Free-standing, ion-selective composite membranes including sulfonated aromatic polymers

Abstract

This disclosure relates to free-standing, composite membranes that include an ion-selective polymer coating that covers at least one surface and partially penetrates into the pore structure of a polyolefin substrate. The ion-selective polymer coating includes an ion-selective, sulfonated aromatic polymer containing aromatic groups in the backbone of the polymer. While the composite membranes do not have open, interconnected pores that connect each major surface, ion transport can take place through wetting of available pores and swelling of the ion-selective polymer coating accompanied by ion migration from one membrane surface to the opposite surface. Such composite membranes are useful for separating the anolyte and catholyte in a flow battery.

Description

FREE-STANDING, ION-SELECTIVE COMPOSITE MEMBRANES INCLUDING SULFONATED AROMATIC POLYMERSRELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 668,069, filed on July 5, 2024, and titled FREE-STANDING, ION-SELECTIVE COMPOSITE MEMBRANES INCLUDING SULFONATED AROMATIC POLYMERS, which is incorporated herein by reference in its entirety.COPYRIGHT NOTICE

[0002] © 2025 Amtek Research International LLC. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1 71 (d).TECHNICAL FIELD

[0003] This disclosure relates to free-standing, composite membranes that include an ion- selective polymer coating that covers at least one surface and partially penetrates into the pore structure of a polyolefin substrate. While the composite membranes do not have open, interconnected pores that connect each major surface, ion transport can still take place through wetting of available pores and swelling of the ion-selective polymer coating accompanied by ion migration from one membrane surface to the opposite surface Such composite membranes are useful for separating the anolyte and catholyte in a flow battery.BACKGROUND

[0004] Energy storage from renewable resources such as wind and solar power is becoming increasingly important to the electric utility industry. Large-scale energy storage applications can help to mitigate climate change and allow utilities to improve system reliability and performance, smooth out power costs, and enable 24 / 7 consumption of renewable energy.

[0005] To achieve the above objectives, utility companies are investigating a variety of battery technologies. Lead-acid batteries have been commonly used because of their low cost, reasonable energy density, and their ability to discharge under high current loads. The cycle life of the lead-acid battery is a disadvantage compared to other battery chemistries. Li-ion batteries are also being utilized in large-scale storage systems. While such batteries have outstanding energy density and excellent cycle life, they suffer from safety issues because the organic electrolyte can result in a fire and explosion. Sodium-sulfur batteries have also been investigated because of their high energy density, yet their operational costs are high because of a required operating temperature at 300-350 C.

[0006] More recently, flow batteries have been investigated for large-scale, renewable power facilities. Flow batteries store electricity in liquid electrolytes that reside in storage tanks and are pumped through the cell during charge and discharge cycles The flow battery consists of two half-cells that are divided by an ion-selective membrane that separates and insulates the two sides from each other. Flow batteries have been demonstrated with a variety of redox couples in multivalent vanadium and iron compounds that are water soluble. The aqueous electrolyte is attractive for safety and otherreasons. While all flow batteries suffer from low energy density because of their large liquid storage tanks, they are an attractive option for wind or solar farms that are typically located in rural areas with low land costs.

[0007] One of the keys to achieving high efficiency and long cycle life in a flow battery is the ion- selective membrane Such membranes must have excellent chemical stability and long-life durability, while preventing cross-over contamination. Furthermore, the membrane must have low specific ionic resistance for transport between the half-cells. In order to prevent cross-over contamination and reduced cycle life, it is desirable to have a composite membrane that exhibits excellent mechanical properties having a porous, aqueous-wettable bulk substrate that is coated on at least one side with an ion-selective polymer-rich, non-porous layer. In some embodiments, the composite membrane is only coated with an ion-selective, polymer-rich, non-porous layer on one side, while the other side remains uncoated, porous, and capable of being thermally, ultrasonically, or adhesively bonded to a frame that can be stacked to form multiple cells in series or parallel. In other embodiments, the composite membrane is coated on both sides with an ion-selective, polymer-rich, non-porous layer. If desired, the ion-selective, polymer-rich, non-porous layer can also optionally be crosslinked.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The embodiments disclosed herein will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. These drawings depict only typical embodiments, which will be described with additional specificity and detail through use of the accompanying drawings in which:

[0009] FIG. 1 is a cross-sectional schematic view of an asymmetric, composite membrane having an ion-selective, non-porous layer on one side of the membrane.

[0010] FIG. 2 is a scanning electron microscopy (SEM) image of a cross-section of a composite membrane made in accordance with an embodiment of the disclosure, depicting an ion-selective, non- porous layer disposed on a microporous web.

