Lithium secondary battery

Abstract

A lithium secondary battery includes a negative electrode, a positive electrode disposed opposite the negative electrode, a separator disposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-based active material. The non-aqueous electrolyte includes a lithium salt, an organic solvent, and additives, the additives include a first additive and a second additive, the first additive includes a coumarin-based compound represented by a specific formula, and the second additive includes at least one of lithium fluoromalonate (difluoro) borate (LiFMDFB), lithium difluoro (oxalate) borate (LiDFOB), lithium difluorophosphate (LiDFP), or lithium difluoro bis (oxalate) phosphate (LiDFOP). In the lithium secondary battery of the present invention, a highly durable flexible film is formed on a negative electrode containing a silicon-based active material, thereby improving high-temperature cycle life performance and high-temperature storage performance.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Korean Patent Application No. 10-2022-0103657, filed on August 18, 2022, the disclosure of which is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a lithium secondary battery. Background Art

[0004] In recent years, the use of lithium secondary batteries in power storage supply for large equipment such as automobiles and power storage equipment, and in power supplies for electronic equipment such as electrical, electronic, communication and computer equipment has rapidly expanded, and the demand for high-capacity, high-output and high-stability secondary batteries has continued to rise.

[0005] Lithium secondary batteries are mainly provided with a positive electrode composed of a lithium-containing transition metal oxide, a negative electrode responsible for storing lithium, an electrolyte used as a medium for transporting lithium ions, and a separator. In this case, the negative electrode may include a negative electrode active material such as a carbon-based active material or a silicon-based active material.

[0006] For lithium secondary batteries, a film (SEI film) is formed on the positive electrode and / or the negative electrode during the initial activation process, which can protect the positive electrode and the negative electrode during battery operation and prevent electrolyte consumption caused by side reactions of the electrolyte. When a strong electrode film is not formed on the positive electrode and / or the negative electrode during the initial activation process, the capacity and life may be reduced.

[0007] In particular, among negative electrode active materials, silicon-based active materials have attracted much attention as materials that exhibit higher capacity than carbon-based active materials, but the material causes a large volume change due to the insertion and extraction of lithium. The volume expansion of silicon-based active materials causes many problems (such as durability degradation of the preformed SEI film, continuous consumption of electrolyte due to the generation of new negative electrode active materials, and increase in the thickness of the SEI film), resulting in reduced capacity and lifespan. Summary of the invention

[0008] [Technical issues]

[0009] One aspect of the present invention provides a lithium secondary battery that can form a flexible and highly durable film on a negative electrode containing a silicon-based active material to reduce electrolyte side reactions and improve high-temperature cycle life performance and high-temperature storage performance.

[0010] [Technical solution]

[0011] According to one aspect of the present invention, a lithium secondary battery is provided, comprising a negative electrode, a positive electrode arranged opposite to the negative electrode, a separator arranged between the negative electrode and the positive electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprises a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, the additive comprises a first additive and a second additive, the first additive comprises a compound represented by the following formula 1, and the second additive comprises at least one of lithium fluoromalonate (difluoro)borate (LiFMDFB), lithium difluoro (oxalate)borate (LiDFOB), lithium difluorophosphate (LiDFP) or lithium difluorobis (oxalate) phosphate (LiDFOP).

[0012] [Formula 1]

[0013]

[0014] In the above formula 1, R each independently includes a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, 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, or a combination of two or more thereof, and n is an integer selected from 0 to 6.

[0015] [Beneficial Effects]

[0016] The lithium secondary battery disclosed in the present invention includes a negative electrode containing a silicon-based active material and a non-aqueous electrolyte, the non-aqueous electrolyte including a first additive as a coumarin compound having a specific structural formula and a second additive containing lithium fluoromalonate (difluoro) borate (LiFMDFB) or lithium difluoro (oxalate) borate (LiDFOB) and the like. When both the first additive and the second additive are included in the non-aqueous electrolyte, a flexible, highly recoverable and highly durable film can be formed on the negative electrode. Therefore, the lithium secondary battery disclosed in the present invention forms the above-mentioned SEI film in a silicon-based negative electrode (which raises concerns about electrolyte side reactions due to large volume expansion) to prevent SEI film rupture, electrolyte side reactions, and resistance increase due to increased thickness of the electrode film, thereby improving the performance of the lithium secondary battery, especially high temperature cycle life performance and high temperature storage performance. DETAILED DESCRIPTION

[0017] It will be understood that the words or terms used in the specification and claims of the present disclosure should not be interpreted as limited to the meanings defined in commonly used dictionaries. It will be further understood that, based on the principle that the inventor can appropriately define the meaning of words or terms to best explain the present disclosure, the words or terms should be interpreted as having a meaning consistent with its meaning in the context of the relevant technology and the technical concept of the present disclosure.

[0018] It will be further understood that when used in this specification, the terms "comprises", "comprising" or "having" specify the presence of stated 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.

[0019] Meanwhile, before describing the present disclosure, unless otherwise described in the present disclosure, “*” represents a portion connecting between the same or different atoms or terminals of a formula.

[0020] In addition, as used herein, "a" and "b" in the description of "a to b carbon atoms" refer to 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, the term "alkyl group having 1 to 5 carbon atoms" refers to an alkyl group including 1 to 5 carbon atoms, i.e., CH 3 -、CH 3 CH 2 -、CH 3 CH 2 CH 2 -、(CH 3 ) 2 CH-、CH 3 CH 2 CH 2 CH 2 -、(CH 3 ) 2 CHCH 2 -、CH 3 CH 2 CH 2 CH 2 CH 2 - or (CH 3 ) 2 CHCH 2 CH 2 -wait.

[0021] In addition, as used herein, both alkyl and aryl groups may be substituted or unsubstituted. Unless otherwise defined, the term "substituted" means that at least one hydrogen bonded to carbon is replaced by an element other than hydrogen, for example, it means substituted with 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 heterocycloalkyl group having 3 to 12 carbon atoms, a heterocycloalkenyl group having 3 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, or a halogenated aryl group having 6 to 20 carbon atoms, etc.

