JP7194541B2 - Capacitor - Google Patents

Description

The present invention relates to capacitors.

Power storage devices such as lithium ion secondary batteries are small and have high energy density, and are widely used as power sources for portable electronic devices. Further, research and development aimed at further improving the energy density as well as improving the reliability and safety are actively carried out.

By the way, it is known that it is important to control the amount of heat generated during charging and discharging in order to improve the reliability and safety of lithium ion batteries. In order to address this problem, Patent Document 1 proposes a lithium ion battery including an electrode including an electrode mixture layer that widens the lithium capacity range in which the entropy heat generated during charging or discharging becomes negative.

WO2014/155496

However, when the lithium ion battery disclosed in Patent Document 1 is rapidly charged, it has been found that the reliability (hardness of deterioration) and safety are insufficient. While the discharge rate of an electricity storage device such as a battery or a capacitor varies depending on the intended use, rapid charging is required for all electricity storage devices. Therefore, there is a need for an electricity storage device that does not cause problems in reliability and safety even after repeated rapid charging. .

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an electricity storage device that does not cause problems in reliability and safety even after repeated rapid charging.

In view of this situation, the present inventors conducted intensive research and found that by using a capacitor with a larger amount of entropy change during charging than a conventional electricity storage device, the temperature rise accompanying the heat generation of the electricity storage device during the entire charging and discharging process can be reduced. can be suppressed and the above problems can be solved, and the present invention has been completed.

A capacitor according to one embodiment of the present invention (hereinafter also referred to as "this capacitor") is characterized in that the average value of entropy change _ΔSE obtained by the following formula (1) is 82 J/K mol or more. .
Although it is a lithium ion battery, the average value of _ΔSE of the lithium ion battery disclosed in Patent Document 1 is about 20 J/K·mol as estimated from FIG. 14 of Patent Document 1.

Average value of _ΔSE = ( _ΔSEa + _ΔSEb + _ΔSEc )/3 (1)
In the above formula (1), ΔS _Ea , ΔS _Eb and ΔS _Ec are each calculated by the following formula (1-1).

ΔS _E =−n×9.65×10 ⁴ (C/mol)×ΔV ₀ /ΔT (1-1)
In the above formula (1-1),
_ΔSE is either _ΔSEa , _ΔSEb or _ΔSEc ;
n is the number of charges involved in the electrode reaction,
ΔV ₀ /ΔT is obtained by adjusting a capacitor adjusted with a current value of 10 C so that the voltage of the capacitor becomes E (V) in a constant temperature chamber at 25° C. Measure the open circuit voltage V ₀ after leaving it in a constant temperature bath at 30 ° C. or 30 ° C. for a predetermined time, and plot the measurement results with T (K) on the horizontal axis and open circuit voltage V ₀ (V) on the vertical axis. It means the slope of the straight line obtained by the method of least squares in the graph.
The _ΔSEa is the entropy change when the E(V) is set to the “upper limit of the rated voltage”, and the _ΔSEb is the entropy when the E(V) is set to the “lower limit of the rated voltage”. The _ΔSEc is the entropy change when the E(V) is set to “(the upper limit value+the lower limit value)/2”.
9.65×10 ⁴ (C/mol) is Faraday's constant.

The predetermined time means the longer of A and B below.
A: In a constant temperature chamber at 25°C, constant current and constant voltage charging is performed at a current value of 10C for 24 hours to adjust the voltage of the capacitor to the upper limit of its rated voltage. The time (minutes) required for the open circuit voltage to reach its maximum value after being left in
B: 30 minutes if the capacitance of the capacitor is less than 2000F, and capacitance (F)/2000 x 60 (minutes) if it is 2000F or more

This capacitor absorbs more heat due to changes in entropy during charging than conventional power storage devices.

≪Capacitor≫
Although the present capacitor is not particularly limited, it is preferably an electrochemical capacitor. An electrochemical capacitor is a capacitor that utilizes the capacitance generated due to a non-faradaic reaction that does not involve electron transfer or a faradaic reaction that involves electron transfer between the electrode and ions in the electrolyte at the electrode interface. is.
Electrochemical capacitors include electric double layer capacitors, hybrid capacitors, redox capacitors and the like, with hybrid capacitors being preferred. Examples of hybrid capacitors include sodium ion capacitors, lithium ion capacitors, and magnesium ion capacitors. Among these, lithium ion capacitors are particularly preferred.

