KR100537366B1 - Hybrid capacitor - Google Patents

Abstract

A hybrid capacitor is disclosed.

Hybrid capacitor according to the present invention is activated carbon as a cathode active material; Graphite carbon containing lithium ions as an anode active material; Separator; Lithium salts; And a non-aqueous organic solvent, and according to the present invention, it is possible to increase the low energy density, which is a disadvantage of the conventional electric double layer capacitor, by about 2.5 times or more, and does not need to use an expensive metal such as a transition metal, Since the active material is activated carbon and graphite-based carbon, it is environmentally friendly, and it is possible to suppress the sudden drop and rise of the anode potential, which has the advantage of enabling electrical energy storage and output over the entire range of the driving voltage.

Description

Hybrid capacitor

The present invention relates to a hybrid capacitor, and more particularly, to a hybrid capacitor capable of obtaining a high energy density through a stable and high driving voltage of 4V or more.

Representative examples of energy storage devices currently in use include lithium secondary batteries and electric double layer capacitors (EDLC). Lithium secondary battery has the advantage of high energy density of 20 ~ 120Wh / kg, but low output density of 50 ~ 250W / kg, has a disadvantage that the cycle life characteristics are low as about 500 times.

On the contrary, the electric double layer capacitor is a device that accumulates electric energy by using electric charges accumulated in the electric double layer generated at the interface between the solid electrode and the electrolyte, and has a low energy density of 1 to 10 Wh / kg compared to the lithium secondary battery. Due to the very short charging time, very high power density of 1000-2000W / kg and almost semi-permanent cycle life characteristics, there is increasing interest in various applications such as electric vehicles.

The lithium secondary battery and the electric double layer capacitor are very similar in structure and operation principle of the unit cell, but show a difference in the storage mechanism of the charge. That is, in lithium secondary batteries, electrons and ions are transferred into the bulk of the electrode material due to charging and discharging, and depending on the Faradaic reaction, a phase change of the electrode material is involved. In an electric double layer capacitor, such a Faraday Since the reaction is not intervened (non-Faradaic process), the charge and discharge reaction occurs only at the interface (electric double layer) of the electrode / electrolyte without phase change of the active material, and the energy stored because the reaction is limited to the surface. It has the disadvantage of low density.

More specifically, the electric double layer capacitor will be described below.

A general electric double layer capacitor is composed of an electrode, a separator, an electrolyte, and a case. Among them, the most important part of the capacitor is the selection of the material used for the electrode, and since the electrode material has to have high electroconductivity, specific surface area, and electrochemical stability, so far the specific surface area is 1000-2000 m ² / Activated carbon or activated carbon fiber, which is g, is known to be most likely. In addition, in order to be used as an electrolyte, an aprotic polar organic electrolyte is used because it must not be decomposed within the driving voltage of the capacitor. The aprotic polar organic electrolyte which does not generate hydrogen ions inside is electrochemically stable. This is because the potential is large. Therefore, ethylene carbonate (EC), propylene carbonate (PC), acetonitrile, and the like are used. Current commercially available electric double layer capacitors are capable of driving voltages from 2.3V up to 3V. However, a driving voltage of 3 V or more is somewhat limited because an irreversible electrolyte decomposition reaction, which is assumed to be an oxidation reaction of the electrolyte at about 4.4 V based on the lithium ion reduction reaction, occurs at the surface of the activated carbon cathode during charging. This is because the irreversible decomposition reaction of the electrolyte, which is estimated to be the reduction reaction of the electrolyte, occurs at 1.5 V compared to the lithium ion reduction reaction.

As mentioned above, for current electric double layer capacitors, the most problematic is to improve the energy density. As shown in Equation 1, the energy density of the electric double-charge capacitor is composed of the term of the capacitance of the electrode material and the square of the driving voltage.

E = 1 / 2CV ²

(E, C and V correspond to energy density, storage capacity and driving voltage, respectively.)

That is, in order to increase the energy density of the capacitor, the storage capacity of the electrode material should be increased or the driving voltage should be increased. When the activated carbon is used as the electrode material, the storage capacity may be increased by increasing the porosity of the surface of the activated carbon. This is limited, and in the case of the driving voltage, the driving voltage of the electric double layer capacitor is limited because the driving voltage is limited to a range in which decomposition of the electrolyte does not occur. Therefore, various attempts have been made to increase the energy density, and most of these attempts have been made to improve the energy density by increasing the storage capacity by changing the electrode material.

