Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery including a non-aqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by Chemical Formula 1, and a second additive represented by Chemical Formula 2; a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material made of silicon; and a separator interposed between the positive electrode and the negative electrode.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0087179 filed July 14, 2022 and Korean Patent Application No. 10-2023-0087210 filed July 5, 2023, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery comprising an electrolyte containing a combination of specific additives and a pure silicon (Pure Si) negative electrode material. [Background technology]

[0003] Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode including a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode including a negative electrode active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting an electrolyte serving as a medium for transferring lithium ions, and then sealing the battery case.

[0004] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and are therefore used in a variety of fields, including mobile devices, electronic products, and electric vehicles. As the range of applications for lithium secondary batteries becomes more diverse, the required physical properties are also gradually increasing. In particular, there is a demand for the development of lithium secondary batteries that can operate stably even under high-temperature conditions.

[0005] On the other hand, when a lithium secondary battery is operated under high temperature conditions, lithium salts such as LiPF6 contained in the electrolyte are converted to PF6 - Thermal decomposition of anions can generate Lewis acids such as PF5, which react with water to generate HF. These decomposition products, such as PF5 and HF, not only destroy the coating formed on the electrode surface, but can also cause decomposition reactions of organic solvents, which can react with the decomposition products of the positive electrode active material to leach transition metal ions. The leached transition metal ions can then be electrodeposited on the negative electrode, destroying the coating formed on the negative electrode surface.

[0006] If the electrolyte decomposition reaction continues on such a destroyed coating, the battery performance will further deteriorate, so there is a demand for the development of a secondary battery that can maintain excellent performance even under high temperature conditions. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to solve the above-mentioned problems by introducing a non-aqueous electrolyte containing a combination of two specific additives to improve the high-temperature performance of lithium secondary batteries containing pure silicon negative electrode active materials. [Means for solving the problem]

[0008] According to one embodiment, the present invention provides a method for manufacturing a semiconductor device comprising: a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2: a positive electrode comprising a positive electrode active material; a negative electrode comprising a negative electrode active material made of silicon; and The lithium secondary battery includes a separator interposed between the positive electrode and the negative electrode.

[0009] [ka]

[0010] In the above Chemical Formula 1, A is a heterocycle having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms; R1 is an alkylene group having 1 to 3 carbon atoms,

[0011] [ka]

[0012] In the above Chemical Formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorine atoms. [Effects of the Invention]

[0013] The lithium secondary battery according to the present invention includes a non-aqueous electrolyte containing a combination of specific additives, thereby improving the high-temperature life and gas generation rate of lithium secondary batteries containing silicon negative electrode active materials. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will now be described in further detail.

[0015] Generally, anions contained in lithium salts such as LiPF6, which are widely used in electrolytes for lithium secondary batteries, form decomposition products such as hydrogen fluoride (HF) and PF5 due to thermal decomposition or moisture, and this phenomenon is exacerbated when the battery is operated under high temperature conditions. The decomposition products have acidic properties, which deteriorate the properties of the electrode surface in the battery.

[0016] Due to decomposition products of the electrolyte and structural changes in the positive electrode caused by repeated charging and discharging, transition metals in the positive electrode are easily dissolved into the electrolyte, and the dissolved transition metals are redeposited on the positive electrode, increasing the resistance of the positive electrode.

[0017] Furthermore, if the dissolved transition metals move to the negative electrode through the electrolyte, they are electrodeposited on the negative electrode, causing destruction of the solid electrolyte interphase (SEI) film and further decomposition of the electrolyte, which leads to problems such as the consumption of lithium ions and increased resistance.

[0018] In particular, when silicon (Si) is used as the negative electrode active material, the SEI film is easily damaged due to large volume changes during the charge / discharge process, and a regeneration reaction occurs continuously, which leads to problems such as deterioration of battery durability and intensified gas generation.

[0019] To solve these problems, the present inventors have found that a first additive represented by the following Chemical Formula 1 and a second additive represented by the following Chemical Formula 2 are contained in a non-aqueous electrolyte solution, and a strong SEI film is formed on the negative electrode through the addition of the first additive and the second additive, thereby improving the battery performance, particularly the amount of high-temperature gas generation.

[0020] Each of the components constituting the present invention will be described in more detail below.

[0021] non-aqueous electrolyte The lithium secondary battery according to the present invention includes a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2.

[0022] Each component of the non-aqueous electrolyte will be specifically described below.

[0023] (1) First additive and second additive The non-aqueous electrolyte of the present invention contains a first additive represented by the following chemical formula 1.

[0024] [ka]

[0025] In the above Chemical Formula 1, A is a heterocycle having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms; R1 is an alkylene group having 1 to 3 carbon atoms.

