JP2005302950A - Method for refining electrolyte - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for removing impurity such as water in electrolyte affecting the performance of an electric double layer capacitor, and also to provide a method for manufacturing the electric double layer capacitor by using the electrolyte. <P>SOLUTION: This method for refining electrolyte for removing low potential active substances in the electrolyte with moisture in the electrolyte as a subject applies a voltage which is lower than the intercalation start voltage of electrolyte ion between a plurality of carbon electrodes prepared by using non-porous carbon, and brings the carbon electrode into contact with the electrolyte. Also, this method for manufacturing an electric double layer capacitor manufactures an electric double layer capacitor by using the electrolyte obtained by the refining method. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for purifying an electrolytic solution that removes impurities such as moisture in the electrolytic solution, and a manufacturing method for producing an electric double layer capacitor using the obtained purified electrolytic solution.

  An electric double layer capacitor that can be charged and discharged with a large current is promising as a power storage device with a high charge / discharge frequency, such as an electric vehicle, a solar battery auxiliary power source, and a wind power auxiliary power source. For this reason, electric double layer capacitors with high energy density, rapid charge / discharge, and excellent durability are desired (for example, the current status and future of symposium-capacitor technology related to the 4th EV / HEV advanced technology) Issue “International Symposium on Advanced Technology of Electric Vehicle Batteries” Executive Committee November 8, 1999, etc.).

  Such an electric double layer capacitor comprises a pair of polarizable electrodes opposed to each other in an electrolyte solution via a separator to constitute a positive electrode and a negative electrode, and an electric double layer formed at the interface between the polarizable electrode and the electrolytic solution. The principle is to accumulate charges in the multilayer. Therefore, based on the idea that the capacitance of the electric double layer capacitor is almost proportional to the area of the polarizable electrode, conventionally, activated carbon having a large specific surface area with a pore diameter of about 2 nm or more has been exclusively used as the active material of the polarizable electrode. It is used (for example, Unexamined-Japanese-Patent No. 2002-15958 etc.).

  On the other hand, such an electric double layer capacitor uses an organic electrolyte from the viewpoint of energy density. As the organic electrolyte, an electrolytic solution in which a tetraalkylammonium salt or the like is dissolved in an organic solvent or a liquid such as an imidazolium salt is used. An electrolyte or a non-aqueous electrolyte solution in which this is dissolved in an organic solvent is used. As the organic solvent, an aprotic organic solvent having a high dielectric constant, such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxymethane, γ-butyl lactone, and acetonitrile, is used.

  In such an electric double layer capacitor using a non-aqueous electrolyte, impurities such as water existing in the electrolyte affect the performance of the electric double layer capacitor, so impurities such as water in the electrolyte are It is preferable to remove as much as possible. As a method for removing impurities in the electrolytic solution, a method of removing moisture and the like in the electrolytic solution using an adsorption dehydration column is known (Japanese Patent No. 3394400).

Under such circumstances, in order to improve the performance of the electric double layer capacitor, a simple method capable of removing impurities such as moisture in the electrolytic solution as much as possible is required.
JP 2002-15958 A Japanese Patent No. 3394400 The 4th EV / HEV Symposium on Advanced Technology for Capacitors-Current Status of Capacitor Technology and Future Issues "International Symposium on Advanced Technology for Electric Vehicle Batteries" Executive Committee November 8, 1999

  The present invention provides a new method for removing impurities such as water in the electrolytic solution that affects the performance of the electric double layer capacitor, and further using the electrolytic solution from which impurities such as water have been removed by the method, An object is to provide a method of manufacturing an electric double layer capacitor.

  The inventors have proposed an electric double layer capacitor excellent in electrostatic capacity and withstand voltage by using a carbon material having characteristics completely different from those of the above-mentioned activated carbon and using a conventionally used electrolyte (Japanese Patent Laid-Open No. 2000). -77273 and JP 2002-25867). The non-porous charcoal used as the polarizable electrode of this electric double layer capacitor, that is, the non-porous charcoal that is a carbon material having completely different characteristics from activated carbon, consumes electrolyte ions that contribute to the generation of capacitance. In addition, the present invention was found to function extremely effectively in removing impurities such as moisture in the electrolytic solution, and the present invention was completed.

  That is, in the present invention, the carbon electrode and the electrolytic solution are brought into contact with each other while applying a voltage lower than the intercalation start voltage of the electrolyte ions between the plurality of carbon electrodes made using nonporous charcoal. This is a method for purifying an electrolytic solution that removes a low-potential reactive substance in an electrolytic solution mainly composed of moisture in the electrolytic solution.

  Further, the present invention provides an electric double layer capacitor using an organic electrolyte while applying a voltage lower than the intercalation start voltage of electrolyte ions between a plurality of carbon electrodes prepared using non-porous coal in advance. In this method, the low-potential reactive substance in the electrolytic solution mainly containing moisture is removed by contacting the electrolytic solution. The electric double layer capacitor using the organic electrolyte is not only one made using the non-porous charcoal used in the present invention as the polarizable electrode, but any one made using conventional activated carbon. All of the electric double layer capacitors using a so-called non-aqueous organic electrolyte may be included.

The non-porous coal used in the present invention has an average interlayer distance d ₀₀₂ having graphite-like microcrystalline carbon as described in JP-A No. 2002-25867 and JP-A No. 2002-362912. There are no pores having a size of about 350 to 0.380 nm and a specific surface area according to the BET method of 270 m ² / g, preferably 100 m ² / g or less, capable of taking in various electrolyte ions, solvents, N ₂ gas, and the like. It means non-porous carbon. Such non-porous coal is activated by treating coke or carbonized carbon obtained by calcining oil coke or needle coke, a kind of coal coke, or infusible petroleum or coal pitch. An activated non-porous charcoal obtained by heat treatment (calcination) at 650 to 850 ° C. with the development of multilayer graphite microcrystals such as needle coke and infusibilized pitch. Carbon (calcined charcoal) is treated with caustic such as KOH at 800 to 900 ° C., and residual alkali components are removed with heated steam, and if necessary, transition metal catalysts such as Ni, Fe, Co, etc. In the presence or without using a catalyst, heat treatment is performed in a reducing gas stream such as hydrogen, and active hydrogen oxide (such as COOH, CHO, OH, etc.) present on the carbon surface ) Can be obtained by removing the. Further, the average interlayer distance d ₀₀₂ of the non-porous coal varies depending on the type of the carbon material used as a raw material and the temperature of the heat treatment in a reducing gas stream such as hydrogen, and by increasing the heat treatment temperature. _Nonporous charcoal having a small average interlayer distance _d002 is obtained.

