JP4731974B2 - Lithium ion capacitor - Google Patents

Description

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

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

  On the other hand, while environmental problems have been highlighted, the development of power storage devices (main power and auxiliary power) for electric vehicles or hybrid vehicles replacing gasoline vehicles has been actively carried out. Until now, lead batteries have been used. However, with the enhancement of in-vehicle electrical equipment and equipment, new power storage devices are being demanded in terms of energy density and output density.

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

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

  In recent years, a power storage device called a hybrid capacitor, which combines the power storage principles of a lithium ion secondary battery and an electric double layer capacitor, has attracted attention as a power storage device corresponding to applications requiring such high energy density and high output characteristics. In hybrid capacitors, a polarizable electrode is usually used for the positive electrode and a non-polarizable electrode is used for the negative electrode, which is attracting attention as a power storage device that combines high energy density of the battery and high output characteristics of the electric double layer. . On the other hand, in this hybrid capacitor, a negative electrode capable of inserting and extracting lithium ions is brought into contact with lithium metal, and lithium ions are stored and supported (hereinafter also referred to as doping) by a chemical method or an electrochemical method in advance. There has been proposed a capacitor intended to increase the withstand voltage and greatly increase the energy density by lowering the potential. (See Patent Document 1 to Patent Document 4)

  Although this type of hybrid capacitor is expected to have high performance, when 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 a through hole is formed in the front and back of the negative electrode current collector and the positive electrode current collector that constitute the cell, and lithium ions move through the through hole, and at the same time, lithium metal that is a lithium ion supply source and the negative electrode By short-circuiting, it was possible to do so all at once by arranging the lithium metal at the end of the cell and doping all the negative electrodes in the cell with lithium ions (see Patent Document 5). In addition, although doping of lithium ion is normally performed with respect to a negative electrode, it is described in patent document 5 that it is the same also when performing with a negative electrode with a positive electrode instead of a negative electrode.

  Thus, even in a large-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 that can be doped and the withstand voltage is increased dramatically, and the high output density inherent in the electric double layer capacitor is expected to realize a high-capacity capacitor.

However, in order to put such a high-capacity capacitor into practical use, it is further required to have a high capacity, a high energy density, and a high output density.
Japanese Patent Laid-Open No. 8-1007048 JP-A-9-55342 Japanese Patent Laid-Open No. 9-232190 JP-A-11-297578 International Publication 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. To provide a capacitor having a high discharge capacity and a high energy density in a lithium ion capacitor of a type in which lithium ions are electrochemically brought into contact with lithium metal which is a source of ions and lithium ions are previously doped into the negative electrode before charging. Let it be an issue. It is another object of the present invention to provide an inexpensive capacitor with a small amount of lithium ion source such as lithium metal used for doping lithium ions.

In order to solve the above problems, the present inventors have conducted intensive research. As a result, the negative electrode and / or the negative electrode and / or the negative electrode potential before charging so that the positive electrode and negative electrode potential after short-circuiting the positive electrode and the negative electrode is 0.95 V or less. Or in a lithium ion capacitor in which lithium ions are pre-doped with respect to the positive electrode, the physical properties of the negative electrode active material used there are closely related to the charge / discharge efficiency and discharge capacity of the capacitor, and as the negative electrode active material, The present inventors have found that the above-mentioned problems can be solved by forming from a carbon of an easily graphitizable carbon precursor, and have reached the present invention.

Thus, the present invention is characterized by having the following gist.
(1) A lithium ion capacitor comprising a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, wherein the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions. The negative electrode active material is a material capable of reversibly carrying lithium ions, and the potential of the positive electrode after short-circuiting the positive electrode and the negative electrode is 0.95 V or less so that A lithium ion capacitor, wherein ions are doped before charging, and the negative electrode active material is made of a carbide of a graphitizable carbon precursor.
(2) The positive electrode and / or the negative electrode each provided with a current collector having holes penetrating the front and back surfaces, and lithium ions are doped by electrochemical contact between the negative electrode and a lithium ion supply source ( The lithium ion capacitor as described in 1).
(3) The negative electrode active material has a capacitance per unit weight of 3 times or more as compared with the positive electrode active material, and the positive electrode active material weight is larger than the weight of the negative electrode active material. The lithium ion capacitor as described in 2).
(4) The lithium ion capacitor according to any one of (1) to (3), wherein the graphitizable carbon precursor is coke, tar, or vinyl chloride resin.
(5) The average particle diameter (D50) of the negative electrode active material particles is 0.5 to 30 μm, and the specific surface area is 0.1 to 1000 m ² / g according to any one of (1) to (4) above. Lithium ion capacitor.

