thus the positive electrode energy density further increases, which helps to realize a large-capacity battery.

[0074] At the same time, the first positive electrode active material and the second positive electrode active material of the present invention have different average particle sizes D 50 . Specifically, the average particle size D 50 of the second positive electrode active material is smaller than that of the first positive electrode active material. 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.

[0075] The first positive electrode active material can be secondary particles formed by aggregation of primary particles. For example, the secondary particles can be particles formed by aggregation of at least 50, specifically 50 to 80, and more specifically 50 to 70 primary particles.​

[0076] The average particle size D of the first positive electrode active material 50 may be 8 μm to 12 μm, specifically 8 μm to 10 μm, and more specifically 8 μm to 9 μm. When the average particle size D of the first positive electrode active material 50 meets the above numerical range, the charging capacity of the battery can be ensured to reach an appropriate level.

[0077] The BET specific surface area of the first positive electrode active material may be 0.38 m 2 / g to 0.60 m 2 / g, specifically 0.49 m 2 / g to 0.58 m 2 / g, and more specifically 0.5 m 2 / g to 0.57 m 2 / g. When the BET specific surface area of the first positive electrode active material meets the above numerical range, there is an effect of maintaining the output level even when the battery discharges at a high rate.

[0078] The first positive electrode active material can be formed by washing a lithium transition metal oxide with water and then drying and heat-treating it. Through the water washing treatment, boron (B), doped zirconium (Zr), and / or doped strontium (Sr) can be coated on the surface of the particles, thereby improving the capacity and stability of the battery.

[0079] The second positive electrode active material is single particles and / or quasi-single particles. A single particle can be composed of one primary particle. A quasi-single particle can be a particle formed by agglomeration of at least 2 to 30, specifically 2 to 20, and more specifically 2 to 10 primary particles.

[0080] The average particle size D of the second positive electrode active material 50 is smaller than that of the first positive electrode active material. More particularly, the average particle size D of the second positive electrode active material 50 is 1 μm to 7 μm, specifically 2 μm to 6 μm, and more specifically 4 μm to 5 μm. When the average particle size D of the second positive electrode active material 50 meets 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.

[0081] The BET specific surface area of the second positive electrode active material may be 0.54 m 2 / g to 0.74 m 2 / g, specifically 0.59 m 2 / g to 0.69 m 2 / g, and 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, thereby improving the number of reaction sites for lithium ions, and thus improving the output characteristics of the battery.

[0082] The second positive electrode active material can be obtained without washing the lithium transition metal oxide with water. By omitting the water washing treatment in the production of the second positive electrode active material, there is an effect of reducing the initial resistance.

[0083] 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 99% 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.

[0084] The weight ratio of the first positive electrode active material to the second positive electrode active material can be 7.5:2.5 to 9.5:0.5, specifically 7.5:2.5 to 8.5:1.5, more specifically 7.8:2.2 to 8.2:1.8. If the weight ratio of the first positive electrode active material to the second positive electrode active material satisfies the above range, the voltage drop during high-rate discharge can be reduced.

[0085] In this case, when the content of the first positive electrode active material is less than 75% by weight or the content of the second positive electrode active material is greater than 25% by weight, the capacity characteristics of the battery may deteriorate. In addition, when the content of the first positive electrode active material is greater than 95% by weight or the content of the second positive electrode active material is less than 5% by weight, the total specific surface area of the positive electrode active material decreases, so the resistance and output characteristics deteriorate.

[0086] The positive electrode conductive agent is a component for further improving the conductivity of the positive electrode active material, and the conductive agent is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery.

[0087] For example, as the positive electrode conductive agent, the following conductive materials can be used, for example: carbon powder, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or pyrolytic carbon black; graphite powder, such as natural graphite, artificial graphite or graphite with a very developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powder, aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0088] The positive electrode conductive agent of the present invention includes a dot-shaped conductive agent and a linear conductive agent. Specifically, the conductive agent may include a dot-shaped conductive agent and a linear conductive agent in a weight ratio of 10:1 to 50:1, preferably 10:1 to 40:1, more preferably 20:1 to 40:1. When the weight ratio of the dot-shaped conductive agent to the linear conductive agent is less than 10:1, as the specific surface area of the conductive agent increases, side reactions increase, so there are limitations in excessive gas generation during the hot box test. When the weight ratio of the dot-shaped conductive agent to the linear conductive agent is greater than 50:1, there are limitations in that the conductive network in the positive electrode cannot be sufficiently ensured, thereby increasing the resistance value of the positive electrode.

