Polybenzimidazole-based separator for secondary battery and method for producing same

Abstract

The method for producing a polybenzimidazole-based film according to the present invention includes the steps of dissolving polybenzimidazole in an amide-based organic solvent to form a polybenzimidazole solution; impregnating a porous membrane with the polybenzimidazole solution; and drying the porous membrane impregnated with the polybenzimidazole solution at a temperature of 80°C or less.

Description

[Technical Field]

[0001] The present invention relates to a polybenzimidazole-based separator, a method for producing the same, and a secondary battery including the separator. [Background technology]

[0002] As the global economy grows alongside global warming, the need for renewable and sustainable energy systems based on renewable energy, such as solar and wind energy, is increasing. To improve grid network stability in the face of fluctuations due to the intermittent availability of these forms of energy, advanced energy storage systems (ESS) can be used to store excess power and transmit it to end users or the power grid when needed. Among these, electrochemical energy-based ESSs, such as rechargeable secondary batteries, can provide a cost-effective and clean form of energy storage solution. Electrochemical energy storage systems are broadly classified as secondary batteries, and examples include lithium-ion batteries, fuel cells, and redox flow batteries. Various types of electrochemical energy storage systems have different physical and / or chemical properties. Typically, secondary batteries comprise two electrodes, an electrolyte, and a separator (or ion-exchange membrane). Each of these components can affect the performance of the secondary battery.

[0003] Meanwhile, polybenzimidazole (PBI), a polymer material for manufacturing separators, is a glassy thermoplastic resin with high thermal stability and chemical resistance, and is known to have properties suitable for transporting cations, hydrogen, and water. Therefore, there have been attempts to use PBI as a separator material for secondary batteries.

[0004] As secondary batteries are used under increasingly harsh conditions, they are required to have excellent durability while operating for long periods of time. Research is ongoing to improve the mechanical strength of separators. Accordingly, there is a need to further improve the mechanical strength of polybenzimidazole-based separators while maintaining the charge / discharge efficiency and extending the lifespan of secondary batteries. Accordingly, there is also a need to develop a method for improving the process efficiency of polybenzimidazole-based separators. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide a method for producing a polybenzimidazole-based separation membrane, which can improve the mechanical strength of the polybenzimidazole-based separation membrane without using a backup film (or substrate film) in the production of the polybenzimidazole-based separation membrane.

[0006] In addition, the polybenzimidazole-based separator prepared according to the present invention not only has excellent ion exchange properties but also can improve the performance (charge / discharge efficiency and lifespan) of a secondary battery including the separator, thereby contributing to stable operation of the secondary battery even under harsh conditions.

[0007] Although a redox flow battery or a sealed redox battery using the redox couple of the present invention has been described as an example of a secondary battery, the application of the separator of the present invention is not limited thereto. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided a method for producing a polybenzimidazole-based separation membrane, the method including: dissolving a polybenzimidazole-based compound in an amide-based organic solvent to form a polybenzimidazole solution; impregnating a porous membrane with the polybenzimidazole solution; and drying the porous membrane impregnated with the polybenzimidazole solution at a temperature of 80°C or less to obtain a polybenzimidazole-based separation membrane.

[0009] The polybenzimidazole-based separator does not require a backing film to be formed.

[0010] The step of impregnating the porous membrane may include impregnating the cross section or both sides of the porous membrane.

[0011] The porous membrane may be made of materials including polypropylene, polyethylene, or a combination thereof.

[0012] The thickness of the porous membrane may be in the range of 1 to 30 μm, and the thickness of the polybenzimidazole-based separation membrane may be in the range of 2 to 40 μm.

[0013] In forming the polybenzimidazole solution, a surfactant may be mixed in. The surfactant may be contained in an amount of more than 0.1 wt % and less than 5.0 wt % relative to 100 wt % of the polybenzimidazole solution.

[0014] The surfactant may include one or more of an ionic surfactant, a non-ionic surfactant, and an organic surfactant.

[0015] The polybenzimidazole solution may be prepared by dissolving the polybenzimidazole in an amide-based organic solvent at a temperature of 130° C. or higher and / or a pressure of 0.1 MPa or higher.

[0016] When the amide organic solvent is taken as 100% by weight, the maximum solubility of the polybenzimidazole compound may be 8 to 20% by weight.

[0017] The viscosity adjusting solvent may be contained in an amount of 10 to 25% by weight relative to 100% by weight of the polybenzimidazole solution.

