Electrolyte for lithium secondary battery with improved high-temperature stability and lithium secondary battery including the same

Abstract

An electrolyte for a lithium secondary battery according to exemplary embodiments may comprise a lithium salt; an organic solvent; a first additive comprising a lactone-based compound represented by a specific chemical formula; and a second additive comprising a fluorine-containing phosphate-based compound, a fluorine-containing carbonate-based compound, a sulfone-based compound, and a sulfate-based compound.

Description

Technology Field

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same. More specifically, the invention relates to an electrolyte for a lithium secondary battery comprising a lithium salt, an organic solvent, and an additive, and a lithium secondary battery comprising the same. Background Technology

[0002] Rechargeable batteries are batteries capable of repeated charging and discharging, and are widely used as power sources for portable electronic devices such as mobile phones and laptop PCs.

[0003] Lithium-ion batteries are being actively developed and applied due to their high operating voltage and energy density per unit weight, as well as their advantages in charging speed and weight reduction.

[0004] For example, a lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and an electrolyte impregnating the electrode assembly.

[0005] For example, the anode may include a lithium metal oxide capable of reversible insertion and extraction of lithium as an anode active material.

[0006] Meanwhile, during repeated charging and discharging of lithium secondary batteries, structural deformation of the lithium metal oxide and adverse reactions of the electrolyte may occur. In this case, the lifespan characteristics of the lithium secondary battery (e.g., capacity retention rate) may deteriorate.

[0007] In particular, lithium secondary batteries are exposed to high-temperature environments during repeated charging and discharging and overcharging. In this case, the aforementioned problems are accelerated, leading to battery expansion (increase in internal battery gas, increase in battery thickness), increased internal resistance of the battery, and deterioration of battery life characteristics.

[0008] Korean Patent Publication No. 10-2019-0119615 improves the performance of a lithium secondary battery by adding an additive to an electrolyte for a lithium secondary battery. Prior art literature

[0009] Republic of Korea Published Patent Application No. 10-2019-0119615 The problem to be solved

[0010] One objective of the present invention is to provide an electrolyte for a lithium secondary battery having high-temperature stability.

[0011] One objective of the present invention is to provide a lithium secondary battery with improved high-temperature storage characteristics. means of solving the problem

[0012] An electrolyte for a lithium secondary battery according to exemplary embodiments may comprise a lithium salt; an organic solvent; a first additive comprising a lactone-based compound represented by Formula 1; and a second additive comprising a fluorine-containing phosphate-based compound, a fluorine-containing carbonate-based compound, a sulfone-based compound, and a sulfate-based compound.

[0013] [Chemical Formula 1]

[0014]

[0015] (In Chemical Formula 1, X is a substituted or unsubstituted C2-C5 alkylene group).

[0016] In one embodiment, the lactone-based compound may include γ-butyrolactone.

[0017] In one embodiment, the first additive may be included in an amount of 0.1 to 10 weight percent with respect to the total weight of the electrolyte.

[0018] In one embodiment, the second additive may be included in an amount of 0.1 to 10 weight percent based on the total weight of the electrolyte.

[0019] In one embodiment, the ratio of the weight of the second additive to the weight of the first additive in the electrolyte may be 1 to 10.

[0020] In one embodiment, the first additive is included in an amount of 0.5 to 3.5 weight% with respect to the total weight of the electrolyte, and the second additive is included in an amount of 1 to 5 weight% with respect to the total weight of the electrolyte, and the ratio of the weight of the second additive to the weight of the first additive in the electrolyte may be 1.25 to 7.

[0021] In one embodiment, the fluorine-containing carbonate-based compound may have a cyclic structure.

[0022] In one embodiment, the sulfonate compound may include an alkyl sulfonate compound and an alkenyl sulfonate compound.

[0023] In one embodiment, the sulfate-based compound may have a cyclic structure.

[0024] In one embodiment, the fluorine-containing phosphate-based compound is included in an amount of 0.1 to 1.5 weight% based on the total weight of the electrolyte, the fluorine-containing carbonate-based compound is included in an amount of 0.1 to 1.5 weight% based on the total weight of the electrolyte, the sulfone-based compound is included in an amount of 0.1 to 2 weight% based on the total weight of the electrolyte, and the sulfate-based compound may be included in an amount of 0.1 to 0.5 weight% based on the total weight of the electrolyte.

[0025] In one embodiment, the organic solvent may include a cyclic carbonate-based solvent and a linear carbonate-based solvent.

[0026] A lithium secondary battery according to exemplary embodiments may include a positive electrode; a negative electrode facing the positive electrode; and an electrolyte for the lithium secondary battery described above. Effects of the invention

[0027] The electrolyte for a lithium secondary battery according to exemplary embodiments can have improved high-temperature stability (e.g., an effect of suppressing gas generation rate in a high-temperature environment).

