JP2008166309A - Lithium ion capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion capacitor, having a high energy density and a high output density and exhibiting high breakdown voltage and high durability. <P>SOLUTION: In the lithium ion capacitor having a positive electrode, a negative electrode, and an aprotic organic solvent electrolytic solution of lithium salt as electrolyte, where the negative electrode and/or the positive electrode is doped with lithium ions; the positive electrode active material can dope/undope lithium ions and/or anion reversibly; the negative electrode active material can reversibly dope lithium ions; and the quantity of protons per unit weight of active material in the surface functional group of the positive electrode active material and/or the negative electrode active material is reduced. <P>COPYRIGHT: (C)2008,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 has a high voltage and a high capacity.

  On the other hand, while environmental problems are being highlighted, development of power storage devices (main power supply and auxiliary power supply) for electric vehicles or hybrid vehicles replacing gasoline vehicles has been actively conducted. In addition, lead batteries have been used as power storage devices for automobiles. 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 widespread use.

  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 capacitor, a negative electrode capable of occluding and desorbing lithium ions is brought into contact with metallic lithium, and lithium ions are occluded and supported (hereinafter also referred to as dope) 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. (See Patent Document 1 to Patent Document 4)

  Although this type of capacitor is expected to have high performance, when lithium ions are doped into the negative electrode, it is necessary to apply metallic lithium to all the negative electrodes, or locally in a part of the cell. Although it is possible to place metallic lithium in contact with the negative electrode, there is a problem in doping that requires a very long time and that the dope has uniformity over the entire negative electrode. In particular, it has been difficult to put into practical use in a large-capacity cell such as a cylindrical device in which electrodes are wound or a square battery in which a plurality of electrodes are stacked.

  However, this problem is that the negative electrode current collector and the positive electrode current collector that constitute the cell are provided with holes penetrating the front and back surfaces, and lithium ions are moved through the through holes, and at the same time, the lithium metal source and the lithium metal source are negatively charged. By short-circuiting, all the negative electrodes in the cell can be doped with lithium ions simply by arranging metallic lithium at the end of the cell, and the solution has been solved all at once (see Patent Document 5). In addition, although dope of lithium ion is normally performed with respect to a negative electrode, it is described in patent document 5 that it is the same also when performing with a negative electrode with a negative electrode instead of a negative electrode.

Thus, even in a large-sized cell such as a cylindrical device in which electrodes are wound or a square battery in which a plurality of electrodes are stacked, lithium ions are uniformly distributed over the entire negative electrode in a short time with respect to all the negative electrodes in the device. The energy density can be drastically increased by doping and improving the withstand voltage. As a result, it was possible to realize a high-capacity capacitor in combination with the large power density inherent in the electric double layer capacitor.
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

  On the other hand, according to the knowledge of the present inventors, in the case of a high energy density lithium ion capacitor in which lithium ions are doped to the negative electrode and / or the positive electrode as described above, the thickness of the cell in the obtained capacitor increases in some cases. As a result, it has been found that problems such as leakage of the electrolytic solution and an increase in internal resistance may occur, causing a serious problem in practical use.

  The present invention is a high-capacity lithium ion capacitor in which lithium ions are doped into the negative electrode and / or the positive electrode, which reliably prevents the problems associated with the increase in cell thickness as described above and has high resistance to resistance. An object is to provide a lithium capacitor having high voltage and high capacity.

