Electrolyte for lithium secondary battery, and lithium secondary battery comprising same

Abstract

The present invention relates to: an electrolyte for a lithium secondary battery, wherein the electrolyte can improve the ion conductivity, capacity characteristics, and lifespan characteristics of the lithium secondary battery; and a lithium secondary battery comprising same.

Description

Electrolyte for lithium secondary batteries and lithium secondary batteries including the same

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0129850 filed September 25, 2024 and Korean Patent Application No. 10-2025-0128429 filed September 9, 2025, and all contents disclosed in the documents of said Korean patent applications are incorporated herein as part of this specification.

[0003] The present invention relates to an electrolyte for a lithium secondary battery capable of improving the ion conductivity, capacity characteristics, and lifespan characteristics of a lithium secondary battery, and a lithium secondary battery including the same.

[0004] Lithium metal batteries using lithium metal as the anode are attracting attention as next-generation rechargeable batteries that can overcome the low capacity of conventional lithium-ion batteries. When these lithium metal batteries are charged and discharged at high speeds at high current densities, the lithium metal is stabilized, which can improve the battery's lifespan. However, when currently used carbonate-based electrolytes are charged and discharged at high speeds, lithium dendrites grow on the surface of the lithium metal, and a high-resistance solid electrolyte interphase (SEI) layer is formed due to corrosion, which causes a rapid decrease in the battery's lifespan.

[0005] Meanwhile, alkylnitrile-based electrolytes such as acetonitrile have high ionic conductivity and can significantly improve the mobility of lithium ions. Due to these characteristics, their use as electrolytes for lithium metal batteries has been considered, but there are limitations to their actual application due to the rapid corrosion reaction with lithium metal.

[0006] Accordingly, there is a need to develop an electrolyte that is applicable to lithium secondary batteries and can simultaneously secure high ionic conductivity and stable interfacial characteristics.

[0007] The present invention aims to provide an electrolyte for a lithium secondary battery capable of improving the ion conductivity, capacity characteristics, and lifespan characteristics of a lithium secondary battery, and a lithium secondary battery including the same.

[0008] The present invention provides an electrolyte for a lithium secondary battery comprising a lithium salt; an alkyl nitrile-based solvent; and a fluorine-substituted carbonate-based additive, wherein the volume ratio of the alkyl nitrile-based solvent to the fluorine-substituted carbonate-based additive (alkyl nitrile-based solvent : fluorine-substituted carbonate-based additive) is 4 : 1 to 19 : 1.

[0009] In addition, the present invention provides a lithium secondary battery comprising: a positive electrode including a positive active material layer; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte of the present invention.

[0010] The electrolyte of the present invention exhibits excellent ionic conductivity by including an alkylnitrile-based solvent.

[0011] In addition, the electrolyte of the present invention includes a lithium salt and a predetermined fluorine-substituted carbonate-based additive together with an alkylnitrile-based solvent, thereby exhibiting excellent oxidation stability and lithium metal corrosion prevention effects. Due to these characteristics, when the electrolyte is applied to a lithium secondary battery, for example, a lithium metal secondary battery, the capacity characteristics and lifespan characteristics of the battery can be improved.

[0012] Figure 1 shows the results of measuring ion conductivity for cells containing electrolytes of the examples and comparative examples.

[0013] Figure 2 shows the results of measuring the Coulomb efficiency for a cell containing the electrolyte of the example.

[0014] Figure 3 shows the results of evaluating the corrosion reactivity of lithium to the electrolytes of the examples and comparative examples.

[0015] Figures 4 and 5 show the results of measuring the interfacial resistance for cells containing the electrolytes of the examples and comparative examples.

[0016] Figure 6 shows the results of calculating the interfacial resistance increase rate for cells containing the electrolytes of the examples and comparative examples.

[0017] Figure 7 shows the results of evaluating oxidation stability for a cell containing the electrolyte of the example.

[0018] Figure 8 shows the results of the component analysis of the SEI layer of the cell containing the electrolyte of the example and comparative example.

[0019] Figures 9 and 10 show the results of evaluating the lifespan characteristics for cells containing the electrolytes of the examples and comparative examples.

[0020] Figures 11 to 14 show the results of evaluating the performance of cells containing electrolytes of the examples and comparative examples.

[0021] Hereinafter, an electrolyte for a lithium secondary battery according to a specific embodiment of the invention and a lithium secondary battery including the same will be described.

