Non-aqueous alkali metal energy storage element

Abstract

To provide a nonaqueous alkali metal power storage element, suppressing increase in resistance in high-temperature high-voltage cycles and having a high capacity retention rate.SOLUTION: There is provided, a nonaqueous alkali metal power storage element including a positive electrode, a negative electrode, a separator and a nonaqueous electrolytic solution. A positive electrode active material layer contains: a positive electrode active material containing a carbon material; and an alkali metal carbonate. Concentration of hexafluorophosphate ions contained in the nonaqueous electrolytic solution is 0.00 mol / L or more and 0.05 mol / L or less. When C (mAh / cm2) represents single-pole capacity of the positive electrode, and D (mAh / cm2) represents single-pole capacity of the negative electrode, 0.08≤C / D≤0.15 is satisfied. When A(mAh / g) represents alkali metal ion concentration per unit area at an outer periphery of the negative electrode, and B (mAh / g) represents an alkali metal ion concentration per unit area at a central part of the negative electrode, 1.02≤B / A≤1.45 is satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous alkali metal electricity storage element. [Background technology]

[0002] In recent years, from the perspective of preserving the global environment and effectively utilizing energy to conserve resources, attention has been focused on systems such as wind power generation power smoothing systems or late-night power storage systems, distributed home energy storage systems based on solar power generation technology, and energy storage systems for electric vehicles. The primary requirement for batteries used in these energy storage systems is high energy density. Lithium-ion batteries are being actively developed as a promising candidate for high-energy-density batteries that can meet these requirements. The second requirement is high output characteristics. For example, in combinations of high-efficiency engines and energy storage systems (e.g., hybrid electric vehicles) or fuel cells and energy storage systems (e.g., fuel cell electric vehicles), energy storage systems that exhibit high output discharge characteristics during acceleration are required. Currently, electric double-layer capacitors, nickel-metal hydride batteries, and the like are being developed as high-output energy storage devices.

[0003] Among electric double layer capacitors, those that use activated carbon for the electrodes have output characteristics of approximately 0.5 to 1 kW / L. These electric double layer capacitors not only have high output characteristics but also high durability (cycle characteristics and high-temperature storage characteristics), and have been considered to be the optimal device for the above-mentioned fields that require high output. However, their energy density is only approximately 1 to 5 Wh / L, so further improvement in energy density is necessary.

[0004] Nickel-metal hydride batteries, which are currently commonly used in hybrid electric vehicles, have high output equivalent to that of electric double-layer capacitors and an energy density of approximately 160 Wh / L. However, vigorous research is being conducted to further improve their energy density and output characteristics, as well as to increase their durability (especially their stability at high temperatures).

[0005] Research into increasing the output of lithium-ion batteries is also underway. For example, lithium-ion batteries have been developed that can achieve high output of over 3 kW / L at a discharge depth of 50% (i.e., the ratio (%) of the discharged amount to the discharge capacity of the storage element). However, their energy density is below 100 Wh / L, a design that deliberately suppresses the high energy density that is the greatest feature of lithium-ion batteries. Furthermore, their durability (cycle characteristics and high-temperature storage characteristics) is inferior to that of electric double-layer capacitors. Therefore, to ensure practical durability, such lithium-ion batteries are used within a discharge depth range narrower than the range of 0 to 100%. Since the capacity of lithium-ion batteries that can actually be used will be even smaller, research is being actively conducted to further improve their durability.

[0006] As described above, there is a strong demand for practical application of energy storage devices that combine high energy density, high output characteristics, and high durability. However, the existing energy storage devices described above each have their own advantages and disadvantages, and new energy storage devices that satisfy these technical requirements are needed. A storage device called a lithium-ion capacitor has attracted attention as a promising candidate, and development is actively underway. A lithium-ion capacitor is a type of energy storage device that uses a non-aqueous electrolyte solution containing lithium salt (hereinafter also referred to as a "nonaqueous alkali metal energy storage device"). At approximately 3 V or higher, the positive electrode is charged and discharged through a non-Faradic reaction involving the adsorption and desorption of anions, similar to that of an electric double-layer capacitor, and the negative electrode is charged and discharged through a Faradic reaction involving the absorption and release of lithium ions, similar to that of a lithium-ion battery.

[0007] To summarize the electrode materials commonly used in the above-mentioned energy storage elements and their characteristics, generally, when materials such as activated carbon are used for the electrodes and charging and discharging is performed by the adsorption and desorption of ions on the surface of the activated carbon (non-Faradaic reaction), high output and durability are obtained, but the energy density is low (for example, 1x). On the other hand, when oxide or carbon materials are used for the electrodes and charging and discharging is performed by the Faradaic reaction, the energy density is high (for example, 10x that of non-Faradaic reactions using activated carbon), but there are issues with durability and output characteristics.

[0008] As a combination of these electrode materials, electric double layer capacitors use activated carbon (energy density 1x) for the positive and negative electrodes, and are characterized by charging and discharging through non-Faradic reactions at both the positive and negative electrodes.As a result, they have high output and high durability, but are characterized by low energy density (positive electrode 1x x negative electrode 1x = 1).

[0009] Lithium-ion secondary batteries use lithium transition metal oxides (10 times the energy density) for the positive electrode and carbon materials (10 times the energy density) for the negative electrode, and are characterized by charging and discharging via the Faraday reaction at both the positive and negative electrodes. Therefore, they have a high energy density (10 times the positive electrode × 10 times the negative electrode = 100), but they have issues with output characteristics and durability. Furthermore, to meet the high durability required for hybrid electric vehicles and other applications, the depth of discharge must be limited, and lithium-ion secondary batteries can only use 10-50% of their energy.

[0010] Lithium-ion capacitors use activated carbon (energy density 1x) for the positive electrode and carbon material (energy density 10x) for the negative electrode, and are characterized by charging and discharging through a non-Faradaic reaction at the positive electrode and a Faradic reaction at the negative electrode, making them asymmetric capacitors that combine the features of electric double-layer capacitors and lithium-ion secondary batteries. Lithium-ion capacitors are characterized by high output and durability, as well as a high energy density (1x positive electrode x 10x negative electrode = 10), and by the absence of the need to limit the depth of discharge as with lithium-ion secondary batteries.

[0011] Various studies have been conducted to further improve the high-temperature durability of the lithium-ion capacitor (Patent Documents 1 to 3). Patent Document 1 discloses a lithium-ion capacitor that maintains capacity and suppresses resistance increase in an 85°C environment by optimizing parameters obtained from the combination of binder and electrolyte. Patent Document 2 discloses a nonaqueous lithium storage element that suppresses corrosion of the aluminum foil in a high-voltage environment and has high durability when stored in a high-temperature environment of 85°C or higher. Patent Document 3 discloses a nonaqueous lithium storage element that generates less gas due to decomposition of the lithium compound in a high-temperature environment and suppresses resistance increase during high-load charge / discharge cycles. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-17299 [Patent Document 2] International Publication No. 2019 / 156090 [Patent Document 3] International Publication No. 2017 / 126686 [Non-patent literature]

[0013] [Non-Patent Document 1] EP Barrett, LG Joyner and P. Halenda, J. Am. Chem. Soc., 73, 373 (1951) [Non-patent document 2] BCLippens, J. Hde Boer, J. Catalysis, 4319 (1965) [Non-patent document 3] RS Mikhail, S. Brunauer, EE Bodor, J. Colloid Interface Sci., 26, 45 (1968) Summary of the Invention [Problem to be solved by the invention]

[0014] However, neither of these documents takes into consideration the corrosion of the positive electrode current collector due to the in-plane potential variation of the electrode during high-temperature, high-voltage cycling, nor the occurrence of side reactions with excess electrolyte at the end of the negative electrode.Furthermore, they do not consider the deactivation of alkali metal ions doped in the negative electrode in a high-temperature environment and the accompanying reduction in capacity retention.

[0015] In view of the above-mentioned current situation, the problem to be solved by the present invention is to provide a non-aqueous alkali metal electricity storage element that suppresses an increase in resistance during high-temperature, high-voltage cycling and has a high capacity retention rate. [Means for solving the problem]

[0016] The above problems can be solved by the following technical means. That is, the present invention is as follows. [1] A non-aqueous alkali metal electric storage element includes an electrode assembly including a positive electrode including a positive electrode active material layer disposed on a positive electrode current collector, a negative electrode including a negative electrode active material layer disposed on a negative electrode current collector, and a separator, and a non-aqueous electrolyte solution containing alkali metal ions, wherein the positive electrode active material layer includes a positive electrode active material including a carbon material, and an alkali metal carbonate, and the non-aqueous electrolyte solution contains hexafluorophosphate ions at a concentration of 0.00 mol / L or more and 0.05 mol / L or less, and the single electrode capacity of the positive electrode is C (mAh / cm 2 ), and the single electrode capacity of the negative electrode is D (mAh / cm 2 ), 0.08≦C / D≦0.15, and 1.02≦B / A≦1.45, where A (mAh / g) is the alkali metal ion concentration per unit area of ​​the outer periphery of the negative electrode and B (mAh / g) is the alkali metal ion concentration per unit area of ​​the central portion. [2] Item 2. The nonaqueous alkali metal storage element according to item 1, wherein 0.65≦A1 / D1≦0.90 is satisfied, where A1 (mAh) is the charge capacity of the central portion of the negative electrode and D1 (mAh) is the discharge capacity of the negative electrode half-cell of the negative electrode. [3] 3. The nonaqueous alkali metal storage element according to item 1 or 2, wherein the negative electrode has the negative electrode active material layer on both sides of the negative electrode current collector, the outermost layer of the electrode body is the negative electrode, and when E (mAh / g) is an alkali metal ion concentration per unit area in a central part of the negative electrode active material layer that does not face the positive electrode, B>E. [4] 4. The nonaqueous alkali metal electricity storage element according to any one of items 1 to 3, wherein the positive electrode and / or the negative electrode contain carbon nanotubes. [5] Item 5. An electricity storage module comprising the nonaqueous alkali metal electricity storage element according to any one of items 1 to 4. [6] Item 6. The energy storage module according to item 5, wherein the energy storage module is incorporated into at least one system selected from the group consisting of a power regeneration assist system, a power load leveling system, an uninterruptible power supply system, a contactless power supply system, an energy harvesting system, an energy storage system, a solar power generation and storage system, an electric power steering system, an emergency power supply system, an in-wheel motor system, an idling stop system, an electric vehicle, a plug-in hybrid vehicle, a hybrid vehicle, an electric motorcycle, a quick charging system, and a smart grid system. [7] The nonaqueous alkali metal storage element according to any one of items 1 to 4, Lead-acid batteries, nickel-metal hydride batteries, lithium-ion secondary batteries, or fuel cells A storage system in which these are connected in series or parallel. [Effects of the Invention]

[0017] According to the present invention, a non-aqueous alkali metal storage element is provided which has a small resistance increase rate by suppressing corrosion of the positive electrode current collector and side reactions at the negative electrode end during high-temperature, high-voltage cycles, and a high capacity retention rate by suppressing deactivation of alkali metal ions in the negative electrode. [Brief explanation of the drawings]

[0018] [Figure 1]Figure 1 is an image diagram showing the outer periphery and central part of the negative electrode, and shows a schematic top view of the negative electrode (a) and a schematic diagram for explaining the outer periphery, central part, and end part of the negative electrode (b). DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail, but the present invention is not limited to this embodiment. The upper and lower limits of each numerical range in the present embodiment can be arbitrarily combined to form any numerical range.

[0020] <<Non-aqueous alkali metal storage element>> The nonaqueous alkali metal storage element of this embodiment mainly comprises a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution, which is a solution of alkali metal ions dissolved in an organic solvent.

[0021] <Negative electrode> The negative electrode in this embodiment has a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector.

[0022] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material, and may further contain optional components such as a dispersant, a conductive filler, and a binder, as necessary.

[0023] (Negative electrode active material) The negative electrode active material may be a material capable of absorbing and releasing alkali metal ions. Specific examples include carbon materials, titanium oxides, silicon, silicon oxides, silicon alloys, silicon compounds, tin, and tin compounds. The content of the negative electrode active material in the negative electrode active material layer of the negative electrode precursor is preferably 70% by mass or more, and more preferably 80% by mass or more, based on the total mass of the negative electrode active material layer. The negative electrode active material is preferably in particulate form.