[0026] The first additive represented by Formula 1 contains a propargyl functional group. When added to an electrolyte, it undergoes reductive decomposition via a radical reaction to form a PEO (poly(ethylene oxide))-based polymer SEI film on the surface of the negative electrode. This not only improves the high-temperature durability of the negative electrode itself but also prevents electrodeposition of transition metals on the negative electrode surface. Furthermore, the propargyl group has the ability to adsorb metal ions, preventing the elution of metallic impurities contained in the positive electrode by adsorbing to the surface of the positive electrode and preventing the eluted metal ions from being deposited on the negative electrode, thereby suppressing internal short circuits. Furthermore, the first additive binds with PF5, a decomposition product of the electrolyte, to inhibit the generation of HF, thereby preventing the destruction of the cathode electrolyte interphase (CEI) film formed on the surface of the positive electrode and further inhibiting the decomposition of the electrolyte.

[0027] In one embodiment of the present invention, A in the above Chemical Formula 1 may be a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms, preferably an imidazole group.

[0028] As a preferred example, the first additive may be represented by the following chemical formula 1-1.

[0029] [ka]

[0030] In the above chemical formula 1-1, R1 is as defined in Chemical Formula 1 above.

[0031] In one embodiment of the present invention, R1 in Chemical Formula 1 may be a linear or branched alkylene group having 1 to 3 carbon atoms, preferably a linear alkylene group having 1 to 3 carbon atoms, and more preferably a methylene group.

[0032] As a preferred example, the first additive may be represented by the following chemical formula 1A.

[0033] [ka]

[0034] In one embodiment of the present invention, the content of the first additive may be 0.05 wt % to 10 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.1 wt % to 0.5 wt %, based on the total weight of the non-aqueous electrolyte. If the content of the first additive is excessive, it may excessively participate in the decomposition reaction at the interface between the electrode and the electrolyte, causing excessive film resistance and increasing the resistance of the battery. Considering this, the content of the first additive is preferably 10 wt % or less.

[0035] The non-aqueous electrolyte of the present invention also contains a second additive represented by the following chemical formula 2.

[0036] [ka]

[0037] In the above Chemical Formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorine atoms.

[0038] The second additive represented by Formula 2 contains a cyclic phosphate structure, and when it is added to an electrolyte, it can form an SEI film of a polyphosphoester (PPE) component on the surface of the negative electrode through a ring-opening reaction, which can impart elasticity to the silicon negative electrode. In other words, it strengthens the durability of silicon against volume changes, thereby preventing the deterioration of the negative electrode during cycling and in high-temperature environments. In addition, the second additive contains a fluoroalkyl group, and can form a LiF-based inorganic SEI film on the positive and negative electrodes through a reaction with lithium ions. LiF has physically strong properties and is easily oxidized by lithiated silicon (Lix Since it has a strong bond with silicon, it has the effect of suppressing deterioration of the battery due to reactions at the interface between the electrolyte and the electrode.

[0039] In one embodiment of the present invention, R2 in Formula 2 is -(CR2) n CF3, wherein R is hydrogen or fluorine, and n may be an integer from 0 to 9. Preferably, R2 in Chemical Formula 2 is -(CH2) n CF3, wherein n is an integer of 0 to 5, and more preferably, n is an integer of 1 to 3.

[0040] As a preferred example, the second additive may be represented by the following chemical formula 2A.

[0041] [ka]

[0042] In one embodiment of the present invention, the content of the second additive may be 0.1 wt % to 15 wt % based on the total weight of the non-aqueous electrolyte.

[0043] Specifically, the content of the second additive may be 0.5 wt % or more, preferably 2 wt % or more, more preferably 5 wt % or more, and most preferably 10 wt % or more, based on the total weight of the non-aqueous electrolyte. However, considering that an excessive amount of the second additive may increase the resistance of the battery due to decomposition thereof, the content of the second additive is preferably 15 wt % or less.

[0044] When the first and second additives are used together in the nonaqueous electrolyte according to the present invention, the lone pair present in the A substituent of Chemical Formula 1, specifically the imidazole, further promotes the ring-opening reaction of Chemical Formula 2 of the second additive, allowing the reduction reaction to occur smoothly with less energy. This can further increase the polymerized coating component when forming an SEI layer on a silicon anode, and can suppress HF generation by chelating PF5. In particular, since Si anode materials experience large volume changes, the phenomenon of coating damage and durability degradation due to HF in high-temperature environments is exacerbated. Therefore, the combination of these additives can dramatically improve durability and reduce gas generation.

