Non-aqueous electrolyte storage element and method for manufacturing the same

Abstract

To provide: a non-aqueous electrolyte storage element having a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and a low alternating-current resistance after charge-discharge cycling; and a method for manufacturing such a non-aqueous electrolyte storage element.SOLUTION: A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing a sulfur-based active material, and a non-aqueous electrolyte containing a non-aqueous solvent, where the non-aqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 10 vol.% or more and 70 vol.% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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 in the positive electrode (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 in the positive electrode are expected to have high energy density. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-95390 Summary of the Invention [Problem to be solved by the invention]

[0005] In a nonaqueous electrolyte storage element using a sulfur-based active material in the positive electrode, when a nonaqueous electrolyte containing an ether is used as the nonaqueous solvent, charge-discharge reactions proceed through a shuttle reaction caused by the elution of polysulfides formed at the positive electrode into the nonaqueous electrolyte. In contrast, in a nonaqueous electrolyte storage element using a sulfur-based active material in the positive electrode, when a nonaqueous electrolyte containing a carbonate is used as the nonaqueous solvent, a coating is formed on the surface of the sulfur-based active material in the positive electrode during initial discharge due to a reaction between the positive electrode and the nonaqueous electrolyte (such as carbonate). Subsequently, charge-transport ions diffuse through this coating, resulting in the progress of charge-discharge reactions. Thus, it is believed that the reaction mechanisms at the positive electrode during charge and discharge differ between a nonaqueous electrolyte containing an ether as the nonaqueous solvent and a nonaqueous electrolyte containing a carbonate as the nonaqueous solvent in a nonaqueous electrolyte storage element using a sulfur-based active material in the positive electrode.

[0006] According to the findings of the inventors, among nonaqueous electrolyte energy storage elements having a positive electrode using a sulfur-based active material, those using a nonaqueous electrolyte containing a cyclic carbonate such as fluoroethylene carbonate or vinylene carbonate are promising as energy storage elements with a high energy density per mass. Furthermore, in general, nonaqueous electrolyte energy storage elements are desirable for having a large discharge capacity, a high capacity retention rate after charge-discharge cycling, and low resistance after charge-discharge cycling. However, nonaqueous electrolyte energy storage elements having a positive electrode using a conventional sulfur-based active material containing a cyclic carbonate do not have a sufficiently high capacity retention rate after charge-discharge cycling. The inventors previously investigated the use of a nonaqueous solvent containing a specific fluorinated ether together with the cyclic carbonate, but were unable to achieve both a high capacity retention rate after charge-discharge cycling and low AC resistance after charge-discharge cycling.

[0007] An object of the present invention is to provide a nonaqueous electrolyte storage element including a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing a cyclic carbonate, the nonaqueous electrolyte storage element having a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and a low AC resistance after charge-discharge cycling, and a method for manufacturing such a nonaqueous electrolyte storage element. [Means for solving the problem]

[0008] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing a sulfur-based active material and a non-aqueous electrolyte containing a non-aqueous solvent, wherein the non-aqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 10% by volume or more and 70% by volume or less.

[0009] A method for manufacturing a nonaqueous electrolyte storage element according to another aspect of the present invention includes preparing a positive electrode containing a sulfur-based active material and preparing a nonaqueous electrolyte containing a nonaqueous solvent, wherein the nonaqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the nonaqueous solvent is 10% by volume or more and 70% by volume or less. [Effects of the Invention]

[0010] According to one aspect of the present invention, there can be provided a nonaqueous electrolyte storage element including a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing a cyclic carbonate, the nonaqueous electrolyte storage element having a large initial discharge capacity, a high capacity retention rate after charge-discharge cycles, and a low AC resistance after charge-discharge cycles, and a method for manufacturing such a nonaqueous electrolyte storage element. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an electricity storage device including a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] [1] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing a nonaqueous solvent, wherein the nonaqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the nonaqueous solvent is 10% by volume or more and 70% by volume or less.

[0014] The nonaqueous electrolyte storage element described in [1] above is a nonaqueous electrolyte storage element comprising a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing a cyclic carbonate. It exhibits a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and low AC resistance after charge-discharge cycling. While the reasons for this are unclear, the following are presumed. First, in the nonaqueous electrolyte storage element described in [1] above, a fluorinated carboxylic acid ester is used together with a cyclic carbonate as the nonaqueous solvent, which is thought to result in a sufficiently large initial discharge capacity due to the appropriate viscosity of the nonaqueous electrolyte. However, when a fluorinated ether is used together with a cyclic carbonate as the nonaqueous solvent, the capacity retention rate after charge-discharge cycling is low or the AC resistance after charge-discharge cycling is high. This is thought to be because the coating derived from the cyclic carbonate and the fluorinated ether that coats the sulfur-based active material is thin and weak. In other words, in such cases, the coating cannot keep up with the large volume changes of the sulfur-based active material that occur during charge-discharge, and the coating repeatedly collapses and reforms, resulting in a decrease in discharge capacity or an increase in AC resistance. In contrast, when a cyclic carbonate and a predetermined amount of a fluorinated carboxylic acid ester are used as the nonaqueous solvent, the coating formed from the cyclic carbonate and the fluorinated carboxylic acid ester is thought to have sufficient thickness and strength. In such cases, the coating can adapt to large volume changes in the sulfur-based active material and is less likely to collapse. For these reasons, the nonaqueous electrolyte energy storage element described in [1] above is presumed to have a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and a low AC resistance after charge-discharge cycling.

[0015] The content of each component constituting the non-aqueous solvent is determined by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS). Specifically, this is done as follows. Note that the LC-MS and GC-MS measurements are carried out consecutively under the same conditions. (1) Collection of non-aqueous electrolyte First, the 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 poured into the nonaqueous electrolyte storage element, and the nonaqueous electrolyte diluted with the extraction solvent is removed. (2) LC-MS The components of the collected non-aqueous electrolyte are analyzed by LC-MS. The LC-MS analysis is performed in the following order: qualitative analysis and quantitative analysis. The LC-MS analysis is performed using Waters' "Acquity H" and "Xevo G2-5QTof" instruments. Water is used as the eluent. (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, described below, is performed instead of LC-MS analysis. If the peaks can be separated, the components contained in the measurement sample are predicted from the MS spectrum of each peak. A known sample of the predicted components (hereinafter referred to as "predicted components") is subjected to LC-MS analysis. The retention time and MS spectrum of the peaks corresponding to each predicted component in the measurement sample are compared with the retention time and MS spectrum of the peaks 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. First, a known sample of the predicted component with a known concentration is measured by LC-MS, and the peak area is calculated to create a calibration curve. The calibration curve is calculated using the coefficient of determination (r 2) is created so that it is between 0.999 and 1. The amount of the predicted component in the measurement sample is calculated from the calibration curve and the area of ​​the peak of the predicted component in the measurement sample. This process is performed for all peaks detected in the LC-MS analysis of the measurement sample, and the amount of each predicted component is calculated. (3) GC-MS The GC-MS analysis is performed in the following order: qualitative analysis and quantitative analysis. The GC-MS analysis equipment used is the Agilent 5975C. 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 times and MS spectrum of the peaks corresponding to each predicted component in the measurement sample are compared with the retention times and MS spectrum of the peaks 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 a calibration curve method. Quantitative analysis by GC-MS is performed using the same procedure as the quantitative analysis by LC-MS described above, and the content of each predicted component is determined. (4) Calculation of the content of each component The total amount of nonaqueous solvent is calculated by summing the content of each nonaqueous solvent in each predicted component (i.e., each component) measured by LC-MS or GC-MS. The content of each nonaqueous solvent (volume %) relative to the total amount of nonaqueous solvent is calculated by converting the mass content of each nonaqueous solvent measured by LC-MS or GC-MS into a volume at 20°C, and the total amount of nonaqueous solvent is calculated by summing the volumetrically converted content of each nonaqueous solvent. When an extraction solvent is used, the extraction solvent is excluded from the calculation.

