JP2007067088A - Lithium ion capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion capacitor high in energy density and low in resistance. <P>SOLUTION: The lithium ion capacitor is provided with an anode, a cathode, and an aprotic organic solvent electrolytic solution of lithium salt as an electrolyte. An anode active substance is a substance capable of reversibly carrying lithium ions and/or anions. A cathode active substance is a substance capable of reversibly carrying lithium ions. The lithium ions are doped to the cathode and/or the anode before charging, so that potential of the anode becomes ≤2.0 V after short-circuiting the anode and cathode. The cathode is composed of the anode active substance bound with a binder including a (meta) acrylate polymer having a nitrile group. <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 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. 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. 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)

  Although this type of hybrid capacitor is expected to have high performance, when lithium ions are doped into the negative electrode, it is necessary to attach metallic lithium to all the negative electrodes, or to a part of the cell. Although it is possible to place lithium metal locally and contact with the negative electrode, there is a problem in doping that requires a very long time and uniform doping with respect to the whole negative electrode, and in particular, a cylinder in which the electrode is wound. It has been considered difficult to put into practical use in large-sized high-capacity cells such as a type device or a square battery in which a plurality of electrodes are laminated.

  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 surfaces, and lithium ions are moved through the through holes. By short-circuiting, the invention of being able to dope lithium ions to all the negative electrodes in the cell only by arranging lithium metal at the end of the cell has led to a solution (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-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. The energy density can be drastically increased by doping and the withstand voltage is improved, and it is expected that a high-capacity capacitor will be realized in combination with the large output density inherent in the electric double layer capacitor.

However, such a high-capacitance capacitor is required to have a higher withstand voltage and to have a low resistance in order to further improve output characteristics and low-temperature characteristics.
Japanese Patent Laid-Open No. 8-1007048 JP-A-9-55342 Japanese Patent Laid-Open No. 9-232190 JP-A-11-297578 International Publication No. 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. A lithium ion capacitor that is electrochemically contacted with an ion source and previously doped with lithium ions, and has a high energy density and a high output density, as well as an excellent capacitor with a high withstand voltage and a low resistance. The task is to do.

  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 with respect to the positive electrode, the characteristics of the negative electrode or the negative electrode and the positive electrode used therein are closely related to the characteristics of the obtained capacitor. By using a binder containing a (meth) acrylate polymer having a nitrile group as a binder (also referred to as a binder or a binder) for binding or binding an active substance during formation, the above problem is solved. We have found out that we can do it 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. 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 becomes 2.0 V or less after the positive electrode and the negative electrode are short-circuited. A lithium ion capacitor wherein the negative electrode is made of a negative electrode active material bound with a binder containing a (meth) acrylate polymer having a nitrile group.
(2) 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 positive electrode and / or the negative electrode and a lithium ion supply source. The lithium ion capacitor according to (1) above.
(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 greater than the weight of the negative electrode active material (1) or The lithium ion capacitor according to (2).
(4) The lithium ion capacitor according to any one of the above (1) to (3), wherein the positive electrode is made of a positive electrode active material bound with a binder containing a (meth) acrylate polymer having a nitrile group.
(5) The lithium ion according to any one of (1) to (4), wherein the (meth) acrylate polymer having the nitrile group is a copolymer of (meth) acrylic acid ester and (meth) acrylonitrile. Capacitor.
(6) The above (1) to (4), wherein the (meth) acrylate polymer having a nitrile group is a copolymer of (meth) acrylic acid ester, a vinyl monomer having a carboxylic acid group, and (meth) acrylonitrile. The lithium ion capacitor according to any one of the above.
(7) The (meth) acrylate polymer having the nitrile group is a graph obtained by grafting (meth) acrylonitrile to a polymer containing a (meth) acrylic acid ester and the above (1) to ( 4) The lithium ion capacitor according to any one of
(8) The negative electrode active material is a heat-treated product of graphite, non-graphitizable carbon, graphitizable carbon or aromatic condensation polymer, and the atomic ratio of hydrogen atoms / carbon atoms is 0.50-0. The lithium ion capacitor according to any one of the above (1) to (7), which is any one of the polyacene organic semiconductors having a polyacene skeleton structure of 05.
(9) The polyacene organic material having a polyacene skeleton structure in which the positive electrode active material is activated carbon or a heat-treated product of an aromatic condensation polymer and the atomic ratio of hydrogen atom / carbon atom is 0.50 to 0.05 The lithium ion capacitor according to any one of (1) to (8), which is any one of semiconductors.

