WO2015093289A1

WO2015093289A1 - Lithium ion capacitor

Info

Publication number: WO2015093289A1
Application number: PCT/JP2014/081932
Authority: WO
Inventors: 奥野　一樹; 真嶋　正利
Original assignee: 住友電気工業株式会社
Priority date: 2013-12-17
Filing date: 2014-12-03
Publication date: 2015-06-25
Also published as: JP2015135947A

Abstract

A lithium ion capacitor which comprises a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator arranged between the positive electrode and the negative electrode, and an electrolyte. The positive electrode active material contains at least a material that reversibly supports anions. The negative electrode active material contains a carbonaceous material having a graphite-type crystal structure. The ratio of the capacitance (C_n) per unit projected area of the negative electrode to the capacitance (C_p) per unit projected area of the positive electrode, namely C_n/C_p is 20 or more.

Description

Lithium ion capacitor

The present invention relates to a lithium ion capacitor using a negative electrode active material containing a carbonaceous material having a graphite type crystal structure.

中 Amid the close-up of environmental issues, systems that convert clean energy such as solar or wind power into electric power and store it as electric energy are being actively developed. As such an electricity storage device, a lithium ion secondary battery, an electric double layer capacitor, a lithium ion capacitor, and the like are known. Recently, capacitors such as an electric double layer capacitor and a lithium ion capacitor have been attracting attention from the viewpoints of being excellent in instantaneous charge / discharge characteristics, high output characteristics, and excellent handleability.

The problem with capacitors is that they have smaller capacities than lithium ion secondary batteries, among others, but lithium ion capacitors have the advantages of both lithium ion secondary batteries and electric double layer capacitors, and provide relatively large capacities. Therefore, it is expected to be used for various purposes. A lithium ion capacitor generally includes a positive electrode including activated carbon as a positive electrode active material, a negative electrode including a carbonaceous material that absorbs and releases lithium ions as a negative electrode active material, and an electrolyte. For example, in Patent Document 1, graphite is used as the negative electrode active material, and in Patent Document 2, a polyacene-based material having an amorphous structure is used as the negative electrode active material. In a lithium ion capacitor, in order to increase the capacity of the capacitor, lithium ions are previously supported (or pre-doped) on the negative electrode (see Patent Document 1).

JP 2007-294539 A International Publication WO2003 / 003395

In a lithium ion capacitor, if lithium ions are previously supported on the negative electrode, the potential of the negative electrode can be lowered, so that the capacitance of the negative electrode can be increased and the capacity of the capacitor can be increased.

Unlike an electric double layer capacitor, a lithium ion capacitor uses a material that occludes and releases (or inserts and desorbs) lithium ions as a negative electrode active material.
When the capacitor is repeatedly charged and discharged, the negative electrode active material is deteriorated by repeating the volume change of the negative electrode active material with insertion and extraction of lithium ions. Accordingly, a high capacity cannot be maintained, and the cycle characteristics are deteriorated.
Then, it aims at providing the lithium ion capacitor which is excellent in cycling characteristics.

One aspect of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.
The positive electrode active material includes at least a material that reversibly supports anions,
The negative electrode active material includes a carbonaceous material having a graphite-type crystal structure,
The ratio of the capacitance C _n per projected unit area of the negative electrode to the capacitance C _p per projected unit area of the positive electrode relates to a lithium ion capacitor in which C _n / C _p is 20 or more.

According to the above configuration, even when charging / discharging is repeated, deterioration of the negative electrode active material due to insertion and extraction of lithium ions can be suppressed, and thereby cycle characteristics can be significantly improved.

1 is a longitudinal sectional view schematically showing a lithium ion capacitor according to an embodiment of the present invention.

[Description of Embodiment of the Invention]
First, the contents of the embodiment of the present invention will be listed and described.
One embodiment of the present invention includes (1) a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.
The positive electrode active material includes at least a material that reversibly supports anions,
The negative electrode active material includes a carbonaceous material having a graphite-type crystal structure,
The ratio of the capacitance C _n per projected unit area of the negative electrode to the capacitance C _p per projected unit area of the positive electrode relates to a lithium ion capacitor in which C _n / C _p is 20 or more.

In a lithium ion capacitor, a material that occludes and releases (or inserts and desorbs) lithium ions is used as a negative electrode active material. When a carbonaceous material having a graphite type crystal structure (first carbonaceous material) is used as the negative electrode active material, lithium ions are occluded and released (or inserted and desorbed) between layers of the layered graphite type crystal structure. The For example, when an amorphous material is used as the negative electrode active material, lithium ions are occluded and released at the grain boundary. When charging / discharging of the lithium ion capacitor is repeated, the negative electrode active material repeats expansion and contraction associated with insertion and extraction of lithium ions, and deteriorates.

When lithium ion is previously supported on the negative electrode active material, the potential of the negative electrode can be lowered, so that the capacitance of the negative electrode can be increased and the capacity of the lithium ion capacitor can be improved. However, when the negative electrode active material containing the first carbonaceous material is used, the ratio of the capacitance C _n per unit projected area of the negative electrode to the capacitance C _p per unit projected area of the positive electrode: C _n / C _p Depending on the value, the deterioration of the negative electrode active material during charge / discharge becomes significant, and the cycle characteristics may deteriorate.

In the embodiment of the present invention, by using a negative electrode active material containing a first carbonaceous material and setting the C _n / C _p ratio to 20 or more, the potential of the negative electrode during charge / discharge can be stabilized. Therefore, when charging / discharging is repeated, deterioration of the negative electrode active material is suppressed, and a decrease in capacity is suppressed. Therefore, a lithium ion capacitor having excellent cycle characteristics can be provided.

Although details are not certain, when the first carbonaceous material is used as the negative electrode active material, for example, the cycle characteristics greatly change depending on the value of the C _n / C _p ratio as compared with the case where an amorphous material is used. This is thought to be due to the difference in the occlusion and release mechanisms of lithium ions. In the first carbonaceous material, lithium ions are occluded and desorbed between layers of the graphite-type crystal structure. Occlusion of lithium ions into the layered structure during charging proceeds in stages from stage 4 to stage 1. If lithium ions are occluded in the first carbonaceous material in a range across a plurality of stages, it is considered that the deterioration of the first carbonaceous material increases. However, in the amorphous material, lithium ions are occluded in the grain boundary, and thus it is considered that the behavior of the cycle characteristics is different from that of the first carbonaceous material. Specifically, the influence of the depth of charge / discharge on the cycle characteristics as seen in the first carbonaceous material is small in the amorphous material.

