Nonaqueous electrolyte and lithium secondary battery comprising same

Abstract

The present disclosure relates to a non-aqueous electrolyte comprising a lithium salt, an organic solvent, and an additive, in which the additive comprises a cyclic siloxane compound represented by a specific chemical formula.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from Korean Patent Application No. 10-2023-0052976, filed on April 21, 2023, the disclosure of which is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a non-aqueous electrolyte and a lithium secondary battery including the same. Background Art

[0004] Due to the recent development of personal IT devices and computer networks with the information society, and the accompanying increase in the dependence of society as a whole on electric energy, there is a need to develop technologies for efficiently storing and utilizing electric energy.

[0005] Secondary batteries are the most suitable technology for various applications among developed technologies, and among secondary batteries, lithium secondary batteries that can be miniaturized to be suitable for personal IT equipment and have the highest energy density have come into focus.

[0006] Generally, a lithium secondary battery is prepared by injecting or impregnating a non-aqueous electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a porous separator.

[0007] As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides, LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, lithium-containing nickel oxide (LiNiO2), and lithium nickel cobalt manganese transition metal oxides are considered.

[0008] On the other hand, as negative electrode active materials, carbon-based active materials such as graphite are used. However, recently, the use of silicon-based active materials is being considered from the viewpoint of higher capacity than carbon-based active materials.

[0009] Although silicon-based active materials have the advantage of having high capacity, they have the problem of having very large volume expansion / contraction during charging and discharging. This large degree of volume expansion / contraction significantly reduces the conductivity of the negative electrode, resulting in a decrease in life performance. In addition, during initial activation, a solid electrolyte interface layer (hereinafter referred to as the "SEI layer") is formed on the surface of the negative electrode, but the volume expansion of silicon-based active materials is large, resulting in problems such as rupture of the SEI film and continuous generation of new negative electrode surfaces. As a result, there is a problem of accelerating the side reactions of the electrolyte due to the continuous occurrence of the reaction to form the SEI film. The thickness of the SEI film also increases, resulting in an increase in resistance. Summary of the Invention

[0010] Technical issues

[0011] An object of the present disclosure is to provide a non-aqueous electrolyte that can solve the above-mentioned problems by forming an SEI film having excellent recovery and improved durability on the negative electrode, thereby realizing a lithium secondary battery with improved lifespan and storage performance.

[0012] Technical Solution

[0013] The present disclosure provides a non-aqueous electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive comprises a compound represented by the following Formula 1.

[0014] [Formula 1]

[0015]

[0016] In the above formula 1, R1 and R2 are each independently F, Br, Cl, I, a nitrile group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 10 carbon atoms, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, a substituent shown in the following formula 2, or a combination of two or more thereof, at least one of R1 and R2 includes a substituent shown in the following formula 2, and n is an integer from 3 to 8.

[0017] [Formula 2]

[0018]

[0019] In the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine groups, and * is a binding site.

[0020] Furthermore, the present disclosure provides a lithium secondary battery including: a negative electrode; a positive electrode opposite to the negative electrode; a separator provided between the negative electrode and the positive electrode; and the above-mentioned nonaqueous electrolyte.

[0021] Beneficial effects

[0022] The non-aqueous electrolyte of the present invention is characterized in that it contains a cyclic siloxane compound having a specific structure containing an alkoxy group substituted with one or more fluorines as an additive. When reduced at the negative electrode, the above-mentioned compound can form a polymeric siloxane SEI film, and the polymeric siloxane SEI film has a high shear modulus and can help form an SEI film with excellent thermal stability and chemical and electrochemical stability. In addition, the alkoxy group substituted with one or more fluorines contained in the cyclic siloxane compound can form an inorganic SEI film such as LiF when the negative electrode is reduced, thereby improving the durability of the SEI film. In particular, since the alkoxy group is a good leaving group, it can strongly induce the SEI film-forming reaction. Therefore, the lithium secondary battery containing the non-aqueous electrolyte of the present invention can improve the life performance and storage performance, especially at high temperatures. DETAILED DESCRIPTION

[0023] It should be understood that the terms or words used in the present specification and claims should not be interpreted as limited to having the meanings defined in commonly used dictionaries, but should be interpreted as having meanings and concepts consistent with the technical ideas of the present disclosure based on the principle that the inventor can appropriately define the concepts of the terms to best explain the present disclosure.

[0024] It should be understood that terms such as “includes,” “comprising,” or “having” as used herein are intended to reflect specific features, quantities, steps, elements, and / or combinations thereof, and do not exclude the existence or addition of other specific features, quantities, steps, elements, and / or combinations thereof.

[0025] Before describing the present disclosure, unless otherwise specifically stated, "*" represents the same or different atoms or moieties connecting between the termini of a formula (bonding site).

