JP5650029B2 - Lithium ion capacitor - Google Patents

Description

  The present invention relates to a lithium ion capacitor, and more particularly to a lithium ion capacitor characterized in that a specific negative electrode material and an electrolytic solution having a specific composition are combined.

  Lithium ion capacitors are attracting attention as power storage devices featuring high energy density, but conventional lithium ion capacitors have problems in that they have high internal resistance and generally have inferior low temperature characteristics compared to electric double layer capacitors. At present, there is a need for a lithium ion capacitor having a smaller internal resistance and improved low-temperature characteristics.

  Patent Document 1 discloses that in a lithium ion capacitor, high power characteristics and cycle durability can be improved by using graphite having a controlled particle size as a negative electrode active material. However, with this technology, although the internal resistance can be reduced, it is difficult to obtain a highly reliable lithium ion capacitor because of large gas generation during pre-charging or float testing. there were.

  Therefore, a method for reducing the internal resistance of a lithium ion capacitor by using a graphite-based material conventionally used in an electrochemical device or the like as a negative electrode active material has been studied, but by itself, the internal resistance is sufficiently low. In addition, the low temperature characteristics were insufficient, and the generation of a large amount of gas during the preliminary charging and the float test could not be prevented.

JP 2008-103596 A

  As shown in Patent Document 1, it is effective to use graphite as a negative electrode active material and to make the particle diameter of the graphite smaller than usual in order to realize low resistance in a lithium ion capacitor. . However, in order to achieve higher output and lower resistance at lower temperatures in lithium ion capacitors, it is necessary to optimize the dielectric constant and viscosity of the electrolyte solvent, and only graphite is used as the negative electrode active material. In the case of use, it has been found that there is a problem that the generation of gas at the time of preliminary charging and the float test cannot be prevented.

  The present invention solves the above-mentioned problems in a lithium ion capacitor, and has low internal resistance, excellent low temperature characteristics, and high compatibility between the electrolyte and the negative electrode active material, so that it can be precharged and float tested. An object of the present invention is to provide a lithium ion capacitor in which gas generation is suppressed.

The lithium ion capacitor of the present invention comprises a positive electrode, a negative electrode formed of a negative electrode active material made of graphite composite particles having a number average particle diameter D50 of 0.1 to 5 μm, and a lithium salt formed of an aprotic organic solvent. A lithium ion capacitor comprising: a negative electrode and a lithium ion supply source, wherein the negative electrode is doped with lithium ions by electrochemical contact ;
The aprotic organic solvent of the electrolytic solution is a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the volume ratio of ethylene carbonate to the total of ethyl methyl carbonate and dimethyl carbonate is 1: 3. It is characterized by 1: 1.

In the above, in the aprotic organic solvent of the electrolytic solution, the volume ratio of ethyl methyl carbonate to dimethyl carbonate is preferably 1: 1 to 9: 1.
The graphite-based composite particles of the negative electrode active material are those in which graphite powder is coated with tar or pitch material, and the ratio of tar or pitch material to 100 parts by weight of graphite powder is preferably 10 to 40 parts by weight. .

According to the present invention, a specific graphite-based material is used as a negative electrode active material, and an internal electrolyte is used in combination with an aprotic organic solvent having a specific composition. It is possible to provide a lithium ion capacitor in which gas generation during the test is suppressed.
According to the present invention, the chain carbonate constituting the aprotic organic solvent has a specific composition, so that the internal resistance is low, the low temperature characteristics are excellent, and the gas at the time of precharging and the float test is obtained. Therefore, it is possible to provide a lithium ion capacitor in which the occurrence of the above is suppressed.

Hereinafter, the present invention will be described in detail.
The lithium ion capacitor of the present invention basically has an electrode unit in which the positive electrode and the negative electrode are alternately stacked or wound via a separator in the outer container. As the outer container, a cylindrical type, a square type, a laminate type, or the like can be used as appropriate, and is not particularly limited.

  In this specification, “dope” also means occlusion, adsorption, or insertion, and is widely a phenomenon in which at least one of lithium ions and anions enters the positive electrode active material, or a phenomenon in which lithium ions enter the negative electrode active material. Say. “De-doping” also means desorption and release, and refers to a phenomenon in which lithium ions or anions are desorbed from the positive electrode active material, or a phenomenon in which lithium ions are desorbed from the negative electrode active material.

