Aqueous electrolyte for proton battery, and proton battery

Abstract

To provide an aqueous electrolyte for a proton battery having good low temperature stability, and a proton battery having such an aqueous electrolyte.SOLUTION: An aqueous electrolyte for a proton battery according to the present disclosure includes water and pyrophosphoric acid (H4P2O7) dissolved in water at a concentration of 6 mol or more per kg of water, and does not have a freezing point at or above -60°C. The proton battery according to the present disclosure also includes the aqueous electrolyte according to the present disclosure. The proton battery according to the present disclosure also includes the aqueous electrolyte according to the present disclosure.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an aqueous electrolyte for a proton battery, and to a proton battery. [Background technology]

[0002] A non-aqueous battery containing a flammable non-aqueous electrolyte solution has a problem that the number of components required for safety measures is large, resulting in a low energy density per volume of the entire battery. In contrast, an aqueous battery containing a non-flammable aqueous electrolyte solution does not require the above-mentioned safety measures, and therefore has various advantages, such as a high energy density per volume. For this reason, aqueous batteries containing an aqueous electrolyte solution have been developed.

[0003] Patent Document 1 discloses a proton conducting polymer battery including a positive electrode, a negative electrode, and an electrolyte, the electrolyte being an aqueous solution containing sulfuric acid as an electrolyte, in which a portion of the water contained therein is replaced with at least one of phosphoric acid and diphosphoric acid, the sulfuric acid concentration in the electrolyte being 3% by weight to 35% by weight, and the water concentration being 65% by weight or less.

[0004] Patent Document 2 discloses an aqueous electrolyte used in an aqueous potassium ion battery, which contains water and potassium pyrophosphate dissolved at a concentration of 2 mol or more per kg of water.

[0005] Patent Document 3 discloses a liquid electrolyte consisting of a base A and phosphoric acid B, in which the molar ratio of A to B is in the range of 1:3 to 1:50, and the solidification temperature is lower than -30°C. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2007-194105 A [Patent Document 2] JP 2019-220294 A [Patent Document 3] JP 2006-164945 A Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, proton batteries, in which protons move between a positive electrode and a negative electrode through an electrolyte, have been developed as secondary batteries to replace lithium-ion secondary batteries. The aqueous electrolyte for such proton batteries is required to have low-temperature stability so that the batteries can be used even at extremely low temperatures, such as in cold regions.

[0008] An object of the present disclosure is to provide an aqueous electrolyte for a proton battery having good low-temperature stability, and a proton battery having such an aqueous electrolyte. [Means for solving the problem]

[0009] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> Water, and having pyrophosphoric acid dissolved in the water at a concentration of 6 moles or more per kilogram of the water; Does not have a freezing point above -60℃ Aqueous electrolyte for proton batteries. <Aspect 2> having the pyrophosphate dissolved in the water at a concentration of 6 mol to 25 mol per kg of the water; or The pyrophosphate and potassium salt are dissolved in the water at a concentration of more than 25 moles per kilogram of water. 2. The aqueous electrolyte of embodiment 1. <Aspect 3> 3. The aqueous electrolyte of claim 2, wherein the potassium salt is a phosphate salt of potassium. <Aspect 4> 4. The aqueous electrolyte of claim 3, wherein the potassium phosphate is potassium pyrophosphate. <Aspect 5> The aqueous electrolyte solution according to any one of aspects 1 to 4 is included. Proton battery. Effect of the Invention

[0010] According to the present disclosure, it is possible to provide an aqueous electrolyte solution for a proton battery having good low-temperature stability, and a proton battery having such an aqueous electrolyte solution. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a phase diagram showing the state of the aqueous electrolyte of each example after being stored in a thermostatic chamber at −60° C. for a predetermined period of time. [Diagram 2] FIG. 2 is a DSC chart showing the results of DSC measurement for water (Reference Example), an aqueous electrolyte solution having 10 mol of pyrophosphate per 1 kg of water (Example 5), and an aqueous electrolyte solution having 40 mol of pyrophosphate per 1 kg of water (Comparative Example 27). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0013] <Aqueous electrolyte for proton batteries> The aqueous electrolyte for a proton battery of the present disclosure contains water and pyrophosphoric acid (H4P2O7) dissolved in water at a concentration of 6 mol or more per kg of water, and does not have a freezing point at -60°C or higher.

