JP2008060479A - Lithium ion capacitor - Google Patents

Abstract

A lithium ion capacitor having a high energy density is provided.
A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, wherein the positive electrode active material can reversibly carry lithium ions and / or anions. And the negative electrode active material is a material capable of reversibly supporting lithium ions, and the negative electrode and / or the negative electrode active material so that the positive electrode potential after the positive electrode and the negative electrode are short-circuited is 2.0 V (vs. Li / Li ⁺ ) or less. Alternatively, the lithium ion capacitor is characterized in that lithium ions are doped in advance with respect to the positive electrode, and the positive electrode active material is activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more.
[Selection figure] None

Description

  The present invention relates to a lithium ion capacitor including a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte containing a lithium salt as an electrolyte.

In recent years, a so-called lithium ion secondary battery using a carbon material such as graphite as a negative electrode and a lithium-containing metal oxide such as LiCoO _{2 as} a positive electrode has a high capacity and is an effective power storage device. It has been put to practical use as the main power source for telephones. The lithium ion secondary battery is a so-called rocking chair type battery in which lithium ions are supplied to the negative electrode from the lithium-containing metal oxide of the positive electrode by charging after the battery is assembled, and the lithium ion of the negative electrode is returned to the positive electrode in the discharge. Yes, it features high voltage and high capacity.

  Until now, lead batteries have been used as power storage devices for automobiles. However, while environmental problems are being highlighted, power storage devices (main power supply and auxiliary power supply) for electric vehicles or hybrid vehicles that replace gasoline vehicles are being used. Development is actively underway. However, with the enhancement of in-vehicle electrical equipment and equipment, new power storage devices are being demanded in terms of energy density and output density.

  As such a new power storage device, the above lithium ion secondary battery and electric double layer capacitor have attracted attention. However, although the lithium ion secondary battery has a high energy density, there are still problems in output characteristics, safety and cycle life. On the other hand, the electric double layer capacitor is used as a power supply for memory backup of IC and LSI, but the discharge capacity per charge is smaller than that of a general battery. However, it has excellent output characteristics and maintenance-free characteristics that are excellent in instantaneous charge / discharge characteristics and withstands charge / discharge of tens of thousands of cycles or more, which is not possible with lithium ion secondary batteries.

  Although the electric double layer capacitor has such advantages, the energy density of the conventional general electric double layer capacitor is about 3 to 4 Wh / l, which is about two orders of magnitude smaller than that of the lithium ion secondary battery. When considering the use for electric vehicles, it is said that an energy density of 6 to 10 Wh / l is required for practical use and 20 Wh / l is necessary for spreading.

In recent years, a power storage device called a hybrid capacitor, which combines the power storage principles of a lithium ion secondary battery and an electric double layer capacitor, has attracted attention as a power storage device corresponding to applications requiring such high energy density and high output characteristics. In hybrid capacitors, a polarizable electrode is usually used for the positive electrode and a non-polarizable electrode is used for the negative electrode, which is attracting attention as a power storage device that combines high energy density of the battery and high output characteristics of the electric double layer. . On the other hand, in this hybrid capacitor, a negative electrode capable of inserting and extracting lithium ions is brought into contact with metallic lithium, and lithium ions are stored and supported (hereinafter also referred to as dope) by a chemical method or an electrochemical method in advance. Capacitors intended to increase the withstand voltage and greatly increase the energy density by lowering the potential have been proposed. (See Patent Document 1 to Patent Document 4)
This type of hybrid capacitor is expected to have high performance, but when doping lithium ions to the negative electrode, it takes a very long time to dope and has a problem with uniform doping to the whole negative electrode. It has been considered difficult to put into practical use in a large-capacity cell such as a cylindrical device in which electrodes are wound or a square battery in which a plurality of electrodes are stacked.

  However, this problem is that the negative electrode current collector and the positive electrode current collector constituting the cell are provided with holes penetrating the front and back, and lithium ions are moved through the through holes, and at the same time, lithium metal and lithium as a lithium ion supply source. By short-circuiting the battery, it was possible to dope lithium ions to all the negative electrodes in the cell only by arranging metallic lithium at the end of the cell. In addition, although dope of lithium ion is normally performed with respect to a negative electrode, it is described in patent document 5 that it is the same also when performing with a negative electrode with a negative electrode instead of a negative electrode.

Thus, even in a large-sized cell such as a cylindrical device in which electrodes are wound or a square battery in which a plurality of electrodes are stacked, lithium ions are uniformly distributed over the entire negative electrode in a short time with respect to all the negative electrodes in the device. It was possible to dope and to improve the withstand voltage, the energy density dramatically increased, and the high output density inherent in the electric double layer capacitor was expected to realize a high capacity capacitor.
However, in order to put such a high-capacity capacitor into practical use, it is further required to have a high capacity, a high energy density, and a high output density.
Japanese Patent Laid-Open No. 8-1007048 JP-A-9-55342 Japanese Patent Laid-Open No. 9-232190 JP-A-11-297578 International Publication WO98 / 033227

  In the present invention, the positive electrode active material is a material capable of reversibly supporting lithium ions and / or anions, and the negative electrode active material is a material capable of reversibly supporting lithium ions. An object of the present invention is to provide an improved capacitor capable of realizing a higher energy density value in a lithium ion capacitor that is electrochemically contacted with an ion supply source and previously doped with lithium ions in a negative electrode.