  ADVANTAGE OF THE INVENTION According to this invention, it is a capacitor | condenser with a high capacity | capacitance which is a negative electrode and / or a positive electrode doped beforehand with lithium ion, and has a high discharge capacity and a high energy density. Furthermore, an inexpensive capacitor with a small amount of lithium ion source used for doping lithium ions is provided. In the present invention, as a negative electrode active material, it is not necessarily clear about the mechanism by which the excellent effect as described above is achieved by forming a capacitor from an easily graphitizable carbon precursor. Is estimated as follows. The characteristics of the lithium ion capacitor in which the negative electrode and / or the positive electrode are previously doped with lithium ions largely depend on the negative electrode material. Therefore, since the charge / discharge potential range of the carbide of the graphitizable carbon precursor which is the negative electrode material used in the present invention has a large capacity in the region of 0 to 0.25 V, the discharge capacity is high and the energy density is high. It is thought that a high characteristic expresses. The reason why the amount of the lithium ion source used may be small is that the initial charge and discharge efficiency of the carbon of the graphitizable carbon precursor as the negative electrode material is high, and even in a small charge, it is in the range of 0 to 0.25 V. This is considered to be enough capacity.

  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, 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 needs to be 0.95 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 short-circuiting the positive electrode and the negative electrode is 0.95 V or less means that the potential of the positive electrode obtained by either of the following two methods (A) or (B) It means the case of 0.95 V or less. That is, (A) After doping with lithium ions, the positive electrode terminal and the negative electrode terminal of the capacitor cell are left in a state of being directly coupled with a conductive wire for 12 hours or more, and then the short circuit is released, and within 0.5 to 1.5 hours Measured positive electrode potential, (B) Charge-discharge tester discharges constant current to 0V over 12 hours and then leaves positive electrode terminal and negative electrode terminal connected with lead wire for 12 hours or more, then releases short circuit Positive electrode potential measured within 0.5 to 1.5 hours.

In addition, the positive electrode potential after short circuit is 0.95 V or less is not limited to just after lithium ions are doped, such as when short-circuiting after repeated charging, discharging or charging / discharging, In either state, the positive electrode potential after a short circuit is 0.95 V or less.

The fact that the positive electrode potential is 0.95 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 for the positive electrode and carbon material such as graphite or non-graphitizable carbon used in the lithium ion secondary battery for the negative electrode. Since it becomes 3V, the cell voltage becomes about 0V, and even if it is short-circuited, the positive electrode potential is not changed and is about 3V. 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. When 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 0.95 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 are adjusted in consideration of the characteristics of the negative electrode material and the positive electrode material so that the positive electrode potential after the short circuit becomes 0.95 V or less. It is necessary.

In the present invention, before charging the capacitor cell, the negative electrode and / or the positive electrode is preliminarily doped with lithium ions, and the positive electrode potential is 0.95 V or less after the positive electrode and the negative electrode are short-circuited. Since the use capacity becomes high, the capacity becomes high and a large energy density can be obtained. As the supply amount of lithium ions increases, the positive electrode potential when the positive electrode and the negative electrode are short-circuited decreases and the energy density improves . The positive electrode potential when the amount of supplied lithium ion to the positive electrode and / or negative electrode is less was short-circuiting between the positive electrode and the negative electrode is higher than 0.95 V, 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 decrease in cell capacity. For this reason, it is preferable that the lithium ions doped in the negative electrode and the positive electrode do not cause these problems in consideration of the respective electrode active materials. In the present invention, since controlling the doping amount of the positive electrode and the doping amount of the negative electrode becomes complicated in the process, the doping of lithium ions is preferably performed on the negative electrode.

