Electrolyte for secondary battery and lithium secondary battery containing same

Abstract

The present invention relates to an electrolyte for a secondary battery and a lithium secondary battery including the same. The electrolyte for a lithium secondary battery of the present invention may include a lithium salt, a non-aqueous organic solvent, and a polymer including a repeating unit derived from a monomer represented by Chemical Formula 1 and a repeating unit derived from a monomer represented by Chemical Formula 2 as an additive.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0097614, filed August 4, 2022, and Korean Patent Application No. 10-2023-0101238, filed August 2, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to an electrolyte for secondary batteries and a lithium secondary battery including the same, and more particularly to an electrolyte for secondary batteries that forms a polymer coating on the surface of an electrode and can suppress the elution of transition metals, and a lithium secondary battery including the same. [Background technology]

[0003] In recent years, with the rapid development of the electrical, electronic, communication, and computer industries, the demand for high-performance, high-stability secondary batteries has been increasing. In particular, with the trend toward smaller and lighter electronic (communication) devices, there is a demand for thinner and smaller lithium secondary batteries, which are core components in this field.

[0004] Lithium secondary batteries have primarily used liquid electrolytes, such as ion-conductive organic liquid electrolytes, which are made by dissolving salts in organic solvents, primarily consisting of carbonate organic solvents. However, liquid electrolytes have drawbacks, including poor stability due to the high volatility of the organic solvent and the generation of gases inside the battery caused by decomposition of the carbonate organic solvent and / or side reactions between the organic solvent and the electrodes during charging and discharging, resulting in battery thickness expansion. In particular, during high-temperature storage, these side reactions accelerate, resulting in the continuous generation of gases, which increases the battery's withstand voltage and can cause prismatic batteries to expand in a specific direction, resulting in deformation or explosion of the center of a specific surface of the battery. Furthermore, this can lead to localized differences in electrode adhesion, resulting in uneven electrode reactions across the entire electrode surface.

[0005] Therefore, in recent years, in order to ensure the stability of lithium secondary batteries, research has been progressing on various additives that can impart various functions to electrolytes.

[0006] However, repeated charge-discharge cycles and the presence of moisture and Lewis acids inside the cell can erode the passive film on the positive electrode surface, causing transition metal ions to leach from the positive electrode, potentially reducing the structural stability of the positive electrode. Furthermore, the leached transition metal ions can be electro-deposited on the negative electrode or precipitated from the negative electrode surface, reducing the passivation ability of the SEI, leading to negative electrode degradation. This negative electrode degradation accelerates the decomposition of the electrolyte solvent, accelerating gas generation and inducing side reactions such as the desorption of lithium ions inserted in the negative electrode, resulting in a decrease in battery capacity.

[0007] Therefore, there is a need for technological development of an electrolyte that has stable passivation ability, can form a stable coating on the surface of the positive electrode that has low interfacial resistance and high ion transfer efficiency, and can suppress the elution of transition metals. Summary of the Invention [Problem to be solved by the invention]

[0008] In order to solve the above problems, an object of the present invention is to provide an electrolyte for a secondary battery that has stable passivation ability and is capable of forming a coating having low interfacial resistance on the surface of a positive electrode.

[0009] Another object of the present invention is to provide a lithium secondary battery that contains the electrolyte for secondary batteries of the present invention, and thereby has high high-temperature durability and improved low-temperature resistance characteristics. [Means for solving the problem]

[0010] In one embodiment of the present invention to achieve the above object, Provided is an electrolyte for a secondary battery, comprising a lithium salt, a non-aqueous organic solvent, and, as an additive, a polymer including a repeating unit derived from a monomer represented by the following chemical formula 1 and a repeating unit derived from a monomer represented by the following chemical formula 2:

[0011] [ka]

[0012] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R'' is H or Li.

[0013] [ka]

[0014] In the above Chemical Formula 2, R1 and R2 are each independently an alkylene group having 1 to 5 carbon atoms.

[0015] Meanwhile, the lithium salt may include LiPF6. Also, the lithium salt may include LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 , LiAlCl4, LiAlO4, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiTFSI (lithium (bis)trifluoromethanesulfonimide, LiN(SO2CF3)2), LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), and LiBETI (lithium bisperfluoroethanesulfonimide, LiN(SO2CF2CF3)2).

[0016] Meanwhile, in the above Chemical Formula 1, R may be an alkylene group having 1 to 4 carbon atoms, and R'' may be H. Specifically, R may be an alkylene group having 1 to 3 carbon atoms, and preferably R may be an alkylene group having 1 or 2 carbon atoms.

[0017] In addition, in the above chemical formula 2, R1 and R2 may each independently be an alkylene group having 1 to 4 carbon atoms, specifically an alkylene group having 2 or 3 carbon atoms.

[0018] Meanwhile, the polymer may further include a repeating unit derived from a monomer represented by the following Formula 3:

[0019] [ka]

[0020] In the above Chemical Formula 3, R' is an alkyl group having 1 to 6 carbon atoms.

[0021] In the above chemical formula 3, R' may be an alkyl group having 1 to 5 carbon atoms, and preferably an alkyl group having 1 to 4 carbon atoms.

[0022] Meanwhile, the polymer may include a unit represented by the following Chemical Formula 4:

[0023] [ka]

[0024] In the above Chemical Formula 4, R, R1, and R2 each independently represent an alkylene group having 1 to 5 carbon atoms; R' is an alkyl group having 1 to 6 carbon atoms, R″ is H or Li; k is an integer from 1 to 15,000, m is an integer between 10 and 12,000; n is any one integer from 10 to 8,500.