The positive electrode and the negative electrode usually have a sheet form. The projected area of the positive electrode or the negative electrode is an area of a shadow formed when the positive electrode or the negative electrode is projected in a direction perpendicular to the surface direction. For example, in a rectangular sheet-like positive electrode (or negative electrode), the area obtained by multiplying the projected area of the positive electrode (or negative electrode) by the vertical size (cm) and the horizontal size (cm) of the positive electrode (or negative electrode) Used in the same meaning as However, when the active material is supported only on a part of the positive electrode or the negative electrode, the projected area of the region where the active material is supported can be the projected area of the positive electrode or the negative electrode.

(2) The C _n / C _p is preferably 20 to 120. When the C _n / C _p ratio is in such a range, in addition to suppressing the deterioration of cycle characteristics, it is easier to suppress the deposition of metallic lithium.

(3) The C _p is 0.1 to 15 F / cm ² , and the mass of the negative electrode active material per projected unit area of the negative electrode is greater than the mass of the positive electrode active material per projected unit area of the positive electrode It is also preferable that there are many. In this case, the effect of suppressing deterioration of cycle characteristics is further increased. Moreover, it is easy to control the C _n / C _p ratio within an appropriate range, and the effect of improving the cycle characteristics can be further enhanced.

(4) the carbonaceous material, X-rays diffraction: Mean spacing d ₀₀₂ of (XRD X-ray diffraction) is measured in the spectrum (002) plane is preferably less than 0.337 nm. Since such a carbonaceous material (first carbonaceous material) has a developed graphite-type crystal structure, the carbonaceous material is excellent in reversible supportability of lithium ions and is influenced by cycle deterioration depending on the size of C _n. easy.

(5) The negative electrode includes a negative electrode current collector and a negative electrode mixture supported on the negative electrode current collector and including the negative electrode active material, and the negative electrode current collector is made of a three-dimensional network metal. It is preferable to have a skeleton of When such a current collector is used, the capacitance C _n of the negative electrode can be easily increased, so that the C _n / C _p ratio can be increased more easily.

[Details of the embodiment of the invention]
Specific examples of the lithium ion capacitor according to the embodiment of the present invention will be described below with reference to the drawings as appropriate. In addition, this invention is not limited to these illustrations, is shown by the attached claim, and is intended that all the changes within the meaning and range equivalent to the claim are included. .

(Lithium ion capacitor)
The lithium ion capacitor includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.
Hereinafter, the components of the lithium ion capacitor will be described in more detail.
(Negative electrode)
The negative electrode includes a negative electrode active material, and the negative electrode active material includes a first carbonaceous material.
A 1st carbonaceous material raise | generates a Faraday reaction in the case of charging / discharging. Such a first carbonaceous material can occlude and release (or insert and desorb) lithium ions serving as charge carriers for charge and discharge reactions.

The graphite-type crystal structure contained in the first carbonaceous material means a layered crystal structure. Examples of the graphite-type crystal structure include a cubic crystal structure and a rhombohedral crystal structure. Examples of the first carbonaceous material include natural graphite (eg, scaly graphite), artificial graphite, and graphitized mesocarbon microspheres. The first carbonaceous material includes a carbonaceous material having a graphite-type crystal structure, such as pitch-coated graphite, which has been subjected to coating treatment. A 1st carbonaceous material can be used individually by 1 type or in combination of 2 or more types. In a lithium ion capacitor, during charging, lithium ions are inserted between the layers of the graphite-type crystal structure of the first carbonaceous material, and during discharge, lithium ions are released from the layers of the graphite-type crystal structure.

As one index of the degree of development of the graphite-type crystal structure in the first carbonaceous material, an average interplanar spacing _d002 of the (002) plane measured by the XRD spectrum of the first carbonaceous material is used. The first carbonaceous material has an average spacing d ₀₀₂ is preferably less than 0.337 nm. The lower limit of the average spacing d ₀₀₂ is not particularly limited, the average spacing d _002, for example, may be equal to or larger than 0.335 nm. By using the first carbonaceous material having an average interplanar spacing _d002 in such a range, lithium ions can be more efficiently inserted into the graphite-type crystal structure during charging, and the graphite-type crystal structure during discharge. Lithium ions can be released smoothly from Further, the first carbonaceous material having such d ₀₀₂ is easily affected by cycle deterioration depending on the size of C _n .

The negative electrode active material can contain an active material other than the first carbonaceous material. From the viewpoint of efficiently inserting and extracting lithium ions, the content of the first carbonaceous material in the negative electrode active material is preferably 80% by mass or more (specifically, 80 to 100% by mass), More preferably, it is 90% by mass or more (specifically, 90 to 100% by mass). The negative electrode active material may be composed of only the first carbonaceous material.

The negative electrode is not particularly limited as long as it includes the negative electrode active material as described above, and may include a negative electrode mixture containing a negative electrode active material and, as an optional component, a binder and / or a conductive additive. The negative electrode can further include a negative electrode current collector. In such a negative electrode, the negative electrode current collector carries a negative electrode active material or a negative electrode mixture.

The type of the conductive auxiliary agent is not particularly limited, and examples thereof include carbon black such as acetylene black and ketjen black; conductive compound such as ruthenium oxide; and conductive fiber such as carbon fiber and metal fiber. . A conductive support agent can be used individually by 1 type or in combination of 2 or more types. From the viewpoint of ensuring high conductivity and high capacity, the amount of the conductive auxiliary is, for example, 1 to 20 parts by mass, preferably 5 to 15 parts by mass with respect to 100 parts by mass of the negative electrode active material.