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

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

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

  In the lithium ion capacitor of the present invention, means for doping lithium ions into the negative electrode and / or the positive electrode in advance is not particularly limited. For example, a lithium ion supply source such as 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 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 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 so as to face the negative electrode or the positive electrode. The form, number, and the like of the through holes are not particularly limited, and can be set so that lithium ions in the electrolyte described later can move between the front and back of the electrode without being blocked by the electrode current collector.

  In the lithium ion capacitor of the present invention, 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 laminated or wound, the lithium electrode can be smoothly and uniformly placed on the negative electrode and / or the positive electrode by arranging the lithium electrode in a part of the outermost or outermost cell. Ions can be doped.

  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, the lithium ion supply source in the case where the negative electrode and / or the positive electrode are doped by electrochemical contact is contained in the cell and contains at least lithium, such as lithium metal or lithium-aluminum alloy, A substance that can be supplied.

  The negative electrode active material in the lithium ion capacitor of the present invention is made of a material that can reversibly carry lithium ions. In the present invention, the negative electrode active material is formed from a carbide of a graphitizable carbon precursor. In the present invention, the “easily graphitizable carbon precursor” means a carbon-containing material such as an organic compound that can be easily graphitized. “Easily graphitized” means that, for example, a graphite structure is formed by carbonization treatment by firing at a relatively low temperature of about 1500 ° C. or less. Formation of the graphite structure can be confirmed, for example, by having a clear peak at 2θ around 25 ° in the X-ray diffraction pattern. In contrast to the graphitizable carbon precursor of the present invention, the non-graphitizable carbon precursor is not easily graphitized, and graphitization requires a graphitization catalyst at a high temperature or in some cases. Is done. Examples of these non-graphitizable carbon precursors include cellulose, phenolic resin, and epoxy resin.

In the present invention, examples of the graphitizable carbon precursor used as the negative electrode active material include coke, tar, sawdust, vinyl chloride resin, polyacrylonitrile resin, and polyimide resin. Among these, petroleum or coal coke, tar, pitch, sawdust or vinyl chloride resin is preferable. These graphitizable carbon precursors are baked in an inert atmosphere such as nitrogen gas or argon gas at a temperature of 400 to 800 ° C. and a heating time of 1 to 3 hours to form carbides. However, for efficient firing, the firing temperature can be raised to 1500 ° C., and a graphitization catalyst may be used.
The carbide obtained by the firing may be activated as necessary.

The carbide thus obtained is then ground. The pulverization is performed using a known pulverizer such as a ball mill. By such pulverization, the carbide particles preferably have a 50% volume cumulative diameter (also referred to as D50) of 0.5 to 30 μm, particularly 0.5 to 15 μm. Further, a specific surface area of 0.1~1000m ² / g, especially 0.1~500m ² / g are preferred.

  The existing means for producing the negative electrode of the present invention from the above carbide powder can be used. That is, a carbide 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 is applied in advance. You may shape | mold in a sheet form and affix this on a collector. As the binder used here, for example, a rubber-based binder such as SBR, a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, or a thermoplastic resin such as polypropylene or polyethylene can be used. Examples of the conductive agent used as necessary in the above include acetylene black, graphite, and metal powder. The amount of the conductive agent used varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, and the like, but it is appropriate to add the conductive agent at a ratio of 2 to 40% with respect to the negative electrode active material.

  As a negative electrode active material of a lithium ion capacitor, it has been reported that a polyacene-based material (hereinafter also referred to as PAS) is more preferable for obtaining a high capacity. PAS is a heat-treated product of a phenol resin that is a non-graphitizable carbon precursor. As an example, when a discharge is performed after supporting 400 mAh / g of lithium ions, a capacitance of 650 F / g or more is obtained. Further, when a lithium ion of 500 mAh / g or more is charged, a capacitance of 750 F / g or more is obtained. On the other hand, as an example of the negative electrode active material used in the present invention, the coke carbide has a capacitance of 650 F / g or more when discharged after charging 220 mAh / g of lithium ions, and 300 mAh / g. As the above lithium ion is charged, a capacitance of 900 F / g or more can be obtained, and by supplying less lithium ions, a capacity equal to or higher than that of PAS can be obtained. As a negative electrode active material for a lithium ion capacitor, Is suitable.