[0014] The present inventors have found that the low-temperature stability of an aqueous electrolyte for a proton battery is improved by having pyrophosphoric acid dissolved in water at a concentration of 6 mol or more per kg of water. Although not intending to be bound by any theory, the reason for this is believed to be that pyrophosphoric acid forms strong hydrogen bonds with water, thereby preventing the freezing of water in the aqueous electrolyte.

[0015] The aqueous electrolyte of the present disclosure may have pyrophosphoric acid dissolved in water at a concentration of 6 to 25 mol per kg of water.

[0016] The aqueous electrolyte of the present disclosure may have pyrophosphate and potassium salts dissolved in water at a concentration of greater than 25 moles per kg of water.

[0017] The present inventors have found that when the aqueous electrolyte contains pyrophosphoric acid dissolved in water at a concentration of more than 25 mol per kg of water, the water in the aqueous electrolyte is likely to freeze. Although it is not intended to be bound by any theory, it is believed that the reason for this is that as the concentration of pyrophosphoric acid increases, pyrophosphoric acid is more likely to form a eutectic with water. In contrast, the present inventors have found that the aqueous electrolyte further contains a potassium salt, which can prevent the water in the aqueous electrolyte from freezing. Although it is not intended to be bound by any theory, it is believed that the reason for this is that the presence of a potassium salt increases the entropy of the aqueous electrolyte, which makes it difficult to form a eutectic of a mixed solution of pyrophosphoric acid and water.

[0018] In the aqueous electrolyte of the present disclosure, the potassium salt may be a potassium phosphate. Examples of potassium phosphate include, but are not limited to, potassium pyrophosphate (K4P2O7), tripotassium phosphate (K3PO4), and potassium triphosphate (K5P3O 10 ) etc. are examples.

[0019] When the aqueous electrolyte of the present disclosure has pyrophosphoric acid dissolved in water at a concentration of more than 25 mol per kg of water, the concentration of the potassium salt, calculated as potassium ion, may be more than 0 mol / L and may be less than the minimum concentration at which the potassium salt is saturated in the aqueous electrolyte. The concentration at which the potassium salt is saturated in the aqueous electrolyte is affected by the concentration of pyrophosphoric acid, so the concentration of the potassium salt as potassium ion may be determined according to the concentration of pyrophosphoric acid.

[0020] The potassium ion concentration when the potassium salt is potassium pyrophosphate is shown below. This potassium ion concentration may be more than 0 mol / L and not more than 3.5 mol / L when the aqueous electrolyte contains pyrophosphoric acid dissolved in water at a concentration of 25 mol per kg of water, may be more than 0 mol / L and not more than 1.6 mol / L when the aqueous electrolyte contains pyrophosphoric acid dissolved in water at a concentration of 30 mol per kg of water, may be more than 0 mol / L and not more than 1.2 mol / L when the aqueous electrolyte contains pyrophosphoric acid dissolved in water at a concentration of 35 mol per kg of water, and may be more than 0 mol / L and not more than 1.0 mol / L when the aqueous electrolyte contains pyrophosphoric acid dissolved in water at a concentration of 40 mol per kg of water.

[0021] The potassium ion concentration can be analyzed by taking 20 μL of each solution, adding 1.5 mL of hydrochloric acid, and then adding 100 mL of ultrapure water to dilute the solution 5000 times using an inductively coupled plasma (ICP) analyzer. Shimadzu's ICPS-8100 can be used as the ICP.

[0022] The fact that the aqueous electrolyte of the present disclosure does not have a freezing point at -60°C or higher can be confirmed by appearance observation and measurement using a differential scanning calorimeter (DSC). That is, for example, it can be confirmed by evaluating the presence or absence of freezing in the appearance observation of the aqueous electrolyte after keeping it in a thermostatic bath at -60°C for a predetermined time. Also, for example, it can be confirmed by measuring the aqueous electrolyte with DSC and evaluating the presence or absence of suggested heat of freezing up to -60°C. Note that, as the DSC, DSC 200 F3 Maia of NETZSCH can be used.

[0023] An example of a method for preparing an aqueous electrolyte solution is to weigh out water and pyrophosphoric acid so as to obtain a predetermined concentration, mix them, and, if necessary, further add a potassium salt and mix them.

[0024] The aqueous electrolyte of the present disclosure is for use in proton batteries.

[0025] Proton Battery The proton battery of the present disclosure has the aqueous electrolyte of the present disclosure. For the aqueous electrolyte, reference can be made to the above description regarding the aqueous electrolyte for the proton battery of the present disclosure.

[0026] The proton battery of the present disclosure may have a current collector and an electrode active material layer formed on the current collector.

