JP2007067188A - Lithium ion capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a doping lithium ion capacitor capable of maintaining a high withstand voltage, a high capacity, a DC resistance, and a high energy density in a wide temperature range of a high temperature to a low temperature, especially, even at a low temperature of -30°C. <P>SOLUTION: The lithium ion capacitor is equipped with a positive electrode, a negative electrode, and aprotic organic solvent electrolytic solution of lithium salt as electrolyte. A positive electrode active substance is a substance capable of reversibly holding lithium ion and/or anion, a negative electrode active substance is a substance capable of reversibly holding the lithium ion, and the lithium ion is doped to the negative electrode and/or the positive electrode so that a potential of the positive electrode after short-circuiting the positive and negative electrodes may be 2V or less. The electrolytic solution is aprotic organic solvent containing a mixture of an annular carbonate and a chain-like carbonate, and a lithium salt concentration is 1.3 mol/l or more. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion capacitor excellent in low temperature characteristics, comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte of 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 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, in order to put such a high-capacity capacitor into practical use, it is further required to have a high withstand voltage, a high capacity, a high energy density, and a high output density. In order to be used in a wide temperature range from low temperature to high temperature, it is required to maintain excellent characteristics in a wide temperature range, particularly in a low temperature region that is usually a problem.
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 supply source and previously doped with lithium ions in a negative electrode, and is high in a wide temperature range from high temperature to low temperature, particularly at a low temperature of −30 ° C. It is an object of the present invention to provide an excellent lithium ion capacitor that can maintain a withstand voltage, a high capacity, a low DC resistance, and a high energy density.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research. As a result, the positive electrode and the negative electrode potential after the positive electrode and the negative electrode are short-circuited are 2.0 V or less with respect to the negative electrode and / or the positive electrode. In a lithium ion capacitor pre-doped with lithium ions, the characteristics of the aprotic organic solvent electrolyte solution used are closely related to the withstand voltage, capacity, direct current resistance and energy density of the capacitor, particularly in the low temperature region, By using a solution which is an aprotic organic solvent containing a mixture of a cyclic carbonate and a chain carbonate and has a lithium salt concentration of 1.3 mol / l or more as the electrolyte solution, the above problem can be solved. And reached the present invention.

Thus, the present invention is characterized by having the following gist.
(1) A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, wherein the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions. Yes, the negative electrode active material is a material capable of reversibly carrying lithium ions, and the negative electrode and / or the positive electrode is doped with lithium ions so that the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2 V or less. A lithium ion capacitor, wherein the electrolyte solution is an aprotic organic solvent containing a mixture of a cyclic carbonate and a chain carbonate, and a lithium salt concentration is 1.3 mol / l or more.
(2) The lithium ion capacitor according to (1), wherein the lithium salt concentration is 1.5 to 2.0 mol / l.
(3) The positive electrode and / or the negative electrode each provided with a current collector having holes penetrating the front and back surfaces, and lithium ions are doped by electrochemical contact between the negative electrode and a lithium ion supply source ( The lithium ion capacitor according to 1) or (2).
(4) 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 (1) to ( The lithium ion capacitor according to any one of 3).
(5) The lithium ion capacitor according to any one of (1) to (4), wherein the aprotic organic solvent is a mixture of ethylene carbonate, propylene carbonate, and diethyl carbonate.
(6) The lithium ion capacitor according to any one of (1) to (5), wherein the lithium salt is LiPF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , or LiN (CF ₃ SO ₂ ) ₂ .

  According to the present invention, a lithium ion capacitor having a particularly high capacity, in which lithium ions are doped into the negative electrode and / or the positive electrode in advance, in a wide temperature range from a low temperature to a high temperature, particularly at a low temperature of −30 ° C., An excellent lithium ion capacitor capable of maintaining a high withstand voltage, a high capacity, a low DC resistance and a high energy density is provided. In the present invention, the electrolyte solution is an aprotic organic solvent containing a mixture of a cyclic carbonate and a chain carbonate, and by using a solution having a lithium salt concentration of 1.3 mol / l or more, why The mechanism of whether the low temperature characteristics of the doping type lithium ion capacitor are improved is not necessarily clear, but is estimated as follows.