  According to the present invention, there is provided a high-capacity lithium ion capacitor, in which a negative electrode and / or a positive electrode are previously doped with lithium ions, having a high energy density and a low resistance. In the present invention, why the above characteristics are improved by forming the negative electrode or the negative electrode and the positive electrode from an active material bound with a binder containing a (meth) acrylate polymer containing a nitrile group. The mechanism is not necessarily clear, but is estimated as follows.

  Conventional electrode binders include rubber binders such as styrene butadiene rubber (SBR) and acrylonitrile butadiene rubber (NBR), fluorine-containing resins such as polytetrafluoroethylene and polyvinylidene fluoride, and thermoplastics such as polypropylene and polyethylene. Resins and the like are used, but when such a binder is used, various problems are likely to occur. Although the problems differ depending on the binder, binders such as SBR and NBR, when used on the negative electrode side, bond the particles by forming a two-dimensional film on the active material particle surface, Therefore, it is considered that the charge transfer resistance at the interface increases and the resistance of the electrode increases. Further, when used on the positive electrode side, it has a double bond remaining in its main chain, so that it is oxidatively decomposed at the time of charging and easily causes gas generation and characteristic deterioration. On the other hand, the (meth) acrylate polymer having a nitrile group used in the present invention does not have such a difficulty, and can bind the active material particles in a form close to point contact so that the resistance of the electrode can be kept small. Seem.

  The lithium ion capacitor of the present invention comprises a positive electrode, a negative electrode, and an aprotic organic electrolyte of lithium salt as an electrolyte, and the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions, 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 with respect to the negative electrode and / or the positive electrode is 2.0 V or less. In a capacitor in which lithium ions are not doped with respect to the negative electrode and / or the positive electrode, the potentials of the positive electrode and the negative electrode are both 3V, and before charging, the potential of the positive electrode after short-circuiting the positive electrode and the negative electrode is 3V. In the present invention, the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2 V or less. The potential of the positive electrode determined by one of the following two methods (A) or (B) is 2 V or less. Refers to cases. That is, (A) After doping with lithium ions, the positive electrode terminal and the negative electrode terminal of the capacitor cell are directly coupled with a conductive wire and left for 12 hours or more, then the short circuit is released, and within 0.5 to 1.5 hours Measured positive electrode potential. (B) After discharging at constant current to 0 V over 12 hours in a charge / discharge tester, leaving the positive electrode terminal and the negative electrode terminal connected with a conductive wire for 12 hours or more, releasing the short circuit, 0.5 to Positive electrode potential measured within 1.5 hours.

  In addition, the positive electrode potential after the short-circuit is 2.0 V or less is not limited to just after the lithium ions are doped, but when the battery is short-circuited after repeated charging, discharging or charging / discharging, etc. In this state, the positive electrode potential after the short circuit is 2.0 V or less.

The fact that the positive electrode potential 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 around 3 V (Li / Li ⁺ ), and when a cell is formed using activated carbon for both the positive electrode and the negative electrode, both potentials are about 3 V. The voltage is about 0V, and even if it is short-circuited, the positive electrode potential is about 3V regardless. The same applies to a so-called hybrid capacitor using activated carbon as the positive electrode and carbon material such as graphite or non-graphitizable carbon used in the lithium ion secondary battery as the negative electrode. Therefore, the cell voltage is about 0V, and 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-described hybrid capacitor in which the positive electrode potential is about 3 V at the time of short circuit, when the upper limit potential of the positive electrode is 4.0 V, for example, the positive electrode potential at the time of discharge is up to 3.0 V, and the potential change of the positive electrode 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. If the efficiency is lowered and the cell is short-circuited after repeated charging and discharging, the positive electrode potential becomes higher than 3 V, and the utilization capacity further decreases. That is, 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 become 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 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, the lithium ions supported on the negative electrode should be adjusted in view of the characteristics of the negative electrode material and the positive electrode material so that the positive electrode potential after short-circuiting is 2.0 V or less. is required.