[0022] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0023] The terms used in this specification are used merely to describe exemplary embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0024] In this specification, terms such as “comprising,” “comprising,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should not be understood as precluding the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0025]

[0026] According to one embodiment of the present invention, an electrolyte for a lithium secondary battery is provided, comprising a lithium salt; an alkyl nitrile-based solvent; and a fluorine-substituted carbonate-based additive, wherein the volume ratio of the alkyl nitrile-based solvent to the fluorine-substituted carbonate-based additive (alkyl nitrile-based solvent : fluorine-substituted carbonate-based additive) is 4 : 1 to 19 : 1.

[0027] The electrolyte of this embodiment comprises a lithium salt and a fluorine-substituted carbonate-based additive together with an alkylnitrile-based solvent, thereby forming a stable SEI layer on the surface of the lithium metal. Accordingly, side reactions between the lithium metal and the electrolyte are suppressed, which can improve the lifespan characteristics of the lithium secondary battery.

[0028] In one embodiment, the volume ratio of the alkylnitrile-based solvent and the fluorine-substituted carbonate-based additive (alkylnitrile-based solvent : fluorine-substituted carbonate-based additive) may be 4:1 to 19:1, more specifically 4:1 to 15:1, and even more specifically 4:1 to 10:1. If the fluorine-substituted carbonate-based additive in the electrolyte is included in an amount exceeding the above range, an unstable SEI layer is formed on the surface of the lithium metal, causing a problem in which the interfacial resistance increases rapidly, and if it is included in an amount less than the above range, a corrosion reaction between the alkylnitrile-based solvent and the lithium metal occurs.

[0029] In an electrolyte according to one embodiment of the present invention, the lithium salt is used as a medium for transferring ions within a lithium secondary battery. The lithium salt is, for example, Li as a cation + It includes, and may include fluorine-containing anions as anions.

[0030] In one embodiment, the fluorine-containing anion may be a bis(fluorosulfonyl)imide (FSI) anion, a bis(trifluoromethanesulfonyl)imide (TFSI) anion, a hexafluorophosphate (PF6) anion, a tetrafluoroborate (BF4) anion, or a difluoro(oxalato)borate (DFOB) anion.

[0031] Specifically, the lithium salt may include one or more selected from the group consisting of LiFSI (Lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiTFSI (Lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2), LiPF6, LiBF4, and LiDFOB (Lithium difluoro(oxalato)borate, LiBF2(C2O4)).

[0032] In one embodiment, one or more types of lithium salts may be included as the lithium salt, and LiFSI may be included as an essential component to consider the performance of the lithium secondary battery. When LiFSI is included as the lithium salt in the electrolyte of the present invention, not only is the best ionic conductivity exhibited, but an inorganic-based SEI layer is formed on the lithium metal surface, thereby improving the oxidation stability and lifespan characteristics of the battery.

[0033] In another embodiment, the lithium salt comprises a first lithium salt and a second lithium salt, wherein the first lithium salt is LiFSI (Lithium bis(fluorosulfonyl) imide) or LiTFSI (Lithium bis(trifluoromethanesulfonyl) imide), and the second lithium salt may be LiDFOB (Lithium difluoro(oxalato)borate). When LiDFOB is included as the second lithium salt together with the first lithium salt, the corrosion reaction between the positive current collector and the electrolyte is suppressed under high voltage conditions, thereby improving the lifespan characteristics of the battery.

[0034] At this time, the molar ratio of the first lithium salt and the second lithium salt (first lithium salt : second lithium salt) may be 4 : 1 to 20 : 1, specifically 4 : 1 to 15 : 1, and more specifically 4 : 1 to 10 : 1.

[0035] In one embodiment of the present invention, the concentration of the lithium salt may be included in the electrolyte at a concentration of 0.5 to 1.2 M, taking into account the performance of the lithium secondary battery. This concentration of the lithium salt may be defined as a molar concentration considering the total volume of the alkylnitrile-based solvent and the fluorine-substituted carbonate-based additive. As the concentration of the lithium salt satisfies the above range, corrosion of the lithium metal can be suppressed, and accordingly, the performance of the lithium secondary battery can be improved.

[0036] The present invention may include an alkyl nitrile-based solvent as a solvent for the electrolyte. Acetonitrile, propionitrile, butyronitrile, isobutyronitrile, etc., may be used as the alkyl nitrile-based solvent. When the electrolyte of the present invention includes an alkyl nitrile-based solvent, specifically acetonitrile, as a solvent, it has the effect of lowering the viscosity of the electrolyte and improving ionic conductivity.

