Electrolyte additive for secondary batteries, non-aqueous electrolyte for lithium secondary batteries containing the same, and lithium secondary battery containing the same

Abstract

The present invention relates to a new electrolyte additive, a nonaqueous electrolyte for lithium secondary batteries containing this new electrolyte additive, and a lithium secondary battery containing this nonaqueous electrolyte. More specifically, the present invention relates to a nonaqueous electrolyte for lithium secondary batteries containing an additive that can form a stable coating on the electrode surface. The present invention also relates to a lithium secondary battery that contains this nonaqueous electrolyte, which prevents deterioration of the high-temperature life of the lithium secondary battery, prevents increase in resistance when the lithium secondary battery is stored at high temperatures, and improves the ability to suppress expansion of the secondary battery's volume (thickness) when the lithium secondary battery is stored at high temperatures.

Description

[Technical Field]

[0001] This invention relates to an electrolyte additive for secondary batteries. More specifically, this invention relates to a non-aqueous electrolyte additive that is excellent in removing decomposition products generated from lithium salts, and a non-aqueous electrolyte for lithium secondary batteries containing the same. This invention also relates to a non-aqueous electrolyte additive that can form a strong solid electrolyte interface film (SEI) on the surface of the negative electrode, and a non-aqueous electrolyte for lithium secondary batteries containing the same. This invention also relates to a lithium secondary battery containing such a non-aqueous electrolyte. The lithium secondary battery of this invention, by containing the non-aqueous electrolyte, has improved high-temperature performance. [Background technology]

[0002] Lithium-ion batteries are used not only as portable power sources for mobile phones and laptops, but their applications are expanding to medium and large-scale power sources for electric bicycles and electric vehicles (EVs). With this expansion of application areas, there is a growing demand for lithium-ion batteries that can maintain excellent performance not only at room temperature, but also in more demanding external environments such as high and low temperatures.

[0003] Currently widely used lithium-ion secondary batteries generally consist of a carbon-based negative electrode capable of inserting and deintercalating lithium ions, a transition metal oxide-based positive electrode containing lithium, a non-aqueous electrolyte in which lithium salt is dissolved in a mixed carbonate-based organic solvent, and a separation membrane to prevent contact between the positive and negative electrodes. During charging, lithium atoms in the positive electrode are ionized into lithium ions and electrons. The electrons move to the negative electrode through an external circuit, and the lithium ions move to the negative electrode across the non-aqueous electrolyte and separation membrane, where they are intercalated into the carbon negative electrode. During discharge, the electrons move to the positive electrode through an external circuit, and at the same time, lithium ions are deintercalated from the carbon negative electrode and move to the positive electrode across the non-aqueous electrolyte and separation membrane, where lithium ions and electrons meet to form stable lithium atoms. Lithium-ion secondary batteries generate electrical energy by repeatedly performing this charging and discharging process.

[0004] During charging and discharging, lithium-ion batteries can experience structural breakdown of the positive electrode active material, potentially releasing metal ions from the positive electrode surface. These released metal ions can then be electrodeposited onto the negative electrode, causing it to degrade. This degradation of the negative electrode tends to accelerate if the positive electrode potential is high or if the battery is exposed to high temperatures.

[0005] To solve these problems, a method has been proposed in which a compound capable of forming a film (solid electrolyte interphase film, SEI) on the negative electrode surface is added to the non-aqueous electrolyte. However, such electrolyte additives cause other side effects such as a decrease in the lifespan of the secondary battery and deterioration of high-temperature safety, while also leading to another problem: a decrease in the overall performance of the lithium secondary battery.

[0006] LiPF6 is the primary lithium salt used in lithium secondary batteries to achieve the appropriate characteristics of the batteries. - Anions are highly susceptible to heat, and it is known that when secondary batteries are exposed to high temperatures, they undergo thermal decomposition, generating Lewis acids such as PF5. The PF5 thus produced not only causes the decomposition of organic solvents such as ethylene carbonate, but also generates hydrofluoric acid (HF), accelerating the leaching of transition metals from the positive electrode active material. These leached transition metals can electrodeposit onto the positive electrode, increasing its resistance, or electrodeposit onto the negative electrode, causing self-discharge of the negative electrode. They can also destroy the solid electrolyte interface (SEI) film on the negative electrode, leading to further decomposition of the electrolyte and consequently increasing the resistance and degrading the lifespan of the secondary battery. Such electrolyte decomposition reactions also lead to gas generation inside the secondary battery.