 U.S. Patent No. 6,383,363 discloses a high capacity capacitor using amorphous ruthenium oxide and sulfuric acid aqueous solution, which is classified as pseudo-capacitor or redox capacitor. It is accompanied by a change in the oxidation number of the metal ion and the charge is stored. Such pseudocapacitors store electric charge only in the double layer formed on the electrode surface, whereas electric double layer capacitors can store charges up to the bulk near the surface of the electrode material, and thus the storage capacity is about 5 times larger than that of the electric double layer capacitor, and thus energy density. Is even bigger. Metal oxides that can be used as electrode materials for such pseudocapacitors are representative of RuOx, IrOx, TaOz, and the like. All of the metal ions constituting the metal oxide should be transition metals and should not be dissolved in such strong acids because strong acids are used as electrolytes. There is a limitation that the electrode material is expensive because of the constraint.

In addition, US Pat. No. 6,222,723 discloses a hybrid capacitor using different materials as cathode and anode electrode materials. That is, a strong acid or a strong basic electrolyte or a non-aqueous electrolyte is used as the electrolyte, and the first electrode material is Ru, Pd, Ni or an oxide thereof having a large capacitance, and the activated carbon is used as the second electrode material. The first electrode is designed to have a mechanism in which charge is accumulated by the Faraday reaction and at the second electrode by the electric double layer reaction, and an attempt is made to increase the energy density by increasing the storage capacity as a whole. However, the metal or metal oxide used as the first electrode material is expensive and not environmentally friendly. When using sulfuric acid aqueous solution or KOH aqueous solution as an electrolyte, the driving voltage is about 2V, which is very low. Even in the case of using the electrolytic solution, the driving voltage is limited to less than 3 V because of the chemical stability of the metal oxide used as the first electrode material, so that the improvement in the overall energy density is weak.

On the other hand, in order to overcome the driving voltage limit of the electric double layer capacitor using activated carbon, an activated carbon / graphite carbon hybrid that replaces activated carbon used in the anode of the electric double layer capacitor with graphite carbon which can insert and extract lithium ions. Capacitors have been proposed. The proposed system is able to combine the potential stability of activated carbon (1.5V to 4.4V compared to lithium reduction) and lithium ion insertion / extraction reaction (0.2V compared to lithium reduction). You can get it. However, despite these high driving voltages, activated carbon / graphite carbon hybrid capacitors have limited energy density due to the limited charge / discharge voltage range. In other words, unlike electric double layer capacitors, in which electric energy is charged and discharged over the entire range of driving voltages, activated carbon / graphite carbon hybrid capacitors have a charge / discharge voltage range of about 4V to about 2.7V. The problem is that the effect of increasing the energy density is insignificant because it does not utilize the voltage as a whole and uses only a limited area. This is the electricity generated before the initial lithium ion insertion reaction and after the lithium ion desorption reaction in the graphite carbon used as the anode. This is due to a sharp potential change due to the bilayer reaction.

The present invention solves the problems of the conventional electric double layer capacitor as described above, and extends the range of the driving voltage to provide a hybrid capacitor having a higher energy density and no sudden potential change at the beginning and end of the discharge.

The present invention to achieve the above technical problem

Activated carbon as a cathode active material; Lithium Intercalated Graphite (LIG) in which lithium ions are inserted as an anode active material; Separator; Lithium salts; And it provides a hybrid capacitor comprising a non-aqueous organic solvent.

According to one embodiment of the present invention, the lithium-ion-inserted graphite-based carbon is preferably prepared by the constant current and the potentiostatic method.

According to a preferred embodiment of the present invention, the lithium salt is any one selected from the group consisting of LiPF _6, LiBF ₄ , LiClO ₄ , Li (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiSbF ₆ and LiAsF ₆ desirable.

In addition, the non-aqueous organic solvent is preferably ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetone, acetonitrile, n-methyl-2-pyrrolidone (NMP) or a mixture thereof.

In addition, the concentration of the lithium salt in the non-aqueous organic solvent is preferably 0.4M to 1.5M.

Hereinafter, the present invention will be described in more detail.