[0089] In this case, the content of the linear conductive agent may be 0.5% by weight or less, specifically 0.05% by weight to 0.3% by weight, more specifically 0.05% by weight to 0.2% by weight, based on the total weight of the positive electrode active material layer.

[0090] The content of the positive electrode conductive agent may be 1% by weight to 20% by weight, preferably 1% by weight to 15% by weight, 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 agent 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.

[0091] The positive electrode binder is a component that helps the binding between the active material and the conductive agent and the binding with the current collector.

[0092] 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, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, various copolymers, etc., and any one of them can be used alone or a mixture of two or more of them can be used.

[0093] The content of the positive electrode binder may be 0.5% by weight to 5.0% by weight, specifically 1.0% by weight to 4.0% by weight, more specifically 1.0% by weight to 3.5% by weight, based on the total weight of the positive electrode active material layer. 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.

[0094] At the same time, the loading amount of the positive electrode active material layer may be less than 11.0 mg / cm 2 , specifically 8.0 mg / cm 2 to 10.0 mg / cm 2 , more specifically 8.0 mg / cm 2 to 9.4 mg / cm 2. When the loading amount of the positive electrode active material layer is at least 11.0 mg / cm 2 , the following limitations exist: as the resistance value of the positive electrode increases, the resistance of the battery increases, resulting in deterioration of the output characteristics.

[0095] The porosity of the positive electrode active material layer can be greater than 30%, specifically 32% to 38%, 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 is reduced, thereby improving the output characteristics of the battery.

[0096] The packing density of the positive electrode active material layer can be at least 2.8 g / cc, specifically 2.8 g / cc to 3.5 g / cc, more specifically 2.9 g / cc to 3.2 g / cc. Thus, the porosity of the positive electrode active material layer can have the above-mentioned appropriate numerical range.

[0097] Meanwhile, the positive electrode can be prepared according to the conventional preparation method of the positive electrode, except that the above-mentioned positive electrode active material is used. Specifically, the positive electrode can be prepared in the following manner: preparing a positive electrode paste composition containing the above-mentioned positive electrode active material, a positive electrode conductive agent, and / or a positive electrode binder, applying the positive electrode paste composition to a positive electrode current collector, and drying and rolling the positive electrode paste composition.

[0098] In addition, as another method, the positive electrode can be prepared in the following manner: casting the positive electrode paste composition on a separate carrier, and then laminating the film separated from the carrier on the positive electrode current collector.

[0099] (2) Negative electrode

[0100] The negative electrode can include a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

[0101] 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. 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.

[0102] The thickness of the negative electrode current collector can generally be 8 μm to 500 μm, and similar to the positive electrode current collector, micro-concavo-convex objects can be formed on the surface of the negative electrode current collector to improve the binding force of the negative electrode active material. The negative electrode current collector, for example, can be used in various shapes such as film, sheet, foil, net, porous body, foam body, and non-woven fabric body.

[0103] The negative electrode active material layer can contain a negative electrode active material and can also contain a conductive agent, a binder, etc. as needed.

[0104] The negative electrode active material is a material capable of reversibly inserting / extracting lithium metal and lithium ions.

[0105] The negative electrode active material may include at least one selected from the group consisting of carbonaceous materials, metals or alloys of lithium and the metals, metal composite oxides, materials capable of doping and undoping lithium, and transition metal oxides.

[0106] The carbonaceous active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon may include graphite, such as irregular, planar, flaky, 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, and the like.

[0107] As the metal or the alloy of lithium and the metal, 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 lithium and the metal may be used.

[0108] One selected from the group consisting of PbO, PbO 2 , Pb 2 O 3 , Pb 3 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) may be used as the metal composite oxide.