In order to solve the above problems, the present inventors have conducted intensive research. As a result, the increase in cell thickness that occurs in the above lithium ion capacitor is that gas is generated in the process of doping the negative electrode and / or positive electrode with lithium ions. It was found by analysis that the main component of this gas was hydrogen gas. The generation of gas mainly composed of hydrogen gas can be reliably reduced by reducing the amount of dissociable hydrogen ions present in the surface functional groups of the active material constituting the negative electrode and / or the positive electrode. Was found.
The present invention is based on such knowledge and has 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, and doped with lithium ions with respect to the negative electrode and / or the positive electrode, the positive electrode active material Is a material capable of reversibly doping and dedoping lithium ions and / or anions, the negative electrode active material is a material capable of reversibly doping and dedoping lithium ions, and the positive electrode active material and / or A lithium ion capacitor characterized in that the amount of dissociable hydrogen ions per unit weight of active material present in a surface functional group of a negative electrode active material is reduced.
(2) The positive electrode and / or the negative electrode each include a current collector having holes penetrating the front and back surfaces, and the positive electrode and the negative electrode are short-circuited by electrochemical contact between the negative electrode and the lithium ion supply source. The lithium ion capacitor according to (1), wherein lithium ions are doped so that the potential of the positive electrode is 2.0 V (vs. Li / Li ⁺ ) or less.
(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 as described in 2).
(4) Any of the above (1) to (3), wherein the amount of dissociable hydrogen ions per unit weight of the active material present in the surface functional group is reduced by cation substitution, esterification or amidation The lithium ion capacitor according to 1.
(5) The lithium ion capacitor according to (4), wherein the cation is an alkali metal ion.
(6) A polyacene skeleton structure in which the positive electrode active material is a heat-treated product of activated carbon, a conductive polymer, or an aromatic condensation polymer, and the hydrogen atom / carbon atom ratio is 0.50 to 0.05. The lithium ion capacitor according to any one of (1) to (5), wherein the lithium ion capacitor is any one of polyacene-based organic semiconductors.
(7) A polyacene system in which the negative electrode active material is a carbon material or a heat-treated product of an aromatic condensation polymer and has a polyacene skeleton structure in which the atomic ratio of hydrogen atoms / carbon atoms is 0.50 to 0.05 The lithium ion capacitor according to any one of (1) to (6), which is an organic semiconductor.

  According to the present invention, in a lithium ion capacitor of a type in which lithium ions are doped into the negative electrode and / or the positive electrode in advance, hydrogen gas is generated at the negative electrode and / or the positive electrode, thereby increasing the cell thickness in the capacitor and various types associated therewith. Provided is a capacitor having a high energy density and a high output density that does not cause a problem, and has a high withstand voltage and excellent durability. In the present invention, by reducing the amount of dissociable hydrogen ions (hereinafter referred to as protons) per unit weight of the active material present in the surface functional group of the active material constituting the negative electrode and / or positive electrode, Although the mechanism why the above characteristics are obtained by the capacitor is not necessarily clear, it is estimated as follows.

  Lithium ion doping in the lithium ion capacitor of the present invention is usually performed when the electrolyte is injected into the system in a state where the metal lithium disposed in the capacitor is in electrochemical contact with the negative electrode and / or the positive electrode. Be started. Moreover, the reaction in which metallic lithium becomes lithium ions starts to proceed simultaneously. At this time, the lithium ion concentration is increased in the electrolyte solution near the metal lithium. As a result, the protons of the surface functional groups of the active material are ion-exchanged with the lithium ions, and the desorbed protons are the negative electrode and the metal lithium having a low reduction potential. It is considered that hydrogen gas is generated by reduction on the electrode.

  Meanwhile, according to the present invention, the absolute amount of protons desorbed from the positive electrode active material and / or the negative electrode active material is reduced by reducing the amount of protons present on the surface functional group of the positive electrode active material and / or the negative electrode active material. To do. As a result, it is considered that the amount of gas generated at the time of doping lithium ions can be reduced.

  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 can be reversibly doped / dedoped with lithium ions and / or anions And the negative electrode active material is a material capable of reversibly doping and dedoping 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 present invention, dope means occlusion, support, and adsorption, and refers to a phenomenon in which anions and / or lithium ions enter an active material. De-doping also means desorption and desorption and refers to a phenomenon in which anions and / or lithium ions are emitted from an active material.

