Nonaqueous electrolyte for lithium secondary battery and lithium secondary battery containing same

Abstract

The present invention provides a non-aqueous electrolyte for lithium secondary batteries that contains a phosphonium salt-based ionic liquid as an additive, and by using the non-aqueous electrolyte for lithium secondary batteries, the present invention can provide a lithium secondary battery that suppresses resistance at low temperatures and has excellent flame retardancy and high-temperature stability.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0040774 filed on March 31, 2022 and Korean Patent Application No. 10-2023-0041707 filed on March 30, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery containing a phosphonium salt-based ionic liquid, and a lithium secondary battery containing the same. [Background technology]

[0003] As modern society becomes increasingly dependent on electrical energy, the amount of electrical energy produced is also increasing. To address environmental issues arising from the production of electrical energy, renewable energy power generation is gaining attention as a next-generation power generation system. Because renewable energy sources exhibit intermittent power generation characteristics, a large-capacity energy storage device is essential for a stable power supply. Among these energy storage devices, lithium-ion batteries are gaining attention as they offer the highest energy density among currently commercially available devices.

[0004] The lithium ion battery mainly comprises a positive electrode made of a transition metal oxide containing lithium, a negative electrode capable of storing lithium, a non-aqueous electrolyte that serves as a medium for transferring lithium ions, and a separator.

[0005] In particular, non-aqueous electrolytes are known to be components that have a significant impact on the stability and safety of batteries, and much research has been conducted on this topic.

[0006] Generally, non-aqueous electrolytes have mainly been liquid electrolytes in which electrolyte salts are dissolved in organic solvents. However, such liquid electrolytes have drawbacks such as low stability due to the high volatility of the organic solvent in high-temperature atmospheres, the generation of large amounts of gas through side reactions with the electrodes, and the risk of combustion due to the temperature rise of the battery itself. Furthermore, performance degradation occurs due to the increase in resistance at the interface between the electrodes and the non-aqueous electrolyte in low-temperature atmospheres. Therefore, there is a demand for the development of a lithium secondary battery that is flame-retardant and has improved performance in high-temperature and low-temperature atmospheres. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above problems, and has an object to provide a non-aqueous electrolyte solution for a lithium secondary battery containing a phosphonium salt-based ionic liquid.

[0008] Another object of the present invention is to provide a lithium secondary battery that contains the nonaqueous electrolyte solution for a lithium secondary battery, thereby improving flame retardancy, output characteristics in a low-temperature atmosphere, and stability and storage characteristics in a high-temperature atmosphere. [Means for solving the problem]

[0009] In order to achieve the above object, one embodiment of the present invention comprises: The present invention provides a non-aqueous electrolyte solution for a lithium secondary battery, which comprises a lithium salt, a non-aqueous organic solvent, and an ionic liquid represented by the following chemical formula 1:

[0010] [ka]

[0011] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R1 to R3 each independently represent an alkyl group having 1 to 8 carbon atoms or an aryl group having 6 to 12 carbon atoms; X- is BF4 - , PF6 - , ClO4 - , PO2F2 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , SO3CF3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , B(C2O4)2 - , and BF2(C2O4) - and at least one anion selected from the group consisting of:

[0012] Another embodiment of the present invention is The present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte solution, wherein the non-aqueous electrolyte solution comprises the non-aqueous electrolyte solution for lithium secondary batteries of the present invention. [Effects of the Invention]

[0013] The phosphonium salt-based ionic liquid represented by Chemical Formula 1 contained in the non-aqueous electrolyte of the present invention contains a propargyl group in its structure, and thus can form a low-resistance, robust coating on the surface of an electrode through an oxidation / reduction reaction. Therefore, when the non-aqueous electrolyte of the present invention containing the phosphonium salt-based ionic liquid represented by Chemical Formula 1 is used, it is possible to improve flame retardancy and minimize the increase in resistance at low temperatures, thereby improving capacity characteristics and realizing a lithium secondary battery that can ensure excellent high-temperature storage stability.

