JP2006338963A - Lithium ion capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved lithium ion capacitor capable of providing high values of energy density and output density. <P>SOLUTION: This lithium ion capacitor comprises a positive electrode, a negative electrode and an aprotic organic solvent electrolytic solution of a lithium salt as an electrolyte. The lithium ion capacitor is characterized in that a positive electrode active material is a substance capable of reversibly supporting lithium ions and/or anions; a negative electrode active material is a substance capable of supporting lithium ions; lithium ions are doped in the negative electrode and/or the positive electrode so as to set the potential of the positive electrode below 2.0 V after the positive electrode is short-circuited to the negative electrode; and the positive electrode is composed by integrating a sheet-like electrode formed of activated carbon and having a thickness of 50-400 μm with a collector. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion capacitor including a positive electrode, a negative electrode, and an aprotic organic solvent electrolytic solution of 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 is a high-capacity and powerful 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. It is characterized by high voltage, high capacity, and high safety.

  On the other hand, while environmental problems have been highlighted, the development of power storage devices (main power and auxiliary power) for electric vehicles or hybrid vehicles replacing gasoline vehicles has been actively carried out. Until now, lead batteries have been used. 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, electric double layer capacitors are used as memory backup power sources for ICs and LSIs, but their discharge capacity per charge is smaller than batteries. 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 lithium metal, and lithium ions are stored and supported (hereinafter also referred to as doping) by a chemical method or an electrochemical method in advance. There has been proposed a capacitor intended to increase the withstand voltage and greatly increase the energy density by lowering the potential. (See Patent Document 1 to Patent Document 4)

  Although this type of hybrid capacitor is expected to have high performance, when doping lithium ions to the negative electrode, it takes a very long time for doping, and there is a problem in uniform doping with respect to the whole negative electrode. It has been considered difficult to put into practical use in large-sized high-capacity cells such as a cylindrical battery in which electrodes are wound or a square battery in which a plurality of electrodes are stacked.

  However, this problem is that a through hole is formed in the front and back of the negative electrode current collector and the positive electrode current collector constituting the cell, and lithium ions are moved through the through hole, and at the same time, lithium metal that is a lithium ion supply source and the negative electrode By short-circuiting, it was possible to do so all at once by arranging the lithium metal at the end of the cell and doping all the negative electrodes in the cell with lithium ions (see Patent Document 5). In addition, although doping 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 positive electrode instead of a negative electrode.

  Thus, even in a large cell such as a cylindrical cell in which electrodes are wound or a square cell 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 apparatus. It was possible to dope and increase 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. Provided is an improved capacitor capable of realizing higher values of energy density and output density in a lithium ion capacitor in which lithium ions are doped in the negative electrode in advance before charging by electrochemical contact with an ion source. This is the issue.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research. As a result, the negative electrode and / or the negative electrode and / or the negative electrode and / or the negative electrode potential is 2.0 V or less after the positive electrode and the negative electrode are short-circuited. In a lithium ion capacitor in which lithium ions are pre-doped to the positive electrode, the material and shape of the positive electrode used therein are related to the energy density and output density of the obtained capacitor, and the positive electrode is preferably a specific surface area. The present inventors have found that the above-mentioned problems can be solved by using a sheet-like electrode having a thickness of 50 to 400 μm including activated carbon particles having a thickness integrated with a current collector, and 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. Yes, the negative electrode active material is a material capable of reversibly carrying lithium ions, and the lithium ion with respect to the negative electrode and / or the positive electrode so that the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2.0 V or less And the positive electrode is an electrode in which a sheet-like electrode having activated carbon particles and a thickness of 50 to 400 μm is integrated with a current collector.
(2) The lithium ion capacitor according to (1), wherein the sheet-like electrode is a mixture containing a conductive agent and a binder.
(3) The lithium ion capacitor according to (1) or (2), wherein the binder is a fluororesin.
(4) The lithium according to any one of (1) to (3), wherein the binder is polytetrafluoroethylene (PTFE) or a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). Ion capacitor.
(5) The positive electrode and / or the negative electrode each including 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)-(4).
(6) 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 greater than the weight of the negative electrode active material. The lithium ion capacitor as described in 5).
(7) The lithium ion capacitor according to any one of (1) to (6), wherein the activated carbon particles have a specific surface area of 600 to 3000 m ² / g.
(8) The lithium ion capacitor according to any one of (1) to (7), wherein the negative electrode active material is a carbon material or a polyacene-based material.
(9) The above (A), wherein the negative electrode active material is an insoluble infusible material having a hydrogen atom / carbon atom of 0.05 to 0.5 by heat-treating an aromatic condensation polymer in a non-oxidizing atmosphere at 400 to 800 ° C. The lithium ion capacitor according to any one of 1) to (8).