In the lithium ion capacitor of the present invention, 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 preferably 2.0 V (Li / Li ⁺ , the same applies hereinafter) or less. . In a capacitor in which the negative electrode and / or the positive electrode is not doped with lithium ions, 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 short-circuiting the positive electrode and the negative electrode is 2.0 V or less means that the potential of the positive electrode determined by one of the following two methods (A) or (B) is 2. The case of 0.0V or less. That is, (A) After doping with lithium ions, the positive electrode terminal and the negative electrode terminal of the capacitor cell are directly connected 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 short circuit is 2.0 V or less is not limited to just after the lithium ions are doped, and 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 meaning that the positive electrode potential is doped to 2.0 V or less, which is a preferred embodiment of the present invention, will be described in detail below. As described above, activated carbon and carbon materials usually have a potential of about 3 V (vs. Li / Li ⁺ ), and when the cells are assembled using activated carbon for both the positive and negative electrodes, both potentials are about 3 V. The cell 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 a carbon material such as graphite or non-graphitizable carbon used in the lithium ion secondary battery as the negative electrode, and each potential is about 3 V. 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, and the capacitor has a high voltage and a high energy density. In general, the upper limit of the charging voltage is determined to be a voltage at which the electrolyte does not decompose at the positive electrode. Therefore, when the positive electrode potential is set to the upper limit, the charging voltage can be increased as the negative electrode potential decreases. . 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 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 the high voltage and high energy density but also the high capacity and energy density of the hybrid capacitor, it is necessary to improve the utilization capacity of the positive electrode.

  If the positive electrode potential after the short circuit falls below 3.0V, the utilization capacity increases and the capacity increases. In order to make it 2.0V or less, it is preferable to charge not only the amount charged by charging / discharging the cell but also separately charging lithium ions from metallic lithium 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 potential of the positive electrode, the negative electrode, and the metallic lithium is reached, so that both the positive electrode potential and the negative electrode potential are 3.0 V or less. As the amount of metallic lithium increases, the equilibrium potential decreases. If the negative electrode material and the positive electrode material change, the equilibrium potential also changes. Therefore, in view of the characteristics of the negative electrode material and the positive electrode material, adjustment of lithium ions to be carried on the negative electrode is preferably performed so that the positive electrode potential after the short circuit is preferably 2.0 V or less. It is necessary to.

  In the present invention, the capacitor cell is previously doped with lithium ions in the negative electrode and / or the positive electrode, and the positive electrode potential after the positive electrode and the negative electrode are short-circuited is preferably set to 2.0 V or less, so that 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 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, controlling the dope amount of the positive electrode and the dope amount of the negative electrode is complicated in the process, so that doping of lithium ions is preferably performed on the negative electrode.

  In the lithium ion capacitor of the present invention, in particular, the electrostatic capacity per unit weight of the negative electrode active material has more than three times the electrostatic capacity per unit weight of the positive electrode active material, and the positive electrode active material weight is the negative electrode active material When larger than the weight, a capacitor having a high voltage and a high capacity can be obtained. At the same time, when using a negative electrode having a capacitance per unit weight that is larger than the capacitance per unit weight of the positive electrode, the negative electrode active material weight is reduced without changing the potential change amount of the negative electrode. Therefore, the filling amount of the positive electrode active material is increased, and the capacitance and capacity of the cell are increased.

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

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

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

  A large-capacity multilayer capacitor cell is provided with a positive electrode current collector and a negative electrode current collector for receiving and distributing electricity at the positive electrode and the negative electrode, respectively. The positive electrode current collector and the negative electrode current collector are used, and the lithium electrode In the case of a cell provided with a lithium electrode, 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, the lithium ion can be uniformly doped even when the 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 or the like can be used. The lithium ion supply source arranged in the cell means a substance that contains at least lithium element and can supply lithium ions, such as metallic lithium or lithium-aluminum alloy.

  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, such as activated carbon or a heat-treated product of an aromatic condensation polymer, and a polyacene system having a hydrogen atom / carbon atom ratio of 0.50 to 0.05. A polyacene organic semiconductor (PAS) having a skeleton structure is preferred.

A wide range of particle sizes can be used for the positive electrode active material. For example, the 50% volume cumulative diameter (also referred to as D50) is 2 μm or more, preferably 2 to 50 μm, particularly 2 to 20 μm. The average pore diameter is preferably 10 nm or less, and the specific surface area is preferably 600 to 3000 m ² / g, particularly 1300 to 2500 m ² / g.

  Since the PAS used as the positive electrode active material has an amorphous structure, there is no structural change such as swelling / shrinkage with respect to doping / dedoping of lithium ions, so that cycle characteristics are excellent, and against doping / dedoping of lithium ions. 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.