Confirmation of whether or not the active hydrogen oxide present on the carbon surface has been removed appears as a short relaxation time component T ₂ = 20 to 50 μsec (Gauss type) by observing ¹ H resonance of powdered carbon by pulse NMR. The amount of hydrogen directly bonded to the carbon skeleton and the amount of hydrogen present as chemically bonded adsorbed water such as COOH, CHO, OH, etc., expressed as a medium relaxation time component T ₂ = 50 to 400 μsec (Lorentze type), Long relaxation time component T ₂ = 500 to 2000 μsec or more (Lorentze type) and the amount of hydrogen present as physically adsorbed water can be determined and determined by the amount of hydrogen present in carbon in each state. invention as used nonporous carbon or hydrogenated nonporous carbon (hydrogenated nonporous carbon) shows the difference in the bonding state of residual hydrogen in the carbon tissue, ¹ by pulse NMR method Short relaxation time component of T ₂ = 20 to 50 μsec (Gauss type) observed by H resonance, medium relaxation time component of T ₂ = 50 to 400 μsec (Lorentze type), and T ₂ = 500 to 2000 μsec (Lorentze type) When the long relaxation time component is determined, the long relaxation time component is completely absent, and the ratio of the medium relaxation time component to the short relaxation time component is preferably 1/3 or less, and more preferably 1/5 or less. It is more preferable.

  On the other hand, in an electric double layer capacitor test cell prepared using such non-porous charcoal as an electrode, unlike the activated carbon electrode in a conventional capacitor, the interface forming the electric double layer at the beginning of assembly of the electric double layer capacitor is Although there is virtually no, when the applied voltage exceeds a certain threshold during the initial charge, electrolyte ions enter the carbon structure with the solvent (referred to as solvent co-intercalation). Generates an interface to form. Thereafter, this interface is maintained by the hysteresis effect, and a phenomenon that functions effectively as an electric double layer capacitor is recognized. In this way, in the nonporous coal, the voltage at which the electrolyte ions in the electrolytic solution or the electrolyte ions begin to enter the carbon structure of the nonporous coal with the solvent is expressed as “intercalation” in the present invention. Called “starting voltage”.

The intercalation start voltage will vary with the average interlayer distance d ₀₀₂ and electrolytes and the type of the organic solvent constituting the electrolytic solution of nonporous carbon, the larger the average interlayer distance d _002, also the size of the electrolyte ions The smaller the molecular weight (molecular weight / density) of the organic solvent, the lower the intercalation start voltage. The intercalation start voltage is generally about 1.5 to 3.2V. is there. In the present invention, the voltage applied between the carbon electrodes is lower than the intercalation start voltage, and the decomposition voltage of impurities such as water present in the electrolyte is about 1.2 V. In order to complete the process, it is preferable to apply a voltage lower than the intercalation start voltage of the carbon electrode (polarizable electrode) using non-porous charcoal and higher than the water decomposition voltage. In general, it is desirable to set the voltage to about 1.8 to 3.2 V, more preferably about 2.5 to 3.0 V.

The low-potential reactive substance mainly composed of water to be removed in the present invention is caused by, for example, a reaction between water in the electrolyte and BF ₄ ⁻ which is an anion of the electrolyte in addition to water. And impurities such as HF and other fluorinated acids, or impurities originally contained in the electrolyte or organic solvent.

  Further, the present invention is a method for producing an electric double layer capacitor, characterized by using an electrolytic solution from which a low-potential reactive substance mainly composed of water obtained as described above is removed. It is obtained in an electrolytic solution water removing step in which the carbon electrode is brought into contact with the electrolytic solution while applying a voltage lower than an intercalation start voltage between carbon electrodes made using charcoal, and an electrolytic solution water removing step. And an electrolytic solution impregnation step of impregnating the electrolytic solution between the polarizable electrodes of the electric double layer capacitor.

  Furthermore, the present invention provides a plurality of carbon electrodes produced using non-porous charcoal separately from the polarizable electrode constituting the electric double layer capacitor, and an intercalation start voltage of electrolyte ions between the carbon electrodes. The method for producing an electric double layer capacitor comprising the step of contacting the electrolyte while applying a lower voltage and removing the low potential reactive substance in the electrolyte mainly composed of moisture. The electric double layer capacitor to be used is not limited to those produced using the non-porous charcoal used in the present invention as the polarizable electrode, but may be any one produced using conventional activated carbon. This includes all electric double layer capacitors using an aqueous organic electrolyte.

  In the method for removing moisture in the electrolytic solution of the present invention, a voltage lower than the intercalation start voltage is applied between the carbon electrodes made of nonporous charcoal, so that the electrolyte ions in the electrolytic solution are nonporous. Almost no impurities such as moisture are intercalated into the non-porous carbon electrode and retained in the non-porous carbon electrode without intercalating between the carbon structures in the porous carbon electrode. Compared with the adsorption method by the method, the concentration of impurities such as moisture in the electrolytic solution can be made extremely low, and the loss due to contamination and adsorption of the electrolytic solution can be prevented. In addition, an electric double layer capacitor produced using the electrolytic solution from which moisture has been removed according to the present invention has extremely excellent cycle characteristics.