In order to overcome the disadvantage that the hybrid capacitor according to the present invention has a low energy density of a conventional electric double layer capacitor (EDLC), the cathode uses activated carbon as an active material, but graphite based carbon as an anode active material and lithium salt as an electrolyte. It is characterized by using this dissolved organic solvent. As such, when graphite-based carbon is used as the anode active material, as shown in FIG. 1, since the capacitor can be operated at a driving voltage of 1.5 V or less, a driving voltage of 4.2 to 4.3 V can be obtained as a whole. The reason is that even when graphite-based carbon is used as the anode active material, an irreversible decomposition reaction occurs due to the reduction reaction of the electrolyte at 1.5 V or less, but the Faraday reaction occurring at the anode is not affected by the decomposition reaction.

In more detail, the cathode of the capacitor according to the present invention is the same as the electric double layer capacitor, the anode takes the form similar to the lithium secondary battery. That is, the cathode charges and discharges energy by a Faraday reaction such that charges are accumulated in the electric double layer and lithium ions are inserted and desorbed by oxidation-reduction. When graphite-based carbon is used as the anode active material, the specific capacitance of the anode is reduced, so that the storage capacity is reduced. In order to overcome this, the far side of the anode, rather than the accumulation of charge by the electric double layer, allows the Faraday reaction to intervene to reduce the overall reduction in capacitance. In other words, when graphite-based carbon is used as the anode active material, lithium ions must be used in the electrolyte solution.

As such, when the graphite-based active material is used for the anode, charging and discharging of electricity does not occur at the interface between the anode and the electrolyte, but occurs in the bulk inside the anode. In the existing EDLC as well as the lithium secondary battery, when charging and discharging is performed at a driving voltage of 1.5 V or less, a part of the electrolyte is decomposed to form a thin film on the anode surface, which is called a SEI (Solid Electrolyte Interface) film. When the SEI film is formed in the conventional EDLC, a problem occurs because an electric double layer cannot be formed at the interface between the anode and the electrolyte, but according to the present invention, since the reaction occurring at the anode is the insertion and desorption of lithium ions into the anode bulk. There is no problem even if the SEI film is formed. Thus, the width of the overall driving voltage can be expanded, thereby increasing the energy density.

On the other hand, as described above, when only graphite-based carbon is used as the anode active material in the hybrid capacitor, the charge / discharge voltage range is limited to about 2.7V to 4V. The limitation of the charge / discharge voltage range of the activated carbon / graphite carbon hybrid capacitor is due to the rapid potential change caused by the electric double layer occurring in the graphite used as the anode active material before the lithium ion insertion reaction during the initial charge and after the lithium ion desorption reaction during discharge. Is caused. Referring to FIG. 2, the activated carbon / graphite-based carbon hybrid capacitor has an electrostatic orientation of anions in the electrolyte, that is, an electric double layer reaction occurs in the active carbon of the cathode during charging, and after some electric double layer reaction occurs in the graphite of the anode. Insertion of lithium ions occurs. On the other hand, during discharging, the electrostatic orientation of the anion and the cathode in the electrolyte disappears in the active carbon of the cathode and some electrical double layer reaction occurs after the desorption of lithium ions occurs in the graphite of the anode. In this case, the problem is that the electric double layer reaction on the anode surface occurring at the beginning of charge and at the end of discharge is accompanied by a sudden drop and rise of the potential because the capacity is very small, which appears to be a limitation of the charge / discharge region.

Therefore, the graphite-based carbon used in the anode is replaced with the lithium-ion-inserted graphite-based carbon in order to minimize the electric double layer reaction that causes a sudden change in potential at the anode. If the active material used for the anode uses graphite carbon in which lithium ions are inserted instead of graphite carbon, the initial potential of the anode can be maintained at 0.2V compared to the reduction reaction in which the lithium insertion / extraction reaction occurs. Insertion of lithium ions can be proceeded without change, and in addition, during discharging, an extraction reaction of initially inserted lithium ions may be additionally performed in addition to the extraction reaction of lithium ions inserted in the charging process, thereby preventing a rapid increase in potential. For this reason, when an activated carbon / lithium-containing graphite-based carbon hybrid capacitor is formed by using activated carbon as a cathode active material and graphite-based carbon with lithium ions inserted therein as an anode active material, electrical energy is applied over the entire range of driving voltage. Storage and output are possible.

