Aqueous electrolyte for proton batteries, and proton batteries

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 a proton battery.

Background Art

[0002] A non-aqueous battery including a flammable non-aqueous electrolyte has a problem that the energy density per volume of the entire battery becomes small as the number of components increases for safety measures. On the other hand, an aqueous battery including a non-flammable aqueous electrolyte has various advantages such as being able to increase the energy density per volume because the above safety measures are not required. Therefore, an aqueous battery including an aqueous electrolyte has been developed.

[0003] Patent Document 1 discloses a proton-conducting polymer battery including a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte is an aqueous solution having sulfuric acid as an electrolyte, a part of the contained water is substituted by at least one of phosphoric acid and diphosphoric acid, and the sulfuric acid concentration in the electrolyte is 3% by weight to 35% by weight and the contained water concentration is 65% by weight or less.

[0004] Patent Document 2 discloses an aqueous electrolyte used for an aqueous potassium ion battery, which includes water and potassium pyrophosphate dissolved at a concentration of 2 mol or more per 1 kg of water.

[0005] Patent Document 3 discloses a liquid electrolyte composed of a base A and phosphoric acid B, wherein the molar ratio A:B of A and B is in the range of 1:3 to 1:50, and the freezing temperature is less than -30°C.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

[0007] In recent years, proton batteries, in which protons move between the positive and negative electrodes through an electrolyte, have been developed as an alternative to lithium-ion batteries. Such aqueous electrolytes for proton batteries require low-temperature stability so that the batteries can be used even at extremely low temperatures, such as in cold regions.

[0008] This disclosure aims to provide an aqueous electrolyte for a proton battery with good low-temperature stability, and a proton battery having such an aqueous electrolyte. [Means for solving the problem]

[0009] The Disclosing Party has found that the above-mentioned problems can be solved by the following means. <Aspect 1> Water, and The aforementioned water contains pyrophosphate dissolved in the water at a concentration of 6 mol or more per 1 kg of water, and It does not have a freezing point above -60°C. Aqueous electrolyte for proton batteries. <Aspect 2> The pyrophosphate is dissolved in the water at a concentration of 6 mol or more and 25 mol or less per 1 kg of water, or The pyrophosphate and potassium salt are dissolved in the water at a concentration of more than 25 mol per 1 kg of water. The aqueous electrolyte according to Embodiment 1. <Aspect 3> The aqueous electrolyte according to embodiment 2, wherein the potassium salt is a potassium phosphate. <Aspect 4> The aqueous electrolyte according to embodiment 3, wherein the potassium phosphate is potassium pyrophosphate. <Aspect 5> Having an aqueous electrolyte as described in any one of the embodiments 1 to 4, Proton battery.

Advantages of the Invention

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

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 constant temperature bath at -60°C for a predetermined time. [Figure 2] FIG. 2 is a DSC chart showing the results of DSC measurements for water (reference example), an aqueous electrolyte having 10 mol of pyrophosphoric acid per 1 kg of water (Example 5), and an aqueous electrolyte having 40 mol of pyrophosphoric acid per 1 kg of water (Comparative Example 27).

Modes for Carrying Out the Invention

[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 gist of the disclosure.

[0013] 《Aqueous Electrolyte for Proton Battery》 The aqueous electrolyte for a proton battery of the present disclosure has water and pyrophosphoric acid (H4P2O7) dissolved in water at a concentration of 6 mol or more per 1 kg of water, and has no freezing point at -60°C or higher.

[0014] The present inventors have found that when the aqueous electrolyte for a proton battery has pyrophosphoric acid dissolved in water at a concentration of 6 mol or more per 1 kg of water, the low-temperature stability is improved. Although not intending to be bound by any theory, this is presumably because pyrophosphoric acid forms a strong hydrogen bond with water, thereby suppressing 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 mol or more and 25 mol or less per 1 kg of water.

[0016] The aqueous electrolyte of the present disclosure may have pyrophosphoric acid dissolved in water at a concentration of more than 25 mol per 1 kg of water, and a potassium salt.

[0017] The present inventors have found that when the aqueous electrolyte has pyrophosphoric acid dissolved in water at a concentration of more than 25 mol per 1 kg of water, the water in the aqueous electrolyte is likely to freeze. Although not intending to be bound by any theory, this is presumably because as the concentration of pyrophosphoric acid increases, pyrophosphoric acid is likely to form a eutectic with water. In contrast, the present inventors have found that by having the aqueous electrolyte further contain a potassium salt, it is possible to suppress the freezing of the water in the aqueous electrolyte. Although not intending to be bound by any theory, this is presumably because the presence of the potassium salt increases the entropy of the aqueous electrolyte, thereby making it difficult to form a eutectic of the mixed solution of pyrophosphoric acid and water.