The method for removing moisture in the electrolytic solution of the present invention includes non-porous charcoal or hydrogenated non-porous charcoal (hydrogenated non-porous charcoal) between carbon electrodes (polarizable electrodes) prepared using By applying a voltage higher than the water decomposition voltage (about 1.2 V) and lower than the intercalation start voltage, and contacting the carbon electrode (polarizable electrode) with the electrolytic solution, Impurities such as moisture in the electrolyte solution are removed by intercalating impurities such as moisture in the carbon electrode (polarizable electrode). That is, moisture in the electrolytic solution is intercalated or taken into the carbon electrode (polarizable electrode) in the form of H ⁺ and OH ⁻ , and is an anion of water and the electrolyte in the electrolytic solution, for example, BF ₄ ^−. Impurities such as HF and other fluorinated acids generated by the reaction with the benzene, and impurities originally contained in the electrolyte and organic solvent are also intercalated or taken into the carbon electrode (polarizable electrode). As a result, high purity of the electrolytic solution is achieved.

  The non-porous charcoal or hydrogenated non-porous charcoal (hydrogenated non-porous charcoal) used in the present invention is the following when an electric double layer capacitor test cell is assembled using these non-porous charcoal. Behaves like this. That is, there is substantially no interface forming the electric double layer at the beginning of the cell assembly for the electric double layer capacitor test, but when the applied voltage exceeds a certain threshold at the time of initial charging, the electrolyte ion itself and the fluid that carries this ( The organic solvent as a carrier) and ionic molecules (in the case of a liquid electrolyte alone) enter the carbon structure together (Co-intercalation). At this time, an interface that forms an electric double layer is generated for the first time. Thereafter, this interface is maintained by the hysteresis effect, and until a certain voltage is reached, an electric double layer is formed at this interface, and electrical energy is accumulated.

The carbon electrode (polarizable electrode) used as a negative electrode and a positive electrode used for removing impurities such as water in the electrolyte solution of the present invention is non-porous charcoal or hydrogenated non-porous having the above characteristics. Charcoal (hydrogenated non-porous charcoal) is contained as an electrode active material. This non-porous charcoal or hydrogenated non-porous charcoal (hydrogenated non-porous charcoal) is a carbon material as described in JP-A-2002-25867 and JP-A-2002-362912, Specifically, the average interlayer distance d ₀₀₂ having graphite-like microcrystalline carbon is about 0.350 to 0.380 nm, and the specific surface area by the BET method is 270 m ² / g, preferably 100 m ² / g or less. Non-porous carbon having no pores large enough to incorporate various electrolyte ions, solvents, N ₂ gas, and the like, and is obtained by the following method.

  That is, such non-porous coal or hydrogenated non-porous coal (hydrogenated non-porous coal) includes petroleum coke-based or coal coke-based needle coke, infusible petroleum-based or coal-based pitch, etc. These are obtained by activating the graphitized charcoal obtained by calcination, but these needle cokes used as raw materials, infusibilized pitch, etc. are themselves 300 to 500. This solid material, ie, raw coke, which has been heat-treated in a temperature range of about 0 ° C. and divided into gas, oil and solid material, is pulverized to 120 μm or less to obtain “coking coal”. ”In an inert atmosphere, for example, in a nitrogen atmosphere, at 650 ° C. to 850 ° C., preferably 700 ° C. to 800 ° C., for 2 to 4 hours, and pre-heat treated to obtain“ calcined charcoal ”. Next, the obtained “calcined charcoal” is mixed with caustic such as KOH in a weight ratio of 1.8 to 2.2 times, preferably about 2 times, and again in an inert atmosphere, for example, nitrogen. Under an atmosphere, heating is performed at 800 ° C. to 900 ° C., preferably about 800 ° C. for 2 to 4 hours, activation treatment with caustic alkali is performed, and then the alkali remaining in the carbon is removed as follows.

  The alkali is removed by washing the carbon after the alkali activation obtained. For washing, for example, carbon particles having a size of 1 μm or more are collected from the carbon after the alkali treatment, filled in a stainless steel column, and pressurized steam of 120 to 150 ° C., 10 to 100 kgf, preferably 10 to 50 kgf is introduced into the column. However, it can be carried out by continuing to introduce pressurized steam until the pH of the drainage reaches about 7 (usually 6 to 10 hours). After completion of the alkali removal step, an inert gas such as argon or nitrogen is passed through the column and dried to obtain the desired non-porous charcoal.

For further thorough alkali removal, preferably using a pressurizable Soxhlet extractor, (1) boiling (reflux) an aqueous solution of a volatile acid (eg, HCl, HNO _3, etc.) Residual alkali is extracted from the carbon material powder with acid heating steam having a relatively high concentration. (2) Next, a non-volatile alkaline aqueous solution (for example, NaOH, KOH, etc.) is boiled (refluxed), and with steam, The remaining acid is washed out and neutralized and trapped with an alkali. (3) Thereafter, the obtained carbon material is heated and dried.

If necessary, the nonporous charcoal obtained as described above is further reduced to 500 ° C. to 900 ° C. in a reducing atmosphere such as 3H ₂ + N ₂ mixed gas or hydrogen gas obtained by decomposing NH _3. Heat treatment at 4 ° C. for 4 to 6 hours, or using transition metal or transition metal compound such as Fe, Co, Ni as a catalyst in the presence of these at 200 ° C. to 850 ° C. for 2 to 6 hours in a reducing atmosphere By performing heat treatment, active hydrogen oxide other than hydrogen directly bonded to the carbon skeleton (COOH, CHO, OH, etc.) is removed, and non-porous charcoal (hydrogenated nonporous charcoal) blocked with hydrogen may be obtained. it can.

In the preparation method of the non-porous coal described above, the treatment conditions such as the activation treatment temperature and the heat treatment temperature in a reducing atmosphere affect the average interlayer distance d ₀₀₂ of the obtained non-porous coal, and the treatment temperature is For example, by increasing the activation treatment temperature to about 900 ° C., or by increasing the heat treatment temperature in the reducing airflow to about 700 to 900 ° C., the average interlayer distance d ₀₀₂ is 0.350 to 0.370 nm. You can get something about.