[0025] Another embodiment of the present invention provides a lithium secondary battery including the electrolyte for a secondary battery of the present invention. [Effects of the Invention]

[0026] The secondary battery electrolyte of the present invention contains, as an additive, a polymer containing a cyano group, which is a thermally polymerizable functional group at its terminal, and thus has stable passivation ability and can form a coating having low interfacial resistance on the surface of the positive electrode. Therefore, a lithium secondary battery containing the electrolyte can achieve the effects of having high high-temperature durability and improved low-temperature resistance characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in more detail below.

[0028] 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.

[0029] On the other hand, in the present invention, unless otherwise specified, "*" means a bond between the same or different atoms or terminal portions of a chemical formula.

[0030] Also, as used herein, the term "alkylene group" refers to a branched or unbranched divalent unsaturated hydrocarbon group. In one embodiment, the alkylene group may be substituted or unsubstituted. Examples of the alkylene group include, but are not limited to, methylene, ethylene, propylene, isopropylene, butylene, isobutylene, tert-butylene, pentylene, and 3-pentylene groups.

[0031] In this specification, the term "repeating unit" refers to a monomer unit that constitutes a polymer.

[0032] In addition, in this specification, the term "thermally polymerizable functional group" refers to a functional group that can undergo a polymerization reaction between the same functional groups or with other functional groups by heating in the absence of a polymerization initiator.

[0033] Furthermore, in this specification, unless otherwise defined, "substituted" means that at least one hydrogen bonded to a carbon atom is substituted with an element other than hydrogen, specifically, an alkyl group having 1 to 5 carbon atoms.

[0034] [Electrolyte for secondary batteries] Specifically, one embodiment of the present invention is a lithium salt and a non-aqueous organic solvent; The present invention provides an electrolyte for a secondary battery, comprising, as an additive, a polymer including a repeating unit derived from a monomer represented by the following chemical formula 1 and a repeating unit derived from a monomer represented by the following chemical formula 2:

[0035] [ka]

[0036] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R'' is H or Li.

[0037] [ka]

[0038] In the above Chemical Formula 2, R1 and R2 are each independently an alkylene group having 1 to 5 carbon atoms.

[0039] (1) Lithium salt First, the electrolyte for a secondary battery of the present invention contains at least one lithium salt.

[0040] Such lithium salts can be used without any particular limitation as long as they are compounds that can provide lithium ions. Representative examples include compounds containing Li as a cation. + and as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO4 - , 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 - , C6HF6N - (LiTDI), and SCN - Examples of compounds include compounds containing at least one selected from the group consisting of:

[0041] Specifically, the lithium salt may include, as a representative example, LiPF6.

[0042] The lithium salt includes LiPF6 as the first lithium salt, and LiCl, LiBr, LiI, LiBF4, LiClO4, LiBF6, etc. as the second lithium salt. 10 Cl 10 , LiAlCl4, LiAlO4, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiTFSI (lithium (bis)trifluoromethanesulfonimide, LiN(SO2CF3)2), LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), and LiBETI (lithium bisperfluoroethanesulfonimide, LiN(SO2CF2CF3)2).

[0043] When the lithium salt contains two kinds of lithium salts, the molar ratio of the first lithium salt to the second lithium salt may be 1:0.01 to 1:50, specifically 1:1 to 1:30, and more specifically 1:1 to 1:10.

[0044] Meanwhile, the second lithium salt has excellent ion transfer effect, but may react with moisture in the battery to generate amine or alkaline components, as compared with LiPF6. Therefore, if the molar ratio of the second lithium salt to 1 mole of the first lithium salt exceeds 50, the amine or alkaline component generated by the side reaction may cause corrosion of metal components, such as the current collector or battery case, or the cross-linking reaction rate between polymers during the thermal polymerization reaction may decrease.

[0045] Meanwhile, in the secondary battery electrolyte, the total concentration of the lithium salt may be 0.1 M to 5 M, specifically 0.5 M to 5 M, more specifically 1 M to 4.5 M. If the total mixed concentration of the lithium salt in the secondary battery electrolyte is less than 0.1 M, the ionic conductivity of the electrolyte may be reduced, resulting in a deterioration in electrolyte performance, while if the mixed concentration of the lithium salt exceeds 5 M, the viscosity of the electrolyte may increase, resulting in a deterioration in electrolyte impregnation, and a decrease in lithium ion mobility, resulting in a deterioration in capacity characteristics.

[0046] (2) Non-aqueous organic solvent The non-aqueous organic solvent may be any of various organic solvents commonly used in lithium electrolytes, including, for example, a cyclic carbonate organic solvent, a linear carbonate organic solvent, or a mixture thereof.

[0047] The cyclic carbonate organic solvent is a highly viscous organic solvent that has a high dielectric constant and easily dissociates the lithium salt in the electrolyte. Specific examples thereof 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 particularly, at least one of ethylene carbonate and propylene carbonate (PC) may be included.

[0048] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and a representative example thereof may be at least one organic solvent selected from the group consisting of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, and specifically may include at least one of dimethyl carbonate and ethyl methyl carbonate.

[0049] In the present invention, in order to ensure high ionic conductivity, the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed and used. In this case, the cyclic carbonate organic solvent and the linear carbonate organic solvent may be contained in a volume ratio of 10:90 to 50:50, specifically 15:85 to 30:70.

[0050] In addition, in the present invention, in order to improve the ionic conductivity of the secondary battery electrolyte, at least one organic solvent selected from the group consisting of a linear ester organic solvent and a cyclic ester organic solvent, which have a lower melting point and higher stability at high temperatures than the cyclic carbonate organic solvent and / or the linear carbonate organic solvent, may be further included. Specifically, the secondary battery electrolyte may further include a linear ester organic solvent, which can reduce the viscosity of the secondary battery electrolyte and increase the degree of dissociation of the lithium salt by chelation of the lithium cation due to its ether symmetric structure. When the linear ester solvent is further used, the ionic conductivity of the secondary battery electrolyte can be further improved.

