Electrochemical capacitor

Abstract

This electrochemical capacitor is provided with: a positive electrode including a positive electrode current collector and a positive electrode mixture layer that contains activated carbon and is supported on the positive electrode current collector; a negative electrode containing a negative electrode current collector and a negative electrode mixture layer that contains a negative electrode active material reversibly doped with lithium ions and is supported by the negative electrode current collector; and an electrolyte solution containing a nonaqueous solvent and an electrolyte containing lithium ions and anions, the ratio of the amount of anions to the surface area of the positive electrode mixture layer being 1.5-2.4 [mu] mol / m2.

Description

[0001] Cross-referencing of related applications

[0002] This disclosure asserts the priority interest of Japanese Patent Application No. 2023-050796 filed by the Japanese Patent Office on March 28, 2023, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] This invention relates to electrochemical capacitors. Background Technology

[0004] In recent years, electrochemical capacitors, which combine the energy storage principles of lithium-ion batteries and electric double-layer capacitors, have attracted attention. Such electrochemical capacitors typically use a polarized electrode at the positive electrode and a non-polarized electrode at the negative electrode. The goal is to combine the high energy density of lithium-ion batteries with the high output characteristics of electric double-layer capacitors.

[0005] Patent document 1 proposes a positive electrode for a lithium-ion capacitor, characterized in that the pore volume of the positive electrode layer with a pore size of 1.0 nm or more and less than 1.4 nm, calculated by the HK method, is 0.11 cc / g or more, and the total pore volume calculated by the BET method is 1.1 cc / g or less.

[0006] Patent document 2 discloses a lithium-ion capacitor having a positive electrode, a negative electrode, and an electrolyte in contact with the positive and negative electrodes. The electrolyte contains an organic solvent and a lithium salt electrolyte having an imide structure. The organic solvent contains ethylene carbonate and propylene carbonate.

[0007] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 6422483 Patent Document 2: Japanese Patent No. 7103376 Summary of the Invention

[0008] The problem that the invention aims to solve However, in the electrochemical capacitors described above, further improvements are required to achieve higher output at an even higher level.

[0009] Electrochemical capacitors, especially at low temperatures, are prone to increased internal resistance (DCR). Furthermore, when float charging an electrochemical capacitor at high temperatures using an external DC power supply with a constant voltage, the internal resistance tends to rise. To obtain high output characteristics, it is necessary to reduce the initial internal resistance and the internal resistance after the float charge test.

[0010] Methods for solving problems This invention relates to an electrochemical capacitor comprising: a positive electrode including a positive current collector and a positive electrode flux layer containing activated carbon supported on the positive current collector; a negative electrode including a negative current collector and a negative electrode flux layer containing a negative electrode active material reversibly doped with lithium ions supported on the negative current collector; and an electrolyte comprising an electrolyte and a non-aqueous solvent containing lithium ions and anions, wherein the ratio of the amount of anions to the surface area of ​​the positive electrode flux layer is 1.5 μmol / m². 2 Above and 2.4 μmol / m 2 the following.

[0011] Invention Effects According to the present invention, an electrochemical capacitor with low internal resistance and high output can be provided.

[0012] The novel features of the invention are set forth in the appended claims, but the invention, in terms of both its composition and content, along with its other objects and features, can be better understood from the following detailed description taken in conjunction with the accompanying drawings. Attached Figure Description

[0013] Figure 1 This is a longitudinal cross-sectional view of an electrochemical capacitor according to one embodiment of the present invention. Detailed Implementation

[0014] The following examples illustrate embodiments of this disclosure, but this disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc., are sometimes illustrated, but other numerical values, materials, etc., can be applied as long as the effects of this disclosure are achieved. Furthermore, known constituent elements can be applied to constituent elements other than the characteristic parts of this disclosure. In this specification, when referring to "the range of numerical values ​​A to B," that range includes both numerical values ​​A and B.

[0015] In the following description, where lower and upper limits of values ​​related to specific physical properties, conditions, etc., are illustrated, any of the illustrated lower limits can be arbitrarily combined with any of the illustrated upper limits, provided that the lower limit is not above the upper limit. When multiple materials are illustrated, unless otherwise specified, one can be selected for use alone, or two or more can be combined.

[0016] Furthermore, this disclosure includes combinations of matters recited in two or more claims selected from the plurality of claims recited in the appended claims. That is, as long as no technical contradiction arises, it is possible to combine matters recited in two or more claims selected from the plurality of claims recited in the appended claims.

[0017] The electrochemical capacitor according to embodiments of the present invention includes a positive electrode, a negative electrode, and an electrolyte. Typically, the positive and negative electrodes, together with a separator between them, constitute an electrode body. For example, the electrode body can be constructed as a cylindrical wound body by winding strip-shaped positive and negative electrodes through a separator. Alternatively, the electrode body can be constructed as a laminated body by stacking plate-shaped positive and negative electrodes through a separator.

[0018] The positive electrode, for example, comprises a positive current collector and a positive electrode flux layer containing activated carbon supported on the current collector. The positive electrode flux layer contains a positive electrode active material such as activated carbon that is reversibly doped with anions. If anions are adsorbed onto the positive electrode active material such as activated carbon, an electrical double layer is formed, exhibiting capacity. The positive electrode can be a polarized electrode, or an electrode that has the properties of a polarized electrode and whose Faraday reaction also contributes to capacity.

[0019] The doping of anions into positive electrode active materials such as activated carbon refers to the following concept: it includes at least the adsorption of anions into positive electrode active materials such as activated carbon, and may also include the intercalation of anions into positive electrode active materials such as activated carbon, the chemical interaction between positive electrode active materials such as activated carbon and anions, etc.

[0020] The negative electrode comprises a negative current collector and a negative electrode additive layer. The negative electrode additive layer contains a negative electrode active material reversibly doped with lithium ions and is supported on the negative current collector. In the negative electrode active material, a Faraday reaction of reversible lithium ion insertion and extraction occurs, thereby exhibiting capacity. Lithium ion doping into the negative electrode active material refers to at least the phenomenon of lithium ion insertion into the negative electrode active material, and may also include concepts such as lithium ion adsorption into the negative electrode active material and chemical interactions between the negative electrode active material and lithium ions.

[0021] Hereinafter, the positive and negative electrodes will sometimes be referred to collectively as electrodes. Additionally, the positive and negative current collectors will sometimes be referred to collectively as current collectors (or electrode current collectors). Furthermore, the positive and negative flux layers will sometimes be referred to collectively as flux layers (or electrode flux layers). Finally, the positive and negative active materials will sometimes be referred to collectively as active materials (or electrode active materials).