[0016] [2] In the nonaqueous electrolyte storage element according to the above [1], the content of the cyclic carbonate in the nonaqueous solvent may be 30% by volume or more and 90% by volume or less.

[0017] The nonaqueous electrolyte storage element described in [2] above has an optimized composition of the nonaqueous solvent, and as a result, the effects relating to the initial discharge capacity, the capacity retention rate after charge-discharge cycling, and the AC resistance after charge-discharge cycling are further improved.

[0018] [3] In the nonaqueous electrolyte storage element according to the above [1] or [2], the cyclic carbonate may include a fluorinated cyclic carbonate and an unsaturated cyclic carbonate.

[0019] The nonaqueous electrolyte storage element described in [3] above has an optimized composition of the nonaqueous solvent, and as a result, the effects relating to the initial discharge capacity, the capacity retention rate after charge-discharge cycling, and the AC resistance after charge-discharge cycling are further improved.

[0020] [4] The nonaqueous electrolyte storage element according to any one of [1] to [3] above may further include a negative electrode containing metallic lithium.

[0021] The nonaqueous electrolyte electricity storage element described in [4] above has a negative electrode containing metallic lithium, and thus has further advantages such as a high energy density.

[0022] [5] In the nonaqueous electrolyte storage element according to any one of [1] to [4] above, the fluorinated carboxylic acid ester may include a fluorinated acetic acid ester.

[0023] [6] In the nonaqueous electrolyte storage element according to the above item [3], the fluorinated cyclic carbonate may include fluoroethylene carbonate, and the unsaturated cyclic carbonate may include vinylene carbonate.

[0024] [7] In the nonaqueous electrolyte storage element according to any one of [1] to [6] above, the total content of the cyclic carbonate and the fluorinated carboxylic acid ester in the nonaqueous solvent may be 80% by volume or more.

[0025] [8] In the nonaqueous electrolyte storage element according to any one of [1] to [7] above, the nonaqueous electrolyte may contain an imide salt.

[0026] [9] In the nonaqueous electrolyte storage element according to any one of [1] to [8] above, the positive electrode may further contain porous carbon, and the sulfur-based active material and the porous carbon may form a composite.

[0027] The nonaqueous electrolyte storage elements described in [5] to [9] above are also suitable embodiments of the present invention.

[0028]

[10] A method for manufacturing a nonaqueous electrolyte storage element according to another aspect of the present invention includes preparing a positive electrode including a sulfur-based active material and preparing a nonaqueous electrolyte including a nonaqueous solvent, wherein the nonaqueous solvent includes a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the nonaqueous solvent is 10% by volume or more and 70% by volume or less.

[0029] According to the method for producing a nonaqueous electrolyte storage element described in

[10] above, it is possible to produce a nonaqueous electrolyte storage element that includes a positive electrode containing a sulfur-based active material and a nonaqueous electrolyte containing a cyclic carbonate, and that has a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and a low AC resistance after charge-discharge cycling.

[0030] A nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, a method for manufacturing a nonaqueous electrolyte electricity storage element, an electricity storage device, and other embodiments will be described in detail below.

[0031] <Non-aqueous electrolyte energy storage element> A nonaqueous electrolyte storage element according to one embodiment of the present invention comprises a positive electrode, a negative electrode, a nonaqueous electrolyte, and a container for accommodating these. The nonaqueous electrolyte storage element may further comprise a separator interposed between the positive electrode and the negative electrode to electrically insulate the positive electrode from the negative electrode. The positive electrode, the negative electrode, and any separator typically constitute an electrode assembly. At least a portion of the nonaqueous electrolyte typically exists in a state of being impregnated into the electrode assembly. The nonaqueous electrolyte storage element according to one embodiment of the present invention may further comprise other components.

[0032] For example, a nonaqueous electrolyte storage element 1 according to one embodiment of the present invention shown in FIG. 1 includes an electrode assembly 2, a nonaqueous electrolyte (not shown), and a rectangular parallelepiped container 3 that accommodates these. The nonaqueous electrolyte storage element 1 of FIG. 1 further includes a positive electrode lead 4, a positive electrode external terminal 5, a negative electrode lead 6, and a negative electrode external terminal 7. The positive electrode lead 4 and the negative electrode lead 6 are accommodated in the container 3 together with the electrode assembly 2 and the like. The positive electrode external terminal 5 and the negative electrode external terminal 7 are provided outside the container 3. The positive electrode constituting the electrode assembly 2 is electrically connected to the positive electrode external terminal 5 via the positive electrode lead 4. The negative electrode constituting the electrode assembly 2 is electrically connected to the negative electrode external terminal 7 via the negative electrode lead 6.

[0033] The nonaqueous electrolyte storage element of the present invention may be a nonaqueous electrolyte secondary battery. Below, the main components constituting the nonaqueous electrolyte storage element according to one embodiment of the present invention will be described in detail, mainly in the case where the nonaqueous electrolyte storage element is a nonaqueous electrolyte secondary battery (particularly a lithium ion secondary battery), but this is not intended to limit the application of the present invention.

[0034] The lower and upper limits of each numerical range described in the embodiments of the present invention can be combined in any manner.

[0035] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer laminated on the positive electrode substrate directly or via an intermediate layer. Usually, the positive electrode has a portion where the positive electrode substrate is exposed. This exposed portion of the positive electrode substrate is usually connected to the above-mentioned positive electrode lead. The positive electrode may have a shape such as a sheet, plate, or strip.

[0036] The thickness of the positive electrode is appropriately set depending on the application of the nonaqueous electrolyte storage element. The average thickness of the positive electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the positive electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the positive electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the positive electrode is the average thickness of the portion where the positive electrode active material layer is laminated on the positive electrode substrate directly or via an intermediate layer. In the case of a positive electrode in which positive electrode active material layers are laminated on both sides of the positive electrode substrate, the average thickness is the average thickness of the portion where the positive electrode active material layers are laminated on both sides. In addition, in this specification, "average thickness" means the average value of thicknesses measured at any five positions.

[0037] The positive electrode substrate has electrical conductivity. In this specification, "having electrical conductivity" means that the volume resistivity is 10 -2 The volume resistivity is a value measured in accordance with JIS-H-0505 (1975). On the other hand, in this specification, "not having electrical conductivity" or "having (electrical) insulation" means that the volume resistivity is 10 7 This means that the resistance is Ω·cm or more.

[0038] Examples of materials for the positive electrode substrate include metals such as aluminum, titanium, iron, and alloys thereof (stainless steel, etc.). Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high electronic conductivity, and cost.