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

[0019] When the aqueous electrolyte of the present disclosure has pyrophosphoric acid dissolved in water at a concentration of more than 25 mol per 1 kg of water, the concentration of the potassium salt may be more than 0 mol / L in terms of potassium ions, and may be less than the minimum concentration at which the potassium salt is saturated in the aqueous electrolyte. Since the concentration at which the potassium salt is saturated in the aqueous electrolyte is affected by the concentration of pyrophosphoric acid, the concentration of the potassium salt as potassium ions 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 greater than 0 mol / L and less than or equal to 3.5 mol / L if the aqueous electrolyte contains pyrophosphate dissolved in water at a concentration of 25 mol per 1 kg of water, greater than 0 mol / L and less than or equal to 1.6 mol / L if the aqueous electrolyte contains pyrophosphate dissolved in water at a concentration of 30 mol per 1 kg of water, greater than 0 mol / L and less than or equal to 1.2 mol / L if the aqueous electrolyte contains pyrophosphate dissolved in water at a concentration of 35 mol per 1 kg of water, and greater than 0 mol / L and less than or equal to 1.0 mol / L if the aqueous electrolyte contains pyrophosphate dissolved in water at a concentration of 40 mol per 1 kg of water.

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

[0022] The absence of a freezing point at temperatures above -60°C in the aqueous electrolyte of this disclosure can be confirmed by visual observation and measurement using a differential scanning calorimeter (DSC). Specifically, for example, this can be confirmed by visually observing the aqueous electrolyte after it has been kept in a -60°C constant temperature bath for a predetermined time and evaluating whether or not it has frozen. Alternatively, for example, this can be confirmed by measuring the aqueous electrolyte with a DSC and evaluating whether or not the generation of heat suggestive of freezing up to -60°C has occurred. A NETZSCH DSC 200 F3 Maia can be used as the DSC.

[0023] One example of a method for preparing an aqueous electrolyte is to weigh water and pyrophosphate to a predetermined concentration, mix them, and, if necessary, add and mix potassium salt further.

[0024] The aqueous electrolyte disclosed herein is for use in proton batteries.

[0025] Proton battery The proton battery of this disclosure comprises the aqueous electrolyte of this disclosure. For details regarding the aqueous electrolyte, refer to the above description relating to the aqueous electrolyte for proton batteries of this disclosure.

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

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

[0028] Examples of electrode active materials 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; 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 (where the quinone oxygen can become a hydroxyl group through conjugation); and proton-conducting polymers obtained by copolymerization of two or more monomers that give the polymers. By doping these compounds, redox pairs are formed, and conductivity is exhibited. These compounds can be selectively used as positive electrode active materials and negative electrode active materials by appropriately adjusting the difference in their oxidation-reduction potentials.

[0029] The proton battery of this disclosure may further have a separator. The separator may be impregnated in 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 between the positive electrode active material layer and the negative electrode active material layer, and is ion-permeable. The separator may be, for example, a polyolefin material such as polyethylene and polypropylene, polytetrafluoroethylene (PTFE), a cellulose material, an aramid material, an amide material, or a glass fiber material.

[0030] In this disclosure, a proton battery is defined as a battery in which protons (H) are transmitted between a positive electrode active material layer and a negative electrode active material layer. + This refers to a secondary battery in which charging and discharging occur as protons move, and as protons are inserted and removed in the positive electrode active material layer and the negative electrode active material layer.

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

[0032] Preparation of aqueous electrolyte solution Water and pyrophosphate were weighed and placed in a container to achieve the concentrations shown in Table 1 below, and the container was shaken to mix. If the pyrophosphate concentration was greater than 25 mol per kg of water, potassium pyrophosphate was weighed to achieve the potassium ion concentration shown in Table 1 below, and added to the mixture of water and pyrophosphate, and mixed. After mixing, the aqueous electrolytes for each example were prepared by standing in a 25°C constant temperature bath for at least 3 days. The potassium ion concentration was analyzed using an emission spectrometer (ICP). Specifically, using a Shimadzu ICPS-8100, 20 μL of each solution was taken, 1.5 mL of hydrochloric acid was added, and then 100 mL of ultrapure water was added to dilute it 5000 times, and the potassium ion concentration was analyzed.

[0033] Low-temperature stability evaluation <Exterior Evaluation> After keeping each example of the aqueous electrolyte in a -60°C constant temperature bath for at least 8 hours, the appearance was visually inspected to check for freezing.

[0034] <Evaluation by DSC> Water (reference example), an aqueous electrolyte containing 10 mol of pyrophosphate per 1 kg of water (Example 5), and an aqueous electrolyte containing 40 mol of pyrophosphate per 1 kg of water (Comparative Example 27) were evaluated by DSC using the following procedure. 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. A DSC chart was obtained as a result. A NETZSCH DSC 200 F3 Maia and a gold (Au) container were used for the measurements.

[0035] "result" The results of the visual evaluation are shown in Table 1 and Figure 1, and the results of the DSC evaluation are shown in Figure 2.

