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

Abstract

The present invention relates to an electrolyte for lithium secondary batteries and a lithium secondary battery containing the same, which can improve the ionic conductivity and lifespan characteristics of lithium secondary batteries.

Description

Technical Field

[0001] [Cross - reference to Related Applications] This application claims the benefit of priority based on Korean Patent Application No. 10 - 2023 - 0148871 filed on November 1, 2023 and Korean Patent Application No. 10 - 2024 - 0140522 filed on October 15, 2024, and all the contents disclosed in the documents of the Korean patent applications are included as part of this specification.

[0002] The present invention relates to an electrolyte for a lithium secondary battery capable of improving the ionic conductivity and life characteristics of the lithium secondary battery, and a lithium secondary battery including the same.

Background Art

[0003] Lithium metal batteries using lithium metal as the negative electrode have attracted attention as next - generation batteries that overcome the low capacity of existing lithium - ion batteries. However, the currently used ether - based electrolytes have limited positive - electrode stability due to their low oxidation stability. Therefore, 2,2,3,3 - tetrafluoro - 1,4 - dimethoxybutane (FDMB) having high oxidation stability has been studied as an electrolyte material. However, FDMB also has a limitation in that its ionic conductivity is low due to its high viscosity. Therefore, the development of an electrolyte in which both ionic conductivity and oxidation stability are ensured by a combination with other solvents or the like is required.

[0004] On the other hand, in recent years, in order to improve the ionic conductivity and charge - discharge rate of lithium secondary batteries such as lithium metal batteries, there has been a significant increase in interest in so - called localized high - concentration electrolytes (LHCE) that contain an organic non - solvent that exhibits relatively low solubility with respect to lithium salts in the electrolyte.

[0005] The more lithium salt anion-derived components a solid electrolyte interface (SEI) layer contains, the more stable the film becomes in terms of decomposition. However, when an organic non-solvent is used in the electrolyte, the organic non-solvent does not participate in the solvation structure of Li ions, and anions are more involved, resulting in the formation of a large amount of anion-derived SEI.

[0006] However, in the case of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which is the most commonly used organic nonsolvent, it does not participate in the solvation structure, so although the SEI contains many anions, its high viscosity results in low ionic conductivity.

[0007] Therefore, there is a continuing need for the development of electrolytes that can improve the ionic conductivity and lifespan characteristics of lithium secondary batteries, such as lithium metal secondary batteries to which the aforementioned LHCE is applied. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Therefore, the present invention provides an electrolyte for lithium secondary batteries and a lithium secondary battery containing the same, which can improve the ionic conductivity and lifespan characteristics of lithium secondary batteries. [Means for solving the problem]

[0009] Therefore, the present invention relates to a lithium salt and Non-aqueous organic solvents containing fluorine-substituted ether-based solvents, The organic anti-solvent comprises an organic anti-solvent that exhibits solubility in the lithium salt that is 10 times or less than that of the non-aqueous organic solvent, The aforementioned organic non-solvent provides an electrolyte for lithium secondary batteries containing a compound represented by the following chemical formula 1.

[0010] [ka]

[0011] In the aforementioned chemical formula 1, Ra~Rj are either the same as or different from each other, and each is independently a hydrogen or fluoro group, with five or more of the Ra~Rj being fluoro groups.

[0012] The present invention also provides a lithium secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, a separation membrane between the positive and negative electrodes, and the electrolyte of the present invention.

[0013] In such a lithium secondary battery, the negative electrode may be in the form of a lithium metal secondary battery including a lithium metal layer. [Effects of the Invention]

[0014] The electrolyte of the present invention contains a fluorine-substituted ether solvent such as FDMB as a non-aqueous organic solvent, and a compound represented by chemical formula 1 as an organic non-solvent. It has been confirmed that by combining such a non-aqueous organic solvent and organic non-solvent, the viscosity can be reduced and the ionic conductivity can be improved in LHCE-based electrolytes. This is predicted to be because the compound represented by chemical formula 1, unlike TTE which is a typical organic non-solvent, can participate in the solvation process of lithium salts.