[0022] The electrolyte contains an electrolyte with lithium ions and anions, and a non-aqueous solvent. The electrolyte exhibits lithium-ion conductivity, allowing anions to reversibly and repeatedly dop and dedope to the positive electrode. Lithium ions can reversibly insert into and extract from the negative electrode.

[0023] The ratio of anion concentration to the surface area of ​​the positive electrode binder layer is 1.5 μmol / m². 2 Above and 2.4 μmol / m 2The following is a summary of the points discussed. This allows for a reduction in the initial internal resistance of the electrochemical capacitor. Furthermore, since the degradation of the positive electrode can be suppressed, the internal resistance remains low even after repeated charge-discharge cycles, enabling high output characteristics.

[0024] The ratio of anion quantity to the surface area of ​​the cathode flux layer is an indicator representing the ratio of the amount of anions that are reversibly and repeatedly doped and dedoped on the surface of the cathode flux layer to the surface area of ​​the cathode flux layer. This ratio is expressed as the amount of anions divided by the surface area of ​​the cathode flux layer.

[0025] The amount of anions is expressed as the molar amount A of anions contained in each cell of the electrochemical capacitor. Additionally, the surface area of ​​the positive electrode flux layer is expressed as the surface area B of the positive electrode flux layer in each cell of the electrochemical capacitor, calculated from the specific surface area b of the positive electrode flux layer.

[0026] The ratio of the amount of anions to the surface area of ​​the positive electrode mixture layer is expressed as A / B, which is the amount of anions A divided by the surface area B of the positive electrode mixture layer.

[0027] Here, the degradation of the positive electrode is typically evaluated by the rate of increase in the low-temperature DCR of the electrochemical capacitor during float charging at high temperature using an external DC power supply to apply a constant voltage. The rate of increase in low-temperature DCR refers to the ratio of the difference between the initial low-temperature DCR and the low-temperature DCR after float charging (ΔDCR) to the initial low-temperature DCR of the electrochemical capacitor. In other words, the smaller the rate of increase in low-temperature DCR, the smaller the degradation of the positive electrode.

[0028] If the ratio of anion quantity to the surface area of ​​the cathode flux layer (A / B) is small, the amount of anions (A) that can be reversibly and repeatedly doped and dedoped onto the surface of the cathode flux layer is small, thus failing to exhibit sufficient capacity and consequently increasing the initial internal resistance. On the other hand, if A / B is large, the capacity increases, but cathode degradation progresses more easily. For example, the internal resistance tends to rise after a float charge test, and the difference between the initial and float charge internal resistances becomes larger, resulting in a higher float charge internal resistance. A / B less than 1.5 μmol / m 2 At this point, the initial internal resistance increases significantly. If A / B exceeds 2.4 μmol / m 2 If this occurs, the degradation of the positive electrode proceeds rapidly, and the internal resistance increases excessively after float charging. However, when the A / B ratio is 1.5–2.4 μmol / m 2 Within a certain range, it can balance low initial internal resistance and low internal resistance after float charge, thus maintaining output characteristics to a high degree.

[0029] The ratio A / B of the anion content to the surface area of ​​the positive electrode mixture layer is preferably 1.7 μmol / m². 2 Above and 2.2 μmol / m2 The following applies. In this case, the initial internal resistance and the internal resistance after float charging are significantly reduced, which can further improve the output characteristics of the electrochemical capacitor.

[0030] The amount of anions A (mmol) contained in each unit of an electrochemical capacitor is determined by the molar concentration (M) of the electrolyte in the electrolyte, the mass (g) of the electrolyte, and the specific gravity (g / cm³) of the electrolyte. 3 It is represented by the following formula.

[0031] Anion content A = (molar concentration of electrolyte in electrolyte) × (mass of electrolyte) / (specific gravity of electrolyte) The surface area B (m²) of the positive electrode layer contained in each unit of an electrochemical capacitor 2 The surface area per unit mass of the positive electrode mixture layer (specific surface area b of the positive electrode mixture layer) is expressed by the following formula. 2 The product of (g) and the mass of the positive electrode mixture layer (mass of the positive electrode - mass of the positive electrode current collector) is used to calculate the mass.

[0032] The surface area B of the positive electrode flux layer = (specific surface area b of the positive electrode flux layer) × (mass of the positive electrode - mass of the positive electrode current collector) The ratio A / B (μmol / m²) of anion quantity to the surface area of ​​the positive electrode mixture layer. 2 The following formula represents the ratio of anion quantity A (mmol) to the surface area B (m²) of the positive electrode mixture layer. 2 ) of the commerce.

[0033] The ratio of anion quantity to the surface area of ​​the positive electrode mixture layer, A / B = (anion quantity A) / (surface area of ​​the positive electrode mixture layer B) × 1000 It should be noted that when electrode flux layers are provided on both sides of the current collector, the surface area B of the positive electrode flux layer is derived from the above definition and is calculated by multiplying the total mass of the positive electrode flux layers on both sides of the current collector by the surface area per unit mass of the positive electrode flux layer (the specific surface area b of the positive electrode flux layer).

[0034] The specific surface area b of the positive electrode coating layer is the BET specific surface area determined using a measuring apparatus according to JIS Z8830 (e.g., the Tristar II3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical capacitor is disassembled, and the positive electrode is removed. Then, the positive electrode coating layer is peeled off from the positive electrode current collector, and a sample of approximately 0.5 g of the positive electrode coating layer is collected.

[0035] Next, the collected sample was heated at 150°C for 12 hours under reduced pressure below 95 kPa. Then, nitrogen gas was adsorbed onto the sample of known mass, and adsorption isotherms were obtained within a relative pressure range of 0–1. The surface area of ​​the sample was then calculated based on the monolayer adsorption amount of the gas obtained from the adsorption isotherms. Here, the specific surface area b was determined using the BET one-point method (relative pressure 0.3) according to the following BET formula.

[0036] P / V(P0-P)={1 / (Vm·C)}+{(C-1) / (Vm·C)}(P / P0)…(1) S=kVm…(2) P0: Saturated vapor pressure P: Adsorption equilibrium pressure V: Adsorption amount under adsorption equilibrium pressure P Vm: Monolayer adsorption capacity C: Parameters related to adsorption heat, etc. S: Specific surface area b k: Nitrogen single molecule area is 0.162 nm 2 The specific surface area of ​​the positive electrode additive layer roughly reflects the specific surface area of ​​the positive electrode active material. However, the specific surface area depends not only on the specific surface area of ​​the positive electrode active material, but also on the specific surface area, content, and distribution of materials other than the positive electrode active material, such as conductive additives and binders, that may be included in the positive electrode additive layer.

