Lithium secondary battery

Abstract

The lithium secondary battery according to the present invention includes a battery case, an electrode assembly and an electrolyte housed in the battery case, and has a voltage retention rate V at 40 C discharge represented by the following formula 1: d-40 is 74% or more. [Formula 1]V d-40 (%)=(V f-40 / V i-40 ) × 100 (V in the above [Formula 1] 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.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0140773, filed October 27, 2022, Korean Patent Application No. 10-2022-0140774, filed October 27, 2022, and Korean Patent Application No. 10-2023-0143338, filed October 24, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

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

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

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

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

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

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

[0008] According to the present invention, a battery includes a battery case, an electrode assembly housed in the battery case, and an electrolyte, and has a voltage retention rate V at 40 C discharge represented by the following formula 1: d-40 The present invention provides a lithium secondary battery having a capacity of 74% or more, preferably 74% to 85%. [Formula 1] V d-40 (%)=(V f-40 / V i-40 ) x 100 In the formula 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.

[0009] On the other hand, the lithium secondary battery according to the present invention has a voltage retention rate V' during continuous discharge, which is expressed by the following formula 2: d、5 can be 72% or more, preferably 72% to 82%. [Formula 2] V'd、5 (%)=(V' f、5 / V' i ) x 100 In the formula 2, V' f、5 is the voltage of the lithium secondary battery after applying five discharge pulses at a rate increasing by 4C, and V' i is the initial voltage of a fully charged lithium secondary battery before applying a discharge pulse. Here, the first discharge pulse was applied at a rate of 24C, and the fifth discharge pulse was applied at a rate of 40C.

[0010] According to the present invention, the electrode assembly includes a positive electrode, a separator, and a negative electrode, and the positive electrode may include, as a positive electrode active material, a lithium-nickel-based oxide containing nickel in an amount of 80 mol % or more, preferably 85 mol % to 90 mol %, based on the total number of moles of transition metals other than lithium.

[0011] Preferably, the lithium nickel-based oxide may be a compound represented by the following Chemical Formula 1: [Chemical formula 1] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y In Chemical Formula 1, 0.8≦a≦0.95, 0≦b≦0.2, 0≦c≦0.2, 0≦d≦0.1, a+b+c+d=1, 0≦x≦0.3, and 0≦y≦0.2; M1 is Mn, Al, or a combination thereof; M2 is one metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; and X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0012] Meanwhile, in the present invention, the positive electrode may include a positive electrode active material layer having a porosity of more than 30%, specifically 32% to 38%.

[0013] According to the present invention, the electrolyte may include ethyl propionate (EP), and the ethyl propionate may be included in an amount of 80 wt % or less, specifically 40 wt % to 70 wt %, based on the total weight of the electrolyte.

[0014] The negative electrode may include artificial graphite as a negative electrode active material.

[0015] Meanwhile, in the present invention, the battery case may be a pouch-type battery case. [Effects of the Invention]

[0016] When a discharge pulse is applied at a rate of 40 C, the lithium secondary battery according to the present invention has a voltage maintenance rate V d-40 As a result, even when the lithium secondary battery of the present invention is subjected to high-rate discharge, the voltage drop of the battery is not large, and therefore, when the battery of the present invention is used as a power source for a power tool, the power supply termination phenomenon of the power tool due to high-rate discharge can be prevented.

[0017] Furthermore, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it has a relatively light weight and is easy to handle.

[0018] Furthermore, when the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it has a large number of electrode tabs, which means that it has lower resistance and exhibits superior output characteristics compared to cylindrical batteries, making it suitable for use as a power source for power tools.

[0019] The drawings attached to the specification illustrate preferred embodiments of the present invention and, together with the above-described content of the invention, serve to further understand the technical concept of the present invention, and the present invention should not be interpreted as being limited solely to the matters depicted in such drawings. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a graph showing the change in battery voltage over time when a continuous discharge test was carried out on the lithium secondary batteries produced in Examples 1 and 2 and Comparative Example 1. [Figure 2] 1 is a graph showing the change in battery voltage over time when a continuous discharge test was carried out on the lithium secondary batteries produced in Example 2 and Comparative Example 2. [Figure 3] 1 is a graph showing the change in battery voltage over time when a continuous discharge test was carried out on the lithium secondary batteries produced in Examples 1 and 3, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0021] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various different forms. However, the embodiments are provided to fully disclose the present invention and to fully convey the scope of the invention to those skilled in the art to which the present invention pertains, and the present invention is defined by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0022] Unless otherwise defined, all terms (including technical and scientific terms) used in this specification can be used in the sense that can be commonly understood by a person having ordinary knowledge in the technical field to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless they are clearly and specifically defined.