[0026] Before describing the present invention, "a" and "b" in the description of "a to b carbon atoms" in the specification respectively represent the number of carbon atoms contained in a particular functional group. That is, the functional group may include "a" to "b" carbon atoms. For example, "an alkyl group having 1 to 5 carbon atoms" means an alkyl group containing 1 to 5 carbon atoms, i.e., CH3-, CH3CH2-, CH3CH2CH2-, (CH3)2CH-, CH3CH2CH2CH2-, (CH3)2CHCH2-, CH3CH2CH2CH2CH2-, (CH3)2CHCH2-, CH3CH2CH2CH2CH2-, (CH3)2CHCH2CH2-, etc.

[0027] In addition, in the present specification, an alkyl group or an aryl group may be substituted or unsubstituted. Unless otherwise defined in the specification, the expression "substituted" means that at least one hydrogen bonded to a carbon is replaced by an element other than hydrogen, for example, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, a cycloalkenyl group having 3 to 12 carbon atoms, a cycloalkynyl group having 3 to 12 carbon atoms, a heterocycloalkyl group having 3 to 12 carbon atoms, a heterocycloalkenyl group having 3 to 12 carbon atoms, a heterocycloalkynyl group having 2 to 12 carbon atoms, an aryloxy group having 6 to 12 carbon atoms, a halogen atom, a fluoroalkyl group having 1 to 20 carbon atoms, a nitro group, an aryl group having 6 to 20 carbon atoms, a heteroaryl group having 2 to 20 carbon atoms, a halogenated aryl group having 6 to 20 carbon atoms, etc.

[0028] Hereinafter, the present disclosure will be described in more detail.

[0029] non-aqueous electrolytes

[0030] The present disclosure relates to a non-aqueous electrolyte.

[0031] Specifically, the non-aqueous electrolyte disclosed herein includes a lithium salt, an organic solvent, and an additive, wherein the additive includes a compound represented by the following Formula 1.

[0032] [Formula 1]

[0033]

[0034] In the above formula 1, R1 and R2 are each independently F, Br, Cl, I, a nitrile group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 10 carbon atoms, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, a substituent shown in the following formula 2, or a combination of two or more thereof, at least one of R1 and R2 includes a substituent shown in the following formula 2, and n is an integer from 3 to 8.

[0035] [Formula 2]

[0036]

[0037] In the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine groups, and * is a binding site.

[0038] (1) Lithium salt

[0039] As the lithium salt used in the present disclosure, any lithium salt commonly used in electrolyte solutions for lithium secondary batteries may be used without limitation. For example, the lithium salt may include Li + as cations, and may contain a compound selected from the group consisting of F - 、Cl - Br - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - 、AlO4 - 、AlCl4 - PF6 - 、SbF6 - 、AsF6 - 、B 10 Cl 10 - 、BF2C2O4 - BC4O8 - PF4C2O4 - PF2C4O8 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - 、C4F9SO3 - CF3CF2SO3 - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、CH3SO3 - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - At least one of the group consisting of is used as an anion.

[0040] Specifically, the lithium salt may include a salt selected from the group consisting of LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 , LiBOB (LiB (C2O4) 2), LiCF3SO3, LiFSI (LiN (SO2F) 2), LiCH3SO3, LiCF3CO2, LiCH3CO2 and LiBETI (LiN (SO2CF2CF3) 2). Specifically, the lithium salt may include at least one selected from the group consisting of LiBF4, LiClO4, LiPF6, LiBOB (LiB (C2O4) 2), LiCF3SO3, LiTFSI (LiN (SO2CF3) 2), LiFSI ((LiN (SO2F) 2) and LiBETI (LiN (SO2CF2CF3) 2).

[0041] The lithium salt may be appropriately varied within a generally usable range, but may be present in the electrolyte solution at a concentration of 0.5 M to 5.0 M, specifically 0.8 M to 4.0 M, more specifically 0.8 M to 2.0 M. When the concentration of the lithium salt satisfies the above range, this may increase the efficiency of Li + The transfer number and the dissociation degree of lithium ions are increased, thereby improving the output characteristics of the battery.

[0042] (2) Organic solvents

[0043] The organic solvent is a non-aqueous solvent commonly used in lithium secondary batteries and is not limited as long as it can minimize decomposition caused by oxidation reaction or the like during charge and discharge of the battery.

[0044] Specifically, the organic solvent may include at least one organic solvent selected from the group consisting of a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, a linear ester-based organic solvent, and a cyclic ester-based organic solvent.

[0045] Specifically, the organic solvent may include a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixture thereof.

