lithium-ion capacitor

Abstract

To provide a lithium-ion capacitor with sufficient float characteristics.SOLUTION: A lithium ion capacitor 10 includes a cathode 2; an anode 3; a separator 4 placed between the cathode and the anode; and an electrolyte containing a non-aqueous solvent and a lithium salt. The cathode contains activated carbon particles with a specific surface area in the range of 1200-2500 m2 / g. The electrolyte contains additives containing isocyanate groups. The additive concentration in the electrolyte is 0.01% by mass or more and less than 3.0% by mass.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to lithium ion capacitors. [Background technology]

[0002] Lithium ion capacitors have been attracting attention in recent years. Patent Document 1 (WO 2015 / 163254) discloses "a battery additive (B) containing a compound (A) that is composed of 3 to 5 atoms, has 2 to 4 atoms with an electronegativity of 3 or more, has an atomic group (X) having at least one non-polymerizable double bond, and has an ion-dissociating functional group (b), and has no polymerizable unsaturated bonds." Patent Document 1 discloses -N=C=O as an example of the atomic group (X). Patent Document 1 also discloses an electrolyte solution containing the battery additive (B). Patent Document 1 also discloses a lithium ion capacitor containing the electrolyte solution. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 163254 Summary of the Invention [Problem to be solved by the invention]

[0004] Currently, there is a demand for improving the float characteristics of lithium ion capacitors. In this situation, one of the objects of the present disclosure is to provide a lithium ion capacitor with good float characteristics. [Means for solving the problem]

[0005] One aspect of the present disclosure relates to a lithium ion capacitor including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution containing a non-aqueous solvent and a lithium salt, wherein the positive electrode has a specific surface area of ​​1200 to 2500 m 2 / g, the electrolyte solution contains an additive containing an isocyanate group, and the concentration of the additive in the electrolyte solution is 0.01% by mass or more and less than 3.0% by mass. [Effects of the Invention]

[0006] According to the present disclosure, a lithium ion capacitor with good float characteristics can be obtained. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a longitudinal cross-sectional view schematically illustrating a configuration of a lithium ion capacitor according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments according to the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In this specification, the expression "numeric value A to numerical value B" includes numerical value A and numerical value B, and can be read as "numerical value A or more and numerical value B or less." In the following description, when a lower limit and an upper limit are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0009] (lithium ion capacitor) The lithium ion capacitor according to this embodiment includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution containing a non-aqueous solvent and a lithium salt. 2The electrolyte solution contains activated carbon particles in the range of 0.01% by mass to 3.0% by mass. The electrolyte solution contains an additive containing an isocyanate group (-N=C=O). The concentration of the additive in the electrolyte solution is 0.01% by mass or more and less than 3.0% by mass. This additive may be referred to as "additive (A)" below. Furthermore, the lithium ion capacitor may be referred to simply as "capacitor" below. The components of the lithium ion capacitor according to this embodiment will be described below.

[0010] The capacitor of this embodiment uses activated carbon particles with a relatively large specific surface area (activated carbon particles with a relatively small particle size). Activated carbon with a large specific surface area is advantageous in terms of increasing capacity. On the other hand, a large specific surface area makes it easier for the activated carbon particles to react with the electrolyte. When the activated carbon particles react with the electrolyte, gas is generated, which may increase the internal pressure of the capacitor.

[0011] The electrolyte of the capacitor of this embodiment contains an additive (A). The use of such an electrolyte can significantly improve the float characteristics. The reason for this is currently unclear, but it can be considered as follows: At least a portion of the additive (A) containing an isocyanate group may be adsorbed onto the surface of the activated carbon particles. This suppresses the reaction between the activated carbon particles and the electrolyte, thereby suppressing gas generation and decomposition of the electrolyte. Suppressing gas generation and decomposition of the electrolyte improves the float characteristics.