In order to solve the above problems, the present inventors have conducted intensive research. As a result, the negative electrode and / or the negative electrode and / or the positive electrode potential after being short-circuited between the positive electrode and the negative electrode is 2.0 V (vs. Li / Li ⁺ ) or less. Or, in a lithium ion capacitor in which lithium ions are pre-doped to the positive electrode, the physical properties of the activated carbon used as the positive electrode active material are related to the energy density of the obtained capacitor, and the positive electrode active material is phenolic. The present inventors have found that the above problems can be solved by forming from activated carbon particles having a hydroxyl functional group of 0.2 mmol / g or more, and have reached the present invention.

Thus, the present invention is characterized by having the following gist.
(1) A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, wherein the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions. And the negative electrode active material is a material capable of reversibly supporting lithium ions, and the negative electrode and / or the negative electrode and / or the positive electrode potential after being short-circuited between the positive electrode and the negative electrode is 2.0 V (vs. Li / Li ⁺ ) or less. A lithium ion capacitor, wherein the positive electrode is pre-doped with lithium ions, and the positive electrode active material is activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more.
(2) The positive electrode and / or the negative electrode each provided with a current collector having holes penetrating the front and back surfaces, and lithium ions are doped by electrochemical contact between the negative electrode and a lithium ion supply source ( The lithium ion capacitor as described in 1).
(3) The negative electrode active material has a capacitance per unit weight of 3 times or more as compared with the positive electrode active material, and the positive electrode active material weight is larger than the negative electrode active material weight (1) or The lithium ion capacitor according to (2).
(4) The lithium ion capacitor according to any one of claims 1 to 3, wherein the activated carbon particles are subjected to an alkali activation treatment and have a specific surface area of 600 to 3000 m ² / g.
(5) The lithium ion capacitor according to any one of (1) to (4), wherein an average particle diameter (D50) of the activated carbon particles is 1 μm or more.
(6) The lithium ion capacitor according to any one of (1) to (5), wherein the activated carbon particles are phenol resin activated carbon, petroleum pitch activated carbon, petroleum coke activated carbon, or coal coke activated carbon.

According to the present invention, there is provided a capacitor having a high energy density, in particular, a large-capacity capacitor in which lithium ions are doped into the negative electrode and / or the positive electrode in advance. When the same activated carbon is used for both electrodes, the capacity of the entire cell is C / 2 from C _{(positive electrode)} ≈ C _{(negative electrode)} , and the total capacity is only half of one electrode, but by using the present invention, Since 1 / C _{(positive electrode)} >> 1 / C _{( negative electrode)} , C _(whole) ≈ C _{(positive electrode)} is approached, and the capacity of the positive electrode activated carbon becomes the capacity of the entire cell. In the present invention, it is not necessarily clear why the capacitor obtained by forming a positive electrode from activated carbon particles having a phenolic hydroxyl group of 0.2 mmol / g or more has a high energy density. Is estimated as follows.

  That is, it is generally said that activated carbon with a large amount of acidic functional groups has many edge surfaces involved in the reaction. When a lithium ion capacitor using positive activated carbon with a large amount of functional groups is charged and discharged, it is considered that some reversible chemical reaction occurs on the edge surface and the capacity increases. One of the chemical reactions may be that the capacity is improved by the ketoenol reaction of acidic functional groups attached to the edge surface. Moreover, since the wettability with respect to electrolyte solution improved when the functional group existed on the activated carbon surface, electrolyte solution penetrate | infiltrated into the pore which was insufficiently impregnated by wettability improvement, and it is thought that the capacity | capacitance improved.

  In the present invention, “supporting” also means dope, occlusion, or insertion, and refers to a phenomenon in which lithium ions or anions enter the positive electrode active material, or a phenomenon in which lithium ions enter the negative electrode active material.

  The lithium ion capacitor of the present invention includes a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, and the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions. In addition, the negative electrode active material is a material capable of reversibly supporting lithium ions. Here, the “positive electrode” is an electrode on the side where current flows out during discharge, and the “negative electrode” is an electrode on the side where current flows in during discharge.

In the lithium ion capacitor of the present invention, it is necessary that the positive electrode potential after the positive electrode and the negative electrode are short-circuited by doping lithium ions to the negative electrode and / or the positive electrode is 2.0 V (vs. Li / Li ⁺ ) or less. is there. In a capacitor that is not doped with lithium ions with respect to the negative electrode and / or the positive electrode, the potential of the positive electrode and the negative electrode is both about 3 V (vs. Li / Li ⁺ ), and before charging, after the positive electrode and the negative electrode are short-circuited The potential of the positive electrode is about 3 V (vs. Li / Li ⁺ ). In the present invention, the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2.0 V (vs. Li / Li ⁺ ) or less. (A) After doping with lithium ions, the positive electrode terminal and the negative electrode of the capacitor cell After leaving the terminal directly coupled with a lead wire for 12 hours or more, the short circuit is released, and the positive electrode potential measured within 0.5 to 1.5 hours, or (B) 12 hours with a charge / discharge tester Discharge at a constant current to 0 V (vs. Li / Li ⁺ ) over the above, leave the positive electrode terminal and negative electrode terminal for 12 hours or more in a state where they are connected with a conductive wire, and then release the short circuit, within 0.5 to 1.5 hours The positive electrode potential measured by either of the two methods (A) or (B) is 2.0 V (vs. Li / Li ⁺ ) or less.