[0037] The negative electrode active material preferably contains non-graphitized carbon. Non-graphitized carbon is also known as hard carbon. By using non-graphitized carbon, high cycle characteristics can be obtained even under repeated fast charge and discharge conditions. In addition, the internal resistance (DCR) is low even at low temperatures, enabling high-output electrochemical capacitors.

[0038] A surface layer is formed on the surface of the negative electrode mixture layer. The surface layer may have a first layer containing lithium carbonate (a lithium carbonate layer). The first layer is mainly formed on the surface of the negative electrode active material. By forming a first layer containing lithium carbonate, the degradation of the negative electrode can be significantly suppressed.

[0039] The surface layer formed on the negative electrode flux layer may further have a second layer (solid electrolyte layer) comprising a solid electrolyte layer. The second layer may be formed in such a way that it covers at least a portion of the surface of the first layer. The second layer has a different composition from the first layer and can be distinguished from the first layer. In an electrochemical capacitor utilizing lithium ions, a solid electrolyte interfacial film (i.e., SEI film) is formed on the negative electrode flux layer during charge and discharge. The second layer may also be formed as an SEI film. The SEI film plays an important role in the charge and discharge reaction, but if the SEI film is formed too thickly, the deterioration of the negative electrode becomes greater. In contrast, the first layer containing lithium carbonate has the function of promoting the formation of a good SEI film and maintaining the SEI film in a good state under repeated charge and discharge. That is, the first layer is located between the surface of the negative electrode active material and the second layer, becoming the base layer of the second layer, thereby forming a second layer as a well-formed SEI film. Therefore, by having a first layer on the surface, even when the specific surface area of ​​the negative electrode compound layer is increased in order to suppress the increase of low-temperature DCR, the degradation of the negative electrode can be significantly suppressed.

[0040] The second layer can also contain lithium carbonate. When the second layer contains lithium carbonate, the amount of lithium carbonate in the second layer is less than that in the first layer. Using the first layer, which contains a relatively high amount of lithium carbonate, as the substrate layer is a necessary condition for the second layer to form a well-formed SEI film.

[0041] Before assembling the electrochemical capacitor, a first layer is formed as at least a portion of the surface layer on the surface of the negative electrode compound layer. In the electrochemical capacitor assembled using this negative electrode, a homogeneous second layer (SEI film) of appropriate thickness is formed on the surface of the negative electrode active material through subsequent charging and discharging. The SEI film is formed, for example, by the reaction of the electrolyte with the negative electrode in the electrochemical capacitor. The electrolyte can pass through not only the second layer but also the first layer; therefore, the entire surface layer including the first and second layers can also be referred to as the SEI film. However, for convenience, in this specification, the second layer is referred to as the SEI film to distinguish it from the first layer.

[0042] The presence of such a lithium carbonate-containing region in the first layer can be confirmed, for example, by analysis of the surface portion based on X-ray photoelectron spectroscopy (XPS). However, the analytical method is not limited to XPS.

[0043] The thickness of the first layer can be, for example, 1 nm or more. If a longer-term effect is desired, it can be set to 5 nm or more. If a more reliable effect is desired, it can be set to 10 nm or more. However, if the thickness of the first layer exceeds 50 nm, the first layer itself may become a resistive component. Therefore, the thickness of the first layer can be less than 50 nm or less than 30 nm.

[0044] The thickness of the second layer can be as thin as 1 nm or more, or as thin as 3 nm or more, or as thin as 5 nm or more. However, if the thickness of the second layer exceeds 20 nm, the second layer itself may become a resistive component. Therefore, the thickness of the second layer can be less than 20 nm or less than 10 nm.

[0045] From the viewpoint of reducing the initial low-temperature DCR, the ratio of the thickness T1 of the first layer to the thickness T2 of the second layer, T1 / T2, is preferably 1 or less. In this case, the thickness of the second layer is preferably 20 nm or less, and may also be 10 nm or less. However, from the viewpoint of forming a well-formed second layer, T1 / T2 is preferably 0.1 or more, for example, the T1 / T2 ratio may also be 0.2 or more.

[0046] The thicknesses of the first and second layers are determined by analyzing the surface portion formed on the surface of the negative electrode mixture layer at multiple locations (at least five locations). The average thickness of the first or second layer obtained at multiple locations is taken as the thickness of the first or second layer. It should be noted that the negative electrode mixture layer used for the test sample can be peeled off from the negative electrode current collector. In this case, the film formed on the surface of the negative electrode active material near the surface portion in the negative electrode mixture layer is analyzed. Specifically, the negative electrode active material covered by the film can be collected from the region of the negative electrode mixture layer disposed on the side opposite to the surface bonded to the negative electrode current collector and used for analysis.

[0047] In XPS analysis of the surface layer formed on the surface of the negative electrode reactant layer, for example, an argon beam is irradiated onto the film formed on the surface layer or the surface of the negative electrode active material within the chamber of an X-ray photoelectron spectrometer. The changes in the spectra attributed to C1s, O1s electrons, etc., with respect to irradiation time are observed and recorded. From the viewpoint of avoiding analytical errors, the spectrum of the outermost surface layer can be ignored. The thickness of the region where the peak attributed to lithium carbonate is consistently observed corresponds to the thickness of the first layer.

[0048] In the case of a negative electrode removed from an electrochemical capacitor that has undergone specified aging or at least one charge-discharge cycle, the surface portion formed on the surface of the negative electrode mixture layer has an SEI film (i.e., a second layer) containing a solid electrolyte. The thickness of the region attributable to the bond peaks of the compounds contained in the SEI film corresponds to the thickness of the SEI film (i.e., the thickness of the second layer).

[0049] The compounds contained in the SEI coating are selected from those containing elements that can serve as markers for the second layer. Elements that can serve as markers for the second layer can be, for example, elements contained in the electrolyte but not substantially contained in the first layer (e.g., F). For example, LiF can be selected as a compound containing elements that can serve as markers for the second layer.

[0050] When the second layer contains LiF, X-ray photoelectron spectroscopy is used to measure the second layer, and a peak attributable to the LiF bond, essentially an F1s peak, is observed. In this case, the thickness of the region where the peak attributable to the LiF bond is consistently observed corresponds to the thickness of the second layer.

[0051] On the other hand, the first layer typically does not contain LiF, and even when the first layer is measured using X-ray photoelectron spectroscopy, no substantial F1s peak attributable to LiF bonds is observed. Therefore, the thickness of the region where no peak attributable to LiF bonds can be stably observed can be taken as the thickness of the first layer.