[0028] The electrolyte for a lithium secondary battery according to exemplary embodiments can realize a lithium secondary battery with improved high-temperature storage characteristics (e.g., effect of preventing increase in battery thickness, effect of preventing increase in resistance, and capacity retention rate in a high-temperature environment).

[0029] The electrolyte for a lithium secondary battery according to exemplary embodiments can significantly reduce the rate of increase in battery thickness in a high-temperature environment. Brief explanation of the drawing

[0030] FIG. 1 is a schematic plan view of a lithium secondary battery according to exemplary embodiments. FIG. 2 is a schematic cross-sectional view of a lithium secondary battery according to exemplary embodiments. Specific details for implementing the invention

[0031] In this specification, "~-system compound" may mean a compound to which "~-system compound" is attached, and derivatives of the compound.

[0032] In this specification, "Ca-Cb" may mean "a to b number of carbon (C) atoms."

[0033] Electrolyte for Lithium Secondary Batteries

[0034] An electrolyte for a lithium secondary battery according to exemplary embodiments may include a lithium salt; an organic solvent; and an additive.

[0035] In one embodiment, the additive may include a lactone-based compound.

[0036] In one embodiment, the additive may include a first additive comprising a lactone-based compound; and a second additive comprising a fluorine-containing phosphate-based compound, a fluorine-containing carbonate-based compound, a sulfone-based compound, and a sulfate-based compound.

[0037] The electrolyte for a lithium secondary battery according to exemplary embodiments may have improved high-temperature stability (e.g., an effect of suppressing gas generation rate in a high-temperature environment).

[0038] The electrolyte for a lithium secondary battery according to exemplary embodiments can realize a lithium secondary battery with improved high-temperature storage characteristics (e.g., effect of preventing increase in battery thickness, effect of preventing increase in resistance, and capacity retention rate in a high-temperature environment).

[0039] For example, the above electrolyte for a lithium secondary battery can realize a lithium secondary battery in which the rate of increase in battery thickness is significantly reduced in a high-temperature environment.

[0040] The components of the present invention will be described in more detail below.

[0041] First additive

[0042] The electrolyte for a lithium secondary battery according to exemplary embodiments may include a first additive comprising a lactone-based compound represented by the following chemical formula 1.

[0043] [Chemical Formula 1]

[0044]

[0045] In Chemical Formula 1, X may be a substituted or unsubstituted C2-C5 alkylene group.

[0046] For example, the carbon atom of the C=O bond and the oxygen atom of the CO bond can be connected by the alkylene group to form a 4- to 7-membered ring.

[0047] For example, the above alkylene group is an alkane (-C n H 2n -) can refer to a form in which one hydrogen atom is removed from each of the carbon atoms at both ends. For example, -CH2-CH2-CH2- can refer to a propylene group.

[0048] For example, the meaning of "substituted" may be that a hydrogen atom of the alkylene group is substituted with a substituent, so that a substituent may be further bonded to a carbon atom of the alkylene group. For example, the substituent may be at least one of a halogen, a C1-C6 alkyl group, a C2-C6 alkenyl group, an amino group, a C1-C6 alkoxy group, a C3-C7 cycloalkyl group, and a 5-7 heterocycloalkyl group. In some embodiments, the substituent may be a halogen or a C1-C6 alkyl group.

[0049] In one embodiment, X may be a substituted or unsubstituted propylene group.

[0050] In one embodiment, X may be an unsubstituted propylene group. For example, the lactone compound may include gamma-butyrolactone (GBL; γ-butyrolactone).

[0051] For example, the above-mentioned lactone-based compound can form a robust solid electrolyte film (SEI) on the cathode under conditions combined with the additives described below. In this case, the decomposition of organic solvents (e.g., EC, EMC, etc.) can be effectively prevented. Accordingly, gas generation and an increase in battery thickness can be significantly reduced.

[0052] In one embodiment, the first additive may be included in an amount of 0.1 to 10 weight%, 0.25 to 5 weight%, or 0.5 to 3.5 weight% with respect to the total weight of the electrolyte. In this case, a lithium secondary battery with improved high-temperature storage characteristics can be realized.

[0053] Second additive

[0054] The electrolyte for a lithium secondary battery according to exemplary embodiments may further include a second additive comprising a fluorine-containing phosphate-based compound, a fluorine-containing carbonate-based compound, a sulfone-based compound, and a sulfate-based compound, together with the first additive described above.

[0055] In one embodiment, the second additive may be included in an amount of 0.1 to 10 weight% or 1 to 5 weight% with respect to the total weight of the electrolyte. In this case, a lithium secondary battery with improved high-temperature storage characteristics can be realized.

