Electrolyte for electricity storage device containing ion-modified cellulose

Abstract

Provided is an electrolyte for an electricity storage device, such as a capacitor or a secondary battery, which contains a substance having a low environmental burden and high safety as a charge carrier. An ion-modified cellulose is used as a charge carrier for an electrolyte for an electricity storage device. The ion-modified cellulose is preferably an anion-modified cellulose, or an anion-modified cellulose nanofiber.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrolyte for an electricity storage device containing ionically modified cellulose. BACKGROUND

[0002] Electric double layer capacitors (EDLCs) do not contain the above-mentioned metals and the like, and are attracting attention in terms of low environmental load. However, as a problem of EDLCs, environmental load caused by the use of organic solvents and the like as liquid electrolytes can be cited.

[0003] In fact, from the viewpoints of solubility and voltage resistance, in EDLCs, organic solvents such as carbonate-based and nitrile-based solvents are often used as electrolytes. Systems using organic solvents are preferable in terms of voltage resistance, but in addition to concerns about the safety of organic solvents themselves and environmental load, management and manufacturing in a dry room are required in order to prevent the effects of moisture in the air, and as a result, there is a problem that manufacturing costs become high.

[0004] Therefore, in order to reduce environmental load, it is preferable to use water-based electrolytes. As a general electrolyte in water-based electrolytes, aqueous sulfuric acid and potassium hydroxide solutions can be cited. However, although they do not require management in a dry room or the like, they have high polarity and are a concern in terms of safety such as leakage at the time of breakage. In addition, in EDLCs that do not contain rare metals, the substances used in electrolytes are also mainly synthetic materials. If the reduction of environmental load and the improvement of safety are considered, it is preferable to use mainly materials derived from nature.

[0005] In Patent Literature 1, it is described that leakage of electrolyte is suppressed by using a gel electrolyte. However, a polar solvent is used as an electrolyte, and the environmental load is high. In addition, it is described that cellulose nanofiber is used as a thickening agent, but the cellulose nanofiber does not have ionicity and does not function as a charge carrier in the electrolyte.

[0006] In Patent Literature 2, an electrolyte layer containing water is disclosed, and although it is described that it is preferable to contain a polysaccharide such as cellulose or cellulose nanofiber in the electrolyte layer, they are used in order to improve mechanical strength, do not have ionicity, and do not function as a charge carrier in the electrolyte layer.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent Application Publication No. 2019-62166

[0010] Patent Literature 2: Japanese Patent Application Publication No. 2021-145120 SUMMARY

[0011] The present application provides an electrolyte for an electric power storage device such as a capacitor or a secondary battery, which contains a substance having low environmental load and high safety as a charge carrier.

[0012] The present inventors have intensively studied the above object, and as a result, found that a modified cellulose (ionically modified cellulose) formed by introducing an ionic group into cellulose can be used as a charge carrier in an electrolyte for an electric power storage device. Cellulose from nature itself does not have ionicity and does not have a function as a charge carrier, but by introducing an ionic group into cellulose through chemical treatment, it can be used as a charge carrier in an electrolyte for an electric power storage device. The ionically modified cellulose can be produced using cellulose from nature as a raw material, and is a material having low environmental load. In addition, an electrolyte using ionically modified cellulose as a charge carrier can use water as a main medium, and can be said to have high safety. The present application includes the following.

[0013] [1] An electrolyte for an electric power storage device, containing ionically modified cellulose as a charge carrier.

[0014] [2] The electrolyte for an electric power storage device according to [1], wherein the ionically modified cellulose is ionically modified cellulose nanofiber.

[0015] [3] The electrolyte for an electric power storage device according to [1], wherein the ionically modified cellulose is anionically modified cellulose.

[0016] [4] The electrolyte for an electric power storage device according to [1], wherein the ionically modified cellulose is anionically modified cellulose nanofiber.

[0017] [5] The electrolyte for an electric power storage device according to [3], wherein the anionically modified cellulose is carboxymethyl cellulose.

[0018] [6] The electrolyte for an electric power storage device according to [4], wherein the anionically modified cellulose nanofiber is carboxylated cellulose nanofiber.

[0019] [7] The electrolyte for an electric power storage device according to any one of [1] to [4], wherein the content of the ionically modified cellulose is 0.01 to 90 mass%.

[0020] [8] The electrolyte for an electric power storage device according to any one of [1] to [4], wherein the electric power storage device is a capacitor or a secondary battery.

[0021] According to the present application, by using ion-modified cellulose, which is a natural cellulose as a raw material, as a charge carrier in an electrolyte for an electricity storage device, an electricity storage device with low environmental load can be provided. In addition, water can be used as a medium of the electrolyte, and a safe device can be provided. DETAILED DESCRIPTION

[0022] The present application is an invention of an electrolyte for an electricity storage device, which contains ion-modified cellulose, in which an ionic group is introduced into cellulose, as a charge carrier. An electricity storage device such as a capacitor or a secondary battery generally has a positive electrode and a negative electrode and an electrolyte between them. The electrolyte functions to suppress direct contact of the positive electrode with the negative electrode while performing transport of electric charges between the positive electrode and the negative electrode. The present application is based on the finding that ion-modified cellulose can be used as a substance (charge carrier) that performs transport of electric charges in an electrolyte. Ion-modified cellulose can be produced using natural cellulose as a raw material, and thus an electrolyte containing ion-modified cellulose as a charge carrier has the advantage of low environmental load. In addition, an electrolyte containing ion-modified cellulose as a charge carrier can form an aqueous electrolyte using water as a main medium, and thus has the advantage of safety in addition to low environmental load.

