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

Abstract

[Problem] The purpose of the present invention is to provide a means capable of improving ionic conductivity in an electrolyte solution for a lithium secondary battery. [Solution] 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 which is characterized in that: 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 0.4-2.0 (inclusive); the electrolyte solution for a lithium secondary battery shows a predetermined profile in a Raman spectrum measured by microscopic Raman spectroscopy.

Description

Technical Field

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

[0002] In recent years, the widespread adoption of various electric vehicles has been anticipated to address environmental and energy issues. Secondary batteries, such as those used to power motors and other onboard power sources, are being actively developed as a key to the widespread adoption of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium secondary batteries, are attracting significant attention due to their promise of high energy density and high output.

[0003] Here, Japanese Patent Gazette No. 6569883 (corresponding to U.S. Patent Application Publication No. 2019 / 131658) discloses a technology related to an electrolyte for the purpose of increasing the capacity of a lithium secondary battery. Specifically, the electrolyte contains: an electrolyte comprising a lithium salt having a specific structure, an organic solvent comprising a chain carbonate having a specific structure, and an unsaturated cyclic carbonate. Furthermore, it 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 in a concentration of 1.1 to 3.8 mol / L. Summary of the Invention

[0004] Problems to be solved by the invention

[0005] However, the present inventors conducted research and found that the electrolyte solutions described in the above-mentioned documents have a problem of low ion conductivity.

[0006] Therefore, an object of the present invention is to provide a means for improving the ion conductivity in an electrolyte solution for lithium secondary batteries.

[0007] Solutions for solving problems

[0008] The present inventors conducted intensive research to address the above-mentioned issues. They discovered that preparing an electrolyte solution by dissolving a lithium salt in a carbonate-based solvent at a predetermined concentration and then diluting the solution with a fluorinated ether-based solvent in a predetermined amount relative to the lithium salt significantly improves the ionic conductivity of the electrolyte solution. Furthermore, analysis of the electrolyte solution using micro-Raman spectroscopy revealed a predetermined profile in the Raman spectrum, leading to the completion of the present invention.

[0009] That is, one embodiment of the present invention relates to an electrolyte for a lithium secondary battery, comprising a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent, wherein 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 or more and 2.0 or less. Furthermore, the electrolyte for a lithium secondary battery has the following characteristics: in a Raman spectrum measured by micro-Raman spectroscopy, the peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium is U Peak intensity I relative to the peak derived from the carbonate-based first solvent coordinated with lithium S The ratio (I U / I S ) is 0.2 or more and 0.7 or less; The peak intensity I representing the peak of the aggregation of anions G Relative to the peak intensity I S The ratio (I G / I S ) is 0.01 or more and 0.3 or less; The peak area A of the peak of the first carbonate solvent not coordinated with lithium U Peak area A relative to the peak derived from the carbonate-based first solvent S0 The ratio (A U / A S0 ) is 0.15 or more and 0.3 or less; The peak area A representing the peak of the aggregation of anions G Peak area A relative to the peak derived from anions A The ratio (A G / A A ) is greater than or equal to 0.01 and less than or equal to 0.4. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a cross-sectional view schematically showing a flat (stacked) non-bipolar (internal parallel connection) lithium secondary battery (hereinafter also simply referred to as a “stacked secondary battery”) as one embodiment of the present invention. DETAILED DESCRIPTION

[0011] The following describes embodiments of the present invention. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. "X to Y" indicating a range means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties were performed at room temperature (20°C to 25°C) and a relative humidity of 40% to 50%.