[0045] In one embodiment of the present invention, the weight ratio of the second additive to the first additive, i.e., the ratio of the weight of the second additive (W2) to the weight of the first additive (W1) in the non-aqueous electrolyte (W2 / W1), may be 1.5 or more, preferably 6 or more, more preferably 16 or more, and most preferably 30 or more. As the relative proportion of the second additive increases, not only is the formation of a polymerized coating due to the ring-opening reaction of the cyclic phosphate structure more favorable, but also LiF formed by bonding -CF3 with lithium ions can strengthen the durability of the coating, which is favorable for improving the high-temperature performance of batteries using pure Si as an anode material. However, considering that excessive decomposition of the additive can result in an excessively thick coating of the polymer component, increasing battery resistance and generating side reactions and by-products during the decomposition process, the weight ratio of the second additive to the first additive is preferably 40 or less.

[0046] (2) Third additive The non-aqueous electrolyte may include, as a third additive, at least one selected from the group consisting of a cyclic carbonate-based compound, a sultone-based compound, a sulfate-based compound, a phosphorus-based compound, a nitrile-based compound, an amine-based compound, a silane-based compound, a benzene-based compound, and a lithium salt-based compound.

[0047] The cyclic carbonate compound may be at least one selected from the group consisting of vinylene carbonate (VC), vinylethylene carbonate (VEC), and fluoroethylene carbonate (FEC), and specifically may be vinylene carbonate.

[0048] The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be at least one selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and may be specifically 1,3-propane sultone (PS).

[0049] The sulfate-based compound is a material that can be electrically decomposed on the surface of the negative electrode to form a stable SEI film without cracks even during high-temperature storage, and may be at least one selected from the group consisting of ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0050] The phosphorus-based compound may be a phosphate-based or phosphite-based compound, and specifically may be at least one selected from the group consisting of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0051] The nitrile compound may be at least one selected from the group consisting of succinonitrile (SN), adiponitrile (ADN), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), and 1,2,3-tris(2-cyanoethyl)propane (TCEP).

[0052] The amine-based compound may be at least one selected from the group consisting of triethanolamine and ethylenediamine, and the silane-based compound may be tetravinylsilane.

[0053] The benzene-based compound may be at least one selected from the group consisting of monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

[0054] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution, and may be at least one selected from the group consisting of lithium difluorophosphate (LiDFP; LiPOF), lithium bis(oxalato)borate (LiBOB; LiB(C0)), lithium tetrafluoroborate (LiBF), lithium tetraphenylborate, lithium difluoro(oxalato)borate (LiDFOB), and lithium difluoro(bis(oxalato)phosphate (LiDFOP).

[0055] Preferably, the nonaqueous electrolyte according to one embodiment of the present invention may further include at least one third additive selected from the group consisting of vinylene carbonate (VC), 1,3-propane sultone (PS), 1,3-propene sultone (PRS), and lithium difluorophosphate (LiDFP). In this case, a stronger SEI film may be formed on the negative electrode surface during the initial activation process of the secondary battery, thereby suppressing gas generation due to electrolyte decomposition at high temperatures and reducing the rate of resistance increase due to increased cycles during battery operation.

[0056] In one embodiment of the present invention, the content of the third additive may be 0.05 wt % to 5 wt %, preferably 0.1 wt % to 3 wt %, based on the total weight of the non-aqueous electrolyte, and it is preferable that the content of the third additive is 5 wt % or less in terms of reducing the initial resistance.

[0057] (3) Organic solvent The non-aqueous electrolyte of the present invention contains an organic solvent.

[0058] The organic solvent may be any of various organic solvents commonly used in lithium electrolytes, without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a chain carbonate solvent, a chain ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof. Preferably, the organic solvent may be a mixture of a cyclic carbonate solvent and a chain carbonate solvent, and more preferably, a mixture of a fluorinated cyclic carbonate solvent and a chain carbonate solvent.

[0059] The cyclic carbonate solvent is a highly viscous organic solvent with a high dielectric constant, which can effectively dissociate the lithium salt in the electrolyte. The cyclic carbonate solvent may be at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and preferably includes fluoroethylene carbonate (FEC). When a fluorinated cyclic carbonate solvent such as FEC is used, it is possible to form a LiF-based inorganic SEI film, which has the advantage of allowing the formation of a hybrid film together with the organic film of the polymer component formed by the additive.

[0060] The chain carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate. Preferably, it may include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or a mixture thereof, and more preferably, it may include diethyl carbonate and ethyl methyl carbonate. DMC is an organic solvent effective in increasing ionic conductivity and thereby improving room-temperature resistance. However, when DMC is used alone as the chain carbonate solvent, it is unstable at high temperatures, resulting in increased gas generation due to reduction reactions, and its high freezing point significantly reduces low-temperature performance. Therefore, it is preferable to use DEC, EMC, or a combination thereof to reduce gas generation, thereby enhancing the durability of the coating formed on the electrode and improving the low-temperature performance of the battery. In particular, EMC has a lower viscosity than DEC, which is advantageous for improving ionic conductivity.

