Lithium secondary battery

Abstract

A lithium secondary battery according to the present invention comprises: a battery case; and an electrode assembly and an electrolyte accommodated in the battery case, the electrode assembly including a positive electrode including a positive electrode active material and a conductive agent including a dot-type conductive agent and a linear conductive agent in a weight ratio of 4: 1 to 14: 1, and the electrolyte includes a sultone-based compound and a phosphate-based compound in a weight ratio of 1: 0.1 to less than 1: 2 as additives.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0140777, filed on October 27, 2022, and Korean Patent Application No. 10-2023-0143341, filed on October 24, 2023, the disclosures of which are incorporated herein by reference. Technical Field

[0004] The invention relates to a lithium secondary battery. Background Art

[0005] With the recent technological development and increased demand for power tools, electric vehicles, and energy storage systems (ESS), the demand for batteries as energy has increased significantly, and therefore various studies have been conducted on batteries that can meet various needs. As the market demand for lithium secondary batteries with high capacity as power sources for such devices increases, research on improving battery energy density has been actively conducted. In addition, as a battery that can be applied to devices requiring high output such as power tools, the demand for batteries with excellent capacity characteristics and excellent rate characteristics has been increasing.

[0006] However, when the secondary battery embedded in the power tool is discharged at a high output, the system operation of the power tool is terminated due to the voltage drop of the secondary battery. That is, since the potential value of the secondary battery is greatly reduced during high-rate discharge, even if the battery capacity remains unchanged, the system determines that the battery capacity is insufficient, resulting in the power of the power tool being turned off.

[0007] In addition, cylindrical batteries are generally used as batteries for power tools. In the case of cylindrical batteries, since the weight of the battery can is heavy, when the cylindrical batteries are used, the total weight of the power tool increases, so there is a limitation that the convenience of work is reduced. Summary of the invention

[0008] Technical issues

[0009] One aspect of the present invention provides a lithium secondary battery capable of preventing a power-off phenomenon of an electric tool due to a lower degree of battery voltage drop during high-rate discharge by controlling the weight ratio of a dot-type conductive agent and a linear conductive agent and the weight ratio of a sultone-based compound and a phosphate-based compound as electrolyte additives.

[0010] Another aspect of the present invention provides a pouch-type lithium secondary battery having a lower degree of voltage drop during high-rate discharge, having a relatively light weight to have excellent working convenience, and having low resistance and excellent output characteristics.

[0011] Technical Solution

[0012] According to one aspect of the present invention, a lithium secondary battery is provided, comprising: a battery case; and an electrode assembly and an electrolyte contained in the battery case, wherein the electrode assembly comprises a positive electrode, the positive electrode comprises a positive electrode active material and a conductive agent, the conductive agent comprises a point-type conductive agent and a linear conductive agent in a weight ratio of 4:1 to 14:1, and the electrolyte comprises a sultone-based compound and a phosphate-based compound in a weight ratio of 1:0.1 to less than 1:2 as additives.

[0013] According to the present invention, the positive electrode active material may include a lithium nickel-based oxide containing at least 80 mol %, specifically 85 mol % to 90 mol % of nickel relative to the total moles of transition metals excluding lithium.

[0014] Preferably, the nickel-based lithium transition metal oxide may be a compound represented by Formula 1 below.

[0015] [Formula 1]

[0016] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y

[0017] Wherein, 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, 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 X is at least one element selected from the group consisting of F, Cl, Br, I, At, P and S.

[0018] According to the present invention, the point-type conductive agent may be carbon black, and the linear conductive agent may be multi-walled carbon nanotubes.

[0019] According to the present invention, the sultone-based compound may be included in an amount of less than 1 wt %, specifically 0.3 wt % to 0.9 wt %, with respect to the total weight of the electrolyte.

[0020] According to the present invention, the phosphate-based compound may be included in an amount of less than 1 wt %, specifically 0.6 wt % to 0.9 wt %, with respect to the total weight of the electrolyte.

[0021] According to the present invention, the sultone-based compound may be at least one compound selected from the group consisting of 1,3-propane sultone and 1,3-propene sultone.

[0022] According to the present invention, the phosphate-based compound may be at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tri(2,2,2-trifluoroethyl)phosphate, and tri(trifluoroethyl)phosphite.

[0023] According to the present invention, during 24C discharge, the voltage holding ratio V represented by the following equation 1 is d It may be at least 81%, specifically 81% to 87%.

[0024] [Equation 1]

[0025] V d (%)=(V f / V i )×100

[0026] In the above equation 1, V f is the voltage of the lithium secondary battery after a discharge pulse is applied at a 24C rate, V i is the voltage of a fully charged lithium secondary battery before a discharge pulse at a 24C rate is applied.

[0027] According to the present invention, the battery case may be a pouch-type battery case.

[0028] Beneficial Effects

[0029] The lithium secondary battery according to the present invention includes a point-type conductive agent and a linear conductive agent in a weight ratio of 4:1 to 14:1 as a positive electrode conductive agent, and a sultone-based compound and a phosphate-based compound in a weight ratio of 1:0.1 to less than 1:2 as an electrolyte additive. Therefore, even when the lithium secondary battery of the present invention is discharged at a high rate, the voltage drop of the battery is not large. Therefore, when the battery according to the present invention is used as a power source for an electric tool, the power failure of the electric tool caused by high-rate discharge can be prevented.