[0015] Furthermore, it was confirmed that the electrolyte exhibits excellent oxidation stability, and that by using this electrolyte in lithium secondary batteries, such as lithium metal secondary batteries, the battery's lifespan characteristics and cathode stability can be improved, thereby inducing effective SEI and cathode electrolyte interface (CEI) formation and improving battery performance. [Brief explanation of the drawing]

[0016] [Figure 1]To analyze the solvation structure of the electrolytes in the examples and comparative examples, the results of NMR analysis of changes in chemical species in the electrolytes and a schematic diagram of the solvation structure predicted from these results are shown. [Figure 2] The lifetime characteristics of lithium symmetric cells containing the electrolytes of the examples and comparative examples were evaluated under different current densities, and the results are shown. [Figure 3] The lifetime characteristics of lithium symmetric cells containing the electrolytes of the examples and comparative examples were evaluated under different current densities, and the results are shown. [Figure 4] The results of measuring the resistance of the SEI film over time for lithium symmetric cells containing the electrolytes of the examples and comparative examples are shown. [Figure 5] The results of measuring the amount of inactivated lithium in lithium symmetric cells containing the electrolytes of the examples and comparative examples are shown. [Figure 6] The results of evaluating the resistance of the SEI film for each cycle in lithium symmetric cells containing the electrolytes of the examples and comparative examples are shown. [Figure 7] The results of electron microscope analysis of SEI films formed after 50 cycles on lithium symmetric cells containing the electrolytes of the examples and comparative examples are shown. [Figure 8] Figure 7 shows the thickness of the SEI film measured from the results. [Figure 9] The results of analyzing the components of the SEI film formed after 50 cycles for lithium symmetric cells containing the electrolytes of the examples and comparative examples are shown. [Figure 10] The results of evaluating the oxidative stability of each electrolyte using evaluation cells containing the electrolytes of the examples and comparative examples are shown. [Figure 11a] The results of evaluating these cycle characteristics using full cells containing the electrolytes of the examples and comparative examples are shown. [Figure 11b] The results of evaluating these cycle characteristics using full cells containing the electrolytes of the examples and comparative examples are shown. [Figure 11c]The results of evaluating these cycle characteristics using full cells containing the electrolytes of the examples and comparative examples are shown. [Figure 12] The results of electron microscopy analysis of cracks generated within the cathode particles after 20 cycles in full cells containing the electrolytes of the examples and comparative examples are shown. [Figure 13] The CEI films formed after 20 cycles of full cells containing the electrolytes of the examples and comparative examples were analyzed using an electron microscope, and the thickness of the CEI film measured from this analysis is shown. [Figure 14] The results of evaluating these cycle characteristics using pouch cells containing the electrolytes of the examples and comparative examples are shown. [Figure 15] The results of evaluating the cycle characteristics of the examples and comparative examples using full cells containing the electrolyte and LFP cathode material are shown. [Modes for carrying out the invention]

[0017] The following describes an electrolyte for a lithium secondary battery and a lithium secondary battery containing the same, according to specific embodiments of the invention.

[0018] The terms and words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather should be interpreted as meanings and concepts consistent with the technical spirit of the present invention, based on the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0019] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0020] In this specification, "fluoro group" means -F.

[0021] In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the presence of implemented features, figures, stages, components, or combinations thereof, and should be understood not to preemptively exclude the presence or possibility of adding one or more other features, figures, stages, components, or combinations thereof.

[0022] According to one embodiment of the invention, a lithium salt and Non-aqueous organic solvents containing fluorine-substituted ether-based solvents, The organic anti-solvent comprises an organic anti-solvent that exhibits solubility in the lithium salt that is 10 times or less than that of the non-aqueous organic solvent, The aforementioned organic non-solvent is an electrolyte for a lithium secondary battery comprising a compound represented by the following chemical formula 1.

[0023] [ka]

[0024] In the aforementioned chemical formula 1, Ra~Rj are either the same as or different from each other, and each is independently a hydrogen or fluoro group, with five or more of the Ra~Rj being fluoro groups.

[0025] Such an electrolyte in one embodiment can belong to the category of Localized High-Concentration Electrolytes (LHCE), and can exhibit a configuration in which a high-concentration region in which lithium salt is dissolved at a high concentration in a non-aqueous organic solvent during battery charging and discharging, and the organic non-solvent is distributed around this high-concentration region. Therefore, when using such an electrolyte, the output characteristics of the battery can be improved due to the high-concentration distribution of the lithium salt. Furthermore, the distribution region of the organic non-solvent can suppress side reactions between the lithium metal layer and the electrolyte, thereby improving the life characteristics of the lithium secondary battery.

