JP4705404B2 - 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 electrolyte 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. One of them is that a negative electrode capable of occluding and desorbing lithium ions is brought into contact with lithium metal, and lithium ions are occluded and supported (hereinafter also referred to as doping) by a chemical method or an electrochemical method in advance. Capacitors intended to significantly increase the energy density by lowering the value 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 takes a very long time and uniform doping with respect to the whole negative electrode. It has been considered difficult to put into practical use in large-sized high-capacity cells such as a type device or a square battery in which a plurality of electrodes are laminated. However, this problem is caused by providing holes through the front and back of the negative electrode current collector and positive electrode current collector that constitute the cell, and lithium ions can move through the through holes, so that the negative electrode located at the end of the cell Thus, the present invention was able to solve all at once by the invention that lithium ions can be occluded and doped in all the negative electrodes in the cell not in contact with lithium metal only by electrochemically contacting lithium metal (see Patent Document 5). .

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-capacitance capacitor into practical use, there are still various problems, and there is an urgent need to solve these problems.
Japanese Patent Laid-Open No. 8-1007048 JP-A-9-55342 Japanese Patent Laid-Open No. 9-232190 JP-A-11-297578 International Publication Number WO98 / 033227

  The present inventor has advanced research on a large-sized capacitor in which the negative electrode and / or positive electrode is brought into contact with lithium metal which is a lithium ion supply source, and the negative electrode and / or positive electrode is previously doped with lithium ions. It has been found that the capacitance at room temperature has no problem in the type of capacitor, but at the low temperature of −20 ° C. to −10 ° C., its characteristics, in particular, the phenomenon in which the capacitance decreases significantly. . As a power storage device for automobiles and the like, it is also used in cold regions and the like, and the characteristics at the low temperature are also regarded as extremely important.

  Thus, the present invention relates to a lithium ion capacitor of a type in which a negative electrode and / or a positive electrode is brought into contact with lithium metal, and the negative electrode is previously doped with lithium ions. It is an object of the present invention to provide an improved capacitor that does not deteriorate.

In order to solve the above problems, the present inventors have conducted intensive research. As a result, the positive electrode active material is a material capable of reversibly supporting lithium ions and / or anions, and the negative electrode active material is reversible with lithium ions. In a lithium ion capacitor having a positive electrode potential of 0.95 V or less after the negative electrode and / or the positive electrode are doped with lithium ions and the positive electrode and the negative electrode are short-circuited, the negative electrode used therein The physical properties of the active material are related to characteristics at low temperature, particularly capacitance, and the above problem can be solved by forming the negative electrode from a negative electrode active material having a specific particle size distribution smaller than that of the conventional material. And reached the present invention.

Thus, the present invention is characterized by having the following gist.
(1) A positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, 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 substance capable of reversibly carrying lithium ions, and is a lithium ion capacitor having a positive electrode potential of 0.95 V or less after the positive electrode and the negative electrode are short-circuited, wherein the negative electrode has a 50% volume cumulative diameter ( D50) is formed from negative electrode active material particles having a size of 0.5 to 1.9 μm , and the negative electrode active material particles are polyacene-based materials .
(2) In the above (1), the negative electrode and / or the positive electrode are previously doped with lithium ions by electrochemical contact with a lithium ion supply source disposed to face the negative electrode and / or the positive electrode, respectively. The lithium ion capacitor described.
(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 each of the positive electrode and the negative electrode includes a current collector having a hole penetrating the front and back surfaces.
( 5 ) The above-mentioned (1), wherein the polyacene material is an insoluble infusible substrate obtained by heat-treating an aromatic condensation polymer at 400 to 800 ° C. in a non-oxidizing atmosphere and having an H / C of 0.05 to 0.5. )-( 4 ) The lithium ion capacitor in any one of.

  According to the present invention, a lithium ion is previously doped into the negative electrode, in particular, a large-capacity capacitor, which is an improved capacitor that does not lower the characteristics at low temperature, particularly the capacitance, in terms of energy density and output density. Is provided. In the present invention, it is not necessarily clear why the characteristics at a low temperature of the capacitor are improved by forming the negative electrode from a negative electrode active material having a specific particle size distribution smaller than that of the prior art. Is estimated as follows.