[0023] (Ion-modified cellulose)

[0024] "Ion-modified cellulose" refers to ion-modified cellulose in which an ionic group is introduced into a molecular chain of cellulose by chemical treatment of a cellulose raw material. The kind of cellulose raw material used to obtain ion-modified cellulose is not particularly limited. For example, bleached or unbleached mechanical pulp (e.g., thermomechanical pulp (TMP), groundwood pulp) or chemical pulp (e.g., sulfite pulp, kraft pulp) using coniferous trees, broad-leaved trees, cotton, rice straw, bamboo, hemp, jute, kenaf, or the like as a raw material, in addition to dissolving pulp, regenerated cellulose, microfine cellulose, microcrystalline cellulose other than non-crystalline regions, and the like can be used as the cellulose raw material. Any one of these can be used as the cellulose raw material.

[0025] Ion-modified cellulose can be produced by introducing an ionic group into a cellulose raw material. The ionic group can be an anionic group or a cationic group. The method of introducing these ionic groups is not particularly limited. For example, the method described later can be used.

[0026] (Anion-modified cellulose)

[0027] Cellulose with anionic groups introduced as ionic groups is called "anionic modified cellulose". There are no particular limitations on the method of introducing anionic groups, but for example, as described later, methods such as directly oxidizing the hydroxyl groups of the pyranose ring of cellulose to carboxyl groups or introducing anionic groups into the hydroxyl portion of the pyranose ring through etherification or esterification reactions can be cited.

[0028] (Carboxylated cellulose)

[0029] As an example of anionic modified cellulose, carboxylated cellulose with a carboxyl group can be cited. The carboxyl group (-COOH) in the electrolyte can be -COOM (metal salt type) (where M is a metal ion) or -COO ionized from a metal ion. － The form of cellulose. Carboxylated cellulose can be obtained using known methods that oxidize the hydroxyl groups of the pyranose ring of cellulose to carboxyl groups. Examples of cellulose oxidation methods include, for instance, oxidizing cellulose in water using an oxidizing agent in the presence of N-oxygen compounds such as 2,2,6,6-tetramethylpiperidine-1-oxygen radical (TEMPO) and bromides and / or iodides, and oxidizing cellulose by contacting the cellulose raw material with a gas containing ozone as an oxidizing agent.

[0030] The amount of carboxyl groups in carboxylated cellulose is preferably 0.4–3.0 mmol / g relative to the oven-dry weight of carboxylated cellulose, more preferably 0.6–2.0 mmol / g, more preferably 1.0–2.0 mmol / g, and more preferably 1.1–2.0 mmol / g. The amount of carboxyl groups in carboxylated cellulose can be adjusted by controlling reaction conditions such as the amount of oxidant added and the reaction time. The amount of carboxyl groups can be determined by the following methods:

[0031] Prepare 60 ml of 0.5% by mass slurry (aqueous dispersion) of carboxylated cellulose. Add 0.1 M hydrochloric acid aqueous solution to set the pH to 2.5, then add 0.05 N sodium hydroxide aqueous solution dropwise. Measure the conductivity until the pH reaches 11. Calculate the amount of sodium hydroxide consumed (a) during the neutralization phase of the weak acid with a slow change in conductivity using the following formula:

[0032] Carboxyl group content [mmol / g carboxylated cellulose] = a [ml] × 0.05 / mass of carboxylated cellulose [g].

[0033] (Carboxyalkylated cellulose)

[0034] As an example of anion-modified cellulose, carboxyalkylated cellulose having a carboxyalkyl group can be given. In the present specification, a carboxyalkyl group (-RCOOH) (in the formula, R is an alkylene group such as methylene, ethylene, or the like) can be -RCOOM (metal salt type) (in the formula, M is a metal ion) in an electrolyte or -RCOO" ionized by a metal ion. － As the carboxyalkylated cellulose, carboxymethylated cellulose (hereinafter, "carboxymethyl" is referred to as "CM") in which R is methylene is most preferable. The carboxyalkylated cellulose can be obtained using a publicly known method in which a carboxyalkyl group is introduced by treating a cellulose raw material with a mercerizing agent and then treating with a carboxyalkylating agent.

[0035] The degree of substitution of the carboxyalkyl group in the anhydroglucose unit of the carboxyalkylated cellulose (hereinafter, also simply referred to as "degree of substitution") is not particularly limited. If the degree of substitution is 0.40 or more, carboxyalkylated cellulose that is easily soluble in water can be obtained. In addition, if it is less than 0.40, carboxyalkylated cellulose that easily maintains a fibrous form in water can be obtained.

[0036] Carboxyalkylated cellulose having a high degree of substitution is easily ionized and is preferable from the viewpoint of performance as a charge carrier. The degree of substitution of carboxyalkylated cellulose that is easily soluble in water and has a high degree of substitution is more preferably 0.50 or more, further preferably 0.60 or more, further preferably 0.70 or more, and further preferably 0.80 or more.

[0037] On the other hand, for carboxyalkylated cellulose that has a low degree of substitution and easily maintains a fibrous form in water, the fiber diameter is reduced by fibrillation described later, whereby it is possible to convert to fibrillated cellulose, cellulose nanofiber. Fibrillated cellulose, cellulose nanofiber having a small fiber diameter is easily dispersed uniformly in an electrolyte, and thus is preferable from the viewpoint of ionic conductivity. The degree of substitution of carboxyalkylated cellulose that easily maintains a fibrous form in water and has a low degree of substitution is preferably 0.10 or more and less than 0.40, more preferably 0.10 to 0.35, and further preferably 0.15 to 0.30.