[0061] The chain ester solvent may be any one or more selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, and may be preferably methyl propionate, ethyl propionate, or propyl propionate.

[0062] The cyclic ester solvent may be at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0063] The nitrile solvent may be any one or more selected from the group consisting of succinonitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, and may be preferably succinonitrile.

[0064] The remaining components of the total weight of the non-aqueous electrolyte solution, excluding the organic solvent, for example, the first to third additives and the lithium salt, may be an organic solvent unless otherwise specified.

[0065] (4) Lithium salt The non-aqueous electrolyte of the present invention contains a lithium salt.

[0066] The lithium salt may be any one that is commonly used in electrolytes for lithium secondary batteries without any limitation. Specifically, the lithium salt may contain Li as a cation. + and the anion is F - , Cl - , Br - , I - , NO3 -, N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO4 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , BF2C2O4 - , BC4O8 - , BF2C2O4CHF - , PF4C2O4 - , PF2C4O8 - , PO2F2 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - and SCN - It may include any one or more selected from the following.

[0067] Specifically, the lithium salt may be LiPF6, LiClO4, LiBF4, LiN(FSO2)2 (LiFSI), LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO3CF3, LiPO2F2, lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiTFOP). The lithium fluoride may be any one or more selected from the group consisting of fluoromalonato(difluoro)borate, LiFMDFB, and LiPF6, and is preferably LiPF6.

[0068] In one embodiment of the present invention, the concentration of the lithium salt in the non-aqueous organic solution containing the lithium salt and an organic solvent, i.e., the mixed solution of the lithium salt and the organic solvent, may be 0.5 M or more, specifically 1.0 M or more, and more specifically 1.5 M or more. When the lithium salt concentration is within this range, the effects of improving low-temperature output and cycle characteristics can be sufficiently ensured. In particular, when the lithium salt concentration is high, such as 1.5 M or more, the participation of anions in the solvation shell of lithium ions in the electrolyte can be increased. As a result, the anions in the lithium salt, such as PF6 -The addition of an inorganic component such as LiF to the anode SEI film can enhance the resistance of the silicon anode to volume changes. Meanwhile, to prevent excessive increases in viscosity and surface tension and achieve appropriate electrolyte impregnation, the lithium salt concentration is preferably 4.0 M or less, specifically 3.0 M or less, and more specifically 2.0 M or less.

[0069] positive electrode The positive electrode according to the present invention includes a positive electrode active material and may be prepared by coating a positive electrode current collector with a positive electrode slurry including the positive electrode active material, a binder, a conductive material, and a solvent, followed by drying and rolling.

[0070] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, 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 may be used.

[0071] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and may be at least one selected from the group consisting of LCO (LiCoO), LNO (LiNiO), LMO (LiMnO), LiMnO, LiCoPO, LFP (LiFePO), and lithium composite transition metal oxides containing nickel (Ni), cobalt (Co), and manganese (Mn).

[0072] On the other hand, the positive electrode active material may be a lithium transition metal composite oxide in which the molar ratio of nickel among the transition metals is 70 mol % or more, preferably 80 mol % or more, and more preferably 85 mol % or more.

[0073] High-nickel (High-Ni) positive electrode active materials, in which the nickel content of the total transition metals is 70 mol% or more, are structurally unstable and may disintegrate or crack at high temperatures. In such disintegrating active materials, the surface area increases, increasing the number of sites where side reactions with the electrolyte can occur. As a result, when exposed to high temperatures, a large amount of gas is generated. If gas generation continues, the internal pressure of the battery increases, causing premature venting and resulting in premature battery termination. Therefore, it is important to prevent these side reactions and gas generation. This can be prevented by strengthening film formation using the nonaqueous electrolyte containing the first and second additives of the present invention.

[0074] In one embodiment of the present invention, the lithium composite transition metal oxide may be a compound represented by the following Chemical Formula 3. That is, the positive electrode active material according to one embodiment of the present invention may include a lithium composite transition metal oxide represented by the following Chemical Formula 3.

[0075] [Chemical formula 3] Li 1+x (Ni a Co b Mn c M d )O2

[0076] In the above Chemical Formula 3, M is at least one 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; 1 + x, a, b, c, and d are the atomic fractions of each independent element, -0.2≦x≦0.2, 0.60≦a<1, 0 <b≦0.30、0<c≦0.30、0≦d≦0.10、a+b+c+d=1である。

[0077] Since the above 1 + x indicates the molar ratio of lithium in the lithium composite transition metal oxide, it may be -0.1 ≦ x ≦ 0.2, or 0 ≦ x ≦ 0.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium composite transition metal oxide can be stably formed.

[0078] Since the above a indicates the molar ratio of nickel among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0.70 ≦ a < 1, 0.80 ≦ a < 1, or 0.85 ≦ a < 1. When the molar ratio of nickel satisfies the above range, it shows a high energy density and enables the realization of a high capacity.