In the present invention, the positive electrode potential after short-circuiting the positive electrode and the negative electrode is 2.0 V (vs. Li / Li ⁺ ) or less is not limited to just after the lithium ions are doped, The positive electrode potential after a short circuit is 2.0 V (vs. Li / Li ⁺ ) or less in any state, such as when a short circuit occurs after repeated charging, discharging or charging / discharging.

In the present invention, the fact that the positive electrode potential after the positive electrode and the negative electrode are short-circuited is 2.0 V (vs. Li / Li ⁺ ) or less will be described in detail below. As described above, activated carbon and carbon materials usually have a potential of about 3 V (vs. Li / Li ⁺ ), and when the cells are assembled using activated carbon for both the positive and negative electrodes, both potentials are about 3 V. Even if a short circuit occurs, the positive electrode potential is about 3 V regardless. The same applies to so-called hybrid capacitors using activated carbon for the positive electrode and carbon materials such as graphite and non-graphitizable carbon used in lithium ion secondary batteries for the negative electrode. Since it becomes 3V (vs. Li / Li ⁺ ), the positive electrode potential is about 3V even if short-circuited. Although depending on the weight balance between the positive electrode and the negative electrode, when charged, the potential of the negative electrode transitions to around 0 V, so that the charging voltage can be increased, so that the capacitor has a high voltage and a high energy density. Generally, the upper limit of the charging voltage is determined to be a voltage at which the electrolyte solution does not decompose due to the increase in the positive electrode potential. Therefore, when the positive electrode potential is set as the upper limit, the charging voltage can be increased by the amount of decrease in the negative electrode potential. It is. However, in the above hybrid capacitor in which the positive electrode potential is about 3 V (vs. Li / Li ⁺ ) at the time of short circuit, when the upper limit potential of the positive electrode is 4.0 V (vs. Li / Li ⁺ ), the positive electrode potential at the time of discharge is It is up to 3.0 V (vs. Li / Li ⁺ ), and the potential change of the positive electrode is about 1.0 V, and the capacity of the positive electrode cannot be fully utilized. Furthermore, when lithium ions are inserted (charged) and desorbed (discharged) into the negative electrode, the initial charge / discharge efficiency is often low, and it is known that there are lithium ions that cannot be desorbed during discharge. This is explained when it is consumed in the decomposition of the electrolyte solution on the negative electrode surface or trapped in the structural defect part of the carbon material. In this case, the charge / discharge of the negative electrode is compared with the charge / discharge efficiency of the positive electrode. When the efficiency is lowered and the cell is short-circuited after repeated charging and discharging, the positive electrode potential becomes higher than 3.0 V (vs. Li / Li ⁺ ), and the utilization capacity further decreases. That is, the positive electrode can be discharged from 4.0 V (vs. Li / Li ⁺ ) to 2.0 V (vs. Li / Li ⁺ ), and from 4.0 V (vs. Li / Li ⁺ ) to 3.0 V (vs. Li / Li). When only Li ⁺ ) can be used, it means that only half of the capacity is used, and the voltage is high but not high.

  In order to increase not only the high voltage and high energy density but also the high capacity and energy density of the hybrid capacitor, it is necessary to improve the utilization capacity of the positive electrode.

If the positive electrode potential after the short-circuit is lower than 3.0 V (vs. Li / Li ⁺ ), the use capacity increases and the capacity increases. In order to be 2.0 V (vs. Li / Li ⁺ ) or less, not only the amount charged by charging / discharging of the cell, but also lithium ion from the lithium ion supply source such as metallic lithium to the negative electrode by charging. Is preferred. Since lithium ions are supplied from other than the positive electrode and the negative electrode, when they are short-circuited, the equilibrium potential of the positive electrode, the negative electrode, and the metallic lithium is reached, so both the positive electrode potential and the negative electrode potential are 3.0 V (vs. Li / Li ⁺ ) or less. . As the amount of metallic lithium increases, the equilibrium potential decreases. If the negative electrode material and the positive electrode material change, the equilibrium potential also changes. Therefore, the negative electrode material and the positive electrode material are supported on the negative electrode in consideration of the characteristics of the negative electrode material and the positive electrode material so that the positive electrode potential after the short circuit becomes 2.0 V (vs. Li / Li ⁺ ) or less. It is necessary to adjust the amount of lithium ions.

In the present invention, the capacitor cell is previously doped with lithium ions in the negative electrode and / or the positive electrode, and the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is set to 2.0 V (vs. Li / Li ⁺ ) or less. Since the capacity of use increases, the capacity becomes high and a large energy density can be obtained. As the supply amount of lithium ions increases, the positive electrode potential when the positive electrode and the negative electrode are short-circuited becomes lower and the energy density is improved. In order to obtain a higher energy density, 1.5 V (vs. Li / Li ⁺ ) or less, particularly 1.0 V (vs. Li / Li ⁺ ) or less is more preferable. If the amount of lithium ions supplied to the positive electrode and / or the negative electrode is small, the positive electrode potential becomes higher than 2.0 V (vs. Li / Li ⁺ ) when the positive electrode and the negative electrode are short-circuited, and the energy density of the cell is reduced. In addition, when the positive electrode potential falls below 1.0 V (vs. Li / Li ⁺ ), although depending on the positive electrode active material, problems such as gas generation and irreversible consumption of lithium ions occur. It becomes difficult. On the other hand, when the positive electrode potential becomes too low, the negative electrode weight is excessive, and conversely the energy density decreases. Generally, it is 0.1 V (vs. Li / Li ⁺ ) or higher, preferably 0.3 V (vs. Li / Li ⁺ ) or higher.