[0038] Note that the anhydroglucose unit refers to each anhydroglucose (glucose residue) constituting cellulose, and the degree of substitution indicates the proportion of the portion (the number of carboxyalkyl groups in one glucose residue) in which the hydroxyl group (-OH) in the glucose residue constituting cellulose is substituted with a carboxyalkyl group (-ORCOOH or -ORCOOM). The degree of substitution can be adjusted by controlling the amount of mercerizing agent, the reaction time, and the like. For example, the CM degree of substitution can be measured by the following method:

[0039] Precisely weigh about 2.0 g of CM cellulose (absolute dry) and put it into a 300 mL capacity flask with a stopper. Add 100 mL of liquid prepared by adding 100 mL of special grade concentrated nitric acid to 900 mL of methanol, and shake for 3 hours to convert the salt type CM cellulose into the hydrogen type CM cellulose. Precisely weigh 1.5 g to 2.0 g of the hydrogen type CM cellulose (absolute dry) and put it into a 300 mL capacity flask with a stopper. Wet the hydrogen type CM cellulose with 15 mL of 80 mass% methanol, add 100 mL of 0.1 N NaOH, and shake for 3 hours at room temperature. Use phenothalin as an indicator and back-titrate the excess NaOH with 0.1 N H2SO4. Calculate the CM degree of substitution (DS) by the following formula:

[0040] A = [(100 x F' - (mL of 0.1 N H2SO4) x F) x 0.1] / (absolute dry mass of hydrogen type CM cellulose (g))

[0041] DS = 0.162 x A / (1 - 0.058 x A)

[0042] A: 1 N NaOH amount (mL) required for 1 g of hydrogen type CM cellulose to be neutralized

[0043] F: factor of 0.1 N H2SO4

[0044] F': factor of 0.1 N NaOH

[0045] The degree of substitution of the carboxyalkyl group other than the CM group can also be measured by the same method as described above.

[0046] (phosphoric acid esterified cellulose)

[0047] As an example of anionic modified cellulose, phosphoric acid esterified cellulose can be given. The phosphoric acid esterified cellulose can be obtained by mixing a powder or an aqueous solution of a phosphoric acid compound or adding an aqueous solution of a phosphoric acid compound to a slurry of a cellulose raw material, and the like, in the above-described cellulose raw material, thereby introducing a phosphoric acid group from the phosphoric acid compound into the cellulose. As the phosphoric acid compound, phosphoric acid, polyphosphoric acid, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, or an ester or a salt thereof can be given. Specifically, although not limited thereto, for example, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate, and the like can be given. One or two or more of them can be used in combination to introduce a phosphoric acid group from the phosphoric acid compound into the cellulose. In the present specification, the phosphoric acid group from the phosphoric acid compound includes a phosphoric acid group, a phosphorous acid group, a hypophosphorous acid group, a pyrophosphoric acid group, a metaphosphoric acid group, a polyphosphoric acid group, a phosphonic acid group, and a polyphosphonic acid group. The phosphoric acid esterified cellulose includes one or two or more of the phosphoric acid esterified celluloses in which one or two or more of these phosphoric acid groups are introduced into the molecular chain of the cellulose. In the reaction of the cellulose raw material with the phosphoric acid compound, the phosphoric acid compound is preferably used as an aqueous solution from the viewpoint of enabling the reaction to proceed uniformly and increasing the efficiency of introduction of the above-described group, and in this case, the pH of the aqueous solution is preferably pH 3 to 7. In addition, a nitrogen-containing compound such as urea can be added.

[0048] The degree of substitution of the phosphoric acid group among the glucose units of the phosphoric acid esterified cellulose (hereinafter, simply referred to as "phosphoric acid group substitution degree") is preferably 0.001 or greater and less than 0.40. The phosphoric acid group substitution degree among the glucose units can be measured by the following method.

[0049] A slurry of the phosphoric acid esterified cellulose having a solid content of 0.2 mass% was prepared. After adding a strongly acidic ion exchange resin (Amberjet 1024; manufactured by ORGANO Co., Ltd., conditioning was completed) at 1 / 10 of the volume of the slurry, the resin was separated from the slurry by pouring onto a 90-μm mesh sieve after being treated by shaking for 1 hour, thereby obtaining a hydrogen-type phosphoric acid esterified cellulose. Subsequently, while adding 0.1 N sodium hydroxide aqueous solution at 30 seconds per time, 50 μL each, the change in the value of the electric conductivity of the slurry was measured. In the measurement results, the amount of alkali (mmol) required in the region where the electric conductivity sharply decreased was divided by the solid content (g) in the slurry to be titrated, thereby calculating the amount of phosphoric acid group (mmol / g) in 1 g of the hydrogen-type phosphoric acid esterified cellulose. Furthermore, the phosphoric acid group substitution degree (DS) of the phosphoric acid esterified cellulose among the glucose units was calculated using the following formula:

[0050] DS = 0.162 x A / (1 - 0.079 x A)

[0051] A: The amount of phosphoric acid group (mmol / g) in 1 g of the hydrogen type phosphoric acid esterified cellulose.

[0052] (sulfated cellulose)

[0053] As an example of the anion-modified cellulose, sulfated cellulose can be given. The sulfated cellulose can be obtained by reacting a sulfuric acid compound with the above-described cellulose raw material, thereby introducing a sulfuric acid group from the sulfuric acid compound into the cellulose. As the sulfuric acid compound, for example, sulfuric acid, sulfamic acid, chlorosulfonic acid, sulfur trioxide, or an ester or a salt thereof can be given. Among these, sulfamic acid is preferably used from the viewpoint of the solubility of the cellulose and the low acidity.