[0023] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular form includes the plural form unless otherwise specified in the context. The terms "comprise", "include", and "have" used in this specification do not exclude the presence or addition of one or more other components in addition to the components mentioned.

[0024] In this specification, when a part is said to include a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified to the contrary.

[0025] In this specification, the phrase "A and / or B" means A, or B, or A and B.

[0026] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0027] In this specification, D 50 means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0028] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mino II manufactured by BEL Japan.

[0029] The present invention will now be described in further detail.

[0030] Lithium secondary battery The lithium secondary battery according to the present invention includes a battery case, an electrode assembly and an electrolyte housed in the battery case, and has a voltage retention rate V at 40 C discharge represented by the following formula 1: d-40 is over 74%.

[0031] [Formula 1] V d-40 (%)=(V f-40 / V i-40 ) x 100

[0032] In the formula 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.

[0033] The voltage maintenance rate V during 40C discharge d-40 can be 74% or more, specifically 74% to 85%, and more specifically 74% to 80%. d-40 If V is less than 74%, the battery voltage after high-rate discharge will be excessively low, and even if there is still some battery capacity remaining, the power tool system may determine that the battery capacity is insufficient and shut down the power supply of the power tool. However, in the case of the lithium secondary battery of the present invention, V d-40 The battery has a high capacity of over 74%, and the voltage drop is not large even when the battery is discharged at a high rate, making it suitable for use as a power source for power tools.

[0034] On the other hand, in the lithium secondary battery according to the present invention, the voltage maintenance rate V' during continuous discharge, which is represented by the following formula 2, d、5 can be 72% or more, specifically 72% to 82%, and more specifically 74% to 82%.

[0035] [Formula 2] V' d、5 (%)=(V' f、5 / V' i ) x 100

[0036] In the formula 2, V' f、5is the voltage of the lithium secondary battery after applying five discharge pulses at a rate increasing by 4C, and V' i is the initial voltage of a lithium secondary battery that was fully charged before applying a discharge pulse. The first discharge pulse was applied at a rate of 24C, and the fifth discharge pulse was applied at a rate of 40C.

[0037] Voltage maintenance rate V' during continuous discharge d、5 When satisfies the above numerical range, the battery voltage does not become excessively low even when the battery is continuously discharged, and therefore the battery of the present invention is suitable for use as a power source for power tools that require continuous discharge.

[0038] The discharge characteristics of a lithium secondary battery can vary depending on the electrode composition, electrode design factors such as loading amount and porosity, and / or electrolyte composition, particularly the composition and design of the positive electrode and electrolyte. Therefore, by adjusting the electrode composition and design factors and the electrolyte composition, a lithium secondary battery with desired discharge characteristics can be manufactured. For example, a lithium secondary battery with the above-described discharge characteristics can be manufactured by using a positive electrode containing a lithium-nickel-based oxide containing 80 mol % or more of nickel as the positive electrode active material and having a positive electrode active material layer with a porosity of 30% or more, a negative electrode containing artificial graphite as the negative electrode active material, and an electrolyte containing ethyl propionate, but this is not limiting.

[0039] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices such as power tools, but also as a unit battery in medium- to large-sized battery modules containing a large number of battery cells. Preferred examples of such medium- to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems (ESS).

[0040] Each component of the lithium secondary battery according to the present invention will be described in more detail below.

[0041] <Electrode assembly> The electrode assembly according to the present invention may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0042] Hereinafter, each component of the electrode assembly will be described in more detail.

[0043] (1) Positive electrode The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0044] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0045] The thickness of the positive electrode current collector may be 8 μm to 500 μm, specifically 8 μm to 300 μm, and more specifically 10 μm to 50 μm. When the loading amount of the positive electrode active material layer is reduced to lower the resistance of the positive electrode in order to improve the output characteristics of the battery, the positive electrode active material may become embedded in the positive electrode current collector, which may cause the positive electrode current collector to break. However, when the thickness of the positive electrode current collector according to the present invention satisfies the above numerical range, the positive electrode current collector is thicker than conventional ones, and therefore breakage of the current collector can be prevented.

