Electrochemical Devices

Abstract

This electrochemical device is capable of suppressing deterioration of floating characteristics while increasing capacitance. The electrochemical device is provided with a pair of electrodes and an electrolytic solution. At least one of the pair of electrodes contains porous carbon particles. Regarding the pore distribution of the porous carbon particles, the cumulative volume B of pores having a pore size of 20-60 Å is 0.15 cm3 / g or more, and the cumulative volume C of pores having a pore size larger than 60 Å but not larger than 500 Å is 0.25 cm3 / g or less.

Description

[Technical field]

[0001] The present invention relates to an electrochemical device comprising an electrode comprising porous carbon particles. [Background technology]

[0002] The electrochemical device includes a pair of electrodes and an electrolyte, and at least one of the pair of electrodes contains an active material capable of adsorbing and desorbing ions. An electric double layer capacitor, which is an example of an electrochemical device, has a longer life, is capable of rapid charging, and has superior output characteristics compared to a secondary battery, and is widely used as a backup power source, etc.

[0003] For example, porous carbon particles (activated carbon) obtained by carbonizing and activating raw materials such as coconut shells are used as active materials for electrochemical devices. Various studies have been conducted on activated carbon. For example, Patent Document 1 proposes that the total surface functional group density D in the average cross section of the pores of activated carbon be set to a specific range. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6357639 specification Summary of the Invention

[0005] Depending on the pore distribution of the porous carbon particles, the capacity may become small and the float characteristics may be easily deteriorated. The relationship between the pore distribution of the porous carbon particles and the performance of the electrochemical device has not yet been sufficiently investigated.

[0006] In view of the above, one aspect of the present invention provides a method for producing a porous carbon particle-containing electrolyte comprising the steps of: providing a porous carbon particle-containing electrolyte having a pore size distribution of 20 Å or more and 60 Å or less; and 3 / g or more, and the cumulative volume C of pores having a pore diameter of more than 60 Å and not more than 500 Å is 0.25 cm 3 / g or less.

[0007] According to the present invention, it is possible to increase the capacity of an electrochemical device while suppressing the deterioration of the float characteristics. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a partially cutaway perspective view of an electrochemical device according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing the pore distribution of porous carbon particles contained in the electrodes of the electrochemical devices of Example 1 and Comparative Examples 1 and 2 of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] An electrochemical device according to one embodiment of the present invention comprises a pair of electrodes and an electrolyte. At least one of the pair of electrodes contains porous carbon particles. When ions are adsorbed to the porous carbon particles in the electrolyte, an electric double layer is formed, and capacitance is generated. When ions are desorbed from the porous carbon particles, a non-Faradic current flows. The electrodes included in the electrochemical device according to this embodiment utilize this phenomenon. In the pore distribution of the porous carbon particles, the cumulative volume B of pores having a pore diameter of 20 Å or more and 60 Å or less (hereinafter also referred to as mesopores) is 0.15 cm 3 / g or more, and the cumulative volume C of pores having a pore diameter of more than 60 Å and not more than 500 Å (hereinafter also referred to as macropores) is 0.25 cm 3 / g or less.

[0010] The cumulative volume B above is 0.15 cm 3When the capacitance C is 0.25 cm3 / g or more, an electrochemical device having a large capacity (initial capacity) and excellent float characteristics can be obtained even in a low temperature environment. The float characteristics are an index of the degree of deterioration of an electrochemical device when float charging is performed by maintaining a constant voltage using an external DC power source. The smaller the capacity loss during float charging and the more suppressed the increase in internal resistance, the better the float characteristics can be said to be. However, when the cumulative volume C is 0.25 cm3 or more, the above-mentioned 3 If it is larger than / g, the proportion of macropores will be large, the electrode density will be small, and the capacity may decrease.

[0011] Mesopores mainly contribute to the mobility of ions in the electrolyte within the pores, and mainly affect the float characteristics and internal resistance. Mesopores also contribute to the specific surface area of ​​the porous carbon particles, and affect the capacity (initial capacity). When the pore diameter is 20 Å or more, ions in the electrolyte are easily diffused within the pores, and the pores are less likely to become clogged. In pores with a diameter of 20 Å or more, good ion movement is ensured even at low temperatures. When the pore diameter is 60 Å or less, it is easy to increase the specific surface area and obtain a large initial capacity.