The type of the binder is not particularly limited. For example, polyvinylidene fluoride (PVDF) and fluorine resin such as polytetrafluoroethylene; chlorine-containing vinyl resin such as polyvinyl chloride; polyolefin resin; rubber-like material such as styrene butadiene rubber Polymers; polyvinyl pyrrolidone; polyvinyl alcohol; and cellulose derivatives [for example, cellulose ether (carboxyalkyl cellulose such as carboxymethyl cellulose and its sodium salt and salts thereof (such as alkali metal salt and ammonium salt)]] and the like can be used. . A binder can be used individually by 1 type or in combination of 2 or more types. The amount of the binder is not particularly limited, but can be selected from the range of, for example, 0.1 to 15 parts by mass per 100 parts by mass of the negative electrode active material from the viewpoint of easily ensuring high binding properties and high capacity, and preferably 0. .5 to 10 parts by mass.

The material of the negative electrode current collector is preferably copper, copper alloy, nickel, nickel alloy, and / or stainless steel. The negative electrode current collector may be a metal foil or a metal porous body (metal fiber nonwoven fabric, metal porous sheet, etc.). The thickness of the metal foil is, for example, 10 to 50 μm. The thickness of the metal porous body is, for example, 100 to 2000 μm, preferably 700 to 1500 μm.

As the metal porous body, one having a three-dimensional network metal skeleton (in particular, a hollow skeleton) can be used. A porous metal body having a three-dimensional network skeleton constitutes a current collector by, for example, plating a resin porous body (resin foam and / or resin nonwoven fabric) having continuous voids. It may be formed by covering with a metal (specifically, the above-exemplified material). A porous metal body having a hollow skeleton can be formed by removing the resin in the skeleton by heat treatment or the like.

When a metal porous body having a three-dimensional network skeleton is used, a large amount of negative electrode active material or negative electrode mixture can be filled in the voids of the metal porous body, so that the capacitance C _n of the negative electrode is increased. Can do. Therefore, when such a metal porous body is used as the negative electrode current collector, C _n / C _p can be easily adjusted to a large value.

The porosity (or porosity) of the metal porous body having a three-dimensional network skeleton is, for example, 30 to 99% by volume, preferably 50 to 98% by volume, more preferably 80 to 98% by volume, and particularly preferably 90%. ~ 98% by volume. The specific surface area of the porous metal body having a three-dimensional reticulated skeleton (BET specific surface area), for example, 100 ~ 700cm ² / g, is preferably 150 ~ 650cm ² / g, more preferably 200 ~ 600cm ² / g . When the porosity and / or specific surface area of the metal porous body is in such a range, it is easy to carry a sufficient amount of the negative electrode active material and to ensure a high capacity.

The negative electrode can be formed by supporting at least a negative electrode active material on a negative electrode current collector. More specifically, it is obtained by applying or filling a negative electrode mixture containing at least a negative electrode active material on a negative electrode current collector, drying, and compressing (or rolling) the dried product as necessary.

The negative electrode mixture is usually used in the form of a slurry containing the components of the negative electrode mixture (negative electrode active material, conductive additive, binder, etc.). The negative electrode mixture slurry is obtained by dispersing the components of the negative electrode mixture in a dispersion medium. As the dispersion medium, for example, an organic solvent such as N-methyl-2-pyrrolidone (NMP) and / or water is used. The dispersion medium is removed by drying during the production process of the negative electrode (after filling the current collector with the slurry and / or after rolling).

In the negative electrode used for the lithium ion capacitor, it is desirable that lithium ion is supported (pre-doped) on the negative electrode active material. By pre-doping lithium ions into the negative electrode active material, the potential of the negative electrode can be sufficiently lowered, and the capacitance C _n of the negative electrode can be increased. The pre-doping of lithium ions can be performed by a known method. The lithium ion pre-doping may be performed before the lithium ion capacitor is assembled, or may be performed in the lithium ion capacitor.

The thickness of the negative electrode can be appropriately selected from the range of 50 to 2000 μm, for example. When a metal foil is used as the negative electrode current collector, the thickness of the negative electrode is, for example, 50 to 500 μm, preferably 50 to 300 μm. When a porous metal body having a three-dimensional network skeleton is used as the negative electrode current collector, the thickness of the negative electrode is, for example, 150 to 2000 μm, preferably 200 to 1500 μm.

The mass M _n of the negative electrode active material per projected unit area of the negative electrode is, for example, 3 to 100 mg / cm ² , preferably 4 to 80 mg / cm ² , more preferably 5 to 70 mg / cm ² . When a three-dimensional mesh-like metal porous body is used as a current collector, the mass M _n is, for example, 20 to 100 mg / cm ² , preferably 25 to 80 mg / cm ² , more preferably 25 to 70 mg / cm ² . .
When the mass M _n is in such a range, the negative electrode capacitance C _n can be easily increased, so that the C _n / C _p ratio can be adjusted more easily.

The capacitance C _n per unit projected area of the negative electrode, for example, 10 ~ 300F / cm ^2, preferably 10 ~ 250F / cm ^2, more preferably 10 ~ 150F / cm ^2. When a three-dimensional network-like porous metal is used as a current collector, the capacitance C _n is preferably 30 to 300 F / cm ² , more preferably 50 to 250 F / cm ² , and still more preferably 70 to 230 F / cm. ² .

When C _n is in the above range, the C _n / C _p ratio can be easily adjusted to a large value, and the effect of suppressing deterioration of cycle characteristics can be further enhanced. C _n can be adjusted by adjusting the mass M _{n of} the negative electrode active material, the thickness of the negative electrode, and / or the amount of lithium supported (pre-doping amount).
Capacitance C _n is obtained by determining the discharge capacity when the potential of the negative electrode changes by a predetermined value when the cell is assembled with metal lithium as a counter electrode and the cell is discharged at a constant current, and the discharge capacity at this time is determined as the potential of the negative electrode. It is calculated by dividing by the amount of change and the projected area of the negative electrode.

(Positive electrode)
A positive electrode contains a positive electrode active material, and a positive electrode active material contains the material which carry | supports an anion reversibly at least. The positive electrode can include a positive electrode active material and a positive electrode current collector carrying the positive electrode active material. The positive electrode may include a positive electrode mixture containing a positive electrode active material and a positive electrode current collector carrying the positive electrode mixture.