[0046] The positive electrode active material layer may contain a positive electrode active material, and may further contain a conductive material, a binder, and the like, as needed.

[0047] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and may include a lithium metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum.

[0048] Specifically, the positive electrode active material may include a lithium-nickel-based oxide. In this case, the lithium-nickel-based oxide may contain nickel in an amount of 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%, based on the total moles of transition metals other than lithium. When the nickel content satisfies the above numerical range, sufficient positive electrode energy density can be ensured, thereby ensuring the capacity of the positive electrode battery and minimizing the voltage drop of the battery during high-rate discharge.

[0049] The lithium nickel-based oxide may be a compound represented by the following Chemical Formula 1:

[0050] [Chemical formula 1] Li 1+x [Ni a Co b M1 c M2 d ] 1-x O 2-y X y

[0051] In Chemical Formula 1, 0.8≦a≦0.95, 0≦b≦0.2, 0≦c≦0.2, 0≦d≦0.1, a+b+c+d=1, 0≦x≦0.3, and 0≦y≦0.2.

[0052] In the above Chemical Formula 1, M1 can be Mn, Al, or a combination thereof.

[0053] In Chemical Formula 1, M2 may be one metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo.

[0054] In the above Chemical Formula 1, X may be one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

[0055] The positive electrode active material may be contained in an amount of 60 to 99 wt %, preferably 70 to 99 wt %, and more preferably 80 to 98 wt %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode active material satisfies the above range, sufficient positive electrode energy density can be ensured, thereby improving the capacity of the battery.

[0056] The positive electrode conductive material is a component for further improving the conductivity of the positive electrode active material, and such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity.

[0057] For example, the positive electrode conductive material may be carbon powder such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fiber or metal fiber; carbon fluoride powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

[0058] The positive electrode conductive material according to the present invention may contain both dot-like conductive materials and linear conductive materials. Specifically, the positive electrode conductive material may contain the dot-like 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. When the ratio of the dot-like conductive materials to the linear conductive materials satisfies the above range, the problem of excessive gas generation in a hot box test due to an increase in side reactions caused by an increase in the specific surface area of ​​the conductive material can be prevented, and the conductive network within the positive electrode can be sufficiently secured, thereby reducing the resistance of the positive electrode.

[0059] In this case, 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, relative to the total weight of the positive electrode active material layer.

[0060] The positive electrode conductive material may be contained in an amount of 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode conductive material in the positive electrode active material layer satisfies the above range, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0061] The positive electrode binder is a component that serves to bind the active material and conductive material together and to bind them to the current collector.

[0062] Examples of such positive electrode binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers, and one or more of these may be used alone or in combination.

[0063] The positive electrode binder may be contained in an amount of 0.5 wt % to 5.0 wt %, specifically 1.0 wt % to 4.0 wt %, and more specifically 1.0 wt % to 3.5 wt %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode binder satisfies this range, the contact area between the positive electrode binder and the positive electrode active material is increased, ensuring excellent positive electrode adhesive strength.

[0064] On the other hand, the loading amount of the positive electrode active material layer was 11.0 mg / cm 2 Less than 8.0 mg / cm 2 ~10.0mg / cm 2 , more specifically 8.0 mg / cm 2 ~9.4mg / cm 2 The loading amount of the positive electrode active material layer can be 11.0 mg / cm 2 If the positive electrode resistance is increased, the battery resistance increases and the output characteristics deteriorate.

[0065] The porosity of the positive electrode active material layer may be greater than 30%, specifically 32% to 38%, more specifically 32% to 37%. When the porosity of the positive electrode active material layer satisfies this range, the interface resistance of the positive electrode may be reduced, improving the output characteristics of the battery, and the density between the current collector and the positive electrode active material may not be excessively reduced, improving electronic conductivity.

[0066] The packing density of the positive electrode active material layer may be 2.8 g / cc or more, specifically 2.8 g / cc to 3.5 g / cc, more specifically 2.9 g / cc to 3.2 g / cc, thereby allowing the porosity of the positive electrode active material layer to fall within the above-mentioned appropriate numerical range.