[0039] The positive electrode substrate has a shape such as a sheet, plate, or strip. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The positive electrode substrate may be, for example, aluminum foil or aluminum alloy foil.

[0040] The average thickness of the positive electrode substrate may be, for example, 3 μm or more and 50 μm or less. The lower limit of the average thickness of the positive electrode substrate may be 5 μm, 8 μm, 10 μm, or 15 μm. The upper limit of the average thickness of the positive electrode substrate may be 40 μm, 30 μm, 20 μm, or 15 μm.

[0041] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains, for example, a conductive agent and a binder. When the intermediate layer contains a conductive agent, the contact resistance between the positive electrode substrate and the positive electrode active material layer can be reduced. Examples of the conductive agent and binder used in the intermediate layer include the same conductive agent and binder used in the positive electrode active material layer described below.

[0042] 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 may be formed from a positive electrode mixture containing a sulfur-based active material and other optional components. The positive electrode active material layer may be provided on only one side or on both sides of a positive electrode substrate having a shape such as a sheet.

[0043] 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.

[0044] The content of the sulfur-based active material in the positive electrode active material layer is preferably 50% by mass to 80% by mass, more preferably 55% by mass to 75% by mass, and even more preferably 60% by mass to 70% by mass. The content of elemental sulfur in the positive electrode active material layer is preferably 50% by mass to 80% by mass, more preferably 55% by mass to 75% by mass, and even more preferably 60% by mass to 70% by mass. By ensuring that the content of the sulfur-based active material or elemental sulfur in the positive electrode active material layer is within the above range, it is possible to further increase the initial discharge capacity, etc.

[0045] 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.

[0046] In one embodiment of the present invention, the sulfur-based active material may be present in the positive electrode active material layer in the form of a composite with porous carbon. That is, the positive electrode active material layer may further contain porous carbon, and the sulfur-based active material and the porous carbon may form a composite. In a composite of a sulfur-based active material and porous carbon (hereinafter simply referred to as a "composite"), the sulfur-based active material is typically supported in the pores of the porous carbon. This form of the composite ensures sufficient electronic conductivity. The composite may be substantially composed of a sulfur-based active material and porous carbon, or may be substantially composed of a sulfur-based active material and porous carbon. A composite substantially composed of a 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.

[0047] 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 increase the initial discharge capacity, etc.

[0048] 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 by mass. The lower limit of the carbon 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 content in porous carbon may be 100 mass% or 99.9 mass%. Porous carbon may contain elements other than carbon, such as oxygen and nitrogen.

[0049] 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.

[0050] The content of the composite in the positive electrode active material layer is preferably 60% by mass to 97% by mass, more preferably 80% by mass to 96% by mass, and even more preferably 90% by mass to 95% by mass. By setting the content of the composite within the above range, it is possible to increase the initial discharge capacity, etc.

[0051] The conductive agent is usually a component made of a material with electrical conductivity. Note that this conductive agent does not include the porous carbon that constitutes the composite. Even if the volume resistivity of the conductive agent cannot be measured directly, it is possible to estimate the volume resistivity by measuring the volume resistivity of the conductive agent. -2Conductive agents are materials known to have a resistivity of Ω·cm or less. Examples of conductive agents include carbon materials, metals, and conductive ceramics. Carbon materials are materials whose primary constituent element is carbon. The primary constituent element refers to the element with the highest content by mass. For example, the carbon content in a carbon material may be 80% by mass or more, 90% by mass or more, 95% by mass, 99% by mass, or 99.9% by mass or more. The carbon material is preferably a carbon material other than an uncarbonized polymer compound. 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. Conductive agents may be in the form of powder, fiber, or the like. The conductive agent may be one or more types. A composite of these materials may be used as the conductive agent. For example, a composite material of carbon black and CNT may be used. It is also preferable to use a combination of carbon black (preferably acetylene black) and CNT.

[0052] 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 to 15% by mass, more preferably 1% by mass to 10% by mass. 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 within the above range, the energy density per mass of the positive electrode can be increased.

[0053] Examples of the binder include a water-based binder and an organic solvent-based binder.

[0054] The aqueous binder is a binder that dissolves or disperses in water. The aqueous binder may be a binder that dissolves or disperses at 1 part by mass or more in 100 parts by mass of water at 20°C. When a positive electrode active material layer is formed using a positive electrode mixture paste whose dispersion medium is water or a mixed solvent mainly composed of water, an aqueous binder (a water-soluble or water-dispersible polymer material) can be used. Examples of aqueous binders include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polytetrafluoroethylene, styrene-butadiene rubber, polyethylene, polypropylene, nitrile-butadiene rubber, and cellulose.

[0055] The organic solvent-based binder is a binder that dissolves or disperses in an organic solvent (e.g., N-methylpyrrolidone). The organic solvent-based binder may be a binder that dissolves or disperses at 1 part by mass or more in 100 parts by mass of an organic solvent (e.g., N-methylpyrrolidone) at 20°C. When a positive electrode active material layer is formed using a positive electrode mixture paste whose dispersion medium is an organic solvent or a mixed solvent mainly containing an organic solvent, an organic solvent-based binder (a polymer material that is soluble or dispersible in an organic solvent) can be used. Examples of organic solvent-based binders include polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyimide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, chitosan derivatives, and the like.

[0056] The binder may be a fluororesin (polytetrafluoroethylene, polyvinylidene fluoride, etc.), a polyolefin (polyethylene, polypropylene, etc.), an elastomer (ethylene propylene diene rubber, styrene butadiene rubber, fluororubber, etc.), a polysaccharide polymer (cellulose, chitosan derivatives, etc.), etc. As the binder, a fluororesin or an elastomer is preferred, an elastomer is more preferred, and a styrene butadiene rubber is even more preferred. One or more types of binders can be used.

[0057] The content of the binder in the positive electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 1% by mass to 9% by mass, and even more preferably 2% by mass to 8% by mass. The upper limit of the binder content may be 6%, 4%, or 3% by mass. By setting the binder content within the above range, it is possible to stably hold the sulfur-based active material, etc. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a binder.

[0058] 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.

[0059] Examples of thickeners 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.

[0060] The filler is not particularly limited. The filler may be a component other than the sulfur-based active material, other positive electrode active material, porous carbon, conductive agent, binder, dispersant, and thickener, and may be an intentionally contained component. The filler may be contained as a component to fill gaps in the positive electrode active material layer, or may be contained for other purposes. The filler may be an organic substance such as polyolefin, or an inorganic substance such as an inorganic oxide, hydroxide, or carbonate. One or more fillers may be used. When the positive electrode active material layer contains a filler, the content of the filler in the positive electrode active material layer may be 0.1% by mass or more and 8% by mass or less, typically preferably 5% by mass or less, and more preferably 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0061] The positive electrode active material layer may further contain other components in addition to the sulfur-based active material, other positive electrode active material, porous carbon, conductive agent, binder, dispersant, thickener, and filler. The other components may include those unintentionally generated in the positive electrode active material layer. The positive electrode active material layer may also contain unintentionally contained impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally generated components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of impurities unintentionally contained in the positive electrode active material layer may be 10% by mass, 5% by mass, 2% by mass, 1% by mass, 0.1% by mass, or 0.01% by mass.

