JP7103376B2 - Lithium ion capacitor - Google Patents

Description

The present invention relates to a lithium ion capacitor.

A lithium ion capacitor is a so-called hybrid capacitor, and typically, an electrode in which an active material such as activated carbon is held in a current collecting foil to physically absorb and desorb ions is used as a positive electrode, and lithium ions are oxidized and reduced as a negative electrode. An electrode that absorbs and desorbs by the reaction is used. Lithium-ion capacitors are expected to be used in various applications as storage devices capable of achieving both large capacity and high output. For example, in-vehicle applications are being studied.

In relation to this, Patent Document 1 describes an active material as a current collector foil in consideration of developing a battery including a capacitor for in-vehicle use, although the invention focuses on a lithium ion battery instead of a lithium ion capacitor. An electrode binder for binding to is disclosed. Styrene-butadiene rubber (SBR), which is conventionally used as a binder for electrodes, is a water-based binder and is desirable in that it can easily reduce the environmental load in the electrode manufacturing process. On the other hand, in Patent Document 1, after pointing out the problem of oxidative deterioration when SBR is used as the positive electrode of a lithium ion battery, a specific structural unit derived from a monomer having a hydroxyl group and a polyfunctional (meth) An aqueous binder containing a polymer having a highly crosslinked structure containing a specific structural unit derived from an acrylate monomer does not cause oxidative deterioration and affects performance even in a relatively high temperature usage environment of 60 ° C. It is said that there is no such thing.

Japanese Unexamined Patent Publication No. 2014-160638

Considering that the lithium ion capacitor is used even in a low temperature environment, it is desired that the coagulation of the electrolytic solution and the precipitation of the solvent component do not occur as much as possible in the low temperature environment of about -40 ° C. Therefore, an object of the present invention is to provide a lithium ion capacitor in which coagulation of an electrolytic solution and precipitation of solvent components do not occur as much as possible in a low temperature environment.

The present invention takes the following means.
(1) A lithium ion capacitor having a positive electrode, a negative electrode, and an electrolytic solution in contact with the positive electrode and the negative electrode, wherein the electrolytic solution contains an organic solvent and a lithium salt electrolyte having an imide structure, and is said to be organic. The solvent contains ethylene carbonate and propylene carbonate, and the total amount of the ethylene carbonate and the propylene carbonate is 20 to 30 vol% with respect to the total amount of the organic solvent, and the organic solvent contains ethyl methyl carbonate and diethyl carbonate. The total of the ethylene carbonate, the propylene carbonate, the ethylmethyl carbonate and the diethyl carbonate is 100 vol% with respect to the total amount of the organic solvent.
(2) In the lithium ion capacitor of (1), the organic solvent contains the ethylene carbonate in a larger amount than the propylene carbonate.
(3) In the lithium ion capacitor of (1) or (2), the ratio of the ethyl methyl carbonate to the diethyl carbonate is 2: 1 .
(4) In the lithium ion capacitor of (3), the ethylene carbonate is 20 vol% and the propylene carbonate is 10 vol% with respect to the total amount of the organic solvent.

According to the lithium ion capacitor of the present invention , coagulation of the electrolytic solution and precipitation of solvent components can be suppressed as much as possible in a low temperature environment of about -40 ° C.

It is a figure which shows the Hansen solubility parameter of the polymer contained in an electrolytic solution and a binder in a Hansen space. It is a figure explaining the method of determining the dissolution sphere of the polymer contained in a binder. It is a graph which shows the capacity retention rate after leaving the lithium ion capacitor in a high temperature (85 ° C.) environment. It is a graph which shows the internal resistance increase rate after leaving the lithium ion capacitor in a high temperature (85 ° C.) environment. It is a graph which shows the relationship between the content ratio of cyclic carbonate in an organic solvent, and ionic conductivity. It is a graph which shows the relationship between the content ratio of cyclic carbonate in an organic solvent which does not contain dimethyl carbonate, and ionic conductivity.

[Main components of lithium-ion capacitors]
The lithium ion capacitor of the present invention has at least a positive electrode, a negative electrode, and an electrolytic solution in contact with the positive electrode and the negative electrode.

[Electrolytic solution]
The electrolytic solution contains a solvent and an electrolyte.

