Electrolyte for lithium secondary batteries, method for manufacturing the same, and lithium secondary batteries using the said electrolyte

Abstract

To provide means for improving the ionic conductivity in an electrolyte for a lithium secondary battery.SOLUTION: An electrolyte for a lithium secondary battery includes a lithium salt, a carbonate-based first solvent, and an ether-based second solvent, and the ether-based second solvent is at least one selected from the group consisting of hydrocarbon-based ethers having a linear symmetric structure, 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 greater than 0 and equal to or less than 0.70, and in a Raman spectrum obtained by micro-Raman spectroscopy measurement, the ratio (AU / AS0) of the peak area AU of the peak derived from the carbonate-based first solvent that is not coordinated with lithium to the peak area AS0 of the peak derived from the carbonate-based first solvent is greater than 0.24 and less than 0.60.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to an electrolyte for lithium secondary batteries, a method for producing the same, and a lithium secondary battery using the same electrolyte. [Background technology]

[0002] In recent years, the widespread adoption of various electric vehicles has been anticipated as a way to address environmental and energy problems. As a key element in the proliferation of these electric vehicles, intensive development efforts are underway to develop rechargeable batteries for on-board power sources, such as motor drive power supplies. Among rechargeable batteries, non-aqueous electrolyte batteries, such as lithium-ion batteries, which offer high energy density and high output, are attracting particular attention.

[0003] Here, Patent Document 1 discloses a technology relating to an electrolyte for the purpose of increasing the capacity of lithium secondary batteries. Specifically, the electrolyte comprises an electrolyte containing a lithium salt having a specific structure, an organic solvent containing a chain-like carbonate having a specific structure, and an unsaturated cyclic carbonate. Furthermore, the chain-like 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. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6569883 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, our inventors' investigation revealed that the electrolyte described in Patent Document 1 has the problem of low ionic conductivity.

[0006] Therefore, the present invention aims to provide a means for improving the ionic conductivity of an electrolyte for lithium secondary batteries. [Means for solving the problem]

[0007] The inventors diligently conducted research to solve the above problems. In the process, they found that by dissolving a lithium salt in a carbonate-based solvent at a predetermined concentration to obtain a solution, and then diluting the lithium salt with a predetermined amount of an ether-based solvent having a predetermined structure, the ionic conductivity of the electrolyte was significantly improved. Furthermore, when the electrolyte was analyzed by micro-Raman spectroscopy, it was found to exhibit a predetermined profile in the Raman spectrum, leading to the completion of the present invention.

[0008] In other words, 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 an ether-based second solvent, wherein the ether-based second solvent is at least one selected from the group consisting of hydrocarbon ethers having a linear symmetric structure, and 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 greater than 0 and less than or equal to 0.70. Furthermore, in the Raman spectrum measured by micro-Raman spectroscopy, the peak area A of the peak originating from the carbonate-based first solvent is greater than 0. S0 Peak area A of the peak originating from the first carbonate-based solvent that is not coordinated to lithium. U The ratio (A U / A S0 ) is characterized by being greater than 0.24 and less than 0.60. [Effects of the Invention]

[0009] According to the present invention, the ionic conductivity of the electrolyte for lithium secondary batteries can be improved. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described below, but the technical scope of the present invention should be determined based on the claims and is not limited to the following forms. The range "X~Y" means "X or more and Y or less". Unless otherwise specified, operations and measurements of physical properties, etc., will be performed under room temperature (20~25°C) and relative humidity of 40~50%RH.

[0011] <Electrolyte for lithium secondary batteries> One embodiment of the present invention relates to an electrolyte for a lithium secondary battery (hereinafter also simply referred to as "electrolyte") comprising a lithium salt, a carbonate-based first solvent, and an ether-based second solvent, wherein the ether-based second solvent is at least one selected from the group consisting of hydrocarbon ethers having a linear symmetric structure, and 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 greater than 0 and less than or equal to 0.70. Furthermore, in the Raman spectrum measured by micro-Raman spectroscopy, the peak area A of the peak originating from the carbonate-based first solvent is greater than 0. S0 Peak area A of the peak originating from the first carbonate-based solvent that is not coordinated to lithium. U The ratio (A U / A S0 This embodiment is characterized by the fact that the ionic conductivity is greater than 0.24 and less than 0.60. According to this embodiment, the ionic conductivity of the electrolyte for lithium secondary batteries can be improved. This embodiment will be described in detail below.

[0012] [Lithium salts] The electrolyte according to this embodiment contains a lithium salt as the electrolyte. The type of lithium salt is not particularly limited, but examples include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, etc. Among these, Li(FSO2)2N is preferred from the viewpoint of being able to further improve the battery input / output and charge / discharge cycle characteristics.