  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 after the positive electrode and the negative electrode are short-circuited is set to 2 V or less, whereby the available capacity of the positive electrode is reduced. Higher capacity results in higher capacity and higher energy density. As the supply amount of lithium ions increases, the positive electrode potential when the positive electrode and the negative electrode are short-circuited decreases and the energy density improves. In order to obtain a higher energy density, 1.5 V or less, particularly 1 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 V when the positive electrode and the negative electrode are short-circuited, and the energy density of the cell is reduced.

  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 reduction 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.

  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 in the negative electrode and / or the positive electrode in advance is not particularly limited. For example, a lithium 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 supply source (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 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 current collector, a porous metal 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, it is preferable that the lithium electrode is provided at a position facing the negative electrode current collector and lithium ions are 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 to face the negative electrode and / 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, lithium ions can be uniformly doped even when a lithium electrode doped in the negative electrode and / or the positive electrode is locally arranged in the cell. Therefore, even in the case of a large-capacity cell in which the positive electrode and the negative electrode are stacked 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 and the like for the negative electrode current collector. Can be used respectively. 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 negative electrode active material constituting the negative electrode in the lithium ion capacitor of the present invention is formed from a material that can reversibly carry lithium ions. The negative electrode active material is preferably formed from negative electrode active material particles having a D50 of 0.5 to 30 μm. D50 is preferably 0.5 to 15 μm, and particularly preferably 0.5 to 6 μm. The negative electrode active material particle of the present invention, the specific surface area preferably is preferred that a 0.1~2000m ² / g, preferably 0.1~1000m ² / g, particularly 0.1 ˜600 m ² / g is preferred.

  As a preferable negative electrode active material used in the present invention, a carbon material such as graphite, non-graphitizable carbon, graphitizable carbon or the like, or a heat-treated product of an aromatic condensation polymer having a hydrogen atom / carbon atom ratio A polyacene-based organic semiconductor (PAS) having a polyacene-based skeleton structure in which is 0.50 to 0.05 is preferable. The graphite may be either artificial graphite or natural graphite. Examples of the non-graphitizable carbon include phenol resin charcoal and furan resin charcoal. Examples of the graphitizable carbon include petroleum coke, coal pitch coke, and polyvinyl chloride charcoal. I can do it.

  Since the PAS used in the present invention has an amorphous structure, there is no structural change such as swelling / shrinkage with respect to the insertion / desorption of lithium ions, and thus the cycle characteristics are excellent. The isotropic molecular structure (higher order structure) is preferable because it is excellent in rapid charge and rapid discharge. 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

（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 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. It is also possible to use a modified aromatic condensation polymer such as a condensate of phenol, xylene and formaldehyde. Furthermore, a modified aromatic polymer substituted with melamine or urea can be used, and a furan resin is also suitable.

  In the present invention, the PAS is preferably produced as follows. 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), the atomic ratio of hydrogen atoms / carbon atoms (hereinafter referred to as “atom number ratio”) H / C) is an insoluble and infusible substrate having 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 negative electrode is formed by binding a negative electrode active material particle as a binder with a (meth) acrylate polymer having a nitrile group. As the (meth) acrylate polymer having a nitrile group used in the present invention, various polymers can be used, but typically, the following three polymers, that is, (meth) acrylic acid ester and (meth) ) Copolymer with acrylonitrile (a), (meth) acrylic acid ester, vinyl monomer having a carboxylic acid group, copolymer with (meth) acrylonitrile (b), heavy weight containing (meth) acrylic acid ester The graft polymer (c) obtained by grafting (meth) acrylonitrile on the coalescence is preferable.