[0051] As such a linear ester compound, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, or a mixture of two or more of these, can be typically used, but is not limited to these.

[0052] The cyclic ester organic solvent may be at least one organic solvent selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0053] In the electrolyte for a lithium secondary battery of the present invention, the remainder excluding the lithium salt, polymer, and other additives may entirely comprise a non-aqueous organic solvent.

[0054] (3) Additives The electrolyte for a secondary battery of the present invention may contain an additive that has stable passivation ability and is capable of forming a film having low interfacial resistance on the surface of the positive electrode.

[0055] The additive may include a polymer or oligomer including a repeating unit derived from a monomer represented by the following Chemical Formula 1 and a repeating unit derived from a monomer represented by the following Chemical Formula 2:

[0056] [ka]

[0057] In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R'' is H or Li.

[0058] [ka]

[0059] In the above Chemical Formula 2, R1 and R2 are each independently an alkylene group having 1 to 5 carbon atoms.

[0060] The polymer used as an additive in the present invention contains, as essential components, repeating units containing a cyano group (CN) as a terminal group, which can easily form coordinate bonds with metals on the positive electrode surface or metal ions eluted from the positive electrode, and repeating units containing a hydroxyl group (-OH) as a terminal group, which has excellent adsorptivity for transition metals on the positive electrode surface. This allows the formation of a low-resistance, robust functional polymer layer (coating) on ​​the electrode surface. In particular, the polymer of the present invention contains a methacrylate group as a main chain unit, which allows the formation of a robust coating on the positive electrode surface that has low fluidity and is capable of coordinating with Li ions, thereby reducing the positive electrode interfacial resistance. This reduces side reactions, enabling the realization of a lithium secondary battery that ensures excellent high-temperature durability and low-temperature resistance characteristics.

[0061] Meanwhile, in the above Chemical Formula 1, R may be an alkylene group having 1 to 4 carbon atoms, and R″ may be H. Specifically, R may be an alkylene group having 1 to 3 carbon atoms, and preferably an alkylene group having 1 or 2 carbon atoms.

[0062] In addition, in Chemical Formula 2, R1 and R2 may each independently be an alkylene group having 1 to 4 carbon atoms. Specifically, R1 and R2 may each independently be an alkylene group having 2 or 3 carbon atoms. Preferably, R1 and R2 may each independently be an unsubstituted alkylene group having 2 or 3 carbon atoms.

[0063] On the other hand, in the R, R1, and R2, at least one hydrogen bonded to a carbon may be substituted with an element other than hydrogen, specifically, an alkyl group having 1 to 5 carbon atoms.

[0064] In addition, the polymer of the present invention may further include a repeating unit derived from a monomer represented by the following Chemical Formula 3.

[0065] [ka]

[0066] In the above Chemical Formula 3, R' is an alkyl group having 1 to 6 carbon atoms.

[0067] The polymer of the present invention can further improve its solubility in organic solvents by including, as a repeating unit, a repeating unit represented by the above chemical formula 3, which contains an alkyl group (R') as a terminal group.

[0068] On the other hand, in the above chemical formula 3, R' may be an alkyl group having 1 to 5 carbon atoms, and preferably an alkyl group having 1 to 4 carbon atoms.

[0069] On the other hand, in the R', at least one hydrogen bonded to a carbon may be substituted with an element other than hydrogen, specifically, an alkyl group having 1 to 5 carbon atoms.

[0070] Meanwhile, the polymer of the present invention may contain a compound represented by the following chemical formula 4.

[0071] [ka]

[0072] In the above Chemical Formula 4, R, R1, and R2 each independently represent an alkylene group having 1 to 5 carbon atoms; R' is an alkyl group having 1 to 6 carbon atoms, R″ is H or Li; k is an integer from 1 to 15,000, m is an integer between 10 and 12,000; n is any one integer from 10 to 8,500.

[0073] Specifically, the compound represented by Chemical Formula 4 may include a compound represented by the following Chemical Formula 4a:

[0074] [ka]

[0075] In the above chemical formula 4a, R' is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms; R″ is H or Li; k is an integer from 1 to 15,000, m is an integer between 10 and 12,000; n is any one integer from 10 to 8,500.

[0076] More specifically, the compound represented by Chemical Formula 4 may include a compound represented by Chemical Formula 4a-1 or 4a-2 below.

[0077] [ka]

[0078] In the above chemical formula 4a-1, k'' is an integer from 1 to 15,000, m'' is an integer between 10 and 12,000; n'' is any one integer from 10 to 8,500.

[0079] [ka]

[0080] In the above chemical formula 4a-2, k'' is an integer from 1 to 15,000, m'' is an integer between 10 and 12,000; n'' is any one integer from 10 to 8,500.

[0081] Meanwhile, in the chemical formula 4, the molar ratio of repeating unit n to (repeating unit k + repeating unit m) may be 1:0.001 to 1:10, specifically 1:0.01 to 1:10, more specifically 1:0.05 to 1:10, and even more specifically 1:0.2 to 1:8.

[0082] If the molar ratio of repeating units (k+m) to n1 moles in the chemical formula 4 is less than 0.001, not only will the lithium ion migration efficiency decrease, but the crosslinking reaction rate for gelation will also decrease, making it difficult to form a stable electrolyte for secondary batteries. Furthermore, the adhesion between the electrode and the separator will be weak, potentially limiting the effectiveness of improving safety against thermal, mechanical, and electrical shocks. Furthermore, if the molar ratio of repeating units (k+m) to n1 moles in the chemical formula 4 is greater than 10, it will be difficult to control the gelation reaction rate.