The electrolytic solution that is subject to moisture removal may be any non-aqueous organic electrolytic solution that is usually used in electric double layer capacitors, and the electrolyte salt dissolved in an organic solvent or the liquid electrolyte salt as it is. Examples thereof include those used or those obtained by dissolving the liquid electrolyte salt in an organic solvent. Examples of the electrolyte salt include quaternary alkylammonium boron tetrafluoride, phosphorous hexafluoride or perchlorate. Examples of the quaternary alkylammonium include Me ₄ N ⁺ , Et _n. Examples include Me _(4-n) N ⁺ , Et ₄ N ⁺ , (n-Bu) ₄ N ^{+, and the} like. Examples of organic solvents for dissolving these electrolyte salts include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dimethoxyethane (DME), diethoxyethane (DEE), γ -Butyllactone (γ-BL), acetonitrile (AN), propionitrile (PN), etc., and these organic solvents are used alone or in combination of two or more. The concentration of the electrolyte salt is usually 0. It is adjusted to about 5 mol / L or more.

On the other hand, as the liquid electrolyte, for example, a cation having a hetero 5-membered ring or a hetero 6-membered ring structure substituted with a linear alkyl group, and for example, BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , ClO ₄ ⁻ , CF Examples thereof include electrolyte salts composed of anions such as ₃ SO ₃ ⁻ , AlCl ₄ ⁻ , SbF ₆ ^—, and the like, for example, 1-ethyl-3-methylimidazolium ⁺ • BF ₄ ⁻ (EMI • BF ₄ ), 1-ethyl-3-methylimidazolium ⁺ · PF ₆ ⁻ , 1-methyl-3-n-propylimidazolium ⁺ · BF ₄ ⁻ , 1-methyl-3-n-propylimidazolium ⁺ · PF ₆ ^-, 1,2-dimethyl -3-n-propyl imidazolium ⁺ · _BF ⁴ -, 1,2-dimethyl -3-n-propyl imidazolium ⁺ · _PF ⁶ -, 1,3-dimethyl - -N- propyl imidazolium ⁺ · _BF ⁴ -, 1,3- dimethyl -2-n-propyl imidazolium ⁺ · PF ₆ ^-, etc. are exemplified.

  In addition, since the above liquid electrolyte is liquid at room temperature, it can be used as an electrolyte (so-called neat) as it is, but if it has a high melting point and is solid at room temperature, it can be dissolved in an organic solvent. Is used as an electrolyte solution. Moreover, even if it is liquid at normal temperature, it can be used by dissolving in an organic solvent. As the organic solvent to be used, generally propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dimethoxyethane (DME), diethoxyethane (DEE), γ-butyllactone ( γ-BL), acetonitrile (AN), propionitrile (PN), etc., and these organic solvents are used alone or in combination of two or more. The concentration of the liquid electrolyte salt is usually 1.0 mol. / L or more.

  The intercalation start voltage in a carbon electrode (polarizable electrode) using non-porous charcoal is a carbon electrode in which electrolyte ions are accompanied by a solvent (in the case of a liquid electrolyte that does not use a solvent, the liquid electrolyte itself). This is the voltage when it starts to be inserted (intercalated), and this is the same as the normal charge / discharge test, with the carbon electrode (polarizable electrode) immediately after assembly using non-porous charcoal at a constant current. It is obtained from the charge / discharge test result of the first cycle in which charging is monitored and the voltage rises as the charge accumulates in the capacitor. Examples thereof are shown in FIGS.

Fig. 1 shows that non-porous coal is calcined “coking charcoal” derived from petroleum coke-derived needle coke (coking coal B) at 750 ° C. for 4 hours, at 800 ° C. for 2 hours together with twice the amount of KOH. Prepared non-porous charcoal [B754802 (the meaning of which will be described later)] prepared by treatment, washing with heated steam to pH = 7, and drying under heat and vacuum, and carbon black and PTFE are blended (non- Porous carbon: carbon black: PTFE = 10: 1: 0.5 (weight ratio)), kneaded and formed into a sheet shape (polarizable electrode). The electrode had a diameter of 20 mmφ and a thickness of about 0.2 mm. The electrode is vacuum impregnated with an electrolyte solution, and a roughened aluminum foil is used as a collector electrode, and a vacuum is applied to a laminated plastic bag via “CTW-GA55-CTW” in which GA-55 glass fiber is sandwiched between hard paper as a separator. An electric double layer capacitor test cell was prepared by packing, and the electrolyte was prepared by dissolving 1 mol / L Et ₄ N · BF ₄ in an equal volume mixed solvent of propylene carbonate and acetonitrile and applying a voltage of 3 The result of the initial charge (1st cycle) set to .75V is shown.

  As can be seen from FIG. 1, in the initial charging, the voltage rises in a short period of time, but the voltage rise suddenly slows from about 2.8V. This about 2.8 V corresponds to the intercalation start voltage in the present invention.

In addition, FIG. 2 shows that non-porous charcoal is obtained by calcining petroleum coke-derived needle coke (coking coal B) at 700 ° C. for 4 hours, together with a double amount of KOH at 800 ° C. Treated for 4 hours, washed to pH = 7 with heated steam, further extracted with hydrochloric acid using a Soxhlet extractor, washed with water and dried under heat in vacuum, followed by hydrogenation at 500 ° C. for 4 hours in the presence of a nickel metal catalyst. A non-porous charcoal [B704804S + 504H] prepared by heat-treating in nickel and removing the nickel catalyst was used to prepare a test cell in the same manner as described above, and the electrolyte was EMI • BF ₄ alone, and the voltage was 3. The result of the initial charge (1st cycle) set to 75V is shown. The obtained non-porous charcoal [B704804S + 504H] has a d ₀₀₂ of 0.360 nm, a specific surface area of about 60 m ² / g, and a ratio of a medium relaxation time component to a short relaxation time component (hereinafter simply referred to as “relaxation component ratio”). Was 0.05 or less.

  According to FIG. 2, it can be seen that in the initial charging, the voltage initially rises in a short time, and the voltage rise suddenly slows from about 2.9V. This about 2.9V corresponds to the intercalation start voltage in the present invention.