[0027] The current collector has a function of holding the electrode active material layer, supplying electric charge to the electrode active material layer, and recovering electric charge from the electrode active material layer. The current collector is not particularly limited as long as it is acid-resistant and conductive, and can be made of a metal foil or a metal plate. Specifically, the current collector may be aluminum, an alloy mainly composed of aluminum, nickel, titanium, SUS, copper, or the like. The current collector may also be a carbon plate, a mixture of a carbon material with a resin, or the like. The current collector may be a base material such as iron that is plated or surface-coated with the above-mentioned materials.

[0028] Examples of the electrode active material include π-conjugated polymers such as manganese oxide, tungsten oxide, molybdenum oxide, Prussian blue derivatives, MXene, polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylenevinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole, and derivatives thereof, and indole-based π-conjugated compounds such as indole trimer compounds, quinone-based compounds such as benzoquinone, naphthoquinone, and anthraquinone, quinone-based polymers such as polyanthraquinone, polynaphthoquinone, and polybenzoquinone (quinone oxygen can become a hydroxyl group through conjugation), and proton-conducting polymers obtained by copolymerization of two or more types of monomers that give the polymers. By doping these compounds, a redox pair is formed, and electrical conductivity is exhibited. These compounds are selectively used as the positive electrode active material and the negative electrode active material by appropriately adjusting the difference in oxidation-reduction potential between them.

[0029] The proton battery of the present disclosure may further include a separator. The separator may be impregnated with an aqueous electrolyte and disposed between the positive electrode active material layer and the negative electrode active material layer. The separator is not particularly limited as long as it is acid-resistant, can insulate the positive electrode active material layer from the negative electrode active material layer, and has ion permeability. The separator may be, for example, a polyolefin-based material such as polyethylene and polypropylene, polytetrafluoroethylene (PTFE), a cellulose-based material, an aramid-based material, an amide-based material, or a glass fiber-based material.

[0030] In this disclosure, a proton battery is a battery in which protons (H + ) moves in the positive electrode active material layer and the negative electrode active material layer, and protons are inserted and removed in the positive electrode active material layer and the negative electrode active material layer, thereby charging and discharging the secondary battery.

[0031] In a proton battery, protons move as follows: during discharge, protons are released from the negative electrode active material layer, the released protons move through the electrolyte from the negative electrode active material layer to the positive electrode active material layer, and the protons are inserted into the positive electrode active material layer. During charge, protons are released from the positive electrode active material layer, the released protons move through the electrolyte from the positive electrode active material layer to the negative electrode active material layer, and the protons are inserted into the negative electrode active material layer. EXAMPLES

[0032] Preparation of aqueous electrolyte Water and pyrophosphoric acid were weighed and placed in a container so as to obtain the concentrations shown in Table 1 below, and the container was shaken to stir. When the concentration of pyrophosphoric acid was more than 25 mol per kg of water, potassium pyrophosphate was weighed and added to the mixture of water and pyrophosphoric acid so as to obtain the potassium ion concentration shown in Table 1 below, and the mixture was stirred. After stirring, the mixture was left to stand in a thermostatic bath at 25°C for 3 days or more to prepare an aqueous electrolyte solution of each example. The potassium ion concentration was analyzed using an emission spectrometer (ICP). Specifically, the potassium ion concentration was analyzed by taking 20 μL of each solution using Shimadzu Corporation's ICPS-8100, adding 1.5 mL of hydrochloric acid, and then adding 100 mL of ultrapure water to dilute and adjust the solution 5000 times.

[0033] <Low temperature stability evaluation> Appearance Evaluation The aqueous electrolyte solution of each example was kept in a thermostatic chamber at -60°C for 8 hours or more, and then the appearance was visually observed to check for the presence or absence of freezing.

[0034] Evaluation by DSC Water (Reference Example), an aqueous electrolyte solution having 10 mol of pyrophosphoric acid per 1 kg of water (Example 5), and an aqueous electrolyte solution having 40 mol of pyrophosphoric acid per 1 kg of water (Comparative Example 27) were evaluated by DSC in the following procedure. That is, first, the temperature was lowered from room temperature to -120°C at a rate of 1°C / min. Then, the temperature was raised from -120°C to 30°C at a rate of 1°C / min. This resulted in a DSC chart. For the measurement, a DSC 200 F3 Maia from NETZSCH and a container made of gold (Au) were used.

[0035] "result" The results of the appearance evaluation are shown in Table 1 and FIG. 1, and the results of the evaluation by DSC are shown in FIG.