The material of the positive electrode current collector is preferably aluminum and / or an aluminum alloy (such as an aluminum-iron alloy and / or an aluminum-copper alloy).
The positive electrode current collector may be either a metal foil or a metal porous body. Examples of the metal porous body include a porous metal foil and a metal porous body having a three-dimensional network skeleton similar to those described for the negative electrode current collector.

The positive electrode active material includes a material that reversibly carries at least an anion. The positive electrode active material is preferably a material that reversibly supports anions and cations. The material that reversibly carries at least an anion includes a material that adsorbs and desorbs at least an anion, and a material that absorbs and desorbs (or inserts and desorbs) an anion. The former is a material that causes a non-Faraday reaction during charging and discharging, and the latter is a material that causes a Faraday reaction during charging and discharging. Of these, materials that adsorb and desorb at least anions (preferably anions and cations) can be preferably used.
In addition, an anion and a cation are an anion and a cation contained in the electrolyte of a lithium ion capacitor.

As the material for reversibly supporting at least anions, for example, porous carbon materials (also referred to as second carbonaceous materials) such as activated carbon, nanoporous carbon, mesoporous carbon, microporous carbon, and carbon nanotube are preferably used. The porous carbon material may be activated or may not be activated. These porous carbon materials can be used individually by 1 type or in combination of 2 or more types. Of the porous carbon materials, activated carbon and / or nanoporous carbon are preferable. Note that porous carbon having fine pores on the order of sub nm to sub μm is referred to as nanoporous carbon.

The positive electrode active material may include an active material other than the second carbonaceous material. The content of the second carbonaceous material in the positive electrode active material is preferably more than 50% by mass, and may be 80% by mass or more or 90% by mass or more. Content of the 2nd carbonaceous material in a positive electrode active material is 100 mass% or less. In particular, the content of activated carbon and nanoporous carbon in the positive electrode active material is preferably within such a range. It is also preferable that the positive electrode active material contains only the second carbonaceous material (particularly activated carbon and / or nanoporous carbon).

As the nanoporous carbon, known ones used for lithium ion capacitors can be used, for example, those obtained by heating metal carbide such as silicon carbide and / or titanium carbide in an atmosphere containing chlorine gas. Can be mentioned. By controlling the heating temperature and the heating time, the pore diameter, the pore depth, and / or the proportion of the pores can be adjusted. The heating temperature can be selected from the range of 1000 to 2000 ° C., for example, and is preferably 1000 to 1500 ° C.

As the activated carbon, known ones used for lithium ion capacitors can be used. Examples of activated carbon materials include wood; coconut shells; pulp waste liquid; coal or coal-based pitch obtained by thermal decomposition thereof; heavy oil or petroleum-based pitch obtained by thermal decomposition thereof; and / or phenol resin. It is done. The carbonized material is generally then activated.

The average particle diameter of the activated carbon is not particularly limited, but is preferably 20 μm or less, more preferably 3 to 15 μm. The specific surface area (BET specific surface area) of the activated carbon is not particularly limited, but is preferably 800 to 3000 m ² / g, more preferably 1500 to 3000 m ² / g. When the specific surface area is in such a range, it is advantageous for increasing the capacitance of the lithium ion capacitor, and the internal resistance can be reduced.
In this specification, the average particle diameter means a volume-based median diameter in a particle size distribution obtained by laser diffraction particle size distribution measurement.

The positive electrode mixture includes a positive electrode active material as an essential component, and may include a conductive additive and / or a binder as an optional component.
In accordance with the case of the negative electrode, for example, the positive electrode is coated or filled with a positive electrode mixture (specifically, positive electrode mixture slurry) containing at least a positive electrode active material on the positive electrode current collector, dried, and if necessary. It is obtained by compressing (or rolling) the dried product.

The dispersion medium and binder contained in the positive electrode mixture (or positive electrode mixture slurry) can be appropriately selected from those exemplified for the negative electrode. The amount of the binder with respect to 100 parts by mass of the positive electrode active material can be appropriately selected from the range of the amount of the binder with respect to 100 parts by mass of the negative electrode active material.

Examples of the conductive auxiliary agent include graphite (natural graphite such as scale-like graphite and earth-like graphite; and / or artificial graphite) in addition to those exemplified for the negative electrode. The amount of the conductive additive relative to 100 parts by mass of the positive electrode active material can be appropriately selected from the range of the amount of the conductive auxiliary relative to 100 parts by mass of the negative electrode active material described above.

The thickness of the positive electrode can be appropriately selected from the range of 50 to 2000 μm, for example. When a metal foil is used as the positive electrode current collector, the thickness of the positive electrode is, for example, 50 to 500 μm, more preferably 50 to 300 μm. When a porous metal body having a three-dimensional network skeleton is used as the positive electrode current collector, the thickness of the positive electrode is, for example, 300 to 2000 μm, preferably 500 to 1700 μm.

The mass M _p of the positive electrode active material per projected unit area of the positive electrode is, for example, 1 to 100 mg / cm ² , preferably 2 to 90 mg / cm ² , and more preferably 3 to 80 mg / cm ² . When the mass M _p is in such a range, the C _n / C _p ratio can be adjusted more easily.

If the C _n / C _p ratio is 20 or more, the mass M _{n of the} negative electrode active material is not particularly limited and may be the same as or smaller than the mass M _{p of the} positive electrode active material, but larger than the mass M _p. Is preferred. When the mass M _n is larger than the mass M _p , the C _n / C _p ratio can be easily controlled within an appropriate range, and as a result, the effect of improving the cycle characteristics can be further enhanced. The M _n / M _p ratio is preferably greater than 1, more preferably 1 <M _n / M _p ≦ 2, and even more preferably 1 <M _n / M _p ≦ 1.8.

The capacitance C _p per projected unit area of the positive electrode is, for example, 0.1 to 15 F / cm ² , preferably 0.1 to 10 F / cm ² , more preferably 0.2 to 9 F / cm ² . When C _p is in such a range, the C _n / C _p ratio can be easily adjusted to a large value, and the effect of suppressing deterioration of cycle characteristics can be further enhanced. C _p can be adjusted by adjusting the type of the positive electrode active material, the mass M _{p of the} positive electrode active material, and / or the thickness of the positive electrode. C _p can be calculated according to the case of C _n .

