Lithium secondary battery

Abstract

The lithium secondary battery of the present invention includes a battery case, and an electrode assembly and an electrolyte housed in the battery case, and has a Vd-40 of 74% or more, the Vd-40 being a voltage holding ratio during 40C discharge represented by Equation 1 below: [Equation 1] Vd-40 (%) = (Vf-40 / Vi-40) * 100. In Equation 1, Vf-40 is the voltage of the lithium secondary battery after the discharge pulse is applied at a rate of 40C, and Vi-40 is the voltage of the lithium secondary battery before the discharge pulse is applied at a rate of 40C.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to Korean Patent Application Nos. 10 - 2022 - 0140773 filed on October 27, 2022, 10 - 2022 - 0140774 filed on October 27, 2022, and 10 - 2023 - 0143338 filed on October 24, 2023 with the Korean Intellectual Property Office, the disclosures of which are hereby incorporated by reference in their entirety. Technical Field

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

[0005] With the recent increase in the technological development and demand for power tools, electric vehicles, and energy storage systems (ESS), the demand for batteries as an energy source has increased significantly, and thus various studies have been conducted on batteries that can meet various requirements. As the market demand for lithium secondary batteries with high capacity as a power source for such devices has increased, research to increase the energy density of the battery 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 continuously increasing.

[0006] However, when the secondary battery embedded in a power tool discharges at 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 - speed 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 using cylindrical batteries, the total weight of the power tool increases, and thus there is a limitation in reducing work convenience. Summary of the Invention

[0008] Technical Problem

[0009] To solve the above problems, the present invention provides a lithium secondary battery having a low degree of voltage drop in the battery during high - rate discharge, thereby preventing the power - off phenomenon of power tools.

[0010] In addition, the present invention provides a pouch-type lithium secondary battery, which has a low degree of voltage drop in the battery during high-rate discharge, has excellent working convenience due to its relatively light weight, and has low resistance and excellent output properties.

[0011] Technical solution

[0012] According to one aspect of the present invention, there is provided a lithium secondary battery, which includes a battery case and an electrode assembly and an electrolyte accommodated in the battery case, and the V of the lithium secondary battery d-40 is 74% or more, specifically 74% to 85%, and the V d-40 is the voltage retention rate during 40C discharge represented by the following Equation 1.

[0013] [Equation 1]

[0014] V d-40 (%) = (V f-40 / V i-40 ) × 100

[0015] In the above Equation 1, V f-40 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 40C, and V i-40 is the voltage of the lithium secondary battery before applying a discharge pulse at a rate of 40C.

[0016] Meanwhile, the V' of the lithium secondary battery of the present invention d,5 can be 72% or more, preferably 72% to 82%, and the V' d,5 is the voltage retention rate during continuous discharge represented by the following Equation 2.

[0017] [Equation 2]

[0018] V' d,5 (%) = (V' f,5 / V' i ) × 100

[0019] In the above Equation 2, V' f,5 is the voltage of the lithium secondary battery after applying 5 discharge pulses while increasing the rate by 40C, and V' i is the initial voltage of the fully charged lithium secondary battery before applying the discharge pulse. At this time, the first discharge pulse is applied at a rate of 24C, and the fifth discharge pulse is applied at a rate of 40C.

[0020] According to the present invention, the electrode assembly includes a positive electrode, a separator, and a negative electrode. The positive electrode may include a lithium nickel-based oxide as a positive electrode active material, and the lithium nickel-based oxide contains 80 mol% or more, preferably 85 mol% to 90 mol%, of nickel based on the total number of moles of transition metals other than lithium.

[0021] Preferably, the lithium nickel-based oxide may be a compound represented by the following Formula 1.

[0022] [Formula 1]

[0023] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X y

[0024] 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, and 0 ≤ y ≤ 0.2. M1 is Mn, Al, or a combination thereof, M2 is a metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0025] Meanwhile, in the present invention, the positive electrode may include a positive electrode active material layer having a porosity greater than 30%, specifically 32% to 38%.

[0026] According to the present invention, the electrolyte may contain ethyl propionate (EP), and at this time, based on the total weight of the electrolyte, the content of ethyl propionate may be 80% by weight or less, specifically 40% by weight to 70% by weight.

[0027] In addition, the negative electrode may include artificial graphite as a negative electrode active material.

[0028] Meanwhile, in 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 when a discharge pulse is applied at a rate of 40C, the voltage retention rate V before and after the application of the discharge pulse d-40 ​is 75% or more. As a result, even when the lithium secondary battery of the present invention is discharged at a high rate, the degree of voltage drop in the battery is not large, so that when the battery of the present invention is used as a power source for power tools, the phenomenon of power-off 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, due to its relatively light weight, the work convenience is excellent.

[0032] In addition, when the lithium secondary battery of the present invention is a pouch-type lithium secondary battery, there are a plurality of electrode tabs. Therefore, the resistance is lower than that of a cylindrical battery, and the output performance is excellent, making the lithium secondary battery suitable as a power source for power tools. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings herein illustrate preferred embodiments of the present invention by way of example, and together with the detailed description of the present invention given below are used to further understand the technical concept of the present invention. Therefore, the present invention should not be construed only by the content in these drawings.