[0013] [Carbonate-based solvent #1] The electrolytic solution according to the present embodiment contains a carbonate-based first solvent as an organic solvent. The type of the carbonate-based first solvent is not particularly limited as long as it has a structure of R1-O-(C=O)-O-R2 (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 the carbonate-based first solvent include 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), and the like. Among them, from the viewpoint of further improving rapid charging characteristics and input / output characteristics, chain carbonates are preferred, at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) and ethyl methyl carbonate (EMC) is more preferred, and dimethyl carbonate (DMC) is even more preferred.

[0014] [Ether-based second solvent] The electrolytic solution according to the present embodiment further contains an ether-based second solvent having a predetermined structure as an organic solvent. Conventionally, when an electrolytic solution containing a carbonate-based solvent is used, there has been a problem that the aluminum foil serving as a current collector corrodes, lowering the durability of the lithium secondary battery. To address this problem, it has been known that adding an ether-based solvent can suppress the corrosion of the aluminum foil. However, it has been found that adding an ether-based solvent to an electrolytic solution containing a carbonate-based solvent causes a new problem of reduced ion conductivity. The present invention has been made by discovering a means for suppressing this reduction in ion conductivity. In the electrolytic solution according to the present embodiment, lithium ions are considered to have a structure solvated by four carbonate-based first solvent molecules. The solvated lithium ions (hereinafter also simply referred to as "solvated Li") move in the electrolytic solution, whereby the lithium ions move between the electrodes. When the electrolytic solution contains the ether-based second solvent having a predetermined structure, the ether-based second solvent enters between adjacent solvated Li, making the solvated Li easier to move, which is considered to improve the ionic conductivity of the electrolytic solution. The state of each molecule in such an electrolytic solution can be confirmed by a Raman spectrum to be described later.

[0015] The ether-based second solvent has a linear symmetric hydrocarbon ether structure. Ether-based solvents having other structures (for example, branched hydrocarbon ethers, cyclic hydrocarbon ethers, and hydrocarbon ethers having an asymmetric structure) cannot achieve the aforementioned effect of improving ionic conductivity. The hydrocarbon ether having a linear symmetric structure is R3-(OR4) n -O-(R4O) nIt preferably has a structure represented by -R3 (wherein R3 is a monovalent alkyl group, R4 is a divalent alkylene group, and n is an integer of 0 or more). R3 is preferably a methyl group, an ethyl group, a propyl group, or a butyl group, more preferably a methyl group or an ethyl group, and even more preferably a methyl group. R4 is preferably a methylene group, an ethylene group, a trimethylene group, or a tetramethylene group, more preferably an ethylene group or a trimethylene group, and even more preferably an ethylene group. n is preferably an integer of 1 to 4, more preferably an integer of 1 to 3, and even more preferably 1 or 2. Specific examples of the hydrocarbon-based ether having a linear symmetric structure include bis(2-methoxyethyl) ether, bis(2-ethoxyethyl) ether, bis[2-(2-methoxyethoxy)ethyl] ether, bis[2-(2-ethoxyethoxy)ethyl] ether, and the like. Among these, at least one selected from bis(2-methoxyethyl) ether and bis[2-(2-methoxyethoxy)ethyl] ether is preferred from the viewpoint of further improving ion conductivity.

[0016] In the electrolytic solution of the present embodiment, 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 (that is, the number of moles of lithium ions contained in the electrolytic solution) (hereinafter also simply referred to as "molar ratio") is required to be more than 0 and 0.70 or less. When the molar ratio is 0 (that is, the ether-based second solvent is not contained in the electrolytic solution), the interaction between solvated Li and anions cannot be reduced. This prevents lithium ions from moving smoothly in the electrolytic solution, so that the effect of improving ion conductivity cannot be obtained. When the molar ratio exceeds 0.70, the proportion of solvated Li in the electrolytic solution becomes relatively small, which may lead to a decrease in ion conductivity. From the viewpoint of further improving ion conductivity, the molar ratio is preferably 0.10 or more and 0.60 or less, and more preferably 0.17 or more and 0.35 or less.

[0017] The lithium ion concentration in the electrolyte 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, even more preferably 1.5 mol / L or more and 2.0 mol / L or less, and particularly preferably 1.8 mol / L or more and 2.0 mol / L or less. If the lithium ion concentration is 1.2 mol / L or higher, the proportion of the carbonate-based first solvent (uncoordinated solvent) that is not coordinated to the lithium ions becomes smaller. This suppresses corrosion of the aluminum foil as a current collector and can improve the durability of the lithium secondary battery. In high-concentration electrolytes, solvated Li and anions may form an aggregate structure (AGG structure), which may make it difficult for lithium ions to move. If the lithium ion concentration is 2.7 mol / L or less, the AGG structure is less likely to form, and lithium ions can move smoothly, so high ionic conductivity can be exhibited.

