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

Abstract

The present disclosure relates to an electrolyte additive for a secondary battery, a non-aqueous electrolyte containing the same, and a lithium secondary battery containing the same. Specifically, the non-aqueous electrolyte includes an electrolyte additive containing a compound represented by Formula 1. In Formula 1, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 5 carbon atoms, and L is a direct bond, -O-, -COO-, -RO-, or -R'COO-, where R and R' are each independently an alkylene group having 1 to 10 carbon atoms. [Formula 1]

Description

[0001] Cross - reference to related applications

[0002] This application claims the priority of Korean Patent Application No. 10 - 2021 - 0042033, filed on March 31, 2021, and Korean Patent Application No. 10 - 2022 - 0031713, filed on March 14, 2022, the disclosures of which are incorporated herein by reference. Technical field

[0003] The present invention relates to an electrolyte additive for a secondary battery, a non - aqueous electrolyte for a lithium secondary battery containing the additive, and a lithium secondary battery. More specifically, it relates to an electrolyte additive for a secondary battery having an excellent effect of scavenging decomposition products generated from a lithium salt, a non - aqueous electrolyte for a lithium secondary battery containing the additive, and a lithium secondary battery. Background art

[0004] As modern society's dependence on electric energy gradually increases, renewable energy power generation, which can increase production without causing environmental problems, has emerged as a next - generation power generation system.

[0005] For renewable energy, a large - capacity energy storage device is essential for stable power supply due to its intermittent power generation characteristics. Lithium - ion batteries have attracted much attention as the devices with the highest energy density currently commercialized in energy storage devices.

[0006] A lithium - ion battery is composed of a positive electrode formed of a lithium - containing transition metal oxide, a negative electrode capable of storing lithium, an electrolyte containing a non - aqueous organic solvent containing a lithium salt, and a separator.

[0007] In a lithium - ion battery, LiPF6 is mainly used as a representative lithium salt to achieve suitable battery characteristics. However, since LiPF6 is very susceptible to heat damage, it undergoes thermal decomposition when the battery is exposed to high temperatures, generating Lewis acids such as PF5. Such Lewis acid materials not only cause decomposition reactions of non - aqueous organic solvents such as ethylene carbonate, but also deteriorate films such as a solid electrolyte interface (SEI) formed on the electrode surface, leading to further decomposition of the electrolyte, an increase in resistance, and the dissolution of transition metals from the positive electrode.

[0008] In addition, the dissolved transition metal ions become a cause of increased positive electrode resistance while redepositing on the positive electrode. On the other hand, the dissolved transition metal ions are transferred to the negative electrode through the electrolyte and then electrodeposited on the negative electrode, causing self - discharge of the negative electrode, and due to the additional consumption of lithium ions caused by the destruction and regeneration of the solid electrolyte interface (SEI), they become a cause of increased resistance and reduced lifespan.

[0009] Therefore, in order to suppress the degradation behavior of the battery when the battery is exposed to high temperatures, there is an increasing interest in methods that can maintain the passivation ability of the SEI while removing by-products (such as HF and PF5) formed by the thermal decomposition of lithium salts. Summary of the Invention

[0010] [Technical Problem]

[0011] One aspect of the present invention provides an electrolyte additive for a secondary battery, which can remove decomposition products generated by lithium salts and can simultaneously achieve a solid electrolyte interface (SEI) strengthening effect.

[0012] Another aspect of the present invention provides a non-aqueous electrolyte for a lithium secondary battery, which can achieve high-temperature stability and high-temperature cycling characteristics by including the electrolyte additive for the secondary battery, and a lithium secondary battery including the electrolyte.

[0013] [Technical Solution]

[0014] According to one aspect of the present invention, there is provided an electrolyte additive for a secondary battery, which includes a compound represented by Formula 1.

[0015] [Formula 1]

[0016]

[0017] In Formula 1,

[0018] R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 5 carbon atoms, and

[0019] L is a direct bond, -O-, -COO-, -RO- or -R'COO-, where R and R' are each independently an alkylene group having 1 to 10 carbon atoms.

[0020] According to another aspect of the present invention, there is provided a non-aqueous electrolyte for a lithium secondary battery, which includes the electrolyte additive for the secondary battery.

[0021] According to another aspect of the present invention, there is provided a lithium secondary battery, which includes:

[0022] a positive electrode including a positive electrode active material;

[0023] a negative electrode including a negative electrode active material;

[0024] a separator disposed between the negative electrode and the positive electrode; and

[0025] the non-aqueous electrolyte for a lithium secondary battery of the present invention.

[0026] [Advantageous Effects]

[0027] The compound represented by Formula 1 as a non-aqueous electrolyte additive of the present invention is a zwitterionic compound, which is a neutral molecule having an anion part and a cation part in one molecular structure, and it can further improve the ionic conductivity of the non-aqueous electrolyte by virtue of the zwitterionic structure. In particular, for the compound represented by Formula 1, since the nitrogen atom in the cation part of the molecular structure acts as a Lewis base, it can effectively scavenge the Lewis acid generated as a decomposition product of the lithium salt. In addition, the sulfate group (-SO4) as the anion part of the compound represented by Formula 1 can form a stable film on the surface of the positive electrode or the negative electrode.

[0028] Therefore, since the non-aqueous electrolyte for a lithium secondary battery of the present invention forms a stable film on the surface of the positive electrode or the negative electrode by containing the compound represented by Formula 1 as an additive, it can effectively inhibit the dissolution of transition metals from the positive electrode, and at the same time can scavenge the by-products generated by the thermal decomposition of the lithium salt to reduce the deterioration of the solid electrolyte interphase (SEI). Therefore, an increase in the initial resistance can be inhibited, and a lithium secondary battery having improved high-temperature durability (such as high-temperature storage characteristics and high-temperature cycle characteristics) can be obtained. Detailed Description

[0029] First, before describing the present invention, it should be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a common dictionary, and it should be further understood that based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the invention, the words or terms should be construed as having meanings consistent with their meanings in the context of the related art and the technical idea of the invention.

[0030] The terms used in this specification are only for describing exemplary embodiments and are not intended to limit the present invention. Unless otherwise specified, terms in the singular form may include the plural form.

[0031] It will be further understood that the terms "comprises", "comprising", or "having" in this specification specify the presence of the described features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0032] In this specification, unless otherwise clearly specified, the expression "%" means weight %.

[0033] Before describing the present invention, in the description of "a to b carbon atoms" in the specification, the expressions "a" and "b" each represent the number of carbon atoms contained in a specific functional group. That is, the functional group may include "a" to "b" carbon atoms.

[0034] In addition, unless otherwise defined in the specification, the term "substituted" means that at least one hydrogen bonded to carbon is substituted with an element other than hydrogen, for example, substituted with an alkyl group having 1 to 5 carbon atoms or a fluorine element.

[0035] In this specification, the following method can be used to calculate the "difference in reduction potential between additives relative to lithium".