[0012] The activated carbon particles contained in the positive electrode reversibly adsorb and release anions. When anions are adsorbed onto the activated carbon particles, an electric double layer is formed, and capacity is generated.

[0013] The negative electrode includes a negative electrode active material that can reversibly dope and undope lithium ions. The doping of lithium ions into the negative electrode active material includes at least the phenomenon of lithium ions being absorbed into the negative electrode active material, and may also include the adsorption of lithium ions into the negative electrode active material and chemical interactions between the negative electrode active material and lithium ions. Capacity is developed as the Faraday reaction in which lithium ions are absorbed into the negative electrode active material progresses.

[0014] When a lithium-ion capacitor is charged, anions are adsorbed onto the positive electrode and lithium ions are doped onto the negative electrode. When a lithium-ion capacitor is discharged, anions are desorbed from the positive electrode and lithium ions are de-doped from the negative electrode.

[0015] (positive electrode) The positive electrode has a specific surface area of ​​1200 to 2500 m 2 The specific surface area of ​​activated carbon particles is in the range of 1350 to 2300 m / g. 2 / g. The specific surface area of ​​the activated carbon particles may be in the range of 1200 m 2 / g or more (e.g., 1350m 2 / g or more), high capacity can be achieved. 2 / g or less (e.g., 2300m 2 / g or less), the area in contact with the electrolyte is reduced, thereby suppressing decomposition of the electrolyte. The specific surface area of ​​activated carbon particles can be determined by the BET method using nitrogen gas.

[0016] The average particle size of the activated carbon particles may be in the range of 0.5 μm to 6 μm (for example, in the range of 1 μm to 5 μm). By setting the average particle size in this range, it becomes easier to control the specific surface area within the above range. The activated carbon particles may be commercially available or may be produced by a known method. Activated carbon particles with various specific surface areas and average particle sizes are commercially available. In this specification, the average particle size is the median diameter (D50) at which the cumulative volume becomes 50% in the volume-based particle size distribution. The median diameter is determined using a laser diffraction / scattering particle size distribution analyzer.

[0017] Activated carbon particles may be produced by heat-treating a raw material to carbonize it, and then activating the resulting carbonized material to make it porous. Examples of raw materials include coal-based pitch, petroleum-based pitch, phenolic resin, petroleum coke, and coal coke. Examples of activation treatments include gas activation using gases such as steam, and chemical activation using alkalis such as potassium hydroxide. The specific surface area and average particle size of the activated carbon particles may be adjusted by pulverizing and / or classifying the activated carbon particles. The pulverization may be performed using a ball mill, jet mill, or the like.

[0018] The positive electrode may include a positive electrode active material layer containing activated carbon particles (positive electrode active material) and a binder. Examples of binders include fluororesins, acrylic resins, rubbers, cellulose derivatives, etc. Examples of fluororesins include polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, etc. Examples of acrylic resins include polyacrylic acid, acrylic acid-methacrylic acid copolymers, etc. Examples of rubbers include styrene butadiene rubber, etc. Examples of cellulose derivatives include carboxymethyl cellulose, etc.

[0019] The positive electrode active material layer may contain a conductive material. Examples of the conductive material include particulate conductive carbon materials other than activated carbon (such as carbon black), carbon fiber, and fibrous carbon materials (such as carbon nanotubes and carbon nanofibers). Examples of carbon black include acetylene black, ketjen black, and furnace black.

[0020] The content of activated carbon in the positive electrode active material layer may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The content may be 98% by mass or less, or 90% by mass or less. The content may be in the range of 60 to 98% by mass (for example, 60 to 90% by mass). The content of conductive material in the positive electrode active material layer may be in the range of 1 to 30% by mass, or in the range of 5 to 15% by mass. The content of binder in the positive electrode active material layer may be in the range of 1 to 10% by mass.