[0011] FIG. 3 is a scanning electron microscopy (SEM) image of a cross-section of a composite membrane made in accordance with an embodiment of the disclosure, depicting an ion-selective, non- porous layer disposed on a microporous web.

[0012] FIG. 4 is a plot of the electrical resistance of a composite membrane sample disclosed herein.

[0013] FIG. 5 is a cross-sectional view of a schematic of an apparatus used to measure electrical resistance of the composite membrane samples disclosed herein.

[0014] FIG. 6 is a perspective view of a schematic of an apparatus used to measure electrical resistance of the composite membrane samples disclosed herein.

[0015] FIG. 7 is an image of an apparatus used to measure the rate of ion crossover through the composite membrane samples disclosed herein.

[0016] FIG 8 is a graph showing Fe3+ion concentration vs time as used herein to determine the rate of crossover through the composite membrane samples.

[0017] FIG. 9 is a graph of the electrical resistivity (mQ-cm2) of a composite membrane sample disclosed herein at various time periods.

[0018] FIG. 10 is a graph of the Fe3+ion diffusion rate through a composite membrane sample at various time periods

[0019] FIG. 11 is a graph of the electrical resistivity (mQ-cm2) comparing various composite membrane samples at various time periods.

[0020] FIG. 12 is a graph of the Fe3+ion diffusion rate comparing various composite membrane samples at various time periods.

[0021] FIG. 13 is a graph of the absorbance comparing cast films of sulfonated aromatic polymers (not on PE-silica substrate).

[0022] FIG. 14 is a graph of a charge / discharge profile for a vanadium redox flow battery using a composite membrane separator described herein.DETAILED DESCRIPTION

[0023] While Nation® and other ion-selective polymers have been cast or extruded into films for fuel cell and other energy storage applications, these materials suffer from poor mechanical properties, particularly when wet. In addition, the ion-selective polymer is expensive; therefore, it is desirable to minimize the thickness of the film or web even though a thinner film or web is more difficult to handle. Furthermore, it is difficult to bond such polymers as Nation® to other surfaces.

[0024] An advantage of the present disclosure is the ability to produce a free-standing ion- selective membrane for applications in flow batteries or other energy storage devices. This advantage is accomplished by combining a microporous polyolefin substrate with an ion-selective coating to form a composite membrane. In particular embodiments, disclosed herein are composite membranes containing a freestanding, microporous polyolefin substrate comprising a polyolefin and a high surface area, hydrophilic filler. The microporous polyolefin substrate has a bulk structure that extends from a first major surface to a second major surface. The bulk structure has 40-75% porosity, is wettable with an aqueous electrolyte, and includes a high surface area, hydrophilic filler distributed throughout the bulk structure. In some instances, the volume fraction of filler divided by the volume fraction of polyolefin is greater than 0.75, or greater than 1.0, such as between 0.75 and 1.3. In some embodiments, the first major surface is uncoated and includes open pores, readily penetrable by aqueous electrolyte into the porosity of the bulk structure. The second major surface can have a non-porous coating of an ion- selective polymer that results in lower air permeability and liquid permeability. In other embodiments, both major surfaces (i e., the first and second major surfaces) have a non-porous coating of an ion- selective polymer that results in reduced air permeability and liquid permeability.

[0025] “Freestanding” refers to a web or membrane having sufficient mechanical properties for use in unwinding, coating, winding, slitting and other web handling operations. The terms “film,” “sheet,” “substrate”, “web,” and “membrane” can be used interchangeably. “Microporous” refers to an average pore size less than about 1 micrometer (e.g., 0.1 to 0.3 microns as measured by mercury porosimetry).

[0026] The microporous polyolefin substrate is freestanding, has 40-75% porosity, and is wettable with aqueous solutions that are commonly used in electrolytes for flow batteries To impart such wettability to the polyolefin substrate, it is desirable to incorporate a large quantity of a high surface area, hydrophilic filler such as precipitated or fumed silica. Because the volume fraction and orientation of the polymer in the microporous substrate impacts tensile strength and puncture strength, it desirableto use ultra-high molecular weight polyethylene (UHMWPE) or a blend that includes it as part of the polymer matrix. Other molecular weights of polyethylenes or blends can also be used, including very high molecular weight polyethylene (VHMWPE). As used herein, UHMWPE generally corresponds to a range of between about 3.1 million g / mol to about 10 million g / mol, and VHMWPE generally corresponds to a range of between about 500,000 g / mol to about 3.1 g / mol

[0027] While lead-acid separators are commonly produced from UHMWPE and precipitated or fumed silica, they typically contain 10-20% of residual process oil to improve the oxidation resistance of the separator Residual oil is less desirable for composite membranes used in flow batteries. As such, it is important to carefully select the process oil such that the process oil is easily extracted to leave behind a minimal residual content in the microporous polyolefin sheet. In flow battery applications, exemplary process oils that can be used include, but are not limited to, paraffinic oils, naphthenic oils, mineral oils, plant-based oils, and mixtures thereof. In a particular embodiment, the resultant microporous polymer substrate, post-extraction of the process oil, contains less than 3% process oil, or even more preferably, less than 2% process oil.