  As a method of previously doping lithium ions into at least one of the negative electrode and the positive electrode, for example, a lithium ion supply source such as metallic lithium is disposed in the capacitor cell as a lithium electrode, and at least one of the negative electrode and the positive electrode and a lithium ion supply source A method of doping lithium ions by electrochemical contact is used.

In the lithium ion capacitor according to the present invention, it is possible to uniformly dope lithium ions into at least one of the negative electrode and the positive electrode also by locally disposing the lithium electrode in the cell and bringing it into electrochemical contact.
Therefore, even when a large-capacity cell is formed by laminating or further winding the positive electrode and the negative electrode, by disposing the lithium electrode in a part of the cell located on the outermost periphery or outermost layer, At least one can be smoothly and uniformly doped with lithium ions.

[Current collector]
The positive electrode and the negative electrode are respectively provided with a positive electrode current collector and a negative electrode current collector that receive and distribute electricity. As the positive electrode current collector and the negative electrode current collector, for example, a material in which a through-hole penetrating the front and back surfaces such as expanded metal is used, and the lithium electrode is disposed to face at least one of the negative electrode and the positive electrode. Therefore, it is preferable to supply lithium ions electrochemically. The form and number of the through holes are not particularly limited, and can be set so that the lithium ions in the electrolytic solution can move between the front and back surfaces of the electrode without being blocked by the electrode current collector.

  As a material of the current collector, a material generally used for a lithium battery can be applied. For example, aluminum, stainless steel, or the like can be used as the positive electrode current collector, while stainless steel, copper, nickel, or the like can be used as the negative electrode current collector. The thickness of each current collector is not particularly limited, but may be usually 5 to 50 μm, preferably 10 to 50 μm, and particularly preferably 20 to 50 μm.

The through holes of each current collector may be formed by any of a method of forming an opening by etching or the like and a method of forming an opening by mechanical driving. The diameter of the through hole of each current collector is, for example, 0.1 μm to 150 μm, preferably 0.5 to 150 μm, and particularly preferably 1 to 100 μm.
Further, the porosity of the current collector is preferably 20 to 80%, more preferably 30 to 70%.

[Separator]
As the separator in the lithium ion capacitor of the present invention, a material having an air permeability measured by a method according to JISP8117 in the range of 1 to 200 sec can be used. Specifically, for example, polyethylene, polypropylene, polyester In addition, a nonwoven fabric composed of cellulose, polyolefin, cellulose / rayon, or the like, a microporous membrane, or the like can be used, and cellulose / rayon is particularly preferable. The thickness of a separator is 5-100 micrometers, for example, and 10-50 micrometers is preferable.

[Electrolyte]
In the present invention, an electrolyte solution of a lithium salt in a specific aprotic organic solvent is used as the electrolyte solution of the lithium ion capacitor.
The aprotic organic solvent constituting the electrolytic solution in the present invention is a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and includes EC, EMC, and The ratio of the DMC (also referred to as “EMC / DMC”) to the total volume ratio is 1: 3 to 1: 1, and the volume ratio of EMC to DMC is 1: 1 to 9: 1. Some are preferred.

In the volume ratio of EC and EMC / DMC in the aprotic organic solvent, when the ratio of EC is smaller than 1: 3, the electric conductivity of the electrolytic solution is decreased, and the output characteristics are deteriorated. When the ratio of EC is larger than 1: 1, the viscosity of the electrolyte is increased, and the low temperature characteristics, particularly the temperature dependency of internal resistance is deteriorated.
Furthermore, in the volume ratio of EMC and DMC, when the ratio of EMC is smaller than 1: 1, it is not preferable because the stability of the electrolyte solution at a low temperature is lowered and it is likely to freeze, while the ratio of EMC is When the ratio is larger than 9: 1, the stability of the electrolytic solution at a low temperature is similarly lowered, freezing is likely to occur, and an increase in internal resistance is caused.

  An additive such as vinylene carbonate (VC), which is known as a SEI (Solid Electrolyte Interface) forming material generally used in lithium ion batteries, can be added to the electrolytic solution using an aprotic organic solvent.