[0079] Since the above b indicates the molar ratio of cobalt among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0 < b ≦ 0.20, 0 < b ≦ 0.15, or 0 < b ≦ 0.10. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0080] Since the above c indicates the molar ratio of manganese among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0 < c ≦ 0.20, 0 < c ≦ 0.15, or 0 < c ≦ 0.10. When the molar ratio of manganese satisfies the above range, the structural stability of the positive electrode active material is excellently shown.

[0081] In one embodiment of the present invention, the lithium composite transition metal oxide may contain any one or more doping 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, and preferably may contain Al as the doping element. In other words, the above d indicating the molar ratio of the doping element among all the metals excluding lithium in the lithium composite transition metal oxide may be 0 < d ≦ 0.10, 0 < d ≦ 0.08, or 0 < d ≦ 0.05.

[0082] Preferably, a, b, c and d in the formula 3 are 0.70≦a<1, 0 <b≦0.20、0<c≦0.20、0≦d≦0.10であってよい。

[0083] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry. If the content of the positive electrode active material is less than 80 wt%, the energy density may be reduced, resulting in a decrease in capacity.

[0084] The binder is a component that aids in binding the active material and conductive material and the current collector, and may be added in an amount of 1 to 30 wt % based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0085] In addition, the conductive material is a material that imparts conductivity to the battery without inducing a chemical change, and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the positive electrode slurry.

[0086] The conductive material may be selected from the group consisting of carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0087] The solvent for the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP) and may be used to achieve a preferred viscosity when containing the positive electrode active material, binder, conductive material, etc. For example, the positive electrode slurry containing the positive electrode active material, binder, and conductive material may be contained so that the solids concentration is 40 wt % to 90 wt %, preferably 50 wt % to 80 wt %.

[0088] negative electrode The lithium secondary battery according to the present invention includes a negative electrode including a negative electrode active material, and the negative electrode may be fabricated by coating a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, and a solvent, followed by drying and rolling.

[0089] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. The negative electrode current collector may be made of any material that has high conductivity and does not induce chemical changes in the battery. Examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or an aluminum-cadmium alloy. Similarly to the positive electrode current collector, the surface may be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be made in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0090] In the present invention, the negative electrode active material is silicon (Si). Silicon as a negative electrode active material has a theoretical capacity approximately 10 times higher than that of graphite, which allows for the realization of a high-capacity cell. Silicon also has a mass loading (mg cm -2) and improve the fast charging performance of the battery. However, there are problems such as a high lithium ion loss rate due to irreversible reactions, which leads to large volume changes and adversely affects the battery life. In particular, in the case of a 100% Si anode, the SEI film easily cracks due to large volume changes during the charge / discharge process, and the reaction of regeneration occurs continuously. However, when the nonaqueous electrolyte according to the present invention is applied, the SEI film can be strengthened as described above, thereby effectively solving these problems.

[0091] The negative electrode active material may be included in an amount of 60 wt % to 99 wt % based on the total weight of the solid content in the negative electrode slurry.

[0092] The binder is a component that helps bind the conductive material, active material, and current collector together, and may be added in an amount of 1 to 30 wt % based on the total weight of the solids in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0093] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solids in the negative electrode slurry. The conductive material is not particularly limited as long as it is conductive without inducing chemical changes in the battery, and may be selected from the following: carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, carbon nanotubes, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives; or combinations thereof.

[0094] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used to achieve a preferred viscosity when containing the negative electrode active material, binder, conductive material, etc. For example, the negative electrode active material, binder, and conductive material may be contained so that the solids concentration in the slurry is 20 wt % to 80 wt %, preferably 20 wt % to 40 wt %.

[0095] separation membrane The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

[0096] The separator can be any separator that is generally used as a separator in a lithium secondary battery by separating the negative electrode and the positive electrode and providing a path for lithium ions to move. In particular, a separator that has low resistance to ion movement in the electrolyte, excellent humidification ability for the electrolyte, and excellent safety is preferred.

[0097] Specifically, the separator may be a porous polymer film, for example, a porous polymer film made of 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. Conventional porous nonwoven fabrics, for example, nonwoven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc., may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may also be used, and may be used in a single-layer or multi-layer structure.

[0098] The lithium secondary battery according to the present invention as described above may be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0099] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0100] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0101] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0102] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a large number of battery cells.

[0103] The present invention will be described in detail below with reference to specific examples.

[0104] <Example> Example 1 (Production of non-aqueous electrolyte) Fluoroethylene carbonate (FEC): diethyl carbonate (DEC): ethyl methyl carbonate (EMC) were mixed in a volume ratio of 10:45:45, and LiPF6 was dissolved therein to prepare a non-aqueous organic solution to a concentration of 1.5 M. 0.3 wt% of the compound represented by Formula 1A, 0.5 wt% of the compound represented by Formula 2A, and the remainder of the non-aqueous organic solution were mixed to prepare a 100 wt% non-aqueous electrolyte.