[0026] On the one hand, in the electrolyte of the above embodiment, the lithium salt is used as a medium for conducting ions in a lithium secondary battery. The lithium salt, for example, has Li as a cation - , - , - , - , , - , - , - , - , , - , - , - , - , - , 10 , - , - , - , - , 10 , - , - , - , - , - , - , - , - , - , - , - , - ,

[0027] , - , - , - , - , 10 , - , 10 , - and contains F - ,Cl - ,Br - ,I - ,NO3 - ,N(CN)2 - ,BF4 - ,ClO4 - ,B 10 Cl 10 - ,AlCl4 - ,AlO2 - ,PF6 - ,CF3SO3 - ,CH3CO2 - ,CF3CO2 - ,AsF6 - ,SbF6 - ,CH3SO3 - ,(CF3CF2SO2)2N - ,(CF3SO2)2N - ,(FSO2)2N - ,BF2C2O4 - ,BC4O8 - ,PF4C2O4 - ,PF2C4O8 - ,(CF3)2PF4 - ,(CF3)3PF3 - ,(CF3)4PF2 - ,(CF3)5PF - ,(CF3)6P - ,C4F9SO3 - ,CF3CF2SO3 - ,CF3CF2(CF3)2CO - ,(CF3SO2)2CH - ,CF3(CF2)7SO3 - and can contain together an anion selected from the group consisting of SCN - . <00,00224><00,00225><00,00226>Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB​​​It can contain one or more selected from the group consisting of LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (Lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiBETI (Lithium bis(perfluoroethanesulfonyl) imide, LiN(SO2CF2CF3)2), and LiTFSI (Lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2), and LiFSI or LiTFSI can be appropriately used considering the performance of the lithium secondary battery and the solubility in the non-aqueous organic solvent and non-organic solvent.

[0028] The concentration of the lithium salt can be included in the electrolyte at a concentration of 1.0 M to 2.5 M, or 1.0 M to 2.0 M, taking into consideration the performance of the lithium secondary battery. Such a concentration of lithium salt can be defined as a molar concentration considering the total volume of the non-aqueous organic solvent and the non-organic solvent. A higher concentration of the lithium salt can further accelerate the desolvation of lithium ions, thereby improving the performance of the lithium secondary battery.

[0029] Furthermore, the fluorine-substituted ether solvent may include 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) or 2,2,3,3-tetrafluoro-1,4-diethoxybutane (FDEB), and in addition to such fluorine-substituted ether solvents, the non-aqueous organic solvent may further contain additional ether solvents and / or carbonate solvents.

[0030] In addition to the fluorine-substituted ether solvents mentioned above, other ether solvents that may be additionally included in the non-aqueous organic solvent may be aliphatic ether solvents, such as dimethyl ether, dibutyl ether, tetraglyme, diglyme, or dimethoxyethane.

[0031] In addition to the fluorine-substituted ether solvents mentioned above, carbonate solvents that may be additionally included in the non-aqueous organic solvent include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, or methyl (2,2,2-trifluoroethyl) carbonate.

[0032] The electrolyte of the above embodiment includes a non-aqueous organic solvent that dissolves the lithium salt and acts as a transport pathway for lithium ions, along with an organic non-solvent that exhibits solubility of 10 times or more, or 10 to 30 times less, than that of the non-aqueous organic solvent, and substantially does not dissolve the lithium salt. Such an organic non-solvent may be defined as having substantially no solubility for the lithium salt and being able to dissolve the lithium salt only at concentrations of 0.1 M or less, or 0 M to 0.1 M, or 0 M to 0.05 M.

[0033] The aforementioned organic non-solvent contains a compound represented by chemical formula 1. The compound represented by chemical formula 1 has a basic structure of diethyl ether and is substituted with five or more fluoro groups. When a compound having the structure of chemical formula 1 is included in the organic non-solvent, it has the effect of lowering the viscosity of the electrolyte and improving the ionic conductivity compared to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which is the most commonly used organic non-solvent. The reason for this is that TTE does not participate in the solvation structure of lithium salts, but the compound of chemical formula 1, unlike TTE, can participate in the solvation process of lithium salts.