[0028] The ion-selective polymer coating prevents or minimizes the migration of electrochemically active species (e.g., metallic cations) from the anolyte to the catholyte, or vice versa. Such migration results in a loss of current efficiency in the battery and can lead to shorter operating lifetimes. In some embodiments, a coating is chosen that does not excessively impede the transport of the charge-carrying ions between the electrodes. Resistance to flow of these ions will result in reduced voltage efficiency of the battery. The polymer coating resists fouling and maintains integrity over the operating life of the battery.

[0029] The optimization of the ion-selective polymer coating is contingent upon flow battery chemistry, but in general, the polymer swells in water and contains anhydride, carboxylic acid, and / or sulfonic acid groups. T raditional ion-selective polymers that have been used include, but are not limited to, perfluorosulfonic acid / polytetrafluoroethylene copolymers (Chemours; Nation®) and tetrafluoroethylene-sulfonyl fluoride vinyl ether copolymers (Solvay; Aquivion®). Other fluoropolymers such as polyvinylidene fluoride and its copolymers can be chemically modified with ion-exchange head groups to render them suitable as ion-selective polymers. Non-fluorinated and / or non-halogenated polymers can also be used as the ion-selective polymer. Such polymers include, but are not limited to, sulfonated aromatic polymers having one or more aromatic groups in the polymer backbone, such as sulfonated polyethersulfone, sulfonated polyetheretherketone, sulfonated polybenzimidazole, sulfonated polyphenylene (including linear sulfonated polyphenylene, kinked sulfonated polyphenylene, side-chain sulfonated polyphenylene, and pre- and post-sulfonated phenylated polyphenylene), sulfonated polyarylene (including sulfonated poly(arylene ether ketone), sulfonated poly(arylene ether sulfone), and sulfonated poly(arylene sulfone sulfide)), sulfonated phenol-formaldehyde resin, sulfonated resorcinol-formaldehyde resin, and derivatives thereof. Additional, non-limiting examples of polymers that can be modified to be ion-selective include poly ether ether ketone (PEEK), poly phenylene oxide (PPO), polyimide (PI), poly benzimidazole (PBI), poly arylene ether sulfone (PAES), and combinations thereof. These polymers can be sulfonated, carboxylated, or otherwise modified to make them ion-selective polymers.

[0030] In certain embodiments, a sulfonated aromatic polymer can be used as the ion-selective polymer, such as sulfonated polyethersulfone, sulfonated polyetheretherketone, or sulfonated polybenzimidazole.

[0031] For instance, in an embodiment, sulfonated polyethersulfone (S-PES) having the following structure is used:where the ratio of m : n is between about 0.6-1 : 04-0.1, or between about 0.7-0.9 : 0.3-0.1.

[0032] In another embodiment, sulfonated polyetheretherketone (S-PEEK) having the following structure is used:where n is greater than 10.

[0033] In yet another embodiment, sulfonated polybenzimidazone (S-PBI) having the following structure is used:where n is greater than 10.

[0034] The ion-selective polymer can also optionally be crosslinked, such as via irradiation, free radicals, or chemical cross-linking. Various types of crosslinking agents or crosslinkers can be used. For instance, the crosslinkers can be activated by functional groups (e g , NH2, OH, etc ) on the ion- selective polymer, other chemical agents, heat, pressure, change in pH, light (e.g., UV light), or irradiation. In particular embodiments, polyfunctional aziridines are used as crosslinking agents. Other types of crosslinkers can also be used including, but not limited to, polyfunctional isocyanates, epoxides, amines, phenolics, and anhydrides, etc.