[0024] The negative electrode active material preferably contains a carbon material. The content of the carbon material relative to the total amount of the negative electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more. The content of the carbon material can be 100% by mass, but from the viewpoint of obtaining the effect of the combined use of other materials, it is preferably, for example, 95% by mass or less, and may be 90% by mass or less. The upper and lower limits of the range of the content of the carbon material can be combined arbitrarily.

[0025] Examples of carbon materials include non-graphitizable carbon materials; graphitizable carbon materials; carbon black; carbon nanoparticles; activated carbon; artificial graphite; natural graphite; graphitized mesophase carbon microspheres; graphite whiskers; amorphous carbonaceous materials such as polyacene-based substances; carbonaceous materials obtained by heat-treating carbon precursors such as petroleum-based pitch, coal-based pitch, mesocarbon microbeads, coke, and synthetic resins (e.g., phenolic resins); pyrolysates of furfuryl alcohol resins or novolac resins; fullerenes; carbon nanofibers; and composite carbon materials thereof.

[0026] Among these, from the viewpoint of reducing the resistance of the negative electrode, a composite carbon material is preferred, which is obtained by subjecting one or more graphitic materials selected from artificial graphite, natural graphite, graphitized mesophase carbon microspheres, graphite whiskers, and high-specific surface area graphite to a heat treatment in the presence of one or more carbonaceous material precursors selected from petroleum-based pitch, coal-based pitch, mesocarbon microbeads, coke, and synthetic resins, such as phenolic resins, to a heat treatment to combine the graphitic material with a carbonaceous material derived from the carbonaceous material precursor. The carbonaceous material precursor is not particularly limited as long as it becomes a carbonaceous material upon heat treatment, but petroleum-based pitch or coal-based pitch is particularly preferred. Before the heat treatment, the graphitic material and the carbonaceous material precursor may be mixed at a temperature higher than the melting point of the carbonaceous material precursor. The heat treatment temperature may be any temperature at which components generated by volatilization or thermal decomposition of the carbonaceous material precursor used become carbonaceous materials, and is preferably 400° C. or higher and 2,500° C. or lower, more preferably 500° C. or higher and 2,000° C. or lower, and even more preferably 550° C. or higher and 1,500° C. The atmosphere in which the heat treatment is performed is not particularly limited, but a non-oxidizing atmosphere is preferred.

[0027] The BET specific surface area of ​​the composite carbon material is preferably 1 m 2 / g or more 50m 2 / g or less, more preferably 1.5m 2 / g or more 40m 2 / g or less, more preferably 2m 2 / g or more 25m 2 / g or less. The BET specific surface area of ​​the composite carbon material is 1m 2 / g or more, a sufficient number of reaction sites with lithium ions in the non-aqueous electrolyte can be secured, resulting in high input / output characteristics. 2 / g or less, the charge / discharge efficiency of lithium ions is improved and the reductive decomposition of the non-aqueous electrolyte solution during charge / discharge is suppressed, so that excellent high-load charge / discharge cycle characteristics can be exhibited.

[0028] The average pore diameter of the composite carbon material is preferably 1.5 nm to 25 nm, more preferably 2 nm to 22 nm, even more preferably 3 nm to 20 nm, and particularly preferably 3.5 nm to 18 nm. If the average pore diameter of the composite carbon material is 1.5 nm or more, there are many pores larger than the size (1.2 nm or less) of the solvated alkali metal ions in the nonaqueous electrolyte, which improves the diffusion of the solvated alkali metal ions within the composite carbon material, and a nonaqueous alkali metal storage element using the composite carbon material can exhibit high input / output characteristics. On the other hand, if the average pore diameter of the composite carbon material is 25 nm or less, the bulk density of the negative electrode active material layer using the composite carbon material can be sufficiently improved, thereby exhibiting a high energy density.

[0029] The composite carbon material may be in a particulate form, with an average particle size preferably of 1 μm to 10 μm, more preferably 2 μm to 8 μm, and even more preferably 3 μm to 6 μm. If the average particle size of the composite carbon material is 1 μm or more, the charge / discharge efficiency of alkali metal ions can be improved, thereby enabling high-load charge / discharge cycle performance to be exhibited. If the average particle size of the composite carbon material is 10 μm or less, the number of reaction sites with alkali metal ions in the nonaqueous electrolyte solution increases, enabling high input / output performance to be exhibited.

[0030] The mass ratio of the carbonaceous material to the graphite material in the composite carbon material is preferably 1% by mass or more and 20% by mass or less, more preferably 1.2% by mass or more and 15% by mass or less, even more preferably 1.5% by mass or more and 10% by mass or less, and even more preferably 2% by mass or more and 5% by mass or less. When the mass ratio of the carbonaceous material is 1% by mass or more, the carbonaceous material can sufficiently increase the reaction sites with the alkali metal ions in the nonaqueous electrolyte and facilitate desolvation of the alkali metal ions, thereby exhibiting high input / output characteristics. When the mass ratio of the carbonaceous material is 20% by mass or less, the solid diffusion of the alkali metal ions between the carbonaceous material and the graphite material can be well maintained, thereby exhibiting high input / output characteristics. Furthermore, the charge / discharge efficiency of the alkali metal ions can be improved, thereby exhibiting high high-load charge / discharge cycle characteristics.

[0031] The doping amount of alkali metal ions per unit mass of the composite carbon material is preferably 50 mAh / g or more and 700 mAh / g or less, more preferably 70 mAh / g or more and 650 mAh / g or less, even more preferably 90 mAh / g or more and 600 mAh / g or less, and even more preferably 100 mAh / g or more and 550 mAh / g or less. Doping with alkali metal ions lowers the negative electrode potential. Therefore, when a negative electrode containing an alkali metal ion-doped composite carbon material is combined with a positive electrode, the voltage of the nonaqueous alkali metal storage element increases and the usable capacity of the positive electrode increases. This increases the capacity and energy density of the resulting nonaqueous alkali metal storage element. When the doping amount of alkali metal ions per unit mass of the composite carbon material is 50 mAh / g or more, the alkali metal ions are effectively doped into irreversible sites in the composite carbon material where they cannot be desorbed once inserted, resulting in a high energy density. The higher the doping amount, the lower the negative electrode potential, and the better the input / output characteristics, energy density, and durability. If the doping amount of alkali metal ions per unit mass of the composite carbon material is 700 mAh / g or less, side effects such as deposition of alkali metals such as lithium metal are less likely to occur.

[0032] The BET specific surface area of ​​the graphite material used in the composite carbon material is preferably 0.5 m 2 / g or more 80m 2 / g or less, more preferably 1m 2 / g or more 70m 2 / g or less, more preferably 1.5m 2 / g or more 60m 2 When the BET specific surface area of ​​the graphite material used in the composite carbon material is within the above range, the BET specific surface area of ​​the composite carbon material can be adjusted to the above range.

[0033] The graphite material used in the composite carbon material may be in the form of particles, and the average particle size thereof is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 8 μm or less. If the average particle size of the graphite material used in the composite carbon material is in the range of 1 μm or more and 10 μm or less, the average particle size of the composite carbon material can be adjusted to the above range.

[0034] The carbonaceous material precursor used as the raw material for the composite carbon material is a solid, liquid, or solvent-soluble organic material that can be composited with a graphitic material by heat treatment. Examples of carbonaceous material precursors include pitch, mesocarbon microbeads, coke, and synthetic resins, such as phenolic resins. Among these carbonaceous material precursors, the use of inexpensive pitch is preferred in terms of production costs. Pitches are broadly divided into petroleum-based pitch and coal-based pitch. Examples of petroleum-based pitches include crude oil distillation residues, fluid catalytic cracking residues (decant oil, etc.), bottom oils derived from thermal crackers, and ethylene tar obtained during naphtha cracking.

[0035] (Other components of the negative electrode active material layer) The negative electrode active material layer in this embodiment may contain optional components such as a conductive filler, a dispersant, and a binder, in addition to the negative electrode active material, as needed.

[0036] The conductive filler is preferably made of a conductive carbonaceous material having higher conductivity than the negative electrode active material. Examples of such conductive fillers include carbon black, carbon nanotubes, graphene, and mixtures thereof. Examples of carbon black include ketjen black and acetylene black. Carbon nanotubes are preferably used as the conductive filler due to their high electronic conductivity.

[0037] (carbon nanotubes) The negative electrode active material layer preferably contains carbon nanotubes. As the carbon nanotubes, single-walled carbon nanotubes and multi-walled carbon nanotubes are preferably used, and single-walled carbon nanotubes are particularly preferred. The average fiber diameter of the carbon nanotubes is preferably 1 nm or more and less than 20 nm, and more preferably 1.5 nm or more and 15 nm or less. When the average fiber diameter is 1 nm or more, the dispersibility of the carbon nanotubes is improved. When the average fiber diameter is less than 20 nm, a nonaqueous alkali metal storage element with even better input / output characteristics can be provided.

[0038] The single-walled carbon nanotubes preferably have a structure in which single-walled carbon nanotubes are bundled together (hereinafter referred to as "bundled" single-walled carbon nanotubes). While the fiber length of a single single-walled carbon nanotube is on the order of several microns, bundled single-walled carbon nanotubes are estimated to have a fiber length of on the order of several microns to several tens of microns, consisting of multiple single-walled carbon nanotubes strung together or twisted together. The carbon nanotubes preferably cover the surface of the active material in a mesh-like structure, and it is also preferable that the carbon nanotubes bridge the active material. Here, "mesh-like" refers to a structure in which multiple carbon nanotubes and / or bundled single-walled carbon nanotubes intersect with each other in various directions on the surface of the active material, forming a mesh-like structure consisting of areas covered with carbon nanotubes and areas where the active material is exposed. The average fiber diameter of the bundled single-walled carbon nanotubes is preferably 5 nm to 20 nm, more preferably 6 nm to 17 nm, and even more preferably 7 nm to 14 nm. Although bundled single-walled carbon nanotubes tend to form aggregates like balls of thread, if the average fiber diameter of the bundled single-walled carbon nanotubes is 5 nm or more, dispersibility is improved and the formation of aggregates like balls of thread tends to be suppressed, and if the average fiber diameter is 20 nm or less, the binding strength between active material particles and electronic conductivity can be improved. Carbon nanotubes can be synthesized by any appropriate method, such as chemical vapor deposition, arc discharge, or laser evaporation.

[0039] The content of carbon nanotubes in the negative electrode active material layer is preferably 0.003% by mass or more and 0.120% by mass or less, when the total mass of the negative electrode active material layer is taken as 100% by mass. A content of 0.003% by mass or more improves the adhesiveness and electronic conductivity between negative electrode active material particles, and can suppress an increase in resistance in a high-temperature environment of 80°C or higher. A content of 0.120% by mass or less prevents the negative electrode active material particles from being excessively coated with carbon nanotubes, and reduces the amount of SEI (solid electrolyte interface) formed on the carbon nanotubes throughout the negative electrode active material layer, thereby reducing ion diffusion resistance and increasing the amount of alkali metal ions pre-doped into the negative electrode by reducing the irreversible capacity derived from the carbon nanotubes.

[0040] The carbon nanotubes are preferably uniformly dispersed on the surface of the negative electrode active material and between the particles. Uniform dispersion of the carbon nanotubes on the surface of the negative electrode active material and between the particles can improve the electronic conductivity and binding strength between the particles of the negative electrode active material, making it possible to reduce the binder content. Since binders gradually decompose in high-temperature environments of 80°C or higher, reducing the binder content can improve durability in high-temperature environments of 80°C or higher.

[0041] The dispersant is not particularly limited, but is preferably one or more selected from the group consisting of carboxymethyl cellulose, polycarboxylic acid, polycarboxylate, polyvinylpyrrolidone, polyvinyl alcohol, and surfactants. In particular, using two or more of the above dispersants can achieve both the dispersibility of carbon nanotubes and the stability of the coating liquid. It is particularly preferable that the dispersant contains, for example, carboxymethyl cellulose and one or more selected from polyvinylpyrrolidone and polyvinyl alcohol. The content of the dispersion stabilizer is preferably 0.1% by mass or more and 7.0% by mass or less, based on 100% by mass of the solid content in the negative electrode active material layer. When the amount of the dispersion stabilizer is 7.0% by mass or less, the ingress and egress of ions into and diffusion from the negative electrode active material are not hindered, resulting in high input / output characteristics.