[0037] In one embodiment, the fluorine-substituted carbonate-based additive may be fluoroethylene carbonate (FEC). The alkylnitrile-based solvent used as the solvent of the present invention exhibits excellent ionic conductivity, but there is a problem in that the battery life is shortened due to a corrosive reaction with lithium metal. Meanwhile, when a fluorine-substituted carbonate-based additive, preferably FEC, is added as an additive, the FEC having low LUMO (Lowest Unoccupied Molecular Orbital) energy is reduced first to form an organic-based SEI layer on the lithium metal surface. This can suppress the growth of lithium dendrites and improve the battery life performance.

[0038] Meanwhile, according to another embodiment of the present invention, a lithium secondary battery comprising the electrolyte of the above-described embodiment is provided. The lithium secondary battery comprises a positive electrode comprising a positive electrode active material; a negative electrode; a separator between the positive electrode and the negative electrode; and the electrolyte of the above-described embodiment.

[0039] According to the above embodiment, the negative electrode may be in the form of a lithium metal secondary battery including a lithium metal layer.

[0040] First, in the lithium secondary battery of the other embodiment above, the negative electrode may have a lithium metal layer formed on one or both sides of a planar negative current collector according to the general configuration of the lithium secondary battery, and may be manufactured by depositing lithium metal on the negative current collector or by rolling a lithium foil.

[0041] The above negative current collector is a metal having high conductivity that does not cause chemical changes in the battery, and can be formed using any metal previously known to be usable as a negative current collector.

[0042] Specific examples of this include metals such as stainless steel, aluminum, nickel, titanium, or copper, or surfaces of copper, aluminum, or stainless steel treated with carbon, nickel, titanium, silver, etc. Such cathode current collectors can be formed in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0043] In addition, the negative current collector may have a thickness of 3 μm to 100 μm, and the lithium metal layer may have a thickness of, for example, 1 μm to 300 μm.

[0044] Meanwhile, the above anode may include an anode current collector and an anode active material layer located on the anode current collector.

[0045] Such anodes can be manufactured by preparing an anode slurry composition by mixing an anode active material and a binder, and in some cases, a conductive material, a filler, etc., in a solvent, and applying the composition to an anode current collector.

[0046] The above positive current collector can generally have a thickness of 3 μm to 500 μm. In addition, the above positive current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The current collector may also form fine irregularities on its surface to increase the adhesion of the positive active material, and various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics are possible.

[0047] In addition, the positive electrode active material may include lithium; and a lithium transition metal oxide comprising one or more transition metals selected from the group consisting of nickel, manganese, cobalt, and iron.

[0048] Specifically, the lithium transition metal oxide is a lithium-manganese oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt oxide (e.g., LiCoO2, etc.), a lithium-nickel oxide (e.g., LiNiO2, etc.), or a lithium-nickel-manganese oxide (e.g., LiNi 1-Y Mn Y O2(here, 0 <Y<1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2(here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2(here, 0 <Y2<1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r )O2(where, 0<p<1, 0<q<1, 0<r<1, p+q+r=1) or Li(Ni p1 Co q1 Mn r1 )O4(where, 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2), etc.), lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r2, and s2 are the atomic fractions of independent elements, respectively, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, p2 + q2 + r2 + s2 = 1), etc.), or lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b(Here, M is one or more selected from Al, Mg and Ti, and X is one or more selected from F, S and N, -0.5≤a≤+0.5, 0≤x≤0.5, 0≤b≤0.1) etc., and any one or more of these oxides may be included.

[0049] The above-described positive active material may be included in an amount of 60 to 99 weight%, or 70 to 99 weight%, or 80 to 98 weight% based on the total weight of the positive active material layer.

[0050] Meanwhile, the conductive material included in the above-mentioned positive electrode active material layer is a component intended to further improve the conductivity of the positive electrode active material. Such conductive material is not particularly limited as long as it possesses conductivity without causing chemical changes in the battery. For example, carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive nanomaterials such as carbon nanofibers or carbon nanotubes; fluorinated carbon powder; conductive powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used. Among these, the conductive material may include conductive nanomaterials such as carbon nanotubes or carbon nanofibers to further lower the resistance of the lithium metal secondary battery and further improve output characteristics.

[0051] Typically, the conductive material may be included in an amount of 1 to 20 weight%, or 1 to 15 weight%, or 1 to 10 weight% based on the total weight of the positive active material layer.

[0052] The binder optionally included in the above positive active material layer is a component that assists in the bonding of the positive active material and the conductive material, and in the bonding to the current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone (PVP), polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile-based rubber, styrene-butadiene rubber, or fluororubber, and a mixture or copolymer of two or more selected from these may also be used.

