Electrochemical device

Abstract

An electrochemical device provided with a pair of electrodes and an electrolyte, at least one of the pair of electrodes comprising porous carbon particles, in the pore distribution of the porous carbon particles, the cumulative volume B of pores having a pore diameter above and below being 0.15 cm 3 / g or more, and the cumulative volume C of pores having a pore diameter greater than and below being 0.25 cm 3 / g or less.

Description

Technical Field

[0001] This invention relates to an electrochemical device having an electrode comprising porous carbon particles. Background Technology

[0002] Electrochemical devices comprise a pair of electrodes and an electrolyte, with at least one of the electrodes containing an active material capable of adsorbing and desorbing ions. As an example of an electrochemical device, the electric double-layer capacitor offers longer lifespan, faster charging, and superior output characteristics compared to a secondary battery, and is widely used in backup power supplies.

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

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 6357639 Summary of the Invention

[0007] Depending on the pore distribution of porous carbon particles, the capacity may decrease, and the floating properties may be easily reduced. Research on the correlation between the pore distribution of porous carbon particles and the performance of electrochemical devices remains insufficient.

[0008] In view of the above, one aspect of the present invention relates to an electrochemical device comprising a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes comprises porous carbon particles, and the porous carbon particles have a fine pore distribution. The above and The cumulative volume B of the following fine-diameter pores is 0.15 cm³. 3 / g or more, and having greater than and The cumulative volume C of the following fine-diameter pores is 0.25 cm³. 3 / g or less.

[0009] According to the present invention, it is possible to improve the capacity of electrochemical devices while suppressing the reduction of floating characteristics.

[0010] Brief description of the attached figures

[0011] [ Figure 1 ] Figure 1 This is a perspective view showing a portion of the electrochemical device according to one embodiment of the present invention cut away.

[0012] [ Figure 2 ] Figure 2 This is a diagram showing the pore distribution of porous carbon particles contained in the electrodes of the electrochemical devices of Embodiment 1 and Comparative Examples 1-2 of the present invention. Detailed Implementation

[0013] An electrochemical device according to one embodiment of the present invention comprises a pair of electrodes and an electrolyte. At least one of the pairs of electrodes comprises porous carbon particles. When ions are adsorbed onto the porous carbon particles in the electrolyte, an electrical double layer is formed, exhibiting capacity. When ions are desorbed from the porous carbon particles, a non-Radida current flows. The electrodes of the electrochemical device of this embodiment utilize this phenomenon. The porous carbon particles have a fine pore distribution... The above and The cumulative volume B of the following fine-diameter pores (hereinafter also referred to as mesopores) is 0.15 cm³. 3 / g or more, and having greater than and The cumulative volume C of the following fine-diameter pores (hereinafter also referred to as macropores) is 0.25 cm³. 3 / g or less.

[0014] The cumulative volume B mentioned above is 0.15 cm³. 3 At concentrations above a certain value (e.g., g), even at low temperatures, the initial capacity remains high, resulting in electrochemical devices with excellent floating characteristics. Floating characteristics refer to the degree of degradation of an electrochemical device during floating charging with an external DC power supply while maintaining a constant voltage. Generally, the smaller the capacity reduction during floating charging and the more suppressed the increase in internal resistance, the better the floating characteristics. However, when the cumulative volume C exceeds 0.25 cm³, the degradation is less pronounced. 3 At a density of / g, the proportion of macropores increases, the electrode density decreases, and sometimes the capacity decreases.

[0015] Mesopores primarily contribute to the mobility of ions within the pores of the electrolyte, mainly affecting float characteristics and internal resistance. Additionally, mesopores also contribute to the specific surface area of ​​porous carbon particles, thus influencing capacity (initial capacity). In pores with a diameter of... Under the above conditions, ions in the electrolyte easily diffuse within the fine pores, making them less prone to clogging. When the pore diameter is... Within the aforementioned fine pores, good ion movement is ensured even at low temperatures. In pores with a diameter of... Under the following circumstances, it is easy to increase the specific surface area and obtain a large initial capacity.

[0016] The aforementioned cumulative volume B can be, for example, 0.15 cm³. 3 / g or more and 0.35m 3Below / g, it can also be 0.25cm 3 / g or more and 0.30cm 3 / g or less. From the perspective of further increasing capacity, the above-mentioned cumulative volume C can be 0.15cm³. 3 / g or less.