[0042] Examples of binders that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), fluororubber, polyimide, latex, styrene-butadiene copolymer, acrylic copolymer, etc. The content of the binder in the negative electrode active material layer is preferably 0 to 20 mass %, more preferably 0.1 to 15 mass %, relative to 100 mass % of the negative electrode active material.

[0043] [Negative electrode current collector] The material constituting the negative electrode current collector according to this embodiment is preferably a material that has high electronic conductivity and is not susceptible to degradation due to leaching into the electrolyte or reaction with the electrolyte or ions, and may be, for example, a metal foil. Such metal foils are not particularly limited, and examples include aluminum foil, copper foil, nickel foil, and stainless steel foil. As the negative electrode current collector in the nonaqueous alkali metal energy storage element according to this embodiment, copper foil is preferred when the nonaqueous electrolyte contains lithium ions, and aluminum foil is preferred when the nonaqueous electrolyte uses an electrolyte containing sodium ions or potassium ions. The metal foil serving as the negative electrode current collector may be a conventional metal foil without irregularities or through-holes, or may be a metal foil with irregularities obtained by embossing, chemical etching, electrolytic deposition, blasting, or the like, or a metal foil with through-holes such as expanded metal, punched metal, or etched foil. The thickness of the negative electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the negative electrode, but is, for example, 1 to 100 μm.

[0044] The thickness of the negative electrode active material layer is preferably 10 μm to 70 μm, more preferably 15 μm to 60 μm, per side of the current collector. A thickness of 10 μm or more allows for a good charge / discharge capacity to be achieved. On the other hand, a thickness of 70 μm or less allows for a reduction in cell volume, thereby increasing energy density. When the negative electrode current collector has holes, the thickness of the negative electrode active material layer refers to the average thickness per side of the portion of the negative electrode current collector that does not have holes.

[0045] <Positive electrode> The positive electrode of this embodiment includes a positive electrode current collector and a positive electrode active material layer provided on one or both sides of the positive electrode current collector. The positive electrode active material layer according to this embodiment includes a positive electrode active material and an alkali metal carbonate.

[0046] As described below, in this embodiment, it is preferable to pre-dope the negative electrode with alkali metal ions during the energy storage device assembly process. The pre-doping method preferably involves assembling the energy storage device using a positive electrode precursor containing an alkali metal carbonate, a negative electrode, a separator, an outer casing, and a non-aqueous electrolyte, and then applying a voltage between the positive electrode precursor and the negative electrode. In this case, the alkali metal carbonate may be contained in the positive electrode precursor in any manner. For example, the alkali metal carbonate may be present between the positive electrode current collector and the positive electrode active material layer, on the surface of the positive electrode active material layer, or in the positive electrode active material layer. The alkali metal carbonate is preferably contained in the positive electrode active material layer formed on the positive electrode current collector of the positive electrode precursor. In this embodiment, pre-doping the alkali metal ions into the negative electrode forms vacancies in the positive electrode active material layer, thereby increasing the effective area of ​​the positive electrode active material layer. In this specification, the positive electrode before the alkali metal doping step is defined as a "positive electrode precursor," and the positive electrode after the alkali metal doping step is defined as a "positive electrode."

[0047] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material and an alkali metal carbonate. In addition to these, the positive electrode active material layer may contain optional components described below, if necessary.

[0048] (Cathode active material) The positive electrode active material preferably contains activated carbon, and may further contain graphene, a conductive polymer, a lithium transition metal oxide, and the like in addition to activated carbon.

[0049] When using activated carbon as the positive electrode active material, there are no particular restrictions on the type of activated carbon and its raw materials. However, in order to achieve both high input / output characteristics and high energy density, it is preferable to optimally control the pores of the activated carbon. Specifically, when the mesopore volume derived from pores with a diameter of 20 Å or more and 500 Å or less calculated by the BJH method is V1 (cc / g), and the micropore volume derived from pores with a diameter of less than 20 Å calculated by the MP method is V2 (cc / g), (1) For high input / output characteristics, activated carbon (hereinafter also referred to as activated carbon 1) that satisfies 0.3 < V1 ≤ 0.8, and 0.5 ≤ V2 ≤ 1.0, and has a specific surface area measured by the BET method of 1,500 m 2 / g or more and 3,000 m 2 / g or less is preferable. Also, (2) To obtain high energy density, activated carbon (hereinafter also referred to as activated carbon 2) that satisfies 0.8 < V1 ≤ 2.5, and 0.8 < V2 ≤ 3.0, and has a specific surface area measured by the BET method of 2,300 m 2 / g or more and 4,000 m 2 / g or less is preferable.

[0050] The BET specific surface area, mesopore volume, micropore volume, and average pore diameter of the active material in this embodiment are values ​​determined by the following method. A sample is vacuum-dried overnight at 200°C, and adsorption / desorption isotherms are measured using nitrogen as the adsorbate. Using the resulting adsorption isotherms, the BET specific surface area is calculated by the multipoint BET method or the single-point BET method, the mesopore volume by the BJH method, and the micropore volume by the MP method. The BJH method is a calculation method commonly used to analyze mesopores and was proposed by Barrett, Joyner, Halenda, et al. (Non-Patent Document 1). The MP method is a method for determining micropore volume, micropore area, and micropore distribution using the "t-plot method" (Non-Patent Document 2), and was devised by RS Mikhail, Brunauer, and Bodor (Non-Patent Document 3). The average pore diameter refers to the total pore volume per mass of the sample, which is obtained by measuring the equilibrium adsorption amount of nitrogen gas at liquid nitrogen temperature under various relative pressures, divided by the BET specific surface area.

[0051] (activated carbon 1) The mesopore volume V1 of activated carbon 1 is preferably greater than 0.3 cc / g in order to improve the input / output characteristics when the positive electrode material is incorporated into an energy storage device. V1 is preferably 0.8 cc / g or less in order to prevent a decrease in the bulk density of the positive electrode. V1 is more preferably 0.35 cc / g to 0.7 cc / g, and even more preferably 0.4 cc / g to 0.6 cc / g. The micropore volume V2 of activated carbon 1 is preferably 0.5 cc / g or more in order to increase the specific surface area of ​​the activated carbon and increase the capacity. V2 is preferably 1.0 cc / g or less in order to reduce the bulk of the activated carbon, increase the density as an electrode, and increase the capacity per unit volume. V2 is more preferably 0.6 cc / g to 1.0 cc / g, and even more preferably 0.8 cc / g to 1.0 cc / g. The ratio of mesopore volume V1 to micropore volume V2 (V1 / V2) is preferably in the range of 0.3≦V1 / V2≦0.9. That is, from the viewpoint of increasing the ratio of mesopore volume to micropore volume to an extent that can suppress a decrease in output characteristics while maintaining high capacity, V1 / V2 is preferably 0.3 or more. On the other hand, from the viewpoint of increasing the ratio of micropore volume to mesopore volume to an extent that can suppress a decrease in capacity while maintaining high output characteristics, V1 / V2 is preferably 0.9 or less. A more preferable range for V1 / V2 is 0.4≦V1 / V2≦0.7, and an even more preferable range for V1 / V2 is 0.55≦V1 / V2≦0.7.

[0052] The average pore diameter of activated carbon 1 is preferably 17 Å or more, more preferably 18 Å or more, and most preferably 20 Å or more, from the viewpoint of maximizing the output of the resulting energy storage element. Furthermore, from the viewpoint of maximizing the capacity, the average pore diameter of activated carbon 1 is preferably 25 Å or less. The BET specific surface area of ​​activated carbon 1 is 1,500 m 2 / g or more 3,000m 2 / g or less, and 1,500m 2 / g or more 2,500m 2 / g or less is more preferable. 2 / g or more, a good energy density is easily obtained, while a BET specific surface area of ​​3,000 m 2 When the electrode has a capacitance of 0.15 or less, it is not necessary to add a large amount of binder to maintain the strength of the electrode, and therefore the performance per volume of the electrode is improved.

[0053] Activated carbon 1 having the above-described characteristics can be obtained, for example, using the raw materials and processing methods described below. In this embodiment, the carbon source used as the raw material for activated carbon 1 is not particularly limited. Examples include plant-based raw materials such as wood, wood flour, coconut shells, pulp manufacturing by-products, bagasse, and blackstrap molasses; fossil-based raw materials such as peat, lignite, brown coal, bituminous coal, anthracite, petroleum distillation residue components, petroleum pitch, coke, and coal tar; various synthetic resins such as phenolic resins, vinyl chloride resins, vinyl acetate resins, melamine resins, urea resins, resorcinol resins, celluloid, epoxy resins, polyurethane resins, polyester resins, and polyamide resins; synthetic rubbers such as polybutylene, polybutadiene, and polychloroprene; other synthetic woods, synthetic pulps, and carbonized versions of these. Among these raw materials, plant-based raw materials such as coconut shells and wood flour, and carbonized versions of these, are preferred from the standpoints of mass production and cost, with coconut shell carbonized versions being particularly preferred.

[0054] Examples of carbonization and activation methods for converting these raw materials into the activated carbon 1 include known methods such as a fixed bed method, a moving bed method, a fluidized bed method, a slurry method, and a rotary kiln method. Carbonization of these raw materials includes a method of firing the raw materials at approximately 400 to 700°C (preferably 450 to 600°C) for approximately 30 minutes to 10 hours using an inert gas such as nitrogen, carbon dioxide, helium, argon, xenon, neon, carbon monoxide, or combustion exhaust gas, or a mixed gas containing these inert gases as the main component. The carbonized material obtained by the carbonization method described above is preferably activated by a gas activation method in which the raw material is fired using an activation gas such as steam, carbon dioxide, or oxygen. Of these, the method of using steam or carbon dioxide as the activation gas is preferred. In this activation method, the obtained carbonized material is preferably activated by heating it to 800 to 1,000°C over 3 to 12 hours (preferably 5 to 11 hours, more preferably 6 to 10 hours) while supplying an activation gas at a rate of 0.5 to 3.0 kg / h (preferably 0.7 to 2.0 kg / h). Furthermore, prior to the activation treatment of the carbonized material described above, the carbonized material may be subjected to primary activation. This primary activation is typically preferably performed by firing the carbon material at a temperature below 900°C using an activation gas such as water vapor, carbon dioxide, or oxygen. The activated carbon 1 that can be used in this embodiment can be produced by appropriately combining the firing temperature and firing time in the carbonization method described above with the activation gas supply rate, heating rate, and maximum activation temperature in the activation method.

[0055] The average particle diameter of the activated carbon 1 is preferably 2 to 20 μm. If the average particle diameter is 2 μm or more, the density of the active material layer is high, and therefore the capacity per electrode volume tends to be high. Note that a small average particle diameter may result in the drawback of low durability, but if the average particle diameter is 2 μm or more, such a drawback is unlikely to occur. On the other hand, if the average particle diameter is 20 μm or less, it tends to be more suitable for high-speed charging and discharging. The average particle diameter of the activated carbon 1 is more preferably 2 to 15 μm, and even more preferably 3 to 10 μm.

[0056] (activated carbon 2) The mesopore volume V1 of the activated carbon 2 is preferably greater than 0.8 cc / g from the viewpoint of improving the output characteristics when the positive electrode material is incorporated into an energy storage device. V1 is preferably 2.5 cc / g or less from the viewpoint of suppressing a decrease in the capacity of the energy storage device. V1 is more preferably 1.00 cc / g or more and 2.0 cc / g or less, and even more preferably 1.2 cc / g or more and 1.8 cc / g or less. The micropore volume V2 of the activated carbon 2 is preferably greater than 0.8 cc / g to increase the specific surface area of ​​the activated carbon and increase the capacity. V2 is preferably 3.0 cc / g or less from the viewpoint of increasing the density of the activated carbon as an electrode and increasing the capacity per unit volume. V2 is more preferably greater than 1.0 cc / g and 2.5 cc / g or less, and even more preferably 1.5 cc / g or more and 2.5 cc / g or less. The activated carbon 2 having the above-mentioned mesopores and micropores has a higher BET specific surface area than activated carbons used in conventional electric double layer capacitors or lithium ion capacitors. The specific value of the BET specific surface area of ​​the activated carbon 2 is 2,300 m 2 / g or more 4,000m 2 / g or less, and 2 / g or more 4,000m 2 / g or less is more preferable, and 3,200m 2 / g or more 3,800m 2 / g or less. 2 / g or more, a good energy density is easily obtained, and the BET specific surface area is 4,000 m 2 When the electrode has a capacitance of 0.15 or less, it is not necessary to add a large amount of binder to maintain the strength of the electrode, and therefore the performance per volume of the electrode is improved.