[0007] Therefore, when lithium secondary batteries are stored at high temperatures in a fully charged state, there is a problem in that the solid electrolyte interface (SEI) gradually breaks down over time. This breakdown of the solid electrolyte interface exposes the surface of the negative electrode. The exposed negative electrode surface decomposes by reacting with the carbonate-based solvent in the electrolyte, causing a continuous side reaction. This side reaction continuously generates gas.

[0008] The gases generated in this way, regardless of their type, increase the internal pressure of the lithium secondary battery, act as a resistive element in lithium movement, expand the volume (thickness) of the secondary battery, create significant problems in reducing the weight of the secondary battery, and degrade the performance of the secondary battery.

[0009] In recent years, as the application fields of lithium-ion batteries have expanded, there has been a steadily growing demand for stability and long lifespan in high-temperature environments. This performance is largely determined by the solid electrolyte interface (SEI) film formed by the initial reaction between the electrode and the electrolyte. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Therefore, in order to improve the high-temperature cycle characteristics and low-temperature output of lithium secondary batteries, there is a constant need for the development of additives that can suppress side reactions between the positive electrode and the electrolyte and form a strong solid electrolyte interface (SEI) film on the surface of the negative electrode.

[0011] To solve the aforementioned problems, the present invention aims to provide a non-aqueous electrolyte for lithium secondary batteries that includes an additive capable of forming a stable solid electrolyte interface (SEI) film on the electrode surface, particularly on the surface of the negative electrode.

[0012] Furthermore, in order to solve the above-mentioned problems, the present invention aims to provide an electrolyte additive for secondary batteries that forms a strong solid electrolyte interface (SEI) on the electrode surface, particularly on the negative electrode surface, and has an excellent effect in removing decomposition products generated from lithium salts.

[0013] Also, in order to solve the problems of the prior art as described above, the present invention intends to provide a non-aqueous electrolyte for a lithium secondary battery capable of improving high-temperature stability without deterioration of high-temperature life and performance of the lithium secondary battery, and a lithium secondary battery including this non-aqueous electrolyte for a lithium secondary battery.

Means for Solving the Problems

[0014] The non-aqueous electrolyte for a lithium secondary battery according to an embodiment of the present invention is

[0015] a compound containing an imidazole group and a fluoro group as an additive;

[0016] a lithium salt;

[0017] an additional additive; and

[0018] a non-aqueous organic solvent. <​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0027] The anode may contain a carbon-based anode active material and a silicon-based anode active material in a weight ratio of 97:3 to 50:50.

[0028] The anode may contain a carbon-based anode active material and a silicon-based anode active material in a weight ratio of 90:10 to 60:40. [Effects of the Invention]

[0029] The compound containing an imidazole group and a fluoro group, particularly the compound represented by Chemical Formula 1, provided as an additive to the non-aqueous electrolyte of the present invention, has a nitrogen atom in the imidazole group that acts as a Lewis base, allowing it to remove Lewis acids such as HF and PF5, which are decomposition products generated by the decomposition of anions when the secondary battery is exposed to high temperatures, from within the electrolyte. Therefore, the compound containing an imidazole group and a fluoro group, particularly the compound represented by Chemical Formula 1, provided as an additive to the non-aqueous electrolyte of the present invention, can suppress the deterioration of the surface film (SEI) of the positive or negative electrode caused by Lewis acids, preventing further electrolyte decomposition of the secondary battery due to the destruction of the film (SEI), and can also suppress the self-discharge of the secondary battery. The imidazole group also helps to form a stable film (SEI) on the surface of the negative electrode. The fluoro group of the compound plays a role in removing CO2 gas generated at the positive electrode, and plays a role in preventing the phenomenon of the secondary battery swelling.