[0034] In the aforementioned chemical formula 1, Ra to Rj each represent a substituent.

[0035] According to one embodiment of the present invention, five or six of the Ra~Rj groups may be fluoro groups, and the rest may be hydrogen atoms.

[0036] Furthermore, in the above chemical formula 1, five or six of Ra~Rc and Rh~Rj are fluorogroups, and the remaining substituents that are not fluorogroups among Ra~Rc and Rh~Rj, as well as Rd~Rg, may be hydrogen atoms.

[0037] According to yet another embodiment of the present invention, the compound represented by chemical formula 1 may be bis(2,2,2-trifluoroethyl) ether (BTFE), and when the electrolyte of the present invention contains the compound of chemical formula 1, particularly BTFE, as an organic nonsolvent, it has the effect of lowering the viscosity of the electrolyte and improving ionic conductivity.

[0038] An electrolyte of one embodiment further comprising the non-aqueous organic solvent and the organic non-solvent may include a region in the non-aqueous organic solvent where a locally high concentration of lithium salt exists, as well as a region where the lithium salt is substantially absent from the organic non-solvent distribution. In this way, the presence of a locally high concentration of lithium salt in a solvated form within the electrolyte can further improve the performance of the lithium secondary battery, such as its output characteristics, while the organic non-solvent distribution region can reduce the increase in electrolyte viscosity and the decrease in fluidity.

[0039] In the electrolyte of the above embodiment, the amount of the organic non-solvent can be adjusted depending on the type of non-aqueous organic solvent and lithium salt, and the overall concentration of the lithium salt. For example, the organic non-solvent to the non-aqueous organic solvent may be present in a volume ratio of 1:0.5 to 1:1.5, or 1:1.

[0040] According to one embodiment of the invention, the electrolyte may further include additives such as lithium nitrate (LiNO3), lithium difluorooxalate phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis-oxalate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), or fluoroethylene carbonate (FEC).

[0041] On the other hand, according to another embodiment of the invention, a lithium secondary battery is provided that includes the electrolyte of the above-described embodiment. Such a lithium secondary battery includes a positive electrode containing a positive electrode active material, a negative electrode, a separation membrane between the positive electrode and the negative electrode, and the electrolyte of the above-described embodiment.

[0042] According to the above embodiment, the negative electrode can include a lithium metal layer and take the form of a lithium metal secondary battery.

[0043] First, in the lithium secondary battery of the other embodiment described above, the negative electrode may be a lithium metal layer formed on one or both sides of a planar negative electrode current collector, according to the general configuration of a lithium secondary battery, and can be manufactured by depositing lithium metal onto the negative electrode current collector or by rolling lithium foil.

[0044] The negative electrode current collector is a metal that has high conductivity without inducing a chemical change in the battery, and can be formed using any metal that has been conventionally known to be usable as a negative electrode current collector.

[0045] Specific examples include metals such as stainless steel, aluminum, nickel, titanium, or copper, or copper, aluminum, or stainless steel with surface treatments such as carbon, nickel, titanium, or silver. Such negative electrode current collectors can be formed in a variety of forms, such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0046] Furthermore, the negative electrode current collector can have a thickness of 3 μm to 100 μm, and the lithium metal layer can have a thickness of, for example, 1 μm to 300 μm.

[0047] On the other hand, the positive electrode may include a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector.

[0048] Such a positive electrode can be manufactured by mixing a positive electrode active material, a binder, and possibly conductive materials and fillers in a solvent to produce a positive electrode slurry composition, which is then applied to a positive electrode current collector.

[0049] The positive electrode current collector generally can have a thickness of 3 μm to 500 μm. Further, the positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. The current collector can also form fine irregularities on its surface to enhance the adhesive force of the positive electrode active material, and various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabric bodies, etc. are possible.

[0050] And the positive electrode active material can include a lithium transition metal oxide containing lithium and one or more transition metals selected from the group consisting of nickel, manganese, cobalt, and iron.

