Electrolyte for secondary battery and lithium secondary battery containing the 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 on August 4, 2022, and Korean Patent Application No. 10-2023-0101238 filed on August 2, 2023, and all the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference in their entirety.

[0002] The present invention relates to an electrolyte for a secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a secondary battery capable of forming a polymer film on the surface of an electrode and suppressing elution of transition metals, and a lithium secondary battery including the same.

Background Art

[0003] In recent years, with the rapid development of the electric, electronic, communication, and computer industries, the demand for secondary batteries having high performance and high stability has been increasing. In particular, with the trend of miniaturization and weight reduction of these electronic (communication) devices, there is a demand for thinning and miniaturization of lithium secondary batteries, which are core components in this field.

[0004] In lithium secondary batteries, as an electrolyte, an ion-conductive organic liquid electrolyte in which a salt is dissolved in an organic solvent using a liquid-state electrolyte, for example, a carbonate organic solvent as a main solvent, has been mainly used. However, the liquid electrolyte not only has a high volatility of the organic solvent, but also generates gas inside the battery due to decomposition of the carbonate organic solvent during charge and discharge and / or side reactions between the organic solvent and the electrode, resulting in problems such as low stability, such as expansion of the battery thickness. In particular, during high-temperature storage, such side reactions are accelerated and continuously generated gas induces an increase in the pressure resistance of the battery, causing deformation or explosion of the center of a specific surface of the battery, such as expansion of a prismatic battery in a specific direction. Furthermore, it causes a problem that local differences occur in the adhesion on the electrode surface, and the electrode reaction does not occur uniformly on the entire electrode surface.

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

[0006] On the other hand, due to the repeated charge-discharge processes and moisture and Lewis acids inside the cell, the passive film on the positive electrode surface is eroded, and transition metal ions are eluted from the positive electrode, which may reduce the structural stability of the positive electrode. In addition, the eluted transition metal ions are electrodeposited on the negative electrode or precipitated from the surface of the negative electrode, reducing the passivation ability of the SEI, resulting in a problem of negative electrode deterioration. Such negative electrode deterioration phenomena promote the decomposition of the electrolyte solvent, accelerate gas generation, and induce side reactions such as desorbing lithium ions inserted into the negative electrode, causing a decrease in battery capacity.

[0007] Therefore, in a situation where there is a need for technological development of an electrolyte that can form a stable film with stable passivation ability, low interfacial resistance, and high ion transfer efficiency on the surface of the positive electrode to suppress the elution of transition metals.

Summary of the Invention

Problems to be Solved by the Invention

[0008] An object of the present invention is to provide an electrolyte for a secondary battery that can form a film with stable passivation ability and low interfacial resistance on the surface of the positive electrode in order to solve the above problems.

[0009] Another object of the present invention is to provide a lithium secondary battery that has high high-temperature durability and improved low-temperature resistance characteristics by including the electrolyte for a secondary battery of the present invention.

Means for Solving the Problems

[0010] In one embodiment of the present invention for achieving the above object, Provided is an electrolyte for a secondary battery, comprising a lithium salt, a non-aqueous organic solvent, and a polymer containing 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 as an additive.

[0011] [Chemical formula]

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

[0013] [Chemical formula]

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

[0015] On the other hand, the lithium salt may contain LiPF6. Further, the lithium salt may further contain at least one lithium salt selected from the group consisting of 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] On the one hand, in Chemical Formula 1, R is 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] Also, in Chemical Formula 2, R1 and R2 may each independently be an alkylene group having 1 to 4 carbon atoms, and specifically, may be an alkylene group having 2 or 3 carbon atoms.

[0018] On the one hand, the polymer may further contain a repeating unit derived from a monomer represented by the following Chemical Formula 3.

[0019]

Chemical Formula

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

[0021] In Chemical Formula 3, R’ may be an alkyl group having 1 to 5 carbon atoms, and preferably, may be an alkyl group having 1 to 4 carbon atoms.

[0022] On the one hand, the polymer may contain a unit represented by the following Chemical Formula 4.

[0023]

Chemical Formula

[0024] In Chemical Formula 4, R, R1, and R2 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 any integer from 1 to 15,000, m is any integer from 10 to 12,000, n is any integer from 10 to 8,500.

[0025] In addition, another embodiment of the present invention provides a lithium secondary battery including the electrolyte for a secondary battery of the present invention.

Advantages of the Invention

[0026] The electrolyte for a secondary battery of the present invention contains, as an additive, a polymer containing a cyano group which is a thermopolymerizable functional group at its terminal, so that it has stable passivation ability and can form a film having low interfacial resistance on the surface of the positive electrode. Therefore, a lithium secondary battery containing the same can have high high-temperature durability and can achieve the effect of improving low-temperature resistance characteristics.