[0012] <Electrolyte for lithium secondary batteries>

[0013] One embodiment of the present invention relates to an electrolyte for a lithium secondary battery (hereinafter also referred to as "electrolyte"), comprising a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent, wherein 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 or more and 2.0 or less. Furthermore, the electrolyte for a lithium secondary battery has the following characteristics: in a Raman spectrum measured by micro-Raman spectroscopy, the peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium is U Peak intensity I relative to the peak derived from the carbonate-based first solvent coordinated with lithium S The ratio (I U / I S ) is 0.2 or more and 0.7 or less; The peak intensity I representing the peak of the aggregation of anions G Relative to the peak intensity I S The ratio (I G / I S ) is 0.01 or more and 0.3 or less; The peak area A of the peak of the first carbonate solvent not coordinated with lithium U Peak area A relative to the peak derived from the carbonate-based first solvent S0 The ratio (A U / A S0 ) is 0.15 or more and 0.3 or less; The peak area A representing the peak of the aggregation of anions G Peak area A relative to the peak derived from anions A The ratio (A G / A A ) is 0.01 or more and 0.4 or less. According to this embodiment, the ion conductivity can be improved in the electrolyte for lithium secondary batteries. Hereinafter, this embodiment will be described in detail.

[0014] [Lithium salt]

[0015] The electrolyte of this embodiment contains a lithium salt as an electrolyte. The type of lithium salt is not particularly limited, and examples thereof include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), LiN(CF3SO2)2, Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. Among them, Li(FSO2)2N is preferred from the perspective of further improving the battery's input / output and charge / discharge cycle characteristics.

[0016] [Carbonate-based first solvent]

[0017] The electrolyte of this embodiment includes a carbonate-based first solvent as an organic solvent. As long as the carbonate-based first solvent has an R1-O-(C=O)-O-R2 structure (wherein R1 and R2 are each independently a monovalent organic group, optionally connected to each other to form a ring), its type is not particularly limited. As specific examples of the carbonate-based first solvent, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), etc. can be cited. Among them, from the viewpoint of being able to further improve the fast charging characteristics and input-output characteristics, chain carbonates are preferred, more preferably at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) and ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) is further preferred.

[0018] [Fluorinated ether-based second solvent]

[0019] The electrolyte of this embodiment also includes a fluorinated ether-based second solvent as an organic solvent. It is believed that the electrolyte of this embodiment has a structure in which lithium ions and four carbonate-based first solvent molecules are solvated. The lithium ions (hereinafter also referred to as "solvated Li") move in the electrolyte and the lithium ions move between the electrodes. It is believed that the electrolyte includes a fluorinated ether-based second solvent, and the fluorinated ether-based second solvent enters between adjacent solvated Li, solvated Li becomes easier to move, thereby improving the ionic conductivity of the electrolyte. It should be noted that the state of each molecule in such an electrolyte can be confirmed by Raman spectroscopy described later.

[0020] The type of the fluorinated ether-based 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 the ether (R3-O-R4 (herein, R3 and R4 are each independently a monovalent organic group, optionally linked to each other to form a ring) are fluorinated. Specific examples of the fluorinated ether-based 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-pentafluoropropyldifluoromethyl 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 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-hexafluorobutyldifluoromethyl ether, propyl 1,1,2,2-tetrafluoroethyl ether, butyl (1,1,2,2-tetrafluoroethyl) ether, bis (2,2,2-trifluoroethyl) ether, etc. Among them, from the viewpoint of further improving ion 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.

[0021] In the electrolyte of this embodiment, the ratio of the number of moles of the fluorinated ether 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) (hereinafter also referred to as the "molar ratio") must be greater than 0.4 and less than 2.0. If the molar ratio is less than 0.4, fewer molecules of the fluorinated ether second solvent enter between adjacent solvated Li, and therefore the effect of improving ionic conductivity may not be fully exerted. If the molar ratio exceeds 2.0, the proportion of solvated Li in the electrolyte becomes relatively small, and therefore the ionic conductivity may be reduced. From the viewpoint of further improving ionic conductivity, the molar ratio is preferably greater than 0.4 and less than 1.5, and more preferably greater than 0.5 and less than 1.3.

[0022] The concentration of lithium ions in the electrolyte of this embodiment is not particularly limited, and 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 further preferably 1.5 mol / L or more and 2 mol / L or less. If the concentration of lithium ions 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 becomes smaller. Thus, the corrosion of the aluminum foil as a current collector can be suppressed, and the durability of the lithium secondary battery can be improved. In a high-concentration electrolyte, the solvated Li and the anions form an aggregate structure (AGG structure), and the lithium ions are likely to become difficult to move. If the concentration of lithium ions is 2.7 mol / L or less, it is not easy to form an AGG structure, and the lithium ions can move smoothly, so that high ionic conductivity can be exerted.

