Improved safety of lithium secondary batteries

Abstract

The present invention relates to a lithium secondary battery, which has an excellent energy density due to the positive electrode containing an NCM-based positive electrode active material with a high nickel (Ni) content, and an excellent effect of improving safety issues caused by the positive electrode active material at high temperatures by controlling the heat flow between the positive electrode active material and the electrolyte composition to a predetermined low level.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery having excellent energy density and improved high-temperature safety.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0080465 filed on June 22, 2023, and all the contents disclosed in the document of the Korean patent application are included as part of this specification.

Background Art

[0003] In recent years, secondary batteries have been widely applied not only to small devices such as portable electronic devices but also to medium and large-sized devices such as battery packs or power storage devices for hybrid vehicles and electric vehicles.

[0004] Such a secondary battery is manufactured by applying and drying a composition containing an electrode active material on a current collector with a suitable thickness and length, or by forming the electrode active material itself into a film shape to produce a positive electrode and a negative electrode, winding or laminating them together with a separator, which is an insulator, in between to form an electrode assembly, and then placing it in a can or a similar container and injecting an electrolyte.

[0005] To apply such a secondary battery to medium and large-sized devices, a high energy density is required. Therefore, by using a layered lithium nickel metal oxide such as LiNi a Co b Mn c O2 (0.6 < a ≤ 0.9, a + b + c = 1, hereinafter referred to as "NCM-based compound") as a positive electrode active material, high capacity is realized.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007] The object of the present invention is to provide a lithium secondary battery that not only has excellent energy density by including an NCM-based positive electrode active material with a high content of nickel (Ni) in the positive electrode, but also has improved safety by suppressing structural collapse of the NCM-based active material at high temperatures or improving gas generation due to structural collapse. [Means for solving the problem]

[0008] To solve the above problem, In one embodiment, the present invention comprises: an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; the electrode assembly includes an electrolyte composition impregnated therein; The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and containing a lithium metal oxide represented by the following chemical formula 1 as a positive electrode active material: The electrolyte composition includes a lithium salt and a non-aqueous organic solvent containing 60 wt % or more of a cyclic ester solvent represented by the following Chemical Formula 2 based on the total weight: Provided is a lithium secondary battery that exhibits a heat flow rate of 6.0 W / g or less within a temperature range of 200°C to 300°C when measuring the heat flow rate of a mixture containing a positive electrode active material and an electrolyte composition in a 1:1 weight ratio at a 100% state of charge:

[0009] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0010] [ka]

[0011] In the above Chemical Formula 1 and Chemical Formula 2, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are 0.9≦x≦1.30, 0.6≦y<1, 0 <z≦0.2、0<w≦0.2、0≦v≦0.1であり、かつ、y+z+w+v=1であり、 [ka] is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5.

[0012] At this time, the positive electrode active material is LiNi 0.95 Co 0.03 Mn 0.02 O2, LiNi 0.9 Co 0.6 Mn 0.4 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.85 Co 0.1 Mn 0.05 O2, LiNi 0.85 Co 0.05 Mn 0.1 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.75 Co 0.2 Mn 0.15 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.15 Mn 0.15 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.6 Co 0.2 Mn0.2 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 It may contain one or more of O2.

[0013] The cyclic ester solvent represented by Chemical Formula 2 above may include one or more of dihydrofuranone, vinyldihydrofuranone, fluorodihydrofuranone, furanone, tetrahydropyranone, methyldihydrofuranone, propyltetrahydropyranone, and oxepanone.

[0014] The above lithium salts contain Li as the cation. + and as an anion, BF4 - , B 10 Cl 10 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , BF2C2O4 - , B(C2O4)2 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO -, (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , (CF3CF2SO2)2N - and ((C(CN))2NC(CF3))N - It may contain one or more of the following.

[0015] As an example, the lithium salt has Li as the cation. + Contains BF2C2O4 as an anion - , B(C2O4)2 - , (CF3SO2)2N - , (FSO2)2N - , (CF3CF2SO2)2N - , or ((C(CN))2NC(CF3))N - may include:

[0016] As another example, the lithium salt may have one Li as the cation. + Contains BF4 as an anion - , B 10 Cl 10 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO -, (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - and SCN - It may contain two or more of the above.

[0017] In addition, the non-aqueous organic solvent may further contain one or more auxiliary solvents selected from the group consisting of fluorine-containing ether solvents, fluorine-containing cyclic carbonate solvents, linear carbonate solvents, phosphate solvents, and sulfone solvents, together with the ester solvent.

[0018] The electrolyte composition may further include an electrolyte additive represented by the following Chemical Formula 3:

[0019] [ka]

[0020] In the above chemical formula 3, R1 is [ka] [ka] or [ka] and R1' and R1'' are each hydrogen or a methyl group; R2 is an arylene group having 6 to 20 carbon atoms, an aryleneoxy group having 6 to 20 carbon atoms, a heteroarylene group having 5 to 10 carbon atoms containing one or more heteroatoms selected from N, S, and O, a heteroaryleneoxy group having 5 to 10 carbon atoms containing one or more heteroatoms selected from N, S, and O, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, and [ka] Includes one or more of the following: R3 is a fluoro group, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or [ka] and the alkyl group, alkoxy group, cycloalkyl group, [ka] and [ka] One or more of the hydrogen atoms contained in may be substituted with a fluorine atom, M includes at least one selected from the group consisting of lithium, sodium, potassium, tetraalkylammonium having 1 to 4 carbon atoms, and tetraalkylphosphonium having 1 to 4 carbon atoms; l is an integer from 1 to 6; m and n are each an integer of 2 to 20.

[0021] Specifically, the electrolyte additive represented by Chemical Formula 3 may include one or more compounds selected from the following <Structural Formula 1> to <Structural Formula 17>: [ka]

[0022] In this case, the electrolyte additive may be contained in an amount of 5 wt % or less based on the total weight of the electrolyte composition.

[0023] On the other hand, the negative electrode includes a negative electrode active layer provided on at least one surface of a negative electrode current collector and containing a carbon-based negative electrode active material and a silicon-based negative electrode active material; The carbon-based negative electrode active material may contain, as a carbon material, one or more of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, acetylene black, and ketjen black.

[0024] The silicon-based negative electrode active material includes silicon (Si), silicon carbide (SiC), and silicon oxide (SiO q , where 0.8≦q≦2.5).

[0025] The silicon-based negative electrode active material may be contained in an amount of 0.1 wt % to 30 wt % based on the total weight of the negative electrode active material.

[0026] Furthermore, in one embodiment, the present invention provides preparing an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; A method for manufacturing a lithium secondary battery, comprising the step of impregnating the electrode assembly with an electrolyte composition, The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and containing a lithium metal oxide represented by the following chemical formula 1 as a positive electrode active material: The electrolyte composition includes a lithium salt and a non-aqueous organic solvent, and the non-aqueous organic solvent contains a cyclic ester solvent represented by the following chemical formula 2 in an amount of 60 wt % or more based on the total weight of the non-aqueous organic solvent: Provided is a method for producing a lithium secondary battery that, when heat flow is measured for a mixture containing a positive electrode active material and an electrolyte composition in a 1:1 weight ratio at a 100% state of charge, exhibits a heat flow of 6.0 W / g or less within the range of 200°C to 300°C:

[0027] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0028] [ka]

[0029] In the above Chemical Formula 1 and Chemical Formula 2, M 1is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are 0.9≦x≦1.30, 0.6≦y<1, 0 <z≦0.2、0<w≦0.2、0≦v≦0.1であり、かつ、y+z+w+v=1であり、 [ka] is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5. [Effects of the Invention]

[0030] The lithium secondary battery according to the present invention has an excellent energy density due to the positive electrode containing an NCM-based positive electrode active material having a high nickel (Ni) content, and has an excellent effect of improving safety issues caused by the positive electrode active material at high temperatures by controlling the heat flow rate between the positive electrode active material and the electrolyte composition to a low level within a predetermined range at high temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0031] Because the present invention is susceptible to various modifications and embodiments, specific embodiments will be described in detail.