[0053] Typically, the binder may be included in an amount of 1 to 20 weight%, or 1 to 15 weight%, or 1 to 10 weight% based on the total weight of the positive active material layer.

[0054] In addition, a filler may be optionally added to the anode as a component that inhibits expansion. Such a filler is not particularly limited as long as it can inhibit the expansion of the electrode without causing chemical changes in the battery, and for example, olefin-based polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; etc. may be used.

[0055] The above-described anode can be manufactured by dispersing and mixing the anode active material, binder, and conductive material, etc., in a dispersion medium (solvent) to form a slurry, applying the slurry onto a metal current collector, and then drying and rolling. At this time, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof may be used as the dispersion medium, but are not necessarily limited thereto.

[0056] Meanwhile, the above-described lithium secondary battery may further include a porous separator interposed between the positive electrode and the negative electrode.

[0057] Such porous membranes may be used in the form of sheets, multilayer membranes, microporous films, woven fabrics, and nonwoven fabrics, using olefin-based polymers such as polyethylene (PE) and polypropylene (PP), glass fibers, etc., but are not necessarily limited thereto. However, it may be preferable to use porous polyethylene or porous glass fiber nonwoven fabric (glass filter) as the membrane, and it may be even more preferable to use porous glass filter (glass fiber nonwoven fabric) as the membrane. The membrane may be an insulating thin film having high ion permeability and mechanical strength, and the pore diameter of the membrane may generally be in the range of 0.01 μm to 10 μm, and the thickness may generally be in the range of 5 μm to 300 μm, but is not limited thereto.

[0058] Meanwhile, the above-described lithium secondary battery can be manufactured according to conventional methods in the field. For example, it can be manufactured by housing an electrode assembly including a positive electrode, a negative electrode, and a separator in a case and injecting and impregnating the above-described electrolyte.

[0059] These lithium secondary batteries can be applied to battery cells used as power sources for small devices, and are particularly suitable for use as unit cells in battery modules that serve as power sources for medium to large devices. Furthermore, considering the appropriate discharge rate for each application, the battery of the above-mentioned embodiment or another embodiment can be selectively used.

[0060]

[0061] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily practice the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0062]

[0063] <Experimental Example 1. Measurement of Ionic Conductivity and Coulomb Efficiency>

[0064] (1) Preparation of electrolyte

[0065] The electrolytes of Examples 1 to 5 were prepared by mixing acetonitrile and FEC in a volume ratio of 9:1 and dissolving a lithium salt of LiPF6, LiFSI, LiTFSI, LiDFOB, or LiBF4 at a concentration of 1M.

[0066] In addition, the electrolyte of Comparative Example 1 was prepared by mixing a solution of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio with FEC in a 9:1 volume ratio, and then dissolving the lithium salt of LiPF6 at a concentration of 1M.

[0067] (2) Ionic conductivity measurement

[0068] Stainless steel (area 1.5394cm² 2 Six ion conductivity cells were prepared by placing electrodes at intervals of 0.2 cm and injecting 0.4 mL of the electrolyte prepared above. While maintaining the ion conductivity cells at a constant temperature of 25 ℃, the impedance was measured using an AC impedance analyzer (EIS) in a frequency range from 0.5 MHz to 1 Hz, and the ion conductivity of each electrolyte was calculated. The calculated ion conductivity is shown in Fig. 1.

[0069] Referring to FIG. 1, it can be seen that the electrolytes of the present invention have superior ionic conductivity compared to the electrolyte of Comparative Example 1 (7.36 mS / cm) containing a carbonate-based solvent. In particular, when comparing Example 1 and Comparative Example 1 using the same lithium salt, it can be seen that the ionic conductivity of Example 1 is about 5 times higher than that of Comparative Example 1.

[0070] In addition, Example 2, which used LiFSI as the lithium salt, exhibited the highest ionic conductivity.

[0071] (3) Measurement of Coulomb efficiency in Examples 2 to 4

[0072] An evaluation cell was prepared by combining 80 μL of the electrolyte from Examples 2 to 4, Cu as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator, with a output of 5 mAh / cm² 2 Initial activation was performed by charging to the capacity. Subsequently, for the same electrode, 5mAh / cm 2 Electrodepositing lithium with a capacity of 1mAh / cm 2 The process of lithium detachment was repeated 10 times at a capacity of . Next, 0.5 mA / cm 2 Lithium was desorbed at a current density until the voltage reached 1V. At this time, the Coulombic efficiency was calculated by dividing the total capacity of the desorbed lithium by the total capacity of the electrodeposited lithium and multiplying by 100. The results are shown in Figure 2 and Table 1.