[0062] The mass per unit area of ​​one positive electrode active material layer is not particularly limited, but is preferably 1 mg / cm 2 More than 30mg / cm 2 Less than 5 mg / cm is preferred 2 More than 25mg / cm 2 Less than 10 mg / cm is more preferable. 2More than 20mg / cm 2 The following is even more preferable: When the mass per unit area of ​​one positive electrode active material layer is within the above range, the initial discharge capacity can be made larger, for example.

[0063] The porosity of the positive electrode active material layer is not particularly limited, but is preferably 15% to 50%, more preferably 20% to 40%, and even more preferably 25% to 35%. When the porosity of the positive electrode active material layer is in the above range, the initial discharge capacity can be increased.

[0064] The "porosity (%)" of the positive electrode active material layer and the negative electrode active material layer described below is calculated by the formula (1-V2 / V1) × 100, where V1 is the apparent volume (volume including voids) of the positive (negative) electrode active material layer and V2 is the sum of the actual volumes of the materials constituting the positive (negative) electrode active material layer. The sum V2 of the actual volumes of the materials constituting the positive (negative) electrode active material layer can be calculated from the content of each material in the positive (negative) electrode active material layer and the true density of each material.

[0065] (Positive electrode manufacturing method) The positive electrode can be manufactured by a known method. The positive electrode can be manufactured, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) to a positive electrode substrate directly or via an intermediate layer, and then drying the paste to form a positive electrode active material layer. The positive electrode mixture paste typically contains a sulfur-based active material, other optional components, and a dispersion medium. After drying, the positive electrode active material layer may be pressed, etc.

[0066] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer laminated on the negative electrode substrate directly or via an intermediate layer. Usually, the negative electrode has a portion where the negative electrode substrate is exposed. This exposed portion of the negative electrode substrate is usually connected to the above-mentioned negative electrode lead. The negative electrode may have a shape such as a sheet, plate, or strip.

[0067] The thickness of the negative electrode is appropriately set depending on the application of the nonaqueous electrolyte storage element, etc. The average thickness of the negative electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the negative electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the negative electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the negative electrode is the average thickness of the portion where the negative electrode active material layer is laminated on the negative electrode substrate directly or via an intermediate layer. In the case of a negative electrode in which negative electrode active material layers are laminated on both sides of the negative electrode substrate, the average thickness is the average thickness of the portion where the negative electrode active material layers are laminated on both sides.

[0068] The negative electrode substrate is conductive. Examples of materials for the negative electrode substrate include metals such as copper, nickel, iron, and alloys thereof (such as stainless steel), and carbon materials. Among these, nickel or a nickel alloy is preferred.

[0069] The negative electrode substrate has a shape such as a sheet, plate, or strip. Examples of the form of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The negative electrode substrate may be, for example, nickel foil or nickel alloy foil.

[0070] The average thickness of the negative electrode substrate may be, for example, 2 μm or more and 35 μm or less. The lower limit of the average thickness of the negative electrode substrate may be 3 μm, 4 μm, 5 μm, or 10 μm. The upper limit of the average thickness of the negative electrode substrate may be 30 μm, 20 μm, 15 μm, or 10 μm.

[0071] The structure of the intermediate layer of the negative electrode is not particularly limited, and can be selected from the structures exemplified for the intermediate layer of the positive electrode, for example.

[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 thickener, and a filler as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above. The negative electrode active material layer may be formed from a negative electrode mixture containing a negative electrode active material and other optional components. The negative electrode active material layer may be provided on only one side or on both sides of a negative electrode substrate having a shape such as a sheet.

[0073] The negative electrode active material can be a known negative electrode active material. A material capable of absorbing and releasing lithium ions is typically used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic lithium; metals or semimetals such as silicon and tin; metal oxides or semimetal oxides such as silicon oxide, titanium oxide, and tin oxide; and Li4Ti5O. 12 , LiTiO 2、 Examples of the negative electrode active material include titanium-containing oxides such as TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic 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. One or more types of negative electrode active materials can be used.

[0074] Metallic lithium is preferred as the negative electrode active material. In other words, the negative electrode or negative electrode active material layer preferably contains metallic lithium at least in a charged state. It is more preferred that the negative electrode or negative electrode active material layer contains metallic lithium in all states, including a charged state and a discharged state. 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 lithium alloys include lithium-silver alloys, lithium-zinc alloys, lithium-calcium alloys, lithium-aluminum alloys, lithium-magnesium alloys, and lithium-indium alloys. The lithium alloy may contain multiple metal elements other than lithium element.

[0075] The negative electrode active material layer may contain a negative electrode active material other than metallic lithium. However, 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% or more. The upper limit of the lithium element content in the negative electrode active material layer may be 100 mass%.

[0076] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a conductive agent.

[0077] When the negative electrode active material layer contains a binder, the content of the binder in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass. The content of the binder in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a binder.

[0078] When the negative electrode active material layer contains a thickener, the content of the thickener in the negative electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The content of the thickener in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a thickener.

[0079] The filler in the negative electrode active material layer may be a component other than the negative electrode active material, conductive agent, binder, and thickener, and may be an intentionally contained component. The filler may be contained as a component to fill gaps in the negative electrode active material layer, or may be contained for other purposes. When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer can be 0.1% by mass or more and 8% by mass or less, and typically 5% by mass or less is preferred, and 2% by mass or less is more preferred. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a filler.

[0080] The negative electrode active material layer may further contain other components in addition to the negative electrode active material, conductive agent, binder, thickener, and filler. The other components include those unintentionally present in the negative electrode active material layer. The negative electrode active material layer may also contain unintentionally present impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally present components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally present impurities in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0081] 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 to 2,000 μm, more preferably 30 μm to 1,200 μm, and even more preferably 100 μm to 800 μm.

[0082] (Method of manufacturing negative electrode) The negative electrode can be manufactured by a known method. The negative electrode can be manufactured, for example, in the same manner as in the above-described method for manufacturing a positive electrode, by applying a paste-like negative electrode mixture (negative electrode mixture paste) to a negative electrode substrate directly or via an intermediate layer, and then drying to form a negative electrode active material layer. The negative electrode mixture paste usually contains a negative electrode active material, other optional components, and a dispersion medium. After drying, the negative electrode active material layer may be pressed, etc. When the negative electrode active material is a metal such as metallic lithium, it can also be manufactured by laminating a metal foil on the negative electrode substrate directly or via an intermediate layer, and then pressing, etc.

[0083] (separator) The separator may be a known separator, such as a separator consisting of only a base layer, or a separator having an inorganic layer containing inorganic particles and a binder formed on one or both surfaces of a base layer.

[0084] Examples of the form of the substrate layer of the separator include woven fabric, nonwoven fabric, porous resin film, etc. Among these forms, porous resin film is preferred from the viewpoint of strength, etc., and nonwoven fabric is preferred from the viewpoint of non-aqueous electrolyte retention, etc. The material of the substrate layer of the separator is not particularly limited as long as it has insulating properties, but resins such as polyolefin (polyethylene, polypropylene, etc.), polyimide, aramid, etc. are preferred.