[0012] The cumulative volume B above is, for example, 0.15 cm 3 / g or more, 0.35m 3 / g or less, and 3 / g or more, 0.30cm 3 From the viewpoint of further increasing the capacity, the cumulative volume C may be 0.15 cm 3 / g or less.

[0013] In the pore distribution of the porous carbon particles, the ratio B / A of the cumulative volume B to the cumulative volume A of pores having a pore diameter of 10 Å or more and less than 20 Å (hereinafter also referred to as micropores) may be 0.5 or more, 0.5 or more and 0.65 or less, or 0.5 or more and 0.6 or less. When B / A is within the above range, many mesopores are secured together with micropores, and large capacity is obtained while further improving float characteristics. Micropores mainly contribute to the specific surface area and tend to mainly affect the capacity (especially the initial capacity).

[0014] In the pore distribution of the porous carbon particles, the total ratio of the integrated volumes A and B to the total pore volume (the sum of the volumes of all pores in the range of 10 Å to 3000 Å) is preferably, for example, 60% to 85%. In this case, a large number of micropores and mesopores are distributed, and large capacity and excellent float characteristics are easily obtained.

[0015] The above-mentioned cumulative volumes A to C can be determined by disassembling a completely discharged unused or initial electrochemical device to remove the electrodes, peeling off the active layer from the current collector and pulverizing it, heating and drying the pulverized material at 160°C to obtain a sample (particle group), and measuring the pore distribution of the sample.

[0016] The pore distribution is measured by a gas adsorption method using nitrogen gas. For example, an automatic specific surface area / pore distribution measuring device "Tristar II 3020" manufactured by Shimadzu Corporation is used as the measuring device. Note that, in order to remove impurities, the sample is pretreated by heating and evacuating to a vacuum (for example, 250°C and 50 mTorr or less) before the measurement. The BJH method (Barrett-Joyner-Halenda method) is used to analyze the pore distribution, and the Harkins & Jura formula is used in the BJH method. The cumulative pore volume distribution obtained by the BJH method is used to calculate the total volume (cm) of micropores, mesopores, and macropores per 1 g of the above sample. 3 ) are calculated as the above accumulated volumes A to C, respectively.

[0017] In addition, the above sample may contain a binder and a conductive agent in addition to the porous carbon particles, but the amount of the binder, etc. is small, and the effect on the pore distribution of the porous carbon particles is small. The shape of the cumulative pore volume distribution curve is almost the same as in the case of only porous carbon particles, and the cumulative pore volume distribution curve is only slightly shifted downward (the cumulative volume is slightly smaller) compared to the case of only porous carbon particles.

[0018] In the log differential pore volume distribution of porous carbon particles, the log differential pore volume V when the pore diameter is 20 Å is 20 (Hereafter, simply V 20) is 0.5 cm 3 / g Å or more and the log differential pore volume V when the pore diameter is 60 Å 60 (Hereafter, simply V 60 Also called.) is 0.3 cm 3 / g Å or less is preferable. The log differential pore volume distribution is a pore distribution curve with the pore diameter D on the horizontal axis and the log differential pore volume expressed as dVp / d(logD) on the vertical axis. Vp is the pore volume per unit mass. The log differential pore volume distribution is obtained from the data of the cumulative pore volume distribution obtained above.

[0019] V 20 and V 60 When V is within the above range, the number of mesopores is likely to increase and the number of macropores is likely to decrease, in addition to the number of micropores. Therefore, it is easy to obtain a large capacity and excellent float characteristics in a low temperature environment. 20 and V 60 When is within the above range, the log differential pore volume distribution (pore distribution curve) may have a region in which the log differential pore volume decreases as the pore diameter increases in the range of pore diameters from 20 Å to 60 Å, and the tangent at any point within the range has a certain degree of inclination. As in the log differential pore volume distribution of x1 shown in FIG. 2, it is preferable that the tangent has a certain degree of inclination on the side of the pore diameter closer to 60 Å within the above range. If the above region shifts to the side of a pore diameter larger than 60 Å, the number of macropores increases, and the electrode density may decrease. If the above region shifts to the side of a pore diameter smaller than 20 Å, the number of mesopores decreases, and the float characteristics may decrease.