  In the present invention, lithium ion doping may be one or both of the negative electrode and the positive electrode. For example, when activated carbon is used for the positive electrode, the lithium ion becomes irreversible when the amount of lithium ion doping increases and the positive electrode potential decreases. May cause problems such as a decrease in cell capacity. For this reason, it is preferable that the lithium ions doped in the negative electrode and the positive electrode do not cause these problems in consideration of the respective electrode active materials. In the present invention, controlling the dope amount of the positive electrode and the dope amount of the negative electrode is complicated in the process, so that doping of lithium ions is preferably performed on the negative electrode.

  In the lithium ion capacitor of the present invention, in particular, the electrostatic capacity per unit weight of the negative electrode active material has more than three times the electrostatic capacity per unit weight of the positive electrode active material, and the positive electrode active material weight is the negative electrode active material When larger than the weight, a capacitor having a high voltage and a high capacity can be obtained. At the same time, when using a negative electrode having a capacitance per unit weight that is larger than the capacitance per unit weight of the positive electrode, the negative electrode active material weight is reduced without changing the potential change amount of the negative electrode. Therefore, the filling amount of the positive electrode active material is increased, and the capacitance and capacity of the cell are increased. The weight of the positive electrode active material is preferably larger than the weight of the negative electrode active material, but more preferably 1.1 times to 10 times. If it is less than 1.1 times, the capacity difference is small, and if it exceeds 10 times, the cell capacity may be small, and the thickness difference between the positive electrode and the negative electrode becomes too large, which is not preferable in terms of the cell structure.

  In the present invention, the capacitance and capacity of a capacitor cell (hereinafter also simply referred to as a cell) are defined as follows. The capacitance of a cell indicates the amount of electricity flowing through the cell per unit voltage of the cell (the slope of the discharge curve), and the unit is F (farad). The capacitance per unit weight of the cell is expressed by dividing the total weight of the positive electrode active material weight and the negative electrode active material weight filled in the cell with respect to the cell capacitance, and the unit is F / g. The electrostatic capacity of the positive electrode or the negative electrode indicates the amount of electricity flowing through the cell per unit voltage of the positive electrode or the negative electrode (the slope of the discharge curve), and the unit is F. The capacitance per unit weight of the positive electrode or the negative electrode is expressed by dividing the positive electrode or negative electrode capacitance in the cell by the weight of the positive electrode or negative electrode active material, and the unit is F / g.

  Furthermore, the cell capacity is the difference between the cell discharge start voltage and the discharge end voltage, that is, the product of the voltage change amount and the cell capacitance, and the unit is C (coulomb). 1C is 1A per second. Since it is the amount of charge when current flows, it is converted into mAh in the present invention. The positive electrode capacity is the product of the difference between the positive electrode potential at the start of discharge and the positive electrode potential at the end of discharge (amount of change in positive electrode potential) and the electrostatic capacity of the positive electrode. The unit is C or mAh. The product of the difference between the negative electrode potential and the negative electrode potential at the end of discharge (negative electrode potential change amount) and the negative electrode capacitance, and the unit is C or mAh. These cell capacity, positive electrode capacity, and negative electrode capacity coincide.

  In the lithium ion capacitor of the present invention, means for doping lithium ions into the negative electrode and / or the positive electrode in advance is not particularly limited. For example, a lithium ion supply source such as metallic lithium capable of supplying lithium ions can be disposed in the capacitor cell as a lithium electrode. The amount of the lithium ion supply source (the weight of metallic lithium or the like) may be as long as a predetermined negative electrode capacity can be obtained. In this case, the negative electrode and the lithium electrode may be in physical contact (short circuit) or may be electrochemically doped. The lithium ion supply source may be formed on a lithium electrode current collector made of a conductive porous body. As the conductive porous body serving as the lithium electrode current collector, a metal porous body that does not react with a lithium ion supply source such as a stainless mesh can be used.

  A large-capacity multilayer capacitor cell is provided with a positive electrode current collector and a negative electrode current collector for receiving and distributing electricity at the positive electrode and the negative electrode, respectively. The positive electrode current collector and the negative electrode current collector are used, and the lithium electrode In the case of a cell provided with a lithium electrode, the lithium electrode is preferably provided at a position facing the negative electrode current collector, and lithium ions are preferably supplied to the negative electrode electrochemically. In this case, as the positive electrode current collector and the negative electrode current collector, for example, a material having holes penetrating the front and back surfaces such as expanded metal is used, and the lithium electrode is disposed so as to face the negative electrode or the positive electrode. The form, number, and the like of the through holes are not particularly limited, and can be set so that lithium ions in the electrolyte described later can move between the front and back of the electrode without being blocked by the electrode current collector.

  In the lithium ion capacitor of the present invention, the lithium ion can be uniformly doped even when the lithium electrode doped in the negative electrode is locally disposed in the cell. Therefore, even in the case of a large-capacity cell in which the positive electrode and the negative electrode are laminated or wound, the lithium electrode can be smoothly and uniformly doped with lithium ions by arranging the lithium electrode in a part of the outermost or outermost cell. .