<Average value of entropy change ΔS _E >
In the present invention, the rated voltage means the upper and lower limits of the voltage that can be stably used when using a capacitor. For lithium ion capacitors, the rated voltage is typically 2.2V-3.8V.

When measuring the _ΔSE , first, the predetermined time (leaving time) is measured by the following method, and the longer one of the times obtained in A and B below is determined.
A: Constant current constant voltage (hereinafter also referred to as “CCCV”) charging is performed for 24 hours at a current value of 10 C in a constant temperature bath at 25 ° C., so that the voltage of the capacitor reaches the upper limit of the rated voltage of the capacitor. After adjusting the temperature of the constant temperature chamber to 20° C., leave it in the constant temperature chamber at 20° C., and measure the time (minutes) required from the start of standing until the open circuit voltage reaches the maximum value. .
B: 30 minutes when the capacitance of the capacitor is less than 2000F, and capacitance (F)/2000×60 (minutes) when the capacitance is 2000F or more.

The capacitance of a capacitor is the capacitance obtained when the capacitor is discharged from the upper limit to the lower limit of its rated voltage.

After measuring or determining the predetermined time (leaving time), ΔV ₀ /ΔT is calculated as follows.
In a constant temperature chamber at 25° C., the voltage of the capacitor was adjusted at a current value of 10 C so that the voltage of the capacitor became a predetermined voltage E (V). Measure the open circuit voltage V ₀ after leaving it in a constant temperature bath at 28 ° C. or 30 ° C. for a predetermined time, and plot the measurement result with T (K) on the horizontal axis and open circuit voltage V ₀ (V) on the vertical axis. In this graph, the slope (ΔV ₀ /ΔT) of the straight line obtained by the method of least squares is calculated.

More specifically, first, CCCV charging is performed for 24 hours at a current value of 10 C in a constant temperature bath at 25° C., thereby adjusting E(V) to the upper limit of the rated voltage. Then, V ₀ is measured after the adjusted capacitor is left in a constant temperature bath at T=30° C. for a predetermined time. Subsequently, the temperature in the constant temperature bath is changed to 25° C., and CCCV discharge is performed at a current value of 10 C for 24 hours, thereby adjusting E(V) to the lower limit of the rated voltage. Then, V ₀ is measured after the adjusted capacitor is left in a constant temperature bath at T=30° C. for a predetermined time. By repeating the same operation at T=28° C., 26° C., 24° C., 22° C. and 20° C., ΔV ₀ / ΔT is calculated.

Next, by changing the temperature in the constant temperature chamber to 25° C. and performing CCCV charging at a current value of 10 C for 24 hours, E (V) becomes “(maximum value of rated voltage + minimum value of rated voltage)/2 ”. Then, V ₀ is measured after the adjusted capacitor is left in a constant temperature bath at T=30° C. for a predetermined time. Subsequently, CCCV discharge is performed at a current value of 10 C for 24 hours to adjust E(V) to the lower limit of the rated voltage. After that, by changing the temperature in the thermostat to 25°C and performing CCCV charging for 24 hours at a current value of 10C, E (V) is "(upper limit value of rated voltage + lower limit value of rated voltage) / 2". Adjust so that Then, the capacitor after adjustment is left in a constant temperature bath at T=28° C. for a predetermined time, and then V ₀ is measured. By repeating the same operation at T=26° C., 24° C., 22° C. and 20° C., E(V) is set to “(upper limit value of rated voltage + lower limit value of rated voltage)/2”, ΔV ₀ /ΔT is calculated.

Like batteries, capacitors generate heat or absorb heat during charging and discharging reactions.
Takamasa Oshima et al., "Heat generation behavior during rapid charging and discharging of a small lithium-ion secondary battery", The Institute of Electrical Engineers of Japan B, 2004, Vol. 124, No. 12, p. 1521-1527, the endothermic heat in this reaction is related to TΔS in the thermodynamic relational expression ΔG=ΔH−TΔS under constant temperature and pressure in the case of ideal reversible conditions. Therefore, in the present invention, the entropy change ΔS is taken into consideration as an endothermic and exothermic factor.
Here, from the relational expression between Gibbs energy change and entropy change [-ΔS=δ(ΔG)/ΔT] and the relational expression between Gibbs energy change and battery electromotive force [-ΔG=nFE], ΔS is represented by the following formula: be able to.
ΔS = n F (ΔE/ΔT)
[n: number of charges involved in electrode reaction, F: Faraday constant (C/mol), E: electromotive force of battery (V)]

