Electrolyte for lithium secondary battery and lithium secondary battery comprising same

Abstract

The present disclosure relates to an electrolyte for a lithium secondary battery capable of improving ion conductivity and lifespan characteristics of the lithium secondary battery, and a lithium secondary battery comprising the same.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority and benefit to Korean Patent Application No. 10-2023-0148871, filed with the Korean Intellectual Property Office on November 1, 2023, and Korean Patent Application No. 10-2024-0140522, filed with the Korean Intellectual Property Office on October 15, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0003] This disclosure relates to an electrolyte for lithium secondary batteries that can improve the ion conductivity and lifespan characteristics of lithium secondary batteries, and a lithium secondary battery containing the electrolyte. Background Technology

[0004] Lithium metal batteries, using lithium metal as the negative electrode, have attracted attention as a next-generation battery to overcome the low capacity of existing lithium-ion batteries. However, the currently used ether-based electrolytes have low oxidation stability, which limits their positive electrode stability. Therefore, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB), which has high oxidation stability, has been investigated as an electrolyte material. However, FDMB also has the limitation of low ionic conductivity due to its high viscosity. Therefore, it is necessary to develop electrolytes that ensure both ionic conductivity and oxidation stability by combining different solvents.

[0005] On the other hand, in recent years, in order to improve the ion 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 locally high concentration electrolytes (LHCEs) that contain organic non-solvents that exhibit relatively low solubility for lithium salts in the electrolyte.

[0006] The more lithium salt anions the SEI layer contains, the better the film quality is, and the more stable it is against decomposition. However, when organic non-solvents are used in the electrolyte, they do not participate in the solvation structure of Li ions, and many anions participate, thus forming a large amount of anion-derived SEI.

[0007] However, in the case of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which is most commonly used as an organic non-solvent, it does not participate in the solvation structure, so many anions are contained in the SEI, but the ionic conductivity is low due to the high viscosity.

[0008] Therefore, there is a continued need to develop electrolytes that can improve the ion conductivity and lifespan characteristics of lithium secondary batteries (e.g., lithium metal secondary batteries) that utilize LHCE. Summary of the Invention

[0009] [Technical Issues]

[0010] The purpose of this disclosure is to provide an electrolyte for lithium secondary batteries that can improve the ion conductivity and lifespan characteristics of lithium secondary batteries, and a lithium secondary battery containing the electrolyte.

[0011] [Technical Solution]

[0012] This invention provides an electrolyte for lithium secondary batteries, comprising:

[0013] Lithium salts;

[0014] Non-aqueous organic solvents containing fluorinated ether solvents; and

[0015] Organic non-solvents that exhibit at least 10 times less solubility for the lithium salt than the non-aqueous organic solvents.

[0016] The organic non-solvent mentioned herein comprises a compound represented by the following chemical formula 1:

[0017] [Chemical Formula 1]

[0018]

[0019] In chemical formula 1,

[0020] Ra to Rj may be the same as or different from each other, and each independently represents a hydrogen or fluorine group, and at least five of Ra to Rj are fluorine groups.

[0021] The present invention also provides a lithium secondary battery comprising: a positive electrode containing a positive electrode active material; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte as disclosed in this disclosure.

[0022] In this type of lithium secondary battery, the negative electrode can contain a lithium metal layer, thus presenting the form of a lithium metal secondary battery.

[0023] [Beneficial Effects]

[0024] The electrolyte disclosed herein comprises a fluorinated ether solvent, such as FDMB, as a non-aqueous organic solvent, and a compound represented by Formula 1 as an organic non-solvent. It has been confirmed that by combining the non-aqueous organic solvent and the organic non-solvent, viscosity can be reduced and ionic conductivity improved in LHCE-based electrolytes. This is presumably because the compound represented by Formula 1, unlike TTE which is a typical organic non-solvent, is capable of participating in the solvation process of lithium salts.