Modes for Carrying Out the Invention

[0027] Hereinafter, the present invention will be described in more detail.

[0028] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings. The inventors should interpret them in accordance with the meaning and concept consistent with the technical idea of the present invention, in accordance with the principle that they can appropriately define the concept of the terms in order to explain their invention in the best way.

[0029] On the other hand, in the present invention, unless otherwise specified, "*" means a portion connected to each other between the ends of the same or different atoms or chemical formulas.

[0030] In addition, in this specification, the term "alkylene group" means a branched or unbranched divalent unsaturated hydrocarbon group. In one aspect, the alkylene group may or may not be substituted. The alkylene group includes, but is not limited to, a methylene group, an ethylene group, a propylene group, an isopropylene group, a butylene group, an isobutylene group, a tert-butylene group, a pentylene group, a 3-pentylene group, and the like.

[0031] Also, in this specification, the term "repeating unit" represents the monomer unit constituting the polymer.

[0032] Also, in this specification, the term "thermopolymerizable functional group" is a functional group capable of undergoing a polymerization reaction between the same functional groups or with other functional groups by heating in the absence of a polymerization initiator.

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

[0034] 〔Electrolyte for secondary battery〕 Specifically, one embodiment of the present invention is a lithium salt, a non-aqueous organic solvent, and, as an additive, a polymer containing 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, to provide an electrolyte for a secondary battery.

[0035]

Chem.

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

[0037]

Chem.

[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 particular limitation as long as they are compounds capable of providing lithium ions. Typical examples thereof 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 - and include at least any one selected from the group consisting of.

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

[0042] Further, the lithium salt includes LiPF6 as the first lithium salt, and as the second lithium salt, at least one selected from the group consisting of 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) may be included.

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

[0044] On the other hand, although the second lithium salt has excellent ion transfer effect, compared with LiPF6, it can react with moisture in the battery to generate amine or alkali components. Therefore, when the molar ratio of the second lithium salt to 1 mol of the first lithium salt exceeds 50, corrosion of metal components such as current collectors or battery cases may occur due to the amine or alkali components generated by side reactions, or the cross-linking reaction rate between polymers during the thermal polymerization reaction may decrease.

[0045] On the one hand, in the electrolyte for the secondary battery, the total concentration of the lithium salt may be 0.1 M to 5 M, specifically 0.5 M to 5 M, and more specifically 1 M to 4.5 M. When the overall mixing concentration of the lithium salt in the electrolyte for the secondary battery is less than 0.1 M, the ionic conductivity of the electrolyte decreases and the electrolyte performance deteriorates. When the mixing concentration of the lithium salt exceeds 5 M, the impregnability of the electrolyte decreases due to the increase in the viscosity of the electrolyte, and the mobility of lithium ions decreases, which may lead to a decrease in the capacity characteristics.

[0046] (2) Non-aqueous organic solvent As the non-aqueous organic solvent, various organic solvents commonly used in lithium electrolytes can be used without limitation. For example, the organic solvent may include a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixed organic solvent thereof.

[0047] The cyclic carbonate-based organic solvent is a high-viscosity organic solvent with a high dielectric constant and is likely to dissociate the lithium salt in the electrolyte. Specific examples thereof may 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. Among them, at least one of ethylene carbonate and propylene carbonate (PC) may be included.

[0048] In addition, the linear carbonate-based organic solvent is an organic solvent having a low viscosity and a low dielectric constant. Representative examples thereof may include at least one organic solvent selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. Specifically, at least one of dimethyl carbonate and ethyl methyl carbonate may be included.

[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] Further, in the present invention, in order to improve the ionic conductivity of the electrolyte for secondary batteries, at least one organic solvent of a linear ester-based organic solvent and a cyclic ester-based organic solvent having a lower melting point and higher stability at high temperature than the cyclic carbonate-based organic solvent and / or the linear carbonate-based organic solvent may be further included. Specifically, a linear ester-based organic solvent that can reduce the viscosity of the electrolyte for secondary batteries and chelate lithium cations by an ether symmetric structure to increase the dissociation degree of lithium salts may be further included. When the linear ester solvent is further used, the ionic conductivity of the electrolyte for secondary batteries 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 thereto.

[0052] Further, examples of the cyclic ester-based organic solvent include 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 lithium secondary batteries of the present invention, the remainder excluding lithium salts, polymers, and other additives may all contain non-aqueous organic solvents.