[0030] Furthermore, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, the lithium secondary battery has a relatively light weight and thus has excellent working convenience.

[0031] In addition, when the lithium secondary battery according to the present invention is a pouch type lithium secondary battery, there are a plurality of electrode tabs. Therefore, compared with a cylindrical battery, the lithium secondary battery has low resistance and excellent output characteristics, and is therefore suitable for use as a power source for electric tools. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings attached to the specification illustrate preferred embodiments of the present invention by way of example and are used to enable the technical concept of the present invention to be further understood together with the detailed description of the present invention given below, and therefore the present invention should not be interpreted solely by the contents in these drawings.

[0033] Figure 1 : is a graph showing the change in voltage of the battery over time when the lithium secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 5 were subjected to a continuous discharge test, respectively.

[0034] Figure 2 It is based on Figure 1 FIG. 4 is an enlarged view of a graph of the application of the first discharge pulse. DETAILED DESCRIPTION

[0035] The advantages and features of the present invention and their implementation methods will be explained by the following embodiments described with reference to the accompanying drawings. However, the present invention can be embodied in different forms and should not be construed as being limited to the embodiments described herein. On the contrary, these embodiments are provided to make the present disclosure thorough and complete and to fully convey the scope of the present invention to those skilled in the art. In addition, the present invention is limited only by the scope of the claims. The same reference numerals refer to the same elements throughout.

[0036] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be intended to have meanings understood by those skilled in the art. In addition, unless clearly and obviously defined in the specification, terms defined in commonly used dictionaries are not ideally or excessively interpreted as having formal meanings.

[0037] The terms used herein are only used to describe specific example embodiments and are not intended to limit the present invention. In the specification, terms in the singular form may include plural forms unless otherwise specified. It should be further understood that when used in this specification, the terms "comprises" and / or comprising specifically describe the presence of the components, but do not exclude the presence or addition of one or more other components.

[0038] In this specification, when a component is referred to as “comprising” an element, unless otherwise stated, the component does not exclude other elements but may further include other elements.

[0039] In the present specification, "A and / or B" means A, or B, or A and B.

[0040] In the present specification, unless otherwise specifically stated, the expression "%" means % by weight.

[0041] The expression "D 50 " refers to the particle diameter at which the cumulative volume accounts for 50% in the particle size distribution curve. For example, D 50 The measurement can be performed by using the laser diffraction method, which is capable of measuring particle diameters from the submicron range to the several millimeter range, thereby obtaining results with high reproducibility and high resolution.

[0042] In the present specification, the expression "specific surface area" is measured by the BET method, wherein, specifically, the specific surface area can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II from Bell Japan Co., Ltd.

[0043] Hereinafter, the present invention will be described in more detail.

[0044] Lithium secondary battery

[0045] The lithium secondary battery according to the present invention includes a battery case, and an electrode assembly and an electrolyte contained in the battery case, the electrode assembly includes a positive electrode, the positive electrode includes a positive electrode active material and a conductive agent, the conductive agent includes a point-type conductive agent and a linear conductive agent in a weight ratio of 4:1 to 14:1, and the electrolyte includes a sultone-based compound and a phosphate-based compound in a weight ratio of 1:0.1 to less than 1:2 as additives.

[0046] Generally, when a lithium secondary battery is discharged at a high rate, the battery voltage drops significantly. When the battery is used as a power source for a power tool, the power of the power tool may be turned off if the power tool system determines that the battery capacity is insufficient even if the battery capacity still exists.

[0047] As a result of numerous repeated studies to address this limitation, the present inventors have discovered that when a point-type conductive agent and a linear conductive agent in a weight ratio of 4:1 to 14:1 are used as a positive electrode conductive agent, and a sultone-based compound and a phosphate-based compound in a weight ratio of 1:0.1 to less than 1:2 are used as electrolyte additives, the voltage drop is not large even when the battery is discharged at a high rate, and therefore when the battery is used as a power source for an electric tool, power-off of the electric tool can be prevented, thereby completing the present invention.

[0048] Hereinafter, each component of the lithium secondary battery according to the present invention will be described in more detail.

[0049] <Electrode assembly>

[0050] 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.

[0051] Hereinafter, each component of the electrode assembly will be described in more detail.

[0052] (1) Positive electrode

[0053] The positive electrode may include a positive electrode current collector, and a positive electrode active material layer disposed on the positive electrode current collector.

[0054] 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, as the current collector, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like can be used.

[0055] The thickness of the positive electrode collector may be 8 μm to 500 μm, specifically 8 μm to 300 μm, and more specifically 10 μm to 50 μm. In order to improve the output characteristics of the battery, when the loading amount of the positive electrode active material layer is reduced to reduce the resistance of the positive electrode, the positive electrode active material is embedded in the positive electrode collector, and thus the positive electrode collector may no longer be coherent. However, when the thickness of the positive electrode collector according to the present invention satisfies the above numerical range, the thickness of the positive electrode collector is thicker than that of the conventional positive electrode collector, so the collector can be prevented from being incoherent.

[0056] The positive electrode active material layer may include a positive electrode active material, and may further include a conductive agent, a binder, and the like as necessary.

