Diaphragm, battery cell, cell stack, and redox-flow battery system

Abstract

This diaphragm is used in a battery cell of a redox flow battery system, and comprises: a base material having cation permeability; and a coating layer that covers at least part of a first surface and a second surface of the base material. The coating layer contains a nitrogen-containing polymer and a fluorine-containing polymer. The fluorine-containing polymer has cation permeability.

Description

Diaphragms, battery cells, cell stacks, and redox flow battery systems

[0001] This disclosure relates to diaphragms, battery cells, cell stacks, and redox flow battery systems. This application claims priority under Japanese Patent Application No. 2024-167938 dated 26 September 2024, incorporating all the provisions contained herein.

[0002] Patent Document 1 discloses a redox flow battery system as one type of storage battery. The redox flow battery system comprises a battery cell. The battery cell comprises a positive electrode cell and a negative electrode cell separated by a diaphragm. In the redox flow battery system, charging or discharging is performed by circulating the positive electrode electrolyte to the positive electrode cell and the negative electrode electrolyte to the negative electrode cell. These electrolytes contain active material ions that perform charging or discharging by changing their valence. The positive electrode electrolyte and the negative electrode electrolyte are separated by a diaphragm to prevent mixing, and the exchange of charge between the negative electrode cell and the positive electrode cell is carried out by carriers that permeate the diaphragm.

[0003] During operation of a redox flow battery system, active material ions may permeate the diaphragm and move from the positive electrode cell to the negative electrode cell or vice versa. This movement of active material ions alters the balance between the concentration of positive electrode active material ions in the positive electrode electrolyte and the concentration of negative electrode active material ions in the negative electrode electrolyte, resulting in a decrease in the battery characteristics of the redox flow battery system. In the redox flow battery system described in Patent Document 1, the positive electrode electrolyte and the negative electrode electrolyte are mixed to correct the above-mentioned change in balance.

[0004] Japanese Patent Publication No. 2020-187939

[0005] The diaphragm of this disclosure is used in a battery cell of a redox flow battery system and comprises a cation-permeable substrate and a coating layer covering at least a portion of the first and second surfaces of the substrate, wherein the coating layer contains a nitrogen-containing polymer and a fluorine-containing polymer. The fluorine-containing polymer is cation-permeable.

[0006] Figure 1 is a schematic diagram of a redox flow battery system including a battery cell according to an embodiment. Figure 2 is a schematic diagram of a battery cell comprising a positive electrode, a negative electrode, and a diaphragm. Figure 3 is a schematic diagram of a battery cell comprising a diaphragm different from that in Figure 2.

[0007] When the positive electrode electrolyte and the negative electrode electrolyte are mixed, some of the stored electricity is wasted due to self-discharge. Therefore, there is a need to reduce the amount of active material ions that permeate the membrane and to decrease the frequency of mixing the positive electrode electrolyte and the negative electrode electrolyte.

[0008] One of the objectives of this disclosure is to provide a diaphragm that is impermeable to active material ions in a redox flow battery system. Another objective of this disclosure is to provide a battery cell and a redox flow battery system equipped with a diaphragm that is impermeable to active material ions.

[0009] The diaphragm of this disclosure can suppress the movement of active material ions in a redox flow battery system.

[0010] First, the embodiments of this disclosure will be listed and described.

[0011] <1> A diaphragm according to one aspect of the present disclosure is a diaphragm used in a battery cell of a redox flow battery system, comprising a cation-permeable substrate and a coating layer covering at least a portion of the first and second surfaces of the substrate, wherein the coating layer comprises a nitrogen-containing polymer and a fluorine-containing polymer. The fluorine-containing polymer is cation-permeable.

[0012] Here, cation permeability in a substrate refers to its ability to permeate cations as carriers. Typically, carriers are protons. On the other hand, nitrogen-containing polymers are materials that selectively exchange specific anions. Fluorine-containing polymers with cation permeability are materials that selectively exchange specific cations.

[0013] In the diaphragm described in <1> above, the nitrogen-containing polymer contained in the coating layer of the diaphragm is thought to reduce the permeation of positively charged active material ions. On the other hand, the fluorine-containing polymer contained in the coating layer is thought to contribute to maintaining the effect obtained by the nitrogen-containing polymer over a long period of time. The diaphragm described in <1> above, by having a substrate independently of the coating layer, is less likely to hinder the permeation of carriers such as protons compared to the case without a substrate.

[0014] Reducing the amount of active material ions that permeate the membrane in a battery cell makes it easier to improve the current efficiency of the redox flow battery system, including the battery cell. Current efficiency is an indicator that shows the proportion of the current supplied to the redox flow battery system from an external source that is actually stored as chemical energy in the redox flow battery system and can be extracted again as electrical energy. In addition, by reducing the amount of active material ions that permeate the membrane, the frequency of mixing the positive electrode electrolyte and the negative electrode electrolyte to correct the balance between positive electrode active material ions and negative electrode active material ions can be reduced. As a result, less electricity is wasted in the redox flow battery system.

[0015] <2> In the diaphragm described in <1> above, the covering layer may be formed only on the first surface.

[0016] The coating layer may be formed on a portion of the first surface of the substrate, or on the entire first surface. This configuration reduces the effort required to form the coating layer and the amount of material used to form the coating layer compared to forming the coating layer on both the first and second surfaces. Therefore, the diaphragm described in <2> above offers superior productivity, including cost.