[0052] O1s peaks attributable to lithium carbonate can also be observed in the SEI coating. However, the SEI coating formed within the electrochemical capacitor has a different composition than the pre-formed first layer, thus allowing for differentiation. For example, XPS analysis of the SEI coating reveals F1s peaks attributable to LiF bonds, while substantial F1s peaks attributable to LiF bonds are not observed in the first layer. Furthermore, the amount of lithium carbonate in the SEI coating is trace. It should be noted that peaks from compounds such as ROCOLi and ROLi can be detected as Li1s peaks.

[0053] When analyzing the first layer using XPS, in addition to the first O1s peak attributed to the C=O bond, a second O1s peak attributed to the Li-O bond can also be observed. The coated region near the surface of the negative electrode active material may contain small amounts of LiOH or Li2O.

[0054] Specifically, when analyzing the first layer contained in the surface portion of the negative electrode compound layer in the depth direction, following the order of increasing depth from the outermost surface of the surface portion, a first region can be observed where a first peak (attributed to O1s of C=O bonds) and a second peak (attributed to O1s of Li-O bonds) are observable, and the intensity of the first peak is greater than that of the second peak; and a second region where both the first and second peaks are observable, and the intensity of the second peak is greater than that of the first peak. Furthermore, a third region may also exist that is closer to the outermost surface of the surface portion than the first region, where the first peak is observable but the second peak is not. The third region is easily observed when the thickness of the lithium carbonate-containing region is large. It should be noted that the magnitude of the peak intensity can be determined by the height of the peak from the baseline.

[0055] In the center of the thickness direction of the first layer, C1s peaks attributable to CC bonds are usually not observed, or even if they are observed, they are less than half the intensity of peaks attributable to C=O bonds.

[0056] Next, a method for forming a first layer containing lithium carbonate on the surface of the negative electrode binder layer will be described. The process of forming the first layer can be performed, for example, by vapor phase method, coating method, transfer printing, etc.

[0057] As a vapor-phase method, examples include chemical vapor deposition, physical vapor deposition, and sputtering. For instance, lithium carbonate can be deposited onto the surface of the negative electrode mixture layer using a vacuum vapor deposition apparatus. The pressure within the apparatus chamber during vapor deposition is, for example, 10... -2 ~10 -5 Pa is sufficient; the temperature of the lithium carbonate evaporation source can be 400–600℃; and the temperature of the negative electrode binder layer can be -20–80℃.

[0058] As a coating method, for example, a microgravure coating machine can be used to coat a solution or dispersion containing lithium carbonate onto the surface of the negative electrode binder layer and then dry it to form the first layer. The lithium carbonate content in the solution or dispersion is, for example, 0.3 to 2% by mass. In the case of using a solution, a concentration below the solubility (for example, about 0.9 to 1.3% by mass if it is an aqueous solution at room temperature) is sufficient.

[0059] Furthermore, by performing a process of forming a second layer containing a solid electrolyte in a manner that covers at least a portion of the first layer, a negative electrode can be obtained. That is, a surface layer can be formed on the surface of the negative electrode mixture layer. The obtained surface layer has a first layer and a second layer. The second layer is formed such that at least a portion of it covers at least a portion (preferably the entire surface) of the negative electrode active material, with the first layer in between (i.e., the first layer serving as a base layer).

[0060] The process of forming the second layer is carried out while the negative electrode binder layer is in contact with the electrolyte, thus it can also serve as at least part of the pre-doping process of lithium ions into the negative electrode binder layer. For example, metallic lithium can be used as the lithium ion source for pre-doping.

[0061] Lithium metal can be attached to the surface of the negative electrode flux layer. It should be noted that by exposing the negative electrode with the lithium metal flux layer to a carbon dioxide atmosphere, for example, a first layer containing lithium carbonate with a thickness of more than 1 nm and less than 50 nm can also be formed.

[0062] The process of attaching metallic lithium to the surface of the negative electrode binder layer can be performed, for example, by vapor phase deposition or transfer. Examples of vapor phase deposition methods include chemical vapor deposition, physical vapor deposition, and sputtering. For instance, metallic lithium can be formed into a film on the surface of the negative electrode binder layer using a vacuum vapor deposition apparatus. The pressure within the apparatus chamber during vapor deposition is, for example, 10... -2 ~10 -5 Pa is sufficient, the temperature of the lithium evaporation source is 400-600℃, and the temperature of the negative electrode binder layer is -20-80℃.

[0063] The carbon dioxide atmosphere is preferably a dry atmosphere free of moisture, for example, with a dew point below -40°C or -50°C. The carbon dioxide atmosphere may contain gases other than carbon dioxide, but the molar fraction of carbon dioxide is preferably 80% or more, more preferably 95% or more. It is preferably free of oxidizing gases, and the molar fraction of oxygen is preferably 0.1% or less.

[0064] To make the first layer thicker, the partial pressure of carbon dioxide needs to be, for example, greater than 0.5 atmospheres (5.05 × 10⁻⁶). 4 Pa), is effective, and can also be 1 atmosphere (1.01 × 10⁻⁶). 5 Pa) or above.

[0065] The temperature of the negative electrode exposed to the carbon dioxide atmosphere can be, for example, in the range of 15°C to 120°C. The higher the temperature, the thicker the first layer.

[0066] The thickness of the first layer can be easily controlled by varying the exposure time of the negative electrode to the carbon dioxide atmosphere. The exposure time can be, for example, more than 12 hours or less than 10 days.

[0067] The process of forming the first layer is preferably performed before forming the electrode body, but it is not excluded that it may be performed after forming the electrode body. That is, a positive electrode can be prepared, a negative electrode with a negative electrode mixture layer with attached lithium metal can be prepared, a separator can be placed between the positive electrode and the negative electrode to form the electrode body, the electrode body can be exposed to a carbon dioxide atmosphere, and the first layer can be formed on the surface of the negative electrode mixture layer.

[0068] It should be noted that the pre-doping process of lithium ions into the negative electrode flux layer can be performed, for example, by contacting the negative electrode flux layer with the electrolyte and allowing it to stand for a specified time. This process can be a process of forming a second layer in a manner that covers at least a portion of the first layer. For example, by performing at least one charge-discharge cycle on an electrochemical capacitor, a second layer can be formed on the negative electrode flux layer, thus completing the pre-doping of lithium ions into the negative electrode. Alternatively, for example, pre-doping of lithium ions into the negative electrode can also be completed by applying a specified charging voltage (e.g., 3.4 to 4.0 V) for a specified time (e.g., 1 to 75 hours) between the terminals of the positive and negative electrodes.