<Solvent>
As the solvent, an organic solvent conventionally used for lithium ion capacitors is used, and as this kind of organic solvent, a carbonate-based organic solvent, a nitrile-based organic solvent, a lactone-based organic solvent, an ether-based organic solvent, and an alcohol-based organic solvent , Ester-based organic solvent, amide-based organic solvent, sulfone-based organic solvent, ketone-based organic solvent, aromatic-based organic solvent can be exemplified. As the solvent, one kind or two or more kinds can be mixed and used at an appropriate composition ratio. Here, as the carbonate-based organic solvent, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and the like Chain carbonate can be exemplified.

Further, examples of the nitrile-based organic solvent include acetonitrile, acrylonitrile, adiponitrile, valeronitrile, and isobutyronitril. Further, as the lactone-based organic solvent, γ-butyrolactone and γ-valerolactone can be exemplified. Examples of the ether-based organic solvent include cyclic ethers such as tetrahydrofuran and dioxane, and chain ethers such as 1,2-dimethoxyethane, dimethyl ether and triglyme. Further, as the alcohol-based organic solvent, ethyl alcohol and ethylene glycol can be exemplified. Examples of the ester-based organic solvent include phosphoric acid esters such as methyl acetate, propyl acetate and trimethyl phosphate, sulfate esters such as dimethyl sulfate, and sulfite esters such as dimethyl sulfate. Examples of the amide-based organic solvent include N-methyl-2-pyrrolidone and ethylenediamine. Examples of the sulfone-based organic solvent include chain sulfones such as dimethyl sulfone and cyclic sulfones such as 3-sulfolene. Methyl ethyl ketone can be exemplified as the ketone-based organic solvent, and toluene can be exemplified as the aromatic organic solvent. The various organic solvents other than the carbonate-based organic solvent are preferably used in combination with a cyclic carbonate (particularly ethylene carbonate (EC)), and are expected to be used as a substitute for the chain carbonate described later, for example.

In the electrolytic solution of the lithium ion capacitor, it is preferable to use a carbonate-based organic solvent having excellent oxidation resistance in consideration of a usable voltage range (for example, about +4.0 V based on Li). Further, the electrolytic solution preferably has high ionic conductivity (S / m, S: Siemens, m: meter), which is an index of internal resistance, and this ionic conductivity lowers the viscosity of the electrolytic solution and the electrolyte (described later). It can be improved by increasing the degree of dissociation of. For this reason, it is more preferable to use a carbonate-based organic solvent obtained by mixing a cyclic carbonate that enhances the dissociation degree of the electrolyte and a low-viscosity chain carbonate as the organic solvent (see [Table 6] below).

Here, various chain carbonates can be used as the chain carbonate, but from the viewpoint of improving the heat resistance of the electrolytic solution, it is preferable not to use dimethyl carbonate (DMC) having a low boiling point and inferior heat resistance ([Table] below]. 6]). That is, when dimethyl carbonate (DMC) is contained in the organic solvent, dimethyl carbonate (DMC) is thermally decomposed into diethyl carbonate (DEC), and the decomposition by-product at that time causes an increase in internal resistance and deterioration of heat resistance. (Note that this speculation does not limit the present invention). Considering the use of a lithium ion capacitor in a high temperature environment, it is possible to use ethyl methyl carbonate (EMC) having a relatively high boiling point and low viscosity or diethyl carbonate (DEC) having a higher boiling point as the chain carbonate. preferable. From the viewpoint of improving both the ionic conductivity and the heat resistance of the electrolytic solution, it is more preferable to use a mixture of ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). The ratio of ethyl methyl carbonate (EMC) to diethyl carbonate (DEC) in the organic solvent is not particularly limited, but can be set in the range of, for example, EMC: DEC = 2: 1 to 1: 2.

Although various cyclic carbonates can be used as the cyclic carbonate, it is preferable to use ethylene carbonate (EC) having a solid electrolytic solution phase-to-phase (SEI) film forming ability from the viewpoint of improving the oxidation resistance of the electrolytic solution. When ethylene carbonate (EC) and another cyclic carbonate (for example, PC) are mixed and used as the cyclic carbonate, it is preferable that ethylene carbonate (EC) is contained in a larger amount than the other cyclic carbonate (for example, PC). .. By containing a relatively large amount of ethylene carbonate (EC) in this way, the ability to form an SEI film is suitably exhibited, and after the ethylene carbonate (EC) is reduced and decomposed, a protective film called SEI is formed on the surface of the negative electrode. , The electrolyte is no longer directly exposed to the potential of lithium (Li).

