Electrolyte for lithium secondary battery and lithium secondary battery using the same

Abstract

[Problem] The purpose of the present invention is to provide a means for improving the ion conductivity of a lithium secondary battery electrolytic solution. [Solution] Provided is a lithium secondary battery electrolytic solution comprising a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent. The ratio of the moles of the fluorinated ether-based second solvent to the moles of lithium atoms included in the lithium salt is 0.4 to 2.0. The electrolytic solution exhibits a prescribed Raman spectrum profile, which is obtained by micro-Raman spectroscopy.

Description

[Technical Field]

[0001] The present invention relates to an electrolyte solution for a lithium secondary battery and a lithium secondary battery using the same. [Background technology]

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other applications, which hold the key to the widespread use of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium secondary batteries, are attracting attention as they are expected to offer high energy density and high output.

[0003] Japanese Patent Publication No. 6569883 (corresponding to the specification of U.S. Patent Application Publication No. 2019 / 131658) discloses a technology related to an electrolyte solution aimed at increasing the capacity of lithium secondary batteries. Specifically, the electrolyte solution contains an electrolyte containing a lithium salt having a specific structure, an organic solvent containing a chain carbonate having a specific structure, and an unsaturated cyclic carbonate. The electrolyte solution is characterized in that the chain carbonate is contained in a molar ratio of 3 to 6 relative to the lithium salt, and / or the lithium salt is contained at a concentration of 1.1 to 3.8 mol / L. Summary of the Invention [Problem to be solved by the invention]

[0004] However, the inventors have conducted studies and found that the electrolyte solution described in the above document has a problem of low ionic conductivity.

[0005] Therefore, an object of the present invention is to provide a means for improving the ionic conductivity of an electrolyte solution for a lithium secondary battery. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems. In the course of their research, they have found that the ionic conductivity of an electrolyte solution can be significantly improved by preparing an electrolyte solution by dissolving a lithium salt in a carbonate-based solvent at a predetermined concentration to obtain a solution, and then diluting the solution with a fluorinated ether-based solvent in a predetermined amount relative to the lithium salt. They then analyzed the electrolyte solution using micro-Raman spectroscopy and found that the Raman spectrum showed a predetermined profile, which led to the completion of the present invention.

[0007] That is, one aspect of the present invention relates to an electrolyte solution for a lithium secondary battery, which contains a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent, and the ratio of the number of moles of the fluorinated ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 to 2.0. The lithium secondary battery electrolyte solution has a peak intensity I of a peak derived from the carbonate-based first solvent coordinated to lithium in a Raman spectrum measured by microscopic Raman spectroscopy. S The peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (I U / I S ) is 0.2 or more and 0.7 or less; and the peak intensity I S The peak intensity I of the peak representing the aggregation of anions, G The ratio (I G / I S ) is 0.01 or more and 0.3 or less; the peak area A of the peak derived from the carbonate-based first solvent S0 The peak area A of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (A U / A S0 ) is 0.15 or more and 0.3 or less; the peak area A of the peak attributable to the anion A The peak area A of the peak representing the aggregation of anions G The ratio (A G / A A ) is between 0.01 and 0.4. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating a flat (laminated) non-bipolar (internal parallel connection) lithium secondary battery (hereinafter also simply referred to as a "laminated secondary battery") according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described, but the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. The range "X to Y" means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0010] <Electrolyte for lithium secondary batteries> One aspect of the present invention relates to an electrolyte solution for a lithium secondary battery (hereinafter also simply referred to as "electrolyte solution") that contains a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent, and the ratio of the number of moles of the fluorinated ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 to 2.0. The lithium secondary battery electrolyte solution has a peak intensity I of a peak derived from the carbonate-based first solvent coordinated to lithium in a Raman spectrum measured by microscopic Raman spectroscopy. S The peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (I U / I S ) is 0.2 or more and 0.7 or less; and the peak intensity I S The peak intensity I of the peak representing the aggregation of anions, G The ratio (I G / I S ) is 0.01 or more and 0.3 or less; the peak area A of the peak derived from the carbonate-based first solvent S0 The peak area A of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (AU / A S0 ) is 0.15 or more and 0.3 or less; the peak area A of the peak attributable to the anion A The peak area A of the peak representing the aggregation of anions G The ratio (A G / A A ) is 0.01 or more and 0.4 or less. According to this embodiment, the ionic conductivity of the electrolyte solution for a lithium secondary battery can be improved. Hereinafter, this embodiment will be described in detail.