[0035] The ion-selective polymer can further include nanoparticulate fillers. Examples of the nanoparticulate fillers include: metal oxides such as SiO2, TiO2, ZrO2, SnO2, and AI2O3; metal phosphates such as zirconium phosphate, titanium phosphate, and boron phosphate; phosphosilicates such as P2O5-SiO2 and Metal oxide-P205-Si02; zeolites such as natural (chabazite, clinoptilolite,mordenite) and synthetic; inorganic solid acid particles or polyacids such as phosphotungstic acid, phosphomolybdic acid, and silicotungstic acid; carbon materials such as carbon nanotubes, activated carbon, and graphene oxide; metal-organic frameworks (MOF’s); and combinations of any of the foregoing. Many of these fillers can be further modified by sulfonation, carboxylation, phosphonation, amination, hydrolysis / condensation reactions and reactions with silanes to add functionality that improves wettability and / or ion conductivity. For instance, zirconia (ZrO2) and other metal oxides can be sulfonated and used as a nanoparticulate filler. In some embodiments, the ion-selective polymer includes between about 1% and about 10% by weight of the one or more nanoparticulate fillers.

[0036] When nanoparticulate fillers are present, adhesive and / or binder polymers can be present in the ion-selective coating as well. Non-limiting examples of adhesive and / or binder polymers include polyvinyl alcohol (PVOH), acrylates, styrene-butadiene (SBR) emulsions, and combinations thereof.

[0037] Additional polymers or agents can also be blended with the ion-selective polymer (regardless of whether nanoparticulate fillers are present). For instance, in some embodiments, the ion- selective polymer can be blended with a chelating agent. Illustrative chelating agents include, but are not limited to, polyvinylpyrrolidone (PVP), iminodiacetic acid (IDA), ethylenediamine triacetic acid (EDTA), diethylenetriaminepentaacetate (DTPA), nitrilotriacetic acid (NTA), salicyladloxime, citric acid, ethylenediamine di(o-hydroxyphenylacetic acid) (EDDHA), and derivatives thereof.

[0038] As previously mentioned, the microporous polymer substrate or web is wettable with the aqueous electrolyte of the energy storage device to promote or assist proton transport. For instance, the microporous polymer substrate can include a high surface area, hydrophilic filler distributed throughout the polymer matrix such that the volume fraction of filler divided by the volume fraction of polymer exceeds 0.75 or 1.0, such as between 0.75 and 1.3. In some embodiments, the high surface area, hydrophilic filler has a surface area greater than 50 m2 / g. Examples of the hydrophilic fillers that can be used include an inorganic oxide, carbonate, or hydroxide, such as, for example, alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, boron oxides, and mixtures thereof. Hydrotalcites can also be used as hydrophilic fillers. A preferred high-surface area, hydrophilic filler is precipitated or fumed silica.

[0039] For flow battery applications, the ion-selective composite membrane is chemically inert in the electrolyte of the flow battery To that end, in some embodiments the microporous polymer substrate does not include a surfactant to aid in wettability of the polymer substrate. In other embodiments, the microporous polymer substrate does include a surfactant. Furthermore, any residual process oil should not be leached from the substrate over extended periods of use.

[0040] In some embodiments, the microporous polyolefin substrate comprises a thickness of 100 microns to 350 microns. The ion-selective coating comprises a thickness of 1 micron to 25 microns, or from 1 micron to 10 microns.

[0041] The ion-selective composite membranes disclosed herein can have excellent thermal stability as demonstrated through accelerated aging in highly oxidized electrolyte (charged state) For example, in some embodiments, the ion-selective composite membranes have shown stability (e.g., doesn’t fall apart) and are functional after having been disposed in Fe3+solutions at 70 °C for more than 3 months.

[0042] The composite membranes disclosed herein may provide enhanced durability, due at least in part from the presence of UHMWPE in the freestanding microporous polyolefin substrate. Accordingly, methods of making a battery separator with enhanced durability include providing or having provided a microporous polyolefin substrate having two major surfaces and comprising ultrahigh molecular weight polyethylene and coating at least one of the two major surfaces of the microporous polyolefin substrate with an ion-selective polymer material. The coating may be applied by spray coating, knife-over-roll coating, dip coating, rod coating, slot die coating, or gravure coating Other coating methods may also be employed.

[0043] In another embodiment, a precursor material can be coated on at least one of the two major surfaces of the microporous polyolefin substrate, after which the precursor material can be sulfonated to yield an ion-selective polymer material. For instance, a precursor material such as polybenzimidazole can be coated on at least one of the two major surfaces of the microporous polyolefin substrate, after which the polybenzimidazole can be sulfonated.

[0044] An illustrative composite membrane that can be made in accordance with the present disclosure is depicted in FIG. 1. FIG. 1 is a cross-sectional schematic view of an asymmetric, composite membrane 100 having an ion-selective, non-porous coating layer 112 on one side of the membrane 100. As shown in FIG. 1, the composite membrane 100 includes a microporous polymer substrate 102 having a first major surface 104 and a second major surface 106. As further depicted in FIG. 1 , the composite membrane 100 includes a first ion-selective, non-porous coating layer 112 disposed on one side (e g., the first major surface 104) of the microporous polymer substrate 102.