[0046] The cyclic carbonate organic solvent is an organic solvent with high viscosity and high dielectric constant, and is therefore an organic solvent capable of dissociating the lithium salt in the electrolyte well. Specific examples thereof may include at least one organic solvent selected from ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate. More specifically, the cyclic carbonate organic solvent may include at least one selected from the group consisting of ethylene carbonate (EC) and fluoroethylene carbonate (FEC), and more specifically, may include fluoroethylene carbonate (FEC) in terms of facilitating the formation of an SEI film containing an inorganic substance (LiF).

[0047] In addition, the linear carbonate organic solvent is an organic solvent with low viscosity and low dielectric constant. Specifically, it can include at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methylpropyl carbonate and ethylpropyl carbonate. More specifically, it can include at least one selected from ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). More specifically, from the perspective of further improving the oxidation stability of the non-aqueous electrolyte, diethyl carbonate (DEC) can be included.

[0048] The organic solvent may be a mixture of a cyclic carbonate organic solvent and a linear carbonate organic solvent, wherein the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed in a volume ratio of 5:95 to 40:60, specifically 7:93 to 25:75. If the mixing ratio of the cyclic carbonate organic solvent to the linear carbonate organic solvent satisfies the above range, both high dielectric constant and low viscosity characteristics can be achieved, and excellent ion conduction characteristics can be achieved.

[0049] In addition, in order to prepare an electrolyte with high ion conductivity, in addition to at least one carbonate organic solvent selected from cyclic carbonate organic solvents and linear carbonate organic solvents, the organic solvent may also contain at least one ester organic solvent selected from linear ester organic solvents and cyclic ester organic solvents.

[0050] The linear ester organic solvent may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0051] In addition, the cyclic ester organic solvent may include at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0052] Meanwhile, an organic solvent commonly used in non-aqueous electrolytes may be added as needed to use an organic solvent without limitation. For example, it may further contain at least one organic solvent selected from ether organic solvents, glyme solvents, and nitrile organic solvents.

[0053] As the ether solvent, any one selected from dimethyl ether, diethyl ether, dipropyl ether, ethyl methyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL) and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL) and a mixture of two or more thereof may be used, but the invention is not limited thereto.

[0054] Compared with linear carbonate organic solvents, glycol dimethyl ether solvents have a high dielectric constant and a low surface tension, and may include at least one selected from dimethoxyethane (glycol dimethyl ether, DME), diethoxyethane, diglycol dimethyl ether, triglycol dimethyl ether and tetraglycol dimethyl ether (TEGDME) as a solvent with low reactivity with metals, but is not limited thereto.

[0055] The nitrile solvent may include one or more selected from acetonitrile, propionitrile, butyronitrile, valeronitrile, octanonitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile and 4-fluorophenylacetonitrile, but is not limited thereto.

[0056] (3) Additives

[0057] The non-aqueous electrolyte contains additives.

[0058] The additive includes a compound represented by Formula 1 below.

[0059] [Formula 1]

[0060]

[0061] In the above formula 1, R1 and R2 are each independently F, Br, Cl, I, a nitrile group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 10 carbon atoms, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, a substituent shown in the following formula 2, or a combination of two or more thereof, at least one of R1 and R2 includes a substituent shown in the following formula 2, and n is an integer from 3 to 8.

[0062] [Formula 2]

[0063]

[0064] In the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine groups, and * is a binding site.

[0065] The compound represented by Formula 1 is characterized in that it is a cyclic siloxane compound, wherein R1 and / or R2 substituted on Si is an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine groups, and specifically includes the substituent represented by Formula 2.

[0066] When the compound represented by Formula 1 is used as a non-aqueous electrolyte additive, the cyclic siloxane structure opens during negative electrode reduction, thereby forming a polymeric siloxane SEI film. This polymeric siloxane SEI film not only has excellent flexibility and resilience, but also has a high shear modulus and excellent thermal, chemical, and electrochemical stability.

[0067] In addition, the compound represented by Formula 1 contains a substituent represented by Formula 2, which is reduced at the negative electrode to form an inorganic SEI film containing an inorganic material such as LiF. This inorganic SEI film can significantly improve the durability of the SEI film. In particular, the compound represented by Formula 1 of the present disclosure forms the above-mentioned polymer / inorganic composite SEI film, thereby simultaneously improving the durability, flexibility, and stability of the SEI film.

[0068] In addition, the substituents (R1 and / or R2) contained in the compound represented by the above formula 1 are characterized by comprising a fluorine-substituted alkoxy group, specifically, a substituent represented by the above formula 2. The fluorine-substituted alkoxy group is a slightly weak electron-withdrawing group and serves as a good leaving group. This can induce and promote the formation of inorganic SEI films such as LiF.

