Non-aqueous electrolyte power storage element and production method for same

Abstract

A non-aqueous electrolyte power storage element according to one aspect of the present invention comprises: a positive electrode containing a sulfur-based active material; and a non-aqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent. The fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester. The total content of the ionic liquid and the fluorinated solvent with respect to all components other than the electrolyte salt in the non-aqueous electrolyte is 90 mol% or more.

Description

Nonaqueous electrolyte storage element and method for manufacturing same

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and a method for producing the same.

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring charge-transporting ions such as lithium ions between the electrodes. Other non-aqueous electrolyte energy storage elements besides non-aqueous electrolyte secondary batteries include capacitors such as lithium ion capacitors and electric double layer capacitors.

[0003] Known nonaqueous electrolyte storage elements include lithium-sulfur batteries (Li-S batteries) and other nonaqueous electrolyte storage elements that use a sulfur-based active material as the positive electrode active material (see Patent Document 1). Sulfur-based active materials have a large theoretical capacity, and nonaqueous electrolyte storage elements that use a sulfur-based active material as the positive electrode active material are expected to have high energy density.

[0004] JP 2010-95390 A

[0005] Non-aqueous electrolyte storage elements using a sulfur-based active material as the positive electrode active material generally use a negative electrode containing a negative electrode active material capable of supplying charge-transporting ions capable of charging and discharging. The initial charge / discharge cycle of the non-aqueous electrolyte storage element after assembly begins with discharge. Hereinafter, this initial discharge without charging is also referred to as chemical conversion treatment. Some non-aqueous electrolyte storage elements have a low ratio of the amount of electricity discharged during chemical conversion treatment (chemical conversion capacity) to the amount of electricity charged during the initial subsequent charge (initial charge capacity). Hereinafter, the ratio of the initial charge capacity to the chemical conversion capacity is also referred to as the "initial reverse Coulombic efficiency." For example, when a negative electrode containing metallic lithium is used, a low initial reverse Coulombic efficiency means that a large amount of rechargeable lithium ions released from the negative electrode during chemical conversion treatment is consumed, resulting in a decrease in discharge capacity during subsequent charge / discharge. Therefore, a low initial reverse Coulombic efficiency requires the preparation of a negative electrode containing excess metallic lithium that is not used during normal charge / discharge, resulting in increased production costs. Therefore, in a nonaqueous electrolyte energy storage element using a sulfur-based active material as the positive electrode active material, it is desirable that the initial reverse Coulomb efficiency be high. The same is true when using a negative electrode other than a negative electrode containing metallic lithium, from the viewpoint of suppressing consumption of charge-transporting ions (lithium ions, etc.) that are released from the negative electrode during chemical conversion treatment.

[0006] An object of the present invention is to provide a nonaqueous electrolyte storage element that uses a sulfur-based active material as a positive electrode active material and has high initial reverse Coulomb efficiency, and a method for producing such a nonaqueous electrolyte storage element.

[0007] A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the nonaqueous electrolyte other than the electrolyte salt is 90 mol % or more.

[0008] A method for producing a nonaqueous electrolyte storage element according to one aspect of the present invention includes preparing a positive electrode containing a sulfur-based active material, and preparing a nonaqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the nonaqueous electrolyte other than the electrolyte salt is 90 mol % or more.

[0009] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte storage element in which a sulfur-based active material is used as a positive electrode active material and which has high initial reverse Coulomb efficiency, and a method for manufacturing such a nonaqueous electrolyte storage element.

[0010] Fig. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element, and Fig. 2 is a schematic view showing one embodiment of an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements.

[0011] First, an outline of the nonaqueous electrolyte storage element and the method for manufacturing the nonaqueous electrolyte storage element disclosed in this specification will be described.

[0012] [1] A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the nonaqueous electrolyte other than the electrolyte salt is 90 mol % or more.

[0013] The nonaqueous electrolyte storage element described in [1] above is a nonaqueous electrolyte storage element that uses a sulfur-based active material as the positive electrode active material and has a high initial reverse Coulombic efficiency. While the reason for this is unclear, the following is presumed. One factor contributing to the low initial reverse Coulombic efficiency of conventional nonaqueous electrolyte storage elements that use a sulfur-based active material as the positive electrode active material is that a specific solvent irreversibly reacts on the positive electrode surface during chemical conversion treatment, forming a coating on the positive electrode surface. In contrast, such irreversible reactions are unlikely to occur with ionic liquids, fluorinated ethers, and fluorinated carboxylic acid esters. In the nonaqueous electrolyte storage element described in [1] above, these components account for a large portion of the nonaqueous electrolyte, thereby suppressing irreversible reactions during chemical conversion treatment and presumably resulting in a high initial reverse Coulombic efficiency. Furthermore, in the nonaqueous electrolyte storage element described in [1] above, since the nonaqueous electrolyte contains a fluorinated solvent in addition to an ionic liquid, the viscosity of the nonaqueous electrolyte is relatively low, enabling the element to exhibit sufficient charge / discharge performance. By reducing the viscosity of the non-aqueous electrolyte, it is possible to exhibit good charge / discharge performance even when used at a relatively high current density, for example.

[0014] In the present invention, the content of each component contained in the non-aqueous electrolyte is a value measured at 20°C under 1 atmosphere or a value converted to a numerical value at 20°C under 1 atmosphere. The types of ionic compounds such as electrolyte salts and ionic liquids contained in the non-aqueous electrolyte can be determined by ion chromatography (IC), liquid chromatography-mass spectrometry (LC-MS), or other methods. １ The content of ionic compounds such as electrolyte salts and ionic liquids contained in the non-aqueous electrolyte is determined by IC. However, the content of cations in the ionic liquid contained in the non-aqueous electrolyte is determined by LC-MS. If the content cannot be determined by LC-MS, １The IC is determined by the internal standard method using H-NMR. Specifically, the IC is measured as follows: (A1) Collection of Non-Aqueous Electrolyte: First, the non-aqueous electrolyte storage element is charged at a constant current of 0.1 C to the end-of-charge voltage during normal use, resulting in a fully charged state. Here, "normal use" refers to the case where the non-aqueous electrolyte storage element is used under the charge / discharge conditions recommended or specified for the non-aqueous electrolyte storage element, and if equipment for using the non-aqueous electrolyte storage element is available, the non-aqueous electrolyte storage element is used with that equipment. "Current of 1 C" refers to a current that can discharge the theoretical capacity of the non-aqueous electrolyte storage element in one hour. When the theoretical capacity of the positive electrode is smaller than that of the negative electrode, the theoretical capacity of the positive electrode is used as the theoretical capacity of the non-aqueous electrolyte storage element; when the theoretical capacity of the negative electrode is smaller than that of the positive electrode, the theoretical capacity of the negative electrode is used. The theoretical capacity of the positive electrode is calculated from the product of the capacity density per unit area of ​​one positive electrode active material layer and the area of ​​the positive electrode active material layer. The capacity density per unit area of ​​one positive electrode active material layer is calculated by multiplying the mass (g / cm) per unit area of ​​the positive electrode active material layer. ２The theoretical capacity of the negative electrode is calculated from the product of the mass % of the positive electrode active material in the positive electrode active material layer, the mass % of the positive electrode active material, and the theoretical capacity of the positive electrode active material (mAh / g). The theoretical capacity of the negative electrode is calculated according to the method for calculating the theoretical capacity of the positive electrode. Next, the fully charged nonaqueous electrolyte storage element is disassembled to remove the nonaqueous electrolyte. If the nonaqueous electrolyte cannot be removed, the nonaqueous electrolyte storage element is centrifuged to remove the nonaqueous electrolyte. If the nonaqueous electrolyte cannot be removed even after centrifugation, an appropriate extraction solvent (e.g., acetonitrile) is injected into the nonaqueous electrolyte storage element, and the nonaqueous electrolyte diluted with the extraction solvent is removed. (A2) IC Analysis The components of the collected nonaqueous electrolyte are analyzed by IC. The IC analysis is performed in the following order: qualitative analysis and quantitative analysis. (Qualitative Analysis) The measurement sample (nonaqueous electrolyte) is subjected to IC analysis. The components contained in the measurement sample are predicted from the peak positions of each peak in the obtained ion chromatogram. A known sample of the predicted component (hereinafter referred to as the "predicted component") is subjected to IC analysis. The retention time of the peak corresponding to each predicted component of the measured sample is compared with the retention time of the peak of a known sample of each predicted component, and if they match, the prediction is assumed to be correct. (Quantitative Analysis) Quantitative analysis is performed using the calibration curve method. First, a known sample of a predicted component with a known concentration is subjected to IC analysis, and the peak area is determined to create a calibration curve. The calibration curve is calculated using the coefficient of determination (r ２ ) is created so that the difference is between 0.999 and 1. The content of the predicted component in the measurement sample is determined from the calibration curve and the area of ​​the peak of the predicted component in the measurement sample. The above procedure is performed for all peaks detected in the IC analysis of the measurement sample to determine the content of each predicted component. In addition, in order to reduce the variation in measurement error between measurements, quantitative analysis is performed on the same day using the same equipment and under the same conditions unless there are special circumstances, and consumables should not be replaced or the equipment adjusted until all measurements are completed, and the same applies below.