Considering that the lithium ion capacitor is used in a low temperature environment, it is desired that the coagulation of the electrolytic solution and the precipitation of the solvent component do not occur as much as possible in the low temperature environment of about -40 ° C. Therefore, when ethylene carbonate (EC) having a high melting point is used as the cyclic carbonate, the ratio of ethylene carbonate (EC) to the total amount of the organic solvent is preferably less than 35 vol%. By using a mixture of low melting point propylene carbonate (PC) and ethylene carbonate (EC), the content of cyclic carbonate in the organic solvent is increased without causing coagulation of the electrolytic solution and precipitation of solvent components as much as possible. be able to. At this time, the total ratio of ethylene carbonate (EC) and propylene carbonate (PC) to the total amount of the organic solvent is preferably less than 55 vol%, more preferably less than 40 vol%. When the total ratio of ethylene carbonate (EC) and propylene carbonate (PC) is 55 vol% or more, the possibility of coagulation of the electrolytic solution and precipitation of solvent components in a low temperature environment is extremely increased. If the total ratio of ethylene carbonate (EC) and propylene carbonate (PC) is 40 vol% or more, the desired ionic conductivity may not be obtained, for example, in a low temperature environment. When ethylene carbonate (EC) and propylene carbonate (PC) are mixed and used, the total ratio to the total amount of these organic solvents is set in the range of 35 vol% to 20 vol% to coagulate the electrolytic solution and precipitate the solvent component. The desired ionic conductivity can be obtained in a wide temperature range (for example, in the range of 25 ° C. to −40 ° C.) without causing the above.

<Electrolyte>
The electrolyte contributes to the charge / discharge reaction by ionizing lithium ions and anions, and mainly contains a lithium salt having an imide structure. Examples of the lithium salt having an imide structure include lithium bis (fluorosulfonyl) imide [LiFSI], lithium bis (trifluoromethanesulfonyl) imide [LiTFSI], and lithium bis (pentafluoroethanesulfonyl) imide [LiBETI]. As the lithium salt having an imide structure, only one kind may be used or two or more kinds may be used. Further, other lithium salt electrolytes may be used in combination as long as the effects of the present invention are not impaired. The electrolyte is preferably contained in the electrolytic solution at 0.5 to 10.0 mol / L. If the concentration of the electrolyte is less than 0.5 mol / L, the ionic conductivity tends to decrease due to insufficient ion concentration. On the other hand, when the concentration of the electrolyte exceeds 10.0 mol / L, the viscosity of the electrolytic solution increases and the ionic conductivity tends to decrease. More preferably, the concentration of the electrolyte is 0.5 to 2.0 mol / L. In this case, the viscosity of the electrolytic solution is optimum, and the ionic conductivity is unlikely to decrease.

Additives may be appropriately added to the electrolytic solution, and examples of the additives include vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[Positive electrode]
At the positive electrode, an electric double layer is formed by physical adsorption and desorption of ions during charging and discharging. The positive electrode includes at least a current collecting foil, a positive electrode active material, and a binder, and if necessary, a conductive auxiliary agent. The positive electrode is formed by holding a positive electrode active material or the like on the current collecting foil via a binder.

<Current collector foil>
As the current collecting foil, those conventionally used for lithium ion capacitors can be applied, and for example, a metal foil having perforated conductivity such as aluminum or stainless steel can be used.

<Positive electrode active material>
As the positive electrode active material, particles having a high specific surface area capable of adsorbing and desorbing ions can be used. Examples of such a positive electrode active material include activated carbon and carbon nanotubes.

<Binder>
Binders are used to bind the materials that make up the positive electrode. The binder is mainly composed of a polymer, which is an adhesive component. The polymer has a RED value (relative energy difference) of more than 1 based on the Hansen solubility parameter (HSP) with respect to the electrolytic solution. Examples of such a polymer include polyacrylic acid. The term "polyacrylic acid" as used herein is a broad concept including not only unneutralized polyacrylic acid but also neutralized salts of polyacrylic acid and crosslinked products. Polyacrylic acid may be used alone or in combination of two or more. Water or an organic solvent can be used as the solvent for dissolving the polymer. An aqueous binder that uses water as a solvent is preferable because it can reduce the environmental load in the manufacturing process. Polyacrylic acid is also suitable in that it can form an aqueous binder using water as a solvent.