[Non-prototronic organic solvent electrolyte in electrolyte]
As described above, in the present invention, the aprotic organic solvent constituting the electrolytic solution needs to have a volume ratio of 1: 3 to 1: 1 in the mixing ratio of EC and EMC / DMC. As cyclic carbonates other than EC, propylene carbonate (PC) and butylene carbonate (BC) are representative, but when the organic solvent of the electrolyte contains PC or BC, graphite or When a graphite-based material is used, gas generation cannot be suppressed during pre-charging and during a float test, so that there is a problem that practical reliability is lowered. Furthermore, since BC has a high viscosity, it is not possible to obtain an electrolytic solution having a high conductivity. According to an electrolytic solution using such an organic solvent, a lithium ion capacitor having excellent output characteristics and temperature characteristics can be obtained. I can't.

On the other hand, examples of the chain carbonate include diethyl carbonate (DEC), methylpropyl carbonate (MPC), and the like in addition to EMC and DMC. Of these, according to the electrolytic solution using an organic solvent containing DEC, Gas is generated at the time of precharging and at the time of the float test, so it is not appropriate. Also, the electrolyte containing MPC is not suitable because its conductivity is lowered and the internal resistance of the capacitor is increased.
For the above reasons, in the present invention, two types of chain carbonates, EMC and DMC, are used. By using a mixture of these two types of chain carbonates, that is, EMC / DMC, and cyclic carbonate, good conductivity and stability over a wide temperature range can be obtained. Specifically, EC as cyclic carbonate can be obtained. Is preferably used. Therefore, in the present invention, a mixed solvent of three kinds of EC, EMC and DMC (EC / EMC / DMC) is used as the aprotic organic solvent constituting the electrolytic solution.

[Lithium salt]
Examples of the lithium salt of the electrolyte in the electrolytic solution include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) _2, etc. LiPF ₆ is preferably used because of its high ion conductivity and low resistance. The concentration of the lithium salt in the electrolytic solution is preferably 0.1 mol / L or more, more preferably 0.5 to 1.5 mol / L, because low internal resistance is obtained.

[Positive electrode active material]
As the positive electrode active material, a material capable of reversibly doping and dedoping at least one kind of anion such as lithium ion and tetrafluoroborate is used, and examples thereof include activated carbon powder. As for the particle size of the active powder, the value of the number average particle diameter D50 is preferably 2 μm or more, more preferably 2 to 50 μm, and particularly preferably 2 to 20 μm. Further, those having an average pore diameter of 10 nm or less are preferable, and the specific surface area is preferably 600 to 3000 m ² / g, more preferably 1300 to 2500 m ² / g.

[Negative electrode active material]
In the present invention, graphite-based composite particles are used as the negative electrode active material among materials that can be reversibly doped and dedoped with lithium ions. The graphite composite particles are the following (1) or (2).
(1) A carbon substance obtained by a method in which the surface of graphite (graphite) is coated with tar, pitch, etc., and heat treatment is performed to composite the surface tar and pitch.
(2) A carbon electrode material obtained by mixing natural graphite or artificial graphite, a low crystalline carbon powder and a binder, calcining at 800 ° C. or lower, pulverizing, and refiring at 900-1500 ° C.
Examples of the low crystalline carbon powder in the above (2) include mesophase pitch, raw coke, and calcine coke. Examples of the binder include binder pitch and phenol resin.

  In such graphite-based composite particles, the presence or absence of coating with a graphitized substance derived from tar or pitch on the particle surface can be confirmed by measurement of Raman spectrum, XRD or the like. And, in the lithium ion capacitor of the present invention configured by using the above-mentioned graphite-based composite particles as a negative electrode active material and combining with the electrolytic solution having the above specific composition, a large amount of gas is used at the time of preliminary charging and at the time of float test. Can be suppressed.

As the negative electrode active material, a powdery material is used as in the case of the positive electrode active material, and the particle size thereof has a number average particle diameter D50 of 0.1 to 5 μm. When the number average particle diameter D50 is less than 0.1 μm, it is difficult to produce the lithium ion capacitor. The negative electrode active material preferably has a specific surface area of 0.1 to 200 m ² / g, more preferably 0.5 to 50 m ² / g.