  As the material of the electrode current collector, various materials generally proposed for lithium batteries can be used, such as aluminum and stainless steel for the positive electrode current collector, stainless steel, copper, Nickel or the like can be used. The lithium supply source arranged in the cell refers to a substance that contains at least a lithium element and can supply lithium ions, such as metallic lithium or a lithium-aluminum alloy.

  The positive electrode active material in the lithium ion capacitor of the present invention is made of a material that can reversibly carry lithium ions and / or anions such as tetrafluoroborate. In the present invention, the positive electrode active material is formed from activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more. When the phenolic hydroxyl functional group is less than 0.2 mmol / g, the target capacitance and energy density cannot be achieved as will be shown later in Comparative Examples. Especially, it is suitable that it is 0.2-0.5 mmol / g. Control of the phenolic hydroxyl functional group in the activated carbon is performed by heat-treating the activated carbon at an arbitrary temperature. Although the temperature in that case changes with kinds of raw materials, heating time, etc., when heating time shall be about 1 to 20 hours normally, it is set to 500-1000 degreeC. The heating atmosphere is performed with an inert gas such as nitrogen gas or argon gas.

  In the present invention, the raw material for the activated carbon particles is preferably phenol resin, petroleum pitch, petroleum coke, coconut husk, or coal coke, but is preferably suitable because phenol resin and coal coke can increase the specific surface area. is there. These raw materials are baked, carbonized, alkali activated, washed, and then pulverized. The carbonization treatment is performed by storing the current material in a heating furnace or the like and heating it for a required time at a temperature at which the raw material is carbonized. Although the temperature in that case changes with kinds of raw materials, heating time, etc., when heating time shall be about 1 to 20 hours normally, it is set to 500-1000 degreeC. The heating atmosphere is preferably an inert gas such as nitrogen gas or argon gas.

  The alkali activator used for activation of the resulting carbide is preferably a salt or hydroxide of an alkali metal ion such as lithium, sodium or potassium, and potassium hydroxide is particularly preferred. Alkaline activation methods include, for example, a method in which a carbide and an activator are mixed and then heated in an inert gas stream, an activated carbon raw material previously loaded with an activator, and then heated for carbonization and activation. And a method in which the carbide is activated by a gas activation method such as water vapor and then surface-treated with an alkali activator.

  When a monovalent base such as potassium hydroxide is used as the alkali activator, the ratio of “carbide: alkali activator” is preferably 1: 1 to 1:10, and more preferably 1: 1-1: 5 is more preferable, and 1: 2 to 1: 4 is particularly preferable. If the ratio of the activator to 1 part by weight of the carbide is less than 1 part by weight, the activation does not proceed sufficiently. On the other hand, if it exceeds 4 parts by weight, the capacitance per volume may decrease.

  The alkali activation temperature is preferably 400 to 800 ° C, more preferably around 600 to 800 ° C. When the activation temperature is less than 400 ° C., the activation does not proceed and the capacitance becomes small. On the other hand, when it exceeds 900 degreeC, an activation rate falls extremely and is unpreferable. The activation time is preferably 1 to 10 hours, and more preferably 1 to 5 hours. When the activation time is less than 1 hour, the internal resistance when used as the positive electrode is increased. On the other hand, when the activation time is longer than 10 hours, the capacitance per unit volume is decreased. Further, after the activation treatment, it is necessary to remove the alkali activator contained in a large amount by sufficiently washing with acid. The method of acid cleaning is not particularly limited, but it is usually necessary to sufficiently remove the alkali component by repeating acid cleaning with hydrochloric acid of about 1 to 3 N at 80 ° C. several times. Furthermore, it is necessary to carry out neutralization washing sufficiently using ammonia water or the like.

The acid activated alkali activated carbon is then pulverized. The pulverization is performed using a known pulverizer such as a ball mill. By such pulverization, a wide range of particle sizes of activated carbon can be used. For example, the 50% volume cumulative diameter (also referred to as D50) is 1 μm or more, preferably 2 to 50 μm, and most preferably 2 to 20 μm. When the particle size is smaller than this, the positive electrode cannot be formed. The average pore diameter is preferably 10 nm or less, and the specific surface area is preferably 600 to 3000 m ² / g. The activated carbon that is the positive electrode active material of the present invention preferably has a specific surface area of 600 m ² / g or more. If it is smaller than this, it expands to twice or more of the volume when charged and discharged as a positive electrode in a cell, so the capacity per volume is halved. Among them, 800 m ² / g or more, and particularly it is preferable that a 1300~2500m ² / g.

  The positive electrode in the present invention is formed from the above activated carbon powder, and the existing means can be used. That is, activated carbon powder, a binder, and optionally a conductive material and a thickener (CMC (carboxymethylcellulose), etc.) are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is used as needed. Alternatively, the slurry may be applied to the electric body, or the slurry may be formed into a sheet shape in advance and attached to the current collector. Examples of the binder used here include rubber-based binders such as SBR, fluorine-containing resins such as polytetrafluoroethylene and polyvinylidene fluoride, thermoplastic resins such as polypropylene and polyethylene, and acrylic copolymers. Can be used.

  Examples of the conductive material used as necessary in the above include acetylene black, graphite, ketjen black, VGCF (vapor grown carbon fiber), metal powder, and the like. The amount of the conductive material used varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, and the like, but it is appropriate to add the conductive material in a proportion of 1 to 40% by weight with respect to the positive electrode active material.