The Hansen solubility parameter was published by Charles M Hansen and is known as an index of solubility between substances. The Hansen solubility parameter is composed of the following three numerical values D, P, and H, and these three parameters are represented as coordinates in a three-dimensional space (Hansen space). In FIG. 1, in the Hansen space, the Hansen solubility parameter of the polymer is indicated by a reference numeral a, and the Hansen solubility parameter of the electrolytic solution is indicated by a reference numeral b.
D: Dispersion force (atom) P: Polarization force (molecular) H: Hydrogen bond force (molecular) Solubility between substances is estimated by the distance between the coordinates indicating the Hansen solubility parameter of each substance, and the coordinates are It is said that the closer they are to each other, the easier it is to dissolve, and the farther they are from each other, the more difficult it is to dissolve.

The Hansen solubility parameter of the electrolytic solution can be calculated from the chemical structure and composition ratio of the components. In that case, it can be obtained by using the software HSPiP (Hansen Solubility Parameter in Practice: software for Windows [registered trademark] for efficiently handling HSP) developed by Hansen et al.

To determine the Hansen solubility parameter of a polymer, the polymer is dissolved (mixed) in a plurality of solvents having known Hansen solubility parameters, and the Hansen solubility parameters of the solvent in which the polymer dissolves and the solvent in which the polymer does not dissolve are plotted in the Hansen space. The center of the sphere (Hansen solubility parameter) formed by a collection of plots of the solvent in which the polymer dissolves is defined as the Hansen solubility parameter of the polymer. The polymer lysate and Hansen solubility parameters can also be calculated using the software HSPiP. In FIG. 2, the case where polyacrylic acid (sodium neutralized salt of polyacrylic acid) is used as the polymer is illustrated, and the Hansen solubility parameter of the solvent that dissolves polyacrylic acid is a black circle, and the Hansen solubility parameter of the solvent that does not dissolve is Is indicated by a black square, and the calculated dissolution sphere s is indicated. The coordinate at the center of the dissolution sphere s indicated by the white circle is defined as the Hansen solubility parameter a of the polymer.

The RED value of the polymer with respect to the electrolytic solution is RED = Ra / R ₀ , where Ra is the distance between the Hansen solubility parameter of the polymer and the Hansen solubility parameter of the electrolytic solution, and the radius of interaction, which is the radius of the dissolved sphere of the polymer, is R ₀ . It is represented by. When the RED value of the polymer with respect to the electrolytic solution is larger than 1, the Hansen solubility parameter b of the electrolytic solution is located outside the dissolving spheres of the polymer, and the electrolytic solution and the polymer are soluble in each other, as shown in FIG. Hateful. In FIG. 1, lithium bis (fluorosulfonylimide) (LiFSI) is used as a solubility parameter of the electrolytic solution in a mixed solvent of ethylene carbonate (EC) 30 vol%, dimethyl carbonate (DMC) 30 vol% and ethyl methyl carbonate (EMC) 40 vol%. ) Is illustrated as an example of the Hansen solubility parameter of the electrolytic solution containing 1 mol / L.

The binder is preferably added in an amount of 1 to 10% by mass based on the positive electrode active material. If it is less than 1% by mass, the binding force tends to be insufficient. On the other hand, if it exceeds 10% by mass, it may cause an increase in conductor resistance.

<Conductive aid>
The conductive auxiliary agent can be appropriately added in order to reduce the conductor resistance inside the positive electrode and at the interface of the current collector foil. As the conductive auxiliary agent, those conventionally used for lithium ion capacitors can be applied, and examples thereof include acetylene black, graphite fine particles and fine fibers.

<How to make a positive electrode>
The positive electrode is produced by applying a slurry for a positive electrode, which is a mixture of other constituent materials using water as a solvent, to a current collector foil to form a coating film. If the binder thickening action is insufficient in the positive electrode slurry, a thickening material such as carboxycellulose can be added as appropriate. In that case, the total amount of the binder and the thickener added is preferably 1 to 10% by mass with respect to the positive electrode active material. Water, which is a solvent, is applied to the current collector foil when the addition amount is adjusted so that the measured value of 2s ^-1 when the viscosity of the slurry is measured with a B-type viscometer is in the range of 1000 to 10000 mPa · s. It is preferable because it easily forms a film.