[0085] Examples of inorganic compounds constituting the inorganic particles include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, and magnesium oxide; 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; mineral-derived substances such as talc, zeolite, kaolin, bentonite, and mica, or artificial products thereof. One or more types of inorganic particles can be used. The average particle size of the inorganic particles is preferably, for example, from 0.5 μm to 10 μm. The content of the inorganic particles in the inorganic layer is preferably from 50% to 99% by mass, and more preferably from 80% to 98% by mass.

[0086] Examples of binders used in the inorganic layer include the same binders as those exemplified for the positive electrode active material layer.

[0087] The porosity of the separator may be, for example, 20% or more and 80% or less. The lower limit of the porosity of the separator may be 30%, 40%, or 50% from the viewpoint of discharge performance, etc. The upper limit of the porosity of the separator may be 70%, 60%, or 50% from the viewpoint of strength, etc. In this specification, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0088] The average thickness of the separator may be, for example, 10 μm or more and 40 μm or less, or 15 μm or more and 30 μm or less.

[0089] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte, or may be a combination of the porous resin film, nonwoven fabric, or the like described above and a polymer gel.

[0090] (electrode body) As the electrode body, for example, a wound type electrode body, a laminated type electrode body, or other electrode body having a known structure can be used.

[0091] A wound electrode body has a structure in which a positive electrode and a negative electrode are wound in an insulated state. The wound electrode body may be cylindrical (columnar) or flat. The electrode body 2 provided in the nonaqueous electrolyte storage element 1 of FIG. 1 is a flat wound electrode body. The wound electrode body can be produced, for example, by the following procedure. First, a positive electrode, a separator, and a negative electrode, each formed in a strip shape, are stacked together to obtain a laminate. This laminate is then wound to obtain a wound electrode body.

[0092] A laminated electrode body has a structure in which one or more positive electrodes and one or more negative electrodes are stacked in an insulated state. For example, a laminated electrode body can be obtained by stacking a positive electrode, a separator, and a negative electrode, each of which is formed in a rectangular shape.

[0093] As the electrode body, for example, one having a structure in which at least one of the positive electrode and the negative electrode is folded in an accordion-like manner and stacked can also be used.

[0094] (non-aqueous electrolyte) The non-aqueous electrolyte is a medium that transports charge-transporting ions (e.g., lithium ions) between the positive electrode and the negative electrode and is substantially free of water. The water content of the non-aqueous electrolyte may be, for example, 10,000 ppm or less, or 5,000 ppm or less.

[0095] The nonaqueous electrolyte contains a nonaqueous solvent. As the nonaqueous electrolyte, a nonaqueous electrolytic solution containing a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent is usually suitably used. In one embodiment of the present invention, the nonaqueous electrolyte storage element may be a nonaqueous electrolyte storage element.

[0096] The non-aqueous solvent includes a cyclic carbonate and a fluorinated carboxylic acid ester.

[0097] The cyclic carbonate refers to a carbonate having a ring structure containing a carbonate group (-OC(=O)-O-). Examples of the cyclic carbonate include fluorinated cyclic carbonates, unsaturated cyclic carbonates, and non-fluorinated saturated cyclic carbonates. The cyclic carbonate preferably includes fluorinated cyclic carbonates and unsaturated cyclic carbonates.

[0098] The fluorinated cyclic carbonate refers to a compound in which some or all of the hydrogen atoms of a cyclic carbonate have been substituted with fluorine atoms. The fluorinated cyclic carbonate is preferably a fluorinated saturated cyclic carbonate. The fluorinated saturated cyclic carbonate refers to a fluorinated cyclic carbonate that does not have a carbon-carbon double bond or a carbon-carbon triple bond in the molecule.

[0099] Examples of the fluorinated cyclic carbonate include fluorinated ethylene carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), fluorinated propylene carbonate, fluorinated butylene carbonate, and fluorinated vinylene carbonate. Among these, fluorinated ethylene carbonate is preferred, and FEC is more preferred. One or more types of fluorinated cyclic carbonates can be used.

[0100] The lower limit of the content of the fluorinated cyclic carbonate in the non-aqueous solvent is preferably 10% by volume, more preferably 15% by volume, even more preferably 20% by volume, and even more preferably 25% by volume. By having the content of the fluorinated cyclic carbonate be equal to or greater than the above lower limit, it is possible to increase the initial discharge capacity and reduce the AC resistance after charge-discharge cycling. The upper limit of the content of the fluorinated cyclic carbonate in the non-aqueous solvent is preferably 90% by volume, more preferably 70% by volume, and even more preferably 50%, 40%, or 35% by volume. By having the content of the fluorinated cyclic carbonate be equal to or less than the above upper limit, it is possible to increase the capacity retention rate after charge-discharge cycling.

[0101] The content of the fluorinated cyclic carbonate in the cyclic carbonate is preferably 20% by volume or more and 80% by volume or less, more preferably 30% by volume or more and 70% by volume or less, and even more preferably 40% by volume or more and 60% by volume or less.

[0102] The unsaturated cyclic carbonate refers to a cyclic carbonate having a carbon-carbon double bond or a carbon-carbon triple bond in the molecule. The unsaturated cyclic carbonate preferably has a carbon-carbon double bond in the molecule. The unsaturated cyclic carbonate may have a carbon-carbon double bond or a carbon-carbon triple bond in the ring structure or in a part other than the ring structure, but preferably has the bond in the ring structure. The unsaturated cyclic carbonate may be fluorinated or non-fluorinated, but is preferably non-fluorinated. That is, the unsaturated cyclic carbonate may be a non-fluorinated unsaturated cyclic carbonate.

[0103] Examples of unsaturated cyclic carbonates include vinylene carbonate (VC), methyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, butyl vinylene carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, dipropyl vinylene carbonate, and vinyl ethylene carbonate, with VC being preferred. One or more unsaturated cyclic carbonates can be used.

[0104] The lower limit of the content of the unsaturated cyclic carbonate in the non-aqueous solvent is preferably 10% by volume, more preferably 15% by volume, even more preferably 20% by volume, and even more preferably 25% by volume. When the content of the unsaturated cyclic carbonate is equal to or greater than the above lower limit, it is possible to increase the initial discharge capacity and reduce the AC resistance after charge-discharge cycling. The upper limit of the content of the unsaturated cyclic carbonate in the non-aqueous solvent is preferably 90% by volume, more preferably 70% by volume, and even more preferably 50%, 40%, or 35% by volume. When the content of the unsaturated cyclic carbonate is equal to or less than the above upper limit, it is possible to increase the capacity retention rate after charge-discharge cycling.

[0105] The content of the unsaturated cyclic carbonate in the cyclic carbonate is preferably 20% by volume or more and 80% by volume or less, more preferably 30% by volume or more and 70% by volume or less, and even more preferably 40% by volume or more and 60% by volume or less.

[0106] The lower limit of the total content of the fluorinated cyclic carbonate and the unsaturated cyclic carbonate in the cyclic carbonate is preferably 60% by volume, more preferably 80% by volume, even more preferably 90% by volume, and even more preferably 95% by volume, and the upper limit of this total content may be 100% by volume.

[0107] Examples of non-fluorinated saturated cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate.