[0017] <3> In the diaphragm described in <1> above, the covering layer may be formed on both the first surface and the second surface.

[0018] In the configuration described in <3> above, the coating layer is formed on part or all of the first surface and part or all of the second surface of the substrate. A diaphragm with a coating layer formed on both the first and second surfaces is more likely to inhibit the permeation of active material ions compared to a diaphragm with a coating layer formed on only one surface. Furthermore, if the coating layer is formed on both the first and second surfaces, there is no need to consider the orientation of the diaphragm when manufacturing the battery cell. This simplifies the manufacturing of the battery cell and reduces variations in battery cell performance caused by the orientation of the diaphragm.

[0019] <4> In the diaphragm described in any of <1> to <3> above, the average thickness of the coating layer may be 2 nm or more.

[0020] If the average thickness of the coating layer is 2 nm or more, the active material ions will have difficulty penetrating the membrane.

[0021] <5> In the diaphragm described in <4> above, the average thickness of the coating layer may be 1000 nm or less.

[0022] If the average thickness of the coating layer is 1000 nm or less, the coating layer allows protons to pass through easily. Therefore, the increase in cell resistance caused by the coating layer can be reduced.

[0023] <6> In the diaphragm described in any of <1> to <5> above, when the total mass of the nitrogen-containing polymer and the fluorine-containing polymer is 100% by mass, the mass ratio of the nitrogen-containing polymer may be 90% by mass or more.

[0024] The higher the mass percentage of nitrogen-containing polymer in the coating layer, the easier it is for the coating layer to restrict the movement of active material ions. Therefore, if the mass percentage of nitrogen-containing polymer in the coating layer is 90% by mass or more, the movement of active material ions can be restricted even with a thin coating layer.

[0025] <7> In the diaphragm described in any of <1> to <6> above, the nitrogen-containing polymer may be one or more materials selected from the group consisting of materials containing primary to tertiary amino groups, materials containing quaternary ammonium groups, and materials containing heterocyclic molecules having nitrogen.

[0026] The nitrogen-containing polymers listed in <7> above are effective in reducing the amount of active material ions that permeate the membrane by inhibiting the permeation of active material ions.

[0027] <8> In the diaphragm described in <7> above, the nitrogen-containing polymer may be a polyazole compound.

[0028] Polyazole compounds can easily reduce the amount of active material ions that permeate the membrane by inhibiting the permeation of active material ions.

[0029] <9> In the diaphragm described in <8> above, the polyazole compound may be one or more selected from the group consisting of a polymer of a heterocyclic compound containing one or more nitrogen atoms in the ring, and a polymer of a heterocyclic compound containing one or more nitrogen atoms and at least one of oxygen and sulfur in the ring.

[0030] The polyazole compounds listed in <9> above can easily reduce the amount of active material ions that permeate the membrane by inhibiting the permeation of active material ions.

[0031] <10> In the diaphragm described in <9> above, the polyazole compound may be one or more selected from the group consisting of polyimidazole compounds, polybenzimidazole compounds, polybenzobisimidazole compounds, polybenzoxazole compounds, polyoxazole compounds, polythiazole compounds, and polybenzothiazole compounds.

[0032] The polyazole compounds listed in <10> above are effective in reducing the amount of active material ions that permeate the membrane by inhibiting the permeation of active material ions.

[0033] <11> In the diaphragm described in any of <1> to <10> above, the fluorine-containing polymer may be one or more materials selected from the group consisting of materials containing sulfonic acid groups, materials containing carboxylic acid groups, and materials containing phosphate groups.

[0034] In the membrane described in <11> above, the fluorine-containing polymer in the coating layer allows carriers such as protons to permeate while the nitrogen-containing polymer is easily maintained within the coating layer.

[0035] <12> The average thickness of the separator described in any one of <1> to <11> above may be 150 μm or less.

[0036] If the average thickness of the separator including the coating layer is 150 μm or less, carriers can easily permeate through the separator.

[0037] <13> The average thickness of the separator described in <12> above may be 3 μm or more.

[0038] If the average thickness of the separator including the coating layer is 3 μm or more, the separator is less likely to be damaged.

[0039] <14> A battery cell according to one embodiment of the present disclosure includes a positive electrode, a negative electrode, and the separator described in any one of <1> to <13> above disposed between the positive electrode and the negative electrode.

[0040] In the above battery cell, the amount of active material ions permeating through the separator is reduced by the separator according to one embodiment of the present disclosure. Therefore, the current efficiency of a redox flow battery including this battery cell is likely to be improved.

[0041] <15> In the battery cell described in <14> above, the separator may be arranged such that the coating layer faces the negative electrode.

[0042] When the coating layer provided on the separator is arranged to face the negative electrode, the permeation of active material ions is more likely to be inhibited than when the separator is arranged to face the positive electrode.

[0043] <16> A cell stack according to one embodiment of the present disclosure is a cell stack in which a plurality of battery cells are stacked, and at least a part of the plurality of battery cells is the battery cell described in <14> or <15> above.

[0044] Since the above cell stack includes a battery cell according to one embodiment of the present disclosure, the current efficiency of a redox flow battery system including this cell stack is likely to be improved.

[0045] <17> A redox flow battery system according to one embodiment of the present disclosure comprises a cell stack as described in <16> above, a first circulation mechanism for circulating a positive electrode electrolyte through the cell stack, and a second circulation mechanism for circulating a negative electrode electrolyte through the cell stack.