[0036] For example, LiPF6 is dissolved in a non-aqueous organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 30:70 such that the concentration of LiPF6 is 1.0 M. Then, 0.5 wt% of the electrolyte additive for a secondary battery of the present invention is added to prepare a non-aqueous electrolyte. Then, a negative electrode obtained by coating a negative electrode current collector (Cu) with graphite, a porous polyethylene separator, and a lithium metal (Li metal) as a positive electrode are stacked, and the above-prepared non-aqueous electrolyte is injected to assemble a 2 Ah stacked cell. Then, when charging the cell under CC (constant current) conditions of 0.1 C, a capacity-voltage curve during constant current charging is obtained, the capacity-voltage curve is differentiated to obtain a differential capacity-voltage curve, and the voltage at which a reduction peak identified from this curve appears is defined as the reduction potential of the electrolyte additive for a secondary battery of the present invention. Next, except that other additives are used instead of the electrolyte additive for a secondary battery of the present invention to prepare a non-aqueous electrolyte, a cell is prepared in the same manner as above to obtain a differential capacity-voltage curve, and the voltage at which a reduction peak identified from this curve appears is defined as the reduction potential of the other additive. Finally, other additives whose absolute value of the difference in reduction potential between the other additive and the electrolyte additive of the present invention is in the range of 0.0 V to 2.2 V are used as other additives of the present invention.

[0037] Hereinafter, the present invention will be described in more detail.

[0038] Generally, for a lithium secondary battery, during initial charge and discharge, a film with passivation ability is formed on the surfaces of the positive and negative electrodes when the non-aqueous electrolyte decomposes, which can ensure high-temperature storage characteristics. However, this film may be decomposed by Lewis acid materials (such as HF and PF5) formed by the thermal decomposition of lithium salts (LiPF6, etc.) widely used in lithium-ion batteries. That is, if transition metal elements dissolve out from the positive electrode due to the erosion of Lewis acid materials, the surface resistance of the electrode will increase due to the change in surface structure, and the theoretical capacity will decrease with the loss of metal elements as the redox centers. Therefore, the capacity may decrease. In addition, since the dissolved transition metal ions will be electrodeposited on the negative electrode that reacts in a strong reduction potential range, not only will electrons be consumed, but the film will also be damaged during electrodeposition to expose the negative electrode surface, so additional non-aqueous electrolyte decomposition reactions may be caused. As a result, the capacity of the battery may continue to decrease, while the resistance and irreversible capacity of the negative electrode will increase.

[0039] Therefore, an object of the present invention is to provide an additive having excellent effects of scavenging decomposition products generated by lithium salts and strengthening the solid electrolyte interface (SEI), and also to provide a non-aqueous electrolyte and a lithium secondary battery containing the additive.

[0040] Electrolyte Additive for Secondary Battery

[0041] The present invention provides an electrolyte additive for a secondary battery, which contains a compound represented by the following formula 1.

[0042] [Formula 1]

[0043]

[0044] In formula 1,

[0045] R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 5 carbon atoms, and

[0046] L is a direct bond, -O-, -COO-, -RO- or -R'COO-, where R and R' are each independently an alkylene group having 1 to 10 carbon atoms.

[0047] In addition, in formula 1, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 3 carbon atoms, and L is -O-, -COO- or -R'COO-, where R' may be an alkylene group having 1 to 5 carbon atoms.

[0048] In addition, in formula 1, L may be -O- or -COO-.

[0049] Specifically, the compound represented by formula 1 may be at least one selected from the compounds represented by the following formula 1A or formula 1B.

[0050] [Formula 1A]

[0051]

[0052] [Formula 1B]

[0053]

[0054] In Formula 1A or 1B,

[0055] R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 4 carbon atoms. Preferably, the compound represented by Formula 1 may be a compound represented by Formula 1A-1 or Formula 1B-1 below.

[0056] [Formula 1A-1]

[0057]

[0058] [Formula 1B-1]

[0059]

[0060] In particular, since the compound represented by Formula 1A-1 contains a carbonate group in its structure, it has better Li ion transfer ability. Therefore, from the viewpoint of further improving the ionic conductivity of the non-aqueous electrolyte, the compound represented by Formula 1A-1 is more preferable than the compound represented by Formula 1B-1.

[0061] The compound represented by Formula 1 is an amphoteric ionic compound having both an anion part and a cation part in one molecular structure. Among them, since the nitrogen atom of the cation part acts as a Lewis base, it has a high binding force with the Lewis acid material which is a decomposition product of the lithium salt. Therefore, it can easily scavenge by-products that cause deterioration of the secondary battery at high temperatures, for example, decomposition products generated by thermal decomposition of the lithium salt. In addition, the material based on the nitrogen (N) atom can form a film (SEI) based on the nitrogen (N) atom on the negative electrode surface while undergoing electrochemical reduction decomposition. The film based on the nitrogen (N) atom has the property of not being easily decomposed and being maintained when the battery is exposed to high temperatures. Therefore, for the non-aqueous electrolyte of the present invention containing the compound represented by Formula 1, since the property that the SEI does not decompose and is stably maintained on the negative electrode surface is provided, the negative electrode reduction reaction of additional transition metals caused by the decomposition of the SEI can be controlled, and the electrodeposition of the dissolved transition metal on the negative electrode during high-temperature storage can be prevented.

[0062] In addition, since the compound represented by Formula 1 includes a readily reducible propargyl functional group as a terminal group, a passivation film with enhanced durability can be formed when it undergoes reductive decomposition on the surface of the negative electrode. That is, since a stable film can be formed at the interface between the electrolyte and the positive and negative electrodes, stability can be ensured by suppressing side reactions when using a high-nickel positive electrode active material. Therefore, it is effective in improving the high-temperature durability, long life, and initial performance of lithium secondary batteries.

[0063] Regarding the nickel (Ni) element contained in the positive electrode formed of a lithium composite metal oxide, it exists in the form of stable nickel ions (Ni 2+ ) before charge and discharge and transforms into Ni 3+ ions or Ni 4+ ions due to an increase in the oxidation number after charge and discharge. Different from the stable Ni 2+ ions, Ni 3+ ions or Ni 4+ ions undergo side reactions of being reduced to Ni 2+ ions and simultaneously undergo rapid oxygen desorption due to instability. Since the desorbed oxygen reacts with the electrolyte to change the surface properties of the electrode or increase the charge transfer resistance on the electrode surface, the capacity or high-rate performance is reduced, so there is a problem of reduced energy density. This phenomenon is further aggravated on the surface of high-Ni positive electrodes. Therefore, in the process of manufacturing secondary batteries, it is very important to form a stable film on the surface of high-Ni positive electrodes that can prevent side reactions with the electrolyte and provide surface stability. The compound represented by Formula 1 used as an electrolyte additive in the present invention can form a lithium-containing alkyl sulfonate / salt-type stable film on the surfaces of the positive and negative electrodes by binding to the lithium ions contained in the lithium salt. Therefore, by preventing the desorbed oxygen or Ni 4+ ions from contacting the electrolyte, side reactions can be reduced, and the dissolution of transition metals from the positive electrode can be effectively suppressed. Therefore, the capacity degradation or high-rate performance degradation of secondary batteries can be improved.