[0022] Hereinafter, various aspects of the present invention will be described in more detail.

[0023] Lithium secondary battery

[0024] The present disclosure relates to a lithium secondary battery.

[0025] Specifically, the lithium secondary battery of an embodiment of the present invention includes a negative electrode, a positive electrode arranged opposite to the negative electrode, a separator arranged between the negative electrode and the positive electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, the non-aqueous electrolyte includes a lithium salt, an organic solvent and an additive, the additive includes a first additive and a second additive, the first additive includes a compound represented by the following formula 1, and the second additive includes at least one of lithium fluoromalonate (difluoro)borate (LiFMDFB), lithium difluoro (oxalate) borate (LiDFOB), lithium difluorophosphate (LiDFP) or lithium difluorobis (oxalate) phosphate (LiDFOP).

[0026] [Formula 1]

[0027]

[0028] In the above formula 1, R each independently includes a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, 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, or a combination of two or more thereof, and n is an integer selected from 0 to 6.

[0029] The lithium secondary battery disclosed herein includes a negative electrode containing a silicon-based active material and a non-aqueous electrolyte, the non-aqueous electrolyte including a first additive and a second additive containing lithium fluoromalonate (difluoro) borate (LiFMDFB) or lithium difluoro (oxalate) borate (LiDFOB) and the like. When both the first additive and the second additive are included in the non-aqueous electrolyte, a flexible, highly recoverable and highly durable film can be formed on the negative electrode. Therefore, the lithium secondary battery disclosed herein forms the above-mentioned SEI film in a silicon-based negative electrode (which raises concerns about electrolyte side reactions due to large volume expansion) to prevent SEI film rupture, electrolyte side reactions, and resistance increase due to increased thickness of the electrode film, thereby improving the performance of the lithium secondary battery, especially high temperature cycle life performance and high temperature storage performance.

[0030] In this case, the lithium secondary battery of the present disclosure can be manufactured according to a typical method known in the art. For example, the negative electrode, the positive electrode, and the separator between the negative electrode and the positive electrode are stacked in sequence to form an electrode assembly, which is then inserted into a battery case and injected with a nonaqueous electrolyte of the present disclosure to manufacture the lithium secondary battery of the present disclosure.

[0031] (1) Negative electrode

[0032] The negative electrode includes a negative electrode active material.

[0033] The negative electrode active material includes a silicon-based active material.

[0034] The advantage of silicon-based active materials is that they have higher capacity and higher energy density than carbon-based active materials such as graphite, but the disadvantage is that they show volume changes of active materials during charging and discharging. The expansion and contraction of the volume of silicon-based active materials leads to conductive disconnection in the negative electrode, resulting in increased resistance and reduced life performance. In addition, the SEI film of the negative electrode formed during the activation process of the lithium secondary battery may be broken due to the volume change of the silicon-based active material, and this will promote the side reaction of the electrolyte, resulting in increased resistance due to the increase in the thickness of the SEI film and electrolyte depletion, resulting in reduced life performance and storage characteristics.

[0035] In order to solve these limitations, the present disclosure uses a non-aqueous electrolyte in which a first additive and a second additive to be described later are used as additives in combination with a negative electrode including a silicon-based active material. The combined use of the first and second additives allows the SEI film formed on the negative electrode to have improved flexibility (recoverability) and durability, thereby having excellent recoverability in the volume change of the silicon-based active material, and preventing the SEI film from rupturing to significantly prevent electrolyte side reactions and electrolyte depletion, thereby obtaining a lithium secondary battery with excellent life performance and storage performance.

[0036] Silicon-based active materials may include SiO x(0≤x<2) represented by the compound. 2 It does not react with lithium ions and therefore cannot store lithium, so x is preferably within the above range. Specifically, the silicon-based active material may include Si or SiO x At least one of (0.7≤x≤1.2, specifically x=1).

[0037] In terms of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte, the average diameter (D 50 ) can be 1 μm to 30 μm, preferably 2 μm to 15 μm.

[0038] The negative electrode active material may further include a carbon-based active material in addition to the silicon-based active material.

[0039] The carbon-based active material may include at least one of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, or fibrous carbon, and may preferably include at least one of artificial graphite or natural graphite.

[0040] In terms of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte, the average diameter (D 50 ) can be 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0041] When the negative electrode active material includes a silicon-based active material and a carbon-based active material, a weight ratio of the silicon-based active material to the carbon-based active material may be 1:99 to 50:50, specifically 3:97 to 20:80, and more specifically 3:97 to 10:90.

[0042] In one embodiment, the negative electrode active material may be free of carbon-based active materials.

[0043] 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. In this case, the negative electrode active material may be included in the negative electrode active material layer.

[0044] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or copper or stainless steel surface-treated with carbon, nickel, titanium or silver, etc., can be used as the negative electrode current collector.

[0045] The thickness of the negative electrode current collector may generally be 3 μm to 500 μm.

[0046] The negative electrode current collector may have fine irregularities formed on its surface to improve the bonding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.

[0047] 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 may be disposed on one surface or both surfaces of the negative electrode current collector.

[0048] The negative electrode active material layer may include the negative electrode active material in an amount of 60 wt % to 99 wt %, preferably 75 wt % to 95 wt %.

[0049] The negative electrode active material layer may further include a binder and a conductive material in addition to the negative electrode active material.

[0050] The binder is used to improve the performance of the battery by improving the adhesion between the negative electrode active material layer and the negative electrode current collector, and may include, for example, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or at least one of its hydrogen substituted by Li, Na or Ca, or may also include various copolymers thereof.

[0051] The negative active material layer may include the binder in an amount of 0.5 to 30 wt %, preferably 1 to 15 wt %, and more preferably 5 to 10 wt %.

[0052] The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and for example, a conductive material such as: graphite, such as natural graphite or artificial graphite; carbon black, such as 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 powder, nickel powder; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives can be used.

[0053] The negative electrode active material layer may include the conductive material in an amount of 0.5 to 30 wt %, preferably 1 to 25 wt %.

[0054] The thickness of the negative electrode active material may be 10 μm to 100 μm, preferably 50 μm to 80 μm.