[0021] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. There are no particular limitations on the positive electrode current collector, and a positive electrode current collector used in known lithium ion capacitors may be used. The positive electrode current collector may be a sheet metal. Examples of sheet metals include metal foils, porous metals, and etched metals. Examples of metals include aluminum, aluminum alloys, nickel, and titanium. The thickness of the positive electrode current collector may be in the range of 10 to 100 μm. The surface of the sheet metal may be roughened. The roughening may be performed by a known method (e.g., electrolytic etching).

[0022] There is no particular limitation on the method for producing the positive electrode, and it may be produced by a known method. The positive electrode may be produced by the following method. First, a material constituting the positive electrode active material layer is mixed with a dispersion medium (water or an organic solvent) to obtain a slurry. A dispersant may be added to the slurry, if necessary. Examples of dispersants include carboxymethyl cellulose (CMC). Next, the obtained slurry is applied to a current collector to form a coating film. Next, the coating film is dried and / or rolled, if necessary. In this way, a positive electrode including a positive electrode active material layer is obtained. There is no particular limitation on the thickness of the positive electrode active material layer, and it may be in the range of 10 μm to 300 μm.

[0023] (electrolyte) The electrolyte solution has lithium ion conductivity, contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent, and further contains the additive (A) described above.

[0024] (Additive (A)) The additive (A) contains an isocyanate group. Preferably, the additive (A) does not contain an ion dissociative functional group (b). By not containing the ion dissociative functional group (b), excess metal impurities are prevented from entering, and the absence of an ion dissociative functional group increases reactivity, which is thought to facilitate the formation of a coating of isocyanate groups on the surface of the activated carbon particles of the positive electrode.

[0025] A preferred example of additive (A) comprises an atomic group X having a carbon number in the range of 3 to 10 and at least one isocyanate group bonded to the atomic group X. In this case, additive (A) may satisfy at least one selected from the group consisting of the following conditions (1), (2), and (3). For example, additive (A) may satisfy two or three of the following conditions (1) to (3). For example, additive (A) may comprise a hydrocarbon group having a carbon number in the range of 3 to 10 and two isocyanate groups bonded to the hydrocarbon group. (1) The number of carbon atoms in the atomic group X is in the range of 3 to 8, 3 to 7, or 4 to 6. (2) The atomic group X may contain or be a hydrocarbon group. Examples of the hydrocarbon group include an alkylene group. (3) Two isocyanate groups are bonded to the atomic group X.

[0026] The additive (A) may be a compound (polymethylene diisocyanate) in which an isocyanate group is bonded to each of the carbon atoms at both ends of a polymethylene chain. The number of carbon atoms in the polymethylene chain may be in the range of 3 to 10 (e.g., in the range of 3 to 8, 3 to 7, or 4 to 6). The additive (A) may be at least one selected from the group consisting of trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, and heptamethylene diisocyanate. The use of these additives provides high effects. A preferred example of the additive (A) is hexamethylene diisocyanate.

[0027] The concentration (content) of the additive (A) in the electrolytic solution is 0.01% by mass or more, and may be 0.1% by mass or more, 0.2% by mass or more, or 0.25% by mass or more. The concentration may be less than 3.0% by mass, and may be 2.5% by mass or less, 2.0% by mass or less, 1.0% by mass or less, or 0.5% by mass or less. The concentration of the additive (A) in the electrolytic solution is preferably in the range of 0.1% by mass to 2.0% by mass (for example, 0.1 to 1.0% by mass or 0.1 to 0.5% by mass). This range provides a greater effect.

[0028] (lithium salts) A lithium salt is a salt of a lithium ion and an anion. The anion derived from the lithium salt is adsorbed onto the positive electrode during charging of the lithium ion capacitor.

[0029] Examples of lithium salts include LiN(SO2F)2, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 , LiCl, LiBr, LiI, LiBCl4, LiN(CF3SO2)2, etc. These may be used alone or in combination of two or more. The lithium salt preferably contains at least one selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), and lithium bis(fluorosulfonyl)imide (LiN(SO2F)2). These lithium salts have a high degree of dissociation and excellent ionic conductivity. Hereinafter, lithium bis(fluorosulfonyl)imide (LiN(SO2F)2) may be referred to as "LIFSI." The lithium salt preferably contains LIFSI, or it is also possible to use only LIFSI as the lithium salt.

