Lithium secondary battery

Abstract

A lithium secondary battery according to an embodiment of the present invention includes a negative electrode, a positive electrode disposed opposite the negative electrode, and an electrolyte. The negative electrode includes a lithium metal layer and a solid electrolyte interface phase (SEI) layer disposed on the lithium metal layer. The negative electrode has a first peak ratio of 1 to 7.

Description

Technical Field

[0001] This invention provides a lithium secondary battery. Background Technology

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as portable cameras, mobile phones, and laptops (PCs). In addition, in recent years, battery packs that include rechargeable batteries have been developed for use as power sources for environmentally friendly vehicles such as electric vehicles.

[0003] Among secondary batteries, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design, so they are being actively researched and developed.

[0004] In recent years, in order to realize batteries with high energy density and fast charging characteristics, lithium secondary batteries including lithium metal electrodes are being researched and developed.

[0005] When a lithium-ion secondary battery, including a lithium metal electrode, is repeatedly charged and discharged, lithium dendrites may form due to uneven deposition of lithium on the surface of the lithium metal electrode. Lithium dendrites protrude from the lithium metal surface and may damage or penetrate the separator and come into contact with the positive electrode, thereby significantly reducing battery life characteristics.

[0006] Therefore, modifications, treatments, property improvements, or changes in the composition of the electrolyte may be necessary to improve the lifespan characteristics and stability of lithium metal batteries. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] One technical problem of the present invention is to provide a lithium secondary battery with improved electrochemical properties.

[0009] (II) Technical Solution

[0010] A lithium secondary battery according to an exemplary embodiment of the present invention includes a negative electrode, a positive electrode disposed opposite to the negative electrode, and an electrolyte. The negative electrode includes a lithium metal layer and a solid electrolyte interphase (SEI) layer disposed on the lithium metal layer, and the negative electrode has a first peak ratio of 1 to 7 as defined by the following formula 1.

[0011] [Formula 1]

[0012] R1=I F / I CC

[0013] In Equation 1, R1 is the proportion of the first peak, I CCTo determine the intensity of the peak in the range of 283.0 eV to 286.5 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the XPS analysis.

[0014] In an exemplary implementation, the first peak ratio can be 2 to 6.

[0015] In an exemplary embodiment, the electrolyte may comprise lithium salt and fluorine-based organic solvent.

[0016] In an exemplary embodiment, the fluorinated organic solvent may comprise linear fluorinated organic solvents and cyclic fluorinated organic solvents.

[0017] In an exemplary embodiment, the ratio of the content of the linear fluorinated organic solvent to the content of the cyclic fluorinated organic solvent in the total volume of the fluorinated organic solvent can be 3 to 6.

[0018] In an exemplary embodiment, the linear fluorinated organic solvent may include at least one selected from fluorinated linear ether solvents, fluorinated linear carbonate solvents, and fluorinated linear ester solvents.

[0019] In an exemplary embodiment, the cyclic fluorinated organic solvent may include a fluorinated cyclic carbonate solvent.

[0020] In an exemplary embodiment, the lithium salt may include a first lithium salt comprising LiPF6 and a second lithium salt comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0021] In an exemplary embodiment, the ratio of the molar concentration of the first lithium salt to the molar concentration of the second lithium salt in the electrolyte can be greater than 1 and less than 3.

[0022] In an exemplary embodiment, the molar concentration of the first lithium salt can be from 0.5M to 1.0M, and the molar concentration of the second lithium salt can be from 0.01M to 0.5M.

[0023] A lithium secondary battery according to an exemplary embodiment of the present invention includes a negative electrode, a positive electrode disposed opposite to the negative electrode, and an electrolyte. The negative electrode includes a lithium metal layer and a solid electrolyte interphase (SEI) layer disposed on the lithium metal layer, and the negative electrode has a second peak ratio of 3 to 15 as defined by Formula 2 below.

[0024] [Equation 2]

[0025] R2=I F / I CO

[0026] In Equation 2, R2 is the proportion of the second peak, I CO To determine the intensity of the peak in the range of 287.5 eV to 292.0 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the XPS analysis.

[0027] According to an exemplary implementation, the second peak ratio can be 5 to 10.