[0108] The lower limit of the cyclic carbonate content in the non-aqueous solvent is preferably 30% by volume, more preferably 40% by volume, even more preferably 45% by volume, and even more preferably 50% by volume. By setting the cyclic carbonate content to the above lower limit or more, it is possible to increase the initial discharge capacity and reduce the AC resistance after charge-discharge cycling. The upper limit of the cyclic carbonate content in the non-aqueous solvent is preferably 90% by volume, more preferably 80% by volume, even more preferably 70% by volume, and even more preferably 60% by volume. By setting the cyclic carbonate content to the above upper limit or less, it is possible to increase the capacity retention rate after charge-discharge cycling.

[0109] The fluorinated carboxylic acid ester refers to a compound in which some or all of the hydrogen atoms in the hydrocarbon group constituting the carboxylic acid ester are substituted with fluorine atoms. The fluorinated carboxylic acid ester can be used alone or in combination of two or more.

[0110] The fluorinated carboxylic acid ester is preferably a chain fluorinated carboxylic acid ester. By using the chain fluorinated carboxylic acid ester, the viscosity of the non-aqueous electrolyte is reduced, thereby achieving a particularly sufficient effect of improving the initial discharge capacity. The fluorinated carboxylic acid ester may be a compound represented by the following formula (1): R 1 -COO-R 2 ···(1) In formula (1), R 1 and R 2 are each independently a hydrocarbon group or a fluorinated hydrocarbon group, provided that R 1 and R 2 At least one of the groups is a fluorinated hydrocarbon group.

[0111] The hydrocarbon group may be either an aliphatic hydrocarbon group or an aromatic hydrocarbon group, but is preferably an aliphatic hydrocarbon group, and more preferably an alkyl group. The number of carbon atoms in the hydrocarbon group is preferably 1 to 10, more preferably 1 to 5, and even more preferably 1 to 3. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, and a butyl group, and an alkyl group having 1 to 3 carbon atoms is preferred.

[0112] The fluorinated hydrocarbon group includes groups in which some or all of the hydrogen atoms of the above hydrocarbon groups have been substituted with fluorine atoms. The number of carbon atoms in the fluorinated hydrocarbon group is preferably 1 to 10, more preferably 1 to 5, and even more preferably 1 to 3. The number of fluorine atoms in the fluorinated hydrocarbon group is preferably 1 to 5, and more preferably 2 to 3. The fluorinated hydrocarbon group is preferably a fluorinated alkyl group. Specific examples of the fluorinated alkyl group include fluorinated methyl groups such as a difluoromethyl group and a trifluoromethyl group, fluorinated ethyl groups such as a 2,2-difluoroethyl group, a 2,2,2-trifluoroethyl group, and a 1,1,2,2,2-pentafluoroethyl group, fluorinated propyl groups such as a 3,3,3-trifluoropropyl group and a 2,2,3,3,3-pentafluoropropyl group, and fluorinated butyl groups such as a 4,4,4-trifluorobutyl group.

[0113] Above R 1 The preferred groups for R are the hydrocarbon groups and fluorinated hydrocarbon groups described above, and among these, a methyl group or a fluorinated methyl group is preferred, a fluorinated methyl group is more preferred, and a difluoromethyl group is even more preferred. 1 When R is a hydrocarbon group, the capacity retention rate after charge-discharge cycling tends to be higher, and 1 When R is a fluorinated hydrocarbon group, the initial discharge capacity tends to be larger. 1 In some cases, a hydrocarbon group is preferred, and in other cases, a fluorinated hydrocarbon group is preferred.

[0114] Above R 2 The preferred groups for R are the hydrocarbon groups and fluorinated hydrocarbon groups described above, and among these, an ethyl group or a fluorinated ethyl group is preferred, with an ethyl group being more preferred. 2 When R is a hydrocarbon group, the initial discharge capacity tends to be larger, and 2 When R is a fluorinated hydrocarbon group, the capacity retention rate after charge-discharge cycling tends to be higher. 2 In some cases, a hydrocarbon group is preferred, and in other cases, a fluorinated hydrocarbon group is preferred.

[0115] Above R 1 and R 2 It is also preferred that one of R is a fluorinated hydrocarbon group and the other is a hydrocarbon group. 1 is a fluorinated hydrocarbon group, and R 2 may be a hydrocarbon group.

[0116] Specific examples of the fluorinated carboxylic acid ester include methyl difluoroacetate, ethyl difluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, methyl 3,3,3-trifluoropropionate, ethyl 3,3,3-trifluoropropionate, trifluoromethyl acetate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, trifluoromethyl propionate, and 2,2,2-trifluoroethyl propionate.

[0117] As the fluorinated carboxylic acid ester, a fluorinated acetic acid ester is preferred, difluoroethyl acetate (EDFA) and 2,2,2-trifluoroethyl acetate (TFEA) are more preferred, and EDFA is even more preferred.

[0118] The lower limit of the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 10% by volume, preferably 20% by volume, more preferably 30% by volume, and even more preferably 40% by volume. By setting the content of the fluorinated carboxylic acid ester at or above the lower limit, it is possible to increase the capacity retention rate after charge-discharge cycling. The upper limit of the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 70% by volume, preferably 60% by volume, more preferably 55% by volume, and even more preferably 50% by volume. By setting the content of the fluorinated carboxylic acid ester at or below the upper limit, it is possible to increase the initial discharge capacity, increase the capacity retention rate after charge-discharge cycling, and reduce the AC resistance after charge-discharge cycling.

[0119] The lower limit of the total content of the cyclic carbonate and the fluorinated carboxylic acid ester in the non-aqueous solvent is preferably 80% by volume, more preferably 90% by volume, and may be 95%, 98%, or 99% by volume. The upper limit of the total content of the cyclic carbonate and the fluorinated carboxylic acid ester in the non-aqueous solvent may be 100% by volume, and the non-aqueous solvent may be composed essentially of the cyclic carbonate and the fluorinated carboxylic acid ester. When the non-aqueous solvent is composed essentially of the cyclic carbonate and the fluorinated carboxylic acid ester, the effects of a large initial discharge capacity, a high capacity retention rate after charge-discharge cycling, and a low AC resistance after charge-discharge cycling can be particularly well achieved.

[0120] The non-aqueous solvent may further contain a solvent other than the cyclic carbonate and the fluorinated carboxylic acid ester, such as a chain carbonate, an ether, an ester other than the fluorinated carboxylic acid ester, an amide, or a nitrile.

[0121] The electrolyte salt may be a known electrolyte salt. Examples of the electrolyte salt include lithium salt, sodium salt, potassium salt, magnesium salt, and onium salt. Among these, lithium salt is preferred. One or more types of electrolyte salt may be used.

[0122] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, and LiClO4; imide salts such as LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9); and lithium oxalates such as LiB(C2O4)2, LiBF2(C2O4), and LiPF2(C2O4)2. LiN(SO2F)2 also falls under the category of inorganic lithium salts. As the electrolyte salt, imide salts are preferred, and LiN(SO2CF3)2 is more preferred.

[0123] The content of the electrolyte salt in the non-aqueous electrolyte solution or non-aqueous electrolyte is 0.1 mol / dm 3 More than 2.5mol / dm 3Preferably less than 0.3 mol / dm 3 More than 2.0mol / dm 3 Less than 0.5 mol / dm is more preferable. 3 More than 1.7mol / dm 3 More preferably, 0.7 mol / dm 3 More than 1.5mol / dm 3 The following is particularly preferred: By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolytic solution or non-aqueous electrolyte can be increased.