[0054] For example, in the case where sulfamic acid is used as the sulfuric acid compound, the amount of the sulfamic acid used can be appropriately adjusted in consideration of the amount of introduction of the anionic group into the cellulose chain. For example, among 1 mol of the glucose unit in the cellulose molecule, the amount of 0.01 to 50 mol can be preferably used, and the amount of 0.1 to 3.0 mol can be more preferably used.

[0055] The amount of the sulfuric acid group (hereinafter, simply referred to as "sulfuric acid group amount") in the glucose unit of the sulfated cellulose is preferably 0.1 to 3.0 mmol / g. The sulfuric acid group amount in the glucose unit can be measured by the following method.

[0056] After solvent replacement of the aqueous dispersion of the sulfated cellulose with ethanol, t-butanol, and then freeze-drying, 15 ml of ethanol and 5 ml of water were added to 200 mg of the obtained sample, and stirred for 30 minutes. Then, 10 ml of 0.5 N sodium hydroxide aqueous solution was added, and stirred at 70°C for 30 minutes, and further stirred at 30°C for 24 hours. Next, phenothalin was added as an indicator, and titrated with hydrochloric acid, and calculated using the following formula:

[0057] Sulfuric acid group amount [mmol / g sample] = (5 - (0.1 x amount of hydrochloric acid titration [ml] x 2)) / 0.2.

[0058] (cation-modified cellulose)

[0059] The cellulose into which a cationic group is introduced as an ionic group is referred to as "cationic modified cellulose". The method of introducing a cationic group is not particularly limited. For example, it can be obtained by a known method in which glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydroxide or a halogenated alcohol type thereof or the like cationizing agent and an alkali metal hydroxide (sodium hydroxide, potassium hydroxide or the like) as a catalyst are reacted with the above-mentioned carboxylated cellulose in the presence of water or an alcohol having 1 to 4 carbon atoms.

[0060] The cationic substitution degree in the glucose unit of the cationic modified cellulose is preferably 0.02 to 0.50. The cationic substitution degree can be adjusted according to the added amount of the cationizing agent of the reaction, the composition ratio of water or an alcohol having 1 to 4 carbon atoms. The cationic substitution degree in the glucose unit can be measured by the following method:

[0061] After the cationic modified cellulose is dried, the nitrogen content is measured using a total nitrogen analyzer (TN-10 manufactured by Mitsubishi Chemical Corporation), and the cationic substitution degree (average value of the number of moles of the substituent group in 1 mole of anhydrous glucose unit) is calculated using the following formula:

[0062] Cationic substitution degree = (162 x N) / (1 - 151.6 x N)

[0063] N: nitrogen content.

[0064] (defibrillation)

[0065] The cationic modified cellulose obtained by the above can be used in the electrolyte in the state as it is, but can also be subjected to a treatment for reducing the diameter of the cellulose fiber, that is, defibrillation treatment. As the defibrillation method, there are known methods of treating by a high-pressure homogenizer, a microfluidization method, a refiner, a grinding machine grinding method, a shear-type blender, a colloid mill, a beater, a kneader, a disperser, freeze-dried pulverization, ultrasonic defibrillation, and the like by wet or dry methods, but are not limited thereto.

[0066] The degree of defibration is not particularly limited. For example, the ion-modified cellulose before defibration (pulp on which ion modification is performed) is not limited thereto, but is mostly a fiber having a fiber diameter of 10 to 50 μm and a fiber length of about 0.5 to 5.0 mm, and by defibrating the same or by partially fibrillating the surface of the fiber (fibrillating), fibrillated cellulose having an average fiber diameter of about 1 to 10 μm can be produced. The average fiber diameter of the fibrillated cellulose is more preferably about 5 to 10 μm. The average fiber length of the fibrillated cellulose is not particularly limited, and is about 0.2 to 3.0 mm, more preferably about 0.3 to 2.5 mm, and further preferably about 0.5 to 2.0 mm. The average fiber diameter and the average fiber length of the fibrillated cellulose can be measured, for example, by an image analysis type fiber analyzer such as L&W Fiber Tester Plus manufactured by ABB Corporation, Fractionator manufactured by Valmet Corporation, or the like. Specifically, the measurement can be performed by the following methods:

[0067] The water dispersion of the fibrillated cellulose having a solid content concentration of 0.25 mass% was supplied to the Fractionator, and the length-weighted fiber width and the length-weighted average fiber length (n = 2) were measured, respectively.

[0068] In the present specification, the cellulose after fibrillating the ion-modified cellulose is sometimes referred to as ion-modified fibrillated cellulose. Similarly, for example, the cellulose after fibrillating the anion-modified cellulose is sometimes referred to as anion-modified fibrillated cellulose.

[0069] The ionically modified cellulose can be defibrillated to a diameter smaller than that of the original fibrillated cellulose, i.e., to an average fiber diameter of less than 1 μm, to form ionically modified cellulose nanofiber (hereinafter, the cellulose nanofiber is referred to as "CNF"). In obtaining the ionically modified CNF, a wet-type high-pressure homogenizer is preferably used. The average fiber diameter of the ionically modified CNF is more preferably about 3 to 500 nm, further preferably about 3 nm to 150 nm, and further preferably about 3 nm to 20 nm. The aspect ratio of the ionically modified CNF is preferably 30 or more, more preferably 50 or more, and further preferably 100 or more. The upper limit of the aspect ratio is not limited, but is about 500 or less. For the average fiber diameter and the average fiber length of the CNF, the atomic force microscope (AFM) can be used for a diameter of less than 20 nm, and the field emission type electron scanning microscope (FE-SEM) can be used for a diameter of 20 nm or more. The average is calculated by analyzing 200 fibers randomly selected, and thus measured. In addition, the aspect ratio can be calculated by the following equation:

[0070] Aspect ratio = average fiber length / average fiber diameter.