[0023] (Raman spectroscopy)

[0024] The electrolyte solution of this embodiment is also characterized by exhibiting a predetermined profile in the Raman spectrum obtained by micro-Raman spectroscopy. The Raman spectrum can be used to determine whether the carbonate-based first solvent is coordinated with lithium ions and whether anions are aggregated. Therefore, by confirming whether the Raman spectrum exhibits a predetermined profile, it is possible to determine whether molecules with a predetermined structure are present in a predetermined proportion.

[0025] The electrolyte solution of this embodiment is characterized in that, in the Raman spectrum measured by micro-Raman spectroscopy,

[0026] (1) Peak intensity I of the peak derived from the carbonate-based first solvent not coordinated with lithium U Peak intensity I relative to the peak derived from the carbonate-based first solvent coordinated with lithium S The ratio (I U / I S ) is 0.2 or more and 0.7 or less;

[0027] (2) Peak intensity I representing the peak of anion aggregation G Peak intensity I relative to the peak derived from the carbonate-based first solvent coordinated with lithium S The ratio (I G / I S ) is 0.01 or more and 0.3 or less;

[0028] (3) Peak area A of the peak derived from the carbonate-based first solvent not coordinated with lithium U Peak area A relative to the peak derived from the carbonate-based first solvent S0 The ratio (A U / A S0 ) is 0.15 or more and 0.3 or less; and

[0029] (4) Peak area A representing the peak of anion aggregationG Peak area A relative to the peak derived from anions A The ratio (A G / A A ) is 0.01 or more and 0.4 or less. As the values of these ratios, the values measured by the methods described in the Examples described later are adopted.

[0030] The electrolyte of the present embodiment that satisfies these characteristics has a smaller proportion of anions having an aggregate structure (AGG structure) with solvated Li than conventional electrolytes. As a result, lithium ions can move smoothly in the electrolyte, and thus can exhibit high ion conductivity. In addition, the electrolyte of the present embodiment that 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 than conventional electrolytes. As a result, the corrosion of the aluminum foil as a current collector can be suppressed, and the durability of the lithium secondary battery can be improved.

[0031] From the same perspective, the electrolyte solution of this embodiment preferably satisfies the following characteristics:

[0032] Than(I U / I S ) is 0.25 or more and 0.6 or less;

[0033] Than(I G / I S ) is 0.05 or more and 0.2 or less;

[0034] Ratio (A U / A S0 ) is 0.2 or more and 0.3 or less;

[0035] Ratio (A G / A A ) is greater than or equal to 0.05 and less than or equal to 0.3.

[0036] <Method for producing electrolyte for lithium secondary battery>

[0037] The manufacture method of the lithium secondary battery electrolyte of this mode is not particularly limited, and is preferably manufactured by the following method. First, lithium salt is dissolved in carbonate-based first solvent to prepare electrolyte precursor (first process). Then, fluorinated ether-based second solvent is added to the electrolyte precursor, and the ratio of the mole number of the ether-based second solvent to the mole number of lithium atoms contained in the lithium salt becomes 0.4 or more and 2.0 or less to dilute, and prepare electrolyte (second process). According to such manufacture method, after forming the structure formed by the solvation of lithium ion and 4 carbonate-based first solvent molecules in the first process, in the second process, fluorinated ether-based second solvent can enter between adjacent solvated Li, and therefore the electrolyte of this mode can be easily obtained. Therefore, according to another embodiment of the present invention, a method for manufacturing an electrolyte for a lithium secondary battery is provided, which is the method for manufacturing the electrolyte for a lithium secondary battery described above, comprising: a first step of dissolving the lithium salt in the carbonate-based first solvent to prepare an electrolyte precursor; and a second step of adding the fluorinated ether-based second solvent to the electrolyte precursor, diluting the electrolyte precursor in such a manner that the ratio of the molar number of the ether-based second solvent to the molar number of lithium atoms contained in the lithium salt is 0.4 or more and 2.0 or less, thereby preparing an electrolyte. It should be noted that the lithium ion concentration of the electrolyte precursor is not particularly limited, but is preferably 1.5 mol / L or more and 3.2 mol / L or less, more preferably 1.8 mol / L or more and 3.0 mol / L or less, and even more preferably 2.0 mol / L or more and 2.5 mol / L or less.