[0045] The following examples are illustrative in nature and not intended to be limited in any way.

[0046] Example 1

[0047] Ion-selective polymer solutions containing either sulfonated polyetheretherketone (S- PEEK), sulfonated polyethersulfone (S-PES), or sulfonated polybenzimidazole (S-PBI ) were prepared by dissolving the particular polymer at 15 wt% in an aprotic polar solvent (dimethylacetamide (DMAc)). Samples of microporous SiO2 / polyethylene webs were then coated with one of the ion-selective polymer solutions by applying a thin layer of the polymer solution to one side of the flat web using a doctor blade. After coating, the composite structures were suspended in a vented oven where they dried / cured at 130°C for 60 seconds. The resulting coatings were approximately 1 pm to 10 pm in thickness, depending on the doctor blade height. If desired, a second coating can be applied to the other side of the composite structures. A scanning electron microscopy (SEM) image of a cross-section of a composite membrane, depicting the ion-selective, non-porous S-PEEK layer disposed on the microporous web is shown in FIG. 2, and a scanning electron microscopy (SEM) image of a crosssection of a composite membrane, depicting the ion-selective, non-porous S-PES layer disposed on the microporous web is shown in FIG. 3.

[0048] After the coatings were dried, the coating weights (g / m2) were determined by weighing a known area of the sample and subtracting the basis weight for the SiO2 / PE base film Exemplary coating weights that were measured, included the following:TABLE 1

[0049] Subsequently, the through-membrane electrical resistance (ER) was measured using the following procedure: A 1” x 4” strip of the membrane was soaked in 2M KCI at 70°C for 2 hours while stirring. The strip was then cut into thirds and remained immersed in the KCI. Each third was placed separately between two stainless steel electrodes that were interfaced with an electrochemical impedance analyzer. The impedance was recorded between 100 kHz and 1 kHz. The real component of the impedance at 100 kHz was recorded as the resistance of the membrane Subsequently two pieces were stacked on one another, placed in the fixture, and impedance measured. Finally, all three pieces were stacked and the impedance measured. The average impedance of the single pieces plus the two and three stacks were plotted, as shown in FIG. 4. The slope of this line determines the resistance of the material. With reference to FIGS. 5 and 6, the schematic representation of the testing apparatus depicts the top electrode 320, bottom electrode 322, polytetrafluoroethylene (PTFE) insulator 324, sample 330, and leads R,W,B,G.

[0050] In addition to resistance, the diffusional cross-over of vanadium ions and / or iron ions was measured in a two-chamber cell separated by a piece of the sample membrane. An ion-rich electrolyte solution containing either Fe3+or V5+ / V4+was placed in one solution (right side) while the lean side (left side) only contained supporting solution (KCI for iron or H2SC>4 / MgSO4 for vanadium). A picture of the apparatus is shown in FIG. 7. Aliquots were withdrawn from the lean side in periodic intervals and the UV-Vis spectra were recorded. Both Fe3+, V4+, and V5+all have characteristic absorptions, so the rate of diffusional cross-over can be determined by determining the slope of the concentration vs. time plot. An exemplary graph showing Fe3+concentration vs time on the lean side of a diffusion cell for an S- PES-coated membrane at 10 g / m2is shown in FIG. 8. Such a graph can be used to determine the cross-over rate (mol / hr / m2).

[0051] The sample membranes were then submerged in electrolyte solution at 70°C for 2-4 weeks to determine the stability of the coated membranes. The resistance and crossover can then be measured again to determine the stability of the membranes. The data shown in FIGS. 9 and 10 is representative of the resistance and cross-over for the coated samples in Table 1

[0052] Example 2

[0053] Samples of microporous SiCh / polyethylene webs were coated with various ion-selective polymer formulations. The coating weights were from 5 g / m2to 20 g / m2. All samples were then dried / cured at 130 °C for 1 min The following samples were prepared:TABLE 2aCross-linked Kraton Nexar - sulfonated pentablock copolymer dissolved in 1 : 1 n-propanol:toluene at 15 wt% blended with Bakelite LB7575 etherified bisphenol-A crosslinking resin and bisphenol A diglycidylether (BADGE) crosslinker. Solid content: Kraton MD9204 - 60.6%; LB7575 - 34.6%; Badge - 4 8%bS-PES (Konishi Chemicals) - Sulfonated polyethersulfone dissolved 60:40 ratio of dimethylacetamide (DMAc):n-propanol at 15 wt%.cS-PES (Konishi Chemicals) - Sulfonated polyethersulfone dissolved 1 : 1 ratio of acetone: water at 15 wt%.dS-PEEK (Syensqo) - sulfonated polyetheretherketone dissolved in DMAc at 15 wt%eS-PEEK (Syensqo) - sulfonated polyetheretherketone dissolved in dimethylformamide (DMF) at 15 wt%fS-PES / S-PEEK - 3: 1 ratio of S-PES:S-PEEK in 70:30 DMAcm-propanol at 15 wt%.gS-PES / pvp - 90% S-PES with 10% polyvinylpyrrolidinone (PVP) dissolved in 60:40 ratio DMAcm- propanol.