[0020] V 20 is 0.5 cm 3 / g·Å or more, 2.0cm 3 / g Å or less, and 1.0 cm 3 / g·Å or more, 1.5cm 3 / g Å or less. V 60 is 0.05 cm 3 / g·Å or more, 0.3cm 3 / g Å or less, and 3 / g Å or less.

[0021] V 20 and V 60 Difference:V 20 -V 60 But 0.5 cm 3 / g·Å or more. In this case, the absolute value of the slope of the tangent line is large, the proportion of macropores is likely to be small, and the electrode density is likely to be large. As shown in the log differential pore volume distribution of x1 in FIG. 2, it is preferable that the absolute value of the slope of the tangent line is large on the side of the pore diameter close to 60 Å within the above range. In this case, there are more mesopores and fewer macropores, and a large capacity can be obtained in a low-temperature environment while the float characteristics are further improved. V 20 -V 60 is 0.5 cm 3 / g·Å or more, 1.5cm 3 / g Å or less, and 3 / g·Å or more, 1.0cm 3 / g Å or less.

[0022] The log differential pore volume distribution may have one peak (maximum value of the log differential pore volume) in the pore diameter range of 10 Å or more and 20 Å or less. 20 , V 60 , and V 20 -V 60 It is easy to control the diameter of the porous membrane within the above range, and it is easy to ensure a large number of mesopores as well as micropores.

[0023] The porous carbon particles can be produced, for example, by carbonizing the raw material through heat treatment, and activating the resulting carbonized material to make it porous. Examples of the raw material include wood, coconut shells, pulp waste liquid, coal or coal-based pitch obtained by thermal decomposition thereof, heavy oil or petroleum-based pitch obtained by thermal decomposition thereof, phenolic resin, petroleum coke, and coal coke. Examples of the activation treatment include gas activation using gas such as steam, and chemical activation using alkali such as potassium hydroxide. The porous carbon particles obtained by the activation treatment may be subjected to a pulverization treatment. After the pulverization treatment, a classification treatment may be performed. For example, a ball mill, a jet mill, or the like is used for the pulverization treatment.

[0024] The porous carbon particles can be obtained, for example, by heat-treating coconut shells, pulverizing and sieving the resulting carbonized material, and activating the material. Alternatively, the porous carbon particles can be obtained, for example, by adding a binder such as coal tar or pitch to finely pulverized coal, kneading the mixture, compression-molding the mixture, pulverizing and sieving the molded mixture, heat-treating the pulverized mixture, and activating the mixture.

[0025] The pore distribution of the porous carbon particles can be adjusted by the raw material, the heat treatment temperature, the activation temperature in gas activation, the degree of pulverization, etc. One type of porous carbon particles may be used alone, or two or more types may be used in combination.

[0026] At least one of the pair of electrodes may include an active layer and a current collector supporting the active layer. The active layer includes at least the porous carbon particles described above as an active material. The active layer may include a mixture (mixture) of the porous carbon particles and a small amount of a binder and / or a conductive agent. The proportion of the porous carbon particles in the active layer (mixture) is, for example, 88 mass % or more.

[0027] Examples of the binder include resin materials such as polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC) (including alkali metal salts and ammonium salts of CMC), and styrene-butadiene rubber (SBR). Examples of the conductive agent include carbon black such as acetylene black.

[0028] The above-mentioned electrode can be obtained, for example, by applying a slurry containing porous carbon particles, a binder and / or a conductive agent, and a dispersion medium to the surface of a current collector, drying the coating, and rolling it to form an active layer. For the current collector, for example, a metal foil such as an aluminum foil is used.