[0064] As described above, when the compound of Formula 1 is used as an additive, a stronger film can be formed on the surfaces of the positive and negative electrodes. Correspondingly, the effect of suppressing the dissolution of transition metals from the positive electrode at high temperatures can be further improved, and the high-temperature storage and cycling performance can be improved by reducing the self-discharge of secondary batteries.

[0065] The sulfate group (SO4 - ) as one of the terminal groups of the compound represented by Formula 1 is a substituent that easily forms hydrogen bonds. Therefore, a part of the compound represented by Formula 1 can exist in the non-aqueous electrolyte in the form of the compound represented by the following Formula 2.

[0066] [Formula 2]

[0067]

[0068] In Formula 2,

[0069] R1, R2, and L are as defined in Formula 1.

[0070] Non-aqueous Electrolyte

[0071] In addition, the non-aqueous electrolyte of the embodiment of the present invention includes an electrolyte additive containing a compound represented by Formula 1.

[0072] The non-aqueous electrolyte may further include a lithium salt, an organic solvent, and optionally a compound as other additives, wherein the absolute value of the difference in reduction potential relative to lithium between the compound and the compound represented by Formula 1 is in the range of 0.0 V to 2.2 V.

[0073] (1) Electrolyte additive for secondary battery

[0074] The non-aqueous electrolyte of the present invention includes an electrolyte additive for secondary battery containing a compound represented by Formula 1. Since its description is repetitive with the above, its description is omitted.

[0075] Considering the effect of forming a stable film on the electrode surface and the effect of scavenging the thermal decomposition products of the lithium salt, based on the total weight of the non-aqueous electrolyte, the amount of the electrolyte additive for secondary battery may be 0.05% by weight to 5% by weight.

[0076] If the amount of the electrolyte additive for secondary battery is within the above range, the dissolution of transition metals in the positive electrode active material at high temperatures can be effectively suppressed by forming a firm film on the positive electrode surface, while minimizing adverse factors such as side reactions caused by the additive, capacity reduction, and resistance increase. And by effectively scavenging the thermal decomposition products of the lithium salt, excellent high-temperature durability can be achieved.

[0077] That is, if the amount of the electrolyte additive for secondary battery is 0.05% by weight or more, the effect of scavenging the thermal decomposition products of the lithium salt can be maintained even when the operation time is increased, and the effect of suppressing the dissolution of transition metals can be further improved by forming a stable film on the electrode surface. In addition, if the amount of the electrolyte additive for secondary battery is 5% by weight or less, side reactions caused by a slightly large amount of the additive can be prevented.

[0078] Specifically, based on the total weight of the non-aqueous electrolyte, the amount of the electrolyte additive for secondary battery may be 0.05% by weight to 5% by weight, more preferably 0.05% by weight to 4% by weight, 0.1% by weight to 3% by weight, for example 0.5% by weight to 3% by weight.

[0079] (2) Lithium salt

[0080] Any lithium salt commonly used in the electrolyte of a lithium secondary battery can be used as the lithium salt without limitation. For example, the lithium salt can include Li + as a cation, and can include at least one selected from the group consisting of: F - 、Cl - 、Br - 、I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - 、B 10 Cl 10 - 、AlCl4 - 、AlO2 - 、PF6 - 、CF3SO3 - 、CH3CO2 - 、CF3CO2 - 、AsF6 - 、SbF6 - 、CH3SO3 - 、(CF3CF2SO2)2N - 、(CF3SO2)2N - 、(FSO2)2N - 、BF2C2O4 - 、BC4O8 - 、PF4C2O4 - 、PF2C4O8 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - 、C4F9SO3 - 、CF3CF2SO3 - 、CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、CF3(CF2)7SO3 - and SCN - 。

[0081] Specifically, the lithium salt can include a single material selected from the group consisting of or a mixture of two or more thereof: LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10, LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiN(SO2F)2 (lithium bis(fluorosulfonyl)imide, LiFSI), LiN(SO2CF2CF3)2 (lithium bis(perfluoroethylsulfonyl)imide, LiBETI), and LiN(SO2CF3)2 (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI). In addition, lithium salts commonly used in electrolytes for lithium secondary batteries can be used without limitation.

[0082] The lithium salt can be appropriately changed within the range of normal use, but can be included in the electrolyte at a concentration of 0.8 M to 4.0 M, for example, 1.0 M to 3.0 M, to obtain the best effect of forming an anti-corrosion film on the electrode surface.

[0083] When the concentration of the lithium salt satisfies the above range, due to the increased mobility of lithium ions, the low-temperature output characteristics and the cycle characteristics during high-temperature storage can be improved, and the viscosity of the non-aqueous electrolyte can be controlled to achieve the best impregnation property.

[0084] (3) Non-aqueous organic solvents

[0085] In addition, non-aqueous organic solvents will be described below.

[0086] Various organic solvents commonly used in non-aqueous electrolytes can be used as non-aqueous organic solvents without limitation. Among them, the type of non-aqueous organic solvent is not limited as long as the non-aqueous organic solvent can minimize the decomposition caused by the oxidation reaction during the charge and discharge of the secondary battery and can exhibit the desired characteristics together with the additives.

[0087] Specifically, the non-aqueous organic solvent can include cyclic carbonate organic solvents, linear carbonate organic solvents, or a mixed organic solvent thereof.

[0088] Cyclic carbonate organic solvents are highly viscous organic solvents that dissociate lithium salts well in non-aqueous electrolytes due to their high dielectric constants. Specific examples thereof can be at least one non-aqueous 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, the cyclic carbonate organic solvent can include ethylene carbonate.

[0089] Linear carbonate organic solvents are organic solvents with low viscosity and low dielectric constant. Typical examples thereof may be at least one non-aqueous organic solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and the linear carbonate organic solvent may specifically include ethyl methyl carbonate (EMC).

[0090] The cyclic carbonate non-aqueous organic solvent and the linear carbonate non-aqueous organic solvent can be mixed and used as the non-aqueous organic solvent of the present invention. In this case, the cyclic carbonate non-aqueous organic solvent and the linear carbonate non-aqueous organic solvent can be mixed and used in a volume ratio of 10:90 to 50:50, for example, 20:80 to 30:70.

[0091] In addition, in order to prepare an electrolyte with high ionic conductivity, the non-aqueous organic solvent may further include a linear ester non-aqueous organic solvent and / or a cyclic ester non-aqueous organic solvent with a low melting point and high high-temperature stability.

[0092] As a representative example, the linear ester non-aqueous organic solvent may include at least one non-aqueous organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0093] In addition, the cyclic ester non-aqueous organic solvent may include at least one non-aqueous organic solvent selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0094] The remainder of the components other than the non-aqueous organic solvent in the non-aqueous electrolyte of the present invention (for example, electrolyte additives, lithium salts, and other additives for secondary batteries of the present invention), unless otherwise specified, may all be non-aqueous organic solvents.