[0109] The material capable of doping and undoping lithium may include 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, 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 SiO can also be used 2 and a mixture of at least one of them. Element Y 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.

[0110] The transition metal oxide can include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

[0111] Specifically, the negative electrode active material of the present invention can be artificial graphite, and more specifically, it can be artificial graphite coated with hard carbon on the 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 lithium precipitation caused by side reactions is prevented by reducing the overvoltage during the charging process, thereby improving the cycle life characteristics of the battery.

[0112] Relative to the total weight of the negative electrode active material layer, the content of the negative 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 99% by weight. When the content of the negative electrode active material satisfies the above range, the contents of the negative electrode conductive agent and the negative electrode binder can be maintained at the required levels, and sufficient negative electrode energy density can be ensured, thereby improving the battery capacity.

[0113] The negative electrode conductive agent is a component for further improving the conductivity of the negative electrode active material, and there is no particular limitation on the conductive agent as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, the following conductive materials can be used, such as: carbon powder, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or pyrolytic carbon black; graphite powder, such as natural graphite, artificial graphite, or graphite with a very developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0114] Based on the total weight of the negative electrode active material layer, the content of the negative electrode conductive agent can be from 1 wt% to 20 wt%, preferably from 1 wt% to 15 wt%, more preferably from 1 wt% to 10 wt%. 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.

[0115] The negative electrode binder is a component that helps the binding between the negative electrode conductive agent, the negative electrode active material, and the negative electrode current collector. Examples of the binder can 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, various copolymers thereof, etc., and any one of them can be used alone or a mixture of two or more of them can be used.

[0116] Based on the total weight of the negative electrode active material layer, the content of the negative electrode binder can be from 1 wt% to 20 wt%, preferably from 1 wt% to 15 wt%, more preferably from 1 wt% to 10 wt%. When the content of the negative electrode binder satisfies the above range, the negative electrode active material particles can be smoothly bound, thereby minimizing the volume expansion problem of the negative electrode active material as much as possible, and the negative electrode active material can be well attached to the negative electrode current collector.

[0117] (3) Separator

[0118] The separator can be used without particular limitation as long as it is commonly used as a separator in lithium secondary batteries, and in particular, it is preferable that the separator has a high water retention capacity for the electrolyte solution and a low resistance to the transfer of electrolyte ions.

[0119] As the separator, for example, a porous polymer film containing polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer can be used, or a laminated structure with two or more layers can be used. In addition, conventional 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.

[0120] The thickness of the separator can be from 5 μm to 20 μm, preferably from 5 μm to 15 μm, more preferably from 6 μm to 13 μm. When the thickness of the separator satisfies the above range, the short circuit between the positive electrode and the negative electrode can be prevented while minimizing the cell resistance value as much as possible. As a result, the life characteristics and output characteristics of the lithium secondary battery can be improved.

[0121] <Electrolyte>

[0122] Meanwhile, the lithium secondary battery of the present invention can include an electrolyte.

[0123] The electrolyte may contain organic solvents and lithium salts commonly used in the art, and there is no particular limitation.

[0124] Any organic solvent can be used as the organic solvent without particular limitation as long as it can serve as a medium through which the ions involved in the electrochemical reaction of the battery can migrate. Specifically, as the organic solvent, the following can be used alone or in combination: 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), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC).

[0125] 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.

[0126] The cyclic carbonate solvent can be at least one of ethylene carbonate and propylene carbonate. The linear carbonate solvent can be at least one of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0127] The ester solvent can be ethyl propionate (EP). Since the viscosity of ethyl propionate is lower than that of conventional electrolyte components, when ethyl propionate is included, the effect of increasing the ionic conductivity of the electrolyte can be obtained.

[0128] 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.

[0129] Lithium salts can be used without particular limitation as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. Specifically, LiN(SO 2 F) 2 、LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF 6 、LiAlO 4 、LiAlCl 4 、LiCF3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5 SO 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 concentration of the lithium salt contained in the electrolyte is preferably about 0.6 mol% to about 2 mol%.

[0130] The electrolyte of the present invention may further contain an additive to further improve the physical properties of the secondary battery.

