JP7784643B2 - lithium-ion capacitor - Google Patents

Description

This disclosure relates to lithium ion capacitors.

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 greater, has an atomic group (X) with at least one non-polymerizable double bond, and has an ion-dissociating functional group (b), and does not have a polymerizable unsaturated bond." 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.

International Publication No. 2015/163254

Currently, there is a demand for improving the float characteristics of lithium ion capacitors. In this situation, one of the objectives of this disclosure is to provide a lithium ion capacitor with good float characteristics.

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 includes activated carbon particles having a specific surface area in the range of 1200 to 2500 m ² /g, and the electrolyte solution includes 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.

This disclosure provides a lithium-ion capacitor with excellent float characteristics.

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.

The following describes examples of embodiments of the present disclosure, 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 "greater than or equal to numerical value A and less than or equal to numerical value B." In the following description, when lower and upper limits are given for numerical values of specific physical properties or conditions, any of the exemplified lower limits can be combined with any of the exemplified upper limits, as long as the lower limit is not greater than or equal to the upper limit.

(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 nonaqueous solvent and a lithium salt. The positive electrode includes activated carbon particles having a specific surface area in the range of 1200 to 2500 m ² /g. The electrolyte solution includes 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 below as “additive (A).” Hereinafter, the lithium ion capacitor may be simply referred to as “capacitor.” The components of the lithium ion capacitor according to this embodiment are described below.

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. However, 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 can increase the internal pressure of the capacitor.

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

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, thereby generating capacity.

The negative electrode contains a negative electrode active material that can be reversibly doped and dedoped with 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 generated when the Faraday reaction, in which lithium ions are absorbed into the negative electrode active material, progresses.

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.

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

The average particle size of the activated carbon particles may be in the range of 0.5 μm to 6 μm (e.g., 1 μm to 5 μm). Setting the average particle size in this range makes it easier to control the specific surface area within the above range. The activated carbon particles may be commercially available or may be manufactured using known methods. Activated carbon particles with a variety of 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 reaches 50% in the volume-based particle size distribution. The median diameter is determined using a laser diffraction/scattering particle size distribution analyzer.

Activated carbon particles may be produced by heat-treating raw materials to carbonize them, 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 processes 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. Pulverization may be performed using a ball mill, jet mill, or the like.

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, rubber, and cellulose derivatives. Examples of fluororesins include polyvinylidene fluoride, polytetrafluoroethylene, and tetrafluoroethylene-hexafluoropropylene copolymers. Examples of acrylic resins include polyacrylic acid and acrylic acid-methacrylic acid copolymers. Examples of rubber include styrene-butadiene rubber. Examples of cellulose derivatives include carboxymethyl cellulose.

The positive electrode active material layer may contain a conductive material. Examples of conductive materials 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.

The activated carbon content 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. This content may be 98% by mass or less, or 90% by mass or less. This content may be in the range of 60 to 98% by mass (e.g., 60 to 90% by mass). The conductive material content 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 binder content in the positive electrode active material layer may be in the range of 1 to 10% by mass.

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 foil, porous metal, and etched metal. 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. Roughening may be performed by a known method (e.g., electrolytic etching).

There are no particular limitations on the method for manufacturing the positive electrode, and it may be manufactured using known methods. The positive electrode may be manufactured using the following method. First, the materials that make up the positive electrode active material layer are mixed with a dispersion medium (water or an organic solvent) to obtain a slurry. If necessary, a dispersant may be added to the slurry. Examples of dispersants include carboxymethyl cellulose (CMC). Next, the obtained slurry is applied to a current collector to form a coating film. If necessary, the coating film is then dried and/or rolled. In this way, a positive electrode including a positive electrode active material layer is obtained. There are no particular limitations on the thickness of the positive electrode active material layer, and it may be in the range of 10 μm to 300 μm.

(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.