[0083] The molar ratio of the repeating unit k to the repeating unit m may be 1:0.01 to 1:10, specifically 1:0.02 to 1:9, and more specifically 1:0.02 to 1:5.

[0084] In the formula 4, if the molar ratio of repeating unit m to repeating unit k is less than 0.1, the crosslinking reaction rate for gelation may decrease, and if the molar ratio of repeating unit n to repeating unit k is more than 10, it may be difficult to control the gelation reaction rate.

[0085] Meanwhile, the weight average molecular weight (Mw) of the polymer of the present invention can be adjusted by the number of repeating units, and may be specifically 1,500,000 g / mol or less, more specifically 5,000 g / mol to 500,000 g / mol, and even more specifically 5,000 g / mol to 400,000 g / mol.

[0086] When the weight-average molecular weight of the polymer is within the above range, a crosslinking reaction can be carried out by heating at a temperature of 60°C or higher without a polymerization initiator, and the viscosity of the secondary battery electrolyte can be adjusted, thereby improving the impregnation of the secondary battery electrolyte. In particular, when the weight-average molecular weight of the polymer of the present invention is 5,000 g / mol to 400,000 g / mol, the electrolyte impregnation of electrodes and separators can be further improved.

[0087] Meanwhile, the weight-average molecular weight of the polymer of the present invention can be measured using gel permeation chromatography (GPC). For example, after preparing a sample of a certain concentration, the GPC measurement system Alliance 4 instrument is stabilized. After the instrument is stabilized, a standard sample and a sample are injected into the instrument to obtain a chromatogram, and the molecular weight can be calculated from the results obtained by the analytical method (system: Alliance 4, column: Agilent PL mixed B, eluent: THF, flow rate: 0.1 mL / min, temperature: 40°C, injection: 100 μL).

[0088] The viscosity of the polymer of the present invention may be from 4.0 cPs to 100 cPs, more specifically from 4.0 cPs to 20 cPs.

[0089] When the viscosity of the polymer of the present invention satisfies the above range, the impregnation characteristics of the electrolyte can be more easily ensured. In this case, if the viscosity of the polymer is 100 cPs or more, it is difficult to ensure the impregnation characteristics of a large-area and thick electrode. Therefore, in order to ensure the electrolyte impregnation characteristics above a certain range, it is preferable that the viscosity of the polymer is 100 cPs or less.

[0090] The viscosity was measured using a Brookfield LV DV-II+Pro Viscometer (cone-plate type) at 25° C. after dissolving the polymer in a non-aqueous organic solvent at a concentration of 3 wt %. The spindle speed was S40, rpm was 15, and the sample loading amount was 1 mL.

[0091] On the other hand, the polymer of the present invention may be contained in an amount of less than 55% by weight, for example, 0.1% by weight to 50% by weight, specifically 0.1% by weight to 30% by weight, more specifically 0.1% by weight to 20% by weight, and even more specifically 0.1% by weight to 10% by weight, based on the total weight of the electrolyte for secondary batteries.

[0092] When the content of the polymer of the present invention is 0.1 wt% or more, the gel reaction formation effect is improved, ensuring sufficient mechanical strength of the secondary battery electrolyte and effectively controlling side reactions with the positive electrode, resulting in the formation of a strong coating. Furthermore, when the content of the polymer of the present invention is less than 55 wt%, specifically 50 wt% or less, it is possible to prevent increased resistance and side reactions due to excessive polymer and improve the wettability of the secondary battery electrolyte. If the content of the polymer in the secondary battery electrolyte is 55 wt% or more, the viscosity increases, reducing ionic conductivity and increasing resistance, resulting in a decrease in battery operating performance. Meanwhile, the secondary battery electrolyte of the present invention can form a liquid electrolyte or a (gel) polymer electrolyte depending on the polymer content within this range.

[0093] (4) Other additives Meanwhile, the secondary battery electrolyte of the present invention may further contain other additives as needed to prevent the secondary battery electrolyte from being decomposed in a high-power environment, causing the collapse of the negative electrode, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effect of suppressing battery expansion at high temperatures.

[0094] Examples of such other additives include at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0095] Examples of the cyclic carbonate compounds include vinylene carbonate (VC) and vinylethylene carbonate.

[0096] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0097] The sultone compound may be, for example, 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.

[0098] The sulfate compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0099] The phosphate-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphate, and tris(2,2,2-trifluoroethyl)phosphate.

[0100] The borate-based compound may be, for example, tetraphenylborate, lithium oxalyl difluoroborate, or the like.

[0101] The nitrile compound may be, for example, 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.

[0102] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0103] The lithium salt-based compound is a compound different from the lithium salt contained in the electrolyte of the present invention, 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).

[0104] When vinylene carbonate, vinylethylene carbonate, or succinonitrile is contained among such other additives, a stronger SEI coating can be formed on the surface of the negative electrode during the initial activation step of the secondary battery.

[0105] When the battery contains LiBF4, it is possible to suppress the generation of gas that may be generated by decomposition of the electrolyte during high-temperature storage, thereby improving the high-temperature stability of the secondary battery.

[0106] Meanwhile, two or more of the other additives may be mixed and used, and may be included in an amount of 0.01 wt % to 10 wt %, specifically 0.01 wt % to 8 wt %, and preferably 0.05 wt % to 5 wt %, based on the total weight of the secondary battery electrolyte. If the content of the other additives is less than 0.01 wt %, the effect of improving the low-temperature output of the battery, or the high-temperature storage characteristics and high-temperature life characteristics of the battery, is minimal. However, if the content of the other additives is more than 10 wt %, excessive side reactions may occur during battery charge and discharge due to the addition of an excessive amount of additive. In particular, if the SEI film-forming additive is added in an excessive amount, it may not be sufficiently decomposed at high temperatures, and unreacted products may be generated or remain precipitated in the electrolyte at room temperature. This may result in side reactions that reduce the life or resistance characteristics of the secondary battery.