[0011] [Lithium salt] The electrolytic solution according to this embodiment contains a lithium salt as an electrolyte. The type of lithium salt is not particularly limited, but examples include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), LiN(CF3SO2)2, Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. Among these, Li(FSO2)2N is preferred from the viewpoint of further improving the battery's input / output and charge / discharge cycle characteristics.

[0012] [Carbonate-based first solvent] The electrolytic solution according to this embodiment contains a carbonate-based first solvent as an organic solvent. The type of carbonate-based first solvent is not particularly limited as long as it has an R1-O-(C=O)-O-R2 structure (where R1 and R2 are each independently a monovalent organic group and may be linked to each other to form a ring). Specific examples of carbonate-based first solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and fluoroethylene carbonate (FEC). Among these, from the viewpoint of further improving the rapid charging characteristics and the input / output characteristics, chain carbonates are preferred, and at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) and ethyl methyl carbonate (EMC) is more preferred, with dimethyl carbonate (DMC) being even more preferred.

[0013] [Fluorinated ether-based second solvent] The electrolyte solution according to this embodiment further contains a fluorinated ether-based second solvent as an organic solvent. In the electrolyte solution according to this embodiment, it is believed that lithium ions have a structure in which they are solvated by four carbonate-based first solvent molecules. The solvated lithium ions (hereinafter also referred to simply as "solvated Li") move through the electrolyte solution, causing the lithium ions to move between the electrodes. It is believed that the inclusion of a fluorinated ether-based second solvent in the electrolyte solution allows the fluorinated ether-based second solvent to enter between adjacent solvated Li ions, making it easier for the solvated Li to move, thereby improving the ionic conductivity of the electrolyte. The state of each molecule in the electrolyte solution can be confirmed by Raman spectroscopy, which will be described later.

[0014] The type of the fluorinated ether second solvent is not particularly limited as long as it is a solvent (hydrofluoroether) having a structure in which at least a portion of the hydrogen atoms contained in an ether (R3-O-R4 (wherein R3 and R4 are each independently a monovalent organic group and may be linked to each other to form a ring) are fluorinated. Specific examples of the fluorinated ether second solvent include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, ethyl nonafluorobutyl ether, methyl tridecafluorohexyl ether, bis(2,2,2-trifluoroethyl) ether, 2,2,3,3,3-pentafluoropropyl difluoromethyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 2,2,3,4,4,4-hexafluorobutyl difluoromethyl ether, propyl 1,1,2,2-tetrafluoroethyl ether, butyl(1,1,2,2-tetrafluoroethyl)ether, bis(2,2,2-trifluoroethyl)ether, etc. Among these, from the viewpoint of being able to further improve ionic conductivity, at least one selected from 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,1,3,3,3-hexafluoroisopropyl methyl ether is preferred.

[0015] In the electrolyte solution of this embodiment, the ratio of the number of moles of the fluorinated ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt (i.e., the number of moles of lithium ions contained in the electrolyte solution) (hereinafter simply referred to as the "molar ratio") must be 0.4 or more and 2.0 or less. If the molar ratio is less than 0.4, fewer fluorinated ether-based second solvent molecules will be inserted between adjacent solvated Li atoms, which may result in an insufficient improvement in ionic conductivity. If the molar ratio exceeds 2.0, the proportion of solvated Li atoms in the electrolyte solution will be relatively small, which may result in a decrease in ionic conductivity. From the viewpoint of further improving ionic conductivity, the molar ratio is preferably 0.4 or more and 1.5 or less, more preferably 0.5 or more and 1.3 or less.