[Binder]
Production of the positive electrode by the positive electrode active material and the negative electrode by the negative electrode active material can be performed by a commonly used known method.
For example, the negative electrode is prepared by adding a negative electrode active material powder, a binder, and, if necessary, a conductive material, a thickener such as carboxymethyl cellulose (CMC) to water or an organic solvent, and mixing the resulting slurry. It can be produced by applying to a current collector or by pasting the slurry formed into a sheet shape to the current collector.

In the production of the negative electrode, as the binder, 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.
Examples of the conductive material include acetylene black, graphite, and metal powder.
The amount of each of the binder and the conductive material added varies depending on the electrical conductivity of the active material to be used, the shape of the electrode to be produced, and the like, but it is usually preferable that both are 2 to 40% by mass with respect to the active material. .

[Active material layer]
The positive electrode active material layer or the negative electrode active material layer is formed by attaching the positive electrode active material or the negative electrode active material to the current collector by coating or adhesion. The film thickness of each active material layer should just be 5-400 micrometers, 10-300 micrometers is preferable and 15-200 micrometers is more preferable.

[Structure of lithium ion capacitor]
As the structure of the lithium ion capacitor according to the present invention, in particular, a wound type cell in which a strip-like positive electrode and a negative electrode are wound via a separator, and a plate-like or sheet-like positive electrode and negative electrode are each provided via a separator. Examples include a laminated cell in which more than one layer is laminated, a film type cell in which units having such a laminated structure are enclosed in an exterior film, and the like.
The structure of these capacitor cells is known from Japanese Patent Application Laid-Open No. 2004-266091 and the like, and can have the same configuration as those capacitor cells.

  EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example, this invention is not restrict | limited by these.

[Example 1: Cell S1]
(1) Production of positive electrode sheet 1-1. Preparation of Conductive Paint A conductive paint was prepared by adding ion-exchanged water to 95 parts by mass of carbon powder (average particle size 4.5 μm) and 5 parts by mass of carboxymethyl cellulose and mixing them. This conductive paint is referred to as “conductive paint (1)”.

1-2. Preparation of positive electrode slurry 87 parts by mass of phenol-based activated carbon having a specific surface area of 2030 m ² / g and a number average particle diameter D50 of 4 μm, 4 parts by mass of acetylene black powder, 6 parts by mass of an SBR binder (“TRD2001” manufactured by JSR) And ion-exchange water was added to and mixed with 3 parts by mass of carboxymethyl cellulose to prepare a positive electrode slurry having a solid concentration of 35%. This is designated as “positive electrode slurry (1)”.

1-3. Formation of electrode layer Aperture ratio having a configuration in which a plurality of circular through-holes each having an opening area of 0.79 mm ² are arranged in a staggered manner on a band-shaped aluminum foil having a width of 200 mm and a thickness of 15 μm by a punching method A 42% positive electrode current collector was produced.
A portion of this positive electrode current collector is coated with the conductive paint (1) using a vertical die type double-sided coating machine under a coating condition of a coating width of 130 mm and a coating speed of 8 m / min. A double-sided coating was carried out with the total target value of the coating thickness on each of the front and back sides of the body being 20 μm, and then dried under reduced pressure at 200 ° C. for 24 hours, thereby forming conductive layers on both sides of the positive electrode current collector.
Thereafter, on the conductive layers formed on both surfaces of the positive electrode current collector, the positive electrode slurry (1) is applied to the positive electrode using a vertical die type double-side coating machine under a coating condition of a coating speed of 8 m / min. It is formed on the front and back surfaces of the positive electrode current collector by performing double-sided coating with a total target thickness of 150 μm on each of the front and back sides of the current collector, followed by drying at 200 ° C. for 24 hours under reduced pressure. An electrode layer was formed on each conductive layer.

In the above, in the application of the positive electrode slurry, the positive electrode slurry is applied to both surfaces of the positive electrode current collector by passing the positive electrode current collector having a conductive layer formed between two slit dies. By adjusting the gap between the slit die and the positive electrode current collector, the coating thickness on the front surface side and back surface side of the positive electrode current collector was adjusted.
And, the positive electrode current collector in which the conductive layer and the electrode layer are laminated on each of the front surface and the back surface, the plane size of the portion where the conductive layer and the electrode layer are formed is 98 × 128 mm, and no layer is formed. The positive electrode sheet was manufactured by cutting into a planar size of 98 × 143 mm so that the planar size of the portion was 98 × 15 mm. This is designated as “positive electrode sheet (1)”.
When the thickness of the obtained positive electrode sheet (1) was measured, it was 167 μm including the thickness of the current collector.