[0067] Meanwhile, the positive electrode can be manufactured by a conventional method for manufacturing a positive electrode, except for using the above-described positive electrode active material. Specifically, the positive electrode can be manufactured by preparing a positive electrode slurry composition containing the above-described positive electrode active material, a positive electrode conductive material, and / or a positive electrode binder, applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling the composition.

[0068] Alternatively, the positive electrode can be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0069] (2) Negative electrode The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, or may be a graphite electrode made of carbon (C), or may be a metal itself.

[0070] The negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like can be used.

[0071] The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to strengthen the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0072] The negative electrode active material layer may contain a negative electrode active material, and may further contain a conductive material, a binder, and the like, as needed.

[0073] The negative electrode active material is a material that can reversibly intercalate / deintercalate lithium metal and lithium ions.

[0074] The negative electrode active material may include at least one selected from the group consisting of a carbonaceous material, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0075] The carbon-based active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

[0076] As the metal or an alloy of these metals 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 these metals and lithium can be used.

[0077] As the metal composite oxide, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1) and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1 element of the periodic table, Group 2 element, Group 3 element, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) can be used.

[0078] As the substance capable of doping and undoping lithium, Si, SiO x (0 < x ≦ 2), Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element and a combination thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element and a combination thereof, and is not Sn), etc. can be mentioned, and at least one of these can also be used in mixture with SiO2. As the element Y, it can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po and a combination thereof).

[0079] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

[0080] Specifically, the negative electrode active material according to the present invention may be artificial graphite, more specifically, artificial graphite surface-coated with hard carbon. When artificial graphite is used as the negative electrode active material, it reduces the resistance of the negative electrode, thereby improving the capacity characteristics of the battery, and reduces overvoltage during charging, thereby preventing lithium deposition due to side reactions, thereby improving the cycle life characteristics of the battery.

[0081] The negative electrode active material may be contained in an amount of 60 wt% to 99 wt%, preferably 70 wt% to 99 wt%, and more preferably 80 wt% to 99 wt%, based on the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies this range, the contents of the negative electrode conductive material and the negative electrode binder can be maintained at preferred levels, and sufficient negative electrode energy density can be secured, thereby improving the battery capacity.

[0082] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. Examples of such a conductive material include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with highly developed crystal structures; conductive fibers such as carbon fibers and metal fibers; carbon fluoride powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0083] The negative electrode conductive material may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies the above range, a negative electrode conductive network is ensured, thereby improving the electrical conductivity of the negative electrode.

[0084] The negative electrode binder is a component that helps bind the negative electrode conductive material, negative electrode active material, and negative electrode current collector together. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

[0085] The negative electrode binder may be included in an amount of 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer. When the content of the negative electrode binder satisfies this range, the negative electrode active material particles are smoothly bound together, minimizing the problem of volume expansion of the negative electrode active material and allowing the negative electrode active material to be well attached to the negative electrode current collector.

[0086] On the other hand, when a metal is used without forming a negative electrode active material layer on the negative electrode, the negative electrode can be manufactured by physically bonding, rolling, or depositing a metal thin film on the negative electrode current collector or the metal thin film itself. The deposition method can be an electrochemical deposition method or a chemical vapor deposition method.

[0087] For example, the metal to be bonded / rolled / deposited onto the metal thin film itself or the negative electrode current collector may include one metal or an alloy of two metals selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0088] (3) Separator The separator can be any separator that is normally used as a separator in a lithium secondary battery, and is particularly preferred to have low resistance to ion migration of the electrolyte and excellent ability to retain moisture in the electrolyte solution.

[0089] For example, the separator may be a porous polymer film containing a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Also, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used as the separator.

[0090] The thickness of the separator can be 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 6 μm to 13 μm. When the thickness of the separator satisfies the above range, short-circuiting between the positive electrode and the negative electrode can be prevented and the cell resistance can be minimized. As a result, the life and output characteristics of the lithium secondary battery can be improved.

[0091] <Electrolytes> Meanwhile, the lithium secondary battery according to the present invention may include an electrolyte.

[0092] The electrolyte may include an organic solvent and a lithium salt commonly used in the art, and is not particularly limited.