[0055] A negative electrode slurry including a negative electrode active material and / or a binder, a conductive material, and a solvent for forming a negative electrode slurry may be applied to at least one surface of a negative electrode collector, dried, and roll-pressed to prepare a negative electrode.

[0056] In terms of promoting dispersion of, for example, negative electrode active materials, binders and / or conductive materials, the negative electrode slurry forming solvent may include at least one of distilled water, N-methyl-2-pyrrolidone (NMP), ethanol, methanol or isopropanol, preferably distilled water.

[0057] (2) Positive electrode

[0058] The positive electrode and the negative electrode are arranged opposite to each other.

[0059] The positive electrode may include a positive electrode collector and a positive electrode active material layer disposed on at least one surface of the positive electrode collector.

[0060] The positive electrode collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. Specifically, the positive electrode collector may include at least one of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum-cadmium alloy, and may preferably include aluminum.

[0061] The thickness of the positive electrode current collector may generally be 3 μm to 500 μm.

[0062] The positive electrode current collector may have fine irregularities formed on its surface to improve the bonding strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.

[0063] The positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one surface or both surfaces of the positive electrode current collector.

[0064] The positive electrode active material layer may include a positive electrode active material.

[0065] The positive electrode active material is a compound capable of reversibly inserting and deinserting lithium, and may specifically include a lithium transition metal composite oxide containing lithium and at least one transition metal selected from nickel, cobalt, manganese or aluminum, preferably a lithium transition metal composite oxide containing lithium and a transition metal containing nickel, cobalt or manganese.

[0066] For example, the lithium transition metal composite oxide may include a lithium-manganese-based oxide (eg, LiMnO 2 、LiMn 2 O 4 etc.), lithium-cobalt oxides (e.g., LiCoO 2 etc.), lithium-nickel oxides (e.g., LiNiO2 etc.), lithium-nickel-manganese oxides (e.g., LiNi 1-Y Mn Y O 2 (where 0 < Y < 1), LiMn 2-z Ni z O 4 (where 0 < z < 2) etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O 2 (where 0 < Y1 < 1) etc.), lithium-manganese-cobalt oxides (e.g., LiCo 1-Y2 Mn Y2 O 2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O 4 (where 0 < z1 < 2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r1 )O 2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1 and p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O 4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2 and p1 + q1 + r2 = 2) etc.) or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O 2 (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 each the atomic fraction of an independent element, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < S2 < 1 and p2 + q2 + r3 + S2 = 1) etc.), and may include any one or two or more of these compounds. Among these materials, in terms of improving the capacity and stability of the battery, the lithium transition metal composite oxide can be LiCoO 2 , LiMnO 2 , LiNiO 2 , lithium nickel-manganese-cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 , Li(Ni 0.5 Mn0.3 Co 0.2 ) 2 、Li(Ni 0.7 Mn 0.15 Co 0.15 ) 2 Or Li(Ni 0.8 Mn 0.1 Co 0.1 ) 2 etc.), or lithium-nickel-cobalt-aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 ) 2 etc.), and when considering the significant improvement effect according to controlling the type and content ratio of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide may be Li(Ni 0.6 Mn 0.2 Co 0.2 ) 2 、Li(Ni 0.5 Mn 0.3 Co 0.2 ) 2 、Li(Ni 0.7 Mn 0.15 Co 0.15 ) 2 Or Li(Ni 0.8 Mn 0.1 Co 0.1 ) 2 etc., and any one of them or a mixture of two or more thereof may be used.

[0067] More specifically, the positive electrode active material may be a lithium transition metal composite oxide, and the content of nickel may be 60 mol% or more relative to the total moles of transition metals contained in the lithium transition metal composite oxide. Specifically, the positive electrode active material is a lithium transition metal composite oxide, and the transition metal includes nickel and at least one selected from manganese, cobalt or aluminum, and the content of nickel may be 60 mol% or more, specifically 60 mol% to 90 mol% relative to the total moles of transition metals. When using such a lithium transition metal composite oxide with a high nickel content together with the above-mentioned nonaqueous electrolyte, it is preferred for the purpose of reducing gas phase byproducts generated by structural collapse.

[0068] In addition, the positive electrode active material may include a lithium composite transition metal oxide represented by Formula A below.

[0069] [Formula A]

[0070] Li 1+x (Ni a Co bMn c M d ) 2

[0071] In the above formula A, M is at least one of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B or Mo, and 1+x, a, b, c and d are each the atomic fractions of the independent elements (0≤x≤0.2, 0.50≤a<1, 0 <b≤0.25,0<c≤0.25,0≤d≤0.1,a+b+c+d=1)。

[0072] 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.

[0073] In addition, 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.

[0074] 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.

[0075] The positive electrode active material layer may include the positive electrode active material in a content of 80 wt % to 99 wt %, preferably 92 wt % to 98.5 wt %, in consideration of exhibiting sufficient capacity of the positive electrode active material.

[0076] The positive electrode active material layer may further include a binder and / or a conductive material in addition to the above-mentioned positive electrode active material.

[0077] The binder is a component that supports the bonding of the active material and the conductive material and the bonding with the current collector, and may specifically include at least one of 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 or fluororubber, and may preferably include polyvinylidene fluoride.

[0078] For the purpose of sufficiently ensuring the bonding strength 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 %.

[0079] Conductive materials can be used to support and improve the conductivity of secondary batteries, and there is no particular limitation as long as they have conductivity without causing chemical changes in the battery. Specifically, the positive electrode conductive material may include at least one of the following: graphite, such as natural graphite or artificial graphite; carbon black, such as 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 powder or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives, and for the purpose of improving conductivity, carbon black may be preferably included.

[0080] For the purpose of sufficiently ensuring electron conductivity, the content of the conductive material in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0081] The thickness of the positive electrode active material may be 30 μm to 400 μm, preferably 40 μm to 110 μm.

[0082] A positive electrode slurry including a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming the positive electrode slurry may be coated on a positive electrode collector and then dried and roll-pressed to prepare a positive electrode.