(2) Production of negative electrode sheet 2-1. Preparation of slurry for negative electrode 87 parts by mass of graphite composite particles having a number average particle diameter D50 of 4 μm, 4 parts by mass of acetylene black powder, 6 parts by mass of SBR binder (“TRD2001” manufactured by JSR) and 3 parts by mass of carboxymethyl cellulose A negative electrode slurry having a solid content concentration of 35% was prepared by adding ion-exchanged water to and mixing. This is designated as “negative electrode slurry (1)”.
The graphite-based composite particles described above are prepared by mixing 40 parts by mass of a precursor pitch with a kneader with respect to 100 parts by mass of fine graphite powder having a number average particle diameter D50 of 2.5 μm, and in a nitrogen atmosphere, It was obtained by heating at a rate of 5 ° C./min, firing by holding at 1000 ° C. for 6 hours, and pulverizing the obtained fired product until the value of the number average particle diameter D50 was 4 μm. Is.

2-2. Formation of electrode layer Aperture ratio having a configuration in which a plurality of circular through-holes each having an opening area of 0.79 mm ² are arranged in a staggered manner by a punching method on a strip-shaped copper foil having a width of 200 mm and a thickness of 25 μm A 42% negative electrode current collector was produced.
For a part of this negative electrode current collector, the negative electrode slurry (1) was applied to the surface of the negative electrode current collector and the surface of the negative electrode current collector using a vertical die type double-sided coating machine under coating conditions of a coating speed of 8 m / min. Double-side coating was performed with the total target value of the coating thickness on each of the back surfaces being set to 60 μm, and then dried under reduced pressure at 200 ° C. for 24 hours, thereby forming electrode layers on the front and back surfaces of the negative electrode current collector.

Then, in the negative electrode current collector in which the electrode layer is formed on each of the front surface and the back surface, the planar size of the portion where the electrode layer is formed is 100 × 130 mm, and the planar size of the portion where the electrode layer is not formed is 100 × The negative electrode sheet was manufactured by cutting into a plane size of 100 × 145 mm so as to be 15 mm. This is designated as “negative electrode sheet (1)”.
When the thickness of the electrode layer in the obtained negative electrode sheet (1) was measured, it was 86 μm including the thickness of the current collector. The mass (weight per unit area) of the negative electrode active material contained in the electrode layer was 5.7 mg / cm ² .

(3) Production of Separator A film made of a cellulose / rayon composite material having a thickness of 50 μm and an air permeability of 100 sec was cut into 102 mm × 130 mm to produce a separator.

(4) Production of Lithium Ion Capacitor Element First, 10 positive electrode sheets, 11 negative electrode sheets, and 22 separators were prepared, and the positive electrode sheet and the negative electrode sheet were overlapped with each other, but each uncoated The electrode stacking unit was manufactured by stacking the separator, the negative electrode sheet, the separator, and the positive electrode sheet in this order so that the parts would be on the opposite side and not overlapping, and fixing the four sides of the stack with tape.
Next, a lithium electrode having a thickness of 100 μm is cut into a foil shape and bonded to a copper lath having a thickness of 40 μm and a planar size of 100 × 145 mm, thereby producing a lithium ion supply member. Was arranged to face the negative electrode on the upper side.
And the sealant film was heat-sealed in advance to the uncoated part of each of the 10 positive electrode sheets of the produced electrode laminate unit, and was made of aluminum having a width of 50 mm, a length of 50 mm, and a thickness of 0.2 mm. The power tab for positive electrode was overlapped and welded. On the other hand, a sealant film was previously heat-sealed on the seal portion to each uncoated part of each of the eleven negative electrode sheets of the electrode laminate unit and the lithium ion supply member, width 50 mm, length 50 mm, thickness 0.2 mm The copper negative electrode power supply tabs were stacked and welded to prepare a lithium ion capacitor element.