[0093] The organic solvent may be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester-based solvents such as methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Carbonate-based solvents such as propylene carbonate (PC) can be used alone or in combination.

[0094] Preferably, the electrolyte can use a mixed solution of a carbonate-based solvent and an ester-based solvent as the organic solvent, and specifically, a mixed solution of a cyclic carbonate-based solvent and an ester-based solvent, or a mixed solution of a cyclic carbonate-based solvent, a linear carbonate-based solvent and an ester-based solvent can be used.

[0095] The ester solvent may be ethyl propionate (EP), the cyclic carbonate solvent may be at least one of ethylene carbonate and propylene carbonate, and the linear carbonate solvent may be at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0096] Ethyl propionate has a lower viscosity than conventional electrolyte components, and when it is included, it can have the effect of increasing the ionic conductivity of the electrolyte. The ethyl propionate may be included in an amount of 80 wt % or less, specifically 5 wt % to 80 wt %, more specifically 40 wt % to 70 wt %, based on the total weight of the electrolyte. When the content of ethyl propionate satisfies the above numerical range, the viscosity of the electrolyte can be optimized to achieve excellent ionic conductivity of the electrolyte.

[0097] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiN(SO2F)2, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt is preferably contained in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0098] The electrolyte according to the present invention may further include an additive to further improve the physical properties of the secondary battery.

[0099] Examples of such additives include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, nitrile-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0100] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinyl ethylene carbonate (VEC).

[0101] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0102] The nitrile compound may be, for example, succinonitrile, adiponitrile, hexanetricyanide, 1,4-dicyano-2-butene, or the like.

[0103] The sultone compound may be, for example, 1,3-propane sultone, 1,3-propene sultone, or the like.

[0104] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0105] The phosphate-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0106] The borate-based compound may be, for example, tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), or the like.

[0107] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0108] The lithium salt-based compound is a compound different from the lithium salt contained in the bis-electrolyte solution, and may be one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bis(oxalatoborate) (LiB(C2O4)2)), and LiBF4.

[0109] Meanwhile, the additives may be used alone or in combination of two or more.

[0110] The total amount of the additives may be 1 wt % to 25 wt %, preferably 1 wt % to 20 wt %, and more preferably 5 wt % to 20 wt %, based on the total weight of the electrolyte. When the additives are contained within this range, a stable coating is formed on the electrode, which can suppress ignition during overcharge, and can prevent side reactions from occurring during the initial activation process of the secondary battery, or the additives from remaining or being precipitated.

[0111] <Battery case> The lithium secondary battery according to the present invention may include a cylindrical, prismatic, pouch, or coin-type battery case, and preferably includes a pouch-type battery case.

[0112] The pouch-type battery case includes a barrier layer, a substrate layer formed on one side of the barrier layer, and a sealant layer formed on the other side of the barrier layer, and includes at least one cup portion recessed in one direction.

[0113] Specifically, the pouch-type battery case can be manufactured by a method in which a flexible pouch film laminate having a base layer, a barrier layer, and a sealant layer laminated in that order is inserted into a press molding device, and pressure is applied to a portion of the pouch film laminate with a punch to stretch it, thereby forming a cup portion having a recessed shape in one direction.

[0114] The base layer is disposed on the outermost layer of the pouch to protect the electrode assembly from external impact and to provide electrical insulation.

[0115] The substrate layer may be made of a polymer material, for example, one or more polymer materials selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark).

[0116] The substrate layer may have a single layer structure or a multi-layer structure in which different polymer films are laminated together. When the substrate layer has a multi-layer structure, an adhesive layer may be interposed between the polymer films.

[0117] Meanwhile, the substrate layer may have a total thickness of 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the substrate layer has a multilayer structure, the thickness includes the thickness of the adhesive layer. When the substrate layer satisfies the above range, it exhibits excellent durability, insulating properties, and moldability. If the thickness of the substrate layer is too thin, durability may be poor and the substrate layer may be damaged during the molding process. If the thickness of the substrate layer is too thick, moldability may be impaired, the overall thickness of the pouch may increase, the battery storage space may be reduced, and the energy density may be reduced.

[0118] The barrier layer is intended to ensure the mechanical strength of the pouch-type battery case, to block the entry and exit of gases or moisture outside the secondary battery, and to prevent leakage of the electrolyte.