[0032] However, this is not intended to limit the invention to any particular embodiment, but rather to be understood as including all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0033] In the present invention, terms such as "comprise" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts or combinations thereof described in the specification, and may be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0034] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly under" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0035] Additionally, in the present invention, "comprising as a main component" may mean containing 50 wt% or more (or 50 vol% or more), 60 wt% or more (or 60 vol% or more), 70 wt% or more (or 70 vol% or more), 80 wt% or more (or 80 vol% or more), 90 wt% or more (or 90 vol% or more), or 95 wt% or more (or 95 vol% or more) of a defined component relative to the total weight (or volume). For example, "comprising a carbon-based negative electrode active material as a main component as a negative electrode active material" may mean containing 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, or 95 wt% or more of the carbon-based negative electrode active material relative to the total weight of the negative electrode active material. In some cases, it may mean that the entire negative electrode active material is composed of the carbon-based negative electrode active material, accounting for 100 wt%.

[0036] Lithium secondary batteries are used as power sources for a variety of electronic devices, including electric vehicles, due to their high energy density and excellent output performance. To be used as a power source for such a variety of electronic devices, they must have electrochemical properties at high temperatures. Since the electrodes of lithium secondary batteries undergo an exothermic reaction with the electrolyte when exposed to high temperatures, it is important that they have thermal stability.

[0037] NCM-based compounds are considered the most promising candidate to replace cathode active materials such as LiCoO2 due to their overall excellent properties, including high capacity, high rate characteristics, and excellent stability, as well as their low cost and applicability to small, medium, and large batteries. However, NCM-based compounds have drawbacks such as low ionic conductivity, low cycle performance, and tap density, as well as a complicated manufacturing process. Increasing the proportion of nickel (Ni) to achieve high capacity results in rate-limiting characteristics and a decrease in capacity during charge and discharge.

[0038] The two main causes of the decrease in electrochemical properties due to the increase in nickel (Ni) content are side reactions and structural degradation. Of these, side reactions occur when lithium salts present on the particle surface of NCM-based compounds used as the positive electrode active material react with the battery electrolyte to form non-conductive materials on the surface of the NCM-based compounds, or when Ni 4+ This refers to the reaction in which ions accelerate the decomposition of the electrolyte and form a thick solid-state electrolyte interface (SEI). 2+ and Li + This refers to a structural change (or collapse) of the layered structure caused by a cation exchange reaction (cation mixing) and oxygen loss due to the removal of oxygen radicals from the lattice. This structural collapse hinders the movement of Li ions during charging and discharging, limiting the electrochemical properties. In particular, when the structure of NCM-based active materials collapses at high temperatures, electrolyte decomposition occurs at the collapsed sites. Carbon (CO or CO2) oxidized during the electrolyte decomposition process combines with LiOH and other materials to produce water molecules. The water molecules thus generated react with HF and LiF, rapidly generating large amounts of gas inside the battery and causing the battery to explode, thereby reducing battery safety.

[0039] To solve these problems, a surface coating technique for NCM-based compounds has been developed. However, this surface coating requires an additional calcination process after the preparation of the NCM-based active material, which increases production costs, makes it difficult to obtain a uniform coating layer, and makes it difficult to effectively prevent structural collapse.

[0040] Therefore, there is a need to develop a technology that can improve the safety of secondary batteries by applying NCM-based active materials with a high nickel (Ni) content, thereby achieving excellent energy density and preventing structural collapse of NCM-based active materials at high temperatures.

[0041] The lithium secondary battery according to the present invention has excellent energy density due to the inclusion of an NCM-based positive electrode active material with a high nickel (Ni) content in the positive electrode, and also has the advantage of improving safety issues due to the positive electrode active material at high temperatures by suitably adjusting the electrolyte material to reduce the heat flow between the positive electrode active material and the electrolyte composition within a predetermined range at high temperatures.

[0042] The present invention will now be described in more detail.

[0043] <Lithium secondary battery>

[0044] In one embodiment, the present invention comprises: an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and an electrolyte composition impregnated into the electrode assembly; The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and containing a lithium metal oxide represented by the following chemical formula 1 as a positive electrode active material: Provided is a lithium secondary battery that exhibits a heat flow of 6.0 W / g or less in the range of 200°C to 300°C when measuring the heat flow of a mixture containing a positive electrode active material and an electrolyte composition in a 1:1 weight ratio at a 100% state of charge:

[0045] [Chemical formula 1] Lix [Ni y Co z Mn w M 1 v ]O2

[0046] In the above chemical formula 1, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are 0.9≦x≦1.30, 0.6≦y<1, 0 <z≦0.2、0<w≦0.2、0≦v≦0.1であり、かつ、y+z+w+v=1である。

[0047] The lithium secondary battery according to the present invention includes an electrode assembly having a structure in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked and a separator is disposed between the electrodes, and an electrolyte composition impregnated into the electrode assembly.

[0048] Here, the positive electrode included in the electrode assembly includes a positive electrode active layer on at least one surface of a positive electrode current collector, and the positive electrode active layer is a material that can realize electrical activity by electrochemically reacting on the positive electrode current collector, and contains, as a main component, lithium metal oxide represented by Chemical Formula 1, which is capable of reversibly intercalating and deintercalating lithium ions.

[0049] The lithium metal oxide represented by Chemical Formula 1 is an oxide in which lithium (Li) and transition metals nickel (Ni), cobalt (Co) and manganese (Mn) are mixed, and is characterized in that the content of nickel (Ni) is 60% or more (i.e., 60 mol% or more) of the total mole fraction of the transition metals.

[0050] The ternary NCM-based cathode active material, which is mainly composed of nickel (Ni), cobalt (Co), and manganese (Mn), has the advantages of high capacity of LiNiO2 (LNO), excellent electrochemical performance of LiCoO2 (LCO), and stability of LiMn2O4 (LMO). However, despite these advantages, as the content of nickel (Ni) increases, the Li content decreases during the synthesis process in a high-temperature environment and / or during charge and discharge. + Ni, similar in size to (0.76Å) 2+ Cation mixing occurs, which is a phenomenon in which the lithium layer (0.69Å) occupies the layered structure. 2+ This results in the formation of a locally electrochemically inert Ni-O rock salt structure, which hinders the movement of lithium ions and adversely affects electrochemical performance such as high-rate characteristics and discharge capacity. Furthermore, due to this structural instability, when a secondary battery is charged and discharged under high-temperature conditions, water molecules are generated as the electrolyte decomposes, rapidly generating a large amount of gas inside the battery, limiting the high-temperature safety of the secondary battery.

[0051] However, the present invention can solve this problem by including a nickel metal oxide represented by Chemical Formula 1 as a positive electrode active material, in which the content of nickel (Ni) is 60% or more (i.e., 60 mol% or more) relative to the total molar fraction of transition metals, and by reducing the heat flow rate between the positive electrode active material and the electrolyte composition at high temperatures to a predetermined range.

[0052] "Heat flow" refers to the amount of heat flowing per unit weight and may particularly indicate the degree of heat generation in a secondary battery. The electrolyte composition according to the present invention may have high thermal stability due to a difference in the reaction between the positive electrode active material and the electrolyte composition under high-temperature conditions, as compared to conventional electrolyte compositions, as a result of a thermal analysis that simulates the degradation phenomenon in which a secondary battery spontaneously generates heat due to exposure to high temperatures or external mechanical factors, resulting in a decrease in performance. Specifically, the heat flow measured according to the thermal analysis results indicates the degree of heat (e.g., the amount of heat) generated by the high-temperature reaction between the electrolyte composition and the charged positive electrode active material when simulating the high-temperature degradation phenomenon of a secondary battery.

[0053] The heat flow rate can be increased or decreased depending on the type and content ratio of each component constituting the electrolyte composition, and can also be increased or decreased depending on whether an inorganic positive electrode film containing lithium ions is formed on the surface of the positive electrode active layer. The reduction in the heat flow rate means that the amount of heat generated between the positive electrode active material and the electrolyte composition is reduced. As a result, the positive electrode active material contained in the secondary battery under high temperature conditions can be reduced by: i) Ni 2+ and Li + 1) the cation exchange reaction (cation mixing) and 2) the structural change (or collapse) of the layered structure due to oxygen loss caused by the departure of oxygen radicals from the lattice can be delayed.