[0071] In the present specification, the ionically modified cellulose is defibrillated to CNF having an average fiber diameter of less than 1 μm, and the obtained CNF is referred to as ionically modified CNF. Similarly, for example, CNF formed by defibrillating anionically modified cellulose is referred to as anionically modified CNF, and CNF formed by defibrillating carboxylated cellulose is referred to as carboxylated CNF.

[0072] (Electrolyte)

[0073] The electrolyte of the present application contains the above ionically modified cellulose (hereinafter, when referred to simply as "ionically modified cellulose", unless otherwise specified, not only ionically modified cellulose which has not been subjected to defibrillation treatment (ionically modified cellulose which has not been defibrillated) but also ionically modified fibrillated cellulose and ionically modified CNF obtained by defibrillating ionically modified cellulose) as a charge carrier. The electrolyte of the present application can be used for capacitors, secondary batteries, and other power storage devices.

[0074] The electrolyte is generally in a state where the ionically-modified cellulose is contained in a medium (i.e., a dispersion of the ionically-modified cellulose), and can be in a liquid state or can be in a gel state as a whole. In the present specification, the "gel state" refers to both a state where the flowability is reduced compared to the liquid state and thus the electrolyte can be scooped up with a spoon or the like, or a state where the electrolyte can be cut to a predetermined size when the gel strength is high. As a criterion for the flowability of the "liquid state" and the "gel state", although not limited thereto, for example, the "liquid state" is less than 1000 mPa-s in terms of the B-type viscosity (60 rpm, 25°C), and the "gel state" is 1000 mPa-s or more in terms of the B-type viscosity (60 rpm, 25°C). From the viewpoint that the ionically-modified cellulose easily functions as a good charge carrier in the electrolyte, the electrolyte is preferably a dispersion system containing a dispersed phase containing the ionically-modified cellulose and a medium, and preferably the dispersed phase is continuously extended (uniformly dispersed) together with the medium throughout the entire electrolyte. Further, it is more preferable that the electrolyte exhibit the characteristics of a solid such as a gel. In the case of the electrolyte in the "gel state", the B-type viscosity is preferably 1000 mPa-s or more, more preferably 5000 mPa-s or more, and further preferably 10000 mPa-s or more.

[0075] Note that, as described later, the ionically-modified cellulose such as CM cellulose can be used not only as a charge carrier in the electrolyte, but also as a binder in the electrode. The ionically-modified cellulose used as a binder in the electrode strengthens the binding between the electrode materials, imparts mechanical strength, and since the form is independent between the electrode materials, does not exist continuously between the positive and negative electrodes, and thus does not function as a charge carrier for the entire power storage device. It is assumed that even in the case where the material as a binder is in contact with the electrolyte and the medium, its function is the binding of the electrode materials and the imparting of mechanical strength, and not the existence as a charge carrier. Therefore, a power storage device in which the ionically-modified cellulose is contained not in the electrolyte but only as a binder of the electrode is not within the scope of the present application. In the electrolyte of the present application, the ionically-modified cellulose used as a charge carrier functions continuously as a charge carrier between the positive and negative electrodes.

[0076] The medium in the electrolyte can be any medium in which the ionically-modified cellulose can be dispersed and ionized, but from the viewpoints of low environmental load and safety, water is most preferably used. As examples of the medium other than water, water-soluble organic solvents such as methanol, ethanol, isopropanol, 2-propanol, butanol, glycerol, acetone, methyl ethyl ketone, 1,4-dioxane, dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone can be given. alkanes, N-methyl-2-pyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, ethylene glycol, ethylene carbonate, propylene glycol, propylene carbonate, glyme such as tetraethylene glycol dimethyl ether, sulfone such as sulfolane, and combinations thereof. The proportion of water in the medium is preferably 80% by mass or more, further preferably 90% by mass or more, further preferably 95% by mass or more, and most preferably 100% by mass (the entire medium is water).

[0077] The electrolyte is preferably composed of the medium and the ion-modified cellulose, but within a range that does not interfere with the function of the ion-modified cellulose as a charge carrier, it can contain, alone or in combination with two or more, substances other than these. As such substances, for example, non-ionic celluloses, cellulose derivatives, chitin, chitosan, polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, polyethylene terephthalate, polymethyl methacrylate resin, polyethylene, polypropylene, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene, poly-p-hydroxy benzoate resin, polyether resin, polyester resin, polyether derivatives, and polyimine resin, polyamide resin, polycarbonate resin, polymers added for the purpose of improving mechanical strength, ionic liquids, electrolytes other than ion-modified cellulose (for example, organic acids and salts thereof, inorganic acids and salts thereof, high-molecular electrolytes such as perfluorosulfonic acid polymers, and inorganic electrolytes such as glass ceramics), electrolyte salts (for example, lithium salts such as lithium hexafluorophosphate, and quaternary ammonium salts), ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, vinylene carbonate, 1,3-propane sultone, (R)-l-methyl-2-propynyl methane sulfonate, cyclohexylbenzene, tertiary amylbenzene, adiponitrile, phthalate, adipate, trimellitate, polyester, phosphate, citrate, epoxidized vegetable oil, sebacate, azelaic acid ester, maleic acid ester, and benzoic acid ester, and the like can be mentioned.