[0015] The type and content of nonionic compounds such as fluorinated solvents contained in the nonaqueous electrolyte are identified by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS). However, when it is difficult to identify the type of nonionic compound by the above-mentioned analysis, １The components are identified by combining the necessary analyses, including H-NMR, multinuclear NMR, and other analyses. LC-MS and GC-MS measurements are specifically performed as follows: (B1) Collection of non-aqueous electrolyte: The non-aqueous electrolyte is extracted using the same procedure as in "(A1) Collection of non-aqueous electrolyte" above. (B2) LC-MS: The components of the extracted non-aqueous electrolyte are analyzed by LC-MS. LC-MS analysis is performed in the following order of qualitative and quantitative analysis. Waters' "Acquity H" and "Xevo G2-5QTof" LC-MS analyzers are used. Water or a non-aqueous solvent is used as the eluent. Examples of non-aqueous solvents include acetonitrile and tetrahydrofuran. If it is difficult to perform measurements using the above-mentioned measuring devices, another model that is expected to produce equivalent measurement results may be used, as follows. (Qualitative Analysis) The measurement sample (non-aqueous electrolyte) is subjected to LC-MS analysis. If the peaks in the obtained liquid chromatogram cannot be separated, GC-MS analysis, as described below, is performed instead of LC-MS analysis. If the peaks can be separated, the components contained in the measured sample are predicted from the MS spectrum of each peak obtained by LC-MS analysis. A known sample of the predicted components is subjected to LC-MS analysis. The retention time and MS spectrum of the peak corresponding to each predicted component in the measured sample are compared with the retention time and MS spectrum of the peak in a known sample of each predicted component, and if they match, the prediction is assumed to be correct. (Quantitative Analysis) Quantitative analysis is performed using the calibration curve method. Quantitative analysis by LC-MS is performed using the same procedure as the quantitative analysis by IC described above, and the content of each predicted component is determined. (B3) GC-MS GC-MS analysis is performed in the following order: qualitative analysis and quantitative analysis. The GC-MS analysis is performed using an Agilent "5975C" analyzer. Argon is used as the carrier gas. (Qualitative Analysis) A measurement sample (non-aqueous electrolyte) is subjected to GC-MS analysis. The components contained in the sample are predicted from the MS spectrum of each peak in the obtained gas chromatogram. A known sample of the predicted components is subjected to GC-MS analysis.The retention time and MS spectrum of the peak corresponding to the predicted component of the measured sample are compared with the retention time and MS spectrum of the peak of a known sample of each predicted component, and if they match, the prediction is assumed to be correct. (Quantitative Analysis) Quantitative analysis is performed using the calibration curve method. Quantitative analysis by GC-MS is performed using the same procedure as the quantitative analysis by IC described above, and the content of each predicted component is determined.

[0016] [2] In the nonaqueous electrolyte storage element according to the above [1], the fluorinated solvent may be a chain solvent.

[0017] When the fluorinated solvent is a chain solvent, the viscosity tends to be lower, and therefore the nonaqueous electrolyte storage element described in [2] above has a lower viscosity nonaqueous electrolyte, and can exhibit better charge / discharge performance, etc.

[0018] [3] In the nonaqueous electrolyte storage element according to [1] or [2] above, the molar ratio of the ionic liquid to the fluorinated solvent (ionic liquid / fluorinated solvent) may be within a range of 5 / 95 to 99 / 1.

[0019] The nonaqueous electrolyte storage element described in [3] above can exhibit a higher initial reverse Coulomb efficiency and better charge / discharge performance by having a ratio of the ionic liquid to the fluorinated solvent within a suitable range.

[0020] [4] In the nonaqueous electrolyte storage element according to any one of [1] to [3] above, the ionic liquid may contain at least one cation selected from the group consisting of quaternary ammonium cations, imidazolium cations, pyrrolidinium cations, piperidinium cations, quaternary phosphonium cations, and sulfonium cations.

[0021] [5] In the nonaqueous electrolyte storage element according to any one of [1] to [4] above, the ionic liquid may have an imide anion.

[0022] [6] In the nonaqueous electrolyte storage element according to any one of [1] to [5] above, the electrolyte salt may be an imide salt.

[0023] The nonaqueous electrolyte storage elements described in [4] to [6] above are all suitable embodiments of the present invention, and can exhibit higher initial reverse Coulomb efficiency and better charge / discharge performance.

[0024] [7] In the nonaqueous electrolyte storage element according to any one of [1] to [6] above, the viscosity of the nonaqueous electrolyte at 25° C. may be 100 mPa·s or less.

[0025] The nonaqueous electrolyte storage element described in the above [7] can exhibit better charge / discharge performance because the viscosity of the nonaqueous electrolyte is particularly sufficiently reduced.

[0026] [8] The nonaqueous electrolyte storage element according to any one of [1] to [7], wherein the positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of ​​the positive electrode active material layer is 15 mg / cm ２ It may be the following:

[0027] The nonaqueous electrolyte storage element according to the above [8] has a positive electrode active material layer having a mass per unit area of ​​15 mg / cm ２ or less, it is possible to improve the charge / discharge performance when used at a relatively high current density.

[0028] "Mass per unit area of ​​the positive electrode active material layer (mg / cm ２ ")" is the area of ​​the positive electrode active material layer (1 cm ２ )). The area of ​​the positive electrode active material layer refers to the area of ​​one of the front and back surfaces (two surfaces other than the side surfaces) of one positive electrode active material layer. That is, for example, when the positive electrode active material layer is provided by coating, the area of ​​the positive electrode active material layer is equal to the area where the positive electrode active material layer is coated. When the positive electrode active material layer is provided on both surfaces of the positive electrode substrate, the area and mass of the positive electrode active material layer refer to the area and mass of one positive electrode active material layer. For example, when the positive electrode active material layer is provided on both surfaces of the positive electrode substrate with a thickness of 10 mg / cm, the area and mass of the positive electrode active material layer are equal to the area where the positive electrode active material layer is coated. ２ When the positive electrode active material layer is formed with a coating amount (solid content equivalent) of 10 mg / cm ２ The positive electrode active material layer is formed on one side of the positive electrode substrate in an amount of 10 mg / cm ２Even when the coating amount (solid content equivalent) of the positive electrode active material layer is 10 mg / cm ２ "The mass per unit area of ​​the positive electrode active material layer (mg / cm ２ ")" is a value measured when the nonaqueous electrolyte electricity storage element is in a fully charged state. However, if it is difficult to measure the nonaqueous electrolyte electricity storage element in a fully charged state for technical or safety reasons, the value obtained by converting the measurement result in a discharged state to a fully charged state is used.