[0046] In the redox flow battery system described above, the movement of active material ions through the diaphragm is less likely to occur. Therefore, the redox flow battery system has excellent current efficiency. In addition, the frequency of correcting the balance of active material ion concentrations in the positive electrode electrolyte and the negative electrode electrolyte can be reduced.

[0047] <18> In the redox flow battery system described in <17> above, the positive electrode electrolyte may contain vanadium ions as the positive electrode active material, and the negative electrode electrolyte may contain vanadium ions as the negative electrode active material.

[0048] Vanadium ions have a proven track record as an active material in redox flow battery systems. Therefore, redox flow battery systems that utilize vanadium ions as an active material offer superior reliability.

[0049] Specific examples of diaphragms, battery cells, cell stacks, and redox flow battery systems according to the embodiments of this disclosure will be described below with reference to the drawings. In the embodiments, a redox flow battery will be referred to as an RF battery. Identical reference numerals in the figures indicate the same or corresponding parts. The dimensions of the components shown in each drawing are for illustrative purposes only and do not necessarily represent actual dimensions. The present invention is not limited to these examples, but is indicated by the claims, and all modifications within the meaning and scope equivalent to the claims are intended. It should be understood that at least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways.

[0050] <Embodiment 1> <RF Battery System> The RF battery system 1 shown in Figure 1 is an energy storage system that charges with external power as needed, or discharges stored power. The charging power is power generated by the power generation unit 510. The discharge power is supplied to the load 530. The power generation unit 510 is, for example, a solar power generation device, a wind power generation device, or other general power plants. The load 530 is, for example, a power consumer. Typically, the RF battery system 1 is connected to the power generation unit 510 and the load 530 via an AC / DC converter 500 and a substation 520. In Figure 1, the solid arrow between the AC / DC converter 500 and the substation 520 represents charging, and the dashed arrow represents discharging. The RF battery system 1 is used, for example, for load leveling, momentary sag compensation or emergency power supply, and for output smoothing of natural energy such as solar power generation or wind power generation, which are being introduced on a large scale.

[0051] RF battery system 1 uses a positive electrode electrolyte and a negative electrode electrolyte. Typically, the positive electrode electrolyte and the negative electrode electrolyte contain metal ions whose valency changes upon oxidation-reduction as active material ions. Charging or discharging in RF battery system 1 is performed by utilizing the difference between the oxidation-reduction potential of the active material ions in the positive electrode electrolyte and the oxidation-reduction potential of the active material ions in the negative electrode electrolyte.

[0052] The RF battery system 1 of this example, shown in Figure 1, comprises a battery cell 10, a first circulation mechanism 10P, and a second circulation mechanism 10N. The battery cell 10 comprises a positive electrode cell and a negative electrode cell. The positive electrode cell and the negative electrode cell are separated by a diaphragm 3, which will be described later. The first circulation mechanism 10P and the second circulation mechanism 10N circulate the electrolyte in the battery cell 10. One of the features of the RF battery system 1 of this embodiment lies in the configuration of the diaphragm 3.

[0053] The positive electrode cell contains a positive electrode 14. The positive electrode electrolyte is circulated in the positive electrode cell by a first circulation mechanism 10P. The first circulation mechanism 10P comprises a positive electrode electrolyte tank 18, a supply pipe 20, a discharge pipe 22, and a pump 24. The positive electrode electrolyte tank 18 stores the positive electrode electrolyte. The positive electrode electrolyte flows through the supply pipe 20 and the discharge pipe 22. The supply pipe 20 connects the positive electrode electrolyte tank 18 to the positive electrode cell. The discharge pipe 22 connects the positive electrode cell to the positive electrode electrolyte tank 18. The pump 24 pumps the positive electrode electrolyte from the positive electrode electrolyte tank 18. The pump 24 is located in the middle of the supply pipe 20.

[0054] The negative electrode cell contains a negative electrode 15. The negative electrode electrolyte is circulated in the negative electrode cell by a second circulation mechanism 10N. The second circulation mechanism 10N comprises a negative electrode electrolyte tank 19, a supply pipe 21, a discharge pipe 23, and a pump 25. The negative electrode electrolyte tank 19 stores the negative electrode electrolyte. The negative electrode electrolyte flows through the supply pipe 21 and the discharge pipe 23. The supply pipe 21 connects the negative electrode electrolyte tank 19 to the negative electrode cell. The discharge pipe 23 connects the negative electrode cell to the negative electrode electrolyte tank 19. The pump 25 pumps the negative electrode electrolyte from the negative electrode electrolyte tank 19. The pump 25 is located in the middle of the supply pipe 21.

[0055] During operation of the RF battery system 1, the positive electrode electrolyte and negative electrode electrolyte pumped by pumps 24 and 25 flow as follows: The positive electrode electrolyte is supplied from the positive electrode electrolyte tank 18 to the positive electrode cell through the supply pipe 20. This positive electrode electrolyte is then discharged from the positive electrode cell to the positive electrode electrolyte tank 18 through the discharge pipe 22. The negative electrode electrolyte is supplied from the negative electrode electrolyte tank 19 to the negative electrode cell through the supply pipe 21. This negative electrode electrolyte is then discharged from the negative electrode cell to the negative electrode electrolyte tank 19 through the discharge pipe 23. When charging and discharging are not taking place, pumps 24 and 25 are stopped.