[0017] In the fine pore distribution of porous carbon particles, the aforementioned cumulative volume B is relative to the... Above and less than The ratio of the cumulative volume A of the fine-diameter micropores (hereinafter also referred to as micropores) is: B / A can be 0.5 or higher, 0.5 or higher and 0.65 or lower, or 0.5 or higher and 0.6 or lower. When B / A is within the above range, combining mesopores and micropores ensures greater capacity and further improves floating characteristics. Micropores primarily contribute to the relative surface area and are most likely to affect capacity (especially initial capacity).

[0018] In the micropore distribution of porous carbon particles, the aforementioned cumulative volumes A and B account for a significant portion of the total micropore volume ( The above and The total proportion of the volume of all micropores in the following range is preferably 60% or more and 85% or less. In this case, micropores and mesopores are abundantly distributed, making it easy to obtain large capacity and excellent floating characteristics.

[0019] The cumulative volumes A to C mentioned above are obtained by the following method: the electrodes of an unused or initial electrochemical device in a fully discharged state are decomposed, the active layer is peeled off from the current collector and crushed, the crushed material is heated and dried at 160°C to obtain a sample (particle group), and the pore distribution of the sample is measured.

[0020] The pore size distribution was determined using a nitrogen gas adsorption method. For example, the Shimadzu TriStar II 3020 automatic surface area / pore size distribution measuring device was used. It should be noted that, to remove impurities, the sample was pretreated by heating and vacuum degassing (e.g., 250°C and below 50 mTorr) before measurement. In the analysis of the pore size distribution, the BJH method (Barrett-Joyner-Halenda method) was used, employing the formula of Harkins & Jura. Using the cumulative pore volume distribution obtained by the BJH method, the total volume (cm³) of micropores, mesopores, and macropores per 1 g of the sample was calculated. 3 The above cumulative volumes A to C are represented by these volumes.

[0021] It should be noted that the above samples may contain binders and conductive agents in addition to porous carbon particles, but the binders are in small amounts and have little impact on the pore distribution of the porous carbon particles. The shape of the cumulative pore volume distribution curve is almost the same as that of the case with only porous carbon particles, and compared with the case with only porous carbon particles, the cumulative pore volume distribution curve is slightly shifted downward (the cumulative volume is slightly smaller).

[0022] In the log differential pore volume distribution of porous carbon particles, preferably, the pore diameter is [missing value]. The log differential orifice volume V at that time 20 (Hereafter referred to as V) 20 .)for The above, and the fine pore diameter is The log differential orifice volume V at that time 60 (Hereafter referred to as V) 60 .)for The following is a log differential fine pore volume distribution curve, where the horizontal axis is set to the fine pore diameter D and the vertical axis is set to the log differential fine pore volume represented by dVp / d(logD). Vp is the fine pore volume per unit mass. The log differential fine pore volume distribution is obtained from the cumulative fine pore volume distribution data obtained above.

[0023] In V 20 and V 60 Within the aforementioned range, mesopores tend to increase along with micropores, while macropores tend to decrease. Therefore, large capacity and excellent floating characteristics are easily achieved at low temperatures. In V 20 and V 60 Within the above range, the log differential pore volume distribution (pore distribution curve) at a pore diameter of [value missing] The above and The following range can contain regions where, as the pore size increases, the log differential pore volume decreases, and the tangent at any point within this range has a certain degree of inclination. For example... Figure 2 As shown in the log differential fine pore volume distribution of x1, it is preferable to have fine pore diameters close to the aforementioned range. On one side, the aforementioned tangent has a certain degree of inclination. When the aforementioned region is closer to a smaller aperture... When the larger side is moved, the number of large pores increases, and the electrode density sometimes decreases. This occurs when the aforementioned region moves towards a smaller pore size. When the smaller side moves, the number of mesopores decreases, and the floating properties may sometimes be reduced.

[0024] V 20 It can be The above and The following can also be The above and The following. V 60 It can be The above and The following can also be the following.

[0025] V 20 With V 60 Difference: V 20 -V 60 Preferred The above. In this case, the larger the absolute value of the slope of the aforementioned tangent, the smaller the proportion of large holes and the larger the electrode density. For example... Figure 2 As shown in the log differential fine pore volume distribution of x1, it is preferable to have fine pore diameters close to the aforementioned range. On one side, the absolute value of the slope of the aforementioned tangent is larger. In this case, there are more mesopores and fewer macropores, resulting in a large capacity at low temperatures and further improved floating characteristics. V 20 -V 60 It can be The above and The following can also be The above and the following.

[0026] The log differential pore volume integral distribution is applied to a pore diameter of [value missing]. The above and Within the following range, it can have one peak (the maximum value of the log differential pore volume). It is easy to convert V... 20 V 60 and V 20 -V 60 By keeping the control within the above range, it is much easier to ensure that mesopores and micropores are kept together.