[0069] According to the above effects, the non-aqueous electrolyte of the present disclosure can improve the life performance and storage performance of lithium secondary batteries, especially at high temperatures. In particular, the compound shown in Formula 1 can be more preferably applied to a negative electrode using a silicon-based active material. Due to the activation of the negative electrode containing the silicon-based active material, the Li of the lithiated Si and the F derived from the compound shown in Formula 1 can have a strong interaction (glue effect) with each other, and therefore can be more conducive to forming a durable and elastic SEI film on the silicon-based active material with extreme volume expansion during charge and discharge.

[0070] In the above formula 1, R1 and R2 are each independently F, Br, Cl, I, a nitrile group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 10 carbon atoms, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, a substituent shown in the following formula 2, or a combination of two or more thereof, and at least one of R1 and R2 includes a substituent shown in the following formula 2.

[0071] [Formula 2]

[0072]

[0073] In the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with at least one fluorine group, and * is a binding site.

[0074] Specifically, from the viewpoint of preventing a decrease in reactivity due to steric hindrance, either R1 or R2 may include a substituent as shown in Formula 2. For example, in Formula 1, R1 may include a substituent as shown in Formula 2, and R2 may not include a substituent as shown in Formula 2. Specifically, when R1 in Formula 1 includes a substituent as shown in Formula 2, R2 may be an alkyl group having 1 to 5 carbon atoms, more specifically an ethyl group or a methyl group, and even more specifically a methyl group.

[0075] In the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with at least one fluorine group, and * is a binding site.

[0076] In the above formula 2, L1 can specifically be an alkylene group having 1 to 10 carbon atoms, a sulfone group or a combination thereof, more specifically can be an alkylene group having 1 to 5 carbon atoms, and even more specifically can be a methylene group or an ethylene group, because it is easy to accept electrons, which can further improve the reduction characteristics of being easy to accept electrons during negative electrode reduction.

[0077] In the above formula 2, R3 can be an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine groups, specifically an alkoxy group having 1 to 5 carbon atoms substituted with one or more fluorine groups, more specifically one selected from -OCF3, -OCF2CF3 and -OCF2CF2CF3. From the viewpoint of preventing the reduction in reactivity caused by steric hindrance, R3 can be -OCF3.

[0078] In Formula 1 above, n may be an integer of 3 to 8, specifically, an integer of 3 or 4, and more specifically, an integer of 3. When n is 3 to 8, R1 and / or R2 in each repeating unit may be the same as or different from each other.

[0079] Specifically, the compound represented by the above formula 1 may include at least one selected from the group consisting of compounds represented by the following formulas 1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11 and 1-12, more specifically may include at least one selected from the group consisting of compounds represented by the following formulas 1-1, 1-3, 1-9 and 1-10, more specifically may include at least one selected from the group consisting of compounds represented by the following formulas 1-1 and 1-3, and even more specifically may include a compound selected from the group consisting of compounds represented by the following formula 1-1.

[0080] [Formula 1-1]

[0081]

[0082] [Formula 1-2]

[0083]

[0084] [Formula 1-3]

[0085]

[0086] [Formula 1-4]

[0087]

[0088] [Formula 1-5]

[0089]

[0090] [Formula 1-6]

[0091]

[0092] [Formula 1-7]

[0093]

[0094] [Formula 1-8]

[0095]

[0096] [Formula 1-9]

[0097]

[0098] [Formula 1-10]

[0099]

[0100] [Formula 1-11]

[0101]

[0102] [Formula 1-12]

[0103]

[0104] The compound represented by Formula 1 may be present in an amount of 0.01 to 10% by weight, specifically 0.3 to 7% by weight, more specifically 0.5 to 5% by weight, and even more specifically 1 to 3% by weight, based on the weight of the non-aqueous electrolyte. When the compound represented by Formula 1 is used within the above content range, an increase in resistance when excessively added can be prevented, while a flexible and highly durable SEI film can be formed on the negative electrode.

[0105] In addition to the compound represented by Formula 1, the additives may further include other additives. The other additives may be included in the non-aqueous electrolyte to prevent the non-aqueous electrolyte from decomposing under high output conditions and causing negative electrode collapse, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge protection, and battery swelling suppression at high temperatures.

[0106] Specifically, the additional additive may include at least one selected from the group consisting of lithium difluorophosphate (LiDFP), vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, propane sultone, propylene sultone, succinonitrile, adiponitrile, ethylene sulfate, LiBOB (lithium bis(oxalatoborate)), TMSPa (3-trimethoxysilyl-propyl-N-aniline), and TMSPi (tris(trimethylsilyl)phosphite).

[0107] The content of the additional additive in the non-aqueous electrolyte may be 0.1 wt % to 15 wt %, and more specifically 0.3 wt % to 3.0 wt %.

[0108] When the non-aqueous electrolyte further contains the above-mentioned additional additives, the weight ratio of the compound represented by Formula 1 above to the additional additives may be 45:55 to 99:1, specifically 50:50 to 95:5, and more specifically 70:30 to 85:15. When this ratio is within the above range, the lifespan performance and storage performance at high temperatures can be improved to a more desirable level.