[0056] The battery cell 10 is typically housed inside a structure called a cell stack 200. The cell stack 200 comprises a laminate in which multiple battery cells 10 are stacked, two end plates 220, and a clamping mechanism 230. In this example, the laminate is constructed by stacking multiple cell frames 16, positive electrodes 14, diaphragms 3, and negative electrodes 15 in that order. The two end plates 220 sandwich the laminate from the outside. The clamping mechanism 230 tightens both end plates 220. The cell stack 200 may also have a structure comprising multiple sub-cell stacks. Each sub-cell stack is mainly composed of the above-mentioned laminate.

[0057] The cell frame 16 comprises a bipolar plate 161 and a frame 162. The cell frame 16 has a recess for arranging a positive electrode 14 or a negative electrode 15. The recess is formed by the surface of the bipolar plate 161 and the inner circumferential surface of the frame 162. One battery cell 10 is formed between the bipolar plates 161 of two adjacent cell frames 16. The positive electrode cell and the negative electrode cell of adjacent battery cells 10 are formed with one bipolar plate 161 in between. The bipolar plate 161 and the frame 162 can have known configurations. An annular sealing member 167 is arranged between each frame 162. This sealing member 167 reduces leakage of electrolyte from the battery cell 10.

[0058] The positive electrode active material ions contained in the positive electrode electrolyte are, for example, one or more selected from the group consisting of vanadium ions, manganese ions, iron ions, polyacids, quinone derivatives, amines, organic compounds, and organometallic complexes. The negative electrode active material ions contained in the negative electrode electrolyte are, for example, one or more selected from the group consisting of vanadium ions, titanium ions, chromium ions, polyacids, quinone derivatives, amines, and organometallic complexes. Charging and discharging occur in accordance with the valence change reaction of the active material ions. The solvent of the electrolyte is, for example, an aqueous solution containing one or more acids or salts selected from the group consisting of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid.

[0059] When the positive electrode active material ions and negative electrode active material ions are vanadium (V) ions, the V ions constituting the positive electrode active material ions are V ４＋ and V ５＋ That is. V during charging４＋ is oxidized to V ５＋ During discharge, V ５＋ is reduced to V ４＋ The negative electrode active material is V ２＋ and V ３＋ During charging, V ３＋ is reduced to V ２＋ During discharge, V ２＋ is oxidized to V ３＋ Vanadium ions have a track record as active material ions in the RF battery system 1.

[0060] The separator 3 disposed between the positive electrode 14 and the negative electrode 15 has cation permeability that allows carriers composed of cations to permeate. In addition to permeating protons, the separator 3 may permeate metal ions such as V ions which are cations. In the separator 3 having cation permeability, V ２＋ tends to permeate the separator 3 more easily than V ions other than ２＋ When the positive electrode active material ions move to the negative electrode electrolyte through the separator 3, or when the negative electrode active material ions move to the positive electrode electrolyte, a difference occurs in the amount of active material ions in both electrolytes, and there is a risk of a decrease in current efficiency. The separator 3 of this example has a configuration that inhibits the active material ions from permeating the separator 3.

[0061] <<Separator>> Based on FIG. 2 which is a schematic configuration diagram of the battery cell 10, the detailed configuration of the separator 3 will be described. In FIG. 2, a gap is formed between the separator 3 and the positive electrode 14, and between the separator 3 and the negative electrode 15. In the actual battery cell 10, the positive electrode 14 and the negative electrode 15 are in close contact with the separator 3. The separator 3 includes a base material 4 and a coating layer 5. The base material 4 corresponds to the separator in a conventional battery cell. The base material 4 includes a first surface 41 and a second surface 42 on the opposite side of the first surface 41. The coating layer 5 is formed on at least a part of the first surface 41 and the second surface 42 of the base material 4. In this example, the coating layer 5 is formed only on the first surface 41. The coating layer 5 is disposed so as to face the negative electrode 15. As shown in FIG. 3, the coating layer 5 may be formed on both the first surface 41 and the second surface 42.

[0062] The substrate 4 is a sheet that is permeable to cations. In this example, the substrate 4 is permeable to protons, i.e., permeable to hydrogen ions. A known diaphragm from conventional battery cells can be used for the substrate 4. The substrate 4 is formed, for example, from a perfluorosulfonic acid polymer.

[0063] The average thickness of the diaphragm 3 is, for example, 150 μm or less. The average thickness of the diaphragm 3 can be determined, for example, as follows: First, images of five or more different cross-sections of the diaphragm 3 are obtained. The maximum thickness of the diaphragm 3 is measured in each of the obtained cross-sectional images. The average of these maximum thicknesses is taken as the average thickness of the diaphragm 3. A diaphragm 3 with an average thickness of 150 μm or less is permeable to hydrogen ions, making it easier to reduce the cell resistance of the battery cell 10. The average thickness of the diaphragm 3 may be 100 μm or less, 50 μm or less, or 30 μm or less. The cross-section of the diaphragm 3 may be observed using an SEM (Scanning Electron Microscope), a TEM (Transmission Electron Microscope), or an optical microscope.

[0064] Regarding the lower limit of the diaphragm 3, the average thickness of the diaphragm 3 is, for example, 3 μm or more. A diaphragm 3 with an average thickness of 3 μm or more is less likely to be damaged. The average thickness of the diaphragm 3 may also be, for example, 4 μm or more, or 5 μm or more.

[0065] The average thickness range of the diaphragm 3 is, for example, 3 μm to 150 μm, 3 μm to 100 μm, 5 μm to 100 μm, 4 μm to 50 μm, 5 μm to 50 μm, 6 μm to 30 μm, or 8 μm to 30 μm.