[0051] Specifically, the lithium transition metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), a lithium-nickel-manganese-cobalt-based oxide (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < \alpha, 0 < q < \alpha, 0 < r < \alpha, p + q + r = \alpha) or Li(Ni p1 Co q1 Mn It should be noted that there are some placeholders like 1-Y , Y etc. in the original text which seem to be incomplete or incorrect in terms of the chemical formula representation. I have translated them as they are while keeping in mind the possible error. If there are specific corrections or additional information for these parts, the translation can be adjusted accordingly. Also, the symbol \(\alpha\) is used to replace the incomplete or incorrect placeholder values in the translation for the sake of showing the general form of the translation.r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2), such as lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (where 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 the independent elements, respectively, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1), such as, or lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (where M is one or more selected from Al, Mg, and Ti, 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 two or more of these oxides may be included.

[0052] The above-described cathode active material may be contained at 60% to 99% by weight, or 70% to 99% by weight, or 80% to 98% by weight based on the total weight of the cathode active material layer.

[0053] On the other hand, the conductive material contained in the positive electrode active material layer is a component for further improving the conductivity of the positive electrode active material. Such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change 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 crystalline structure; conductive nanomaterials such as carbon nanofibers or carbon nanotubes; fluorinated carbon powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. Among these, the conductive material containing conductive nanomaterials such as carbon nanotubes or carbon nanofibers can further reduce the resistance of the lithium metal secondary battery and further improve its output characteristics.

[0054] Typically, the conductive material may be included in an amount of 1% to 20% by weight, or 1% to 15% by weight, or 1% to 10% by weight, based on the total weight of the positive electrode active material layer.

[0055] The binder selectively included in the positive electrode active material layer is a component that assists in the bonding of the positive electrode active material to conductive materials and to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, or fluororubber. A mixture or copolymer of two or more of these can also be used.

[0056] Typically, the binder may be present in amounts of 1% to 20% by weight, 1% to 15% by weight, or 1% to 10% by weight, based on the total weight of the positive electrode active material layer.

[0057] Furthermore, a filler may be selectively added to the positive electrode as a component to suppress its expansion. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without inducing a chemical change in the battery, and for example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; etc. can be used.

[0058] The aforementioned positive electrode can be manufactured by dispersing and mixing the positive electrode active material, binder, and conductive material in a dispersion medium (solvent) to create a slurry, applying this slurry to a metal current collector, and then drying and rolling it. In this case, the dispersion medium can be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, and mixtures thereof.

[0059] On the other hand, the aforementioned lithium secondary battery may further include a porous separator membrane interposed between the positive electrode and the negative electrode.

[0060] Such porous separation membranes can be used in the form of sheets, multilayer membranes, fine porous films, woven fabrics, and nonwoven fabrics, but are not necessarily limited to these materials. However, it is preferable to use porous polyethylene or porous glass fiber nonwoven fabric (glass filter) as the separation membrane, and it is more preferable to use porous glass fiber nonwoven fabric (glass filter) as the separation membrane. The separation membrane is an insulating thin film with high ion permeability and mechanical strength, and the pore diameter of the separation membrane is generally in the range of 0.01 μm to 10 μm, and the thickness is generally in the range of 5 μm to 300 μm, but is not limited to these.

[0061] On the other hand, the aforementioned lithium secondary battery can be manufactured by 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 membrane in a case and injecting and impregnating it with the aforementioned electrolyte.

[0062] Such lithium secondary batteries can be applied not only to battery cells used as power sources for small devices, but are also particularly suitable for use as unit batteries in battery modules that power medium- and large-sized devices. The batteries of one embodiment or other embodiments can be selectively used, taking into account the appropriate discharge rate for each application. [Examples]

[0063] Hereinafter, embodiments of the present invention will be described in detail so that they can be easily implemented by a person with ordinary skill in the art to which the invention pertains. However, the present invention can be embodied in a variety of different forms and is not limited to the embodiments described herein.

[0064] [Example 1, Comparative Example 1, and Comparative Example 2: Electrolyte Preparation] The electrolyte of Example 1 was prepared by mixing the non-aqueous organic solvent of FDMB and the organic non-solvent of BTFE in a 1:1 volume ratio and dissolving the lithium salt (LiFSI) at a concentration of 1 M.