  In addition, 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, for example, 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 manufactured as follows. That is, the aromatic condensation polymer is gradually heated to a suitable temperature of 400 to 800 ° C. in a non-oxidizing atmosphere (including a vacuum), whereby a hydrogen atom / carbon atom ratio (hereinafter referred to as H). / C) is 0.5 to 0.05, preferably 0.35 to 0.10. When used as a negative electrode, it can be used as it is, but when used as a positive electrode, activation treatment is performed with an oxidizing atmosphere gas such as H ₂ O, CO ₂ , and Air at a temperature of about 750 to 900 ° C. for an appropriate time. It is preferable. 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.

  On the other hand, 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. Preferable negative electrode active materials include carbon materials such as graphite, non-graphitizable carbon, and graphitizable carbon, or PAS described as the positive electrode active material.

The negative electrode active material of the present invention is 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. Moreover, the negative electrode active material particles of the present invention preferably have a specific surface area of preferably 0.1 to 2000 m ² / g, preferably 0.1 to 1000 m ² / g, and particularly 0.1. ˜600 m ² / g is preferred.

  In the present invention, the positive electrode active material and / or the negative electrode active material described above needs to reduce the amount of protons present in the surface functional group of the positive electrode active material and / or negative electrode active material. These electrode active materials have surface functional groups such as a carboxyl group, a phenolic hydroxyl group, and a lactone group. In the present invention, the amount of protons per unit weight of the active material present in the surface functional group is preferably 0.60 mmol / g or less, particularly preferably 0.45 mmol / g or less, more preferably 0.36 mmol / g or less. It is preferable to lower it. In the present invention, the amount of protons per unit weight of the active material present in the surface functional groups of the electrode active material can be determined by an acidic surface functional group content measurement method for activated carbon. In the present invention, it is preferable that both the positive electrode active material and the negative electrode active material are reduced in the amount of protons present on the surface functional group of the electrode active material, but in general, protons present on the surface functional group of the negative electrode active material. The amount is lower than the amount of protons present in the surface functional group of the positive electrode active material, and the amount of gas generated is small, so that it is particularly effective to carry out the positive electrode active material, especially the positive electrode active material made of activated carbon. is there.

  In the present invention, the means for reducing the amount of protons present on the surface functional group of the electrode active material is not particularly limited, but the proton of the surface functional group may be replaced by a cation, esterified or amidated, or subjected to high heat treatment. It is mentioned as a preferable means. The substitution with a cation preferably involves reducing the proton by substituting the proton of the surface functional group with a metal ion by contacting an aqueous solution containing a metal ion such as an alkali metal or alkaline earth metal. As the metal ion, since the proton to be substituted is a monovalent ion, an alkali metal ion, in particular, an electrolyte used for a lithium ion capacitor is a lithium salt, and thus lithium ion is preferable. Contact with the aqueous solution containing metal ions is usually carried out at 10 to 60 ° C. for 0.5 to 24 hours. In this case, means for promoting substitution with metal ions can be used in combination. Examples of such promoting means include ultrasonic irradiation and stirring of an aqueous solution. In addition, the active material which substituted the proton of the surface functional group with the metal ion can be easily returned to the state before substitution by making it contact with strong acidic aqueous solution.

  When protons are reduced by the esterification or amidation treatment of protons on the surface functional groups, the esterification treatment is brought into contact with alcohols in the presence of an acid such as sulfuric acid, nitric acid or hydrochloric acid, or a dehydrating agent such as dicyclohexylcarbodiimide (DCC). Implemented. The amidation treatment is carried out by reacting with an amine compound in the presence of DCC or the like in the presence of DCC or the like in a solvent such as triethylene glycol with heating with a nitrogen-containing compound such as hydrazine. These esterification or amidation is carried out at a temperature of 40 to 300 ° C. or 0.5 to 24 hours according to a conventional method. Moreover, when performing by high heat processing, it is normally implemented at 200-900 degreeC for 1 to 10 hours. When this high heat treatment is performed, the basic structure of the active material changes, so that the capacitance of the obtained capacitor may be reduced.

  In the present invention, the positive electrode and / or the negative electrode are formed from the positive electrode active material and / or the negative electrode active material, respectively, in which the amount of protons present in the surface functional group is reduced, but known means can be used. That is, an 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 used as necessary, or the slurry May be previously formed into a sheet shape and attached to the 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. Although the usage-amount of a binder changes with electric conductivity, an electrode shape, etc. of an electrode active material, it is suitable to add in the ratio of 2-40 weight part with respect to 100 weight part of electrode active materials. Examples of the conductive powder include carbon black, graphite, carbon nanotube, carbon nanofiber, and metal powder.