[0095] (4) Other additives

[0096] In addition, the non-aqueous electrolyte for lithium secondary batteries of the present invention may further contain other additives, so as to produce a synergistic effect with the compound represented by Formula 1, thereby enabling a more stable film to be formed on the surface of the positive electrode.

[0097] Other additives may include compounds with an absolute value of the difference in reduction potential relative to lithium between the compounds represented by Formula 1 in the range of 0.0 V to 2.2 V, particularly 0.0 V to 2.0 V. That is, due to the difference in reduction potential between the compound represented by Formula 1 of the present invention and the other additives, the thermal stability and ion transferability of the film can be further enhanced due to the change in the SEI or cathode electrolyte interface (CEI) components. For compounds with an absolute value of the difference in reduction potential relative to lithium greater than 2.2 V from the compound represented by Formula 1, since it does not participate in the reaction of initially forming SEI beyond the solvent decomposition reaction region, it may cause an increase in the volume of the battery during high-temperature storage.

[0098] Therefore, the other additives of the present invention may include at least one selected from the group consisting of non-halogenated or halogenated carbonate compounds, sultone compounds, sulfate / salt compounds, phosphate / salt or phosphite / salt compounds, borate / salt compounds, nitrile compounds, amine compounds, silane compounds, and lithium salt compounds, except for phenyl-containing overcharge inhibitors with an absolute value of the difference in reduction potential relative to lithium greater than 2.2 V, such as monofluorobenzene; more specifically, non-halogenated or halogenated carbonate compounds may be included.

[0099] Typical examples of non-halogenated or halogenated carbonate compounds may be vinylene carbonate (VC), vinylethylene carbonate, or fluoroethylene carbonate (FEC).

[0100] Sultone compounds may be at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethyl sultone, and 1,3-propene sultone (PRS).

[0101] Sulfate / salt compounds may be, for example, ethylene sulfite (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0102] Phosphate / salt or phosphite / salt compounds may be at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphite, and tris(2,2,2-trifluoroethyl)phosphate.

[0103] Borate / salt compounds may be tetraphenylborate, lithium oxalyl difluoroborate (LiODFB), or lithium bis(oxalato)borate (LiB(C2O4)2; LiBOB).

[0104] The nitrile compound can be, for example, at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0105] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte. Among them, the lithium salt compound may include LiPO2F2 or LiBF4.

[0106] Two or more compounds can be mixed and used as the other additives. Based on the total weight of the non-aqueous electrolyte, the content of the other additives can be 0.01% by weight to 10% by weight, particularly 0.05% by weight to 7% by weight, preferably 0.1% by weight to 5% by weight.

[0107] If the content of the other additives is within the above range, the effects of improving the low-temperature output characteristics, high-temperature storage characteristics, and high-temperature life characteristics of the secondary battery can be obtained, and the battery side reactions caused by excessive additives can be prevented. In addition, the formation of unreacted materials in the electrolyte at room temperature can be prevented, or the precipitation of other additives due to insufficient decomposition at high temperatures can be prevented.

[0108] Lithium Secondary Battery

[0109] Next, the lithium secondary battery of the present invention will be described.

[0110] The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In this case, the non-aqueous electrolyte is the non-aqueous electrolyte of the present invention. Since the non-aqueous electrolyte has been described above, its description is omitted, and the following describes other components.

[0111] (1) Positive electrode

[0112] The positive electrode of the present invention may include a positive electrode active material layer containing a positive electrode active material. If necessary, the positive electrode active material layer may further include a conductive agent and / or a binder.

[0113] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium. Among them, the positive electrode active material may specifically include a lithium composite metal oxide, which includes lithium and at least one metal selected from nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and aluminum (Al); and may particularly include a high-nickel (Ni) lithium composite metal oxide in which the Ni content is as high as 0.55 or more.

[0114] For the lithium composite metal oxide with a high Ni content, although it has the advantage of enabling a high-capacity battery, its disadvantage is that Ni2+ Cations dissolve from the positive electrode into the electrolyte, Ni 2+ The cations react with the passivation film (SEI) of the negative electrode to decompose the SEI. As a result, side reactions occur due to a part of the negative electrode active material being exposed to the electrolyte, deteriorating the capacity and life characteristics, and increasing the resistance. In addition, for high-Ni positive electrode active materials, the dissolution of transition metals may be exacerbated by high-temperature exposure accelerating the collapse of the structure of the positive electrode. Especially when HF is present in the electrolyte, the dissolution of transition metals may be accelerated.

[0115] Therefore, to solve this problem, the lithium secondary battery of the present invention is characterized in that it uses a non-aqueous electrolyte containing a compound represented by Formula 1 as an additive. That is, due to this additive contained in the non-aqueous electrolyte, not only can the stabilization of the film on the positive electrode surface be achieved, but also the Lewis acid in the non-aqueous electrolyte can be scavenged to prevent the degradation of the film, and by interacting with the Ni 2+ cations dissolved in the electrolyte to stabilize them, the effect of reducing the resistance can be achieved. Therefore, the deterioration of the high-temperature durability, high-temperature capacity, and life characteristics of the lithium secondary battery can be prevented.

[0116] Typical examples of the lithium composite metal oxide can be Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.7 Mn 0.2 Co 0.1 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, Li[Ni 0.8 Co 0.15 Al 0.05 O2, Li[Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 O2 or Li(Ni 0.9 Mn 0.05 Co 0.05 )O2.

[0117] In addition, in addition to the above lithium composite metal oxides, the positive electrode active material may further include: lithium manganese-based oxides (for example, LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (for example, LiCoO2, etc.), lithium nickel-based oxides (for example, LiNiO2, etc.), lithium nickel manganese-based oxides (for example, LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni zO4 (where 0 < Z < 2)), lithium nickel cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1)), lithium manganese cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co z1 O4 (where 0 < Z1 < 2)), lithium nickel manganese cobalt-based oxide (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, and p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, and p1 + q1 + r2 = 2), or lithium nickel cobalt transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M s2 )O2 (where M is selected from the group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and p2, q2, r3, and s2 are atomic fractions of each independent element, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1).

[0118] Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 80% by weight to 98% by weight, for example 85% by weight to 98% by weight. When the positive electrode active material is present in the above range of amounts, excellent capacity characteristics can be exhibited.

[0119] Next, a conductive agent is used to provide conductivity to the electrode, where any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not cause adverse chemical changes to the battery. Specific examples of the conductive agent can be, for example, the following conductive materials: carbon powder, such as carbon black, acetylene black (or Denka black), Ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; graphite powder, such as natural graphite, artificial graphite, or graphite with a well-developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives, and any one or a mixture of two or more of them can be used.

[0120] Based on the total weight of the positive electrode active material layer, the content of the conductive agent can be from 0.1% by weight to 10% by weight, for example, from 0.1% by weight to 5% by weight.

[0121] Next, the binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector.