[0124] The non-aqueous electrolyte may contain an additive in addition to the non-aqueous solvent and the electrolyte salt. One or more additives may be used. When an additive is used in the non-aqueous electrolyte, the content of the additive in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass.

[0125] The non-aqueous electrolyte may be a combination of a non-aqueous electrolytic solution and a solid electrolyte. The solid electrolyte may be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc., and is solid at room temperature (e.g., 20°C). Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, halide solid electrolytes, and polymer solid electrolytes. One or more solid electrolytes may be used.

[0126] (container) The container accommodates the electrode assembly and the non-aqueous electrolyte in its internal space. The container may be made of a metal material such as aluminum or stainless steel, or a resin material. Metal materials are preferred from the viewpoint of strength, etc. Composite materials of metal and resin may also be used.

[0127] The shape of the container is not particularly limited, and may be cylindrical, rectangular (square), disk-like, etc. The container may also be in the shape of a sheet formed from a metal resin composite film.

[0128] (shape, use, etc. of non-aqueous electrolyte storage element) The shape of the nonaqueous electrolyte storage element according to one embodiment of the present invention is not particularly limited, and may be, for example, a cylindrical battery, a prismatic battery, a flat battery, a coin battery, a button battery, or the like.

[0129] The use of the nonaqueous electrolyte electricity storage element according to one embodiment of the present invention is not particularly limited, and the nonaqueous electrolyte electricity storage element can be used, for example, as a power source for automobiles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, a power source for electronic devices such as personal computers and communication terminals, a power source for power storage, etc.

[0130] The nonaqueous electrolyte electricity storage element of the present invention may be used singly or in plural. When the required output and required voltage are small, the nonaqueous electrolyte electricity storage element may be used singly. On the other hand, when at least one of the required output and required voltage is large, the nonaqueous electrolyte electricity storage element may be used as an electricity storage device in combination with other nonaqueous electrolyte electricity storage elements. In an electricity storage device in which a plurality of nonaqueous electrolyte electricity storage elements are combined, at least one nonaqueous electrolyte electricity storage element included in the electricity storage device may be the nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. The electricity storage device will be described in detail later.

[0131] In a nonaqueous electrolyte energy storage element according to one embodiment of the present invention, for example, the container may be constrained so as to maintain a constant thickness, or may not be constrained in this manner. Alternatively, the container may be constrained so as to apply a constant load to it. When the container is constrained, expansion of the container due to charge / discharge cycles, etc., may be suppressed, and deterioration of charge / discharge performance may be suppressed. When the container is constrained, a load may or may not be applied to the electrode assembly within the container. For example, a constraining member that performs such constraining may be provided in the nonaqueous electrolyte energy storage element or the energy storage device.

[0132] <Method of manufacturing nonaqueous electrolyte energy storage element> A method for producing a nonaqueous 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 nonaqueous electrolyte containing a nonaqueous solvent, wherein the nonaqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the nonaqueous solvent is 10% by volume or more and 70% by volume or less. The method for producing a nonaqueous electrolyte storage element may include, for example, preparing a negative electrode, preparing a separator, producing an electrode assembly using the positive electrode, the negative electrode, and the separator, and housing the positive electrode, the negative electrode, and the nonaqueous electrolyte in a container. Housing the positive electrode, the negative electrode, and the nonaqueous electrolyte in a container may mean housing the electrode assembly and the nonaqueous electrolyte in the container.

[0133] The prepared positive electrode contains a sulfur-based active material. Specific and preferred embodiments of the prepared positive electrode are the same as those of the positive electrode included in the nonaqueous electrolyte energy storage element according to one embodiment of the present invention described above. Preparing the positive electrode may also mean fabricating the positive electrode.

[0134] Preparing a negative electrode may mean fabricating a negative electrode. Specific and preferred embodiments of the prepared negative electrode are the same as those of the negative electrode included in the nonaqueous electrolyte electricity storage element according to one embodiment of the present invention described above.

[0135] The prepared non-aqueous electrolyte includes a non-aqueous solvent. The non-aqueous solvent includes a cyclic carbonate and a fluorinated carboxylic acid ester, and the content of the fluorinated carboxylic acid ester in the non-aqueous solvent is 10% by volume or more and 70% by volume or less. Specific and preferred aspects of the prepared non-aqueous electrolyte are the same as those of the non-aqueous electrolyte included in the non-aqueous electrolyte storage element according to one embodiment of the present invention described above. Preparing the non-aqueous electrolyte may mean preparing the non-aqueous electrolyte. The non-aqueous electrolyte can be prepared, for example, by mixing an electrolyte salt, a non-aqueous solvent, and the like.

[0136] The electrode assembly (or the positive electrode and negative electrode) and the non-aqueous electrolyte can be placed in the container by a known method. When the non-aqueous electrolyte is a non-aqueous electrolyte solution, for example, the electrode assembly (or the positive electrode and negative electrode) is first placed in the container, and then the non-aqueous electrolyte solution is poured into the container through an inlet provided in the container. The inlet is then sealed after the non-aqueous electrolyte solution is poured into the container.

[0137] The manufacturing method may include performing initial charging and discharging on an undischarged nonaqueous electrolyte storage element. In a nonaqueous electrolyte storage element according to one embodiment of the present invention, the initial charging and discharging typically begins with discharging. The first discharge or the like may be referred to as a chemical conversion treatment. The number of times of charging and discharging in the initial charging and discharging is not particularly limited. By undergoing the initial charging and discharging or chemical conversion treatment, a good coating is formed on the surface of the sulfur-based active material due to a reaction between a portion of the positive electrode and the nonaqueous electrolyte.

[0138] <Electricity storage device> 2 includes a plurality of energy storage units 20. Each energy storage unit 20 includes a plurality of electrically connected nonaqueous electrolyte energy storage elements 1. The energy storage device 30 may include a bus bar (not shown) that electrically connects the plurality of nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects the plurality of energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more nonaqueous electrolyte energy storage elements 1.

[0139] <Other embodiments> The nonaqueous electrolyte storage element of the present invention is 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 may be added to the configuration of another embodiment, or part of the configuration of one embodiment may be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0140] 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 nonaqueous electrolyte storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0141] In the above embodiment, the electrode assembly is described in which a separator is interposed between the positive electrode and the negative electrode. However, 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, with a non-conductive layer formed on the active material layer of the positive electrode or the negative electrode. In this way, the positive electrode and the negative electrode may further include layers other than the substrate, the intermediate layer, and the active material layer. Furthermore, the positive electrode and the negative electrode may not have a layer structure. [Example]

[0142] 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.