[0066] The coating layer 5 contains a nitrogen-containing polymer and a cation-permeable fluorine-containing polymer. When the total mass of the nitrogen-containing polymer and the fluorine-containing polymer in the coating layer 5 is 100% by mass, the mass percentage of the nitrogen-containing polymer is, for example, 50% by mass or more. The nitrogen-containing polymer is thought to reduce the permeation of positively charged active material ions. The higher the mass percentage of the nitrogen-containing polymer in the coating layer 5, the easier it is for the coating layer 5 to restrict the movement of active material ions. From this viewpoint, the mass percentage of the nitrogen-containing polymer in the coating layer 5 is, for example, 90% by mass or more. The mass percentage of the nitrogen-containing polymer may be 95% by mass or more, or 98% by mass or more.

[0067] A fluorine-containing polymer with cation permeability is thought to contribute to maintaining the effects obtained by the nitrogen-containing polymer over a long period of time. When the total mass of the nitrogen-containing polymer and the fluorine-containing polymer in the coating layer 5 is set to 100% by mass, the mass percentage of the fluorine-containing polymer is, for example, 0.01% by mass or more and less than 50% by mass. If the mass percentage of the fluorine-containing polymer is within the above range, the effects obtained by the nitrogen-containing polymer are more easily maintained.

[0068] Nitrogen-containing polymers are polymers that do not contain fluorine. Examples of nitrogen-containing polymers include materials containing primary to tertiary amino groups, materials containing quaternary ammonium groups, or materials containing heterocyclic molecules having nitrogen. If the coating layer 5 contains multiple nitrogen-containing polymers, the coating layer 5 may contain one or more materials selected from the group consisting of the three materials described above.

[0069] Materials containing primary to tertiary amino groups include, for example, polyethyleneimine or its salts, polyaniline or its salts, polyallylamine or its salts, polystyrene-based tertiary amines, or polyphenol-based tertiary amines.

[0070] Materials containing quaternary ammonium groups include, for example, diallyldimethylammonium chloride polymer, diallylamine hydrochloride-sulfur dioxide copolymer, tetrabutylammonium, benzyltriethylammonium, polystyrene-based quaternary ammonium, or benzethonium.

[0071] Materials containing nitrogen-containing heterocyclic molecules include, for example, polypyrrole or its salts, diallylamine polymer or its salts, poly(4-vinylpyridine) or its salts, vinylpyridine / divinylbenzene copolymer or its salts, vinylpyridine / styrene copolymer or its salts, poly(diallyldimethylammonium) or its salts, or cetylpyridinium.

[0072] The material containing a nitrogen-containing heterocyclic molecule may be, for example, a polyazole compound. The polyazole compound may be one or more selected from the group consisting of, for example, polymers of heterocyclic compounds containing one or more nitrogen atoms in the ring, and polymers of heterocyclic compounds containing one or more nitrogen atoms and at least one of oxygen and sulfur in the ring. More specifically, the polyazole compound may be one or more selected from the group consisting of polyimidazole compounds, polybenzimidazole compounds, polybenzobisimidazole compounds, polybenzoxazole compounds, polyxazole compounds, polythiazole compounds, and polybenzothiazole compounds. The polybenzimidazole compound may be polybenzimidazole or a salt thereof.

[0073] The nitrogen-containing polymer may also contain hydrocarbon-based ion exchange materials. Alternatively, the nitrogen-containing polymer may be any other known anion exchange resin.

[0074] Fluorine-containing polymers include, for example, materials containing sulfonic acid groups, materials containing carboxylic acid groups, and materials containing phosphate groups. When the coating layer 5 contains multiple fluorine-containing polymers, it may also contain one or more materials selected from the group consisting of the three materials described above. Specific examples of materials containing sulfonic acid groups are sulfonated polymers, perfluorocarbon sulfonic acid resins, polystyrene sulfonic acid resins, sulfonated polyether sulfones, sulfonated polyetheretherketones, sulfonated polyimides, sulfonated polyethers, sulfonated polybenzimidazoles, or sulfonated polyarylenes. Specific examples of materials containing carboxylic acid groups are methacrylic acid-based weakly acidic cation exchange resins and acrylic acid-based weakly acidic cation exchange resins. Specific examples of materials containing phosphate groups are phosphate ester polymers or polyphosphates. Fluorine-containing polymers may also contain hydrocarbon-based ion exchange materials. The specific examples of sulfonated polymers mentioned above may be homopolymers or copolymers containing homopolymers.

[0075] The method for forming the coating layer 5 is as follows: A raw material solution containing the above-mentioned ion exchange material is prepared, and the raw material solution is applied to the substrate 4, or the substrate 4 is immersed in the raw material solution. The solvent in the raw material solution is, for example, dimethylacetamide, N-methyl-2-pyrrolidone, or dimethyl sulfoxide. The ion exchange material contained in the raw material solution may be in the form of parts dispersed in the solution, or it may be dissolved in the solvent. The raw material solution may contain a crosslinking agent. Depending on the type of solvent, the substrate 4 may be pretreated. Examples of pretreatment include corona treatment, plasma treatment, or ozone treatment. The above pretreatment may improve the adhesion between the substrate 4 and the coating layer 5. The coating layer 5 is formed by drying the substrate 4 with the raw material solution attached and evaporating the liquid components of the raw material solution. If the raw material solution contains a crosslinking agent, the ion exchange material is crosslinked by heat treatment or the like for crosslinking, making it difficult for the coating layer 5 to peel off from the substrate 4.