  On the other hand, the negative electrode active material in the present invention is formed from a material capable of reversibly carrying lithium ions. Examples of preferable substances include carbon materials such as graphite, hard carbon, and coke, and polyacene-based substances (hereinafter also referred to as PAS). PAS is obtained by carbonizing a phenol resin or the like, activated as necessary, and then pulverized. The carbonization treatment is performed by housing in a heating furnace or the like and heating for a required time at a temperature at which the phenol resin or the like is carbonized, as in the case of the activated carbon in the positive electrode. Although the temperature in that case changes with heating time etc., it is normally set to 400-800 degreeC. The pulverization step is performed using a known pulverizer such as a ball mill.

  Among these, as the negative electrode active material of the present invention, PAS is more preferable in that a high capacity can be obtained. Capacitance of 650 F / g or more can be obtained by discharging after charging (charging) 400 mAh / g of lithium ions on PAS, and static electricity of 750 F / g or more can be obtained by charging lithium ions of 500 mAh / g or more. Capacitance can be obtained. Since PAS has an amorphous structure and the potential decreases as the amount of lithium ions carried increases, the withstand voltage (charging voltage) of the obtained capacitor increases, and the rate of voltage rise during discharge (the slope of the discharge curve) ) Is low, the capacity is slightly high. Therefore, it is desirable to set the amount of lithium ions within the range of the lithium ion storage capacity of the active material according to the required working voltage of the capacitor.

  In addition, since PAS has an amorphous structure, there is no structural change such as swelling / shrinkage with respect to insertion / extraction of lithium ions, so that cycle characteristics are excellent, and isotropic to insertion / extraction of lithium ions. Since it has a molecular structure (higher order structure), it is suitable for rapid charging and rapid discharging. The aromatic condensation polymer that is a precursor of PAS is a condensate of an aromatic hydrocarbon compound and an aldehyde. As the aromatic hydrocarbon compound, so-called phenols such as phenol, cresol, xylenol and the like can be suitably used. For example, the following formula

(Wherein x and y are each independently 0, 1 or 2), or may be a hydroxy biphenyl or a hydroxynaphthalene. Of these, phenols are preferred.

  As the aromatic condensed polymer, a part of the aromatic hydrocarbon compound having a phenolic hydroxyl group is substituted with an aromatic hydrocarbon compound having no phenolic hydroxyl group, such as xylene, toluene, aniline, etc. Modified aromatic condensation polymers such as a condensate of phenol, xylene and formaldehyde can also be used. Furthermore, a modified aromatic polymer substituted with melamine or urea can be used, and a furan resin is also suitable.

  In the present invention, PAS is produced by gradually heating the above aromatic condensation polymer to a suitable temperature of 400 to 800 ° C. in a non-oxidizing atmosphere (including vacuum). It is preferably an insoluble and infusible substrate having an atomic ratio of hydrogen atoms / carbon atoms (hereinafter referred to as H / C) of 0.5 to 0.05, preferably 0.35 to 0.10.

  According to X-ray diffraction (CuKα), the insoluble and infusible substrate described above has a main peak position represented by 2θ of 24 ° or less, and between 41 and 46 ° in addition to the main peak. There are other broad peaks. That is, the insoluble infusible substrate has a polyacene skeleton structure in which an aromatic polycyclic structure is appropriately developed, has an amorphous structure, and can be stably doped with lithium ions.

The particle size characteristic of the negative electrode active material in the present invention is formed from negative electrode active material particles having a 50% volume cumulative diameter (also referred to as D50) of 0.5 to 30 μm, preferably 0.5 to 15 μm, particularly 0.5-6 micrometers is suitable. Moreover, the negative electrode active material particles of the present invention preferably have a specific surface area of preferably 0.1 to 2000 m ² / g, preferably 0.1 to 1000 m ² / g, and particularly 0.1. ˜600 m ² / g is preferred.

  The negative electrode in the present invention is formed from the above negative electrode active material powder, and the existing means can be used as in the case of the positive electrode. That is, a negative electrode active material powder, a binder, and, if necessary, a conductive material and a thickener (CMC (carboxymethylcellulose), etc.) are dispersed in an aqueous system or an organic solvent to form a slurry, and the slurry is the above-described current collector. Alternatively, the slurry may be formed into a sheet in advance and attached to a current collector. As the binder used here, for example, a rubber-based binder such as SBR, a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, a thermoplastic resin such as polypropylene or polyethylene, an acrylic resin, or the like may be used. it can. The amount of the binder used varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, etc., but it is appropriate to add it at a ratio of 2 to 4% by weight with respect to the negative electrode active material.

  Examples of the aprotic organic solvent forming the aprotic organic solvent electrolyte solution in the lithium ion capacitor of the present invention include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, acetonitrile, and dimethoxy. Examples include ethane, tetrahydrofuran, dioxolane, methylene chloride, sulfolane and the like. Furthermore, a mixed solution in which two or more of these aprotic organic solvents are mixed can also be used.

Any electrolyte can be used as long as it is an electrolyte capable of generating lithium ions as the electrolyte dissolved in the single or mixed solvent. Examples of such an electrolyte include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) _{2 and the} like. The electrolyte and the solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. The concentration of the electrolyte in the electrolyte solution is at least 0.1 mol / l in order to reduce the internal resistance of the electrolyte solution. The above is preferable, and the range of 0.5 to 1.5 mol / l is more preferable.