The C _n / C _p ratio is 20 or more, but when the C _n / C _p ratio is less than 20, it is difficult to stabilize the potential of the negative electrode during charge / discharge. Therefore, as charging and discharging are repeated, the deterioration of the negative electrode active material becomes significant, and the cycle characteristics deteriorate. In order to further improve the cycle characteristics, the C _n / C _p ratio is more preferably 30 or more, and further preferably 50 or more.
From the viewpoint of suppressing the deposition of metallic lithium in the negative electrode and suppressing the decrease in capacity due to the deposition, the C _n / C _p ratio is preferably 120 or less, and more preferably 100 or less, for example.
These lower limit values and upper limit values can be arbitrarily combined. For example, a preferable range of C _n / C _p ratio can be 20 to 120, or 30 to 120.

(Separator)
The separator has ion permeability, is interposed between the positive electrode and the negative electrode, and physically separates them to prevent a short circuit. The separator has a porous structure and allows ions to pass through by holding an electrolyte in the pores. Examples of the material of the separator include polyolefin such as polyethylene and polypropylene; polyester such as polyethylene terephthalate; polyamide; polyimide; cellulose; and / or glass fiber.

The average pore diameter of the separator is not particularly limited and is, for example, about 0.01 to 5 μm.
The thickness of the separator is not particularly limited, and is about 10 to 100 μm, for example.
The porosity of the separator is not particularly limited and is, for example, 40 to 80% by volume, preferably 50 to 70% by volume.

(Electrolytes)
The electrolyte includes cations and anions. The electrolyte is preferably a non-aqueous electrolyte having lithium ion conductivity. Such a non-aqueous electrolyte contains at least a cation containing lithium ions and an anion. Examples of the non-aqueous electrolyte include an electrolyte (organic electrolyte) obtained by dissolving a salt (lithium salt) of lithium ions and anions in a non-aqueous solvent (or organic solvent), and at least a cation and an anion containing lithium ions. An ionic liquid or the like is used.

The organic electrolyte can contain an ionic liquid and / or an additive in addition to the non-aqueous solvent (organic solvent) and the lithium salt. The total content of the non-aqueous solvent and the lithium salt in the electrolyte is, for example, It is 60% by mass or more, preferably 75% by mass or more, and more preferably 85% by mass or more. The total content of the nonaqueous solvent and the lithium salt in the electrolyte is, for example, 100% by mass or less, and preferably 95% by mass or less. These lower limit values and upper limit values can be arbitrarily combined. The total content of the nonaqueous solvent and the lithium salt in the electrolyte may be, for example, 60 to 100% by mass, or 75 to 95% by mass.

In the present specification, the term “ionic liquid” is synonymous with a molten salt (molten salt), and means a liquid ionic substance (liquid having ion conductivity) composed of an anion and a cation. use.
When an ionic liquid is used for the electrolyte, the electrolyte can contain a nonaqueous solvent and / or an additive in addition to the ionic liquid containing a cation and an anion containing lithium ions, but the content of the ionic liquid in the electrolyte Is preferably 60% by mass or more, and more preferably 70% by mass or more. The content of the ionic liquid in the electrolyte may be 80% by mass or more, or 90% by mass or more. The content of the ionic liquid in the electrolyte is 100% by mass or less.

From the viewpoint of low temperature characteristics and the like, it is preferable to use an electrolyte containing a non-aqueous solvent (organic solvent). From the viewpoint of suppressing the decomposition of the electrolyte as much as possible, an electrolyte containing an ionic liquid is preferably used, and an electrolyte containing an ionic liquid and a nonaqueous solvent may be used.
The concentration of the lithium salt or lithium ion in the electrolyte can be appropriately selected from the range of 0.3 to 5 mol / L, for example.

The kind of the anion (first anion) constituting the lithium salt is not particularly limited. For example, an anion of a fluorine-containing acid (anion of fluorine-containing phosphate such as hexafluorophosphate ion; fluorine containing such as tetrafluoroborate ion) Anions of boric acid), anions of chlorine-containing acids (such as perchlorate ions), oxalates such as oxylate anions [bis (oxalato) borate ions (B (C ₂ O ₄ ) ₂ ⁻ )] Borate ions; and oxalate phosphate ions such as tris (oxalato) phosphate ions (P (C ₂ O ₄ ) ₃ ⁻ ), anions of fluoroalkanesulfonic acids [trifluoromethanesulfonate ions (CF ₃ SO ₃ ⁻ ), etc. And bissulfonylamide anions.
A lithium salt may be used individually by 1 type, and may use it combining 2 or more types of lithium salts from which the kind of 1st anion differs.

Examples of the bissulfonylamide anion include bis (fluorosulfonyl) amide anion (FSA ⁻ : bis (fluorosulfonyl) amide anion), bis (trifluoromethylsulfonyl) amide anion (TFSA ⁻ : bis (trifluoromethylsulfamide) amide anion. ), (Fluorosulfonyl) (perfluoroalkylsulfonyl) amide anion [(FSO ₂ ) (CF ₃ SO ₂ ) N ⁻ etc.], and / or bis (perfluoroalkylsulfonyl) amide anion [N (SO ₂ CF ₃ ) _{^{_{2 -, N (SO 2 C}}} 2 F 5) 2 - ], and others.
Of these, FSA ^- is preferred.

The non-aqueous solvent is not particularly limited, and a known non-aqueous solvent used for a lithium ion capacitor can be used. Non-aqueous solvents include, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and γ-butyrolactone. The cyclic carbonate of the above can be preferably used. A non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

The ionic liquid contains a molten salt of a cation and an anion (second anion). The ionic liquid may contain a kind of molten salt, or may contain two or more kinds of molten salts having different types of cations and / or second anions.
As the second anion, a bissulfonylamide anion is preferably used. The bissulfonylamide anion can be selected from those similar to those exemplified for the first anion.