According to the present invention, there is provided a capacitor having a high energy density and a high output density, in which a negative electrode and / or a positive electrode are preliminarily doped with lithium ions before charging. The capacity of the electric double layer capacitor using the same activated carbon for both _electrodes is C _{(positive electrode)} ≒ C _{(negative electrode)} , so the total cell capacity is C / 2, and the total capacity is only half of one electrode. In a capacitor in which the negative electrode and / or the positive electrode are previously doped with lithium ions, 1 / C _{(positive electrode)} >> 1 / C _{negative electrode)} , so that C _(whole) ≈ C _{(positive electrode)} approaches, It becomes the capacity of the whole cell. In the present invention, as the positive electrode of this capacitor, an electrode in which a sheet-like electrode having a thickness of 50 to 400 μm containing the above activated carbon particles is integrated with a current collector is used. Although it is not necessarily clear whether it has energy density or power density, it is estimated as follows.

  The positive electrode in the capacitor of the present invention is an electrode in which a sheet-like electrode having a specific thickness is integrated with a current collector, and a positive electrode active material is coated with a slurry-like electrode containing a thickener and a binder. Different from the coated electrode. In the case of a coated electrode, a thickener is indispensable, and since the binder amount is generally large and the binding force between the positive electrode active material particles is weak, the positive electrode is thickened or the packing density of the positive electrode active material is increased. There is a limit. Further, in the coated electrode, since the thickener covers the periphery of the positive electrode active material particles, the contact resistance between the particles and the contact resistance with the current collector are increased.

  However, in the positive electrode in which the specific sheet-like electrode of the present invention is integrated with the current collector, the thickener is obtained by entwining the activated carbon particles, which are positive electrode active materials, and carrying them on the current collector and integrating them. Since it is unnecessary and the amount of the binder can be reduced, the packing density of the positive electrode active material is increased and the capacitor has a high energy density. Furthermore, since the sheet-like electrode of the present invention does not use a thickener, the contact resistance is low, and thus a capacitor having a high output density can be obtained. In particular, in the present invention, when a fluororesin such as tetrafluoroethylene is used as the binder, the binder is in a thin fiber shape, and the active carbon particles can be closely entangled and supported on the current collector. It becomes possible to shape | mold and to make the packing density of a positive electrode active material very high.

  The positive electrode in which the sheet-like electrode of the present invention is integrated with a current collector as described above is particularly 3.0 V or less due to the uniform presence of interparticle voids with a size that facilitates smooth diffusion of lithium ions. Has a large capacity and low resistance. Therefore, in the lithium ion capacitor of the present invention in which lithium ions are doped with respect to the negative electrode and / or the positive electrode so that the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2.0 V or less, 3. Lithium ion doping occurs in the region of 0 V or less, and the positive electrode using the sheet-like electrode seems to provide a capacitor having a high output density and a high capacity.

  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 potential of the positive electrode 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 or less. 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. is there. 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 or less. The potential of the positive electrode determined by either of the following two methods (A) or (B) is 2 It means the case of 0V or less. That is, (A) After doping with lithium ions, the positive electrode terminal and the negative electrode terminal of the capacitor cell are left in a state of being directly coupled with a conductive wire for 12 hours or more, and then the short circuit is released, and within 0.5 to 1.5 hours Measured positive electrode potential, (B) Charge-discharge tester discharges constant current to 0V over 12 hours and then leaves positive electrode terminal and negative electrode terminal connected with lead wire for 12 hours or more, then releases short circuit Positive electrode potential measured within 0.5 to 1.5 hours.

  In the present invention, the positive electrode potential after the positive electrode and the negative electrode are short-circuited is 2.0 V or less, not only immediately after the lithium ions are doped, but in the charged state, discharged state or charged state. The positive electrode potential after short-circuiting is 2.0 V or less in any state, such as when short-circuiting after repeating discharge.