[0029] Examples of electrochemical devices include electric double layer capacitors (EDLCs) and lithium ion capacitors (LICs). When the electrochemical device is an EDLC, an electrode containing the above-mentioned porous carbon particles can be used for at least one of a pair of electrodes. When the electrochemical device is a LIC, an electrode containing the above-mentioned porous carbon particles can be used for one of the pair of electrodes (positive electrode), and a negative electrode used in a lithium ion secondary battery can be used for the other of the pair of electrodes (negative electrode). The negative electrode used in a lithium ion secondary battery contains, for example, a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

[0030] The electrolyte contains a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and contains a cation and an anion. The ionic substance may contain a low-melting-point compound (ionic liquid) that can exist as a liquid at, for example, around room temperature. The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol / L or more, 2.0 mol / L or more. below It is.

[0031] The solvent is preferably a high boiling point solvent, for example, lactones such as γ-butyrolactone, carbonates such as propylene carbonate, polyhydric alcohols such as ethylene glycol and propylene glycol, cyclic sulfones such as sulfolane, amides such as N-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methyl acetate, ethers such as 1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde can be used.

[0032] Ionic substances include, for example, organic salts. An organic salt is a salt in which at least one of the anion and cation contains an organic substance. An example of an organic salt in which the cation contains an organic substance is a quaternary ammonium salt. An example of an organic salt in which the anion (or both ions) contains an organic substance is trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazolinium phthalate, mono-1,3-dimethyl-2-ethylimidazolinium phthalate, etc.

[0033] From the viewpoint of improving the voltage resistance characteristic, the anion preferably contains an anion of a fluorine-containing acid. Examples of the anion of a fluorine-containing acid include BF 4 - and / or P.F. 6 - The organic salt preferably contains, for example, a cation of tetraalkylammonium and an anion of a fluorine-containing acid. Specifically, diethyldimethylammonium tetrafluoroborate (DEDMABF 4 ), triethylmethylammonium tetrafluoroborate (TEMABF 4 ) etc.

[0034] It is desirable to interpose a separator between the pair of electrodes. The separator has ion permeability and serves to physically separate the pair of electrodes to prevent short circuit. For the separator, for example, a nonwoven fabric mainly composed of cellulose, a glass fiber mat, or a microporous film of a polyolefin such as polyethylene is used.

[0035] Hereinafter, an electrochemical device according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a partially cutaway perspective view of an electrochemical device according to an embodiment of the present invention. Note that the present invention is not limited to the electrochemical device of Fig. 1.

[0036] The electrochemical device 10 in Fig. 1 is an electric double layer capacitor, and includes a wound type capacitor element 1. The capacitor element 1 is configured by winding a sheet-like first electrode 2 and a second electrode 3 with a separator 4 interposed therebetween. The first electrode 2 and the second electrode 3 each have a first current collector and a second current collector made of metal, respectively, and a first active layer and a second active layer carried on the surfaces thereof, and exhibit capacitance by adsorbing and desorbing ions.

[0037] The current collector is made of, for example, aluminum foil. The surface of the current collector may be roughened by etching or other methods. The separator 4 is made of, for example, a nonwoven fabric mainly composed of cellulose. The first electrode 2 and the second electrode 3 are connected to a first lead wire 5a and a second lead wire 5b as lead-out members, respectively. The capacitor element 1 is housed in a cylindrical exterior case 6 together with an electrolyte (not shown). The material of the exterior case 6 may be, for example, a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the exterior case 6 is sealed by a sealing member 7. The lead wires 5a and 5b are led out to the outside so as to pass through the sealing member 7. The sealing member 7 is made of, for example, a rubber material such as butyl rubber.

[0038] In the above embodiment, a wound type capacitor has been described, but the scope of application of the present invention is not limited to the above, and the present invention may also be applied to capacitors of other structures, such as stacked type or coin type capacitors.

[0039] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0040] Examples 1-2 and Comparative Examples 1-2 The electrochemical device produced was a wound-type electric double layer capacitor with a rated voltage of 2.7 V. A specific method for producing the electrochemical device will be described below.