  The conductive powder used as necessary in the above varies depending on the electric conductivity of the electrode active material, the electrode shape, and the like, but is preferably 2 to 40 parts by weight, particularly preferably 5 to 100 parts by weight of the electrode active material. It is preferred to use 10 to 10 parts by weight.

  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 or more in order to reduce the internal resistance due to the electrolyte. It is preferable to set it within a range of 0.5 to 1.5 mol / l.

  In addition, the lithium ion capacitor of the present invention is particularly a wound type cell in which a belt-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 desirable to apply to a laminated cell, or a large-capacity cell such as 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.

(Method for measuring the amount of acidic surface functional groups)
A weighing bottle containing 5 g of active material is allowed to stand in a dryer set at 110 ° C. and dried for 3 hours or more. After completion of drying, the weighing bottle containing the active material is allowed to stand in a desiccator containing silica gel that has been dried in advance, and allowed to cool for 1 hour or longer. After precisely weighing 1 g of the active material thus well dried and adding it to an Erlenmeyer flask, the surface of the active material is brought into contact with a basic aqueous solution such as 0.1 mol / l sodium hydroxide and subjected to ultrasonic irradiation. Protons present in the functional group are sufficiently reacted. After completion of the reaction, the amount of proton consumed by the neutralization reaction with the base, that is, the active material, is obtained by back titrating the filtrate obtained by the filtration operation with an acidic aqueous solution such as 0.05 mol / l hydrochloric acid. The amount of protons present on the surface functional group is determined.

  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
(Positive electrode 1 manufacturing method)
After adding 20 g of alkali-activated activated carbon having a specific surface area of 1910 m ² / g and a proton amount of 0.620 mmol / g per unit weight of the active material existing in the surface functional group to 200 g of distilled water and dispersing well, the concentration is reduced to 0.000 per g of activated carbon. 5.6 ml of 1 mol / l lithium hydroxide aqueous solution was added so that it might become 280 mmol, and ultrasonic irradiation was performed for 1 hour, and the proton which exists in the surface functional group which activated carbon fully substituted was replaced. A positive electrode active material was obtained by subjecting the suspension to suction filtration, washing, and drying. At this time, the proton amount per unit weight of the active material present in the surface functional group was 0.340 mmol / g.
A slurry is obtained by sufficiently mixing the positive electrode active material obtained by the above method in a composition of 92 parts by weight, 6 parts by weight of acetylene black powder, 7 parts by weight of an acrylate binder, 4 parts by weight of carboxymethyl cellulose, and 200 parts by weight of water. Got.

  The aluminum expanded metal (made by Nippon Metal Industry Co., Ltd.) with a thickness of 38 μm (porosity 47%) was coated on both sides with a non-aqueous carbon-based conductive paint (made by Nippon Atson Co., Ltd .: EB-815) with a die coater, A positive electrode current collector having a conductive layer formed thereon was obtained by drying. 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 slurry is applied to both sides of the positive electrode current collector with a roll coater, and after vacuum drying, the total thickness of the 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) The positive electrode 1 having a total thickness of 180 μm was obtained.

(Negative electrode manufacturing method)
Put a 0.5 mm thick phenolic resin molded plate in a siliconite electric furnace, heat up and heat-treat at a rate of 50 ° C./hour up to 550 ° C. and further at a rate of 10 ° C./hour up to 670 ° C. in a nitrogen atmosphere. PAS was synthesized by The PAS plate thus obtained was pulverized with a ball mill to obtain a PAS negative electrode powder having a D50 of 4 μm. H / C of this PAS negative electrode powder was 0.22.

  Next, a negative electrode slurry was obtained by sufficiently mixing with 92 parts by weight of the above PAS negative electrode 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 negative electrode slurry was applied to 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 die coater. Negative electrode 1 having a total thickness of negative electrode layers on both sides and negative electrode current collector thickness of 75 μm was obtained.