  The lithium ion capacitor in which the negative electrode and / or the positive electrode are doped in advance with lithium ions uses an organic solvent solution containing lithium ions as an electrolyte, and the low temperature characteristics of the electrolyte are low because the electrolyte has low ion conductivity at low temperatures. It was inferior to the electric double layer capacitor. In particular, since it was strongly influenced by the negative electrode through which only lithium ions enter and exit, by controlling the particle size of the negative electrode active material to a specific small particle size distribution, the interface with the electrolyte increased, and lithium ions even at low temperatures. It is considered that the above-mentioned excellent characteristics can be obtained by improving the characteristics at a low temperature.

The lithium ion capacitor of the present invention includes a positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of a lithium salt as an electrolytic solution, and the positive electrode active material is a substance capable of reversibly supporting lithium ions and / or anions. and the negative electrode active material is reversibly carried substance lithium ions, a positive electrode collector position after short-circuiting the positive electrode and the negative electrode has the following 0.95 V. 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) 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 the present invention, the positive electrode potential of 0.95 V or less after the positive electrode and the negative electrode are short-circuited is not limited to just after lithium ions are doped. The positive electrode potential after short-circuiting is 0.95 V or less in any state, such as when short-circuiting after repeating.

In the present invention, the fact that the positive electrode potential after the positive electrode and the negative electrode are short-circuited is set to 0.95 V or less will be described in detail below. As described above, activated carbon and carbon materials usually have a potential of about 3 V (Li / Li ⁺ ), and when the cell is assembled using activated carbon for both the positive electrode and the negative electrode, both potentials are about 3 V. Even if it is short-circuited, the positive electrode potential is about 3 V regardless. The same applies to 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, even if a short circuit occurs, the positive electrode potential is about 3 V regardless. Although depending on the weight balance between the positive electrode and the negative electrode, when charged, the potential of the negative electrode transitions to around 0 V, so that the charging voltage can be increased, so that the capacitor has a high voltage and a high energy density. Generally, the upper limit of the charging voltage is determined to be a voltage at which the electrolyte solution does not decompose due to the increase in the positive electrode potential. Therefore, when the positive electrode potential is set as the upper limit, the charging voltage can be increased by the amount of decrease in the negative electrode potential. . However, in the above-described hybrid capacitor in which the positive electrode potential is about 3V at the time of short circuit, when the upper limit potential of the positive electrode is 4.0V, for example, the positive electrode potential at the time of discharge is up to 3.0V. 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 3.0 V, the use 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 a lithium ion supply source such as lithium metal to the negative electrode. Since lithium ions are supplied from other than the positive electrode and the negative electrode, when they are short-circuited, the equilibrium potentials of the positive electrode, the negative electrode, and the lithium metal are reached, so that both the positive electrode potential and the negative electrode potential are 3.0 V or less. As the amount of lithium metal increases, the equilibrium potential decreases. If the negative electrode material and the positive electrode material change, the equilibrium potential also changes. Therefore, the amount of lithium ions supported on the negative electrode can 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 circuit becomes 0.95 V or less. 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 . 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 below 0.95 V, the energy density of the cell is reduced. On one general it is at 0.1V or more, preferably 0.3V or higher.

  In the present invention, lithium ion doping may be either one or both of the negative electrode and the positive electrode. For example, when activated carbon is used for the positive electrode, the lithium ion becomes irreversible when the amount of lithium ion doping increases and the positive electrode potential decreases. May cause problems such as a decrease in cell capacity. For this reason, it is preferable that the lithium ions doped in the negative electrode and the positive electrode do not cause these problems in consideration of the respective electrode active materials. In the present invention, since controlling the doping amount of the positive electrode and the doping amount of the negative electrode becomes complicated in the process, the doping of lithium ions is preferably performed on the negative electrode.

  In the lithium ion capacitor of the present invention, in particular, the electrostatic capacity per unit weight of the negative electrode active material has more than three times the electrostatic capacity per unit weight of the positive electrode active material, and the positive electrode active material weight is the negative electrode active material When larger than the weight, a capacitor having a high voltage and a high capacity can be obtained. At the same time, when using a negative electrode having a capacitance per unit weight that is larger than the capacitance per unit weight of the positive electrode, the negative electrode active material weight is reduced without changing the potential change amount of the negative electrode. Therefore, the filling amount of the positive electrode active material is increased, and the capacitance and capacity of the cell are increased. The weight of the positive electrode active material is preferably larger than the weight of the negative electrode active material, but more preferably 1.1 times to 10 times. If it is less than 1.1 times, the capacity difference becomes small, and if it exceeds 10 times, the capacity may be reduced, and the thickness difference between the positive electrode and the negative electrode becomes too large, which is not preferable in terms of the cell structure.

