KR100851371B1 - Electric double layer capacitor - Google Patents

Abstract

The present invention relates to an electric double layer capacitor, and more particularly, to an electric double layer capacitor having a high energy density and improved lifespan characteristics. The present invention includes at least one cell, wherein the cell is disposed between the anode and the cathode opposite to each other with a separator interposed therebetween, the electrolyte is impregnated with the electrolyte, wherein the electrode, the oxidation of standard hydrogen The present invention provides an electric double layer capacitor composed of activated carbon whose redox potential is controlled to + 0.5V or less based on the reduction potential. According to the present invention, activated carbon having a controlled redox potential (V) is applied as an electrode, and thus an electrolyte decomposition voltage (operating voltage) is increased to have a high energy density while improving life characteristics.

Electric double layer, capacitor, voltage, activated carbon, redox potential, energy density

Description

Electric Double Layer Capacitors {ELECTRIC DOUBLE LAYER CAPACITOR}

1 is a view for explaining the operating voltage-life characteristics of a cell according to the prior art and the present invention.

The present invention relates to an electric double layer capacitor, and more particularly, to an electric double layer capacitor having a high energy density and improved lifetime characteristics by controlling a redox potential of the activated carbon constituting the electrode. It is about.

An electric double layer capacitor (EDLC) is an energy storage device using a pair of charge layers (electric double layers) having different signs, which are capable of continuous charge / discharge. Compared with the general battery, the energy efficiency and output are high, and durability and stability are excellent. EDLC generally has a cell constructed by disposing two electrodes of a cathode and an anode facing each other with a separator interposed therebetween, and then impregnating an electrolyte. The capacitance of the EDLC is determined by the amount of charge accumulated in the electric double layer, and the amount of charge becomes larger as the surface area of the electrode is larger. Therefore, as shown in the following equation, the surface area of the electrode can be increased to increase the capacitance.

                              C = εS / d

(In the above formula, C: capacitance, ε: dielectric constant, S: surface area of electrode, d: distance between opposite electrode and electrode.)

Since activated carbon has a high specific surface area, it is suitable as an electrode material for EDLC requiring a large surface area. However, since activated carbon has low electrical conductivity, a conductive material is added for this purpose. Specifically, the electrode (electrode active material composition) of the EDLC may include porous activated carbon particles capable of adsorption and desorption of electrolyte ions on the surface of countless pores; A conductive material which electrically connects between these porous activated carbon particles and between the activated carbon particles and the metal current collector; And a binder for bonding them.

EDLC, however, has better output characteristics than batteries, but has a lower operating voltage per cell due to a gradual drop in voltage at the same time of discharge. In general, conventional EDLC cells have an operating voltage of 1.3V to 2.5V, and 2.7V per unit.

Almost all electronics, including ICs and backup power products, require use over a wide voltage range from 1.8V, usefully above 3V to as high as 48V for electric vehicles. As a result, two or more cells are connected in series to increase the operating voltage to at least 5V, and 10 to 100 cells from 10V to 48V for use in industrial equipment, electric vehicles, and UPS. Is connected in series.

However, when two or more cells are used to increase the operating voltage of the EDLC through an external series connection, there is another problem to solve the balance problem between each cell. Specifically, in consideration of cell capacity, equivalent series resistance (ESR), leakage current, etc., a voltage balance protection circuit such as a resistor, diode, or other IC is required so that the entire operating voltage of the EDLC is not concentrated in one cell. .

On the other hand, energy density (energy storage amount), which is a data on energy storage, is a good indicator for comparing the amount of energy in EDLC as well as storage batteries. Energy density can be obtained through the following equation.

Energy density (J) = 1/2 CV ²

(C is the capacitance (F) per cell, V is the voltage that can be applied to the cell.)

As shown in the above equation, the energy density is proportional to the capacity (C), but proportional to the square of the voltage (V2). In other words, if the voltage of a cell having the same capacity is doubled, the energy density (energy storage amount) is theoretically increased by four times.

Therefore, the best way to increase the energy density (energy storage) of EDLC is to increase the voltage. As described above, however, the EDLC according to the prior art has a high operating voltage per cell, which is 2.7 V at maximum, which is connected in series to increase the operating voltage. As the voltage balance between the cells is broken by repetitive cycles such as the rate of change of current, there is a problem that a high voltage is applied to any one cell. This in turn causes problems such as decomposition of the electrolyte material, lowering of internal resistance and capacitance.

The present invention has been invented to solve the problems of the prior art as described above, and an object thereof is to provide an electric double layer capacitor (EDLC) having a high energy density and improved lifetime characteristics due to an increased operating voltage.