[0083] The positive electrode slurry forming solvent may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP). The positive electrode slurry may have a solid content of 40 wt % to 90 wt %, specifically 50 wt % to 80 wt %.

[0084] (3) Diaphragm

[0085] Commonly used porous polymer films, for example, porous polymer films prepared by polyolefin polymers (such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers or ethylene / methacrylate copolymers) can be used alone or laminated to each other as a separator. Moreover, a typical porous non-woven fabric can be used, for example, a non-woven fabric formed by high melting point glass fiber or polyethylene terephthalate fiber, but the present disclosure is not limited thereto. In addition, a coated separator including a ceramic component or a polymer component can be used to ensure heat resistance or mechanical strength, and a separator with a single layer or multilayer structure can be used.

[0086] (4) Non-aqueous electrolytes

[0087] The nonaqueous electrolyte disclosed herein includes a lithium salt, an organic solvent, and an additive, the additive including a first additive and a second additive, the first additive including a compound represented by the following formula 1, and the second additive including at least one of lithium fluoromalonate (difluoro)borate (LiFMDFB), lithium difluoro (oxalate)borate (LiDFOB), lithium difluorophosphate (LiDFP), or lithium difluorobis (oxalate) phosphate (LiDFOP).

[0088] [Formula 1]

[0089]

[0090] In the above formula 1, R each independently includes a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, 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, or a combination of two or more thereof, and n is an integer selected from 0 to 6.

[0091] The non-aqueous electrolyte disclosed in the present invention includes a first additive containing a coumarin compound having a specific structural formula and a second additive containing lithium fluoromalonate (difluoro) borate (LiFMDFB) or lithium difluoro (oxalate) borate (LiDFOB) as an additive. When the first additive and the second additive are both included in the non-aqueous electrolyte, a flexible, highly recoverable and highly durable film can be formed on the negative electrode. Therefore, the lithium secondary battery disclosed in the present invention forms the above-mentioned SEI film in the silicon-based negative electrode (which raises concerns about electrolyte side reactions due to large volume expansion) to prevent SEI film rupture, electrolyte side reactions, and resistance increase due to increased thickness of the electrode film, thereby improving the performance of the lithium secondary battery, especially high temperature cycle life performance and high temperature storage performance.

[0092] At the same time, in the case where a silicon-based active material is not used as a negative electrode active material, for example, when the above-mentioned non-aqueous electrolyte is applied to a negative electrode containing only a carbon-based active material, the combined use of the first and second additives may instead lead to an increase in resistance, so the effect may not be significantly improved or may even be reduced compared to the case where the first additive or the second additive is used alone.

[0093] 1) Lithium salt

[0094] As the lithium salt used in the present disclosure, various lithium salts commonly used in non-aqueous electrolytes for lithium secondary batteries can be used without limitation. For example, the lithium salt may include Li + as cations and may contain F - , Cl - Br -、I - 、NO 3 - 、N(CN) 2 - 、BF 4 - 、ClO 4 - 、AlO 4 - 、AlCl 4 - 、PF 6 - 、SbF 6 - 、AsF 6 - 、B 10 Cl 10 - 、BF 2 C 2 O 4 - 、BC 4 O 8 - 、PF 4 C 2 O 4 - 、PF 2 C 4 O 8 - 、(CF 3 ) 2 PF 4 - 、(CF 3 ) 3 PF 3 - 、(CF 3 ) 4 PF 2 - 、(CF 3 ) 5 PF - 、(CF 3 ) 6 P - 、CF 3 SO 3 - 、C 4 F 9 SO 3 - 、CF 3 CF 2 SO 3 - 、(FSO 2 ) 2N - CF 3 CF 2 (CF 3 ) 2 CO - ,(CF 3 SO 2 ) 2 CH - , CH 3 SO 3 - CF 3 (CF 2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - or (CF 3 CF 2 SO 2 ) 2 N - as an anion.

[0095] Specifically, lithium salts may include LiCl, LiBr, LiI, LiBF 4 、LiClO 4 、LiAlO 4 、LiAlCl 4 、LiPF 6 、LiSbF 6 、LiAsF 6 , LiB 10 Cl 10 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiFSI(LiN(SO 2 F) 2 )、LiCH 3 SO 3 、LiCF 3 CO 2 、LiCH 3 CO 2 or LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ) at least one of. Specifically, the lithium salt may include LiBF4 、LiClO 4 、LiPF 6 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 ) 2 )、LiFSI((LiN(SO 2 F) 2 ) or LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ) at least one of.

[0096] The non-aqueous electrolyte may include a lithium salt having a concentration of 0.5 M to 5 M, specifically 0.8 M to 4 M, more specifically 0.8 M to 2.0 M. When the concentration of the lithium salt satisfies the above range, the lithium ion yield (Li + The migration number) and the dissociation degree of lithium ions are improved, and therefore, the output characteristics of the battery can be improved.

[0097] 2) Organic solvents

[0098] The organic solvent is a non-aqueous solvent commonly used in lithium secondary batteries, and is not particularly limited as long as its decomposition due to oxidation reaction or the like during charge / discharge of the secondary battery can be minimized.

[0099] Specifically, the organic solvent may include at least one of a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, a linear ester-based organic solvent, or a cyclic ester-based organic solvent.

[0100] Specifically, the organic solvent may include a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixture thereof.

[0101] The cyclic carbonate organic solvent is an organic solvent that is highly viscous and can dissociate the lithium salt in the electrolyte well due to its high dielectric constant, and specifically, may include at least one of 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 or vinylene carbonate, and may more specifically include at least one of ethylene carbonate (EC) or fluoroethylene carbonate (FEC), and may more specifically include fluoroethylene carbonate (FEC). Fluoroethylene carbonate (FEC) may form a SEI film with a high content of inorganic components such as LiF on the negative electrode, thereby improving the durability of the SEI film, which may improve the life performance and storage characteristics, especially when fluoroethylene carbonate (FEC) is applied to the negative electrode including a silicon-based active material.