  The (meth) acrylic acid ester constituting the copolymer (a) is preferably an ester of (meth) acrylic acid and an alcohol having 1 to 12 carbon atoms. Preferred examples thereof include ester butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, nonyl methacrylate, decyl methacrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate , Heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, etc., preferably heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate. Moreover, as (meth) acrylonitrile, acrylonitrile is preferable.

  A preferred composition constituting the copolymer (a) is (meth) acrylic acid ester of 50 to 95 mol%, particularly 60 to 90 mol%, and (meth) acrylonitrile is 5 to 50 mol%. In particular, the content is preferably 10 to 40 mol%. In the copolymer (a), a monomer other than (meth) acrylic acid ester and (meth) acrylonitrile may be copolymerized. These other monomers include monofunctional monomers such as styrene, α-methylstyrene, chlorostyrene, acrylamide, vinyl benzyl alcohol, styrene sulfinate, styrene sulfonate, divinylbenzene, ethylene glycol dimethacrylate, isopropylene glycol diester. Bifunctional monomers such as acrylate and tetramethylene glycol dimethacrylate may be mentioned. However, in this case, it is preferable not to use an ethylenic hydrocarbon monomer or a diene monomer such as butadiene.

  Preferred examples of the (meth) acrylic acid ester and (meth) acrylonitrile constituting the copolymer (b) are the same as those in the copolymer (a). On the other hand, the vinyl monomer which has a carboxylic acid group is acrylic acid, methacrylic acid, and maleic acid, Preferably they are acrylic acid and methacrylic acid. A preferred composition for forming the copolymer (b) is (meth) acrylic acid ester of 50 to 95 mol%, particularly 60 to 90 mol%, (meth) acrylonitrile is 3 to 40 mol%, Is preferably from 5 to 300 mol%, and preferably from 1 to 25 mol%, particularly preferably from 3 to 20 mol%, of the vinyl monomer having a carboxylic acid group. In the copolymer (b), in addition to the above three components, other monomers exemplified in the case of the copolymer (a) may be copolymerized as in the case of the copolymer (a). It is.

  Preferable specific examples of the copolymer (b) include the following copolymers. 2-ethylhexyl acrylate, copolymer of acrylic acid and acrylonitrile (copolymerization molar ratio 80: 5: 15), nonyl acrylate, acrylic acid, copolymer of acrylonitrile and styrene (copolymerization molar ratio 82: 3: 10: 5) ), Butyl acrylate, acrylic acid, copolymer of acrylonitrile and divinylbenzene (copolymerization molar ratio 80: 5: 10: 5), 2-ethylhexyl acrylate, copolymer of methacrylic acid and acrylonitrile (copolymerization molar ratio 80) : 5: 15), a copolymer of ethyl acrylate, acrylic acid and acrylonitrile (copolymerization molar ratio 80: 5: 15).

  The polymer containing (meth) acrylic acid ester in the graft polymer constituting the copolymer (c) is a homopolymer of (meth) acrylic acid ester, or (meth) acrylic acid ester and a carboxylic acid group. Preferred are vinyl monomers and copolymers having Preferred examples of the (meth) acrylic acid ester and (meth) acrylonitrile are the same as those in the copolymer (a). In the graft polymer of the copolymer (c), the branched polymer of (meth) acrylonitrile is preferably contained in 10 to 80 parts by weight, particularly 20 to 60 parts by weight with respect to 100 parts by weight of the backbone polymer. Is preferred.

  The binder made of the (meth) acrylate polymer having a nitrile group of the present invention can be used by mixing with other binders. When other binders are used in combination, the use ratio of the binder made of the (meth) acrylate polymer having a nitrile group is 30% by weight or more, preferably 50% by weight or more. Other binders include starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, polyvinyl chloride, polyvinyl pyrrolidone, tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene-diene terpolymer. (EPDM), sulfonated EPDM, styrene butadiene rubber, polybutadiene, fluororubber, polyethylene oxide and other polysaccharides, thermoplastic resins, rubber-elastic polymers, and the like may be used alone or as a mixture thereof. Of these, carboxymethyl cellulose is preferable. Carboxymethyl cellulose preferably has a molecular weight of 200,000 to 5,000,000, more preferably 500,000 to 2,000,000. The degree of etherification is preferably from 0.5 to 1.0, more preferably from 0.6 to 0.8.