[0122] As an example of the binder, any one of the following can be used: fluororesin binders, which include polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber binders, which include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, or styrene-isoprene rubber; cellulose binders, which include carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; polyol binders, which include polyvinyl alcohol; polyolefin-based binders, which include polyethylene or polypropylene; polyimide binders; polyester binders; and silane binders, or a mixture of two or more of them.

[0123] Based on the total weight of the positive electrode active material layer, the content of the binder can be from 0.1% by weight to 15% by weight, for example, from 0.1% by weight to 10% by weight.

[0124] The positive electrode of the present invention as described above can be prepared by a method for preparing a positive electrode known in the art. For example, the positive electrode can be prepared by coating a positive electrode slurry prepared by dissolving or dispersing a positive electrode active material, a binder, and / or a conductive agent in a solvent on a positive electrode current collector, drying, and then roll-pressing to form an active material layer, or can be prepared by casting a positive electrode active material layer on a separate support and then laminating a film separated from the support on the positive electrode current collector.

[0125] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes to the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collector is usually 3 μm to 500 μm. In order to improve the adhesion of the positive electrode material, minute irregularities can be formed on the surface of the current collector. For example, the positive electrode current collector can be used in various shapes, such as a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0126] The solvent can be a solvent commonly used in the art, and can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and any one or a mixture of two or more of them can be used. If the positive electrode material mixture can be adjusted to an appropriate viscosity considering the coating thickness, manufacturing yield, and processability of the positive electrode material mixture, the amount of the solvent used can be sufficient and is not particularly limited.

[0127] (2) Negative electrode

[0128] Next, the negative electrode will be described.

[0129] The negative electrode of the present invention includes a negative electrode active material layer containing a negative electrode active material, and if necessary, the negative electrode active material layer may further include a conductive agent and / or a binder.

[0130] Various negative electrode active materials used in the art, such as carbon-based negative electrode active materials, silicon-based negative electrode active materials, or mixtures thereof, can be used as the negative electrode active material.

[0131] According to one embodiment, the negative electrode active material may include a carbon-based negative electrode active material, and various carbon-based negative electrode active materials used in the art can be used as the carbon-based negative electrode active material. For example, graphite-based materials such as natural graphite, artificial graphite, and Kish graphite; pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch, soft carbon, and hard carbon. The shape of the carbon-based negative electrode active material is not particularly limited, and materials of various shapes such as irregular shapes, planar shapes, flake shapes, spherical shapes, or fibrous shapes can be used.

[0132] 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 natural graphite and artificial graphite are used simultaneously, the adhesion to the current collector can be improved, thereby suppressing the peeling of the active material.

[0133] According to another embodiment, the negative electrode active material may include a carbon-based negative electrode active material and a silicon-based negative electrode active material.

[0134] Specific examples of the carbon-based negative electrode active material are the same as those described above.

[0135] For example, the silicon-based negative electrode active material may include at least one selected from the group consisting of 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). The element Y can be selected from the group consisting of: Mg, calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), scandium (Sc), yttrium (Y), Ti, zirconium (Zr), hafnium (Hf), (Rf), V, niobium (Nb), Ta, (Db), Cr, Mo, tungsten (W), (Sg), technetium (Tc), rhenium (Re), (Bh), Fe, lead (Pb), ruthenium (Ru), osmium (Os), (Hs), rhodium (Rh), iridium (Ir), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), zinc (Zn), cadmium (Cd), boron (B), Al, gallium (Ga), tin (Sn), indium (In), germanium (Ge), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and combinations thereof.

[0136] Since the silicon-based negative electrode active material has higher capacity characteristics than the carbon-based negative electrode active material, better capacity characteristics can be obtained when the silicon-based negative electrode active material is further included. However, for the negative electrode containing the silicon-based negative electrode active material, the oxygen-rich (O-rich) component in the SEI is more than that in the graphite negative electrode. When there are Lewis acids such as HF or PF5 in the electrolyte, the SEI containing the O-rich component is more likely to decompose. Therefore, for the negative electrode containing the silicon-based negative electrode active material, it is necessary to suppress the generation of Lewis acids such as HF and PF5 in the electrolyte, or it is necessary to remove (or scavenge) the formed Lewis acids to stably maintain the SEI. Since the non-aqueous electrolyte of the present invention contains an electrolyte additive capable of forming a stable film on the positive electrode and the negative electrode, when using a negative electrode containing a silicon-based negative electrode active material, it can effectively suppress the decomposition of the SEI.

[0137] The mixing ratio of the silicon-based negative electrode active material and the carbon-based negative electrode active material can be in the range of 3:97 to 99:1. For example, the weight ratio is 5:95 to 15:85. When the mixing ratio of the silicon-based negative electrode active material and the carbon-based negative electrode active material satisfies the above range, excellent cycle performance can be ensured because the volume expansion of the silicon-based negative electrode active material is suppressed while the capacity characteristics are improved.

[0138] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 80% by weight to 99% by weight. When the amount of the negative electrode active material satisfies the above range, excellent capacity characteristics and electrochemical characteristics can be obtained.

[0139] Next, the conductive agent is a component used to further improve the conductivity of the negative electrode active material. Based on the total weight of the negative electrode active material layer, the addition amount of the conductive agent can be 10% by weight or less, for example, 5% by weight or less. Any conductive agent can be used without particular limitation as long as it has conductivity and does not cause adverse chemical changes to the battery. For example, the following conductive materials can be used: carbon powder, such as carbon black, acetylene black (or Denka black), Ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; graphite powder, such as natural graphite with a well-developed crystal structure, artificial graphite, or graphite; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powder, aluminum powder, and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; or polyphenylene derivatives.

[0140] The binder is a component that helps to bond the conductive agent, active material, and current collector. Based on the total weight of the negative electrode active material layer, the addition amount of the binder is generally 0.1% by weight to 10% by weight. Examples of the binder can be fluororesin-based binders, which include polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); rubber-based binders, which include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, or styrene-isoprene rubber; cellulose-based binders, which include carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; polyol-based binders, which include polyvinyl alcohol; polyolefin-based binders, which include polyethylene or polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders.

[0141] Based on the total weight of the negative electrode active material layer, the content of the binder can be 0.1% by weight to 15% by weight, for example, 0.1% by weight to 10% by weight.

[0142] The negative electrode can be prepared by a method known in the art for preparing a negative electrode. For example, the negative electrode can be prepared by coating a negative electrode slurry prepared by dissolving or dispersing the negative electrode active material and optional binder and conductive agent in a solvent on a negative electrode current collector, rolling and drying to form a negative electrode active material layer, or by casting the negative electrode active material layer on a separate support and then laminating the film separated from the support on the negative electrode current collector.

[0143] The negative electrode current collector is not particularly limited as long as it has high electrical conductivity and does not cause adverse chemical changes to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., or an aluminum-cadmium alloy can be used. Additionally, the thickness of the negative electrode current collector is generally 3 μm to 500 μm, and, similar to the positive electrode current collector, in order to improve the adhesion of the negative electrode active material, minute irregularities can also be formed on the surface of the current collector. For example, the negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0144] The solvent can be a commonly used solvent in the art, and can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and any one or a mixture of two or more of them can be used. If the viscosity of the negative electrode slurry can be adjusted to an appropriate level considering the coating thickness, manufacturing yield, and processability of the negative electrode material mixture, the amount of the solvent used can be sufficient and is not particularly limited.