  In the present invention, the capacitance and capacity of a capacitor cell (hereinafter also simply referred to as a cell) are defined as follows. The capacitance of a cell indicates the amount of electricity flowing through the cell per unit voltage of the cell (the slope of the discharge curve), the unit is F (farad), and the capacitance per unit weight of the cell is the capacitance of the cell It is shown by the division of the total weight of the weight of the positive electrode active material and the weight of the negative electrode active material filled in the cell (unit: F / g). The capacitance 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. The value is shown by dividing the positive electrode or negative electrode capacitance by the weight of the positive electrode or negative electrode active material filling the cell, 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 a current flows, in this patent it is converted to mAh. 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 lithium ion 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 / or the positive electrode and the lithium electrode may be in physical contact (short circuit) or may be electrochemically doped. The lithium ion supply source may be formed on a lithium electrode current collector made of a conductive porous body. As the conductive porous body serving as the lithium electrode current collector, a metal porous body that does not react with a lithium ion supply source such as a stainless mesh can be used.

  A large-capacity multilayer capacitor cell is provided with a positive electrode current collector and a negative electrode current collector for receiving and distributing electricity at the positive electrode and the negative electrode, respectively. The positive electrode current collector and the negative electrode current collector are used, and the lithium electrode In the case of the cell, the lithium electrode is preferably provided at a position facing the negative electrode current collector, and lithium ions are preferably supplied to the negative electrode and / or the positive 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, the lithium ion can be uniformly doped even when the lithium electrode doped in the negative electrode and / or the positive electrode is locally arranged in the cell. Therefore, even in the case of a large-capacity cell in which the positive electrode and the negative electrode are 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, 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.

Negative electrode active material in the lithium ion capacitor of the present invention is important, negative electrode active material, lithium ions reversibly carrying it, such a material Lupolen Riasen based material (hereinafter, also referred to as PAS). P AS is the phenol resin is carbonized, is activated as needed, and then those obtained by pulverizing are used. A carbonization process is performed by accommodating phenol resin etc. in a heating furnace etc., and heating for a required time at the temperature which phenol resin etc. carbonize. The temperature at that time varies depending on the heating time and the like, but in the case of PAS, it is usually set to 500 to 1000 ° C. when the heating time is about 1 to 3 hours. The pulverization step is performed using a known pulverizer such as a ball mill.

As the negative electrode active material of the present invention, P AS is a need in that high capacity is obtained. Capacitance of 650 F / g or more can be obtained by discharging after charging (charging) 400 mAh / g of lithium ions on PAS, and static electricity of 750 F / g or more can be obtained by charging lithium ions of 500 mAh / g or more. Capacitance can be obtained. Since PAS has an amorphous structure and the potential decreases as the amount of lithium ions carried increases, the withstand voltage (charging voltage) of the obtained capacitor increases, and the rate of voltage rise during discharge (the slope of the discharge curve) ) Is low, the capacity is slightly increased. Therefore, it is desirable to set the amount of lithium ions within the range of the lithium ion storage capacity of the active material according to the required working voltage of the capacitor.

  In addition, since PAS has an amorphous structure, there is no structural change such as swelling / shrinkage with respect to insertion / extraction of lithium ions, so that cycle characteristics are excellent, and isotropic to insertion / extraction of lithium ions. Since it has a molecular structure (higher order structure), it is suitable for rapid charging and rapid discharging. The aromatic condensation polymer that is a precursor of PAS is a condensate of an aromatic hydrocarbon compound and an aldehyde. As the aromatic hydrocarbon compound, so-called phenols such as phenol, cresol, xylenol and the like can be suitably used. For example, the following formula

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

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

  In the present invention, PAS is manufactured as follows. That is, by gradually heating the aromatic condensation polymer to a suitable temperature of 400 to 800 ° C. in a non-oxidizing atmosphere (including vacuum), the atomic ratio of hydrogen atoms / carbon atoms (hereinafter referred to as “atom number ratio”) H / C) is an insoluble and infusible substrate having 0.5 to 0.05, preferably 0.35 to 0.10.

  According to X-ray diffraction (CuKα), the insoluble and infusible substrate described above has a main peak position represented by 2θ of 24 ° or less, and between 41 and 46 ° in addition to the main peak. There are other broad peaks. That is, the insoluble infusible substrate has a polyacene skeleton structure in which an aromatic polycyclic structure is appropriately developed, has an amorphous structure, and can be stably doped with lithium ions.