[0028] (III) Beneficial Effects

[0029] A lithium secondary battery according to an exemplary embodiment of the present invention includes a solid electrolyte interface phase (SEI) layer firmly attached to the surface of a lithium metal layer, thereby suppressing the formation of lithium dendrites.

[0030] The solid electrolyte interface layer can induce uniform lithium deposition during repeated charge-discharge cycles. Therefore, the power characteristics can be maintained even with repeated charge-discharge cycles. Furthermore, the dense solid electrolyte interface layer blocks direct contact between the electrolyte and the lithium metal layer, thereby preventing electrolyte decomposition or side reactions on the lithium metal layer surface.

[0031] A lithium secondary battery according to an exemplary embodiment of the present invention includes a negative electrode having a first peak ratio or a second peak ratio that satisfies a predetermined range. Therefore, capacity reduction during repeated charge-discharge cycles can be prevented, and battery life characteristics can be improved. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a cross-section of a lithium secondary battery according to an exemplary embodiment.

[0033] Figure 2 The XPS analysis results are for the SEI layer in Example 1.

[0034] Explanation of reference numerals in the attached figures:

[0035] 100: Diaphragm

[0036] 200: Negative electrode

[0037] 210: Lithium metal layer

[0038] 205: Solid electrolyte interface phase layer

[0039] 300: Positive electrode

[0040] 310: Positive current collector

[0041] 320: Positive electrode active material layer Detailed Implementation

[0042] According to an exemplary embodiment of the present invention, a lithium secondary battery including a solid electrolyte interface layer is provided.

[0043] The present invention will now be described in detail with reference to the accompanying drawings. However, this is merely exemplary, and the present invention is not limited to the specific embodiments described herein.

[0044] Figure 1 This is a schematic diagram of a cross-section of a lithium secondary battery according to an exemplary embodiment.

[0045] Reference Figure 1 The lithium secondary battery includes a negative electrode 200 and a positive electrode 300 disposed opposite to the negative electrode 200. The negative electrode 200 may include a lithium metal layer 210 and a solid electrolyte interface layer 205.

[0046] The lithium metal layer 210 is composed solely of lithium, thus exhibiting a higher energy density compared to typical graphite and / or silicon-based anode active materials. Therefore, by including an anode 200 with the lithium metal layer 210, a high-energy-density battery with improved power characteristics can be achieved.

[0047] The lithium metal layer 210 can essentially be composed of metallic bonds between lithium atoms. The lithium metal layer 210 can be, for example, composed of lithium foil. The lithium metal layer 210 can contain more than 90% by weight of lithium and can essentially be composed of lithium.

[0048] The thickness of the lithium metal layer 210 can be, for example, from 10 μm to 50 μm.

[0049] A solid electrolyte interphase (SEI) layer 205 can be formed on the surface of the lithium metal layer 210 through battery formation and charging / discharging. According to an exemplary embodiment, a fluorine-based organic solvent contained in the electrolyte described later can decompose on the surface of the lithium metal layer 210 to form the solid electrolyte interphase (SEI) layer 205.

[0050] The solid electrolyte interphase (SEI) layer 205 can be in direct contact with the lithium metal layer 210.

[0051] The solid electrolyte interphase (SEI) layer 205 may contain lithium fluoride (LIF). In addition, the solid electrolyte interphase (SEI) layer 205 may also contain lithium compounds such as lithium carbonate, and may further contain byproducts such as lithium phosphorus oxides generated from the solvent of the electrolyte or the decomposition of lithium salts.

[0052] A solid electrolyte interphase (SEI) layer 205 can be formed on the surface opposite to the positive electrode 300 of the lithium metal layer 210. Therefore, when lithium ions migrate from the positive electrode 300 to the lithium metal layer 210, the solid electrolyte interphase (SEI) layer 205 can induce uniform lithium deposition on the surface of the lithium metal layer 210 and can suppress the formation of lithium dendrites.

[0053] The thickness of the solid electrolyte interface phase layer 205 can be, for example, 1 μm to 5 μm.

[0054] According to an exemplary embodiment, the negative electrode 200 has a first peak ratio of 1 to 7 as defined by Formula 1 below. According to some embodiments, the negative electrode 200 may have a first peak ratio of 2 to 6 as defined by Formula 1 below.

[0055] [Formula 1]

[0056] R1=I F / I CC

[0057] In Equation 1, R1 is the proportion of the first peak, I CC To determine the intensity of the peak in the range of 283.0 eV to 286.5 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the XPS analysis.