[0016] The lithium ion concentration in the electrolyte solution of this embodiment is not particularly limited, but is preferably 1.2 mol / L or more and 2.7 mol / L or less, more preferably 1.3 mol / L or more and 2.4 mol / L or less, and even more preferably 1.5 mol / L or more and 2 mol / L or less. If the lithium ion concentration is 1.2 mol / L or more, the proportion of the carbonate-based first solvent (uncoordinated solvent) that is not coordinated with the lithium ions is reduced. This suppresses corrosion of the aluminum foil used as a current collector and can improve the durability of the lithium secondary battery. In a high-concentration electrolyte solution, solvated Li and anions may form an aggregate structure (AGG structure), which may hinder lithium ion migration. If the lithium ion concentration is 2.7 mol / L or less, the AGG structure is less likely to form and lithium ions can migrate smoothly, resulting in high ionic conductivity.

[0017] (Raman spectrum) The electrolyte solution according to the present embodiment is also characterized by exhibiting a predetermined profile in a Raman spectrum obtained by micro-Raman spectroscopy. Information such as whether or not the carbonate-based first solvent is coordinated to lithium ions and whether or not anions are aggregated can be obtained from the Raman spectrum. Therefore, by checking whether or not the Raman spectrum has a predetermined profile, it is possible to determine whether or not molecules having a predetermined structure are present in a predetermined ratio.

[0018] The electrolyte solution according to the present embodiment has the following characteristics in the Raman spectrum measured by microscopic Raman spectroscopy: (1) Peak intensity I of the peak derived from the carbonate-based first solvent coordinated to lithium S The peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (I U / I S ) is between 0.2 and 0.7; (2) Peak intensity I of the peak derived from the carbonate-based first solvent coordinated to lithium S The peak intensity I of the peak representing the aggregation of anions, G The ratio (I G / I S ) is between 0.01 and 0.3; (3) Peak area A of the peak derived from the carbonate-based first solvent S0 The peak area A of the peak derived from the carbonate-based first solvent that is not coordinated with lithium U The ratio (A U / A S0 ) is greater than or equal to 0.15 and less than or equal to 0.3; and (4) Peak area A of the peak derived from anions A The peak area A of the peak representing the aggregation of anions G The ratio (A G / A A ) is between 0.01 and 0.4; These ratios are measured by the methods described in the Examples below.

[0019] The electrolyte solution according to the present embodiment, which satisfies these characteristics, has a smaller proportion of anions that have an aggregate structure (AGG structure) with solvated Li compared to conventional electrolyte solutions. This allows lithium ions to move smoothly in the electrolyte solution, thereby enabling high ionic conductivity to be exhibited. Furthermore, the electrolyte solution according to the present embodiment, which satisfies the above (1) to (4), has a smaller proportion of the carbonate-based first solvent (uncoordinated solvent) that is not coordinated with lithium ions compared to conventional electrolyte solutions. This suppresses corrosion of the aluminum foil used as a current collector, and can improve the durability of the lithium secondary battery.

[0020] From the same viewpoint, it is preferable that the electrolyte solution according to the present embodiment satisfies the following characteristics: Ratio (I U / I S ) is greater than or equal to 0.25 and less than or equal to 0.6; Ratio (I G / I S ) is greater than or equal to 0.05 and less than or equal to 0.2; Ratio(A U / A S0 ) is greater than or equal to 0.2 and less than or equal to 0.3; Ratio(A G / A A ) is between 0.05 and 0.3.