[0054] In this case, the heat flow rate between the positive electrode active material and the electrolyte composition according to the present invention may be 6.0 W / g or less within a range of 200°C to 300°C. More specifically, the upper limit of the heat flow rate may be 5.5 W / g or less, 5.0 W / g or less, 4.0 W / g or less, or 3.0 W / g or less, and the lower limit may be greater than 0 W / g, 0.5 W / g or more, 0.8 W / g or more, 1.0 W / g or more, or 2.0 W / g or more. As one example, the heat flow rate between the positive electrode active material and the electrolyte composition may be 0.5 W / g to 6.0 W / g, 0.5 W / g to 5 W / g, 0.5 W / g to 4.5 W / g, 0.5 W / g to 4.0 W / g, or 0.5 W / g to 3.5 W / g within a range of 200°C to 300°C. Here, the heat flow rate may be measured under predetermined conditions. Each material has its own thermal conductivity constant, and the positive electrode active material and the electrolyte composition may also have different thermal conductivity constants. Due to this difference, the amount of heat flow transferred between the positive electrode active material and the electrolyte composition may vary depending on the temperature conditions to which they are exposed. That is, the heat flow between the positive electrode active material and the electrolyte composition may vary depending on the temperature conditions to which they are exposed. Therefore, the heat flow according to the present invention may be measured under predetermined measurement conditions, specifically, under a heating rate.

[0055] As an example, the heat flow between the positive electrode active material and the electrolyte composition can be measured by mixing the positive electrode active material charged to 100% SOC at 4.25 V with the electrolyte composition in a 1:1 weight ratio, and then using a differential scanning calorimeter (DSC) at a temperature increase rate of 10±1°C / min in the range of 100°C to 400°C.

[0056] The positive electrode active material can be applied without any particular limitation as long as it is a lithium metal oxide represented by Chemical Formula 1. Specifically, LiNi 0.95 Co 0.03 Mn 0.02 O2, LiNi 0.9 Co 0.6 Mn 0.4 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.85 Co 0.1 Mn 0.05 O2, LiNi 0.85 Co 0.05 Mn 0.1 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.75 Co 0.2 Mn 0.15 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.15 Mn 0.15 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2, and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 O2.

[0057] The electrolyte composition includes a lithium salt as a liquid electrolyte and a non-aqueous organic solvent, and the composition of the non-aqueous organic solvent may be adjusted so that the lithium salt and the non-aqueous organic solvent contain specific components to minimize the transfer of thermal energy from outside the secondary battery to the positive electrode active material inside the secondary battery.

[0058] Specifically, the non-aqueous organic solvent may contain, as a main component, a cyclic ester solvent represented by the following chemical formula 2:

[0059] [ka]

[0060] In the above chemical formula 2, [ka] is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5.

[0061] Specifically, the cyclic ester solvent represented by the above chemical formula 2 may include one or more of the ester cyclic compounds shown below: [ka]

[0062] Generally, cyclic ester solvents such as dihydrofuranone have high solubility for lithium salts, but lithium bis(oxalato)borate (LiBOB, cation: Li + , anion: BF2C2O4 -When used in combination with boron-based lithium salts or imide-based lithium salts such as cyclic ester-based cations, the cycle characteristics of the secondary battery deteriorate, limiting its application to lithium secondary batteries. However, the present invention includes an excess amount of a cyclic ester-based solvent represented by Chemical Formula 2 as a nonaqueous solution of the electrolyte composition, thereby minimizing the transfer of external heat to the positive electrode active material when the lithium secondary battery is exposed to high temperatures, and thus has the advantage of being excellent in improving the high-temperature safety of the lithium secondary battery.

[0063] The cyclic ester solvent may be included in an amount of 60 wt% or more, more specifically, 70 wt% or more, 75 wt% or more, 80 wt% or more, 85 wt% or more, 90 wt% or more, 60 wt% to 99 wt%, 70 wt% to 99 wt%, 80 wt% to 99 wt%, 90 wt% to 99 wt%, 95 wt% to 99 wt%, 60 wt% to 80 wt%, or 70 wt% to 90 wt%. In some cases, the cyclic ester solvent may be used as the nonaqueous solvent itself and included in an amount of 100 wt% based on the total weight of the nonaqueous solvent. By adjusting the content of the cyclic ester solvent in the nonaqueous solvent as described above, the present invention can prevent the problem of insufficient reduction in heat transfer to the positive electrode active material when a lithium secondary battery is exposed to high temperatures, even when the content is less than 60 wt%. In addition, since the cyclic ester-based solvent has excellent heat resistance, when its content in the non-aqueous organic solvent is adjusted to the above range, the electrolyte composition is less likely to be decomposed at high temperatures, and when combined with the predetermined lithium salt according to the present invention, the amount of heat transferred to the positive electrode active material can be minimized.

[0064] The electrolyte composition may also contain a specific lithium salt to reduce the amount of heat transferred to the positive electrode active material. Specifically, the lithium salt contains Li as a cation. + Contains BF4 as an anion - , B 10 Cl 10 - , ClO4 - , AlO4 - , AlCl4- , PF6 - , SbF6 - , AsF6 - , BF2C2O4 - , B(C2O4)2 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , (CF3CF2SO2)2N - and ((C(CN))2NC(CF3))N - It may contain one or more of the following.

[0065] Lithium salts essentially impart conductivity to the organic solvent constituting the electrolyte while inducing passivation of the negative electrode by forming an SEI. In the present invention, the lithium salt is dissolved in a cyclic ester-based solvent and decomposed on the positive electrode surface when the lithium secondary battery is exposed to high temperatures, thereby delaying structural collapse of the positive electrode active material. For this reason, the lithium salt used in the present invention may selectively include the lithium salts described above.

[0066] As an example, the lithium salt has Li as the cation. + Contains BF2C2O4 as an anion - , BC4O8 -, (CF3SO2)2N - , (FSO2)2N - , (CF3CF2SO2)2N - , or ((C(CN))2NC(CF3))N - In this case, the lithium salt may contain a borate group or a sulfonamide group in the anion, which may enhance the heat resistance of the electrolyte composition containing the lithium salt. In addition, the lithium salt may contain a borate group or a sulfonamide group, which may form a protective film upon reaction between the positive electrode active material and the electrolyte composition, thereby delaying the structural collapse of the positive electrode and significantly reducing the heat flow.

[0067] As another example, the lithium salt may have one Li as the cation. + Contains BF4 as an anion - , B 10 Cl 10 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - and SCN- In this case, the lithium salt may be applied in the form of a basalt in which different types of salts are used in combination, thereby exhibiting a synergistic effect that significantly enhances the heat flow reduction effect of the electrolyte composition, which was only slight when applied as a single component.

[0068] For example, PF6 as an anion upon dissociation - LiPF6, which represents LiBF, is a lithium salt commonly used in electrolyte compositions for lithium secondary batteries and is known for its ability to impart high conductivity to carbonate-based electrolytes. However, when LiPF6 is used alone, it decomposes to PF5 at temperatures above 200°C when exposed to high temperatures. This is highly unstable and removes the cathode film formed on the surface of the cathode active layer and organic materials such as solvents and the cathode active material, thereby reducing the high-temperature durability of the cathode active material. However, LiBF4 decomposes to BF3 at temperatures above 300°C, which has a higher decomposition temperature than the decomposition temperature of the cathode active material. That is, LiBF4 does not damage or remove the cathode film formed on the surface of the cathode active layer inside the secondary battery when exposed to high temperatures, thereby maintaining the high-temperature durability of the cathode active material. However, LiBF4 only has a limited effect on increasing the conductivity of the electrolyte composition, and when an electrolyte containing LiBF4 is used in a secondary battery, it can reduce the performance of the anode. Although LiPF6 and LiBF4 have limitations when used alone in an electrolyte composition, when they are used together with a lithium salt in the form of a bi-salt, the poor high-temperature stability of LiPF6 can be significantly improved, and the heat flow rate at which the electrolyte composition transfers heat to the positive electrode active material can be significantly reduced.