[0056] In one embodiment, the ratio of the weight of the second additive to the weight of the first additive in the electrolyte may be 1 to 10, greater than 1 and less than or equal to 10, 1.25 to 10, or 1.25 to 7. In this case, a lithium secondary battery with improved high-temperature storage characteristics can be realized.

[0057] In some embodiments, the first additive is included in an amount of 0.5 to 3.5 weight percent with respect to the total weight of the electrolyte, and the second additive is included in an amount of 1 to 5 weight percent with respect to the total weight of the electrolyte, and the ratio of the weight of the second additive to the weight of the first additive in the electrolyte may be 1.25 to 7. In this case, a lithium secondary battery can be realized that has an excellent capacity retention rate, a low resistance increase rate, and a significantly low battery thickness increase rate in a high-temperature environment.

[0058] For example, the above fluorine-containing phosphate compound may have a fluorine (F) atom directly bonded to a phosphorus (P) atom, or an alkyl group (e.g., -CF3) bonded to a fluorine atom.

[0059] In one embodiment, the fluorine-containing phosphate-based compound may be a fluorine-containing lithium phosphate-based compound and may be represented by the following chemical formula 2.

[0060] [Chemical Formula 2]

[0061]

[0062] In chemical formula 2, R1 and R2 are independently halogens, or substituted or unsubstituted C1-C6 alkyl groups, and at least one of R1 and R2 may be F.

[0063] In some embodiments, the fluorine-containing phosphate compound may include at least one of lithium difluorophosphate (LiPO2F2), lithium tetrafluorooxalate phosphate, and lithium difluoro(bisoxalato)phosphate.

[0064] In one embodiment, the fluorine-containing phosphate-based compound may be included in an amount of 0.1 to 2 weight%, 0.1 to 1.5 weight%, 0.1 to 1 weight%, or 0.1 to 0.5 weight% based on the total weight of the electrolyte.

[0065] For example, the above-mentioned fluorine-containing carbonate compound may have a fluorine atom directly bonded to at least one carbon (C) atom, or an alkyl group bonded to a fluorine atom may be bonded.

[0066] In one embodiment, the fluorine-containing carbonate compound may have a cyclic structure. For example, the fluorine-containing carbonate compound may have at least one atom of the carbonate group arranged within the ring. For example, the fluorine-containing carbonate compound may have a cyclic structure of 5 to 7 members.

[0067] In one embodiment, the fluorine-containing carbonate compound can be represented by Chemical Formula 3.

[0068] [Chemical Formula 3]

[0069]

[0070] In the chemical formula 3, R3 and R4 may independently be hydrogen, a halogen, and a C1-C6 alkyl group, and at least one of R3 and R4 may be F.

[0071] In some embodiments, the fluorinated carbonate compound may include fluoroethylene carbonate (FEC).

[0072] In one embodiment, the fluorinated carbonate-based compound may be included in an amount of 0.1 to 2 weight%, 0.1 to 1.5 weight%, 0.1 to 1 weight%, or 0.1 to 0.5 weight% based on the total weight of the electrolyte.

[0073] In one embodiment, the sulfonate compound may be represented by Chemical Formula 4.

[0074] [Chemical Formula 4]

[0075]

[0076] In Chemical Formula 4, R5 may be a substituted or unsubstituted C2-C5 alkylene group, or a substituted or unsubstituted C3-C5 alkenylene group.

[0077] In one embodiment, the sulfonate compound may include at least one of an alkyl sulfonate compound and an alkenyl sulfonate compound.

[0078] In one embodiment, The above sulfonate compound may include alkyl sulfonate compounds and alkenyl sulfonate compounds together.

[0079] For example, the above alkyl sulfone compound may have only saturated bonds within the ring, and the above alkenyl sulfone compound may have unsaturated bonds within the ring (e.g., C=C double bond).

[0080] For example, the above alkyl sulfone compound may include at least one of 1,3-propane sulfone (PS) and 1,4-butane sulfone.

[0081] For example, the alkenyl sulfone compound may include at least one of ethensulfone, 1,3-propenesulfone (PRS), 1,4-butenesulfone, and 1-methyl-1,3-propenesulfone.

[0082] In one embodiment, the sulfonate compound may be included in an amount of 0.1 to 2 weight%, 0.1 to 1.5 weight%, 0.1 to 1 weight%, or 0.1 to 0.5 weight% based on the total weight of the electrolyte.

[0083] In one embodiment, the sulfate-based compound may have a cyclic structure.

[0084] For example, the above sulfate-based compound may have at least one atom of the sulfate group arranged within the ring. For example, the above sulfate-based compound may have a cyclic structure of 5 to 7 members.

[0085] In one embodiment, the sulfate-based compound may be represented by Chemical Formula 5.