[0030] The additives contained in the non-aqueous electrolyte of the secondary battery of the present invention, namely compounds containing an imidazole group and a fluoro group, and especially the functional group of the compound represented by chemical formula 1, have the effects described above, and even when the lithium secondary battery is exposed to high temperatures, its lifespan does not deteriorate, and the increase in resistance and gas generation during storage at high temperatures are suppressed, thereby reducing the volume expansion of the secondary battery and realizing a secondary battery with improved performance. [Modes for carrying out the invention]

[0031] The present invention will be described in more detail below with reference to examples. These examples are merely illustrative and should not be construed as limiting the scope of the present invention.

[0032] Terms such as “contains” and “possess” as used herein should be understood as open-ended terms that imply the possibility of including other components, unless otherwise specified in the phrase or sentence in which they are used.

[0033] In this specification, "%" means weight percent unless otherwise explicitly indicated.

[0034] The following describes in detail the electrolyte additive for lithium secondary batteries, the non-aqueous electrolyte for lithium secondary batteries, and the lithium secondary battery containing this non-aqueous electrolyte according to the present invention.

[0035] <Electrolyte additive for lithium secondary batteries>

[0036] The present invention provides compounds containing an imidazole group and a fluoro group, particularly compounds containing an imidazole group and a fluoro group represented by the following chemical formula 1, as additives for electrolytes in lithium secondary batteries.

[0037] [Chemical formula 1]

[0038] JPEG0007833195000002.jpg2970

[0039] <Electrolyte for lithium secondary batteries>

[0040] This invention

[0041] Compounds containing an imidazole group or a fluoro group;

[0042] Additives;

[0043] Lithium salts; and

[0044] This invention provides an electrolyte for lithium secondary batteries containing a non-aqueous organic solvent.

[0045] This invention

[0046] A compound comprising an imidazole group and a fluoro group represented by the aforementioned chemical formula 1;

[0047] Additives;

[0048] Lithium salts; and

[0049] This invention provides an electrolyte for lithium secondary batteries containing a non-aqueous organic solvent.

[0050] The compound containing the imidazole group and the fluoro group may be present in an amount of 0.05 to 20% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0051] The compound containing the imidazole group and the fluoro group may preferably be present in an amount of 0.05 to 10% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0052] The compound containing the imidazole group and the fluoro group may more preferably be present in an amount of 0.05 to 5% by weight, 0.05 to 3% by weight, or 0.05 to 2% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0053] The compound containing the imidazole group and the fluoro group may be present in an amount of 0.1 to 20% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0054] The compound containing the imidazole group and the fluoro group may preferably be present in an amount of 0.1 to 10% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0055] The compound containing the imidazole group and the fluoro group may more preferably be present in an amount of 0.1 to 5% by weight, 0.1 to 3% by weight, or 0.1 to 2% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0056] If the compound containing the imidazole group and fluoro group is present in an amount of less than 0.05% by weight of the total weight of the electrolyte for the lithium secondary battery, the effect of preventing volume expansion and reducing internal resistance of the lithium secondary battery is insufficient. Conversely, if the compound containing the imidazole group and fluoro group is present in an amount exceeding 20% ​​by weight of the total weight of the electrolyte for the lithium secondary battery, problems arise such as an increase in the internal resistance and decrease in capacity of the secondary battery, leading to a deterioration in high-temperature life characteristics and high-temperature storage characteristics.

[0057] The electrolyte for the lithium secondary battery may further contain at least one additive selected from the group consisting of halogen-substituted or unsubstituted carbonate compounds, nitrile compounds, borate compounds, lithium salt compounds, phosphate compounds, sulfite compounds, sulfone compounds, sulfate compounds, and sultone compounds.

[0058] Representative examples of the aforementioned additives include lithium difluorophosphate, lithium tetrafluoro(oxalate)phosphate, lithium bis(fluorosulfonyl)imide, 1,3-propane sultone, 1,3-propene sultone, fluoroethylene carbonate, vinylene carbonate, and vinyl ethylene carbonate.