[0034] Figure 1 is a graph showing the change in voltage of the battery over time when continuous discharge tests are performed on the lithium secondary batteries manufactured in Example 1 and 2 and Comparative Example 1, respectively.

[0035] Figure 2 is a graph showing the change in voltage of the battery over time when continuous discharge tests are performed on the lithium secondary batteries manufactured in Example 2 and Comparative Example 2, respectively.

[0036] Figure 3 is a graph showing the change in voltage of the battery over time when continuous discharge tests are performed on the lithium secondary batteries manufactured in Example 1 and 3, respectively. DETAILED DESCRIPTION

[0037] The advantages and features of the present invention and the methods for realizing them will be clarified by the following embodiments described with reference to the drawings. However, the present invention can be implemented in different forms and should not be construed as limited to the embodiments set forth 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 defined only by the scope of the claims. Throughout the specification, the same reference numerals denote the same elements.

[0038] Unless otherwise defined, all terms (including technical and scientific terms) used herein will be used in the sense commonly understood by those of ordinary skill in the art to which the concept of the present invention belongs. In addition, terms defined in commonly used dictionaries are not to be interpreted ideally or overly, unless specifically defined.

[0039] The terms used herein are for the purpose of describing embodiments and are not intended to limit the present invention. In the present disclosure, the singular forms include the plural forms unless the context clearly indicates otherwise. As used herein, the terms "comprising" and / or "including" are intended to include the stated elements and do not preclude the possibility of the presence or addition of one or more other elements.

[0040] In this specification, when a part is referred to as including a certain component, this means that the part may further include another component rather than excluding another component, unless otherwise specified.

[0041] In this specification, the description of "A and / or B" means A, or B, or A and B.

[0042] In this specification, unless otherwise specified, "%" refers to weight %.

[0043] In this specification, D 50 refers to the particle size corresponding to 50% of the volume accumulation in the particle size distribution curve of the particles. D 50 can be measured by, for example, the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters, and thus high reproducibility and high-resolution results can be obtained.

[0044] In this specification, the "specific surface area" is measured by the BET method, and specifically, it can be calculated from the nitrogen adsorption amount at the liquid nitrogen temperature (77K) using Belsorp-mino II of BEL Japan Co., Ltd.

[0045] Hereinafter, the present invention will be described in more detail.

[0046] Lithium secondary battery

[0047] The lithium secondary battery of the present invention includes a battery case and an electrode assembly and an electrolyte accommodated in the battery case, and the V d-40 of the lithium secondary battery is 74% or more, and the V d-40 is the voltage retention rate during discharge at 40C represented by the following Equation 1.

[0048] [Equation 1]

[0049] V d-40 (%) = (V f-40 / V i-40 ) × 100

[0050] In the above [Equation 1], V f-40 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 40C, and V i-40 is the voltage of the lithium secondary battery before applying a discharge pulse at a rate of 40C.

[0051] During discharge at 40C, the voltage retention rate V d-40 can be 74% or more, particularly 74% to 85%, and more particularly 74% to 80%. If V d-40 is less than 74%, the battery voltage becomes too low after high-rate discharge, such that even if there is actually remaining battery capacity, the power tool system may determine that the battery capacity is insufficient and shut off the power tool. However, in the case of the lithium secondary battery of the present invention, V d-40 is up to 74% or more, such that even when the battery undergoes high-rate discharge, the degree of voltage drop is not significant, which makes the battery suitable as a power source for power tools.

[0052] Meanwhile, the V' of the lithium secondary battery of the present invention d,5 can be 72% or more, specifically 72% to 82%, and more specifically 74% to 82%, where the V' d,5 is the voltage retention rate during continuous discharge represented by the following Equation 2.

[0053] [Equation 2]

[0054] V'd ,5 (%) = (V'f ,5 / V'i) × 100

[0055] In the above [Equation 2], V' f,5 is the voltage of the lithium secondary battery after applying 5 discharge pulses while increasing the rate by 40C, V' i is the initial voltage of the fully charged lithium secondary battery before applying the discharge pulses, and the first discharge pulse is applied at a rate of 24C and the fifth discharge pulse is applied at a rate of 40C.

[0056] If the voltage retention rate V' during continuous discharge d,5 meets the above numerical range, then even when the battery discharges continuously, the battery voltage will not become too low, such that the battery of the present invention is suitable as a power source for power tools that require continuous discharge.

[0057] The discharge properties of a lithium secondary battery can vary depending on the following factors: the electrode composition; details of the electrode design such as the loading and porosity; and / or the electrolyte composition, and in particular, may be affected by the composition and design of the positive electrode and the electrolyte. Accordingly, the composition and design details of the electrode and the composition of the electrolyte can be controlled to manufacture a lithium secondary battery having desired discharge performance. For example, a lithium secondary battery having the above-described discharge properties can be manufactured by applying the following positive electrode, negative electrode, and electrolyte, the positive electrode including a lithium nickel-based oxide containing 80 mol% or more nickel as a positive electrode active material, and the porosity of the positive electrode active material layer being 30% or more, the negative electrode including artificial graphite as a negative electrode active material, and the electrolyte including ethyl propionate, but the embodiments are not limited thereto.