[0107] [Lithium secondary battery] The present invention also provides a lithium secondary battery containing the secondary battery electrolyte of the present invention.

[0108] The lithium secondary battery of the present invention can be manufactured by a method including the steps of: (a) housing an electrode assembly formed by sequentially stacking a positive electrode, a separator, and a negative electrode in a battery case; (b) injecting the secondary battery electrolyte of the present invention into the battery case; and (c) activating the secondary battery electrolyte at a temperature of 25°C to 100°C to form an electrolyte coating on the surface of the electrode.

[0109] In this case, the activation step may be performed for about 2 minutes to 48 hours, specifically, 1 hour to 24 hours.

[0110] The activation step is a step of forming a solid electrolyte interface (SEI) film on the surface of the negative electrode by partially charging and discharging the battery, and can be performed by a method known in the art. Specifically, the activation step can be performed by repeating charge and discharge once or repeatedly at a constant current or voltage within a certain range. Specifically, the activation step can be performed once at a voltage range of 2.5V to 4.8V. The activation step can be performed at a state of charge (SOC) of 30 to 70%.

[0111] After the activation step, an aging step may further be included.

[0112] The aging step is a step in which the activated battery is left to stand for a certain period of time to stabilize it, and can be carried out at a temperature range of 19°C to 25°C.

[0113] On the other hand, the positive electrode, negative electrode, and separator used in the lithium secondary battery of the present invention are not particularly limited and may be any electrode that is produced by a conventional method and used in the production of lithium secondary batteries.

[0114] (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 the positive electrode active material layer may further include a conductive material and / or a binder, as necessary.

[0115] As the positive electrode active material, a lithium iron phosphate-based positive electrode active material, which is structurally very stable, may be used.

[0116] The lithium iron phosphate-based positive electrode active material may be represented by the following chemical formula I:

[0117] [Chemical formula I] Life 1-x M x PO4

[0118] In the above chemical formula I, M is any one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, and 0≦x<1.

[0119] In Chemical Formula I, M is a metal element that can replace Fe to improve the structural stability and conductivity of the lithium iron phosphate-based positive electrode active material, and specifically, M may include any one or more elements selected from the group consisting of Ni, Co, and Mn.

[0120] In the above formula I, x may be 0≦x≦0.5.

[0121] The lithium iron phosphate-based positive electrode active material, particularly LiFePO4 having an olivine structure, has a capacitance of 3.6 g / cm 3It has a high volume density, generates a high potential of 3.4 V, and has a high theoretical capacity of approximately 170 mAh / g. Furthermore, in its initial state, LiFePO4 contains one Li atom per Fe atom, which can be electrochemically undoped, making it a promising material for use as a positive electrode active material in secondary batteries. However, LiFePO4 has low bulk ionic conductivity, and low electrical conductivity leads to high interfacial resistance at the surface, resulting in poor output characteristics. In particular, Fe ions due to Fe oxide impurities are eluted into the electrolyte and precipitated on the surface of the negative electrode.

[0122] In the present invention, by using an electrolyte containing the polymer having a cyano group (CN) as a terminal group as an additive, a coordinate bond is formed with the metal ions eluted from the positive electrode, increasing the metal ion removal effect, and by forming a functional polymer layer (coating) on ​​the surface of the positive electrode, the elution of metal ions can be effectively suppressed, thereby reducing side reactions caused by the eluted metal ions.

[0123] Meanwhile, the positive electrode active material of the present invention may further contain, in addition to the lithium iron phosphate-based positive electrode active material, a lithium nickel cobalt manganese-based oxide represented by the following chemical formula II, if necessary.

[0124] [Chemical formula II] Li x [Ni y Co z Mn w M 1 v ]O2

[0125] In chemical formula II, Said M 1 is a doping element substituted at the transition metal site, and may be one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and y+z+w+v=1.

[0126] The above x means the ratio of lithium atoms to all transition metals in the lithium nickel cobalt manganese oxide, and may be 0.8 to 1.2, preferably 1 to 1.2.

[0127] The y represents the ratio of nickel atoms among the transition metals in the lithium nickel cobalt manganese oxide, and is 0.5 or more and less than 1, preferably 0.7 to less than 1, and more preferably 0.75 to 0.98. The higher the content of nickel among the transition metals, the higher the capacity that can be achieved, so a nickel content of 0.5 or more is advantageous in achieving high capacity.

[0128] The above z means the ratio of cobalt atoms among the transition metals in the lithium nickel cobalt manganese oxide, and is greater than 0 and less than 0.5, preferably 0.01 to 0.3, and more preferably 0.01 to 0.25.

[0129] The w refers to the ratio of manganese atoms to the transition metals in the lithium nickel cobalt manganese oxide, and is greater than 0 and less than 0.5, preferably 0.01 to 0.3, and more preferably 0.01 to 0.25.

[0130] The v is a dopant element M doped into the transition metal site in the lithium nickel cobalt manganese-based oxide. 1 The doping element M 1 When added, the structural stability of the lithium nickel cobalt manganese oxide is improved. However, if the content of the doping element increases, the capacity may decrease. Therefore, it is preferable that the content is 0.2 or less.

[0131] Specific examples of the lithium nickel cobalt manganese oxide include LiNi 0.5 Co 0.2 Mn 0.3 O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 ) O2, etc., but are not limited to these.

[0132] The positive electrode active material may be contained in an amount of 80 wt % to 98 wt %, more specifically 85 wt % to 98 wt %, based on the total weight of the positive electrode active material layer. When the positive electrode active material is contained in the above range, excellent capacity characteristics can be exhibited.