[0105] (Lithium secondary battery manufacturing) Li(Ni) as the positive electrode active material 0.85 Co 0.05 Mn 0.08 Al 0.02 A positive electrode slurry (solid content 75.5 wt%) was prepared by adding 02, carbon black as a conductive material, and polyvinylidene fluoride as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.74:0.7:1.56. The positive electrode slurry was applied to a positive electrode current collector (Al thin film) with a thickness of 15 μm, dried, and roll-pressed to prepare a positive electrode.

[0106] A negative electrode active material slurry with a solid content of 26.0 wt% was prepared by mixing distilled water as a solvent with Si particles (Wacker) with an average particle size of 5 μm as a negative electrode active material, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as a binder, and carbon black as a conductive material in a weight ratio of 70:9.7:20.3. The negative electrode active material slurry was applied to a negative electrode current collector (Cu thin film) with a thickness of 15 μm, dried, and roll-pressed to prepare a negative electrode.

[0107] The positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a negative electrode were sequentially stacked to prepare an electrode assembly.

[0108] The assembled electrode assembly was placed in a pouch-type battery case, and the prepared non-aqueous electrolyte was poured into the case to manufacture a lithium secondary battery.

[0109] Example 2. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 2A was changed to 2 wt % when preparing the non-aqueous electrolyte.

[0110] Example 3. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 2A was changed to 5 wt % when preparing the non-aqueous electrolyte.

[0111] Example 4. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 2A was changed to 10 wt % when preparing the non-aqueous electrolyte.

[0112] Example 5. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 1A was changed to 10 wt % when preparing the non-aqueous electrolyte.

[0113] Example 6 A lithium secondary battery was fabricated in the same manner as in Example 1, except that 2.5 wt % of vinylene carbonate (VC) was further added during the preparation of the non-aqueous electrolyte.

[0114] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Formula 1A and the compound represented by Formula 2A were not added when preparing the non-aqueous electrolyte solution.

[0115] Comparative Example 2 A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula 2A was not added during the preparation of the non-aqueous electrolyte.

[0116] Comparative Example 3. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula 1A was not added during the preparation of the non-aqueous electrolyte.

[0117] Comparative Example 4. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula Z1 (5-Ethynyl-1-methyl-1H-imidazole) was used instead of the compound represented by Formula 1A when preparing the non-aqueous electrolyte.

[0118] [ka]

[0119] Comparative Example 5. A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula Z2 (2-ethoxy-1,3,2-dioxaphospholane 2-oxide) was used instead of the compound represented by Formula 2A when preparing the non-aqueous electrolyte.

[0120] [ka]

[0121] Comparative Example 6-1. A lithium secondary battery was fabricated in the same manner as in Example 1, except that instead of the slurry described in Example 1, a negative active material slurry was prepared by blending SiO and artificial graphite as the negative active material in a weight ratio of 2:8 in distilled water as a solvent, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as a binder, carbon black as a conductive material, and sodium carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 96.0:2.3:0.7:1.

[0122] Comparative Example 6-2. A lithium secondary battery was fabricated in the same manner as in Comparative Example 6-1, except that the compound represented by Formula 2A was not added during the preparation of the non-aqueous electrolyte.

[0123] Comparative Example 6-3. A lithium secondary battery was fabricated in the same manner as in Comparative Example 6-1, except that the compound represented by Formula 1A was not added during the preparation of the non-aqueous electrolyte.

[0124] <Experimental Example> Experimental Example 1: Evaluation of gas generation amount after high temperature (45°C) cycle The lithium secondary batteries prepared in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3 were subjected to an activation process, and then constant current / constant voltage (CC / CV) charging (0.05C cut off) was performed at 45°C at a rate of 1C up to 4.2V, and constant current (CC) discharging was performed at a rate of 0.5C down to 3.0V.

[0125] In addition, the batteries charged after 200 cycles were transferred to a charger / discharger at room temperature (25°C), and the gas collected in the pouch was analyzed using GC-TCD (gas chromatography-thermal conductivity detector). The gas generation amount measured in Comparative Example 1 was set to 100%, and the relative gas generation amount of each battery was calculated and shown in Table 1 below.