[0025] Furthermore, the electrolyte has been confirmed to exhibit excellent oxidation stability, and due to this oxidation stability, the use of the electrolyte in lithium secondary batteries (e.g., lithium metal secondary batteries) can improve battery life characteristics and cathode stability, and can induce the effective formation of SEI and CEI, thereby improving battery performance. Attached Figure Description

[0026] Figure 1 The NMR analysis results of the changes in chemical substances in the electrolytes used to analyze the solvation structure of the electrolytes of the examples and comparative examples are shown, along with a schematic diagram of the solvation structure predicted by the results.

[0027] Figure 2 and Figure 3 The evaluation results of the lifetime characteristics of lithium symmetric cells containing electrolytes of examples and comparative examples are shown.

[0028] Figure 4 The results show the resistance of the SEI film measured over time in lithium symmetric single cells containing the electrolytes of the examples and comparative examples.

[0029] Figure 5 The results show the measurement of the amount of deactivated lithium in lithium-symmetric single cells containing the electrolytes of the examples and comparative examples.

[0030] Figure 6 The evaluation results of the SEI film resistance of lithium symmetric single cells containing the electrolytes of the examples and comparative examples are shown for each cycle.

[0031] Figure 7 The results of electron microscopy analysis of the SEI film formed in lithium-symmetric single cells containing the electrolytes of the examples and comparative examples after 50 cycles are shown. Figure 8 The thickness of the SEI film, as measured from this result, is shown.

[0032] Figure 9 The results of compositional analysis of the SEI film formed after 50 cycles of lithium symmetric single cells containing the electrolytes of the examples and comparative examples are shown.

[0033] 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.

[0034] Figures 11a-11c The evaluation results of the cycling characteristics of full cells containing the electrolytes of the examples and comparative examples are shown.

[0035] Figure 12 The results of electron microscopy analysis of cracks appearing in the cathode particles after 20 cycles in full cells containing the electrolytes of the examples and comparative examples are shown.

[0036] Figure 13 The thickness of the CEI film formed after 20 cycles in a full cell containing the electrolytes of the examples and comparative examples is shown by electron microscopy analysis.

[0037] Figure 14 The evaluation results of the cycling characteristics of pouch cells containing electrolytes from the examples and comparative examples are shown.

[0038] Figure 15 The evaluation results of the cycling characteristics of full cells containing the electrolytes of the examples and comparative examples, as well as the LFP cathode material, are shown. Detailed Implementation

[0039] In the following, specific embodiments of an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the electrolyte will be described.

[0040] The terms or words used in this specification and claims should not be construed as limited to their ordinary or dictionary meanings, and this disclosure should be interpreted based on the principle that inventors can appropriately define the concepts of terms to best describe their own inventions, in accordance with the meanings and concepts consistent with the technical ideas of this disclosure.

[0041] The terminology used herein is provided to describe exemplary embodiments but is not intended to limit the inventive concept. Unless the context clearly indicates otherwise, the singular forms include the plural forms.

[0042] As used in this article, the term "fluorine group" refers to -F.

[0043] It should be understood that the terms “comprising,” “including,” “having,” etc., are used herein to specify the presence of the stated features, numbers, steps, constituent elements, or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, constituent elements, or combinations thereof.

[0044] According to embodiments of this disclosure, an electrolyte for lithium secondary batteries is provided, comprising:

[0045] Lithium salts;

[0046] Non-aqueous organic solvents containing fluorinated ether solvents; and

[0047] Organic non-solvents that exhibit at least 10 times less solubility for the lithium salt than the non-aqueous organic solvents.

[0048] The organic non-solvent mentioned herein comprises a compound represented by the following chemical formula 1:

[0049] [Chemical Formula 1]

[0050]

[0051] In chemical formula 1,

[0052] Ra to Rj may be the same as or different from each other, and each independently represents a hydrogen or fluorine group, and at least five of Ra to Rj are fluorine groups.