[0014] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the above-described content of the invention, serve to further understand the technical concept of the present invention. Therefore, the present invention should not be interpreted as being limited to only the matters depicted in the drawings. [Brief explanation of the drawings]

[0015] [Figure 1] 10 is a graph showing the evaluation results of the rate of increase in thickness during high-temperature storage of the lithium secondary battery of Experimental Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will now be described in more detail. The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0017] Nonaqueous electrolyte for lithium secondary batteries Specifically, one embodiment of the present invention is The present invention provides a non-aqueous electrolyte solution for a lithium secondary battery, which comprises a lithium salt, a non-aqueous organic solvent, and an ionic liquid represented by the following chemical formula 1:

[0018] [ka]

[0019] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R1 to R3 each independently represent an alkyl group having 1 to 8 carbon atoms or an aryl group having 6 to 12 carbon atoms; X - is BF4 - , PF6 - , ClO4 - , PO2F2 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , SO3CF3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N -, CF3CF2SO3 - , CF3CF2(CF3)2CO - , B(C2O4)2 - , and BF2(C2O4) - and at least one anion selected from the group consisting of:

[0020] (1) Lithium salt First, the lithium salt will be described as follows. As the lithium salt, any salt commonly used in non-aqueous electrolytes for lithium secondary batteries can be used without limitation. For example, the cation may be Li + 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 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO -, (CF3SO2)2CH - , CF3(CF2)7SO3 - , and SCN - The composition may include at least one selected from the group consisting of:

[0021] Specifically, the lithium salts include LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 The lithium salt may include a single substance or a mixture of two or more substances selected from the group consisting of LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(pentafluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2), and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2). In addition to the above-mentioned lithium salts, lithium salts commonly used in electrolytes for lithium secondary batteries can be used without limitation. Specifically, the lithium salt may include LiPF6.

[0022] The lithium salt may be varied as appropriate within a generally usable range, but in order to obtain an optimal effect of forming a coating for preventing corrosion on the electrode surface, it may be contained in the electrolyte at a concentration of 0.8 M to 3.0 M, specifically 1.0 M to 3.0 M. When the concentration of the lithium salt satisfies the above range, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, and the mobility of lithium ions can be improved, resulting in improved capacity characteristics and cycle characteristics of the lithium secondary battery.

[0023] (2) Non-aqueous organic solvent The non-aqueous organic solvent is described below. The non-aqueous organic solvent may be any of various organic solvents commonly used in non-aqueous electrolytes, and the type of organic solvent is not limited as long as it can minimize decomposition due to oxidation reactions during the charge / discharge process of the secondary battery and can exhibit the desired properties together with the additives.

[0024] Specifically, the non-aqueous organic solvent may include a cyclic carbonate organic solvent, a linear carbonate organic solvent, a linear ester organic solvent, or a mixture thereof.

[0025] The cyclic carbonate organic solvent is a high-viscosity organic solvent that has a high dielectric constant and effectively dissociates the lithium salt in the non-aqueous electrolyte solution. Specific examples of the cyclic carbonate organic solvent include at least one organic solvent selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and among these, ethylene carbonate may be included.

[0026] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and specific examples thereof may include at least one organic solvent 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, and specifically may include ethyl methyl carbonate (EMC).

[0027] The linear ester-based organic solvent is a solvent that is relatively more stable at high temperatures and high voltages than cyclic carbonate-based organic solvents, and can improve the drawback of cyclic carbonate-based organic solvents, which cause gas generation during high voltage operation, while also achieving high ionic conductivity.

[0028] Specific examples of the linear ester organic solvent include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, and specifically, at least one of ethyl propionate and propyl propionate.

[0029] Furthermore, the non-aqueous electrolyte solution of the present invention may further contain a cyclic ester organic solvent, if necessary. The cyclic ester organic solvent may contain at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0030] Meanwhile, the remainder of the non-aqueous electrolyte solution of the present invention, excluding the lithium salt and the compound represented by Chemical Formula 1 as an additive, may be a non-aqueous organic solvent unless otherwise specified.