[Negative electrode]
At the negative electrode, lithium ions are occluded and desorbed during charging and discharging. As the negative electrode, a negative electrode constituting a conventional lithium ion capacitor can be used. Typically, a negative electrode active material such as graphite capable of occluding and desorbing lithium ions is held in a current collector foil via a binder. Examples of the current collecting foil of the negative electrode include copper, aluminum, stainless steel and the like. As the binder, those conventionally used can be applied, but from the viewpoint of environmental protection and the like, an aqueous binder such as styrene-butadiene rubber (SBR) is preferable.

[Construction of lithium ion capacitor]
The lithium ion capacitor of the present invention can take a cell form having the same configuration as the conventional lithium ion capacitor. For example, it can be a laminated cell in which a positive electrode and a negative electrode are laminated via a separator, or a wound cell in which a separator is interposed between a positive electrode and a negative electrode and wound.

The amount of the positive electrode and negative electrode slurry applied according to the above-described embodiment is adjusted by the discharge capacity, the size of the positive electrode and the negative electrode, and the like. Further, the thickness of the current collector foil is a thickness that is not damaged during manufacturing, and is adjusted by the internal resistance performance of the lithium ion capacitor or the like. In addition, the solvent is changed according to the performance of the lithium ion capacitor.
Further, the lithium ion capacitor of another embodiment has a positive electrode, a negative electrode, and an electrolytic solution in contact with the positive electrode and the negative electrode, and the electrolytic solution contains an organic solvent and a lithium salt electrolyte having an imide structure. The positive electrode has a current collector foil and a positive electrode active material, and the positive electrode active material is held in the current collector foil via a binder containing a polymer having a RED value greater than 1 based on the Hansen solubility parameter for the electrolytic solution. .. According to this lithium ion capacitor, the capacity can be maintained and the increase in internal resistance can be suppressed to be small in a high temperature environment of 85 ° C.

[Creation of positive electrode]
First, powdered activated carbon as a positive electrode active material, polyacrylic acid (sodium neutralized salt of polyacrylic acid) as a binder, acrylic acid ester or styrene-butadiene rubber [SBR], acetylene black as a conductive auxiliary agent, and a thickener. Using carboxymethyl cellulose [CMC] and water as a solvent, the positive electrode slurry A to C containing the positive electrode active material were prepared with the compositions shown in Table 1. In addition, "part" in Table 1 indicates mass part, and "%" indicates mass%.

The positive electrode slurry A using polyacrylic acid as a binder was prepared by the following procedure.
(1) All the materials and water were mixed with a mixer a (Awatori Rentaro ARE-310 manufactured by Shinky Co., Ltd.) to prepare a pre-slurry.
(2) The pre-slurry obtained in (1) was further mixed with a mixer b (Filmix 40-L manufactured by Primix Corporation) to prepare an intermediate slurry.
(3) The intermediate slurry obtained in (2) was mixed again with the mixer a to prepare a slurry A for a positive electrode.

Slurries B and C for positive electrodes using acrylic acid ester or SBR as a binder were prepared by the following procedure.
(1) A pre-slurry was prepared by mixing the material excluding the binder and water with a mixer a.
(2) The pre-slurry obtained in (1) was further mixed with a mixer b to prepare an intermediate slurry.
(3) A binder was added to the intermediate slurry obtained in (2) and mixed with a mixer a to prepare a slurry B or C for a positive electrode.

Next, using an aluminum foil (porous foil) having a thickness of 15 μm as the current collecting foil, the positive electrode slurries A to C were applied to the current collecting foils and dried to prepare positive electrodes A to C. The amount of the positive electrode slurry applied was adjusted so that the mass of the activated carbon after drying was 3 mg / cm ² . A blade coater or a die coater was used to apply the positive electrode slurry to the current collector foil.

[Creation of negative electrode]
First, 95 parts by mass of graphite as a negative electrode active material, 1 part by mass of SBR as a binder, 1 part by mass of CMC as a thickener, and 100 parts by mass of water as a solvent were mixed to prepare a slurry for a negative electrode by the following procedure. ..
(1) A pre-slurry was prepared by mixing the material excluding the binder and water with a mixer a.
(2) The pre-slurry obtained in (1) was further mixed with a mixer b to prepare an intermediate slurry.
(3) A binder was added to the intermediate slurry obtained in (2) and mixed with a mixer a to prepare a slurry for a negative electrode.

Next, a copper foil (porous foil) having a thickness of 10 μm was used as the current collector foil, and a slurry for the negative electrode was applied to the current collector foil and dried to prepare a negative electrode. The amount of the negative electrode slurry applied was adjusted so that the mass of graphite after drying was 3 mg / cm ² . A blade coater was used to apply the negative electrode slurry to the current collector foil.