[0018] The viscosity adjusting solvent may include one or more of acetone, methyl ethyl ketone, methyl isobutyl ketone, methanol, ethanol, isopropanol, butanol, and isobutanol.

[0019] According to a second aspect of the present invention, there is provided a polybenzimidazole-based separation membrane produced according to the first aspect of the present invention.

[0020] According to a third aspect of the present invention, there is provided a secondary battery including the polybenzimidazole-based separator produced according to the first aspect of the present invention.

[0021] The secondary battery includes a redox battery involving oxidation and reduction reactions of a vanadium redox couple. [Effects of the Invention]

[0022] According to the method for manufacturing a polybenzimidazole-based separator according to the present invention, a backing film that must be removed after forming the separator is not used, and therefore, process efficiency can be significantly improved.

[0023] Furthermore, the polybenzimidazole-based separator prepared according to the present invention has high mechanical strength, which can contribute to improving the performance of secondary batteries. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic diagram of an exemplary redox flow battery. [Figure 2a]1 is a schematic diagram of a sealed redox battery according to an embodiment. [Figure 2b] 1 is a schematic diagram of a sealed redox battery including multiple sealed redox battery cells in a stacked configuration, according to some embodiments. [Figure 2c] 1 is a schematic diagram of a sealed redox battery including multiple sealed redox battery cells in a stacked configuration according to some other embodiments. [Figure 2d] 1 is a schematic diagram of a sealed redox battery including a plurality of sealed redox battery cells in a cylindrically stacked configuration according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0025] The above-mentioned objects, features, and advantages will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily implement the technical concept of the present invention. In describing the present invention, if a detailed description of known technologies relating to the present invention is deemed to obscure the gist of the present invention, the detailed description will be omitted. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings are used to indicate the same or similar components.

[0026] When the elements in this specification are described as "comprising," "having," "consisting of," "arranged," "comprising," etc., other parts may be added unless "only" is used. When an element is expressed in the singular, it also includes the plural unless otherwise expressly stated.

[0027] When interpreting the elements in this specification, they are interpreted as including a margin of error unless otherwise explicitly stated.

[0028] In this specification, when an arbitrary structure is arranged "on (or under)" a component or "above (or below)" a component, it means that the arbitrary structure is not only arranged in contact with the upper surface (or lower surface) of the component, but also that other structures may be interposed between the component and the arbitrary structure arranged above (or below) the component.

[0029] As previously mentioned, competitive factors considered in selecting and designing an electrochemical energy storage system suitable for a particular application include, among others, investment cost, power, energy, lifespan, recyclability, efficiency, scalability, and maintenance costs. Among the various electrochemical energy storage systems, redox flow batteries (RFBs) are considered preferable for stationary energy storage. RFBs are electrochemical energy conversion devices that utilize the redox process of redox species dissolved in a solution. The solution is stored in an external tank and introduced into the RFB cell when needed. Some of the advantageous features of RFB technology are the independent scalability of power and energy, high depth of discharge (DOD), and reduced environmental impact. These features allow for a wide range of operating power and discharge times, making RFBs preferable for storing electricity generated from renewable sources.

[0030] Specific disadvantages of some secondary batteries known in the art, such as lithium-ion batteries, include the generation of excessive heat and internal pressure during operation. To mitigate these effects, some secondary batteries use spacing between battery cells and / or a separate cooling device. Preferably, batteries according to embodiments disclosed herein generate significantly less heat and pressure, which ultimately reduces the risk of explosion and eliminates the need for spacing between battery cells or a cooling device, allowing for compact integration of the battery cells and the battery itself.

[0031] Various batteries use bus bars to electrically connect battery cells and / or the batteries themselves. For compact integration, the bus bars must be efficiently arranged to reduce the amount of space they occupy. In addition to electrically connecting battery cells and batteries, it is also necessary to physically and mechanically secure the battery cells or batteries together in an efficient manner. To address these and other needs, various embodiments disclosed herein provide bus bars that enable high-density integration of battery cells and / or batteries, and energy storage devices including the same. Furthermore, embodiments disclosed herein provide batteries and energy storage devices including the same that are easy to maintain after installation.