[0029] [9] The nonaqueous electrolyte storage element according to any one of [1] to [7], wherein the positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of ​​the positive electrode active material layer is 5 mg / cm ２ It may be more than that.

[0030] The nonaqueous electrolyte storage element according to the above [9] has a positive electrode active material layer having a mass per unit area of ​​5 mg / cm ２ As a result, the energy density per volume of the nonaqueous electrolyte electricity storage element can be increased.

[0031]

[10] The nonaqueous electrolyte storage element according to any one of [1] to [7], wherein the positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of ​​the positive electrode active material layer is 10 mg / cm ２ It may be more than that.

[0032] The nonaqueous electrolyte storage element according to the above

[10] has a positive electrode active material layer having a mass per unit area of ​​10 mg / cm ２ As a result, the energy density per volume of the nonaqueous electrolyte electricity storage element can be increased.

[0033]

[11] The nonaqueous electrolyte storage element according to any one of [1] to

[10] above may further comprise a negative electrode containing metallic lithium at least in a charged state.

[0034] The nonaqueous electrolyte storage element described in

[11] above has advantages such as a particularly high energy density due to the use of metallic lithium in the negative electrode.

[0035]

[12] In the nonaqueous electrolyte storage element according to any one of [1] to

[11] above, at least one of the electrolyte salt and the ionic liquid may have bis(fluorosulfonyl)imide as an anion.

[0036] The nonaqueous electrolyte electricity storage element described in the above

[12] can improve the charge / discharge performance when used at a relatively high current density.

[0037]

[13] In the nonaqueous electrolyte storage element according to any one of [1] to

[12] above, the fluorinated solvent may be a fluorinated ether, and the molar ratio of the ionic liquid to the fluorinated ether (ionic liquid / fluorinated ether) may be within a range of 10 / 90 to 99 / 1.

[0038]

[14] In the nonaqueous electrolyte storage element according to any one of [1] to

[12] above, the fluorinated solvent may be a fluorinated carboxylic acid ester, and the molar ratio of the ionic liquid to the fluorinated carboxylic acid ester (ionic liquid / fluorinated carboxylic acid ester) may be within a range of 10 / 90 to 99 / 1.

[0039]

[15] In the nonaqueous electrolyte storage element according to any one of [1] to

[14] above, the molecular weight of the fluorinated solvent may be 60 or more and 300 or less.

[0040]

[16] The nonaqueous electrolyte storage element according to any one of [1] to

[15] , wherein the content of the electrolyte salt in the nonaqueous electrolyte is 0.3 mol / dm ３ 3.0mol / dm or more ３ It may be the following:

[0041] The nonaqueous electrolyte storage elements described in the above

[13] to

[16] are also suitable embodiments of the present invention.

[0042]

[17] A method for producing a nonaqueous electrolyte storage element according to one aspect of the present invention includes: preparing a positive electrode including a sulfur-based active material; and preparing a nonaqueous electrolyte including an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the nonaqueous electrolyte other than the electrolyte salt is 90 mol % or more.

[0043] According to the method for producing a nonaqueous electrolyte storage element described in

[17] above, it is possible to produce a nonaqueous electrolyte storage element that uses a sulfur-based active material as the positive electrode active material and that has high initial reverse Coulomb efficiency. Furthermore, the nonaqueous electrolyte storage element obtained by the method for producing a nonaqueous electrolyte storage element described in

[17] above can exhibit sufficient charge-discharge performance.

[0044] A nonaqueous electrolyte electricity storage element, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, and other embodiments will be described in detail below. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0045] <Non-aqueous electrolyte storage element> A non-aqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container that accommodates the electrode assembly and the non-aqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. At least a portion of the non-aqueous electrolyte exists in a state in which it permeates the voids between the positive electrode, the negative electrode, and the separator. As an example of a non-aqueous electrolyte storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0046] It should be noted that the lower and upper limits of each numerical range described in the embodiments of the present invention can be combined in any way (with the proviso that the upper limit is greater than the lower limit). Unless otherwise specified, the lower and upper limits of each numerical range include the lower and upper limits. In other words, when the lower limit is "A," it means that the range is equal to or greater than A. Similarly, when the upper limit is "B," it means that the range is equal to or less than B. Unless otherwise specified, the lower and upper limits of each numerical range are the values ​​when the nonaqueous electrolyte storage element is in a fully charged state.

[0047] (Positive Electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0048] The positive electrode substrate has electrical conductivity. Whether or not it has electrical conductivity is determined by whether or not the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 －２ The resistance is determined using Ω cm as a threshold value. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0049] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, it is possible to increase the strength of the positive electrode substrate and increase the energy density per volume.

[0050] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0051] The positive electrode active material layer contains a sulfur-based active material. The positive electrode active material layer contains optional components such as a conductive agent, a binder, a dispersant, a thickener, and a filler, as needed. The positive electrode active material layer is usually formed from a positive electrode mixture containing the sulfur-based active material and other optional components.

[0052] The sulfur-based active material is a component that functions as a positive electrode active material. The sulfur-based active material may be elemental sulfur, a sulfur compound, or a mixture thereof. Examples of sulfur compounds include metal sulfides such as lithium sulfide, organic disulfide compounds, and organic sulfur compounds such as carbon sulfide compounds. Sulfur-based active materials have advantages such as a large theoretical capacity and low cost.

[0053] The content of the sulfur-based active material in the positive electrode active material layer is preferably 50% by mass to 90% by mass, more preferably 60% by mass to 80% by mass. By having the content of the sulfur-based active material in this range, charge / discharge performance can be improved.

[0054] The positive electrode active material layer preferably further includes porous carbon forming a composite with the sulfur-based active material. In other words, the sulfur-based active material is preferably contained in the positive electrode active material layer as a composite with porous carbon. Hereinafter, the composite of the sulfur-based active material and porous carbon will also be simply referred to as a "composite." In the composite, the sulfur-based active material is typically supported within the pores of the porous carbon. This form of the composite ensures sufficient electronic conductivity. The composite may be substantially composed of only the sulfur-based active material and porous carbon, or may be substantially composed of only the sulfur-based active material and porous carbon. A composite substantially composed of only the sulfur-based active material and porous carbon means, for example, that the total content of the sulfur-based active material (sulfur elemental substance and sulfur compound) and porous carbon in the composite is 90% by mass or more, and may be 95% by mass or more, 98% by mass or more, or 99% by mass or more.

[0055] The content of the sulfur-based active material in the composite is preferably 50% by mass to 90% by mass, more preferably 60% by mass to 80% by mass. By setting the content of the sulfur-based active material in the composite within this range, it is possible to improve charge / discharge performance, etc.

[0056] Porous carbon has electrical conductivity. Porous carbon is generally a porous inorganic material whose main constituent element is carbon. The main constituent element refers to the element that is most abundant on a mass basis. The lower limit of the carbon element content in porous carbon is preferably 70 mass%, more preferably 80 mass%, 90 mass%, 95 mass%, or 97 mass%. The upper limit of the carbon element content in porous carbon may be 100 mass% or 99.9 mass%. The carbon element content in porous carbon can be within a range that combines any of the above-mentioned lower limits and any of the above-mentioned upper limits. Porous carbon may contain elements other than carbon, such as oxygen and nitrogen.

[0057] The composite can be produced by a conventional method, for example, by heating a mixture of a sulfur-based active material and porous carbon to a temperature equal to or higher than the melting point of the sulfur-based active material, and then cooling the mixture.

[0058] The content of the composite in the positive electrode active material layer is preferably 60% by mass or more and 97% by mass or less, more preferably 80% by mass or more and 96% by mass or less, and even more preferably 90% by mass or more and 95% by mass or less. By setting the content of the composite in the above range, charge / discharge performance can be improved.