[0057] The positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium, wherein the positive electrode active material may include a lithium composite metal oxide including lithium and at least one metal such as cobalt, manganese, nickel, or aluminum.

[0058] Specifically, the positive electrode active material may include a lithium nickel-based oxide. In this case, the lithium nickel-based oxide may contain at least 80 mol%, specifically 80 mol% to 95 mol%, more specifically 80 mol% to 90 mol%, and most specifically 85 mol% to 90 mol% nickel relative to the total moles of transition metals excluding lithium. When the content of nickel satisfies the above numerical range, the positive electrode energy density is increased to ensure sufficient battery capacity, so the battery of the present invention is suitable for use as a battery for electric tools requiring large capacity.

[0059] The lithium nickel-based oxide may be a compound represented by Formula 1 below.

[0060] [Formula 1]

[0061] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y

[0062] 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 may be satisfied.

[0063] In Formula 1, M1 may be Mn, Al, or a combination thereof.

[0064] In the above formula 1, M2 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.

[0065] In the above Formula 1, X may be at least one element selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0066] The content of the positive electrode active material may be 60 wt % to 99 wt %, preferably 70 wt % to 99 wt %, and more preferably 80 wt % to 98 wt % relative to the total weight of the positive electrode active material layer. When the content of the positive electrode material satisfies the above range, the battery capacity can be increased by ensuring sufficient positive electrode energy density.

[0067] The positive electrode conductive agent is a component for further improving the conductivity of the positive electrode active material, and has conductivity without causing adverse chemical changes in the battery.

[0068] The positive electrode conductive agent according to the present invention includes both a point-type conductive agent and a linear conductive agent. Specifically, the conductive agent may include a point-type conductive agent and a linear conductive agent, and the weight ratio thereof is 4:1 to 14:1, preferably 5:1 to 10:1, and more preferably 5:1 to 7:1. When the ratio of the point-type conductive agent to the linear conductive agent satisfies the above numerical range, the side reaction increases with the increase of the specific surface area of ​​the conductive agent, thereby preventing excessive gas generation in the hot box test, and the positive electrode resistance can also be reduced by fully ensuring the conductive network in the positive electrode.

[0069] The dot type conductive agent may include at least one of carbon black and acetylene black, but is not limited thereto. Preferably, carbon black may be used as the dot type conductive agent, and in this case, conductivity is increased compared to the case of using other dot type conductive agents.

[0070] The linear conductive agent may include at least one of multi-walled carbon nanotubes and single-walled carbon nanotubes, but is not limited thereto. Preferably, multi-walled carbon nanotubes may be used as the linear conductive agent, and in this case, conductivity is increased compared to the case of using other linear conductive agents.

[0071] The content of the positive electrode conductor may be 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt % relative to the total weight of the positive electrode active material layer. When the content of the positive electrode conductor in the positive electrode active material layer satisfies the above range, the conductivity of the positive electrode can be improved by ensuring a positive electrode conductive network.

[0072] The positive electrode binder is a component that facilitates bonding between the active material and the conductive agent and bonding with the current collector.

[0073] Examples of positive electrode binders may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, various copolymers, and the like, and any one of them may be used alone or a mixture of two or more thereof may be used.

[0074] The content of the positive electrode binder may be 0.5 wt % to 5.0 wt %, specifically 1.0 wt % to 4.0 wt %, and more specifically 1.0 wt % to 3.5 wt % relative to 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.

[0075] At the same time, the loading amount of the positive electrode active material layer can be less than 11.0 mg / cm 2 , specifically 8.0mg / cm 2 Up to 10.0mg / cm 2 , more specifically 8.0 mg / cm 2 Up 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 When the positive electrode resistance increases, the battery resistance increases, which leads to a limitation of deteriorating the output characteristics.

[0076] The porosity of the positive electrode active material layer may be greater than 30%, specifically 32% to 38%, and more specifically 32% to 37%. When the porosity of the positive electrode active material layer satisfies the above range, the interface resistance of the positive electrode is reduced, thereby improving the output characteristics of the battery.

[0077] The packing density of the positive active material layer may be at least 2.8 g / cc, specifically 2.8 g / cc to 3.5 g / cc, and more specifically 2.9 g / cc to 3.2 g / cc, and thus the porosity of the positive active material layer may have the above appropriate numerical range.

[0078] Meanwhile, in addition to using the above-mentioned positive electrode active material, the positive electrode can be prepared according to a typical method for preparing a positive electrode. Specifically, the positive electrode can be prepared by the following method, that is, preparing a positive electrode slurry composition including the above-mentioned positive electrode active material, a positive electrode conductor, and / or a positive electrode binder, applying the positive electrode slurry composition on a positive electrode current collector, and drying and rolling the positive electrode slurry composition.

[0079] Furthermore, as another method, the positive electrode may be prepared by casting the positive electrode slurry composition on a separate support and then laminating a film separated from the support on a positive electrode current collector.

[0080] (2) Negative electrode

[0081] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. Alternatively, the negative electrode may be a graphite electrode made of carbon (C). Alternatively, the negative electrode may be a metal itself.

[0082] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery, and for example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys can be used.

[0083] The thickness of the negative electrode current collector may generally be 8 μm to 500 μm, and similar to the positive electrode current collector, microscopic irregularities may 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 may be used in various shapes such as films, sheets, foils, nets, porous bodies, foams, and nonwoven objects.