[0073] Classification Example 2 (LiFSI) Example 3 (LiTFSI) Example 4 (LiDFOB) CE (%) 98.8 98 91.38

[0074] Referring to Figure 2 and Table 1, it can be seen that Examples 2 to 4 all have a high Coulomb efficiency of 90% or more, and in particular, in the case of Example 2 using LiFSI as the lithium salt, it can be seen that the Coulomb efficiency is the highest.

[0075]

[0076] <Experimental Example 2. Measurement of Corrosion Reactivity 1>

[0077] (1) Preparation of electrolyte

[0078] Example 2: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0079] Example 6: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 0.5 M to prepare an electrolyte.

[0080] Comparative Example 2: An electrolyte was prepared using acetonitrile alone.

[0081] Comparative Example 3: An electrolyte was prepared by mixing acetonitrile and FEC in a volume ratio of 9:1.

[0082] (2) Measurement of corrosion reactivity

[0083] The electrolytes of Example 2, Example 6, Comparative Example 2, or Comparative Example 3 and lithium pieces were placed into a cylindrical container, and the lid was closed to seal it. After 24 hours, the degree of lithium corrosion was determined by checking the color change of the electrolyte, and this is shown in FIG. 3.

[0084] Referring to Fig. 3, it was confirmed that the electrolytes of Examples 2 and 6 remained in a transparent solution state even after 24 hours, whereas the electrolytes of Comparative Examples 2 and 3, which do not contain lithium salt or FEC, turned yellow due to lithium corrosion.

[0085]

[0086] <Experimental Example 3. Measurement of Corrosion Reactivity 2>

[0087] (1) Preparation of electrolyte

[0088] Example 2: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0089] Example 2-1: Acetonitrile and FEC were mixed in a volume ratio of 4:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0090] Comparative Example 2-1: Acetonitrile and FEC were mixed in a volume ratio of 7:3, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0091] (2) Manufacturing of symmetric cells

[0092] A symmetric cell was prepared by combining 60 μL of the electrolyte of Example 2, Example 2-1, or Comparative Example 2-1, 150 μm thick Li as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator.

[0093] (3) Measurement of corrosion reactivity

[0094] The impedance of the symmetric cell of Example 2, Example 2-1, or Comparative Example 2-1 was measured using an AC impedance analyzer (EIS) in a frequency range from 1 MHz to 0.1 Hz while maintaining the symmetric cell at a constant temperature of 25 ℃. The initial interface resistance of the cell and the interface resistance at 10 hours of operation were evaluated using this method, and the results are shown in FIGS. 4 and 5.

[0095] Referring to FIGS. 4 and 5, it can be seen that Comparative Example 2-1, in which the volume ratio of acetonitrile to FEC is 7:3, shows a rapid increase in interfacial resistance after 10 hours of cell operation compared to Example 2 and Example 2-1. This is judged to be the result of the formation of an unstable SEI layer on the lithium surface. From this, it is predicted that the electrolyte of the present invention, having a predetermined volume ratio of an alkylnitrile-based solvent and a fluorine-substituted carbonate-based additive, will exhibit excellent performance when applied to a battery.

[0096]

[0097] <Experimental Example 4. Measurement of Corrosion Reactivity 3 and Measurement of Lithium Ion Transition Number>

[0098] (1) Preparation of electrolyte

[0099] Example 7: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1.2 M to prepare an electrolyte.

[0100] Example 8: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiDFOB) was dissolved at a concentration of 1.2 M to prepare an electrolyte.

[0101] Example 9: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiTFSI) was dissolved at a concentration of 1.2 M to prepare an electrolyte.

[0102] Comparative Example 4: An electrolyte was prepared by mixing a solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) with FEC in a volume ratio of 9:1 and dissolving the lithium salt of LiPF6 at a concentration of 1.2M.

[0103] (2) Manufacturing of symmetric cells

[0104] A symmetric cell was prepared by combining 50 μL of the electrolyte of Example 7, Example 8, Example 9 or Comparative Example 4, 150 μm thick Li as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator.

[0105] (3) Measurement of corrosion reactivity

[0106] The impedance of the symmetric cells of Examples 7, 8, and Comparative Example 4 was measured using an AC impedance analyzer (EIS) in the frequency range from 0.5 MHz to 1 Hz while maintaining the temperature at a constant 25 ℃. Through this, the SEI layer interface resistance at 1 hour and 100 hours of operation of the symmetric cells was calculated, respectively, and the interface resistance increase rate (R) was used. film The increase rate, %) was calculated. The calculated increase rate of interfacial resistance is shown in Fig. 6.