[0053] One embodiment of the electrolyte may belong to the category of locally high concentration electrolytes (LHCE), which, during battery charge and discharge, exhibit a form in which lithium salts are dissolved in a high concentration in a non-aqueous organic solvent, and an organic non-solvent is distributed around such a high concentration region. Therefore, when using this electrolyte, the battery's output characteristics can be improved due to the high concentration distribution of lithium salts. Furthermore, based on the organic non-solvent distribution region, side reactions between the lithium metal layer and the electrolyte can be suppressed, and the lifespan characteristics of the lithium secondary battery can be improved.

[0054] On the other hand, in one embodiment of the electrolyte, a lithium salt is used as a medium for transferring ions within a lithium secondary battery. For example, the lithium salt contains Li. + As a cation, and can contain F selected from... - 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 SCN - The anions that make up the group.

[0055] Specifically, lithium salts may include those selected from LiCl, LiBr, LiI, LiBF4, LiClO4, and LiB. 10 Cl 10 At least one of the following groups: 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 may be appropriately used, taking into account the performance of lithium secondary batteries and their solubility in non-aqueous organic solvents and organic non-solvents.

[0056] Considering the performance of lithium-ion secondary batteries, lithium salts can be included in the electrolyte at concentrations of 1.0 M to 2.5 M or 1.0 M to 2.0 M. The concentration of the lithium salt can be defined as the molar concentration taking into account the total volume of non-aqueous organic solvents and organic non-solvents. Due to the high concentration of lithium salts, the desolvation of lithium ions can be further accelerated, and the performance of the secondary battery can be further improved.

[0057] Furthermore, the fluorinated ether solvent may contain 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) or 2,2,3,3-tetrafluoro-1,4-diethoxybutane (FDEB), and in addition to containing the fluorinated ether solvent, the non-aqueous organic solvent may further contain other ether solvents and / or carbonate solvents.

[0058] Based on the fluorinated ether solvent, the non-aqueous organic solvent may further include an aliphatic ether solvent, and examples include dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, or dimethoxyethane.

[0059] In addition to the fluorinated ether solvents, the carbonate solvents that may be further included in the non-aqueous organic solvents include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butyl carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, methyl (2,2,2-trifluoroethyl) carbonate, etc.

[0060] One embodiment of the electrolyte comprises a non-aqueous organic solvent that dissolves lithium salts and acts as a lithium-ion migration pathway, and an organic non-solvent that exhibits a solubility for the lithium salt that is more than 10 times or 10 to 30 times less than that of the non-aqueous organic solvent and is substantially insoluble in the lithium salt. This organic non-solvent can be defined as an organic non-solvent that is substantially insoluble in lithium salts, for example, an organic non-solvent capable of dissolving lithium salts only at concentrations below 0.1 M, or 0 to 0.1 M, or 0 to 0.05 M.

[0061] The organic nonsolvent comprises a compound represented by Formula 1. The compound represented by Formula 1 has a structure based on diethyl ether and substituted with five or more fluorine groups. When the organic nonsolvent contains a compound having the structure of Formula 1, it has the effect of reducing the viscosity of the electrolyte and improving ionic conductivity compared to the most commonly used organic nonsolvent, such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). Therefore, it is predicted that TTE will not participate in the solvation structure of lithium salts, but the compound of Formula 1, unlike TTE, can participate in the solvation process of lithium salts.

[0062] In chemical formula 1, Ra to Rj each represent a substituent.

[0063] According to one embodiment of this disclosure, five or six of Ra to Rj may be fluorine groups, and the remainder may be hydrogen groups.

[0064] Furthermore, in Formula 1, five or six of Ra to Rc and Rh to Rj can be fluorine groups, and the remaining substituents in Ra to Rc and Rh to Rj that are not fluorine groups, as well as Rd to Rg, can be hydrogen.

[0065] According to another embodiment of this disclosure, the compound represented by Formula 1 can be bis(2,2,2-trifluoroethyl) ether (BTFE), and when the electrolyte of this disclosure contains a compound of Formula 1, specifically BTFE as an organic non-solvent, it has the effect of reducing electrolyte viscosity and improving ionic conductivity.