(5) Production of Lithium Ion Capacitor A film made of a cellulose / rayon composite material having a thickness of 50 μm, an air permeability of 100 sec, and vertical and horizontal dimensions of 204 mm × 130 mm is wound so as to cover the peripheral surface of the lithium ion capacitor element. A polyimide tape having a width of 50 μm and a width of 19 mm was cut into a size of 30 mm × 19 mm, and two portions at both ends of the film were fixed to each other.
Next, one exterior film in which a polypropylene layer, an aluminum layer, and a nylon layer are laminated, the dimensions are 125 mm in length, 160 mm in width, 0.15 mm in thickness, and 105 mm in length and 147 mm in width are applied to the central portion. In addition, a polypropylene layer, an aluminum layer, and a nylon layer were laminated to produce the other exterior film having dimensions of 125 mm in length, 160 mm in width, and 0.15 mm in thickness.

Next, the lithium ion capacitor element in which the film is wound is placed at a position serving as a housing portion on the other exterior film, and each of the positive electrode terminal and the negative electrode terminal is outward from the end of the other exterior film. 3 which includes two sides on which the positive electrode terminal and the negative electrode terminal protrude at the outer peripheral edge of one outer film and the other outer film. The sides were heat-sealed.
On the other hand, using a mixed solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) at a volume ratio of 1: 1: 1, LiPF ₆ having a concentration of 1.2 mol / L is used. An electrolytic solution containing was prepared.
And after inject | pouring the said electrolyte solution between one exterior film and the other exterior film, the other one side in the outer periphery part of one exterior film and the other exterior film can be reheat-fused. A space where re-bonding was possible with the end of the lithium capacitor element was heat sealed.
In the manner as described above, a total of 10 test laminate outer lithium ion capacitors (hereinafter referred to as “cells”) were produced.

[Preliminary charging evaluation]
After the cells were produced as described above, they were left for 20 days, but no gas generation was observed in all 10 cells. When 10 cells were opened, it was confirmed that lithium metal was completely lost in any cell. From this, it was determined that the negative electrode active material was doped with lithium ions and precharged.
Further, the presence or absence of gas generation during the preliminary charging was examined. The evaluation criteria are as shown below.
○: No cell expansion was observed and no gas was generated.
Δ: Slight expansion of the cell due to gas generation was observed, but re-fusion was possible by degassing by opening.
X: Due to gas generation, the cell was greatly expanded and deformed, and re-fusion was impossible.

[Capacitance measurement]
Each of the 10 cells is re-fused, charged under a 20 A environment with a constant current of 10 A until the cell voltage reaches 3.8 V, and then a constant current of 3.8 V is applied. -Constant voltage charging was performed for 0.5 hour and then discharged at a constant current of 10A until the cell voltage was 2.2V. By repeating this cycle of 3.8V-2.2V, the electrostatic capacity in the 10th discharge was measured, and the average value of 10 cells was obtained. As a result, it was as high as 1066F.

[DC internal resistance measurement]
The above-described charging and discharging are performed for each of the 10 cells, and an AC internal resistance of 1 KHz in an environment at a temperature of 25 ° C. is measured using an “AC milliohm high tester 3560” manufactured by Hioki Electric Co., Ltd. When an average value was determined, it was 2.8 mΩ.

[Temperature characteristics evaluation by low temperature / room temperature internal resistance ratio]
Each of the produced 10 cells is charged and discharged as described above, and the average value of the DC internal resistance is obtained in the same manner as described above in an environment at a temperature of −20 ° C., and the ratio to the average value of the DC internal resistance at 20 ° C. When the ratio of low temperature / room temperature internal resistance was determined, it was 2.7 times on average.

[Float test]
Each of the 10 cells evaluated for temperature characteristics was charged with a constant current of 10 A until the voltage reached 3.8 V, and then a float test in which a constant voltage of 3.8 V was continuously applied for 500 hours in a 60 ° C. environment. When implemented and evaluated for the presence or absence of gas generation, no cell expansion was observed and it was confirmed that there was no gas generation. Evaluation criteria are shown below.
In addition, about the cell whose evaluation of precharge is "x", since it did not have the performance as a lithium ion capacitor, the float test was not performed.
(Double-circle): Cell expansion is not recognized and there is no gas generation.
○: Slight expansion of the cell was observed, but there was no practical problem.
Δ: Expansion due to gas generation was observed, but the deformation was not large.
X: Expansion due to gas generation is observed, and deformation is large.