[0018] (Raman spectrum) The electrolyte according to this embodiment is also characterized by exhibiting a predetermined profile in the Raman spectrum obtained by micro-Raman spectroscopy. From the Raman spectrum, information such as whether or not the carbonate-based first solvent is coordinated to the lithium ions can be read. Therefore, by confirming whether or not the Raman spectrum has a predetermined profile, it is possible to determine whether or not molecules with a predetermined structure are present in a predetermined proportion.

[0019] In the Raman spectrum obtained by micro-Raman spectroscopy, the electrolyte in this embodiment shows a peak area A originating from the carbonate-based first solvent. S0 Peak area A of the peak originating from the first carbonate-based solvent that is not coordinated to lithium. U The ratio (A U / A S0 The ratio (A) is characterized by being greater than 0.24 and less than 0.60. U / A S0 The value of ) shall be the value measured by the method described in the examples below.

[0020] The electrolyte according to this embodiment, which satisfies the above characteristics, maintains solvated Li better than conventional electrolytes. As a result, lithium ions can move smoothly within the electrolyte, and high ionic conductivity can be achieved. Furthermore, the electrolyte according to this embodiment has a smaller proportion of carbonate-based primary solvent (uncoordinated solvent) that is not coordinated to lithium ions compared to conventional electrolytes. As a result, corrosion of the aluminum foil used as a current collector is suppressed, and the durability of the lithium secondary battery can be improved. From a similar viewpoint, ratio (A U / A S0 The value is preferably 0.30 or more and less than 0.60, and more preferably 0.40 or more and 0.57 or less.

[0021] <Method for manufacturing electrolyte for lithium secondary batteries> The method for producing the electrolyte for lithium secondary batteries according to this embodiment is not particularly limited, but it is preferable to produce it by the following method. First, a lithium salt is dissolved in a carbonate-based first solvent to prepare an electrolyte precursor (first step). Next, an ether-based second solvent is added to the electrolyte 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 greater than 0 and less than or equal to 0.70 to prepare the electrolyte (second step). With this production method, in the first step a structure is formed in which lithium ions are solvated by four carbonate-based first solvent molecules, and in the second step the ether-based second solvent can enter between adjacent solvated Li molecules, so that the electrolyte according to this embodiment can be easily obtained. Therefore, according to another embodiment of the present invention, a method for producing an electrolyte for a lithium secondary battery is provided, 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 ether-based second solvent to the electrolyte precursor to dilute it 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 greater than 0 and 0.70 or less, thereby preparing an electrolyte. 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.

[0022] <Lithium-ion secondary battery> The electrolyte for lithium secondary batteries according to this embodiment has excellent ionic conductivity, and by applying it to a lithium secondary battery, the input and output characteristics of the battery can be improved. Furthermore, by applying it to a lithium secondary battery having 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 above-mentioned electrolyte for lithium secondary batteries is provided. A lithium secondary battery according to a preferred embodiment has an aluminum current collector. [Examples]

[0023] 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 performed under room temperature (20-25°C) and relative humidity of 40-50%RH.

[0024] <Example of preparation of electrolyte for lithium secondary batteries> The electrolyte was prepared under conditions with a dew point of -40°C or lower.

[0025] [Comparative Example 1] The electrolyte for lithium secondary batteries in this comparative example was obtained by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI), a lithium salt, in dimethyl carbonate (DMC), a carbonate-based primary solvent, to a lithium ion concentration of 2.0 mol / L (2.0 M).

[0026] [Example 1] An electrolyte 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 2.1 mol / L (2.1 M). This electrolyte precursor was diluted with bis(2-methoxyethyl) ether, an ether-based second solvent, to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this example. In this electrolyte, the ratio of the number of moles of bis(2-methoxyethyl) ether to the number of moles of lithium atoms contained in LiFSI was 0.17.

[0027] [Example 2] An electrolyte 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 2.2 mol / L (2.2 M). Bis(2-methoxyethyl) ether, an ether-based second solvent, was added to this electrolyte precursor to dilute it to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this example. In this electrolyte, the ratio of the number of moles of bis(2-methoxyethyl) ether to the number of moles of lithium atoms contained in LiFSI was 0.32.