[0133] The conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it does not cause chemical changes in the battery that is constructed and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used.

[0134] The conductive material may be contained in an amount of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the total weight of the positive electrode active material layer.

[0135] Next, the binder serves to improve adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector.

[0136] Examples of the binder 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, each of which may be used alone or in combination. The binder may be present in an amount of 0.1 to 15 wt %, preferably 0.1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0137] The positive electrode of the present invention as described above may be manufactured by a method known in the art. For example, the positive electrode may be manufactured 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, or by casting the positive electrode slurry on a separate support, peeling off the support, and laminating the resulting film on a positive electrode current collector.

[0138] 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 surface-treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and the surface of the current collector may be provided 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.

[0139] The solvent may be any 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 sufficient to adjust the viscosity of the cathode composite to an appropriate level, taking into consideration the coating thickness of the cathode composite, production yield, workability, etc.

[0140] (2) Negative electrode Next, the negative electrode will be described.

[0141] 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.

[0142] 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.

[0143] 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 be any of 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 various shapes such as amorphous, plate-like, flake-like, spherical, and fibrous may be used.

[0144] Preferably, the carbon-based negative electrode active material may include at least one of natural graphite and artificial graphite. More preferably, the carbon-based negative electrode active material may include natural graphite and artificial graphite. When both natural graphite and artificial graphite are used, the adhesive strength with the current collector is increased, and detachment of the active material can be suppressed.

[0145] According to another embodiment, the negative electrode active material may include a silicon-based negative electrode active material. The silicon-based negative electrode active material may be, for example, silicon metal (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 include one or more selected from the group consisting of. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0146] 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, the negative electrode containing the silicon-based negative electrode active material contains more O-rich components in the SEI film than the graphite negative electrode, and the SEI film containing the O-rich component tends to be more easily decomposed when Lewis acids such as HF or PF5 are present in the electrolyte. Therefore, in order for the negative electrode containing the silicon-based negative electrode active material to maintain a stable SEI film, it is necessary to suppress the generation of Lewis acids such as HF and PF5 in the electrolyte or to remove (or collect) the generated Lewis acids. The non-aqueous electrolyte according to the present invention contains, as an additive, a compound of Chemical Formula II containing an N atom that acts as a Lewis base and an F atom that improves the stability of the SEI film on the surface of the negative electrode, so that when using a negative electrode containing a silicon-based active material, the decomposition of the SEI film can be effectively suppressed.

[0147] According to another embodiment, the negative electrode active material may include a mixture of a carbon-based negative electrode active material and a silicon-based negative electrode active material.

[0148] Specific examples of the carbon-based negative electrode active material and the silicon-based negative electrode active material are as described above.

[0149] The mixing ratio of the silicon-based negative electrode active material to the carbon-based negative electrode active material may be, by weight, 3:97 to 99:1, preferably 5:95 to 30:70, and more preferably 5:95 to 15:85. When the mixing ratio of the silicon-based negative electrode active material to the carbon-based negative electrode active material satisfies the above range, the capacity characteristics are improved, and volume expansion of the silicon-based negative electrode active material is suppressed, ensuring excellent cycle performance.

[0150] 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 the above range, excellent capacity characteristics and electrochemical characteristics can be obtained.

[0151] 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 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0152] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 0.1 to 10 wt % based on the total weight of the negative electrode active material layer. 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.

[0153] The negative electrode may be manufactured by a method known in the art, for example, by coating 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 onto a negative electrode current collector, followed by rolling and drying, or by casting the negative electrode slurry onto a separate support, peeling off the support, and laminating the resulting film onto the negative electrode current collector.

[0154] The negative electrode current collector usually has a thickness of 3 μm to 500 μm.

[0155] Such a negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel whose surface has been surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. As with the positive electrode current collector, the bonding force of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0156] 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 sufficient to adjust the viscosity of the negative electrode slurry to an appropriate level, taking into consideration the coating thickness of the negative electrode composite, production yield, workability, etc.

[0157] (3) Separator The separator serves to prevent internal short circuits between the electrodes and to impregnate the electrolyte. The separator may be prepared by mixing a polymer resin, a filler, and a solvent to prepare a separator composition, and then directly coating and drying the separator composition on the top of the electrode to form a separator film. Alternatively, the separator may be formed by casting the separator composition on a support, drying it, and then peeling the separator film from the support and laminating it on the top of the electrode.

[0158] The separator may be a commonly used 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, either alone or in a laminate thereof, or a commonly used porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, or the like, but is not limited thereto.

[0159] In this case, the porous separator may have a pore diameter of generally 0.01 μm to 50 μm, a porosity of 5% to 95%, and a thickness of generally 5 μm to 300 μm.

[0160] 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.

[0161] [Example] Example 1 (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent containing a 30:40:30 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate to a concentration of 1.0 M. Then, 0.5 wt% of a polymer represented by chemical formula 4a-1 (k:m:n'' molar ratio of 25:20:55, weight average molecular weight (Mw): 15,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propane sultone were added to prepare a secondary battery electrolyte.

[0162] (Secondary battery manufacturing) A cathode active material slurry (solid content 48 wt%) was prepared by adding a cathode active material (LiFePO4), a conductive material (carbon black), and a binder (polyvinylidene fluoride (PVDF)) in a weight ratio of 94:3:3 to a solvent, N-methyl-2-pyrrolidone (NMP). The cathode active material slurry was applied to a cathode current collector (Al thin film) with a thickness of 100 μm, dried, and then roll-pressed to prepare a cathode.