[0030] (non-aqueous solvent) Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, lactones, chain ethers, cyclic ethers, and other non-aqueous solvents. The non-aqueous solvent of the electrolyte may be composed of only one type of non-aqueous solvent. Alternatively, the non-aqueous solvent of the electrolyte may be a mixed solvent of multiple types of non-aqueous solvents.

[0031] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, etc. Examples of linear carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of aliphatic carboxylic acid esters include methyl formate, methyl acetate, methyl propionate (PC), ethyl propionate, etc. Examples of lactones include γ-butyrolactone, γ-valerolactone, etc. Examples of linear ethers include 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), ethoxymethoxyethane (EME), etc. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, etc. Examples of other non-aqueous solvents include dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, trimethoxymethane, sulfolane, methyl sulfolane, 1,3-propane sultone, and the like.

[0032] The non-aqueous solvent preferably contains a chain carbonate and a cyclic carbonate, or may be composed only of a chain carbonate and a cyclic carbonate. In these cases, the electrolyte preferably satisfies at least one of the following conditions (1) to (3), and more preferably two or three. By satisfying at least one of the following conditions (1) to (3), the float characteristics can be particularly improved. (1) The mixing ratio of the chain carbonate to the cyclic carbonate is in the range of chain carbonate:cyclic carbonate=3:7 to 7:3 on a volume basis. (2) The cyclic carbonate constituting the electrolytic solution contains propylene carbonate and is substantially free of ethylene carbonate. Here, "substantially free of ethylene carbonate" means that the ethylene carbonate content in the cyclic carbonate is 1% by mass or less. In a preferred example of the electrolytic solution in this case, the propylene carbonate content in the cyclic carbonate is 90% by mass or more (e.g., 95% by mass or more or 99% by mass or more), and the ethylene carbonate content is 1% by mass or less. The cyclic carbonate constituting the electrolytic solution may be composed solely of propylene carbonate. (3) The chain carbonate constituting the electrolytic solution contains ethyl methyl carbonate and is substantially free of diethyl carbonate and dimethyl carbonate. Here, "substantially free of diethyl carbonate and dimethyl carbonate" means that the total content of these carbonates in the chain carbonate is 1% by mass or less. In a preferred example of the electrolytic solution in this case, the content of ethyl methyl carbonate in the chain carbonate is 90% by mass or more (e.g., 95% by mass or more or 99% by mass or more), and the total content of diethyl carbonate and dimethyl carbonate is 1% by mass or less. The chain carbonate constituting the electrolytic solution may be composed solely of ethyl methyl carbonate.

[0033] If necessary, additives other than the additive (A) may be added to the electrolyte solution. For example, unsaturated carbonates such as vinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate may be used as additives that form a coating film with high lithium ion conductivity on the surface of the negative electrode.

[0034] (Negative electrode) As described above, the negative electrode includes a negative electrode active material that can reversibly dope and dedope lithium ions. The negative electrode and negative electrode active material may be the same as those used in known lithium ion secondary batteries and known lithium ion capacitors.

[0035] Examples of negative electrode active materials include carbonaceous materials, metal compounds, alloys, and ceramic materials. Carbonaceous materials are preferred because they can lower the potential of the negative electrode. Examples of carbonaceous materials include graphite, hard carbon (non-graphitizable carbon), and soft carbon (easily graphitizable carbon). The negative electrode active material preferably contains hard carbon having an interlayer distance d(002) of 0.38 nm or more as determined from the results of X-ray diffraction measurement. The X-ray diffraction measurement is performed using CuKα radiation. Hard carbon is preferred because it has lower resistance and higher capacity than graphite.