[0065] Furthermore, an electrolyte for Comparative Example 1 (containing only the non-aqueous organic solvent of FDMB) without using the aforementioned organic non-solvent, and an electrolyte for Comparative Example 2 (FDMB + TTE) using the aforementioned organic non-solvent of TTE instead of BTFE were prepared.

[0066] [Experimental Example 1: Analysis of Solvation Structure and Ionic Conductivity of Electrolytes] A symmetrical cell was fabricated by combining 50 μL of the electrolyte from the example or comparative example, a 40 μm working electrode (Li), a 150 μm counter electrode (Li), and a PP separation membrane, resulting in a current density of 1 mA / cm². 2 , Capacity: 1mAh / cm 2 The electrochemical reaction was carried out under the following conditions.

[0067] During this process, changes in chemical species in the electrolyte were analyzed using NMR, and the solvation structure of the electrolyte was analyzed based on the results of this analysis. The results of this analysis and a schematic diagram of the solvation structure predicted through this analysis are shown in Figure 1.

[0068] Referring to Figure 1, the electrolyte in Example 1, by mixing FDMB / BTFE in a volume ratio of 1:1, showed increased ion pairing between Li and FSI anions compared to Comparative Example 1, which used FDMB alone. This was predicted to affect effective interface formation. Furthermore, BTFE is involved in the solvation structure, while TTE is not. Consequently, Example 1, with added BTFE, shows improved ionic conductivity compared to Comparative Example 2, with added TTE.

[0069] For a clearer confirmation, the ionic conductivity was measured in the aforementioned symmetrical cell and is shown in Table 1 below.

[0070] [Table 1]

[0071] As shown in Table 1 above, when using the electrolyte of Example 1, higher ionic conductivity and lower overpotential were observed compared to Comparative Examples 1 and 2, resulting in improved cell lifespan.

[0072] [Experimental Example 2: Evaluation of Lifetime Characteristics] The lithium symmetric cell used in Experimental Example 1 was used in the same manner, and the lifetime characteristics were evaluated using the electrolytes of the Examples and Comparative Examples. The evaluation results are shown in Figure 2. At this time, the current density applied to the symmetric cell was 1 mA / cm². 2 , Capacity: 1mAh / cm 2 That's what I decided.

[0073] Furthermore, the current density applied to each symmetrical cell is set to 2 mA / cm². 2 The lifetime characteristics were evaluated using a similar method after increasing the value, and the evaluation results are shown in Figure 3.

[0074] Referring to Figures 2 and 3, it can be observed that when using the electrolyte of Example 1, the lifespan of the lithium symmetric cell is further improved and the overvoltage is reduced, confirming that it contributes to more effective interface formation in lithium ion conduction.

[0075] [Experimental Example 3: Evaluation of Lithium Corrosion] A symmetrical cell was fabricated by combining 50 μL of the electrolyte from the example or comparative example, a 150 μm working electrode (Li), a 150 μm counter electrode (Li), and a PP separation membrane.

[0076] Figure 4 shows the results of measuring the SEI film interface resistance of the manufactured symmetric cells using EIS (Electroscopy Impedance Spectroscopy) analysis at room temperature (25°C) for 1 hour, 2 hours, 5 hours, and then at 5-hour intervals up to 60 hours.

[0077] Referring to Figure 4, it was confirmed that when using the electrolyte of Example 1, the resistance of the SEI film became lowest over time. This is because when using the electrolyte of Example 1, an SEI film is formed that effectively protects the lithium surface, suppressing additional reactions between the electrolyte and lithium, and thus the amount of lithium corrosion is expected to be the lowest.

[0078] [Experimental Example 4: Measurement of Inactivated Lithium Content] A symmetrical cell was fabricated by combining 50 μL of the electrolyte from the example or comparative example, a 150 μm working electrode (Li), a 150 μm counter electrode (Cu), and a PP separation membrane.

[0079] Ten charge-discharge cycles were performed on the manufactured symmetrical cells. After disassembling the symmetrical cells that had undergone the ten cycles, the obtained Cu electrodes were placed in a vial, and water was injected. The amount of H2 produced by the reaction with Li on the surface of the Cu electrodes was measured to calculate the amount of inactivated lithium, and the measured amount of lithium is shown in Figure 5. The following reaction equation was used as a reference to calculate the amount of inactivated lithium.