[0058] 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 including a plurality of battery cells. Preferred examples of the above medium- and large-sized devices can include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems (ESS), and the like.

[0059] Hereinafter, each component of the lithium secondary battery of the present invention will be described in more detail.

[0060] <Electrode assembly>

[0061] The electrode assembly of the present invention can include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0062] Hereinafter, each component of the electrode assembly will be described in more detail.

[0063] (1) Positive electrode

[0064] The positive electrode can include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0065] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause a chemical change in the battery. For example, as the current collector, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0066] The thickness of the positive electrode current collector can be 8 μm to 500 μm, specifically 8 μm to 300 μm, and more specifically 10 μm to 50 μm. If the loading amount of the positive electrode active material layer is reduced to lower the resistance of the positive electrode and thus improve the output performance of the battery, the positive electrode active material can be embedded in the positive electrode current collector and the positive electrode current collector can be disconnected. However, if the thickness of the positive electrode current collector of the present invention satisfies the above numerical range, the positive electrode current collector is thicker than a typical positive electrode current collector, and thus disconnection of the current collector can be prevented.

[0067] The positive electrode active material layer may include a positive electrode active material, and if necessary, may also include a conductive material, a binder, and the like.

[0068] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium, and may include a lithium metal oxide including one or more metals such as cobalt, manganese, nickel, and aluminum and lithium.

[0069] Specifically, the positive electrode active material may include a lithium nickel-based oxide. In this case, based on the total molar amount of transition metals other than lithium, the lithium nickel-based oxide may contain 80 mol% or more, specifically 80 mol% to 95 mol%, more specifically 80 mol% to 90 mol%, and even more specifically 85 mol% to 90 mol% of nickel. If the content of nickel satisfies the above range, the battery capacity can be ensured by ensuring sufficient positive electrode energy density, while minimizing the voltage drop during high-rate discharge of the battery.

[0070] The lithium nickel-based oxide may be a compound represented by the following Formula 1.

[0071] [Formula 1]

[0072] Li 1+x [Ni a Co b M1 c M2 d 1-x O 2-y X y

[0073] 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.

[0074] In the above Formula 1, M1 may be Mn, Al, or a combination thereof.

[0075] In the above Formula 1, M2 may be a 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.

[0076] In the above Formula 1, X may be one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0077] ​Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 60% to 99% by weight, preferably 70% to 99% by weight, and more preferably 80% to 98% by weight. If the content of the positive electrode active material satisfies the above range, the battery capacity can be increased by ensuring sufficient positive electrode energy density.

[0078] The positive electrode conductive material is a component for further improving the conductivity of the positive electrode active material, and there is no particular limitation on the conductive material as long as it has conductivity without causing chemical changes in the battery.

[0079] For example, as the positive electrode conductive material, the following can be used: carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal cracking carbon black; graphite powder such as natural graphite, artificial graphite or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; fluorocarbon powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives.

[0080] The positive electrode conductive material of the present invention can include dot-shaped conductive materials and linear conductive materials. Specifically, the positive electrode conductive material can include dot-shaped conductive materials and linear conductive materials in a weight ratio of 1:1 to 50:1, preferably 2:1 to 20:1, and more preferably 2:1 to 10:1. If the ratio of the dot-shaped conductive material and the linear conductive material satisfies the above numerical range, the problem of excessive gas generation in the hot box test due to the increase in side reactions as the specific surface area of the conductive material increases can be prevented, and the positive electrode resistance can be reduced by sufficiently fixing the conductive network within the positive electrode.

[0081] In this case, based on the total weight of the positive electrode active material layer, the content of the linear conductive material can be 2.0% by weight or less, specifically 0.05% to 1.0% by weight, and more specifically 0.05% to 0.5% by weight.

[0082] Based on the total weight of the positive electrode active material layer, the content of the positive electrode conductive material can be 1% to 20% by weight, preferably 1% to 15% by weight, and more preferably 1% to 10% by weight. If the content of the positive electrode conductive material in the positive electrode active material layer satisfies the above range, the conductivity of the positive electrode can be improved by ensuring the positive electrode conductive network.

[0083] The positive electrode binder is a component that helps the binding between the active material, the conductive material, etc. and the binding with the current collector.

[0084] 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, or various copolymers thereof, etc., and any one of them or a mixture of two or more thereof may be used.

[0085] Based on the total weight of the positive electrode active material layer, 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, and more specifically 1.0% by weight to 3.5% by weight. If the content of the positive electrode binder satisfies the above range, excellent positive electrode adhesion can be ensured due to the increased contact area between the positive electrode binder and the positive electrode active material.

[0086] Meanwhile, 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 , and more specifically 8.0 mg / cm 2 to 9.4 mg / cm 2 . If the loading amount of the positive electrode active material layer is 11.0 mg / cm 2 or more, the problem is that as the positive electrode resistance value increases, the battery has an increased resistance, resulting in a reduced output performance.