In order to achieve the above object, the present invention comprises at least one cell, wherein the cell is disposed between the anode and the cathode two electrodes facing each other with a separator therebetween in the electric double layer capacitor comprising an electrolyte impregnated,

The electrode provides an electric double layer capacitor composed of activated carbon whose redox potential is controlled to + 0.5V or less based on the redox potential of standard hydrogen.

The redox potential of the activated carbon can be controlled by physical or chemical methods. For example, a method of heat treatment in an inert or reducing atmosphere, and a chemical method of controlling the content of oxygen (O) by reacting with a reducing agent may be mentioned.

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

The electric double layer capacitor according to the present invention typically has at least one or more unit cells, and the cell is composed of two electrodes arranged opposite to each other with a separator interposed therebetween, followed by impregnation of an electrolyte solution. . In this case, the electrode is manufactured from an electrode active material composed of activated carbon as a main material.

More specifically, the electrode is formed by rolling the electrode active material onto a sheet, and then punching or cutting the electrode active material into an appropriate size, or coating and fixing the electrode active material onto a thin foil-shaped metal current collector, It is constructed by punching or incision to the appropriate size. In this case, the electrode active material is a porous active carbon particles as a main material, the conductive material for electrically connecting between the activated carbon particles and between the activated carbon particles and the metal current collector; And a binder for bonding them. In addition, an additive such as a dispersion medium may be further added to the electrode active material.

Additives such as the binder, the conductive material and the dispersion medium can be used that is commonly used.

For example, the binder may be polytetrafluoroethylene (PTFE; poly-tetrafluoroethylene), polyvinylidene fluoride (PVdF; polyvinylidenefloride), carboxymethylcellulose (CMC; carboxymethylcellulose), polyvinyl alcohol (PVA; poly vinyl alcohol) , One or two or more selected from polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP; poly-N-vinylpyrrolidone), styrene butadiene rubber (SBR), etc. Can be used. As the conductive material, particulate conductive materials such as carbon black, acetylene black, fine graphite powder, or fibrous conductive materials such as carbon whiskers, fibers, and nanofibers may be used alone or in combination. Can be. As the dispersion medium, an organic solvent such as ethanol (EtOH), methyl pyrrolidone (NMP), or water may be used.

At this time, according to the present invention, as the activated carbon constituting the electrode (electrode active material), a redox potential (V) having a value of +0.5 V or less is used. Activated carbon is included in the present invention as long as it is controlled to have a redox potential (V) value in the above range by performing a separate treatment process using activated activated carbon 'as a starting material. Here, the 'activated activated carbon' refers to activated carbon activated through a conventional manufacturing process, activated carbon sold for electrode materials, etc., and refers to activated carbon having a redox potential (V) of more than 0.5V. Preferably the redox potential (V) is controlled in the range of -0.5V to + 0.5V, more preferably -0.35V to + 0.35V. In the present invention, the redox potential (V) is based on the redox potential value (= 0) of standard hydrogen, and may have a corresponding value when the standard is changed to Ag / Ag +.

Generally, activated carbon is composed of carbon (C) as a main material, and has a porous structure including hetero-atom such as oxygen (O), hydrogen (H), nitrogen (N), and sulfur (S). Have Among these heterogeneous elements, oxygen (O) and hydrogen (H) are mainly introduced in the process of activation such as steam activation or chemical activation, and nitrogen (N) and sulfur (S) are mainly derived from raw materials (petroleum, coal, etc.). do. These heteroatoms act as functional groups and affect the electrical properties of the cell. In particular, oxygen (O) decomposes and desorbs during electrochemical charging and discharging to generate gases such as CO and CO ₂ , thereby increasing the internal pressure of the cell, or causing electrolyte decomposition to deteriorate electrical and life characteristics. Becomes

According to the present invention, the redox potential (V) can be controlled by increasing or decreasing (removing) the content of functional groups contained in activated carbon through physical and chemical treatments. Preferably it is a method of reducing the content of oxygen (O) that inhibits the electrical properties of the cell. For example, a method of reducing the oxygen content by heat-treating activated carbon under an inert or reducing atmosphere, a dry method; And a wet method for controlling the content of oxygen (O) by reacting with a reducing agent (Wet Method); Etc., the redox potential V can be controlled. In this case, the reducing agent used in the wet method may include hydrazine (N ₂ H ₄ ), LiAlH ₄ , NaBH ₄ .

For example, the heat treatment is preferably performed by heat treating activated carbon at a temperature of 300 ° C. to 900 ° C. in an inert atmosphere (nitrogen atmosphere, etc.) or a reducing atmosphere (hydrogen atmosphere, etc.). At this time, if the heat treatment temperature is too low as less than 300 ℃, it is difficult to control the functional group content, if it exceeds 900 ℃ it is undesirable because the electrical properties of the activated carbon due to the reduction of the specific surface area is lowered. In addition, activated carbon is preferably heat-treated for 20 minutes to 12 hours in the above temperature range (300 ℃ ~ 900 ℃).