(Capacitance measurement per unit weight of negative electrode active material)
One piece was cut out so that the area of the negative electrode 1 was 1.5 × 2.0 cm ² , and used as an evaluation negative electrode. As a counter electrode of the negative electrode, a metal lithium plate having a size of 1.5 × 2.0 cm ² and a thickness of 200 μm was arranged on both surfaces of the negative electrode 1 with a 50 μm-thick polyethylene non-woven fabric as a separator, and a simulated cell was assembled. Metallic lithium was used as a reference electrode. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 was used.

  A lithium ion of 600 mAh / g was doped 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 capacitance per unit weight of the active material of the negative electrode 1 was determined 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, and was 912 F / g.

(Small film cell creation method)
For each cell, 10 positive electrodes 1 are cut into 2.4 cm × 3.8 cm, 11 negative electrodes 1 are cut into 2.5 cm × 3.9 cm, and the separator is passed through a cellulose / rayon mixed nonwoven fabric having a thickness of 35 μm. After laminating and 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 1. As the metallic lithium for 600 mAh / g with respect to the negative electrode active material weight of the negative electrode, a metal lithium foil having a thickness of 80 μm bonded to a stainless steel net having a thickness of 100 μm is used. Two sheets were arranged on the outermost part. The negative electrode (11 sheets) and the stainless steel mesh bonded with metallic lithium were welded and brought into contact with each other. As a reference electrode, a metal lithium foil having a thickness of 120 μm bonded to a stainless steel net having a thickness of 100 μm was laminated on the outermost part of the electrode lamination unit via a separator made of a cellulose / rayon mixed nonwoven fabric having a thickness of 35 μm. The outer periphery was wound with a cellulose / rayon mixed nonwoven fabric separator and taped to obtain an electrode laminate unit 2. An aluminum terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm, in which a sealant film is heat-sealed to the seal portion in advance, is stacked on the terminal welded portion (10 sheets) of the positive electrode current collector of the electrode laminate unit 2. Ultrasonic welding. Similarly, a nickel terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm in which a sealant film is heat-sealed in advance to the terminal welded portion (11 sheets) of the negative electrode current collector and the terminal welded portion of the reference electrode. Each was supersonic welded. Thus, the electrode lamination unit in which the terminals were welded to the positive electrode and the negative electrode was placed between one exterior laminate film that was deep-drawn 60 mm long, 30 mm wide, and 5 mm deep and one exterior laminate film that was not deep-drawn.

After heat-sealing the two sides of the terminal portion of the exterior laminate film and the other side, 1 mol / l of a mixed solvent in which ethylene carbonate, diethyl carbonate and propylene carbonate were used at a weight ratio of 3: 4: 1 as an electrolytic solution. After the electrode laminate unit was vacuum impregnated with a solution in which LiPF ₆ was dissolved in concentration, the remaining one side was heat-sealed under reduced pressure and vacuum sealing was performed to assemble three film-type lithium ion capacitor cells 1 It was.

Example 2
30 g of the alkali activated carbon of Example 1 was added to 400 ml of methanol and dispersed well, and then 10 ml of concentrated sulfuric acid was added. Then, it stirred for 24 hours, heating at 50 degreeC, and was made to fully react, and the surface acidic functional group of activated carbon was esterified. Next, a positive electrode 2 having a thickness of 180 μm was produced in the same manner as in Example 1 except that activated carbon obtained by subjecting the suspension to suction filtration, washing, and drying was used as a positive electrode active material. The proton amount per unit weight of the active material present in the surface functional group of the obtained activated carbon was 0.230 mmol / g. Three film-type lithium ion capacitor cells 2 similar to those in Example 1 were assembled using the positive electrode 2 and the negative electrode 1.

Example 3
A phenol resin molded plate having a thickness of 0.5 mm was 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 920 ° C. in a nitrogen atmosphere. PAS was synthesized by steam activation at 10 ° C. for 10 hours. The PAS plate thus obtained was pulverized with a ball mill to obtain a PAS positive electrode powder having a specific surface area of 1930 m ² / g, a proton amount per unit weight existing in the surface functional group of 0.480 mmol / g, and a D50 of 4 μm. It was. The H / C of this PAS positive electrode powder was 0.09.