[0059] The aforementioned additive may be present in an amount of 0.05 to 20% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0060] The aforementioned additive may preferably be present in an amount of 0.05 to 10% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

[0061] The aforementioned additive may more preferably be present in an amount of 0.05 to 5% by weight, specifically 0.05 to 3% by weight, relative to the total weight of the electrolyte for the lithium secondary battery.

[0062] If the additive is present in an amount of less than 0.05% by weight relative to the total weight of the electrolyte for the lithium secondary battery, the film-forming effect on the electrodes may be minimal, and the effect of suppressing side reactions between the electrodes and the electrolyte may decrease. If the electrolyte additive is present in an amount exceeding 20% ​​by weight relative to the total weight of the electrolyte for the lithium secondary battery, an excessively thick film may be formed on the electrode surface, increasing interfacial resistance and potentially causing a decrease in capacity.

[0063] The lithium salt may include at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiBF6, LiSbF6, LiAlO4, LiAlCl4, LiClO4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, and LiB(C2O4)2.

[0064] It is preferable to use a lithium salt that has a high degree of lattice energy dissociation, excellent ionic conductivity, and excellent thermal stability and oxidation resistance. The lithium salt functions as a pathway for the movement of lithium ions within the secondary battery, enabling the basic operation of the lithium secondary battery.

[0065] The concentration of the lithium salt may be 0.1 to 2.5 M (mol / L) relative to the total amount of the electrolyte for the lithium secondary battery.

[0066] The concentration of the lithium salt may preferably be 0.3 to 2.5 M (mol / L) relative to the total amount of the electrolyte for the lithium secondary battery, taking into consideration the properties related to electrical conductivity and the viscosity related to the mobility of lithium ions.

[0067] The concentration of the lithium salt may more preferably be 0.7 to 1.6 M (mol / L), taking into consideration the properties related to electrical conductivity and the viscosity related to the mobility of lithium ions.

[0068] If the concentration of the lithium salt is less than 0.1 M, the electrical conductivity of the electrolyte for the lithium secondary battery decreases, and the performance of the non-aqueous electrolyte that rapidly transfers ions between the positive and negative electrodes of the lithium secondary battery deteriorates. If the concentration of the lithium salt exceeds 2.5 M, the viscosity of the electrolyte for the lithium secondary battery increases, the mobility of lithium ions decreases, and the performance of the secondary battery deteriorates at low temperatures.

[0069] The non-aqueous organic solvent may be a linear carbonate solvent, a cyclic carbonate solvent, or a mixture thereof.

[0070] The linear carbonate solvent may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC).

[0071] Furthermore, the cyclic carbonate solvent may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate (BC), vinylene carbonate (VC), and fluoroethylene carbonate (FEC).

[0072] In some cases, it is desirable to use a mixture of a cyclic, high-dielectric-constant carbonate-based organic solvent with high ionic conductivity that can improve the charge-discharge performance of secondary batteries, and a low-viscosity, linear carbonate-based organic solvent that can appropriately adjust the viscosity of the high-dielectric-constant carbonate-based organic solvent.

[0073] Specifically, a high dielectric constant carbonate-based organic solvent selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), and mixtures thereof, which are cyclic carbonate-based solvents, can be used in combination with a low viscosity carbonate-based organic solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and mixtures thereof, which are linear carbonate-based solvents.

[0074] While the aforementioned cyclic carbonate solvent has high polarity and can sufficiently dissociate lithium ions, it has the disadvantage of high viscosity and low ionic conductivity. Therefore, by mixing the cyclic carbonate solvent with a linear carbonate solvent, which has low polarity but low viscosity, the characteristics of the lithium secondary battery can be optimized.

[0075] Therefore, it may be preferable to use a mixture of at least one solvent selected from the cyclic carbonate solvents and at least one solvent selected from the linear carbonate solvents as the non-aqueous organic solvent.

[0076] The mixed solvent of the linear carbonate solvent and the cyclic carbonate solvent can be used by mixing the linear carbonate solvent and the cyclic carbonate solvent in a volume ratio of 9:1 to 1:9.