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 or less will be described in detail below. As described above, activated carbon and carbon materials usually have a potential of about 3 V (Li / Li ⁺ ), and when the cell is assembled using activated carbon for both the positive electrode and the negative electrode, both potentials are about 3 V. Even if it is short-circuited, 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, even if it short-circuits, a positive electrode potential is about 3V regardless. 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-described hybrid capacitor in which the positive electrode potential is about 3 V when short-circuited, when the upper limit potential of the positive electrode is 4.0 V, for example, the positive electrode potential during discharge is up to 3.0 V, and the positive electrode potential change is 1. The capacity of the positive electrode of about 0 V is not 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, and the utilization capacity further decreases. In other words, the positive electrode can be discharged from 4.0 V to 2.0 V. However, when only 4.0 V to 3.0 V can be used, only half of the usage capacity is used. It will not be.

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

  If the positive electrode potential after the short circuit falls below 3.0V, the utilization capacity increases and the capacity increases. In order to be 2.0 V or less, it is preferable to charge not only the amount charged by charging / discharging the cell but also separately charging lithium ions from a lithium ion supply source such as lithium metal to the negative electrode. Since lithium ions are supplied from other than the positive electrode and the negative electrode, when they are short-circuited, the equilibrium potentials of the positive electrode, the negative electrode, and the lithium metal are reached, so that both the positive electrode potential and the negative electrode potential are 3.0 V or less. As the amount of lithium metal increases, the equilibrium potential decreases. If the negative electrode material and the positive electrode material change, the equilibrium potential also changes. Therefore, it is necessary to adjust the amount of lithium ions supported on the negative electrode in view 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 or less. It is.

  In the present invention, before charging the capacitor cell, the negative electrode and / or the positive electrode is previously doped with lithium ions, and the positive electrode potential is 2.0 V or less after the positive electrode and the negative electrode are short-circuited. Since the capacity is increased, the capacity is increased and a large energy density is 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 or less, particularly 1.0 V or less is more preferable. When 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 when the positive electrode and the negative electrode are short-circuited, and the energy density of the cell is reduced. Further, when the positive electrode potential is lower than 1.0 V, although depending on the positive electrode active material, problems such as gas generation and irreversible consumption of lithium ions occur, so that it is difficult to measure the positive electrode potential. Further, when the positive electrode potential becomes too low, the weight of the negative electrode is excessive, and the energy density may be decreased. Generally, it is 0.1 V or more, preferably 0.3 V or more.

  In the present invention, lithium ion doping may be either 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, since controlling the doping amount of the positive electrode and the doping amount of the negative electrode becomes complicated in the process, the 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 becomes small, and if it exceeds 10 times, the capacity may be reduced, 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 this is the amount of charge when current flows, it is converted into mAh in this patent. 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 lithium metal that can supply lithium ions can be disposed in the capacitor cell as a lithium electrode. The amount of the lithium ion supply source (the weight of lithium metal 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 and number 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 periphery or outermost cell. .

  Various materials can be used as the material of the electrode current collector. The positive electrode current collector is preferably made of aluminum or stainless steel, and the negative electrode current collector is preferably made of stainless steel, copper or nickel. Each can be used. In addition, when doping is performed by electrochemical contact with a lithium supply source disposed in the cell, lithium means that at least lithium is contained and lithium ions are supplied like lithium metal or lithium-aluminum alloy. A substance that can be used.

  The positive electrode active material in the lithium ion capacitor of the present invention is made of a material capable of reversibly carrying lithium ions and anions such as tetrafluoroborate.

  An example of the positive electrode active material in the present invention is activated carbon, and the positive electrode is a sheet electrode having a thickness of 50 to 400 μm including activated carbon particles. It is generally difficult in the process to make the thickness of the sheet-like electrode thinner than 50 μm. On the other hand, when the thickness of the sheet-like electrode exceeds 400 μm, the electrode becomes brittle and the detached particles increase. Here, the thickness of the sheet-like electrode does not include the thickness of the conductive adhesive and the current collector.