[0058] According to an exemplary embodiment, the negative electrode 200 has a second peak ratio of 3 to 15 as defined by Formula 2 below. According to some embodiments, the negative electrode 200 may have a second peak ratio of 5 to 10 as defined by Formula 2 below.

[0059] [Equation 2]

[0060] R2=I F / I CO

[0061] In Equation 2, R2 is the proportion of the second peak, I CO To determine the intensity of the peak in the range of 287.5 eV to 292.0 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the XPS analysis.

[0062] When analyzing the solid electrolyte interfacial phase layer 205 using X-ray photoelectron spectroscopy (XPS), multiple peaks can be observed in the XPS spectrum. For example, 3 to 10 peaks can be observed in the XPS spectrum of the solid electrolyte interfacial phase layer 205.

[0063] In the XPS spectrum of the solid electrolyte interfacial phase layer 205, peaks appear in the ranges of 283.0 eV to 286.5 eV, 287.5 eV to 292.0 eV, and 682.0 eV to 686.5 eV. The peak in the 283.0 eV to 286.5 eV range can be attributed to carbon-carbon bonds contained in the solid electrolyte interfacial phase layer 205. The peak in the 287.5 eV to 292.0 eV range can be attributed to carbon-oxygen double bonds contained in the solid electrolyte interfacial phase layer 205. The peak in the 682.0 eV to 686.5 eV range can be attributed to lithium fluoride contained in the solid electrolyte interfacial phase layer 205.

[0064] The intensity of a peak within a predetermined range can refer to the maximum intensity of a peak when only one peak appears within that range. When multiple peaks appear within that range, it can refer to the maximum intensity of the peak with the greatest intensity.

[0065] In some implementations, the first peak ratio can be 2 to 6. In some implementations, the first peak ratio can be 2.5 to 5.5 or 2.7 to 4.

[0066] Within the aforementioned range, the solid electrolyte interface layer 205 contains an appropriate amount of lithium fluoride, thereby improving the physical properties of the solid electrolyte interface layer 205, and the solid electrolyte interface layer 205 can be firmly attached to the surface of the lithium metal layer 210.

[0067] For example, when the first peak ratio is less than 1 or greater than 7, the adhesion between the solid electrolyte interface phase layer 205 and the lithium metal layer 210 will be relatively reduced, which may not be able to sufficiently suppress the formation of lithium dendrites.

[0068] In some implementations, the second peak ratio can be 5 to 10. In some implementations, the second peak ratio can be 5.5 to 9.7.

[0069] The solid electrolyte interface layer 205 contains an appropriate amount of lithium fluoride, which can improve the physical properties of the solid electrolyte interface layer 205, and the solid electrolyte interface layer 205 can be firmly attached to the surface of the lithium metal layer 210.

[0070] For example, when the proportion of the second peak is less than 3 or greater than 15, the adhesion between the solid electrolyte interfacial phase layer 205 and the lithium metal layer 210 will be relatively reduced, which may not be able to sufficiently suppress the formation of lithium dendrites. X-ray photoelectron spectroscopy analysis can be performed using a PHI Genesis model (Ulvac PHI, inc., Japan) under monochromatic Al Kα irradiation (approximately 1486.6 eV).

[0071] The positive electrode 300 may include a positive electrode current collector 310 and a positive electrode active material layer 320 disposed on at least one side of the positive electrode current collector 310.

[0072] The positive electrode current collector 310 may comprise stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector 310 may also comprise aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The thickness of the positive electrode current collector may be, for example, from 10 μm to 50 μm.

[0073] The positive electrode active material layer 320 may contain a positive electrode active material. The positive electrode active material may contain compounds that enable reversible insertion and extraction of lithium ions.

[0074] According to an exemplary embodiment, the positive electrode active material may comprise a lithium-nickel metal oxide. The lithium-nickel metal oxide may further comprise at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0075] In some embodiments, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 1.

[0076] [Chemical Formula 1]

[0077] Li x Ni a M b O 2+z

[0078] In chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can include Co, Mn, and / or Al.

[0079] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude other additional elements. For example, M may include Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements and should be understood as including the introduction and substitution of additional elements.