[0069] When the lithium salts used in combination are a first lithium salt and a second lithium salt, the mixing ratio of the first lithium salt to the second lithium salt may be 1:0.5 to 2.0, specifically 1:0.5 to 1.5 or 1:0.8 to 1.2, based on the molar concentration. By adjusting the ratio of the first lithium salt to the second lithium salt forming the bi-salt as described above, the present invention can maximize the effect of reducing the heat flow of the electrolyte composition while minimizing the disadvantages of each lithium salt.

[0070] The concentrations of these lithium salts are not particularly limited, but a preferred concentration is 0.5 mol / L or more, specifically 0.7 mol / L or more, more specifically 0.9 mol / L or more, with an upper limit of 2.5 mol / L or less, specifically 2.0 mol / L or less, more specifically 1.5 mol / L or less. If the lithium salt concentration is below 0.5 mol / L, the ionic conductivity decreases, which may result in a decrease in the cycle characteristics and output performance of the nonaqueous electrolyte battery. Furthermore, if the lithium salt concentration exceeds 2.5 mol / L, the viscosity of the electrolyte for the nonaqueous electrolyte battery increases, which may also result in a decrease in ionic conductivity, which may result in a decrease in the cycle characteristics and output performance of the nonaqueous electrolyte battery.

[0071] Furthermore, when a large amount of lithium salt is dissolved in a non-aqueous organic solvent at once, the liquid temperature may rise due to the heat of dissolution of the lithium salt. If the temperature of the non-aqueous organic solvent rises significantly due to the heat of dissolution of the lithium salt, decomposition of the fluorine-containing lithium salt may be accelerated, resulting in the production of hydrogen fluoride (HF). Hydrogen fluoride (HF) is undesirable because it can cause deterioration of battery performance. Therefore, the temperature at which the lithium salt is dissolved in the non-aqueous organic solvent is not particularly limited, but may be adjusted to −20° C. to 80° C., specifically, 0° C. to 60° C.

[0072] In addition, the non-aqueous organic solvent may further contain one or more auxiliary solvents selected from the group consisting of fluorine-containing ether solvents, fluorine-containing cyclic carbonate solvents, linear carbonate solvents, phosphate solvents, and sulfone solvents, in addition to the ester solvent.

[0073] Specifically, the fluorine-containing ether solvent may be 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the fluorine-containing cyclic carbonate solvent may be fluoroethylene carbonate (FEC). The linear carbonate solvent may be ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and the like. The phosphate solvent may be trimethyl phosphate (TMP), and the sulfone solvent may be sulfolane, methyl sulfolane, dimethyl sulfoxide, and the like. In some cases, a cyclic carbonate solvent such as ethylene carbonate (EC) may be further included in addition to the non-aqueous organic solvent.

[0074] The co-solvent can improve the low-temperature performance of the electrolyte composition and prevent gas generation due to side reactions induced in the electrolyte composition when exposed to high temperatures. Therefore, from the viewpoints of electrochemical stability against oxidation-reduction and chemical stability against reactions with heat and solutes, the co-solvent may be used singly with the ester-based solvent, or two or more co-solvents may be used in any combination with the ester-based solvent depending on the application.

[0075] The cosolvent may be mixed with the ester solvent at a certain volume ratio. Specifically, the cosolvent may be contained in an amount of less than 40% by volume based on the total volume of the nonaqueous organic solvent, and more specifically, the cosolvent may be contained in an amount of 30% by volume or less, 20% by volume or less, 1% to 30% by volume, 1% to 20% by volume, 1% to 10% by volume, 5% to 15% by volume, 5% to 20% by volume, 10% to 20% by volume, 20% to 30% by volume, 1% to 10% by volume, or 0.5% to 5% by volume based on the total volume of the nonaqueous organic solvent.

[0076] By adjusting the content of the co-solvent in the total non-aqueous organic solvent to the above ratio, the present invention can maintain high compatibility between the ester-based solvent and the co-solvent, and at the same time, can improve the charge mobility and / or ion mobility of the battery, thereby improving the performance of the battery.

[0077] In addition, the non-aqueous organic solvent may be further mixed with an organic solvent commonly used in non-aqueous electrolytes in the art to reduce the viscosity of the electrolyte composition at 25°C and improve the compatibility of the ester-based solvent represented by Chemical Formula 2 with the co-solvent. In this case, the content of the organic solvent may be less than 10 wt% based on the total weight of the non-aqueous organic solvent in the electrolyte composition. Examples of miscible non-aqueous organic solvents include aprotic organic solvents such as N-methyl-2-pyrrolidinone, ethylene carbonate (EC), propylene carbonate, butylene carbonate, 1,2-dimethoxyethane (DME), tetrahydrofuran, 2-methyltetrahydrofuran, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, 1,3-dimethyl-2-imidazolidinone, ether, methyl propionate, and ethyl propionate.

[0078] The electrolyte composition may further contain additives in addition to the basic components described above. Additives generally used in the non-aqueous electrolyte solution of the present invention may be added in any proportion as long as the gist of the present invention is not impaired.

[0079] Specifically, the electrolyte composition may further include an electrolyte additive represented by the following Chemical Formula 3, together with the non-aqueous organic solvent and the lithium salt:

[0080] [ka]

[0081] In the above chemical formula 3, R1 is [ka] [ka] or [ka] and R1' and R1'' are each hydrogen or a methyl group; R2 is an arylene group having 6 to 20 carbon atoms, an aryleneoxy group having 6 to 20 carbon atoms, a heteroarylene group having 5 to 10 carbon atoms containing one or more heteroatoms selected from N, S, and O, a heteroaryleneoxy group having 5 to 10 carbon atoms containing one or more heteroatoms selected from N, S, and O, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, and [ka] Includes one or more of the following: R3 is a fluoro group, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or [ka] and the alkyl group, alkoxy group, cycloalkyl group, [ka] and [ka] One or more of the hydrogen atoms contained in may be substituted with a fluorine atom, M includes at least one selected from the group consisting of lithium, sodium, potassium, tetraalkylammonium having 1 to 4 carbon atoms, and tetraalkylphosphonium having 1 to 4 carbon atoms; l is an integer from 1 to 6; m and n are each an integer of 2 to 20.

[0082] The electrolyte additive includes an ionic compound having a mother nucleus in which a vinyl group, a (meth)acrylate group, or an acrylamide group is bonded to one side of a sulfonylimide group as represented by Chemical Formula 3, either i) a structure containing a saturated hydrocarbon group or an oxygen atom introduced into the saturated hydrocarbon group, or ii) a structure in which a conjugation of the sulfonylimide group is extended or a structure having a polyglycol unit is used.

[0083] The electrolyte additive has a chemical structure as shown in Chemical Formula 3, and thus can uniformly form organic and / or inorganic coatings on the surfaces of the positive and negative electrodes upon activation of the secondary battery. That is, the electrolyte additive can be directly incorporated in a monomolecular form into the organic and / or inorganic coatings formed on the surfaces of the positive and / or negative electrodes upon activation of the secondary battery. As a result, the electrolyte additive can suppress side reactions between the positive and / or negative electrodes and the electrolyte composition, thereby improving the electrical performance of secondary batteries containing the electrolyte additive.

[0084] For this purpose, in the compound represented by the above chemical formula 1, R2 is an ethylene group, a propylene group, a cyclohexylene group, a phenylene group, an oxymethylene group, a pyrrole group, an oxyphenylene group, an oxynaphthalene group, an oxypyrrole group, an oxythiophenylene group, an oxyfuranyl group, or [ka] and R3 is a fluoro group, a methyl group, a fluoromethyl group, a methoxy group, a fluoromethoxy group, or [ka] and the above [ka] One or more of the hydrogen atoms contained in may be substituted with a fluorine atom, M is lithium, l is an integer of 1 or 2, and m can be an integer of 2-10.