[0036] The negative electrode typically includes a negative electrode active material layer containing a negative electrode active material. The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. There are no particular limitations on the negative electrode current collector, and a negative electrode current collector used in known lithium ion capacitors or known lithium ion secondary batteries may be used. The negative electrode current collector may be a sheet metal. Examples of sheet metals include metal foils, porous metals, and etched metals. Examples of metals include copper, copper alloys, nickel, and stainless steel. The thickness of the negative electrode current collector may be in the range of 10 to 100 μm.

[0037] The negative electrode active material layer may contain a binder, a conductive material, etc. There are no particular limitations on the binder and conductive material, and binders and conductive materials used in known lithium ion secondary batteries and known lithium ion capacitors may be used, or the binders and conductive materials described for the positive electrode may be used.

[0038] The method for producing the negative electrode is not particularly limited, and it may be produced by a known method. The negative electrode may be produced by the following method. First, a material constituting the negative electrode active material layer is mixed with a dispersion medium (water or an organic solvent) to obtain a slurry. Next, the obtained slurry is applied to a current collector to form a coating film. Next, the coating film is dried and / or rolled as necessary. In this way, a negative electrode including a negative electrode active material layer is obtained. There is no particular limit to the thickness of the negative electrode active material layer, and it may be in the range of 10 μm to 300 μm.

[0039] It is desirable to pre-dope the negative electrode active material layer with lithium ions, which reduces the potential of the negative electrode, increasing the difference between the potential of the positive electrode and the potential of the negative electrode (i.e., voltage), and improving the energy density of the lithium ion capacitor.

[0040] The pre-doping of lithium ions into the negative electrode may be performed by the following method. First, a metallic lithium layer serving as a lithium ion supply source is formed on the surface of the negative electrode active material layer. Next, the negative electrode having the metallic lithium layer is immersed in an electrolyte solution having lithium ion conductivity. At this time, lithium ions are eluted from the metallic lithium layer into the non-aqueous electrolyte solution, and the eluted lithium ions are absorbed into the negative electrode active material. This progresses the pre-doping of lithium ions into the negative electrode. The amount of pre-doped lithium ions can be controlled by the mass of the metallic lithium layer. The amount of pre-doped lithium may be approximately 50% to 95% of the maximum amount of lithium ions that can be absorbed into the negative electrode active material layer.

[0041] The step of pre-doping the negative electrode with lithium ions may be carried out before assembling the electrode group, or the pre-doping may be carried out after the electrolyte and the electrode group are housed in a case of a lithium ion capacitor.

[0042] (separator) The separator is not particularly limited, and separators used in known lithium ion capacitors may be used. The separator may be a woven fabric, nonwoven fabric, or microporous membrane made of an insulating material. Examples of materials for the woven fabric and nonwoven fabric include cellulose fiber, glass fiber, and polyolefin. Materials for the microporous membrane include polyolefin. The thickness of the separator may be in the range of 10 to 300 μm (e.g., 10 to 40 μm).

[0043] (exterior body) A lithium ion capacitor includes an electrode group and an exterior housing that houses an electrolyte. The electrode group is composed of a positive electrode, a negative electrode, and a separator. There are no particular limitations on the exterior housing, and exterior housings used in known lithium ion capacitors may be used. The exterior housing may be formed of, for example, metal and / or resin. The exterior housing of the capacitor may include a metal case and a sealing member. In the case of a cylindrical capacitor, a cylindrical metal case with a bottom may be used. In the case of a rectangular tubular capacitor, a rectangular tubular metal case with a bottom may be used. Alternatively, the exterior housing of the capacitor may be a laminate type exterior housing.

[0044] The exterior body may or may not include a gas vent valve. The gas vent valve is a valve for releasing gas inside the exterior body when the pressure inside the exterior body rises excessively. There are no particular limitations on the gas vent valve, and gas vent valves used in known lithium ion secondary batteries may be used. The capacitor of this embodiment uses additive (A), so gas generation is suppressed. Therefore, the capacitor of this embodiment is preferably used in lithium ion capacitors that do not have a gas vent valve.