Assuming that the electromotive force E of the battery can be approximated to the open circuit voltage V ₀ (V), the amount of heat associated with entropy change during charging of the capacitor can be approximately expressed by the following equation (1-1).
ΔS _E =−n·9.65×10 ⁴ ·(ΔV ₀ /ΔT) (1-1)

n in formula (1-1) is, for example, n=1 when lithium ions and sodium ions, which are monovalent ions, are involved in the electrode reaction, and magnesium ions, which are divalent ions, are involved in the electrode reaction. case, n=2. ΔS _E at a predetermined voltage E (V) can be calculated by substituting ΔV ₀ /ΔT calculated above into the equation (1-1).

In the present invention, as the predetermined voltage E (V), "the upper limit value of the rated voltage", "the lower limit value of the rated voltage" and "(the upper limit value + the lower limit value)/2" are adopted, and the voltage E Using _ΔSEa , _ΔSEb , and _ΔSEc in each case, the average value was calculated by the following formula (1). The present inventors have found that by setting the average value to 82 J/K·mol or more, it is possible to provide a capacitor that does not cause problems in reliability and safety even after repeated rapid charging.
Average value of _ΔSE = ( _ΔSEa + _ΔSEb + _ΔSEc )/3 (1)

When the present capacitor is a lithium-ion capacitor, the above-mentioned "upper limit value of rated voltage", "lower limit value of rated voltage" and "(above-mentioned upper limit value + above-mentioned lower limit value)/2" are respectively 3.8V, 2.8V, and 2.8V. 2V and 3.0V are preferred. In this case, _ΔSEa = ΔS _3.8 , _ΔSEb = ΔS _2.2 , and _ΔSEc = ΔS _3.0 , and the above formula (1) becomes the following formula.
Average value of _ΔSE = (ΔS _3.8 + ΔS _2.2 + ΔS _3.0 )/3

The lower limit of the average value of _ΔSE is preferably 83 J/K·mol, more preferably 84 J/K·mol, and particularly preferably 85 J/K·mol. On the other hand, the upper limit of the average value of _ΔSE is preferably 200 J/K·mol, particularly preferably 150 J/K·mol.
If the average value is less than 82 J/K·mol, the desired effect may not be obtained. On the other hand, if the average value is too large, the heat generated during discharge becomes too large, and repeated charging and discharging may easily deteriorate the capacitor cell.

Factors that greatly affect the average value include a combination of a positive electrode active material, a negative electrode active material, and an electrolyte, which will be described later. By adjusting these factors, the average value can be set within a predetermined range.

<Capacitor Configuration>
The capacitor typically comprises a positive electrode, a negative electrode and an electrolyte.

[Positive electrode]
The positive electrode generally includes a positive electrode active material.
Although the positive electrode active material is not particularly limited, it is preferably a positive electrode active material capable of absorbing and releasing anions, and more preferably activated carbon and carbon nanotubes.
The positive electrode active material used for the positive electrode may be one kind, or two or more kinds.

Examples of the positive electrode include a laminate in which a positive electrode active material layer containing the positive electrode active material is formed on a positive electrode current collector.
The positive electrode active material layer can usually be produced by preparing a slurry containing a positive electrode active material, a binder, etc., applying the slurry on a current collector, and drying the slurry.
Moreover, the positive electrode may have a conductive layer.

The positive electrode current collector used for the positive electrode is preferably a plate-like body made of aluminum, stainless steel, or the like, and may have a through hole penetrating through the front and back surfaces.
The thickness of the positive electrode current collector is usually 10 to 50 μm.

Examples of the binder include rubber-based binders such as styrene-butadiene rubber (SBR) and NBR; polytetrafluoroethylene, polyvinylidene fluoride (PVDF); fluorine-based binders such as modified (meth)acrylic binders; polypropylene; and polyethylene.
1 type may be used for the said binder and 2 or more types may be used for it.

The positive electrode active material layer further includes a conductive agent such as carbon black, carbon fiber, graphite, and metal powder; carboxymethylcellulose (CMC), its Na salt or ammonium salt, methylcellulose, hydroxymethylcellulose, ethylcellulose, hydroxypropylcellulose, polyvinyl alcohol, Thickeners such as oxidized starch, phosphorylated starch, and casein may also be included.
One of these may be used, or two or more thereof may be used.