Further, FIG. 3 shows a test cell made of non-porous coal [A704804 + 504H] prepared using infusible petroleum pitch-based carbon (raw coal A) as non-porous coal, and EMI was used as an electrolyte. The results of initial charging (first cycle) in which an electrolyte solution in which BF ₄ and an organic solvent are mixed at a volume ratio of 1: 1 are used as the electrolyte and the voltage is set to 3.5 V are shown in the figure. (A) shows the case where the organic solvent is propylene carbonate (PC), and (b) shows the case where the organic solvent is acetonitrile (AN). The obtained non-porous charcoal [A704804 + 504H] had d ₀₀₂ of 0.360 nm, a specific surface area of about 100 m ² / g, and a relaxation component ratio of 0.17 or less.

  According to FIG. 3, in the case of using an organic solvent, in both (a) and (b), at the initial charging, the voltage initially rises in a short time and starts from about 2.4 V and about 2.2 V, respectively. It can be seen that the voltage rise suddenly slows down. These about 2.4 V and about 2.2 V correspond to the intercalation start voltage in the present invention.

  Thus, when an organic solvent is used, an organic solvent having a small molecular volume is inserted (intercalated) into a carbon electrode (polarizable electrode) using non-porous charcoal along with electrolyte ions. Therefore, the intercalation start voltage is lower than that in the case of the liquid electrolyte alone, and the molecular volume of the organic solvent used together with the electrolyte salt (PC: 70.90, AN: 52. 48) that the intercalation start voltage changes.

As described above, the intercalation start voltage can be obtained, and this intercalation start voltage is determined based on the average interlayer distance d ₀₀₂ of the used non-porous coal and the composition of the electrolyte (electrolyte salt or organic solvent). Depending on the type). This relationship is shown in FIGS.

FIG. 4 shows non-porous coal obtained by activating various petroleum-based needle cokes or infusible petroleum pitch-based carbon under various treatment conditions (800 to 900 ° C., 2 to 4 hours), and Hydrogenated non-porous carbon obtained by post-heat-treating these non-porous charcoal in hydrogen under various treatment conditions (500 to 900 ° C., 4 to 6 hours) was subjected to an average interlayer distance d ₀₀₂ by XRD measurement. The results of measurement of the intercalation start voltage when the test cell is assembled using these non-porous carbons or hydrogenated non-porous carbons are shown. The electrolytic solution is a 1 mol / L propylene carbonate solution of Et ₄ N · BF ₄ . As is apparent from FIG. 4, it can be seen that the intercalation is started at a higher voltage as the average interlayer distance _d002 is narrower.

In FIG. 5, the horizontal axis represents the molecular volume of the organic solvent (molecular weight / specific gravity), and the intercalation start voltage in a test cell using nonporous carbon or hydrogenated nonporous carbon is plotted. is there. In FIG. 5, the non-porous charcoal is non-porous charcoal [F754804] (average interlayer distance d ₀₀₂ is 0.370 nm) prepared using petroleum-based needle coke F, and hydrogenated non-porous charcoal. [F754804 + 654H] (average interlayer distance d ₀₀₂ is 0.364 nm), and as an electrolyte, an equivalent volume mixed solvent of acetonitrile (AN), ethylene carbonate / diethyl carbonate (volume ratio, 1: 1), γ-butyrolactone ( GBL) and shows the measurement results in the case of using an electrolyte prepared by dissolving at a concentration of 1 mol / L of Et _{4 N} · BF 4 in the organic solvent of propylene carbonate (PC). According to FIG. 5, the intercalation start voltage is inversely proportional to the molecular volume of the solvent molecules used for the electrodes having the same average interlayer distance and the same electrolyte ions, and the intercalation is increased as the molecular volume is smaller. It can be seen that the start voltage of the operation decreases. In addition, when an organic solvent is mixed and used, it can be understood from an example of a mixed solvent of ethylene carbonate (EC: molecular volume 68.7) and diethyl carbonate (DEC: molecular volume 121.2) in a volume ratio of 1: 1. In addition, it is understood that an organic solvent having a small molecular volume dominates the intercalation onset voltage in the case of a mixed solvent since it is linear when plotted with the molecular volume value of ethylene carbonate having a small molecular volume.

Further, FIG. 6 shows a case where a liquid electrolyte is used. The non-porous coal used is non-porous coal [B704804 + 504H] (average interlayer distance d ₀₀₂ is 0.360 nm) prepared using needle coke derived from petroleum coke (coking coal B) as the raw coal. assembling the test cell using, as an electrolytic solution, in the case of using EMI · BF ₄ alone, the EMI · BF _4, propylene carbonate (PC), .gamma.-butyrolactone (GBL), ethylene carbonate (EC) and The result of having calculated | required the intercalation start voltage at the time of using the electrolyte solution melt | dissolved in acetonitrile (AN) by the equal volume ratio, respectively is shown. Incidentally, EMI ⁺ molecular volume is the molecular volume of the salt itself EMI ⁺ ions and BF ₄ ^- and is calculated by prorated to the ratio of the van der Waals volume of ions (respectively, 118A ³ and 48 Å ³⁾ .

  According to FIG. 6, it can be seen that, even when a liquid electrolyte is used, the intercalation start voltage decreases in the same manner by dissolving in an organic solvent and the smaller the molecular volume of the organic solvent.

As described above, the intercalation start voltage is determined by the average interlayer distance d ₀₀₂ of the nonporous coal if the composition of the electrolytic solution to be treated is determined. The start voltage is about 1.5 to 3.2V.

In such non-porous charcoal, when using, for example, Et ₄ N · BF ₄ , Et ₃ MeN · BF ₄ , EMI · BF ₄ or the like as a supporting electrolyte in an organic solvent at a low potential, a specific voltage ( At a potential equal to or lower than the intercalation start voltage), ions are not inserted (intercalated) into the carbon electrode, and electrolyte ions are not adsorbed. This intercalation start voltage is determined by the average interlayer distance d ₀₀₂ determined by the raw coal used and its heat treatment conditions, and the solvent (solvent associated with the electrolyte ions when the electrolyte ions intercalate with the solvent as the carrier. In the case of a liquid electrolyte that does not use, it depends on the molecular volume (molecular weight / density) of the liquid electrolyte itself. At this time, in the case of the mixed solvent, the solvent having the smallest molecular volume among the constituent solvents intercalates with the electrolyte ions, so that the intercalation start voltage is the smallest molecular volume in the mixed solvent. Will depend on the solvent having

That is, if the composition of the electrolyte (the type of electrolyte and the composition of the organic solvent) is determined, non-porous coal having an average interlayer distance d ₀₀₂ can be selected, and the intercalation start voltage is set to 2 to 3 V. Can be set in the range.