[0084] The negative electrode active material layer may include a negative electrode active material, and may further include a conductive agent, a binder, and the like as needed.

[0085] The negative electrode active material is a material that can reversibly intercalate / deintercalate lithium metal and lithium ions.

[0086] The negative electrode active material may include at least one selected from the group consisting of carbon-based materials, metals or alloys of lithium and the metal, metal composite oxides, materials that can be doped or undoped with lithium, and transition metal oxides.

[0087] The carbon-based active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon may include graphite, such as natural graphite or artificial graphite that is irregular, planar, flake-shaped, spherical, or fibrous, and examples of amorphous carbon may include soft carbon (low-temperature sintered carbon) or hard carbon, mesophase pitch carbide, sintered coke, or the like.

[0088] 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.

[0089] 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.

[0090] The materials that can be doped or undoped with lithium may include Si, SiOx (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 Si), and SiO can also be used 2 and mixtures of at least one of these materials. The 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.

[0091] Transition metal oxides can include lithium titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

[0092] Specifically, the negative electrode active material according to the present invention can be artificial graphite, and more specifically, can be artificial graphite coated with hard carbon on the surface. When artificial graphite is used as the negative electrode active material, the cycle life characteristics of the battery are improved by reducing the resistance of the negative electrode to improve the capacity characteristics of the battery and by reducing the overvoltage during charging to prevent lithium precipitation due to side reactions.

[0093] Relative to the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 60 wt% to 99 wt%, preferably 70 wt% to 99 wt%, and more preferably 80 wt% to 99 wt%. 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 desired levels, and sufficient negative electrode energy density can be ensured, thereby improving the battery capacity.

[0094] The negative electrode conductive agent is a component for further improving the conductivity of the negative 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, and for example, a conductive material such as: 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 well-grown crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as carbon fluoride 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.

[0095] The content of the negative electrode conductive agent may be 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt % relative to the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies the above range, the conductivity of the negative electrode can be improved by ensuring a negative electrode conductive network.

[0096] The negative electrode binder is a component that helps to bind between the negative electrode conductor, the negative electrode active material and the negative electrode collector. Examples of binders may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, various copolymers thereof, and the like, and any one thereof may be used alone or a mixture of two or more thereof may be used.

[0097] The content of the negative electrode binder may be 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt % relative to the total weight of the negative electrode active material layer. When the content of the negative electrode binder satisfies the above range, the negative electrode active material particles may be smoothly combined to minimize the volume expansion problem of the negative electrode active material, and the negative electrode active material may be well attached to the negative electrode current collector.

[0098] Meanwhile, when the metal itself is used without forming a negative electrode active material layer on the negative electrode, the negative electrode can be prepared by physically combining, rolling, or depositing the metal on the metal film itself or the negative electrode current collector. The deposition method can use electrodeposition of the metal or chemical vapor deposition.

[0099] For example, the metal bonded / rolled / deposited on the metal film itself or the negative electrode current collector may include one metal selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In), or an alloy of two metals thereof.

[0100] (3) Partition

[0101] The separator may be used without particular limitation as long as it is generally used as a separator in a lithium secondary battery, and in particular, it is preferred that the separator have high moisture retention capability for an electrolyte solution and low resistance to transfer of electrolyte ions.

[0102] As separator, for example, can use the porous polymer that comprises polyolefin-based polymer, such as ethylene homopolymer, propylene homopolymer, ethylene / butylene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer, or have the two-layer or multilayer laminated structure of these materials.In addition, can use typical porous nonwoven fabric for example the nonwoven fabric that is formed by high melting point glass fiber or polyethylene terephthalate fiber as separator.

[0103] The thickness of the separator may 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, the battery resistance value can be minimized while preventing a short circuit between the positive electrode and the negative electrode. As a result, the life characteristics and output characteristics of the lithium secondary battery can be improved.

[0104] <Electrolytes>

[0105] Meanwhile, the lithium secondary battery according to the present invention may include an electrolyte.

[0106] The electrolyte may include an organic solvent and a lithium salt commonly used in the art, and is not particularly limited.

[0107] Any organic solvent may be used as the organic solvent without particular limitation as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, the following can be used alone or in combination: ester-based solvents such as methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene or fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC).

[0108] Preferably, in the electrolyte, a mixed solution of a carbonate-based solvent and an ester-based solvent may be used as the organic solvent, 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 may be used.

[0109] The cyclic carbonate-based solvent may be at least one of ethylene carbonate and propylene carbonate. The linear carbonate-based solvent may be at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0110] The ester-based solvent may be ethyl propionate (EP). Since ethyl propionate has a lower viscosity than conventional electrolyte components, when ethyl propionate is included, an effect of increasing the ion conductivity of the electrolyte may be obtained.

[0111] Relative to the total weight of the electrolyte, the content of ethyl propionate may be 80 wt% or less, specifically 5 wt% to 80 wt%, and more specifically 40 wt% to 70 wt%. When the content of ethyl propionate satisfies the above numerical range, the viscosity of the electrolyte may be optimized to achieve excellent ionic conductivity of the electrolyte.