[0059] The positive electrode active material layer may contain a positive electrode active material other than the sulfur-based active material, provided that the content of the sulfur-based active material in the total positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass.

[0060] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Note that this conductive agent does not include the porous carbon that constitutes the composite. Examples of such conductive agents include carbon materials, metals, conductive ceramics, and the like. Examples of carbon materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofiber, pitch-based carbon fiber, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerene. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred. It is also preferable to use carbon black (preferably acetylene black) in combination with CNT.

[0061] The content of the conductive agent (excluding the porous carbon in the composite) in the positive electrode active material layer is preferably 0.5% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less. The upper limit of the content of the conductive agent in the positive electrode active material layer may be 5% by mass or 3% by mass. By setting the content of the conductive agent in the above range, it is possible to increase the energy density, etc.

[0062] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0063] The content of the binder in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 9% by mass or less. The upper limit of the content of the binder in the positive electrode active material layer may be 6% by mass or 4% by mass. By setting the content of the binder in the above range, the sulfur-based active material and the like can be stably held.

[0064] Examples of dispersants include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the dispersant has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. The content of the dispersant in the positive electrode active material layer is preferably 0.05% by mass or more and 5% by mass or less, and more preferably 0.1% by mass or more and 2% by mass or less. The polysaccharide polymer may function as a thickener or a binder.

[0065] Examples of the thickener include polyacrylic acid (PAA). The content of the thickener in the positive electrode active material layer is preferably 0.05% by mass or more and 5% by mass or less, and more preferably 0.1% by mass or more and 4% by mass or less. Polyacrylic acid may function as a binder.

[0066] The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof. When the positive electrode active material layer contains a filler, the content of the filler in the positive electrode active material layer is preferably, for example, 0.1% by mass or more and 10% by mass or less. In one embodiment of the present invention, the positive electrode active material layer may not contain a filler.

[0067] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the sulfur-based active material, other positive electrode active materials, porous carbon, binder, conductive agent, dispersant, thickener, and filler.

[0068] The mass per unit area of ​​the positive electrode active material layer is not particularly limited, but is, for example, 1 mg / cm ２ 30mg / cm or more ２ In one embodiment of the present invention, the upper limit of the mass per unit area of ​​the positive electrode active material layer is 20 mg / cm ２ and 15 mg / cm ２ and 10 mg / cm ２ 7 mg / cm ２ 5 mg / cm ２By setting the mass per unit area of ​​the positive electrode active material layer to the above upper limit or less, it is possible to improve the charge / discharge performance when used at a relatively high current density. In one embodiment of the present invention, the lower limit of the mass per unit area of ​​the positive electrode active material layer is 2 mg / cm ２ and 3 mg / cm ２ 5 mg / cm ２ 7 mg / cm ２ and 10 mg / cm ２ By setting the mass per unit area of ​​the positive electrode active material layer to be equal to or greater than the above-mentioned lower limit, it is possible to increase the energy density per volume of the nonaqueous electrolyte storage element, etc. The mass per unit area of ​​the positive electrode active material layer can be set within a range that combines any one of the above-mentioned upper limits and any one of the above-mentioned lower limits (with the proviso that the upper limit is greater than the lower limit).

[0069] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0070] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, alloys thereof, and carbon materials are used as the material of the negative electrode substrate. Among these, nickel and nickel alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, nickel foil or nickel alloy foil is preferred as the negative electrode substrate.

[0071] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the nonaqueous electrolyte storage element.

[0072] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a dispersant, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the dispersant, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above. The negative electrode active material layer does not necessarily contain optional components such as the conductive agent, the binder, the dispersant, the thickener, and the filler.

[0073] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, dispersant, thickener, and filler.

[0074] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for a lithium ion secondary battery. Examples of the negative electrode active material include metallic lithium; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; Li ４ Ti ５ O １２ , LiTiO ２ , TiNb ２ O ７ Examples of the negative electrode active material include titanium-containing oxides such as those mentioned above, polyphosphate compounds, silicon carbide, and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or non-graphitizable carbon). In the case of a negative electrode active material that does not contain charge transport ions such as lithium ions, a material doped with charge transport ions such as lithium ions can be used.

[0075] The negative electrode active material is preferably metallic lithium. In other words, the negative electrode or the negative electrode active material layer preferably contains metallic lithium at least in a charged state. It is more preferable that the negative electrode or the negative electrode active material layer contains metallic lithium in all states, including a charged state and a discharged state.

[0076] The metallic lithium may be pure metallic lithium consisting essentially of lithium element alone, or may be a lithium alloy containing other metal elements. Examples of the lithium alloy include a lithium-silver alloy, a lithium-zinc alloy, a lithium-calcium alloy, a lithium-aluminum alloy, a lithium-magnesium alloy, and a lithium-indium alloy. The lithium alloy may contain multiple metal elements other than lithium element.

[0077] The negative electrode active material layer is preferably a layer consisting essentially of metallic lithium (pure metallic lithium or a lithium alloy). The lower limit of the lithium element content in the negative electrode active material layer is preferably 80 mass%, more preferably 90 mass%, and even more preferably 99 mass%. The upper limit of the lithium element content in the negative electrode active material layer may be 100 mass%. The lithium element content in the negative electrode active material layer can be within a range that combines any of the above-mentioned lower limits and the above-mentioned upper limit.

[0078] The negative electrode active material layer may be a non-porous layer (solid layer) or a porous layer, but is preferably a non-porous layer. The negative electrode active material layer may be a layer of metallic lithium. The negative electrode active material layer may be a layer made of metallic lithium foil (pure metallic lithium foil or lithium alloy foil). The average thickness of the negative electrode active material layer in a charged state is preferably 5 μm or more and 2,000 μm or less, more preferably 30 μm or more and 1,200 μm or less, and even more preferably 100 μm or more and 800 μm or less.

[0079] (Separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0080] The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere, and more preferably have a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials that have a mass loss of a predetermined amount or less include inorganic compounds. That is, the heat-resistant layer may be an inorganic particle layer containing inorganic particles and a binder. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As inorganic compounds, these substances may be used alone or in the form of a complex, or two or more may be mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of the safety of nonaqueous electrolyte storage elements.

[0081] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0082] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0083] (Non-aqueous electrolyte) The non-aqueous electrolyte contains an electrolyte salt, an ionic liquid, and a fluorinated solvent. The non-aqueous electrolyte may be a non-aqueous electrolyte solution. In one embodiment of the present invention, the non-aqueous electrolyte storage element may be a non-aqueous electrolyte solution storage element.

[0084] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0085] The anions constituting the electrolyte salt include N(CF ３ SO ２ ) ２ － (Bis(trifluoromethanesulfonyl)imide anion: TFSI － ), N(SO ２ F) ２ － (bis(fluorosulfonyl)imide anion: FSI － ), N(C ２ F ５ SO ２ ) ２ － (bis(pentafluoroethanesulfonyl)imide anion), N(C ４ F ９ SO ２ ) ２ －(bis(nonafluorobutanesulfonyl)imide anion), N(POF ２ ) ２ － (bis(difluorophosphonyl)imide anion), N(CF ３ SO ２ ) (CF ３ CO) － ((trifluoromethanesulfonyl)(trifluoromethanecarbonyl)imide anion), N(CN) ２ － (dicyanoimide anion), CF ３ -SO ２ -N-SO ２ -N-SO ２ CF ３ － , FSO ２ -N-SO ２ -C ４ F ９ － , C.F. ３ -SO ２ -N-SO ２ -C ４ F ９ － , C.F. ３ -SO ２ -N-SO ２ -CF ２ -SO ２ -N-SO ２ -CF ３ ２－ , C.F. ３ -SO ２ -N-SO ２ -CF ２ -SO ３ ２－ , C.F. ３ -SO ２ -N-SO ２ -CF ２ -SO ２ -C(-SO ２ CF ３ ) ２ ２－ Examples of anions constituting the electrolyte salt include imide anions such as PF ６ － , P.O. ２ F ２ － , B.F. ４－ , ClO ４ － , NO ２ － , NO ３ － , I － , S.O. ３ CF ３ － , C(SO ２ CF ３ ) ３ － , C(SO ２ C ２ F ５ ) ３ － Anions other than imide anions such as the above can also be used.