[0069] Predoping of the negative electrode with lithium ions can be performed by contacting the negative electrode with an electrolyte that is conductive to lithium ions before assembling the electrochemical capacitor. For example, metallic lithium can be used as the lithium ion source for predoping. For instance, the negative electrode and a working electrode (e.g., a metal plate made of SUS) with the lithium ion source mounted on them are immersed in an electrolyte filled with a lithium ion-conductive solution, with a membrane sandwiched between them. The working electrode is used as the positive electrode, and a voltage is applied between the positive and negative electrodes, thereby enabling predoping (liquid-phase doping). The voltage can be applied, for example, under conditions where a specified constant current flows between the positive and negative electrodes. The voltage application time is, for example, 1 to 75 hours.

[0070] Lithium metal, which serves as a lithium-ion source, can be pre-attached to the surface of the negative electrode mixture layer. The negative electrode with attached lithium metal is then placed into the electrode cell, and a voltage is applied between the negative electrode and the working electrode to perform pre-doping (solid-phase doping method).

[0071] In liquid-phase doping, reducing the concentration of lithium salt in the electrolyte decreases the ionic conductivity of the electrolyte, leading to a longer pre-doping process. On the other hand, solid-phase doping pre-attaches metallic lithium to the surface of the negative electrode binder layer, thus the concentration of lithium salt in the electrolyte has less impact on the processing time of the pre-doping process.

[0072] Furthermore, in the process of forming the second layer (SEI film) in the electrochemical capacitor, the amount of lithium salt required to react with the negative electrode flux layer can be less when the first layer is formed using solid-phase doping compared to when liquid-phase doping is used. This is believed to be because the first layer formed by solid-phase doping covers the entire surface of the negative electrode flux layer, which is advantageous in ensuring sufficient film thickness. Therefore, it is possible to suppress excessive increases in the amount of electrolyte and electrolyte concentration in the electrochemical capacitor, making it easier to obtain an electrochemical capacitor with low internal resistance and high output.

[0073] Figure 1The structure of an electrochemical capacitor 200 according to one embodiment of the present invention is shown in schematic. The electrochemical capacitor 200 includes an electrode body 100, a non-aqueous electrolyte (not shown), a bottomed metal unit housing 210 that houses the electrode body 100 and the non-aqueous electrolyte, and a sealing plate 220 that seals the opening of the unit housing 210. A gasket 221 is disposed around the periphery of the sealing plate 220, and the interior of the unit housing 210 is sealed by riveting the open end of the unit housing 210 to the gasket 221. A positive current collector plate 13 with a through hole 13h in the center is welded to the exposed portion 11x of the positive current collector. The other end of a tab 15 connected to the positive current collector plate 13 is connected to the inner surface of the sealing plate 220. Therefore, the sealing plate 220 functions as an external positive terminal. On the other hand, a negative current collector plate 23 is welded to the exposed portion 21x of the negative current collector. The negative current collector 23 is directly welded to a welding component provided on the inner bottom surface of the unit housing 210. Therefore, the unit housing 210 functions as an external negative terminal.

[0074] The constituent elements of the electrochemical capacitor according to embodiments of the present invention will be described in more detail below.

[0075] (negative electrode) The negative electrode comprises a negative current collector and a negative electrode additive layer supported on the negative current collector. The negative electrode additive layer contains a negative electrode active material reversibly doped with lithium ions. The negative electrode active material preferably contains non-graphitizable carbon (i.e., hard carbon). The thickness of the negative electrode additive layer on each side of the negative current collector is, for example, 10–300 μm.

[0076] The negative electrode current collector uses a sheet-like metal material. This sheet-like metal material can be any material such as metal foil, porous metal, or etched metal. Suitable metal materials include copper, copper alloys, nickel, and stainless steel.

[0077] The negative current collector plate is a roughly disc-shaped metal plate. The material of the negative current collector plate can be, for example, copper, copper alloy, nickel, or stainless steel. The material of the negative current collector plate can be the same as that of the negative current collector itself.

[0078] The interplanar spacing (i.e., the interplanar spacing between carbon layers) d002 of the (002) plane, as measured by X-ray diffraction, of the non-graphitized carbon can be 3.8 Å or more. The theoretical capacity of the non-graphitized carbon is preferably, for example, 150 mAh / g or more. By using non-graphitized carbon, it is easy to obtain a negative electrode with low-temperature DCR and minimal expansion and contraction during charging and discharging. The non-graphitized carbon preferably constitutes 50% or more by mass of the negative electrode active material, more preferably 80% or more by mass, and even more preferably 95% or more by mass. Furthermore, the non-graphitized carbon preferably constitutes 40% or more by mass of the negative electrode binder layer, more preferably 70% or more by mass, and even more preferably 90% or more by mass.

[0079] As a negative electrode active material, it can also be used in combination with materials other than difficult-to-graphitize carbon. Examples of materials other than difficult-to-graphitize carbon that can be used as a negative electrode active material include easily graphitizeable carbon (soft carbon), graphite (natural graphite, artificial graphite, etc.), lithium titanium oxide (spinel-type lithium titanium oxide, etc.), silicon oxide, silicon alloy, tin oxide, tin alloy, etc.

[0080] From the viewpoint that the negative electrode active material has high filling capacity and can easily suppress side reactions with the electrolyte, the average particle size of the negative electrode active material (especially the non-graphitized carbon) is preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm.

[0081] It should be noted that, in this specification, the average particle size refers to the median particle size (D50) of the volume reference in the particle size distribution obtained by laser diffraction particle size distribution measurement.

[0082] The negative electrode binder layer contains a negative electrode active material as an essential component, and conductive agents (conductive additives), binders, etc., as optional components. Examples of conductive agents include carbon black and carbon fiber. Examples of binders include fluoropolymers, acrylic resins, rubber materials, and cellulose derivatives.

[0083] The negative electrode binder layer is formed, for example, by mixing the negative electrode active material, conductive agent and binder with a dispersion medium to prepare a negative electrode binder slurry, and then coating the negative electrode binder slurry onto the negative electrode current collector and drying it.

[0084] Lithium ions are pre-doped into the negative electrode flux layer. This lowers the potential of the negative electrode, thus increasing the potential difference (i.e., voltage) between the positive and negative electrodes, and improving the energy density of the electrochemical capacitor. The amount of pre-doped lithium can be set, for example, to approximately 50% to 95% of the maximum amount that can be embedded in the negative electrode flux layer.

[0085] The potential of the negative electrode is, for example, below 0.2V based on lithium. This negative electrode potential is the potential of the negative electrode when lithium-ion predoping is complete (or during charging) (25°C). Lithium ions are predoped in the first and second regions of the negative electrode (negative electrode flux layer). This lowers the negative electrode potential, increases the potential difference (i.e., voltage) between the positive and negative electrodes, and improves the energy density of the electrochemical capacitor. The amount of predoped lithium is set such that the potential of the negative electrode in the electrolyte after predoping is below 0.2V relative to metallic lithium.