On the other hand, the positive electrode active material in the lithium ion capacitor of the present invention is formed from a material that can reversibly carry lithium ions and / or anions. As a positive electrode active material, materials, such as activated carbon, a conductive polymer, and PAS, can be used, for example. In the present invention, the particle size characteristic of the positive electrode active material is such that the average particle diameter (also referred to as D50) is preferably 2 to 50 μm, particularly preferably 2 to 20 μm. The specific surface area is preferably 600 to 3000 m ² / g.

  The positive electrode in the present invention is formed from the above positive electrode active material powder, and the existing means can be used as in the case of the negative electrode. That is, the positive electrode active material powder, the binder and, if necessary, the conductive powder are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is applied to the current collector, or the slurry is previously formed into a sheet shape. You may shape | mold and stick this to a collector. As the binder used here, for example, a rubber-based binder such as SBR, a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, or a thermoplastic resin such as polypropylene or polyethylene can be used. The amount of the binder used varies depending on the electrical conductivity of the positive electrode active material, the electrode shape, and the like, but it is appropriate to add 1 to 20% by weight based on the positive electrode active material.

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

Any electrolyte can be used as long as it is an electrolyte capable of generating lithium ions as the electrolyte dissolved in the single or mixed solvent. Examples of such an electrolyte include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF _{6 and the} like. The above electrolyte and solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. The concentration of the electrolyte in the electrolyte is at least 0.1 mol / liter in order to reduce the internal resistance of the electrolyte. It is preferable to set it as the above, and it is still more preferable to set it in the range of 0.5-1.5 mol / liter.

  In addition, as the lithium ion capacitor of the present invention, in particular, a wound type cell in which a strip-like positive electrode and a negative electrode are wound through a separator, and a plate-like positive electrode and a negative electrode are laminated in three or more layers through a separator. It is suitable for a large-capacity cell such as a laminated cell or a film-type cell in which a laminate of three or more layers each having a plate-like positive electrode and negative electrode through a separator is enclosed in an exterior film. The structures of these cells are already known from lithium ion secondary batteries and large electric double layer capacitors developed for automobiles. The capacitor cell of the present invention or the amount of binder used is that of the positive electrode active material. Although it varies depending on the electric conductivity, the electrode shape, etc., it is appropriate to add at a ratio of 1 to 20 wt% with respect to the positive electrode active material.

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

Any electrolyte can be used as long as it is an electrolyte capable of generating lithium ions as the electrolyte dissolved in the single or mixed solvent. Examples of such an electrolyte include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF _{6 and the} like. The above electrolyte and solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. The concentration of the electrolyte in the electrolyte is at least 0.1 mol / liter in order to reduce the internal resistance of the electrolyte. It is preferable to set it as the above, and it is still more preferable to set it in the range of 0.5-1.5 mol / liter.

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

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

(Laminate cell single electrode evaluation)
Example 1
A coke material having an average particle size (D50) of 14 μm used as the negative electrode active material was carbonized by heat treatment at 780 ° C. for 2 hours in a nitrogen atmosphere in order to eliminate variation in thermal history. This heat-treated coke had a hydrogen / carbon atom number ratio of 0.10 in CHN elemental analysis. Since the powder was agglomerated by the heat treatment, the powder was pulverized by a vibration mill to obtain graphitizable carbon powder made of carbide having an average particle diameter (D50) of 7 μm.

  To 92 parts by weight of this graphitizable carbon powder, 6 parts by weight of acetylene black powder, 5 parts by weight of acrylate copolymer binder, 4 parts by weight of carboxymethyl cellulose (CMC), and 200 parts by weight of ion-exchanged water were added. The negative electrode slurry 1 was obtained by sufficiently mixing with a mixing stirrer.

The obtained negative electrode slurry was applied to one side of a 18 μm thick copper foil so that the solid content was 2.5 mg / cm ^2, and vacuum dried at 200 ° C. for 20 hours to obtain a negative electrode. The negative electrode was cut into a size of 2.4 × 3.8 cm ² to prepare a negative electrode foil electrode 1.

Two simulated laminate cells of the negative electrode foil electrode 1 were assembled with metallic lithium having the same size and thickness of 250 μm as a counter electrode, and a non-woven fabric made of polyethylene having a thickness of 50 μm as a separator. Moreover, metallic lithium was used for the 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.