[0086] The anion constituting the electrolyte salt is preferably an imide anion, and more preferably a bis(trifluoromethanesulfonyl)imide anion (TFSI － ) and bis(fluorosulfonyl)imide anion (FSI － ), and bis(fluorosulfonyl)imide anion (FSI － ) is more preferred. That is, the electrolyte salt is preferably an imide salt, more preferably at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), and even more preferably lithium bis(fluorosulfonyl)imide (LiFSI). In addition, the anion constituting the electrolyte salt preferably has a fluorine atom. When the anion constituting the electrolyte salt is such an anion, the ionic conductivity of the non-aqueous electrolyte is increased, and charge / discharge performance can be improved. One or more electrolyte salts can be used.

[0087] The content of the electrolyte salt in the non-aqueous electrolyte is 0.3 mol / dm at 20°C and 1 atmosphere. ３ 3.0mol / dm or more ３ The lower limit of the content of the electrolyte salt is preferably 0.5 mol / dm ３ is preferred, and 0.8 mol / dm ３ More preferably, 1.2 mol / dm３ 1.6 mol / dm ３ The upper limit of the content of the electrolyte salt is 2.6 mol / dm ３ is preferred, and 2.2 mol / dm ３ 1.8 mol / dm ３ By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be optimized, and charge / discharge performance can be improved, etc. The content of the electrolyte salt can be set within a range that combines any of the above-mentioned lower limits and any of the above-mentioned upper limits.

[0088] An ionic liquid is an ionic compound that is at least partially liquid at room temperature (20° C.) under 1 atmosphere.

[0089] Examples of cations constituting the ionic liquid include quaternary ammonium cations, imidazolium-based cations, pyrrolidinium-based cations, piperidinium-based cations, quaternary phosphonium cations, sulfonium cations, pyridinium-based cations, pyrrolium-based cations, pyrazolium-based cations, and pyrrolinium-based cations.

[0090] Examples of imidazolium cations include 1,3-dimethylimidazolium cation, 1-ethyl-3-methylimidazolium cation, 1,3-diethylimidazolium cation, 1-butyl-3-methylimidazolium cation, 1,2,3-trimethylimidazolium cation, 1,2-dimethyl-3-ethylimidazolium cation, 1,2-dimethyl-3-propylimidazolium cation, and 1-butyl-2,3-dimethylimidazolium cation.

[0091] Examples of tetraalkylammonium cations include trimethylethylammonium cation, trimethylpropylammonium cation, trimethylbutylammonium cation, trimethylhexylammonium cation, and tetrapentylammonium cation.

[0092] Examples of pyridinium cations include 1-methylpyridinium cation, 1-ethylpyridinium cation, 1-propylpyridinium cation, 1-butylpyridinium cation, 1-ethyl-2-methylpyridinium cation, 1-butyl-4-methylpyridinium cation, and 1-butyl-2,4-dimethylpyridinium cation.

[0093] Examples of pyrrolium-based cations include 1,1-dimethylpyrrolium cation, 1-ethyl-1-methylpyrrolium cation, 1-methyl-1-propylpyrrolium cation, and 1-butyl-1-methylpyrrolium cation.

[0094] Examples of pyrazolium cations include 1,2-dimethylpyrazolium cation, 1-ethyl-2-methylpyrazolium cation, 1-propyl-2-methylpyrazolium cation, and 1-butyl-2-methylpyrazolium cation.

[0095] Examples of pyrrolinium cations include 1,2-dimethylpyrrolinium cation, 1-ethyl-2-methylpyrrolinium cation, 1-propyl-2-methylpyrrolinium cation, and 1-butyl-2-methylpyrrolinium cation.

[0096] Examples of pyrrolidinium cations include 1,1-dimethylpyrrolidinium cation, 1-ethyl-1-methylpyrrolidinium cation, 1-methyl-1-propylpyrrolidinium cation, and 1-butyl-1-methylpyrrolidinium cation.

[0097] Examples of piperidinium cations include 1,1-dimethylpiperidinium cation, 1-ethyl-1-methylpiperidinium cation, 1-methyl-1-propylpiperidinium cation, and 1-butyl-1-methylpiperidinium cation.

[0098] Examples of the quaternary phosphonium cation include a tetramethylphosphonium cation, a tetraethylphosphonium cation, a trimethylethylphosphonium cation, a trimethylpropylphosphonium cation, a trimethylbutylphosphonium cation, a tetraphenylphosphonium cation, and a trimethylmethoxymethylphosphonium cation.

[0099] Examples of the sulfonium cation include a trimethylsulfonium cation, a triethylsulfonium cation, and a tributylsulfonium cation.

[0100] The cation constituting the ionic liquid is preferably at least one selected from the group consisting of quaternary ammonium cations, imidazolium cations, pyrrolidinium cations, piperidinium cations, and quaternary phosphonium cations, and more preferably at least one selected from the group consisting of imidazolium cations, pyrrolidinium cations, and piperidinium cations. When the cation constituting the ionic liquid is such a cation, it is possible to further increase the initial reverse Coulomb efficiency and improve charge / discharge performance. One or more of these cations may be contained.

[0101] Examples of anions constituting the ionic liquid include the same anions as those constituting the electrolyte salt. As an anion constituting the ionic liquid, imide anions are preferred, and bis(trifluoromethanesulfonyl)imide anion (TFSI) is preferred. － ) and bis(fluorosulfonyl)imide anion (FSI － ), and bis(fluorosulfonyl)imide anion (FSI － ) is more preferred. Furthermore, the anion constituting the ionic liquid preferably has a fluorine atom. When the anion constituting the ionic liquid is such an anion, it is possible to further increase the initial reverse Coulomb efficiency and improve the charge / discharge performance. One or more of these anions may be contained.

[0102] At least one of the electrolyte salt and the ionic liquid contains a bis(fluorosulfonyl)imide anion (FSI) as an anion. － It is preferable that the non-aqueous electrolyte contains a bis(fluorosulfonyl)imide anion (FSI) as an anion. － In this case, it is possible to improve the charge / discharge performance when used at a relatively high current density.

[0103] In addition, it is preferable that the anions present in the non-aqueous electrolyte are substantially only imide anions, and the anions present in the non-aqueous electrolyte are substantially bis(trifluoromethanesulfonyl)imide anions (TFSI － ) and bis(fluorosulfonyl)imide anion (FSI － For example, the content of these anions relative to all anions in the non-aqueous electrolyte is preferably 90 mol % or more, more preferably 99 mol % or more, and even more preferably 99.9 mol % or more. By configuring the anions in the non-aqueous electrolyte in this way, the ionic conductivity of the non-aqueous electrolyte can be increased, and charge / discharge performance can be improved.

[0104] The fluorinated solvent may be a solvent having a hydrocarbon group in which some or all of the hydrogen atoms in the hydrocarbon group have been substituted with fluorine atoms. The fluorinated solvent is at least one selected from the group consisting of fluorinated ethers and fluorinated carboxylic acid esters, and fluorinated ethers are preferred. Use of such a fluorinated solvent can improve the initial Coulombic efficiency of the nonaqueous electrolyte storage element. One or more fluorinated solvents can be used.