[0086] The capacitance per unit mass of the negative electrode active material should be, for example, 1000 F / g or higher. However, from the viewpoint of increasing the capacitance density of the electrochemical capacitor, the capacitance per unit mass of the negative electrode active material should be, for example, 30000 F / g or lower. The capacitance per unit mass of the negative electrode active material is typically greater than that of the positive electrode active material, for example, 20 to 800 times greater. It should be noted that the capacitance per unit mass of the negative electrode active material can be determined by the following method.

[0087] First, an evaluation negative electrode was prepared, cut to a size of 31mm × 41mm. As the counter electrode of the negative electrode, a 100μm thick lithium metal foil, cut to a size of 40mm × 50mm, was prepared. Using 25μm thick cellulose paper (e.g., product number TF4425) manufactured by Japan Kokkuri Paper Industry Co., Ltd. as a separator, the negative electrode mixture layer and the lithium metal foil were placed opposite each other to form an electrode body. The electrode body was then immersed in the electrolyte of Example 1 described later to form a cell.

[0088] The cell was charged with a constant current (CC) of 0.5mA until the cell voltage reached 0.01V, then charged with a constant voltage (CV) for 1 hour, and then discharged with 0.5mA until the cell voltage reached 1.5V. The capacitance per unit mass of the negative electrode active material was calculated from the discharge time during which the potential of the negative electrode changed by 0.1V from 1 minute after the start of discharge.

[0089] (positive electrode) The positive electrode comprises a positive current collector and a positive electrode flux layer containing activated carbon supported on the positive current collector. The positive electrode flux layer contains a positive electrode active material reversibly doped with anions, and the positive electrode active material contains activated carbon. The thickness of the positive electrode flux layer is, for example, 10–300 μm on each single side of the positive current collector.

[0090] The positive current collector uses sheet-like metal materials. These sheet-like metal materials can be metal foils, porous metal bodies, etched metals, etc. Suitable metal materials include aluminum, aluminum alloys, nickel, and titanium.

[0091] The positive current collector is a generally disc-shaped metal plate. Preferably, a through hole is formed in the center of the positive current collector to provide a passage for non-aqueous electrolyte. The material of the positive current collector is, for example, aluminum, aluminum alloy, titanium, stainless steel, etc. The material of the positive current collector can be the same as that of the positive current collector.

[0092] As the carbon material used as the positive electrode active material, porous carbon materials are preferred, such as activated carbon, and carbon materials exemplified as negative electrode active materials (e.g., non-graphitized carbon). Examples of raw materials for activated carbon include wood, coconut shells, coal, asphalt, and phenolic resins. Activated carbon that has undergone activation treatment is preferred.

[0093] The average particle size of activated carbon is not particularly limited, but it is preferably less than 20 μm, and more preferably 3 μm to 15 μm.

[0094] The specific surface area of ​​the positive electrode additive layer roughly reflects the specific surface area of ​​the positive electrode active material. For example, the specific surface area of ​​the positive electrode additive layer might be 1000 m². 2 / g or more and 2000m 2 Below / g is acceptable, preferably 1200m 2 / g or more and 1800m 2 / g or less. The specific surface area of ​​the positive electrode mixture layer is the BET specific surface area determined using a measuring apparatus according to JIS Z8830 (e.g., Shimadzu Corporation Tristar II 3020). Specifically, the electrochemical capacitor is disassembled, and the positive electrode is removed. Next, the positive electrode is cleaned with DMC and dried. Then, the positive electrode mixture layer is peeled off from the positive electrode current collector, and a sample of approximately 0.5g of the positive electrode mixture layer is collected. Next, the specific surface area of ​​the collected sample is determined according to the method for determining the specific surface area of ​​the negative electrode mixture layer described above.

[0095] Activated carbon preferably accounts for 50% or more of the positive electrode active material, more preferably 80% or more, and even more preferably 95% or more. Furthermore, activated carbon preferably accounts for 40% or more of the positive electrode additive layer, more preferably 70% or more, and even more preferably 90% or more.

[0096] The positive electrode binder layer contains a positive electrode active material as an essential component, and conductive agents, binders, etc., as optional components. Examples of conductive agents include carbon black and carbon fibers. Examples of binders include fluoropolymers, acrylic resins, rubber materials, and cellulose derivatives.

[0097] For example, a positive electrode additive layer is formed by mixing a positive electrode active material, a conductive agent, and a binder with a dispersion medium to prepare a positive electrode additive slurry, and then coating the positive electrode additive slurry onto a positive electrode current collector and drying it.

[0098] (Septum) As the diaphragm, nonwoven fabrics made of cellulose fibers, nonwoven fabrics made of glass fibers, microporous membranes made of polyolefins, woven fabrics, or nonwoven fabrics can be used. The thickness of the diaphragm is, for example, 8 to 300 μm, preferably 8 to 40 μm.

[0099] (electrolyte) Electrolytes are lithium-ion conductive and, for example, contain lithium salts and solvents that dissolve the lithium salts. The lithium salt dissolved in the solvent contains lithium ions and anions. The lithium salt anions can reversibly and repeatedly dop and dedopate into the positive electrode. Lithium ions derived from the lithium salt can reversibly intercalate and deintercalate into the negative electrode.

[0100] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, and LiB. 10 Cl 10 LiCl, LiBr, LiI, LiBCl4, LiN(FSO2)2, LiN(CF3SO2)2, etc. These can be used individually or in combination of two or more. Salts containing fluoride anions are preferred, and lithium bis(fluorosulfonyl)imide, i.e., LiN(SO2F)2, is particularly preferred. The concentration of the lithium salt in the electrolyte at the state of charge (SOC) of 90–100% is, for example, 0.2–5 mol / L. Hereinafter, LiN(SO2F)2 will be referred to as LiFSI. For example, 80% by mass or more of the lithium salt can be LiFSI.

[0101] By using LiFSI, the increase rate of low-temperature DCR tends to be significantly reduced. LiFSI is believed to have the effect of reducing the degradation of both positive and negative electrode active materials. It is believed that in salts containing fluoride anions, the stability of FSI anions is excellent, thus reducing the likelihood of byproduct formation and preventing damage to the surface of the active materials, which facilitates smooth charge and discharge. Especially when increasing the capacity of the positive electrode and the specific surface area of ​​the negative electrode binder layer, the degradation inhibition effect (the inhibition of low-temperature DCR increase) brought about by using LiFSI, which significantly reduces the impact of byproducts on each active material, becomes significant.

[0102] As solvents, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butyl carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate, and ethyl propionate; lactones such as γ-butyrolactone and γ-valerolactone; chain ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), and ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; dimethyl sulfoxide; 1,3-dioxolane; formamide; acetamide; dimethylformamide; dioxolane; acetonitrile; propionitrile; nitromethane; ethyl monoethylene glycol dimethyl ether; trimethoxymethane; sulfolane; methyl sulfolane; and 1,3-propane sulpholactone can be used. These can be used alone or in combination of two or more.