[0109] lithium secondary batteries

[0110] In addition, the present disclosure provides a lithium secondary battery including the non-aqueous electrolyte.

[0111] Specifically, the lithium secondary battery of the present disclosure includes a negative electrode, a positive electrode facing the negative electrode, a separator provided between the negative electrode and the positive electrode, and the above-mentioned nonaqueous electrolyte.

[0112] After accommodating an electrode assembly including a negative electrode, a positive electrode opposite to the negative electrode, and a separator disposed between the negative electrode and the positive electrode in a battery case, a lithium secondary battery may be prepared by injecting a nonaqueous electrolyte therein.

[0113] Since the nonaqueous electrolyte has been described above, the negative electrode, the positive electrode, and the separator will be described below.

[0114] (1) Negative electrode

[0115] The negative electrode may include a negative electrode active material.

[0116] Any negative electrode active material used in the art may be used as the negative electrode active material without limitation. The negative electrode active material may include at least one selected from silicon-based active materials and carbon-based active materials, and more specifically, may include a silicon-based active material.

[0117] Silicon-based active materials exhibit higher capacity than carbon-based active materials, but suffer from significant volume expansion and contraction during charge and discharge. However, when silicon-based active materials are used together with the aforementioned non-aqueous electrolytes, a SEI film with improved flexibility and durability can be formed on the negative electrode, thereby preventing electrolyte side reactions and achieving lithium secondary batteries with high lifespan performance.

[0118] The silicon-based active material may include a compound represented by Formula A below.

[0119] [Formula A]

[0120] SiOx(0≤x<2)

[0121] In Formula A, SiO2 may not react with lithium ions and thus does not store lithium. Therefore, it is preferred that x is within the above range. More specifically, the silicon-based active material may be Si.

[0122] The average particle size of silicon-based active materials (D 50 ) can be 1 μm to 20 μm.

[0123] The carbon-based active material may include at least one selected from graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and may preferably include graphite. The graphite may include at least one selected from artificial graphite and natural graphite.

[0124] From the perspective of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte solution, the average particle size (D 50 ) can be 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0125] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material may be contained in the negative electrode active material layer.

[0126] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery. Specifically, the negative electrode current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy.

[0127] The thickness of the negative electrode current collector may be 3 μm to 500 μm.

[0128] The negative electrode current collector may have microscopic irregularities formed on its surface to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector may be used in various shapes such as a film, sheet, foil, mesh, porous body, foam, or non-woven fabric.

[0129] The negative electrode active material layer is disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer can be disposed on one surface or both surfaces of the negative electrode current collector.

[0130] From the viewpoint of exhibiting sufficient capacity in the secondary battery while minimizing the influence of volume expansion / contraction on the battery, the content of the negative electrode active material in the negative electrode active material layer may be 60 wt % to 99 wt %.

[0131] In addition to the silicon-based active material, the negative electrode active material layer may further include a conductive agent and / or a binder.

[0132] The binder may be used to improve adhesion between the negative electrode active material layer and the negative electrode current collector described below or bonding between silicon-based active materials.

[0133] Specifically, from the viewpoint of further improving the electrode adhesion and providing sufficient tolerance to the volume expansion / contraction of the silicon-based active material, the binder may include at least one selected from styrene-butadiene rubber (SBR), nitrile rubber (NBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN) and polyacrylamide (PAM).

[0134] The binder content in the negative electrode active material layer may be 1% to 30% by weight. When the content is within this range, the binder can better bind the negative electrode active material to minimize the volume expansion problem of the active material, and the binder can be easily dispersed and improve the coating property and phase stability of the slurry when preparing the slurry for forming the negative electrode active material layer.

[0135] The conductive agent is not particularly limited, as long as it can be used to assist and improve the conductivity in the secondary battery and has conductivity without causing adverse chemical changes. Specifically, the conductive agent can include at least one selected from the following: graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum or nickel powders; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives.

[0136] The content of the conductive agent in the negative electrode active material layer may be 1 wt % to 20 wt %, and when the content of the conductive agent is within this range, it is preferable from the viewpoint of being able to form an excellent conductive network while alleviating an increase in resistance caused by the binder.

[0137] The thickness of the negative electrode active material layer may be 5 μm to 500 μm, preferably 5 μm to 100 μm.

[0138] The negative electrode may be prepared by coating a negative electrode collector with a negative electrode slurry including a negative electrode active material and optionally a binder, a conductive agent, and a solvent for forming the negative electrode slurry, followed by drying and roll pressing.

[0139] From the viewpoint of promoting dispersion of the negative electrode active material, the binder and / or the conductive agent, the solvent used to form the negative electrode slurry may include, for example, at least one selected from distilled water, ethanol, methanol and isopropyl alcohol, preferably distilled water.