[0076] The average thickness of the coating layer 5 is, for example, 2 nm or more. The average thickness of the coating layer 5 can be determined, for example, as follows: First, images of five or more different cross-sections are obtained from the diaphragm 3. The maximum thickness of the region containing the fluorine-containing polymer and the nitrogen-containing polymer is measured in each of the obtained cross-sectional images. The average of these maximum thicknesses is taken as the average thickness of the coating layer 5. A coating layer 5 of 2 nm or more tends to inhibit the permeation of active material ions. The average thickness of the coating layer 5 may be 10 nm or more, or 50 nm or more. The cross-section of the diaphragm 3 is observed, for example, with a TEM.

[0077] Regarding the upper limit of the coating layer 5, the average thickness of the coating layer 5 is, for example, 5 μm or less. A coating layer 5 of 5 μm or less allows protons to pass through easily. The average thickness of the coating layer 5 may also be, for example, 3 μm or less, or 1 μm (1000 nm) or less. The thinner the coating layer 5, the less the increase in cell resistance caused by the coating layer 5 can be reduced. The average thickness of the coating layer 5 may also be 800 nm or less, 500 nm or less, or 300 nm or less.

[0078] The average thickness range of the coating layer 5 is, for example, 2 nm to 5 μm, 10 nm to 3 μm, 50 nm to 1000 nm, 50 nm to 500 nm, 50 nm to 300 nm, or 100 nm to 300 nm.

[0079] <Test Example 1> In Test Example 1, the effect of the coating layer 5 on the diaphragm 3 on the battery performance of the RF battery system 1 was investigated. Specifically, RF battery systems 1 from sample No. 1 to sample No. 15 and from sample No. 101 to sample No. 104 were examined. Each RF battery system 1 is equipped with a single cell battery that differs only in the configuration of the diaphragm 3. A single cell battery is a battery consisting of one battery cell 10. The above effect was investigated by measuring the cell resistivity and current efficiency of each sample.

[0080] ≪Sample No. 1 to Sample No. 10≫ In the single-cell batteries of Sample No. 1 to Sample No. 10, the diaphragm 3 has a two-layer structure in which a coating layer 5 is formed only on the first surface 41 of the substrate 4. The coating layer 5 is positioned to face the negative electrode 15. The main components of the diaphragm 3 are as follows: ・Material of substrate 4: Perfluorosulfonic acid polymer ・Average thickness of substrate 4: 20 μm ・Method of forming the coating layer 5: Coating method in which a raw material solution containing ion exchange material is applied to the substrate 4 and then dried ・Solvent of raw material solution: Dimethylacetamide ・Type of ion exchange material: Nitrogen-containing polymers are shown in Table 1. Nitrogen-containing polymers are polyethyleneimine, poly(diallyldimethylammonium chloride), polybenzimidazole, poly(4-vinylpyridine), or polyaniline. Fluorine-containing polymers were perfluorosulfonic acid polymer, divinylbenzene-acrylic acid copolymer, or parastyrenephosphonic acid polymer, which are types of sulfonated polymers. • Blend ratio in the raw material liquid: Table 1 shows the blend ratio in the raw material liquid for the coating layer 5 applied to the substrate 4. The blend ratio is the mass ratio when the total of nitrogen-containing polymer and fluorine-containing polymer is set to 100. For example, '99:1' in the table means that the mass percentage of nitrogen-containing polymer is 99% by mass and the mass percentage of fluorine-containing polymer is 1% by mass. • Average thickness of the coating layer 5: 1000 nm

[0081] ≪Sample No. 11≫ In the single-cell battery of Sample No. 11, the diaphragm 3 has a two-layer structure in which a coating layer 5 is formed only on the second surface 42 of the substrate 4. In other words, the coating layer 5 of Sample No. 11 is positioned to face the positive electrode 14. In the "Diaphragm Structure" section of the table, a "*" is added below "Two-layer structure" to distinguish the structure of the diaphragm 3 of Sample No. 11 from the structure of the diaphragm 3 of Samples No. 1 to No. 10.

[0082] ≪Sample No. 12≫ The diaphragm 3 in the single cell battery of sample No. 12 has a three-layer structure in which a coating layer 5 is formed on both the first surface 41 and the second surface 42 of the base material 4.

[0083] <<Sample No. 13 to Sample No. 15>> The single-cell batteries of Sample No. 13 to Sample No. 15 differ from the single-cell batteries of Sample No. 1 to Sample No. 12 in the thickness of the coating layer 5. The thickness of the coating layer 5 in Sample No. 13 and Sample No. 14 is 500 nm and 100 nm, respectively. The single-cell batteries of Sample No. 13 and Sample No. 14 are the same as the single-cell battery of Sample No. 4 in all other aspects except for the thickness of the coating layer 5. The thickness of the coating layer 5 in the single-cell battery of Sample No. 15 is 100 nm. The single-cell battery of Sample No. 15 is the same as the single-cell battery of Sample No. 3 in all other aspects except for the thickness of the coating layer 5.

[0084] ≪Sample No. 101≫ The diaphragm in the single cell battery of sample No. 101 is a single-layer structure formed only by the substrate 4 and does not have a coating layer 5.