  The raw material of the activated carbon which is an example of the present invention is preferably phenol resin, petroleum pitch, petroleum coke, coconut husk, or coal-based coke, but preferably phenol resin and coal-based coke can increase the specific surface area. Therefore, it is preferable. These activated carbon materials are calcined, carbonized, activated, and then pulverized. The carbonization treatment is performed by storing the raw 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 activation method includes alkali activation activated using an alkaline agent such as KOH, gas activation using CO ₂ , steam activation treated in water vapor, and the like. Preferably, the alkali activated carbon can increase the capacity. Therefore, it is preferable. The activation temperature is preferably 400 to 800 ° C, and more preferably 600 to 800 ° C. When the activation temperature is less than 400 ° C., the activation does not proceed and the electrostatic capacity becomes small. On the other hand, when it exceeds 900 ° C., the activation rate is extremely lowered, which is not preferable. 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 it exceeds 10 hours, the capacitance per unit volume is decreased.

  The activated charcoal is then crushed. The pulverization is performed using a known pulverizer such as a ball mill. The average particle diameter D50 of the activated carbon particles obtained is, for example, 2 μm or more, preferably 2 to 50 μm, particularly preferably 2 to 20 μm. The average particle diameter D50 is determined by the X-ray microtrack method or the like.

The specific surface area of the activated carbon particles is preferably 600 to 3000 m ² / g. When the specific surface area is smaller than 600 m ² / g, in the capacitor cell using the activated carbon as the positive electrode, the volume of the positive electrode expands twice when charged / discharged, so that the capacity per volume is reduced to half. The target effect cannot be achieved. The specific surface area is, among others, 800 m ² / g or more, and particularly it is preferable that a 1300~2500m ² / g.

  The sheet-like positive electrode in the present invention is formed using the above activated carbon particles, and the means is preferably adopted as follows. Activated carbon particles are dispersed in an aqueous system or an organic solvent together with a conductive agent and a dispersing agent, if necessary, to form a slurry. A binder is further added to the slurry, and after kneading into a lump, the kneaded product is formed into a sheet having a thickness of 50 to 400 μm, preferably 50 to 200 μm, using a roller. This sheet-like material is bonded to and integrated with both or one side of the current collector using a conductive adhesive or the like. A mixer, a kneader, a disper or the like is used for kneading the slurry. The thickness of the sheet-like material can be measured using a micro gauge or the like, and can be set to a predetermined thickness using a rolling roller.

  In the sheet-like electrode of the present invention, examples of the conductive agent used as necessary include acetylene black, graphite, ketjen black, VGCF, and metal powder. The amount of the conductive agent used varies depending on the electrical conductivity of the positive electrode active material, but is preferably 0.5 to 40% by weight with respect to the activated carbon particles. If the amount of the conductive agent is less than 0.5% by weight, the conductivity of the electrode is low, which is not preferable. Conversely, if the amount exceeds 40% by weight, the amount of the positive electrode active material per volume is not preferable.

  As the binder used in the sheet-like electrode of the present invention, a fluororesin or polyvinyl alcohol is preferable. Among these, polytetrafluoroethylene (PTFE) or tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) is preferable. When polyvinylidene fluoride, SBR, or the like is used as a binder in the production of the sheet-like electrode, it is difficult to produce the sheet-like electrode. When a binder such as polyvinylidene fluoride or SBR is used, activated carbon particles are dispersed in water or an organic solvent to form a slurry, and the slurry is applied to a current collector to obtain an electrode, 400 μm per side When a thick electrode is applied, for example, the coating material flows, causing unevenness in thickness, or the electrode is warped during drying. In addition, when the electrode is cut after drying, the electrode is dropped and causes a short circuit. Therefore, the method of applying slurry is not suitable for producing a thick electrode. Moreover, the density | concentration of the electrode which disperse | distributed activated carbon particle | grains in water or an organic solvent to make a slurry, and apply | coated this slurry to the electrical power collector tends to become low compared with a sheet-like electrode.

  Although the usage-amount of a binder changes with shapes, electrical conductivity, etc. of a sheet-like electrode, Preferably it is 0.5 to 40 weight% with respect to the activated carbon particle which is a positive electrode active material. If the binder amount is less than 0.5% by weight, the sheet shape cannot be maintained. Conversely, when it exceeds 40% by weight, the amount of the positive electrode active material per volume decreases, the resistance increases, and the energy density decreases.