[0087] The porosity of the positive electrode active material layer may be greater than 30%, specifically 32% to 38%, and more specifically 32% to 37%. If the porosity of the positive electrode active material layer satisfies the above range, the positive electrode has a reduced interfacial resistance, which can improve the output characteristics of the battery, and the density between the current collector and the positive electrode active material will not be excessively reduced, which can improve the electron conductivity.

[0088] The packing density of the positive electrode active material layer may be greater than 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, and thus the porosity of the positive electrode active material layer may have the above appropriate numerical range.

[0089] Meanwhile, in addition to using the above positive electrode active material, the positive electrode may be manufactured according to a typical method for manufacturing a positive electrode. Specifically, a positive electrode paste composition containing the above positive electrode active material, positive electrode conductive material, and / or positive electrode binder may be prepared, and then the positive electrode paste composition may be applied to the positive electrode current collector, and then the composition may be dried and roll-pressed to manufacture the positive electrode.

[0090] In addition, in another method, a positive electrode can be manufactured by casting a positive electrode paste composite on a separate carrier and then laminating a film obtained by peeling from the carrier on a positive electrode current collector.

[0091] (2) Negative electrode

[0092] The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. Alternatively, the negative electrode may be a graphite electrode composed of carbon (C). Alternatively, the negative electrode itself may be a metal.

[0093] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used.

[0094] The negative electrode current collector usually has a thickness of 3 μm to 500 μm, and as in the case of the positive electrode current collector, microscopic irregularities can be formed on the surface of the negative electrode current collector to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven body, etc.

[0095] The negative electrode active material layer may include a negative electrode active material, and if necessary, may also include a conductive material, a binder, etc.

[0096] The negative electrode active material is a material capable of reversibly inserting / extracting lithium metal and / or lithium ions.

[0097] The negative electrode active material may include at least one selected from the group consisting of carbon-based materials, metals or alloys of metals and lithium, metal composite oxides, materials capable of doping and dedoping lithium, and transition metal oxides.

[0098] The carbon-based 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 fired carbon) or hard carbon, mesophase pitch carbide, fired coke, etc.

[0099] As the metal or the alloy of the metal and lithium, 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 the metal and lithium can be used.

[0100] As the metal composite oxide, PbO, PbO 2 , Pb 2 O3 , 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 1, 2, and 3 of the periodic table, or halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) at least one in the group consisting of.

[0101] Materials that can be doped and de-doped with lithium can 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, but 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, but not Sn), etc., or one of them can be mixed with SiO 2 and used. 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.

[0102] Transition metal oxides can be lithium-containing titanium composite oxides (LTO), vanadium oxides, lithium vanadium oxides, etc.

[0103] Specifically, the negative electrode active material of the present invention may be artificial graphite, and more specifically, it may be artificial graphite coated with hard carbon on the surface. When artificial graphite is used as the negative electrode active material, it has the effect of improving the capacity characteristics of the battery by reducing the resistance of the negative electrode, while reducing the overvoltage during charging to prevent lithium precipitation caused by side reactions, thereby improving the cycle life characteristics of the battery.

[0104] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material may be 60% by weight to 99% by weight, preferably 70% by weight to 99% by weight, and more preferably 80% by weight to 99% by weight. If the content of the negative electrode active material satisfies the above range, the contents of the negative electrode conductive material and the negative electrode binder can be maintained at desired levels, and the battery capacity can be increased by ensuring sufficient negative electrode energy density.

[0105] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material, and there is no particular limitation on the conductive material as long as it has conductivity without causing chemical changes in the battery. For example, the following can be used: carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal cracking carbon black; graphite powders such as natural graphite, artificial graphite or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; fluorocarbon powders; conductive powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives.

[0106] Based on the total weight of the negative electrode active material layer, the content of the negative electrode conductive material may be 1% by weight to 20% by weight, preferably 1% by weight to 15% by weight, and more preferably 1% by weight to 10% by weight. If the content of the negative electrode conductive material satisfies the above range, the conductivity of the negative electrode can be improved by ensuring a negative electrode conductive network.

[0107] The negative electrode binder is a component that helps the binding between the negative electrode conductive material, the negative electrode active material and the negative electrode current collector. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, various copolymers thereof, etc., and any one of them or a mixture of two or more of them can be used.

[0108] Based on the total weight of the negative electrode active material layer, 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%. If the content of the negative electrode binder satisfies the above range, the negative electrode active material particles can be smoothly bonded to minimize the volume expansion problem of the negative electrode active material, and the negative electrode active material can be well adhered to the negative electrode current collector.

[0109] Meanwhile, when a metal itself is used as the negative electrode without forming a negative electrode active material layer, the negative electrode can be manufactured by using the metal itself, or by physically bonding, roll-pressing, or depositing the metal on the negative electrode current collector. The deposition method may be electrodeposition or chemical vapor deposition.

[0110] For example, the metal thin film itself or the metal bonded / roll-pressed / deposited on the negative electrode current collector may be a metal selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In), or an alloy of two of these metals.