[0112] The lithium salt can be used without particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. Specifically, LiPF 6 、LiClO 4 、LiAsF 6 , LiBF 4 、LiSbF 6 、LiAl0 4 、LiAlCl 4 、LiCF 3 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 , or the like as the lithium salt. The lithium salt is preferably included in the electrolyte at a concentration of about 0.6 mol % to about 2 mol %.

[0113] The electrolyte according to the present invention may include a sultone-based compound and a phosphate-based compound as additives to further improve the physical properties of the secondary battery. In this case, the electrolyte may include a sultone-based compound and a phosphate-based compound as additives in a weight ratio of 1:0.1 to less than 1:2, specifically 1:0.3 to 1:1.9, and more specifically 1:0.5 to 1:1.8. When the weight ratio of the sultone-based compound and the phosphate-based compound included as an electrolyte additive is 1:2 or higher, the film cannot be stably formed on the electrode, so the voltage drop increases during high-rate discharge, and the output characteristics of the battery deteriorate. In addition, when the battery is driven at high temperature, the oxidation reaction of the electrolyte cannot be fully prevented, and the heat released cannot be controlled to an appropriate level, so the high-temperature cycle performance may deteriorate.

[0114] The content of the sultone-based compound may be less than 1 wt %, specifically 0.3 wt % to 0.9 wt %, and more specifically 0.5 wt % to 0.8 wt %, relative to the total weight of the electrolyte. When the content of the sultone-based compound in the electrolyte satisfies the above numerical range, the output characteristics of the battery can be improved, the initial resistance of the lithium secondary battery can be reduced, and thus the degree of voltage drop during high rate discharge is reduced.

[0115] Relative to the total weight of the electrolyte, the content of the phosphate-based compound may be less than 1 wt%, specifically 0.6 wt% to 0.9 wt%, and more specifically 0.7 wt% to 0.8 wt%. When the content of the phosphate-based compound in the electrolyte satisfies the above numerical range, the high temperature cycle performance of the battery can be improved while reducing the initial resistance of the lithium secondary battery, thereby reducing the degree of voltage drop during high rate discharge.

[0116] The sultone-based compound may be, for example, at least one compound selected from the group consisting of 1,3-propane sultone and 1,3-propene sultone, but is not limited thereto.

[0117] The phosphate-based compound may be, for example, at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tri(2,2,2-trifluoroethyl)phosphate, and tri(trifluoroethyl)phosphite, but is not limited thereto.

[0118] Meanwhile, in addition to the sultone-based compounds and phosphate-based compounds as described above, the electrolyte may further include cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, nitrile-based compounds, sulfate-based compounds, borate-based compounds, phenyl compounds, amine-based compounds, silane-based compounds, and / or lithium salt-based compounds as additives.

[0119] The cyclic carbonate-based compound may be, for example, vinylene carbonate (VC) or vinyl ethylene carbonate.

[0120] The halogen-substituted carbonate-based compound may be, for example, fluoroethylene carbonate (FEC) or the like.

[0121] The nitrile compound may be, for example, succinonitrile, adiponitrile, adiponitrile, 1,4-dicyano-2-butene, or the like.

[0122] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), or the like.

[0123] The borate ester compound may be, for example, tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), or the like.

[0124] The phenyl compound may include, for example, fluorobenzene, the amine compound may include triethanolamine or ethylenediamine, and the silane compound may include tetravinylsilane.

[0125] The lithium salt-based compound is a compound other than the lithium salt included in the non-aqueous electrolyte solution, wherein the lithium salt-based compound may include at least one compound 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 .

[0126] The total amount of the additive may be 1 wt % to 20 wt %, specifically 1 wt % to 15 wt %, and more specifically 2 wt % to 10 wt % relative to the total weight of the electrolyte solution. When the content of the additive is within the above range, a film may be stably formed on the electrode, an ignition phenomenon may be suppressed when the battery is overcharged, and a side reaction may be prevented from occurring during the initial activation process of the secondary battery, or the additive may be prevented from remaining or precipitating.

[0127] <Battery Case>

[0128] The lithium secondary battery according to the present invention may include a cylindrical, prismatic, pouch, or coin-type battery case, and may preferably include a pouch-type battery case.

[0129] 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.

[0130] Specifically, the pouch-type battery case has flexibility and can be manufactured by inserting a bag film stack in which a base layer, a barrier layer and a sealant layer are stacked sequentially into a press-molding device, and stretching the bag film stack by applying pressure to a partial area of ​​the bag film stack with a punch, thereby forming a cup portion recessed in one direction.

[0131] The base layer is disposed on the outermost layer of the pouch to protect the electrode assembly from external impact and to electrically insulate it.

[0132] The base layer may 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 polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyacrylate, and Teflon.

[0133] The base layer may have a single-layer structure or a multi-layer structure in which different polymer films are stacked. When the base layer has a multi-layer structure, an adhesive layer may be interposed between the polymer films.

[0134] At the same time, the total thickness of the substrate layer can be 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the substrate layer has a multilayer structure, the thickness is the thickness including the adhesive layer. When the substrate layer meets the above range, durability, insulation and moldability are excellent. If the substrate layer is too thin, the durability will deteriorate, and damage to the substrate layer may occur during the formation process. If the substrate layer is too thick, the moldability will deteriorate, the total thickness of the bag will increase, and the battery accommodating space will decrease, thereby reducing the energy density.