[0028] [Example 3] An electrolyte 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 2.4 mol / L (2.4 M). This electrolyte precursor was then diluted with bis(2-methoxyethyl) ether, an ether-based second solvent, to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this example. In this electrolyte, the ratio of moles of bis(2-methoxyethyl) ether to moles of lithium atoms in LiFSI was 0.59.

[0029] [Comparative Example 2] An electrolyte 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 2.6 mol / L (2.6 M). This electrolyte precursor was diluted with bis(2-methoxyethyl) ether, an ether-based second solvent, to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of bis(2-methoxyethyl) ether to the number of moles of lithium atoms contained in LiFSI was 0.81.

[0030] [Comparative Example 3] An electrolyte 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). This electrolyte precursor was diluted with bis(2-methoxyethyl) ether, an ether-based second solvent, to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of bis(2-methoxyethyl) ether to the number of moles of lithium atoms contained in LiFSI was 1.76.

[0031] [Comparative Example 4] An electrolyte precursor was prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI), a lithium salt, in dimethyl carbonate (DMC), a carbonate-based primary solvent, to a lithium ion concentration of 2.4 mol / L (2.4 M). Diethylene glycol ethyl methyl ether was added to this electrolyte precursor to dilute it to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of diethylene glycol ethyl methyl ether to the number of moles of lithium atoms contained in LiFSI was 0.52.

[0032] [Example 4] An electrolyte 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 2.4 mol / L (2.4 M). To this electrolyte precursor, bis[2-(2-methoxyethoxy)ethyl] ether, an ether-based second solvent, was added to dilute it to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this example. In this electrolyte, the ratio of the number of moles of bis[2-(2-methoxyethoxy)ethyl] ether to the number of moles of lithium atoms contained in LiFSI was 0.38.

[0033] [Example 5] An electrolyte 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 2.0 mol / L (2.0 M). To this electrolyte precursor, bis[2-(2-methoxyethoxy)ethyl] ether, an ether-based second solvent, was added to dilute it to a lithium ion concentration of 1.8 mol / L (1.8 M) to obtain the electrolyte for the lithium secondary battery of this example. In this electrolyte, the ratio of the number of moles of bis[2-(2-methoxyethoxy)ethyl] ether to the number of moles of lithium atoms contained in LiFSI was 0.25.

[0034] [Comparative Example 5] An electrolyte 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). To this electrolyte precursor, bis[2-(2-methoxyethoxy)ethyl] ether, an ether-based second solvent, was added to dilute it to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of bis[2-(2-methoxyethoxy)ethyl] ether to the number of moles of lithium atoms contained in LiFSI was 1.76.

[0035] [Comparative Example 6] An electrolyte precursor was prepared by dissolving lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI), a lithium salt, in dimethyl carbonate (DMC), a carbonate-based primary solvent, to a lithium ion concentration of 2.4 mol / L (2.4 M). 15-Crown-5 was added to this electrolyte precursor to dilute it to a lithium ion concentration of 2.0 mol / L (2.0 M) to obtain the electrolyte for the lithium secondary battery of this comparative example. In this electrolyte, the ratio of the number of moles of 15-Crown-5 to the number of moles of lithium atoms in LiFSI was 0.42.

[0036] [Comparative Example 7] The electrolyte for lithium secondary batteries of this comparative example was obtained using the same formulation amounts as in Example 3, except that dimethyl carbonate (DMC), a carbonate-based first solvent, bis(2-methoxyethyl) ether, an ether-based second solvent, and lithium bis(fluorosulfonyl)imide (Li(FSO2)2N, LiFSI), a lithium salt, were mixed simultaneously. In this electrolyte, the ratio of the number of moles of bis(2-methoxyethyl) ether to the number of moles of lithium atoms contained in LiFSI was 0.59.

[0037] (Micro-Raman spectroscopy measurement of electrolyte) For each electrolyte prepared in the examples and comparative examples, micro-Raman spectroscopy measurements were performed using the following method to obtain Raman spectra. First, the electrolyte was dropped into an aluminum container in a glove box with an argon atmosphere at a dew point of -68°C or lower. The container was then placed in a stainless steel jig with a glass window, which allows measurement in an air-free environment, and sealed under an argon atmosphere. The jig was removed from the glove box and placed in a micro-Raman spectrometer (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 cm². -1 The measurement time was set to 10 seconds, and the number of integrations was set to 24. From the Raman spectrum obtained in this way, 912 cm⁻¹ was obtained. -1 A peak (peak U) originates from a carbonate-based first solvent that is not coordinated to lithium in the vicinity, at 932 cm⁻¹. -1 The peak (peak S) originating from the carbonate-based first solvent coordinated to the nearby lithium was fitted with a normal distribution. Then, for each peak after fitting, the peak area A was determined. U and A S The following was calculated: Peak area A of the peak originating from the first carbonate-based solvent. S0 The peak area A of peak S S And the peak area A of peak U U It was calculated by adding the values. Then, from the obtained value, the ratio (A U / A S0 ) was calculated.