[0111] (3) Separator

[0112] The separator can be used without particular limitation as long as it is generally used as a separator in a lithium secondary battery, and in particular, a separator having excellent moisture retention ability for the electrolyte solution and low resistance to ion movement in the electrolyte is preferred.

[0113] For example, as the separator, a porous polymer film including a polyolefin-based polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc.) or a stacked structure having two or more layers thereof can be used. Additionally, typical porous non-woven fabrics, such as non-woven fabrics made of glass fibers with a high melting point, polyethylene terephthalate fibers, etc., can be used.

[0114] The separator may have a thickness of 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 6 μm to 13 μm. If the thickness of the separator satisfies the above range, short circuit between the positive electrode and the negative electrode can be prevented while minimizing the battery resistance value. As a result, the life characteristics and output characteristics of the lithium secondary battery can be improved.

[0115] <Electrolyte>

[0116] Meanwhile, the lithium secondary battery of the present invention may include an electrolyte.

[0117] The electrolyte may include organic solvents and lithium salts commonly used in the art, without particular limitation.

[0118] Any organic solvent can be used as the organic solvent without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. 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, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic 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), etc.

[0119] Preferably, the electrolyte can use a mixed solution of a carbonate solvent and an ester solvent 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.

[0120] The ester solvent can be ethyl propionate (EP), and the cyclic carbonate solvent can be at least one of ethylene carbonate and propylene carbonate, and the linear carbonate solvent can be one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0121] Ethyl propionate has a lower viscosity than typical electrolyte components. Therefore, when ethyl propionate is included, there is an effect of increasing the ionic conductivity of the electrolyte. Based on the total weight of the electrolyte, the content of ethyl propionate can be 80% by weight or less, specifically 5% to 80% by weight, and more specifically 40% to 70% by weight. If 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.

[0122] Any compound can be used as the lithium salt without particular limitation as long as it can provide lithium ions used in the lithium secondary battery. Specifically, as the lithium salt, LiN(SO 2 F) 2 、LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF 6 、LiAlO 4 、LiAlCl 4 、LiCF 3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5 SO3 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , LiCl, LiI or LiB(C 2 O 4 ) 2 . Preferably, the lithium salt is included in the electrolyte at a concentration of about 0.6 mol% to about 2 mol%.

[0123] The electrolyte of the present invention may further include an additive to further improve the physical properties of the secondary battery.

[0124] Examples of the additive may include at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted 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.

[0125] The cyclic carbonate compound may be, for example, vinylene carbonate (VC), vinyl ethyl carbonate (VEC), etc.

[0126] The halogen-substituted carbonate compound may be, for example, fluoroethyl carbonate (FEC), etc.

[0127] The nitrile compound may be, for example, succinonitrile, adiponitrile, adiponitrile, 1,4-dicyano-2-butene, etc.

[0128] The sultone compound may be, for example, 1,3-propane sultone, 1,3-propene sultone, etc.

[0129] The sulfate / salt compound may be, for example, ethylene sulfite (ESA), trimethylene sulfate (TMS), methyltrimethylene sulfate (MTMS), etc.

[0130] The phosphate / salt compound may be, for example, one or more compounds selected from lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, trimethylsilyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphite.

[0131] The borate / salt compound may be, for example, tetraphenyl borate, lithium oxalyl difluoroborate (LiODFB), etc.

[0132] The benzene compound can be, for example, fluorobenzene, etc., the amine compound can be triethanolamine, ethylenediamine, etc., and the silane compound can be tetravinylsilane, etc.

[0133] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution, and can be selected from the group consisting of LiPO 2 F 2 , LiODFB, lithium bis(oxalato)borate (LiB(C 2 O 4 )(LiBOB), and LiBF 2 ), and one or more compounds selected from the group consisting of 4 .

[0134] Meanwhile, the additives can be used alone, or two or more of them can be used in combination.

[0135] Based on the total weight of the electrolyte solution, the total amount of the additives can be 1 wt% to 25 wt%, preferably 1 wt% to 20 wt%, and more preferably 5 wt% to 20 wt%. If the content of the additives is within the above range, a film can be stably formed on the electrode, the ignition phenomenon during overcharging can be suppressed, side reactions during the initial activation process of the secondary battery can be prevented, or the residue or precipitation of the additives can be prevented.

[0136] <Battery case>

[0137] The lithium secondary battery of the present invention may include a cylindrical, prismatic, pouch-type, or coin-type battery case, and preferably includes a pouch-type battery case.

[0138] The pouch-type battery case includes a barrier layer, a base layer formed on one surface of the barrier layer, and a sealing layer formed on the other surface of the barrier layer, and includes at least one cup portion recessed in one direction.

[0139] Specifically, the pouch-type battery case has flexibility and can be manufactured by inserting a pouch film laminate in which the base layer, the barrier layer, and the sealing layer are sequentially stacked 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 to form a cup portion having a shape recessed in one direction.

[0140] The base layer is provided on the outermost layer of the pouch to protect the electrode assembly from external shocks and electrically insulate the electrode assembly.