[Adjustment of electrolyte]
As a solvent, a mixed solvent of ethylene carbonate (EC) 30 vol%, dimethyl carbonate (DMC) 30 vol% and ethyl methyl carbonate (EMC) 40 vol% was used, and 1 mol / L of lithium bis (fluorosulfonylimide) (LiFSI) was used as the mixed solvent. The electrolyte I was prepared by adding. Further, lithium hexafluorophosphate (LiPF ₆ ) was added to the mixed solvent to prepare the electrolytic solution P. As a solvent, a mixed solvent of ethylene carbonate (EC) 30 vol%, ethyl methyl carbonate (EMC) 46.7 vol%, diethyl carbonate (DEC) 23.3 vol%, and propylene carbonate (PC) 10 vol% was used, and lithium was used as the mixed solvent. The electrolytic solution I2 was prepared by adding 1 mol / L of bis (fluorosulfonylimide) (LiFSI).

[Preparation of lithium-ion capacitor cells for evaluation]
The evaluation lithium ion capacitor cells of Examples and Comparative Examples were prepared by the following procedure with the combination of the positive electrode and the electrolyte shown in Table 2.
(1) The positive electrode and the negative electrode are punched out to form a rectangle with a size of 60 mm × 40 mm, and the coating film in the 20 mm × 40 mm region on one end side of the long side is peeled off while leaving the coating film of 40 mm × 40 mm to collect the current. Was installed.
(2) A laminated body was prepared by facing the coating film portions of the positive electrode and the negative electrode with a cellulose separator having a thickness of 20 μm interposed therebetween.
(3) The laminate prepared in (2) and a metallic lithium foil for lithium predoping were encapsulated in an aluminum laminate foil, and an electrolytic solution was injected and sealed to prepare a lithium ion capacitor cell for evaluation.

[Measurement of initial performance]
After performing lithium pre-doping, charging / discharging, and aging in each of the manufactured lithium-ion capacitor cells for evaluation, the cutoff voltage is 2.2 to 3.8 V and the internal resistance is measured at 10 C at room temperature (25 ° C.). And the discharge capacity was measured, and the result was used as the initial performance.

[Durability test (85 ° C float test)]
The evaluation lithium ion capacitor cell in a state where an external power source was connected and the voltage was maintained at 3.8 V was left in a constant temperature bath at 85 ° C. The leaving time corresponds to an 85 ° C., 3.8 V float time. After a lapse of a predetermined time, the evaluation lithium ion capacitor cell is taken out from the constant temperature bath, returned to room temperature, and then the internal resistance and the discharge capacity are measured under the same conditions as the above initial performance measurement, and the capacity retention rate (initial discharge capacity is 100). The percentage of discharge capacity when% was used) and the internal resistance increase rate (internal resistance increase rate from the initial performance) were calculated. The results are shown in Table 3 and FIG. 3 or FIG.

As shown in FIG. 3 and the like, when left in a high temperature environment of 85 ° C., in Comparative Example 3 using an electrolytic solution containing lithium fluoride phosphate having no imide structure as an electrolyte, the capacity retention rate was short. While the volume was halved, the capacity retention rate was kept high for a long time in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 using an electrolytic solution containing a lithium salt having an imide structure as an electrolyte. However, as shown in FIG. 4 and the like, it was clarified that there is a difference in the internal resistance increase rate depending on the composition of the binder of the positive electrode even when an electrolytic solution containing a lithium salt having an imide structure is used as the electrolyte. .. Therefore, when the RED values (see Table 2) of the polymer constituting the binder of the positive electrode with respect to the electrolytic solution were compared, in Comparative Example 1 using an acrylic acid ester having a RED value of 1 or less and Comparative Example 2 using SBR, It was found that the rate of increase in internal resistance was high. On the other hand, in Examples 1 and 2, an electrolytic solution containing a lithium salt having an imide structure is used as the electrolyte, and polyacrylic acid having a RED value greater than 1 with respect to the electrolytic solution is used as the polymer constituting the binder of the positive electrode. Is used. In this case, it is clear that the polymer constituting the binder of the positive electrode is difficult to dissolve in the electrolytic solution, the capacity retention rate is kept high even when left in a high temperature environment of 85 ° C., and the internal resistance increase rate can be suppressed to a small value. became. In particular, in Example 2, it was found that the rate of increase in internal resistance was further suppressed. The result of Example 2 is that the carbonate-based organic solvent as the organic solvent does not contain dimethyl carbonate (DMC) and contains more ethylene carbonate (EC) than propylene carbonate (PC). It was easily inferred that there was. In order to calculate the RED value, the Hansen solubility parameter of the polymer and the interaction radius and the Hansen solubility parameter of the electrolytic solution were calculated by the following procedure.