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

[0055] The additive may include a polymer or oligomer containing 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]

Chemical formula

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

[0058]

Chemical formula

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

[0060] The polymer contained as the additive of the present invention contains, as essential components, a repeating unit having a cyano group (CN) capable of easily forming a coordination bond with a metal on the surface of the positive electrode or a metal ion eluted from the positive electrode as a terminal group, and a repeating unit having a hydroxyl group (-OH) excellent in the adsorption force with the transition metal on the surface of the positive electrode as a terminal group. Therefore, a functional polymer layer (film) with low resistance and strong strength can be formed on the surface of the electrode. In particular, the polymer of the present invention contains a methacrylate group as a main chain unit, so that a stronger film with low fluidity and capable of coordinating with Li ions can be formed on the surface of the positive electrode, thereby reducing the positive electrode interfacial resistance. As a result, side reactions are reduced, and a lithium secondary battery capable of ensuring excellent high-temperature durability and low-temperature resistance characteristics can be realized.

[0061] On the other hand, in Chemical Formula 1, R is 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 may be an alkylene group having 1 or 2 carbon atoms.

[0062] Also, 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, at least one hydrogen bonded to carbon in R, R1, and R2 may be substituted with an element other than hydrogen, specifically, an alkyl group having 1 to 5 carbon atoms.

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

[0065]

Chemical Formula

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

[0067] By including, as a repeating unit, the repeating unit represented by Chemical Formula 3 having an alkyl group (R') as a terminal group, the solubility of the polymer of the present invention in an organic solvent can be further improved.

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

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

[0070] On the other hand, the polymer of the present invention may contain a compound represented by the following Chemical Formula 4.

[0071]

Chemical formula

[0072] In Chemical Formula 4, R, R1, and R2 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 any integer from 1 to 15,000, m is any integer from 10 to 12,000, n is any integer from 10 to 8,500.

[0073] Specifically, the compound represented by Chemical Formula 4 may contain a compound represented by the following Chemical Formula 4a.

[0074]

Chemical formula

[0075] In Chemical Formula 4a, R' is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, R'' is H or Li, k is any integer from 1 to 15,000, m is any integer from 10 to 12,000, n is any integer from 10 to 8,500.

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

[0077]

Chem.

[0078] In the chemical formula 4a-1, k’’ is any integer from 1 to 15,000, m’’ is any integer from 10 to 12,000, n’’ is any integer from 10 to 8,500.

[0079]

Chem.

[0080] In the chemical formula 4a-2, k’’ is any integer from 1 to 15,000, m’’ is any integer from 10 to 12,000, n’’ is any integer from 10 to 8,500.

[0081] On the other hand, in the chemical formula 4, the molar ratio of the repeating unit n to (the repeating unit k + the 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 more specifically 1:0.2 to 1:8.

[0082] If the molar ratio of the repeating unit (k + m) to 1 mole of the repeating unit n1 in the chemical formula 4 is less than 0.001, not only does the migration efficiency of lithium ions decrease, but also the cross-linking reaction rate for gelation decreases, and it is difficult to form a stable electrolyte for a secondary battery. In addition, since the adhesion between the electrode and the separator becomes low, the effect of improving safety against thermal, mechanical, and electrical shocks may be negligible. Further, when the molar ratio of the repeating unit (k + m) to 1 mole of the repeating unit n1 in the chemical formula 4 exceeds 10, there is a drawback that it is difficult to control the gelation reaction rate.

[0083] In addition, 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 chemical formula 4, when the molar ratio of the repeating unit m to 1 mol of the repeating unit k1 is less than 0.1, the crosslinking reaction rate for gelation may decrease. Further, when the molar ratio of the repeating unit n to 1 mol of the repeating unit k1 exceeds 10, there is a drawback that it is difficult to control the gelation reaction rate.

[0085] On the other hand, the weight average molecular weight (Mw) of the polymer of the present invention can be adjusted by the number of repeating units, specifically, it may be 1,500,000 g / mol or less, more specifically 5,000 g / mol to 500,000 g / mol, and 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, at a temperature of 60 °C or higher, a crosslinking reaction can be carried out by heating without a polymerization initiator, the viscosity of the electrolyte for a secondary battery can be adjusted, and the impregnation property of the electrolyte for a secondary battery can be improved. 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 impregnation property of the electrolyte into the electrode and the separator can be further improved.

[0087] On the one hand, 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 solution of a certain concentration, the GPC measurement system, Alliance 4 instrument, is stabilized. After the instrument is stabilized, a standard sample and a sample solution are injected into the instrument to obtain a chromatogram, and then the molecular weight can be calculated from the results obtained by the analysis method (system: Alliance 4, column: PL mixed B of Agilent, eluent: THF, flow rate: 0.1 mL / min, temp: 40 °C, injection: 100 μL).