  In a lithium ion capacitor, since the lithium ion doping and dedoping reaction is used for the charge / discharge mechanism of the negative electrode, the main component of the DC resistance at low temperatures is considered to be the charge transfer resistance of the negative electrode. As the lithium salt concentration in the electrolyte solution increases, the lithium ion concentration near the negative electrode interface also increases. It is presumed that the lithium ion concentration in the vicinity of the negative electrode interface affects the charge transfer resistance at a low temperature, thereby reducing the charge transfer resistance of the negative electrode, and as a result, reducing the DC resistance at a low temperature of the lithium ion capacitor cell. In addition, when only cyclic carbonate is used as the electrolytic solution, the characteristics at normal temperature are deteriorated, but by using a mixed solvent of cyclic carbonate and chain carbonate, the characteristics from low temperature to high temperature are maintained in a balanced manner.

  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 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 left in a state of being directly coupled with a conductive wire for 12 hours or more, and then the short circuit is released, and within 0.5 to 1.5 hours Measured positive electrode potential. (B) 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, the negative electrode and / or the positive electrode is doped with lithium ions in advance, and the positive electrode potential after the positive electrode and the negative electrode are short-circuited is set to 2 V or less, so that the use capacity of the positive electrode is increased and the capacity is increased. Energy density is obtained. As the supply amount of lithium ions increases, the positive electrode potential when the positive electrode and the negative electrode are short-circuited 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). 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 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 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 a polyacene skeleton having an atomic ratio of hydrogen atom / carbon atom of 0.5 to 0.05, which is a heat-treated product of activated carbon or an aromatic condensation polymer. Examples thereof include a polyacene organic semiconductor (PAS) having a structure.

  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. As a preferable negative electrode active material used in the present invention, a carbon material such as graphite, non-graphitizable carbon, and graphitizable carbon, or a polyacene organic semiconductor (PAS) also used as the positive electrode active material 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 carbon. Etc.

  Since the PAS used as the positive electrode and / or negative electrode active material has an amorphous structure, there is no structural change such as swelling / shrinkage with respect to insertion / desorption of lithium ions, and cycle characteristics are excellent. -The molecular structure (higher order structure) isotropic with respect to desorption is suitable 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

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

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

In the present invention, 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), a hydrogen atom / carbon atom ratio (hereinafter referred to as H). / C) is 0.5 to 0.05, preferably 0.35 to 0.10. The insoluble infusible substrate is gradually heated to a temperature of 350 to 800 ° C., preferably 400 to 750 ° C. in a non-oxidizing atmosphere (including vacuum), thereby An insoluble infusible substrate having H / C and having a specific surface area by the BET method of 600 m ² / g or more can also be obtained by, for example, activation treatment.

  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 particle diameter of the positive electrode active material is such that D50 is 2 μm or more, preferably 2 to 50 μm, and most preferably 2 to 20 μm. When the particle size is smaller than this, the positive electrode cannot be formed. The average pore diameter is preferably 10 nm or less, and the specific surface area is preferably 600 to 3000 m ² / g. Moreover, as a particle diameter of a negative electrode active material, D50 is 0.5-30 micrometers, Preferably it is 0.5-15 micrometers, Especially 0.5-6 micrometers is suitable. The negative electrode active material particle of the present invention has a specific surface area preferably 0.1~2000m ² / g, preferably 0.1~1000m ² / g, particularly 0.1~600m ² / g Is preferred.

  The positive electrode and negative electrode in the present invention are formed from the above positive electrode active material and negative electrode active material, respectively, and known means can be used. That is, active material particles, a binder and, if necessary, 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 is It may be formed into a sheet in advance, and this may be attached to the current collector, preferably using a conductive adhesive. 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 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.

  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 binder used varies depending on the electrical conductivity of the active material particles, the electrode shape, etc., but is preferably contained in an amount of 1 to 20% by weight, particularly preferably 2 to 10% by weight, based on 100 parts by weight of the active material particles. Is preferable.

  In this invention, when forming a negative electrode from the said positive electrode and negative electrode active material particle, 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 electric conductivity of the active material particles, 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 active material particles. It is.