[0086] [Chemical Formula 5]

[0087]

[0088] In chemical formula 5, R6 may be a substituted or unsubstituted C2-C5 alkylene group.

[0089] For example, the sulfate-based compound may include at least one of ethylene sulfate (ESA), trimethylene sulfate (TMS), and methyltrimethylene sulfate (MTMS).

[0090] For example, the content of the sulfate-based compound may be 0.1 to 2 weight%, 0.1 to 1.5 weight%, 0.1 to 1 weight%, or 0.1 to 0.5 weight% based on the total weight of the electrolyte.

[0091] In one embodiment, the electrolyte may not contain a vinylidene carbonate-based compound. For example, the vinylidene carbonate-based compound may be vinylene carbonate (VC), vinylethylene carbonate (VEC), etc. In this case, a lithium secondary battery having a significantly reduced battery thickness increase rate can be realized in a high-temperature environment.

[0092] For example, the above electrolyte may not contain lithium bis(oxalate)borate (LiBOB). In this case, a lithium secondary battery having a significantly reduced battery thickness increase rate can be realized in a high-temperature environment.

[0093] Organic solvents and lithium salts

[0094] The above organic solvent may include, for example, an organic compound that has sufficient solubility for the lithium salt, the additive, and the auxiliary additive and does not have reactivity in the battery.

[0095] For example, the organic solvent may include at least one of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, and an aprotic solvent.

[0096] In one embodiment, the organic solvent may include a carbonate-based solvent.

[0097] In some embodiments, the carbonate-based solvent may include a linear carbonate-based solvent and a cyclic carbonate-based solvent.

[0098] For example, the linear carbonate-based solvent may include at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and dipropyl carbonate.

[0099] For example, the cyclic carbonate-based solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate.

[0100] In some embodiments, the organic solvent may contain more of the linear carbonate-based solvent than the cyclic carbonate-based solvent by volume.

[0101] For example, the mixing volume ratio of the linear carbonate-based solvent and the cyclic carbonate-based solvent may be 1:1 to 9:1, and preferably 1.5:1 to 4:1.

[0102] For example, the ester-based solvent may include at least one of methyl acetate (MA; methyl acetate), ethyl acetate (EA; ethyl acetate), n-propyl acetate (n-PA; n-propyl acetate), 1,1-dimethylethyl acetate (DMEA; 1,1-dimethylethyl acetate), methyl propionate (MP; methyl propionate), and ethyl propionate (EP; ethyl propionate).

[0103] For example, the above ether-based solvent may include at least one of dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), and 2-methyltetrahydrofuran.

[0104] For example, the above ketone-based solvent may include cyclohexanone.

[0105] For example, the above alcohol-based solvent may include at least one of ethyl alcohol and isopropyl alcohol.

[0106] For example, the aprotic solvent may include at least one of a nitrile-based solvent, an amide-based solvent (e.g., dimethylformamide), a dioxolane-based solvent (e.g., 1,3-dioxolane), and a sulfolane-based solvent.

[0107] The above electrolyte includes a lithium salt, and the lithium salt is Li + X - It can be expressed as.

[0108] For example, the anion (X) of the above lithium salt - ) is F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 -, CH3CO2 - , SCN - and (CF3CF2SO2)2N - It can be any one selected from the back.

[0109] In some embodiments, the lithium salt may include at least one of LiBF4 and LiPF6.

[0110] In one embodiment, the lithium salt may be included in the organic solvent at a concentration of 0.01 to 5 M, more preferably 0.01 to 2 M. Within the concentration range, lithium ions and / or electrons can be smoothly moved during charging and discharging of the battery.

[0111] Lithium secondary battery

[0112] A lithium secondary battery according to exemplary embodiments may comprise a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte comprising an organic solvent and a lithium salt.

[0113] Hereinafter, a lithium secondary battery according to exemplary embodiments will be described in more detail with reference to the drawings. FIGS. 1 and 2 are a schematic plan view and a cross-sectional view, respectively, showing a lithium secondary battery according to exemplary embodiments. FIGS. 2 is a cross-sectional view taken along line II' of FIGS. 1.

[0114] Referring to FIGS. 1 and 2, a lithium secondary battery may include a positive electrode (100) and a negative electrode (130) facing the positive electrode (100).

[0115] For example, the positive electrode (100) may include a positive electrode current collector (105) and a positive electrode active material layer (110) on the positive electrode current collector (105).

[0116] For example, the positive active material layer (110) may include a positive active material, a positive binder and a conductive material as needed.

[0117] For example, the anode (100) can be manufactured by mixing and stirring an anode active material, an anode binder, a conductive material, a dispersion medium, etc. to produce an anode slurry, and then applying, drying, and rolling the anode slurry onto an anode current collector (105).