[0045] When aging (pre-doping of lithium ions) is performed after housing an electrode group and an electrolyte solution in an exterior body, a large amount of gas is generated in conventional lithium ion secondary batteries and conventional lithium ion capacitors, so exterior bodies having a gas vent valve have been preferably used. On the other hand, in the capacitor according to the present disclosure, gas generation during aging can be suppressed even when aging (pre-doping of lithium ions) is performed after housing an electrode group and an electrolyte solution in an exterior body.

[0046] There are no particular limitations on the configuration of the electrode group of the lithium ion capacitor. The electrode group may be a wound type or a stacked type. A wound type electrode group is formed by winding a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. A stacked type electrode group is formed by stacking a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode.

[0047] An example of a lithium ion capacitor according to the present disclosure will be specifically described below with reference to the drawings. The components described above can be applied to the components of the example capacitor described below. Furthermore, the components of the example capacitor described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the embodiment described below, components that are not essential for the capacitor of the present disclosure may be omitted.

[0048] (Embodiment 1) The lithium ion capacitor of the first embodiment will be described with reference to Fig. 1. Fig. 1 is a perspective view of the lithium ion capacitor with a portion cut away.

[0049] The lithium ion capacitor 10 in FIG. 1 includes a wound electrode group 1. The electrode group 1 is formed by winding a positive electrode 2 and a negative electrode 3 with a separator 4 disposed between them. The positive electrode 2, the negative electrode 3, and the separator 4 are each strip-shaped. The positive electrode 2 and the negative electrode 3 each include a metal current collector and an active material layer carried on the surface of the current collector. Lead wires 5a and 5b are connected to the positive electrode 2 and the negative electrode 3, respectively, as lead members.

[0050] The electrode group 1 is housed in a cylindrical exterior case (metal case) 6 together with an electrolyte (not shown). An additive (A) is dissolved in the electrolyte. The opening of the exterior case 6 is sealed with a sealing member 7. Lead wires 5a and 5b are led out to the outside so as to pass through the sealing member 7. The exterior case 6 and the sealing member 7 constitute an exterior body. [Example]

[0051] The capacitor according to the present disclosure will be described in more detail below based on examples.

[0052] Example 1 In Example 1, as will be described below, several types of capacitors (lithium ion capacitors) were manufactured and evaluated by changing the non-aqueous solvent and the electrolyte solution.

[0053] (Capacitor A1) The capacitor A1 was fabricated in the following manner. (1) Preparation of the positive electrode

[0054] A positive electrode slurry was prepared by dispersing 88 parts by weight of activated carbon particles, 2 parts by weight of polytetrafluoroethylene (PTFE, binder), 4 parts by weight of carboxymethyl cellulose (thickener), and 6 parts by weight of acetylene black (conductive material) in water. The activated carbon particles had a specific surface area of ​​2000 m. 2 / g and an average particle size of 0.5 μm.

[0055] The obtained slurry was applied to both sides of a positive electrode current collector (aluminum foil) to form a coating film. The coating film was dried and rolled to form a positive electrode active material layer on both sides of the positive electrode current collector. In this way, a positive electrode was obtained.

[0056] (2) Preparation of the negative electrode A 20 μm thick copper foil was prepared as a negative electrode current collector. A negative electrode slurry containing a negative electrode mixture and water in a mass ratio of 40:60 was prepared. The negative electrode mixture was a mixture of 90 parts by mass of hard carbon, 5 parts by mass of Ketjen black, 1.5 parts by mass of carboxymethyl cellulose, and 3 parts by mass of styrene butadiene rubber. Next, the negative electrode slurry was applied to both sides of the negative electrode current collector to form a coating. The coating was dried to form a negative electrode active material layer (35 μm thick) on both sides of the negative electrode current collector. In this way, a negative electrode was obtained.