[0080] In one embodiment, in addition to the primary active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure and form bonds; this should be understood to also include the chemical structures represented by Formula 1.

[0081] The auxiliary element may include at least one of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may act as an auxiliary active element, together with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0082] For example, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 2.

[0083] [Chemical Formula 2]

[0084] Li x Ni a M1 b M2 c O 2+z

[0085] In chemical formula 2, M1 may include Co, Mn, and / or Al. M2 may include the aforementioned auxiliary elements. In chemical formula 2, the elements can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b+c≤0.4, and -0.5≤z≤0.1.

[0086] The positive electrode active material may further include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, one or more combinations of the elements described above can be used as coating elements or doping elements.

[0087] The coating element or dopant element may exist on the surface of the lithium-nickel metal oxide particles, or may penetrate through the surface of the lithium-nickel metal oxide particles and be contained within the bonding structure represented by chemical formula 1 or chemical formula 2.

[0088] The positive electrode active material may contain nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0089] Ni can be provided as a transition metal related to the power and capacity of a lithium secondary battery. Thus, as described above, by using a high-content (high nickel (High-Ni)) composition for the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0090] However, as the Ni content increases, the long-term storage stability and life stability of the positive electrode or the secondary battery may be relatively reduced, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, conductivity can be maintained by including Co, and life stability and capacity retention characteristics can be improved by Mn.

[0091] The content of Ni in the NCM-based lithium oxide (for example, the mole fraction of Ni in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the content of Ni can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0092] In some embodiments, the positive electrode active material may further include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (for example, LiFePO4).

[0093] In some embodiments, the positive electrode active material may include, for example, a Mn-rich-based active material having a chemical structure or crystal structure represented by Chemical Formula 3, a Li rich layered oxide (LLO) / Over Lithiated Oxide (OLO)-based active material, and a Co-less-based active material.

[0094] [Chemical Formula 3]

[0095] p[Li2MnO3]·(1-p)[Li q JO2]

[0096] In Chemical Formula 3, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0097] For example, a solvent and the positive electrode active material can be mixed to prepare a positive electrode slurry. The positive electrode slurry can be coated onto the positive electrode current collector 310 and then dried and calendered to produce the positive electrode active material layer 320. The coating process can be performed using methods such as gravure coating, slot die coating, multilayer simultaneous die coating, embossing, doctor blade coating, dip coating, bar coating, and casting, and is not limited to these methods. The positive electrode active material layer may further contain a binder and may further contain conductive materials, thickeners, etc.

[0098] Non-limiting examples of solvents used in the preparation of the positive electrode slurry include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0099] The adhesive may include polyvinylidene fluoride (PVDF), vinylidene fluoride-co-hexafluoropropylene copolymer (poly(vinylidene fluoride-co-hexafluoropropylene)), polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. In one embodiment, the positive electrode adhesive may be a PVDF-based adhesive.

[0100] The conductive material can be added to enhance the conductivity of the positive electrode active material layer and / or the mobility of lithium ions or electrons. For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), and carbon fiber, and / or metal-based conductive materials including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3, but is not limited thereto.

[0101] In some embodiments, the positive electrode slurry may further contain thickeners and / or dispersants. In one embodiment, the positive electrode slurry may contain a thickener such as carboxymethyl cellulose (CMC).

[0102] The lithium secondary battery includes an electrolyte. The electrolyte impregnates the negative electrode and the positive electrode, and can serve as a migration medium for lithium ions during battery charging and discharging.

[0103] In an exemplary embodiment, the electrolyte may comprise a lithium salt and a fluorine-based organic solvent. The lithium salt may include, for example, LiF, LiCl, LiBr, LiI, LiNO3, LiN(CN)2, LiBF4, LiClO4, LiPF6, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiCF3CF2SO3, Li(CF3SO2)2N, Li(FSO2)2N, LiCF3CF2(CF3)2CO, LiCF3C(CF3)2COO, Li(CF3SO2)2CH, Li(SF5)3C, Li(CF3SO2)3C, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, LiSCN, Li(CF3CF2SO2)2N, etc. These may be used alone or in combination of two or more.

[0104] In an exemplary embodiment, the molar concentration of the lithium salt in the electrolyte can be from 0.1M to 2M. In some embodiments, the molar concentration of the lithium salt in the electrolyte can be from 0.5M to 1.5M. Within these ranges, the battery can be charged and discharged smoothly without lithium deposition.