[0102] Straight-chain carbonate organic solvents are organic solvents with low viscosity and low dielectric constant, and may specifically include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate or ethyl propyl carbonate, and may more specifically include at least one of ethyl methyl carbonate (EMC) or diethyl carbonate (DEC), and may more specifically include diethyl carbonate (DEC). Diethyl carbonate is a symmetrical straight-chain carbonate, and no transesterification reaction occurs, so it is preferably possible to prevent the reduction stability caused by the generation of by-products by the transesterification reaction from being reduced. Diethyl carbonate is particularly preferred when applied to the negative electrode containing a silicon-based active material.

[0103] The organic solvent can be a mixture of a cyclic carbonate organic solvent and a straight-chain carbonate organic solvent. In this case, the cyclic carbonate organic solvent and the straight-chain carbonate organic solvent can be mixed with a volume ratio of 5:95 to 40:60, specifically a volume ratio of 7:93 to 30:70 or a volume ratio of 8:92 to 30:70, more specifically a volume ratio of 8:92 to 20:80. When the mixing ratio of the cyclic carbonate organic solvent and the straight-chain carbonate organic solvent meets the above range, high dielectric constant and low viscosity can be met simultaneously, and excellent ionic conductivity is achieved.

[0104] In addition, in order to prepare an electrolyte with high ionic conductivity, in addition to at least one carbonate organic solvent selected from cyclic carbonate organic solvents or linear carbonate organic solvents, the organic solvent may further include at least one ester organic solvent selected from linear ester organic solvents or cyclic ester organic solvents.

[0105] The linear ester organic solvent may specifically include at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate or butyl propionate.

[0106] In addition, the cyclic ester organic solvent may specifically include at least one of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, or ε-caprolactone.

[0107] Meanwhile, if necessary, the organic solvent may be used by adding an organic solvent commonly used in the non-aqueous electrolyte without limitation. For example, at least one organic solvent selected from an ether organic solvent, a glyme organic solvent or a nitrile organic solvent may be further included.

[0108] As the ether solvent, any one of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL) or 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL), or a mixture of two or more thereof can be used, but the present embodiment is not limited thereto.

[0109] Glyme solvents, which are solvents having a higher dielectric constant and lower surface tension than linear carbonate organic solvents and lower reactivity with metals, may include at least one of dimethoxyethane (glyme dimethyl ether, DME), diethoxyethane, diglyme, triglyme or tetraglyme (TEGDME), but are not limited thereto.

[0110] The nitrile solvent may include at least one of acetonitrile, propionitrile, butyronitrile, valeronitrile, octanonitrile, heptanecarbonitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, benzyl cyanide, 2-fluorobenzoyl cyanide or 4-fluorobenzoyl cyanide, but is not limited thereto.

[0111] 3) Additives

[0112] The additive includes a first additive and a second additive.

[0113] The first additive includes a compound represented by Formula 1 below.

[0114] [Formula 1]

[0115]

[0116] In the above formula 1, R each independently includes a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, 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, or a combination of two or more thereof, and n is an integer selected from 0 to 6.

[0117] The first additive includes a coumarin compound having the above structural formula, and the coumarin compound has strong reducing property in the negative electrode, and the ring structure is opened during the initial activation of the lithium secondary battery, so that a polyethylene oxide-based polymer SEI film can be obtained. The advantage of the polymer SEI film is that it has excellent flexibility and recoverability. However, the SEI film derived from the first additive has poor durability, so it is necessary to use the second additive to be described later in combination with the first additive to achieve the effect of improving the high temperature cycle life performance and high temperature storage performance as the purpose of the present disclosure.

[0118] In one embodiment, in the above formula 1, R may each independently include a propargyl group, an ester group, an ether group, a ketone group, a carboxyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, 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, or a combination of two or more thereof.

[0119] In one embodiment, in the above formula 1, R may specifically each independently include a halogen (the halogen may be selected from F, Cl, Br and I, and may specifically be F), a nitrile group, a propargyl group, an ester group, an ether group or a combination of two or more thereof. These substituents exhibit excellent reducibility, are conducive to the formation of a polymeric SEI film, and exhibit excellent performance in transporting lithium ions, and are therefore preferred.

[0120] In the above Formula 1, n may be an integer selected from 0 to 6, specifically an integer selected from 1 to 6, and more specifically, n may be 1. In the above Formula 1, when n is 2 or more, R may be each the same as or different from each other.

[0121] Specifically, the compound represented by the above Formula 1 may include at least one of the compound represented by the following Formula 2 or the compound represented by the following Formula 3.

[0122] [Formula 2]

[0123]

[0124] [Formula 3]

[0125]

[0126] In the above Formulae 2 and 3, R is the same as defined in the above Formula 1.

[0127] The compounds represented by formulas 2 and 3 each have a structure in which a substituent is present at the 3 and 7 positions (according to IUPAC nomenclature) of the ring structure, and in this case, it is preferred that the synthesis of the compound at the above position is favorable compared to other substitution positions. In particular, for the compound represented by formula 2 having a substituent at the 3 position, it is more preferred that the uniformity of the reaction is improved when reduced to the negative electrode.

[0128] 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 formulae 4 to 12. Considering smoother reduction to the negative electrode and more conducive to the formation of a polymeric SEI film, the compound represented by formula 1 may specifically include at least one selected from the group consisting of compounds represented by the following formulae 4 to 7, and may more specifically include at least one selected from the group consisting of the following formulae 4, 5 and 7, and in terms of having high reducibility and having an excellent effect of inhibiting the dissolution of transition metals as well as the above-mentioned effects, the compound represented by formula 1 may even more specifically include a compound represented by the following formula 4.

[0129] [Formula 4]

[0130]

[0131] [Formula 5]

[0132]

[0133] [Formula 6]

[0134]

[0135] [Formula 7]

[0136]

[0137] [Formula 8]

[0138]

[0139] [Formula 9]

[0140]

[0141] [Formula 10]

[0142]

[0143] [Formula 11]

[0144]

[0145] [Formula 12]

[0146]

[0147] The non-aqueous electrolyte may include a first additive in an amount of 0.01 wt % to 10 wt %, specifically 0.05 wt % to 7 wt %, more specifically 0.1 wt % to 5 wt %, more specifically 0.3 wt % to 2 wt %. When the amount of the compound represented by Formula 1 satisfies the above range, it is preferred in terms of fully providing SEI film flexibility and recoverability and preventing the resistance of the lithium secondary battery from increasing due to excessive addition, thereby causing a decrease in life performance.