[0105] Examples of fluorinated ethers include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, methyl heptafluoropropyl ether, methyl nonafluorobutyl ether, etc. The fluorinated ether is preferably at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0106] Examples of the fluorinated carboxylic acid ester include methyl 2,2-difluoroacetate, methyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoroacetate, ethyl 2,2,2-trifluoroacetate, methyl 3,3,3-trifluoropropionate, ethyl 3,3,3-trifluoropropionate, trifluoromethyl acetate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, trifluoromethyl propionate, 2,2,2-trifluoroethyl propionate, 2,2,2-trifluoroethyl 3,3,3-trifluoropropionate, etc. As the fluorinated carboxylic acid ester, ethyl 2,2-difluoroacetate is preferred.

[0107] The fluorinated solvent is preferably a chain solvent. A chain solvent is a solvent (molecule) having a molecular structure without a ring structure. The molecular weight of the fluorinated solvent is preferably 60 to 300, and more preferably 100 to 250. The number of carbon atoms in the fluorinated solvent is preferably 2 to 7, more preferably 3 to 6, and even more preferably 4 or 5. When the fluorinated solvent is such a solvent, the viscosity of the non-aqueous electrolyte is further reduced, which can improve the charge / discharge performance of the non-aqueous electrolyte storage element.

[0108] The lower limit of the total content of the ionic liquid and at least one fluorinated solvent selected from the group consisting of fluorinated ethers and fluorinated carboxylic acid esters relative to all components other than the electrolyte salt in the non-aqueous electrolyte is 90 mol%, preferably 95 mol%, and more preferably 99 mol%. The lower limit of the total content of the ionic liquid and at least one fluorinated solvent selected from the group consisting of fluorinated ethers and fluorinated carboxylic acid esters relative to all components other than the electrolyte salt in the non-aqueous electrolyte is preferably 90 vol%, more preferably 95 vol%, and even more preferably 99 vol%. In this way, when the non-aqueous electrolyte is primarily composed of an electrolyte salt, an ionic liquid, and at least one fluorinated solvent selected from the group consisting of fluorinated ethers and fluorinated carboxylic acid esters, charge / discharge cycle performance tends to be improved. For example, if the non-aqueous electrolyte contains an excess of non-fluorinated solvents, internal short circuits may be more likely to occur during charge / discharge cycles. The upper limit of the total content may be 100 mol% or 100 vol%. The total content can be within a range that combines any of the above lower limits and the above upper limit.

[0109] The molar ratio of the ionic liquid to the fluorinated solvent (ionic liquid / fluorinated solvent) is preferably within the range of 5 / 95 to 99 / 1. By setting the molar ratio of the ionic liquid to the fluorinated solvent within the above range, the initial reverse Coulomb efficiency is further increased and the charge / discharge performance is also improved. The threshold A of the above molar ratio (ionic liquid / fluorinated solvent) range may be 10 / 90, 20 / 80, 30 / 70, 40 / 60, 50 / 50, 55 / 45, 60 / 40, 65 / 35, or 70 / 30. The threshold B of the above molar ratio (ionic liquid / fluorinated solvent) range may be 95 / 5, 90 / 10, 80 / 20, 70 / 30, 65 / 35, 60 / 40, 55 / 45, or 50 / 50. The molar ratio (ionic liquid / fluorinated solvent) can be within a range that combines any one of the threshold values ​​A and any one of the threshold values ​​B (wherein the molar ratio of the fluorinated solvent is greater in threshold value A than in threshold value B). When the fluorinated solvent is a fluorinated ether, the molar ratio of the ionic liquid to the fluorinated ether (ionic liquid / fluorinated ether) can be within a range of 10 / 90 to 99 / 1. The threshold value C of the range of the molar ratio (ionic liquid / fluorinated ether) can be 20 / 90, 30 / 70, 40 / 60, 50 / 50, 55 / 45, 60 / 40, 65 / 35, or 70 / 30. The threshold value D of the range of the molar ratio (ionic liquid / fluorinated ether) can be 95 / 5, 90 / 10, 80 / 20, 70 / 30, 65 / 35, 60 / 40, 55 / 45, or 50 / 50. The molar ratio (ionic liquid / fluorinated ether) can be within a range that is a combination of any of the threshold values ​​C and D described above (wherein the molar ratio of the fluorinated ether is greater in threshold value C than in threshold value D). When the fluorinated solvent is a fluorinated carboxylic acid ester, the molar ratio of the ionic liquid to the fluorinated carboxylic acid ester (ionic liquid / fluorinated carboxylic acid ester) may be within a range of 10 / 90 to 99 / 1. The threshold value E of the molar ratio (ionic liquid / fluorinated carboxylic acid ester) may be 20 / 90, 30 / 70, 40 / 60, 50 / 50, 55 / 45, 60 / 40, 65 / 35, or 70 / 30.The threshold F of the molar ratio (ionic liquid / fluorinated carboxylic acid ester) may be 95 / 5, 90 / 10, 80 / 20, 70 / 30, 65 / 35, 60 / 40, 55 / 45, or 50 / 50. The molar ratio (ionic liquid / fluorinated carboxylic acid ester) may be within a range that is a combination of any of the thresholds E and F (wherein the molar ratio of the fluorinated carboxylic acid ester is greater in threshold E than in threshold F).

[0110] The non-aqueous electrolyte may contain other components in addition to the electrolyte salt, the ionic liquid, and the fluorinated solvents, ie, the fluorinated ether and the fluorinated carboxylic acid ester. Examples of the other components include non-aqueous solvents other than the fluorinated ether and the fluorinated carboxylic acid ester, additives, and the like.

[0111] Examples of other non-aqueous solvents include cyclic carbonates, chain carbonates, ethers other than fluorinated ethers, carboxylic acid esters other than fluorinated carboxylic acid esters, phosphoric acid esters, amides, and nitriles.

[0112] Examples of additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, and busulfa. Examples of the additives include methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, and lithium nitrate. These additives may be used alone or in combination of two or more.

[0113] The content of the other components or the content of the additives in the non-aqueous electrolyte can be 0.01 mol % to 10 mol %, or even 0.1 mol % to 5 mol %, based on all components in the non-aqueous electrolyte other than the electrolyte salt. The content of the other components or the additives in the non-aqueous electrolyte can be 0.01 vol % to 10 vol %, or even 0.1 vol % to 5 vol %, based on all components in the non-aqueous electrolyte other than the electrolyte salt. For example, by setting the content of the additives within the above ranges, it is possible to improve capacity retention or cycle performance after high-temperature storage, and further improve safety.

[0114] The upper limit of the viscosity of the non-aqueous electrolyte at 25°C may be, for example, 500 mPa·s, 400 mPa·s, 300 mPa·s, or 200 mPa·s, but is preferably 100 mPa·s, more preferably 80 mPa·s, and even more preferably 70 mPa·s, and may be 60 mPa·s, 50 mPa·s, or 40 mPa·s. When the viscosity of the non-aqueous electrolyte is equal to or less than the above upper limit, charge / discharge performance can be improved, and good charge / discharge performance can be exhibited, for example, even at relatively high current densities. The lower limit of the viscosity may be 5 mPa·s, or may be 10 mPa·s, 20 mPa·s, or 30 mPa·s. The viscosity can be within a range that combines any of the above upper limits and any of the above lower limits. The viscosity is measured using a LOVIS2000ME measuring device manufactured by Anton Paar, with the liquid temperature of the measurement target set to 25°C.

[0115] The viscosity of the non-aqueous electrolyte tends to decrease, for example, by increasing the content of the fluorinated solvent, decreasing the content of the electrolyte salt, etc. The viscosity of the non-aqueous electrolyte can also be adjusted by the types of the electrolyte salt, ionic liquid, and fluorinated solvent, etc.

[0116] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0117] Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0118] <Electricity Storage Device> The nonaqueous electrolyte energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in automotive power sources such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for aircraft such as electric aircraft and HAPS (High Altitude Platform Stations), power sources for electronic devices such as personal computers and communication terminals, or power storage power sources. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit. The nonaqueous electrolyte energy storage element of the present embodiment combines an extremely high mass energy density and a high initial back Coulombic efficiency, which are particularly required for power sources for aircraft, and is therefore suitable as a power source for aircraft, and is particularly suitable as a power source for eVTOL (Electric Vertical Take-off and Landing). "Aircraft-specific" means permanently incorporated into the design of an air vehicle.