[0088] Also, the viscosity of the polymer of the present invention may be 4.0 cPs to 100 cPs, more specifically 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. At this time, when the viscosity of the polymer is 100 cPs or more, it is difficult to ensure the impregnation property for a large area and thick electrode. Therefore, in order to ensure the electrolyte impregnation property above a certain range, the viscosity of the polymer is preferably 100 cPs or less.

[0090] The viscosity was measured with a Brookfield LV DV-II+Pro Viscometer (cone-plate type) at a temperature of 25 °C after dissolving the polymer in a non-aqueous organic solvent at a concentration of 3% by weight. During the measurement, the spindle 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 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 a secondary battery.

[0092] When the content of the polymer of the present invention is 0.1% by weight or more, the gel reaction formation effect is improved, and not only can sufficient mechanical strength of the electrolyte for secondary batteries be ensured, but also an electrolyte for secondary batteries can be formed that can effectively control side reactions with the positive electrode and form a strong film. Further, when the content of the polymer of the present invention is less than 55% by weight, specifically 50% by weight or less, an increase in resistance and side reactions due to an excessive amount of the polymer can be prevented, and the wettability of the electrolyte for secondary batteries can be improved. If the content of the polymer in the electrolyte for secondary batteries is 55% by weight or more, the ionic conductivity decreases due to an increase in viscosity, the resistance increases, causing a decrease in battery driving performance. On the other hand, the electrolyte for secondary batteries of the present invention can form a liquid electrolyte or a (gel) polymer electrolyte depending on the content range of the polymer.

[0093] (4) Other additives On the other hand, the electrolyte for secondary batteries of the present invention may further contain other additives as necessary in order to prevent the electrolyte for secondary batteries from being decomposed in a high-output environment and causing the collapse of the negative electrode, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, battery expansion suppression effect at high temperatures, etc.

[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) or vinyl ethylene 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 (Ethylene Sulfate; Esa), trimethylene sulfate (Trimethylene sulfate; TMS), or methyl trimethylene sulfate (Methyl trimethylene sulfate; MTMS), etc.

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

[0100] The borate compound may be, for example, tetraphenylborate, lithium oxalyldifluoroborate, etc.

[0101] The nitrile compound may be, for example, at least one or more compounds 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, etc., the amine-based compound may be triethanolamine or ethylenediamine, etc., and the silane-based compound may be tetravinylsilane, etc.

[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 bisoxalate borate (LiB(C2O4)2)), and LiBF4.

[0104] When vinylene carbonate, vinyl ethylene carbonate, or succinonitrile is included among such other additives, a stronger SEI film can be formed on the surface of the negative electrode during the initial activation process of the secondary battery.

[0105] When LiBF4 is included, the generation of gas that can be generated by the decomposition of the electrolyte during high-temperature storage can be suppressed, and the high-temperature stability of the secondary battery can be improved.

[0106] On the other hand, two or more of the other additives can be mixed and used, and may be included in an amount of 0.01% by weight to 10% by weight, specifically 0.01% by weight to 8% by weight, preferably 0.05% by weight to 5% by weight, based on the total weight of the electrolyte for the secondary battery. When the content of the other additives is less than 0.01% by weight, the improvement effects on the low-temperature output of the battery and the high-temperature storage characteristics and high-temperature life characteristics are negligible. When the content of the other additives exceeds 10% by weight, side reactions may occur excessively during the charge and discharge of the battery due to the excessive additives. In particular, when the additive for forming the SEI film is added in excess, it may not be sufficiently decomposed at high temperatures, and unreacted substances may be generated or remain precipitated in the electrolyte at room temperature. Therefore, side reactions that reduce the life or resistance characteristics of the secondary battery may occur.

[0107] 〔Lithium secondary battery〕 Furthermore, the present invention provides a lithium secondary battery including the electrolyte for a secondary battery of the present invention.

[0108] The lithium secondary battery of the present invention can be manufactured by a method including: (a) a step of housing an electrode assembly in which a positive electrode, a separator, and a negative electrode are laminated in this order in a battery case; (b) a step of injecting the electrolyte for a secondary battery of the present invention into the battery case; and (c) a step of activating the electrolyte for a secondary battery under the conditions of 25°C to 100°C to form an electrolyte film on the surface of the electrode.

[0109] At this time, the activation step can 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 SEI (Solid electrolyte interface) film on the surface of the negative electrode by performing partial charge and discharge, and can be performed by a method known in this field. Specifically, by way of example, charge and discharge can proceed once or repeatedly at a constant current or constant voltage within a certain range. Specifically, charge and discharge can be performed once in a voltage range of 2.5V to 4.8V. Further, the charging for the activation can be performed at a SOC (state of charge) in the range of 30 to 70%.