[0107] [Calculation Formula]

[0108]

[0109] (R film ,1h is the SEI layer interface resistance value after 1 hour of symmetric cell operation, and Rfilm ,100h refers to the SEI layer interface resistance value after 100 hours of symmetric cell operation.)

[0110] Referring to Fig. 6, it can be seen that the interfacial resistance of the SEI layer increases sharply in the electrolyte of Comparative Example 4 containing a carbonate-based solvent compared to the electrolytes of Examples 7 and 8.

[0111] (4) Measurement of lithium ion transition number

[0112] For the symmetric cells of Examples 7, 8, 9 and Comparative Example 4, the current was measured at 0.1-second intervals for 30 minutes while a voltage of 10 mV (0.01 V) was applied. At this time, the initial current value was defined as I (0), and the average value of the current measured during the 300 seconds prior to the end of the measurement was defined as I (ss). In addition, the interface resistance before the voltage was applied was defined as R (0), and the interface resistance after 30 minutes had elapsed after the voltage was applied was defined as R (ss). Based on these measured values, the lithium-ion transfer number was calculated according to Equation A below.

[0113] [Essence A]

[0114] Lithium-ion transition number = [I(ss) × (Voltage(0.01V)-I(0)R(0))] / [I0 × (Voltage(0.01V)-I(ss)R(ss))]

[0115] Classification Example 7 (LiFSI) Example 8 (LiDFOB) Example 9 (LiTFSI) Comparative Example 4 Lithium ion transition number 0.8 10.5 7 40.6 5 10.31

[0116] Referring to Table 2, it can be seen that the lithium ion transfer number of the electrolyte in the present embodiment is about twice as high as that of Comparative Example 4, which used a carbonate-based solvent. In a lithium secondary battery, the higher the lithium ion transfer number, the more efficient the energy transfer becomes. If the lithium ion transfer number is low, the proportion of current carried by anions increases, which can cause chemical reactions inside the battery to become unstable, and consequently, the battery's lifespan and stability may decrease.

[0117]

[0118] <Experimental Example 5. Measurement of Oxidation Stability of Examples 7 to 9>

[0119] An evaluation cell was prepared by combining 60 μL of the electrolytes from Examples 7 to 9, carbon-coated aluminum as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator, and the oxidation stability of each electrolyte was evaluated (scan rate: 0.1 mV / s) and is shown in Fig. 7.

[0120] Referring to FIG. 7, Example 7 (4.71 V vs. Li / Li + ), Example 8 (4.76 V vs. Li / Li + ) and Example 9 (4.97 V vs. Li / Li + The oxidation initiation voltage is 4.5 V for both Li / Li + Therefore, it can be confirmed that it possesses high oxidation stability. This high oxidation stability indicates that it can be reliably applied even to anodes requiring high voltage.

[0121]

[0122] <Experimental Example 6. Analysis of SEI Layer Components>

[0123] (1) Preparation of electrolyte

[0124] Example 2: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0125] Comparative Example 3: An electrolyte was prepared by mixing acetonitrile and FEC in a volume ratio of 9:1.

[0126] (2) SEI layer component analysis

[0127] A symmetric cell was prepared by combining 50 μL of the electrolyte of Example 2 or Comparative Example 3, a 150 μm thick Li working electrode, a 150 μm thick Li counter electrode, and a PP separator. After assembling the cell, it was stabilized in a resting state for 10 hours, and then the composition of the SEI layer was analyzed using XPS (X-ray Photoelectron Spectroscopy), and the results are shown in Fig. 8.

[0128] Referring to Fig. 8, it can be seen that Comparative Example 3, which does not contain lithium salt, shows a higher proportion of N component compared to Example 2. This is predicted to be the result of the corrosion reaction between lithium metal and acetonitrile. Additionally, it can be seen that Example 2, which contains lithium salt and FEC, shows a lower proportion of N component. This is predicted to be the result of a stable SEI layer being formed on the lithium surface by the FEC decomposition reaction before the reaction between lithium metal and acetonitrile.

[0129]

[0130] <Experimental Example 7. Evaluation of Life Characteristics>

[0131] (1) Preparation of electrolyte

[0132] Example 2: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiFSI) was dissolved at a concentration of 1M to prepare an electrolyte.

[0133] Example 3: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and a lithium salt (LiTFSI) was dissolved at a concentration of 1 M to prepare an electrolyte.