[0131] Examples of the additive may include at least one selected from the group consisting of cyclic carbonate compounds, halogenated carbonate compounds, nitrile compounds, sultone compounds, sulfate / salt compounds, phosphate / salt compounds, borate / salt compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0132] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinyl ethyl carbonate (VEC).

[0133] The halogenated carbonate compound may be, for example, fluoroethylene carbonate (FEC), etc.

[0134] The nitrile compound may be, for example, succinonitrile, adiponitrile, tricyanohexane, 1,4-dicyano-2-butene, etc.

[0135] The sultone compound may be, for example, 1,3-propane sultone, 1,3-propene sultone, etc.

[0136] The sulfate / salt compound may be, for example, ethylene sulfite (Esa), trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), etc.

[0137] The phosphate / salt compound may include, for example, at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, trimethylsilyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphite.

[0138] The borate / borate compound can be, for example, tetraphenyl borate, lithium oxalyl difluoroborate (LiODFB), etc.

[0139] Benzene compounds, for example, can include fluorobenzene, amine compounds can include triethanolamine or ethylenediamine, and silane compounds can include tetravinylsilane.

[0140] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution. Among them, the lithium salt compound can include those selected from the group consisting of 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 from the group consisting of.

[0141] At the same time, the additive can be used alone or as a mixture of two or more of them.

[0142] Relative to the total weight of the electrolyte solution, the total amount of the additive can be 1 wt% to 20 wt%, specifically 1 wt% to 15 wt%, and more specifically 2 wt% to 10 wt%. When the additive is included within the above range, a film can be stably formed on the electrode, the fire phenomenon can be suppressed during battery overcharging, side reactions can be prevented during the initial activation process of the secondary battery, or the residue or precipitation of the additive can be prevented.

[0143] <Battery case>

[0144] The lithium secondary battery of the present invention can include a cylindrical, prismatic, pouch-type or button-type battery case, and preferably can include a pouch-type battery case.

[0145] The pouch-type battery case includes a barrier layer, a base 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.

[0146] Specifically, the pouch-type battery case has flexibility and can be manufactured by the following method: inserting a pouch film laminate formed by sequentially laminating a base layer, a barrier layer, and a sealant layer into a compression molding device, and stretching the pouch film laminate by applying pressure to a partial area of the pouch film laminate with a punch, thereby forming a cup portion recessed in one direction.

[0147] The base layer is disposed on the outermost layer of the pouch to protect the electrode assembly from external shocks and electrically insulate the electrode assembly.

[0148] The base layer can be made of a polymer material, for example, at least one polymer material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, polyaramide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyarylate, and Teflon.

[0149] The base layer can have a single-layer structure or a multi-layer structure formed by laminating different polymer films. When the base layer has a multi-layer structure, an adhesive layer can be inserted between the polymer films.

[0150] Meanwhile, the total thickness of the base layer can be 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the base layer has a multi-layer structure, the thickness includes the thickness of the adhesive layer. When the base layer meets the above range, the durability, insulation, and formability are excellent. If the base layer is too thin, the durability may deteriorate, and the base layer may be damaged during the forming process. If it 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.

[0151] The barrier layer is used to ensure the mechanical strength of the pouch-type battery case, prevent the entry of gases, moisture, etc. from outside the secondary battery, and prevent the leakage of the electrolyte.

[0152] The thickness of the barrier layer can be 40 μm to 100 μm, more preferably 50 μm to 80 μm, and even more preferably 60 μm to 80 μm. When the thickness of the barrier layer meets the above range, the formability is improved to increase the forming depth of the cup part, or even when forming two cups, fewer cracks and / or pinholes are generated, thereby improving the resistance to external stress after forming.

[0153] Meanwhile, the barrier layer can be made of a metal material, specifically, it can be made of an aluminum alloy thin film.

[0154] The aluminum alloy thin film can contain aluminum and metal elements other than aluminum, for example, one or more selected from the group consisting of iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0155] The sealant layer is attached by thermal compression to seal the bag and is located on the innermost layer of the bag film laminate.