[0032] FIG. 1 is a schematic diagram of an exemplary redox flow battery (RFB). The RFB 100 includes a battery cell 104. The battery cell 104 has a first half-cell 104A and a second half-cell 104B separated by a separator or ion exchange membrane 112. The first half-cell 104A includes a positive electrode electrolyte reservoir 106A containing a first positive electrode electrolyte and a positive electrode electrolyte, and the second half-cell 104B includes a negative electrode electrolyte reservoir 106B containing a second negative electrode electrolyte and a negative electrode electrolyte. The positive electrode is electrically connected to a positive electrode current collector 108A, and the negative electrode is electrically connected to a negative electrode current collector 108B. Positive electrode electrolyte reservoir 106A is physically connected in fluid communication with positive electrode electrolyte tank 116A, and negative electrode electrolyte reservoir 106B is physically connected in fluid communication with negative electrode electrolyte tank 116B. In operation, positive electrode electrolyte is circulated between positive electrode electrolyte tank 116A and positive electrode electrolyte reservoir 106A via outlet and inlet conduits 120A and 124A, as represented by arrows, using positive electrode electrolyte pump 128A. Similarly, negative electrode electrolyte is circulated between negative electrode electrolyte tank 116B and negative electrode electrolyte reservoir 106B via outlet and inlet conduits 120B and 124B.

[0033] In some configurations, multiple battery cells 104-1, 104-2, ..., 104-n are stacked to form an RFB cell 150, where each cell is configured in a manner similar to battery cell 104. The multiple battery cells 104-1, 104-2, ..., 104-n include respective positive electrode electrolyte reservoirs 106A in fluid communication with one another and respective negative electrode electrolyte reservoirs 106B in fluid communication with one another. The multiple positive electrode electrolyte reservoirs 106A are coupled to one another and in fluid communication with a positive electrode electrolyte tank 116A, and the multiple negative electrode electrolyte reservoirs 106B are coupled to one another and in fluid communication with a negative electrode electrolyte tank 116B.

[0034] Compared to other electrochemical storage technologies, such as lithium-ion, lead-acid, and sodium-sulfur batteries, RFBs offer several advantages, enabling independent power and energy scalability by separating power conversion from energy storage. For example, RFBs are flexible and can be scaled in a distributed manner depending on the application, providing power and energy ranging from a few kW / kWh for home storage to systems up to tens of MW / MWh for grid storage. Also, unlike fuel cells, reactions in RFBs are reversible, allowing the same cell to serve as a converter of electricity to chemical energy and vice versa. RFBs operate by changing the valence of metal ions without consuming ionic metals, allowing for long life cycles. Cell temperature can be relatively easily controlled by adjusting electrolyte flow, due in part to the electrolyte's relatively high thermal mass. State of charge (SOC) can be easily monitored via cell voltage, and very deep depths of discharge (DOD) can be achieved.

[0035] Despite the many advantages of RFBs, and despite decades of relatively large capital, research, and development investments in this technology, RFBs have not been widely commercialized compared to other electrochemical storage technologies. In particular, the recent surge in battery demand for ESS applications and the increasing need for safety due to frequent fires and explosions indicate the suitability of RFBs, but widespread commercialization has yet to be achieved. This suggests that while there has long been a need for RFB commercialization, there are significant obstacles. The inventors have identified several of these obstacles, including relatively low reliability, low efficiency, large system footprint, and high system complexity.

[0036] A primary obstacle to widespread commercialization of RFBs relates to the relatively high complexity and associated reliability issues of RFBs, such as the RFB 100 described above with reference to FIG. 1. As previously described, an RFB includes multiple conduits 120A, 120B, 124A, and 124B for transporting electrolyte to and from the battery cells 104, pumps 128A and 128B for circulating the electrolyte, and tanks 116A and 116B for storing the electrolyte. Due to the relatively high complexity, the various connection points associated with the conduits 120A, 120B, 124A, and 124B between the battery cells 104 and the tanks 116A and 116B can create problems, such as leaks. The likelihood and frequency of failures increases proportionally with the number of these conduits, which scales with the size of the ESS. Failures pose stability risks as well as unscheduled repairs. Furthermore, reducing the likelihood of these failures through preventative maintenance to ensure uninterrupted operation adds to operating costs.

[0037] A second obstacle to the widespread commercialization of RFBs relates to their relatively low efficiency. One factor behind this relatively low efficiency is the energy consumed in circulating the electrolyte. For example, the electrolyte for vanadium-based RFBs may contain sulfuric acid and have a relatively high viscosity. Circulating an electrolyte, especially an electrolyte with a relatively high viscosity, through the microporous structure of electrodes based on randomly oriented carbon fiber felt can reduce the external efficiency of RFBs by consuming a relatively large amount of external energy. The low external efficiency of RFB systems is one of the main reasons why they are less commercially competitive than competing secondary battery technologies, such as lithium-ion batteries (LIBs).