[0036] [Table 1]

[0037] FIG. 1 is a phase diagram showing the state of the aqueous electrolyte of each example after being stored in a thermostatic bath at −60° C. for a predetermined time. As shown in Table 1 and FIG. 1, the aqueous electrolyte having pyrophosphoric acid dissolved in water at a concentration of 6 mol to 25 mol per kg of water had a region (non-freezing region) that did not freeze at −60° C. On the other hand, when the concentration of pyrophosphoric acid was 30 mol to 40 mol per kg of water, the aqueous electrolyte not having potassium pyrophosphate froze at −60° C. In contrast, when the aqueous electrolyte had potassium pyrophosphate, the aqueous electrolyte did not freeze at −60° C. even if the concentration of pyrophosphoric acid was 30 mol to 40 mol per kg of water. From the above, it was suggested that the concentration range of pyrophosphoric acid capable of preparing an aqueous electrolyte not having a freezing point at −60° C. can be expanded to a higher concentration side by the aqueous electrolyte having a potassium salt.

[0038] 2 is a DSC chart showing the results of DSC measurement of water (Reference Example), an aqueous electrolyte having 10 mol of pyrophosphate per 1 kg of water (Example 5), and an aqueous electrolyte having 40 mol of pyrophosphate per 1 kg of water (Comparative Example 27). As shown in FIG. 2, the aqueous electrolyte of Example 5 did not exhibit a suggested heat of freezing at -60°C or higher. That is, this aqueous electrolyte did not have a freezing point up to -60°C or higher. Furthermore, this aqueous electrolyte did not exhibit a suggested heat of freezing even at -100°C.

[0039] Next, the aqueous electrolyte solutions of each example were prepared in the same manner as in the case of using potassium pyrophosphate, except that the potassium salt was changed from potassium pyrophosphate to tripotassium phosphate and the concentration was set as shown in Table 2 below. The aqueous electrolyte solutions obtained were subjected to the same appearance evaluation as above. The results are shown in Table 2.

[0040] [Table 2]

[0041] As shown in Table 2, when tripotassium phosphate was used as the potassium salt, the aqueous electrolyte solution of the Example having 30 mol of pyrophosphate per kg of water did not freeze at -60°C. In contrast, the aqueous electrolyte solution of the Comparative Example having 10 mol of pyrophosphate per kg of water or 20 mol of pyrophosphate per kg of water froze at -60°C even though it contained tripotassium phosphate. This result is considered to support the above inference that the concentration range of pyrophosphate capable of preparing an aqueous electrolyte solution that does not have a freezing point at -60°C can be expanded to a higher concentration side by the aqueous electrolyte solution containing a potassium salt.

[0042] Further, the aqueous electrolyte solution of each example was prepared in the same manner as in the case of using potassium pyrophosphate, except that the potassium salt was changed from potassium pyrophosphate to potassium triphosphate and the concentration was set as shown in Table 3 below. The aqueous electrolyte solution obtained was subjected to the same appearance evaluation as above. The results are shown in Table 3.

[0043] [Table 3]

[0044] As shown in Table 3, when potassium triphosphate was used as the potassium salt, the aqueous electrolyte of the Example having 20 mol to 40 mol of pyrophosphate per 1 kg of water did not freeze at -60°C. In contrast, the aqueous electrolyte of the Comparative Example having 10 mol of pyrophosphate per 1 kg of water froze at -60°C even though it contained potassium triphosphate. This result, like the case of using tripotassium phosphate as the potassium salt, is considered to support the above inference that the concentration range of pyrophosphate capable of preparing an aqueous electrolyte that does not have a freezing point at -60°C can be expanded to a higher concentration side by the aqueous electrolyte having a potassium salt.

Claims

1. Water, and having pyrophosphoric acid dissolved in the water at a concentration of 6 mol or more per kg of the water; Does not have a freezing point above -60°C; Aqueous electrolyte for proton batteries.

2. The pyrophosphate is dissolved in the water at a concentration of 6 mol to 25 mol per kg of the water; or The pyrophosphate and potassium salt are dissolved in the water at a concentration of more than 25 mol / kg of water. The aqueous electrolyte according to claim 1 .

3. 3. The aqueous electrolyte of claim 2, wherein the potassium salt is a phosphate of potassium.

4. 4. The aqueous electrolyte of claim 3, wherein the potassium phosphate is potassium pyrophosphate.

5. The aqueous electrolyte according to any one of claims 1 to 4 is included. Proton battery.

Images