[0156] Since the sealant layer is the surface that comes into contact with the electrolyte and the electrode assembly after the bag is formed, the sealant layer should have insulation and corrosion resistance. And since it should be completely sealed inside to prevent material movement between the inside and the outside, the sealant layer should have high sealing performance.

[0157] The sealant layer can be made of a polymer material, for example, at least one selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, polyaramide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyarylate, and Teflon. And particularly preferably, the sealant layer contains polypropylene (PP) having excellent mechanical properties (such as tensile strength, stiffness, surface hardness, abrasion resistance, and heat resistance) and excellent chemical properties (such as corrosion resistance).

[0158] The sealant layer can have a single-layer structure, or can have a multi-layer structure including two or more layers made of different polymer materials.

[0159] The total thickness of the sealant layer can be 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 sealing durability and insulation may deteriorate, and if it is too thick, the flexibility may deteriorate and the total thickness of the pouch film laminate may increase, resulting in a decrease in the energy density per volume.

[0160] Meanwhile, the above-mentioned pouch film laminate can be prepared by a method known in the art for preparing a pouch film laminate. For example, the pouch film laminate can be prepared by the following method: attaching a base layer to the upper surface of a barrier layer through an adhesive, and forming a sealant layer on the lower surface of the barrier layer by coextrusion or through an adhesive, but not limited thereto.

[0161] The pouch-type battery case can be sealed in a state of accommodating the electrode assembly, thereby exposing a part of the electrode lead (i.e., the terminal part). Specifically, when the electrode lead is connected to the electrode tab of the electrode assembly and an insulating part is formed on a part of the electrode lead, the electrode assembly is accommodated in the accommodation space provided in the cup part, the electrolyte is injected, and then the pouch-type battery case can be sealed.

[0162] The thickness of the electrode lead can 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 satisfies the above numerical range that is thicker than the existing electrode lead, there are the following effects: the resistance of the battery decreases, and at the same time, the heat generation decreases when an external short circuit occurs, thereby improving the heat resistance.

[0163] When the lithium secondary battery of the present invention is a pouch-type lithium secondary battery, the lithium secondary battery is lighter than a cylindrical secondary battery, and the resistance becomes smaller due to the presence of multiple tabs. Therefore, the lithium secondary battery has excellent output characteristics and can thus be suitably used as a battery for power tools.

[0164] Meanwhile, the lithium secondary battery of the present invention may have a battery resistance (DC-IR) of 7 mΩ or less, specifically 1 mΩ to 7 mΩ, more specifically 1 mΩ to 6.5 mΩ at SOC 100. When the battery resistance (DC-IR) satisfies the above numerical range, since the voltage drop is not significant even when the battery is discharged at a high rate, the battery of the present invention can be suitably used as a power source for power tools.

[0165] The lithium secondary battery of 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 containing multiple 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.

[0166] Hereinafter, the present invention will be described in more detail with reference to specific embodiments. However, the following embodiments are provided 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 substitutions can be made within the scope and spirit of the present invention. Such modifications and changes fall within the scope of the claims included herein.

[0167] Examples and comparative examples

[0168] Example 1 (Preparation of Lithium Secondary Battery)

[0169] Artificial graphite (average particle size D 50 = 18.8 μm, BET specific surface area = 1.0 m 2 / g) coated with hard carbon, 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 wt%.

[0170] The negative electrode slurry was applied to 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 vacuum dried. Then, the dried negative electrode slurry was roll-pressed, dried in a vacuum oven at 130 °C for 12 hours, and then punched to prepare a negative electrode.

[0171] Li[Ni 50 having an average particle size D of 10 μm and a BET specific surface area of 0.57 m 2 / g and used as a first positive electrode active material, and the average particle size D 0.83 Co 0.12 Mn 0.05 ]O 2 was used as the first positive electrode active material, and the average particle size D50 is 4 μm and the BET specific surface area is 0.65 m 2 / g as single particles and / or quasi-single particles of Li[Ni 0.86 Co 0.08 Mn 0.06 O 2 is used as the second positive electrode active material.