In the present invention, the particle size characteristic of the negative electrode active material is important, and the 50% volume cumulative diameter (also referred to as D50) is 0.5 to 1.9 μm. When D50 is smaller than 0.5 μm, the active material particles become bulky, and the density when formed into an electrode becomes small, so that the energy density per unit volume tends to decrease, which is not preferable. Furthermore, since the amount of binder necessary for binding the particles increases, the internal resistance may increase. On the other hand, when D50 is larger than 1.9 μm, the rate at which the solvated lithium ions diffuse into and out of the negative electrode material particles becomes slow, which is not preferable. In the present invention, D50 of the negative electrode active material, JP, 0.5 to 1.0 [mu] m is preferred.

  In general, in a lithium ion secondary battery, it is known that when the negative electrode material is pulverized, the specific surface area increases, the initial charge / discharge efficiency decreases, and the cell capacity decreases. Accordingly, the main particle size is around 20 μm. On the other hand, in the lithium ion capacitor of the present invention, since the negative electrode and / or the positive electrode are preliminarily doped with lithium ions, the cell capacity does not change due to the initial charge / discharge efficiency. A power storage device suitable for the above.

The negative electrode active material particle of the present invention, the total mesopore volume is 0.005~1.0cc / g, specific surface area is preferably 0.01~1000m ² / g. When the total mesopore volume is less than 0.005 cc / g, the mobility of solvated lithium ions decreases, so the lithium ion concentration near the negative electrode active material interface is less likely to follow at high output and low temperatures. It is not preferable. On the other hand, if it is larger than 1.0 cc / g, the true density of the active material is lowered, and the capacity per electrode volume is decreased, which is not preferable. The total mesopore volume is preferably 0.006 to 0.8 cc / g. On the other hand, if the specific surface area exceeds 1000 m ² / g, the irreversible capacity of lithium ions increases, which is not preferable. On the other hand, if it is less than 0.01 m ² / g, the amount of electrolyte solution retained is reduced and the resistance is increased, which is not preferable. The specific surface area is preferably 0.1 to 600 m ² / g.

  The negative electrode in the present invention is formed from the negative electrode active material powder having the above specific particle size characteristics, and existing means can be used. That is, a negative electrode active material powder, a binder and, if necessary, a conductive powder and a thickener (such as CMC) are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is applied to the above-described current collector, Alternatively, the slurry may be formed into a sheet shape in advance and attached to a current collector. As the binder used here, for example, a rubber-based binder such as SBR, a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, a thermoplastic resin such as polypropylene or polyethylene, an acrylic resin, or the like is used. Can do. The amount of the binder used varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, and the like, but it is appropriate to add it at a ratio of 2 to 40% by weight with respect to the negative electrode active material.

  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.

On the other hand, the positive electrode active material used for forming the positive electrode in the present invention is not particularly limited as long as it can reversibly carry lithium ions and anions such as tetrafluoroborate. Examples of the positive electrode active material include activated carbon, conductive polymer, polyacene-based material, and the like. In the positive electrode active material of the present invention, for example, activated carbon having a wide range of particle sizes can be used. For example, the 50% volume cumulative diameter (also referred to as D50) is 2 μm or more, preferably 2 to 50 μm, particularly 2 to 20 μm. The average pore diameter is preferably 10 nm or less, and the specific surface area is preferably 600 to 3000 m ² / g, particularly 1300 to 2500 m ² / g.

  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, a negative electrode active material powder, a binder and, if necessary, a conductive powder and a thickener (such as CMC) are dispersed in an aqueous or organic solvent to form a slurry, and the slurry is applied to the above-described current collector, Alternatively, the slurry may be formed into a sheet shape in advance and attached to a current collector. As the binder used here, for example, a rubber-based binder such as SBR, a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, a thermoplastic resin such as polypropylene or polyethylene, an acrylic resin, or the like is used. Can do. The amount of the binder used varies depending on the electrical conductivity of the negative electrode active material, the electrode shape, and the like, but it is appropriate to add it at a ratio of 2 to 40% by weight with respect to the negative electrode active material.

  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.