  One simulated laminate cell was charged at 25 ° C. with a constant current of 30 mA until the negative electrode potential became 25 mV, and then subjected to constant current-constant voltage charging in which a constant voltage of 25 mV was applied for 12 hours. Next, discharging was performed at a constant current of 3 mA until the negative electrode potential reached 1.5 V, and the initial discharge capacity was measured. The results are shown in Table 1.

  The remaining 1 cell was charged with 220 mAh / g of lithium ions with respect to the amount of the negative electrode active material at a charging current of 3 mA, and then discharged to 1.5 V at 3 mA. The capacitance per unit weight of the negative electrode foil electrode 1 was determined from the discharge time during which the potential of the negative electrode changed by 0.2 V from the potential of the negative electrode one minute after the start of discharge, and it was 672 F / g.

(Comparative Example 1)
As a negative electrode active material material, a granular phenol resin (Bellpearl R700; manufactured by Air Water Bellpearl) having an average particle diameter (D50) of 17 μm was used instead of coke. This phenol resin powder was placed in a static electric furnace, heated to 750 ° C. at a heating rate of 50 ° C./hour in a nitrogen atmosphere, and further maintained at 750 ° C. for 5 hours to obtain a PAS powder. Furthermore, this was pulverized with an alumina ball mill pulverizer for 6 hours to obtain a PAS powder having an average particle diameter (D50) of 7 μm.

  Using this PAS powder, a negative electrode slurry 2 was prepared in the same manner as in Example 1, and then coated on one side of a copper foil having a thickness of 18 μm to obtain a negative electrode foil electrode 2. Further, two simulated laminate cells were prepared using the negative electrode foil electrode 2, and initial discharge capacity was measured for one cell. Table 1 shows the evaluation results together with the results of Example 1.

  Further, the remaining 1 cell was charged with 380 mAh / g of lithium ions with respect to the amount of the negative electrode active material at a charging current of 3 mA, and then discharged to 1.5 V at 3 mA. The capacitance per unit weight of the negative electrode foil electrode 2 was determined from the discharge time during which the potential of the negative electrode changed by 0.2 V from the potential of the negative electrode one minute after the start of discharge, and was 671 F / g.

  As can be seen from Table 1, when a carbonized coke of an easily graphitizable carbon precursor is used as an active material particle as a negative electrode active material, a PAS powder made from a non-graphitizable carbon precursor as a raw material is used. It can be seen that the charge / discharge efficiency of the initial characteristics is higher and the discharge capacity can be made larger than when it is used. That is, it can be seen that a large discharge capacity can be obtained with a small amount of charge (a small amount of lithium ions).

(Example 2)
(Method for producing positive electrode activated carbon slurry)
By using sawdust as a raw material, putting it in an electric furnace and raising the temperature to 950 ° C. at a rate of temperature increase of 50 ° C./hour in a nitrogen stream, steam activation with a mixed gas of nitrogen / steam 1: 1 for 12 hours Activated carbon having a surface area of 2450 m ² / g was produced. The activated carbon was pulverized with an alumina ball mill pulverizer for 5 hours to obtain activated carbon powder having an average particle diameter (D50) of 7 μm.

92 parts by weight of the above activated carbon powder for positive electrode, 6 parts by weight of acetylene black powder, 7 parts by weight of acrylate copolymer binder, 4 parts by weight of carboxymethyl cellulose (CMC) and 200 parts by weight of ion-exchanged water are sufficiently mixed with a mixing stirrer. Thus, positive electrode slurry 1 was obtained. The obtained positive electrode slurry 1 was applied to a solid surface weight of 2.5 mg / cm ² on one side of an aluminum foil with a thickness of 20 μm coated with a carbon-based conductive paint, and the coating was carried out at 200 ° C. for 20 hours. Vacuum drying was performed to obtain a positive electrode. This positive electrode was cut into a size of 2.4 × 3.8 cm ² to produce a positive electrode foil electrode 1.