[0163] Anode active material (carbon powder), binder (PVDF), and conductive material (carbon black) were mixed in a weight ratio of 96:3:1 with NMP solvent to prepare anode active material slurry (solid content: 70 wt%). The anode active material slurry was applied to a 90 μm-thick anode current collector (Cu thin film), dried, and then roll-pressed to prepare anodes.

[0164] The positive electrode, the negative electrode, and a three-layer separator of polypropylene / polyethylene / polypropylene (PP / PE / PP) were sequentially stacked to produce an electrode assembly, which was then housed in a pouch-shaped secondary battery case. The above-prepared secondary battery electrolyte was then poured into the assembly, followed by heat treatment at 60°C for 1 hour to produce a lithium secondary battery containing the secondary battery electrolyte.

[0165] Example 2. (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent containing a 30:40:30 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate to a concentration of 1.0 M. Then, 1.0 wt% of the polymer represented by chemical formula 4a-1 (k:m:n'' molar ratio of 25:20:55, weight average molecular weight (Mw): 15,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propane sultone were added to prepare a secondary battery electrolyte.

[0166] (Secondary battery manufacturing) A lithium secondary battery was produced in the same manner as in Example 1, except that the produced secondary battery electrolyte was used.

[0167] Example 3. (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent containing a 30:40:30 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate to a concentration of 1.0 M. Then, 2.0 wt% of the polymer represented by chemical formula 4a-1 (k:m:n'' molar ratio of 25:20:55, weight average molecular weight (Mw): 15,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propane sultone were added to prepare a secondary battery electrolyte.

[0168] (Secondary battery manufacturing) A lithium secondary battery was produced in the same manner as in Example 1, except that the produced secondary battery electrolyte was used.

[0169] Example 4. (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent containing a 30:40:30 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate to a concentration of 1.0 M. Then, 1.0 wt% of the polymer represented by chemical formula 4a-2 (k:m:n'' molar ratio of 25:20:55, weight average molecular weight (Mw): 16,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propane sultone were added to prepare an electrolyte for a secondary battery.

[0170] (Secondary battery manufacturing) A lithium secondary battery was produced in the same manner as in Example 1, except that the produced secondary battery electrolyte was used.

[0171] Comparative Example 1 (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate in a volume ratio of 30:40:30 to a concentration of 1.0 M, and then 2.5 wt% of vinylene carbonate (VC) and 0.5 wt% of 1,3-propane sultone were added to prepare a secondary battery electrolyte.

[0172] (Secondary battery manufacturing) A lithium secondary battery was produced in the same manner as in Example 1, except that the produced secondary battery electrolyte was used.

[0173] Comparative Example 2 (Manufacturing electrolytes for secondary batteries) LiPF6 was dissolved in a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate in a volume ratio of 30:40:30 to a concentration of 1.0 M, and then 0.5 wt% of a polymer represented by the following chemical formula 5 (o:p:q molar ratio of 30:1:59, weight average molecular weight (Mw): 51,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propane sultone were added to prepare an electrolyte for a secondary battery.

[0174] (Secondary battery manufacturing) A lithium secondary battery was produced in the same manner as in Example 1, except that the produced secondary battery electrolyte was used.

[0175] [ka]

[0176] [Experimental Example] Experimental example 1: Evaluation of resistance increase rate after high-temperature cycle The secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 were charged at room temperature (25°C) under constant current / constant voltage conditions at a rate of 0.33C, and then discharged for 10 seconds at a rate of 0.33C. The initial resistances were measured using a PNE-0506 charger / discharger (manufacturer: PNE solution), and the results are shown in Table 1 below.

[0177] Thereafter, the battery was charged to 3.2 V at a constant current / constant voltage at a rate of 0.33 C at a high temperature (45°C), and then discharged to 2.5 V at a constant current rate of 0.33 C, counting as one cycle. After 300 charge / discharge cycles, the resistance was measured and the resistance increase rate (%) was calculated using the following formula 1. The calculated resistance increase rate (%) is shown in Table 1 below.

[0178] [Formula 1] Resistance increase rate (%) = {(resistance after 300 cycles - initial resistance) / initial resistance} x 100

[0179] Experimental Example 2: Evaluation of capacity retention rate after high-temperature cycling The secondary batteries prepared in Example 1 and the secondary batteries prepared in Comparative Examples 1 and 2 were charged at room temperature (25°C) under constant current / constant voltage conditions at a rate of 0.33C, and then discharged for 10 seconds at a rate of 0.33C, and the initial discharge capacity was measured using a PNE-0506 charger / discharger (manufacturer: PNE solution).

[0180] Thereafter, the battery was charged to 3.2 V at a high temperature (45°C) under constant current / constant voltage conditions at a rate of 0.33 C, and then discharged to 2.5 V under constant current conditions at a rate of 0.33 C. One cycle was defined as 300 charge / discharge cycles, after which the discharge capacity was measured and the capacity retention rate (%) was calculated using the following formula 2. The results are shown in Table 1 below.

[0181] [Formula 2] Capacity retention rate (%) = (discharge capacity after 300 cycles / initial discharge capacity) x 100

[0182] [Table 1]

[0183] Referring to Table 1, it can be seen that the resistance increase rate (%) and capacity retention rate (%) of the secondary battery of Example 1 of the present invention after 300 cycles at high temperature were improved compared to the secondary batteries of Comparative Examples 1 and 2.

[0184] Experimental Example 3: Evaluation of metal elution amount The secondary battery of Example 1 and the secondary batteries of Comparative Examples 1 and 2, whose cycle characteristics had been evaluated in Experimental Example 2, were disassembled to recover the positive electrode plates, which were then sealed in 20 ml of the electrolyte prepared in Example 1 and stored in an oven at 45°C for one week.

[0185] Thereafter, the electrolyte was recovered, and the amount of the eluted transition metal (Fe) was measured by ICP analysis. The results are shown in Table 2 below.