[0038] <Lithium Secondary Battery>

[0039] The electrolyte for lithium secondary batteries of this embodiment has excellent ionic conductivity, so by applying it to lithium secondary batteries, the input and output characteristics of the battery can be improved. In addition, by applying it to lithium secondary batteries having aluminum foil as a current collector, the corrosion of the aluminum foil can be suppressed, thereby improving the durability of the lithium secondary battery. Therefore, according to another embodiment of the present invention, a lithium secondary battery is provided, which includes the electrolyte for lithium secondary batteries. A preferred embodiment of the lithium secondary battery has an aluminum current collector.

[0040] Figure 1 This is a cross-sectional view schematically showing a flat (stacked) non-bipolar (internal parallel connection) lithium secondary battery (hereinafter also simply referred to as a “stacked secondary battery”) as one embodiment of the present invention.

[0041] like Figure 1As shown, the stacked secondary battery 10a of this embodiment has a structure in which a generally rectangular power generation element 21, which actually performs charge and discharge reactions, is sealed within a laminate film 29. Here, the power generation element 21 has a stacked structure comprising a positive electrode having positive electrode active material layers 13 disposed on both sides of a positive electrode current collector 11', an electrolyte layer 17 formed of a separator containing an electrolyte solution, and a negative electrode having negative electrode active material layers 15 disposed on both sides of a negative electrode current collector 12. Specifically, the positive electrode, electrolyte layer, and negative electrode are stacked in this order, with one positive electrode active material layer 13 and an adjacent negative electrode active material layer 15 facing each other via the electrolyte layer 17.

[0042] Thus, the positive electrode, the electrolyte layer, and the negative electrode constitute one single cell layer 19. Figure 1 The stacked secondary battery 10a shown can also be said to have a structure formed by stacking a plurality of single cell layers 19 and electrically connecting them in parallel. It should be noted that, in the outermost positive electrode collectors located at the two outermost layers of the power generation element 21, the positive electrode active material layer 13 is only arranged on one side, but the active material layer can also be provided on both sides. That is, instead of a collector dedicated to the outermost layer having an active material layer only on one side, a collector having active material layers on both sides can be directly used as the outermost collector. In addition, it is also possible to make the configuration of the positive and negative electrodes different from that of the positive electrode. Figure 1 In contrast, the outermost negative electrode current collector is positioned at both outermost layers of the power generating element 21 , and the negative electrode active material layer is disposed on one or both surfaces of the outermost negative electrode current collector.

[0043] The structure is as follows: a positive electrode collector plate 25 and a negative electrode collector plate 27 are attached to the positive electrode collector 11' and the negative electrode collector 12, respectively, and are electrically connected to the respective electrodes (positive and negative). The plates are sandwiched between the ends of a laminate film 29 and are led out of the laminate film 29. The positive electrode collector plate 25 and the negative electrode collector plate 27 can be attached to the positive electrode collector 11' and the negative electrode collector 12 of each electrode via positive and negative terminal leads (not shown) as needed by ultrasonic welding, resistance welding, or the like.

[0044] Hereinafter, main components of the lithium secondary battery of this embodiment will be described.

[0045] [Current Collector]

[0046] The current collector has the function of mediating the movement of electrons from the electrode active material layer. The material constituting the current collector is not particularly limited, and for example, metals and conductive resins can be used.

[0047] Specifically, as metal, aluminum, nickel, iron, stainless steel, titanium, copper, etc. can be cited. In addition, it is preferred to use a cladding material of nickel and aluminum, a cladding material of copper and aluminum, or a plating material of a combination of these metals. In addition, it is also possible to use a foil formed by covering aluminum on the metal surface, or a carbon-coated aluminum foil. Among them, from the viewpoint of electronic conductivity and battery operating potential, aluminum, stainless steel, copper, and nickel are preferred. In addition, by applying aluminum foil as a current collector to the lithium secondary battery of this mode, the corrosion of the aluminum foil can be suppressed, and the durability of the lithium secondary battery can be improved. Therefore, it is more preferred to use aluminum foil as a current collector.