[0057] Next, a metallic lithium foil was attached to the negative electrode active material layer in an amount calculated so that the negative electrode potential in the electrolyte after the pre-doping was 0.2 V or less relative to metallic lithium.

[0058] (3) Preparation of electrode groups Lead tabs were connected to the positive and negative electrodes, respectively. Next, the positive and negative electrodes and a separator were wound together to form an electrode assembly. A nonwoven fabric (thickness: 35 μm) made of cellulose fiber was used as the separator.

[0059] (4) Preparation of electrolyte Under a dry argon atmosphere, lithium bis(fluorosulfonylimide) (LIFSI) was dissolved in a non-aqueous solvent at a concentration of 1.2 mol / L, and hexamethylene diisocyanate (additive (A)) was added to prepare an electrolyte solution. The hexamethylene diisocyanate was added so that its concentration in the electrolyte solution was 0.1 mass%. The non-aqueous solvent used was a mixed solvent of 50% by volume of propylene carbonate (PC) and 50% by volume of ethyl methyl carbonate (EMC).

[0060] (5) Fabrication of lithium ion capacitor The electrode group and the electrolyte were placed in a cylindrical container with a bottom, and a lithium ion capacitor as shown in Figure 1 was assembled. Then, a charging voltage of 3.8 V was applied between the positive and negative electrode terminals, and the capacitor was aged at 25°C for 24 hours to allow pre-doping of lithium ions into the negative electrode. Capacitor A1 was thus produced.

[0061] (Capacitors A2 to A13, C1 to C6) Capacitors A2 to A13 and C1 to C6 were fabricated under the same conditions and by the same method as for the fabrication of capacitor A1, except that the electrolyte solution was changed. Specifically, the composition of the solvent in the electrolyte solution and the type and concentration of the additive were changed as shown in Table 1.

[0062] (Evaluation of float characteristics) The resulting lithium-ion capacitor was evaluated for float characteristics using the following method. First, the cell was charged at a voltage of 3.8 V in a 25°C environment, and then discharged at a current of 1.0 A until the discharge voltage reached 2.2 V. During the discharge, the amount of discharge charge flowing while the discharge voltage dropped from 3.3 V to 3.0 V was measured. The amount of discharge charge was then divided by the voltage change ΔV (=0.3 V) to determine the initial capacity C0 (F).

[0063] Next, the cell was continuously charged for 1000 hours at 85°C and 3.8V. After that, it was discharged at a current of 1.0A until the discharge voltage reached 2.2V. During the discharge, the amount of discharge charge flowing while the discharge voltage dropped from 3.3V to 3.0V was measured. The amount of discharge charge was then divided by the voltage change ΔV (=0.3V) to obtain the capacity C1 (F). The float characteristic was then calculated using the following formula: Float characteristic (%) = 100 x C1 / C0

[0064] The float characteristic is the ratio of the capacity C1 after continuous charging to the initial capacity C0. The larger this value (closer to 100%), the better the float characteristic. The evaluation results of the float characteristic are shown in Table 1. Capacitors A2 to A13 are capacitors of the present disclosure, and C1 to C6 are capacitors of comparative examples.

[0065] [Table 1]

[0066] As shown in Table 1, capacitors A1 to A13, in which the concentration of additive (A) in the electrolyte was 0.01 mass% or more and less than 3.0 mass% (for example, 0.1 to 2.0 mass%), had good float characteristics. On the other hand, capacitors C1 to C6 had significantly reduced float characteristics. When the concentration of additive (A) was in the range of 0.1 to 0.5 mass%, the float characteristics were particularly high. In capacitor C3, in which a large amount of additive (A) was added, the initial internal resistance increased, which may have reduced the float characteristics. The initial capacitance C0 of capacitors A1 to A13 was all sufficiently high.