  Production and evaluation of cells S2 to S6 according to the example and cells C1 to C10 according to the comparative example were performed as follows.

[Example 2: Cell S2]
10 cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 1.5: 0.5 was used, and the same evaluation was performed. did.

[Example 3: Cell S3]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 1.5: 1.5 was used, and the same evaluation was performed. did.

[Example 4: Cell S4]
Ten cells were prepared and evaluated in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 2.7: 0.3 was used. did.

[Example 5: Cell S5]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 0.5: 0.5 was used, and the same evaluation was performed. did.

[Example 6: Cell S6]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 0.9: 0.1 was used, and the same evaluation was performed. did.

[Comparative Example 1: Cell C1]
Ten cells were produced in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 2: 2 was used, and the same evaluation was performed.

[Comparative Example 2: Cell C2]
Ten cells were prepared and evaluated in the same manner as in Example 1 except that a mixed solvent in which EC, EMC, and DMC were mixed at a volume ratio of 1: 0.25: 0.25 was used. did.

[Comparative Example 3: Cell C3]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC and EMC were mixed at a volume ratio of 1: 4 was used, and the same evaluation was performed.

[Comparative Example 4: Cell C4]
Ten cells were produced in the same manner as in Example 1 except that a mixed solvent in which EC and EMC were mixed at a volume ratio of 1: 0.5 was used, and the same evaluation was performed.

[Comparative Example 5: Cell C5]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which propylene carbonate (PC), EMC, and DMC were mixed at a volume ratio of 1: 0.5: 0.5 was used. Evaluation was conducted.

[Comparative Example 6: Cell C6]
Ten cells were prepared in the same manner as in Example 1 except that a mixed solvent in which EC and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 1 was used, and the same evaluation was performed.

[Comparative Example 7: Cell C7]
Ten cells were prepared in the same manner as in Example 1 except that non-graphitizable carbon A having a specific surface area of 16 m ² / g and an average particle diameter D50 of 4 μm was used as the negative electrode active material. .

[Comparative Example 8: Cell C8]
As the negative electrode active material, non-graphitizable carbon A having a specific surface area of 16 m ² / g and an average particle diameter D50 of 4 μm is used, and EC, PC, and DEC are used in a volume ratio of 3: 1: 4 as a solvent for the electrolytic solution. Ten cells were prepared in the same manner as in Example 1 except that the mixed solvent was mixed, and the same evaluation was performed.

[Comparative Example 9: Cell C9]
Ten cells were prepared in the same manner as in Example 1 except that fine graphite powder B having a number average particle diameter D50 of 2.5 μm was used as the negative electrode active material, and the same evaluation was performed.

[Comparative Example 10: Cell C10]
Mixing in which particulate graphite powder B having a number average particle diameter D50 of 2.5 μm is used as the negative electrode active material, and EC, PC, and DEC are mixed at a volume ratio of 3: 1: 4 as a solvent for the electrolytic solution. Ten cells were prepared in the same manner as in Example 1 except that the solvent was used, and the same evaluation was performed.

  The results are shown in Table 1.

Claims (3)

It includes a positive electrode, a negative electrode a value of number average particle diameter D50 is formed by the negative electrode active material made of graphite-based composite particles of 0.1 to 5 [mu] m, and an electrolyte consisting of a solution of a lithium salt with a non-protonic organic solvents A lithium ion capacitor in which the negative electrode is doped with lithium ions by electrochemical contact between the negative electrode and a lithium ion supply source ,
The aprotic organic solvent of the electrolytic solution is a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the volume ratio of ethylene carbonate to the total of ethyl methyl carbonate and dimethyl carbonate is 1: 3. A lithium ion capacitor characterized by being 1: 1.   2. The lithium ion capacitor according to claim 1, wherein a volume ratio of ethyl methyl carbonate to dimethyl carbonate is 1: 1 to 9: 1 in the aprotic organic solvent of the electrolytic solution. The graphite-based composite particles of the negative electrode active material are obtained by coating graphite powder with tar or pitch material, and the ratio of tar or pitch material to 100 parts by weight of graphite powder is 10 to 40 parts by weight. The lithium ion capacitor according to claim 1 or 2.