[0031] (3) Ionic liquid represented by chemical formula 1 The nonaqueous electrolyte solution for a lithium secondary battery of the present invention may contain an ionic liquid represented by the following chemical formula 1 as an additive.

[0032] [ka]

[0033] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R1 to R3 each independently represent an alkyl group having 1 to 8 carbon atoms or an aryl group having 6 to 12 carbon atoms; X - is BF4 - , PF6 - , ClO4 - , PO2F2 - , CF3SO3 - , CH3CO2 - , CF3CO2 -, SO3CF3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , B(C2O4)2 - , and BF2(C2O4) - and at least one anion selected from the group consisting of:

[0034] The phosphonium salt-based ionic liquid represented by Chemical Formula 1 contains a propargyl group in its structure, which provides more stable oxidation / reduction reactivity than phosphonium salt-based ionic liquids containing double bonds. This can further promote the reaction of coating additives such as ethylene carbonate (EC), vinylene carbonate (VC), or vinylethylene carbonate (VEC). This allows for the formation of a robust, low-resistance copolymerized coating on the electrode surface, further enhancing the durability of the coating. Furthermore, as a non-volatile compound, the phosphonium salt-based ionic liquid represented by Chemical Formula 1 has higher flame retardancy than organic solvents such as ethylene carbonate (EC), which are the main components of conventional non-aqueous electrolytes. This improves the flame retardancy of lithium secondary batteries at high temperatures, thereby reducing the risk of fire. Therefore, when the non-aqueous electrolyte of the present invention containing the phosphonium salt-based ionic liquid represented by Chemical Formula 1 is used, it can improve flame retardancy and minimize resistance increases at low temperatures, thereby achieving lithium secondary batteries with improved low-temperature output characteristics, stability, and storage characteristics at high temperatures.

[0035] Specifically, in the above Chemical Formula 1, R is an alkylene group having 1 to 3 carbon atoms, R1 to R3 are each independently an alkyl group having 1 to 5 carbon atoms or a phenyl group, and X - is BF4 - , PF6 - , ClO4 - , PO2F2 - , CF3SO3 - , SO3CF3 -, (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , and BF2(C2O4) - The anion may be at least one of the following: In addition, in the above chemical formula 1, R1 to R3 may each independently be a methyl group, an ethyl group, or a phenyl group.

[0036] More specifically, the ionic liquid represented by Chemical Formula 1 may be at least one of the compounds represented by Chemical Formulas 1-1 to 1-4 below.

[0037] [ka]

[0038] [ka]

[0039] [ka]

[0040] [ka]

[0041] The ionic liquid represented by Chemical Formula 1 may be included in an amount of 0.3 wt % to 50 wt % based on the total weight of the non-aqueous electrolyte solution. When the content of the ionic liquid represented by Chemical Formula 1 satisfies the above range, a stable passivation film with low resistance is formed on the surfaces of the positive and negative electrodes, ensuring safe high-temperature and low-temperature capacity characteristics, and enhancing flame retardancy, thereby further improving the performance of the lithium secondary battery under high-temperature and low-temperature atmospheres.

[0042] Specifically, the ionic liquid represented by Chemical Formula 1 may be contained in an amount of 0.5 wt % to 30 wt %, preferably 0.5 wt % to 20 wt %, based on the total weight of the non-aqueous electrolyte.

[0043] (4) Other additives In addition, the nonaqueous electrolyte solution for a lithium secondary battery of the present invention may further contain other additives as needed to prevent the negative electrode from collapsing due to decomposition of the nonaqueous electrolyte solution in a high-power environment, and to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effect of suppressing battery expansion at high temperatures.

[0044] Representative examples of such other additives may include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, nitrile-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0045] Examples of the cyclic carbonate compounds include vinylene carbonate (VC) and vinylethylene carbonate. The halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC).

[0046] The sultone compound includes at least one compound 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.