[0077] From the viewpoint of the lifespan and storage characteristics of secondary batteries, it may be more preferable to use a mixed solvent of the linear carbonate solvent and the cyclic carbonate solvent in a volume ratio of 2:8 to 8:2.

[0078] The non-aqueous organic solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC).

[0079] The non-aqueous organic solvent may include 5 to 40% by weight of ethylene carbonate (EC), 5 to 20% by weight of propylene carbonate (PC), 10 to 70% by weight of ethyl methyl carbonate (EMC), and 10 to 60% by weight of diethyl carbonate (DEC).

[0080] Specifically, in the cyclic carbonate solvent, ethylene carbonate (EC) or propylene carbonate (PC), which have high dielectric constants, can be used. When artificial graphite is used as the negative electrode active material, it is preferable to use ethylene carbonate (EC). Among the linear carbonate solvents, it is preferable to use dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC), which have low viscosity.

[0081] The non-aqueous organic solvent may be present in an amount of 5% to 80% of the total amount of the electrolyte for the lithium secondary battery. The non-aqueous organic solvent may be present in an amount of 5% to 70% of the total amount of the electrolyte for the lithium secondary battery.

[0082] <Lithium-ion secondary battery>

[0083] The lithium secondary battery containing the aforementioned non-aqueous electrolyte exhibits excellent performance in that its lifespan characteristics do not deteriorate at high temperatures, its resistance does not increase during storage at high temperatures, and it suppresses expansion of the secondary battery volume (thickness).

[0084] The lithium secondary battery of the present invention will be described in detail below.

[0085] The lithium secondary battery of the present invention

[0086] positive electrode;

[0087] negative electrode;

[0088] Separation membrane; and

[0089] Contains a non-aqueous electrolyte.

[0090] The positive electrode is LiCoO2, LiFePO4, LiMnO2, LiMn2O4, LiNiO 2, Or Li Limited 1-x-y Co x M y The cathode active material may include at least one selected from the group consisting of lithium metal oxides such as O2 (0≦x≦, 0≦y≦1, 0≦x+y≦1, where M is Al, Sr, Mg, Mn, or La).

[0091] The negative electrode may contain at least one negative electrode active material selected from the group consisting of silicon, silicon compounds, tin, tin compounds, lithium titanate, crystalline carbon, amorphous carbon, artificial graphite, natural graphite, and mixtures of artificial and natural graphite.

[0092] The separation membrane may consist solely of a porous polymer film made from at least one polyolefin polymer selected from ethylene polymer, propylene polymer, ethylene / butene copolymer, and ethylene / hexene copolymer, or it may consist of a laminate thereof. The separation membrane may include a coating film coated with ceramic or polymeric material.

[0093] The non-aqueous electrolyte is a compound containing an imidazole group and a fluoro group, particularly a compound represented by the following formula 1;

[0094] Additives;

[0095] Lithium salts; and

[0096] It may contain non-aqueous organic solvents.

[0097] [Chemical formula 1]

[0098] JPEG0007833195000003.jpg2970

[0099] Examples of the lithium secondary battery include, but are not limited to, lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

[0100] More specifically, the positive electrode active material is preferably a composite metal oxide of one or more substances selected from cobalt, manganese, and nickel and lithium. The solid solubility ratio between cobalt, manganese, and nickel metals in the composite metal oxide can vary, and in addition to these cobalt, manganese, and nickel metals, elements selected from the group consisting of Mg, Al, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Cr, Fe, Sr, V, and rare earth elements can be further included.

[0101] Specifically, as the positive electrode active material, lithium metal oxides such as LiCoO2, LiFePO4, LiMnO2, LiMn2O4, LiNiO 2, or LiNi 1-x-y Co x M y O2 (0 ≦ x ≦ , 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1, M is Al, Sr, Mg, Mn, or La), etc., or lithium intercalation compounds such as lithium chalcogenide compounds can be used, but are not limited thereto, and any substance that can be used as a positive electrode active material in a secondary battery can be used.

[0102] The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer can include a positive electrode active material capable of occluding and releasing lithium, a binder, a conductive material, and the like.