[0103] The electrolyte can contain various additives as needed. For example, as an additive to form a lithium-ion conductive film on the negative electrode surface, unsaturated carbonates such as vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate can be added.

[0104] (Postscript) The following technology is disclosed through the above description.

[0105] (Technology 1) An electrochemical capacitor comprises: a positive electrode including a positive current collector and a positive electrode flux layer containing activated carbon supported on the positive current collector; a negative electrode including a negative current collector and a negative electrode flux layer containing reversibly doped lithium ions supported on the negative current collector; and an electrolyte comprising an electrolyte and a non-aqueous solvent containing lithium ions and anions, wherein the ratio of the amount of anions to the surface area of ​​the positive electrode flux layer is 1.5 μmol / m². 2 Above and 2.4 μmol / m 2 the following.

[0106] (Technology 2) According to the electrochemical capacitor of technology 1, the ratio of the amount of anions to the surface area of ​​the positive electrode coating layer is 1.7 μmol / m². 2 Above and 2.2 μmol / m 2 the following.

[0107] (Technology 3) The electrochemical capacitor according to technique 1 or 2, wherein the electrolyte comprises lithium bis(fluorosulfonyl)imide.

[0108] (Technology 4) The electrochemical capacitor according to any one of techniques 1 to 3, wherein the negative electrode active material comprises non-graphitized carbon.

[0109] (Technology 5) The electrochemical capacitor according to any one of techniques 1 to 4, wherein the potential of the negative electrode is less than 0.2V based on lithium.

[0110] (Technology 6) According to the electrochemical capacitor of technology 1, a surface layer having a first layer containing lithium carbonate is formed on the surface of the negative electrode compound layer.

[0111] (Technology 7) According to the electrochemical capacitor of the technology 6, the surface portion formed on the surface of the negative electrode mixture layer has a second layer comprising a solid electrolyte, at least a portion of the second layer covering at least a portion of the surface of the negative electrode mixture layer through the first layer.

[0112] (Technology 8) According to the electrochemical capacitor of technology 7, the second layer contains lithium carbonate, and the content of the lithium carbonate in the second layer is less than the content of the lithium carbonate in the first layer.

[0113] (Technology 9) The electrochemical capacitor according to any one of techniques 6 to 8, wherein the thickness of the first layer is 1 nm or more and 50 nm or less.

[0114] (Technology 10) According to the electrochemical capacitor of technique 7 or 8, when the first layer is measured by X-ray photoelectron spectroscopy, no substantial F1s peak belonging to the LiF bond is observed, but when the second layer is measured by X-ray photoelectron spectroscopy, a substantial F1s peak belonging to the LiF bond is observed.

[0115] The present invention has been described with respect to the currently preferred embodiments, but such disclosure should not be interpreted as limiting. Various modifications and alterations will be readily apparent to those skilled in the art to which this invention pertains upon reading the above disclosure. Therefore, it should be understood that the appended claims encompass all modifications and alterations without departing from the true spirit and scope of the invention.

[0116] [Example] The present invention will now be described in more detail based on embodiments, but the present invention is not limited to the embodiments. Furthermore, the structural outlines of the various electrochemical capacitors fabricated below are shown in Table 1.

[0117] (Electrochemical capacitors A1~A5, B1~B2) (1) Production of the positive electrode A 20 μm thick aluminum foil (positive electrode current collector) was prepared. Meanwhile, 88 parts by mass of activated carbon (average particle size 5.5 μm) as the positive electrode active material, 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder, 4 parts by mass of carboxymethyl cellulose (CMC) as a thickener, and 6 parts by mass of acetylene black (AB) as a conductive aid were dispersed in water to prepare a positive electrode slurry. The obtained positive electrode slurry was coated on both sides of the aluminum foil, and the coating was dried and calendered to form a positive electrode slurry layer, thus obtaining the positive electrode. A 10 mm wide exposed portion of the positive electrode current collector was formed at the end of the positive electrode current collector along its length. The weight of the positive electrode slurry layer contained in each unit of the electrochemical capacitor in the obtained positive electrode was 2.3 g. Furthermore, the specific surface area b of the positive electrode slurry layer was 1407 m². 2 / g, the surface area B of the positive electrode mixture layer is 3285m² per unit of the electrochemical capacitor.2 .

[0118] (2) Fabrication of the negative electrode Prepare a copper foil (negative electrode current collector) with a thickness of 8 μm. Meanwhile, disperse non-graphitized carbon (HC) (average particle size 5 μm), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) as the negative electrode active materials in water to prepare a negative electrode slurry. Coat the obtained negative electrode slurry onto both sides of the copper foil, dry and calender the coating to form a negative electrode layer, thus obtaining the negative electrode.

[0119] Then, a thin film of pre-doped lithium metal is formed on the entire surface of the negative electrode flux layer by vacuum evaporation. The amount of pre-doped lithium is set such that the potential of the negative electrode in the non-aqueous electrolyte after pre-doping is below 0.2V relative to the lithium metal.

[0120] Then, the chamber of the device is purged with carbon dioxide to create a carbon dioxide atmosphere, thereby forming a first layer containing lithium carbonate on the surface of the negative electrode binder layer. The dew point of the carbon dioxide atmosphere is -40°C, the molar fraction of carbon dioxide is 100%, and the pressure inside the chamber is 1 atmosphere (1.01 × 10⁻⁶). 5 Pa). The temperature of the negative electrode exposed to a carbon dioxide atmosphere at 1 atmosphere was set to 25°C. The exposure time of the negative electrode to the carbon dioxide atmosphere was set to 22 hours. The first layer substantially contained no F (or LiF).

[0121] The mixing ratio of non-graphitizable carbon (HC), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in the negative electrode slurry is HC:CMC:SBR = 97:1:2 by mass.

[0122] (3) Fabrication of the electrode body The positive and negative electrodes are separated by a 25μm thick cellulose nonwoven fabric separator and wound into a cylindrical shape to form an electrode body. The positive current collector protrudes from one end face of the wound body, and the negative current collector protrudes from the other end face. Disc-shaped positive and negative current collector plates are then welded to the exposed positive and negative current collector portions, respectively.

[0123] (4) Preparation of non-aqueous electrolyte Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 3:5:2, and then 0.2% by mass of vinylene carbonate (VC) was added to prepare a solvent. LiFSI, as a lithium salt, was dissolved in the obtained solvent at the molar concentrations shown in Table 1 to prepare a non-aqueous electrolyte.