[0047] Examples of the sulfate-based compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0048] The phosphate-based compound may be one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphate. Examples of the borate compounds include tetraphenylborate and lithium oxalyl difluoroborate.

[0049] The nitrile compound may be at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0050] The benzene-based compound may be fluorobenzene, the amine-based compound may be triethanolamine or ethylenediamine, and the silane-based compound may be tetravinylsilane.

[0051] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and may be one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bis(oxalato)borate (LiB(C2O4)2)), and LiBF4.

[0052] Among these other additives, when vinylene carbonate, vinyl ethylene carbonate, or succinonitrile is contained, a stronger SEI film can be formed on the surface of the negative electrode during the initial activation process of the secondary battery.

[0053] Meanwhile, two or more of the other additives may be mixed and used, and may be included in an amount of 50 wt % or less, specifically 0.01 wt % to 10 wt %, and preferably 0.05 wt % to 5.0 wt %, based on the total weight of the nonaqueous electrolyte. If the content of the other additives is less than 0.01 wt %, the effects of improving the low-temperature output, high-temperature storage characteristics, and high-temperature life characteristics of the battery are negligible. If the content of the other additives is more than 50 wt %, excessive side reactions may occur in the electrolyte during battery charge and discharge. In particular, if the SEI film-forming additive is added in excess, it may not be sufficiently decomposed at high temperatures and may remain in the electrolyte at room temperature as an unreacted product or precipitate. This may result in side reactions that reduce the life or resistance characteristics of the secondary battery.

[0054] Lithium secondary battery Another embodiment of the present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte solution of the present invention.

[0055] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly in which a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode are sequentially stacked, housing the electrode assembly in a battery case, and then injecting the nonaqueous electrolyte solution of the present invention into the battery case.

[0056] The method for manufacturing the lithium secondary battery of the present invention may be manufactured and applied by a conventional method well known in the art, and will be described in detail below.

[0057] (1) Positive electrode The positive electrode according to the present invention may include a positive electrode active material layer containing a positive electrode active material, and, if necessary, the positive electrode active material layer may further include a conductive material and / or a binder.

[0058] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium composite metal oxide represented by the following Chemical Formula 2 containing at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and aluminum (Al) and lithium.

[0059] [Chemical Formula 2] Li 1+a Ni x Co y M 1 z M 2 w O2

[0060] In Chemical Formula 2, M 1 is Mn, Al, or a combination thereof, M 2 is at least one selected from the group consisting of Al, Zr, W, Ti, Mg, Ca, and Sr, and 0 ≦ a ≦ 0.5, 0 < x ≦ 1.0, 0 < y ≦ 0.4, 0 < z ≦ 0.4, 0 ≦ w ≦ 0.1.

[0061] The 1 + a indicates the atomic fraction of lithium in the lithium transition metal oxide, and at this time, 0 ≦ a ≦ 0.5, preferably 0 ≦ a ≦ 0.2, more preferably 0 ≦ a ≦ 0.1 may be satisfied.

[0062] The x indicates the atomic fraction of nickel among all transition metal elements in the lithium transition metal oxide, and 0 < x ≦ 1.0, specifically 0.55 < x < 1.0, more specifically 0.6 ≦ x ≦ 0.98, and even more specifically 0.6 ≦ x ≦ 0.95 may be satisfied.

[0063] Said y represents the atomic fraction of cobalt among all transition metal elements in the lithium transition metal oxide, where 0 < y ≦ 0.4, specifically 0 < y ≦ 0.3, and more specifically 0.05 ≦ y ≦ 0.3 may also be applicable.