[0027] Porous carbon particles can be produced, for example, by carbonizing raw materials through heat treatment and then activating the resulting carbides to create porous particles. Examples of raw materials include wood, coconut shells, pulp waste liquid, coal or coal-based pitch obtained through its thermal decomposition, heavy oil or petroleum-based pitch obtained through its thermal decomposition, phenolic resin, petroleum coke, and coal char. Examples of activation treatments include gas activation using gases such as steam and chemical activation using alkalis such as potassium hydroxide. The porous carbon particles obtained from the above activation treatment can be pulverized. After pulverization, they can be graded. For example, ball mills or spray mills can be used in the pulverization process.

[0028] Porous carbon particles can be obtained, for example, by heat-treating coconut shells, crushing and granulating the resulting carbides, and then activating them. Alternatively, porous carbon particles can be obtained, for example, by adding binders such as coal tar and pitch to finely pulverized coal, mixing and compressing it, crushing and granulating the shaped material, and then heat-treating and activating the pulverized material.

[0029] The pore distribution of porous carbon particles can be adjusted by factors such as raw materials, heat treatment temperature, activation temperature during gas activation, and the degree of pulverization. Porous carbon particles can be used alone or in combination with two or more types.

[0030] At least one of the pairs of electrodes may have an active layer and a current collector supporting the active layer. The active layer contains at least the aforementioned porous carbon particles as the active material. The active layer may contain a mixture (alloy) of the porous carbon particles and a small amount of binder and / or conductive agent. The proportion of porous carbon particles in the active layer (alloy) is, for example, 88% by mass or more.

[0031] As a binder, resin materials such as polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC) (including alkali metal salts or ammonium salts of CMC), and styrene-butadiene rubber (SBR) can be used, for example. As a conductive agent, carbon black such as acetylene black can be used, for example.

[0032] The aforementioned electrode can be obtained, for example, by coating a slurry containing porous carbon particles, a binder and / or a conductive agent, and a dispersion medium onto the surface of a current collector, and then drying and calendering the coating to form an active layer. The current collector can be, for example, a metal foil such as aluminum foil.

[0033] Examples of electrochemical devices include electric double-layer capacitors (EDLCs) and lithium-ion capacitors (LICs). In the case of an EDLC, at least one of the electrodes can be an electrode containing the aforementioned porous carbon particles. In the case of a LIC, one electrode (positive electrode) can be an electrode containing the aforementioned porous carbon particles, and the other electrode (negative electrode) can be a negative electrode used in lithium-ion secondary batteries. The negative electrode used in lithium-ion secondary batteries, for example, contains a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

[0034] The electrolyte comprises a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and includes both cations and anions. The ionic substance may include, for example, low-melting-point compounds (ionic liquids) that can exist in liquid form near room temperature. The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0035] High-boiling-point solvents are preferred. For example, lactones such as γ-butyrolactone, carbonates such as propylene carbonate, polyols 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.

[0036] Ionic substances include, for example, organic salts. An organic salt is a salt in which at least one of its anion and cation contains an organic compound. Examples of organic salts containing an organic compound as a cation include quaternary ammonium salts. Examples of organic salts containing an organic compound as anions (or bis-ions) include trimethylamine maleate, triethylamine borosalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazoline phthalate, and mono-1,3-dimethyl-2-ethylimidazoline phthalate.

[0037] From the perspective of improving voltage withstand characteristics, anions containing fluorinated acids are preferred. Examples of fluorinated acid anions include BF4. - and / or PF6 - Organic salts, for example, preferably contain tetraalkylammonium cations and fluorinated acid anions. Specifically, examples include diethyldimethylammonium tetrafluoroborate (DEDMABF4) and triethylmethylammonium tetrafluoroborate (TEMABF4).

[0038] The desired outcome is to place a spacer between a pair of electrodes. The spacer should be ion-permeable and function to physically separate the electrodes, preventing short circuits. Examples of spacers that can be used include nonwoven fabrics with cellulose as the main component, fiberglass mats, and microporous membranes of polyolefins such as polyethylene.

[0039] The following is for reference Figure 1 The electrochemical device according to an embodiment of the present invention will be described. Figure 1 This is a perspective view showing a portion of the electrochemical device according to an embodiment of the present invention removed. It should be noted that the present invention is not limited to... Figure 1 Electrochemical devices.

[0040] Figure 1 The electrochemical device 10 is a double-layer capacitor and includes a wound capacitor element 1. The capacitor element 1 is constructed by winding sheet-shaped first electrode 2 and second electrode 3 with spacers 4. The first electrode 2 and second electrode 3 each have a first current collector and a second current collector made of metal, and a first active layer and a second active layer supported on their surfaces, respectively, and exhibit capacity by adsorbing and desorbing ions.