  In the present invention, the binder is preferably used as an emulsion or suspension emulsified or suspended in water. The content of the binder in the emulsion or suspension is preferably from 30 to 50% by weight, particularly preferably from 35 to 45% by weight, as the solid content. The amount of the binder used varies depending on the electrical conductivity of the positive electrode active material, the electrode shape, etc., but is preferably 1 to 20% by weight, particularly preferably 2 to 10% by weight, based on the positive electrode active material. It is.

  In this invention, when forming a negative electrode from negative electrode active material particles and a binder, a electrically conductive agent is used as needed. Examples of the conductive agent include acetylene black, graphite, and metal powder. The conductive agent varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, and the like, but is preferably 2 to 40 parts by weight, particularly preferably 5 to 10 parts by weight, based on 100 parts by weight of the positive electrode active material. It is. Formation of the negative electrode from the negative electrode active material particles, the binder, and the conductive agent used as necessary is produced by known means. For example, negative electrode active material particles, a binder, and a conductive agent used as necessary are dispersed in an aqueous medium to form a slurry, and the slurry is applied to the current collector, or the slurry is preliminarily sheeted. It is formed into a shape and is preferably attached to a current collector using a conductive adhesive.

  On the other hand, 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. As such a positive electrode active material, various materials can be used, and activated carbon, polyacene-based material (PAS) described as the negative electrode active material, and the like can be given. These carbon materials and PAS are obtained by carbonizing a phenol resin or the like, activated as necessary, and then pulverized.

In the present invention, the positive electrode active material is formed from positive electrode active material particles having a D50 of 0.5 to 30 μm. D50 is preferably 0.5 to 15 μm, and particularly preferably 0.5 to 6 μm. Moreover, the positive 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 positive electrode in the present invention is formed from the positive electrode active material described above, and the same means as in the case of the negative electrode can be used. That is, a positive electrode active material powder, a binder and, if necessary, a conductive powder are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is applied to a current collector as necessary, or the slurry May be formed into a sheet shape in advance and attached to a current collector. Examples of the binder used here include rubber-based binders such as SBR and NBR, fluorine-containing resins such as polytetrafluoroethylene and polyvinylidene fluoride, and thermoplastic resins such as polypropylene, polyethylene and polyacrylate. Can be used. Especially, as a binder, use of the (meth) acrylate polymer which has a nitrile group used in the case of manufacture of the said negative electrode is preferable. Moreover, as the (meth) acrylate polymer having a nitrile group, the following three polymers, that is, a copolymer (a) of (meth) acrylic acid ester and (meth) acrylonitrile, (meth) acrylic acid ester And a copolymer (b) of a vinyl monomer having a carboxylic acid group and (meth) acrylonitrile (b), a polymer containing (meth) acrylic acid ester, and a graft polymer obtained by grafting (meth) acrylonitrile ( c) is preferred.

  The amount of binder used in the production of the positive electrode varies depending on the electrical conductivity of the positive electrode active material, the electrode shape, and the like, but it is appropriate to add 2-40 parts by weight with respect to 100 parts by weight of the positive electrode active material. In the production of the positive electrode, the additives described in the production of the negative electrode can be added in the same manner, and the specific production means can be the same as the means described in the production of the negative electrode.

  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, γ-butyrolactone, acetonitrile, dimethoxyethane, Tetrahydrofuran, dioxolane, methylene chloride, sulfolane and the like can be mentioned. 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 , and LiN (CF ₃ SO ₂ ) ₂ . 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 is at least 0.1 mol / l 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 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 WO 00/07255, International Publication WO 03/003395, 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.