The amount of polymer components such as binders and thickeners that can be contained in the positive electrode active material layer is preferably 1 to 20 parts by mass, particularly preferably 2 to 15 parts by mass, based on 100 parts by mass of the positive electrode active material. be.

[Negative electrode]
The negative electrode in this capacitor usually contains a negative electrode active material.
The negative electrode active material is not particularly limited, but is preferably a negative electrode active material capable of absorbing and releasing cations, and particularly preferably a negative electrode active material capable of absorbing and releasing lithium ions, sodium ions, and magnesium ions. More specifically, graphite, graphitizable carbon, non-graphitizable carbon, carbon materials such as composite carbon materials in which graphite particles are coated with pitch or resin carbide; metals that can be alloyed with lithium, sodium or magnesium, or Materials containing metalloids or oxides of these metals or metalloids; lithium titanate is preferred.
One type of negative electrode active material may be used for the negative electrode, or two or more types may be used.

Similar to the positive electrode, the negative electrode includes, for example, a laminate in which a negative electrode active material layer containing the negative electrode active material is formed on a negative electrode current collector.
The negative electrode current collector used for the negative electrode is preferably a plate-like body made of copper, nickel, stainless steel, or the like, and may have a through hole penetrating through the front and back surfaces.
The thickness of the negative electrode current collector is usually 10 to 50 μm.
Like the positive electrode, the negative electrode active material layer may contain a binder and other components, and specific examples and amounts used thereof are the same as in the case of the positive electrode.

[Electrolytes]
The electrolyte is not particularly limited, but includes lithium salts, sodium salts, magnesium salts, organic onium salts and the like, among which lithium salts, sodium salts and magnesium salts are preferred.
Examples of the anion portion constituting the electrolyte include _PF6- ^, _PF3 ( _C2F5 ) _3- , _{PF3 ( CF3} ) _{3- ,} ^{BF4- ,} BF2 ₍ CF) ^{2- ,} _{BF3 ( CF3} ) ^- , B(CN) _4- ^, N( _FSO2 ) _2- ^, N _{( CF3SO2} ) _2- , _{N ( C2F5SO2} ) _2- ^{, Cl- ,} Br- ^, _ClO4 ^- are listed.
One type of the electrolyte may be used, or two or more types may be used.

The electrolyte is usually used in the form of an electrolytic solution dissolved in a solvent. As a solvent for dissolving the electrolyte, an aprotic organic solvent is preferable. Aprotic organic solvents include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride, and sulfolane. These solvents may be used alone or in combination of two or more.
As the solvent, it is preferable to use a mixed solvent of a cyclic carbonate and a chain carbonate, because an electrolytic solution having a low viscosity and a high ionic conductivity can be obtained.

The concentration of the electrolyte in the electrolytic solution is preferably 0.1 mol/L or more, more preferably 0.5 to 1.5 mol/L, in order to reduce the internal resistance of the electrolytic solution. Note that the electrolyte may be gel or solid. In this case, electrolyte leakage can be suppressed.

When the present capacitor is a lithium ion capacitor, it is preferable to use an electrolytic solution containing lithium fluorophosphate (at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate). The lower limit of the content of lithium fluorophosphate is preferably 0.01% by mass, particularly preferably 0.05% by mass, in the electrolytic solution. On the other hand, the upper limit of the content of lithium fluorophosphate is preferably 10% by mass, more preferably 5% by mass, and particularly preferably 3% by mass in the electrolytic solution.

When the present capacitor is a lithium ion capacitor, it is preferable to further add sultones to the electrolytic solution. One kind of sultones may be used, or two or more kinds thereof may be used.
Note that sultones may decompose as the capacitor is charged and discharged, but in this specification, if the electrolytic solution used for manufacturing the capacitor contains sultones, the electrolytic solution contains sultones.

Examples of the sultones include compounds represented by the following formulas (B) and (C).

In formula (B), R ^B1 to R ^B6 are each independently a hydrogen atom, a halogen atom, or an alkyl group having 1 to 6 carbon atoms optionally substituted by a halogen atom, and n is an integer of 0 to 3 is.
The halogen atom is preferably a fluorine atom, the alkyl group having 1 to 6 carbon atoms is preferably an alkyl group having 1 to 3 carbon atoms, n is preferably 1 to 3, more preferably 1 to 2, and 1 is particularly preferred.