[0141] The base layer can be made of a polymer material, for example, it can be made of one or more polymer materials 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 benzoxazole), polyarylate, and polytetrafluoroethylene (Teflon).

[0142] The base layer can have a single-layer structure or a multi-layer structure in which different polymer films are stacked. If the base layer has a multi-layer structure, an adhesive layer can be placed between the polymer films.

[0143] 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. If the base layer has a multi-layer structure, the above thickness includes the thickness of the adhesive layer. When the base layer meets the above range, the durability, insulation, and moldability are excellent. If the base layer is too thin, the durability may be reduced, and the base layer may be damaged during the molding process. And if it is too thick, the moldability may be reduced, the total thickness of the bag may increase, and the space for accommodating the battery may decrease, resulting in a reduction in energy density.

[0144] The barrier layer is used to ensure the mechanical strength of the pouch-type battery case, block the entry / exit of gas or moisture from the outside of the secondary battery, and prevent electrolyte leakage.

[0145] The thickness of the barrier layer can be 40 μm to 100 μm, preferably 50 μm to 80 μm, and more preferably 60 μm to 80 μm. If the thickness of the barrier layer meets the above range, the moldability is improved, thereby increasing the molding depth of the cup part, or the resistance to external stress after molding is improved because the generation of cracks and / or pinholes is reduced even during double-cup molding.

[0146] Meanwhile, the barrier layer can be made of a metal material and can specifically be composed of an aluminum alloy thin film.

[0147] The aluminum alloy thin film can include 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).

[0148] The sealing layer is used to seal the bag by thermocompression bonding and is located in the innermost layer of the bag film laminate.

[0149] The sealing layer is the surface that comes into contact with the electrolyte and the electrode assembly after the bag is molded. Therefore, it needs to have insulating properties and corrosion resistance, and it needs to completely seal the inside to block the movement of substances between the inside and the outside. Therefore, it needs to have high sealing performance.

[0150] The sealing layer can be made of a polymer material and can be made of, for example, one or more 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 benzoxazole), polyarylate, and polytetrafluoroethylene. Among the above, polypropylene (PP) is particularly preferred, which has excellent mechanical properties such as tensile strength, stiffness, surface hardness, abrasion resistance, and heat resistance, as well as excellent chemical properties such as corrosion resistance.

[0151] The sealing layer can have a single-layer structure or can have a multi-layer structure including two or more layers composed of different polymer materials.

[0152] The total thickness of the sealing layer can be 60 μm to 100 μm, preferably 60 μm to 90 μm, and more preferably 70 μm to 90 μm. If the sealing layer is too thin, the sealing durability and insulation performance may be reduced. If it is too thick, the flexibility may be reduced, and the total thickness of the pouch film laminate may increase, resulting in a decrease in the energy density per volume.

[0153] Meanwhile, the pouch film laminate can be manufactured by methods known in the art for manufacturing pouch film laminates. For example, the pouch film laminate can be manufactured by a method in which a base layer is attached to the upper surface of a barrier layer using an adhesive, and then a sealing layer is formed on the lower surface of the barrier layer using coextrusion or an adhesive, but the embodiments are not limited thereto.

[0154] The pouch-type battery case can be sealed while accommodating the electrode assembly so that a part of the electrode lead, that is, the terminal part, is exposed. 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 will be accommodated in the accommodation space provided in the cup part, and after injecting the electrolyte, the pouch-type battery case can be sealed.

[0155] 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. If the thickness of the electrode lead satisfies the above numerical range greater than its typical thickness, there is an effect of reducing the battery resistance, and at the same time, the heat resistance is improved by reducing the heat during an external short circuit.

[0156] When the lithium secondary battery of the present invention is a pouch-type lithium secondary battery, the lithium secondary battery is lighter in weight than a cylindrical secondary battery and has a reduced resistance due to the presence of multiple tabs, thus having excellent output performance, making the lithium secondary battery suitable for use as a battery for power tools.

[0157] Hereinafter, the present invention will be described in more detail with reference to specific embodiments. However, the following embodiments are merely for facilitating the understanding of the present invention and do not limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope and spirit of the present invention, and it is apparent that these changes and modifications are within the scope of the appended claims.

[0158] Examples and Comparative Examples

[0159] Example 1 (Manufacture of Lithium Secondary Battery)

[0160] Artificial graphite coated with hard carbon (average particle size D 50 = 19 μm, BET specific surface area = 0.8 m 2 / g), carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed at a weight ratio of 94.3:2.0:1.2:2.5, and distilled water was added thereto to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 44 wt%.

[0161] 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. Thereafter, the dried negative electrode slurry was roll-pressed, dried in a vacuum oven at 130 °C for 12 hours, and then punched to manufacture a negative electrode.

[0162] Li[Ni 50 having an average particle size D 0.83 Co 0.11 Mn 0.06 O 2 as the positive electrode active material, multi-walled carbon nanotubes (CNT) having a BET specific surface area of 254 m 2 / g, polyvinylidene fluoride (PVdF), and hydrogenated acrylonitrile-butadiene rubber (H-NBR) were added to an N-methylpyrrolidone (NMP) solvent at a weight ratio of 97.0:1.0:1.5:0.5 and stirred to prepare a positive electrode slurry. The solid content of the positive electrode slurry was 70 wt%.