[0134] Example 10: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and an electrolyte was prepared by dissolving LiFSI at a concentration of 0.9 M and LiDFOB at a concentration of 0.1 M as lithium salts.

[0135] Comparative Example 1: An electrolyte was prepared by mixing a solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) with FEC in a volume ratio of 9:1 and dissolving the lithium salt of LiPF6 at a concentration of 1M.

[0136] (2) Life Characteristics Evaluation 1

[0137] A symmetric cell was prepared by combining 50 μL of the electrolyte of Example 2, Example 3, or Comparative Example 1, 150 μm thick Li as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator.

[0138] The lifetime characteristics of the fabricated symmetric cell were evaluated, and the results are shown in FIG. 9 and Table 3. Here, the current density of the symmetric cell is 1 mA / cm² 2 , capacity is 1mAh / cm 2 Authorized it.

[0139] Ionic conductivity [mS·cm²] -1 ] Overvoltage η [mV] Cycles [h] Example 2 36.9 12.5500 Example 3 31.2 25.8 302 Comparative Example 17.36 34.5 176

[0140] Referring to FIG. 9 and Table 3, it can be observed that the lifespan of the symmetric cell and the overvoltage are improved when using the electrolytes of Examples 2 and 3. (2) Lifespan characteristic evaluation 2

[0141] A symmetric cell was prepared by combining 60 μL of the electrolyte of Example 2 or Example 10, 150 μm thick Li as the working electrode, 150 μm thick Li as the counter electrode, and PP as the separator.

[0142] The lifetime characteristics of the fabricated symmetric cell were evaluated, and the results are shown in FIG. 10. Here, the current density of the symmetric cell is 1 mA / cm² 2 , capacity is 1mAh / cm 2 Authorized it.

[0143] Referring to Fig. 10, it can be seen that the lifespan of the symmetric cell is excellent when using the electrolytes of Example 2 and Example 10, and in particular, when using the electrolyte of Example 10 containing LiFSI and LiDFOB as lithium salts, it can be seen that the lifespan characteristics are even better than those of Example 2.

[0144]

[0145] <Experimental Example 8. Cell Performance Evaluation>

[0146] (1) Preparation of electrolyte

[0147] Example 10: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and an electrolyte was prepared by dissolving LiFSI at a concentration of 0.9 M and LiDFOB at a concentration of 0.1 M as lithium salts.

[0148] Example 11: Acetonitrile and FEC were mixed in a volume ratio of 9:1, and an electrolyte was prepared by dissolving LiFSI at a concentration of 1 M and LiDFOB at a concentration of 0.05 M as lithium salts.

[0149] Comparative Example 1: An electrolyte was prepared by mixing a solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) with FEC in a volume ratio of 9:1 and dissolving the lithium salt of LiPF6 at a concentration of 1M.

[0150] Comparative Example 5: An electrolyte was prepared by dissolving LiFSI as a lithium salt in dimethoxyethane (DME) at a concentration of 1 M.

[0151] Comparative Example 6: Diethyl carbonate (DEC) and FEC were mixed in a volume ratio of 2:1, and an electrolyte was prepared by dissolving 0.6 M of lithium salt and 0.6 M of LiBF4.

[0152] Comparative Example 7: Dimethoxyethane (DME) and FEC were mixed in a volume ratio of 9:1, and an electrolyte was prepared by dissolving LiFSI at a concentration of 0.9 M and LiDFOB at a concentration of 0.1 M as lithium salts.

[0153] Comparative Example 8: Dimethyl carbonate (DMC) and FEC were mixed in a volume ratio of 9:1, and an electrolyte was prepared by dissolving LiFSI at a concentration of 0.9 M and LiDFOB at a concentration of 0.1 M as lithium salts.

[0154] (2) Cell Performance Evaluation 1

[0155] 60 μL of the electrolyte of Example 11, Comparative Example 1, Comparative Example 5, or Comparative Example 6, and NCM811 (LiNi) as the working electrode 0·8 Co 0·1 Mn 0·1 A coin cell was fabricated by combining O2, a 40㎛ thick Li counter electrode, and a PP separator. The area capacity of the NCM811 is 2mAh / cm². 2 , The cathode capacity / anode capacity ratio (N / P) was set to 4.

[0156] For manufactured coin cells, 3.0-4.2V, 0.1C cutoff (0.2 mA / cm²) 2 After performing initial charging and discharging under the conditions, the performance of the cell was evaluated, and the measurement results are shown in FIGS. 11 to 13. FIG. 12 shows the results of measuring the charging and discharging performance at a speed of 1C, and FIG. 13 shows the results of measuring the charging and discharging performance at a speed of 2C.