Example 1
(Method for producing positive electrode 1)
6 parts by weight of acetylene black powder, copolymer of methyl acrylate and acrylonitrile (former / latter copolymer molar ratio = 7) with respect to 92 parts by weight of commercially available activated carbon (D50 = 6 μm, specific surface area 1950 m ² / g) / 3 emulsion, solid content 40 wt%, Tg-40 ° C., viscosity 50 mPa · s, pH 8.0) 5 parts, carboxymethyl cellulose (CMC) 4 parts, and ion-exchanged water 200 parts by weight were added to the mixing stirrer. And mixed well to obtain a slurry. The positive electrode 1 was obtained by applying the slurry to a solid content of about 7 mg / cm ² on one side of an aluminum foil having a thickness of 20 μm coated with a carbon-based conductive paint, and drying and pressing.

(Capacitance measurement per unit weight of positive electrode 1)
Four positive electrodes were cut out into a size of 1.5 cm in length and 2.0 cm in width (thickness: 150 μm), and used as evaluation positive electrodes. A simulation cell was assembled with this positive electrode, a metallic lithium plate having a size of 1.5 cm in length and 2.0 cm in width (thickness: 200 μm) as a counter electrode, and a non-woven fabric made of polyethylene having a thickness of 50 μm as a separator. Metallic lithium was used as a reference electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in propylene carbonate at a concentration of 1 mol / liter was used.

  The battery was charged to 3.6 V at a charging current of 1 mA and then charged at a constant voltage. After a total charging time of 1 hour, the battery was discharged to 2.5 V at 1 mA. The capacitance per unit weight of the positive electrode 1 was determined from the discharge time between 3.5 V and 2.5 V and found to be 92 F / g.

(Method for producing positive electrode 2)
A 35 μm thick (50% porosity) aluminum expanded metal (manufactured by Nippon Metal Industries Co., Ltd.) is coated with a nonaqueous carbon-based conductive paint (manufactured by Nippon Atsson Co., Ltd .: EB-815) with a die coater and dried. Thus, a current collector for a positive electrode on which a conductive layer was formed 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. The positive electrode 1 slurry is formed on both sides of the positive electrode current collector with a die coater and pressed to obtain the total thickness (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 current collector). Of the positive electrode 2 was 312 μm.

(Production method of negative electrode 1)
A 0.5 mm thick phenolic resin molded plate is placed in a siliconite electric furnace, heated to 500 ° C. at a rate of 50 ° C./hour, and further at a rate of 10 ° C./hour to 650 ° C. in a nitrogen atmosphere, followed by heat treatment. PAS was synthesized. The PAS thus obtained was pulverized with a disk mill to obtain a PAS (average particle size 5 μm) powder. This PAS powder had an H / C atomic ratio of 0.22.

Next, with respect to 92 parts by weight of the PAS powder, 6 parts by weight of acetylene black powder, a copolymer of methyl acrylate and acrylonitrile, which is the same binder as that used for the positive electrode 1, (the former / the latter copolymer molar ratio) = 7/3 emulsion, solid content 40 wt%, Tg-40 ° C., viscosity 50 mPa · s, pH 8.0) 5 parts, carboxymethylcellulose (CMC) 3 parts, ion exchange water 200 parts by weight A slurry was obtained by sufficiently mixing with a stirrer. The slurry was applied on one side of a 18 μm thick copper foil to a solid content of about 7 mg / cm ² , dried and pressed to obtain negative electrode 1.

(Capacitance measurement per unit weight of negative electrode 1)
Four negative electrodes were cut into a size of 1.5 cm in length and 2.0 cm in width (thickness 70 μm), and used as negative electrodes for evaluation. A simulated cell was assembled with this negative electrode and a metallic lithium plate having a size of 1.5 cm in length and 2.0 cm in width (thickness of 200 μm) as a counter electrode, and a non-woven fabric made of polyethylene having a thickness of 50 μm as a separator. Metallic lithium was used as a reference electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in propylene carbonate at a concentration of 1 mol / liter was used.