[0048] Examples of the latter conductive resin include conductive polymer materials or non-conductive polymer materials to which a conductive filler is added as needed.

[0049] It should be noted that the current collector may be a single-layer structure formed from a single material, or may be a laminated structure formed by appropriately combining layers formed from these materials. From the perspective of lightweighting the current collector, it is preferred to include at least a conductive resin layer formed from a conductive resin. In addition, from the perspective of blocking the movement of lithium ions between single cell layers, a metal layer may be provided on a portion of the current collector.

[0050] [Positive Electrode Active Material Layer]

[0051] The positive electrode active material layer contains a positive electrode active material and may further contain other additives such as a conductive auxiliary agent, a conductive member, and a binder as needed. As the positive electrode active material, for example, lithium-transition metal composite oxides, lithium-transition metal phosphate compounds, lithium-transition metal sulfate compounds, etc. such as LiMn2O4, LiCoO2, LiNiO2, Li(Ni-Mn-Co)O2 and materials in which a portion of these transition metals is replaced by other elements can be cited. Depending on the circumstances, two or more positive electrode active materials can also be used in combination. From the viewpoint of capacity and input-output characteristics, lithium-transition metal composite oxides are preferably used as the positive electrode active material. More preferably, a composite oxide containing lithium and nickel is used. It is further preferred to use Li(Ni-Mn-Co)O2 and materials in which a portion of these transition metals is replaced by other elements (hereinafter also referred to as "NMC composite oxide"), or lithium-nickel-cobalt-aluminum composite oxides (hereinafter also referred to as "NCA composite oxide"), etc. NMC composite oxides have a layered crystal structure composed of alternating layers of lithium atoms and transition metal (Mn, Ni, and Co) atoms, interposed with oxygen atoms. Furthermore, since each atom of the transition metal (M) contains one lithium atom, the amount of lithium that can be extracted is twice that of spinel-based lithium manganese oxides, meaning the supply capacity is doubled, enabling high capacity.

[0052] From the perspective of high output, the average particle size (D50) of the positive electrode active material is preferably 1 to 100 μm, more preferably 1 to 20 μm. In this specification, the average particle size (D50) is the particle size measured by a particle size distribution analyzer using a laser diffraction / scattering method.

[0053] 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.

[0054] [Negative Electrode Active Material Layer]

[0055] The negative electrode active material layer contains a negative electrode active material and may contain other additives such as a conductive auxiliary agent, a conductive member, and a binder as needed. Examples of the negative electrode active material include carbon materials such as graphite (black lead), soft carbon, hard carbon, lithium-transition metal composite oxides (such as Li4Ti5O 12 ), metal materials (silicon, tin), lithium alloy-based negative electrode materials (such as lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). Depending on the circumstances, two or more negative electrode active materials can also be used in combination. From the viewpoint of capacity and input-output characteristics, it is preferred to use carbon materials, metal materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials as negative electrode active materials.

[0056] From the viewpoint of increasing output, the average particle size (D50) of the negative electrode active material is preferably 1 to 100 μm, more preferably 1 to 20 μm.

[0057] 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.

[0058] [Electrolyte layer]

[0059] The electrolyte layer is disposed adjacent to the electrode active material layer constituting the electrode, and has a structure in which the separator is impregnated with the above-mentioned electrolyte solution for lithium secondary batteries.

[0060] The separator has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and also functions as a partition wall between the positive electrode and the negative electrode. Examples of the separator include a porous sheet separator formed of a polymer or fiber that absorbs and retains the electrolyte, and a non-woven fabric separator.

[0061] [Positive electrode collector plate and negative electrode collector plate]

[0062] As the constituent material of the collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. It should be noted that the same material or different materials may be used for the positive collector plate and the negative collector plate.