[0064] Said z represents the atomic fraction of element M among all transition metal elements in the lithium transition metal oxide, where 0 < z ≦ 0.4, preferably 0 < z ≦ 0.3, and more preferably 0.01 ≦ z ≦ 0.3 may also be applicable. 1

[0065] Said w represents the atomic fraction of element M among all transition metal elements in the lithium transition metal oxide, where 0 < w ≦ 0.1, preferably 0 < w ≦ 0.05, and more preferably 0 < w ≦ 0.02. 2

[0066]

[0067] Specifically, in order to realize a high-capacity battery, the positive electrode active material contains Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.7 Mn 0.2 Co 0.1 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2, or may contain a lithium composite transition metal oxide such as Li(Ni 0.90 Mn 0.05 Co 0.05 )O2.

[0067] In addition, the positive electrode active material of the present invention, together with the lithium composite metal oxide represented by the chemical formula 2, includes lithium-manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (e.g., LiCoO2, etc.), lithium-nickel-based oxides (e.g., LiNiO2, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi 1-Y Mn Y O2(0 < Y < 1), LiMn 2-z Ni z O4(0 < Z < 2), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2(0 < Y1 < 1), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2(0 < Y2 < 1), LiMn 2-z1 Co z1 O4(0 < Z1 < 2), or Li(Ni p1 Co q1 Mn r2 )O4(0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc., may be used in combination.

[0068] The positive electrode active material may be contained in an amount of 80% to 99% by weight, specifically 90% to 99% by weight, based on the total weight of the solid content in the positive electrode slurry. At this time, when the content of the positive electrode active material is 80% by weight or less, the energy density may decrease and the capacity may decline.

[0069] The conductive material is used to impart conductivity to the electrodes, and can be any material that does not cause chemical changes in the resulting battery and has electronic conductivity. Specific examples include carbon powders such as carbon black, acetylene black (or Denka Black), Ketjen Black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite, which have highly developed crystalline structures; conductive fibers such as carbon fiber and metal fiber; 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 conductive materials such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may be contained in an amount of 0.1% by weight to 10% by weight, preferably 0.1% by weight to 5.0% by weight, based on the total weight of the positive electrode active material layer.

[0070] Next, the binder serves to improve adhesion between particles of the positive electrode active material and the adhesive strength between the positive electrode active material and the current collector. Examples of such binders include fluororesin-based binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based binders including styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol-based binders including polyvinyl alcohol; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders, which may be used alone or in combination of two or more. The binder may be contained in an amount of 0.1% by weight to 15% by weight, preferably 0.1% by weight to 10% by weight, based on the total weight of the positive electrode active material layer.

[0071] The positive electrode of the present invention may be manufactured by a method known in the art, such as by dissolving or dispersing a positive electrode active material, a binder, and / or a conductive material in a solvent to prepare a positive electrode slurry, which is then coated on a positive electrode current collector, followed by drying and rolling to form an active material layer, or by casting the positive electrode slurry on a separate support, peeling off the support, and laminating the resulting film on the positive electrode current collector.

[0072] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and the surface of the current collector may be formed with fine irregularities to enhance the adhesive strength of the positive electrode material. It may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0073] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of the solvent used is not particularly limited as long as it can be adjusted to an appropriate viscosity for the cathode composite, taking into consideration the coating thickness of the cathode composite, production yield, workability, etc.

[0074] (2) Negative electrode Next, the negative electrode will be described. The negative electrode according to the present invention includes a negative electrode active material layer containing a negative electrode active material, and the negative electrode active material layer may further include a conductive material and / or a binder, as necessary.

[0075] The negative electrode active material may be any of various negative electrode active materials used in the art, such as a carbon-based negative electrode active material, a silicon-based negative electrode active material, or a mixture thereof.

[0076] According to one embodiment, the negative electrode active material may include a carbon-based negative electrode active material, and the carbon-based negative electrode active material may include various carbon-based negative electrode active materials used in the art, such as graphite-based materials such as natural graphite, artificial graphite, and Kish graphite; pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes; soft carbon; and hard carbon. The shape of the carbon-based negative electrode active material is not particularly limited, and materials of various shapes, such as amorphous, plate-like, flake-like, spherical, or fibrous, may be used.