  In addition, the aprotic organic solvent electrolyte solution used in the lithium ion capacitor of the present invention is important, and the aprotic organic solvent that forms the solution needs to contain a mixture of a cyclic carbonate and a chain carbonate. It is. As the cyclic carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and the like are preferably used. The chain carbonate is preferably dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate or the like. The mixing ratio of the cyclic carbonate and the chain carbonate is preferably from 1:99 to 80:20, particularly preferably from 10:90 to 60:40, in terms of volume ratio.

  As the aprotic organic solvent of the present invention, a mixture of ethylene carbonate, propylene carbonate, and diethyl carbonate is particularly preferable among a mixture of cyclic carbonate and chain carbonate, and the combination suppresses an increase in the viscosity of the electrolyte. In addition, the degree of dissociation of the electrolyte can be increased, the conductivity can be increased, and a capacitor cell in which the characteristics at normal temperature and the characteristics at low temperature are balanced can be manufactured.

  As the aprotic organic solvent of the present invention, it is essential to contain the above-mentioned carbonate, but other solvents can be contained if necessary. Even when such other solvents are included, the carbonate is preferably contained in an aprotic organic solvent in an amount of 50 mol% or more, particularly preferably 80 mol% or more, on a molar basis. . On the other hand, examples of the other solvent contained in the aprotic organic solvent of the present invention include γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride, sulfolane and the like.

In the present invention, an electrolyte capable of generating lithium ions can be used as the electrolyte dissolved in the aprotic organic solvent. Examples of such an electrolyte include lithium salts such as LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , and LiN (CF ₃ SO ₂ ) ₂ . No. ₆ is particularly preferable because it can obtain high conductivity and is excellent in oxidation stability.

  The lithium salt and the aprotic organic solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. In the present invention, the concentration of the lithium salt in the electrolyte solution is 1.3 mol / l or more. It is necessary to be. When the concentration of the lithium salt is smaller than 1.3 mol / l, the obtained capacitor has poor characteristics such as internal resistance and capacitance at low temperature, and it is difficult to achieve the object of the present invention. The concentration of the lithium salt is preferably 1.4 mol / l or more, particularly preferably 1.5 mol / l or more. On the other hand, it is preferable that the lithium salt concentration is large so that it can be dissolved in a solvent. If it is excessively large, it is difficult to dissolve in a solvent, and the properties at room temperature may deteriorate. Therefore, it is usually 2.5 mol / l or less, particularly 2.0 mol / l or less. Is preferred.

  As the lithium ion capacitor of the present invention, in particular, a wound cell in which a strip-like positive electrode and a negative electrode are wound through a separator, and a laminate in which three or more layers each of a plate-like positive electrode and a negative electrode are laminated via a separator. It is suitable for large-capacity cells such as a type 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. (Negative electrode manufacturing method)
A 0.5 mm-thick phenolic resin molded plate is placed in a siliconite electric furnace, heated to 550 ° C. at a rate of 50 ° C./hour, and further at a rate of 10 ° C./hour to 670 ° C. in a nitrogen atmosphere, followed by heat treatment. PAS was synthesized. The PAS plate thus obtained was pulverized with a ball mill to obtain a PAS powder having an average particle size of 4 μm. The H / C ratio of this PAS powder was 0.2.

  Next, a slurry was obtained by sufficiently mixing the composition of 92 parts by weight of the PAS powder, 6 parts by weight of acetylene black powder, 5 parts by weight of an acrylic binder, 3 parts by weight of carboxymethyl cellulose, and 200 parts by weight of water. .

  A slurry of negative electrode is formed on both sides of a copper expanded metal (manufactured by Nippon Metal Industry Co., Ltd.) having a thickness of 32 μm (porosity 57%) on both sides of the negative electrode current collector with a roll coater, vacuum dried, and then the total thickness A negative electrode having a total thickness of negative electrode layers on both sides, conductive layer thickness on both sides, and negative electrode current collector thickness of 89 μm was obtained.