[0135] The barrier layer serves to ensure the mechanical strength of the pouch type battery case, block gas, moisture, etc. from entering the outside of the secondary battery, and prevent leakage of the electrolyte.

[0136] The thickness of the barrier layer may be 40 to 100 μm, more preferably 50 to 80 μm, and more preferably 60 to 80 μm. When the thickness of the barrier layer satisfies the above range, moldability is improved to increase the molding depth of the cup portion, or even when two cups are molded, cracks and / or pinholes are less likely to be generated, thereby improving resistance to external stress after molding.

[0137] Meanwhile, the barrier layer may be made of a metal material, specifically, may be made of an aluminum alloy thin film.

[0138] In addition to aluminum, the aluminum alloy film may include aluminum and metal elements, for example, one or two 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).

[0139] The sealant layer is attached by heat and pressure to seal the bag and is located at the innermost layer of the bag film stack.

[0140] Since the sealant layer is the surface in contact with the electrolyte and the electrode assembly after the bag is molded, the sealant layer should have insulation and corrosion resistance, and since its interior should be completely sealed to prevent material movement between the inside and the outside, the sealant layer should have high sealing performance.

[0141] 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, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyacrylate, and Teflon, and among these materials, it is particularly preferred that the sealant layer includes polypropylene (PP), which has excellent mechanical properties such as tensile strength, stiffness, surface hardness, wear resistance and heat resistance, as well as excellent chemical properties such as corrosion resistance.

[0142] The sealant layer may have a single-layer structure, or may have a multi-layer structure including two or more layers made of different polymer materials.

[0143] The total thickness of the sealant layer may be 60 to 100 μm, preferably 60 to 90 μm, and more preferably 70 to 90 μm. If the sealant layer is too thin, the sealing durability and insulation properties may deteriorate, and if it is too thick, the bendability may deteriorate and the total thickness of the bag film stack may increase, resulting in a decrease in energy density relative to volume.

[0144] Meanwhile, the above bag film stack can be prepared by a method for preparing a bag film stack known in the art. For example, the bag film stack can be prepared by attaching the base layer to the upper surface of the barrier layer by an adhesive, and forming a sealant layer on the lower surface of the barrier layer by coextrusion or an adhesive, but is not limited thereto.

[0145] The pouch-type battery case may be sealed in a state where the electrode assembly is accommodated so that a portion of the electrode lead, i.e., the terminal portion, is exposed. Specifically, when the electrode lead is connected to the electrode tab of the electrode assembly and an insulating portion is formed on a portion of the electrode lead, the electrode assembly is accommodated in an accommodation space provided in the cup portion, an electrolyte is injected, and then the pouch-type battery case may be sealed.

[0146] 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 satisfies the above numerical range, i.e., is thicker than the existing numerical range, there is an effect that when an external short circuit occurs, the resistance of the battery is reduced, and the heat generation is reduced, thereby improving the heat resistance.

[0147] When the lithium secondary battery according to the present invention is a pouch type lithium secondary battery, the lithium secondary battery is lighter than a cylindrical type secondary battery and has a smaller resistance due to the presence of a plurality of tabs, so the lithium secondary battery has excellent output characteristics and is suitable for use as a battery for power tools.

[0148] Meanwhile, in the lithium secondary battery according to the present invention, during 24C discharge, the voltage holding ratio V represented by the following equation 1 is d It may be at least 81%, specifically 81% to 87%, and more specifically 81% to 85%.

[0149] [Equation 1]

[0150] V d (%)=(V f / V i )×100

[0151] In the above equation 1, V f is the voltage of the lithium secondary battery after a discharge pulse is applied at a 24C rate, V i is the voltage of a fully charged lithium secondary battery before a discharge pulse is applied at a 24C rate. d When the above numerical range is satisfied, the degree of voltage drop is not large even when the battery is discharged at a high rate, and thus the battery of the present invention can be suitably used as a power source for an electric tool.

[0152] The lithium secondary battery according to the present invention can be used in a battery cell used as a power source for a small device such as an electric tool, and can also be used as a unit cell in a medium-sized and large-sized battery module including a plurality of battery cells. Preferred examples of medium-sized and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems (ESS), and the like.

[0153] Hereinafter, the present invention will be described in more detail with reference to specific embodiments. However, the following embodiments are only used to illustrate the present invention, and the scope of the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications and changes are possible within the scope and technical spirit of the present invention. Such modifications and changes fall within the scope of the claims included herein.

[0154] Examples and Comparative Examples

[0155] Example 1 (Preparation of lithium secondary battery)

[0156] The artificial graphite (average particle size D 50 =18.8μm, BET specific surface area =1.0m 2 / g), carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a weight ratio of 94.3:2.0:1.2:2.5, and distilled water was added thereto to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 44% by weight.

[0157] The negative electrode slurry was heated to 6.96 mAh / cm 2 A loading amount of 10 μm was applied to one surface of a copper (Cu) metal film having a thickness of 10 μm, and then dried in a vacuum. The dried negative electrode slurry was then roll-pressed, dried in a vacuum oven at 130° C. for 12 hours, and then stamped to prepare a negative electrode.