[0063] [Battery exterior]

[0064] As the battery outer body, in addition to the known metal can case, the battery outer body can also be used. Figure 1 As shown, a bag-shaped case is formed using a laminate film 29 containing aluminum that can cover the power generation element. For example, a three-layer laminate film of PP, aluminum, and nylon laminated in this order can be used as the laminate film, but it is not limited to these.

[0065] The lithium secondary battery of this embodiment can exhibit excellent input-output characteristics (rate characteristics) and sufficient cycle durability. Therefore, the lithium secondary battery of this embodiment is suitable for use as a driving power source for EVs and HEVs.

[0066] It should be noted that the following embodiments are also included in the scope of the present invention: the electrolyte for lithium secondary batteries according to claim 1 having the features of claim 2; the electrolyte for lithium secondary batteries according to claim 1 or 2 having the features of claim 3; the electrolyte for lithium secondary batteries according to any one of claims 1 to 3 having the features of claim 4; the electrolyte for lithium secondary batteries according to any one of claims 1 to 4 having the features of claim 5; the electrolyte for lithium secondary batteries according to any one of claims 1 to 5 having the features of claim 6; and a lithium secondary battery comprising the electrolyte for lithium secondary batteries according to any one of claims 1 to 6.

[0067] Example

[0068] The present invention is described in more detail below by way of examples. However, the technical scope of the present invention is not limited to the following examples. It should be noted that, unless otherwise specified, operations and measurements of physical properties were performed at room temperature (20°C to 25°C) and a relative humidity of 40% to 50%.

[0069] <Preparation Example of Electrolyte for Lithium Secondary Batteries>

[0070] The electrolyte was prepared at a dew point below -40°C.

[0071] [Example 1]

[0072] The electrolyte precursor is prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt in dimethyl carbonate (DMC) as a carbonate-based first solvent to a lithium ion concentration of 2.4 mol / L (2.4 M). 1,1,1,3,3,3-hexafluoroisopropyl methyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted to a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this embodiment. 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 is 0.64.

[0073] [Example 2]

[0074] The electrolyte precursor is prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt in dimethyl carbonate (DMC) as a carbonate-based first solvent to a lithium ion concentration of 3.0 mol / L (3.0 M). 1,1,1,3,3,3-hexafluoroisopropyl methyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted to a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this embodiment. 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 is 1.28.

[0075] [Example 3]

[0076] Lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt is dissolved in dimethyl carbonate (DMC) as a carbonate-based first solvent in a manner to obtain a lithium ion concentration of 2.4 mol / L (2.4 M) to prepare an electrolyte precursor. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted in a manner to obtain a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain an electrolyte for a lithium secondary battery of this embodiment. 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 is 0.55.

[0077] [Example 4]

[0078] The electrolyte precursor is prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt in dimethyl carbonate (DMC) as a carbonate-based first solvent to a lithium ion concentration of 2.4 mol / L (2.4 M). 1,1,1,3,3,3-hexafluoroisopropyl methyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted to a final lithium ion concentration of 1.5 mol / L (1.5 M) to obtain the electrolyte for the lithium secondary battery of this embodiment. 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 is 1.91.

[0079] [Comparative Example 1]

[0080] Lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt is dissolved in dimethyl carbonate (DMC) as a carbonate-based first solvent to a lithium ion concentration of 4.0 mol / L (4.0 M) to prepare an electrolyte precursor. 1,1,1,3,3,3-hexafluoroisopropyl methyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted to a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain an electrolyte for a lithium secondary battery of this comparative 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 is 1.91.

[0081] [Comparative Example 2]

[0082] Lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt is dissolved in dimethyl carbonate (DMC) as a carbonate-based first solvent in a manner to obtain a lithium ion concentration of 4.0 mol / L (4.0 M) to prepare an electrolyte precursor. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as a fluorinated ether-based second solvent is added to the electrolyte precursor and diluted to a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain 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 is 1.65.

[0083] [Comparative Example 3]

[0084] Lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI) as a lithium salt is dissolved in sulfolane (SL) to a lithium ion concentration of 4.0 mol / L (4.0 M) to prepare an electrolyte precursor. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is added to the electrolyte precursor and diluted to a final lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the lithium secondary battery electrolyte 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 is 2.62.