In formula (C), R ^C1 to R ^C4 are each independently a hydrogen atom, a halogen atom, or an alkyl group having 1 to 6 carbon atoms optionally substituted by a halogen atom, and n is an integer of 0 to 3 is.
Preferred examples of these halogen atoms, alkyl groups having 1 to 6 carbon atoms and n are the same as those in formula (B).

As the sultones, propane sultone and 1-propene 1,3-sultone are preferable.

When sultones are used, the lower limit of the amount of the sultones to be added to the electrolytic solution is preferably 0.01% by mass with respect to 100% by mass of the electrolytic solution. On the other hand, the upper limit of the amount of sultones to be added to the electrolytic solution is preferably 10% by mass, more preferably 5% by mass, and particularly preferably 2% by mass with respect to 100% by mass of the electrolytic solution. It should be noted that this amount of addition refers to the amount of addition to the electrolytic solution used when producing the capacitor.

When the electrolyte is used in the form of an electrolytic solution, a separator is usually used between the positive electrode and the negative electrode so that the positive electrode and the negative electrode do not come into physical contact with each other.
Examples of the separator include nonwoven fabrics and porous films made from cellulose rayon, polyethylene, polypropylene, polyamide, polyester, polyimide, and the like.

Capacitor structures include, for example, a laminated cell in which a plate-shaped positive electrode and a negative electrode are laminated with a separator or a gel-like or solid electrolyte interposed therebetween, and a laminate is enclosed in an exterior film or the like; A wound-type cell in which a laminate obtained by winding a negative electrode with a separator or a gel-like or solid-state electrolyte interposed therebetween is housed in a rectangular or cylindrical container can be exemplified.
In addition, as the structure of the capacitor, a bipolar cell is used, in which electrodes with a positive electrode formed on one surface of a plate-shaped current collector and a negative electrode formed on the other surface are stacked with a separator interposed therebetween. can also be mentioned.
In particular, when the structure of the capacitor is a wound cell housed in a square or cylindrical container, the present capacitor is suitable because it is easily affected by heat generation.

When the capacitor is a lithium ion capacitor, it is preferred that the negative electrode is pre-doped with lithium ions. For methods of pre-doping lithium ions, see, for example, WO 1998/033227 and WO 2000/007255.

EXAMPLES The embodiments of the present invention will now be described more specifically with reference to Examples. However, the present invention is not limited to the following examples.

[Example 1]
<Preparation of positive electrode>
92 parts by mass of commercially available activated carbon powder having a specific surface area of 2000 m ² /g, 6 parts by mass of acetylene black powder, 7 parts by mass of acrylic resin binder, 4 parts by mass of CMC and 200 parts by mass of water were stirred and mixed to prepare a slurry.
After applying a non-aqueous carbon-based conductive paint to both sides of an aluminum expanded metal (positive electrode current collector) with a porosity of 47% and a thickness of 38 μm with a roll coater, it is dried in a vacuum to form a conductive layer with a total thickness of 52 μm. A positive electrode current collector was obtained. The slurry was applied to both sides of the positive electrode current collector with a conductive layer using a roll coater, and then vacuum dried to obtain a laminate having a total thickness of 200 μm. In addition, on both the front and back surfaces of the positive electrode current collector, a portion where the conductive layer and the positive electrode active material layer are laminated (hereinafter also referred to as "coating portion") and a portion where no layer is formed (hereinafter, " The conductive layer and the electrode layer were formed so as to form an uncoated portion. The through-holes of the positive electrode current collector in the coated portion were almost blocked by the conductive layer. The laminate thus obtained is cut into a size of 98 mm×141 mm so that the coated portion has a size of 98 mm×126 mm and the uncoated portion has a size of 98 mm×15 mm, thereby producing a positive electrode. did.