According to the present invention, when the heat treatment process as described above, at least the content of oxygen (O) in the hetero-functional group contained in the activated carbon is reduced. At this time, the content of oxygen is preferably controlled to within 5.0% by weight. As such, when the content of oxygen (O) is reduced, the redox potential (V) of the activated carbon is controlled to be low.

According to the present invention, when the redox potential (V) of the activated carbon is controlled to + 0.5V or less as described above, the cell to which it is applied as an electrode has an electrolyte decomposition voltage, that is, an operating voltage is increased and lifespan characteristics are improved. At this time, in configuring the electrode of the cell, activated carbon having a redox potential (V) of + 0.5V or less is applied to both the positive electrode (+) and the negative electrode (+), or any one selected from the positive electrode (+) and the negative electrode (+) Activated carbon having a redox potential (V) of + 0.5V or less may be applied to one electrode.

1 is a view for comparing the operating voltage-life characteristics of a conventional cell and a cell according to the present invention. In the figure, ① denotes an operating voltage region of a cell according to the prior art, and ② and ③ denote an operating voltage region of a cell according to the present invention.

Referring to FIG. 1, the conventional cell ① generally has an operating voltage of 0 to 2.5V and a maximum of 2.7V. However, the cells (②, ③) to which the activated carbon according to the present invention is applied may have a redox potential (V) controlled to be lower than 2.7 V, preferably 3.0 V or higher.

Specifically, as in the case of ②, the redox potential (V) is controlled to be low on both the anode (dotted line) and the cathode (dotted line), or as in the case of ③, the anode (solid line) is the redox potential as in the prior art. When the one having (V) is used, but the cathode (dotted line) having a low redox potential (V) is used, the reach to the electrolyte decomposition voltage is widened, and the operating voltage range is widened. In addition, as shown in Figure 1, the use time can be increased as in the case of ② (time ① <②).

Accordingly, commercially available cells are generally commercialized at an operating voltage of 2.5V. However, when activated carbon according to the present invention is applied, the cells may be supplied with an operating voltage of at least 2.7V, preferably at least 3.0V. As the operating voltage increases as described above, the energy density which is proportional to the square of the voltage V is improved as shown in the equation below.

Energy density (J) = 1/2 CV ²

(C is the capacitance (F) per cell, V is the voltage that can be applied to the cell.)

Hereinafter, specific test examples and comparative examples of the present invention will be described. The following examples are provided only to explain the present invention in more detail, whereby the technical scope of the present invention is not limited.

Example 1

<Activated Carbon Manufacturing>

Activated carbon activated from coal-based raw material was used as the activated carbon specimen (Sample 1) of the present embodiment, and the activated carbon specimen (Sample 1) was introduced into a furnace in a nitrogen atmosphere, and then thermally irradiated at 400 ° C. for 1 hour. Heat treatment process was performed. The activated carbon treated as described above was subjected to component analysis according to an elemental analysis (EA), and the results are shown in the following [Table 1].

<Electrode and Cell Manufacturing>

Carbon black (KETJENBLACK EC 600JD, manufactured by Mitsubishi Chemical Corporation) as a conductive material and polytetrafluoroethylene (PTFE; poly-tetrafluoroethylene) as a binder were uniformly mixed in a mass ratio of 8: 1: 1 to the heat treated activated carbon, and then rolled. An electrode sheet having a thickness of 250 μm was prepared. After drying the obtained sheet at 150 ° C. for 15 hours, an electrode specimen having a diameter of 12 mm was obtained using a circular punching machine.

Next, one sheet of cellulose-type separator (manufactured by Kodo Kogyo Co., Ltd.) having a thickness of 200 µm was inserted between the two sheets of electrodes, and then it was fixed inside a coin-type cell having a diameter of 18 mm. After impregnating the electrode with a propylene carbonate solution containing 1 mol (M) of tetraethylammonium tetrafluoroborate, the cell top was covered to complete cell assembly.

<Property Evaluation>

For the manufactured cell, capacity per unit mass (F / g), equivalent series resistance (ESR, Ohm), redox potential (V), electrolyte decomposition voltage (V), capacity reduction rate (%) after high temperature load test, and Voltage reduction (ΔV) was measured and shown in the following [Table 2].

Example 2

The activated carbon specimen (Sample 1) used in Example 1 was used as the activated carbon specimen in this Example, and the activated carbon specimen (Sample 1) was put into a furnace in a nitrogen atmosphere, and then heated at 500 ° C. for 1 hour. Irradiation was carried out for the heat treatment process. The results of component analysis according to EA of the heat treated activated carbon are shown in the following [Table 1].