3 shows a schematic diagram of a hybrid capacitor according to the invention. The activated carbon used as the cathode active material of the hybrid capacitor according to the present invention is preferably activated carbon in which a carbon material is modified by steam activation, molten KOH activation, or the like. For example, coconut shell activated carbon, phenol-based activated carbon, petroleum And coke-based activated carbon, and one of these may be used alone or in combination of two or more thereof.

On the other hand, the graphite-based carbon that can be used as the anode active material is not particularly limited as long as it is a graphite-based carbon material used for the anode of a lithium ion battery and capable of inserting and extracting lithium ions.

The separator usable in the hybrid capacitor according to the present invention is polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyester nonwoven fabric, polyacrylonitrile porous separator, poly (vinylidene fluoride) hexafluoropropane copolymer porous separator, cellulose porous separator, kraft If the separator is generally used in the field of batteries and capacitors, such as paper or rayon fibers, it is not particularly limited.

The lithium salt used in the present invention is not particularly limited as lithium salts commonly used in general lithium secondary batteries, and for example, LiPF _6, LiBF ₄ , LiClO ₄ , Li (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiSbF ₆ or LiAsF ₆ and the like.

In addition, the non-aqueous organic solvent used in the present invention is not particularly limited as long as it is an organic liquid electrolyte commonly used in batteries and capacitors. For example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetone, acetonitrile, n -Methyl-2-pyrrolidone (NMP) or a mixture thereof.

The concentration of the lithium salt in the non-aqueous organic solvent used in the present invention is preferably 0.4M to 1.5M, since the ion conductivity of the lithium salt is the highest in the above range.

Lithium ion-inserted graphite carbon electrode used as an anode in the present invention can be prepared by an electrochemical method. In detail, a three-electrode system is constructed by using a graphite carbon electrode, a non-aqueous organic electrolyte, and lithium metal, wherein the graphite carbon electrode is used as a working electrode, and the reference electrode and the counter electrode are made of lithium metal. do. Next, using a charger and discharger, the graphite-based carbon electrode is discharged at a current density of 0.05C relative to the theoretical capacity of the graphite-based carbon material to induce the insertion reaction of lithium ions generated at 0.2V compared to the lithium ion reduction reaction. When the graphite-based carbon electrode is recovered from the cell formed immediately after this process, a lithium-ion-inserted graphite-based carbon electrode can be obtained.

Hereinafter, the present invention will be described in more detail with reference to preferred examples and comparative examples, but the scope of the present invention is not limited to these examples.

LiBF ₄ used in the following Examples and Comparative Examples was used as a battery reagent grade product of Hashimoto, Japan without purification, and the solvent used in the preparation of the organic electrolyte solution was subjected to distillation, followed by molecular sieve of Fluka Co. Moisture was removed using.

Example 1

Preparation of Graphite Carbon Anode Inserted with Lithium Ion

Artificial graphite (manufactured by Osaka Gas Co., Ltd.) and acetylene black as a conductive agent were dried for 48 hours or more under a vacuum atmosphere at 80 ° C. to remove moisture contained in the compound as much as possible. 95 parts by weight of the artificial graphite and 5 parts by weight of acetylene black as a conductive agent were sufficiently mixed in a high speed mixer for 30 minutes. When 85 parts by weight of the mixed powder was added to 12 parts by weight of a mixed solution of 6.6 parts by weight of a vinylidene fluoride-hexafluoropropylene copolymer as a binder and added to an N-methylpyrrolidone solution, the resultant was a slurry having a predetermined viscosity. Stir for about 2 hours intermittently. The slurry was cast using a doctor blade on a copper foil having a thickness of 20 μm with a blade gap of 300 μm and then dried in an oven at 100 ° C. for 12 hours to completely remove N-methylpyrrolidone. A three-phase electrode system was constructed using lithium metal and an electrolyte in which 1M LiBF ₄ was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed in a 1: 1 ratio. The working electrode was a graphite carbon electrode, and both the reference electrode and the counter electrode were lithium metal. Next, the graphite-based carbon anode was discharged at a current density of 0.05 C using a charger and a discharger, and lithium ions corresponding to the capacity of about 200 mAh / g based on the initial capacity of the graphite-based carbon were inserted into the lattice of the graphite-based carbon. This intercalated graphite carbon anode was prepared.

Preparation of Activated Carbon Cathodes

5 wt% of CMC (carboxy methyl cellulose) as a binder is added to MSC30 (manufactured by Kurea, Japan) as activated carbon, mixed with water for about 12 hours, coated on an aluminum current collector, and subjected to vacuum at 120 ^o C. Drying for hours produced the cathode.