[0172] The positive electrode paste is prepared as follows: The 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 8:2, the positive electrode conductive agent obtained by mixing carbon black and multi-walled carbon nanotubes (MWCNT) 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. The solid content of the positive electrode paste is 70% by weight.

[0173] The positive electrode paste is applied to one surface of an aluminum 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 roll-pressed, dried in a vacuum oven at 130 °C for 12 hours, and then punched to prepare the positive electrode.

[0174] The negative electrode and the positive electrode prepared as described above and a porous polyethylene separator with a thickness of 10 μm are assembled in a laminated manner to prepare an electrode assembly.

[0175] The electrolyte is prepared as follows: LiPF 6 and LiFSi are dissolved in a solvent obtained by mixing ethylene carbonate (EC), ethyl propionate (EP), and ethyl methyl carbonate (EMC) in a weight ratio of 20:40:40, such that LiPF 6 is 0.7 M and LiFSi is 0.7 M.

[0176] The lithium secondary battery is prepared by accommodating the electrode assembly in a pouch-type battery case, injecting the electrolyte therein, and sealing the case.

[0177] Example 2 (Preparation of Lithium Secondary Battery)

[0178] The positive electrode is prepared in the same manner as in Example 1, except that the conductive agent obtained by mixing the positive electrode active material, carbon black, and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 15:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) are mixed in a weight ratio of 95.16:3.2:1.3:0.34, and the loading amount of the positive electrode is 9.24 mg / cm 2 .

[0179] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above-described positive electrode was used.

[0180] Comparative Example 1 (Preparation of Lithium Secondary Battery)

[0181] A negative electrode was prepared in the same manner as in Example 1, except that the loading amount of the negative electrode was 6.52 mg / cm 2 .

[0182] A positive electrode was prepared in the same manner as in Example 1, except that the first positive electrode active material, carbon black, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed and used at a weight ratio of 94.85:3.5:1.3:0.35, and the loading amount of the positive electrode was 9.64 mg / cm 2 .

[0183] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above-described negative electrode and the above-described positive electrode were used.

[0184] Comparative Example 2 (Preparation of Lithium Secondary Battery)

[0185] A negative electrode was prepared in the same manner as in Example 1, except that the loading amount of the negative electrode was 6.52 mg / cm 2 .

[0186] A positive electrode was prepared in the same manner as in Example 1, except that the conductive agent obtained by mixing the first positive electrode active material, carbon black, and multi-walled carbon nanotubes (MWCNT) at a weight ratio of 6:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed at a weight ratio of 94.8:3.5:1.3:0.4, and the loading amount of the positive electrode was 9.64 mg / cm 2 .

[0187] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above-described negative electrode and the above-described positive electrode were used.

[0188] Comparative Example 3 (Preparation of Lithium Secondary Battery)

[0189] A negative electrode was prepared in the same manner as in Example 1, except that the loading amount of the negative electrode was 6.56 mg / cm 2 .

[0190] A positive electrode was prepared in the same manner as in Example 1, except that the conductive agent obtained by mixing the first positive electrode active material, carbon black, and multi-walled carbon nanotubes (MWCNT) at a weight ratio of 2:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed at a weight ratio of 94.8:3.5:1.3:0.4, and the loading amount of the positive electrode was 9.64 mg / cm2 .

[0191] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above negative electrode and the above positive electrode were used.

[0192] Comparative Example 4 (Preparation of Lithium Secondary Battery)

[0193] A negative electrode was prepared in the same manner as in Example 1, except that the loading amount of the negative electrode was 6.32 mg / cm 2 .

[0194] A positive electrode was prepared in the same manner as in Example 1, except that a conductive agent obtained by mixing a first positive electrode active material, carbon black, and multi-walled carbon nanotubes (MWCNT) at a weight ratio of 30:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed at a weight ratio of 95.28:3.1:1.3:0.32, and the loading amount of the positive electrode was 9.28 mg / cm 2 .

[0195] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above negative electrode and the above positive electrode were used.

[0196] Comparative Example 5 (Preparation of Lithium Secondary Battery)

[0197] A positive electrode was prepared in the same manner as in Example 1, except that a conductive agent obtained by mixing a positive electrode active material, carbon black, and multi-walled carbon nanotubes (MWCNT) at a weight ratio of 2:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed at a weight ratio of 93.88:4.5:1.3:0.32, and the loading amount of the positive electrode was 9.23 mg / cm 2 .