[0105] In an exemplary embodiment, the lithium salt may include a first lithium salt comprising LiPF6 and a second lithium salt comprising lithium bis(trifluoromethanesulfonyl)imide (Li(CF3SO2)2N, LiTFSI). When the lithium salt comprises both the first and second lithium salts, the power characteristics of the battery can be improved.

[0106] In an exemplary embodiment, the molar concentration ratio of the first lithium salt to the second lithium salt in the electrolyte can be greater than 1 and less than 3. In some embodiments, the molar concentration ratio of the first lithium salt to the second lithium salt in the electrolyte can be from 1.2 to 2.

[0107] In an exemplary embodiment, the molar concentration of the first lithium salt can be from 0.5 M to 1.0 M. In some embodiments, the molar concentration of the first lithium salt can be from 0.6 M to 1.0 M.

[0108] In an exemplary embodiment, the molar concentration of the second lithium salt can be from 0.01M to 0.5M. In some embodiments, the molar concentration of the second lithium salt can be from 0.1M to 0.4M.

[0109] Within the aforementioned range, the power characteristics and lifespan characteristics of the battery can be improved.

[0110] In an exemplary embodiment, the fluorinated organic solvent may include linear fluorinated organic solvents and cyclic fluorinated organic solvents. "Fluorinated organic solvent" can refer to an organic solvent in which at least one hydrogen atom is replaced by fluorine.

[0111] In an exemplary embodiment, the linear fluorinated organic solvent may include at least one selected from fluorinated linear ether solvents, fluorinated linear carbonate solvents, and fluorinated linear ester solvents.

[0112] The fluorinated linear ether solvent may include, for example, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane, etc.

[0113] The fluorinated linear carbonate solvent may include, for example, methyl 2,2,2-trifluoroethyl carbonate, methyl trifluoromethyl carbonate, ethyl 2,2,2-trifluoroethyl carbonate, etc.

[0114] The fluorinated linear ester solvent may include, for example, 2,2-difluoroacetic acid ester, methyl difluoroacetate, ethyl difluoroacetate, ethyl 2,2-difluoroacetate, etc.

[0115] The examples of the above-mentioned linear fluorinated organic solvents can be used alone or in combination of two or more.

[0116] In an exemplary embodiment, the cyclic fluorinated organic solvent may include a fluorinated cyclic carbonate solvent. The fluorinated cyclic carbonate solvent may include, for example, fluoroethylene carbonate or difluoroethylene carbonate.

[0117] In an exemplary embodiment, the ratio of the content of the linear fluorinated organic solvent to the content of the cyclic fluorinated organic solvent in the total volume of the fluorinated organic solvent can be 3 to 6. In some embodiments, the ratio of the content of the linear fluorinated organic solvent to the content of the cyclic fluorinated organic solvent in the total volume of the fluorinated organic solvent can be 3 to 4.5.

[0118] Within the aforementioned range, with an appropriate ratio of cyclic solvents to linear solvents, the solubility of lithium salts can be sufficiently high, and byproducts with low chemical stability can be avoided during battery charging and discharging.

[0119] The electrolyte may further comprise a non-fluorinated organic solvent. The non-fluorinated organic solvent may include, for example, propylene carbonate (PC), ethylene carbonate (EC), butene carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, vinylene carbonate, methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), ethyl fluoroacetate (FEA), ethyl difluoroacetate (DFEA), ethyl trifluoroacetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), and diethylene glycol dimethyl ether. Dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, diethoxyethane, sulfolane, γ-butyrolactone, propylene sulfite, etc. These can be used alone or in combination of two or more.

[0120] The electrolyte may further contain additives.

[0121] The additives may include, for example, unsaturated cyclic carbonate compounds, sulfonyl lactone compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, and borate compounds.

[0122] The unsaturated cyclic carbonate-based compounds may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc.

[0123] The sulfonyl compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0124] The cyclic sulfate-based compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0125] The cyclic sulfite-based compound may include ethylene sulfite, butylene sulfite, etc.

[0126] The phosphate-based compounds may include lithium difluorobis-oxalato phosphate, lithium difluorophosphate, etc.

[0127] The borate-based compounds may include lithium bis(oxalate) borate, etc.