  Lithium ions for 280 mAh / g, 350 mAh / g, 400 mAh / g, and 500 mAh / g were charged with respect to the weight of the negative electrode active material at a charging current of 1 mA, and then discharged to 1.5 V at 1 mA. The electrostatic capacity per unit weight of the negative electrode 1 was determined from the discharge time during which the potential of the negative electrode changed by 0.2 V from the potential of the negative electrode one minute after the start of discharge. The results are shown in Table 1. The amount of charge here is a value obtained by dividing the integrated value of the charging current flowing through the negative electrode by the weight of the negative electrode active material.

(Production method of negative electrode 2)
A slurry of the negative electrode 1 is formed on both sides of a copper expanded metal (manufactured by Nippon Metal Industry Co., Ltd.) having a thickness of 32 μm (porosity 50%) with a die coater and pressed to obtain a total thickness (negative electrode layers on both sides). Negative electrode 2 having a thickness of 148 μm was obtained.

(Production of laminated unit)
The negative electrode 2 having a thickness of 148 μm and the positive electrode 2 having a thickness of 312 μm were each cut to 6.0 cm × 7.5 cm (excluding the terminal welded portion). Using a cellulose / rayon mixed nonwoven fabric with a thickness of 35 μm as a separator, the terminal welds of the positive electrode current collector and the negative electrode current collector are arranged on opposite sides as shown in FIG. Lamination was performed so that the surface had 20 layers. The uppermost part and the lowermost part are provided with separators and taped on the four sides. The terminal welded part (10 sheets) of the positive electrode current collector and the terminal welded part (11 sheets) of the negative electrode current collector are each 50 mm wide and long. An electrode laminated unit was obtained by ultrasonic welding to an aluminum positive electrode terminal and a copper negative electrode terminal having a thickness of 50 mm and a thickness of 0.2 mm. Note that 10 positive electrodes and 11 negative electrodes were used. The weight of the positive electrode active material is 1.4 times the weight of the negative electrode active material.

(Production of capacitor cell 1)
As shown in FIG. 1, a 6.5 mm deep-drawn exterior film is provided with a lithium metal foil (82 μm, 6.0 × 7.5 cm ² , equivalent to 400 mAh / g) as a lithium electrode on a 80 μm thick stainless steel mesh. A crimped product was used, one on each of the upper and lower parts of the electrode laminate unit so as to completely face the negative electrode, to obtain a three-electrode laminate unit. In addition, the terminal welding part (two sheets) of the lithium electrode current collector was resistance welded to the negative electrode terminal welding part.

The tripolar laminated unit is placed inside the deeply drawn exterior film, covered with the exterior laminate film, and fused on the three sides, and then ethylene carbonate, diethyl carbonate, and propylene carbonate as the electrolyte solution at a weight ratio of 3: 4: 1. A solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / l was vacuum impregnated in the mixed solvent, and the remaining one side was fused to assemble 8 film capacitor cells.

(Initial evaluation of the cell)
When two cells were disassembled after being left for 20 days after cell assembly, all the lithium metal was completely lost, so lithium was precharged to obtain a capacitance of 650 F / g per unit weight of the negative electrode active material. Judged that.

(Characteristic evaluation of cells)
6 cells were charged with a constant current of 1000 mA until the cell voltage reached 3.6 V, and then a constant current-constant voltage charge in which a constant voltage of 3.6 V was applied 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 cycle of 3.6V-1.9V was repeated, and the cell capacity and energy density were evaluated in the third discharge. In addition, the DC resistance of the cell was determined from the iR drop of the discharge curve. Table 2 shows the average of each cell evaluation.

Example 2
In the manufacturing method of the negative electrode 2 of Example 1 above, 2-ethylhexyl acrylate / acrylic acid / methacrylonitrile copolymer (copolymerization weight ratio of 80 parts / 8 parts / 15 parts, respectively) was used as the negative electrode binder. A negative electrode 3 was prepared in the same manner as in Example 1 except that the film capacitor cell 2 was assembled in an 8-cell assembly and evaluated in the same manner. Table 2 shows the results of the initial cell capacity, energy density, and DC resistance measurement. The values in the table are average values of 6 cells for each test.