[0118] For example, the positive current collector (105) may include stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0119] For example, the positive electrode active material may include lithium metal oxide particles capable of reversible insertion and extraction of lithium ions.

[0120] In one embodiment, the positive electrode active material may include lithium metal oxide particles containing nickel.

[0121] In some embodiments, the lithium metal oxide particles may contain 83 mol% or more of nickel relative to the total moles of all elements excluding lithium and oxygen. In this case, a lithium secondary battery with a high capacity can be realized.

[0122] In some embodiments, the lithium metal oxide particles may contain at least 83 mol%, at least 85 mol%, at least 90 mol%, or at least 95 mol% of nickel with respect to the total number of moles of all elements excluding lithium and oxygen.

[0123] In some embodiments, the lithium metal oxide particles may further include at least one of cobalt and manganese.

[0124] In some embodiments, the lithium metal oxide particles may further include cobalt and manganese. In this case, a lithium secondary battery with excellent output characteristics and penetration stability can be realized.

[0125] In one embodiment, the lithium metal oxide particles may be represented by the following chemical formula 7.

[0126] [Chemical Formula 7]

[0127] Lix Ni a Co b M c O y

[0128] In Chemical Formula 7, M is at least one of Al, Zr, Ti, Cr, B, Mg, Mn, Ba, Si, Y, W, and Sr, and may be 0.9≤x≤1.2, 1.9≤y≤2.1, 0.83≤a≤1, 0≤c / (a+b) ≤0.13, and 0≤c≤0.11.

[0129] In some embodiments, a may be 0.85≤a≤1, 0.9≤a≤1, or 0.95≤a≤1.

[0130] In some embodiments, a may be 0.85≤a<1, 0.9≤a<1, or 0.95≤a<1.

[0131] In some embodiments, M is Mn and c is 0 <c<0.17, 0<c<0.15, 0<c<0.1, 또는 0<c<0.05일 수 있다.

[0132] In one embodiment, the lithium metal oxide particles may further include a coating element or a doping element. For example, the coating element or doping element may include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, alloys thereof, or oxides thereof. In this case, a lithium secondary battery with improved lifespan characteristics can be realized.

[0133] For example, the anode binder may include organic binders such as polyvinylidenefluoride (PVDF), vinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, and polymethylmethacrylate; and water-based binders such as styrene-butadiene rubber (SBR). Additionally, for example, the anode binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0134] For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, graphene, and carbon nanotubes; and metal-based conductive materials such as perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3.

[0135] For example, the cathode (130) may include a cathode current collector (125) and a cathode active material layer (120) on the cathode current collector (125).

[0136] For example, the negative electrode active material layer (120) may include a negative electrode active material, a negative electrode binder and a conductive material as needed.

[0137] For example, the cathode (130) can be manufactured by mixing and stirring a cathode active material, a cathode binder, a conductive material, a solvent, etc. to produce a cathode slurry, and then applying, drying, and rolling the cathode slurry onto a cathode current collector (125).

[0138] For example, the negative current collector (125) may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and more preferably, may include copper or a copper alloy.

[0139] For example, the above-mentioned negative electrode active material may be a material capable of absorbing and extracting lithium ions. For example, the above-mentioned negative electrode active material may include a lithium alloy, a carbon-based material, a silicon-based material, etc.

[0140] For example, the lithium alloy may include metallic elements such as aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, and indium.

[0141] For example, the carbon-based material may include crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.

[0142] For example, the amorphous carbon may be hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500°C or lower, mesophase pitch-based carbon fiber (MPCF), etc. The crystalline carbon may be, for example, natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0143] In one embodiment, the negative electrode active material may include a silicon-based material. For example, the silicon-based material may be Si, SiO x (0 <x<2), Si / C, SiO / C, Si-Metal 등을 포함할 수 있다. 이 경우, 고용량을 갖는 리튬 이차 전지를 구현할 수 있다.

[0144] For example, if the negative electrode active material includes a silicon-based material, there may be a problem where the battery thickness increases during repeated charging and discharging. A lithium secondary battery according to exemplary embodiments can mitigate the rate of increase in battery thickness by including the electrolyte described above.

[0145] In some embodiments, the content of silicon source in the negative electrode active material may be 1 to 20 weight%, 1 to 15 weight%, or 1 to 10 weight%.

[0146] The above-described cathode binder and conductive material may be materials substantially identical or similar to the anode binder and conductive material described above. For example, the cathode binder may be a water-based binder such as styrene-butadiene rubber (SBR). Additionally, for example, the cathode binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0147] For example, a separator (140) may be interposed between the anode (100) and the cathode (130).