[0054] For each sample, resistance measurements were taken using the stacked membrane technique and equipment described in Example 1 , where-by the through-membrane resistance was measured for each of three pieces of membrane and averaged. Subsequently the resistance was measured for two membranes stacked and then three membranes stacked The slope of the Resistance vs layer determines the resistance of the material.

[0055] In measuring the resistance, the un-aged samples were soaked in 2 M KCI for 2 hours at 70 “C to wet the membranes priorto taking measurements. Aged samples immersed in 1.5 M FeCl3 / 1 5 M KCI at 70 °C for 2 weeks and 4 weeks. The aged samples, immediately prior to measurements, were soaked in 2 M KCI for 1 hour to remove excess electrolyte. A graph of the measured resistances is set forth in FIG. 11 , with the target resistance being less than 600 mfl-cm2over time.

[0056] In addition to resistance, the diffusional cross-over of Fe3+ ions was also determined in a two-chamber diffusion cell (as described in Example 1 ) with FeCh on the rich side of the membrane and supporting electrolyte (no Fe) on the lean side. The rate of diffusion was determined by measuring the concentration of Fe3+ in the lean side vs time using UV-Vis spectroscopy. A graph of the measured diffusion rates is set forth in FIG. 12, with the target cross-over Fe3+diffusion rate (mol / hr / m2) being less than 0.2.

[0057] The durability / stability of the coating polymers in vanadium electrolytes was also measured. The results in FIG 13 show UV-Vis absorbance of the coating material (cast films of the sulfonatedaromatic polymers (not on PE-silica substrate)) aged in V5+electrolyte at 50 °C. The presence of V4+indicates oxidation / depredation of the membrane material, thus reducing the amount of V5+.

[0058] Example 3

[0059] A composite membrane battery separator was prepared by coating a microporous Si02 / polyethylene web with an ion-selective polymer formulation including S-PES at a coat weight of about 10.4 g / m2. The coated microporous web was then dried / cured at 130 °C. The composite membrane battery separator was then incorporated into a vanadium redox flow battery, and the battery’s voltage and current was measured during several charge / discharge cycles. Typical results of the measurements are depicted in in the charge / discharge curve shown in FIG. 14.

[0060] As can be appreciated, this disclosure pertains to structures and methods of making the same Any methods disclosed or contemplated herein comprise one or more steps or actions for performing the described method. The method steps and / or actions may be interchanged with one another In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and / or use of specific steps and / or actions may be modified.

[0061] Reference throughout this specification to “an embodiment” or “the embodiment” means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, the quoted phrases, or variations thereof, as recited throughout this specification are not necessarily all referring to the same embodiment.

[0062] Similarly, in the above description of embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure This method of disclosure, however, is not to be interpreted as reflecting an intention that any claim requires more features than those expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment.

[0063] References to approximations are made throughout this specification, such as by use of the terms “substantially” and “about.” For each such reference, it is to be understood that, in some embodiments, the value, feature, or characteristic may be specified without approximation. For example, where qualifiers such as “about” and “substantially” are used, these terms include within their scope the qualified words in the absence of their qualifiers.

[0064] Unless otherwise stated, all ranges include both endpoints and all numbers between the endpoints

[0065] Recitation in the claims of the term “first” with respect to a feature or element does not necessarily imply the existence of a second or additional such feature or element.

[0066] The claims following this written disclosure are hereby expressly incorporated into the present written disclosure, with each claim standing on its own as a separate embodiment. This disclosure includes all permutations of the independent claims with their dependent claims. Moreover, additional embodiments capable of derivation from the independent and dependent claims that follow are also expressly incorporated into the present written description

[0067] Without further elaboration, it is believed that one skilled in the art can use the preceding description to utilize the invention to its fullest extent. The claims and embodiments disclosed hereinare to be construed as merely illustrative and exemplary, and not a limitation of the scope of the present disclosure in any way. It will be apparent to those having ordinary skill in the art, with the aid of the present disclosure, that changes may be made to the details of the above-described embodiments without departing from the underlying principles of the disclosure herein. In other words, various modifications and improvements of the embodiments specifically disclosed in the description above are within the scope of the appended claims. Moreover, the order of the steps or actions of the methods disclosed herein may be changed by those skilled in the art without departing from the scope of the present disclosure. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order or use of specific steps or actions may be modified. The scope of the invention is therefore defined by the following claims and their equivalents.