[0077] Preferably, the negative electrode active material may be at least one carbon-based negative electrode active material selected from natural graphite and artificial graphite, or both natural graphite and artificial graphite may be used to increase adhesive strength with the current collector and suppress detachment of the active material.

[0078] According to another embodiment, the negative electrode active material may be a combination of the carbon-based negative electrode active material and the silicon-based negative electrode active material. The silicon-based negative electrode active material is, for example, metal silicon (Si), silicon oxide (SiO x, where 0 < x < 2), silicon carbide (SiC), and Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si) may be included. As the element Y, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof may be selected.

[0079] Since the silicon-based negative electrode active material exhibits higher capacity characteristics than the carbon-based negative electrode active material, when the silicon-based negative electrode active material is further included, even more excellent capacity characteristics can be obtained. However, in the case of a negative electrode containing a silicon-based negative electrode active material, the SEI film contains a greater amount of an oxygen (O)-rich component than in the case of a graphite negative electrode, and the SEI film containing an oxygen-rich component tends to be more easily decomposed when a Lewis acid such as HF or PF5 is present in the electrolytic solution. Therefore, in the case of a negative electrode containing a silicon-based negative electrode active material, in order to maintain a stable SEI film, it is necessary to suppress the generation of Lewis acids such as HF and PF5 in the electrolytic solution or to remove (or scavenge) the generated Lewis acids. Since the non-aqueous electrolytic solution according to the present invention contains an electrolytic solution additive capable of forming a stable film on the positive electrode and the negative electrode, it is possible to effectively suppress the decomposition of the SEI film when using a negative electrode containing a silicon-based active material.

[0080] On the other hand, the mixing ratio of the silicon-based negative electrode active material and the carbon-based negative electrode active material may be 3:97 to 99:1, preferably 5:95 to 15:85, by weight. When the mixing ratio of the silicon-based negative electrode active material and the carbon-based negative electrode active material satisfies the above range, while improving the capacity characteristics, the volume expansion of the silicon-based negative electrode active material can be suppressed, and excellent cycle performance can be ensured.

[0081] The negative electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode active material layer. When the content of the negative electrode active material satisfies this range, excellent capacity characteristics and electrochemical characteristics can be obtained.

[0082] The conductive material may be added to further improve the conductivity of the negative electrode active material layer. It is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, the conductive material may be carbon powder such as carbon black, acetylene black (or denka black), ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fiber or metal fiber; 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; or conductive materials such as polyphenylene derivatives. The conductive material may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer.

[0083] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and examples thereof include fluororesin-based binders including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based binders including styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol-based binders including polyvinyl alcohol; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders. The binder may be contained in an amount of usually 0.1% by weight to 15% by weight, preferably 0.1% by weight to 10% by weight, based on the total weight of the negative electrode active material layer.

[0084] The negative electrode may be manufactured by a method known in the art, for example, by coating a negative electrode current collector with a negative electrode slurry prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent, followed by rolling and drying to form an active material layer, or by casting the negative electrode slurry on a separate support, peeling off the support, and laminating the resulting film on the negative electrode current collector.

[0085] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and, like the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0086] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of the solvent used is not particularly limited as long as it is adjusted to an appropriate viscosity for the negative electrode slurry, taking into consideration the coating thickness of the negative electrode composite, production yield, workability, etc.

[0087] (3) Separator The separator included in the lithium secondary battery of the present invention may be a commonly used conventional porous polymer film, for example, a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminate thereof, or a conventional porous nonwoven fabric, for example, a nonwoven fabric made from a high-melting point glass fiber, polyethylene terephthalate fiber, or the like, but is not limited thereto.

[0088] 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.

[0089] [Example] Example 1 (Production of non-aqueous electrolyte) LiPF6 was dissolved in a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 to a concentration of 1.0 M, and then 0.5 wt% of the compound represented by Formula 1-1 and other additives, such as 1.0 wt% vinylene carbonate, 1.0 wt% 1,3-propane sultone, and 1.0 wt% ethylene sulfate, were added to prepare a non-aqueous electrolyte.