[0143] [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 rate of 5°C / min and held for 5 hours. After cooling to 80°C, the temperature at which elemental sulfur solidifies, the mixture was again heated to 300°C at a rate of 5°C / min and held for 2 hours to produce a composite (sulfur-porous carbon composite: SPC). A positive electrode mixture paste containing the composite obtained above, acetylene black and CNT as a conductive agent, CMC as a dispersant, PAA as a thickener, and SBR as a binder, was applied to an aluminum positive electrode substrate (average thickness 15 μm) and dried. The dispersion medium was removed so that the mass per unit area of ​​the positive electrode active material layer after drying was 15 mg / cm. 2The amount of the positive electrode mixture paste applied was adjusted so that the positive electrode active material layer was formed on the positive electrode substrate through the above steps. The porosity of the positive electrode active material layer in the resulting positive electrode was 30%.

[0144] (Preparing the negative electrode) A pure metallic lithium foil (average thickness 600 μm) was prepared as the negative electrode.

[0145] (Preparation of non-aqueous electrolyte) Fluoroethylene carbonate (FEC) as a fluorinated cyclic carbonate, vinylene carbonate (VC) as an unsaturated cyclic carbonate, and ethyl difluoroacetate (EDFA) as a fluorinated carboxylic acid ester were mixed in a volume ratio of 38.5:38.5:23.0 in a non-aqueous solvent. LiN(SO2CF3)2 was added as an electrolyte salt at a concentration of 1.0 mol / dm 3 The non-aqueous electrolyte was prepared by adding the above-mentioned components in an amount of 1:1.

[0146] (Assembly of non-aqueous electrolyte energy storage element) The separator was prepared by laminating inorganic particle layers on both sides of a polyethylene microporous membrane. The nonaqueous electrolyte storage element of Example 1 was obtained using the positive electrode, negative electrode, separator, and nonaqueous electrolyte described above.

[0147] [Examples 2 to 7, Comparative Examples 1 to 7] The nonaqueous electrolyte storage elements of Examples 2 to 7 and Comparative Examples 1 to 7 were obtained in the same manner as in Example 1, except that the compositions of the nonaqueous solvents were as shown in Table 1. The nonaqueous solvents used were as follows. FEC: Fluoroethylene carbonate VC: vinylene carbonate EDFA: Ethyl difluoroacetate TFEA: 2,2,2-trifluoroethyl acetate ETFEE: Ethyl-1,1,2,2-tetrafluoroethyl ether

[0148] [evaluation] (Measurement of initial discharge capacity) Each of the obtained nonaqueous electrolyte storage elements was first subjected to constant current discharge at a discharge current of 0.1 C and a discharge cut-off voltage of 1.0 V as a chemical conversion treatment. Subsequently, constant-current / constant-voltage charging was performed with a charging current of 0.1 C and a cut-off voltage of 3.0 V. Charging was terminated until the total charging time reached 30 hours. Next, constant-current discharging was performed with a discharging current of 0.1 C and a cut-off voltage of 1.0 V. A 10-minute pause was provided after each discharge and charge. Discharge, charge, and pause were all performed in a thermostatic chamber at 25°C. The quantity of electricity per mass of the sulfur-based active material (sulfur elemental substance) during discharge after the above charge was determined as the initial discharge capacity. The results are shown in Table 1.

[0149] (Charge-discharge cycle test) A charge-discharge cycle test was carried out in the following manner for each of the nonaqueous electrolyte storage elements of Examples 1 to 7 and Comparative Examples 1 to 5 and 7, for which the above-mentioned "measurement of initial discharge capacity" was carried out. Note that the nonaqueous electrolyte storage element of Comparative Example 6 was not subjected to a charge-discharge cycle test because its initial discharge capacity was very small. Constant current / constant voltage charging was performed with a charging current of 0.1 C and a charge cut-off voltage of 3.0 V. Charging was terminated until the total charging time reached 15 hours. Next, constant current discharging was performed with a discharge current of 0.1 C and a discharge cut-off voltage of 1.0 V. A 10-minute pause was provided after discharge and charge. Discharge, charge, and pause were all performed in a thermostatic chamber at 25°C. 1.0 C was defined as the current that could charge the initial discharge capacity measured in the "Measurement of Initial Discharge Capacity" section above in 1 hour. The above charge and discharge cycles were performed 50 times. The capacity retention rate after charge-discharge cycling was calculated as the percentage of the discharge capacity at the 50th cycle relative to the discharge capacity at the 1st cycle in the "charge-discharge cycle test." The results are shown in Table 1. After the charge-discharge cycle test, the alternating current resistance (AC resistance) of each nonaqueous electrolyte storage element was measured at 1 kHz at 25° C. The results are shown in Table 1.

[0150] [Table 1]

[0151] In comparison with the nonaqueous electrolyte storage element of Comparative Example 1, which used only a cyclic carbonate as the nonaqueous solvent, the nonaqueous electrolyte storage elements of Comparative Examples 2 to 5, which added the fluorinated ether ETFEE as the nonaqueous solvent, either exhibited a reduced capacity retention rate after charge-discharge cycling as in Comparative Examples 4 and 5, or exhibited a high AC resistance after charge-discharge cycling as in Comparative Examples 2 and 3. Furthermore, the nonaqueous electrolyte storage element of Comparative Example 6, which used only the fluorinated carboxylic acid ester EDFA as the nonaqueous solvent, exhibited a very small initial discharge capacity. The nonaqueous electrolyte storage element of Comparative Example 7, which used a nonaqueous solvent containing a cyclic carbonate and a fluorinated carboxylic acid ester with a fluorinated carboxylic acid ester content of less than 10% by volume, did not exhibit a high capacity retention rate after charge-discharge cycling. In contrast to these, the nonaqueous electrolyte storage elements of Examples 1 to 7, which used a nonaqueous solvent containing a cyclic carbonate and a fluorinated carboxylic acid ester with a fluorinated carboxylic acid ester content of 10% by volume or more and 70% by volume or less, showed a large initial discharge capacity (1,000 mAh / g or more), a high capacity retention rate after charge-discharge cycling (70% or more), and a low AC resistance after charge-discharge cycling (less than 15 Ω). [Industrial Applicability]

[0152] 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, automobiles, industrial equipment, and the like. [Explanation of symbols]

[0153] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive lead 5 Positive external terminal 6 Negative lead 7 Negative external terminal 20 Energy storage unit 30 Electricity storage device

Claims

1. a positive electrode containing a sulfur-based active material; and Non-aqueous electrolyte containing a non-aqueous solvent Equipped with the non-aqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester; The nonaqueous electrolyte electricity storage element has a content of the fluorinated carboxylic acid ester in the nonaqueous solvent of 10% by volume or more and 70% by volume or less.

2. 2. The nonaqueous electrolyte storage element according to claim 1, wherein the content of the cyclic carbonate in the nonaqueous solvent is 30% by volume or more and 90% by volume or less.

3. 3. The nonaqueous electrolyte storage element according to claim 1, wherein the cyclic carbonate comprises a fluorinated cyclic carbonate and an unsaturated cyclic carbonate.

4. 3. The nonaqueous electrolyte electricity storage element according to claim 1, further comprising a negative electrode containing metallic lithium.

5. Providing a positive electrode including a sulfur-based active material; and Preparing a non-aqueous electrolyte containing a non-aqueous solvent Equipped with the non-aqueous solvent contains a cyclic carbonate and a fluorinated carboxylic acid ester; The method for producing a nonaqueous electrolyte storage element, wherein the content of the fluorinated carboxylic acid ester in the nonaqueous solvent is 10% by volume or more and 70% by volume or less.

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