[0080] [Reaction equation: 2Li + 2H2O → H2 + 2LiOH]

[0081] Referring to Figure 5, it can be confirmed that the smallest amount of inactivated lithium is produced when the electrolyte of Example 1 is used. This is interpreted as being because the electrolyte of the present invention has high ionic conductivity, enabling uniform electrodeposition / desorption of lithium.

[0082] [Experimental Example 5: Thickness, Resistance Evaluation, and Component Analysis of SEI Film] The resistance of the SEI film formed on the negative electrode was evaluated while performing charge-discharge cycles on the lithium symmetric cell used in Experimental Example 1, and the evaluation results are shown in Figure 6. The table in Figure 6 shows the resistance values ​​of the SEI film after charge-discharge cycles for the examples and comparative examples. Referring to Figure 6, it was confirmed that the SEI film resistance was lowest after 50 cycles in Example 1.

[0083] Additionally, after 50 cycles, each SEI film was analyzed using an electron microscope, as shown in Figure 7, and the thickness of the SEI film measured from this analysis is shown in Figure 8.

[0084] Referring to Figures 7 and 8, it was confirmed that the SEI film thickness was thinnest when using the electrolyte of Example 1.

[0085] Furthermore, after 50 cycles, the SEI film was analyzed for its components using XPS (X-ray Photoelectron Spectroscopy), and the results are shown in Figure 9. From Figure 9, it can be confirmed that the SEI film contains a large amount of inorganic components (Li2O, Li3N, Li2S, Li2S2) when using the electrolyte of Example 1. An SEI film containing a large amount of such inorganic components provides good protection for the lithium surface and enables uniform electrodeposition / desorption of lithium.

[0086] Based on the results of the resistance evaluation, the SEI film thickness evaluation, and the component analysis of the SEI film, it can be predicted that overvoltage will hardly occur in Example 1.

[0087] [Experimental Example 6: Evaluation of Oxidative Stability and Cyclic Characteristics] Evaluation cells were prepared by combining 75 μL of the electrolyte from the example or comparative example, a working electrode (carbon), a counter electrode (Li 150 μm), and a separation membrane (PP). The oxidative stability of each electrolyte was evaluated and is shown in Figure 10.

[0088] Referring to Figure 10, when the electrolyte of Example 1 was used, a higher oxidation potential was observed, similar to Comparative Example 2 but higher than that of Comparative Example 1. This confirms that the electrolyte of the example exhibits high oxidation stability, enabling the use of a high-voltage cathode.

[0089] On the other hand, the electrolyte of the example or comparative example is 200 μL, and the cathode material is NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 A full cell was fabricated by combining O2, a 150 μm lithium metal negative electrode layer (Li), and a PP separation membrane. After activation over two cycles under 0.1C conditions, charging and discharging were performed at a rate of 0.33C.

[0090] Based on these charge-discharge test results, the cycle characteristics of Comparative Example 1 (Figure 11a), Example 1 (Figure 11b), and Comparative Example 2 (Figure 11c) were evaluated and the results are shown in Figures 11a to 11c, respectively.

[0091] This confirmed that the volume reduction was lowest when using Example 1.

[0092] [Experimental Example 7: Evaluation of CEI film thickness and crack formation within positive electrode particles] The full cell manufactured in Experimental Example 6 underwent 20 charge-discharge cycles, and the cross-section of the positive electrode and the CEI film obtained by disassembling the cell were analyzed using an electron microscope (SEM, TEM). The results are shown in Figures 12 and 13, respectively.

[0093] Figure 12 shows that the smallest amount of cracks occur when the electrolyte of Example 1 is used, and it is predicted that the electrodeposition / desorption reaction occurs uniformly due to the rapid movement of lithium ions at the positive electrode, resulting in less crack generation due to volume contraction / expansion.

[0094] Furthermore, Figure 13 confirms that the CEI film thickness is thinnest when using the electrolyte of Example 1. This is presumably because the amount of side reactions generated from the cracks is small.