  In addition, as the lithium ion capacitor of the present invention, in particular, a wound cell in which a strip-like positive electrode and a negative electrode are wound through a separator, and a plate-like positive electrode and a negative electrode are laminated in three or more layers via a separator. It is suitable for a large-capacity cell such as a laminated cell or a film-type cell in which a laminate of three or more layers each having a plate-like positive electrode and negative electrode through a separator is enclosed in an exterior film. The structures of these cells are already known from International Publication No. WO00 / 07255, International Publication No. WO03 / 003395, Japanese Patent Application Laid-Open No. 2004-266091, etc., and the capacitor cell of the present invention is similar to such an existing cell. It can be set as a simple structure.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

(Positive electrode manufacturing method)
The following three types of activated carbon were used. About the alkali activation treatment goods, what was sufficiently acid-washed and neutralized and washed was used as a sample. Further, the specific surface area of all the samples was 1990 m ² / g.
(A) A phenol resin activated carbon having a phenolic hydroxyl functional group content of 0.253 mmol / g that has been alkali-activated (Example 1);
(B) The amount of phenolic hydroxyl functional group subjected to alkali activation treatment is 0.332 mmol / g phenol resin activated carbon (Example 2),
(C) The amount of phenolic hydroxyl functional group subjected to alkali activation treatment is 0.193 mmol / g phenol resin activated carbon (Comparative Example 1),
Each of the activated carbons (a) to (c) is sufficiently mixed in a composition of 10 parts by weight, 0.5 parts by weight of ketjen black powder, and 1 part by weight of a tetrafluoroethylene binder, and 20 parts by weight of isopropanol is added. And kneaded. The obtained lump was rolled to obtain positive electrode sheet electrodes (a1), (b1) and (c1).

  An aluminum expanded metal having a thickness of 38 μm (porosity 47%) (manufactured by Nippon Metal Industry Co., Ltd.) was coated with a water-based carbon conductive adhesive on both sides of the current collector, and the positive electrode sheet electrode (a1), (b1 ) And (c1) were attached to both sides of the current collector, and the current collector and the positive electrode sheet electrode were brought into close contact with a rolling roller. Next, vacuum drying is performed to obtain positive electrodes (a2), (b2), and (c2) having a total thickness of the positive electrode (total thickness of the positive electrode layers on both sides, the thickness of the conductive layers on both sides, and the thickness of the positive electrode collector) of 165 μm. Obtained.

(Capacitance measurement per unit weight of positive electrode)
The positive electrode sheet electrodes (a1), (b1) and (c1) are bonded and fixed to one side of an aluminum foil having a thickness of 20 μm using a carbon-based conductive adhesive, and dried to obtain positive electrode foil electrodes (a3), (b3 ) And (c3) were obtained.
Two pieces of the positive electrode foil electrodes (a3), (b3) and (c3) were cut into 2.0 × 2.0 cm ² sizes, respectively, and used as a positive electrode for evaluation and a counter electrode. A capacitor simulation cell was assembled with a positive electrode and a counter electrode made of a 60 μm thick paper nonwoven fabric as a separator. Metallic lithium was used as a reference electrode. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 was used.
The reference electrode (vs. Li / Li ⁺ ) was charged to 3.825 V at a charging current of 8 mA, then constant voltage charging was performed, and after 1 hour of total charging time, the battery was discharged to 8.425 V at 8 mA. . Table 1 shows the capacitance per unit weight of the positive electrode determined from the discharge time between 3.825V and 2.425V.

As shown in Table 1, the capacitance of the positive electrode was increased by using activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more for the positive electrode.

(Negative electrode manufacturing method)
A 0.5 mm thick phenolic resin molded plate was placed in a siliconite electric furnace, heated to 550 ° C. at a rate of 50 ° C./hour in a nitrogen atmosphere, and further heated to 670 ° C. at a rate of 10 ° C./hour. Thereafter, heat treatment was performed to synthesize a PAS plate. The PAS plate thus obtained was pulverized with a ball mill to obtain a PAS powder having an average particle size of 4 μm. The H / C of this PAS powder was 0.2.
Next, a slurry was obtained by sufficiently mixing the composition of 92 parts by weight of the PAS powder, 6 parts by weight of acetylene black powder, 5 parts by weight of SBR, 4 parts by weight of carboxymethylcellulose, and 200 parts by weight of water.
A slurry of negative electrode was formed on both sides of a copper expanded metal (manufactured by Nippon Metal Industry Co., Ltd.) having a thickness of 32 μm (porosity 57%) on both sides of the negative electrode current collector with a roll coater. A negative electrode (a) having a thickness of 134 μm (the total thickness of the negative electrode layers on both sides and the thickness of the negative electrode current collector) was obtained.

(Capacitance measurement per unit weight of negative electrode)
Each of the negative electrodes (a) was cut into a size of 1.5 × 2.0 cm ² and used as a negative electrode for evaluation. As a negative electrode and a counter electrode, a simulation cell was assembled with a metallic lithium having a size of 1.5 × 2.0 cm ² and a thickness of 200 μm as a separator and a nonwoven fabric made of polyethylene having a thickness of 50 μm. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 was used.
This simulated cell was doped with 600 mAh / g of lithium ions by charging at a charging current of 1 mA with respect to the weight of the negative electrode active material of the negative electrode (a), and then discharged to 1.5 V at 1 mA. The electrostatic capacitance per unit weight of the negative electrode (a) was 912 F / g based on the discharge time during which the potential of the negative electrode changed 0.2 V from the potential of the negative electrode 1 minute after the start of discharge.