[0066] An electrolyte comprising, in addition to the aforementioned non-aqueous organic solvent, also includes the aforementioned organic non-solvent. This electrolyte may include regions where a high concentration of lithium salt is locally present in the non-aqueous organic solvent, and regions where a substantially non-lithium salt is present in the organic non-solvent. In this way, because the locally high concentration of lithium salt exists in a solvated form within the electrolyte, the performance of the lithium secondary battery, such as its output characteristics, can be further improved, while the increase in electrolyte viscosity and the decrease in fluidity can be reduced through the organic non-solvent distribution regions.

[0067] In one embodiment of the electrolyte, the amount of the organic non-solvent can be adjusted according to the type of the non-aqueous organic solvent and the lithium salt or the total concentration of the lithium salt. For example, the volume ratio of the included organic non-solvent to the non-aqueous organic solvent can be 1:0.5 to 1:1.5 or 1:1.

[0068] According to one embodiment of this disclosure, the electrolyte may also contain additives such as lithium nitrate (LiNO3), lithium difluorooxalate phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(oxalate borate) (LiBOB), lithium difluorooxalate borate (LiDFOB), or ethylene fluorocarbonate (FEC).

[0069] On the other hand, according to another embodiment of this disclosure, a lithium secondary battery comprising the electrolyte of the above embodiment is provided. The lithium secondary battery comprises a positive electrode containing a positive electrode active material; a negative electrode; a separator between the positive electrode and the negative electrode; and a lithium secondary battery comprising the electrolyte of one embodiment.

[0070] According to one embodiment, the negative electrode may include a lithium metal layer to form a lithium metal secondary battery.

[0071] First, in another embodiment of the lithium secondary battery, the negative electrode may be an electrode with a lithium metal layer formed on one or both surfaces of a planar negative electrode current collector, according to the general structure of a lithium secondary battery, and may be manufactured by depositing lithium metal or rolling lithium foil on the negative electrode current collector.

[0072] The negative electrode current collector is a metal with 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 electrode current collector.

[0073] Specific examples include metals such as stainless steel, aluminum, nickel, titanium, or copper, or copper, aluminum, or stainless steel with surfaces treated with carbon, nickel, titanium, silver, etc. The negative electrode current collector can be formed in various forms, such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven structures.

[0074] In addition, the negative electrode current collector may have a thickness of 3 to 100 μm, and the lithium metal layer may have a thickness of, for example, 1 to 300 μm.

[0075] 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.

[0076] Such a positive electrode can be manufactured by mixing an active material, a binder, and optionally a conductive material, a filler, etc. in a solvent to prepare a positive electrode paste composition, and coating the paste composition onto the positive electrode current collector.

[0077] The positive electrode current collector generally may have a thickness of 3 to 500 μm. The positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause any chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the current collector may have fine concavities and convexities formed on its surface to enhance the adhesion of the positive electrode active material, and the current collector can be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric structure.

[0078] In addition, the positive electrode active material may include lithium; and a lithium transition metal oxide containing at least one transition metal selected from the group consisting of nickel, manganese, cobalt, and iron.

[0079] Specifically, the lithium transition metal oxide may include lithium manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (e.g., LiCoO2, etc.), lithium nickel-based oxides (e.g., LiNiO2, etc.), lithium nickel manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), lithium nickel cobalt-based oxides (e.g., LiNi 1- Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium manganese cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2- Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), lithium nickel manganese cobalt-based oxides (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, and p + q + r = 1) or Li(Ni p1 Coq1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, and p1 + q1 + r1 = 2), etc.), or 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 atomic fractions of each independent element, where 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1), etc.), lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (where M is at least one selected from Al, Mg, and Ti, X is at least one selected from F, S, and N, -0.5 ≤ a ≤ +0.5, 0 ≤ x ≤ 0.5, 0 ≤ b ≤ 0.1), etc. Any one or a mixture of two or more thereof can be used.