[0128] The embodiments of the present invention will be further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only for illustrating the present invention and are not intended to limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and the technical concept, which will be obvious to those skilled in the art, and these variations and modifications also fall within the scope of the claims.

[0129] Example 1

[0130] An electrolyte comprising a mixed solvent and 1 M of LiPF6 was prepared, wherein the mixed solvent comprises fluoroethylene carbonate and 2,2,3,3-tetrafluoro-1,4-dimethoxybutane in a volume ratio of 1:4.

[0131] A positive electrode slurry was prepared by adding 96 wt% lithium-nickel / cobalt / manganese oxide as the positive electrode active material, 2.0 wt% polyvinylidene fluoride (PVDF) as the binder, and 2.0 wt% Super-P as the conductive material to N-methylpyrrolidone (NMP). The positive electrode slurry was then uniformly coated onto an aluminum substrate with a thickness of 12 μm, dried, and calendered to manufacture the positive electrode.

[0132] The negative electrode uses a lithium foil with a thickness of 100μm.

[0133] The positive and negative electrodes are notched to predetermined sizes, and a porous polyethylene membrane (13 μm thick) is placed in the middle as a separator and then stacked to manufacture an electrode assembly.

[0134] The electrode assembly is placed in a soft package, and the three sides except for the electrolyte injection surface are sealed. At this time, the part with the tabs is included in the sealed part. The electrolyte is injected through the electrolyte injection surface, and the electrolyte injection surface is also sealed. Then, it is immersed for more than 12 hours to manufacture a 2000mAh lithium secondary battery.

[0135] The lithium secondary battery is charged at 0.5C and discharged at 0.5C to complete the formation charge and discharge process.

[0136] Example 2

[0137] The lithium secondary battery was manufactured using the same method as in Example 1, except that an electrolyte comprising a mixed solvent and 0.6 M of LiPF6 and 0.4 M of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was prepared and used, wherein the mixed solvent comprised fluoroethylene carbonate and methyl-2,2,2-trifluoroethyl carbonate in a volume ratio of 1:4.

[0138] Example 3

[0139] The lithium secondary battery was manufactured using the same method as in Example 1, except that an electrolyte comprising a mixed solvent and 0.6 M of LiPF6 and 0.4 M of LiTFSI was prepared and used, wherein the mixed solvent comprised fluoroethylene carbonate and 2,2,3,3-tetrafluoro-1,4-dimethoxybutane in a volume ratio of 1:4.

[0140] Example 4

[0141] The lithium secondary battery was manufactured using the same method as in Example 1, except that an electrolyte comprising a mixed solvent and 0.6 M of LiPF6 and 0.4 M of LiTFSI was prepared and used, wherein the mixed solvent comprised fluoroethylene carbonate and methyl-2,2,2-trifluoroethyl carbonate in a volume ratio of 25:75.

[0142] Comparative Example 1

[0143] The lithium secondary battery was manufactured using the same method as in Example 1, except that an electrolyte was prepared comprising 1 M LiPF6 dissolved in a mixed solvent containing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7, and 2% by weight of fluoroethylene carbonate.

[0144] Comparative Example 2

[0145] The lithium secondary battery was manufactured using the same method as in Example 1, except that an electrolyte containing 2,2,3,3-tetrafluoro-1,4-dimethoxybutane as a solvent and 1 M of LiPF6 was prepared and used.

[0146] Experimental Example

[0147] The lithium secondary batteries of the examples and comparative examples were evaluated using the following methods, and the results are shown in Table 1.

[0148] (1) XPS Analysis

[0149] The lithium secondary batteries of the examples and comparative examples were disassembled to confirm the formation of a solid electrolyte interphase (SEI) layer on the surface of the lithium foil negative electrode, and the SEI was analyzed by X-ray photoelectron spectroscopy. Peaks were confirmed to appear in the XPS spectra at 283.0 eV to 286.5 eV, 287.5 eV to 292.0 eV, and 682.0 eV to 686.5 eV. The proportions of the first and second peaks were calculated according to Equations 1 and 2, respectively, based on the intensity of each peak.

[0150] Figure 2 The XPS analysis results are for the SEI layer in Example 1. Specifically, from... Figure 2 The XPS analysis results for fluorine, carbon, and lithium are shown sequentially from the left, along with the chemical bonds originating from each peak and the peak positions. In each XPS analysis result, the solid line represents the sum of the curves represented by dashed lines, dotted lines, single-dash lines, or double-dash lines.