[0163] The positive electrode slurry was applied to one surface of an aluminum film having a thickness of 15 μm at a loading amount of 10.2 mg / cm 2 , and then vacuum dried 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 punched to manufacture a positive electrode. The porosity of the positive electrode active material layer contained in the positive electrode was 35%.

[0164] The positive electrode, negative electrode, and porous polyethylene separator having a thickness of 10 μm manufactured as described above were assembled in a stacked manner to manufacture an electrode assembly.

[0165] In a solvent in which ethylene carbonate (EC) and ethyl propionate (EP) are mixed at a weight ratio of 35:65, LiPF 6 is dissolved to 0.7 M, and LiFSi is dissolved to 0.7 M to prepare an electrolyte.

[0166] An electrode assembly is accommodated in a pouch-type battery case, the electrolyte is injected therein, and the battery case is sealed to fabricate a lithium secondary battery.

[0167] Example 2 (Fabrication of a Lithium Secondary Battery)

[0168] An electrolyte is prepared in the same manner as in Example 1, except that a solvent in which ethylene carbonate (EC), ethyl propionate (EP), and ethyl methyl carbonate (EMC) are mixed at a weight ratio of 20:40:40 is used.

[0169] A lithium secondary battery is fabricated in the same manner as in Example 1, except that the above electrolyte is used.

[0170] Example 3 (Fabrication of a Lithium Secondary Battery)

[0171] A positive electrode is fabricated in the same manner as in Example 1, except that Li[Ni 0.88 Co 0.07 Mn 0.05 O 2 is used as a positive electrode active material, and the loading amount is 9.6 mg / cm 2 .

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

[0173] Comparative Example 1 (Fabrication of a Lithium Secondary Battery)

[0174] An electrolyte is prepared in the same manner as in Example 1, except that a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a weight ratio of 30:70 is used.

[0175] A lithium secondary battery is fabricated in the same manner as in Example 1, except that the above electrolyte is used.

[0176] Comparative Example 2 (Fabrication of a Lithium Secondary Battery)

[0177] A positive electrode is fabricated in the same manner as in Example 1, except that the porosity of the positive electrode active material layer is 30%.

[0178] The electrolyte was produced in the same manner as in Example 1, except that a solvent in which ethylene carbonate (EC), ethyl propionate (EP), and ethyl methyl carbonate (EMC) were mixed at a weight ratio of 20:40:40 was used.

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

[0180] Comparative Example 3 (Production of lithium secondary battery)

[0181] A positive electrode was produced in the same manner as in Example 1, except that the porosity of the positive electrode active material layer was 30%.

[0182] A lithium secondary battery was produced in the same manner as in Example 1, except that the above-described positive electrode was used.

[0183] Comparative Example 4 (Production of lithium secondary battery)

[0184] A positive electrode was produced in the same manner as in Example 1, except that the porosity of the positive electrode active material layer was 40%.

[0185] A lithium secondary battery was produced in the same manner as in Example 1, except that the above-described positive electrode was used.

[0186] Comparative Example 5 (Production of lithium secondary battery)

[0187] The electrolyte was prepared in the same manner as in Example 1, except that a solvent in which ethylene carbonate (EC) and ethyl propionate (EP) were mixed at a weight ratio of 15:85 was used.

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

[0189] In Examples 1 to 3 and Comparative Examples 1 to 5, the nickel content in the positive electrode active material, the porosity of the positive electrode active material layer, and the composition of the electrolyte are shown in Table 1 below.

[0190] [Table 1]

[0191]

[0192] Experimental Example 1 - Continuous Discharge Test Evaluation

[0193] At a temperature of 23 °C, each of the lithium secondary batteries of the examples and comparative examples was fully charged to 4.2 V under an initial 0.6C condition. Thereafter, using the PNE Cycle program (CTSMonPro, PNE SOLUTION CO.LTD), a continuous discharge test was performed on the lithium secondary batteries by applying a discharge pulse to the lithium secondary batteries under the conditions described in Table 2 below and setting a rest period between each pulse application.

[0194] At this time, in Figures 1 to 3 is a graph showing the change in voltage of the lithium secondary battery over time during the continuous discharge test. Specifically, Figure 1 is a graph showing the change in voltage of the battery over time when the lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Example 1 are subjected to a continuous discharge test. Figure 2 is a graph showing the change in voltage of the battery over time when the lithium secondary batteries manufactured in Example 2 and Comparative Example 2 are subjected to a continuous discharge test. Figure 3 is a graph showing the change in voltage of the battery over time when the lithium secondary batteries manufactured in Examples 1 and 3 are subjected to a continuous discharge test. As Figures 1 to 3 shown, it can be seen that when a discharge pulse is applied to the lithium secondary battery, a voltage drop occurs, and the voltage steadily decreases when the pulse is repeatedly applied. In addition, it can be seen that when the application of the discharge pulse ends, the voltage of the lithium secondary battery recovers.