  The conductive adhesive used for carrying the sheet electrode on the current collector and integrating it is a carbon agent such as graphite or a conductive agent, and if necessary, a dispersant or an adhesive in an aqueous or organic solvent. A dispersed slurry-like conductive paint can be used. As the current collector, meshes such as aluminum and stainless steel, expanded metal, and punching metal are preferably used. The thickness of the current collector is preferably 5 to 100 μm, particularly preferably 10 to 30 μm, and the porosity is preferably 15 to 75%, particularly preferably 35 to 65%.

  When a sheet-like electrode is supported on a current collector and integrated, apply the above slurry-like conductive paint to one or both of the current collector or the sheet-like electrode, and crimp the application surface. Thus, the sheet-like electrode and the current collector are integrated. In this case, it is preferable to use a roll press, and the pressure is preferably 10 to 1000 kg as a linear pressure.

  On the other hand, the negative electrode active material in the lithium ion capacitor of 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 storing 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 carrying (charging) lithium ions of 400 mAh / g or more on PAS, and 750 F / g or more can be obtained by charging lithium ions of 500 mAh / g or more. Capacitance is 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 increased. 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 is isotropic with respect 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 preferably used. For example, the following formula

（ここで、ｘ及びｙはそれぞれ独立に、０、１または２である）で表されるメチレン・ビスフェノール類であることができ、あるいはヒドロキシ・ビフェニル類、ヒドロキシナフタレン類であることもできる。なかでも、フェノール類が好適である。

(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 condensation 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. A modified aromatic condensation polymer 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 preferably used as an insoluble and infusible substrate. That is, by gradually heating the aromatic condensation polymer to a suitable temperature of 400 to 800 ° C. in a non-oxidizing atmosphere (including vacuum), a hydrogen atom / carbon atom ratio (hereinafter referred to as H). / C) is 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.

In the present invention, the average particle diameter D50 of the negative electrode active material is preferably 0.5 to 30 μm, more preferably 0.5 to 15 μm, and particularly preferably 0.5 to 6 μm. Also, preferably the specific surface area of the anode active material particles are 0.1~2000m ² / g, more preferably from 0.1~1000m ² / g, particularly preferably at 0.1~600m ² / g .

  The negative electrode in the present invention is formed from the above negative electrode active material powder, and the existing means can be used. That is, whether a negative electrode active material powder, a binder, and, if necessary, a conductive agent and a thickener (CMC, etc.) are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is applied to the current collector. Alternatively, the slurry may be previously formed into a sheet shape and integrated with the 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 polymer, or the like is used. Can do. The amount of the binder 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 it at a ratio of 2 to 10% by weight with respect to the negative electrode active material.

  Examples of the aprotic organic solvent for 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, ethyl methyl 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 above electrolyte and solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. The concentration of the electrolyte in the electrolyte is at least 0.1 mol / liter in order to reduce the internal resistance of the electrolyte. 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-type 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 through a separator. It is suitable for a large-capacity cell such as a rectangular 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 structure of these cells is already known from International Publication WO00 / 07255, International Publication 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)
(Production of sheet electrode)
Slurry in which alkali activated charcoal (material: petroleum coke charcoal, D50: 5 μm, specific surface area: 1500 m ² / g) is sufficiently mixed in a composition of 10 parts by weight, acetylene black powder 0.5 parts by weight, and isopropanol 20 parts by weight. After that, 1 part by weight of a polytetrafluoroethylene binder was added to prepare a kneaded product, which was formed into a sheet-like electrode using a rolling roller to obtain a positive electrode (a1) having a thickness of 60 μm.

An aluminum expanded metal (made by Nippon Metal Industry Co., Ltd.) current collector with a thickness of 38 μm (porosity 47%) is coated on both sides with a water-based carbon conductive adhesive, and the sheet electrode is immediately attached to both sides of the current collector. Pasted on. Next, the current collector and the positive electrode sheet-like electrode were brought into close contact with a rolling roller, and then vacuum-dried. The thickness of the whole positive electrode (the thickness of the positive electrode layer on both sides, the thickness of the conductive layer on both sides, and the thickness of the positive electrode collector) A positive electrode (a2) having a total of 165 μm was obtained.
Moreover, the positive electrode sheet electrode (a3) was obtained by bonding and fixing the positive electrode sheet electrode (a2) to one side of an aluminum foil having a thickness of 20 μm using a carbon-based conductive adhesive and drying.