[0158] By dividing the average diameter D 50 10 μm Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 The positive electrode slurry was prepared by mixing carbon black and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 6:1 and hydrogenated nitrile rubber (H-NBR) in a weight ratio of 94.8:3.5:1.3:0.4 into an N-methylpyrrolidone (NMP) solvent, followed by stirring. The solid content of the positive electrode slurry was 70% by weight.

[0159] The positive electrode slurry was heated to 9.24 mg / cm 2 A loading amount of 200 μm was applied to one surface of an aluminum thin film having a thickness of 15 μm, and then dried in a vacuum at 130° C. for 10 hours. The dried positive electrode slurry was roll pressed, dried in a vacuum oven at 130° C. for 12 hours, and then stamped to prepare a positive electrode.

[0160] The negative electrode and the positive electrode prepared as described above and a porous polyethylene separator having a thickness of 10 μm were assembled in a stacking manner to prepare an electrode assembly.

[0161] The electrolyte was prepared by the following method, namely, LiPF 6 and LiFSi are dissolved in a solvent to make LiPF 6The electrolyte is changed to 0.7M and LiFSi is changed to 0.7M, ethylene carbonate (EC), ethyl propionate (EP) and ethyl methyl carbonate (EMC) are mixed in a weight ratio of 20:40:40 in the solvent, 0.5 wt% of 1,3-propane sultone (PS) relative to the total weight of the electrolyte and 0.8 wt% of lithium difluorophosphate (LiDFP) relative to the total weight of the electrolyte are mixed as additives, and the mixture is stirred.

[0162] The lithium secondary battery is prepared by housing an electrode assembly in a pouch-type battery case, injecting an electrolyte therein, and sealing the case.

[0163] Example 2 (Preparation of lithium secondary battery)

[0164] An electrolyte was prepared in the same manner as in Example 1, except that 1.0 wt % of 1,3-propane sultone relative to the total weight of the electrolyte and 0.5 wt % of lithium difluorophosphate relative to the total weight of the electrolyte were used as additives.

[0165] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above electrolyte was used.

[0166] Comparative Example 1 (Preparation of lithium secondary battery)

[0167] A positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material, a conductive agent obtained by mixing carbon black and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 15:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed in a weight ratio of 95.16:3.2:1.3:0.34, and the loading amount of the positive electrode was 9.24 mg / cm 2 .

[0168] An electrolyte was prepared in the same manner as in Example 1, except that 1.0 wt % of 1,3-propane sultone relative to the total weight of the electrolyte and 0.5 wt % of lithium difluorophosphate relative to the total weight of the electrolyte were used as additives.

[0169] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above cathode and the above electrolyte were used.

[0170] Comparative Example 2 (Preparation of lithium secondary battery)

[0171] A positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material, a conductive agent obtained by mixing carbon black and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 15:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed in a weight ratio of 95.16:3.2:1.3:0.34, and the loading amount of the positive electrode was 9.24 mg / cm 2 .

[0172] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0173] Comparative Example 3 (Preparation of lithium secondary battery)

[0174] A positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material, a 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) were mixed in 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 .

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

[0176] Comparative Example 4 (Preparation of lithium secondary battery)

[0177] A positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material, a conductive agent obtained by mixing carbon black and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 3:1, polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed in a weight ratio of 94.3:4.0:1.3:0.4, and the loading amount of the positive electrode was 9.64 mg / cm 2 .

[0178] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above positive electrode was used.

[0179] Comparative Example 5 (Preparation of lithium secondary battery)

[0180] The positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material, multi-walled carbon nanotubes (MWCNT), polyvinylidene fluoride (PVdF), and hydrogenated nitrile rubber (H-NBR) were mixed and used in a weight ratio of 95.4:3.0:1.3:0.3, and the loading amount of the positive electrode was 9.58 mg / cm 2 .

[0181] An electrolyte was prepared in the same manner as in Example 1, except that 1.0 wt % of 1,3-propane sultone relative to the total weight of the electrolyte and 0.5 wt % of lithium difluorophosphate relative to the total weight of the electrolyte were used as additives.

[0182] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above cathode and the above electrolyte were used.

[0183] Comparative Example 6 (Preparation of lithium secondary battery)

[0184] An electrolyte was prepared in the same manner as in Example 1, except that 0.5 wt % of 1,3-propane sultone relative to the total weight of the electrolyte and 1.0 wt % of lithium difluorophosphate relative to the total weight of the electrolyte were used as additives.

[0185] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above electrolyte was used.

[0186] Comparative Example 7 (Preparation of lithium secondary battery)

[0187] An electrolyte was prepared in the same manner as in Example 1, except that 1 wt % of 1,3-propane sultone relative to the total weight of the electrolyte was used as an additive, and lithium difluorophosphate was not used.

[0188] A lithium secondary battery was prepared in the same manner as in Example 1, except that the above electrolyte was used.

[0189] In Examples 1 to 2 and Comparative Examples 1 to 7, the compositions of the positive electrode conductive agent and the electrolyte are shown in Table 1 below.

[0190] [Table 1]

[0191]

[0192] Experimental Example 1 - Evaluation of Continuous Discharge Test

[0193] The lithium secondary battery of each of the embodiments and comparative examples was fully charged to 4.2 V at an initial 0.6 C condition at 23° C. Then, a continuous discharge test of the lithium secondary battery was performed by applying a discharge pulse under the conditions shown in Table 2 below with a rest time between each pulse application using the PNE Cycle program (CTSMonPro, PNE SOLUTION CO. LTD).