[0021] <Method of manufacturing an electrolyte for lithium secondary batteries> The method for producing the electrolyte solution for lithium secondary batteries according to this embodiment is not particularly limited, but is preferably produced by the following method. First, a lithium salt is dissolved in a carbonate-based first solvent to prepare an electrolyte solution precursor (first step). Next, a fluorinated ether-based second solvent is added to the electrolyte solution precursor and diluted so that the ratio of the number of moles of the ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 or more and 2.0 or less to prepare an electrolyte solution (second step). According to this production method, a structure in which lithium ions are solvated with four carbonate-based first solvent molecules is formed in the first step, and then the fluorinated ether-based second solvent can enter between adjacent solvated Li atoms in the second step, making it possible to easily obtain the electrolyte solution according to this embodiment. According to another aspect of the present invention, there is provided a method for producing an electrolyte solution for a lithium secondary battery, the method comprising: a first step of dissolving the lithium salt in the carbonate-based first solvent to prepare an electrolyte solution precursor; and a second step of adding the fluorinated ether-based second solvent to the electrolyte solution precursor to dilute the electrolyte solution precursor so that the ratio of the number of moles of the ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 to 2.0. The lithium ion concentration of the electrolyte solution precursor is not particularly limited, but is preferably 1.5 mol / L to 3.2 mol / L, more preferably 1.8 mol / L to 3.0 mol / L, and even more preferably 2.0 mol / L to 2.5 mol / L.

[0022] <Lithium secondary battery> The electrolyte solution for lithium secondary batteries according to this embodiment has excellent ionic conductivity, and therefore, when applied to a lithium secondary battery, the output and input characteristics of the battery can be improved. Furthermore, when applied to a lithium secondary battery having an aluminum foil as a current collector, corrosion of the aluminum foil can be suppressed, and the durability of the lithium secondary battery can be improved. Therefore, according to yet another embodiment of the present invention, a lithium secondary battery containing the electrolyte solution for lithium secondary batteries is provided. A lithium secondary battery according to a preferred embodiment has an aluminum current collector.

[0023] FIG. 1 is a cross-sectional view schematically illustrating a flat (laminated) non-bipolar (internal parallel connection) lithium secondary battery (hereinafter also simply referred to as a "laminated secondary battery") according to one embodiment of the present invention.

[0024] 1, the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power generating element 21 has a configuration in which a positive electrode in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12 are laminated. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween.

[0025] As a result, the positive electrode, electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power generating element 21, active material layers may be disposed on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive electrode and negative electrode from FIG. 1, the outermost negative electrode current collectors may be located on both outermost layers of the power generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

[0026] A positive electrode current collector 25 and a negative electrode current collector 27, which are electrically connected to the electrodes (positive and negative electrodes), are attached to the positive electrode current collector 11′ and the negative electrode current collector 12, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11′ and the negative electrode current collector 12 of the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary.

[0027] The main components of the lithium secondary battery according to this embodiment will be described below.

[0028] [Current collector] The current collector has a function of mediating the transfer of electrons from the electrode active material layer. There are no particular restrictions on the material constituting the current collector, but for example, metals and conductive resins can be used.

[0029] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other preferred metals include nickel-aluminum clad materials, copper-aluminum clad materials, and plated materials of a combination of these metals. Foils in which the metal surface is coated with aluminum, or carbon-coated aluminum foils, are also suitable. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity and battery operating potential. Furthermore, the use of aluminum foil as a current collector in the lithium secondary battery according to this embodiment can suppress corrosion of the aluminum foil and improve the durability of the lithium secondary battery, making it more preferable to use aluminum foil as a current collector.

[0030] The latter electrically conductive resin may be a resin obtained by adding an electrically conductive filler to an electrically conductive polymer material or a non-electrically conductive polymer material, if necessary.

[0031] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector include at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector.