(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), it is thought that excess metal impurities are prevented from entering, and the absence of an ion dissociative functional group increases reactivity, making it easier for a coating of isocyanate groups to form on the surface of the activated carbon particles of the positive electrode.

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.

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., 3 to 8, 3 to 7, or 4 to 6). Additive (A) may be at least one selected from the group consisting of trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, and heptamethylene diisocyanate. High effectiveness can be achieved by using these additives. A preferred example of additive (A) is hexamethylene diisocyanate.

The concentration (content) of additive (A) in the electrolyte 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. This 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 additive (A) in the electrolyte solution is preferably in the range of 0.1% by mass to 2.0% by mass (e.g., 0.1 to 1.0% by mass or 0.1 to 0.5% by mass). This range provides better effects.

(lithium salt)
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.

Examples of lithium salts include LiN( _SO2F ) ₂ , LiClO4, _LiBF4 , _LiPF6 , _LiAlCl4 , _LiSbF6 , LiSCN, _LiCF3SO3 , _LiFSO3 , _LiCF3CO2 , _LiAsF6 , _LiB10Cl10 , 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 (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and lithium bis(fluorosulfonyl)imide (LiN(SO ₂ F) ₂ ). These lithium salts have a high degree of dissociation and excellent ionic conductivity. Hereinafter, lithium bis(fluorosulfonyl)imide (LiN(SO ₂ F) ₂ ) may be referred to as "LIFSI." The lithium salt preferably contains LIFSI, or LIFSI alone may be used as the lithium salt.

(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.

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. Other examples of non-aqueous solvents include dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, trimethoxymethane, sulfolane, methyl sulfolane, and 1,3-propane sultone.

The nonaqueous 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 of them. 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 by volume.
(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 electrolyte 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 electrolyte 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 electrolyte solution may be composed solely of ethyl methyl carbonate.

If necessary, additives other than 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 with high lithium ion conductivity on the surface of the negative electrode.

(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.

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 whose interlayer distance d(002) determined from the results of X-ray diffraction measurement is 0.38 nm or greater. X-ray diffraction measurement is performed using CuKα radiation. Hard carbon is preferred because it has lower resistance and higher capacity than graphite.

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 metal include metal foil, porous metal, and etched metal. Examples of metal include copper, copper alloy, nickel, and stainless steel. The thickness of the negative electrode current collector may be in the range of 10 to 100 μm.

The negative electrode active material layer may contain a binder, a conductive material, and the like. There are no particular limitations on the binder and conductive material; 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.

There are no particular limitations on the method for manufacturing the negative electrode, and it may be manufactured by any known method. The negative electrode may be manufactured by the following method. First, the materials that make up the negative electrode active material layer are 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 are no particular limitations on the thickness of the negative electrode active material layer, and it may be in the range of 10 μm to 300 μm.

It is desirable to pre-dope the negative electrode active material layer with lithium ions. This lowers 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., the voltage), and improving the energy density of the lithium-ion capacitor.

Pre-doping of lithium ions into the negative electrode may be performed using 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 with the metallic lithium layer is impregnated with an electrolyte solution with lithium ion conductivity. During this process, 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 allows 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.

The process of pre-doping the negative electrode with lithium ions may be carried out before assembling the electrode group. Alternatively, the pre-doping process may be carried out after the electrolyte and electrode group are placed in the lithium ion capacitor case.

(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).

(Exterior body)
The 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 capacitor exterior housing 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 capacitor exterior housing may be a laminate type exterior housing.

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), which suppresses gas generation. Therefore, the capacitor of this embodiment is preferably used in lithium ion capacitors that do not have a gas vent valve.

When aging (pre-doping of lithium ions) is performed after the electrode group and electrolyte solution are housed in an exterior case, conventional lithium ion secondary batteries and conventional lithium ion capacitors generate a large amount of gas, so exterior cases with gas vent valves have been preferred. In contrast, with the capacitor disclosed herein, gas generation during aging can be suppressed even when aging (pre-doping of lithium ions) is performed after the electrode group and electrolyte solution are housed in an exterior case.