<Calculation of Hansen solubility parameter and interaction radius of polymer>
For each binder, the radius of interaction of the polymer was calculated as _R0 and the Hansen solubility parameter was calculated by the following procedure.
(1) Preparation of test sample A binder dissolved in water is dropped onto a Teflon sheet (Teflon: registered trademark) and dried at 60 ° C. for at least one day to prepare a test sample.
(2) Preparation of test solvent In the composition shown in Table 4, No. 1-29 test solvents were adjusted. Each of the prepared test solvents was placed in a vial having a capacity of 10 ml, and a molecular sieve 3A was also prepared for a highly hygroscopic solvent.
(3) Swelling test The test sample was immersed in each test solvent. The sample was left at room temperature for 1 to 14 days, and the shape change of the sample was observed. When there was no difference in shape change (degree of swelling) due to the test solvent, the mixture was left at 60 ° C.
(4) Evaluation of shape change After (3), the shape change of the test sample with respect to each test solvent was judged and scored in the following four stages. 0: No change 1: Dissolution 2: Swelling 3: Other changes (discoloration, etc.)
(5) Calculation of Hansen solubility parameter and interaction radius Using software HSPiP, the Hansen solubility parameter and interaction radius were calculated based on the evaluation result of (4). The results are shown in Table 5.

<Calculation of Hansen solubility parameter of electrolytic solution>
Calculated using software HSPiP based on the chemical structure and composition ratio of the components. As a result, the Hansen solubility parameter of the electrolytic solution I was D: 16.3 P: 12.5 H: 7.5.

<Effect of organic solvent>
The following [Table 6] shows the physical characteristics of various solvent components used as carbonate-based organic solvents. In addition, [Table 7] to [Table 9] show the characteristics of various electrolytic solutions in which 1 mol / L LiFSI was added to various carbonate-based organic solvents (boiling point (boiling point measurement method will be described later), presence or absence of precipitation in a low temperature environment). (The method for confirming the presence or absence of precipitation will be described later), and the ionic conductivity in a normal temperature and low temperature environment) is shown. In [Table 7], EC was used as the cyclic carbonate, in [Table 8], PC was used as the cyclic carbonate, and in [Table 9], EC and PC were mixed and used as the cyclic carbonate. Further, [Table 10] and [Table 11] show the characteristics (presence or absence of precipitation in an environment of −50 ° C.) of various electrolytic solutions obtained by adding 1 mol / L LiFSI to various carbonate-based organic solvents. The graph of FIG. 5 is a graph prepared based on [Table 7] to [Table 9], and the horizontal axis shows the value of the content ratio (sufficiency ratio) of the cyclic carbonate to the total amount of the organic solvent, and the vertical axis shows the value. Indicates the value of ionic conductivity in normal temperature and low temperature environments. The graph of FIG. 6 is a graph prepared based on [Table 9], and the horizontal axis shows the value of the content ratio (sufficiency ratio) of cyclic carbonate to the total amount of the organic solvent, and the vertical axis shows the normal temperature and low temperature. Shows the value of ionic conductivity under the environment.

<Measuring method of boiling point>
Here, the methods for measuring the boiling points of the various electrolytic solutions shown in [Table 7] to [Table 9] will be described in detail. In this test, a separator or the like is encapsulated in an aluminum laminate foil in an Ar atmosphere, an electrolytic solution is injected and sealed to prepare a lithium ion capacitor cell, and the lithium ion capacitor cell is placed in a constant temperature bath. After raising the temperature to a predetermined temperature in increments of ° C., the mixture was left to stand for about 30 minutes. After leaving it for about 30 minutes, the lithium ion capacitor cell was observed, and when the electrolytic solution exceeded the boiling point, the electrolytic solution was a gas and the lithium ion capacitor cell was swollen. Therefore, in this test, the boiling point of each electrolytic solution was set as the upper limit temperature at which the lithium ion capacitor cell did not swell.