[0038] A third obstacle to the widespread commercialization of RFBs relates to their relatively low power and energy densities compared to other electrochemical storage technologies, hindering their application in mobile applications. As explained herein, power and energy density refer to the power output and energy storage, respectively, of a storage device relative to the total volume of the energy storage device. Therefore, the power and energy densities of an RFB refer to the ratio of power output and energy storage to the total volume, including the cell volume, tank volume, and conduit volume for electrolyte transport. To partially compensate for the low power and energy densities, RFBs often have relatively large cell active areas and membranes, increasing cell size and potentially causing high cross-electrolyte gradients within the electrolyte reservoirs 116A, 116B. As a result, the average current density and nominal current of an RFB may be significantly lower than the theoretical maximum based on a uniform maximum current density. Furthermore, the need for a circulation system, including a separate tank and conduits, further reduces overall system-level space efficiency.

[0039] A fourth obstacle to widespread commercialization of RFBs relates to system complexity, which is comparable to that of a chemical plant. The high complexity of RFB system design results in long development cycles, which significantly slows technology development. System complexity also makes the system labor- and capital-intensive, requiring a high level of expertise for on-site installation, maintenance, and removal. System complexity is a deterrent to consumers due to the potential increase in manpower and training required to deploy and maintain the system, as well as the associated increase in overall costs.

[0040] To address these and other limitations while maintaining most of the advantages offered by RFBs, the present disclosure relates to a sealed redox battery that is not separately connected to an electrolyte tank. The present disclosure also relates to a secondary battery that includes bus bars that allow for efficient integration of multiple redox battery cells that can be further sealed. However, the present disclosure is not limited to only the battery configurations described above.

[0041] Sealed Redox Battery In one aspect, various embodiments of the redox battery disclosed herein relate to a redox battery. The redox battery according to the embodiments overcomes or alleviates at least some of the above-mentioned obstacles to commercialization of RFBs while maintaining the advantages of RFBs. In particular, while using a redox couple that participates in a redox reaction, unlike some RFBs, the embodiments of the redox battery disclosed herein include a sealed redox battery cell and do not have a separate electrolyte tank connected to the redox battery cell or an electrolyte circulation device such as a pump for supplying electrolyte from outside the redox battery cell.

[0042] FIG. 2a is a schematic diagram of a sealed redox battery according to an embodiment. The illustrated sealed redox battery 200A includes a first half-cell 204A and a second half-cell 204B. The first half-cell 204A includes a positive electrode electrolyte reservoir 106A having a first positive electrode electrolyte in contact with a positive electrode disposed therein. The first electrolyte contains a first redox couple dissolved therein, configured for a first redox half reaction to occur. The second half-cell 204B includes a negative electrode electrolyte reservoir 106B having a second negative electrode electrolyte in contact with a negative electrode disposed therein. The second electrolyte contains a second redox couple dissolved therein, configured for a second redox half reaction to occur. The positive and negative electrode electrolyte reservoirs 106A and 106B define reaction spaces for their respective half reactions. The sealed redox battery 200A further includes an ion exchange membrane or separator 112 separating the positive electrode electrolyte reservoir 106A and the negative electrode electrolyte reservoir 106B. The positive electrode is electrically connected to the positive electrode current collector 108A, and the negative electrode is electrically connected to the negative electrode current collector 108B. In some embodiments, a first bipolar plate 208A is interposed between the positive electrode current collector 108A and the positive electrode electrolyte reservoir 106A, and a second bipolar plate 208B is interposed between the negative electrode current collector 108B and the negative electrode electrolyte reservoir 106B.