[0124] (5) Assembly of electrochemical capacitors The electrodes are housed in a bottomed, open-ended unit housing. The lead wires for the positive current collector are connected to the inner surface of a sealing plate. Then, the negative current collector is welded to the inner bottom surface of the unit housing. After adding non-aqueous electrolyte to the unit housing according to the quantities shown in Table 1, the opening of the unit housing is sealed with the sealing plate, and the assembly is performed as follows: Figure 1 The electrochemical capacitor shown.

[0125] Then, while applying a charging voltage of 3.8V between the terminals of the positive and negative electrodes, aging is carried out at 60°C to complete the pre-doping of lithium ions into the negative electrode.

[0126] In this way, while changing the molar concentration of the electrolyte and the mass of the electrolyte, multiple electrochemical capacitors A1 to A5 and B1 to B2 are manufactured, and the following evaluation is performed.

[0127] (6) Evaluation (Determination of the internal resistance of an electrochemical capacitor) For newly aged electrochemical capacitors, at -30℃, with a positive electrode area of ​​2 mA / cm², 2 A constant current was applied until the voltage reached 3.8V, and then the 3.8V voltage was maintained for 10 minutes. Then, at -30°C, the voltage was increased to 2 mA / cm² per unit positive electrode area. 2 The current density is used to discharge at a constant current until the voltage reaches 2.2V.

[0128] In the discharge curve obtained through the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), find an approximate straight line within the range of 0.5 seconds to 2 seconds from the start of the discharge, and calculate the intercept voltage VS of this approximate straight line. Subtract the voltage VS from the voltage V0 at the start of the discharge (0 seconds after the start of the discharge) to obtain the value (V0-VS), and use this value as ΔV. Use ΔV (V) and the discharge current value (current density per unit positive electrode area, 2 mA / cm²) to calculate ΔV. 2 × positive electrode area), the internal resistance (DCR) R1 (Ω) of the electrochemical capacitor can be calculated from the following formula.

[0129] Internal resistance R1 = ΔV / Id (Float charge test of electrochemical capacitors) Next, a float charge test was conducted on the electrochemical capacitor at 85°C with a constant voltage of 3.8V applied for 1000 hours. Then, the internal resistance (DCR) R2 (Ω) of the electrochemical capacitor was calculated at -30°C in the same manner as R1.

[0130] Table 2 shows the capacitance C2 and internal resistance R2 of electrochemical capacitors A1-A5 and B1-B2 after float charge tests. In Table 2, C2 and R2 are expressed as relative values ​​with C2 and R2 of electrochemical capacitor A3 as 100. Electrochemical capacitors A1-A5 are examples, and B1-B2 are comparative examples.

[0131] In addition, Table 2 shows the ratio A / B of the above-mentioned anion content to the surface area of ​​the positive electrode compound layer together with the evaluation results after the float charge test.

[0132] According to Table 2, the A / B ratio is between 1.5 and 2.4 μmol / m 2 Electrochemical capacitors A1 to A5 exhibit low internal resistance R2 after float charging tests, resulting in an A / B ratio of 1.7 to 2.2 μmol / m. 2 In the electrochemical capacitors A2 to A4, the internal resistance R2 is significantly reduced after the float charge test, resulting in excellent output characteristics.

[0133] On the other hand, electrochemical capacitor B1, due to its small A / B ratio, not only failed to exhibit sufficient capacitance, but its internal resistance R2 also increased after the float charge test. This is presumably because the amount of anions A reversibly doped onto the surface of the positive electrode binder layer is insufficient. Conversely, electrochemical capacitor B2, due to its large A / B ratio, exhibited sufficient capacitance, but its internal resistance R2 increased after the float charge test.

[0134] Comparison of electrochemical capacitors A1-A5, B1, and B2 shows that the internal resistance R2 after the float charge test is 2.0 μmol / m in the A / B ratio. 2 The lowest value was found in electrochemical capacitor A3, with an A / B ratio of 1.5–2.4 μmol / m. 2 Within a certain range, the internal resistance R2 can be maintained at a sufficiently low value after the float charge test.

[0135] Industrial applicability The electrochemical capacitor of the present invention is suitable, for example, for automotive applications.

[0136] Figure Labels 100: Electrode body 10: Positive electrode 11x: Exposed portion of the positive current collector 13: Positive current collector 15: Electrode 20: Negative electrode 21x: Exposed portion of the negative current collector 23: Negative current collector 30: Diaphragm 200: Electrochemical capacitor 210: Unitary Shell 220: Sealing board 221: Washer

Claims

1. An electrochemical capacitor comprising: A positive electrode, comprising a positive current collector and a positive electrode additive layer containing activated carbon and supported on the positive current collector; A negative electrode, comprising a negative electrode current collector and a negative electrode active material containing reversibly doped lithium ions and supported on the negative electrode current collector; and Electrolyte, which contains an electrolyte with lithium ions and anions and a non-aqueous solvent. The ratio of the anion content to the surface area of ​​the positive electrode mixture layer is 1.5 μmol / m². 2 Above and 2.4 μmol / m 2 the following.

2. The electrochemical capacitor according to claim 1, wherein, The ratio of the anion content to the surface area of ​​the positive electrode mixture layer is 1.7 μmol / m². 2 Above and 2.2 μmol / m 2 the following.

3. The electrochemical capacitor according to claim 1 or 2, wherein, The electrolyte contains lithium bis(fluorosulfonyl)imide.

4. The electrochemical capacitor according to claim 1 or 2, wherein, The negative electrode active material contains non-graphitized carbon.

5. The electrochemical capacitor according to claim 1 or 2, wherein, The potential of the negative electrode is below 0.2V based on lithium.

6. The electrochemical capacitor according to claim 1, wherein, A surface layer containing a first layer of lithium carbonate is formed on the surface of the negative electrode mixture layer.

7. The electrochemical capacitor according to claim 6, wherein, The surface portion formed on the surface of the negative electrode mixture layer has a second layer containing a solid electrolyte. At least a portion of the second layer is separated from at least a portion of the surface of the first layer covering the negative electrode mixture layer.

8. The electrochemical capacitor according to claim 7, wherein, The second layer contains lithium carbonate. The second layer contains less lithium carbonate than the first layer.

9. The electrochemical capacitor according to any one of claims 6 to 8, wherein, The thickness of the first layer is greater than 1 nm and less than 50 nm.

10. The electrochemical capacitor according to claim 7 or 8, wherein, When the first layer was measured using X-ray photoelectron spectroscopy, no F1s peak belonging to the LiF bond was observed. When the second layer was measured using X-ray photoelectron spectroscopy, the F1s peak, which is essentially attributed to the LiF bond, was observed.

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