[0057] Activated carbon 2 having the above-described characteristics can be obtained, for example, using the raw materials and processing methods described below. The carbonaceous material used as the raw material for activated carbon 2 is not particularly limited as long as it is a carbon source typically used as an activated carbon raw material, and examples include plant-based raw materials such as wood, wood flour, and coconut shells; fossil-based raw materials such as petroleum pitch and coke; and various synthetic resins such as phenolic resin, furan resin, vinyl chloride resin, vinyl acetate resin, melamine resin, urea resin, and resorcinol resin. Among these raw materials, phenolic resin and furan resin are particularly preferred because they are suitable for producing activated carbon with a high specific surface area.

[0058] Methods for carbonizing these raw materials or heating methods during activation treatment include known methods such as fixed bed, moving bed, fluidized bed, slurry, and rotary kiln. The heating atmosphere is an inert gas such as nitrogen, carbon dioxide, helium, or argon, or a mixture of these inert gases with other gases as the main component. Carbonization is typically performed at a temperature of approximately 400 to 700°C for approximately 0.5 to 10 hours. Methods for activating charcoal include gas activation, which involves firing using an activation gas such as steam, carbon dioxide, or oxygen, and alkali metal activation, which involves mixing with an alkali metal compound and then heating. However, alkali metal activation is preferred for producing activated carbon with a high specific surface area. In this activation method, the carbide is mixed with an alkali metal compound such as KOH or NaOH in a mass ratio of 1:1 or greater (the amount of the alkali metal compound is equal to or greater than the amount of the carbide), followed by heating in an inert gas atmosphere at 600 to 900°C for 0.5 to 5 hours. The alkali metal compound is then removed by washing with acid and water, followed by drying. To increase the micropore volume without increasing the mesopore volume, a larger amount of carbide may be mixed with KOH during activation. To increase both the micropore volume and the mesopore volume, a larger amount of KOH may be used. Furthermore, to primarily increase the mesopore volume, it is preferable to perform alkali activation followed by steam activation. The average particle size of the activated carbon 2 is preferably 2 μm to 20 μm, more preferably 3 μm to 10 μm.

[0059] (Use of activated carbon) Activated carbons 1 and 2 may each be a single type of activated carbon or a mixture of two or more types of activated carbons that exhibits each of the above-described characteristic values as the entire mixture. Either one of the activated carbons 1 and 2 may be selected and used, or both may be mixed and used. The positive electrode active material may contain materials other than activated carbons 1 and 2 (for example, activated carbons that do not have the specific V1 and / or V2 described above, or materials other than activated carbons (for example, conductive polymers, etc.)). In an exemplary embodiment, the total content of activated carbons 1 and 2 is preferably 40.0% by mass or more and 70.0% by mass or less, more preferably 45.0% by mass or more and 65.0% by mass or less. If this value is 40.0% by mass or more, the energy density can be increased. If this value is 70.0% by mass or less, the high-temperature durability and high-voltage durability can be improved.

[0060] (Lithium transition metal oxide) It is preferable that the positive electrode active material layer further contains a lithium transition metal compound. By including a lithium transition metal oxide, the non-aqueous alkali metal storage element can be made to have a higher capacity. The lithium transition metal oxide includes a transition metal oxide capable of occluding and releasing lithium. There is no particular limitation on the transition metal oxide used as the positive electrode active material. Examples of the transition metal oxide include oxides containing at least one element selected from the group consisting of cobalt, nickel, manganese, iron, vanadium, and chromium. Specifically, the transition metal oxide has the following formula: Li x CoO2 {where x satisfies 0 ≦ x ≦ 1.} Li x NiO2 {where x satisfies 0 ≦ x ≦ 1.} Li x Ni y M (1-y) O2 {where M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Mg, and Ti, x satisfies 0 ≦ x ≦ 1, and y satisfies 0.05 < y < 0.97.} Li x Ni 1 / 3 Co 1 / 3Mn 1 / 3 O2 {where x satisfies 0 ≦ x ≦ 1.} Li x MnO2 {where x satisfies 0 ≦ x ≦ 1.} α-Li x FeO2 {where x satisfies 0 ≦ x ≦ 1.} Li x VO₂ {where x satisfies 0 ≦ x ≦ 1.} Li x CrO₂ {where x satisfies 0 ≦ x ≦ 1.} Li x FePO₄ {where x satisfies 0 ≦ x ≦ 1.} Li x MnPO₄ {where x satisfies 0 ≦ x ≦ 1.} Li z V₂(PO₄)₃ {where z satisfies 0 ≦ z ≦ 3.} Li x Mn₂O₄ {where x satisfies 0 ≦ x ≦ 1.} Li x M y Mn (2-y) O₄ {where M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Mg, and Ti, x satisfies 0 ≦ x ≦ 1, and y satisfies 0.05 < y < 0.97.} Li x Ni a Co b Al (1-a-b) O₂ {where x satisfies 0 ≦ x ≦ 1, and a and b satisfy 0.02 < a < 0.97 and 0.02 < b < 0.97.} Li x Ni c Co d Mn (1-c-d) O₂ {where x satisfies 0 ≦ x ≦ 1, and c and d satisfy 0.02 < c < 0.97 and 0.02 < d < 0.97.} Compounds represented by the above formula etc. may be mentioned. Among these, from the viewpoints of high capacity, low resistance, cycle characteristics, decomposition of alkali metal carbonate, and suppression of loss of the positive electrode active material during pre-doping, the above formula Li x Ni a Co b Al (1-a-b)O2, Li x Ni c Co d Mn (1-c-d) O2, Li x CoO2, Li x Mn2O4, or Li x FePO4, Li x MnPO4, and Li z Compounds represented by V2(PO4)3 are preferred.

[0061] In this embodiment, if the positive electrode precursor contains an alkali metal carbonate different from the positive electrode active material, the alkali metal carbonate can act as a dopant source for the alkali metal during pre-doping, allowing the negative electrode to be pre-doped. Therefore, even if the transition metal compound does not already contain lithium ions (i.e., even if x = 0 or z = 0), the nonaqueous alkali metal storage element can be electrochemically charged and discharged. The content of the lithium transition metal oxide in the positive electrode active material layer of the positive electrode precursor is preferably 8.0% by mass or more and 30.0% by mass or less. A lithium transition metal oxide content of 8.0% by mass or more can increase the capacity of the nonaqueous alkali metal storage element. A lithium transition metal oxide content of 30.0% by mass or less can decrease the resistance of the nonaqueous alkali metal storage element.

[0062] (alkali metal carbonate) In this embodiment, the positive electrode active material layer of the positive electrode precursor may contain an alkali metal carbonate other than the positive electrode active material. The alkali metal carbonate decomposes in the positive electrode precursor to release cations, which are then reduced at the negative electrode, thereby pre-doping the negative electrode. Since the positive electrode active material layer of the positive electrode precursor contains an alkali metal carbonate, it is preferable that the positive electrode active material layer of the positive electrode obtained after pre-doping also contains an alkali metal carbonate.

[0063] Specific examples of such alkali metal carbonates include lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate. In particular, lithium carbonate, sodium carbonate, or potassium carbonate is more preferably used, and lithium carbonate is particularly preferably used from the viewpoint of high capacity per unit mass.

[0064] The alkali metal carbonate is preferably in the form of fine particles. In this embodiment, the average particle diameter of the alkali metal carbonate is preferably 0.1 μm or more and 10 μm or less. An average particle diameter of 0.1 μm or more provides excellent dispersibility in the positive electrode precursor. An average particle diameter of 10 μm or less increases the surface area of ​​the alkali metal carbonate, thereby efficiently promoting the decomposition reaction. Furthermore, the average particle diameter of the alkali metal carbonate is preferably smaller than the average particle diameter of the activated carbon described above. A smaller average particle diameter of the alkali metal carbonate than the average particle diameter of the activated carbon enhances the electronic conductivity of the positive electrode active material layer, contributing to lower resistance of the electrode body or energy storage device. The method for measuring the average particle diameter of the alkali metal carbonate in the positive electrode precursor is not particularly limited, but it can be calculated from SEM images and SEM-EDX images of the positive electrode cross section. The positive electrode cross section can be formed using a beam-in-beam (BIB) process, in which an Ar beam is irradiated from above the positive electrode and a smooth cross section is created along the edge of a shielding plate placed directly above the sample. Various methods can be used to microparticulate the alkali metal carbonate, for example, using a grinder such as a ball mill, a bead mill, a ring mill, a jet mill, or a rod mill.

[0065] The content of alkali metal carbonate in the positive electrode active material layer of the positive electrode precursor is preferably 25.0% by mass or more and 50.0% by mass or less, when the total mass of the positive electrode active material layer is taken as 100% by mass. If this value is 25.0% by mass or more, a sufficient amount of alkali metal ions can be pre-doped into the negative electrode, increasing the capacity of the nonaqueous alkali metal storage element. If this value is 50.0% by mass or less, electronic conductivity in the positive electrode precursor can be improved, thereby efficiently decomposing the alkali metal carbonate. The alkali metal elements and alkaline earth metal elements can be quantified by ICP-AES, atomic absorption spectrometry, X-ray fluorescence spectrometry, neutron activation analysis, ICP-MS, or the like.

[0066] (Other components of the positive electrode active material layer) In addition to the positive electrode active material and alkali metal carbonate, the positive electrode active material layer of the positive electrode precursor of the present invention may contain other components, such as a dispersant, a conductive filler, and a binder, as needed. The dispersant, conductive filler, and binder in the positive electrode active material layer may be appropriately selected from the dispersants, conductive fillers, and binders exemplified above for the negative electrode active material layer. The contents of the dispersant and binder in the positive electrode active material layer may be within the ranges described above for the contents of the dispersant and binder in the negative electrode active material. The content of the conductive filler in the positive electrode active material layer of the positive electrode precursor is preferably 0 to 20% by mass relative to 100% by mass of the positive electrode active material. From the perspective of high input power, it is preferable to incorporate as much conductive filler as possible. However, if the amount exceeds 20% by mass, the content of the positive electrode active material in the positive electrode active material layer decreases, resulting in a decrease in the energy density per volume of the positive electrode active material layer, which is undesirable.

[0067] (carbon nanotubes) The positive electrode active material layer preferably contains carbon nanotubes as a conductive filler. The carbon nanotubes contained in the positive electrode active material layer may be the same as the carbon nanotubes contained in the negative electrode active material layer. Furthermore, the carbon nanotubes may be contained in the positive electrode and / or the negative electrode.

[0068] The carbon nanotube content in the positive electrode active material layer is preferably 0.010% by mass or more and 0.200% by mass or less, assuming the total mass of the positive electrode active material layer to be 100% by mass. A content of 0.010% by mass or more improves the bonding strength and electronic conductivity between positive electrode active material particles, suppressing increases in resistance in high-temperature environments of 80°C or higher, and promotes decomposition of the alkali metal carbonate contained in the positive electrode active material layer, suppressing decomposition of the electrolyte, thereby increasing the amount of alkali metal ions pre-doped into the negative electrode. A content of 0.200% by mass or less prevents excessive coating of the positive electrode active material particles with carbon nanotubes, and reduces the amount of SEI (solid electrolyte interface) formed on the carbon nanotubes throughout the positive electrode active material layer, thereby reducing ion diffusion resistance.

[0069] The carbon nanotubes are preferably uniformly dispersed on the surface of the positive electrode active material and between the particles. Uniform dispersion of the carbon nanotubes on the surface of the positive electrode active material and between the particles can improve the electronic conductivity and binding strength between the particles of the positive electrode active material, making it possible to reduce the binder content. Since binders gradually decompose in high-temperature environments of 80°C or higher, reducing the binder content can improve durability in high-temperature environments of 80°C or higher.

[0070] Uniform dispersion of carbon nanotubes on the surface and between particles of the positive electrode active material can improve the electronic conductivity and binding strength between particles of the positive electrode active material, making it possible to reduce the amount of binder. Because binders gradually decompose in high-temperature environments of 80°C or higher or high-voltage environments of 4.1V or higher, reducing the binder content can impart high-temperature durability above 80°C and high-voltage durability above 4.1V.