<Production of negative electrode>
100 parts by mass of graphite powder having an average particle size of 4 μm and 50 parts by mass of optically isotropic pitch having a softening point of 110° C. and a mesophase content (QI (quinoline insoluble matter) content) of 13% are kneaded in a heated kneader, and then kneaded. It was fired at 800° C. in an oxidizing atmosphere. The obtained fired product was pulverized to have an average particle size of 8 μm, and fired again at 1000° C. in a non-oxidizing atmosphere to obtain a negative electrode active material. 92 parts by mass of the obtained negative electrode active material, 6 parts by mass of acetylene black powder, 5 parts by mass of SBR, 3 parts by mass of CMC and 200 parts by mass of water were stirred and mixed to prepare a slurry. The obtained slurry was coated on both sides of a negative electrode current collector made of copper expanded metal having a porosity of 57% and a thickness of 32 μm by a roll coater, and then dried in a vacuum to obtain a laminate having a total thickness of 80 μm. . The laminate thus obtained is cut into a size of 100 mm×143 mm so that the coated portion has a size of 100 mm×128 mm and the uncoated portion has a size of 100 mm×15 mm, thereby producing a negative electrode. did.

<Production of lithium ion capacitor element>
First, 10 sheets of the positive electrode, 11 sheets of the negative electrode, and 22 sheets of the separator having a thickness of 50 μm were prepared. , a separator, a negative electrode, a separator, and a positive electrode, and four sides of the resulting stack were fixed with tape to prepare an electrode stack. Next, a lithium ion supply member is produced by cutting a 100 μm thick lithium foil and crimping it to a 40 μm thick stainless mesh, and this lithium ion supply member is placed on one side of the electrode stack facing the negative electrode with a separator interposed therebetween. An electrode laminate unit was obtained by arranging so as to Then, an aluminum positive electrode having a width of 50 mm, a length of 50 mm, and a thickness of 0.2 mm, in which a sealant film was heat-sealed in advance to the uncoated portion of each of the 10 positive electrodes of the fabricated electrode laminate unit. The power tabs were superimposed and ultrasonically welded. On the other hand, a 50 mm wide, 50 mm long, and 0.2 mm thick copper copper plate having a sealant film heat-sealed in advance to the uncoated portion and the lithium ion supply member of each of the 11 negative electrodes of the electrode laminate unit. The power supply tabs for the negative electrode were overlapped and resistance welded. A lithium ion capacitor element was produced as described above.

<Production of lithium ion capacitor>
Next, a polypropylene (PP) layer, an aluminum layer and a nylon layer are laminated, and the dimensions are 125 mm (vertical width) × 160 mm (horizontal width) × 0.15 mm (thickness), and 105 mm (vertical width) × A 140 mm (width) drawn upper exterior film (the width of the outer peripheral edge that becomes the joint is 10 mm), a PP layer, an aluminum layer, and a nylon layer are laminated, and the dimension is 125 mm (vertical width). A lower exterior film of 160 mm (width) x 0.15 mm (thickness) was produced. Next, the lithium ion capacitor element is placed at a position on the lower exterior film to be the accommodation portion so that each of the positive electrode power tab and the negative electrode power tab protrudes outward from the end of the lower exterior film, An upper exterior film is superimposed on the lithium ion capacitor element, and three sides (two sides from which the positive electrode power source tab and the negative electrode power source tab protrude and one side in the longitudinal direction) of the outer peripheral edge of the upper and lower exterior films are laminated. were heat-sealed to form joints surrounding the accommodating portion on the three sides.

A mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (3:4:3 by volume, respectively) was used as the aprotic organic solvent, and a solution containing _LiPF6 at a concentration of 1.2 mol/L was prepared. After that, 0.5% by mass of an additive (lithium difluorophosphate) was added to the electrolytic solution to prepare an electrolytic solution.

Next, after injecting the obtained electrolytic solution into the accommodating portion between the exterior films, the remaining one side of the outer peripheral portion of the exterior film was heat-sealed. In this state, the negative electrode was doped with lithium ions from the lithium foil (lithium ion supply member) by leaving it for 10 days. In this manner, a test laminate-packaged lithium ion capacitor was produced.

<Evaluation of cell characteristics>
The lithium ion capacitor is charged with a constant current of 10 A until the cell voltage reaches the upper limit of the rated voltage of 3.8 V, and then a constant current-constant voltage charge of 3.8 V is applied for 0.5 times. time went Next, the capacitance was measured when the cell was discharged at a constant current of 10 A until the cell voltage reached the lower limit of 2.2 V of the rated voltage. The measured capacitance was 1110F.
At this time, the internal resistance was obtained by dividing the difference between the voltage immediately before discharge and the voltage 100 msec after the start of discharge by the discharge current. The measured internal resistance was 1.2 mΩ.
These results are shown in Table 1. The capacitance and internal resistance results of Example 2 and Comparative Examples 1 and 2 in Table 1 are relative values when the values of capacitance and internal resistance in Example 1 are set to 100, respectively.