[0078] The content of the ion-modified cellulose relative to the mass of the entire electrolyte is preferably 0.01 to 90% by mass, further preferably 0.01 to 80% by mass, further preferably 0.01 to 50% by mass, further preferably 0.5 to 40% by mass, further preferably 0.5 to 30% by mass, and further preferably 1.0 to 30% by mass. It can also be 0.5 to 20% by mass, or 1.0 to 20% by mass. If it is less than 0.01% by mass, there is a tendency for the performance as a charge carrier to be low.

[0079] The electrolyte containing the ion-modified cellulose of the present application as a charge carrier has a thickness-direction conductivity of the electrolyte of 0.01 mS / cm or more, further preferably 0.10 mS / cm or more, and further preferably 1.00 mS / cm or more, as measured by the method described in the Examples below.

[0080] (Storage device)

[0081] The electrolyte of the present application can be used as an electrolyte for a storage device such as a capacitor, a secondary battery, and the like. There is no particular limitation on the constituent elements other than the electrolyte at the time of forming the storage device, and usual constituent elements can be used. For example, in the case of forming an electric double layer capacitor (EDLC), although not limited thereto, as a positive and negative current collector electrode, activated carbon subjected to an activation treatment can be used. As a raw material of the activated carbon, although not limited thereto, examples include coconut shell, coke, phenol resin, and the like. As the shape of the activated carbon, although not limited thereto, it can be in the form of a powder, a fiber, and the like. For example, by making the activated carbon a powder of 10 μm or less and applying it to a current collector base material having a thickness of about 10 to 30 μm, a sheet-shaped current collector electrode can be formed. As the current collector base material, although not limited thereto, a metal, a metal oxide, carbon, an electrically conductive polymer, a polymer imparted with electrical conductivity by dispersing carbon or a metal, and the like can be used. If necessary, an electrically conductive paste called a primer can be thinly applied to the current collector base material in advance. In addition, an electrically conductive aid for improving electrical conductivity and a binder for stably adhering the activated carbon to the current collector base material can be used. As the electrically conductive aid, for example, examples include graphite, carbon black, acetylene black, Ketjen black, and the like. As the binder, in the case of using a water-based binder, for example, examples include styrene butadiene rubber (SBR), polytetrafluoroethylene, and CM cellulose, CNF, and the like. In addition, in the case of using an organic binder, for example, examples include polyvinylidene fluoride. For example, the activated carbon, the binder, the electrically conductive aid, a solvent (water in the case of a water-based binder, and N-methylpyrrolidone and the like in the case of an organic binder), and, optionally, a thickening agent (an auxiliary binder) can be mixed to prepare a slurry, which is applied to the current collector base material and dried, thereby obtaining a sheet-shaped electrode. If necessary, the electrode can be appropriately densified and the like by pressure application, calendering, and the like.

[0082] In the case of forming an EDLC, a separator can be disposed between the electrode sheets of the positive and negative electrodes. The material of the separator is not particularly limited, and a known separator can be used. For example, it can be a cellulose-based separator, a glass-based separator, a resin film-based separator, or a nonwoven fabric-based separator.

[0083] The separator is generally provided for the purpose of preventing direct contact between the positive electrode and the negative electrode, but in the present application, the dispersion of ion-modified cellulose serving as the electrolyte sometimes becomes a high-viscosity gel or is electrodeposited on the electrode to form a film depending on the kind thereof. In this case, the electrolyte (the dispersion of ion-modified cellulose) itself can achieve the same effect as the separator, and therefore the EDLC can also not have a separator. The thickness of the film formed on the electrode can be adjusted by adjusting the kind and content of the ion-modified cellulose, the power-on time, and the like, and for example, a film having a thickness of about 0.1 μm to 100 mm can be formed on the electrode depending on these conditions. In the case where the film of ion-modified cellulose formed on the electrode is used as a separator, if the thickness of the film is 0.1 μm or less, there is a tendency that the two electrodes easily short-circuit, and if the thickness is 100 mm or more, there is a tendency that the volume of the power storage device increases and the electric resistance between the two electrodes becomes high.

[0084] The EDLC can be formed by disposing the electrolyte and optionally the separator between the positive electrode and the negative electrode, and sealing these in an outer case. The shape of the EDLC is not particularly limited, and can be any shape such as a coin type, a cylindrical type, a square type, a laminated type, and the like.

[0085] The secondary battery can also be formed in the same manner as the capacitor, and the constituent elements other than the electrolyte are not particularly limited, and the usual constituent elements can be used. That is, the secondary battery can be formed of a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte disposed therebetween. The positive electrode can be manufactured, for example, by applying a paste in which a positive electrode active material, a binder, a solvent, a conductive aid, and the like are mixed to a positive electrode current collector. Similarly, the negative electrode can be manufactured, for example, by applying a paste in which a negative electrode active material, a binder, a solvent, a conductive aid, and the like are mixed to a negative electrode current collector. As in the case of the capacitor, a separator can be disposed, or can not be disposed depending on the state of the electrolyte. The shape of the secondary battery is not particularly limited, and can be any shape such as a coin type, a cylindrical type, a square type, a laminated type, and the like.