[0043] Unlike conventional RFBs, in the sealed redox battery 200A according to the embodiment, the first half-cell 204A, the second half-cell 204B, and the ion exchange membrane or separator 112 define a redox battery cell that is partially enclosed or sealed by a frame 212 or casing that surrounds at least four sides of the battery cell. The illustrated sealed redox battery 200A is shown in cross section, so only the top and bottom sides of the casing 212 are shown. However, it should be understood that the casing 212 continuously surrounds the top, bottom, front, and rear of the illustrated battery cell. Additionally, first and second separator plates 208A and 208B contact the left and right edges or lips of the casing 212, respectively, to surround / enclose or seal the battery cell within a sealed space defined by the casing 212 and the first and second separator plates 208A and 208B. Thus, the enclosed / sealed or sealed frame or casing 212, first separator 208A, and second separator 208B define an enclosed or sealed volume divided by separator 112 into two spaces: a negative electrode electrolyte reservoir 106B containing the positive electrode and a positive electrode electrolyte reservoir 106A containing the negative electrode. The volume sealed by casing 212 and first and second separators 208A, 208B is physically inaccessible to its internal contents from the outside during normal operation. That is, the positive and negative electrode electrolytes are not in fluid communication with an external container, such as an electrolyte tank. The casing 212 and first and second separators 208A, 208B can hermetically and / or permanently seal redox battery 200A. These configurations are in contrast to conventional redox flow batteries, in which redox battery cells are in fluid communication with an external tank. That is, in sealed redox battery 200A, unlike RFB 100 described above with reference to Figure 1, neither positive electrode electrolyte reservoir 106A nor negative electrode electrolyte reservoir 106B within the sealed cell is in fluid communication with or physically connected to a separate electrolyte tank storing the first or second electrolyte, respectively. Thus, substantially the entire volume of positive and negative electrode electrolytes is stored within the redox battery cell and is sealed and enclosed by casing 212 and first and second separator plates 208A, 208B.That is, first electrolyte reservoir 106A stores substantially the entire volume of the first electrolyte for first half-cell 204A, and second electrolyte reservoir 106B stores substantially the entire volume of the second electrolyte for second half-cell 204B. In part, because sealed redox battery 200A is not separately connected to a storage tank, unlike RFB 100 shown in FIG. 1, sealed redox battery 200A does not include conduits 120A, 120B, 124A, 124B (FIG. 1) for conveying electrolyte to and from the redox battery cells, or pumps 128A, 128B (FIG. 1) for circulating the electrolyte.

[0044] As previously mentioned, a notable structural difference in sealed redox battery 200A is the omission of pumps 128A and 128B (FIG. 1). Instead, sealed redox battery 200A according to an embodiment is configured so that first and second electrolytes circulate spontaneously within positive electrode electrolyte reservoir 106A of first half-cell 204A and negative electrode electrolyte reservoir 106B of second half-cell 204B, respectively. In various configurations, the spontaneous circulation of the first and second electrolytes is caused by one or more of the following: an osmotic pressure difference between the first and second electrolyte reservoirs; density changes of one or both of the first and second electrolytes; diffusion or migration of one or both of the first and second electrolytes; affinity of one or both of the first and second electrolytes for the first and second electrodes, respectively; the first and second redox half reactions; and thermal expansion or contraction of one or both of the first and second electrolytes.

[0045] The inventors have found that self-circulation is effective in providing stability in power and energy output when the thickness of the positive and negative electrolyte reservoirs 106A, 106B in the cross-sectional view of FIG. 2a does not exceed a range defined as 20 cm, 15 cm, 10 cm, 5 cm, 2 cm, 1 cm, or any value therein.

[0046] Referring now to FIG. 2a, the casing 212 is formed of a corrosion-resistant material suitable for containing the positive and negative electrolytes, which may be highly acidic. In addition to providing corrosion resistance, the casing 212 may be rigid to provide mechanical support to the sealed redox battery 200A. In some embodiments, at least a portion of the casing 212 may be formed of a flexible material configured to deform to accommodate changes in internal pressure within the positive and negative electrolyte reservoirs 106A, 106B. Increases in internal pressure may be caused by various effects, for example, as described below for pressure-controlled sealed redox batteries. In configurations in which only a portion of the casing is formed of a flexible material, other portions may be formed of a rigid material. The flexible portion can be configured to expand in response to an increase in pressure, for example, to accommodate a respective volume increase of greater than 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, 10%, 20%, 50% or more for one or both of the positive and negative electrolyte reservoirs 106A, 106B. Suitable materials for the casing 212 may include polyvinyl chloride (PVC), polyethylene (PE), polystyrene (PS), polypropylene (PP), polycarbonate (PC), ABS, reinforced plastics, and the like.