[0032] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material and, if necessary, may further contain other additives such as a conductive aid, a conductive material, and a binder. Examples of positive electrode active materials include lithium-transition metal composite oxides such as LiMn2O4, LiCoO2, LiNiO2, and Li(Ni-Mn-Co)O2, in which a portion of the transition metal in these oxides is substituted with other elements, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. From the viewpoints of capacity and input / output characteristics, a lithium-transition metal composite oxide is preferably used as the positive electrode active material. More preferably, a composite oxide containing lithium and nickel is used. Even more preferably, Li(Ni-Mn-Co)O2 and a portion of these transition metals in which a portion of the transition metal is substituted with other elements (hereinafter also referred to as "NMC composite oxide"), or a lithium-nickel-cobalt-aluminum composite oxide (hereinafter also referred to as "NCA composite oxide") is used. NMC composite oxides have a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co) atomic layers are stacked alternately with oxygen atomic layers interposed between them. Each transition metal M atom contains one Li atom, meaning that the amount of Li that can be extracted is twice that of spinel-type lithium manganese oxides, meaning that the supply capacity is doubled, resulting in high capacity.

[0033] From the viewpoint of achieving high output, the average particle diameter (D50) of the positive electrode active material is preferably 1 to 100 μm, and more preferably 1 to 20 μm. In this specification, the average particle diameter (D50) is measured using a particle size distribution measuring device using a laser diffraction / scattering method.

[0034] The thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm.

[0035] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material, and may further contain other additives such as a conductive aid, a conductive material, a binder, etc., as necessary. Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, lithium-transition metal composite oxides (e.g., Li4Ti5O 12 ), metal materials (silicon, tin), lithium alloy-based negative electrode materials (for example, lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). In some cases, two or more negative electrode active materials may be used in combination. From the viewpoints of capacity and input / output characteristics, carbon materials, metal materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials are preferably used as the negative electrode active material.

[0036] The average particle size (D50) of the negative electrode active material is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm.

[0037] The thickness of the negative electrode active material layer is usually about 1 to 1000 μm, preferably 10 to 800 μm, more preferably 15 to 600 μm, and even more preferably 20 to 200 μm.

[0038] [Electrolyte layer] The electrolyte layer is disposed adjacent to the electrode active material layer that constitutes the electrode, and has a configuration in which a separator is impregnated with the above-mentioned electrolyte solution for lithium secondary batteries.

[0039] The separator has a function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and a function of acting as a partition between the positive electrode and the negative electrode. Examples of the separator include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

[0040] [Positive and negative current collector plates] As a constituent material of the current collector plate, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), or an alloy thereof is preferred. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate and the negative electrode current collector plate may be made of the same material or different materials.

[0041] [Battery exterior] As the battery exterior, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element can be used as shown in Fig. 1. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto.

[0042] The lithium secondary battery according to the present embodiment can exhibit excellent input / output characteristics (rate characteristics) and sufficient cycle durability, and is therefore suitable for use as a driving power source for EVs and HEVs.

[0043] The following embodiments are also included within the scope of the present invention: an electrolyte solution for lithium secondary batteries according to claim 1 having the characteristics of claim 2; an electrolyte solution for lithium secondary batteries according to claim 1 or 2 having the characteristics of claim 3; an electrolyte solution for lithium secondary batteries according to any one of claims 1 to 3 having the characteristics of claim 4; an electrolyte solution for lithium secondary batteries according to any one of claims 1 to 4 having the characteristics of claim 5; an electrolyte solution for lithium secondary batteries according to any one of claims 1 to 5 having the characteristics of claim 6; and a lithium secondary battery comprising the electrolyte solution for lithium secondary batteries according to any one of claims 1 to 6. [Example]

[0044] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, operations and measurements of physical properties were carried out at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0045] <Preparation example of electrolyte for lithium secondary battery> The electrolyte solution was prepared under conditions of a dew point of −40° C. or lower.