[0145] (3) Separator

[0146] The lithium secondary battery of the present invention includes a separator between the positive electrode and the negative electrode.

[0147] The separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Among them, any separator can be used without particular limitation as long as it is generally used as a separator in lithium secondary batteries. In particular, a separator having a high moisture retention capacity for the electrolyte and a low resistance to the ion transfer of the lithium salt can be used.

[0148] Specifically, a porous polymer membrane can be used, for example, a porous polymer membrane prepared from polyolefin-based polymers (such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers), or a laminated structure having two or more layers thereof. In addition, typical porous non-woven fabrics can be used, such as non-woven fabrics formed from high melting point glass fibers or polyethylene terephthalate fibers. In addition, in order to ensure heat resistance or mechanical strength, a coated separator including a ceramic component or a polymer material can be used, and a separator having a single-layer or multi-layer structure can be optionally used.

[0149] The lithium secondary battery of the present invention as described above can be applied to portable devices (such as mobile phones, laptop computers, and digital cameras) and electric vehicles (such as hybrid electric vehicles (HEV)).

[0150] Therefore, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0151] The battery module or battery pack can be used as a power source for at least one of the following medium- and large-sized devices: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0152] The shape of the lithium secondary battery of the present invention is not particularly limited, but a cylindrical, prismatic, pouch-type, or coin-type using a can can be used.

[0153] The lithium secondary battery of the present invention can be used not only in battery cells used as power sources for small devices, but also as unit battery cells in medium- and large-sized battery modules including a plurality of battery cells.

[0154] Hereinafter, the present invention will be described in detail according to specific embodiments.

[0155] Example

[0156] Example 1

[0157] (Preparation of non-aqueous electrolyte)

[0158] LiPF6 was dissolved in a non-aqueous organic solvent in which 99 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70 so that the concentration of LiPF6 was 1.0 M, and 0.5 g of the compound represented by Formula 1A-1 was added as an additive and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential relative to lithium between vinylene carbonate and the compound of Formula 1A-1 was 0.7 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0159] (Preparation of positive electrode)

[0160] Lithium nickel manganese aluminum oxide (Li(Ni 0.86 Mn 0.07 Co 0.05 Al 0.02 )O2), which is a positive electrode active material particle, carbon black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 90:5:5 to prepare a positive electrode active material slurry (solid content 48% by weight). The positive electrode active material slurry was coated on a 100-μm-thick positive electrode current collector (Al film), dried, and then roll-pressed to prepare a positive electrode.

[0161] (Preparation of negative electrode)

[0162] The negative electrode active material (artificial graphite: SiO = 94.5:5.5 by weight ratio), PVDF as a binder, and carbon black as a conductive agent were added to NMP as a solvent at a weight ratio of 95:2:3 to prepare a negative electrode active material slurry (solid content: 70% by weight). A negative electrode current collector (Cu film) with a thickness of 90 μm was coated with the negative electrode active material slurry, dried, and then roll-pressed to prepare a negative electrode.

[0163] (Preparation of secondary battery)

[0164] An electrode assembly was prepared by the conventional method of sequentially stacking a polyethylene porous membrane, the positive electrode and the negative electrode prepared by the above method. Thereafter, the electrode assembly was placed in a pouch-type secondary battery case, and the non-aqueous electrolyte prepared above was injected therein to prepare a lithium secondary battery.

[0165] Example 2

[0166] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 98.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, and 1.0 g of the compound represented by Formula 1A-1 was added as an additive and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between vinylene carbonate and the compound of Formula 1A-1 was 0.7 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0167] Example 3

[0168] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 96.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, and 3.0 g of the compound represented by Formula 1A-1 was added as an additive and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between vinylene carbonate and the compound of Formula 1A-1 was 0.7 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0169] Example 4

[0170] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 94.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, 5.0 g of the compound represented by Formula 1A-1 was added as an additive, and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between vinylene carbonate and the compound of Formula 1A-1 was 0.7 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0171] Example 5

[0172] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 99.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, and 0.5 g of the compound represented by Formula 1A-1 was added as an additive to prepare a non-aqueous electrolyte (see Table 1 below).

[0173] Example 6

[0174] A lithium secondary battery was prepared in the same manner as in Example 1, except that a non-aqueous electrolyte was prepared by adding the compound represented by Formula 1B-1 instead of the compound represented by Formula 1A-1 (see Table 1 below).

[0175] Example 7

[0176] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 99 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, 0.5 g of the compound represented by Formula 1A-1 was added as an additive, and 0.5 g of monofluorobenzene was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between monofluorobenzene and the compound of Formula 1A-1 was 2.2 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0177] Comparative Example 1

[0178] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 99.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, the electrolyte additive of the present invention was not included, and 0.5 g of vinylene carbonate was added as another additive, thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0179] Comparative Example 2

[0180] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 99.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, and 0.5 g of the compound represented by the following Formula 3 was added as an additive to prepare a non-aqueous electrolyte (see Table 1 below).

[0181] For a compound in which a phenyl group is bonded to a nitrogen element, such as the compound represented by the following Formula 3, even if it is an imidazole-structured zwitterionic compound having a structure similar to that of the compound of the present invention, a nitro-phenyl-based SEI is formed. Among them, since the binding energy of this SEI to lithium ions is greater than that of a sulfonate-based SEI, the lithium ion transfer performance may deteriorate. Therefore, the effect of improving battery durability (such as an increase in initial resistance and a decrease in capacity retention rate) may not be significant.

[0182] [Formula 3]

[0183]

[0184] This compound is an imidazole-structured zwitterionic compound having a structure similar to that of the compound of the present invention. However, since a phenyl group is bonded to a nitrogen element, a nitro-phenyl-based SEI is formed. The binding energy of this nitro-phenyl-based SEI to lithium ions is greater than that of a sulfonate-based SEI, and the lithium ion transfer performance may deteriorate. Therefore, the effect of improving battery durability (such as an increase in initial resistance and a decrease in capacity retention rate) may not be significant.

[0185] Comparative Example 3

[0186] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 99 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, and 0.5 g of the compound represented by Formula 3 was added as an additive and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between vinylene carbonate and the compound of Formula 3 was 0.7 V) to prepare a non-aqueous electrolyte (see Table 1 below).

[0187] Comparative Example 4

[0188] A lithium secondary battery was prepared in the same manner as in Example 1, except that LiPF6 was dissolved in a non-aqueous organic solvent in which 94.5 g of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70, the concentration of LiPF6 was made 1.0 M, 5.0 g of the compound represented by Formula 3 was added as an additive, and 0.5 g of vinylene carbonate was added as another additive (the absolute value of the difference in reduction potential with respect to lithium between vinylene carbonate and the compound of Formula 3 was 0.7 V), thereby preparing a non-aqueous electrolyte (see Table 1 below).