[0198] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0199] Comparative Example 6 (Preparation of Lithium Secondary Battery)

[0200] A positive electrode was prepared in the same manner as in Example 1, except that a positive electrode active material, carbon black, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed and used at a weight ratio of 96.88:1.5:1.3:0.32, and the loading amount of the positive electrode was 9.23 mg / cm 2 .

[0201] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0202] Comparative Example 7 (Preparation of Lithium Secondary Battery)

[0203] The positive electrode was prepared in the same manner as in Example 1, except that the first positive electrode active material and the second positive electrode active material were mixed at a weight ratio of 7:3.

[0204] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0205] Experimental example 1 - Evaluation of the internal resistance of a lithium secondary battery

[0206] The internal resistance of the lithium secondary battery was evaluated by measuring the direct current internal resistance (DC-IR) value. Specifically, the DC-IR was calculated as follows: Two lithium secondary batteries prepared in the Examples and Comparative Examples were charged to SOC 100% and SOC 50%, respectively, and then the voltage change and current change of the batteries were measured according to the application of a pulse of 0.1 second and 10 A through the PNE cycle program (CTSMonPro, PNE SOLUTION Co., Ltd.), and the voltage change value was divided by the current change value. The results are shown in Table 1 below.

[0207] [Table 1]

[0208]

[0209] As shown in Table 1, it can be confirmed that the DC-IR values of Examples 1 to 2 measured at SOC 100% were below 7 mΩ, which were significantly lower than the DC-IR values of Comparative Examples 1 to 7. This indicates that the degree of voltage drop of the batteries in Examples 1 and 2 was less than that of the batteries in Comparative Examples 1 to 7. That is, in the case of Examples 1 to 2 where the weight ratio of the first positive electrode active material to the second positive electrode active material was 75:2.5 to 9.5:0.5 and the weight ratio of the dot-shaped conductive agent to the linear conductive agent was 10:1 to 50:1, it can be confirmed that the degree of voltage drop of the battery during high-rate discharge was less than that in Comparative Examples 1 to 7.

Claims

1. A lithium secondary battery, comprising: a battery case; and an electrode assembly and an electrolyte accommodated in the battery case, wherein, the electrode assembly includes a positive electrode, the positive electrode includes a first positive electrode active material, a second positive electrode active material, and a conductive agent, the second positive electrode active material is a single particle composed of one primary particle, a quasi-single particle that is an aggregate of 2 to 30 primary particles, or a combination thereof, the weight ratio of the first positive electrode active material to the second positive electrode active material is 7.5:2.5 to 9.5:0.5, and the conductive agent includes a dot-shaped conductive agent and a linear conductive agent with a weight ratio of 10:1 to 50:

1.

2. The lithium secondary battery according to claim 1, wherein, the first positive electrode active material includes a compound represented by Formula 1: [Formula 1] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X1 y ​ wherein, in Formula 1 above, 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 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 X1 is at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

3. The lithium secondary battery according to claim 1, wherein, the second positive electrode active material includes a compound represented by Formula 2: [Formula 2] Li 1+z [Ni g Co h M3 i M4 j 1-z O 2-w X2 w ​ wherein, in Formula 2 above, 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 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 at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

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

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

6. The lithium secondary battery according to claim 1, wherein, the second positive electrode active material includes a lithium nickel-based oxide, and the lithium nickel-based oxide contains at least 85 mol% of nickel relative to the total molar amount 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 aggregation of at least 50 primary particles.

8. The lithium secondary battery according to claim 1, wherein, The average particle size D of the second positive electrode active material 50 is smaller than that of the first positive electrode active material.

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

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

11. The lithium secondary battery according to claim 1, wherein, the dot-shaped conductive agent is carbon black, and the linear conductive agent is multi-walled carbon nanotube.

12. The lithium secondary battery according to claim 1, wherein, the battery resistance DC-IR at SOC 100 is 7 mΩ or less.

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

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