[0148] The second additive may include at least one of lithium (difluoro)fluoromalonate borate (LiFMDFB), lithium difluoro(oxalate)borate (LiDFOB), lithium difluorophosphate (LiDFP), or lithium difluorobis(oxalate)phosphate (LiDFOP).

[0149] The second additive may include a fluorine-containing lithium salt additive, and the lithium salt additive may form an inorganic SEI film such as LiF during initial activation of the lithium secondary battery. Inorganic SEI films such as LiF have excellent adhesion to the negative electrode surface, but it is difficult to completely cover the negative electrode surface, so it is difficult to fully prevent electrolyte side reactions.

[0150] In this aspect, the non-aqueous electrolyte is characterized by the combined use of a first additive and a second additive capable of forming a polymeric / inorganic composite SEI film. According to the non-aqueous electrolyte of the present disclosure, the polymeric SEI film derived from the first additive covers the entire surface of the negative electrode to form a SEI film with excellent flexibility and recoverability, and the inorganic SEI film derived from the second additive is distributed in the polymeric SEI film, so that the durability of the SEI film can be improved. In addition, the first additive (coumarin compound) forms free radicals during the reduction reaction, and the free radicals promote the release of fluorine contained in the second additive, so that the inorganic SEI film can be formed more smoothly. Therefore, the non-aqueous electrolyte of the present disclosure can form a SEI film with flexibility, excellent recoverability, and improved durability and strength on the surface of the negative electrode to prevent the electrolyte consumption caused by the electrolyte side reaction when the battery is running and the resistance increase caused by the increase in the thickness of the electrode film, thereby improving the performance of the lithium secondary battery, especially the high temperature cycle life performance and high temperature storage performance. In particular, when the nonaqueous electrolyte of the present invention is used together with a negative electrode containing a silicon-based active material, a SEI film having flexibility and improved durability can be formed on the silicon-based active material having a large volume expansion during charge and discharge, and therefore, it is preferred to prevent damage to the SEI film due to the volume expansion of the silicon-based active material, and to prevent an increase in the thickness of the SEI film and electrolyte consumption caused by surface exposure of new silicon-based active materials due to the volume expansion.

[0151] The second additive may specifically include at least one of lithium fluoromalonate (difluoro) borate (LiFMDFB) or lithium difluoro (oxalate) borate (LiDFOB), and in this case, it is preferred in terms of supporting the improvement of the performance of transporting lithium ions by helping to form a polymeric ether and LiF component that is beneficial to physical strength when the negative electrode is reduced and including an ester structure in the SEI film. In one embodiment, the second additive may specifically include lithium fluoromalonate (difluoro) borate (LiFMDFB). In one embodiment, the second additive may specifically include lithium difluoro (oxalate) borate (LiDFOB).

[0152] The nonaqueous electrolyte may include a second additive having a content of 0.01 wt % to 10 wt %, specifically 0.05 wt % to 7 wt %, more specifically 0.1 wt % to 5 wt %, more specifically 0.3 wt % to 2 wt %. When the content of the compound represented by Formula 1 satisfies the above range, it is preferred in terms of fully providing SEI film durability and strength and preventing the resistance of the lithium secondary battery from increasing due to excessive addition, thereby causing a decrease in life performance.

[0153] The weight ratio of the first additive to the second additive is 5:95 to 95:5, specifically 10:90 to 92:8, more specifically 16:84 to 91:9, even more specifically 30:70 to 70:30, even more specifically 40:60 to 60:40, and when the above weight ratio is met, preferably, the flexibility and durability of the SEI film can be improved to the desired level.

[0154] In addition to the first additive and the second additive, the additive may further include an additional additive (or a third additive). The additional additive may be included in the non-aqueous electrolyte to prevent the negative electrode from collapsing due to decomposition of the non-aqueous electrolyte in a high power environment, or to improve low temperature high rate discharge characteristics, high temperature stability and overcharge prevention, and battery expansion suppression effect at high temperature.

[0155] Specifically, the additional additive may include at least one of vinylene carbonate, vinyl ethylene carbonate, propane sultone, propylene sultone, succinonitrile, adiponitrile, ethylene sulfate, lithium bis(oxalato)borate (LiBOB), tris(trimethylsilyl) phosphate (TMSPa) or tris(trimethylsilyl) phosphite (TMSPi), and may specifically be vinylene carbonate.

[0156] The non-aqueous electrolyte may include additional additives in an amount of 0.1 wt % to 15 wt %.

[0157] In one embodiment, the electrolyte may not include a silyl group-containing additive.

[0158] The outer shape of the lithium secondary battery of the present disclosure is not particularly limited, and thus a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape may be used.

[0159] Hereinafter, various embodiments of the present invention will be described in more detail by specific embodiments. However, the examples shown below are only for understanding the present invention, and the scope of the inventive concept is not limited thereto. It will be apparent to those skilled in the art that various modifications and changes may be made within the scope and technical scope of the present invention, and these modifications and changes fall within the scope of the claims included herein.

[0160] Examples and Comparative Examples

[0161] Example 1

[0162] (Preparation of non-aqueous electrolyte)

[0163] As the organic solvent, a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 10:90 was used.

[0164] LiPF as a lithium salt 6 , a compound represented by Formula 4 as a first additive, LiFMDFB as a second additive, and vinylene carbonate (VC) as an additional additive were added to an organic solvent to prepare a nonaqueous electrolyte.

[0165] LiPF 6 It is contained in a non-aqueous electrolyte at a concentration of 1.5 M.

[0166] The content of the compound represented by Formula 4 in the non-aqueous electrolyte was 0.5 wt %, the content of LiFMDFB in the non-aqueous electrolyte was 0.5 wt %, and the content of vinylene carbonate used as an additional additive in the non-aqueous electrolyte was 0.5 wt %.