(Positive electrode manufacturing method)
Slurry by mixing thoroughly with a composition of 92 parts by weight of commercially available activated carbon powder having a specific surface area of 2200 m ² / g, 6 parts by weight of acetylene black powder, 7 parts by weight of acrylic binder, 4 parts by weight of carboxymethylcellulose, and 200 parts by weight of water Got.
A positive electrode with a conductive layer formed by coating a non-aqueous carbon-based conductive paint with a roll coater on both sides of an aluminum expanded metal (made by Nippon Metal Industry Co., Ltd.) with a thickness of 38 μm (porosity 47%). A current collector was obtained. The total thickness (the sum of the current collector thickness and the conductive layer thickness) was 52 μm, and the through-hole was almost blocked by the conductive paint. The positive electrode slurry is formed on both sides of the positive electrode current collector by a roll coater, and after vacuum drying, the thickness of the entire 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 current collector). A positive electrode having a total of 173 μm was obtained.

(Capacitance measurement per unit weight of negative electrode)
Four negative electrodes were cut into 1.5 × 2.0 cm ² sizes, and used as negative electrodes for evaluation. As a negative electrode and a counter electrode, a simulation cell was assembled with a metallic lithium having a size of 1.5 × 2.0 cm ² and a thickness of 200 μm as a separator and a nonwoven fabric made of polyethylene having a thickness of 50 μm. 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 / l was used. Lithium was charged for 600 mAh / g 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. When the electrostatic capacity per unit weight of the negative electrode 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, it was 912 F / g.

(Capacitance measurement per unit weight of positive electrode)
Four positive electrodes were cut into 1.5 × 2.0 cm ² sizes, and used as positive electrodes for evaluation. As a positive electrode and a counter electrode, a simulation cell was assembled with a lithium metal non-woven fabric having a size of 1.5 × 2.0 cm ² and a thickness of 200 μm and a thickness of 50 μm as a separator. Lithium metal 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 / l 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. When the electrostatic capacity per unit weight of the positive electrode was determined from the discharge time between 3.5 V and 2.5 V, it was 160 F / g.

(Film type capacitor cell creation method)
After cutting 5 pieces of the positive electrode to 2.4 cm × 3.8 cm and 6 pieces of the negative electrode to 2.4 cm × 3.8 cm, laminating them via a separator as shown in FIG. 1 and drying at 150 ° C. for 12 hours, Separators were placed on the uppermost and lowermost parts, and four sides were taped to obtain an electrode laminate unit. As the lithium metal for 600 mAh / g with respect to the weight of the negative electrode active material, a lithium metal foil with a thickness of 70 μm is bonded to a copper lath with a thickness of 23 μm, and the outermost part of the electrode stack unit is opposed to the negative electrode. One was placed in the. The negative electrode (six pieces) and the stainless steel mesh bonded with lithium metal were welded and brought into contact with each other to obtain an electrode laminated unit. An aluminum positive terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm, in which a sealant film is heat-sealed in advance to the seal portion, is stacked on the terminal welded portion (five pieces) of the positive electrode current collector of the electrode laminate unit. Ultrasonic welding. Similarly, a nickel negative electrode terminal having a width of 3 mm, a length of 50 mm, and a thickness of 0.1 mm, in which a sealant film is thermally fused in advance to the seal portion, is superposed on the terminal welded portion (six pieces) of the negative electrode current collector, and ultrasonic welding is performed. Then, it was installed between one exterior film deeply drawn to 60 mm in length, 30 mm in width, and 3 mm in depth, and one exterior film not deeply drawn.

  After heat-sealing the two sides of the terminal part of the exterior laminate film and the other side, the film is impregnated with the electrolyte, and then the other side is heat-sealed under reduced pressure and vacuum sealed. Type capacitor cell was assembled.

In Example 1, a solution obtained by dissolving LiPF ₆ at a concentration of 1.3 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 3: 4: 1 as an electrolytic solution was used. Three cells were produced.

In Example 2, a solution obtained by dissolving LiPF ₆ at a concentration of 1.5 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 3: 4: 1 as an electrolytic solution was used. Three cells were produced.

In Example 3, a solution of LiPF ₆ dissolved at a concentration of 1.5 mol / l in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 2: 3 was used as an electrolytic solution. did.

In Comparative Example 1, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 3: 4: 1 as an electrolytic solution was used. Three cells were produced.