[0119] The barrier layer may have a thickness of 40 μm to 100 μm, preferably 50 μm to 80 μm, and more preferably 60 μm to 80 μm. When the thickness of the barrier layer is within this range, moldability is improved, the molding depth of the cup portion is increased, and even during two-cup molding, cracks and / or pinholes are reduced, improving resistance to external stress after molding.

[0120] Meanwhile, the barrier layer may be made of a metal material, specifically, an aluminum alloy thin film.

[0121] The aluminum alloy thin film may contain aluminum and one or more metal elements other than aluminum, such as iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0122] The sealant layer is bonded by heat and pressure to seal the pouch, and is located as the innermost layer of the pouch film laminate.

[0123] The sealant layer is the surface that comes into contact with the electrolyte and the electrode assembly after the pouch is formed, and therefore must have insulating and corrosion resistance. It must also have high sealing properties because it must completely seal the interior and prevent material transfer between the inside and outside.

[0124] The sealant layer may be made of a polymer material, for example, one or more selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark). Among these, it is particularly preferable to use polypropylene (PP), which has excellent mechanical properties such as tensile strength, rigidity, surface hardness, abrasion resistance, and heat resistance, and chemical properties such as corrosion resistance.

[0125] The sealant layer may be a single layer structure or a multi-layer structure including two or more layers made of different polymer materials.

[0126] The sealant layer may have a total thickness of 60 μm to 100 μm, preferably 60 μm to 90 μm, and more preferably 70 μm to 90 μm. If the sealant layer is too thin, the seal durability and insulating properties may decrease, while if it is too thick, the flexibility may be poor, the total thickness of the pouch film laminate may increase, and the energy density per volume may decrease.

[0127] Meanwhile, the pouch film laminate may be manufactured by a method known in the art, for example, by attaching a substrate layer to the upper surface of a barrier layer via an adhesive, and forming a sealant layer on the lower surface of the barrier layer by coextrusion or an adhesive, but the method is not limited thereto.

[0128] The pouch-type battery case may be sealed with the electrode assembly housed therein such that a portion of the electrode lead, i.e., a terminal portion, is exposed. Specifically, the electrode lead may be connected to the electrode tab of the electrode assembly, an insulating portion may be formed on a portion of the electrode lead, the electrode assembly may be housed in a housing space provided in the cup portion, an electrolyte may be injected, and then the pouch-type battery case may be sealed.

[0129] The thickness of the electrode lead may be 0.05 mm to 0.5 mm, specifically 0.08 mm to 0.3 mm, and more specifically 0.1 mm to 0.2 mm. When the thickness of the electrode lead is in this range, which is thicker than conventional thicknesses, the resistance of the battery is reduced, and heat generation during an external short circuit is reduced, resulting in improved heat resistance.

[0130] When the lithium secondary battery according to the present invention is a pouch-type lithium secondary battery, it is lighter in weight than a cylindrical secondary battery, and the presence of many tabs reduces resistance, resulting in excellent output characteristics, making it suitable for use as a battery for power tools.

[0131] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely illustrative for facilitating understanding of the present invention and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present description, and it goes without saying that such changes and modifications are within the scope of the appended claims.

[0132] Examples and Comparative Examples Example 1 (Production of Lithium Secondary Battery) Hard carbon coated artificial graphite (average particle size D 50 =19μm, BET specific surface area=0.8m 2 The negative electrode slurry was prepared by mixing 100% cellulose acetate (CMC) and 100% styrene-butadiene rubber (SBR) in a weight ratio of 94.3:2.0:1.2:2.5, followed by the addition of distilled water. The solid content of the negative electrode slurry was 44 wt%.

[0133] The negative electrode slurry was applied to one surface of a 10 μm thick copper (Cu) metal thin film at 6.96 mAh / cm 2The slurry was applied in a loading amount of 1000 kJ / cm2 and then dried in a vacuum. The dried negative electrode slurry was then rolled and dried in a vacuum oven at 130°C for 12 hours, and then punched out to prepare a negative electrode.