(Small film type capacitor making method)
For each cell, five positive electrodes are cut to 2.4 cm x 3.8 cm, six negative electrodes are cut to 2.4 cm x 3.8 cm, and laminated through a separator (100% rayon) at 150 ° C for 12 hours. After drying, separators were placed on the uppermost part and the lowermost part, and four sides were taped to obtain an electrode laminated unit. Table 2 shows combinations of the positive electrodes (a2) to (c2) and the negative electrode (a). As the metallic lithium for 600 mAh / g with respect to the weight of the negative electrode active material of the negative electrode (a), a metal lithium foil having a thickness of 110 μm bonded to a stainless steel mesh having a thickness of 23 μm is used. One sheet was placed on the outermost part of the laminated unit. The negative electrode (six pieces) and the stainless steel mesh bonded with metallic lithium were welded and brought into contact with each other to obtain an electrode laminated unit. An aluminum positive terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm, in which a sealant film is heat-sealed in advance to the seal portion, is stacked on the terminal welded portion (five pieces) of the positive electrode current collector of the electrode laminate unit. Ultrasonic welding. Similarly, a nickel negative electrode terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm, in which a sealant film is thermally fused in advance to the seal portion, is superposed on the terminal welded portion (six pieces) of the negative electrode current collector, and ultrasonic welding is performed. Then, it was installed between one exterior film deeply drawn to 60 mm in length, 30 mm in width, and 3 mm in depth, and one exterior film not deeply drawn.
After heat-sealing the two sides of the terminal portion of the exterior laminate film and the other side, 1 mol / l of a mixed solvent in which ethylene carbonate, diethyl carbonate and propylene carbonate were used at a weight ratio of 3: 4: 1 as an electrolytic solution. After the solution in which LiPF ₆ was dissolved in a concentration was vacuum impregnated, the remaining one side was heat-sealed under reduced pressure, and vacuum sealing was performed to assemble two film type capacitor cells.

(Characteristic evaluation of cells)
After leaving at room temperature for 14 days, one cell was disassembled for each film type capacitor cell. As a result, all of the lithium metal was completely removed. Therefore, a capacitance of 912 F / g per unit weight of the negative electrode active material was obtained. It was determined that the lithium ions to obtain were pre-doped by charging. The electrostatic capacity of the negative electrode of each film type capacitor was 5.8 times or more that of the positive electrode.

  Each cell of the remaining film type capacitor was left at 25 ° C. for 24 hours, and then charged with a constant current of 100 mA until the cell voltage reached 3.8 V, and then a constant voltage of 3.8 V was applied. Voltage charging was performed for 1 hour. Next, the battery was discharged at a constant current of 100 mA until the cell voltage reached 2.2V. This 3.8V-2.2V cycle was repeated, and the third discharge capacity was measured. Table 2 shows the measurement result of the discharge capacity and the value of the energy density.

Measurement of the potential of the positive electrode are short-circuited between the positive electrode and the negative electrode after the completion of the measurement, both in the range of 0.65～0.95V (relative to Li / Li ⁺⁾ or, 2.0 V (relative to Li / Li ⁺⁾ or less Met. A capacitor having a high energy density is obtained by supporting lithium ions in advance on the negative electrode and / or the positive electrode so that the positive electrode potential when the positive electrode and the negative electrode are short-circuited is 2.0 V (vs. Li / Li ⁺ ) or less. It was.
As shown in Table 2, a lithium ion capacitor having a high capacity and energy density can be obtained by using activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more.

  The lithium ion capacitor of the present invention is extremely effective as a drive or auxiliary storage power source for electric vehicles, hybrid electric vehicles and the like. Further, it can be suitably used as a storage power source for driving such as an electric bicycle or an electric wheelchair, a power storage device for various energy such as solar energy or wind power generation, or a storage power source for household electric appliances.

Claims (6)

A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution containing a lithium salt as an electrolyte, wherein the positive electrode active material is a substance capable of reversibly carrying lithium ions and / or anions, The active material is a material capable of reversibly supporting lithium ions, and the positive electrode potential after the positive electrode and the negative electrode are short-circuited is 2.0 V or less (vs. Li / Li ⁺ ) with respect to the negative electrode and / or the positive electrode. The lithium ion capacitor is characterized in that lithium ions are pre-doped and the positive electrode active material is activated carbon particles having a phenolic hydroxyl functional group of 0.2 mmol / g or more.   The positive electrode and / or the negative electrode each include a current collector having holes penetrating the front and back surfaces, and lithium ions are doped by electrochemical contact between the negative electrode and a lithium ion supply source. Lithium ion capacitor.   3. The negative electrode active material according to claim 1, wherein the negative electrode active material has a capacitance per unit weight of 3 times or more as compared with the positive electrode active material, and the positive electrode active material weight is larger than the negative electrode active material weight. Lithium ion capacitor. The lithium ion capacitor according to claim 1, wherein the activated carbon particles are subjected to an alkali activation treatment and have a specific surface area of 600 to 3000 m ² / g.   The lithium ion capacitor according to claim 1, wherein the activated carbon particles have an average particle diameter (D50) of 1 μm or more.   The lithium ion capacitor according to any one of claims 1 to 5, wherein the activated carbon particles are phenol resin activated carbon, petroleum pitch activated carbon, petroleum coke activated carbon, or coal coke activated carbon.