[0071] [Positive electrode current collector] The material constituting the positive electrode current collector according to this embodiment is not particularly limited as long as it has high electronic conductivity and does not undergo degradation due to leaching into the electrolyte or reaction with the electrolyte or ions, but metal foil is preferred. Aluminum foil is more preferred as the positive electrode current collector in the nonaqueous alkali metal storage element according to this embodiment. The metal foil may be a normal metal foil without irregularities or through-holes, or a metal foil with irregularities obtained by embossing, chemical etching, electrolytic deposition, blasting, or the like, or a metal foil with through-holes such as expanded metal, punched metal, or etched foil. From the viewpoint of the alkali metal doping process described below, non-porous aluminum foil is more preferred, and it is particularly preferred that the surface of the aluminum foil be roughened. The thickness of the positive electrode current collector is not particularly limited as long as it can sufficiently maintain the shape and strength of the positive electrode, but a thickness of 1 to 100 μm is preferred, for example. It is preferable to provide an anchor layer containing a conductive material such as graphite, flake graphite, carbon nanotubes, graphene, ketjen black, acetylene black, or vapor-grown carbon fiber on the surface of the metal foil. By providing the anchor layer, electrical conduction between the positive electrode current collector and the positive electrode active material layer can be improved, resulting in lower resistance. The thickness of the anchor layer is preferably 0.1 μm or more and 5 μm or less per side of the positive electrode current collector.

[0072] The thickness of the positive electrode active material layer according to this embodiment is preferably 10 μm to 200 μm per side of the positive electrode current collector. The thickness of the positive electrode active material layer is more preferably 20 μm to 150 μm per side, and even more preferably 30 μm to 100 μm per side. A thickness of 10 μm or more can achieve sufficient charge / discharge capacity. On the other hand, a thickness of 200 μm or less can maintain low ion diffusion resistance within the electrode. Therefore, sufficient output characteristics can be obtained, and the cell volume can be reduced, thereby increasing the energy density. Note that when the positive electrode current collector has through-holes or irregularities, the thickness of the positive electrode active material layer refers to the average thickness per side of the portion of the positive electrode current collector that does not have through-holes or irregularities.

[0073] <Separator> The positive electrode precursor and the negative electrode are stacked with a separator interposed therebetween, or stacked and wound together, to form an electrode laminate or electrode wound body having the positive electrode precursor, the negative electrode, and the separator. The separator can be a polyethylene microporous membrane or a polypropylene microporous membrane used in lithium-ion secondary batteries, or a cellulose nonwoven paper used in electric double-layer capacitors. A film made of organic or inorganic fine particles may be laminated on one or both sides of these separators. The separator may also contain organic or inorganic fine particles. The thickness of the separator is preferably 5 μm or more and 35 μm or less. A thickness of 5 μm or more is preferred because it tends to reduce self-discharge due to internal micro-short circuits. On the other hand, a thickness of 35 μm or less is preferred because it tends to improve the output characteristics of the energy storage device. The thickness of the film made of organic or inorganic fine particles is preferably 1 μm or more and 10 μm or less. A thickness of 1 μm or more is preferred because it tends to reduce self-discharge due to internal micro-short circuits. On the other hand, by making the thickness of this film 10 μm or less, the output characteristics of the electricity storage element tend to be improved, which is preferable.

[0074] <Exterior body> Examples of the exterior body include metal cans and laminated films. Aluminum metal cans are preferred. Metal cans may be rectangular, round, or cylindrical. Laminated films are preferably films made of a metal foil and a resin film, such as a three-layer structure consisting of an outer resin film, a metal foil, and an interior resin film. The outer resin film is intended to prevent damage to the metal foil due to contact, and resins such as nylon or polyester are suitable. The metal foil is intended to prevent moisture and gas permeation, and foils such as copper, aluminum, and stainless steel are suitable. The interior resin film protects the metal foil from the electrolyte solution contained therein and also serves to melt-seal the exterior body during heat sealing. Polyolefins, acid-modified polyolefins, and the like are suitable.

[0075] <Electrolyte> The electrolyte solution in this embodiment is a non-aqueous electrolyte solution. That is, this electrolyte solution contains a non-aqueous solvent. The non-aqueous electrolyte solution contains 0.5 mol / L or more of an alkali metal salt based on the total amount of the non-aqueous electrolyte solution. That is, the non-aqueous electrolyte solution contains an alkali metal salt as an electrolyte. Examples of non-aqueous solvents contained in the non-aqueous electrolyte solution include cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0076] Examples of electrolyte salts containing alkali metal ions that dissolve in non-aqueous solvents include MFSI, MBF, MPF, MClO, LiB(C0), LiBF(C0), and the like, where M is Li, Na, K, Rb, or Cs.

[0077] The electrolyte salt concentration in the electrolytic solution is preferably in the range of 0.5 to 2.0 mol / L. At an electrolyte salt concentration of 0.5 mol / L or more, sufficient anions are present, and the capacity of the nonaqueous alkali metal energy storage element is maintained. On the other hand, at an electrolyte salt concentration of 2.0 mol / L or less, the salt is sufficiently dissolved in the electrolytic solution, and appropriate viscosity and conductivity of the electrolytic solution are maintained. The electrolyte salt concentration of MPF6 contained in the nonaqueous electrolytic solution in this embodiment is preferably in the range of 0.00 mol / L or more to 0.05 mol / L. A concentration of 0.05 mol / L or less can suppress decomposition of hexafluorophosphate ions in a high-temperature environment, and can suppress an increase in resistance during high-temperature, high-voltage cycles. From the same viewpoint, in the nonaqueous alkali metal energy storage element according to this embodiment, the hexafluorophosphate ions (PF6) contained in the nonaqueous electrolytic solution - The concentration of ) is preferably 0.00 mol / L or more and 0.05 mol / L or less.

[0078] The water content of the non-aqueous electrolyte is preferably 0 ppm or more and 500 ppm or less. If the water content is 0 ppm or more, the alkali metal carbonate in the positive electrode precursor can be slightly dissolved, allowing pre-doping to be performed under mild conditions, resulting in high capacity and low resistance. If the water content is 500 ppm or less, decomposition of the electrolyte is suppressed, improving high-temperature storage characteristics. The water content in the electrolyte can be measured using the Karl Fischer method described above.

[0079] <<Method for manufacturing a non-aqueous alkali metal storage element>> [Manufacturing of negative electrodes] The negative electrode has a negative electrode active material layer on one or both sides of the negative electrode current collector. In a typical embodiment, the negative electrode active material layer is fixed to the negative electrode current collector. The negative electrode can be manufactured using known electrode manufacturing techniques for lithium ion batteries, electric double layer capacitors, etc. For example, various materials including the negative electrode active material are dispersed or dissolved in water or an organic solvent to prepare a slurry-like coating liquid, and this coating liquid is applied to one or both sides of the negative electrode current collector to form a coating film, which is then dried to obtain a negative electrode. The obtained negative electrode may then be pressed to adjust the film thickness or bulk density of the negative electrode active material layer.

[0080] The method for preparing the coating liquid is not particularly limited, but can be suitably carried out using a disperser such as a homodisper, a multi-axis disperser, a planetary mixer, or a thin film rotary high-speed mixer. To obtain a coating liquid in a well-dispersed state, the coating liquid is preferably dispersed at a peripheral speed of 1 m / s or more and 50 m / s or less. A peripheral speed of 1 m / s or more is preferred because various materials are well dissolved or dispersed. A peripheral speed of 50 m / s or less is preferred because various materials are not destroyed by heat or shear force due to dispersion, and re-aggregation is suppressed.

[0081] The degree of dispersion of the coating liquid is preferably 0.1 μm or more and 100 μm or less as measured with a particle gauge. The upper limit of the degree of dispersion is more preferably 80 μm or less, and even more preferably 50 μm or less as measured with a particle gauge. A particle size within this range prevents material from being crushed during preparation of the coating liquid, and nozzle clogging and the occurrence of streaks in the coating film during coating can be suppressed, enabling stable coating.

[0082] The viscosity (ηb) of the coating fluid is preferably 100 mPa·s or more and 10,000 mPa·s or less, more preferably 300 mPa·s or more and 5,000 mPa·s or less, and even more preferably 500 mPa·s or more and 3,000 mPa·s or less. A viscosity (ηb) of 100 mPa·s or more suppresses dripping during coating film formation, allowing for good control of the coating film width and thickness. Furthermore, a viscosity of 10,000 mPa·s or less reduces pressure loss in the coating fluid flow path when using a coating machine, allowing for stable coating and easy control of the coating film thickness.

[0083] The TI value (thixotropy index value) of the coating liquid is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.5 or more. If the TI value is 1.1 or more, the width and thickness of the coating film can be well controlled.

[0084] The coating film of the negative electrode active material layer is not particularly limited, but a coating machine such as a die coater, comma coater, knife coater, or gravure coater can be preferably used. The coating film may be formed by single-layer coating or multi-layer coating. In the case of multi-layer coating, the coating liquid composition may be adjusted so that the content of components in each coating layer varies. When applying the coating film to the negative electrode current collector, it may be multi-line coating, intermittent coating, or multi-line intermittent coating. When forming the negative electrode active material layer on both sides of the negative electrode current collector, sequential coating may be performed by coating one side of the negative electrode current collector and drying, and then coating the other side and drying. Alternatively, simultaneous double-sided coating may be performed by simultaneously coating and drying the coating liquid on both sides of the negative electrode current collector. In this case, the difference in thickness between the negative electrode active material layers on the front and back surfaces of the negative electrode current collector is preferably 10% or less of the average thickness of both surfaces. The closer the mass ratio and film thickness ratio of the negative electrode active material layers on the front and back surfaces is to 1.0, the less the charge / discharge load is concentrated on one surface, improving the high-load charge / discharge cycle characteristics.

[0085] After forming a coating film of the negative electrode active material layer on the negative electrode current collector, the coating film is dried. The coating film of the negative electrode precursor is preferably dried by an appropriate drying method such as hot air drying or infrared (IR) drying, preferably using far infrared rays, near infrared rays, or hot air. The coating film may be dried at a single temperature or by varying the temperature in multiple stages. A combination of drying methods may also be used. The drying temperature is preferably 25°C or higher and 200°C or lower, more preferably 40°C or higher and 180°C or lower, and even more preferably 50°C or higher and 160°C or lower. A drying temperature of 25°C or higher allows the solvent in the coating film to be sufficiently evaporated. On the other hand, a drying temperature of 200°C or lower can prevent cracking of the coating film due to rapid solvent evaporation, uneven distribution of the binder due to migration, and oxidation of the negative electrode current collector or negative electrode active material layer.

[0086] The moisture content of the negative electrode active material layer after drying is preferably 0.1% by mass or more and 10% by mass or less, when the total mass of the negative electrode active material layer is taken as 100% by mass. A moisture content of 0.1% by mass or more can prevent deterioration of the binder due to excessive drying, thereby reducing resistance. A moisture content of 10% by mass or less can prevent deactivation of alkali metal ions, thereby increasing capacity. When N-methyl-2-pyrrolidone (NMP) is used to prepare the coating liquid, the NMP content in the negative electrode active material layer after drying is preferably 0.1% by mass or more and 10% by mass or less, when the total mass of the negative electrode active material layer is taken as 100%. The moisture content of the negative electrode active material layer can be measured, for example, by Karl Fischer titration (JIS 0068 (2001) "Method for measuring moisture content in chemical products"). The amount of NMP contained in the negative electrode active material layer can be determined by immersing the negative electrode active material layer in ethanol with a mass that is 50 to 100 times the mass of the negative electrode active material layer for 24 hours in a 25°C environment to extract NMP, and then measuring the result with GC / MS, and quantifying the amount based on a calibration curve that has been prepared in advance.

[0087] To press the negative electrode active material layer, an appropriate press such as a hydraulic press, a vacuum press, or a roll press can be suitably used. The film thickness, bulk density, and electrode strength of the negative electrode active material layer can be adjusted by the press pressure, the gap between the press rolls, and the surface temperature of the press, as described below. The press pressure is preferably 0.5 kN / cm or more and 20 kN / cm or less, more preferably 1 kN / cm or more and 10 kN / cm or less, and even more preferably 2 kN / cm or more and 7 kN / cm or less. A press pressure of 0.5 kN / cm or more can sufficiently increase the electrode strength. On the other hand, a press pressure of 20 kN / cm or less can prevent bending and wrinkling of the negative electrode, allowing the negative electrode active material layer to be adjusted to the desired film thickness or bulk density. When a roll press is used for pressing, the gap between the press rolls can be appropriately set so that the negative electrode active material layer has the desired thickness and bulk density. The press speed can be appropriately set so that bending and wrinkling do not occur in the negative electrode.