[0134] As a positive electrode active material, the average particle size D 50 is 10μm Li[Ni 0.83 Co 0.11 Mn 0.06 ]O2, BET specific surface area is 254m 2 A positive electrode slurry was prepared by adding multi-walled carbon nanotubes (CNTs) (g), polyvinylidene fluoride (PVDF), and hydrogenated nitrile butadiene rubber (H-NBR) in a weight ratio of 97.0:1.0:1.5:0.5 to N-methylpyrrolidone (NMP) solvent and stirring. The solid content of the positive electrode slurry was 70 wt%.

[0135] The positive electrode slurry was applied to one surface of a 15 μm thick aluminum thin film at a rate of 10.2 mg / cm. 2 The positive electrode slurry was applied in a loading amount of 1000 and then dried in a vacuum at 130°C for 10 hours. The dried positive electrode slurry was rolled and dried in a vacuum oven at 130°C for 12 hours, and then punched out to prepare a positive electrode. The porosity of the positive electrode active material layer included in the positive electrode was 35%.

[0136] The negative and positive electrodes prepared as described above and a porous polyethylene separator having a thickness of 10 μm were stacked to prepare an electrode assembly.

[0137] An electrolyte was prepared by dissolving LiPF6 and LiFSi to a concentration of 0.7M in a solvent in which ethylene carbonate (EC) and ethyl propionate (EP) were mixed in a weight ratio of 35:65.

[0138] The electrode assembly was placed in a pouch-type battery case, the electrolyte was injected, and the case was sealed to manufacture a lithium secondary battery.

[0139] Example 2 (Production of Lithium Secondary Battery) An electrolyte was 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) were mixed in a weight ratio of 20:40:40 was used.

[0140] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the above electrolyte was used.

[0141] Example 3 (Production of Lithium Secondary Battery) As the positive electrode active material, Li[Ni 0.88 Co 0.07 Mn 0.05 ]O2 with a loading of 9.6 mg / cm 2 A positive electrode was produced in the same manner as in Example 1, except that:

[0142] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0143] Comparative Example 1 (Production of Lithium Secondary Battery) An electrolyte was prepared in the same manner as in Example 1, except that a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 30:70 was used.

[0144] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the above electrolyte was used.

[0145] Comparative Example 2 (Production of Lithium Secondary Battery) 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%.

[0146] An electrolyte was 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) were mixed in a weight ratio of 20:40:40 was used.

[0147] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode and electrolyte were used.

[0148] Comparative Example 3 (Production of Lithium Secondary Battery) 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%.

[0149] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0150] Comparative Example 4 (Production of Lithium Secondary Battery) 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%.

[0151] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode was used.

[0152] Comparative Example 5 (Production of Lithium Secondary Battery) An 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 in a weight ratio of 15:85 was used.

[0153] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the electrolyte was used.

[0154] 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 as shown in Table 1 below.

[0155] [Table 1]

[0156] Experimental example 1 - Continuous discharge test evaluation At a temperature of 23°C, each of the lithium secondary batteries of the Example and Comparative Examples was first fully charged to 4.2 V at 0.6 C. Then, using the PNE Cycle program (CTSMonPro, PNE SOLUTION CO., LTD.), discharge pulses were applied under the conditions shown in Table 2 below, with a rest period between each pulse, to perform a continuous discharge test on the lithium secondary batteries.

[0157] Graphs showing the voltage change of the lithium secondary battery over time during the continuous discharge test are shown in FIGS. 1 to 3. Specifically, FIG. 1 is a graph showing the voltage change of the battery over time when a continuous discharge test was performed on the lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Example 1. FIG. 2 is a graph showing the voltage change of the battery over time when a continuous discharge test was performed on the lithium secondary batteries manufactured in Example 2 and Comparative Example 2. FIG. 3 is a graph showing the voltage change of the battery over time when a continuous discharge test was performed on the lithium secondary batteries manufactured in Examples 1 and 3. As shown in FIGS. 1 to 3, when a discharge pulse was applied to the lithium secondary battery, a voltage drop occurred, and the voltage continued to decrease as the pulse was repeatedly applied. It was also confirmed that the voltage of the lithium secondary battery recovered when the application of the discharge pulse was terminated.