[0067] Compared with capacitors containing ethylene carbonate as a cyclic carbonate, capacitors containing only propylene carbonate as a cyclic carbonate had higher floatability.Compared with capacitors containing ethyl methyl carbonate as a chain carbonate, capacitors containing only ethyl methyl carbonate as a chain carbonate had higher floatability.

[0068] As is clear from capacitors A5 to A8, capacitors with a volume ratio of chain carbonate to cyclic carbonate in the range of 3:7 to 7:3 had particularly high float characteristics. The reason for this is not clear, but in the case of capacitor A7, there may be a large amount of unsolvated chain carbonate, which may have resulted in increased gas generation. In the case of capacitor A8, the viscosity of the nonaqueous solvent is high, which increases the internal resistance of the capacitor. This is thought to be why the capacitance decreases and the float characteristics deteriorate.

[0069] Example 2 In Example 2, several types of lithium ion capacitors were fabricated under the same conditions and by the same method as for Capacitor A2, except that the specific surface area and average particle size of the activated carbon used in the positive electrode were changed. The float characteristics of the fabricated capacitors were evaluated by the same method as in Example 1. Table 2 shows the physical properties of the activated carbon used, the type and amount of additive, and the float characteristics. As shown in Table 1, the nonaqueous solvent was a mixed solvent of propylene carbonate (50 vol%) and ethyl methyl carbonate (50 vol%).

[0070] [Table 2]

[0071] As shown in Table 2, the specific surface area of ​​activated carbon is set to 1200 to 2500 m 2 / g range (e.g., 1350-2300m 2 / g), high float characteristics were achieved. The initial capacitances C0 of the capacitors A2, A14 and A15 were higher than the initial capacitances C0 of the capacitors C7 and C8. [Industrial Applicability]

[0072] The present disclosure can be used in lithium ion capacitors. [Explanation of symbols]

[0073] 1: Electrode group 2: Positive electrode 3: Negative electrode 4: Separator 5a: Lead wire 5b: Lead wire 6: Outer case 7: Sealing member 10: Lithium ion capacitor

Claims

1. positive electrode, negative electrode, a separator disposed between the positive electrode and the negative electrode; and A lithium ion capacitor comprising an electrolyte solution containing a non-aqueous solvent and a lithium salt, The positive electrode has a specific surface area of ​​1200 to 2500 m ２ / g of activated carbon particles, the electrolyte solution contains an additive containing an isocyanate group, The concentration of the additive in the electrolytic solution is 0.01% by mass or more and less than 3.0% by mass, the non-aqueous solvent contains a chain carbonate and a cyclic carbonate, In the non-aqueous solvent, the mixing ratio of the chain carbonate to the cyclic carbonate is in the range of 3:7 to 7:3 on a volume basis, the cyclic carbonate comprises propylene carbonate and is substantially free of ethylene carbonate; The additive comprises a hydrocarbon group having a carbon number in the range of 3 to 10 and two isocyanate groups bonded to the hydrocarbon group.

2. 2. The lithium ion capacitor according to claim 1, wherein the activated carbon particles have an average particle size in the range of 0.5 μm to 6 μm.

3. 3. The lithium ion capacitor according to claim 1, wherein the chain carbonate contains ethyl methyl carbonate and is substantially free of diethyl carbonate and dimethyl carbonate.

4. The additive is at least one selected from the group consisting of trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, and heptamethylene diisocyanate. The lithium ion capacitor according to any one of claims 1 to 3.

5. The lithium ion capacitor according to any one of claims 1 to 4, wherein the concentration of the additive in the electrolyte solution is in the range of 0.1% by mass to 2.0% by mass.

6. The lithium ion capacitor according to any one of claims 1 to 5, wherein the negative electrode contains hard carbon having an interlayer distance d(002) of 0.38 nm or more determined from the results of X-ray diffraction measurement.

7. Including an outer casing, The lithium ion capacitor according to any one of claims 1 to 6, wherein the exterior body does not include a gas vent valve.

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