[0148] In some embodiments, the area of ​​the negative electrode (130) may be larger than the area of ​​the positive electrode (100). In this case, lithium ions generated from the positive electrode (100) can move smoothly to the negative electrode (130) without precipitating in the middle.

[0149] For example, the separator (140) may include a porous polymer film made of a polyolefin-based polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc. Additionally, for example, the separator (140) may include a nonwoven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0150] For example, an electrode cell may be formed including an anode (100), a cathode (130), and a separator (140).

[0151] For example, a plurality of electrode cells may be stacked to form an electrode assembly (150). For example, the electrode assembly (150) may be formed by winding, lamination, zigzag-folding, etc. of a separator (140).

[0152] A lithium secondary battery according to exemplary embodiments may include a positive electrode lead (107) connected to a positive electrode (100) and protruding outside of a case (160); and a negative electrode lead (127) connected to a negative electrode (130) and protruding outside of a case (160).

[0153] For example, the positive electrode (100) and the positive electrode lead (107) may be electrically connected. Likewise, the negative electrode (130) and the negative electrode lead (127) may be electrically connected.

[0154] For example, the positive lead (107) can be electrically connected to the positive current collector (105). Additionally, the negative lead (130) can be electrically connected to the negative current collector (125).

[0155] For example, the positive current collector (105) may include a protrusion (positive tab, not shown) on one side. A positive active material layer (110) may not be formed on the positive tab. The positive tab may be integral with the positive current collector (105) or connected by welding or the like. The positive current collector (105) and the positive lead (107) may be electrically connected through the positive tab.

[0156] Likewise, the negative current collector (125) may include a protrusion (negative tab, not shown) on one side. A negative active material layer (120) may not be formed on the negative tab. The negative tab may be integral with the negative current collector (125) or connected by welding or the like. The negative current collector (125) and the negative lead (127) may be electrically connected through the negative tab.

[0157] In one embodiment, the electrode assembly (150) may include a plurality of positive electrodes and a plurality of negative electrodes. For example, the plurality of positive electrodes and negative electrodes may be arranged alternately with respect to each other, and a separator may be interposed between the positive electrodes and the negative electrodes. Accordingly, a lithium secondary battery according to one embodiment of the present invention may include a plurality of positive electrode tabs and a plurality of negative electrode tabs protruding from each of the plurality of positive electrodes and the plurality of negative electrodes.

[0158] In one embodiment, the positive tabs (or negative tabs) may be laminated, pressed, and welded to form a positive tab laminate (or negative tab laminate). The positive tab laminate may be electrically connected to a positive lead (107). Additionally, the negative tab laminate may be electrically connected to a negative lead (127).

[0159] For example, the electrode assembly (150) and the above-described electrolyte can be housed together in a case (160) to form a lithium secondary battery.

[0160] The above lithium secondary battery can be manufactured in, for example, cylindrical, prismatic, pouch, or coin types.

[0161] Preferred embodiments and comparative examples of the present invention are described below. However, the following examples are merely preferred embodiments of the present invention, and the present invention is not limited to the following examples.

[0162] [Example 1]

[0163] (1) Preparation of electrolyte

[0164] A 1 M LiPF6 solution (EC / EMC mixed solvent with a volume ratio of 25:75) was prepared.

[0165] The electrolytes of the examples and comparative examples were prepared by adding and mixing the first additive and the second additive in the amounts (weight%) listed in Table 1 below, based on the total weight of the electrolyte, to the above LiPF6 solution.

[0166] (2) Preparation of lithium secondary battery samples

[0167] Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2 and Li[Ni 0.8 Co 0.1 Mn 0.1 A positive electrode slurry was prepared by mixing O2 in a 6:4 weight ratio, carbon black, and polyvinylidene fluoride (PVDF) in NMP in a 92:5:3 weight ratio.

[0168] The above anode slurry was uniformly applied to an area excluding the protrusion of an aluminum foil (15 μm thick) having a protrusion (anode tab) on one side, and an anode was manufactured by drying and rolling.

[0169] A cathode slurry was prepared by mixing artificial graphite and natural graphite in a weight ratio of 7:3, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in water in a weight ratio of 97:1:2.

[0170] The above cathode slurry was uniformly applied to an area excluding the protrusion of a copper foil (15 μm thickness) having a protrusion (cathode tab) on one side, and the cathode was manufactured by drying and rolling.

[0171] An electrode assembly was formed by interposing a polyethylene separator (thickness 20 μm) between the anode and the cathode. An anode lead and a cathode lead were welded and connected to the anode tab and the cathode tab, respectively.

[0172] The electrode assembly is housed inside a pouch (case) so that a portion of the positive lead and the negative lead are exposed to the outside, and three sides excluding the surface of the electrolyte injection part are sealed.