[0090] (Secondary battery manufacturing) N-methyl-2-pyrrolidone (NMP) was used as the positive electrode active material (Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2), a conductive material (carbon black), and a binder (polyvinylidene fluoride) were mixed in a weight ratio of 97.5:1:1.5 to prepare a positive electrode slurry (solid content: 50 wt%). The positive electrode slurry was applied to a 12 μm-thick aluminum (Al) thin film as a positive electrode current collector, dried, and then roll-pressed to prepare a positive electrode.

[0091] Anode active material (graphite), binder (SBR-CMC), and conductive material (carbon black) were mixed in a weight ratio of 95:3.5:1.5 with water as a solvent to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to a 6 μm-thick copper (Cu) thin film as anode current collector, dried, and then roll-pressed to prepare anode.

[0092] 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, which was then placed in a pouch-type battery case, and the non-aqueous electrolyte for lithium secondary batteries was injected into the case to prepare a pouch-type lithium secondary battery with a driving voltage of 4.45 V or more.

[0093] Example 2. (Production of non-aqueous electrolyte) LiPF6 was dissolved in a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 to a concentration of 1.0 M, and then 2.0 wt% of the compound represented by Formula 1-1 and other additives, such as 1.0 wt% vinylene carbonate, 1.0 wt% 1,3-propane sultone, and 1.0 wt% ethylene sulfate, were added to prepare a non-aqueous electrolyte.

[0094] (Secondary battery manufacturing) A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte solution prepared above was injected.

[0095] Example 3 (Production of non-aqueous electrolyte) LiPF6 was dissolved in a non-aqueous organic solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 to a concentration of 1.0 M, and then the solution was dissolved in the solution of the compound represented by the formula 1. -1 and other additives, 1.0 wt % of vinylene carbonate, 1.0 wt % of 1,3-propane sultone, and 1.0 wt % of ethylene sulfate, to prepare a non-aqueous electrolyte solution.

[0096] (Secondary battery manufacturing) A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte solution prepared above was injected.

[0097] Comparative example. A non-aqueous electrolyte for a lithium secondary battery and a pouch-type lithium secondary battery containing the same were manufactured in the same manner as in Example 1, except that LiPF was dissolved to a concentration of 1.0 M in a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70, and 1.0 wt % of vinylene carbonate, 1.0 wt % of 1,3-propane sultone, and 1.0 wt % of ethylene sulfate were added to prepare a non-aqueous electrolyte.

[0098] [Experimental Example] Experimental example 1: Evaluation of low-temperature output characteristics The lithium secondary batteries prepared in Examples 1 and 2 and the lithium secondary battery prepared in Comparative Example were each charged at a constant current of 0.04 C to 4.2 V at room temperature (25° C.), and then discharged to 3.0 V under constant current (CC) conditions. Each cycle was repeated three times, and the initial discharge capacities were measured and shown in Table 1 below.

[0099] Next, the battery was charged at a constant current of 0.04 C to a state of charge (SOC) of 10%, and then stored at a low temperature (-10°C) for 1 hour. The discharge capacity after 1 hour was calculated as a percentage based on the initial discharge capacity. The results are shown in Table 1 below.

[0100] [Table 1]

[0101] Referring to Table 1, it can be seen that the lithium secondary batteries of Examples 1 and 2 of the present invention have slightly lower initial capacities at room temperature than the lithium secondary battery of the Comparative Example, but have improved capacity retention rates after low-temperature storage compared to the lithium secondary battery of the Comparative Example.

[0102] Experimental Example 2: Evaluation of high-temperature storage characteristics The lithium secondary batteries prepared in Examples 1 and 2 and the lithium secondary battery prepared in Comparative Example were each charged at a constant current of 0.04 C to 4.2 V at room temperature (25° C.), and then discharged to 3.0 V under constant current (CC) conditions. Each cycle was counted as one cycle, and the initial discharge capacity was measured.