[0103] The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material layer may include a negative electrode active material, binder, conductive material, etc., that can insert and remove lithium. As the negative electrode active material, crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, lithium alloy, or carbon-silicon composite can be used, but are not limited to these, and any material that can be used as a negative electrode active material in a secondary battery can be used.

[0104] The positive electrode and / or negative electrode can be manufactured by dispersing an electrode active material, a binder, a conductive material, and optionally a thickener in a solvent to produce an electrode slurry composition, and then applying the slurry composition to an electrode current collector. Aluminum or an aluminum alloy can often be used as the positive electrode current collector, and copper or a copper alloy can often be used as the negative electrode current collector.

[0105] Examples of the positive electrode current collector and the negative electrode current collector include foil or mesh forms.

[0106] The binder is a substance that plays a role in pasteuring the active material, bonding the active materials together, bonding to the current collector, and providing a buffering effect against the expansion and contraction of the active material. Any binder that can be used by a person skilled in the art is acceptable. For example, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVdF), polyhexafluoropropylene-polyvinylidene fluoride copolymer (PVdF / HFP), poly(vinyl acetate), alkylated polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), poly(ethyl acrylate), polyacrylonitrile, polyvinylpyridine, polyethylene, polypropylene, styrene-butadiene rubber, acrylicated styrene-butadiene rubber, acrylonitrile-butadiene rubber, epoxy resin, nylon, etc. can be used, but are not limited to these.

[0107] The conductive material is used to impart conductivity to the electrodes, and any conductive material that does not cause a chemical change in the secondary battery that is constructed can be used. As the conductive material, at least one selected from the group consisting of graphite-based conductive materials, carbon black-based conductive materials, and metal or metal compound-based conductive materials can be used. Examples of graphite-based conductive materials include artificial graphite and natural graphite, while examples of carbon black-based conductive materials include acetylene black, Ketjen black, Denka black, thermal black, and channel black, and examples of metal or metal compound-based conductive materials include perovskite substances such as tin, tin oxide, tin phosphate (SnPO4), titanium oxide, potassium titanate, LaSrCoO3, and LaSrMnO3. However, the conductive material is not limited to those listed above.

[0108] The aforementioned thickening agent is not particularly limited as long as it plays a role in adjusting the viscosity of the active material slurry, and for example, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, etc. can be used.

[0109] The solvent in which the electrode active material, binder, conductive material, etc. are dispersed can be a non-aqueous solvent or an aqueous solvent. Examples of the non-aqueous solvent include N-methyl-2-pyrrolezidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran. Examples of the aqueous solvent include water.

[0110] The lithium secondary battery may include a separator that prevents short circuits between the positive and negative electrodes and provides a pathway for lithium ions to move. The separator can be a polyolefin polymer membrane such as polypropylene, polyethylene, polyethylene / polypropylene, polyethylene / polypropylene / polyethylene, or polypropylene / polyethylene / polypropylene, or a multilayer film thereof, a microporous film, a woven fabric, or a nonwoven fabric. Alternatively, a film in which a porous polyolefin film is coated with a resin with excellent stability can be used as the separator.

[0111] Furthermore, the lithium secondary battery can be made into various shapes, such as rectangular, cylindrical, pouch-shaped, or coin-shaped. [Examples]

[0112] The present invention will be described in more detail below with reference to examples. It should not be construed that the scope of the present invention is limited by these examples.

[0113] <Example of synthesis of 2,2,2-trifluoroethyl 1H-imidazole-1-carboxylate (compound of chemical formula 1)>

[0114] A 500 mL three-necked flask was fitted with an N2 purge line, a dropping funnel, and a thermometer. 0.31 mol of 1,1'-carbonyldiimidazole and 150 mL of dichloromethane were added and stirred. The reactor was filled with a nitrogen atmosphere, and the temperature was cooled from ambient temperature to 10°C. While maintaining the temperature, 0.32 mol of 2,2,2-trifluoroethanol was added dropwise for 30 minutes. After the addition of 2,2,2-trifluoroethanol was complete, the temperature was changed from 10°C to room temperature, and the reaction was allowed to proceed at the same temperature for 3 hours. After the reaction was complete, 150 mL of water was added to extract the organic layer. This process was repeated twice. After treatment with MgSO4 to remove water, the mixture was filtered. The filtrate was concentrated to remove dichloromethane. After drying in a vacuum oven, the final compound, 2,2,2-trifluoroethyl 1H-imidazole-1-carboxylate, was obtained. The yield was 75%.