[0041] (Preparation of electrodes) 88 parts by mass of the active material, 2 parts by mass of polytetrafluoroethylene, 4 parts by mass of ammonium salt of carboxymethylcellulose (solid content ratio 5% by mass) swollen in water, and 6 parts by mass of acetylene black were dispersed in water to prepare a slurry. The obtained slurry was applied to an Al foil (thickness 30 μm), and the coating was vacuum dried at 110°C and rolled to form an active layer (thickness 40 μm), and an electrode was obtained.

[0042] (Preparation of electrolyte) γ-Butyrolactone (GBL) with diethyldimethylammonium tetrafluoroborate (DEDMABF 4 ) was dissolved in the electrolyte to prepare the electrolyte. 4 The concentration was 1.0 mol / L.

[0043] (Fabrication of electrochemical devices) A pair of electrodes was prepared, each was connected to a lead wire, and the electrodes were wound with a cellulose nonwoven separator to form a capacitor element, which was then housed in a specified exterior case together with an electrolyte and sealed with a sealing material to complete an electrochemical device (electric double layer capacitor). After that, the device was aged at 60°C for 16 hours while applying the rated voltage.

[0044] In the preparation of the above electrodes, porous carbon particles with different pore distributions were used as the active material to obtain electrodes x1-x2, y1-y2. Electrochemical devices were prepared using each electrode. The electrochemical devices of Examples 1-2 are electrochemical devices X1-X2 each having a pair of electrodes x1-x2. The electrochemical devices of Comparative Examples 1-2 are electrochemical devices Y1-Y2 each having a pair of electrodes y1-y2.

[0045] Each electrochemical device (fully discharged after aging treatment) was disassembled by the method described above, and the pore distribution of the porous carbon particles contained in the active layer of each electrode was measured. Data on the pore distribution of the porous carbon particles contained in the electrodes of each electrochemical device is shown in Table 1. As an example, the pore distribution of the porous carbon particles contained in the electrodes of the electrochemical devices of Example 1 and Comparative Examples 1 and 2 is shown in Figure 2. x1, y1 to y2 in Figure 2 indicate the log differential pore volume distribution of the porous carbon particles contained in the electrodes x1 and y1 to y2 of the electrochemical devices X1, Y1 to Y2, respectively.

[0046] In the porous carbon particles contained in the electrodes x1 and x2, the total ratio of the integrated volumes A and B to the total pore volume (the sum of the volumes of all pores in the range of 10 Å to 3000 Å) was in the range of 60% to 85%. In addition, the log differential pore volume distribution had one peak (the maximum value of the log differential pore volume) in the pore diameter range of 10 Å to 20 Å. In the porous carbon particles contained in the electrode x1, V 20 is 1.1cm 3 / g Å, V 60 0.2 cm 3 / g Å, V 20 -V 60 0.9cm 3 / g Å. For the porous carbon particles in electrode x2, V 20 0.8cm 3 / g Å, V 60 0.3 cm 3 / g Å, V 20 -V 60 0.5cm 3 / g Å.

[0047] The electrochemical devices obtained above were evaluated as follows. [evaluation] (Measurement of initial capacity and internal resistance of electrochemical devices (before float test)) In an environment of -30°C, the battery was charged at a constant current of 100 mA until the voltage reached 2.7 V, and then the voltage of 2.7 V was maintained for 7 minutes. Thereafter, in an environment of -30°C, the battery was discharged at a constant current of 75 mA until the voltage reached 0 V.

[0048] In the above discharge, the time t (sec) required for the voltage to drop from 2.0 V to 1.5 V was measured. Note that 2.0 V is a voltage equivalent to 74% of 2.7 V (the voltage at full charge), and 1.5 V is a voltage equivalent to 56% of 2.7 V. Using the measured time t, the capacity (initial capacity) C1 (F) of the electrochemical device before the float test was calculated according to the following formula (1). Capacity C1=Id×t / V (1) In addition, in the formula (1), Id is the current value during discharge (0.075 A), and V is the value obtained by subtracting 1.5 V from 2.0 V (0.5 V).