[0103] Next, the battery was charged at a constant current of 0.04 C to 100% SOC (state of charge), and then stored at high temperature (60°C) for 20 weeks. The thickness increase rate was measured every 4 weeks, and the results are shown in Figure 1.

[0104] Referring to FIG. 1, it can be seen that the lithium secondary batteries of Examples 1 and 2 of the present invention have a relatively reduced thickness increase rate compared to the comparative example due to a reduced amount of gas generated during high-temperature storage.

[0105] Experimental Example 3: Evaluation of high temperature stability characteristics The lithium secondary battery prepared in Example 3 and the lithium secondary battery prepared in Comparative Example were each charged at a constant current of 0.04 C to 4.2 V at room temperature (25° C.), and then discharged to 3.0 V under constant current (CC) conditions. Each cycle consisted of three cycles, and the initial discharge capacities were measured.

[0106] Next, the battery was charged at a constant current of 0.04 C to a SOC (state of charge) of 100%, and then the amount of heat generated inside the battery was measured while the temperature was raised to a high temperature (300°C). The results are shown in Table 2 below.

[0107] [Table 2]

[0108] Referring to Table 2, it can be seen that the lithium secondary battery of Example 3 of the present invention has improved high-temperature stability due to a decrease in the amount of heat generated at high temperatures compared to the lithium secondary battery of the Comparative Example.

Claims

1. A non-aqueous electrolyte solution for a lithium secondary battery, comprising a lithium salt, a non-aqueous organic solvent, and an ionic liquid represented by the following chemical formula 1: 【Chemical 1】 In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R １ ~R ３ are each independently an alkyl group having 1 to 8 carbon atoms or an aryl group having 6 to 12 carbon atoms, X － is BF ４ － , P.F. ６ － , ClO ４ － , P.O. ２ F ２ － , C.F. ３ SO ３ － , C.H. ３ CO ２ － , C.F. ３ CO ２ － , S.O. ３ CF ３ － , (CF ３ CF ２ SO ２ ) ２ N － , (CF ３ SO ２ ) ２ N － , (FSO ２ ) ２ N － , C.F. ３ CF ２ SO ３ － , C.F. ３ CF ２ (CF ３ ) ２ CO － , B(C ２ O ４ ) ２ － , and B.F. ２ (C ２ O ４ ) － and at least one anion selected from the group consisting of:

2. R is an alkylene group having 1 to 3 carbon atoms, R １ ~R ３ are each independently an alkyl group having 1 to 5 carbon atoms or a phenyl group, X － is BF ４ － , P.F. ６ － , ClO ４ － , P.O. ２ F ２ － , C.F. ３ SO ３ － , S.O. ３ CF ３ － , (CF ３ CF ２ SO ２ ) ２ N － , (CF ３ SO ２ ) ２ N － , (FSO ２ ) ２ N － , C.F. ３ CF ２ SO ３ － , C.F. ３ CF ２ (CF ３ ) ２ CO － , and B.F. ２ (C ２ O ４ ) － 2. The nonaqueous electrolyte for a lithium secondary battery according to claim 1, wherein the anion is at least one selected from the group consisting of:

3. The R １ ~R ３ and each independently represent a methyl group, an ethyl group, or a phenyl group.

4. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the ionic liquid represented by Chemical Formula 1 is at least one of compounds represented by the following Chemical Formulas 1-1 to 1-4: 【Chemistry 2】 【Chemistry 3】 【Chemistry 4】 【Chemistry 5】

5. 5. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the ionic liquid represented by Chemical Formula 1 is contained in an amount of 0.3 wt % to 50 wt % based on the total weight of the non-aqueous electrolyte solution for a lithium secondary battery.

6. 6. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 5, wherein the ionic liquid represented by Chemical Formula 1 is contained in an amount of 0.5 wt % to 30 wt % based on the total weight of the non-aqueous electrolyte solution for a lithium secondary battery.

7. 2. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, further comprising at least one other additive selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

8. a negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte; The nonaqueous electrolyte solution for lithium secondary batteries according to claim 1 .

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