[0085] (Micro-Raman Spectroscopy of Electrolyte)

[0086] For each electrolyte prepared in the examples and comparative examples, micro-Raman spectroscopy was performed by the following method to obtain Raman spectra. First, in a glove box with an argon atmosphere at a dew point below -68°C, the electrolyte was dripped into an aluminum container, and the container was placed in a stainless steel jig with a glass window that can be measured in an environment without exposure to air, and sealed under an argon atmosphere. The jig was taken out of the glove box and placed in a micro-Raman spectrometer (manufactured by JEOL Ltd.). The measurement conditions used a 100x objective lens, a 532nm wavelength laser for incident light, and a slit width of 0.1mm. The measurement range was set to 0-4000cm -1 The measurement time is set to 10 seconds and the cumulative number of times is set to 24. According to the Raman spectrum obtained in this way, the normal distribution fitting of 912 cm -1 Nearby peaks derived from the carbonate-based first solvent that is not coordinated with lithium (peak U), 932 cm -1 Nearby peaks derived from the carbonate-based first solvent coordinated with lithium (peak S), 730-770 cm -1 The peaks indicating the accumulation of anions (peak G) near 720 cm -1 The peak derived from the anion near the peak (peak A). Then, the peak intensity and peak area are calculated for the fitted peaks. The peak area A of the peak derived from the carbonate-based first solvent is S0 By dividing 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.

[0087] It should be noted that when 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.

[0088] (Evaluation of ionic conductivity σ)

[0089] For each electrolyte prepared in the examples and comparative examples, the ion conductivity at room temperature (25°C) was measured by the following method. First, a measuring jig was used to place a SUS plate with a diameter of 10 mm at a distance of 10 mm. In a glove box with an argon atmosphere at a dew point below -68°C, 1.5 mL of electrolyte was injected into the jig. By electrochemical impedance spectroscopy, the AC impedance was measured while changing the frequency from 100 mHz to 300 kHz under the conditions of open circuit voltage and potential amplitude of 10 mV, and the solution resistance was calculated based on the intersection with the real axis. At this time, correction was performed using the cell constant (cell constant) obtained by measuring a standard solution (a solution obtained by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of EC:DEC=3:7 (volume ratio)). The ion conductivity was calculated from the value of the obtained solution resistance. The results are shown in Table 1 below.

[0090] (Evaluation of cycle durability)

[0091] Using each of the electrolyte solutions prepared in Examples and Comparative Examples, lithium secondary batteries were produced by the following methods, and the cycle durability was evaluated.

[0092] Prepare a positive electrode active material (LiNi 0.80 Mn 0.10 Co 0.10 O2) 95 mass %, carbon black as a conductive aid 3 mass % and polyvinylidene fluoride (PVdF) as a binder 2 mass % solid content. Relative to the solid content, an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjustment solvent is added and mixed to prepare a positive electrode active material slurry. Next, a scraper is used to apply the obtained positive electrode active material slurry to a single side of an aluminum foil (thickness 20 μm) as a collector, and dried on a hot plate at 80°C for 1 hour. Then, a roller press is used to press the obtained laminate, thereby controlling the porosity of the positive electrode active material layer to 25%. Then, it is placed in a vacuum dryer and dried at 130°C under vacuum conditions for 8 hours to prepare a positive electrode (weight per unit area 30 mg / cm 2 ).

[0093] Prepare a solid component containing 95.5% by mass of graphite (average particle size: 20 μm) as a negative electrode active material, 0.5% by mass of carbon black as a conductive additive, and 4% by mass of polyvinylidene fluoride (PVdF) as a binder. To this solid component, an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjustment solvent is added and mixed to prepare a negative electrode active material slurry. Next, the obtained negative electrode active material slurry is applied to one side of a copper foil (thickness 20 μm) as a collector, and dried and pressed in the same manner as above to prepare a negative electrode (weight per unit area 20 mg / cm 2 , porosity 23%).