[0038] Furthermore, if the type (chemical structure) of the lithium salt and the carbonate-based first solvent differs, the positions (Raman shifts) of peaks U and S may change. In this case, by comparing the Raman spectrum of the electrolyte with the Raman spectra of each component, it is easy to identify which peak originates from which component.

[0039] (Evaluation of ionic conductivity σ) The ionic conductivity of each electrolyte prepared in the examples and comparative examples was measured at room temperature (25°C) using the following method. First, using a measuring jig, 10 mm diameter SUS plates were placed 10 mm apart. 1.5 mL of the electrolyte was poured into the jig in a glove box with an argon atmosphere and a dew point of -68°C or lower. By electrochemical impedance spectroscopy, the AC impedance was measured while varying the frequency from 100 mHz to 300 kHz under conditions of an open-circuit voltage and potential amplitude of 10 mV, and the solution resistance was calculated from the intersection with the real axis. At that time, correction was made using the cell constant, which had been determined separately by measuring a standard solution (a solution in which LiPF6 was dissolved 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.

[0040] [Table 1]

[0041] The results shown in Table 1 demonstrate that, according to the present invention, the ionic conductivity of the electrolyte for lithium secondary batteries is improved.

Claims

1. It comprises a lithium salt, a carbonate-based first solvent, and an ether-based second solvent. The ether-based second solvent is at least one selected from the group consisting of hydrocarbon ethers having a linear, symmetrical structure. An electrolyte for a lithium secondary battery, wherein 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 greater than 0 and less than or equal to 0.70, In the Raman spectrum obtained by micro-Raman spectroscopy, the peak area A of the peak originating from the carbonate-based first solvent. Ｓ０ Peak area A of the peak originating from the first carbonate-based solvent that is not coordinated to lithium. Ｕ The ratio (A Ｕ / A Ｓ０ ) is greater than 0.24 and less than 0.60, An electrolyte for lithium secondary batteries having a lithium ion concentration of 1.2 mol / L or more and 2.7 mol / L or less.

2. A lithium salt comprising a carbonate-based first solvent and an ether-based second solvent, The ether-based second solvent is at least one selected from the group consisting of hydrocarbon ethers having a linear, symmetrical structure. An electrolyte for a lithium secondary battery, wherein 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 greater than 0 and less than or equal to 0.70, In the Raman spectrum obtained by micro-Raman spectroscopy, the ratio of the peak area A U of the carbonate-based first solvent that is not coordinated to lithium to the peak area A S0 of the carbonate-based first solvent (A U / A S0) is greater than 0.24 and less than 0.

60. The ether-based second solvent is at least one selected from bis(2-methoxyethyl) ether and bis[2-(2-methoxyethoxy)ethyl] ether, in an electrolyte for a lithium secondary battery.

3. A solution comprising a lithium salt, a carbonate-based first solvent, and an ether-based second solvent, The ether-based second solvent is at least one selected from the group consisting of hydrocarbon ethers having a linear, symmetrical structure. An electrolyte for a lithium secondary battery, wherein 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 greater than 0 and less than or equal to 0.70, In the Raman spectrum obtained by micro-Raman spectroscopy, the ratio of the peak area A U of the carbonate-based first solvent that is not coordinated to lithium to the peak area A S0 of the carbonate-based first solvent (A U / A S0) is greater than 0.24 and less than 0.

60. The lithium salt is lithium bis(fluorosulfonyl)imide, which is the electrolyte for lithium secondary batteries.

4. The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium ion concentration is 1.8 mol / L or more and 2.0 mol / L or less.

5. A method for producing an electrolyte for a lithium secondary battery according to any one of claims 1 to 4, The first step involves dissolving the lithium salt in the carbonate-based first solvent to prepare an electrolyte precursor. A second step involves adding the ether-based second solvent to the electrolyte precursor and diluting it so that the ratio of moles of the ether-based second solvent to moles of lithium atoms contained in the lithium salt is greater than 0 and less than or equal to 0.70, thereby preparing the electrolyte. A method for producing an electrolyte for lithium secondary batteries, comprising the characteristics of:

6. A lithium secondary battery comprising the electrolyte for lithium secondary batteries according to any one of claims 1 to 4.

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