[0126] Experimental Example 2: Evaluation of gas generation rate after high temperature (60°C) storage The lithium secondary batteries prepared in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3 were subjected to an activation process and then charged at 25°C at a rate of 0.33C up to 4.2V under constant current / constant voltage conditions (0.05C cut-off) and fully charged to 100% SOC. The fully charged batteries were stored at 60°C for 8 weeks and then transferred to a charger / discharger at room temperature (25°C). The gas collected in the pouch was analyzed using a GC-TCD (gas chromatography-thermal conductivity detector). The relative gas generation amount of each battery was calculated, assuming the gas generation amount measured in Comparative Example 1 as 100%, and is shown in Table 1 below.

[0127] [Table 1]

[0128] From the results in Table 1, it can be seen that when the first additive and the second additive of the present invention are simultaneously contained as electrolyte additives, it is effective in reducing the amount of gas generated after high-temperature cycling and high-temperature storage of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0129] Specifically, it was confirmed that the gas generation reduction effect was very small when the first additive or the second additive was used alone (Comparative Examples 2 and 3) compared to when neither the first nor the second additive was used (Comparative Example 1). Furthermore, the compound of formula Z1 used in Comparative Example 4 is a propargyl-substituted imidazole compound. When used in place of the first additive of the present invention, it was confirmed that the gas generation reduction effect was significantly reduced. Furthermore, the compound of formula Z2 used in Comparative Example 5 is a cyclic phosphate compound like the second additive of the present invention, but it has a methyl group at the terminal (R2 in Formula 2) instead of a fluoroalkyl group. This resulted in a greater increase in film resistance and a greater increase in gas generation compared to Comparative Example 1, which did not use any additives. This phenomenon occurs because fluoroalkyls contribute to the inclusion of components with excellent durability and ionic conductivity, such as LiF, in the SEI film, while methyl groups only produce polyolefin components that lack conductivity and reduce durability. That is, a portion of the SEI film is lost due to its inability to withstand the volume expansion of the Si anode, and as cycling progresses, another film is continuously formed in the lost portion due to a side reaction of the electrolyte, which exacerbates the increase in film resistance.

[0130] On the other hand, in the case of Comparative Example 6-1, in which a mixture of SiO and graphite was used instead of Si as the negative active material, not only was the amount of gas generated greater than that of Example 1, but the effect of reducing gas generation was also insignificant compared to Comparative Examples 6-2 and 6-3, in which the first additive and the second additive were used alone, respectively.Comparative Examples 6-1 to 6-3 demonstrate that the combination of the first additive and the second additive of the present invention exhibits a particularly significant effect in improving the amount of high-temperature gas generation in batteries containing a pure silicon negative active material.

[0131] Furthermore, it can be confirmed that among Examples 1 to 6, Examples 3 and 4, in which the weight ratio of the second additive to the first additive (W2 / W1) was 15 or more, had the most excellent effect in reducing the amount of gas generated.

[0132] Experimental Example 3: Evaluation of capacity retention rate after high-temperature (45°C) cycling The lithium secondary batteries prepared in Examples 1 to 6 and Comparative Examples 1 to 5 were subjected to an activation process, and then constant current / constant voltage (CC / CV) charging (0.05C cut off) was performed at 45°C at a rate of 1C up to 4.2V, and constant current (CC) discharging was performed at a rate of 0.5C down to 3.0V.

[0133] Each charge / discharge cycle was counted as one cycle, and the same charge / discharge cycle was repeated 200 times. The capacity retention rate after 200 cycles relative to the initial discharge capacity after one cycle was measured, and the results are shown in Table 2 below.

[0134] [Table 2]

[0135] From the results of Table 2, it can be seen that when the first additive and the second additive of the present invention are simultaneously contained as electrolyte additives, it is effective in improving the high-temperature life of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0136] Specifically, it was confirmed that the capacity retention rate was significantly improved compared to the case where neither the first nor the second additive was included (Comparative Example 1), as well as the case where only the first or second additive was used alone (Comparative Examples 2 and 3). Furthermore, it was confirmed that the compound of formula Z1 used in Comparative Example 4 and the compound of formula Z2 used in Comparative Example 5 have similar structures to the first and second additives of the present invention, respectively, but actually worsen the life characteristics.

[0137] On the other hand, it can be confirmed that among Examples 1 to 6, Examples 3 and 4, in which the weight ratio of the second additive to the first additive (W2 / W1) was 15 or more, had the most excellent effect on improving the capacity retention rate.

[0138] Experimental Example 4: Evaluation of rapid charging performance After the activation process, each of the lithium secondary batteries prepared in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3 was subjected to constant current / constant voltage (CC / CV) charging (0.05C cutoff) at 25°C to 4.2V at a rate of 1C, and constant current (CC) discharging at a rate of 0.5C to 3.0V to measure the initial discharge capacity, initial resistance, and initial volume. The volume was measured using a buoyancy method.

[0139] Next, the lithium secondary battery was adjusted to a 15% SOC (State Of Charge) state, and then charged at 25°C while changing the C-rate according to the SOC state as shown in Table 3 below. The voltage profile was measured by checking the voltage value at 1-second intervals for each charging section.