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

[0112] The aging step is a step of stabilizing the battery activated as described above by leaving it for a certain period of time, and can proceed in a temperature range of 19°C to 25°C.

[0113] On the other hand, as the positive electrode, negative electrode, and separator used in the lithium secondary battery of the present invention, those manufactured and used by ordinary methods during the manufacture of lithium secondary batteries can be used without particular limitation.

[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, if necessary, the positive electrode active material layer may further include a conductive material and / or a binder.

[0115] As the positive electrode active material, a lithium iron phosphate-based positive electrode active material with a very stable structure 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 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, it may include any one or two or more elements selected from the group consisting of Ni, Co, and Mn.

[0120] In Chemical 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 density of 3.6 g / cm 3It has a high volume density, generates a high potential of 3.4 V, and also has a high theoretical capacity of about 170 mAh / g. Also, in the initial state, LiFePO4 contains 1 Li that can be electrochemically de-doped per Fe atom, so it is a promising material as a cathode active material for secondary batteries. However, LiFePO4 has low bulk ion conductivity and low electrical conductivity, resulting in high interfacial resistance and low output characteristics on the surface. In particular, there is a drawback that Fe ions caused by Fe oxide-based impurities are eluted into the electrolyte and deposited on the surface of the negative electrode.

[0122] In the present invention, by applying an electrolyte containing the polymer containing a cyano group (CN) as a terminal group as an additive, a coordination bond is formed with metal ions eluted from the cathode, increasing the metal ion removal effect, and by forming a functional polymer layer (coating) on the surface of the cathode, elution of metal ions can be effectively suppressed. Therefore, side reactions caused by eluted metal ions can be improved.

[0123] On the other hand, in addition to the lithium iron phosphate-based cathode active material, the cathode active material of the present invention may further be used in combination with 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, the 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 may hold.

[0126] In the lithium nickel cobalt manganese oxide, x represents the atomic ratio of lithium to all transition metals, which may be 0.8 to 1.2, preferably 1 to 1.2.

[0127] In the lithium nickel cobalt manganese oxide, y represents the atomic ratio of nickel among the transition metals, which is 0.5 or more and less than 1, preferably 0.7 to less than 1, more preferably 0.75 to 0.98. The higher the nickel content among the transition metals, the higher the capacity can be realized. Therefore, a nickel content of 0.5 or more is advantageous for realizing a high capacity.

[0128] In the lithium nickel cobalt manganese oxide, z represents the atomic ratio of cobalt among the transition metals, which is more than 0 and less than 0.5, preferably 0.01 to 0.3, more preferably 0.01 to 0.25.

[0129] In the lithium nickel cobalt manganese oxide, w represents the atomic ratio of manganese among the transition metals, which is more than 0 and less than 0.5, preferably 0.01 to 0.3, more preferably 0.01 to 0.25.

[0130] The above-mentioned v represents the atomic ratio of the doping element M doped into the transition metal sites in the lithium nickel cobalt manganese oxide 1 and may be 0 to 0.2, preferably 0 to 0.1. When the doping element M 1 is added, it has the effect of improving the structural stability of the lithium nickel cobalt manganese oxide. However, as the content of the doping element increases, the capacity may decrease. Therefore, it is preferably contained in a content of 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 and the like can be mentioned, but it is not limited thereto.

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

[0133] Next, the conductive material is used to impart conductivity to the electrode, and in the battery constituted thereby, it can be used without particular limitation as long as it does not cause a chemical change and has electron 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 fibers and metal fibers; 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, etc. 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 0.1% by weight to 10% by weight, preferably 0.1% by weight to 5% by weight, based on the total weight of the positive electrode active material layer.

[0135] Next, the binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive force between the positive electrode active material and the current collector.

[0136] Examples of the binder include fluororesin binders containing polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber binders containing styrene butadiene rubber (SBR), acrylonitrile - butadiene rubber, and styrene - isoprene rubber; cellulose binders containing carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol binders containing polyvinyl alcohol; polyolefin binders containing polyethylene and polypropylene; polyimide binders; polyester binders; and silane binders. Among these, one kind may be used alone, or a mixture of two or more kinds may be used. The binder may be contained in an amount of 0.1% to 15% by weight, preferably 0.1% to 10% by weight, based on the total weight of the positive electrode active material layer.