[0049] Using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained by the above discharge, a linear approximation line was calculated for the range of 0.5 to 2 seconds after the start of discharge of the discharge curve, and the voltage VS at the intercept of the approximation line was calculated. The value (V0-VS) obtained by subtracting the voltage VS from the voltage V0 at the start of discharge (0 seconds after the start of discharge) was calculated as ΔV. The internal resistance (DCR) R1 (Ω) of the electrochemical device before the float test was calculated using ΔV (V) and the current value Id (0.075 A) during discharge according to the following formula (2). Internal resistance R1=ΔV / Id (2)

[0050] (Float Testing of Electrochemical Devices) In an environment of 70°C, constant current charging was performed at a current of 100 mA until the voltage reached 2.7 V, and then the voltage of 2.7 V was maintained for 1300 hours. In this manner, the electrochemical device was stored with a voltage of 2.7 V applied. Thereafter, in an environment of 25°C, constant current discharging was performed at a current of 20 mA until the voltage reached 0 V.

[0051] (Measurement of internal resistance after float test of electrochemical device) After that, the batteries were charged and discharged in an environment of -30°C in the same manner as in the measurement of the internal resistance before the float test. by The internal resistance R2 (Ω) of the capacitor was measured after the float test.

[0052] (Measurement of resistance change rate) The resistance change rate was calculated from the internal resistance R1 and the internal resistance R2 of the electrochemical device before and after the float test according to the following formula (3). Resistance change rate = R2 / R1 x 100 (3)

[0053] The evaluation results of electrochemical devices X1 to X2 and Y1 to Y2 are shown in Table 1. In Table 1, the electrode density is calculated as the density per 1 cm of the active layer. 3 is the mass (g) of porous carbon particles contained per unit area.

[0054] [Table 1]

[0055] In the electrochemical devices X1 and X2, the initial capacity was large, the rate of resistance change was small, and excellent float characteristics were obtained.

[0056] For the electrochemical device Y1, the cumulative volume B is 0.15 cm 3 / g, the resistance change rate increased, and the float characteristics deteriorated. 3 / g, resulting in a decrease in initial capacity. [Industrial Applicability]

[0057] The electrochemical device according to the present invention is suitable for use in applications requiring large capacity and excellent float characteristics. [Explanation of symbols]

[0058] 1: capacitor element, 2: first electrode, 3: second electrode, 4: separator, 5a: first lead wire, 5b: second lead wire, 6: exterior case, 7: sealing member, 10: electrochemical device

Claims

1. A pair of electrodes and an electrolyte solution are provided. At least one of the pair of electrodes contains porous carbon particles, In the pore distribution of the porous carbon particles, The cumulative volume B of pores having a pore diameter of 20 Å or more and 60 Å or less is 0.15 cm ３ / g or more and 0.35 cm 3 / g or less, and The cumulative volume C of pores having a pore diameter of more than 60 Å and less than or equal to 500 Å is 0.25 cm ３ / g or less, In the log differential pore volume distribution of the porous carbon particles, The log differential pore volume V20 when the pore diameter is 20 Å is 0.5 cm 3 / g·Å or more, and An electrochemical device having a log differential pore volume V60 of 0.3 cm 3 / g·Å or less when the pore diameter is 60 Å.

2. The cumulative volume C is 0.15 cm ３ The electrochemical device according to claim 1 , wherein the surface area of ​​the electrochemical device is 0.1 μm or less.

3. 3. The electrochemical device according to claim 1, wherein in a pore distribution of the porous carbon particles, a ratio of an integrated volume B of pores having a pore diameter of 10 Å or more and less than 20 Å to an integrated volume A of the pores: B / A is 0.5 or more.

4. The log differential pore volume V20 is 0.5 cm ３ 4. The electrochemical device according to claim 1, wherein the surface area density is 2.0 cm 3 / g·Å or more and 2.0 cm 3 / g·Å or less.

5. The difference between the log differential pore volume V20 and the log differential pore volume V60: V20-V60 is 0.5 cm ３ The electrochemical device according to any one of claims 1 to 4, wherein the surface area is 1 / g·Å or more.

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