[0080] Based on the total weight of the positive electrode active material layer, the content of the above positive electrode active material can be 60 to 99% by weight, or 70 to 99% by weight, or 80 to 98% by weight.

[0081] 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. There is no particular limitation on such a conductive material as long as it has conductivity and does not cause any chemical change in the battery. For example, conductive materials including: carbon powder, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; graphite powder, such as natural graphite, artificial graphite, or graphite with a well-developed crystal structure; conductive nanomaterials, such as carbon nanofibers or carbon nanotubes; fluorocarbon powder; conductive powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive materials, such as polyphenylene derivatives. Among them, the conductive material includes conductive nanomaterials, such as carbon nanotubes or carbon nanofibers, which can further reduce the resistance of the lithium secondary battery and further enhance output characteristics, etc.

[0082] Generally, based on the total weight of the positive electrode active material layer, the content of the conductive material can be 1 to 20% by weight, or 1 to 15% by weight, or 1 to 10% by weight.

[0083] The adhesive optionally included in the positive electrode active material layer is a component that facilitates adhesion between the positive electrode active material and the conductive material, as well as adhesion to the current collector. Examples of such adhesives may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, nitrile rubber, styrene-butadiene rubber, fluororubber, etc. Mixtures or copolymers selected from these may also be used.

[0084] Typically, based on the total weight of the positive electrode active material layer, the content of the binder can be 1 to 20% by weight, or 1 to 15% by weight, or 1 to 10% by weight.

[0085] In addition, fillers may optionally be added to the positive electrode as a component to suppress its expansion. There are no particular limitations on such fillers, as long as they can suppress the expansion of the electrode without causing any chemical changes in the battery, and examples may include olefin polymers, such as polyethylene and polypropylene; fibrous materials, such as glass fibers and carbon fibers.

[0086] The aforementioned positive electrode can be manufactured, for example, by dispersing and mixing positive electrode active materials, binders, conductive materials, etc., in a dispersion medium (solvent) to form a slurry, coating the slurry onto a metal current collector, and then drying and calendering it. In this case, the dispersion medium can be NMP (N-methyl-2-pyrrolidone), DMF (dimethylformamide), DMSO (dimethyl sulfoxide), ethanol, isopropanol, water, or mixtures thereof, but is not necessarily limited to these.

[0087] On the other hand, the aforementioned lithium secondary battery may also include a porous separator inserted between the positive and negative electrodes.

[0088] Such porous membranes can be made from olefin polymers such as polyethylene (PE) and polypropylene (PP), glass fibers, etc., in the form of sheets, multilayer membranes, microporous membranes, woven fabrics, and nonwoven fabrics, but are not necessarily limited to these. However, porous polyethylene or porous glass fiber nonwoven fabric (glass filter) is preferred as the membrane, and porous glass filter (glass fiber nonwoven fabric) is more preferred as the membrane. The membrane can be an insulating film with high ion permeability and mechanical strength, and the pore size of the membrane is typically in the range of 0.01 to 10 μm, and the thickness is typically in the range of 5 to 300 μm, but is not limited to these.

[0089] On the other hand, the aforementioned lithium secondary battery can be manufactured according to conventional methods in the art. For example, the lithium secondary battery can be manufactured by housing an electrode assembly including a positive electrode, a negative electrode, and a separator in a casing and injecting and impregnating the aforementioned electrolyte into the casing.

[0090] This lithium-ion rechargeable battery can be used not only as a battery cell for powering small devices, but also as a unit battery in a battery module for powering medium to large devices. Batteries from the described embodiment or other embodiments can be selectively used, taking into account the appropriate discharge rate for each application.

[0091] Model for implementing invention

[0092] The embodiments of this disclosure will now be described in detail to the extent that those skilled in the art can readily practice it. This disclosure can be implemented in various different forms and is not limited to the embodiments described herein.