[0137] The positive electrode of the present invention as described above can be manufactured by a method for manufacturing a positive electrode known in the art. For example, the positive electrode can be manufactured by applying a positive electrode slurry prepared by dissolving or dispersing a positive electrode active material, a binder, and / or a conductive material in a solvent onto a positive electrode current collector, followed by drying and rolling, or by casting the positive electrode slurry onto another support and then laminating the film obtained by peeling the support onto the positive electrode current collector.

[0138] The positive current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Further, the positive current collector usually has a thickness of 3 μm to 500 μm, and the adhesion of the positive electrode material may be enhanced by forming fine irregularities on the surface of the current collector. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0139] The solvent may be a solvent generally used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. Among these, one kind can be used alone, or a mixture of two or more kinds can be used. The usage amount of the solvent may be adjusted so that the positive electrode mixture has an appropriate viscosity in consideration of the coating thickness of the positive electrode mixture, production yield, workability, etc., and is not particularly limited.

[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 contain a conductive material and / or a binder as required.

[0142] As the negative electrode active material, various negative electrode active materials used in the industry, such as carbon-based negative electrode active materials, silicon-based negative electrode active materials, or mixtures thereof, may be used.

[0143] According to one embodiment, the negative electrode active material may include a carbon-based negative electrode active material. As the carbon-based negative electrode active material, various carbon-based negative electrode active materials used in the art can be used, for example, graphite-based materials such as natural graphite, artificial graphite, and Kish graphite; pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes, soft carbon, hard carbon, etc. The shape of the carbon-based negative electrode active material is not particularly limited, and substances with various shapes such as amorphous, plate-like, scaly, spherical, or fibrous can 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 force with the current collector increases, and the 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, metallic silicon (Si), silicon oxide (SiO x, where 0 < x < 2), silicon carbide (SiC), and Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si) may be included. 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, a negative electrode containing a silicon-based negative electrode active material contains more O-rich components in the SEI film than a graphite negative electrode, and an SEI film containing an O-rich component tends to be more easily decomposed when a Lewis acid such as HF or PF5 is present in the electrolyte. Therefore, in order for a negative electrode containing a 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 and the carbon-based negative electrode active material may be 3:97 to 99:1, preferably 5:95 to 30:70, more preferably 5:95 to 15:85, by weight. When the mixing ratio of the silicon-based negative electrode active material and the carbon-based negative electrode active material satisfies the above range, the capacity characteristics can be improved, the volume expansion of the silicon-based negative electrode active material can be suppressed, and excellent cycle performance can be ensured.

[0150] The negative electrode active material may be contained in an amount of 80% to 99% by weight 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] Next, 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% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as copper, nickel, aluminum, silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; polyphenylene derivatives and the like may be used.

[0152] The binder is a component that aids in the binding between the conductive material, the active material, and the current collector, and is usually added in an amount of 0.1% by weight to 10% by weight based on the total weight of the negative electrode active material layer. Examples of such binders include fluororesin-based binders containing polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based binders containing styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, or styrene-isoprene rubber; cellulose-based binders containing carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; polyalcohol-based binders containing polyvinyl alcohol; polyolefin-based binders containing polyethylene or polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders, etc.

[0153] The negative electrode can be manufactured by a method for manufacturing a negative electrode known in the art. For example, the negative electrode can be manufactured by applying a negative electrode slurry prepared by dissolving or dispersing a negative electrode active material, 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 another support and then laminating the film obtained by peeling off the support 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 a chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those with a surface treatment of carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. may be used. Also, similar to the positive electrode current collector, the binding force of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0156] The solvent may be a solvent generally used in the technical field, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, etc. One of these can be used alone, or a mixture of two or more can be used. The amount of the solvent used may be adjusted so that the negative electrode slurry has an appropriate viscosity in consideration of the coating thickness of the negative electrode composite material, production yield, workability, etc., and is not particularly limited.

[0157] (3) Separator Also, the separator plays a role in preventing internal short circuits between both electrodes and impregnating the electrolyte. After manufacturing a separator composition by mixing a polymer resin, a filler, and a solvent, a separator film may be formed by directly coating and drying the separator composition on top of the electrode, or after casting and drying the separator composition on a support, a separator film peeled from the support may be laminated on top of the electrode to form it.

[0158] As the separator, a commonly used porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a homopolymer of ethylene, a homopolymer of propylene, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer may be used alone or in a laminated form, or a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used, but it is not limited thereto.

[0159] At this time, the pore diameter of the porous separator is generally 0.01 μm to 50 μm, and the porosity may be 5% to 95%. Also, the thickness of the porous separator may generally be in the range of 5 μm to 300 μm.