[0046] [Example 1] An electrolyte solution precursor was prepared by dissolving lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 2.4 mol / L (2.4 M). The electrolyte solution precursor was diluted with 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 2.0 mol / L (2.0 M), to obtain the lithium secondary battery electrolyte solution of this example. In this electrolyte solution, the ratio of the number of moles of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether to the number of moles of lithium atoms contained in LiFSI was 0.64.

[0047] [Example 2] An electrolyte solution precursor was prepared by dissolving lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 3.0 mol / L (3.0 M). The electrolyte solution precursor was diluted with 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 2.0 mol / L (2.0 M), to obtain the lithium secondary battery electrolyte solution of this example. In this electrolyte solution, the ratio of the number of moles of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether to the number of moles of lithium atoms contained in LiFSI was 1.28.

[0048] [Example 3] An electrolyte solution precursor was prepared by dissolving lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 2.4 mol / L (2.4 M). The electrolyte solution precursor was diluted with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 2.0 mol / L (2.0 M), to obtain the lithium secondary battery electrolyte solution of this example. In this electrolyte solution, the ratio of the number of moles of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether to the number of moles of lithium atoms contained in LiFSI was 0.55.

[0049] [Example 4] An electrolyte precursor was prepared by dissolving lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 2.4 mol / L (2.4M). The electrolyte precursor was diluted with 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 1.5 mol / L (1.5M), to obtain the lithium secondary battery electrolyte of this example. In this electrolyte, the ratio of the number of moles of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether to the number of moles of lithium atoms contained in LiFSI was 1.91.

[0050] [Comparative Example 1] An electrolyte solution precursor was prepared by dissolving lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 4.0 mol / L (4.0 M). The electrolyte solution precursor was diluted with 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 2.0 mol / L (2.0 M), to obtain the lithium secondary battery electrolyte solution of this comparative example. In this electrolyte solution, the ratio of the number of moles of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether to the number of moles of lithium atoms contained in LiFSI was 1.91.

[0051] Comparative Example 2 An electrolyte solution precursor was prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI), a lithium salt, in dimethyl carbonate (DMC), a carbonate-based first solvent, to a lithium ion concentration of 4.0 mol / L (4.0 M). The electrolyte solution precursor was diluted with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, a fluorinated ether-based second solvent, to a final lithium ion concentration of 2.0 mol / L (2.0 M), to obtain the lithium secondary battery electrolyte solution of this comparative example. In this electrolyte solution, the ratio of the number of moles of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether to the number of moles of lithium atoms contained in LiFSI was 1.65.

[0052] Comparative Example 3 An electrolyte precursor was prepared by dissolving the lithium salt lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) in sulfolane (SL) to a lithium ion concentration of 4.0 mol / L (4.0 M). The electrolyte precursor was diluted with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether to a final lithium ion concentration of 2.0 mol / L (2.0 M), yielding an electrolyte for a lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether to the number of moles of lithium atoms contained in LiFSI was 2.62.

[0053] (Microscopic Raman spectroscopy of electrolyte) For each of the electrolyte solutions prepared in the Examples and Comparative Examples, microscopic Raman spectroscopy was performed by the following method to obtain a Raman spectrum. First, in a glove box with an argon atmosphere at a dew point of -68°C or less, the electrolyte solution was dropped into an aluminum container, and the container was set in a stainless steel jig with a glass window that allows measurements in an environment not exposed to the atmosphere, and sealed under an argon atmosphere. The jig was removed from the glove box and set in a microscopic Raman spectroscopy device (manufactured by JEOL Ltd.). The measurement conditions were as follows: a 100x objective lens was used, a laser with a wavelength of 532 nm was used as the incident light, and the slit width was 0.1 mm. The measurement range was 0 to 4000 cm. -1 The measurement time was 10 seconds, and the number of accumulations was 24. From the Raman spectrum obtained in this way, -1 Peak U, which is derived from the carbonate-based first solvent that is not coordinated with the nearby lithium, at 932 cm -1 Peaks (peak S) originating from carbonate-based first solvents coordinated to lithium in the vicinity of 730-770 cm -1 Peak G, which represents the aggregation of anions near 720 cm -1 The peak (peak A) derived from the anion in the vicinity was fitted with a normal distribution. The peak intensity and peak area of ​​each fitted peak were calculated. The peak area A of the peak derived from the carbonate-based first solvent S0 is the peak area A of peak S S and the peak area A of peak U U The results are shown in Table 1 below.