[0167] [Formula 4]

[0168]

[0169] (Preparation of lithium secondary battery)

[0170] The positive electrode active material (LiNi 0.85 Co 0.05 Mn 0.07 Al 0.03 O 2), a conductive material (carbon nanotube) and a binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 97.74:0.70:1.56 to prepare a positive electrode mixture slurry (solid content: 75.5 wt %). The positive electrode mixture slurry was coated on one surface of a positive electrode current collector (Al film) having a thickness of 15 μm, and then dried and rolled to prepare a positive electrode.

[0171] The negative electrode active material (Si), the conductive material (carbon black) and the binder (styrene-butadiene rubber) were added to distilled water as a solvent at a weight ratio of 70.0:20.3:9.7 to prepare a negative electrode mixture slurry (solid content: 26 wt%). The negative electrode mixture slurry was coated on one surface of a negative electrode current collector (Cu film) having a thickness of 15 μm, and then dried and rolled to prepare a negative electrode.

[0172] A polyethylene porous film separator was placed between the prepared positive electrode and negative electrode in a dry room, and then the prepared nonaqueous electrolyte was injected to prepare a secondary battery.

[0173] Example 2

[0174] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive was added to the nonaqueous electrolyte in an amount of 0.1 wt %.

[0175] Example 3

[0176] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive was added to the nonaqueous electrolyte in an amount of 5.0 wt %.

[0177] Example 4

[0178] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that, as a first additive, a compound represented by the following Formula 5 was included in the nonaqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Formula 4.

[0179] [Formula 5]

[0180]

[0181] Example 5

[0182] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that, as a first additive, a compound represented by the following Formula 6 was included in the nonaqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Formula 4.

[0183] [Formula 6]

[0184]

[0185] Example 6

[0186] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that, as a first additive, a compound represented by the following Formula 7 was included in the nonaqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Formula 4.

[0187] [Formula 7]

[0188]

[0189] Example 7

[0190] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that, as the second additive, LiDFP was contained in the nonaqueous electrolyte in an amount of 0.5 wt % instead of LiFMDFB.

[0191] Example 8

[0192] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that, as the second additive, LiDFOB was contained in the nonaqueous electrolyte in an amount of 0.5 wt % instead of LiFMDFB.

[0193] Example 9

[0194] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the second additive was added to the nonaqueous electrolyte in an amount of 0.1 wt %.

[0195] Example 10

[0196] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the second additive was added to the nonaqueous electrolyte in an amount of 5 wt %.

[0197] Embodiment 11

[0198] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 8, except that the second additive was added to the nonaqueous electrolyte in an amount of 0.1 wt %.

[0199] Example 12

[0200] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 8, except that the second additive was added to the nonaqueous electrolyte in an amount of 5 wt %.

[0201] Comparative Example 1

[0202] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive and the second additive were not added.

[0203] Comparative Example 2

[0204] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the second additive was not added.

[0205] Comparative Example 3

[0206] A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive was not added.

[0207] [Table 1]

[0208]

[0209] Experimental example

[0210] Experimental Example 1: Evaluation of high temperature cycle capacity retention

[0211] The lithium secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 3 prepared above were charged to 4.2 V under CC / CV and 0.33 C at 45° C. using an electrochemical charger and discharger, and discharged to 3 V under CC and 0.33 C as one cycle, and 300 charge and discharge cycles were performed, and the capacity retention rate was measured.

[0212] The capacity retention rate was calculated by the following equation, and the results are shown in Table 2.

[0213] Capacity retention rate (%) = {(discharge capacity after 300th cycle) / (discharge capacity after 1st cycle)}×100

[0214] Experimental Example 2: Evaluation of capacity retention after high temperature storage

[0215] The lithium secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 3 prepared above were charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and discharged to 2.5 V at 0.33 C conditions for initial charging and discharging, and then charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and then stored at 60° C. After storage, the secondary batteries were charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and discharged to 2.5 V at 0.33 C conditions to measure the capacity at discharge.

[0216] The capacity retention rate was evaluated according to the following formula, and the results are shown in Table 2 below.

[0217] Capacity retention rate (%) = (discharge capacity after storage for N weeks / initial discharge capacity) × 100

[0218] (wherein N is an integer greater than 1).

[0219] [Table 2]

[0220]

[0221] Referring to Table 2, it is confirmed that the lithium secondary batteries of Examples 1 to 12 using the nonaqueous electrolyte containing the first additive and the second additive of the present disclosure have significantly improved high temperature cycle life performance and high temperature storage performance compared to the lithium secondary batteries of Comparative Examples 1 to 3.

[0222] Reference example

[0223] Reference Example 1

[0224] 1. Preparation of non-aqueous electrolyte

[0225] The same nonaqueous electrolyte as used in Example 1 was used.

[0226] 2. Preparation of lithium secondary batteries

[0227] The positive electrode active material (Li[Ni 0.85 Co 0.03 Mn 0.07 Al 0.03 ]O 2 ), a conductive material (carbon nanotube) and a binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 97.74:0.70:1.56 to prepare a positive electrode mixture slurry (solid content: 75.5 wt %). The positive electrode mixture slurry was coated on one surface of a positive electrode current collector (Al film) having a thickness of 15 μm, and then dried and rolled to prepare a positive electrode.

[0228] The negative electrode active material (natural graphite), the conductive material (carbon black) and the binder (styrene-butadiene rubber (SBR)-carboxymethyl cellulose (CMC)) were added to distilled water as a solvent at a weight ratio of 95.0:1.5:3.5 to prepare a negative electrode mixture slurry (solid content: 60 wt%). The negative electrode mixture slurry was coated on one surface of a negative electrode current collector (Cu film) having a thickness of 15 μm, and then dried and rolled to prepare a negative electrode.

[0229] A polyethylene porous film separator was placed between the prepared positive electrode and negative electrode in a dry room, and then the prepared nonaqueous electrolyte was injected to prepare a secondary battery.