  A positive electrode active material was produced by performing the same treatment as in Example 1 except that the above PAS positive electrode powder was used in place of the alkali-activated activated carbon of Example 1, and a positive electrode 3 having a total thickness of 180 μm was produced. Three film-type lithium ion capacitor cells 3 similar to Example 1 were assembled using the positive electrode 3 and the negative electrode 1.

Comparative Example 1
A film type lithium ion capacitor cell in the same manner as in Example 1 except that alkali-activated activated carbon treated in the same manner as in Example 1 was used as the positive electrode active material, except that 1 mol / l lithium hydroxide aqueous solution was not added. 4 was assembled in 3 cells.

Comparative Example 2
Three film-type lithium ion capacitor cells 5 were assembled in the same manner as in Example 1 except that the alkali-activated activated carbon of Example 1 that was not subjected to any treatment was used as the positive electrode active material.

Comparative Example 3
Three film-type lithium ion capacitor cells 6 were assembled in the same manner as in Example 1 except that the PAS positive electrode powder of Example 3 that was not subjected to any treatment was used as the positive electrode active material.

Example 4
The alkali activated carbon of Example 1 was placed in a static electric furnace, heated to 800 ° C. in 2 hours and 40 minutes in a nitrogen atmosphere, and held at that temperature for 4 hours. Three film type lithium ion capacitor cells 7 were assembled in the same manner as in Example 1 except that the sample taken out after cooling by cooling was used as the positive electrode active material.

(Characteristic evaluation of cells)
After standing at room temperature for 20 days, the thickness of each film capacitor cell was measured, and the rate of change from the capacitor cell thickness immediately after assembly was determined. Moreover, when one cell was disassembled for each film type capacitor cell, all of the metallic lithium was completely removed. Therefore, lithium ions for obtaining a capacitance of 912 F / g per unit weight of the negative electrode active material were obtained. It was judged that it was doped in advance. Moreover, when the external appearance and thickness of the electrode laminated unit were examined, it was not different from the state before the capacitor cell assembly.

  After each cell of the remaining film type capacitor was left at 25 ° C. for 24 hours, it was charged with a constant current of 100 mA until the cell voltage reached 3.8 V, and then a constant current of applying a constant voltage of 3.8 V− Constant voltage charging was performed for 1 hour. Next, the battery was discharged at a constant current of 100 mA until the cell voltage reached 2.2V. This 3.8V-2.2V cycle was repeated, and the third discharge capacity was measured. Table 1 shows the measurement results of the discharge capacity and the volume energy density, the measurement results of the proton amount per unit weight of the active material present in the surface functional group of the positive electrode active material, and the cell thickness change rate.

The cell thickness change rate was a value calculated by the following equation. The volume energy density was calculated from the cell thickness after standing at room temperature for 20 days.

Cell thickness change rate (%) = (cell thickness after standing at room temperature for 20 days−capacitor cell thickness immediately after assembly) ÷ capacitor cell thickness immediately after assembly × 100

  As shown in Table 1, the cell of Example 1 in which lithium hydroxide was added and the protons present on the surface functional groups of the alkali-activated activated carbon were replaced with lithium ions was compared with the cell of Comparative Example 1 in cell thickness. Since the increase was suppressed, it can be said that the gas generated from the inside of the capacitor cell could be reduced. In addition, the cell of Example 2 in which the surface functional group of the positive electrode active material is esterified is generated from the inside of the capacitor as in Example 1 because the increase in cell thickness is suppressed as compared with the cell of Comparative Example 2. It can be said that the amount of gas used was reduced. Moreover, since the increase in cell thickness is also suppressed in the cell of Example 3 using PAS instead of alkali activated activated carbon as the positive electrode active material as compared with the cell of Comparative Example 3, the gas generated from the inside of the capacitor cell is reduced. It can be said that it has been reduced. On the other hand, since the cell of Example 4 using the alkali activated activated carbon heat-treated as the positive electrode active material is suppressed from increasing in cell thickness as compared with the cell of Comparative Example 1, the capacitor is the same as in Examples 1 and 2. Although it can be said that the gas generated from the inside can be reduced, it can be seen that the capacity is small compared to the cells of Examples 1 and 2. The cell of Example 4 is considered to have a reduced capacity due to the heat treatment of the positive electrode active material.