[0085] ≪Sample No. 102 and Sample No. 103≫ The diaphragm in the single-cell batteries of Sample No. 102 and Sample No. 103 has a coating layer 5 on the first surface 41 of the substrate 4, but the coating layer 5 is a diaphragm containing only a nitrogen-containing polymer. The nitrogen-containing polymer is polyethyleneimine or poly(4-vinylpyridine).

[0086] ≪Sample No. 104≫ The diaphragm in the single cell battery of Sample No. 104 differs from that of the single cell battery of Sample No. 101 in the composition of the base material 4. Specifically, the diaphragm in Sample No. 104 has a single-layer structure and does not have a coating layer 5. The base material 4 of Sample No. 104 is formed from poly(4-vinylpyridine) and perfluorosulfonic acid polymer. When the total mass of poly(4-vinylpyridine) and perfluorosulfonic acid polymer is 100% by mass, the mass ratio of poly(4-vinylpyridine) is 5% by mass and the mass ratio of perfluorosulfonic acid polymer is 95% by mass.

[0087] <<Battery Characteristics Test>> In the battery characteristics test of each sample, a vanadium sulfate aqueous solution is used as the positive electrode electrolyte and the negative electrode electrolyte. The vanadium concentration is 1.7 mol / L. Vanadium ions act as active material ions. The battery characteristics test is performed with a current density of 70 mA / cm². ２ The process is carried out with a constant current, and when a predetermined switching voltage is reached, charging and discharging are switched. In this battery characteristic test, five test cycles are performed, each consisting of the aforementioned charging and discharging cycles.

[0088] Based on the data obtained from this test, the cell resistivity (Ω·cm) of each sample was calculated. ２ The cell resistivity (%) and current efficiency (%) were determined. The results are shown in Table 2. • Cell resistivity is calculated by multiplying the cell resistance value by the electrode area. Here, the electrode area refers to the area where the positive electrode 14 and the negative electrode 15 overlap in a single-cell battery. The cell resistance value is obtained by dividing the difference between the intermediate voltage during charging and the intermediate voltage during discharging by 2, and then dividing that value by the current value. 'Intermediate voltage' refers to the voltage value at the midpoint of the time from the start to the end of charging or discharging. • Current efficiency is the ratio of discharge time to charge time in the 5th cycle. Specifically, the current efficiency is calculated by (total discharge time / total charge time) × 100. This current efficiency can be considered the current efficiency of the single-cell battery in its initial stages of manufacture.

[0089] In this test example, to investigate the durability of the diaphragm 3 against the electrolyte, V was used as the active material. ２＋An immersion test is conducted by immersing the diaphragm 3 in an electrolyte containing the specified substance, i.e., a test electrolyte simulating the negative electrode electrolyte. The temperature of the test electrolyte is 75°C, and the immersion time of the diaphragm 3 in the test electrolyte is 4 weeks. Using the diaphragm 3 after immersion in the test electrolyte, an RF battery system is fabricated for each sample, and the same tests as the battery characteristic tests described above are performed. From the test results, the current efficiency of each sample is calculated using the above formula. The calculated current efficiency after the immersion test is also shown in Table 2.

[0090] In this test example, a durability test consisting of 300 test cycles was conducted to investigate the durability of the diaphragm 3 during actual use in the RF battery system. Subsequently, the current efficiency after the 300-cycle durability test was calculated according to the current efficiency calculation method described above. The calculated current efficiency after the durability test is shown in Table 2.

[0091]

[0092]

[0093] ≪Evaluation of Test Results≫ As shown in Table 2, the cell resistivity of both sample No. 101 and sample No. 104, which used a diaphragm 3 without the coating layer 5, was 0.46 Ω·cm. ２ On the other hand, the cell resistivity of samples No. 1 to No. 15 and samples No. 102 to No. 103, which are equipped with the coating layer 5, is 0.44 Ω·cm. ２ From 0.49 Ω·cm ２ This is within the range. In other words, the coating layer 5 may slightly increase the cell resistivity. By increasing the mass ratio of the fluorine-containing polymer contained in the coating layer 5, as in sample No. 7 and sample No. 8, it may be possible to make it less likely for the cell resistivity to increase. Also, from the comparison of sample No. 1 to sample No. 5, it can be seen that by changing the type of nitrogen-containing polymer, it may be possible to make it less likely for the cell resistivity to increase.

[0094] The current efficiencies of samples No. 1 to No. 15, which are equipped with the coating layer 5, are 98% or 99%. On the other hand, the current efficiencies of sample No. 101 and sample No. 104, which are not equipped with the coating layer, are 95% and 96%, respectively. In other words, it can be seen that the current efficiency of the RF battery system 1 is significantly improved by the inclusion of the coating layer 5 in the diaphragm 3. Furthermore, by comparing sample No. 4 and samples No. 6 to No. 8, it can be seen that the current efficiency tends to be higher when the mass proportion of fluorine-containing polymer in the coating layer 5 is lower than when it is higher.

[0095] The current efficiency of samples No. 1 to No. 15 after immersion testing was the same as or almost the same as the current efficiency at the time of fabrication. Similarly, the current efficiency of samples No. 1 to No. 15 after durability testing was the same as or almost the same as the current efficiency at the time of fabrication. The coating layer 5 of the diaphragm 3 in samples No. 1 to No. 15 contains both nitrogen-containing polymer and fluorine-containing polymer. On the other hand, the current efficiency of samples No. 102 and No. 103 after immersion testing decreased by approximately 4% compared to the current efficiency at the time of fabrication. Similarly, the current efficiency of samples No. 102 and No. 103 after durability testing decreased by approximately 4% compared to the current efficiency at the time of fabrication. The coating layer 5 of the diaphragm in samples No. 102 and No. 103 contains nitrogen-containing polymer but does not contain fluorine-containing polymer. Therefore, it is presumed that the inclusion of a fluorine-containing polymer in the coating layer 5, in addition to a nitrogen-containing polymer, reduces the movement of active material ions over a long period of time, thereby maintaining high current efficiency.