[0230] Reference Example 2

[0231] A secondary battery was prepared in the same manner as in Reference Example 1, except that the nonaqueous electrolyte used in Comparative Example 2 was used as the nonaqueous electrolyte.

[0232] Reference Example 3

[0233] A secondary battery was prepared in the same manner as in Reference Example 1, except that the nonaqueous electrolyte used in Comparative Example 3 was used as the nonaqueous electrolyte.

[0234] Reference Experiment 1

[0235] The lithium secondary batteries of Reference Examples 1 to 3 prepared above were charged to 4.2 V under CC / CV and 0.33 C conditions at 45° C. using an electrochemical charger and discharger, and discharged to 3 V under CC and 0.33 C conditions as one cycle, and 300 charge and discharge cycles were performed, and the capacity retention rate was measured.

[0236] The capacity retention rate was calculated by the following equation, and the results are shown in Table 3.

[0237] Capacity retention rate (%) = {(discharge capacity after 300th cycle) / (discharge capacity after 1st cycle)}×100

[0238] Reference Experiment 2

[0239] The lithium secondary batteries of Reference Examples 1 to 3 prepared above were charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and discharged to 2.5 V at 0.33 C conditions for initial charging and discharging, and then charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and then stored at 60° C. After storage, the secondary batteries were charged to 4.2 V / 55 mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions, and discharged to 2.5 V at 0.33 C conditions to measure the capacity at discharge.

[0240] The capacity retention ratio was evaluated according to the following formula, and the results are shown in Table 3 below.

[0241] Capacity retention rate (%) = (discharge capacity after storage for N weeks / initial discharge capacity) × 100

[0242] (wherein N is an integer greater than 1).

[0243] [Table 3]

[0244]

[0245] Referring to Table 3 above, when the nonaqueous electrolyte having the characteristics of the present disclosure is applied to a negative electrode containing only a carbon-based active material (Reference Example 1), performance degradation compared to Reference Examples 2 and 3 (which differ in that only any one of the first additive and the second additive is used) is confirmed. Compared with the silicon-based active material, the volume expansion of the carbon-based active material is smaller or less significant, and therefore, the polymer-type SEI film derived from the first additive acts as a resistor instead, and it is believed that when the first additive and the second additive are used together, this resistance increase is further aggravated.

[0246] Therefore, it is clearly understood that the effects of improving life performance and storage performance of the present disclosure, especially the effects of improving high-temperature life performance and high-temperature storage performance, can be achieved only when the negative electrode including the silicon-based active material is combined with the above-mentioned nonaqueous electrolyte.

Claims

1. A lithium secondary battery, comprising: negative electrode; a positive electrode disposed opposite to the negative electrode; A separator disposed between the negative electrode and the positive electrode; as well as Non-aqueous electrolytes, Wherein, the negative electrode comprises a negative electrode active material, The negative electrode active material includes a silicon-based active material, The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, The additives include a first additive and a second additive, The first additive includes a compound represented by Formula 1, and The second additive includes at least one of lithium fluoromalonate (difluoro) borate (LiFMDFB), lithium difluoro (oxalate) borate (LiDFOB), lithium difluorophosphate (LiDFP) or lithium difluorobis (oxalate) phosphate (LiDFOP), [Formula 1] Wherein, in Formula 1, R each independently includes halogen, nitrile, propargyl, ester, ether, ketone, carboxyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted alkoxy, boron, borate, isocyanate, isothiocyanate, silyl, siloxane, sulfone, sulfonate, sulfate, or a combination of two or more thereof, and n is an integer selected from 0-6.

2. The lithium secondary battery according to claim 1, wherein The compound represented by Formula 1 includes at least one of the compound represented by Formula 2 or the compound represented by Formula 3: [Formula 2] [Formula 3] Wherein, in Formulae 2 and 3, R is the same as defined in Formula 1.

3. The lithium secondary battery according to claim 1, wherein The compound represented by Formula 1 includes at least one selected from the group consisting of compounds represented by Formulae 4 to 12: [Formula 4] [Formula 5] [Formula 6] [Formula 7] [Formula 8] [Formula 9] [Formula 10] [Formula 11] [Formula 12] 4. The lithium secondary battery according to claim 1, wherein The nonaqueous electrolyte includes the first additive in an amount of 0.01 wt % to 10 wt %.

5. The lithium secondary battery according to claim 1, wherein The second additive includes at least one selected from lithium difluoromalonate borate (LiFMDFB) or lithium difluoro(oxalate)borate (LiDFOB).

6. The lithium secondary battery according to claim 1, wherein The nonaqueous electrolyte includes the second additive in an amount of 0.01 wt % to 10 wt %.

7. The lithium secondary battery according to claim 1, wherein The weight ratio of the first additive to the second additive is 10:90 to 90:

10.

8. The lithium secondary battery according to claim 1, wherein The lithium salts include 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 or LiBETI (LiN (SO2CF2CF3) 2).

9. The lithium secondary battery according to claim 1, wherein: The nonaqueous electrolyte includes the lithium salt at a molar concentration of 0.5M to 5.0M.

10. The lithium secondary battery according to claim 1, wherein The organic solvent includes at least one of a cyclic carbonate organic solvent, a linear carbonate organic solvent, a linear ester organic solvent or a cyclic ester organic solvent.

11. The lithium secondary battery according to claim 10, wherein: The organic solvent includes a cyclic carbonate organic solvent and a linear carbonate organic solvent. The cyclic carbonate organic solvent includes fluoroethylene carbonate, and The linear carbonate organic solvent includes diethyl carbonate.

12. The lithium secondary battery according to claim 1, wherein The additive further includes at least one third additive selected from vinylene carbonate, vinyl ethylene carbonate, propane sultone, propylene sultone, succinonitrile, adiponitrile, ethylene sulfate, lithium bis(oxalato)borate (LiBOB), tris(trimethylsilyl)phosphate (TMSPa) or tris(trimethylsilyl)phosphite (TMSPi).

13. The lithium secondary battery according to claim 1, wherein: The silicon-based active material includes SiO x A compound represented by , where 0≤x<2.

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