On the other hand, water, which is an impurity in the electrolytic solution, has a decomposition voltage of 1.2 V. When a voltage higher than this is applied, it decomposes and enters the carbon electrode and is removed from the electrolytic solution. This is because, for example, an electric double layer capacitor test cell using non-porous charcoal as an electrode is constructed, charging is started at a constant current, and a voltage equal to or lower than the intercalation start voltage is applied as the upper limit. It can be understood from the “decay curve”. 7 and 8, respectively, the applied voltage is set to 2.5 V and 1.5 V, which are voltages equal to or lower than the intercalation start voltage, and a constant current of 10 mA (the current density is 3.5 because the electrode diameter is 20 mm). 18 shows the results of measuring changes in voltage and current over time when charged with a current of 18 mA / cm ² .

  According to FIG. 7 and FIG. 8, when charging is started at a constant current and the set voltage is reached, the voltage is maintained to enter a low voltage charging mode (relaxation charging), and the current is attenuated. The attenuation is a curve that gradually decreases. Assuming that all the carbon composition in the electrode is ideally uniform, this indicates that there are carriers that carry current, and that when such carriers run out, the current gradually decays. Such a carrier corresponds to an impurity such as water present in the electrolytic solution. Since a voltage higher than the decomposition voltage of water is applied between the electrodes, water decomposes or is taken into a carbon electrode (polarizable electrode) using nonporous charcoal in the water state. As a result of the exhaustion of impurities such as water in the electrolyte, there are no carriers carrying current, the current value attenuates, and settles to a substantially constant value. Actually, the carbon structure in the electrode is not ideally uniform, and the carbon composition of the electrode is not uniform. Therefore, in some carbons, the potential (rising) propagation is via the capacitor capacitance. Therefore, the charging process of the multi-stage CR circuit is performed, and a characteristic in which a current similar to the above is gradually attenuated is generated. Therefore, although it cannot be determined only from this current attenuation characteristic, the current always converges to 0 when charging such a multi-stage CR circuit, so that the electrochemical reaction such as moisture occurs when 0 convergence is abnormally slow. It can be judged that there is a source.

  Since the applied voltage is smaller than the intercalation start voltage, a phenomenon in which electrolyte ions are inserted (intercalated) into a carbon electrode (polarizable electrode) using nonporous charcoal. Does not happen.

  In this way, impurities such as water in the electrolytic solution are selectively removed. As a result, the treated electrolytic solution has very little impurities such as water and is so-called water-free.

  In the method of removing impurities such as moisture in the electrolytic solution of the present invention, first, the above-mentioned nonporous charcoal or hydrogenated nonporous charcoal is prepared, and a carbon electrode (polarizable electrode) is produced using this. And it can achieve by making electrolyte solution contact, applying the voltage lower than the intercalation start voltage between this carbon electrode (polarizable electrode), preferably higher than the decomposition voltage of water. Of course, a voltage may be applied between the carbon electrodes after contacting the electrolytic solution, or a voltage may be applied between the carbon electrodes and then the electrolytic solution may be contacted.

  Non-porous charcoal or hydrogenated non-porous charcoal can be prepared as described above, specifically as described in JP-A No. 2002-25867 and JP-A No. 2002-362912. Further, the carbon electrode (polarizable electrode) can be produced by the same method as that for a normal polarizable electrode for an electric double layer capacitor using activated carbon or the like. For example, in order to produce a sheet-like electrode, after non-porous charcoal or hydrogenated non-porous charcoal is pulverized and the particle size is adjusted, for example, carbon black as a conductive auxiliary agent and, eg, Tetrafluoroethylene (PTFE) can be added and kneaded and formed into a sheet by pressure drawing.

  FIG. 9 is a basic conceptual diagram for carrying out the method for removing impurities such as moisture in the electrolytic solution of the present invention, in which 1 is a container, 2 and 3 are non-porous charcoal or hydrogenated. Carbon electrodes (polarizable electrodes) using nonporous charcoal, 4 and 5 are an inlet and an outlet for electrolyte solution, respectively, and 6 is a flow path control member for controlling the flow of the electrolyte solution in the container. Is shown. For example, a simple aluminum laminate bag is used as the container 1, carbon electrodes (polarizable electrodes) 2 and 3 are provided therein, and a voltage of about 1.2 to 2.5 V is applied between the carbon electrodes. And electrolyte solution is inject | poured with a syringe from the electrolyte solution injection port 4. FIG. The injected electrolyte is a carbon electrode inside the container as indicated by an arrow by a flow path control member 6 (which can be achieved simply by partially clamping an aluminum laminate bag or partially heat-sealing). Effectively in contact with the entire area, it is taken out from the discharge port 5 as an electrolytic solution from which impurities such as moisture have been removed. Whether or not impurities such as moisture have been removed can be confirmed by monitoring the current flowing between the electrodes.

  That is, as an apparatus for removing impurities such as moisture in the electrolytic solution, a container (electrolytic treatment tank) including a carbon electrode (polarizable electrode) using non-porous charcoal or hydrogenated non-porous charcoal, The liquid supply means includes a liquid supply pump, but it is simple and preferable to use a pressurized reservoir of dry argon gas or helium gas with little contamination. FIG. 10 shows an outline of an apparatus configured using a pressurized reservoir using such argon gas as an example. In FIG. 10, 11 is a gas drying column, 12 is a pressure reservoir, 13 is a check valve, 14 is an electrolytic treatment tank, and 15 and 16 are terminals for application to a carbon electrode (polarizable electrode). The electrolyte solution in the pressurized reservoir 12 is guided to the electrolytic treatment tank 14 by the argon gas, and is applied to a non-porous charcoal or hydrogenated to which a voltage lower than the intercalation start voltage and higher than the water decomposition voltage is applied. Impurities such as water in the electrolytic solution are removed by a carbon electrode (polarizable electrode) using non-porous charcoal, and a so-called water-free electrolytic solution is obtained.