There are no particular limitations on the configuration of the electrode group of a lithium ion capacitor. The electrode group may be wound or stacked. A wound 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 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.

Below, an example of a lithium ion capacitor according to the present disclosure will be described in detail 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 to the capacitor of the present disclosure may be omitted.

(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.

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, negative electrode 3, and separator 4 are each strip-shaped. The positive electrode 2 and 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 negative electrode 3, respectively, as lead-out members.

The electrode group 1 is housed in a cylindrical outer case (metal case) 6 together with an electrolyte (not shown). An additive (A) is dissolved in the electrolyte. The opening of the outer case 6 is sealed with a sealing member 7. Lead wires 5a and 5b extend to the outside, passing through the sealing member 7. The outer case 6 and sealing member 7 form the exterior body.

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

Example 1
In Example 1, as will be described below, a plurality of types of capacitors (lithium ion capacitors) were manufactured and evaluated by changing the nonaqueous solvent and the electrolyte solution.

(Capacitor A1)
The capacitor A1 was fabricated by the following method.
(1) Preparation of the positive electrode

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 used had a specific surface area of 2000 ^m /g and an average particle size of 0.5 μm.

The resulting slurry was applied to both sides of a positive electrode current collector (aluminum foil) to form a coating. The coating was then 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.

(2) Preparation of 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 manner, a negative electrode was obtained.

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

(3) Preparation of Electrode Assembly 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.

(4) Preparation of Electrolyte Solution 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%. A mixed solvent of 50% by volume of propylene carbonate (PC) and 50% by volume of ethyl methyl carbonate (EMC) was used as the non-aqueous solvent.

(5) Preparation 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. In this way, capacitor A1 was prepared.

(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.

(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 an environment of 25° C., 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 decreased 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 C ₀ (F).

Next, the cell was continuously charged for 1000 hours under conditions of 85°C and 3.8 V. Thereafter, it was discharged at a current of 1.0 A until the discharge voltage reached 2.2 V. During this 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 capacity _C1 (F). The float characteristic was then calculated using the following formula:
Float characteristic (%) = 100 × C ₁ /C ₀

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.

As shown in Table 1, capacitors A1 to A13, in which the concentration of additive (A) in the electrolyte solution was 0.01 mass% or more and less than 3.0 mass% (e.g., 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, which had a large amount of additive (A), the initial internal resistance increased, possibly reducing the float characteristics. The initial capacitance _C0 of capacitors A1 to A13 was all sufficiently high.

Compared to capacitors containing ethylene carbonate as a cyclic carbonate, capacitors containing only propylene carbonate as a cyclic carbonate had better float characteristics.Compared to capacitors containing ethyl methyl carbonate as a chain carbonate, capacitors containing only ethyl methyl carbonate as a chain carbonate had better float characteristics.

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 led to increased gas generation. In the case of capacitor A8, the viscosity of the non-aqueous 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.

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%).

As shown in Table 2, high float characteristics were achieved by setting the specific surface area of the activated carbon in the range of 1200 to 2500 ^{m 2} /g (for example, in the range of 1350 to 2300 m ² /g). The initial capacitances C ₀ of capacitors A2, A14, and A15 were higher than the initial capacitances C ₀ of capacitors C7 and C8.

This disclosure can be used in lithium ion capacitors.

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 (7)

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 contains activated carbon particles having a specific surface area in the range of 1200 to 2500 m ² /g;
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 . 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. The lithium ion capacitor according to claim 1 , wherein the chain carbonate comprises ethyl methyl carbonate and is substantially free of diethyl carbonate and dimethyl carbonate. The lithium ion capacitor according to any one of claims 1 to 3 , wherein 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 4 , wherein the concentration of the additive in the electrolytic solution is in the range of 0.1% by mass to 2.0% by mass. 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 as determined from the results of X-ray diffraction measurement. 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.