Claims

CLAIMS1. A composite membrane comprising: a freestanding, microporous polyolefin substrate comprising a polyolefin and a hydrophilic filler, the microporous polyolefin substrate having a porosity of 40-75% that extends from a first major surface to a second major surface, wherein the hydrophilic filler is distributed throughout the substrate and in which a volume fraction of hydrophilic filler divided by a volume fraction of polyolefin is greater than 0.75 thereby making the substrate wettable, and wherein at least one of the first and second major surfaces comprises a non-porous coating of an ion-selective, sulfonated aromatic polymer containing aromatic groups in the backbone of the polymer.

2. The composite membrane of claim 1 , wherein at least one major surface comprises open pores, readily penetrable by an aqueous electrolyte into the porosity of the substrate.

3. The composite membrane of claim 1 or 2, wherein the ion-selective, sulfonated aromatic polymer is crosslinked.

4. The composite membrane of any one of claims 1-3, wherein the ion-selective, sulfonated aromatic polymer mitigates the transfer of metallic cations.

5. The composite membrane of any one of claims 1-4, wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises nanoparticulate fillers.

6. The composite membrane of any one of claims 1-5, wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises inorganic solid acid particles.

7. The composite membrane of any one of claims 1-6, wherein the coating of the ion-selective, sulfonated aromatic polymer comprises a blend.

8. The composite membrane of claim 7, wherein the blend comprises a sulfonated aromatic polymer and a chelating agent.

9. The composite membrane of claim 8, wherein the chelating agent comprises at least one of polyvinylpyrrolidone (PVP), iminodiacetic acid (IDA), ethylenediamine triacetic acid (EDTA), diethylenetriaminepentaacetate (DTPA), nitrilotriacetic acid (NTA), salicyladloxime, citric acid, or ethylenediamine di(o-hydroxyphenylacetic acid) (EDDHA).

10. The composite membrane of any one of claims 1-9, wherein the sulfonated aromatic polymer is sulfonated after being coated on the at least one of the first and second major surfaces.

11. The composite membrane of any one of claims 1-10, wherein the microporous polyolefin substrate further comprises a surfactant.

12. The composite membrane of any one of claims 1-11 , wherein the microporous polyolefin substrate comprises less than 3% of a residual process oil13. The composite membrane of any one of claims 1-12, wherein the microporous polyolefin substrate has a thickness of 100 microns to 350 microns14. The composite membrane of any one of claims 1-14, wherein the coating of the ion-selective, sulfonated aromatic polymer has a thickness of 1 micron to 25 microns, or 1 micron to 10 microns.

15. The composite membrane of any one of claims 1-14, wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, sulfonated polybenzimidazole, sulfonated polyphenylene, sulfonated polyarylene, sulfonated phenol-formaldehyde resin, sulfonated resorcinol-formaldehyde resin, or derivatives thereof.

16. The composite membrane of claim 15, wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, or sulfonated polybenzimidazole.

17. The composite membrane of any one of claims 1-16, wherein the microporous polyolefin substrate comprises ultra-high molecular weight polyethylene and provides extended mechanical strength to the composite membrane.

18. A flow battery, comprising: a composite membrane comprising: a freestanding, microporous polyolefin substrate comprising a polyolefin and a hydrophilic filler, the microporous polyolefin substrate having a porosity of 40-75% that extends from a first major surface to a second major surface, wherein the hydrophilic filler is distributed throughout the substrate and in which a volume fraction of hydrophilic filler divided by a volume fraction of polyolefin is greater than 0.75 thereby making the substrate wettable, and wherein at least one of the first and second major surfaces comprises a non-porous coating of an ion-selective, sulfonated aromatic polymer containing aromatic groups in the backbone of the polymer19. The flow battery of claim 18, wherein at least one major surface comprises open pores, readily penetrable by an aqueous electrolyte into the porosity of the substrate.

20. The flow batter of claim 18 or 19, wherein the ion-selective, sulfonated aromatic polymer is crosslinked.

21. The flow battery of any one of claims 18-20, wherein the ion-selective, sulfonated aromatic polymer mitigates the transfer of metallic cations.

22. The flow battery of any one of claims 18-21 , wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises nanoparticulate fillers.

23. The flow battery of any one of claims 18-22, wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises inorganic solid acid particles.