[0088] The surface temperature of the press part may be room temperature, or may be heated as necessary. When heated, the lower limit of the surface temperature of the press part is preferably at least 60°C below the melting point of the binder used, more preferably at least 45°C below the melting point of the binder, and even more preferably at least 30°C below the melting point of the binder. On the other hand, when heated, the upper limit of the surface temperature of the press part is preferably at most 50°C above the melting point of the binder used, more preferably at most 30°C above the melting point of the binder, and even more preferably at most 20°C above the melting point of the binder. For example, when polyvinylidene fluoride (melting point 150°C) is used as the binder, the press part is preferably heated to a temperature between 90°C and 200°C, more preferably at most 105°C and 180°C, and even more preferably at most 120°C and 170°C. Furthermore, when a styrene-butadiene copolymer (melting point 100°C) is used as the binder, the press section is preferably heated to 40°C to 150°C, more preferably 55°C to 130°C, and even more preferably 70°C to 120°C. The melting point of the binder can be determined from the endothermic peak position in DSC (Differential Scanning Calorimetry). For example, using a PerkinElmer DSC7 differential scanning calorimeter, 10 mg of sample resin is placed in the measurement cell and heated from 30°C to 250°C at a heating rate of 10°C / min in a nitrogen gas atmosphere. The endothermic peak temperature during the heating process is the melting point.

[0089] Pressing may be performed multiple times while changing the conditions of the press pressure, gap, speed, and surface temperature of the press part. When the negative electrode active material layer is multi-line coated, it is preferable to slit it before pressing. If the negative electrode is pressed without slitting the multi-line coated negative electrode active material layer, excessive stress may be applied to the portion of the negative electrode current collector not coated with the negative electrode active material layer, which may cause wrinkles. After pressing, the negative electrode active material layer may be slit again.

[0090] [Production of positive electrode precursor] In this embodiment, the positive electrode precursor serving as the positive electrode of the nonaqueous alkali metal storage element can be produced using known electrode production techniques for lithium ion batteries, electric double layer capacitors, and the like. For example, a positive electrode active material, an alkali metal carbonate, and other optional components used as needed are dispersed or dissolved in water or an organic solvent to prepare a slurry coating liquid. This coating liquid is then applied to one or both sides of a positive electrode current collector to form a coating film, which is then dried to obtain a positive electrode precursor. The resulting positive electrode precursor may then be pressed to adjust the film thickness or bulk density of the positive electrode active material layer. Preparation of the coating liquid for forming the positive electrode active material layer, application of the coating liquid to the positive electrode current collector, drying of the coating film, and pressing can each be performed in accordance with the methods described above for producing a negative electrode.

[0091] [Assembly process] In one embodiment, an electrode laminate is fabricated by connecting a positive electrode terminal and a negative electrode terminal to a laminate formed by stacking, for example, a positive electrode precursor and a negative electrode cut into sheets with a separator interposed therebetween. In another embodiment, an electrode wound body may be fabricated by connecting a positive electrode terminal and a negative electrode terminal to a wound body formed by stacking and winding, for example, a positive electrode precursor and a negative electrode with a separator interposed therebetween. The electrode wound body may be cylindrical or flat. The method for connecting the positive electrode terminal and the negative electrode terminal is not particularly limited, but methods such as resistance welding and ultrasonic welding can be used. The electrode body (electrode laminate or electrode wound body) with the terminals connected is preferably dried to remove the remaining solvent. The drying method is not particularly limited, but vacuum drying or the like can be used. The remaining solvent is preferably 1.5% by mass or less based on the total mass of the positive electrode active material layer or the negative electrode active material layer. Residual solvent exceeding 1.5% by mass is undesirable because the solvent remains in the system and deteriorates the self-discharge characteristics. The dried electrode assembly is preferably housed in an outer package, typically a metal can or a laminate film, in a dry environment with a dew point of -40°C or less, and sealed except for one opening for injecting the nonaqueous electrolyte. A dew point higher than -40°C is undesirable because moisture adheres to the electrode assembly, leaving water in the system and deteriorating self-discharge characteristics. The method for sealing the outer package is not particularly limited, but methods such as heat sealing and impulse sealing can be used.

[0092] [Injection, impregnation, sealing process] After the assembly process, a nonaqueous electrolyte solution is injected into the electrode body housed in the outer casing. After the injection, it is desirable to further impregnate the electrode body, thoroughly immersing the positive electrode, negative electrode, and separator in the nonaqueous electrolyte solution. If the electrolyte solution is not immersed in at least a portion of the positive electrode, negative electrode, and separator, the alkali metal doping process described below will proceed unevenly, resulting in increased resistance and reduced durability of the resulting nonaqueous alkali metal storage element. The impregnation method is not particularly limited, but for example, the electrode body after the injection can be placed in a decompression chamber with the outer casing open, and the chamber can be decompressed using a vacuum pump and then returned to atmospheric pressure. After the impregnation, the electrode body with the outer casing open can be sealed while decompressing.

[0093] [Alkali metal doping process] In the alkali metal doping step, a voltage is applied between the positive electrode precursor and the negative electrode to decompose the alkali metal carbonate preferably contained in the positive electrode precursor to release alkali metal ions, and then the alkali metal ions are reduced at the negative electrode, thereby pre-doping the alkali metal ions into the negative electrode active material layer. In the alkali metal doping step, gases such as CO2 are generated as a result of the oxidative decomposition of the alkali metal carbonate in the positive electrode precursor. Therefore, when applying a voltage, it is preferable to take measures to release the generated gas to the outside of the outer casing. Examples of such measures include applying a voltage with a portion of the outer casing open; or applying a voltage with an appropriate gas release means, such as a gas vent valve or a gas-permeable film, already installed in a portion of the outer casing. Furthermore, in the alkali metal doping step, it is preferable to sandwich and pressurize both sides of the center of the electrode body sealed in the outer casing with buffer material. More preferably, it is preferable to sandwich and pressurize both sides of the center of the electrode body with buffer material from the outside of the outer casing. The pressure during pressing is preferably 10 kPa or more and 1000 kPa or less. If the pressure during pressurization is 10 kPa or more, gas generated during the doping process is discharged from the inside of the electrode body, accelerating the decomposition of the alkali metal carbonate. If the pressure during pressurization is 1000 kPa or less, the nonaqueous electrolyte is sufficiently retained within the electrode body, accelerating the decomposition of the alkali metal carbonate. Examples of buffer materials include silicone sponge, silicone rubber, chloroprene rubber, butyl rubber, urethane rubber, styrene butadiene rubber, and acrylonitrile rubber.

[0094] [Aging process] After the alkali metal doping step, it is preferable to perform aging on the electrode body. In the aging step, the solvent in the electrolyte solution is decomposed at the negative electrode, and an alkali metal ion-permeable solid polymer coating is formed on the surface of the negative electrode. The aging method is not particularly limited, but for example, a method of reacting the solvent in the electrolyte solution in a high-temperature environment can be used.

[0095] [Gas removal process] After the aging step, it is preferable to further degas the electrolyte, the positive electrode, and the negative electrode to completely remove any remaining gas. If gas remains in at least a portion of the electrolyte, the positive electrode, and the negative electrode, ion conduction is inhibited, resulting in an increase in the resistance of the resulting nonaqueous alkali metal storage element. The degassing method is not particularly limited, but for example, a method can be used in which the electrode assembly is placed in a reduced-pressure chamber with the outer casing open and the chamber is reduced in pressure using a vacuum pump. After degassing, the outer casing is sealed to hermetically seal the outer casing, thereby producing a nonaqueous alkali metal storage element.

[0096] [High temperature and high voltage cycle characteristics] Under high-temperature conditions of 85°C or higher and high-voltage conditions of 4.0V or higher, the decomposition reaction of the electrolyte is likely to be accelerated, and the reduction reaction of the electrolyte solvent proceeds at the negative electrode, where the potential is low due to the doping of alkali metal ions. This side reaction is particularly likely to occur at the electrode edges, where a large amount of excess electrolyte is present. Furthermore, under high-temperature conditions, the deactivation of the alkali metal ions doped in the negative electrode active material is accelerated, which increases the potential and results in a decrease in capacity. In other words, by suppressing the side reaction at the negative electrode edges and suppressing the deactivation of the alkali metal ions doped in the negative electrode, it is possible to suppress the increase in resistance during high-temperature, high-voltage cycling and improve the capacity retention rate.

[0097] The side reaction at the negative electrode end correlates with the alkali metal ion concentration at the negative electrode end; that is, the lower the amount of alkali metal ions per unit area at the outer periphery of the negative electrode, the lower the reactivity. On the other hand, by increasing the amount of alkali metal ions at the center of the negative electrode, the increase in potential can be suppressed even if the alkali metal ions doped in the negative electrode are somewhat deactivated. That is, when the alkali metal ion concentration per unit area at the outer periphery of the negative electrode is A (mAh / g) and the alkali metal ion concentration per unit area at the center is B (mAh / g), a ratio of 1.02 ≦ B / A ≦ 1.45 can suppress the increase in resistance during high-temperature, high-voltage cycling and improve the capacity retention rate. A ratio of 1.02 or greater suppresses the side reaction at the negative electrode end, thereby suppressing the increase in resistance during high-temperature, high-voltage cycling. A ratio of 1.45 or less suppresses excessive potential increase at the positive electrode end facing the negative electrode end, and suppresses corrosion of the positive electrode current collector, thereby suppressing the increase in resistance. From this perspective, a ratio of 1.03 ≦ B / A ≦ 1.44 is preferred. FIG. 1 is an image diagram showing the outer and central portions of a negative electrode, showing a schematic top view of the negative electrode (a) and a schematic view illustrating the outer, central, and end portions of the negative electrode (b). By referring to FIG. 1, the outer and central portions (o) of the negative electrode (1) under test can be defined. In the negative electrode (1), a negative electrode active material layer (3) is disposed on a negative electrode current collector (2). The central portion (c) is the area surrounded by a dotted line, located 5% of the length from the electrode end (e), shown by a thick line, toward the center. The outer and central portions (o) are the area surrounded by the dotted and thick lines.

[0098] The method for setting B / A to 1.02 or more and 1.45 or less is not particularly limited, and examples thereof include a method of increasing the concentration of alkali metal carbonate in the center of the positive electrode precursor facing the negative electrode, a method of locally heating the center of the electrode body during alkali metal doping to promote the alkali metal doping reaction in the center, a method of locally pressurizing the center of the electrode body during alkali metal doping to promote the alkali metal doping reaction in the center, and a method of assembling an electrode body using a negative electrode whose center has been doped with an alkali metal in advance.

[0099] Furthermore, the single electrode capacity of the positive electrode is defined as C (mAh / cm 2 ) and the single electrode capacity of the negative electrode is D (mAh / cm 2 ), by satisfying 0.08≦C / D≦0.15, it is possible to suppress the increase in resistance during high-temperature, high-voltage cycling and improve the capacity retention rate. If C / D is 0.08 or more, it is possible to increase the amount of alkali metal ions that can be doped into the negative electrode, thereby suppressing the increase in potential of the negative electrode during high-temperature, high-voltage cycling. If C / D is 0.15 or less, it is possible to suppress the potential fluctuation of the negative electrode during charge and discharge, and it is possible to suppress side reactions. From this perspective, 0.09≦C / D≦0.14 is preferable.