[0085] As an example, the compound represented by Chemical Formula 1 above may be one or more of the compounds represented by the following <Structural Formula 1> to <Structural Formula 17>: [ka]

[0086] A positive electrode active material containing the lithium metal oxide represented by Chemical Formula 1 exhibits a higher potential during charge and discharge compared to a positive electrode active material such as iron phosphate (LiFePO4). In this regard, when the electrolyte additives represented by <Structural Formulas 1> to <Structural Formula 17> are applied to a lithium secondary battery together with a positive electrode active material such as the lithium metal oxide represented by Chemical Formula 1, they can uniformly form organic and inorganic coating layers on the surfaces of the positive and negative electrodes, respectively, due to the high charge and discharge potential of the positive electrode active material during the activation process of the secondary battery. The organic and inorganic coating layers thus formed can effectively suppress side reactions that occur in the electrolyte composition when the secondary battery is exposed to high temperatures. Furthermore, the electrolyte additives can easily increase the oxidation potential window of the electrolyte composition, thereby preventing decomposition of the electrolyte composition during charge and discharge of the secondary battery.

[0087] As one example, the electrolyte composition according to the present invention may have an oxidation potential window of 4.5 V or more. More specifically, the oxidation potential window of the electrolyte composition may be within a range of 4.5 V to 7.0 V, 5.0 V to 6.5 V, 5.0 V to 6.0 V, 5.3 V to 6.5 V, or 5.5 V to 6.0 V.

[0088] The oxidation potential window is the voltage range in which no additional electrochemical reactions occur inside the secondary battery other than the electrochemical oxidation-reduction that occurs between the positive electrode and the negative electrode. Here, the oxidation potential window is defined as the voltage range in which no additional electrochemical reactions occur inside the secondary battery other than the electrochemical oxidation-reduction that occurs between the positive electrode and the negative electrode. +In the present invention, when the oxidation potential window of the electrolyte is outside the above range, the potential window narrows, and therefore the nonaqueous electrolyte itself undergoes electrolysis during battery charge and discharge, shortening the life of the lithium secondary battery and causing safety issues due to the gas generated. Furthermore, when the electrolyte composition according to the present invention satisfies the above-mentioned oxidation potential window range or higher, it is stable against the potential applied during charge and discharge of the secondary battery, thereby not only improving the life of the battery but also reducing the risk of explosion and the like.

[0089] The electrolyte additive may be included in the electrolyte composition at a specific content. Specifically, the electrolyte additive including the compound represented by Formula 1 may be included in an amount of 5 wt % or less, more specifically, 0.01 wt % to 5 wt %, based on the total weight of the electrolyte composition. More specifically, the electrolyte additive may be included in an amount of 0.05 wt % to 3 wt %, or 1.0 wt % to 2.5 wt %, based on the total weight of the electrolyte composition. The present invention prevents the use of an excessive amount of electrolyte additive outside the above range, which increases the viscosity of the electrolyte composition and reduces the wettability of the electrodes and separator, while also preventing a decrease in the ionic conductivity of the electrolyte composition and a decrease in battery performance. The present invention also prevents the use of a small amount of electrolyte additive outside the above range, which results in a slight realization of the additive's effects.

[0090] The lithium secondary battery according to the present invention includes the electrolyte composition having the above-described configuration, and therefore can exhibit excellent high-temperature safety even when it includes a lithium metal oxide having a nickel (Ni) content of 60% or more in total molar fraction as a positive electrode active material of the positive electrode.

[0091] Each component of the lithium secondary battery will now be described in more detail.

[0092] The electrode assembly included in the lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator.

[0093] The positive electrode includes a positive electrode active layer prepared by applying a positive electrode slurry containing a positive electrode active material to at least one surface of a positive electrode current collector, drying the slurry, and pressing the slurry.

[0094] The positive electrode active material contained in the positive electrode active layer includes a lithium metal oxide represented by Chemical Formula 1, in which the content of nickel (Ni) is 60% or more of the total molar fraction, as described above, and has the advantage of being able to stably supply high-capacity and / or high-voltage electricity.

[0095] The positive electrode active material may be included in an amount of 85 parts by weight or more, specifically 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more, based on the weight of the entire positive electrode active layer.

[0096] The positive electrode active layer may further contain a conductive material, a binder, other additives, and the like, together with the positive electrode active material, as needed.

[0097] The conductive material is used to improve the electrical performance of the positive electrode and may be one commonly used in the art, specifically, one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, denka black, ketjen black, Super P, channel black, furnace black, lamp black, thermal black, graphene, and carbon nanotubes.

[0098] The conductive material may be included in an amount of 0.1 to 5 parts by weight based on the weight of the entire positive electrode active layer, specifically 0.1 to 4 parts by weight, 2 to 4 parts by weight, 1.5 to 5 parts by weight, 0.5 to 3.5 parts by weight, 1 to 3 parts by weight, 0.1 to 2.5 parts by weight, 0.1 to 2 parts by weight, or 0.1 to 1 part by weight.

[0099] The binder functions to bind the positive electrode active material, the positive electrode additive, and the conductive material together, and any material having this function may be used without particular limitation. Specifically, the binder may include one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof. As one example, the binder may include polyvinylidene fluoride (PVDF).

[0100] The binder may be included in an amount of 0.1 to 5 parts by weight based on the total weight of the positive electrode active layer, specifically 0.1 to 4 parts by weight, 2 to 4 parts by weight, 1.5 to 5 parts by weight, 0.5 to 3.5 parts by weight, 1 to 3 parts by weight, 0.1 to 2.5 parts by weight, 0.1 to 2 parts by weight, or 0.1 to 1 part by weight.

[0101] The total thickness of the positive electrode active layer is not particularly limited, but may be specifically 50 μm to 300 μm, more specifically 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0102] The positive electrode may use a current collector having high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. may be used. In the case of aluminum or stainless steel, it may also be surface-treated with carbon, nickel, titanium, silver, etc. The average thickness of the current collector may be preferably 3 μm to 500 μm, taking into account the conductivity and total thickness of the positive electrode to be manufactured.

[0103] Furthermore, the negative electrode includes a negative electrode active layer, which is prepared by applying a negative electrode slurry containing a negative electrode active material to at least one surface of a negative electrode current collector, followed by drying and pressing, similar to the positive electrode.

[0104] The negative electrode active layer may have a structure in which two individual layers are stacked depending on the battery model or product application to which the negative electrode of the present invention is applied, but is not limited thereto.

[0105] Specifically, the negative electrode according to the present invention may have a structure in which a first negative electrode active layer is provided on a negative electrode current collector, and a second negative electrode active layer is provided on the first negative electrode active layer. The first and second negative electrode active layers each contain a negative electrode active material, and the negative electrode active materials contained in each layer may be the same or different. When a two-layer negative electrode active layer is provided on a negative electrode current collector, the composition of each negative electrode active layer can be easily controlled. This not only improves the electrical performance of the negative electrode by using an active material with excellent energy efficiency as the negative electrode active material, but also allows for the composition of a negative electrode active layer that can improve and / or prevent problems that may arise from this (e.g., a decrease in interfacial adhesion between the negative electrode current collector and the negative electrode active layer), but also allows for the composition of a negative electrode active layer that can improve and / or prevent problems that may arise from this (e.g., a decrease in interfacial adhesion between the negative electrode current collector and the negative electrode active layer).

[0106] In addition, the negative electrode active layer includes a carbon-based negative electrode active material as a negative electrode active material to realize electrical activity through a reversible oxidation-reduction reaction during charging and discharging of the battery.

[0107] The carbon-based negative electrode active material refers to a material mainly composed of carbon atoms, and may include graphite. The graphite may include at least one of natural graphite and artificial graphite.

[0108] As one example, the carbon-based negative electrode active material may include a mixed graphite, which is a mixture of natural graphite and artificial graphite. In this case, the mixed graphite may be a mixture of natural graphite and artificial graphite in a weight ratio of 10-50:50-90 or 10-30:70-90. By adjusting the content ratio of natural graphite and artificial graphite as described above, the mixed graphite can prevent a decrease in the adhesive strength between the negative electrode current collector and the negative electrode active layer due to less than 10 parts by weight of natural graphite relative to the total weight, and can prevent a decrease in the charge / discharge capacity of the negative electrode due to more than 50 parts by weight of natural graphite.

[0109] The carbon-based negative electrode active material is not particularly limited in shape, but preferably has the form of spherical graphite granules formed by the aggregation of multiple flake graphite particles. Examples of flake graphite include natural graphite, artificial graphite, mesophase calcined carbon (bulk mesophase) made from tar or pitch, and graphitized cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.). In particular, the carbon-based negative electrode active material is preferably one assembled using multiple pieces of highly crystalline natural graphite. Each graphite granule may be formed by the aggregation of 2 to 100, preferably 3 to 20, flake graphite particles.