[0140] (2) Positive electrode

[0141] The positive electrode contains a positive electrode active material.

[0142] The positive electrode active material is a compound capable of reversibly inserting and deinserting lithium, specifically, it can include a lithium transition metal composite oxide containing lithium and at least one transition metal selected from the group consisting of nickel, cobalt, manganese and aluminum, and preferably can include a lithium transition metal composite oxide containing lithium and a transition metal containing nickel, cobalt and manganese.

[0143] For example, the lithium transition metal composite oxide may include lithium manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (e.g., LiCoO2, etc.), lithium nickel-based oxides (e.g., LiNiO2, etc.), lithium nickel manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), lithium nickel cobalt-based oxides (e.g., LiNi 1- Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium manganese cobalt-based oxides (e.g., LiCo 1-Y2 Mn[[ID=1​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium nickel cobalt aluminum oxide (such as Li(Ni 0.8 Co 0.15 Al 0.05 ) O2, etc.). Considering the significant improvement effect produced by controlling the types and content ratios of the components constituting the lithium transition metal composite oxide, the lithium transition metal composite oxide may be Li(Ni 0.6 Mn 0.2 Co 0.2 )O2、Li(Ni 0.5 Mn 0.3 Co 0.2 )O2、Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc., and any one of them or a mixture of two or more thereof can be used.

[0144] More specifically, the positive electrode active material may be a lithium transition metal composite oxide, and may contain 60 mol% or more of nickel based on the total molar number of transition metals contained in the lithium transition metal composite oxide. Specifically, the positive electrode active material is a lithium transition metal composite oxide, the transition metal includes nickel and at least one selected from manganese, cobalt, or aluminum, and the nickel content may be 60 mol% or more, specifically 60 mol% to 90 mol%, based on the total molar number of transition metals. When using a lithium transition metal composite oxide with such a high nickel content together with the above-mentioned non-aqueous electrolyte, it is preferred from the viewpoint of being able to reduce gaseous byproducts generated due to structural damage.

[0145] In addition, the positive electrode active material may include a lithium composite transition metal oxide represented by the following formula B:

[0146] [Formula B]

[0147] Li 1+x (Ni a Co b Mn c M d )O2

[0148] In Formula B, M is at least one selected from 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 1+x, a, b, c, and d are each independent atomic fractions of the element, wherein 0≤x≤0.2, 0.50≤a<1, 0≤b≤0.25, 0≤c≤0.25, 0≤d≤0.1, and a+b+c+d=1.

[0149] Preferably, a, b, c and d may be 0.70≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.20 and 0≤d≤0.05, respectively.

[0150] Preferably, a, b, c and d may be 0.80≤a≤0.95, 0.025≤b≤0.15, 0.025≤c≤0.15 and 0≤d≤0.05, respectively.

[0151] In addition, a, b, c and d may be 0.85≤a≤0.90, 0.05≤b≤0.10, 0.05≤c≤0.10 and 0≤d≤0.03, respectively.

[0152] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer may include the positive electrode active material described above.

[0153] The positive electrode current collector may generally have a thickness of 3 μm to 500 μm.

[0154] The positive electrode current collector may have microscopic irregularities formed on its surface to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various shapes such as a film, sheet, foil, mesh, porous body, foam, or non-woven fabric.

[0155] The positive electrode active material layer may be provided on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be provided on one surface or both surfaces of the positive electrode current collector.

[0156] In consideration of exhibiting sufficient capacity of the positive electrode active material, the content of the positive electrode active material in the positive electrode active material layer may be 80 wt % to 99 wt %.

[0157] As described above, the positive electrode active material layer may further include a binder and / or a conductive agent in addition to the positive electrode active material.

[0158] The binder is a component that helps to bond the active material to the conductive agent and to the current collector. Specifically, it may include at least one selected from polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber and fluororubber, preferably including polyvinylidene fluoride.

[0159] From the perspective of ensuring sufficient bonding between components such as the positive electrode active material, the content of the binder in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0160] Conductive agents can be used to assist and improve the conductivity in secondary batteries, and there are no particular restrictions as long as they have conductivity without causing chemical changes. Specifically, the positive electrode conductive agent may include at least one selected from the following: graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives, and in order to improve conductivity, preferably carbon nanotubes may be included.

[0161] In terms of sufficiently ensuring conductivity, the content of the conductive agent in the positive electrode active material layer may be 1% by weight to 20% by weight, preferably 1.2% by weight to 10% by weight.

[0162] The thickness of the positive electrode active material layer may be 5 μm to 500 μm, preferably 20 μm to 200 μm.

[0163] The positive electrode may be prepared by coating a positive electrode collector with a positive electrode slurry containing a positive electrode active material and optionally a binder, a conductive agent, and a solvent for forming the positive electrode slurry, followed by drying and roll pressing.