[0194] in this case, Figure 1 and Figure 2 , which is a graph showing the voltage of a lithium secondary battery changing with time in a continuous discharge test. Specifically, Figure 1 : is a graph showing the change in voltage of the battery over time when the lithium secondary batteries prepared in Examples 1 to 2 and Comparative Examples 1 to 5 are respectively subjected to the above-mentioned continuous discharge test. Figure 2 It is based on Figure 1 The graph of the first discharge pulse is an enlarged view of the graph of the application of the first discharge pulse. Figure 1 and Figure 2 As shown, it can be confirmed that when a discharge pulse is applied to the lithium secondary battery, a voltage drop occurs, and the voltage steadily decreases while repeatedly applying the pulse. In addition, it can be confirmed that when the application of the discharge pulse is terminated, the voltage of the lithium secondary battery is restored.

[0195] Meanwhile, the battery voltages before and after the first pulse was applied in the continuous discharge test were measured, and the results are shown in Table 3 below. In addition, the voltage holding ratio V during 24C discharge represented by the following equation 1 was calculated from the measured voltage values. d , and the results are shown in Table 3 below.

[0196] [Equation 1]

[0197] V d (%)=(V f / V i )×100

[0198] In the above equation 1, V f is the voltage of the lithium secondary battery after a discharge pulse is applied at a 24C rate, V i is the voltage of a fully charged lithium secondary battery before a discharge pulse at a 24C rate is applied.

[0199] [Table 2]

[0200]

[0201] [Table 3]

[0202]

[0203] As shown in Table 3, it can be confirmed that the voltage retention rate V of the lithium secondary batteries in Examples 1 and 2 during 24C discharge is d is at least 81%, which is higher than the voltage retention ratio V of the lithium secondary batteries of Comparative Examples 1 to 7. d. This means that the degree of voltage drop of the batteries in Examples 1 and 2 is smaller than that of the batteries in Comparative Examples 1 to 7. That is, in the case of Examples 1 and 2 including the point-type conductive agent and the linear conductive agent in a weight ratio of 4:1 to 14:1, and the sultone-based compound and the phosphate-based compound in a weight ratio of 1:0.1 to less than 1:2 as additives, it can be confirmed that the degree of voltage drop of the battery during high-rate discharge is smaller than that of Comparative Examples 1 to 7.

Claims

1. A lithium secondary battery, include: Battery housing; and an electrode assembly and an electrolyte contained in the battery case, The electrode assembly comprises a positive electrode, The positive electrode comprises a positive electrode active material and a conductive agent, The conductive agent includes a point-type conductive agent and a linear conductive agent in a weight ratio of 4:1 to 14:1, and The electrolyte includes a sultone-based compound and a phosphate-based compound as additives in a weight ratio of 1:0.1 to less than 1:

2. 2 . The lithium secondary battery according to claim 1 , wherein the positive electrode active material comprises a lithium nickel-based oxide containing at least 80 mol % of nickel relative to the total moles of transition metals excluding lithium. 3 . The lithium secondary battery according to claim 1 , wherein the positive electrode active material comprises a lithium nickel-based oxide containing 85 mol % to 90 mol % of nickel relative to the total moles of transition metals excluding lithium.

4. The lithium secondary battery according to claim 2, wherein the lithium nickel-based oxide is a compound represented by the following Formula 1: [Formula 1] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y ( in, 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, 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 X is at least one element selected from the group consisting of F, Cl, Br, I, At, P and S. ) 5 . The lithium secondary battery according to claim 1 , wherein the point-type conductive agent is carbon black, and the linear conductive agent is multi-walled carbon nanotube. 6 . The lithium secondary battery according to claim 1 , wherein the sultone-based compound is included in an amount of less than 1 wt % with respect to the total weight of the electrolyte. 7 . The lithium secondary battery according to claim 1 , wherein the sultone-based compound is included in an amount of 0.3 wt % to 0.9 wt % relative to the total weight of the electrolyte. 8 . The lithium secondary battery according to claim 1 , wherein the phosphate-based compound is included in an amount of less than 1 wt % with respect to the total weight of the electrolyte. 9 . The lithium secondary battery according to claim 1 , wherein the phosphate-based compound is included in an amount of 0.6 wt % to 0.9 wt % with respect to the total weight of the electrolyte. 10 . The lithium secondary battery according to claim 1 , wherein the sultone-based compound is at least one compound selected from the group consisting of 1,3-propane sultone and 1,3-propene sultone.

11. The lithium secondary battery according to claim 1, wherein the phosphate-based compound is at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

12. The lithium secondary battery according to claim 1, wherein during 24C discharge, the voltage holding rate V represented by the following equation 1 is d For at least 81%: [Equation 1] V d (%)=(V f / V i )×100 in, In the above equation 1, V f is the voltage of the lithium secondary battery after a discharge pulse is applied at a 24C rate, V i is the voltage of a fully charged lithium secondary battery before a discharge pulse at a 24C rate is applied.

13. The lithium secondary battery according to claim 12, wherein the voltage holding rate V during 24C discharge is d It is between 81% and 87%. 14 . The lithium secondary battery according to claim 1 , wherein the battery case is a pouch type battery case.

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