[0110] The carbon-based negative electrode active material has an average particle size (D 50 ), and specifically, the average particle size (D 50 ) can be shown.

[0111] The average particle size of the carbon-based negative electrode active material is preferably as small as possible to maximize the degree of disorder in the direction of expansion of each particle, thereby preventing particle expansion during charging with lithium ions. However, when the graphite particle size is less than 0.5 μm, a large amount of binder may be required due to the increased number of particles per unit volume. On the other hand, when the maximum particle size exceeds 20 μm, excessive expansion occurs, which reduces the adhesion between particles and between the particles and the current collector during repeated charging and discharging, resulting in a significant decrease in cycle performance.

[0112] Furthermore, the negative electrode according to the present invention may include a predetermined silicon-based negative electrode active material in the negative electrode active layer together with the carbon-based negative electrode active material. The silicon-based negative electrode active material is a material containing silicon (Si) as a main component, and may include silicon (Si), silicon carbide (SiC), silicon monoxide (SiO), or silicon dioxide (SiO2) alone or in combination. When silicon monoxide (SiO) and silicon dioxide (SiO2) are uniformly mixed or composited and included in the negative electrode active layer as the silicon-based negative electrode active material, they are referred to as silicon oxide (SiO q , where 0.8≦q≦2.5).

[0113] The negative electrode active material may be included in an amount of 85 parts by weight or more, specifically 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more, based on the weight of the entire negative electrode active layer.

[0114] The silicon-based negative electrode active material may be included in an amount of 0.1 wt% to 30 wt% of the total weight of the negative electrode active material, specifically 0.5 wt% to 20 wt%, 1 wt% to 9 wt%, 5 wt% to 15 wt%, 3 wt% to 7 wt%, 11 wt% to 19 wt%, 13 wt% to 17 wt%, 15 wt% to 20 wt%, 10 wt% to 30 wt%, 20 wt% to 30 wt%, 15 wt% to 25 wt%, or 9 wt% to 22 wt%. By adjusting the content of the carbon-based negative electrode active material and silicon-based negative electrode active material in the negative electrode active material within the above ranges, the present invention can reduce lithium consumption and irreversible capacity loss during initial charge / discharge of a secondary battery while improving charge capacity per unit mass. Furthermore, volume change of the negative electrode active layer during charge / discharge of the secondary battery can be minimized, improving the structural stability of the negative electrode active layer and thereby improving the lifespan of the secondary battery.

[0115] Meanwhile, the negative electrode active layer according to the present invention may further include, in addition to the carbon-based negative electrode active material as the main component, a conductive material, a binder, and other additives, as needed.

[0116] The conductive material may include, but is not limited to, one or more of carbon black such as acetylene black or ketjen black, carbon nanotubes, carbon fibers, and the like.

[0117] For example, the negative electrode active layer may contain carbon black, carbon nanotubes, carbon fibers, etc., alone or in combination, as a conductive material.

[0118] The content of the conductive material may be 0.1 to 10 parts by weight, specifically 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, 2 to 6 parts by weight, or 0.5 to 2 parts by weight, based on 100 parts by weight of the total negative electrode active layer. By controlling the content of the conductive material within the above range, the present invention can prevent a decrease in charge capacity due to an increase in negative electrode resistance caused by a low content of conductive material, and can prevent problems such as a decrease in charge capacity due to a decrease in the content of negative electrode active material caused by an excessive amount of conductive material, or a decrease in fast charge characteristics due to an increase in the loading amount of the negative electrode active layer.

[0119] The binder is a component that aids in bonding between the negative electrode active material and the conductive material, etc., and between the negative electrode active material and the current collector, and may be suitably used within a range that does not degrade the electrical properties of the electrode. Specifically, the binder may include at least one selected from the group consisting of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber (SBR), and fluororubber.

[0120] The content of the binder may be 0.1 to 10 parts by weight, specifically 0.1 to 8 parts by weight, 0.1 to 5 parts by weight, 0.1 to 3 parts by weight, or 2 to 6 parts by weight, based on 100 parts by weight of the total negative electrode active layer. By controlling the content of the binder contained in the negative electrode active layer within the above range, the present invention can prevent a decrease in adhesive strength of the active layer due to a low content of binder or a decrease in electrical properties of the electrode due to an excessive amount of binder.

[0121] Furthermore, the average thickness of the negative electrode active layer may be 100 μm to 300 μm, specifically 100 μm to 250 μm, 100 μm to 250 μm, or 130 μm to 190 μm, and the average thickness may be the same as the average thickness of the flat region. By adjusting the average thickness of the negative electrode active layer within this range, the present invention can uniformly align the crystal planes of the carbon-based negative electrode active material contained in each region, thereby improving the high-rate charge / discharge performance and energy density of a battery including the negative electrode.

[0122] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery, and may be made of, for example, copper, stainless steel, nickel, titanium, calcined carbon, etc. In the case of copper or stainless steel, it may also be surface-treated with carbon, nickel, titanium, silver, etc. The average thickness of the negative electrode current collector is preferably 1 μm to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0123] Meanwhile, the separator interposed between the positive and negative electrodes of each unit cell is an insulating thin film with high ion permeability and mechanical strength. It may be any material commonly used in the industry, but is not particularly limited thereto. Specifically, it may contain one or more polymers selected from the group consisting of polypropylene, polyethylene, and polyethylene-propylene copolymers, which are chemically resistant and hydrophobic. The separator may have a porous polymer substrate, such as a sheet or nonwoven fabric containing the above-mentioned polymers. In some cases, it may have a composite separator formed by coating organic or inorganic particles on the porous polymer substrate with an organic binder. The separator may have an average pore diameter of 0.01 μm to 10 μm and an average thickness of 5 μm to 300 μm.

[0124] Meanwhile, the lithium secondary battery according to the present invention is not particularly limited, and may be variously applied to a cylindrical type, a square type, a pouch type, a coin type, etc. depending on the application. The lithium secondary battery according to one embodiment of the present invention may be a pouch type secondary battery.

[0125] <Method of manufacturing lithium secondary batteries>

[0126] Furthermore, in one embodiment, the present invention provides 1. A method for manufacturing a lithium secondary battery, comprising: an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and an electrolyte composition impregnated into the electrode assembly, The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and containing a lithium metal oxide represented by the following chemical formula 1 as a positive electrode active material: The electrolyte composition includes a lithium salt and a non-aqueous organic solvent, and the non-aqueous organic solvent contains a cyclic ester solvent represented by the following chemical formula 2 in an amount of 60 wt % or more based on the total weight of the non-aqueous organic solvent: A method for manufacturing a lithium secondary battery is provided, in which a heat flow measurement is performed on a mixture containing a positive electrode active material and an electrolyte composition in a 1:1 weight ratio at a 100% state of charge, and the mixture exhibits a heat flow of 6.0 W / g or less within the range of 200°C to 300°C:

[0127] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0128] [ka]

[0129] In the above Chemical Formula 1 and Chemical Formula 2, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are 0.9≦x≦1.30, 0.6≦y<1, 0 <z≦0.2、0<w≦0.2、0≦v≦0.1であり、かつ、y+z+w+v=1であり、 [ka] is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5.

[0130] The method for manufacturing a lithium secondary battery according to the present invention is a method for manufacturing the lithium secondary battery of the present invention. The method includes a step of impregnating an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode with an electrolyte composition. The positive electrode of the electrode assembly includes an NCM-based positive electrode active material having a high nickel (Ni) content, and the electrolyte composition has an advantage in that it effectively reduces the heat flow between the positive electrode active material and the electrolyte composition at high temperatures within a predetermined range, thereby improving safety issues associated with the positive electrode active material at high temperatures.

[0131] The specific explanations of the positive electrode active material and the electrolyte composition are the same as those described above, and therefore will not be repeated.

[0132] The present invention will be described in more detail below with reference to examples and experimental examples.