The cation constituting the ionic liquid includes at least lithium ions, and may include lithium ions (first cations) and second cations.
As a 2nd cation, the inorganic cation different from a lithium ion, an organic cation, etc. can be illustrated. Examples of the inorganic cation include alkali metal ions (sodium ions, potassium ions, etc.) other than lithium ions, alkaline earth metal ions (magnesium ions, calcium ions, etc.), ammonium ions, and the like. The second cation may be an inorganic cation, but is preferably an organic cation. The ionic liquid may contain one type of second cation, or may contain two or more types in combination.

Organic cations include cations derived from aliphatic amines, alicyclic amines or aromatic amines (for example, quaternary ammonium cations), and cations having nitrogen-containing heterocycles (that is, cations derived from cyclic amines). And nitrogen-containing onium cations; sulfur-containing onium cations; and phosphorus-containing onium cations.
Of the nitrogen-containing organic onium cations, those having pyrrolidine, pyridine, or imidazole as the nitrogen-containing heterocyclic skeleton in addition to the quaternary ammonium cation are particularly preferable.

Specific examples of nitrogen-containing organic onium cations include tetraalkylammonium cations (TEA ⁺ : tetraethylammonium cation), tetraalkylammonium cations such as methyltriethylammonium cation (TEMA ⁺ : methyltriethylammonium cation); 1-methyl-1-propylpyrrolidinium Cations (MPPY ⁺ : 1-methyl-1-propylpyrrolidinium cation), 1-butyl-1-methylpyrrolidinium cation (MBPY ⁺ : 1-butyl-1-methylpyrrolidinium cation); 1-ethyl-3-methylimidazolium cation ^{(EMI +: 1-ethyl-} 3-methylimidazo ium cation), and 1-butyl-3-methylimidazolium cation ^{(BMI +: 1-buthyl-} 3-methylimidazolium cation) and the like.

The lithium ion capacitor according to the embodiment of the present invention includes, for example, (a) a step of forming an electrode group with a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and (b) an electrode group and an electrolyte. It can manufacture by passing through the process of accommodating in a cell case.

FIG. 1 is a longitudinal sectional view schematically showing a lithium ion capacitor according to an embodiment of the present invention. The lithium ion capacitor includes a stacked electrode group, an electrolyte (not shown), and a rectangular aluminum cell case 10 for housing them. The cell case 10 includes a bottomed container body 12 having an upper opening and a lid 13 that closes the upper opening.

When assembling a lithium ion capacitor, first, an electrode group is configured by laminating the positive electrode 2 and the negative electrode 3 with the separator 1 interposed therebetween, and the configured electrode group is formed in the cell case 10. Inserted into the container body 12. Thereafter, a step of injecting an electrolyte into the container body 12 and impregnating the electrolyte in the gaps of the separator 1, the positive electrode 2, and the negative electrode 3 constituting the electrode group is performed. Alternatively, when the electrolyte includes an ionic liquid, the electrode group may be impregnated in the electrolyte, and then the electrode group including the electrolyte may be accommodated in the container body 12.

In the center of the lid 13, a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the cell case 10 rises. An external positive terminal 14 that penetrates the lid 13 is provided near the one side of the lid 13 with the safety valve 16 in the center, and an external that penetrates the lid 13 is located near the other side of the lid 13. A negative terminal is provided.

The stacked electrode group is composed of a plurality of positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed therebetween, all in the form of a rectangular sheet. In FIG. 1, the separator 1 is formed in a bag shape so as to surround the positive electrode 2, but the form of the separator is not particularly limited. The plurality of positive electrodes 2 and the plurality of negative electrodes 3 are alternately arranged in the stacking direction within the electrode group.

A positive electrode lead piece 2 a may be formed at one end of each positive electrode 2. The plurality of positive electrodes 2 are connected in parallel by bundling the positive electrode lead pieces 2 a of the plurality of positive electrodes 2 and connecting them to the external positive terminal 14 provided on the lid 13 of the cell case 10. Similarly, a negative electrode lead piece 3 a may be formed at one end of each negative electrode 3. The plurality of negative electrodes 3 are connected in parallel by bundling the negative electrode lead pieces 3 a of the plurality of negative electrodes 3 and connecting them to the external negative terminal provided on the lid 13 of the cell case 10. The bundle of the positive electrode lead pieces 2a and the bundle of the negative electrode lead pieces 3a are desirably arranged on the left and right sides of one end face of the electrode group with an interval so as to avoid mutual contact.

Both the external positive terminal 14 and the external negative terminal are columnar, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A flange 8 is provided in a portion of each terminal accommodated in the cell case 10, and the flange 8 is fixed to the inner surface of the lid 13 via a washer 9 by the rotation of the nut 7. .
The electrode group is not limited to the laminated type, and may be formed by winding a positive electrode and a negative electrode through a separator.

[Appendix]
Regarding the above embodiment, the following additional notes are disclosed.
(Appendix 1)
A positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and an electrolyte;
The positive electrode active material includes at least a material that reversibly supports anions,
The negative electrode active material includes a carbonaceous material having a graphite-type crystal structure,
The ratio of the capacitance C _n per projected unit area of the negative electrode to the capacitance C _p per projected unit area of the positive electrode: a lithium ion capacitor in which C _n / C _p is 20 or more.
In such a lithium ion capacitor, since the deterioration of the negative electrode active material during charging / discharging can be suppressed, the cycle characteristics can be significantly improved.

(Appendix 2)
In Appendix 1,
The negative electrode includes a negative electrode current collector, and a negative electrode mixture supported on the negative electrode current collector and including the negative electrode active material,
The negative electrode current collector has a three-dimensional network metal skeleton,
The carbonaceous material has an average plane spacing d ₀₀₂ of the measurement by X-ray diffraction spectrum (002) plane is less than 0.337 nm,
The C _n / C _p is preferably 30 to 120.
Such a lithium ion capacitor can further enhance the effect of improving the cycle characteristics.

Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

Test example 1
In the following procedure, a negative electrode and a positive electrode for a lithium ion capacitor were prepared, and a capacitance C _n per unit area of the negative electrode and a capacitance C _p per unit area of the positive electrode were measured.
(1) Production of negative electrode (a) Negative electrode 1
An artificial graphite powder (d ₀₀₂ : 0.3356 nm) as a negative electrode active material, acetylene black as a conductive additive, and an NMP solution of PVDF (binder) (PVDF concentration: 2.3% by mass) using a mixer The negative electrode mixture slurry was prepared by mixing under stirring. The mass ratio of the graphite powder, acetylene black, and PVDF was 100: 10.7: 5.7.

The obtained negative electrode mixture slurry has, on one surface of a copper foil (thickness: 20 μm) as a current collector, the mass M _n of the negative electrode active material per projected unit area of the negative electrode is about 5 mg / cm ^2. Was applied to form a coating film and dried. By compressing the dried product in the thickness direction, negative electrode 1 (negative electrode thickness: 66.4 μm) having a negative electrode mixture layer having a thickness of 46.4 μm was produced. A nickel lead was welded to the other surface of the current collector.

(B) Negative electrode 2
In the negative electrode 2, a copper porous body (porosity 85%, thickness 300 μm) having a three-dimensional network skeleton was used as a current collector. The negative electrode mixture slurry similar to the above (a) was filled in the copper porous body so that M _n was about 35 mg / cm ² and dried. The dried material was compressed in the thickness direction to produce a negative electrode 2 having a thickness of 263 μm. A nickel lead was welded to one surface of the negative electrode 2.

(2) Production of positive electrode (a) Positive electrode 1
Activated carbon powder as a positive electrode active material (specific surface area: 2300 m ² / g), acetylene black as a conductive additive, and NDF solution of PVDF (binder) (PVDF concentration: 2.3 mass%) using a mixer The positive electrode mixture slurry was prepared by mixing under stirring. The mass ratio of the activated carbon powder, acetylene black, and PVDF was 100: 10.7: 5.7.

The mass M _p of the positive electrode active material per unit area of the positive electrode projected on one surface (roughened surface) of an aluminum foil (thickness: 25 μm) as a current collector was obtained from the obtained positive electrode mixture slurry. A coating film was formed by coating so as to be about 3 mg / cm ² and dried. By compressing the dried product in the thickness direction, positive electrode 1 (positive electrode thickness: 89.1 μm) having a positive electrode mixture layer having a thickness of 64.1 μm was produced. An aluminum lead was welded to the other surface of the current collector.

(B) Positive electrode 2
In the positive electrode 2, an aluminum porous body (porosity 95%, thickness 1000 μm) having a three-dimensional network skeleton was used as a current collector. The positive electrode material mixture slurry similar to the above (a) was filled in the porous aluminum body so that M _p was about 30 mg / cm ² and dried. By compressing the dried product in the thickness direction, a positive electrode 2 having a thickness of 620 μm was produced. An aluminum lead was welded to one surface of the positive electrode 2.

(3) Measurement of capacitance (a) using a negative electrode obtained by C _n of measurement (1) the following procedure to measure the C _n.
The negative electrode was cut into a size of 3 cm long × 3 cm wide to prepare a sample negative electrode for measurement.
A sample negative electrode and a counter electrode were opposed to each other with a separator (resin microporous film, thickness 30 μm) interposed therebetween to form an electrode group. As the counter electrode, a sheet-like metallic lithium (length 3 cm × width 3 cm, thickness 50 μm) having a nickel lead welded to one surface thereof was used. The sample negative electrode and the counter electrode were opposed to each other on the surface where the lead was not welded.

A cell was produced by immersing the obtained electrode group and metallic lithium as a reference electrode in an electrolyte. As the electrolyte, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used.

By charging the cell at a current of 0.2 mA, lithium was supported from the counter electrode on the negative electrode sample. The amount of lithium supported at this time was adjusted so as to be equivalent to the capacity (mAh / cm ² ) per projected unit area of the negative electrode shown in Table 1. The cell was then discharged at a current of 0.2 mA until the negative electrode potential was 1.5V. The discharge capacity from the time when 1 minute has elapsed from the start of discharge until the negative electrode potential changes by 0.2 V is obtained, and the discharge capacity is calculated as the negative electrode potential change amount (0.2 V) and the negative electrode projected area (9 cm ² ). The electrostatic capacity C _n per projected unit area of the negative electrode was determined. The results are shown in Table 1. C _{n in} Table 1 is an average value of C _n obtained for a total of five cells prepared in the same manner as described above.

(B) by using the cathode obtained in C _p measurement above (2) by the following procedure to measure the C _p.
First, a cell was produced in the same manner as in the case (a) except that a cellulose nonwoven fabric (thickness 30 μm) was used as the separator.

The battery was charged at a current of 0.2 mA until the positive electrode potential reached 4.2V, and then discharged until the positive electrode potential reached 2.2V. The amount of change in the potential of the positive electrode and the discharge capacity from the time when 1 minute elapsed from the start of discharge until the potential of the positive electrode reached 2.2V were determined. From these values and the projected area (9 cm ² ) of the positive electrode, the capacitance C _p per projected unit area of the positive electrode was determined in the same manner as in the case (a). The results are shown in Table 1. C _{p in} Table 1 is an average value of C _p determined for a total of five cells prepared in the same manner as described above.

Examples 1 to 8 and Comparative Examples 1 to 4
(1) Production of positive electrode and negative electrode Except that M _p and M _n and the thickness of the positive electrode and the negative electrode were changed as shown in Table 2, the negative electrode was prepared in the same manner as in (1) and (2) of Test Example 1. And the positive electrode was produced.
In Examples 1 to 4 (A1 to A4) and Comparative Examples 1 and 2 (B1 and B2), like the negative electrode 1, a copper foil was used for the current collector of the negative electrode, and Examples 5 to 10 (A5 to In A10) and Comparative Examples 3 and 4 (B3 and B4), a copper porous body was used for the current collector of the negative electrode like the negative electrode 2.
In Examples 1 to 4 (A1 to A4) and Comparative Examples 1 and 2 (B1 and B2), an aluminum foil was used for the current collector of the positive electrode as in the positive electrode 1, and Examples 5 to 10 (A5 to In A10) and Comparative Examples 3 and 4 (B3 and B4), a porous aluminum body was used for the current collector of the positive electrode like the positive electrode 2.
When using copper foil or aluminum foil for the current collector, the thicknesses of the positive electrode and the negative electrode were adjusted by adjusting the thicknesses of the positive electrode mixture layer and the negative electrode mixture layer, respectively. In the same manner as in Test Example 1, the positive electrode and the negative electrode were each cut to a size of 3 cm in length and 3 cm in width and welded with leads.