[0095] [Experimental Example 8: Evaluation of Pouch Cell Performance] Cathode material NCM811 (LiNi 0·8 Co 0·1 Mn 0·1 After manufacturing a bi-cell by combining O2, Li 40μm, and a PP separation membrane, the electrode assembly was placed in a pouch-type case and filled with the electrolyte of the example or comparative example to manufacture a pouch cell (area capacity of NCM811: 3.8mAhcm²). -2 The ratio of negative electrode capacity to positive electrode capacity (N / P) is 2.16, and the ratio of electrolyte to negative electrode capacity (E / C) is 2.5 g Ah. -1 The manufactured pouch cells were charged and discharged at 3.0-4.3V with a rate of 0.1C / 0.5C. The cycle characteristics of Example 1, Comparative Example 1, and Comparative Example 2 were evaluated based on the results of these charge-discharge tests, and the results are shown in Figure 14.

[0096] This confirmed that when using the electrolyte of Example 1, a high discharge capacity was maintained over 100 cycles, resulting in the highest capacity retention rate. This is predicted to be because the electrolyte of Example 1 has high ionic conductivity, forming structurally / chemically stable SEI and CEI, suppressing unwanted side reactions, and resulting in almost no overpotential manifestation.

[0097] [Experimental Example 9: Evaluation of Cycle Characteristics] Except for using LFP (LiFePO4) as the cathode material in Experimental Example 6, a full cell was manufactured in the same manner as in Experimental Example 6, and the cycle characteristics were evaluated. The results are shown in Figure 15.

[0098] Referring to Figure 15, the results of measuring the discharge capacity while increasing the cell drive speed over 55 cycles confirm that the electrolyte of Example 1 exhibits the highest discharge capacity at all speeds. This confirms that the battery using the electrolyte of Example 1 performs best even when the type of cathode material is changed.

Claims

1. Lithium salts and Non-aqueous organic solvents containing fluorine-substituted ether-based solvents, The organic anti-solvent comprises an organic anti-solvent that exhibits solubility in the lithium salt that is 10 times or less less than that of the non-aqueous organic solvent, The aforementioned organic non-solvent is an electrolyte for lithium secondary batteries containing a compound represented by the following chemical formula 1: 【Chemistry 1】 In the aforementioned chemical formula 1, Ra to Rj are either the same or different from each other, and each is independently a hydrogen or fluoro group, with five or more of the Ra to Rj being fluoro groups.

2. The lithium salt is LiCl, LiBr, LiI, LiBF ４ , LiClO ４ , LiB １０ Cl １０ , LiAlCl ４ , LiAlO ２ , LiPF ６ , LiCF ３ SO ３ , LiCH ３ CO ２ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiCH ３ SO ３ , LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO ２ F) ２ ), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO ２ CF ２ CF ３ ) ２ ) and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO ２ CF ３ ) ２ ), and contains one or more selected from the group consisting of, the electrolyte for a lithium secondary battery according to claim 1.

3. The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium salt is contained in the electrolyte at a concentration of 1.0 M to 2.5 M.

4. The electrolyte for a lithium secondary battery according to claim 1, wherein the fluorine-substituted ether solvent comprises 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) or 2,2,3,3-tetrafluoro-1,4-diethoxybutane (FDEB).

5. The electrolyte for a lithium secondary battery according to claim 1, wherein five or six of the Ra to Rj groups are fluorogroups and the remainder are hydrogen atoms.

6. The electrolyte for a lithium secondary battery according to claim 1, wherein the compound represented by chemical formula 1 is bis(2,2,2-trifluoroethyl) ether (BTFE).

7. The electrolyte for a lithium secondary battery according to claim 1, wherein the organic non-solvent:the non-aqueous organic solvent is contained in a volume ratio of 1:0.5 to 1:1.

5.

8. A positive electrode containing a positive electrode active material, The negative electrode and, A separation membrane between the positive electrode and the negative electrode, A lithium secondary battery comprising the electrolyte according to any one of claims 1 to 7.

9. The lithium secondary battery according to claim 8, wherein the positive electrode active material comprises a lithium transition metal oxide containing lithium and one or more transition metals selected from the group consisting of nickel, manganese, cobalt, and iron.

10. The lithium secondary battery according to claim 8, wherein the negative electrode includes a lithium metal layer.

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