[0094] Cut the positive electrode obtained above into 12 cm 2 Cut the negative electrode into 13cm squares. 2 Then, an aluminum foil with an aluminum terminal is stacked on the positive electrode current collector (aluminum foil). On the other hand, a copper foil with a nickel terminal is stacked on the negative electrode current collector (copper foil).

[0095] Next, a separator (made of polypropylene (PP) from Celgard) was inserted between the positive and negative electrode active material layers to form a stack. This stack was sandwiched between a heat-welded aluminum laminate film (150 μm thick) serving as an outer casing. After the electrolyte prepared above was injected, the interior of the outer casing was decompressed to a vacuum using a vacuum sealer. After temporarily releasing the pressure to return to atmospheric pressure, the pressure was again reduced to 99.7% and sealed. This produced a pouch-type lithium secondary battery (test cell) with a power generation element in which the positive and negative electrode active material layers were stacked facing each other with the separator interposed therebetween.

[0096] The test battery prepared above was subjected to a charge-discharge cycle durability test. Specifically, first, the battery electrode portion was clamped between rubber plates to uniformly apply pressure to the electrode reaction surface, and then clamped between aluminum flat plates and fixed with bolts.

[0097] Next, the first and second charge and discharge are performed at a rate of 0.1C. At this time, as the battery voltage, charging is performed in CC-CV (1 / 100C cutoff) mode within the range of 2.5 to 4.3V, and discharging is performed in CC mode (rest time between charging and discharging is 8 hours). Then, 100 cycles of charge and discharge are performed at a rate of 0.33C within the same battery voltage range as above. Then, the ratio of the discharge capacity of the 100th cycle to the discharge capacity of the first cycle is calculated as the capacity retention rate [%] during cycle endurance. The results are shown in Table 1 below.

[0098] [Table 1]

[0099]

[0100] The results shown in Table 1 indicate that the present invention improves ion conductivity in the electrolyte for lithium secondary batteries. It is also clear that lithium secondary batteries using this electrolyte have sufficient cycle durability.

[0101] 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.

[0102] Description of Reference Numerals

[0103] 10a stacked secondary battery,

[0104] 11' positive electrode collector

[0105] 12 negative electrode collector

[0106] 13 positive electrode active material layer,

[0107] 15 negative electrode active material layer,

[0108] 17 electrolyte layer,

[0109] 19 single cell layers,

[0110] 21 Power generation elements,

[0111] 25 positive electrode collector plate (positive electrode ear),

[0112] 27 negative electrode collector plate (negative electrode tab),

[0113] 29 laminated films.

Claims

1. An electrolyte for a lithium secondary battery, comprising a lithium salt, a carbonate-based first solvent, and a fluorinated ether-based second solvent, The ratio of the number of moles of the second fluorinated ether 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 of the lithium secondary battery electrolyte measured by micro-Raman spectroscopy, the peak intensity I of the peak derived from the carbonate-based first solvent that is not coordinated with lithium is U Peak intensity I relative to the peak derived from the carbonate-based first solvent coordinated with lithium S Ratio I U / I S is 0.2 or more and 0.7 or less, The peak intensity I representing the peak of anion aggregation G Relative to the peak intensity I S 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 not coordinated with lithium U Peak area A relative to the peak derived from the carbonate-based first solvent S0 A U / A S0 is 0.15 or more and 0.3 or less, The peak area A representing the accumulation of anions G Peak area A relative to the peak derived from anions A A G / A A It is 0.01 or more and 0.4 or less.

2. The electrolyte for lithium secondary batteries according to claim 1, wherein The ratio of the number of moles of the second fluorinated ether 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 electrolyte for lithium secondary batteries according to claim 1 or 2, wherein 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 It is 0.05 or more and 0.3 or less.

4. The electrolyte for lithium secondary batteries according to claim 1 or 2, wherein The lithium salt is lithium bis(fluorosulfonyl)imide.

5. The electrolyte for lithium secondary batteries according to claim 1 or 2, wherein The carbonate-based first solvent is dimethyl carbonate.

6. The electrolyte for lithium secondary batteries according to claim 1 or 2, 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 for a lithium secondary battery according to claim 1 .

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