[0093] Example 1, Comparative Example 1 and Comparative Example 2: Preparation of Electrolytes

[0094] The electrolyte of Example 1 was prepared by mixing FDMB (a non-aqueous organic solvent) and BTFE (an organic non-solvent) in a 1:1 volume ratio and dissolving lithium salt (LiFSI) at a concentration of 1M.

[0095] In addition, electrolytes were prepared for Comparative Example 1 (containing only FDMB non-aqueous organic solvent) without the use of organic non-solvents and Comparative Example 2 (FDMB+TTE) using TTE organic non-solvent instead of BTFE.

[0096] Experimental Example 1: Analysis of the solvation structure and ionic conductivity of electrolytes

[0097] A symmetrical single cell was fabricated by combining a 50 μL electrolyte, a 40 μm Li working electrode, a 150 μm Li counter electrode, and a PP separator, as described in the examples or comparative examples. The cell achieved a performance of 1 mA / cm². 2 Current density and 1mAh / cm 2 Electrochemical reactions are carried out at a capacity of [specific value].

[0098] In this process, changes in the chemical substances in the electrolyte are analyzed using NMR, and the solvation structure of the electrolyte is analyzed based on the analytical results. The analytical results and a schematic diagram of the solvation structure predicted through this analysis are shown in [the diagram]. Figure 1 As shown.

[0099] refer to Figure 1 In the electrolyte of Example 1, FDMB / BTFE was mixed at a 1:1 volume ratio, thereby increasing ion pairing between Li and FSI anions compared to Comparative Example 1, which used FDMB alone, and this was predicted to affect effective interface formation. Furthermore, BTFE participated in the solvation structure, while TTE did not. This indicates that Example 1 with added BTFE has improved ion conductivity compared to Comparative Example 2, which added TTE.

[0100] To confirm this more clearly, ionic conductivity was measured in a symmetrical single cell and is shown in Table 1 below.

[0101]

[0102] Referring to Table 1, when the electrolyte of Example 1 was used, higher ion conductivity and lower overvoltage were observed compared with Comparative Examples 1 and 2, thus improving single-cell life.

[0103] Experimental Example 2: Evaluation of Lifetime Characteristics

[0104] Using the same lithium-symmetric single cell as in Example 1, the lifetime characteristics were evaluated when using the electrolytes of the Examples and Comparative Examples, and the evaluation results were... Figure 2 As shown in the figure. At this time, the current density applied to the above-mentioned symmetrical single cell is 1 mA / cm². 2 And the capacity is 1mAh / cm 2 .

[0105] Furthermore, the current density applied to each symmetrical single cell was increased to 2 mA / cm². 2 And the lifetime characteristics were evaluated using the same method. The evaluation results are shown in... Figure 3 middle.

[0106] refer to Figure 2 and Figure 3 It was observed that when the electrolyte of Example 1 was used, the lifetime of the lithium symmetric single cell was enhanced and the overvoltage was improved, thus confirming that it helps to form a more efficient interface in lithium-ion transfer.

[0107] Experimental Example 3: Evaluation of Lithium Corrosion

[0108] Symmetrical single cells were fabricated by combining 50 μL of electrolyte, 150 μm Li working electrode, 150 μm Li counter electrode, and PP separator as described in the examples or comparative examples.

[0109] The SEI film interfacial resistance of the fabricated symmetrical single cell was measured using EIS (electrochemical impedance spectroscopy) at room temperature (25°C) for 1 hour, 2 hours, 5 hours, and then at 5-hour intervals up to 60 hours, and the results are shown in... Figure 4 middle.

[0110] refer to Figure 4 It was confirmed that when using the electrolyte of Example 1, the resistance of the SEI film became the lowest over time. This is because when using the electrolyte of Example 1, a well-protected SEI film is formed on the lithium surface, thereby inhibiting additional reactions between the electrolyte and lithium, and therefore, the predicted amount of lithium corrosion is also the lowest.