  In addition, the carbon electrode (polarizable electrode) used for removing impurities such as water in the electrolytic solution as described above is washed with a volatile solvent, for example, a volatile solvent such as acetonitrile or isopropanol, It is regenerated by drying and can be reused. For washing and drying, for example, according to the principle of a Soxhlet extractor, acetonitrile is boiled, liquefied by a cooling capacitor provided at the top, and liquefied and dropped onto a carbon electrode (polarizable electrode) storage container provided in the middle. In this way, it is possible to remove impurities of non-volatile components by constantly washing the electrode with acetonitrile purified by distillation and draining impurities and electrolytes contained in the carbon structure of the electrode into the lower container. Thereafter, the polarizable electrode (carbon electrode) is dried by heating, further vacuum dried at about 200 to 250 ° C., and the chemically adsorbed water is removed to regenerate the carbon electrode (polarizable electrode).

  The method for producing an electric double layer capacitor of the present invention is characterized by using the electrolytic solution obtained by removing impurities such as moisture obtained as described above. The electric double layer capacitor itself can be manufactured by a known method, and the electric double layer capacitor to be manufactured may be of any type such as a so-called button type or multilayer type. Needless to say, the so-called water-free electrolyte from which impurities such as water obtained in the present invention have been removed is not limited to an electric double layer capacitor but can be applied to a battery using a non-aqueous electrolyte. .

  Next, the present invention will be described in more detail with reference to examples.

Example 1
First, non-porous charcoal (hydrogenated non-porous charcoal) was prepared by the method described in JP-A-2002-25867 and JP-A-2002-362912. That is, “coking charcoal” obtained by calcining needle coke (coking coal B) derived from petroleum coke pulverized to 10 to 240 μm at 700 ° C. for 4 hours is treated with double amount of KOH at 800 ° C. for 2 hours. , Washed with heated steam to pH = 7 and dried, then heat treated in hydrogen at 700 ° C. for 4 hours in the presence of a nickel metal catalyst to remove the nickel catalyst, and non-porous charcoal (hydrogenated non-porous charcoal) [ B704802 + 704H] was prepared. Next, carbon black and PTFE were blended with the obtained non-porous charcoal (hydrogenated non-porous charcoal) (non-porous charcoal: carbon black: PTFE = 10: 1: 0.5 (weight ratio)). After being kneaded and formed into a sheet having a thickness of about 0.2 mm, it was cut into 20 mmφ with a punching jig to obtain a carbon electrode (polarizable electrode). The average interlayer distance _{d 002} of the nonporous carbon (hydrogenated nonporous carbon) is 0.364Nm, specific surface area was ^68m 2 / g, relaxation component ratio was 0.12 . Moreover, the intercalation start voltage in the electrolyte solution used below was 2.9V.

  Next, this carbon electrode (polarizable electrode) was inserted and heat-sealed to form two electrode parts “R” and “Pre” using an aluminum laminate bag as shown in FIG. A cell was prepared. FIG. 11 shows an outline of a test cell used for removing impurities such as moisture in the electrolytic solution, and “R” in the figure is an electrode of a product to be an electric double layer capacitor. Polar electrode (2 ', 3'), "Pre" is an electrode for trapping impurities such as water, that is, a carbon electrode (polarizable electrode) for removing impurities such as moisture in the electrolytic solution of the present invention ) (2, 3). The electrodes are inserted into a laminated plastic bag via “CTW-GA55-CTW” in which each electrode is made of roughened aluminum foil as a collecting electrode and GA-55 glass fiber is sandwiched between hard paper as a separator. After that, hold it with a strong permanent magnet and iron plate from the outside of the aluminum laminate bag to maintain the "electrode stacking accuracy", and heat seal the electrode insertion part to the aluminum laminate bag with an impulse heater from the outside of the bag. And sealed.

Next, the electrolyte (EMI • BF ₄ , 1.5 ml) is injected with a syringe through the sealing end of the “Pre” part with a syringe, injected into the bag, and the injection port is sealed with heat sealing. The inside of the aluminum laminate bag was a complete sealing system. After injection of the electrolytic solution, the “electrode stacking accuracy” is maintained by “wetting” of the electrolytic solution, so that pressing with a permanent magnet and an iron plate is not necessary.

  Then, the aluminum laminate bag is placed with the “R” part at the top and the “Pre” part at the bottom, and the electrolyte solution in the aluminum laminate bag is collected in the “Pre” part. While applying a voltage of 2.0 V between the electrodes, the application was continued until the current converged to almost zero. As a result, the water present in the above electrolyte solution, the water inevitably infiltrated during electrode preparation, the fluorine-containing acid produced by the reaction between the water and the electrolyte, or a low molecular weight organic solvent or electrolyte that is easily electrolyzed Such impurities migrate in the electrolytic solution and accumulate in the carbon electrode of the “Pre” part, and impurities such as water in the electrolytic solution are removed.

  Thereafter, the electrolyte solution in the aluminum laminate bag is moved to the “R” portion with the “R” portion facing downward. Thereafter, an initial charge (charge in the first cycle, 4.0 V) is performed in order to generate capacitance in the electric double layer capacitor using non-porous charcoal by the electrolytic solution from which impurities such as water are removed ( If there is gas etc. generated at this time, it is collected in the upper part of the aluminum laminate bag, “Pre” part), and then the center part of the dotted line of the aluminum laminate is heat sealed to remove the carbon electrode part of the “Pre” part. Thus, an electric double layer capacitor composed of the polarizable electrode of the “R” part is obtained. The electrostatic capacity at the time of initial charge was 52.2 F / cc.