[0189] Comparative Example 5

[0190] A lithium secondary battery was prepared in the same manner as in Example 1, except that a non-aqueous electrolyte was prepared by adding the compound represented by the following Formula 4 in place of the compound represented by Formula 1A-1 (see Table 1 below).

[0191] [Formula 4]

[0192]

[0193] [Table 1]

[0194]

[0195] In Table 1, the abbreviations of the compounds are as follows:

[0196] VC: Vinylene carbonate

[0197] FB: Monofluorobenzene

[0198] Experimental Example

[0199] Experimental Example 1: Initial resistance evaluation

[0200] After charging the lithium secondary batteries prepared in Examples 1 to 7 and the lithium secondary batteries prepared in Comparative Examples 1 to 4 at a rate of 0.33C to 4.2V under constant current / constant voltage conditions at room temperature (25°C), each lithium secondary battery was discharged to 50% DOD (depth of discharge) to adjust the state of charge (SOC) to 50%, and then discharged at a rate of 2.5C for 10 seconds, and the initial resistance was measured using a PNE-0506 charge / discharge device (manufacturer: PNE solution). The results are shown in Table 2 below.

[0201] [Table 2]

[0202] Initial Resistance Example 1 5.31 Example 2 5.47 Example 3 5.72 Example 4 6.01 Example 5 5.11 Example 6 5.57 Example 7 5.32 Comparative Example 1 6.07 Comparative Example 2 6.24 Comparative Example 3 6.28 Comparative Example 4 7.24 Comparative Example 5 6.75

[0203] Referring to Table 2, it can be seen that the initial resistance of the secondary batteries of Examples 1 to 7 of the present invention is about 6.01 mOhm or less.

[0204] In contrast, for the secondary batteries of Comparative Examples 2 to 4, which contain a non-aqueous electrolyte containing the compound represented by Formula 3 (an amphoteric ionic compound having an imidazole structure), it can be seen that the initial resistance increases as compared with the secondary batteries of the Examples.

[0205] In addition, for the secondary battery of Comparative Example 5, which includes a non-aqueous electrolyte containing the compound represented by Formula 4, it can be understood that the initial resistance increases as compared with the secondary batteries of the Examples.

[0206] Experimental Example 2. Evaluation of High-Temperature Cycle Characteristics

[0207] The lithium secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 5 were charged at a rate of 0.33C to 4.2V under constant current / constant voltage conditions at 45°C, and then discharged at a rate of 0.33C under constant current conditions to 3V. This was defined as 1 cycle. After 100 charge-discharge cycles, the capacity retention rate (%) and the resistance increase rate (%) were measured. The capacity retention rate (%) was calculated according to Equation 1 below, and the resistance increase rate (%) was calculated according to Equation 2 below. The measurement results are shown in Table 3 below.

[0208] [Equation 1]

[0209] Capacity retention rate (%) = (discharge capacity after 100 cycles / discharge capacity after 1 cycle) × 100

[0210] [Equation 2]

[0211] Resistance increase rate (%) = { (resistance after 100 cycles - resistance after 1 cycle) / resistance after 1 cycle} × 100

[0212] [Table 3]

[0213] Capacity Retention Rate (%) after 100 Cycles Resistance Increase Rate (%) after 100 Cycles Example 1 98.9 1.10 Example 2 99.0 1.02 Example 3 99.2 0.94 Example 4 98.0 1.32 Example 5 98.6 1.11 Example 6 95.9 2.67 Example 7 98.7 1.11 Comparative Example 1 78.2 34.5 Comparative Example 2 85.2 15.4 Comparative Example 3 86.4 13.4 Comparative Example 4 87.2 12.1 Comparative Example 5 84.5 16.2

[0214] Referring to Table 3, it can be seen that the capacity retention rate (%) of the secondary batteries of Examples 1 to 7 of the present invention is about 95.9% or more after 100 cycles, and the resistance increase rate (%) is about 2.67% or less.

[0215] In contrast, for the secondary battery of Comparative Example 1 in which the non-aqueous electrolyte does not contain the compound of Formula 1A-1, the secondary batteries of Comparative Examples 2 to 4 in which the non-aqueous electrolyte contains the compound of Formula 3 instead of the compound of Formula 1A-1, and the secondary battery of Comparative Example 5 in which the non-aqueous electrolyte contains the compound of Formula 4, it can be seen that the capacity retention rate (%) and the resistance increase rate (%) after 100 cycles are significantly worse as compared with the secondary batteries of Examples 1 to 7.

[0216] On the other hand, in the case of the secondary battery of Example 7, which includes monofluorobenzene instead of vinylene carbonate as other additives, compared with the secondary battery of Example 1, the capacity retention rate (%) after 100 cycles decreases in a relatively small amount, and the resistance increase rate (%) increases in a relatively small amount. As the cycles are repeated, this difference may further increase.

[0217] Experimental Example 3. Evaluation of Characteristics after High-Temperature Storage (1)

[0218] Each of the lithium secondary batteries prepared in Examples 1 to 7 and the lithium secondary batteries prepared in Comparative Examples 1 to 5 was fully charged at a rate of 0.33C under constant current / constant voltage conditions at room temperature (25°C) to 4.2V (the state of charge (SOC) was 100%), the cut-off current was 50 mA, and then discharged at a rate of 0.33C under constant current conditions to 3V. After that, the discharge capacity before high-temperature storage was measured using a PNE-0506 charge-discharge device (manufacturer: PNE solution).

[0219] Then, after each lithium secondary battery was stored at 60°C for 2 weeks, the capacity of each lithium secondary battery after high-temperature storage was measured, and then the high-temperature capacity retention rate (%) was calculated using the following Equation 3. The results are shown in Table 4 below.

[0220] [Equation 3]

[0221] Capacity retention rate (%) = (Discharge capacity after 2 weeks of high-temperature storage / Discharge capacity before high-temperature storage) × 100

[0222] Experimental Example 4. Evaluation of Characteristics after High-Temperature Storage (2)

[0223] Each of the lithium secondary batteries prepared in Examples 1 to 7 and the lithium secondary batteries prepared in Comparative Examples 1 to 5 was charged to 4.2V at a rate of 0.33C under constant current / constant voltage conditions at room temperature (25°C). After that, each lithium secondary battery was discharged to 50% of the DOD (depth of discharge) to adjust the SOC to 50%, and then discharged at a rate of 2.5C for 10 seconds, and the initial resistance was measured using a PNE-0506 charge-discharge device (manufacturer: PNE solution).

[0224] Then, after each lithium secondary battery was stored at 60°C for 2 weeks, the resistance value of each lithium secondary battery was measured, and then the resistance increase rate (%) was calculated using the following Equation 4. The results are shown in Table 4 below.