Next, using the heat-treated activated carbon, to prepare an electrode and a cell in the same manner as in Example 1, and then to evaluate the physical properties in the same manner as in Example 1 for the cell [Table 2] Shown in

Example 3

Activated carbon activated from petroleum-based raw material was used as the activated carbon specimen (Sample 2), and the NaBH ₄ solution was mixed with the activated carbon specimen (Sample 2) and subjected to reduction treatment at room temperature for 2 hours while stirring. Degassing process to control) was carried out according to the chemical reduction method. The results of component analysis according to EA of the degassed activated carbon are shown in the following [Table 1].

Next, using the degassed activated carbon, an electrode and a cell were manufactured in the same manner as in Example 1, and then the physical properties of the cell were evaluated in the same manner as in Example 1, and the results are shown below. ].

Comparative Example 1

Compared with Example 1, it was carried out in the same manner as in Example 1 except that the heat treatment process was not performed on the activated carbon. That is, the activated carbon specimen (Sample 1) was configured as an electrode without heat treatment, and then a cell was prepared. And the physical properties of the prepared cell in the same manner as in Example 1 was evaluated and the results are shown in the following [Table 2].

Comparative Example 2

Compared with Example 3, it was carried out in the same manner as in Example 3 except that the activated carbon was not subjected to a degassing treatment step. That is, the activated carbon specimen (Sample 2) was configured as an electrode without degassing (chemical reduction), and then a cell was prepared. And the physical properties of the prepared cell in the same manner as in Example 1 was evaluated and the results are shown in the following [Table 2].

                      <Component Analysis Results of Activated Carbon>  Remarks  Process Composition of activated carbon (% by weight) C H O N Etc Example 1 400 ° C, 1 hour heat treatment 91 2.1 3.5 0.21 3.19 Example 2 500 ℃, 1 hour heat treatment 92 2.0 2.9 0.21 2.89 Example 3 Chemical reduction 92 2.0 3.1 0.21 2.69 Comparative Example 1 - 90 0.55 9.0 0 0.45 Comparative Example 2 - 89 0.1 9.1 0.22 1.58

                          <Property Evaluation Results> Remarks Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Measuring conditions Oxygen content (wt%) 3.5 2.9 3.1 5.8 9.1 - Impurity Content (ppm) 198 182 289 190 301 - Capacity per unit mass (F / g) 39 40 38 38 39 - Equivalent series resistance (ESR, Ohm) 2.5 1.2 6.0 7.0 18.0 coin cell 1820 Redox potential (V) 0.34 0.35 0.33 0.63 0.79 Standard hydrogen standard Electrolytic Solution Breakdown Voltage (V) 2.7 2.8 2.7 2.3 2.2 PC with TEABF ₄ Capacity reduction rate (%, 100cycles) 3.0 1.0 2.5 15.0 28.2 Life Voltage Range: 1.5V ~ 3.0V Voltage Reduction (ΔV) 0.05 0.01 0.06 0.31 0.45 Voltage drop after 24hr of 2.7V charge

First, as shown in [Table 1] and [Table 2], in Examples 1 to 3 of the present invention, dry (heat treatment) or wet treatment (reduction treatment) was performed to compare oxygen ( It can be seen that the content of O) is reduced and the redox potential V is controlled.

As shown in Table 2, the cells of Examples 1 to 3 in which the content of oxygen (O) is reduced and the redox potential (V) is controlled, the electrolyte decomposition voltage (operating voltage) is high, and the life characteristics are improved. It can be seen that the improvement. In addition, it can be seen that electrochemical properties such as equivalent series resistance (ESR) also show improved values.

As described above, in the present invention, activated carbon having a controlled redox potential (V) is applied as an electrode, and thus an electrolyte decomposition voltage (operating voltage) is increased, thereby having a high energy density and improving life characteristics.

Claims (5)

In the electric double layer capacitor comprising at least one cell, wherein the cell is disposed so that the positive electrode and the negative electrode opposite to each other with a separator therebetween and then the electrolyte is impregnated, The electrode is an electric double layer capacitor, characterized in that the redox potential on the basis of the redox potential of the standard hydrogen is controlled to + 0.5V or less. The method of claim 1, The activated carbon is heat-treated for 20 minutes to 12 hours at a temperature of 300 ℃ ~ 900 ℃ electric double layer capacitor, characterized in that the redox potential is controlled to + 0.5V or less. The method of claim 1, The activated carbon is an electric double layer capacitor, characterized in that the content of oxygen is reduced by reaction with a reducing agent, the redox potential is controlled to + 0.5V or less. The method according to any one of claims 1 to 3, The activated double carbon is an electric double layer capacitor, characterized in that the oxygen content is 5% by weight or less. delete

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