<Method of confirming the presence or absence of precipitation in a low temperature environment>
Next, a method for confirming the presence or absence of precipitation of the various electrolytic solutions shown in [Table 7] to [Table 11] in a low temperature environment will be described in detail. In this test, a separator or the like is encapsulated in a transparent container in an Ar atmosphere, an electrolytic solution is injected and sealed to prepare a lithium ion capacitor cell, and the lithium ion capacitor cell is placed in a constant temperature bath. It was left at 50 ° C. for 30 minutes. After leaving it for 30 minutes, the lithium ion capacitor cell was observed, and when it became non-transparent (solidified when it became cloudy or the like), it was determined that there was precipitation.

With reference to [Table 7] and [Table 8], it was found that the boiling point of the electrolytic solution was lowered by the inclusion of dimethyl carbonate (DMC) in the organic solvent. From this, it was easily inferred that the use of dimethyl carbonate (DMC) as the organic solvent tends to cause an increase in the internal resistance of the electrolytic solution and a deterioration in heat resistance (this inference does not limit the present invention. do not have). Further, referring to [Table 7] to [Table 9], the boiling point of the electrolytic solution is increased by containing ethyl methyl carbonate (EMC) or diethyl carbonate (DEC) in the electrolytic solution instead of dimethyl carbonate (DMC). It turned out to rise. From this, it was found that it is preferable to use ethyl methyl carbonate (EMC) or diethyl carbonate (DEC) as the chain carbonate, considering that the lithium ion capacitor is used in a high temperature environment. Further, from the viewpoint of improving both the ionic conductivity and the heat resistance of the electrolytic solution, it was found that it is more preferable to use a mixture of ethyl methyl carbonate (EMC) and diethyl carbonate (DEC).

Further, referring to [Table 10], precipitation of solvent components was observed in the electrolytic solution of the organic solvent containing 35 vol% or more of ethylene carbonate (EC). From this, it was found that when ethylene carbonate (EC) is used as the cyclic carbonate, the ratio of ethylene carbonate (EC) in the organic solvent is preferably less than 35 vol%.

Further, referring to [Table 11], by using a mixture of propylene carbonate (PC) and ethylene carbonate (EC), the cyclic mixture in the organic solvent is used without causing coagulation of the electrolytic solution and precipitation of solvent components as much as possible. It was found that the carbonate content could be increased. Further, the electrolytic solution of the organic solvent containing a total of 55 vol% or more of ethylene carbonate (EC) and propylene carbonate (PC) had an extremely high possibility of coagulation. Further, referring to FIGS. 5 and 6, in the electrolytic solution of the organic solvent containing ethylene carbonate (EC) and propylene carbonate (PC) in total of 40 vol% or more, the ionic conductivity tended to decrease in a low temperature environment. From this, it was found that the total ratio of ethylene carbonate (EC) and propylene carbonate (PC) to the total amount of the organic solvent is preferably less than 55 vol%, more preferably less than 40 vol%. By setting the total ratio of ethylene carbonate (EC) and propylene carbonate (PC) to the total amount of the organic solvent in the range of 35 vol% to 20 vol%, a wide temperature range (for example,) without causing coagulation of the electrolytic solution as much as possible. It was found that the desired ionic conductivity can be obtained in the range of 25 ° C. to −40 ° C.).

Claims (4)

A lithium ion capacitor having a positive electrode, a negative electrode, and an electrolytic solution in contact with the positive electrode and the negative electrode.
The electrolytic solution contains an organic solvent and a lithium salt electrolyte having an imide structure.
The organic solvent contains ethylene carbonate and propylene carbonate, and contains
The total amount of the ethylene carbonate and the propylene carbonate is 20 to 30 vol% with respect to the total amount of the organic solvent.
The organic solvent contains ethyl methyl carbonate and diethyl carbonate.
A lithium ion capacitor in which the total of the ethylene carbonate, the propylene carbonate, the ethyl methyl carbonate and the diethyl carbonate is 100 vol% with respect to the total amount of the organic solvent. The lithium ion capacitor according to claim 1.
The organic solvent is a lithium ion capacitor containing the ethylene carbonate in a larger amount than the propylene carbonate. The lithium ion capacitor according to claim 1 or 2.
A lithium ion capacitor having a ratio of the ethyl methyl carbonate to the diethyl carbonate of 2: 1 . The lithium ion capacitor according to claim 3.
A lithium ion capacitor in which the ethylene carbonate is 20 vol% and the propylene carbonate is 10 vol% with respect to the total amount of the organic solvent.

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