[0086] In addition to this, the electrolyte of the present application can also be used for the electrolyte of a primary battery which is mainly used as a disposable power storage device and has a low environmental load. In addition, it can be used as the electrolyte of a power storage device in a wide range including a fuel cell, an air cell. These devices take on the role of a general power storage device by virtue of the ability to directly generate electric power by a chemical reaction and efficiently supply energy.

[0087] Example

[0088] Hereinafter, the present application will be further explained in detail according to examples, but the present application is not limited to the following examples.

[0089] (Example 1)

[0090] (Preparation of ion-modified cellulose)

[0091] A bleached unground kraft pulp (whiteness 85%) 500 g (absolute dry) from coniferous trees was added to an aqueous solution 50 L in which TEMPO (Sigma Aldrich Corporation) (0.025 mmol / g with respect to the cellulose raw material) and sodium bromide (1.0 mmol / g with respect to the cellulose raw material) were dissolved, and the pulp was stirred until it was uniformly dispersed. An aqueous sodium hypochlorite solution was added to the reaction system in an amount of 5.2 mmol / g with respect to the cellulose raw material, and the oxidation reaction was started. The pH in the system during the reaction decreased, but 3M aqueous sodium hydroxide solution was added sequentially to adjust the pH to 10. The reaction was ended at the point at which the sodium hypochlorite was consumed and the pH in the system no longer changed. The mixture after the reaction was filtered with a glass filter to separate the pulp, and the pulp was washed thoroughly with water, thereby obtaining an oxidized pulp. The pulp yield at this time was 90%, and the time required for the oxidation reaction was 100 minutes. The oxidized pulp (carboxylated cellulose) obtained in the above procedure was adjusted to 0.5% (w / v) with water, and subjected to fibrillation treatment 5 times using a high-pressure homogenizer (20°C, 150 MPa), thereby obtaining a dispersion of carboxylated CNF. The carboxyl group amount of the obtained carboxylated CNF was 1.30 mmol / g.

[0092] (Measurement of conductivity of electrolyte)

[0093] A gasket was disposed between graphite electrodes having an area of 2 cm 2 , and a dispersion of the carboxylated CNF obtained above (carboxylated CNF content 0.5 mass%) in water as the electrolyte was filled in the space defined by the electrodes and the gasket, thereby producing a coin-type cell. The obtained coin-type cell was subjected to measurement of the alternating current impedance at 25°C using an electrochemical evaluation device (BioLogic Corporation, SP-300) using an alternating current of 7 MHz to 100 mHz. The amplitude of the alternating current signal was 10 mV. The conductivity (mS / cm) of the electrolyte was calculated from the value of the electrolyte resistance at a frequency of 10 kHz obtained by the measurement using the following equation:

[0094] σ = {L / (R x A)} x 1000

[0095] σ: conductivity (mS / cm)

[0096] L: thickness of electrolyte (thickness of gasket) (cm)

[0097] R: electrolyte resistance (Ω)

[0098] A: electrode area (cm 2 ).

[0099] (Measurement of electrostatic capacitance)

[0100] An activated carbon (manufactured by Kureha Corporation) as a porous electrode, carbon black (manufactured by Denka Corporation: type Li-400) as a conductive aid, and styrene butadiene rubber (SBR) (manufactured by JSR Corporation: type TRD2001) as a binder were mixed in water at a dry mass ratio of 10:1:0.5, and dispersed using a self-rotation and revolution mixer at 2000 rpm for 20 minutes to obtain a carbon dispersion having a concentration of 30 mass%. The carbon dispersion was coated on an etched aluminum foil (25 cm x 20 cm, thickness 20 μm) as a current collector substrate, and then a coating film having a mass of 40 g / m2was obtained by removing water using a drying machine at 80°C. The obtained porous current collector electrode was punched into an electrode having an area of 4 cm2. 2 2 The porous current collector electrode was punched into an electrode having an area of 4 cm2.

[0101] A dispersion of the carboxylated CNF obtained above in water (carboxylated CNF content 0.5 mass%) was prepared, and the dispersion was used as an electrolyte. A gasket having a thickness of 1 mm was disposed between the positive electrode and the negative electrode, and a coin-type battery was produced in a state in which the above electrolyte was filled in a space defined by the positive electrode, the gasket, and the negative electrode. A separator was not used.

[0102] A charge-discharge test was performed on the obtained battery using an electrochemical evaluation device (manufactured by BioLogic Corporation, SP-300), and the electrostatic capacitance (mF) was calculated from a discharge curve of the 3rd cycle. The charge-discharge conditions were as follows:

[0103] (1) CC charging was performed at a charge current of 1 mA to 1.2 V, and CV charging was performed at 1.2 V for 5 minutes.

[0104] (2) Next, CC discharging was performed at a discharge current of 1 mA to 0.1 V.

[0105] (3) Next, the electrostatic capacitance was calculated from a discharge curve in which the vertical axis voltage was plotted against the horizontal axis time.

[0106] (Example 2)

[0107] ​In preparing the ionically modified cellulose, the concentration of the oxidized pulp to be subjected to defibrillation treatment using a high-pressure homogenizer was changed from 0.5% to 2.0% other than that, the same operation as in Example 1 was performed to obtain a dispersion of carboxylated CNF in water as a dispersion medium (carboxylated CNF content 2.0 mass%). The same operation as in Example 1 was performed except that the gelled electrolyte was formed on the electrode without using a gasket and "L: thickness of the electrolyte" was calculated from "distance between electrodes". The conductivity of the obtained dispersion was measured. Further, the electrostatic capacitance when using the electrolyte was measured in the same manner as in Example 1.