[0036] [Table 1]

[0037] Figure 1 is a phase diagram showing the state of each example of aqueous electrolyte after being stored in a constant temperature bath at -60°C for a predetermined time. As shown in Table 1 and Figure 1, aqueous electrolytes containing pyrophosphate dissolved in water at a concentration of 6 mol to 25 mol per kg of water had a region that did not freeze at -60°C (non-freezing region). On the other hand, when the pyrophosphate concentration was 30 mol to 40 mol per kg of water, aqueous electrolytes without potassium pyrophosphate froze at -60°C. In contrast, when the aqueous electrolyte contained potassium pyrophosphate, the aqueous electrolyte did not freeze at -60°C even when the pyrophosphate concentration was 30 mol to 40 mol per kg of water. From the above, it is suggested that the presence of potassium salt in the aqueous electrolyte can expand the range of pyrophosphate concentrations that allow for the preparation of aqueous electrolytes without a freezing point at -60°C to the higher concentration side.

[0038] Figure 2 is a DSC chart showing the results of DSC measurements for water (reference example), an aqueous electrolyte containing 10 mol of pyrophosphate per kg of water (Example 5), and an aqueous electrolyte containing 40 mol of pyrophosphate per kg of water (Comparative Example 27). As shown in Figure 2, no heat suggesting freezing was observed in the aqueous electrolyte of Example 5 above -60°C. In other words, this aqueous electrolyte did not have a freezing point above -60°C. Furthermore, no heat suggesting freezing was observed in this aqueous electrolyte even at -100°C.

[0039] Next, aqueous electrolytes for each example were prepared in the same manner as when potassium pyrophosphate was used, except that the potassium salt was changed from potassium pyrophosphate to tripotassium phosphate and the concentrations were as shown in Table 2 below. The obtained aqueous electrolytes were evaluated for their appearance in the same manner as described 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 of the example containing 30 mol of pyrophosphate per kg of water did not freeze at -60°C. In contrast, the aqueous electrolyte of the comparative example containing 10 mol or 20 mol of pyrophosphate per kg of water froze at -60°C, even with the presence of tripotassium phosphate. This result supports the above reasoning that the presence of a potassium salt in the aqueous electrolyte expands the concentration range of pyrophosphate that allows for the preparation of an aqueous electrolyte that does not freeze at -60°C to the higher concentration side.

[0042] Furthermore, aqueous electrolytes for each example were prepared in the same manner as when potassium pyrophosphate was used, except that the potassium salt was changed from potassium pyrophosphate to potassium triphosphate and the concentrations were as shown in Table 3 below. The obtained aqueous electrolytes were evaluated for their appearance in the same manner as described 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 electrolytes of the examples containing 20 to 40 moles of pyrophosphate per kg of water did not freeze at -60°C. In contrast, the aqueous electrolyte of the comparative example containing 10 moles of pyrophosphate per kg of water froze at -60°C even with potassium triphosphate. This result, similar to the case where tripotassium phosphate was used as the potassium salt, supports the above reasoning that the presence of a potassium salt in the aqueous electrolyte can expand the concentration range of pyrophosphate that allows for the preparation of an aqueous electrolyte without a freezing point at -60°C to the higher concentration side.

Claims

1. Having water, Satisfy any of the following conditions (i) to (vi): (i) The water contains pyrophosphate dissolved in it at a concentration of 6 mol or more and less than 10 mol per 1 kg of water, and does not contain potassium salts; (ii) The water contains pyrophosphate dissolved in it at a concentration of 10 mol or more and less than 15 mol per 1 kg of water, and a potassium salt at a concentration of 0.95 mol / L or less; (iii) The mixture contains pyrophosphate dissolved in the water at a concentration of 15 mol to 25 mol per 1 kg of water, and a potassium salt at a concentration of 3.44 mol / L or less; (iv) The mixture contains pyrophosphate dissolved in the water at a concentration of more than 25 mol and no more than 30 mol per 1 kg of water, and a potassium salt at a concentration of 0.57 mol / L or more and no more than 1.52 mol / L; (v) The mixture contains pyrophosphate dissolved in the water at a concentration of more than 30 mol and 35 mol or less per 1 kg of water, and a potassium salt at a concentration of 0.06 mol / L or more and 1.12 mol / L or less; (vi) A solution comprising pyrophosphate dissolved in water at a concentration of more than 35 mol and 40 mol or less per 1 kg of water, and a potassium salt at a concentration of 0.36 mol / L or more and 0.93 mol / L or less. Aqueous electrolyte for proton batteries.

2. The aqueous electrolyte according to claim 2, wherein the potassium salt is a potassium phosphate.

3. The aqueous electrolyte according to claim 3, wherein the potassium phosphate is potassium pyrophosphate.

4. Having the aqueous electrolyte according to any one of claims 1 to 3, Proton battery.

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