[0158] Meanwhile, in the continuous discharge test, the voltage of the battery was measured before and after the first to fifth pulse applications, and the results are shown in Table 3 below. Here, the voltage of the battery before each pulse application is V i and the battery voltage after each pulse is V f and the battery voltage V before each pulse application. i The battery voltage V after each pulse application f The ratio of V d The voltage retention rate V during 40C discharge, expressed as the following formula 1, is calculated from the measured voltage value. d-40 and the voltage maintenance rate V' during continuous discharge, which is expressed by the following formula 2: d、5 were calculated and are shown in Table 4 below.

[0159] [Formula 1] V d-40 (%)=(V f-40 / V i-40 ) x 100

[0160] In the formula 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.

[0161] [Formula 2] V' d、5 (%)=(V' f、5 / V' i ) x 100

[0162] In the formula 2, V' f、5 is the voltage of the lithium secondary battery after applying five consecutive discharge pulses, and V' i is the voltage of the lithium secondary battery fully charged before the discharge pulse was applied. During the continuous discharge, the discharge pulse was applied at a rate increasing by 4C, with the rate of the first discharge pulse being 24C and the rate of the final discharge pulse being 40C.

[0163] [Table 2]

[0164] [Table 3]

[0165] [Table 4]

[0166] As shown in Tables 3 and 4, in Examples 1 to 3, the voltage retention rate V of the lithium secondary battery at 40 C discharge d-40It can be seen that the voltage retention rate V' during continuous discharge of the lithium secondary batteries in Examples 1 to 3 is 74% or more, which is higher than those in Comparative Examples 1 to 5. d、5 It can be seen that the voltage drop of the battery is 72% or more, which is higher than that of Comparative Examples 1 to 5. This means that the voltage drop of the battery is smaller in Examples 1 to 3 during high-rate discharge and continuous discharge than in Comparative Examples 1 to 5.

Claims

1. A battery case and an electrode assembly and an electrolyte housed in the battery case, The voltage maintenance rate V during 40 C discharge is expressed by the following [Equation 1] ｄ－４０ is 74% or more, [Formula 1] V ｄ－４０ (%)=(V ｆ－４０ / V ｉ－４０ )×100 In the formula 1, V ｆ－４０ is the voltage of the lithium secondary battery after applying a discharge pulse at a rate of 40 C, and V ｉ－４０ is the voltage of the lithium secondary battery before applying a discharge pulse at a rate of 40C.

2. The voltage maintenance rate V ｄ－４０ 2. The lithium secondary battery according to claim 1, wherein the .lambda.

3. The voltage maintenance ratio V' during continuous discharge is expressed by the following [Equation 2] ｄ、５ is 72% or more, [Formula 2] V' ｄ、５ (%)=(V' ｆ、５ / V' ｉ )×100 In the formula 2, V' ｆ、５ is the voltage of the lithium secondary battery after applying a discharge pulse five times while increasing the rate by 4C, and V' ｉ 3. The lithium secondary battery according to claim 1, wherein V is the initial voltage of a fully charged lithium secondary battery before applying a discharge pulse.

4. The voltage maintenance ratio V' during continuous discharge ｄ、５ The lithium secondary battery according to claim 3, wherein the .lambda. / .lambda. ratio is 72% to 82%.

5. the electrode assembly includes a positive electrode, a separator, and a negative electrode; 2. The lithium secondary battery according to claim 1, wherein the positive electrode contains, as a positive electrode active material, a lithium nickel-based oxide containing 80 mol % or more of nickel relative to the total number of moles of transition metals other than lithium.

6. 6. The lithium secondary battery according to claim 5, wherein the positive electrode active material comprises a lithium nickel-based oxide containing 85 mol % to 90 mol % of nickel relative to the total number of moles of transition metals other than lithium.

7. 6. The lithium secondary battery of claim 5, wherein the lithium nickel-based oxide is a compound represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ｘ [Ni ａ Co ｂ M1 ｃ M2 ｄ ] １－ｘ O ２－ｙ X ｙ 6. The lithium secondary battery of claim 5, wherein, in Chemical Formula 1, 0.8≦a≦0.95, 0≦b≦0.2, 0≦c≦0.2, 0≦d≦0.1, a+b+c+d=1, 0≦x≦0.3, and 0≦y≦0.2; M1 is Mn, Al, or a combination thereof; M2 is one metal element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; and X is one or more elements selected from the group consisting of F, Cl, Br, I, At, P, and S.

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

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

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

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

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

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

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

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