[0108] (Example 3)

[0109] In preparing the ionically modified cellulose, the concentration of the oxidized pulp to be subjected to defibrillation treatment using a high-pressure homogenizer was changed from 0.5% to 5.0% other than that, the same operation as in Example 1 was performed to obtain a dispersion of carboxylated CNF in water as a dispersion medium (carboxylated CNF content 5.0 mass%). The conductivity of the obtained dispersion was measured in the same manner as in Example 2. Further, the electrostatic capacitance when using the electrolyte was measured in the same manner as in Example 1.

[0110] (Example 4)

[0111] A dispersion of carboxylated CNF in water as a dispersion medium (carboxylated CNF content 0.5 mass%) was prepared in the same manner as in Example 1. The obtained dispersion was applied to an electrode for electrostatic capacitance measurement and an electrode for conductivity measurement, respectively, and dried at 50°C for 12 hours, whereby an electrode and electrolyte (ionically modified cellulose film, weight per square meter 50 g / m 2 ) were obtained. A solvent (water) was added to the film of the electrolyte on the obtained electrode and electrolyte in such a manner that the content of the charge carrier (carboxylated CNF) was 15 mass% relative to the mass of the entire electrolyte. The film swelled to become gelled. The conductivity of the obtained gelled electrolyte film was measured in the same manner as in Example 2. Further, another electrode was overlapped on the electrolyte film (gel) side of the obtained electrode and electrolyte, and a coin-type battery was produced. Using the obtained battery, the electrostatic capacitance was measured in the same manner as in Example 1.

[0112] (Example 5)

[0113] A solvent (water) was added to the film of the electrolyte in such a manner that the content of the charge carrier (carboxylated CNF) was 30 mass% relative to the mass of the entire electrolyte other than that, the same operation as in Example 4 was performed, and the conductivity of the gelled electrolyte film and the electrostatic capacitance when using the electrolyte film were measured.

[0114] (Example 6)

[0115] The same operation as in Example 4 was performed except that the water was added to the electrolyte film in an amount of 80 mass% relative to the mass of the electrolyte as a whole, and the conductivity of the gel-like electrolyte film and the static capacitance when using the electrolyte film were measured.

[0116] (Example 7)

[0117] Instead of the water dispersion of the carboxylated CNF, a water dispersion of a commercially available sodium salt of carboxymethyl cellulose (produced by Japan Viscose Co., Ltd.: FS350HC) (CM degree of substitution 0.90) (CM-modified cellulose content 0.5 mass%) was used as the electrolyte, and the conductivity was measured in the same manner as in Example 1. In addition, a coin-type battery was produced in the same manner as in Example 1 except that the electrolyte was used, and the static capacitance was measured.

[0118] (Example 8)

[0119] A coin-type battery was produced in the same manner as in Example 1 except that a separator made of cellulose (thickness 50 μm) was disposed in the electrolyte between the positive electrode and the negative electrode, and the static capacitance was measured. Since the electrolyte was the same as in Example 1, the conductivity of the electrolyte was the same value as in Example 1.

[0120] (Comparative Example 1)

[0121] Instead of the water dispersion of the carboxylated CNF, a water dispersion of nonionic fine cellulose containing nonionic CNF (CNF content 0.5 mass%) was used as the electrolyte, and the conductivity was measured in the same manner as in Example 1. In addition, a coin-type battery was produced in the same manner as in Example 1 except that the electrolyte was used, and the static capacitance was measured. The water dispersion of the nonionic fine cellulose was prepared by suspending a commercially available powdered cellulose (produced by Japan Viscose Co., Ltd.: trade name KC flock (registered trademark) W-50) in water to prepare a slurry having a concentration of 0.5 mass%, and defibrating the above slurry 10 times with a high-pressure homogenizer (20°C, 150 MPa).

[0122] The results of the measurement of the static capacitance of each of the examples and the comparative example and the conductivity of each of the electrolytes are shown in Table 1.

[0123] [Table 1]

[0124]

[0125] From the results of Table 1, it is known that the ion-modified cellulose can be used as a charge carrier of an electrolyte for an electricity storage device. If the content of the ion-modified cellulose (charge carrier) in the electrolyte is increased to a certain extent, an increase in the electric conductivity is observed, and in addition, an increase in the static capacitance is confirmed. It means that the greater the electric conductivity, the greater the ion conductivity of the electrolyte. For the electrolyte in which the ion-modified cellulose is increased to form a gel-like electrolyte, the electrolyte itself can achieve the same effect as the separator, and an advantage of reducing the components by omitting the separator is obtained.

Claims

1. An electrolyte for an electrical storage device, comprising ionically modified cellulose as a charge carrier.

2. The electrolyte for power storage devices according to claim 1, wherein The ionically modified cellulose is ionically modified cellulose nanofiber.

3. The electrolyte for power storage devices according to claim 1, wherein, The ionically modified cellulose is anionically modified cellulose.

4. The electrolyte for power storage devices according to claim 1, wherein, The ionically modified cellulose is anionically modified cellulose nanofiber.

5. The electrolyte for power storage devices according to claim 3, wherein, The anionically modified cellulose is carboxymethyl cellulose.

6. The electrolyte for power storage devices according to claim 4, wherein The anionically modified cellulose nanofiber is carboxylated cellulose nanofiber.

7. The electrolyte for power storage devices according to any one of claims 1 to 4, wherein The ionically modified cellulose is contained in an amount of 0.01 to 90 mass%.

8. The electrolyte for power storage devices according to any one of claims 1 to 4, wherein The electrical storage device is a capacitor or a secondary battery.

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