[0041] The current collector can be, for example, aluminum foil. The surface of the current collector can also be roughened by methods such as etching. The spacer 4 can be, for example, a nonwoven fabric with cellulose as the main component. The first lead 5a and the second lead 5b are connected to the first electrode 2 and the second electrode 3 as lead-out components, respectively. The capacitor element 1 and the electrolyte (not shown) are housed together in a cylindrical outer casing 6. The outer casing 6 can be made of metals such as aluminum, stainless steel, copper, iron, or brass. The opening of the outer casing 6 is sealed by a sealing component 7. The leads 5a and 5b are led out to the outside by passing through the sealing component 7. The sealing component 7 can be made of rubber materials such as butyl rubber.

[0042] In the above embodiments, a wound capacitor has been described, but the application scope of the present invention is not limited to the above, and it can also be applied to capacitors of other structures, such as stacked or coin-shaped capacitors.

[0043] The present invention will now be described in more detail based on embodiments, but the present invention is not limited to the embodiments.

[0044] Examples 1-2 and Comparative Examples 1-2

[0045] As an electrochemical device, a wound-type electric double-layer capacitor with a rated voltage of 2.7V is fabricated. The specific manufacturing method of the electrochemical device is described below.

[0046] (Electrode fabrication)

[0047] A slurry was prepared by dispersing 88 parts by mass of active material, 2 parts by mass of polytetrafluoroethylene, 4 parts by mass of ammonium salt of carboxymethyl cellulose swollen in water (solid content ratio 5% by mass), and 6 parts by mass of acetylene black in water. The obtained slurry was coated onto an Al foil (thickness 30 μm), and the coating was vacuum dried and calendered at 110 °C to form an active layer (thickness 40 μm), thus obtaining the electrode.

[0048] (Preparation of electrolyte)

[0049] An electrolyte was prepared by dissolving diethyldimethylammonium tetrafluoroborate (DEDMABF4) in γ-butyrolactone (GBL). The concentration of DEDMABF4 in the electrolyte was set at 1.0 mol / L.

[0050] (Fabrication of electrochemical devices)

[0051] Prepare a pair of electrodes, connect them to leads, and wind them together with a cellulose nonwoven fabric spacer to form a capacitor element. Place this element, along with the electrolyte, in a pre-designated outer casing and seal it with a sealing component, thus completing the electrochemical device (double-layer capacitor). Subsequently, apply the rated voltage while performing an aging treatment at 60°C for 16 hours.

[0052] In the above electrode fabrication, porous carbon particles with different pore distributions were used as the active material to obtain electrodes x1-x2 and y1-y2. Electrochemical devices were fabricated using each electrode. The electrochemical devices of Examples 1-2 are electrochemical devices X1-X2 having a pair of electrodes x1-x2. The electrochemical devices of Comparative Examples 1-2 are electrochemical devices Y1-Y2 having a pair of electrodes y1-y2.

[0053] Using the methods described above, each electrochemical device (an electrochemical device set to a fully discharged state after aging treatment) was disassembled, and the pore distribution of porous carbon particles contained in the active layer of each electrode was measured. Data related to the pore distribution of porous carbon particles contained in the electrodes of each electrochemical device are shown in Table 1. Furthermore, as an example, the pore distribution of porous carbon particles contained in the electrodes of the electrochemical devices of Example 1 and Comparative Examples 1-2 is shown in Table 1. Figure 2 middle. Figure 2 In the figure, x1 and y1 to y2 represent the log differential pore volume distribution of porous carbon particles contained in the electrodes x1 and y1 to y2 of the electrochemical devices X1 and Y1 to Y2, respectively.

[0054] In the porous carbon particles contained in electrodes x1 and x2, the cumulative volumes A and B are in the total micropore volume ( The above and The total volume of all micropores in the following range accounts for 60% to 85% of the total volume. Additionally, the log differential micropore volume distribution in micropores with a diameter of [missing information] The above and The following range exhibits one peak (the maximum value of the log differential pore volume). In the porous carbon particles contained in electrode x1, V 20 for V 60 for V 20 -V 60 for In the porous carbon particles contained in electrode x2, V 20 for V 60 for V 20 -V 60 for

[0055] The following evaluations are made on the electrochemical devices obtained above.