[0111] Experiment Example 4: Measurement of Deactivated Lithium Content

[0112] Symmetrical single cells were fabricated by combining 50 μL of electrolyte, 150 μm Li working electrode, 150 μm Cu counter electrode, and PP separator as described in the examples or comparative examples.

[0113] The fabricated symmetric single cell was subjected to 10 charge-discharge cycles. After 10 cycles, the symmetric single cell was disassembled, the resulting Cu electrode was placed in a vial, water was added, and the amount of H2 generated on the Cu electrode surface (reflecting Li) was measured, and the amount of deactivated lithium was calculated. The measured lithium content is displayed... Figure 5 In order to calculate the amount of deactivated lithium, the following reaction formula is used.

[0114] [Reaction formula: 2Li + 2H₂O → H₂ + 2LiOH]

[0115] refer to Figure 5 It can be confirmed that minimal amounts of deactivated lithium are produced when the electrolyte of Example 1 is used. This is explained as being because the electrolyte of this disclosure has rapid ionic conductivity, thereby enabling uniform lithium deposition / deposition.

[0116] Experimental Example 5: Evaluation of SEI film thickness, resistivity, and composition analysis

[0117] For the lithium-ion symmetric single cell used in Example 1, the resistance of the SEI film formed on the negative electrode was evaluated during charge-discharge cycling, and the evaluation results are shown in... Figure 6 middle. Figure 6 The tables described herein show the resistance values ​​of the SEI film during charge-discharge cycles according to the embodiments and comparative examples. Reference Figure 6 It was confirmed that the SEI film resistance was lowest after 50 cycles in Example 1.

[0118] Furthermore, after 50 cycles, each SEI film was analyzed using electron microscopy and the results were displayed. Figure 7 In this process, the thickness of the SEI film measured thus is shown in... Figure 8 middle.

[0119] refer to Figure 7 and Figure 8 It was confirmed that the SEI film was thinnest when the electrolyte of Example 1 was used.

[0120] Furthermore, after 50 cycles, the composition of the SEI film was analyzed using XPS (X-ray photoelectron spectroscopy), and the results are shown below. Figure 9 From. Figure 9It can be confirmed that when using the electrolyte of Example 1, the SEI film contains a large amount of inorganic components (Li2O, Li3N, Li2S, Li2S2). The SEI film containing a large amount of these inorganic components advantageously protects the lithium surface and enables uniform lithium deposition / deposition.

[0121] Based on the resistance evaluation results, SEI film thickness evaluation results, and SEI film composition analysis results, it can be predicted that almost no overvoltage will occur in Example 1.

[0122] Experimental Example 6: Evaluation of Oxidative Stability and Cyclic Characteristics

[0123] Evaluation batteries were fabricated by combining 75 μL of electrolyte from examples or comparative examples, carbon as the working electrode, 150 μm of Li as the counter electrode, and PP as the separator. The oxidation stability of each electrolyte was evaluated and displayed. Figure 10 middle.

[0124] refer to Figure 10 When the electrolyte of Example 1 was used, it exhibited a higher oxidation potential than that of Comparative Example 1, similar to Comparative Example 2, thus confirming that the electrolyte of the Example exhibits high oxidation stability and can be used with a high-voltage positive electrode.

[0125] On the other hand, by using 200 μL of electrolyte and NCM811 (LiNi) as examples or comparative examples... 0.8 Co 0.1 Mn 0.1 The full cell was fabricated using a combination of O2, a 150μm Li negative electrode metal layer, and a PP separator, and after two cycles of activation at 0.1C, it was charged and discharged at a rate of 0.33C.

[0126] Based on the results of these charge-discharge tests, Comparative Example 1 was evaluated. Figure 11a Example 1 Figure 11b ) and Comparative Example 2 ( Figure 11c The cyclic characteristics of ) are shown, and the results are displayed respectively. Figure 10 a~10c.

[0127] Therefore, it was confirmed that the capacity reduction was minimal when using Example 1.