[0119] 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

[0120] <Method for manufacturing non-aqueous electrolyte storage element> A method for manufacturing a non-aqueous electrolyte storage element according to one embodiment of the present invention includes preparing a positive electrode containing a sulfur-based active material, and preparing a non-aqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the non-aqueous electrolyte other than the electrolyte salt is 90 mol % or more. The production method may also include other steps such as preparing a negative electrode, preparing a separator, fabricating an electrode assembly using the positive electrode, the negative electrode, and the separator, and housing the electrode assembly and the non-aqueous electrolyte in a container.

[0121] Preparing a positive electrode may also mean manufacturing a positive electrode. Manufacturing a positive electrode can be performed, for example, by applying a positive electrode mixture paste containing a sulfur-based active material directly to a positive electrode substrate or via an intermediate layer. The positive electrode mixture paste may further contain, in addition to the sulfur-based active material, components constituting the positive electrode active material layer and a dispersion medium. After application, drying, pressing, etc. may be performed. The applied positive electrode mixture paste is dried to form a positive electrode active material layer. Specific and preferred forms of the prepared positive electrode are the same as those of the positive electrode provided in the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

[0122] Preparing a negative electrode may also mean manufacturing a negative electrode. For example, a negative electrode having metallic lithium at least in a charged state can be manufactured by laminating a negative electrode active material layer containing metallic lithium directly or via an intermediate layer on a negative electrode substrate and pressing the laminate. The negative electrode active material layer containing metallic lithium at least in a charged state may be a pure metallic lithium foil or a lithium alloy foil. The negative electrode can also be manufactured using a negative electrode mixture paste, as in the manufacture of a positive electrode. Specific and preferred forms of the prepared negative electrode are the same as those of the negative electrode included in the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

[0123] Preparing a nonaqueous electrolyte may mean preparing a nonaqueous electrolyte. The nonaqueous electrolyte can be prepared, for example, by mixing an electrolyte salt, an ionic liquid, and a fluorinated solvent. Specific and preferred forms of the prepared nonaqueous electrolyte are the same as those of the nonaqueous electrolyte included in the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

[0124] The electrode assembly can be produced by stacking or winding a positive electrode and a negative electrode with a separator interposed therebetween.

[0125] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0126] The manufacturing method may include initially charging and discharging the undischarged nonaqueous electrolyte storage element. In the nonaqueous electrolyte storage element according to one embodiment of the present invention, the initial charging and discharging usually begins with discharging, i.e., chemical conversion treatment. The number of charging and discharging in the initial charging and discharging is not particularly limited.

[0127] <Other Embodiments> The nonaqueous electrolyte storage element and its manufacturing method of the present invention are not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0128] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium-sulfur battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries, capacitors, etc.

[0129] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

[0130] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0131] The components used in preparing the nonaqueous electrolytes of the examples and comparative examples are listed below. (Electrolyte salt) LiFSI: lithium bis(fluorosulfonyl)imide LiTFSI: lithium bis(trifluoromethanesulfonyl)imide (Ionic liquid) Py13FSI: 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide EMImFSI: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide Py13TFSI: 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide PP13FSI: 1-methyl-1-propylpiperidinium bis(fluorosulfonyl)imide P111(101)FSI: trimethylmethoxymethylphosphonium bis(fluorosulfonyl)imide P1113FSI: trimethylpropylphosphonium bis(fluorosulfonyl)imide (Fluorinated solvent: fluorinated ether or fluorinated carboxylic acid ester) TFEE: 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether TFETFPE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether EDFA: 2,2-difluoroethyl acetate (Other ingredients) FEC: Fluoroethylene carbonate VC: Vinylene carbonate DME: 1,2-dimethoxyethane

[0132] [Example 1] (Preparation of Positive Electrode) Elemental sulfur and porous carbon, which are sulfur-based active materials, were mixed in a mass ratio of 70:30. This mixture was placed in a sealed electric furnace. After argon flow for 1 hour, the mixture was heated to 150°C at a heating rate of 5°C / min and held for 5 hours, then allowed to cool to 80°C, the temperature at which elemental sulfur solidifies. The mixture was then heated again to 300°C at a heating rate of 5°C / min and held for 2 hours to prepare a composite (sulfur-porous carbon composite: SPC). A positive electrode mixture paste containing the composite obtained above, acetylene black and carbon nanotubes as conductive agents, carboxymethyl cellulose as a dispersant, polyacrylic acid as a thickener, and styrene butadiene rubber as a binder, was applied to an aluminum positive electrode substrate (average thickness 15 μm) and dried. The mass per unit area of ​​the positive electrode active material layer after drying the dispersion medium was 3 mg / cm. ２ The amount of the positive electrode mixture paste applied was adjusted so that the following was achieved: By the above steps, a positive electrode in which a positive electrode active material layer was laminated on a positive electrode substrate was obtained.

[0133] (Preparation of Negative Electrode) A pure metallic lithium foil (average thickness: 600 μm) was prepared as a negative electrode.

[0134] (Preparation of non-aqueous electrolyte) A liquid obtained by mixing 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (Py13FSI), an ionic liquid, and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFEE), a fluorinated solvent, in a molar ratio of 70:30 was added with 1.0 mol / dm lithium bis(fluorosulfonyl)imide (LiFSI), an electrolyte salt. ３ The non-aqueous electrolyte was prepared by adding the above-mentioned components in an amount of 1:1.

[0135] (Assembly of non-aqueous electrolyte storage element) A separator was prepared, which was a polyethylene microporous membrane with inorganic particle layers laminated on both sides, and had such high wettability that the non-aqueous electrolyte could penetrate into the pores. The non-aqueous electrolyte storage element of Example 1 was obtained using the positive electrode, negative electrode, separator, and non-aqueous electrolyte.

[0136] [Examples 2 to 19, Comparative Example 3] Nonaqueous electrolyte storage elements of Examples 2 to 19 and Comparative Example 3 were obtained in the same manner as in Example 1, except that the composition of the nonaqueous electrolyte was as shown in Table 1. In Table 1, the contents of the ionic liquid and fluorinated solvent are the contents (mol %) of the ionic liquid and fluorinated solvent relative to the total content of the ionic liquid and fluorinated solvent. The contents of other components are the contents (vol %) of all components in the nonaqueous electrolyte other than the electrolyte salt. In the column for each component, "-" indicates that the corresponding component was not contained.

[0137] Comparative Example 1 A non-aqueous solvent was prepared by mixing fluoroethylene carbonate (FEC) and vinylene carbonate (VC) in a volume ratio of 50:50. The electrolyte salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), was added at a concentration of 1.0 mol / dm ３ A nonaqueous electrolyte storage element of Comparative Example 1 was obtained in the same manner as in Example 1, except that the above nonaqueous electrolyte was used.

[0138] Comparative Example 2 A nonaqueous electrolyte storage element of Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that lithium bis(fluorosulfonyl)imide (LiFSI) was used as the electrolyte salt.

[0139] Comparative Example 4: A liquid obtained by mixing Py13FSI, an ionic liquid, TFEE, a fluorinated solvent, and DME, another component, in a molar ratio of 70:15:15 was mixed, and LiFSI, an electrolyte salt, was added at 2.0 mol / dm ３ A nonaqueous electrolyte storage element of Comparative Example 4 was obtained in the same manner as in Example 1, except that the above nonaqueous electrolyte was used.

[0140] (Viscosity Measurement) The viscosity at 25° C. was measured for the nonaqueous electrolyte in each of the nonaqueous electrolyte storage elements of Examples 1 to 3, 5, 6, 9, and 12 and Comparative Example 1. The results are shown in Table 1.