[0054] If the types (chemical structures) of the lithium salt, carbonate-based first solvent, and fluorinated ether-based second solvent are different, the positions (Raman shifts) of Peak U, Peak S, Peak G, and Peak A may vary. In this case, by comparing the Raman spectrum of the electrolyte with the Raman spectra of each component, it is possible to easily identify which peak originates from which component.

[0055] (Evaluation of ionic conductivity σ) The ionic conductivity of each electrolyte solution prepared in the Examples and Comparative Examples was measured at room temperature (25°C) using the following method. First, a 10 mm diameter SUS plate was placed 10 mm apart using a measurement jig. In a glove box with an argon atmosphere and a dew point of -68°C or lower, 1.5 mL of electrolyte solution was poured into the jig. Using electrochemical impedance spectroscopy, AC impedance was measured at an open circuit voltage and a potential amplitude of 10 mV while varying the frequency from 100 mHz to 300 kHz. The solution resistance was calculated from the intersection with the real axis. This was corrected using a cell constant obtained separately from a standard solution (a solution prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of EC:DEC = 3:7 (volume ratio)). The ionic conductivity was calculated from the obtained solution resistance value. The results are shown in Table 1 below.

[0056] (Evaluation of cycle durability) Using each of the electrolyte solutions prepared in the Examples and Comparative Examples, lithium secondary batteries were fabricated in the following manner, and their cycle durability was evaluated.

[0057] Cathode active material (LiNi 0.80 Mn 0.10 Co 0.10 A solid content consisting of 95% by mass of O2, 3% by mass of carbon black as a conductive additive, and 2% by mass of polyvinylidene fluoride (PVdF) as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), a slurry viscosity adjusting solvent, was added to this solid content and mixed to prepare a positive electrode active material slurry. The resulting positive electrode active material slurry was then applied to one side of an aluminum foil (thickness: 20 μm) current collector using a doctor blade and dried on a hot plate at 80°C for 1 hour. The resulting laminate was then pressed using a roll press to adjust the porosity of the positive electrode active material layer to 25%. The laminate was then placed in a vacuum dryer and dried at 130°C for 8 hours under vacuum conditions to form a positive electrode (basis weight: 30 mg / cm). 2 ) was produced.

[0058] A solid content consisting of 95.5% by mass of graphite (average particle size: 20 μm) as the negative electrode active material, 0.5% by mass of carbon black as the conductive additive, and 4% by mass of polyvinylidene fluoride (PVdF) as the binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent was added to this solid content and mixed to prepare a negative electrode active material slurry. The resulting negative electrode active material slurry was then applied to one side of a copper foil (thickness: 20 μm) as a current collector, and dried and pressed in the same manner as above to form a negative electrode (basis weight: 20 mg / cm). 2 , porosity 23%) was produced.

[0059] The positive electrode obtained above is 12 cm 2 , negative electrode 13cm 2 The positive electrode current collector (aluminum foil) was laminated with aluminum foil with an aluminum terminal, while the negative electrode current collector (copper foil) was laminated with copper foil with a nickel terminal.

[0060] Next, separators (manufactured by Celgard, made of polypropylene (PP)) were inserted on the electrode active material layer sides of the positive and negative electrodes to form a laminate. This laminate was sandwiched between heat-sealed aluminum laminate films (thickness 150 μm) that served as exterior bodies, and the electrolyte solution prepared above was poured into them. After that, the inside of the exterior body was depressurized to a vacuum using a vacuum sealer, the depressurization was temporarily released and returned to atmospheric pressure, and then the vacuum was reduced again to a vacuum degree of 99.7% and sealed, thereby producing a pouch-type lithium secondary battery (test cell) having a power generation element in which the positive electrode active material layer and the negative electrode active material layer were stacked so that they faced each other with the separator interposed therebetween.