Example 3
In Example 1 above, 43 wt% of methyl methacrylate and acrylonitrile in total are grafted on the backbone polymer of ethylene / ethyl acrylate copolymer (copolymerization molar ratio 85% / 15%) as the binder of the negative electrode A negative electrode was prepared in the same manner as in Example 1 except that the graft polymer used was used, and the film-type capacitor cell 3 was evaluated in the same manner as in the 8-cell assembly. Table 2 shows the results of the initial cell capacity, energy density, and DC resistance measurement. The values in the table are average values of 6 cells for each test.

Example 4
In Example 1, except that SBR was used as the positive electrode binder, a positive electrode was prepared in the same manner as in Example 1, and the film-type capacitor cell 4 was assembled and evaluated in the same manner. Table 2 shows the results of the initial cell capacity, energy density, and DC resistance measurement. The values in the table are average values of 6 cells for each test.

Comparative Example 1
In Example 1 above, a negative electrode was prepared in the same manner as in Example 1 except that 5 parts by weight of SBR was used in place of the copolymer of methyl acrylate and acrylonitrile as a binder for producing the negative electrode. Further, eight film capacitor cells 4 were assembled and evaluated in the same manner as in Example 1 except that this negative electrode was used. Table 2 shows the results of the initial cell capacity, energy density, and DC resistance measurement. The values in the table are average values of 6 cells for each test.

表２より、負極バインダーとしては、ニトリル基を有する（メタ）アクリレート重合体を含むバインダーの方がＳＢＲバインダーより良好な特性が得られることがわかる。

From Table 2, it can be seen that, as the negative electrode binder, the binder containing the (meth) acrylate polymer having a nitrile group can obtain better characteristics than the SBR binder.

  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 an accumulator for driving electric bicycles, electric wheelchairs and the like, an energy storage device for various energy such as solar energy and wind power generation, or an accumulator for household appliances.

FIG. 1 is a schematic diagram showing the structure of a capacitor cell used in Example 1. FIG. 1: positive electrode, 1 ′: current collector (positive electrode), 2: negative electrode, 2 ′: current collector (negative electrode), 3: separator, 4: lithium metal, 4 ′: current collector (lithium metal), 5: Conductor

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 carrying lithium ions, and the negative electrode and / or the positive electrode is doped with lithium ions so that the potential of the positive electrode becomes 2.0 V or less after the positive electrode and the negative electrode are short-circuited. And the negative electrode comprises a negative electrode active material bound with a binder containing a (meth) acrylate polymer having a nitrile group.   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 positive electrode and / or the negative electrode and a lithium ion supply source. Item 2. The lithium ion capacitor according to Item 1.   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 weight of the negative electrode active material. Lithium ion capacitor.   The lithium ion capacitor according to claim 1, wherein the positive electrode is made of a positive electrode active material bound with a binder containing a (meth) acrylate polymer having a nitrile group.   The lithium ion capacitor according to claim 1, wherein the (meth) acrylate polymer having a nitrile group is a copolymer of (meth) acrylic acid ester and (meth) acrylonitrile.   The lithium according to any one of claims 1 to 4, wherein the (meth) acrylate polymer having a nitrile group is a copolymer of (meth) acrylic acid ester, a vinyl monomer having a carboxylic acid group, and (meth) acrylonitrile. Ion capacitor.   The (meth) acrylate polymer having a nitrile group is a graft polymer obtained by grafting (meth) acrylonitrile to a polymer containing a (meth) acrylic acid ester. Lithium ion capacitor.   Polyacene in which the negative electrode active material is a heat-treated product of graphite, non-graphitizable carbon, graphitizable carbon, or aromatic condensation polymer, and the atomic ratio of hydrogen atoms / carbon atoms is 0.50 to 0.05 The lithium ion capacitor according to claim 1, wherein the lithium ion capacitor is a polyacene organic semiconductor having a skeleton structure.   The positive electrode active material is activated carbon or a heat-treated product of an aromatic condensation polymer, which is a polyacene organic semiconductor having a polyacene skeleton structure having a hydrogen atom / carbon atom atomic ratio of 0.50 to 0.05. Item 9. The lithium ion capacitor according to any one of Items 1 to 8.

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