(Preparation of coated electrode)
Alkaline activated charcoal (same as used above) is sufficiently mixed in a composition of 92 parts by weight, 6 parts by weight of acetylene black powder, 7 parts by weight of acrylic binder, 4 parts by weight of carboxymethylcellulose, and 200 parts by weight of water. As a result, a positive electrode slurry (b1) was obtained.

  For positive electrode with conductive layer formed by coating non-aqueous carbon conductive paint on both sides of aluminum expanded metal (Nippon Metal Industry Co., Ltd.) with a thickness of 38μm (porosity 47%) with a roll coater and drying A current collector was obtained. The total thickness (the sum of the current collector thickness and the conductive layer thickness) was 52 μm, and the through-hole was almost blocked by the conductive paint.

  Next, the positive electrode slurry (b1) was applied to both surfaces of the positive electrode current collector with a roll coater so that the thickness after drying on one side was 60 μm. After vacuum drying, a positive electrode (b2) having a total positive electrode thickness (total of positive electrode layer thickness on both sides, conductive layer thickness on both sides, and positive electrode current collector thickness) of 172 μm was obtained.

Further, the positive electrode slurry (b1) was coated on a single surface of an aluminum foil having a thickness of 20 μm coated with a carbon-based conductive paint so that the solid weight per unit area was 4.0 mg / cm ² and dried to obtain a positive electrode foil electrode ( b3) was obtained.

(Capacitance measurement per unit weight of positive electrode)
Two pieces of the positive electrode foil electrodes (a3) and (b3) were cut into a size of 2.0 × 2.0 cm ² to obtain a positive electrode for evaluation and a negative electrode. A capacitor simulation cell was assembled with a positive electrode and a negative electrode made of a paper nonwoven fabric having a thickness of 60 μm as a separator. Lithium metal 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 battery was charged to 2.5 V at a charging current of 8 mA and then charged at a constant voltage. After a total charging time of 1 hour, the battery was discharged to 0 V at 8 mA. Table 1 shows the electrostatic capacity per unit weight of the positive electrode from the discharge time between 2.5V and 0V. Similarly, the electrostatic capacity per unit weight of the positive electrode was also shown from the potential difference between the reference electrode (Li / Li ⁺ ) and the positive electrode.

  As shown in Table 1, the sheet-like electrode a3 had a higher capacitance per volume and a lower DC resistance than the coating electrode b3.

(Negative electrode manufacturing method)
A 0.5 mm-thick phenolic resin molded plate is placed in a siliconite electric furnace, heated to 550 ° C. at a rate of 50 ° C./hour, and further at a rate of 10 ° C./hour to 670 ° C. in a nitrogen atmosphere, followed by heat treatment. PAS was synthesized. The obtained PAS plate was pulverized with a ball mill to obtain a PAS powder having an average particle size of 4 μm. The H / C ratio 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 is 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. After vacuum drying, the total thickness ( A negative electrode (a) having a negative electrode layer (a) having a thickness of 113 μm was obtained (total of the negative electrode layer thickness on both sides, the conductive layer thickness on both sides, and the negative electrode current collector thickness).

In addition, by using a commercially available Carbotron P (manufactured by Kureha Chemical Industry Co., Ltd., specific surface area: 9 m ² / g, particle size: 9 μm) as the negative electrode powder, a 113 μm negative electrode (b )created.

(Capacitance measurement per unit weight of negative electrode)
The negative electrodes (a) and (b) were cut into 1.5 × 2.0 cm ² sizes, and used as negative electrodes for evaluation. As a counter electrode and a reference electrode, a simulation cell was assembled through a lithium non-woven fabric having a size of 1.5 × 2.0 cm ² and a thickness of 200 μm and a polyethylene nonwoven fabric having a thickness of 50 μm as a separator. 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 simulated cell was charged with 600 mAh / g of lithium ions with respect to the negative electrode active material weight of the negative electrode (a) at a charging current of 1 mA, 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 from 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. Similarly, lithium ions of 300 mAh / g were charged with respect to the weight of the negative electrode active material of the negative electrode (b), and then discharged to 1.5 V at 1 mA. The electrostatic capacitance per unit weight of the negative electrode (a) was 3526 F / g from the discharge time during which the potential of the negative electrode changed 0.2 V from the potential of the negative electrode one minute after the start of discharge.