[0173] A lithium secondary battery sample was prepared by injecting the electrolyte prepared in (1) above, sealing the surface of the electrolyte injection area, and then impregnating it for 12 hours.

[0174] First additive Second additive Total amount of the second additive Second additive / First additive GBL LiPO2F2 FEC PS PRS ESA VC LiBOB Example 1 2 0.5 0.5 0.5 0.25 0.25 2 1 Example 2 2 1 0.5 0.5 0.5 0.5 2.5 1.25 Example 3 2 1 1 0.5 0.5 0.5 3.5 1.75 Example 4 2 1.5 1.5 1 0.5 0.5 5 2.5 Example 5 2 1.5 1.5 1 1 0.5 5.5 2.75 Example 6 1 1 1 0.5 0.5 0.5 3.5 3.5 Example 7 0.5 1 1 0.5 0.5 0.5 3.5 7 Example 8 4 1 1 0.5 0.5 0.5 3.5 0.86 Example 9 5 1 1 0.5 0.5 0.5 3.5 0.7 Example 10 6 1 1 0.5 0.5 0.5 3.5 0.58 Example 11 7 1 1 0.5 0.5 0.5 3.5 0.5 Comparative Example 1 1 1 0.5 0.5 0.5 3.5 Comparative Example 2 2 0.5 0.5 0.25 Comparative Example 3 2 0.5 0.5 1 0.5 Comparative Example 4 2 1 1 2 1 Comparative Example 5 2 1.5 1.5 3 1.5 Comparative Example 6 2 2 2 4 2 Comparative Example 7 2 1 1 0.5 0.5 0.5 3.5 1.75 Comparative Example 8 2 1 1 0.5 0.5 0.5 3.5 1.75

[0175] The components listed in Table 1 are as follows.

[0176] GBL: Gamma-butyrolactone

[0177] LiPO2F2: Lithium difluorophosphate

[0178] FEC: Fluoroethylene carbonate

[0179] PS: 1,3-propane sulfonate

[0180] PRS: 1,3-propene sulfonate

[0181] ESA: Ethylene sulfate

[0182] VC: Vinylene carbonate

[0183] LiBOB: Lithium bis(oxalate) borate

[0184] Experimental Example: Evaluation of High Temperature (60℃) Storage Characteristics

[0185] (2) Measurement of battery thickness increase rate

[0186] After charging the lithium secondary batteries of the examples and comparative examples at 0.5C CC / CV (4.2V 0.05C CUT-OFF) at 25℃, the battery thickness T1 was measured.

[0187] After leaving the lithium secondary batteries of the charged examples and comparative examples for 3 weeks under exposure conditions to 60°C air (using a constant temperature device), they were left for an additional 30 minutes at room temperature, and the battery thickness T2 was measured.

[0188] The battery thickness was measured using a flat plate thickness measuring device (Mitutoyo, 543-490B).

[0189] The battery thickness increase rate was calculated as follows, and the results are listed in Table 2 below.

[0190] Battery thickness increase rate (%) = (T2-T1) / T1 × 100(%)

[0191] (2) Measurement of the rate of increase in internal resistance (DC-IR)

[0192] After charging the lithium secondary batteries of the examples and comparative examples at room temperature at 25°C with a 0.5C CC / CV charge (4.2V 0.05C CUT-OFF), they were discharged at 0.5C CC to SOC 60.

[0193] At the SOC 60 point, DCIR R1 was measured by varying the C-rate to 0.2C, 0.5C, 1C, 1.5C, 2C, 2.5C, and 3.0C, and discharging and recharging for 10 seconds for each.

[0194] After leaving the lithium secondary batteries of the charged examples and comparative examples exposed to air at 60°C for 3 weeks, they were additionally left at room temperature for 30 minutes, and DCIR R2 was measured in the same manner as described above.

[0195] The rate of increase in internal resistance was calculated as follows, and the result is listed in Table 2 below.

[0196] Internal resistance increase rate (%) = (R2-R1) / R1 × 100(%)

[0197] (2) Measurement of capacity retention rate (Ret.)

[0198] The lithium secondary batteries of the examples and comparative examples were subjected to 0.5C CC / CV charging (4.2V, 0.05C CUT-OFF) and 0.5C CC discharging (2.7V CUT-OFF) three times at 25℃, and the third discharge capacity C1 was measured.

[0199] The lithium secondary batteries of the examples and comparative examples were charged at 0.5C CC / CV (4.2V 0.05C CUT-OFF). After storing the charged lithium secondary batteries at 60°C for 3 weeks, they were left at room temperature for an additional 30 minutes, and then discharged at 0.5C CC (2.75V CUT-OFF) to measure the discharge capacity C2.

[0200] The capacity retention rate was calculated as follows and recorded in Table 2 below.