[0100] [Calculation method for A and B] The alkali metal ion concentration A per unit area in the outer periphery and the alkali metal ion concentration B per unit area in the center can be determined by the following method. First, the voltage of the nonaqueous alkali metal storage element was adjusted to 3.8 V in a 25°C environment, and then disassembled in an argon atmosphere with a dew point of -60°C or lower, and the negative electrode was removed. The resulting negative electrode was washed twice with dimethyl carbonate and air-dried. A region of the electrode within 5% of the electrode edge was cut out to form a peripheral sample. The center point was determined to be the longest distance from the electrode edge, and a region including this center point was cut out to form a central sample. While there is no specific requirement for the area of ​​the electrode to be cut out, it is preferable to cut out the electrode so that the area is 5% to 20% of the total area of ​​the negative electrode including the active material layer. The resulting sample was used as the working electrode, and a separator and metallic lithium were used as the counter electrode and reference electrode. The above-mentioned nonaqueous electrolyte was then poured into a negative electrode half-cell. For the working electrode sample, if active material layers were present on both sides of the current collector, the active material layer on one side was removed using a spatula or brush. The non-aqueous electrolyte solution used is preferably an electrolyte salt containing the same cation as the alkali metal ion species doped in the negative electrode, and a mixed solvent of a chain carbonate and a cyclic carbonate (e.g., a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2). The obtained negative electrode half cell was subjected to a test at 25°C under a current of 0.1 mA / cm. 2 Constant current charging is performed at a current value of 0.01 V until the potential of the working electrode reaches 2.5 V. Regarding the charge capacities at this time, A and B can be calculated by, for example, referring to FIG. 1, defining the evaluation result of the outer peripheral sample as A1 (mAh) and the evaluation result of the central sample as B1 (mAh), and dividing A1 and B1 by the weight of the active material layer of the working electrode sample.

[0101] [Calculation method for C and D] The single electrode capacity C of the positive electrode and the single electrode capacity D of the negative electrode can be determined by the following method. After disassembling the non-aqueous alkali metal storage element as described above, the positive electrode was removed, washed twice with dimethyl carbonate, and air-dried. The center of the resulting positive electrode was cut out to serve as the working electrode, and a separator and metallic lithium were used as the counter electrode and reference electrode. The non-aqueous electrolyte solution described above was then poured into the resulting positive electrode half-cell. The resulting positive electrode half-cell was then subjected to a current of 0.1 mA / cm at 25°C. 2 The battery was charged at a constant current of 0.1 mA / cm until the potential of the working electrode reached 4.0 V, and then the battery was charged at a constant voltage of 4.0 V for 30 minutes. 2 The discharge capacity at this time is divided by the electrode area of ​​the working electrode to obtain C (mAh / cm 2 ) can be calculated.

[0102] In addition, the negative electrode half-cell in which the alkali metal ion concentration in the center of the negative electrode was measured was charged to 2.5 V as described above, and then charged at a constant current of 0.1 mA / cm 2 The discharge capacity at this time is defined as D1 (mAh), which is divided by the electrode area of ​​the working electrode sample to obtain D (mAh / cm 2 ) can be calculated.

[0103] [About A1 / D1] It is preferable that the ratios of A1 and D1 obtained by the above method are 0.65≦A1 / D1≦0.90. If A1 / D1 is 0.65 or more, the potential increase of the negative electrode during high-temperature, high-voltage cycles can be suppressed, and the capacity retention rate can be improved. If A1 / D1 is 0.90 or less, the deposition of alkali metals on the negative electrode can be suppressed.

[0104] [In the case of a negative electrode having an active material layer on both sides of the outermost layer] When the outermost layer of an electrode laminate or electrode wound body is a negative electrode and the negative electrode has negative electrode active material layers on both sides of the negative electrode current collector, it is preferable that B>E, where E (mAh / g) is the alkali metal ion concentration per unit area in the central part of the negative electrode active material layer that does not face the positive electrode. By making B>E, side reactions on the outermost negative electrode layer can be suppressed. E can be calculated in the same manner as the alkali metal ion concentration B described above.

[0105] <<Uses of non-aqueous alkali metal storage elements>> A storage module can be fabricated by connecting multiple nonaqueous alkali metal storage elements according to this embodiment in series or in parallel. The nonaqueous alkali metal storage element and storage module according to this embodiment can achieve both high input / output characteristics and safety at high temperatures. Therefore, they can be used in power regeneration assist systems, power load leveling systems, uninterruptible power supply systems, contactless power supply systems, energy harvesting systems, storage systems, solar power generation and storage systems, electric power steering systems, emergency power supply systems, in-wheel motor systems, idling stop systems, quick charging systems, smart grid systems, and the like. Storage systems are suitable for natural power generation such as solar power generation and wind power generation, load leveling systems for microgrids, and uninterruptible power supply systems for factory production facilities. In contactless power supply systems, non-aqueous alkali metal storage elements are suitable for leveling out voltage fluctuations such as those caused by microwave power transmission or electric field resonance, and for storing energy; and in energy harvesting systems, non-aqueous alkali metal storage elements are suitable for using power generated by vibration power generation, etc.

[0106] In the energy storage system, a plurality of nonaqueous alkali metal energy storage elements are connected in series or parallel to form a cell stack, or a nonaqueous alkali metal energy storage element is connected in series or parallel to a lead battery, a nickel-metal hydride battery, a lithium-ion secondary battery, or a fuel cell. Furthermore, the nonaqueous alkali metal energy storage element according to the present embodiment can achieve both high input / output characteristics and safety at high temperatures, and therefore can be installed in vehicles such as electric vehicles, plug-in hybrid vehicles, hybrid vehicles, and electric motorcycles. The above-described power regeneration assist system, electric power steering system, emergency power supply system, in-wheel motor system, idling stop system, or a combination thereof can be suitably installed in the vehicle. [Example]

[0107] Hereinafter, the embodiments of the present invention will be specifically described with reference to examples and comparative examples. However, the present invention is not limited to the following examples and comparative examples. In the following description, the abbreviations CMC1, CMC2, and CMC3 are used, but CMC1 to CMC3 are the same carboxymethyl cellulose and are used for calculating the blending ratio.

[0108] <<Examples of Manufacturing Negative Electrode, Positive Electrode Precursor, and Electrolyte>> [Preparation of carbon nanotube dispersion] A carbon nanotube dispersion was prepared by mixing 0.400 mass% of commercially available single-walled carbon nanotubes (CNTs), 0.600 mass% of carboxymethyl cellulose (CMC1) as a dispersant, and 99.000 mass% of distilled water, and dispersing the mixture using a thin-film swirling high-speed mixer, Filmix, manufactured by PRIMIX, at a peripheral speed of 17 m / s.

[0109] [Production of negative electrode 1] 94.800% by mass of artificial graphite with an average particle size of 4.5 μm, 3.000% by mass of carbon black, 2.000% by mass of carboxymethyl cellulose (CMC2) as a dispersant, and distilled water were mixed to obtain a mixture with a solids content of 28.0% by mass. The resulting mixture was dispersed for 10 minutes at a rotation speed of 2,000 rpm using a Thinky Planetary Mixer "Awatori Rentaro (registered trademark)." A carbon nanotube dispersion was mixed with the above-prepared mixture so that carbon nanotubes (CNTs) were 0.080% by mass and CMC1 was 0.120% by mass, and the mixture was dispersed for 10 minutes at a rotation speed of 2,000 rpm to prepare negative electrode coating solution 1. The viscosity (ηb) and TI value of the obtained negative electrode coating solution 1 were measured using a Toki Sangyo Co., Ltd. E-type viscometer TVE-35H. The viscosity (ηb) was 1,560 mPa·s and the TI value was 3.7. Negative electrode coating solution 1 was applied to one side of a 10 μm-thick electrolytic copper foil using a doctor blade and dried for 10 minutes on a hot plate heated to 50°C. Negative electrode 1 was then produced by pressing using a roll press under conditions of a pressure of 5 kN / cm and a surface temperature of the press part of 25°C. Negative electrode 1 was cut into a piece measuring 3 cm x 3 cm, and the weight of the negative electrode active material layer was divided by the area of ​​the negative electrode to calculate the weight of the negative electrode active material layer, which was 44.0 g / m 2 It was.

[0110] [Production of negative electrodes 2 to 7] Negative electrodes 2 to 7 were produced in the same manner as in [Production of negative electrode 1], except that the weight per unit area of ​​the negative electrode active material layer was adjusted as shown in Table 1.

[0111] [Table 1]

[0112] [Preparation of positive electrode active material] The crushed coconut shell carbonized material was placed in a small carbonization furnace and carbonized at 500°C for 3 hours under a nitrogen atmosphere to obtain a carbonized material. The resulting carbonized material was placed in an activation furnace, and steam heated in a preheating furnace was introduced into the furnace at 1 kg / h. The temperature was raised to 900°C over 8 hours for activation. The activated carbonized material was removed and cooled under a nitrogen atmosphere to obtain activated carbon. The activated activated carbon was washed with water for 10 hours, drained, dried in an electric dryer maintained at 115°C for 10 hours, and then pulverized in a ball mill for 1 hour to obtain activated carbon 1. The average particle size of activated carbon 1 was measured using a Shimadzu laser diffraction particle size analyzer (SALD-2000J) and found to be 5.5 μm. The pore size distribution of activated carbon 1 was also measured using a Yuasa Ionics pore size analyzer (AUTOSORB-1 AS-1-MP). As a result, the BET specific surface area was 2360m 2 / g, mesopore volume (V1) was 0.52 cc / g, micropore volume (V2) was 0.88 cc / g, and V1 / V2 = 0.59.

[0113] [Production of positive electrode precursor 1] Activated carbon 1 46.200 mass%, carboxymethyl cellulose (CMC3) 2.000 mass%, lithium carbonate 40.000 mass%, carbon black 5.000 mass%, acrylic latex (LTX) 4.500 mass%, PVP (polyvinylpyrrolidone) 2.000 mass%, and distilled water was mixed so that the mass ratio of solids was 34.1 mass%, and this was dispersed for 20 minutes at a rotation speed of 2000 rpm using a Thinky Planetary Mixer "Awatori Rentaro (registered trademark)". The obtained mixture was mixed with a carbon nanotube dispersion so that carbon nanotubes (CNT) were 0.120 mass%, and CMC1 were 0.180 mass%, and dispersed for 10 minutes at a rotation speed of 2,000 rpm to prepare positive electrode coating solution 1. The viscosity (ηb) and TI value of the obtained positive electrode coating solution 1 were measured using a Toki Sangyo Co., Ltd. E-type viscometer TVE-35H. The viscosity (ηb) was 2,340 mPa·s and the TI value was 5.1. The dispersion of the obtained positive electrode coating solution 1 was also measured using a particle gauge manufactured by Yoshimitsu Seiki Co., Ltd. The particle size was found to be 33 μm. Positive electrode coating solution 1 was applied to one side of a 15 μm-thick aluminum foil using a doctor blade, dried on a hot plate heated to 50°C for 10 minutes, and then pressed using a roll press under conditions of a pressure of 6 kN / cm and a surface temperature of the press part of 25°C to obtain positive electrode precursor 1. The basis weight of positive electrode precursor 1 was measured using the same method as above and found to be 48.0 g / m 2 It was.

[0114] [Production Examples of Positive Electrode Precursors 2 to 6] Positive electrode precursors 2 to 6 were produced in the same manner as in [Production of positive electrode precursor 1], except that the amounts of each component used were adjusted as shown in Table 2.

[0115] [Table 2]

[0116] [Preparation of electrolyte 1] A mixed solvent of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 33:67 (volume ratio) was used as the organic solvent, and LiFSI was used as the electrolyte. An electrolyte salt was dissolved therein to give a concentration of 1.0 mol / L, thereby obtaining non-aqueous electrolyte solution 1.

[0117] [Preparation of electrolytes 2 to 5] Non-aqueous electrolyte solutions 2 to 5 were produced in the same manner as in [Preparation of electrolyte solution 1], except that the amounts of each component used were adjusted as shown in Table 3.

[0118] [Table 3]

[0119] Example 1 <Production of non-aqueous alkali metal storage element> [Assembly process] The obtained positive electrode precursor 1 was cut into a single piece so that the positive electrode active material layer measured 4.4 cm × 9.4 cm. Subsequently, the negative electrode 1 was cut into a single piece so that the negative electrode active material layer measured 4.5 cm × 9.5 cm. A 4.7 cm × 9.8 cm polyethylene separator (manufactured by Asahi Kasei Corporation, thickness 15 μm) was also prepared. Using these, the positive electrode precursor 1, separator, and negative electrode 1 were stacked in this order with the positive electrode active material layer and the negative electrode active material layer facing each other with the separator sandwiched between them to obtain an electrode laminate. The positive electrode terminal and negative electrode terminal were ultrasonically welded to the obtained electrode body, which was then placed in an exterior case formed of an aluminum laminate packaging material, and the three sides including the electrode terminal portion were heat-sealed.