Next, a cycle test was performed on the obtained electric double layer capacitor. In the cycle test, as the initial charge, the charge in the first cycle is 10 mA (current density is 3.18 mA / cm ² ) up to 4.0 V, the relaxation charge time is increased (about 5000 seconds or more), and then Discharge to 0V at 10mA, complete the initial charge, charge to 10mA at the same or slightly lower voltage than the initial voltage, and after appropriate moderate charge (about 2000 to 3000 seconds), discharge to 0V at 10mA, This is performed by an automatic measurement method that repeats the specified number of times. The set voltage is 3.5 V for 2 to 10 cycles, 3.0 V for 11 to 500 cycles, and 501 to 1000 cycles. The result in the case of 3.5V is shown in FIG. The capacitance is a value calculated from Q = (1/2) × CV ² using values from the start of discharge to 50% of the total discharge current, and converted per volume when the positive electrode and negative electricity are dried. is there.

  FIG. 12 shows the change in capacitance with respect to the number of cycles, but it can be seen that good linearity on the log scale is obtained. In the conventional cycle test, such linearity was not obtained, and the deterioration in performance was multifactorial and often decreased greatly. That is, by removing impurities such as moisture in the electrolytic solution, the reaction between water and the electrolytic solution or carbon material is suppressed, performance deterioration is small, and an electric double layer capacitor exhibiting excellent cycle characteristics is obtained. I understand.

  In addition, according to the present invention, the influence of impurities such as water in the electrolytic solution can be excluded, so that the cause of an essential part of performance deterioration can be investigated.

The method for removing impurities such as water in the electrolytic solution of the present invention is summarized as follows. The non-porous charcoal or hydrogenated non-porous charcoal used in the present invention itself is one that chemically adsorbs water to form COOH, CHO, OH, etc. (active hydrogen oxide). Has the ability to adsorb. Therefore, a certain degree of moisture removal effect can be expected without applying a voltage as in the present invention. However, when no voltage was applied, water in the electrolyte could not be completely removed. This is because the electrolyte containing ions has a very high affinity with water. Here, when an electric field is applied, water first migrates by Coulomb force, and when a voltage higher than the decomposition voltage is applied to the electrode surface, it is taken into the non-porous coal in the form of H ⁺ and OH ^−. It will be.

  For this reason, the present invention has (1) a completely sealed system, and the electrolyte to be used is purified, so there is no risk of contamination during the work process. (2) Any added products to the product (3) Unlike the simple adsorption method, the selectivity by the electric field is taken into account, so the purification is done thoroughly. (4) The necessary electrolyte is adsorbed. (5) The carbon electrode (polarizable electrode) used for moisture removal is regenerated by being washed and dried with a volatile solvent and can be reused. Is.

It is a diagram showing the intercalation start voltage in the case of using 1 mol / L of Et _{4 N} · BF 4 in propylene carbonate / acetonitrile volume mixture as the electrolyte. Is a diagram showing the intercalation start voltage when using EMI · BF ₄ as the electrolyte. Volume ratio of the organic solvent EMI · BF ₄ as an electrolytic solution 1: a diagram showing the intercalation start voltage in the case of using mixed electrolyte solution in 1, in the drawing, (a) represents the organic solvent is propylene carbonate In the case of (PC), (b) shows the case where the organic solvent is acetonitrile (AN). Is a graph showing the relationship between the average interlayer distance d ₀₀₂ and intercalation start voltage of nonporous carbon or hydrogenated nonporous carbon. As the electrolytic solution, in the case of using a 1 mol / L solution of Et ₄ N _· BF 4, is a graph showing the relationship between the molecular volume and intercalation start voltage of each organic solvent. Graph as an electrolytic solution, in the case of using the electrolyte prepared by dissolving alone and EMI · BF ₄ and EMI · BF ₄ in an organic solvent equal volume, the relationship between the molecular volume and intercalation start voltage of each organic solvent It is. It is a figure which shows the time-dependent change of an electric current and a voltage when a voltage of 2.5V is applied to the carbon electrode (polarizable electrode) using non-porous charcoal. It is a figure which shows the time-dependent change of an electric current and a voltage when a voltage of 1.5V is applied to the carbon electrode (polarizable electrode) using nonporous charcoal. It is a basic conceptual diagram for implementing the method of removing impurities, such as a water | moisture content, in electrolyte solution. It is a figure which shows an example of the apparatus for implementing the method of removing impurities, such as a water | moisture content, in electrolyte solution. In an Example, it is a figure which shows the outline | summary of the aluminum laminated bag used for the removal of the water | moisture content in electrolyte solution, and preparation of an electrical double layer capacitor. It is a graph which shows the result of the cycle test of the electric double layer capacitor of this invention.

Explanation of symbols

1 container 2, 3 carbon electrode (polarizable electrode)
2 ', 3' Polarized electrode 4 Inlet 5 Outlet 6 Flow path control member 11 Gas drying column 12 Pressurized reservoir 13 Check valve 14 Electrolytic treatment tanks 15 and 16 Terminals

Claims (4)

Electrolysis characterized by bringing the carbon electrode into contact with an electrolytic solution while applying a voltage lower than the intercalation start voltage of electrolyte ions between a plurality of carbon electrodes made of non-porous charcoal. Liquid purification method. In an electric double layer capacitor using an organic electrolyte, an electrolyte solution is brought into contact while applying a voltage lower than the intercalation start voltage of the electrolyte ion between a plurality of carbon electrodes previously prepared using nonporous charcoal. A method of removing a low potential reactive substance in an electrolyte mainly composed of moisture. While applying a voltage lower than the intercalation start voltage of the electrolyte ions between the carbon electrodes produced using non-porous charcoal, the carbon electrode and the electrolytic solution are brought into contact with each other, and the electrolytic solution mainly composed of moisture A method for producing an electric double layer capacitor, comprising: obtaining an electrolytic solution from which a low potential reactive substance is removed, and using the electrolytic solution as an electrolytic solution of the electric double layer capacitor. Separately from the polarizable electrode, a plurality of carbon electrodes made of non-porous charcoal are provided, and the electrolyte solution is contacted while applying a voltage lower than the electrolyte ion intercalation starting voltage between the carbon electrodes. And a method for producing an electric double layer capacitor, comprising a step of removing a low potential reactive substance in an electrolytic solution mainly containing moisture.

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