Fabrication of Unit Capacitor Cells

The activated carbon cathode prepared above, the graphite-based carbon anode with lithium ions, and a polypropylene separator were laminated in this order, and then a solvent in which ethylene carbonate and dimethyl carbonate were mixed in a 1: 1 ratio. A hybrid capacitor was prepared as shown in FIG. 3 using an electrolyte in which 1M LiBF ₄ was dissolved in.

Comparative Example 1

A hybrid capacitor was manufactured in the same manner as in Example 1, except that lithium ions were not inserted into the graphite-based carbon anode by an electrochemical method.

Test Example 1

The hybrid capacitors prepared in Example 1 and Comparative Example 1 were charged to 4V at a current density of 10 mA / cm ² using a constant current charger and discharged to 0V repeatedly, and then the storage capacity, energy density and resistance Table 1 and the constant current charge and discharge curves are shown in FIGS. 4 and 5.

Capacity (F / g) Energy Density (Wh / kg) Example 1 28.1 64.4 Comparative Example 1 7.3 14.4

As can be seen in FIG. 5, in the hybrid capacitor according to Comparative Example 1, the voltage increases or decreases with a linear slope in the voltage range between 4V and 3V, and a sudden increase and decrease of the voltage can be confirmed below 3V. . This means that energy storage and output are concentrated only between 4V and 3V, and below 3V very small amounts of electrical energy are stored and output. The capacities and energy densities obtained from the constructed electrochemical capacitors are 7.3 F / g and 14.4 Wh / kg of electrode weight, respectively. In contrast, as shown in FIG. 4, the hybrid capacitor according to the present invention exhibits a capacitor behavior in which the voltage continuously decreases between 4V and 0V, and this behavior can be seen to appear over all driving voltage ranges of 4V. In addition, the capacitance and energy density obtained in the hybrid capacitor according to the present invention are 28.1 F / g and 64.4 Wh / kg of the electrode weight, respectively, which are much better than those of Comparative Example 1. In conclusion, when comparing Example 1 and Comparative Example 1, when using the same active carbon and the electrolyte, using a graphite-based carbon containing lithium ions as the anode active material can be obtained a high energy density of more than four times, When considering that the energy density of the electric double layer capacitor of 1 ~ 10Wh / kg can be seen that according to the present invention can be obtained more than 10 times the energy density.

As described above, the hybrid capacitor according to the present invention can increase the low energy density, which is a disadvantage of the conventional electric double layer capacitor, by about 10 times or more, and does not require the use of expensive metals such as transition metals, and the active material is activated carbon. And graphite-based carbon, which are environmentally friendly, and can abruptly lower and raise anode potentials, thereby enabling storage and output of electrical energy over the entire range of drive voltages.

1 illustrates a difference in driving voltages between a conventional electric double layer capacitor and a hybrid capacitor using a graphite-based carbon anode.

2 shows a typical operating mechanism of a hybrid capacitor using a graphite carbon anode.

3 schematically shows a configuration of a hybrid capacitor according to the present invention.

4 shows a constant current charge / discharge curve of a hybrid capacitor according to Example 1 of the present invention.

5 shows a constant current charge and discharge curve of a hybrid capacitor according to Comparative Example 1.

<Description of the symbols for the main parts of the drawings>

1: cathode 2: separator

3: anode 4: electrolyte

Claims (5)

Activated carbon as a cathode active material; Lithium Intercalated Graphite (LIG) in which lithium ions are inserted as an anode active material; Separator; Lithium salts; And a nonaqueous organic solvent. The hybrid capacitor of claim 1, wherein the graphite-based carbon into which lithium ions are inserted is manufactured by a constant current and a potential potential. The method of claim 1, wherein the lithium salt is LiPF _6, LiBF ₄ , LiClO ₄ , Li (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiSbF ₆ And LiAsF _{6 It} characterized in that any one selected from the group consisting of Hybrid capacitors. The method of claim 1, wherein the non-aqueous organic solvent is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetone, acetonitrile, n-methyl-2-pyrrolidone (NMP) or a mixture thereof Hybrid capacitors. The hybrid capacitor of claim 1, wherein a concentration of lithium salt in the non-aqueous organic solvent is 0.4M to 1.5M.