[0160] The outer shape of the lithium secondary battery of the present invention is not particularly limited, and it may be a cylindrical shape, a rectangular shape, a pouch shape, or a coin shape using a can, etc.

[0161] [Examples] Example 1. (Manufacture of electrolyte for secondary battery) After dissolving LiPF6 to 1.0 M in a non-aqueous organic solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate are mixed at a volume ratio of 30:40:30, 0.5 wt% of the polymer represented by Chemical Formula 4a-1 (molar ratio of k'':m'':n'' is 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 manufacture an electrolyte for a secondary battery.

[0162] (Manufacture of secondary battery) The positive electrode active material (LiFePO4), conductive material (carbon black), and binder (polyvinylidene fluoride: PVDF) were added to N-methyl-2-pyrrolidone (NMP), which is a solvent, at a weight ratio of 94:3:3 to produce a positive electrode active material slurry (solid content: 48% by weight). After applying and drying the positive electrode active material slurry to a positive electrode current collector (Al thin film) with a thickness of 100 μm, roll press was performed to produce a positive electrode.

[0163] The negative electrode active material (carbon powder), binder (PVDF), and conductive material (carbon black) were added to NMP, which is a solvent, at a weight ratio of 96:3:1 to produce a negative electrode active material slurry (solid content: 70% by weight). After applying and drying the negative electrode active material slurry to a negative electrode current collector (Cu thin film) with a thickness of 90 μm, roll press was performed to produce a negative electrode.

[0164] The positive electrode, the negative electrode, and a separator composed of three layers of polypropylene / polyethylene / polypropylene (PP / PE / PP) were laminated in order to produce an electrode assembly. Then, it was housed in a pouch-shaped secondary battery case, and after injecting the electrolyte for the secondary battery manufactured above, heat treatment was performed at 60 °C for 1 hour to produce a lithium secondary battery containing the electrolyte for the secondary battery.

[0165] Example 2. (Manufacture of Electrolyte for Secondary Battery) LiPF6 was dissolved in a non-aqueous organic solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate were mixed at a volume ratio of 30:40:30 so that it became 1.0 M. Then, 1.0% by weight of a polymer represented by Chemical Formula 4a-1 (molar ratio of k'':m'':n'' is 25:20:55, weight average molecular weight (Mw): 15,000), 2.5% by weight of vinylene carbonate (VC), and 0.5% by weight of 1,3-propanesultone were added to produce an electrolyte for a secondary battery.

[0166] (Manufacture of Secondary Battery) A lithium secondary battery was manufactured in the same manner as in Example 1 except that the manufactured electrolyte for the secondary battery was used.

[0167] Example 3. (Manufacture of Electrolyte for Secondary Battery) After dissolving LiPF6 to 1.0 M in a non-aqueous organic solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate are mixed at a volume ratio of 30:40:30, 2.0 wt% of the polymer represented by Chemical Formula 4a-1 (molar ratio of k'':m'':n'' is 25:20:55, weight average molecular weight (Mw): 15,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propanesultone were added to manufacture an electrolyte for a secondary battery.

[0168] (Manufacture of Secondary Battery) A lithium secondary battery was manufactured in the same manner as in Example 1, except that the manufactured electrolyte for a secondary battery was used.

[0169] Example 4. (Manufacture of Electrolyte for Secondary Battery) After dissolving LiPF6 to 1.0 M in a non-aqueous organic solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate are mixed at a volume ratio of 30:40:30, 1.0 wt% of the polymer represented by Chemical Formula 4a-2 (molar ratio of k'':m'':n'' is 25:20:55, weight average molecular weight (Mw): 16,000), 2.5 wt% of vinylene carbonate (VC), and 0.5 wt% of 1,3-propanesultone were added to manufacture an electrolyte for a secondary battery.

[0170] (Manufacture of Secondary Battery) A lithium secondary battery was manufactured in the same manner as in Example 1, except that the manufactured electrolyte for a secondary battery was used.

[0171] Comparative Example 1. (Manufacture of Electrolyte for Secondary Battery) Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate were mixed in a volume ratio of 30:40:30, and LiPF6 was dissolved therein to a concentration of 1.0 M. Then, 2.5 wt% of vinylene carbonate (VC) and 0.5 wt% of 1,3 - propane sultone were added to produce an electrolyte for a secondary battery.

[0172] (Manufacture of secondary battery) A lithium secondary battery was manufactured in the same manner as in Example 1, except that the manufactured electrolyte for a secondary battery was used.

[0173] Comparative Example 2. (Manufacture of electrolyte for secondary battery) Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate were mixed in a volume ratio of 30:40:30, and LiPF6 was dissolved therein to a concentration of 1.0 M. Then, 0.5 wt% of a polymer represented by the following Chemical Formula 5 (molar ratio of o:p:q is 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 produce an electrolyte for a secondary battery.