[0151] (2) Evaluation of capacity retention

[0152] The lithium secondary batteries of the examples and comparative examples were charged and discharged at 0.5C to measure the initial discharge capacity. Then, this charging and discharging cycle was repeated 100 times, and the discharge capacity of the 100th cycle was measured. The percentage of the 100th discharge capacity relative to the initial discharge capacity was calculated as the capacity retention rate.

[0153] [Table 1]

[0154]

[0155] Reference Figure 2 As shown in Table 1, the lithium secondary batteries of the embodiments have a first peak ratio of 1 to 7 and a second peak ratio of 3 to 15, which can maintain high capacity even during repeated charge and discharge cycles. Therefore, the lifespan characteristics of the lithium secondary batteries of the embodiments are improved.

[0156] The comparative example lithium secondary battery has a first peak ratio of less than 1 or greater than 7, or a second peak ratio of less than 3 or greater than 15. Therefore, compared with the lithium secondary battery of the embodiment, the comparative example lithium secondary battery has significantly worse life characteristics.

[0157] The above description is merely an example of applying the principles of this invention, and other configurations may be further included without departing from the scope of this invention.

Claims

1. A lithium secondary battery, wherein, The lithium secondary battery includes: The negative electrode includes a lithium metal layer and a solid electrolyte interphase (SEI) layer disposed on the lithium metal layer, and the negative electrode has a first peak ratio of 1 to 7 as defined by the following formula 1; Positive electrode, wherein the positive electrode and the negative electrode are disposed opposite to each other; and Electrolyte [Formula 1] R1=I F / I CC In Equation 1, R1 is the proportion of the first peak, I CC To determine the intensity of the peak in the range of 283.0 eV to 286.5 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the X-ray photoelectron spectroscopy (XPS).

2. The lithium secondary battery according to claim 1, wherein, The proportion of the first peak is 2 to 6.

3. The lithium secondary battery according to claim 1, wherein, The electrolyte contains lithium salt and fluorine-based organic solvent.

4. The lithium secondary battery according to claim 3, wherein, The fluorinated organic solvents include linear fluorinated organic solvents and cyclic fluorinated organic solvents.

5. The lithium secondary battery according to claim 4, wherein, In the total volume of the fluorinated organic solvent, the ratio of the content of the linear fluorinated organic solvent to the content of the cyclic fluorinated organic solvent is 3 to 6.

6. The lithium secondary battery according to claim 4, wherein, The linear fluorinated organic solvent includes at least one selected from fluorinated linear ether solvents, fluorinated linear carbonate solvents, and fluorinated linear ester solvents.

7. The lithium secondary battery according to claim 4, wherein, The cyclic fluorinated organic solvents include fluorinated cyclic carbonate solvents.

8. The lithium secondary battery according to claim 3, wherein, The lithium salt includes a first lithium salt containing LiPF6 and a second lithium salt containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

9. The lithium secondary battery according to claim 8, wherein, In the electrolyte, the ratio of the molar concentration of the first lithium salt to the molar concentration of the second lithium salt is greater than 1 and less than 3.

10. The lithium secondary battery according to claim 8, wherein, The molar concentration of the first lithium salt is from 0.5 M to 1.0 M, and the molar concentration of the second lithium salt is from 0.01 M to 0.5 M.

11. A lithium secondary battery, wherein, The lithium secondary battery includes: The negative electrode includes a lithium metal layer and a solid electrolyte interphase (SEI) layer disposed on the lithium metal layer, and the negative electrode has a second peak ratio of 3 to 15 as defined by the following formula 2; Positive electrode, wherein the positive electrode and the negative electrode are disposed opposite to each other; and Electrolyte [Equation 2] R2=I F / I CO In Equation 2, R2 is the proportion of the second peak, I CO To determine the intensity of the peak in the range of 287.5 eV to 292.0 eV in the X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interface phase layer, I F The intensity of the peak in the range of 682.0 eV to 686.5 eV in the X-ray photoelectron spectroscopy (XPS) analysis.

12. The lithium secondary battery according to claim 11, wherein, The proportion of the second peak is 5 to 10.

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