  Moreover, 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, all were in the range of 0.65 to 0.95 V, and all were 2.0 V or less. The amount of hydrogen ions that can be dissociated per unit weight of the active material present in the surface functional group of the positive electrode active material is reduced, and the positive electrode potential when the positive electrode and the negative electrode are short-circuited is 2.0 V or less. Alternatively, it was found that a capacitor having a high energy density can be obtained by previously doping the positive electrode with lithium ions.

(Capacitance measurement per unit weight of positive electrode active material)
The positive electrode slurry of Examples 1 to 4 and Comparative Examples 1 to 3 was applied to a single side of an aluminum foil having a thickness of 20 μm coated with a carbon-based conductive paint and dried to thereby form positive electrode foil electrodes (a1), (b1), ( c1), (d1), (e1), (f1), and (g1) were obtained.

One piece was cut out so that the areas of the positive electrode foil electrodes (a1) to (g1) were 1.5 × 2.0 cm ^2, respectively, and used as positive electrodes for evaluation. As a counter electrode of the positive electrode, a simulation cell was assembled by arranging a metal lithium plate of 1.5 × 2.0 cm ² size and thickness of 200 μm on both surfaces of each positive electrode foil electrode through a 50 μm-thick polyethylene non-woven fabric as a separator. Metallic lithium was used as a reference electrode. As the electrolytic solution, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 was used.

  The 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. Table 2 shows the results obtained by determining the capacitance per unit weight of the active material of the positive electrode foil electrodes (a1) to (g1) from the discharge time between 3.5V and 2.5V.

As shown in Table 2, Examples 1-3 in which the amount of protons per unit weight of the active material existing in the surface functional group of the activated carbon was reduced by chemical substitution or modification were compared with Comparative Examples 1-3 in comparison with the positive electrode active While there was no change in the capacitance per unit weight of the substance, Example 4, which was heat-treated at 800 ° C. for 4 hours in a nitrogen atmosphere, could reduce the proton amount per unit weight of the active material present in the surface functional groups of the activated carbon. However, the electrostatic capacity per unit weight of the positive electrode active material was reduced by the heat treatment.
Moreover, the electrostatic capacity per unit weight of the negative electrode active material of each film type capacitor is 5 times or more the electrostatic capacity per unit weight of the positive electrode active material.

Claims (7)

  A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of lithium salt as an electrolyte, and doped with lithium ions for the negative electrode and / or the positive electrode, wherein the positive electrode active material is lithium ion And / or a material capable of reversibly doping / dedoping anions, a negative electrode active material capable of reversibly doping / dedoping lithium ions, and the positive electrode active material and / or negative electrode active material A lithium ion capacitor characterized in that the amount of dissociable hydrogen ions per unit weight of active material present in a surface functional group of the lithium ion capacitor is reduced. The positive electrode and / or the negative electrode each include a current collector having holes penetrating the front and back surfaces, and the positive electrode potential after the positive electrode and the negative electrode are short-circuited by electrochemical contact between the negative electrode and the lithium ion supply source. 2. The lithium ion capacitor according to claim 1, wherein lithium ions are doped so as to be 2.0 V (vs. Li / Li ⁺ ) or less.   3. The lithium according to claim 1, wherein the negative electrode active material has a capacitance per unit weight of at least three times that of 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. Ion capacitor.   The amount of dissociable hydrogen ions per unit weight of the active material present in the surface functional group according to claim 1 is reduced by substitution with a cation, or esterification or amidation. The lithium ion capacitor according to 1.   The lithium ion capacitor according to claim 4, wherein the cation is an alkali metal ion.   The positive electrode active material is activated carbon or a heat-treated product of an aromatic condensation polymer, and is a polyacene organic semiconductor having a polyacene skeleton structure in which the atomic ratio of hydrogen atoms / carbon atoms is 0.50 to 0.05 The lithium ion capacitor in any one of Claims 1-5.   2. The negative electrode active material is a polyacene organic semiconductor having a polyacene skeleton structure which is a heat-treated product of an aromatic condensation polymer and has a hydrogen atom / carbon atom ratio of 0.50 to 0.05. The lithium ion capacitor in any one of -5.