[0174] (Manufacture of secondary battery) A lithium secondary battery was manufactured in the same manner as in Example 1, except that the manufactured electrolyte for a secondary battery was used.

[0175] [Chemical formula]

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

[0177] Thereafter, at a high temperature (45°C), after charging up to 3.2V under constant current / constant voltage conditions at a rate of 0.33C, discharging up to 2.5V under constant current conditions at a rate of 0.33C was defined as one cycle, and after performing 300 cycles of charge and discharge, the resistance was measured, and the resistance increase rate (%) was calculated according to 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} × 100

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

[0180] Thereafter, at a high temperature (45°C), after charging up to 3.2V under constant current / constant voltage conditions at a rate of 0.33C, discharging up to 2.5V under constant current conditions at a rate of 0.33C was defined as one cycle, and after performing 300 cycles of charge and discharge, the discharge capacity was measured, the capacity retention rate (%) was calculated according to the following Formula 2, and the results are shown in Table 1 below.

[0181] [Formula 2] Capacity retention rate (%) = (Discharge capacity after 300 cycles / Initial discharge capacity) × 100

[0182]

Table 1

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

[0184] Experimental Example 3. Evaluation of Metal Elution Amount After disassembling the secondary battery of Example 1 and the secondary batteries of Comparative Examples 1 and 2 for which the cycle characteristics evaluation was completed in Experimental Example 2 to recover the positive electrode plates, they were each sealed in 20 ml of the electrolyte manufactured in Example 1 and then 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, and the results are shown in Table 2 below.

[0186]

Table 2

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

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

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

[0190]

Table 3

[0191] Referring to Table 3 above, 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 were improved compared to the secondary battery of Comparative Example 2.

Claims

1. A lithium salt, a non-aqueous organic solvent, and a polymer containing 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, an electrolyte for a secondary battery. 【Chemical 1】 (In the above Chemical Formula 1, R is an alkylene group having 1 to 5 carbon atoms, R’’ is H or Li. 【Chemical 2】 In the above Chemical Formula 2, R １ and R ２ are each independently an alkylene group having 1 to 5 carbon atoms.)

2. The lithium salt contains LiPF ６ The electrolyte for a secondary battery according to claim 1, which contains ６ .

3. The lithium salt is LiCl, LiBr, LiI, 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 The electrolyte for a secondary battery according to claim 2, further comprising at least one selected from the group consisting of these.

4. In the above Chemical Formula 1, R is an alkylene group having 1 to 4 carbon atoms and R’’ is H, the electrolyte for a secondary battery according to Claim 1.

5. In the above Chemical Formula 1, R is an alkylene group having 1 to 3 carbon atoms, the electrolyte for a secondary battery according to Claim 1.

6. In the above Chemical Formula 1, R is an alkylene group having 1 or 2 carbon atoms, the electrolyte for a secondary battery according to Claim 1.

7. In the chemical formula (2), R １ and R ２ are each independently an alkylene group having 1 to 4 carbon atoms. The electrolyte for a secondary battery according to claim 1.

8. In the chemical formula (2), R １ and R ２ are each independently an alkylene group having 2 or 3 carbon atoms. The electrolyte for a secondary battery according to claim 1.

9. The polymer further contains a repeating unit derived from a monomer represented by the following Chemical Formula 3, the electrolyte for a secondary battery according to Claim 1. 【Chemical Formula 3】 (In the above Chemical Formula 3, R’ is an alkyl group having 1 to 6 carbon atoms.)

10. In the above Chemical Formula 3, R’ is an alkyl group having 1 to 5 carbon atoms, the electrolyte for a secondary battery according to Claim 9.

11. In the above Chemical Formula 3, R’ is an alkyl group having 1 to 4 carbon atoms, the electrolyte for a secondary battery according to Claim 9.

12. The polymer contains a unit represented by the following Chemical Formula 4, the electrolyte for a secondary battery according to Claim 1. [Chemical Formula 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 any integer from 1 to 15,000, m is any integer from 10 to 12,000, n is any integer from 10 to 8,500.)

13. The polymer is contained in an amount of 0.1% by weight to 50% by weight based on the total weight of the electrolyte for a secondary battery, the electrolyte for a secondary battery according to Claim 1.

14. A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte for a secondary battery according to any one of Claims 1 to 13, a lithium secondary battery.

15. The positive electrode contains a lithium iron phosphate-based positive electrode active material represented by the following Chemical Formula I, the lithium secondary battery according to Claim 14. [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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