  Examples of the aprotic organic solvent 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 electrolyte and the solvent are mixed in a sufficiently dehydrated state to form an electrolyte solution. The concentration of the electrolyte in the electrolyte is at least 0.1 mol / l in order to reduce the internal resistance of the electrolyte. The above is preferable, and the range of 0.5 to 1.5 mol / l is more preferable.

  In addition, as the lithium ion capacitor of the present invention, in particular, a cylindrical cell in which a strip-like positive electrode and a negative electrode are wound through a separator, and a plate-like positive electrode and a negative electrode are laminated in three or more layers through a separator. It is suitable for a large-capacity cell such as a rectangular cell or a form-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 outer form. 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.
Example 1
(Method for producing negative electrode active material)
Particulate phenolic resin (Bellpearl R700; manufactured by Air Water Bellpearl Co., Ltd.) having an average particle diameter of 17 μm is placed in a static electric furnace, heated to 720 ° C. at a heating rate of 50 ° C./hour in a nitrogen atmosphere, and further 720 A PAS powder was obtained by holding at 5 ° C. for 5 hours. By changing the pulverization time of the PAS powder thus obtained with an alumina ball mill pulverizer to 6 hours, 40 hours, 72 hours, 170 hours, and 720 hours, the average particle diameter D50% was 7.1 μm (sample 1). ), 2.5 μm (sample 2), 1.9 μm (sample 3), 1.0 μm (sample 4), and 0.5 μm (sample 5).

  Next, 6 parts by weight of acetylene black powder, 5 parts by weight of an acrylate copolymer binder, 4 parts by weight of carboxymethyl cellulose (CMC), and 200 parts by weight of ion-exchanged water with respect to 92 parts by weight of the powders of Samples 1 to 5 above. Were added and sufficiently mixed with a mixing stirrer to obtain negative electrode slurries 1 to 5.

(Measurement of the doping amount of lithium ions required in advance for the capacitance measurement per unit weight of the negative electrode to be 650 F / g)
The negative electrode slurries 1 to 5 were coated on one side of a copper foil having a thickness of 18 μm so as to have a solid content of 2.5 mg / cm ² and vacuum-dried at 200 ° C. for 20 hours. 5 × 2.0 cm ² size was cut out to prepare negative electrode foil electrodes 1 to 5. Simulated cells 1 to 5 were assembled with the negative electrode foil electrode, metallic lithium having a size of 1.5 × 2.0 cm ² 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 cells 1 to 5 were charged at a constant current of 10 mA at 25 ° C. 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, and then 1 at 1 mA. Discharge to 5V. The electrostatic capacity per unit weight of the negative electrode active material of the negative electrode foil electrodes 1 to 5 was determined from the discharge time while the potential of the negative electrode changed 0.2 V from the potential of the negative electrode 1 minute after the start of discharge, and the capacitance was 650 F / g. The charging amount (lithium ion doping amount) was confirmed by controlling the charging time so that the negative electrode foil electrode 1 was 385 mAh / g, the negative electrode foil electrode 2 was 420 mAh / g, and the negative electrode foil electrode 3 was 450 mAh / g. The negative electrode foil electrode 4 was 530 mAh / g, and the negative electrode foil electrode 5 was 750 mAh / g. In other words, the negative electrode active material having a smaller average particle diameter D of 50% has a larger doping amount. The results are shown in Table 1.

(Method for producing positive electrode activated carbon slurry)
Sawdust is used as a raw material, put in an electric furnace, heated to a rate of 950 ° C. at a rate of 50 ° C./hour in a nitrogen stream, and then activated with a mixed gas of nitrogen / water vapor 1: 1 for 12 hours, whereby a specific surface area of 2450 m ² / g of activated carbon 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 size of 7 μm.

  92 parts by weight of the activated carbon powder, 6 parts by weight of acetylene black powder, 7 parts by weight of a water-soluble 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.

(Capacitance measurement per unit weight of positive electrode)
The positive electrode slurry 1 was applied to one side of an aluminum foil having a thickness of 20 μm coated with a carbon-based conductive paint so that the solid content was 4.0 mg / cm ^2, and vacuum-dried at 200 ° C. for 20 hours to form a positive electrode. Obtained. The positive electrode foil electrode 1 was obtained by cutting this positive electrode into 1.5 × 2.0 cm ² size and drying it.