[0141] [Evaluation] (Initial Charge / Discharge) The obtained nonaqueous electrolyte storage elements of Examples 1 to 19 and Comparative Examples 1 to 3 were subjected to the following initial charge / discharge cycles in a constant temperature bath at 25°C. First, as a chemical conversion treatment, constant current discharge was performed under conditions of a discharge current of 0.1 C and a discharge cut-off voltage of 1.0 V. Then, as an initial charge, constant current / constant voltage (CCCV) charging was performed under conditions of a charge current of 0.1 C and a charge cut-off voltage of 3.0 V until a total charge time of 30 hours was reached. Next, as an initial discharge, constant current discharge was performed under conditions of a discharge current of 0.1 C and a discharge cut-off voltage of 1.0 V. Note that a 10-minute pause was provided after the chemical conversion treatment and the initial charge. The quantity of electricity in the chemical conversion treatment (chemical conversion capacity), the quantity of electricity in the initial charge (initial charge capacity), and the quantity of electricity in the initial discharge (initial discharge capacity) were each calculated as the quantity of electricity per mass of the sulfur-based active material (sulfur element). The initial reverse Coulomb efficiency was calculated as the percentage of the initial charge capacity relative to the formation capacity. The results are shown in Table 1.

[0142] [Example 20] A nonaqueous electrolyte storage element of Example 20 was obtained under the same conditions as in Example 9. The resulting nonaqueous electrolyte storage element of Example 20 was subjected to initial charge / discharge under the same conditions as described above, except that the currents for the chemical conversion treatment, initial charge, and initial discharge were all 0.02 C, and the initial charge was terminated after a total charge time of 70 hours. The chemical conversion capacity, initial charge capacity, initial discharge capacity, and initial reverse Coulomb efficiency were determined. The results are shown in Table 2. Table 2 also lists the results of Example 9.

[0143] [Example 21] A positive electrode active material layer having a mass per unit area of ​​15 mg / cm ２ A nonaqueous electrolyte storage element of Example 21 was obtained in the same manner as in Example 5, except that the amount of positive electrode mixture paste applied was adjusted so that the positive electrode mixture paste was 0.02 C. The initial charge / discharge cycle of the obtained nonaqueous electrolyte storage element of Example 21 was performed under the same conditions as described above, except that the currents for the chemical conversion treatment, initial charge, and initial discharge were all 0.02 C, and the initial charge was terminated after a total charge time of 70 hours. The chemical conversion capacity, initial charge capacity, initial discharge capacity, and initial reverse Coulomb efficiency were determined. The results are shown in Table 3. Table 3 also lists the results of Example 5.

[0144]

[0145]

[0146]

[0147] As shown in Table 1, the nonaqueous electrolyte storage elements of Comparative Examples 1 to 3, in which the nonaqueous electrolyte did not contain an ionic liquid, had an initial reverse Coulombic efficiency of 90% or less. In contrast, the nonaqueous electrolyte storage elements of Examples 1 to 19, in which the nonaqueous electrolyte contained an ionic liquid and at least one of a fluorinated ether and a fluorinated carboxylic acid ester as a fluorinated solvent, had an initial reverse Coulombic efficiency of greater than 90%. Comparisons with Examples 1 to 3 also confirmed that the fluorinated solvent functions as a component that reduces the viscosity of the nonaqueous electrolyte. As shown in Table 2, the nonaqueous electrolyte storage element of Example 9, which had small chemical formation capacity, initial charge capacity, and initial discharge capacity, exhibited sufficiently large chemical formation capacity, initial charge capacity, and initial discharge capacity by performing initial charge / discharge at a current of 0.02 C as in Example 20. As shown in Table 3, the nonaqueous electrolyte storage element of Example 21, in which the mass per unit area of ​​the positive electrode active material layer was large, also exhibited sufficiently large formation capacity, initial charge capacity, and initial discharge capacity, and the initial reverse Coulomb efficiency was also high.

[0148] [Evaluation] (Charge-Discharge Cycle Test) The nonaqueous electrolyte storage element of Example 5, which had undergone the initial charge-discharge cycle, was subjected to a charge-discharge cycle test as follows. At 25°C, constant-current charging was performed with a charge current of 0.1 C and a charge cut-off voltage of 3.0 V, followed by constant-voltage charging. The charge termination condition was that the total charge time of the constant-current charging and constant-voltage charging was 15 hours. A 10-minute rest period was then provided. Subsequently, constant-current discharging was performed with a discharge current of 0.1 C and a discharge cut-off voltage of 1.0 V, followed by a 10-minute rest period. After 50 cycles of this charge-discharge cycle, no short circuit occurred. The nonaqueous electrolyte storage element of Comparative Example 4 was subjected to the initial charge-discharge cycle test in the same manner as in Example 5, and then a charge-discharge cycle test was performed. A short circuit occurred at the 12th cycle. It is believed that the nonaqueous electrolyte storage element of Comparative Example 4 experienced an early short circuit during the charge-discharge cycle test because the total content of the ionic liquid and fluorinated solvent relative to all components other than the electrolyte salt in the nonaqueous electrolyte was less than 90 mol%. In contrast, the nonaqueous electrolyte storage element of Example 5, in which the total content of the ionic liquid and the fluorinated solvent relative to all components other than the electrolyte salt in the nonaqueous electrolyte was 90 mol % or more, did not suffer from short circuits even after 50 cycles of charge and discharge in a charge and discharge cycle test, demonstrating good charge and discharge performance.

[0149] The present invention can be applied to nonaqueous electrolyte storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

[0150] REFERENCE SIGNS LIST 1 nonaqueous electrolyte energy storage element 2 electrode body 3 container 4 positive electrode terminal 41 positive electrode lead 5 negative electrode terminal 51 negative electrode lead 20 energy storage unit 30 energy storage device

Claims

1. A non-aqueous electrolyte energy storage element comprising: a positive electrode containing a sulfur-based active material; and a non-aqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components in the non-aqueous electrolyte other than the electrolyte salt is 90 mol % or more.

2. The nonaqueous electrolyte storage element according to claim 1, wherein the fluorinated solvent is a chain solvent.

3. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the molar ratio of the ionic liquid to the fluorinated solvent (ionic liquid / fluorinated solvent) is within the range of 5 / 95 to 99 / 1.

4. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the ionic liquid contains at least one cation selected from the group consisting of quaternary ammonium cations, imidazolium cations, pyrrolidinium cations, piperidinium cations, quaternary phosphonium cations, and sulfonium cations.

5. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the ionic liquid has an imide anion.

6. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the electrolyte salt is an imide salt.

7. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein the viscosity of the nonaqueous electrolyte at 25° C. is 100 mPa·s or less.

8. The positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of the positive electrode active material layer is 15 mg / cm ２ 3. The nonaqueous electrolyte storage element according to claim 1, wherein:

9. The positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of the positive electrode active material layer is 5 mg / cm ２ The nonaqueous electrolyte storage element according to claim 1 or 2, wherein 10. The positive electrode has a positive electrode active material layer containing the sulfur-based active material, and the mass per unit area of the positive electrode active material layer is 10 mg / cm ２ The nonaqueous electrolyte storage element according to claim 1 or 2, wherein 11. The nonaqueous electrolyte storage element according to claim 1 or 2, further comprising a negative electrode containing metallic lithium at least in a charged state.

12. A method for producing a non-aqueous electrolyte storage element, comprising: preparing a positive electrode containing a sulfur-based active material; and preparing a non-aqueous electrolyte containing an electrolyte salt, an ionic liquid, and a fluorinated solvent, wherein the fluorinated solvent is at least one selected from the group consisting of a fluorinated ether and a fluorinated carboxylic acid ester, and the total content of the ionic liquid and the fluorinated solvent relative to all components other than the electrolyte salt in the non-aqueous electrolyte is 90 mol % or more.

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