[0061] The test cell was subjected to a charge-discharge cycle durability test. Specifically, to apply pressure uniformly across the electrode reaction surface, the electrode portion of the cell was sandwiched between rubber plates, which were then sandwiched between aluminum plates and fixed with bolts.

[0062] The first and second charge / discharge cycles were then performed at a rate of 0.1C. The cell voltage ranged from 2.5 to 4.3V, with charging in CC-CV (1 / 100C cutoff) mode and discharging in CC mode (with an 8-hour break between charge and discharge). The battery was then charged / discharged 100 times at a rate of 0.33C within the same cell voltage range. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate [%] during cycling. The results are shown in Table 1 below.

[0063] [Table 1]

[0064] The results shown in Table 1 demonstrate that the present invention improves the ionic conductivity of the electrolyte solution for lithium secondary batteries. It was also found that the lithium secondary battery using the electrolyte solution had sufficient cycle durability.

[0065] This application is based on Japanese Patent Application No. 2022-201025, filed on December 16, 2022, the disclosure of which is incorporated by reference in its entirety. [Explanation of symbols]

[0066] 10a stacked secondary battery, 11' Positive electrode current collector 12 Negative electrode current collector 13 positive electrode active material layer, 15 negative electrode active material layer, 17 electrolyte layer, 19 cell layer, 21 power generation elements, 25 Positive current collector plate (positive tab), 27 negative electrode current collector plate (negative electrode tab), 29 Laminating film.

Claims

1. a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent; an electrolyte solution for a lithium secondary battery, wherein a ratio of the number of moles of the fluorinated ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 or more and 2.0 or less, In the Raman spectrum measured by micro-Raman spectroscopy, the peak intensity I of the peak derived from the carbonate-based first solvent coordinated to lithium Ｓ The peak intensity I of the peak derived from the carbonate-based first solvent not coordinated with lithium Ｕ The ratio (I Ｕ / I Ｓ ) is 0.2 or more and 0.7 or less, The peak intensity I Ｓ The peak intensity I of the peak representing the aggregation of anions Ｇ The ratio (I Ｇ / I Ｓ ) is 0.01 or more and 0.3 or less, Peak area A of the peak derived from the carbonate-based first solvent Ｓ０ The peak area A of the peak derived from the carbonate-based first solvent not coordinated with lithium Ｕ The ratio (A Ｕ / A Ｓ０ ) is 0.15 or more and 0.3 or less, Peak area A of the peak derived from anion Ａ The peak area A of the peak representing the aggregation of the anion Ｇ The ratio (A Ｇ / A Ａ ) is 0.01 or more and 0.4 or less.

2. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein a ratio of the number of moles of the fluorinated ether-based second solvent to the number of moles of lithium atoms contained in the lithium salt is 0.4 or more and 1.5 or less.

3. The ratio (I Ｕ / I Ｓ ) is 0.25 or more and 0.6 or less, The ratio (I Ｇ / I Ｓ ) is 0.05 or more and 0.2 or less, The ratio (A Ｕ / A Ｓ０ ) is 0.2 or more and 0.3 or less, The ratio (A Ｇ / A Ａ 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the value of (a) is 0.05 or more and 0.3 or less.

4. 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the lithium salt is lithium bis(fluorosulfonyl)imide.

5. 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the carbonate-based first solvent is dimethyl carbonate.

6. 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the fluorinated ether-based second solvent is at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,1,3,3,3-hexafluoroisopropyl methyl ether.

7. A lithium secondary battery comprising the electrolyte solution for lithium secondary batteries according to claim 1 or 2.

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