[0225] [Equation 4]

[0226] Resistance increase rate (%) = { (Resistance value after 2 weeks of high-temperature storage - Resistance value before high-temperature storage) / Resistance value before high-temperature storage} × 100

[0227] [Table 4]

[0228] Capacity Retention Rate (%) after High Temperature Storage Resistance Increase Rate (%) after High Temperature Storage Example 1 97.9 3.2 Example 2 98.1 3.0 Example 3 98.2 2.6 Example 4 95.4 5.2 Example 5 97.4 3.5 Example 6 94.1 6.3 Example 7 97.6 3.3 Comparative Example 1 76.4 32.4 Comparative Example 2 85.4 6.9 Comparative Example 3 87.4 6.5 Comparative Example 4 81.1 8.9 Comparative Example 5 83.4 7.5

[0229] Referring to Table 4, it can be seen that the capacity retention rate (%) of the secondary batteries of Examples 1 to 7 of the present invention after storage at high temperature for 2 weeks is about 94.1% or more, and the resistance increase rate (%) is about 6.3% or less.

[0230] On the contrary, for the secondary battery of Comparative Example 1 in which the non-aqueous electrolyte does not contain the compound of Formula 1A-1, the secondary batteries of Comparative Examples 2 to 4 in which the non-aqueous electrolyte contains the compound of Formula 3 instead of the compound of Formula 1A-1, and the secondary battery of Comparative Example 5 in which the non-aqueous electrolyte contains the compound of Formula 4, it can be seen that the capacity retention rate (%) and the resistance increase rate (%) after high temperature storage are significantly worse than those of the secondary batteries of Examples 1 to 7.

[0231] On the other hand, in the case of the secondary battery of Example 7, which includes monofluorobenzene instead of vinylene carbonate as other additives, after storage at high temperature for 2 weeks, compared with the secondary battery of Example 1, the capacity retention rate (%) decreases in a relatively small amount, and the resistance increase rate (%) increases in a relatively small amount. As the storage time at high temperature becomes longer, this difference may further increase.

[0232] Experimental Example 5. Evaluation of Volume Increase Rate after High Temperature Storage

[0233] Each of the lithium secondary batteries prepared in Examples 1 to 7 and the lithium secondary batteries prepared in Comparative Examples 1 to 5 was charged to 4.2V at a rate of 0.33C under constant current / constant voltage conditions at room temperature (25°C). After that, each lithium secondary battery was discharged to 50% DOD (depth of discharge) to adjust the SOC to 50%, and discharged at a rate of 2.5C for 10 seconds, and then the initial thickness was measured.

[0234] Then, after each lithium secondary battery was stored at 60°C for 2 weeks, the thickness (volume increase rate (%)) of each lithium secondary battery after high temperature storage was measured, and the results are listed in Table 5 below.

[0235] [Table 5]

[0236] Volume Increase Rate (%) Example 1 4.5 Example 2 4.2 Example 3 3.4 Example 4 3.2 Example 5 5.9 Example 6 6.5 Example 7 5.7 Comparative Example 1 25.4 Comparative Example 2 7.9 Comparative Example 3 7.2 Comparative Example 4 7.0 Comparative Example 5 8.5

[0237] Referring to Table 5, it can be seen that the volume increase rate (%) of the secondary batteries of Examples 1 to 7 of the present invention after high temperature storage is about 6.5% or less.

[0238] In contrast, for the secondary battery of Comparative Example 1 in which the non-aqueous electrolyte does not contain the compound of Formula 1A-1, the secondary batteries of Comparative Examples 2 to 4 in which the non-aqueous electrolyte contains the compound of Formula 3 instead of the compound of Formula 1A-1, and the secondary battery of Comparative Example 5 in which the non-aqueous electrolyte contains the compound of Formula 4, it can be seen that the volume increase rate (%) after high-temperature storage is significantly increased compared to the secondary batteries of Examples 1 to 7.

[0239] For the secondary battery of Example 7, in which the non-aqueous electrolyte contains monofluorobenzene (FB) instead of vinylene carbonate as another additive, the volume increase rate (%) after high-temperature storage is about 5.7%. Among them, it can be seen that the volume increase rate (%) after high-temperature storage is increased compared to the secondary battery of Example 1.

[0240] As described above, since the non-aqueous electrolyte for a lithium secondary battery of the present invention forms a stable film on the surface of the positive electrode or the negative electrode by containing the compound represented by Formula 1 as an additive, it can effectively suppress the dissolution of transition metals from the positive electrode, and at the same time can scavenge by-products generated by the thermal decomposition of the lithium salt, thereby reducing the degradation of the solid electrolyte interface (SEI). Therefore, an increase in the initial resistance can be suppressed, and a lithium secondary battery having improved high-temperature durability (such as high-temperature storage characteristics and high-temperature cycle characteristics) can be achieved.

Claims

1. A non-aqueous electrolyte for a lithium secondary battery, the non-aqueous electrolyte comprising: An electrolyte additive containing the compound represented by Formula 1: [Formula 1] Among them, In Formula 1, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 5 carbon atoms, and L is a direct bond, -O-, -COO-, -RO- or -R'COO-, wherein R and R' are each independently an alkylene group having 1 to 10 carbon atoms.

2. The non-aqueous electrolyte according to claim 1, wherein, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 3 carbon atoms, and L is -O-, -COO- or -R'COO-, wherein R' is an alkylene group having 1 to 5 carbon atoms.

3. The non-aqueous electrolyte according to claim 1, wherein L is -O- or -COO-.

4. The non-aqueous electrolyte according to claim 1, wherein, The compound represented by Formula 1 is at least one selected from the compounds represented by Formula 1A or Formula 1B: [Formula 1A] [Formula 1B] wherein, in Formulas 1A and 1B, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 4 carbon atoms.

5. The non-aqueous electrolyte according to claim 1, wherein, The compound represented by Formula 1 is at least one selected from the compounds represented by Formula 1A-1 or Formula 1B-1: [Formula 1A-1] [Formula 1B-1] 6. The non-aqueous electrolyte according to claim 1, wherein Based on the total weight of the non-aqueous electrolyte, the amount of the electrolyte additive present is 0.05% by weight to 4% by weight.

7. The non-aqueous electrolyte according to claim 1, further comprising a lithium salt and a non-aqueous organic solvent.

8. The non-aqueous electrolyte according to claim 1, further comprising other additives, wherein, The absolute value of the difference in reduction potential relative to lithium between the other additive and the compound represented by Formula 1 is in the range of 0.0 V to 2.2 V.

9. A lithium secondary battery, comprising: A positive electrode containing a positive electrode active material; A negative electrode containing a negative electrode active material; A separator disposed between the negative electrode and the positive electrode; and The non-aqueous electrolyte according to claim 1.

10. An electrolyte additive for a secondary battery, the electrolyte additive containing the compound represented by Formula 1: [Formula 1] Among them, In Formula 1, R1 and R2 are each independently an unsubstituted or substituted alkylene group having 1 to 5 carbon atoms, and L is a direct bond, -O-, -COO-, -RO- or -R'COO-, wherein R and R' are each independently an alkylene group having 1 to 10 carbon atoms.

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