24. The flow battery of any one of claims 18-23, wherein the coating of the ion-selective, sulfonated aromatic polymer comprises a blend.

25. The flow battery of claim 24, wherein the blend comprises a sulfonated aromatic polymer and a chelating agent.

26. The flow battery of claim 25, wherein the chelating agent comprises at least one of polyvinylpyrrolidone (PVP), iminodiacetic acid (IDA), ethylenediamine triacetic acid (EDTA), diethylenetriaminepentaacetate (DTPA), nitrilotriacetic acid (NTA), salicyladloxime, citric acid, or ethylenediamine di(o-hydroxyphenylacetic acid) (EDDHA).

27. The flow batery of any one of claims 18-26, wherein the sulfonated aromatic polymer is sulfonated after being coated on the at least one of the first and second major surfaces.

28. The flow batery of any one of claims 18-27, wherein the microporous polyolefin substrate further comprises a surfactant.

29. The flow batery of any one of claims 18-28, wherein the microporous polyolefin substrate comprises less than 3% of a residual process oil.

30. The flow battery of any one of claims 18-29, wherein the microporous polyolefin substrate has a thickness of 100 microns to 350 microns.

31. The flow battery of any one of claims 18-30, wherein the coating of the ion-selective, sulfonated aromatic polymer has a thickness of 1 micron to 25 microns, or 1 micron to 10 microns.

32. The flow battery of any one of claims 18-31 , wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, sulfonated polybenzimidazole, sulfonated polyphenylene, sulfonated polyarylene, sulfonated phenolformaldehyde resin, sulfonated resorcinol-formaldehyde resin, or derivatives thereof.

33. The flow battery of claim 32, wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, or sulfonated polybenzimidazole.

34. The flow batery of any one of claims 18-33, wherein the microporous polyolefin substrate comprises ultra-high molecular weight polyethylene and provides extended mechanical strength to the composite membrane.

35. A method of making a separator with enhanced durability, the method comprising: providing or having provided a microporous polyolefin substrate having two major surfaces and comprising ultrahigh molecular weight polyethylene; coating at least one major surface of the microporous polyolefin substrate with an ion-selective, sulfonated aromatic polymer containing aromatic groups in the backbone of the polymer.

36. The method of claim 35, wherein coating comprises spray coating, knife-over-roll coating, dip coating, rod coating, slot die coating, or gravure coating.

37. The method of claim 35 or claim 36, wherein at least one major surface comprises open pores, readily penetrable by an aqueous electrolyte into the porosity of the substrate.

38. The method of any one of claims 35-37, wherein the ion-selective, sulfonated aromatic polymer is crosslinked.

39. The method of any one of claims 35-38, wherein the ion-selective polymer mitigates the transfer of metallic cations.

40. The method of any one of claims 35-39, wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises nanoparticulate fillers.

41. The method of any one of claims 35-40, wherein the coating of the ion-selective, sulfonated aromatic polymer further comprises inorganic solid acid particles42. The method of any one of claims 35-41 , wherein the coating of the ion-selective, sulfonated aromatic polymer comprises a blend.

43. The method of claim 42, wherein the blend comprises a sulfonated aromatic polymer and a chelating agent.

44. The method of claim 43, wherein the chelating agent comprises at least one of polyvinylpyrrolidone (PVP), iminodiacetic acid (IDA), ethylenediamine triacetic acid (EDTA), diethylenetriaminepentaacetate (DTPA), nitrilotriacetic acid (NTA), salicyladloxime, citric acid, or ethylenediamine di(o-hydroxyphenylacetic acid) (EDDHA).

45. The method of any one of claims 35-44, wherein the sulfonated aromatic polymer is sulfonated after being coated on the at least one of the first and second major surfaces.

46. The method of any one of claims 35-45, wherein the microporous polyolefin substrate further comprises a surfactant.

47. The method of any one of claims 35-46, wherein the microporous polyolefin substrate comprises less than 3% of a residual process oil.

48. The method of any one of claims 35-47, wherein the microporous polyolefin substrate has a thickness of 100 microns to 350 microns.

49. The method of any one of claims 35-48, wherein the coating has a thickness of 1 micron to 25 microns, or 1 micron to 10 microns.

50. The method of any one of claims 35-49, wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, sulfonated polybenzimidazole, sulfonated polyphenylene, sulfonated polyarylene, sulfonated phenol-formaldehyde resin, sulfonated resorcinol-formaldehyde resin, or derivatives thereof.

51. The method of claim 50, wherein the ion-selective, sulfonated aromatic polymer comprises at least one of sulfonated polyetheretherketone, sulfonated polyethersulfone, or sulfonated polybenzimidazole.

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