<Measurement of _ΔSE >
First, the following measurements were performed in order to determine the predetermined time for leaving in the constant temperature bath. CCCV charging was performed for 24 hours at a current value of 10C with a charging voltage (upper limit of rated voltage) of 3.8V in a constant temperature chamber at 25°C. After that, when the temperature in the constant temperature bath was changed to 20°C, the open circuit voltage (OCV) of the lithium ion capacitor showed the maximum value after 30 minutes, so the time to leave in the constant temperature bath was 30 minutes.
The _ΔSE during charging of the lithium ion capacitor cell is obtained from Takamasa Oshima et al., "Heat generation behavior during rapid charging and discharging of a small lithium ion secondary battery", The Institute of Electrical Engineers of Japan B, 2004, Vol. 124, No. 12, p. . 1521-1527, it was measured as follows.

CCCV charging was performed for 24 hours at a current value of 10C with E=3.8V (the upper limit of the rated voltage) in a constant temperature bath at 25°C. After that, the temperature T in the constant temperature bath was changed to 30° C., and the OCV of the lithium ion capacitor was measured after 30 minutes had passed. Subsequently, the temperature in the constant temperature bath was changed to 25° C., and CCCV discharge was performed for 24 hours at a current value of 10 C with E=2.2 V (lower limit of rated voltage). After that, the temperature T in the constant temperature bath was changed to 30° C., and the OCV of the lithium ion capacitor was measured after 30 minutes had passed.
These measurements are
E = 3.8V; T = 30°C
E=2.2V; T=30°C
is represented as
Thereafter, OCV measurements were similarly performed in the following order. Note that the temperature of the constant temperature bath during CCCV charging and CCCV discharging other than the OCV measurement was set to 25°C.

E=3.8V; T=28°C
E=2.2V; T=28°C
E=3.8V; T=26°C
E=2.2V; T=26°C
E=3.8V; T=24°C
E=2.2V; T=24°C
E=3.8V; T=22°C
E=2.2V; T=22°C
E = 3.8V; T = 20°C
E=2.2V; T=20°C

The relationship between the OCV value for each temperature T (K) in the constant temperature bath measured as described above is plotted for E = 3.8 V and E = 2.2 V, and the OCV was calculated, and _{ΔS E} (J/K·mol) was calculated by the above formula (1-1). As a result, ΔSE at _E =3.8 V and E=2.2 V were 55.5 J/K·mol and 112.4 J/K·mol, respectively.

CCCV charging was performed for 24 hours at a current value of 10C with E=3.0V in a constant temperature bath at 25°C. After that, the temperature T in the constant temperature bath was changed to 30° C., and after 30 minutes had passed, the OCV (at E=3.0 V; T=30° C.) of the lithium ion capacitor was measured. Subsequently, CCCV discharge was performed at a set voltage of 2.2 V and a current value of 10 C for 24 hours. After that, the temperature in the constant temperature bath was changed to 25° C., and CCCV charging was performed at a current value of 10 C with E=3.0 V for 24 hours. Subsequently, the temperature T in the constant temperature bath was changed to 28° C., and the OCV of the lithium ion capacitor (E=3.0 V; T=28° C.) was measured after 30 minutes had passed. Repeat the same operation, E = 3.0 V; T = 26 ° C., E = 3.0 V; T = 24 ° C., E = 3.0 V; T = 22 ° C., E = 3.0 V; OCV measurements were performed at

As a result, ΔSE at _E =3.0 V was 93.0 J/K·mol. From the above, the average value of ΔSE at the three points of _E =3.8V, E=3.0V and E=2.2V was 87.0J/K·mol. Table 1 shows the results.

<Measurement of calorific value when charging with large current>
A thermocouple attached to the surface of the cell measures the amount of heat generated when the lithium ion capacitor is adiabatically charged from the lower limit of 2.2 V to the upper limit of 3.8 V of the rated voltage at a current of 100 A in an environment of 25 ° C. was calculated from the temperature change measured by and the heat capacity of the cell. As a result, the amount of heat generated is -210 J, and even when charging with such a large current, the charging process is substantially an endothermic process, and the deterioration due to (rapid) charging and discharging is suppressed. was confirmed. Table 1 shows the results.