[0195] Meanwhile, during the continuous discharge test, the battery voltage was measured before and after each of the first to fifth pulses was applied, as shown in Table 3 below. At this time, the battery voltage before each pulse application is represented as V i , the battery voltage after each pulse application is represented as V f , and the ratio of the battery voltage V f after each pulse application to the battery voltage V i before each pulse application is represented as V d (%). In addition, from the measured voltage values, the voltage retention rate V d-40 during the 40C discharge represented by Equation 1 below and the voltage retention rate V d,5 during the continuous discharge represented by Equation 2 below were calculated and shown in Table 4 below.

[0196] [Equation 1]

[0197] V d-40 (%) = (V f-40 / V i-40 ) × 100

[0198] In Equation 1 above, V f-40is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 40C, V i-40 is the voltage of the lithium secondary battery before applying a discharge pulse at a rate of 40C.

[0199] [Equation 2]

[0200] V' d,5 (%) = (V' f,5 / V' i ) × 100

[0201] In the above [Equation 2], V' f,5 is the voltage of the lithium secondary battery after applying a discharge pulse continuously 5 times, and V' i is the voltage of the fully charged lithium secondary battery before applying a discharge pulse. During continuous discharging, a discharge pulse is applied while increasing the rate by 4C. The rate of the initial discharge pulse is 24C, and the rate of the final discharge pulse is 40C.

[0202] [Table 2]

[0203]

[0204] [Table 3]

[0205]

[0206]

[0207] [Table 4]

[0208]

[0209] As shown in Tables 3 to 4, it can be confirmed that in Examples 1 to 3, the voltage retention rate V d-40 of the lithium secondary battery during discharging at 40C is 74% or more, which is higher than the voltage retention rates in Comparative Examples 1 to 5. In addition, it can be confirmed that in Examples 1 to 3, the voltage retention rate V' d,5 of the lithium secondary battery during continuous discharging is 72% or more, which is higher than the voltage retention rates in Comparative Examples 1 to 5. This means that in Examples 1 to 3, compared with Comparative Examples 1 to 5, the degree of voltage drop of the battery during high-rate discharging and continuous discharging is smaller.

Claims

1. A lithium secondary battery, which comprises: a battery case; and An electrode assembly and an electrolyte accommodated in the battery case, where V d-40 is 74% or more, and the V d-40 is the voltage retention rate during 40C discharge represented by the following Equation 1: [Equation 1] V d-40 (%) = (V f-40 / V i-40 ) × 100 Among them, in the above Equation 1, V f-40 is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 40C, and V i-40 is the voltage of the lithium secondary battery before applying a discharge pulse at a rate of 40C.

2. The lithium secondary battery according to claim 1, wherein, The V as the voltage retention rate during 40C discharge d-40 is 74% to 85%.

3. The lithium secondary battery according to claim 1, wherein, V' d,5 is more than 72%, and said V' d,5 is the voltage retention rate during continuous discharge represented by the following Equation 2: [Equation 2] V'd ,5 (%) = (V'f ,5 / V'i) × 100 Among them, in the above Equation 2, V' f,5 is the voltage of the lithium secondary battery after applying 5 discharge pulses while increasing the magnification by 40C, and V' i is the initial voltage of the fully charged lithium secondary battery before applying the discharge pulse.

4. The lithium secondary battery according to claim 3, wherein, The V' as the voltage retention rate during continuous discharge d,5 is 72% to 82%.

5. The lithium secondary battery according to claim 1, wherein, the electrode assembly includes a positive electrode, a separator and a negative electrode, wherein the positive electrode includes a lithium nickel-based oxide as a positive electrode active material, and the lithium nickel-based oxide contains nickel in an amount of 80 mol% or more based on the total number of moles of transition metals other than lithium.

6. The lithium secondary battery according to claim 5, wherein, the positive electrode active material contains a lithium nickel-based oxide, and the lithium nickel-based oxide contains nickel in an amount of 85 mol% to 90 mol% based on the total number of moles of transition metals other than lithium.

7. The lithium secondary battery according to claim 5, 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 ​ 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 and 0 ≤ y ≤ 0.2, M1 is Mn, Al or a combination thereof, M2 is a metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P and S.

8. The lithium secondary battery according to claim 5, wherein, the positive electrode includes a positive electrode active material layer having a porosity greater than 30%.

9. The lithium secondary battery according to claim 8, wherein, the positive electrode active material layer has a porosity of 32% to 38%.

10. The lithium secondary battery according to claim 1, wherein, the electrolyte contains ethyl propionate (EP).

11. The lithium secondary battery according to claim 10, wherein, based on the total weight of the electrolyte, the content of ethyl propionate is 80% by weight or less.

12. The lithium secondary battery according to claim 10, wherein, based on the total weight of the electrolyte, the content of ethyl propionate is 40% by weight to 70% by weight.

13. The lithium secondary battery according to claim 5, wherein, the negative electrode contains artificial graphite as a negative electrode active material.

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

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