A simulated laminate cell was assembled using the positive electrode foil electrode 1 having a metal lithium having the same size and a thickness of 250 μm as a counter electrode and a non-woven fabric made of polyethylene having a thickness of 50 μm as a separator. Moreover, metallic lithium was used for the 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 simulated laminate cell was charged with a constant current of 30 mA until the positive electrode potential reached 3.8 V, and then subjected to constant current-constant voltage charging for applying a constant voltage of 3.8 V for 1 hour. Next, the battery was discharged at a constant current of 3 mA until the positive electrode potential became 2.5V. The capacitance per unit weight of the positive electrode foil electrode 1 was determined from the discharge time between 3.5 V and 2.5 V, and found to be 88 F / g.

(Production method of negative electrode expanded metal electrode)
By applying both sides of the negative electrode slurry 1 of Example 1 to a copper expanded metal (made by Nippon Metal Industry Co., Ltd.) having a thickness of 32 μm (opening ratio: 57%) using a vertical double-sided simultaneous die coater and drying. A negative electrode expanded metal electrode 1 having a total thickness of 144 μm was obtained.

(Production method of positive electrode expanded metal electrode)
Water-based carbon conductive paint is coated on both sides of aluminum expanded metal (manufactured by Nippon Metal Industry Co., Ltd.) with a thickness of 38μm (aperture ratio 45%) on both sides with a vertical double-sided simultaneous die coater and dried to conduct electricity. A positive electrode current collector with a layer formed thereon was obtained. The total thickness (the total of the current collector thickness and the conductive layer thickness) was 52 μm, and the through holes were almost blocked by the conductive paint. A positive electrode expanded metal electrode 1 having a thickness of 266 μm was obtained by coating the positive electrode slurry on both surfaces of the positive electrode current collector with a comma coater one side at a time.

(Production of stacked cells)
The negative electrode expanded metal electrode 1 having a thickness of 144 μm is cut to a size of 2.4 cm × 3.8 cm, and the positive electrode expanded metal electrode 1 having a thickness of 266 μm is cut to 2.4 cm × 3.8 cm, and cellulose having a thickness of 35 μm is used as a separator. Using a non-woven fabric mixed with rayon, the negative electrode current collector and the positive electrode current collector are arranged so that the welded portions (hereinafter referred to as connection terminal welded portions) of the positive electrode current collector are alternately opposite to each other. A cell in which five positive electrodes were laminated was prepared.

  Separators were placed on the uppermost and lowermost parts, and four sides were taped to obtain an electrode laminate unit. In order to obtain a capacitance of 670 F / g or more with respect to the weight of the negative electrode active material, a lithium metal foil having a thickness of 55 μm was prepared by pressure bonding to a stainless steel net having a thickness of 80 μm, and this was opposed to the negative electrode. One sheet was placed on the outermost part of the electrode laminate unit. The negative electrode (six pieces) and the stainless steel mesh to which the lithium metal was pressure bonded were welded and brought into contact with each other to obtain a three-pole laminated unit in which the negative electrode and the lithium metal foil were short-circuited.

  Next, a positive electrode made of aluminum having a width of 10 mm, a length of 30 mm, and a thickness of 0.2 mm, in which a sealant film is heat-sealed in advance to the terminal welded portion (five pieces) of the positive electrode current collector of the three-pole laminated unit. The terminals were superposed and ultrasonically welded. Similarly, a negative electrode terminal made of nickel (width 10mm, length 30mm, thickness 0.2mm), in which a sealant film is heat-sealed to the seal portion in advance, is overlapped and resistance welded to the terminal welds (six pieces) of the negative electrode current collector. And installed in two exterior films deeply drawn to 102 mm in length, 52 mm in width, and 1.3 mm in depth.

After heat-sealing the two sides of the terminal portion of the exterior laminate film and the other side, 1 mol / l in a mixed solvent of ethylene carbonate, diethyl carbonate and propylene carbonate in a weight ratio of 3: 4: 1 as an electrolytic solution. A solution in which LiPF ₆ was dissolved at a concentration of 2 was vacuum impregnated, and the remaining one side was heat-sealed under reduced pressure, and vacuum sealing was performed to assemble two cell capacitors.

(Characteristic evaluation of cells)
When one cell was disassembled after standing at room temperature for 14 days, all the lithium metal was completely lost. Therefore, lithium ions for obtaining a capacitance of 670 F / g or more per unit weight of the negative electrode active material were obtained. Judged to be precharged.