[0128] Experimental Example 7: Evaluation of CEI film thickness and intraparticle crack formation in cathode particles

[0129] The full cell fabricated in Example 6 was subjected to 20 charge-discharge cycles, and the cross-section of the positive electrode and the CEI film obtained by disassembling the cell were analyzed by electron microscopy (SEM, TEM). The results are shown in the figures below. Figure 12 and Figure 13 middle.

[0130] from Figure 12It can be confirmed that the minimum amount of cracks occurs when the electrolyte of Example 1 is used, and this is predicted to be due to the uniform electrodeposition / deposition reaction caused by the rapid movement of lithium ions in the positive electrode, resulting in fewer cracks due to volume shrinkage / expansion.

[0131] In addition, from Figure 13 It can be confirmed that the CEI film thickness becomes the thinnest when the electrolyte of Example 1 is used. This is predicted to be because the amount of side reactions originating from the crack is small.

[0132] Experiment Example 8: Evaluation of Pocket Battery Performance

[0133] By using the cathode material NCM811(LiNi) 0.8 Co 0.1 Mn 0.1 A bi-cell electrode assembly (bi-cell) was fabricated using a combination of O2, Li 40μm, and a PP separator. This assembly was then housed in a pouch-type casing and filled with the electrolyte from the examples or comparative examples to fabricate a pouch-type battery (NCM811 with an areal capacity of 3.8 mAh / cm²). -2 The negative electrode capacity / positive electrode capacity ratio (N / P) is 2.16, and the electrolyte / negative electrode capacity ratio (E / C) is 2.5 g Ah. -1 The manufactured pouch cells were charged and discharged at 3.0-4.3V at 0.1C / 0.5C, and the cycle characteristics of Example 1, Comparative Example 1, and Comparative Example 2 were evaluated based on the results of the charge and discharge tests. The results are shown in [Table data would be inserted here]. Figure 14 middle.

[0134] Thus, it was confirmed that when using the electrolyte of Example 1, the high discharge capacity was maintained for 100 cycles, achieving the highest capacity retention rate. This is predicted to be because the electrolyte of Example 1 has high ionic conductivity, forms structurally / chemically stable SEI and CEI to suppress unwanted side reactions, and exhibits almost no overvoltage.

[0135] Experiment Example 9: Evaluation of Cyclic Characteristics

[0136] The full cell was fabricated in the same manner as in Example 6, except that LFP (LiFePO4) was used as the cathode material in Example 6. Cycle performance was evaluated, and the results are shown below. Figure 15 middle.

[0137] refer to Figure 15 It can be confirmed that when the discharge capacity was measured while increasing the battery operating rate over 55 cycles, the electrolyte of Example 1 exhibited the highest discharge capacity at all rates. This confirms that the battery using the electrolyte of Example 1 maintains superior performance even when the type of cathode material is changed.

Claims

1. An electrolyte for lithium secondary batteries, comprising: Lithium salts; Non-aqueous organic solvents containing fluorinated ether solvents; and Organic non-solvents that exhibit at least 10 times less solubility for the lithium salt than the non-aqueous organic solvents. The organic non-solvent mentioned herein comprises a compound represented by the following chemical formula 1: [Chemical Formula 1] In chemical formula 1, Ra to Rj may be the same as or different from each other, and each independently represents a hydrogen or fluorine group, and at least five of Ra to Rj are fluorine groups.

2. The electrolyte for lithium secondary batteries according to claim 1, wherein the lithium salt comprises selected from LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 At least one of the following groups: 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).

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

4. The electrolyte for lithium secondary batteries 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 lithium secondary batteries according to claim 1, wherein five or six of Ra to Rj are fluorine-based, and the remainder are hydrogen-based.

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

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

5.

8. A lithium secondary battery, comprising: A cathode containing positive electrode active material; negative electrode; The membrane between the positive electrode and the negative electrode; and 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: Lithium; and A lithium transition metal oxide comprising at least one transition metal selected from the group consisting of nickel, manganese, cobalt and iron.

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

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