[0096] A comparison of sample No. 4, sample No. 13, and sample No. 14 revealed that the durability of the diaphragm 3 was maintained even with a thin coating layer 5. Furthermore, in the configuration of sample No. 14, where the coating layer 5 was 100 nm thick, the increase in cell resistivity was reduced compared to the configurations of sample No. 4 and sample No. 13, where the coating layer 5 was thicker. A comparison of sample No. 3 and sample No. 15 revealed that by thinning the coating layer 5, the increase in cell resistivity was reduced while maintaining the durability of the diaphragm 3.

[0097] Sample No. 104, which has a substrate 4 made of poly(4-vinylpyridine) and perfluorosulfonic acid polymer, has low cell resistivity and low current efficiency. Poly(4-vinylpyridine) is a nitrogen-containing polymer, and perfluorosulfonic acid polymer is a fluorine-containing polymer. From these results, it was found that a single-layer diaphragm 3 made of nitrogen-containing polymer and fluorine-containing polymer without a coating layer 5 cannot improve the current efficiency of the RF battery system.

[0098] 1 RF Battery System 10 Battery Cell, 10N Second Circulation Mechanism, 10P First Circulation Mechanism 14 Positive Electrode, 15 Negative Electrode 16 Cell Frame 161 Bipolar Plate, 162 Frame, 167 Sealing Member 18 Positive Electrolyte Tank, 19 Negative Electrolyte Tank 20, 21 Supply Pipe, 22, 23 Discharge Pipe, 24, 25 Pump 3 Diaphragm 4 Substrate 41 First Surface, 42 Second Surface 5 Coating Layer 200 Cell Stack 220 End Plate, 230 Tightening Mechanism 500 AC / DC Converter 510 Power Generation Unit, 520 Substation Equipment, 530 Load

Claims

1. A diaphragm used in a battery cell of a redox flow battery system, comprising: a substrate having cation permeability; and a coating layer covering at least a portion of the first and second surfaces of the substrate, wherein the coating layer contains a nitrogen-containing polymer and a fluorine-containing polymer, and the fluorine-containing polymer is cation permeable.

2. The diaphragm according to claim 1, wherein the coating layer is formed only on the first surface.

3. The diaphragm according to claim 1, wherein the coating layer is formed on both the first surface and the second surface.

4. The diaphragm according to any one of claims 1 to 3, wherein the average thickness of the coating layer is 2 nm or more.

5. The diaphragm according to claim 4, wherein the average thickness of the coating layer is 1000 nm or less.

6. The diaphragm according to any one of claims 1 to 5, wherein when the total mass of the nitrogen-containing polymer and the fluorine-containing polymer is 100% by mass, the mass ratio of the nitrogen-containing polymer is 90% by mass or more.

7. The diaphragm according to any one of claims 1 to 6, wherein the nitrogen-containing polymer is one or more materials selected from the group consisting of materials containing primary to tertiary amino groups, materials containing quaternary ammonium groups, and materials containing a heterocyclic molecule having nitrogen.

8. The diaphragm according to claim 7, wherein the nitrogen-containing polymer is a polyazole compound.

9. The diaphragm according to claim 8, wherein the polyazole compound is one or more selected from the group consisting of a polymer of a heterocyclic compound containing one or more nitrogen atoms in the ring, and a polymer of a heterocyclic compound containing one or more nitrogen atoms and at least one of oxygen and sulfur in the ring.

10. The diaphragm according to claim 9, wherein the polyazole compound is one or more selected from the group consisting of polyimidazole compounds, polybenzimidazole compounds, polybenzobisimidazole compounds, polybenzoxazole compounds, polyoxazole compounds, polythiazole compounds, and polybenzothiazole compounds.

11. The diaphragm according to any one of claims 1 to 10, wherein the fluorine-containing polymer is one or more materials selected from the group consisting of materials containing sulfonic acid groups, materials containing carboxylic acid groups, and materials containing phosphate groups.

12. The diaphragm according to any one of claims 1 to 11, wherein the average thickness is 150 μm or less.

13. The diaphragm according to claim 12, wherein the average thickness is 3 μm or more.

14. A battery cell comprising: a positive electrode; a negative electrode; and a diaphragm according to any one of claims 1 to 13, disposed between the positive electrode and the negative electrode.

15. The battery cell according to claim 14, wherein the diaphragm is arranged such that the coating layer faces the negative electrode.

16. A cell stack comprising a plurality of battery cells stacked on top of each other, wherein at least a portion of the plurality of battery cells are the battery cells described in claim 14 or claim 15.

17. A redox flow battery system comprising: a cell stack according to claim 16; a first circulation mechanism for circulating a positive electrode electrolyte through the cell stack; and a second circulation mechanism for circulating a negative electrode electrolyte through the cell stack.

18. The redox flow battery system according to claim 17, wherein the positive electrode electrolyte contains vanadium ions as a positive electrode active material, and the negative electrode electrolyte contains vanadium ions as a negative electrode active material.

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