[0186] [Table 2]

[0187] Referring to Table 2, it can be seen that the secondary battery of Example 1 of the present invention had a reduced amount of transition metal elution (%) compared to the secondary batteries of Comparative Examples 1 and 2.

[0188] Experimental Example 4: Evaluation of low-temperature discharge capacity The secondary batteries prepared in Examples 1 to 4 and the secondary batteries prepared in Comparative Examples 1 and 2 were charged at room temperature (25°C) under constant current / constant voltage conditions at a rate of 0.33C, and then discharged for 10 seconds at a rate of 0.33C, and the initial discharge capacity was measured using a PNE-0506 charger / discharger (manufacturer: PNE solution).

[0189] Thereafter, after leaving the battery at low temperature (-20°C) for 3 hours, the battery was charged to 3.2 V under constant current / constant voltage conditions at a rate of 1 C, and then discharged to 2.5 V under constant current conditions at a rate of 1 C to measure the discharge capacity retention rate at low temperature. In this case, the low-temperature discharge capacity retention rate (%) of the secondary batteries of Examples 1 to 4 and Comparative Example 2 was converted into a relative rate based on the low-temperature discharge capacity retention rate (%) of the lithium secondary battery of Comparative Example 1, and the results are shown in Table 3 below.

[0190] [Table 3]

[0191] Referring to Table 3, it can be seen that the low-temperature discharge capacity retention rates (%) of the secondary batteries of Examples 1 to 4 of the present invention are improved compared to the secondary battery of Comparative Example 2.

Claims

1. A lithium salt, a non-aqueous organic solvent; An electrolyte for a secondary battery, comprising: a polymer consisting of a repeating unit derived from a monomer represented by the following chemical formula 1, a repeating unit derived from a monomer represented by the following chemical formula 2, and a repeating unit derived from a monomer represented by the following chemical formula 3, a molar ratio m of the repeating unit derived from the monomer represented by Chemical Formula 1, a molar ratio n of the repeating unit derived from the monomer represented by Chemical Formula 2, and a molar ratio k of the repeating unit derived from the monomer represented by Chemical Formula 3 are k:m:n (k is an integer of 1 to 15,000, m is an integer of 10 to 12,000, and n is an integer of 10 to 8,500); The secondary battery electrolyte contains the polymer in an amount of 0.1 wt % to 50 wt % based on the total weight of the secondary battery electrolyte. 【Chemistry 1】 (In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R'' is H or Li. 【Chemistry 2】 In the above Chemical Formula 2, R １ and R ２ are each independently an alkylene group having 1 to 5 carbon atoms. 【Transformation 3】 (In the above chemical formula 3, R' is an alkyl group having 1 to 6 carbon atoms.

2. The lithium salt is LiPF ６ The electrolyte for a secondary battery according to claim 1 , comprising:

3. The lithium salts include LiCl, LiBr, LiI, and LiBF. ４ , LiClO ４ , LiB １０ Cl １０ , LiAlCl ４ , LiAlO ４ , LiCF ３ SO ３ , LiCH ３ CO ２ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiCH ３ SO ３ , LiTFSI (lithium (bis) trifluoromethanesulfonimide, LiN (SO ２ CF ３ ) ２ ), LiFSI (Lithium bis(fluorosulfonyl)imide, LiN(SO ２ F) ２ ), and LiBETI (lithium bisperfluoroethanesulfonimide, LiN(SO ２ CF ２ CF ３ ) ２ 3. The electrolyte for a secondary battery according to claim 2, further comprising at least one selected from the group consisting of:

4. 2. The electrolyte for a secondary battery according to claim 1, wherein in Formula 1, R is an alkylene group having 1 to 4 carbon atoms, and R″ is H.

5. 2. The electrolyte for a secondary battery according to claim 1, wherein R in Formula 1 is an alkylene group having 1 to 3 carbon atoms.

6. 2. The electrolyte for a secondary battery according to claim 1, wherein R in Formula 1 is an alkylene group having 1 or 2 carbon atoms.

7. In the above chemical formula 2, R １ and R ２ and each independently represent an alkylene group having 1 to 4 carbon atoms.

8. In the above chemical formula 2, R １ and R ２ and each independently represent an alkylene group having 2 or 3 carbon atoms.

9. 2. The electrolyte for a secondary battery according to claim 1, wherein R' in Formula 3 is an alkyl group having 1 to 5 carbon atoms.

10. 2. The electrolyte for a secondary battery according to claim 1, wherein R' in Formula 3 is an alkyl group having 1 to 4 carbon atoms.

11. The electrolyte for a secondary battery according to claim 1 , wherein the polymer comprises a unit represented by the following Chemical Formula 4: 【Chemistry 4】 (In the above chemical formula 4, R, R １ , and R ２ are each independently an alkylene group having 1 to 5 carbon atoms, R' is an alkyl group having 1 to 6 carbon atoms; R″ is H or Li; k is an integer from 1 to 15,000, m is any one integer from 10 to 12,000, n is any one integer from 10 to 8,500.

12. 2. The electrolyte for a secondary battery according to claim 1, wherein the polymer is contained in an amount of 0.1% by weight to 10% by weight based on the total weight of the electrolyte for a secondary battery.

13. a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode; A lithium secondary battery comprising the electrolyte for a secondary battery according to claim 1 .

14. The lithium secondary battery according to claim 13 , wherein the positive electrode comprises a lithium iron phosphate-based positive electrode active material represented by the following chemical formula I: [Chemical formula I] LiFe １－ｘ M ｘ PO ４ (In the above chemical formula I, M is any one selected from the group consisting of Mn, Ni, Co, Cu, Sc, Ti, Cr, V, and Zn, and 0≦x<1.)

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