  The remaining film-type capacitor cell is allowed to stand at 25 ° C. for 24 hours, then charged with a constant current of 200 mA until the cell voltage reaches 3.8 V, and then a constant current-constant voltage in which a constant voltage of 3.8 V is applied. Charging was performed for 1 hour. Next, the battery was discharged at a constant current of 20 mA until the cell voltage reached 1.9V. This 3.8V-1.9V cycle was repeated, and the third discharge capacity was measured. The results are shown in Table 2.

(Comparative Example 2)
Similarly to Comparative Example 1, a negative electrode expanded metal electrode 2 having a thickness of 172 μm was prepared by using the negative electrode slurry 2 prepared using PAS powder as the negative electrode active material, in the same manner as in the preparation of the negative electrode expanded metal electrode of Example 2. did. Using this negative electrode expanded metal electrode 2 and the positive electrode expanded metal electrode 1 of Example 2, two cells of a laminated film capacitor having the same configuration as that of Example 2 were assembled. The thickness of the lithium metal foil for obtaining a capacitance of 670 F / g or more with respect to the weight of the negative electrode active material was 95 μm.

(Characteristic evaluation of cells)
When one cell was disassembled after standing at room temperature for 14 days, all the lithium metal was completely lost. Therefore, lithium ions for obtaining a capacitance of 670 F / g or more per unit weight of the negative electrode active material were obtained. Judged to be precharged.

  The remaining film-type capacitor cell was allowed to stand at 25 ° C. for 24 hours, and then charged with a constant current of 200 mA until the cell voltage reached 3.8 V, and then a constant voltage of 3.8 V was applied. The applied constant current-constant voltage charge was performed for 1 hour. Next, the battery was discharged at a constant current of 20 mA until the cell voltage reached 1.9V. This 3.8V-1.9V cycle was repeated, and the third discharge capacity was measured. The results are shown in Table 2 together with the results of Example 2.

(Example 3)
Two cells of a laminated film type capacitor having the same configuration as in Example 2 were assembled except that the thickness of the lithium metal foil was the same as in Comparative Example 2.

(Characteristic evaluation of cells)
When the cell was disassembled after standing at room temperature for 14 days, all of the lithium metal was completely lost.
The remaining film-type capacitor cell was allowed to stand at 25 ° C. for 24 hours, and then charged with a constant current of 200 mA until the cell voltage reached 3.8 V, and then a constant voltage of 3.8 V was applied. The applied constant current-constant voltage charge was performed for 1 hour. Next, the battery was discharged at a constant current of 20 mA until the cell voltage reached 1.9V. This 3.8V-1.9V cycle was repeated, and the third discharge capacity was measured. The results are shown in Table 2 together with the results of Example 2 and Comparative Example 2.

Moreover, 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 0.80 to 0.95 V, and were 0.95 V or less.

As shown in Table 2, since the positive electrode potential after short-circuiting the positive electrode and the negative electrode was 0.95 V or less, a laminated film capacitor having a high energy density was obtained. It can be seen that the same or higher energy density can be obtained with a lithium metal foil in the case where the carbide of the graphitizable carbon precursor is used less than the case of using the carbide of the non-graphitizable carbon precursor. It can also be seen that a larger energy density can be obtained when the same lithium metal foil is used.

  Since an energy density equal to or higher than that can be obtained with a small amount of lithium metal foil, the material cost is low and it is industrially excellent.

Claims (5)

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 substance capable of reversibly carrying lithium ions, and the negative electrode and / or the positive electrode is charged with lithium ions so that the positive electrode potential is 0.95 V or less after the positive electrode and the negative electrode are short-circuited. A lithium ion capacitor that has been previously doped and that the negative electrode active material is made of a carbide of a graphitizable carbon precursor.   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. Lithium ion capacitor.   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 lithium ion capacitor according to claim 1, wherein the graphitizable carbon precursor is coke, tar, or vinyl chloride resin. The lithium ion capacitor according to claim 1, wherein the negative electrode active material particles have an average particle diameter (D50) of 0.5 to 30 μm and a specific surface area of 0.1 to 1000 m ² / g.