<Cycle test with quick charge>
A charge-discharge cycle test was performed in which the lithium ion capacitor was charged at 60° C. at 100 A to the upper limit of the rated voltage of 3.8 V, and then discharged at 10 A to the lower limit of the rated voltage of 2.2 V. This charging and discharging was defined as one cycle, and the thickness of the cell was measured after 10000 cycles. Table 1 shows the results. The results of the cycle tests of Example 2 and Comparative Examples 1 and 2 in Table 1 are relative values when the increase in the thickness of the cell after the cycle test using the cell of Example 1 is set to 100. . The smaller this value is, the more difficult it is to deteriorate even if rapid charging is repeated, and the better the safety is.

[Example 2]
In Example 1, the same procedure as in Example 1 was performed except that the additive added to the electrolytic solution was changed to 1% by mass of lithium difluorophosphate and 0.05% by mass of 1-propene-1,3-sultone. Lithium ion capacitors were produced by the method, and various evaluations were performed. Table 1 shows the results.

[Comparative Example 1]
A lithium ion capacitor was produced in the same manner as in Example 1, except that no additive was added to the electrolytic solution, and various evaluations were performed. Table 1 shows the results.

[Comparative Example 2]
In Example 1, except that the negative electrode had the same structure as the positive electrode in Example 1, and the lithium ion supply member was not used (that is, the negative electrode was not doped with lithium ions). A capacitor cell was produced in the same manner as in the above.
The capacitor obtained in Comparative Example 2 was an electric double layer capacitor, and the rated voltage had an upper limit of 1.6V and a lower limit of 0V. In the measurement of _ΔSE , an average value was obtained at three points of E=1.6V, E=0.8V and E=0V. Evaluation of cell characteristics, measurement of heat generation when charging with a large current, and cycle test of rapid charging were also evaluated at 0-1.6V. Table 1 shows the evaluation results.

Claims (5)

comprising a positive electrode, a negative electrode and an electrolyte,
wherein the negative electrode is a negative electrode pre-doped with lithium ions;
A lithium ion capacitor having an entropy change ΔS _E average value of 82 J/K·mol or more determined by the following formula (1).
Average value of _ΔSE = (ΔS _3.8 + ΔS _3.0 + ΔS _2.2 )/3 (1)
[In equation (1), ΔS _3.8 , ΔS _3.0 and ΔS _2.2 are each calculated by the following equation (1-1). ]
ΔS _E =−9.65×10 ⁴ (C/mol)×ΔV ₀ /ΔT (1-1)
[In formula (1-1),
ΔS _E is either ΔS _3.8 , ΔS _3.0 or ΔS _2.2 ,
ΔV ₀ /ΔT is obtained by adjusting a lithium ion capacitor with a current value of 10 C so that the voltage of the lithium ion capacitor becomes E (V) in a thermostat at 25° C. at temperatures T=20° C., 22° C., 24° C., The open circuit voltage V ₀ after being left in a constant temperature bath at 26° C., 28° C. or 30° C. for a predetermined time was measured, and the horizontal axis was T (K) and the vertical axis was the open circuit voltage V ₀ (V). It means the slope of a straight line obtained by the method of least squares in a graph plotting the measurement results.
The ΔS _3.8 is the entropy change when the E (V) is 3.8 V,
The ΔS _3.0 is the entropy change when the E (V) is 3.0 V,
The ΔS _2.2 is the entropy change when the E(V) is set to 2.2V.
The predetermined time means the longer of A and B below.
A: A lithium ion capacitor adjusted to have a voltage of 3.8 V by performing constant current and constant voltage charging at a current value of 10 C for 24 hours in a constant temperature bath at 25 ° C. The time (minutes) required for the open circuit voltage to reach its maximum value after being left in the tank
B: 30 minutes when the capacitance of the lithium ion capacitor is less than 2000 F, and capacitance (F) / 2000 × 60 (minutes) when it is 2000 F or more] 2. The lithium ion capacitor of claim 1 , comprising an electrolyte comprising lithium fluorophosphate. 3. The lithium ion capacitor according to claim 2 , wherein said electrolytic solution further contains sultones. 4. The lithium ion capacitor according to claim 1 , wherein the entropy change _ΔSE has an average value of 200 J/K·mol or less. The lithium ion capacitor according to any one of claims 1 to 4, wherein the positive electrode contains activated carbon as a positive electrode active material.