[0115] 1 H NMR(Chloroform-d, δ ppm): 1H 8.1ppm, 1H 7.3.ppm, 1H 6.9ppm, 2H 4.7ppm,

[0116] 19 F NMR(Chloroform-d, δ ppm) -74.09 1F, HRMS: C6H5N2O2F3(M+):194.03

[0117] <Preparation of electrolyte for lithium secondary batteries containing 2,2,2-trifluoroethyl 1H-imidazole-1-carboxylate (compound of chemical formula 1)>

[0118] After dissolving LiPF6 to 1.0 M in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 25 / 75 by volume ratio), 1.0 wt% of fluoroethylene carbonate (FEC), 1.0 wt% of lithium difluorophosphate (LiPO2F2), 0.5 wt% of propane sultone (PS), 0.5 wt% of ethylene sulfate (Esa), and 0.5 wt% of 2,2,2-trifluoroethyl 1H-imidazole-1-carboxylate of the above Synthesis Example represented by Chemical Formula 1 were added to the mixed solution to produce an electrolyte for a lithium secondary battery containing the compound of Chemical Formula 1.

[0119] <Manufacture of a Lithium Secondary Battery Containing an Electrolyte Containing the Compound of Chemical Formula 1>

[0120] Li[Ni x Co 1-x-y Mn y O2 (0 < x < 0.5, 0 < y < 0.5), 94 wt% of an NCM-based positive electrode active material, 3 wt% of a conductive material (Super-P), and 3 wt% of a binder (PVdF) were added to N-methyl 2-pyrrolidone (NMP), an organic solvent, to prepare a positive electrode active material slurry. The positive electrode active material slurry was applied to an aluminum thin film as a current collector, dried, and then rolled by a roll press to fabricate a positive electrode. Also, 96 wt% of a graphite-based negative electrode active material containing silicon oxide (SiOx), 1 wt% of a conductive material (Super-P), 1.5 wt% of a binder SBR, and 1.5 wt% of CMC were mixed to produce a negative electrode active material slurry. The negative electrode active material slurry was applied to a copper thin film as a negative electrode current collector and dried to fabricate a negative electrode.

[0121] The positive electrode and negative electrode manufactured as described above were prepared, and a separator was interposed therebetween. Then, an electrolyte for a lithium secondary battery containing the compound of Chemical Formula 1 was injected between the two electrodes with the separator interposed therebetween, and a lithium secondary battery containing an electrolyte containing the compound of Chemical Formula 1 in an aluminum pouch type (Al-Pouch type) was manufactured.

[0122] [Comparative Example]

[0123] <Manufacturing of electrolyte for lithium secondary batteries containing 1,3-propene sultone (PRS) additive>

[0124] Non-aqueous electrolytes for lithium secondary batteries can decompose, weakening their high-temperature stability and causing the battery to expand at high temperatures. Therefore, to improve high-temperature stability and suppress battery expansion at high temperatures, sultone compounds may be included as needed. The sultone compound may be, for example, at least one compound selected from the group consisting of 1,3-propanesultone (PS), 1,4-butanesultone (BS), ethensultone, 1,3-propensultone, 1,4-butensultone, and 1-methyl-1,3-propensultone. In the comparative example, 1,3-propensultone (PRS), which is currently in common use, was used.

[0125] LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 25 / 75 volume ratio) to a concentration of 1.0 M. Then, 1.0 wt% of fluoroethylene carbonate (FEC), 1.0 wt% of lithium difluorophosphate (LiPO2F2), 0.5 wt% of propanesultone (PS), 0.5 wt% of ethylene sulfate (Esa), and, as a comparative example, 0.5 wt% of 1,3-propensultone (PRS) were added to the mixed solution to produce an electrolyte for lithium secondary batteries.