[0157] Referring to Fig. 11, as a result of measuring the discharge capacity while increasing the cell driving speed for 30 cycles, it can be confirmed that the highest discharge capacity is exhibited at all speeds when using the electrolyte of Example 11. Through this, it can be confirmed that using the electrolyte of the present invention exhibits superior performance compared to electrolytes using other solvents.

[0158] Referring to Figures 12 and 13, it was confirmed that when using the electrolyte of Example 11, high discharge capacity and high capacity retention rate were achieved over 300 cycles. Additionally, when using the electrolyte of Example 11, capacity retention rates of 81.3% after 200 cycles and 73.8% after 300 cycles were observed for 1C, while capacity retention rates of 77% after 300 cycles were observed for 2C.

[0159] This suggests that the electrolyte of Example 11 has high ionic conductivity and forms a structurally / chemically stable SEI, thereby suppressing unnecessary side reactions and preventing the manifestation of overpotential.

[0160] (3) Cell Performance Evaluation 2

[0161] A coin cell was prepared using the same method as in Cell Performance Evaluation 1, except that 50 μL of the electrolyte from Example 10, Comparative Example 1, Comparative Example 5, Comparative Example 7, or Comparative Example 8 was used. For the prepared coin cell, 3.0-4.2 V, 0.1 C cutoff (0.2 mA / cm²) 2 After performing initial charging and discharging under the condition, the charging and discharging performance at a speed of 4C was measured, and the results are shown in Fig. 14.

[0162] Referring to Fig. 14, it was confirmed that Example 10, which used acetonitrile as a solvent, had a higher discharge capacity and a higher capacity retention rate when applied to a battery compared to the electrolytes of Comparative Examples 1, 5, 7, and 8, which did not contain alkylnitrile-based solvents. In addition, when using the electrolyte of Example 10, it was confirmed that it showed excellent values, with a capacity retention rate of 92.5% after 100 cycles, 82.1% after 200 cycles, and 72.5% after 300 cycles.

[0163] Similar to the experimental results of Example 11, the electrolyte of Example 10 also has high ionic conductivity and forms a structurally and chemically stable SEI, thereby suppressing unnecessary side reactions and predicting that overpotential generation will hardly occur.

Claims

1. Lithium salt; alkyl nitrile-based solvent; and It includes a fluorine-substituted carbonate-based additive, An electrolyte for a lithium secondary battery in which the volume ratio of the alkylnitrile-based solvent and the fluorine-substituted carbonate-based additive (alkylnitrile-based solvent : fluorine-substituted carbonate-based additive) is 4 : 1 to 19 :

1.

2. In Claim 1, The above lithium salt is an electrolyte for a lithium secondary battery that contains a fluorine-containing anion.

3. In Claim 2, The above fluorine-containing anion is a bis(fluorosulfonyl)imide (FSI) anion, a bis(trifluoromethanesulfonyl)imide (TFSI) anion, a hexafluorophosphate (PF6) anion, a tetrafluoroborate (BF4) anion, or a difluoro(oxalato)borate (DFOB) anion, in an electrolyte for a lithium secondary battery.

4. In Claim 1, The above lithium salt includes a first lithium salt and a second lithium salt, and The first lithium salt is LiFSI (Lithium bis(fluorosulfonyl) imide) or LiTFSI (Lithium bis(trifluoromethanesulfonyl) imide), and The above second lithium salt is an electrolyte for a lithium secondary battery, wherein the second lithium salt is LiDFOB (Lithium difluoro(oxalato)borate).

5. In Claim 4, An electrolyte for a lithium secondary battery in which the molar ratio of the first lithium salt and the second lithium salt (first lithium salt : second lithium salt) is 4 : 1 to 20 :

1.

6. In Claim 1, The above lithium salt is an electrolyte for a lithium secondary battery containing a molar concentration of 0.5 M to 1.2 M.

7. In Claim 1, The above alkylnitrile-based solvent is an electrolyte for a lithium secondary battery, wherein the alkylnitrile-based solvent is an acetonitrile solvent.

8. In Claim 1, The above-mentioned fluorine-substituted carbonate-based additive is an electrolyte for a lithium secondary battery, wherein the fluorine-substituted carbonate-based additive is fluoroethylene carbonate (FEC).

9. Anode comprising a positive active material layer; cathode; A separator disposed between the anode and the cathode; and A lithium secondary battery comprising the electrolyte of any one of claims 1 to 8.

10. In Claim 9, The above negative electrode is a lithium secondary battery comprising a lithium metal layer.

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