[0056] [evaluate]

[0057] (Determination of initial capacity and internal resistance of electrochemical devices (before floating test))

[0058] At -30°C, the circuit is charged with a constant current of 100mA until the voltage reaches 2.7V, and then maintained at 2.7V for 7 minutes. Afterward, at -30°C, the circuit is discharged with a constant current of 75mA until the voltage reaches 0V.

[0059] In the above discharge, the time t (seconds) required for the voltage to drop from 2.0V to 1.5V was measured. It should be noted that 2.0V is equivalent to 74% of 2.7V (the voltage at full charge), and 1.5V is equivalent to 56% of 2.7V. 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).

[0060] Capacity C1 = Id × t / V (1)

[0061] It should be noted that in equation (1), Id is the current value during discharge (0.075A), and V is the value after subtracting 1.5V from 2.0V (0.5V).

[0062] Using the discharge curve obtained through the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), an approximate straight line is obtained within the range of 0.5 seconds to 2 seconds from the start of discharge. The intercept voltage VS of this approximate straight line is then obtained. The value of ΔV is obtained by subtracting the voltage VS from the voltage V0 at the start of discharge (0 seconds after the start of discharge). Using ΔV (V) and the current value Id (0.075A) during discharge, the internal resistance (DCR) R1 (Ω) of the electrochemical device before the float test is obtained according to the following formula (2).

[0063] Internal resistance R1=ΔV / Id (2)

[0064] (Floating test of electrochemical devices)

[0065] The electrochemical device was charged at a constant current of 100mA at 70°C until the voltage reached 2.7V, and then maintained at 2.7V for 1300 hours. This process was repeated to store the electrochemical device under a 2.7V applied voltage. Subsequently, it was discharged at a constant current of 20mA at 25°C until the voltage reached 0V.

[0066] (Determination of internal resistance of electrochemical devices after floating test)

[0067] Subsequently, the internal resistance R2 (Ω) of the electrochemical device after the float test was obtained by charging and discharging at -30°C using the same method as the method used to measure the internal resistance before the float test.

[0068] (Determination of the rate of change of resistance)

[0069] Using the internal resistances R1 and R2 of the electrochemical device obtained above before and after the floating test, the resistance change rate is calculated according to the following formula (3).

[0070] Rate of change of resistance = R2 / R1 × 100 (3)

[0071] The evaluation results of electrochemical devices X1-X2 and Y1-Y2 are shown in Table 1. It should be noted that the electrode density is calculated per 1 cm² of active layer. 3 The mass (g) of porous carbon particles contained in it.

[0072] [Table 1]

[0073]

[0074] In electrochemical devices X1 to X2, the initial capacity is large and the resistance change rate is small, resulting in excellent floating characteristics.

[0075] In electrochemical device Y1, the cumulative volume B is less than 0.15 cm³. 3 / g, the rate of change in resistance increases, and the floating characteristic decreases. In the electrochemical device Y2, the cumulative volume C is greater than 0.25cm³. 3 / g, initial capacity decreases.

[0076] Industrial availability

[0077] The electrochemical device of the present invention is preferably used in applications requiring high capacity and excellent floating characteristics.

[0078] Explanation of reference numerals in the attached figures

[0079] 1: Capacitor element; 2: First electrode; 3: Second electrode; 4: Spacer; 5a: First lead; 5b: Second lead; 6: Outer casing; 7: Sealing component; 10: Electrochemical device

Claims

1. An electrochemical device comprising a pair of electrodes and an electrolyte, At least one of the pair of electrodes comprises porous carbon particles. In the fine pore distribution of the porous carbon particles, The cumulative volume B of the pores with a diameter of 20 Å or more but less than 60 Å is 0.15 cm³. 3 / g or more and 0.35cm 3 / g or less, and The cumulative volume C of the pores with a diameter greater than 60 Å and less than 500 Å is 0.25 cm³. 3 / g or less, In the log differential fine pore volume distribution of the porous carbon particles The log differential pore volume V when the pore diameter is 20 Å 20 0.5cm 3 / g·Å or more, and The log differential pore volume V when the pore diameter is 60 Å 60 0.3cm 3 / g·Å or less.

2. The electrochemical device according to claim 1, wherein, The cumulative volume C is 0.15 cm³. 3 / g or less.

3. The electrochemical device according to claim 1 or 2, wherein, In the micropore distribution of the porous carbon particles, the ratio of the cumulative volume B to the cumulative volume A of the micropores having a micropore diameter of 10 Å or more and less than 20 Å, i.e., B / A, is 0.5 or more.

4. The electrochemical device according to claim 1, wherein, The log differential fine pore volume V 20 With the log differential fine hole volume V 60 The difference, i.e., V 20 -V 60 0.5cm 3 / g·Å or more.

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