[0201] Dose Retention Rate (%) = C2 / C1 × 100(%)

[0202] Thickness increase rate (%) DCIR growth rate (%) Ret.(%) Example 1 11 15.6 91 Example 2 1 1.6 92 Example 3 2 16.1 93 Example 4 3 15.6 92 Example 5 3 35.6 90 Example 6 4 9.2 91 Example 7 5 10.8 90 Example 8 6 26.9 92 Example 9 8 15.7 91 Example 10 8 17.7 91 Example 11 9 21.3 89 Comparative Example 1 16 6.8 88 Comparative Example 2 42 25.5 79 Comparative Example 3 34 32.8 81 Comparative Example 4 13 23.7 83 Comparative Example 5 13 23.4 82 Comparative Example 6 38 29.9 81 Comparative Example 7 13 2.8 90 Comparative Example 8 11 9.7 90

[0203] As can be seen in Table 2 above, the lithium secondary batteries of the examples showed excellent results in high-temperature storage evaluation (thickness increase rate, resistance increase rate, and capacity retention rate).

[0204] For example, the lithium secondary battery of the embodiments showed a significantly improved effect of reducing the thickness increase rate (i.e., reducing the gas generation rate).

[0205] For example, when the content of the second additive in the electrolyte is within a specific range (e.g., 5% by weight or less), the effect of reducing the resistance increase rate is further enhanced.

[0206] For example, when the value of (weight of the second additive / weight of the first additive) in the electrolyte is within a specific range (e.g., 1 or more), the effect of reducing the thickness increase rate and the effect of reducing the resistance increase rate are further enhanced. Explanation of the symbols

[0207] 100: Anode 105: Anode current collector 107: Anode Lead 110: Anode Active Material Layer 120: Cathode active material layer 125: Cathode current collector 127: Cathode Lead 130: Cathode 140: Separator 150: Electrode assembly 160: Case

Claims

Claim 1 An electrolyte for a lithium secondary battery comprising: a lithium salt; an organic solvent; a first additive comprising a lactone-based compound represented by Chemical Formula 1; and a second additive comprising a fluorine-containing phosphate-based compound, a fluorine-containing carbonate-based compound, a sulfone-based compound, and a sulfate-based compound, and not comprising a vinylidene carbonate-based compound and lithium bis(oxalate)borate: [Chemical Formula 1] (In Chemical Formula 1, X is a substituted or unsubstituted C2-C5 alkylene group). Claim 2 An electrolyte for a lithium secondary battery according to claim 1, wherein the lactone-based compound comprises γ-butyrolactone. Claim 3 An electrolyte for a lithium secondary battery according to claim 1, wherein the first additive is included in an amount of 0.1 to 10 weight percent based on the total weight of the electrolyte. Claim 4 An electrolyte for a lithium secondary battery according to claim 1, wherein the second additive is included in an amount of 0.1 to 10 weight percent based on the total weight of the electrolyte. Claim 5 An electrolyte for a lithium secondary battery according to claim 1, wherein the ratio of the weight of the second additive to the weight of the first additive in the electrolyte is 1 to 10. Claim 6 An electrolyte for a lithium secondary battery according to claim 1, wherein the first additive is included in an amount of 0.5 to 3.5 weight% with respect to the total weight of the electrolyte, the second additive is included in an amount of 1 to 5 weight% with respect to the total weight of the electrolyte, and the ratio of the weight of the second additive to the weight of the first additive in the electrolyte is 1.25 to 7. Claim 7 The electrolyte for a lithium secondary battery according to claim 1, wherein the fluorine-containing carbonate-based compound has a cyclic structure. Claim 8 An electrolyte for a lithium secondary battery according to claim 1, wherein the sulfonate compound comprises an alkyl sulfonate compound and an alkenyl sulfonate compound. Claim 9 The electrolyte for a lithium secondary battery according to claim 1, wherein the sulfate-based compound has a cyclic structure. Claim 10 An electrolyte for a lithium secondary battery according to claim 1, wherein the fluorine-containing phosphate-based compound is included in an amount of 0.1 to 1.5 weight% based on the total weight of the electrolyte, the fluorine-containing carbonate-based compound is included in an amount of 0.1 to 1.5 weight% based on the total weight of the electrolyte, the sulfone-based compound is included in an amount of 0.1 to 2 weight% based on the total weight of the electrolyte, and the sulfate-based compound is included in an amount of 0.1 to 0.5 weight% based on the total weight of the electrolyte: Claim 11 An electrolyte for a lithium secondary battery according to claim 1, wherein the organic solvent comprises a cyclic carbonate-based solvent and a linear carbonate-based solvent. Claim 12 A lithium secondary battery comprising: a positive electrode; a negative electrode facing the positive electrode; and an electrolyte for a lithium secondary battery according to claim 1.

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