A simulated cell 6 was assembled using the positive electrode foil electrode 1 having a lithium metal of the same size and a thickness of 250 μm as a counter electrode and a polyethylene non-woven fabric 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 cell 6 was charged with a constant current of 10 mA until the positive electrode potential reached 3.8 V, and then subjected to constant current-constant voltage charging in which a constant voltage of 3.8 V was applied for 1 hour. Next, the battery was discharged at a constant current of 1 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)
With respect to a copper expanded metal (manufactured by Nippon Metal Industry Co., Ltd.) having a thickness of 32 μm (aperture ratio 57%), the negative electrode slurries 1 to 5 are successively applied in the same amount of each active material by a vertical double-sided simultaneous die coater. Thus, negative electrode 1-5 was obtained by adjusting the discharge amount of die | dye, coating both surfaces simultaneously, and drying. The total thickness of each negative electrode was 165 μm for negative electrode 1, 171 μm for negative electrode 2, 173 μm for negative electrode 3, 180 μm for negative electrode 4, and 193 μm for negative electrode 5.

(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 hole was almost blocked by the conductive paint. The positive electrode slurry 1 was applied to both surfaces of the positive electrode current collector on one side with a comma coater and dried to obtain a positive electrode 1 having a total thickness of 260 μm.

(Production of stacked cells)
The negative electrodes 1 to 5 and the positive electrode 1 are cut to a size of 2.4 cm × 3.8 cm, and a cellulose / rayon mixed nonwoven fabric having a thickness of 35 μm is used as a separator. The welded portions (hereinafter referred to as “connecting terminal welded portions”) were alternately arranged on the opposite side, and negative electrodes 1 to 5 were each fabricated by stacking 6 negative electrodes and 5 positive electrodes.

  Separators were placed on the uppermost and lowermost parts, and four sides were taped to obtain an electrode laminate unit. For the adjustment of the required charge amount shown in Table 1 with respect to the weight of the negative electrode active material, a lithium metal foil having a predetermined amount is bonded to a stainless steel net having a thickness of 80 μm, and this is opposed to the negative electrode. One sheet was arranged on the outermost part of the electrode stacking 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. Table 2 shows the thickness of the lithium metal foil that is a predetermined amount with respect to the negative electrodes 1 to 5.

  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 5 was vacuum impregnated, and the remaining one side was heat-sealed under reduced pressure, and vacuum sealing was performed to assemble three film capacitors.

(Characteristic evaluation of cells)
After leaving at room temperature for 14 days, one cell was disassembled for each negative electrode. As a result, all of the lithium metal was completely removed, so that a capacitance of 650 F / g per unit weight of the negative electrode active material was obtained. It was judged that lithium ions were doped.

  The remaining cells of the film type capacitor were left at 25 ° C. and −20 ° C. for 24 hours, 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. Constant current-constant voltage 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. Table 3 shows the discharge capacity and ratio at 25 ° C. and −20 ° C. and the energy density measurement results at 25 ° C.

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 3, by adjusting the amount of lithium ions appropriately, the positive electrode potential after short-circuiting the positive electrode and the negative electrode is 0.95 V or less, so the finely divided negative electrode active material is used. However, the capacity did not decrease, and a multilayer film capacitor having a high energy density was obtained. Among them, samples 3, 4 and 5 whose D50% particle size is in the range of 0.5 to 1.9 μm have a high capacity retention rate of 75% or more with respect to the discharge capacity of 25 ° C. even in a low temperature region of −20 ° C. It can be seen that

Claims (5)

A positive electrode, a negative electrode, and an aprotic organic solvent electrolyte solution of lithium salt as an electrolyte solution, the positive electrode active material is a material capable of reversibly supporting lithium ions and / or anions, and the negative electrode active material is lithium ions Is a lithium ion capacitor having a positive electrode potential of 0.95 V or less after the positive electrode and the negative electrode are short-circuited, and the negative electrode has a 50% volume cumulative diameter (D50). A lithium ion capacitor , wherein the negative electrode active material particles are 0.5 to 1.9 μm , and the negative electrode active material particles are a polyacene-based material .   2. The lithium ion according to claim 1, wherein the negative electrode and / or the positive electrode are previously doped with lithium ions by electrochemical contact with a lithium ion source disposed opposite to the negative electrode and / or the positive electrode, respectively. 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 each of the positive electrode and the negative electrode includes a current collector having holes penetrating the front and back surfaces. Polyacene substance, heat treated at 400 to 800 ° C. The aromatic condensation polymer in a non-oxidizing atmosphere, H / C is according to claim 1-4 which is insoluble and infusible base of 0.05 to 0.5 The lithium ion capacitor in any one.