[0133] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0134] Production Examples 1 to 17. Production of electrolyte compositions for lithium secondary batteries

[0135] Dihydrofuranone (DHF), fluorodihydrofuranone (FDHD), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and ethyl methyl carbonate (EMC) were prepared as non-aqueous solvents, and LiPF6, LiBF4, LiN(FSO2)2, and LiBF2(CO2)2 were prepared as lithium salts.

[0136] Each lithium salt was dissolved in a non-aqueous organic solvent at 30-40°C to a concentration of 1.0M (≒1.0mol / L), and an electrolyte additive was added at 2.0 wt% of the total weight of the electrolyte composition to prepare an electrolyte composition. The composition of the non-aqueous organic solvent, the lithium salt, and the electrolyte additive used are shown in Table 1 below.

[0137] [Table 1]

[0138] Examples 1 to 8 and Comparative Examples 1 to 9. Production of lithium secondary batteries

[0139] LiNi with a particle size of 5 μm is used as the positive electrode active material. 0.86 Co 0.05 Mn 0.07 Al 0.02 O2 was prepared and mixed with N-methylpyrrolidone (NMP) in a weight ratio of 94:3:3 with polyvinylidene fluoride as a carbon-based conductive material and binder to prepare a cathode slurry with a solid content of 45%. The prepared cathode slurry was cast onto an aluminum sheet, dried in a vacuum oven at 120°C, and then rolled to prepare a cathode with a cathode active layer 160 μm thick.

[0140] Separately, a negative electrode active material was prepared by mixing a carbon-based negative electrode active material (natural graphite and artificial graphite mixed in a weight ratio of 3:7) with a silicon-based negative electrode active material containing SiO2 in a weight ratio of 88:12. 97 parts by weight of the prepared negative electrode active material and 3 parts by weight of styrene butadiene rubber (SBR) were mixed with water to prepare a negative electrode slurry with a solids content of 40%. The negative electrode slurry was cast onto a copper sheet, dried in a vacuum oven at 130°C, and then rolled to prepare a negative electrode with a 180 μm-thick negative electrode active layer.

[0141] A separator made of 18 μm polypropylene was placed between the positive electrode and negative electrode obtained above, and the resulting electrode was inserted into a case. Then, the electrolyte composition prepared in the above Preparation Example was injected as shown in Table 2 below to assemble a lithium secondary battery.

[0142] Each assembled lithium secondary battery was initially charged. Specifically, the lithium secondary battery was initially charged at 55±2°C under the conditions shown in Table 2 below to a cut-off voltage of 4.2 V to produce an activated lithium secondary battery.

[0143] [Table 2]

[0144] Experimental example

[0145] In order to evaluate the safety and charge / discharge performance of the lithium secondary battery according to the present invention, the following experiments were carried out.

[0146] 1) Heat flow measurement between the positive electrode active material and the electrolyte composition

[0147] Each lithium secondary battery manufactured in the examples and comparative examples was fully charged at 25°C at a rate of 0.5C up to 4.25V under constant current (CC)-constant voltage (CV) conditions. The fully charged lithium secondary battery was disassembled to measure the positive electrode active material, LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02O2 and the electrolyte composition were weighed and mixed in a 1:1 weight ratio.

[0148] The prepared mixture was poured into a sample can of a differential scanning calorimeter (DSC) to prepare a sample, and the heat flow of the prepared sample was measured in the range of 100°C to 350°C while increasing the temperature at a rate of 10±0.1°C / min.

[0149] The heat flow between the positive electrode active material and the electrolyte composition contained in each lithium secondary battery was evaluated from the measurement results, and when there were two or more valid heat flow peaks within the temperature range of 100°C to 350°C, the peak value showing the largest heat flow value was determined as the heat flow. The results are shown in Table 3 below.

[0150] 2) Thermal runaway test evaluation

[0151] The lithium secondary batteries prepared in the examples and comparative examples were charged at 25°C under constant current (CC) conditions maintained at 1.25 A until they reached 4.2 V, and then maintained at 4.2 V. After charging was completed, they were activated by discharging at constant current (CC) conditions maintained at 1.25 A until they reached 2.85 V. Then, each activated lithium secondary battery was rested for 6 hours at 25°C.

[0152] Next, the batteries were fully charged at a constant current (CC) of 1.25 A at 25°C until the battery voltage reached 4.2 V. A thermal runaway test was then performed using an accelerating rate calorimetry (ARC). The ARC was manufactured by Thermal Hazard Technology (THT), and the thermal runaway test was performed using a heat-wait-search (HWS) method. The temperature of the insulated oven chamber containing each lithium secondary battery was raised from 50°C by 5°C at a rate of 10°C / min, and then heated to 190°C by waiting for 10 minutes. If the lithium secondary battery showed a temperature change of 0.02°C / min or more during the 10-minute waiting period, this was considered to be self-heating, which corresponds to thermal runaway (TR). After this point, no additional thermal energy was applied to allow the temperature to change due to the self-heating of each secondary battery. After the secondary battery self-heated, the self-heating profile of the secondary battery was measured, and the maximum temperature of the secondary battery was calculated from the measured self-heating profile. The results are shown in Table 3 below.

[0153] [Table 3]

[0154] The lithium secondary battery according to the present invention has excellent high-temperature safety and battery life despite containing an NCM-based positive electrode active material in which the nickel (Ni) content is as high as 80% or more of the total molar fraction of transition metals, by adjusting the heat flow rate between the positive electrode active material and the electrolyte composition to a low level within a predetermined range.

[0155] Specifically, the lithium secondary batteries of the examples in which the heat flow rate between the positive electrode active material and the electrolyte composition was 6 W / g or less exhibited a low maximum temperature of less than about 275°C in the thermal runaway test.

[0156] On the other hand, the lithium secondary battery of the comparative example, in which the heat flow rate between the positive electrode active material and the electrolyte composition exceeded 6 W / g, exhibited a high maximum temperature of approximately 299° C. or higher in the thermal runaway test.

[0157] This means that when the heat flow rate between the positive electrode active material and the electrolyte composition is adjusted to a value lower than a predetermined range, even if an NCM-based positive electrode active material containing a high content of nickel (Ni) and having low structural stability at high temperatures is used, damage to the positive electrode active material, such as structural collapse, can be suppressed, and a significant increase in the temperature of the secondary battery at high temperatures can be prevented.

[0158] These results demonstrate that the lithium secondary battery according to the present invention has excellent energy density due to the NCM-based positive electrode active material having a high nickel (Ni) content in the positive electrode, and is effective in improving safety issues caused by the positive electrode active material at high temperatures by controlling the heat flow rate between the positive electrode active material and the electrolyte composition to a low level within a predetermined range.

[0159] Although the present invention has been described above with reference to preferred embodiments, it will be understood that a person skilled in the art or having ordinary knowledge in the art can make various modifications and changes to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims below.

[0160] Therefore, the technical scope of the present invention is not limited to the content described in the Summary of the Invention of the specification, but can be defined by the claims.

Claims

1. an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and an electrolyte composition impregnated into the electrode assembly; The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and including a lithium metal oxide represented by the following Chemical Formula 1 as a positive electrode active material: The electrolyte composition includes a lithium salt and a non-aqueous organic solvent, and the non-aqueous organic solvent includes a cyclic ester-based solvent represented by the following Chemical Formula 2 in an amount of 60 wt % or more based on the total weight of the non-aqueous organic solvent: A lithium secondary battery that exhibits a heat flow of 6.0 W / g or less in the range of 200°C to 300°C when a heat flow measurement is performed on a mixture containing a positive electrode active material and an electrolyte composition in a weight ratio of 1:1 at a 100% charged state: [Chemical formula 1] Li ｘ [Ni ｙ Co ｚ Mn ｗ M １ ｖ ]O ２ 【Chemical 1】 In the above Chemical Formula 1 and Chemical Formula 2, M １ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are in the ranges 0.9≦x≦1.30, 0.6≦y<1, 0<z≦0.2, 0<w≦0.2, 0≦v≦0.1, respectively, and y+z+w+v=1; 【Chemistry 2】 is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5;