[0120] [Injection, impregnation, sealing process] Approximately 2.5 g of nonaqueous electrolyte solution 1 was injected into the exterior housing containing the electrode laminate under a dry air environment at atmospheric pressure, a temperature of 25°C, and a dew point of -40°C or less. The exterior housing containing the electrode laminate and nonaqueous electrolyte solution was then placed in a vacuum chamber, depressurized from atmospheric pressure to -87 kPa, then returned to atmospheric pressure, and allowed to stand for 5 minutes. The process of depressurizing the exterior housing in the chamber from atmospheric pressure to -87 kPa and then returning to atmospheric pressure was repeated four times, and then allowed to stand for 15 minutes under atmospheric pressure. Through these steps, the electrode laminate impregnated with nonaqueous electrolyte solution 1 was then placed in a vacuum sealer, and the exterior housing was sealed by sealing at a pressure of 0.1 MPa at 180°C for 10 seconds under a vacuum of -95 kPa.

[0121] [Alkali metal doping process] The sealed electrode stack was sandwiched between 3.0 cm × 7.0 cm silicone sponge buffers on both sides of the center and pressed at 30 kPa. This was then subjected to constant current charging at 45°C with a current value of 10 mA until a voltage of 4.5 V was reached, followed by initial charging at a constant voltage of 4.5 V for 2 hours, thereby doping the negative electrode with an alkali metal.

[0122] [Aging process] The electrode laminate after alkali metal doping was removed from the dry box and subjected to constant current discharge at 50 mA in an environment of 25°C until the voltage reached 2.0 V, and then constant current discharge at 2.0 V was performed for 1 hour to adjust the voltage to 2.0 V. Subsequently, the electrode laminate sealed in the exterior body was stored in a constant temperature bath at 85°C for 12 hours.

[0123] [Gas removal process] After aging, the exterior was partially opened and the electrode stack was removed in a dry air environment at a temperature of 25°C and a dew point of -40°C. The removed electrode stack was placed in a vacuum chamber, and a diaphragm pump was used to reduce the pressure from atmospheric pressure to -80 kPa over 3 minutes, and then the pressure was returned to atmospheric pressure over 3 minutes. This process was repeated three times. The electrode stack was then placed back into the exterior, and the pressure was reduced to -90 kPa using a vacuum sealer. The exterior was then sealed at 200°C for 10 seconds at a pressure of 0.1 MPa, thereby producing a nonaqueous alkali metal storage element. In addition, for the nonaqueous alkali metal storage element, each value was measured or calculated as described in the above items [Calculation method for A and B], [Calculation method for C and D], and [Regarding A1 / D1], and is shown in Table 4.

[0124] <Evaluation of non-aqueous alkali metal energy storage elements> [Measurement of internal resistance Ra] The internal resistance Ra (mΩ) is a value obtained by the following method. First, a nonaqueous alkali metal storage element is charged at a constant current of 20 C in a thermostatic chamber set at 25°C until the voltage reaches 4.0 V. Subsequently, a constant voltage of 4.0 V is applied and constant voltage charging is performed for a total of 30 minutes. Next, a constant current discharge is performed at a current of 20 C to 2.0 V with a sampling interval of 0.05 seconds to obtain a discharge curve (time-voltage). In this discharge curve, the voltage values ​​at 1 second and 2 seconds of discharge are extrapolated by linear approximation to obtain the voltage at 0 seconds of discharge, Eo. The value Ra is calculated as Ra = ΔE / (current value at 20 C) from the voltage drop ΔE = 4.0 - Eo. The internal resistance Ra of one of the obtained nonaqueous alkali metal storage elements was measured by the above-mentioned method using a charge / discharge device (5 V, 10 A) manufactured by Asuka Electronics Co., Ltd. in a thermostatic chamber set at 25°C. Ra was 80.5 mΩ. This value is shown in Table 5 as the initial internal resistance Ra.

[0125] [Capacitance Qa measurement] The capacity Qa (mAh) is a value obtained by the following method. First, the nonaqueous alkali metal storage element is charged at a constant current of 20 C in a thermostatic chamber set at 25°C until it reaches 4.0 V, and then a constant voltage charge of 4.0 V is applied for a total of 30 minutes. Next, a sampling interval is set to 1.0 second, and a constant current discharge is performed at a current of 1 C until it reaches 2.0 V. The discharge capacity when this is performed is defined as the capacity Qa. The capacity Ra of the above nonaqueous alkali metal storage element was measured by the above method using a charge / discharge device (5 V, 10 A) manufactured by Asuka Electronics Co., Ltd. in a thermostatic chamber set at 25°C. The Qa was 6.58 mAh. This value is shown in Table 5 as the initial capacity Qa.

[0126] [High temperature and high voltage cycle test] The resistance and capacity change rates after the high-temperature, high-voltage cycle test were measured using the following method. First, a cell corresponding to the nonaqueous alkali metal storage element was charged at a constant current of 50 C in a thermostatic chamber set at 85°C until it reached 4.2 V, and then discharged at a constant current of 50 C until it reached 3.0 V. After repeating this charge / discharge cycle 1,000 times, the internal resistance Rb after the high-temperature, high-voltage cycle test was measured using the internal resistance measurement method described above, and the capacity Qb after the high-temperature, high-voltage cycle test was measured using the capacity measurement method described above. Rb / Ra was defined as the resistance change rate after the high-temperature, high-voltage cycle test, and Qb / Qa was defined as the capacity change rate after the high-temperature, high-voltage cycle test. These results are shown in Table 5.

[0127] Examples 2 to 11 and Comparative Examples 1 to 12 A nonaqueous alkali metal storage element was produced and evaluated in the same manner as in Example 1, except that the negative electrode, positive electrode precursor, and nonaqueous electrolyte solution shown in Tables 1, 2, and 3 were used and the production conditions were as shown in Table 4. The evaluation results are shown in Tables 4 and 5.

[0128] [Table 4]

[0129] [Table 5]

[0130] Comparing Examples 1 to 11 with Comparative Examples 1 to 12, it is believed that by applying pressure to the center of the nonaqueous alkali metal storage element during alkali metal doping, the decomposition reaction of the alkali metal carbonate in the center is promoted, making it possible to change the alkali metal ion concentration in the outer periphery and center of the negative electrode, thereby suppressing the increase in resistance during high-temperature, high-voltage cycles and improving the capacity retention rate.

[0131] Example 12 Negative electrode coating solution 1 was applied to one side of a 10 μm thick electrolytic copper foil using a doctor blade and dried for 10 minutes on a hot plate heated to 50° C. Next, negative electrode coating solution 1 was applied to the back side of the electrolytic copper foil and dried in the same manner as above. Next, negative electrode 8 was produced by pressing using a roll press under conditions of a pressure of 5 kN / cm and a surface temperature of the press part of 25° C.

[0132] Using a doctor blade, positive electrode coating solution 1 was applied to one side of a 15 μm thick aluminum foil and dried for 10 minutes on a hot plate heated to 50° C. Next, positive electrode coating solution 1 was applied to the back side of the aluminum foil and dried in the same manner as above. Positive electrode precursor 7 was obtained by pressing using a roll press under conditions of a pressure of 6 kN / cm and a surface temperature of the press part of 25° C.

[0133] A nonaqueous alkali metal storage element was produced in the same manner as in Example 1, except that an electrode laminate was obtained by stacking a negative electrode 8, the same separator as used in Example 1, a positive electrode precursor 7, the same separator as used in Example 1, and a negative electrode 8 in this order, with the positive electrode active material layer and the negative electrode active material layer facing each other with the separator sandwiched between them.

[0134] Comparative Example 13 A negative electrode 9 was produced in the same manner as in Example 12, except that an etched copper foil with a thickness of 12 μm and an aperture ratio of 20% was used. A positive electrode precursor 8 was produced in the same manner as in Example 12, except that an etched aluminum foil with a thickness of 20 μm and an aperture ratio of 20% was used. A nonaqueous alkali metal storage element was produced in the same manner as in Comparative Example 1, except that an electrode laminate was obtained by stacking the negative electrode 9, separator, positive electrode precursor 8, separator, and negative electrode 9 in this order, with the positive electrode active material layer and the negative electrode active material layer facing each other with the separator sandwiched between them.

[0135] The evaluation results of Example 12 and Comparative Example 13 are shown in Table 6.

[0136] [Table 6]

[0137] In Comparative Example 13, the use of an etched foil with openings as the current collector foil allowed the alkali metal ions to be uniformly doped into the outermost negative electrode active material layer, which is thought to have resulted in a decrease in the potential of the outermost negative electrode active material layer, making it more likely to react with excess nonaqueous electrolyte outside the electrode body, resulting in an increase in resistance and a decrease in capacity retention rate during high-temperature, high-voltage cycling. [Industrial Applicability]

[0138] The nonaqueous alkali metal energy storage element of the present invention can be suitably used as a nonaqueous alkali metal energy storage element for power regeneration systems in automotive hybrid drive systems that require high durability in high-temperature, high-voltage environments; power load leveling systems for natural power generation such as solar power generation and wind power generation, and microgrids; uninterruptible power supply systems for factory production facilities; contactless power supply systems for leveling voltage fluctuations and storing energy such as those caused by microwave power transmission and electrolytic resonance; and energy harvesting systems for utilizing power generated by vibration power generation, etc. The nonaqueous alkali metal energy storage element can be used to form an energy storage module, for example, by connecting multiple nonaqueous alkali metal energy storage elements in series or in parallel. The nonaqueous alkali metal energy storage element of the present invention is preferred when used as a lithium ion capacitor, because the effects of the present invention are maximized. [Explanation of symbols]

[0139] 1 negative electrode 2 Negative electrode current collector 3 Negative electrode active material layer c Negative electrode center e Negative end о Negative electrode outer periphery

Claims

1. A non-aqueous alkali metal storage element includes an electrode assembly including a positive electrode including a positive electrode active material layer disposed on a positive electrode current collector, a negative electrode including a negative electrode active material layer disposed on a negative electrode current collector, and a separator, and a non-aqueous electrolyte solution containing alkali metal ions, wherein the positive electrode active material layer includes a positive electrode active material including a carbon material, and an alkali metal carbonate, and the non-aqueous electrolyte solution contains a bis(fluorosulfonyl)imide ion at a concentration of 0.97 mol / L or more and a hexafluorophosphate ion at a concentration of 0.00 mol / L or more and 0.05 mol / L or less, and the single electrode capacity of the positive electrode is C (mAh / cm ２ ) and the single electrode capacity of the negative electrode is D (mAh / cm ２ ), 0.08≦C / D≦0.15, and 1.02≦B / A≦1.45, where A (mAh / g) is the alkali metal ion concentration per unit area of ​​the outer peripheral portion of the negative electrode and B (mAh / g) is the alkali metal ion concentration per unit area of ​​the central portion.

2. 2. The nonaqueous alkali metal storage element according to claim 1, wherein 0.65≦A1 / D1≦0.90 is satisfied, where A1 (mAh) is the charge capacity of the central portion of the negative electrode and D1 (mAh) is the discharge capacity of the negative electrode half-cell of the negative electrode.

3. 3. The nonaqueous alkali metal storage element according to claim 1, wherein the negative electrode has the negative electrode active material layer on both sides of the negative electrode current collector, the outermost layer of the electrode body is the negative electrode, and when the alkali metal ion concentration per unit area of ​​a central part of the negative electrode active material layer that does not face the positive electrode is E (mAh / g), B>E.

4. 4. The nonaqueous alkali metal storage element according to claim 1, wherein the positive electrode and / or the negative electrode contains carbon nanotubes.

5. An electricity storage module comprising the nonaqueous alkali metal electricity storage element according to any one of claims 1 to 4.

6. 6. The power storage module according to claim 5, wherein the power storage module is incorporated into at least one system selected from the group consisting of a power regeneration assist system, a power load leveling system, an uninterruptible power supply system, a contactless power supply system, an energy harvesting system, a power storage system, a solar power generation and storage system, an electric power steering system, an emergency power supply system, an in-wheel motor system, an idling stop system, an electric vehicle, a plug-in hybrid vehicle, a hybrid vehicle, an electric motorcycle, a quick charging system, and a smart grid system.

7. The nonaqueous alkali metal storage element according to any one of claims 1 to 4, Lead-acid batteries, nickel-metal hydride batteries, lithium-ion secondary batteries, or fuel cells A storage system in which these are connected in series or parallel.

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