[0140] [Table 3]

[0141] Then, the end conditions were set based on the voltage values obtained in each section, and the charge amount when charged in CC mode was recorded. Then, the battery was discharged again in CC mode at 0.33C to SOC 15%.

[0142] The charging and discharging was counted as one cycle, and after 100 cycles, the capacity retention rate, resistance increase rate, and volume increase rate were calculated using the following Equations 1 to 3, and the results are shown in Table 4 below.

[0143] [Formula 1] Capacity retention rate (%) = (discharge capacity after 100 cycles / initial discharge capacity) x 100 [Formula 2] Resistance increase rate (%) = {(resistance after 100 cycles - initial resistance) / initial resistance} x 100 [Formula 3] Volume increase rate (%) = {(volume after 100 cycles - initial volume) / initial volume} x 100

[0144] [Table 4]

[0145] From the results of Table 4, it can be seen that when the first additive and the second additive of the present invention are simultaneously contained as electrolyte additives, it is effective in improving the performance after fast charging of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0146] Specifically, the battery according to the present invention exhibited significantly improved capacity, resistance, and volume after fast charging compared to a battery containing neither the first nor second additive (Comparative Example 1) as well as a battery containing only the first or second additive (Comparative Examples 2 and 3). Furthermore, the compound of formula Z1 used in Comparative Example 4 was found to have a minimal effect on improving fast charging performance compared to the first additive of the present invention, while the compound of formula Z2 used in Comparative Example 5 actually worsened fast charging performance. Furthermore, Comparative Example 6-1, which used a mixture of SiO and graphite instead of Si as the anode active material, also exhibited a lower capacity retention rate and a higher resistance increase rate and volume increase rate after fast charging compared to Example 1.

[0147] Meanwhile, among Examples 1 to 6, it can be seen that Examples 3 and 4, in which the weight ratio (W2 / W1) of the second additive to the first additive is 15 or more, exhibited the best performance after fast charging.

Claims

1. a non-aqueous electrolyte solution comprising a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material made of silicon; and A lithium secondary battery comprising a separator interposed between the positive electrode and the negative electrode: 【Chemical 1】 In the above Chemical Formula 1, A is a heterocycle having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms; R1 is an alkylene group having 1 to 3 carbon atoms; 【Chemistry 2】 In the above Chemical Formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorine atoms.

2. 2. The lithium secondary battery according to claim 1, wherein A in Chemical Formula 1 is a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms.

3. R2 in Formula 2 is —(CR ２ ) ｎ CF ３ and wherein R is hydrogen or fluorine; 2. The lithium secondary battery according to claim 1, wherein n is an integer of 0 to 9.

4. The lithium secondary battery of claim 1 , wherein the content of the first additive is 0.05 wt % to 10 wt % based on the total weight of the non-aqueous electrolyte.

5. The lithium secondary battery of claim 1 , wherein the content of the second additive is 0.1 wt % to 15 wt % based on the total weight of the non-aqueous electrolyte.

6. 6. The lithium secondary battery according to claim 1, wherein the weight ratio of the second additive to the first additive is 1.5 or more.

7. 6. The lithium secondary battery according to claim 1, wherein the weight ratio of the second additive to the first additive is 6 or more.

8. 2. The lithium secondary battery of claim 1, wherein the non-aqueous electrolyte further comprises at least one third additive selected from the group consisting of vinylene carbonate, 1,3-propane sultone, 1,3-propene sultone, and lithium difluorophosphate.

9. 2. The lithium secondary battery according to claim 1, wherein the organic solvent comprises a mixture of a fluorinated cyclic carbonate solvent and a chain carbonate solvent.

10. 10. The lithium secondary battery according to claim 9, wherein the chain carbonate solvent contains diethyl carbonate, ethyl methyl carbonate, or a mixture thereof.

11. 2. The lithium secondary battery according to claim 1, wherein the concentration of the lithium salt in the mixed solution of the lithium salt and the organic solvent is 1.5 M or more.

12. 2. The lithium secondary battery according to claim 1, wherein the positive electrode active material comprises a lithium composite transition metal oxide represented by the following chemical formula 3: [Chemical formula 3] Li １＋ｘ (N ａ Co ｂ Mn ｃ M ｄ )O ２ In the above Chemical Formula 3, M is at least one 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; 1 + x, a, b, c, and d are the atomic fractions of each independent element, -0.2≦x≦0.2, 0.60≦a<1, 0<b≦0.30, 0<c≦0.30, 0≦d≦0.10, and a+b+c+d=1.

13. 2. The lithium secondary battery according to claim 1, wherein the positive electrode active material contains a lithium composite transition metal oxide in which the molar ratio of nickel among the transition metals is 70 mol % or more.

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