2. The positive electrode active material is LiNi ０．９５ Co ０．０３ Mn ０．０２ O ２ , LiNi ０．９ Co ０．６ Mn ０．４ O ２ , LiNi ０．９ Co ０．０５ Mn ０．０５ O ２ , LiNi ０．８５ Co ０．１ Mn ０．０５ O ２ , LiNi ０．８５ Co ０．０５ Mn ０．１ O ２ , LiNi ０．８ Co ０．１ Mn ０．１ O ２ , LiNi ０．７５ Co ０．２ Mn ０．１５ O ２ , LiNi ０．７ Co ０．２ Mn ０．１ O ２ , LiNi ０．７ Co ０．１５ Mn ０．１５ O ２ , LiNi ０．７ Co ０．１ Mn ０．２ O ２ , LiNi ０．６ Co ０．２ Mn ０．２ O ２ , LiNi ０．６ Co ０．２ Mn ０．１ Al ０．１ O ２ , LiNi ０．６ Co ０．２ Mn ０．１５ Al ０．０５ O ２ , and LiNi ０．７ Co ０．１ Mn ０．１ Al ０．１ O ２ The lithium secondary battery according to claim 1, comprising one or more of:

3. 2. The lithium secondary battery according to claim 1, wherein the cyclic ester solvent represented by Chemical Formula 2 includes one or more of dihydrofuranone, vinyldihydrofuranone, fluorodihydrofuranone, furanone, tetrahydropyranone, methyldihydrofuranone, propyltetrahydropyranone, and oxepanone.

4. The lithium salt is Li as a cation ＋ Including, アニオンとして、BF ４ － 、B １０ Cl １０ － 、ClO ４ － 、AlO ４ － 、AlCl ４ － 、PF ６ － 、SbF ６ － 、AsF ６ － 、BF ２ C ２ O ４ － 、B(C ２ O ４ ) ２ － 、PF ４ C ２ O ４ － 、PF ２ C ４ O ８ － 、(CF ３ ) ２ 10 ４ － 、(CF ３ ) ３ 10 ３ － 、(CF ３ ) ４ 10 ２ － 、(CF ３ ) ５ 10 － 、(CF ３ ) ６ P － 、(C ２ O ４ ) ２ 10 ２ － 、CF ３ 2000 ３ － 、C ４ F ９ 2000 ３ － 、CF ３ CF ２ 2000 ３ － , (CF ３ SO ２ ) ２ N － , (FSO ２ ) ２ N － , C.F. ３ CF ２ (CF ３ ) ２ CO － , (CF ３ SO ２ ) ２ CH － , (SF ５ ) ３ C － , (CF ３ SO ２ ) ３ C － , C.F. ３ (CF ２ ) ７ SO ３ － , C.F. ３ CO ２ － , C.H. ３ CO ２ － , SCN － , (CF ３ CF ２ SO ２ ) ２ N － and ((C(CN)) ２ NC (CF ３ ))N － The lithium secondary battery according to claim 1, comprising one or more of:

5. The lithium salt is Li as a cation ＋ Including, BF as an anion ２ C ２ O ４ － , B(C ２ O ４ ) ２ － , (CF ３ SO ２ ) ２ N － , (FSO ２ ) ２ N － , (CF ３ CF ２ SO ２ ) ２ N － , or ((C(CN)) ２ NC (CF ３ ))N － The lithium secondary battery according to claim 1 , comprising:

6. The lithium salt is Li as a cation ＋ Including, アニオンとしてBF ４ － 、B １０ Cl １０ － 、ClO ４ － 、AlO ４ － 、AlCl ４ － 、PF ６ － 、SbF ６ － 、AsF ６ － 、PF ４ C ２ O ４ － 、PF ２ C ４ O ８ － 、(CF ３ ) ２ 10 ４ － 、(CF ３ ) ３ 10 ３ － 、(CF ３ ) ４ 10 ２ － 、(CF ３ ) ５ 10 － 、(CF ３ ) ６ P － 、(C ２ O ４ ) ２ 10 ２ － 、CF ３ 2000 ３ － 、C ４ F ９ 2000 ３ － 、CF ３ CF ２ 2000 ３ － 、CF ３ CF ２ (CF ３ ) ２ CO － 、(CF ３ 2000 ２ ) ２ CH － , (SF ５ ) ３ C － , (CF ３ SO ２ ) ３ C － , C.F. ３ (CF ２ ) ７ SO ３ － , C.F. ３ CO ２ － , C.H. ３ CO ２ － and SCN － The lithium secondary battery according to claim 1, comprising two or more of the following:

7. 2. The lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent further comprises one or more auxiliary solvents selected from the group consisting of fluorine-containing ether solvents, fluorine-containing cyclic carbonate solvents, linear carbonate solvents, phosphate solvents, and sulfone solvents.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein the electrolyte composition further comprises an electrolyte additive represented by the following chemical formula 3: 【Chemistry 3】 In the above Chemical Formula 3, R １ teeth 【Chemistry 4】 【Chemistry 5】 or 【Chemistry 6】 and R １ ' and R １ '' are hydrogen or methyl groups, R ２ represents an arylene group having 6 to 20 carbon atoms, an aryleneoxy group having 6 to 20 carbon atoms, a heteroarylene group having 5 to 10 carbon atoms and containing one or more heteroatoms selected from N, S, and O, a heteroaryleneoxy group having 5 to 10 carbon atoms and containing one or more heteroatoms selected from N, S, and O, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, and 【Chemistry 7】 Contains one or more of the following: R ３ represents a fluoro group, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or 【Chemistry 8】 and the alkyl group, alkoxy group, cycloalkyl group, 【Chemistry 9】 and 【Chemistry 10】 One or more hydrogen atoms contained in M includes at least one selected from the group consisting of lithium, sodium, potassium, tetraalkylammonium having 1 to 4 carbon atoms, and tetraalkylphosphonium having 1 to 4 carbon atoms; l is an integer from 1 to 6; m and n are each an integer of 2 to 20.

9. 9. The lithium secondary battery of claim 8, wherein the electrolyte additive represented by Chemical Formula 3 includes at least one compound selected from the following <Structural Formula 1> to <Structural Formula 17>: 【Chemistry 11】

10. The lithium secondary battery according to claim 8 , wherein the electrolyte additive is contained in an amount of 5 wt % or less based on the weight of the entire electrolyte composition.

11. the negative electrode includes a negative electrode active layer provided on at least one surface of a negative electrode current collector and including a carbon-based negative electrode active material and a silicon-based negative electrode active material; 2. The lithium secondary battery according to claim 1, wherein the carbon-based negative electrode active material comprises at least one carbon material selected from the group consisting of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, acetylene black, and ketjen black.

12. The silicon-based negative electrode active material includes silicon (Si), silicon carbide (SiC), and silicon oxide (SiO ｑ 12. The lithium secondary battery according to claim 11, wherein q is at least one of q and ...

13. 12. The lithium secondary battery according to claim 11, wherein the silicon-based negative electrode active material is contained in an amount of 0.1 wt % to 30 wt % based on the total weight of the negative electrode active material.

14. preparing an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and A method for manufacturing a lithium secondary battery, comprising the step of impregnating the electrode assembly with an electrolyte composition, The positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector and containing a lithium metal oxide represented by the following Chemical Formula 1 as a positive electrode active material: The electrolyte composition includes a lithium salt and a non-aqueous organic solvent, and the non-aqueous organic solvent includes a cyclic ester-based solvent represented by the following Chemical Formula 2 in an amount of 60 wt % or more based on the total weight of the non-aqueous organic solvent: A method for producing a lithium secondary battery, wherein a heat flow measurement is performed on a mixture containing a positive electrode active material and an electrolyte composition in a weight ratio of 1:1 at a 100% charged state, and the mixture exhibits a heat flow of 6.0 W / g or less in a range of 200°C to 300°C: [Chemical formula 1] Li ｘ [Ni ｙ Co ｚ Mn ｗ M １ ｖ ]O ２ 【Chemistry 12】 In the above Chemical Formula 1 and Chemical Formula 2, M １ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, y, z, w, and v are in the ranges 0.9≦x≦1.30, 0.6≦y<1, 0<z≦0.2, 0<w≦0.2, 0≦v≦0.1, respectively, and y+z+w+v=1; 【Chemistry 13】 is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group; p is an integer of 1 to 5;

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