[0164] (3) Diaphragm

[0165] The separator is provided between the positive electrode and the negative electrode.

[0166] As a separator, a porous polymer film commonly used as a separator can be used alone or in a stacked manner, for example, a porous polymer film prepared from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer. In addition, a typical porous non-woven fabric can be used, such as a non-woven fabric formed from high-melting point glass fiber or polyethylene terephthalate fiber, but is not limited thereto. In order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can also be used, and a separator having a single-layer or multi-layer structure can be selectively used.

[0167] The shape of the lithium secondary battery of the present disclosure is not particularly limited, and a cylindrical type using a can, a prismatic type, a pouch type, or a coin type can be used.

[0168] Hereinafter, the present disclosure will be described in detail based on specific examples. However, the following examples are provided only to illustrate the present disclosure, and the scope of the present disclosure is not limited thereto. It will be apparent to those skilled in the art that various modifications and changes can be made within the scope and technical spirit of the present disclosure, and such modifications and changes fall within the scope of the claims included herein.

[0169] Examples and Comparative Examples

[0170] Example 1

[0171] (Preparation of non-aqueous electrolyte)

[0172] The following organic solvent was used, in which fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) were mixed in a volume ratio of 10:90.

[0173] The non-aqueous electrolyte is prepared by adding LiPF6 as a lithium salt, the compound represented by the above formula 1-1, and lithium difluorophosphate (LiDFP) as an additive to an organic solvent.

[0174] The non-aqueous electrolyte contains LiPF6 at a molar concentration of 1.5M.

[0175] The non-aqueous electrolyte contained 2 wt % of a compound represented by the following formula 1-1, and the non-aqueous electrolyte contained 0.5 wt % of lithium difluorophosphate.

[0176] (Secondary Battery Preparation)

[0177] The positive electrode active material (Li[Ni 0.85 Co 0.05 Mn 0.07 Al 0.03]O2), a conductive agent (carbon nanotubes), and a binder (polyvinylidene fluoride) were added to prepare a positive electrode mixture slurry (solid content: 75.5 wt%). The positive electrode mixture slurry was applied to one surface of a 12 μm thick positive electrode current collector (Al film), dried, and then roll-pressed to prepare a positive electrode.

[0178] A negative electrode mixture slurry (solid content: 26 wt%) was prepared by adding a negative electrode active material (silicon-based active material, Si), a conductive agent (carbon black), and a binder (styrene-butadiene rubber) to distilled water as a solvent at a weight ratio of 70.0:20.3:9.7. The negative electrode mixture slurry was applied to one surface of a 15 μm thick negative electrode current collector (Cu film), dried, and then roll-pressed to prepare a negative electrode.

[0179] In a dry room, a polyethylene porous film separator was inserted between the positive electrode and the negative electrode prepared above, and then the prepared non-aqueous electrolyte was injected therein to prepare a secondary battery.

[0180] Example 2

[0181] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 wt % of the compound represented by the above Formula 1-1 was added to the nonaqueous electrolyte instead of 2 wt %.

[0182] Example 3

[0183] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 5 wt % of the compound represented by the above Formula 1-1 was added to the nonaqueous electrolyte instead of 2 wt %.

[0184] Example 4

[0185] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by the above Formula 1-9 was added to the nonaqueous electrolyte instead of the compound represented by the above Formula 1-1.

[0186] Comparative Example 1

[0187] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by Formula 1-1 was not included.

[0188] Comparative Example 2

[0189] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt % of the compound represented by the above formula X was added to the nonaqueous electrolyte instead of the compound represented by the above formula 1-1.

[0190] [Formula X]

[0191]

[0192] Comparative Example 3

[0193] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt % of the compound represented by Formula Y was added to the nonaqueous electrolyte instead of the compound represented by Formula 1-1 above.

[0194] [Formula Y]

[0195]

[0196] Comparative Example 4

[0197] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt % of the compound represented by Formula Z was added to the nonaqueous electrolyte instead of the compound represented by Formula 1-1 above.

[0198] [Formula Z]

[0199]

[0200] Experimental example

[0201] Experimental Example 1: Evaluation of High-Temperature Cycling Performance

[0202] The lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were charged to 4.2 V at 0.05 C using an electrochemical charge / discharge apparatus under CC / CV, 1 C conditions at 45° C., and then discharged to 3.0 V under CC, 0.5 C conditions, with this being considered one cycle. 200 charge / discharge cycles were then performed.

[0203] The capacity retention rate was calculated using the following formula, and the results are shown in Table 1 below.