(2) Pre-doping of lithium A negative electrode was obtained in the same manner as in (3) (a) of Test Example 1 except that the negative electrode obtained in (1) above was used and the amount of lithium supported was changed as shown in Table 2. Was supported (pre-doped) with lithium.
(3) Production of Lithium Ion Capacitor A non-woven fabric made of cellulose (thickness 30 μm) as a separator was interposed between the positive electrode obtained in (1) above and the negative electrode pre-doped with lithium in (2) above. The electrode group was formed by laminating in the state. Furthermore, metallic lithium (length 3 cm × width 3 cm, thickness 50 μm) is disposed on the negative electrode side of the electrode group with a resin microporous film (thickness 30 μm) as a separator, and the obtained laminate is made of an aluminum laminate. It accommodated in the cell case produced with the sheet | seat.

Next, an electrolyte was injected into the cell case to impregnate the laminate. As the electrolyte, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used. Finally, a lithium ion capacitor was produced by sealing the cell case while reducing the pressure with a vacuum sealer.

(4) Evaluation The following evaluation was performed using the positive and negative electrodes obtained in (1) above and the lithium ion capacitor obtained in (3) above.
(A) C _p , C _n and C _n / C _p
Using the positive electrode and the negative electrode obtained in (1) above, the capacitances C _p and C _n were determined in the same manner as in (3) (a) and (b) of Test Example 1, and the capacitance ratio C _n / C _p was calculated. For the negative electrode, the amount of lithium supported was adjusted to the capacity per unit area (mAh / cm ² ) shown in Table 2.

(B) Discharge capacity The lithium ion capacitor was charged and discharged 10 times at a current of 0.2 mA / cm ² between 2.2 V and 4.2 V. The discharge capacity (mAh / cm ² ) for the sixth to tenth times was determined, and the average value of these was calculated.

(C) Cycle characteristics (capacity maintenance rate)
Under the same conditions as in (b) above, the lithium ion capacitor was charged and discharged 10,000 times. The capacity retention rate (%) of the discharge capacity at the 100th, 1000th and 10000th discharges when the discharge capacity at the 10th discharge was taken as 100% was determined and used as an index for evaluating the cycle characteristics.
The results of Examples and Comparative Examples are shown in Table 2 and Table 3.

As shown in Table 3, in the lithium ion capacitor of the example having a C _n / C _p ratio of 20 or more, the capacity retention rate was extremely high exceeding 97% even after the charge / discharge was repeated 10,000 times. . On the other hand, in the lithium ion capacitor of the comparative example in which the C _n / C _p ratio is less than 20, when the charge / discharge is repeated 1000 times, the capacity retention rate of 90% or more is maintained, but 10,000 When repeated, the capacity retention rate decreased to 76% or less. When the charge / discharge is repeated 10,000 times, the capacity retention rate of the comparative example is 20% or more lower than the capacity retention rate of the example.

Thus, in the lithium ion capacitor of Example C _n / C _p ratio is 20 or more, a decrease in the cycle characteristics are significantly suppressed. However, in the lithium ion capacitor of the comparative example in which the C _n / C _p ratio is less than 20, when the charge / discharge is repeated 10,000 times, the cycle characteristics are clearly deteriorated. Therefore, it can be seen that when the C _n / C _p ratio is around 20, there is a critical point where the cycle characteristics change remarkably.
From the results of A1, A9, and A10, it can be seen that when the C _n / C _p ratio is 30 or 50 or 50 or more, the cycle characteristics tend to be further improved. Even after 10,000 charge / discharge cycles, a very high capacity retention rate of 98% or more for A1 and A9 and 99% or more for A10 is obtained.

Further, in Table 2, when comparing the comparative example with the examples in which the positive electrode active material amount and the negative electrode active material amount are close to the comparative example (B1 and A4, B2 and A2, B3 and A7, B4 and A9, Compared to each other), the discharge capacity of the lithium ion capacitor is higher in the example. Therefore, even if the positive electrode active material amount and the negative electrode active material amount are constant, it can be said that the discharge capacity of the lithium ion capacitor can be increased by setting the C _n / C _p ratio to 20 or more.

The lithium ion capacitor according to an embodiment of the present invention is excellent in cycle characteristics. Even if charging / discharging is repeated, the high capacity is maintained, and therefore, it can be applied to various uses requiring a long life.

DESCRIPTION OF SYMBOLS 1: Separator 2: Positive electrode 2a: Positive electrode lead piece 3: Negative electrode 3a: Negative electrode lead piece 7: Nut 8: Eaves part 9: Washer 10: Cell case 12: Container body 13: Lid body 14: External positive electrode terminal 16: Safety valve

Claims

A positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and an electrolyte;
The positive electrode active material includes at least a material that reversibly supports anions,
The negative electrode active material includes a carbonaceous material having a graphite-type crystal structure,
The ratio of the capacitance C n per projected unit area of the negative electrode to the capacitance C p per projected unit area of the positive electrode: a lithium ion capacitor in which C n / C p is 20 or more.
The lithium ion capacitor according to claim 1, wherein the C n / C p is 20 to 120.
C p is 0.1 to 15 F / cm 2 ,
The lithium ion capacitor according to claim 1, wherein a mass of the negative electrode active material per projected unit area of the negative electrode is larger than a mass of the positive electrode active material per projected unit area of the positive electrode.
The carbonaceous material, a lithium ion capacitor according to any one of claims 1 to 3 average plane spacing d 002 of the measurement by X-ray diffraction spectrum (002) plane is less than 0.337 nm.
The negative electrode includes a negative electrode current collector, and a negative electrode mixture supported on the negative electrode current collector and including the negative electrode active material,
5. The lithium ion capacitor according to claim 1, wherein the negative electrode current collector has a three-dimensional network metal skeleton.