In Comparative Example 2, three cells were produced using a solution obtained by dissolving LiPF ₆ at a concentration of 1.5 mol / l in a mixed solvent of ethylene carbonate and propylene carbonate in a volume ratio of 2: 3 as an electrolytic solution.

(Characteristic evaluation of cells)
When each cell was disassembled after standing for 14 days after cell assembly, all the lithium metal was completely lost. Therefore, lithium ions for obtaining a capacitance of 912 F / g per unit weight of the negative electrode active material were obtained. Judged to be precharged.

  Then, when the positive electrode and the negative electrode were short-circuited for each cell and the potential of the positive electrode was measured, all the positive electrode potentials were in the range of 0.85 to 0.95 V, and were 2.0 V or less.

  Each cell of the remaining film-type capacitor was charged with a constant current of 200 mA until the cell voltage reached 3.8 V, and then a constant current-constant voltage charge for applying a constant voltage of 3.8 V was performed for 30 minutes. Next, the battery was discharged at a constant current of 200 mA until the cell voltage reached 1.9V. The initial capacitance, energy density, and direct current resistance were calculated from the cell capacity, discharge start voltage, discharge end voltage, average voltage, and IR drop of this 3.8V-1.9V cycle.

  Then, after leaving it to stand in a -30 degreeC thermostat for 3 hours, the same charging / discharging cycle was performed, and the electrostatic capacitance and DC resistance were computed.

As shown in Table 1, in the cells previously doped with lithium ions, each cell has a high energy density. A solution of LiPF ₆ dissolved at a concentration of 2.0 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 causes LiPF ₆ crystals to precipitate during cell preparation. Since the cell having an electrolyte solution solute concentration of 2.0 mol / l could not be prepared, the solute concentration is preferably 2.0 mol / l or less. In addition, when only cyclic carbonate is used as the electrolyte solvent (Comparative Example 2), the electrolyte solvent is a mixed solvent of cyclic carbonate and chain carbonate because DC resistance at room temperature is significantly higher than that of other solvents. It is preferable that

  As shown in Table 2, the capacitor cell of the example of the present invention has a high capacitance at −30 ° C. and a low DC resistance compared to Comparative Example 1, and thus has excellent low temperature characteristics. Recognize. In addition, since the solute concentration of 1.5 mol / l (Example 2) is superior in low temperature characteristics as compared to the case of solute concentration of 1.3 mol / l (Example 1), the solute concentration is 1.5 mol / l. It is particularly preferable to set it to 1 or more. On the other hand, when only the cyclic carbonate is used as the electrolyte solvent (Comparative Example 2), the characteristics at −30 ° C. are excellent, but the characteristics at room temperature are significantly inferior to the capacitor cell of the example (see Table 1). It is unsuitable to comprehensively judge the characteristics of low temperature to high temperature.

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

FIG. 1 is a schematic diagram showing the structure of a capacitor cell used in Example 1. FIG.

Explanation of symbols

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 (6)

  A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, wherein the positive electrode active material is a substance capable of reversibly carrying lithium ions and / or anions, The active material is a material capable of reversibly supporting lithium ions, and the negative electrode and / or the positive electrode is doped with lithium ions so that the potential of the positive electrode after the positive electrode and the negative electrode are short-circuited is 2 V or less. The lithium ion capacitor is characterized in that the electrolyte solution is an aprotic organic solvent containing a mixture of a cyclic carbonate and a chain carbonate, and the lithium salt concentration is 1.3 mol / l or more.   The lithium ion capacitor according to claim 1, wherein the lithium salt concentration is 1.5 to 2.0 mol / l.   The said positive electrode and / or negative electrode are each equipped with the electrical power collector which has the hole which penetrates front and back, respectively, The lithium ion is doped by the electrochemical contact of a negative electrode and a lithium ion supply source. The lithium ion capacitor according to 1.   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. Item 3. The lithium ion capacitor according to Item 1 or 2.   The lithium ion capacitor according to any one of claims 1 to 4, wherein the aprotic organic solvent is a mixture of ethylene carbonate, propylene carbonate, and diethyl carbonate. The lithium ion capacitor according to claim 1, wherein the lithium salt is LiPF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , or LiN (CF ₃ SO ₂ ) ₂ .

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