[0126] <Manufacturing of lithium secondary batteries containing electrolyte with 1,3-propensultone (PRS) additive>

[0127] A lithium secondary battery was manufactured in the same manner as the lithium secondary battery of the above example, but without the addition of the 2,2,2-trifluoroethyl 1H-imidazole-1-carboxylate compound represented by chemical formula 1 as the electrolyte, and instead using an electrolyte for lithium secondary batteries containing 1,3-propensultone (PRS).

[0128] The compositions of the lithium secondary battery electrolytes for the above-mentioned examples and comparative examples are shown in Table 1 below.

[0129] <Composition of electrolyte for lithium secondary batteries>

[0130] [Table 1]

[0131] [Example of experiment]

[0132] <Experimental Example 1> Measurement of lifetime capacity retention rate at high temperature (45°C)

[0133] Pouch-type lithium secondary batteries prepared using the lithium secondary battery electrolytes of the above examples and comparative examples were charged to 4.2V at a high temperature (45°C) at a 1C-rate, then discharged to 2.7V at a 1C-rate after a 10-minute rest period, followed by another 10-minute rest period. This process was repeated 500 times, and the discharge capacity (mAh) and retention rate (%) of the batteries were measured. The measured discharge capacity and retention rate of the secondary batteries were compared, and the results are shown in Table 2.

[0134] [Table 2]

[0135] As shown in Table 2 above, the life evaluation at high temperatures showed that the lithium secondary battery of the above example had a higher discharge capacity of 500 cycles and a higher life capacity retention rate at high temperatures compared to the lithium secondary battery of the comparative example.

[0136] Therefore, it was confirmed that the lithium secondary battery of the above embodiment, by containing an electrolyte containing the compound represented by chemical formula 1, had a higher lifetime capacity retention rate without deterioration in high-temperature life performance compared to the lithium secondary battery of the comparative example. In other words, the compound additive of chemical formula 1 improved the lifetime capacity retention rate at high temperatures without performance degradation due to side reactions with other additives.

[0137] <Experimental Example 2> Measurement of Storage Characteristics at High Temperature (60°C)

[0138] Lithium secondary batteries in pouch form, prepared using the lithium secondary battery electrolytes of the above examples and comparative examples, were stored at a high temperature (60°C) for 6 weeks, and the volume change rate of the secondary batteries was measured. Table 3 below shows the volume change rate of the secondary batteries after 6 weeks of storage compared to 0 weeks at a high temperature (60°C).

[0139] [Table 3]

[0140] As shown in Table 3, the volume increase rate in the examples was lower than that of the comparative examples. Therefore, it was found that the additive of the present invention is an additive with excellent effect in suppressing gas generation.

Claims

1. Additives; Additives; Lithium salts; and A non-aqueous electrolyte for lithium secondary batteries containing a non-aqueous organic solvent, The aforementioned additive is a compound of the following chemical formula 1, [Chemical formula 1] The additive is at least one compound selected from the group consisting of halogen-substituted or unsubstituted carbonate compounds, nitrile compounds, borate compounds, lithium salt compounds, phosphate compounds, sulfite compounds, sulfone compounds, sulfate compounds, and sultone compounds. A non-aqueous electrolyte for lithium secondary batteries, wherein the additive is contained in an amount of 0.05% to 20% by weight based on the total weight of the non-aqueous electrolyte for lithium secondary batteries.

2. Non-aqueous electrolyte for lithium secondary battery according to claim 1; positive electrode; Negative electrode; and A lithium secondary battery containing a separation membrane.

3. The lithium secondary battery according to claim 2, wherein the negative electrode comprises a carbon-based negative electrode active material and a silicon-based negative electrode active material.

4. The lithium secondary battery according to claim 3, wherein the carbon-based anode active material and the silicon-based anode active material are contained in a weight ratio of 97:3 to 50:

50.

5. The lithium secondary battery according to claim 4, wherein the carbon-based anode active material and the silicon-based anode active material are contained in a weight ratio of 90:10 to 60:40.

Images