(Small film cell creation method)
For each cell, five positive electrodes are cut to 2.4 cm x 3.8 cm, and six negative electrodes are cut to 2.4 cm x 3.8 cm, and stacked via a separator (100% rayon) as shown in FIG. Then, after drying at 150 ° C. for 12 hours, 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) and (b2) and the negative electrodes (a) and (b). As lithium metal of 600 mAh / g with respect to the weight of the negative electrode active material of the negative electrode (a), a lithium metal foil having a thickness of 90 μm bonded to a copper lath having a thickness of 23 μm was used, and the negative electrode active of the negative electrode (b) was used. As lithium metal for 300 mAh / g with respect to the material weight, a lithium metal foil with a thickness of 55 μm is bonded to a copper lath with a thickness of 23 μm. Placed. The negative electrode (six pieces) and the stainless steel mesh to which the lithium metal was pressure bonded were welded and brought into contact with each other to obtain a three-layer laminated unit. The aluminum positive electrode terminal with 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 overlaid on the terminal welded portion (five pieces) of the positive electrode current collector of the three-pole laminated unit. And 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 in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 as an electrolytic solution. Then, a solution in which LiPF ₆ was dissolved in a vacuum concentration was vacuum impregnated, and the remaining one side was heat-sealed under reduced pressure, and vacuum sealing was performed to assemble two film 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, 900 F / g or 3500 F / g or more per unit weight of the negative electrode active material. It was judged that lithium ions were pre-doped to obtain the capacitance of 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 3.6 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 1.9V. This 3.6V-1.9V cycle was repeated, and the third discharge capacity was measured. The measurement results are shown in Table 2.

  When the positive electrode and the negative electrode were short-circuited after the measurement was completed and the potential of the positive electrode was measured, both were in the range of 0.65 to 0.95 V, and were 2.0 V or less. A capacitor having a high energy density was obtained by previously supporting lithium ions on the negative electrode and / or the positive electrode so that the positive electrode potential when the positive electrode and the negative electrode were short-circuited was 2.0 V or less.

  As shown in Table 2, the use of a sheet-like electrode for the positive electrode (Example 1) resulted in higher capacitance and energy density and lower DC resistance than Comparative Example 1 using a slurry electrode. . Further, even when a carbon material other than PAS was used for the negative electrode, the capacitance and energy density were increased by using a sheet-like electrode for the positive electrode (Example 2). However, since the capacitor cell using the carbotron P as the negative electrode material has a high DC resistance, it is preferable to use PAS as the negative electrode material in consideration of the DC resistance.

  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 (9)

  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 carrying lithium ions and / or anions, The active material is a material capable of reversibly supporting lithium ions, and the negative electrode and / or the positive electrode is doped with lithium ions so that the positive electrode potential is 2.0 V or less after the positive electrode and the negative electrode are short-circuited. The lithium ion capacitor is characterized in that the positive electrode is an electrode in which a sheet-like electrode having a thickness of 50 to 400 μm containing activated carbon particles is integrated with a current collector.   The lithium ion capacitor according to claim 1, wherein the sheet-like electrode includes a conductive agent and a binder.   The lithium ion capacitor according to claim 1, wherein the binder is a fluororesin.   The lithium ion capacitor according to claim 1, wherein the binder is polytetrafluoroethylene or tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer.   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. 4. The lithium ion capacitor according to any one of 4 above.   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 weight of the positive electrode active material is larger than the weight of the negative electrode active material. The lithium ion capacitor according to 1. The lithium ion capacitor according to claim 1, wherein the activated carbon particles have a specific surface area of 600 to 3000 m ² / g.   The lithium ion capacitor according to claim 1, wherein the negative electrode active material is a carbon material or a polyacene-based material.   The negative electrode active material is an insoluble infusible material having a hydrogen atom / carbon atom of 0.05 to 0.5, which is obtained by heat-treating an aromatic condensation polymer at 400 to 800 ° C in a non-oxidizing atmosphere. The lithium ion capacitor according to any one of the above.