[0204] Capacity retention (%) = (discharge capacity after 200 cycles / discharge capacity after 1 cycle) × 100

[0205] Experimental Example 2: Evaluation of high temperature storage performance

[0206] The lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were initially charged and discharged at a current of 0.05 C at 25°C to 4.2 V under CC / CV conditions at 0.33 C, and then discharged at a current of 0.33 C to 2.5 V. Subsequently, the batteries were charged to 4.2 V under CC / CV conditions at 0.05 C at 25°C and 0.33 C, and then stored at 60°C for 8 weeks.

[0207] After the lithium secondary battery was stored for 8 weeks, it was charged to 4.2 V at a current of 0.05 C at 25° C. under the condition of 0.33 C, and then discharged to 3.0 V at a current of 0.33 C. The capacity was then measured during discharge.

[0208] The capacity retention rate was evaluated using the following formula, and the results are shown in Table 1 below.

[0209] Capacity retention (%) = (discharge capacity after 8 weeks of storage / initial discharge capacity) × 100

[0210] Table 1

[0211] Example Experimental Example 1 Experimental Example 2 Capacity retention rate (%, 200th cycle) Capacity increase rate (%, 60℃, 8 weeks storage) Example 1 93.5 96.9 Example 2 90.3 93.0 Example 3 91.0 94.2 Example 4 88.5 90.5 Comparative Example 1 70.8 76.7 Comparative Example 2 73.3 81.2 Comparative Example 3 71.6 78.2 Comparative Example 4 79.4 86.8

[0212] With reference to Table 1 above, it can be understood that the lithium secondary batteries of Examples 1 to 4 using the nonaqueous electrolyte of the present disclosure exhibited significantly superior capacity retention during cyclic charge and discharge and storage at high temperature, compared to Comparative Examples 1 to 4 not using the nonaqueous electrolyte of the present disclosure.

Claims

1. A non-aqueous electrolyte comprising: a lithium salt, an organic solvent and an additive, wherein: The additive comprises a compound represented by the following formula 1: [Formula 1] Wherein, in the above formula 1, R1 and R2 are each independently F, Br, Cl, I, a nitrile group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkoxy group having 1 to 10 carbon atoms, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, a substituent shown in the following formula 2, or a combination of two or more thereof, At least one of R1 and R2 contains a substituent represented by the following formula 2, and n is an integer from 3 to 8; [Formula 2] Wherein, in the above formula 2, L1 is an alkylene group having 1 to 10 carbon atoms, an ester group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof, R3 is an alkoxy group having 1 to 10 carbon atoms substituted with one or more fluorine atoms, and * indicates the binding site.

2. The nonaqueous electrolyte according to claim 1, wherein In the above formula 1, n is 3 or 4.

3. The nonaqueous electrolyte according to claim 1, wherein R3 includes one selected from the group consisting of -OCF3, -OCF2CF3 and -OCF2CF2CF3.

4. The nonaqueous electrolyte according to claim 1, wherein R1 includes the substituent shown in Formula 2 above, and R2 does not include the substituent shown in Formula 2 above.

5. The nonaqueous electrolyte according to claim 4, wherein R2 is an alkyl group having 1 to 5 carbon atoms. The nonaqueous electrolyte according to claim 1 , wherein The compound represented by the above formula 1 includes at least one selected from the group consisting of compounds represented by the following formulas 1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, and 1-12: [Formula 1-1] [Formula 1-2] [Formula 1-3] [Formula 1-4] [Formula 1-5] [Formula 1-6] [Formula 1-7] [Formula 1-8] [Formula 1-9] [Formula 1-10] [Formula 1-11] [Formula 1-12] 7. The nonaqueous electrolyte according to claim 1, wherein The compound represented by Formula 1 is present in an amount of 0.01 wt % to 10 wt % based on the weight of the non-aqueous electrolyte.

8. The nonaqueous electrolyte according to claim 1, wherein The lithium salt includes a salt selected from the group consisting of LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 , LiBOB (LiB (C2O4) 2), LiCF3SO3, LiFSI (LiN (SO2F) 2), LiCH3SO3, LiCF3CO2, LiCH3CO2 and LiBETI (LiN (SO2CF2CF3) 2) At least one of the group consisting of.

9. The nonaqueous electrolyte according to claim 1, wherein The lithium salt is contained in the non-aqueous electrolyte at a molar concentration of 0.5M to 5.0M.

10. The nonaqueous electrolyte according to claim 1, wherein The organic solvent includes at least one selected from the group consisting of a cyclic carbonate organic solvent, a linear carbonate organic solvent, a linear ester organic solvent, and a cyclic ester organic solvent.

11. A lithium secondary battery comprising: negative electrode; a positive electrode opposite to the negative electrode; a separator, the separator being disposed between the negative electrode and the positive electrode; and The non-aqueous electrolyte according to claim 1.

12. The lithium secondary battery according to claim 11, wherein The negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-based active material.

Images