[0047] The sealed redox battery 200A configured in this manner offers various technical and commercial advantages. For example, various reliability failures associated with conduits between the redox battery cells and the tank, e.g., pipe joints and electrolyte circulation pumps, are substantially reduced or eliminated, reducing unscheduled repairs as well as safety risks and operational costs associated with the operation of the sealed redox battery 200A. Furthermore, as previously described for the RFB 100 (FIG. 1), the need to circulate electrolyte between the redox battery cells and the tank using a pump is eliminated, substantially improving external efficiency. The inventors have found that, depending on the system size, the sealed redox battery 200A can achieve up to 2-50 times the power or energy density compared to conventional RFBs because it does not require electrolyte circulation between the cells and the electrolyte tank. As previously described, power or energy density refers to the power or energy output, respectively, of a storage device relative to the total volume of the energy storage device. Therefore, the power or energy density of a sealed redox battery refers to the ratio of power or energy output, respectively, to the total volume of the sealed redox battery. In addition, space efficiency is greatly improved by omitting a separate circulation system including a tank, pump, and conduits. Furthermore, system complexity is greatly reduced, significantly lowering the barrier to commercial implementation of the sealed redox battery 200A. For example, unlike conventional RFBs, the sealed redox battery 200A can be manufactured into a pack similar to a lithium-ion battery for modular implementation, eliminating the need for the intrusive structure required for the installation of conventional RFBs and making it more suitable for automation and mass production.

[0048] Hereinafter, the operating principles and aspects of the sealed redox battery 200A will be described using an example of a sealed vanadium (V) redox battery based on a vanadium-based redox pair, but the embodiments are not limited thereto, and it will be understood that the principles described herein can be applied to redox batteries based on various other redox pairs.

[0049] In the sealed V redox battery according to the embodiment, the first redox couple dissolved in the first positive electrode electrolyte of the first half-cell 204A is V 4+ / V 5+ The second redox couple dissolved in the second negative electrode electrolyte of the second half-cell 204B may be V 2+ / V 3+ The redox reactions during charging and discharging can be described using the following equations, where → indicates the direction of the discharge reaction and ← indicates the direction of the charge reaction:

[0050] Second half cell / negative electrode: V 2+ ←→V 3+ +e - First half cell / positive electrode: V 5+ +e - ←→V 4+ Overall response: V 2+ +V 5+ ←→V 3+ +V 4+

[0051] During charging, the first half-cell 204A converts tetravalent vanadium ions, V 4+ is the pentavalent vanadium ion V 5+ In the second half-cell 204B, the trivalent ions V 3+ is a divalent ion V 2+ During discharge, in the first half-cell 204A, pentavalent vanadium ions, V 5+ is tetravalent vanadium ion V 4+ In the second half-cell 204B, the divalent ions V 2+ is a trivalent ion V 3+ During these redox reactions, electrons are transferred through an external circuit and specific ions diffuse across the ion exchange or separation membrane 112, balancing the electroneutrality of the positive and negative half-cells, respectively.

[0052] Other redox reactions can be implemented in the sealed redox battery 200A according to various embodiments. In various embodiments, the first or second redox couple includes one or more ions of vanadium (V), zinc (Zn), bromine (Br), chromium (Cr), manganese (Mn), titanium (Ti), iron (Fe), cerium (Ce), and cobalt (Co). In some embodiments, the first and second redox couples include ions of the same metal, as in the sealed V redox battery described above. In these embodiments, mixing of the positive and negative electrode electrolytes preferably does not result in cross-contamination of the electrolytes.

[0053] As described herein, the electrolyte of a redox battery is a solution that conducts electric current through ionization. The electrolyte supports the reduced and oxidized forms of the redox couple and the corresponding cations and anions to balance the ionic charge in the solution during the oxidation and reduction of the redox couple. According to an embodiment, the positive and negative electrode electrolytes include an acidic aqueous solution. In the case of a sealed V redox battery, the concentration of V ions is related to the energy density of the electrolyte. A higher energy density can be advantageous in reducing the volume of the positive and negative electrode electrolyte reservoirs 106A, 106B required for a given positive energy and power output. However, if the V ion concentration is too high, the stability of the V ions may be reduced. Therefore, there is an optimal V ion range for a given application. For example, the vanadium ions dissolved in one or both of the first and second electrolytes may exceed 1.0 M, 1.5 M, 2.0 M, 2.5 M, or any value within the range defined above. On the one hand, V ion concentrations below 1.0 M may result in energy levels that are not suitable for some applications. On the other hand, V ion concentrations above 2.5 M may result in V ion levels above 50° C., for example. 5+ This can reduce the stability of ions, e.g., V in the electrolyte at operating temperatures below -20°C. 2+ and V 3+ The solubility limit of the ion can be reached.

[0054] Preferably, depending on the embodiment, the positive and negative electrode electrolytes may contain the same solvent and / or the same metal ions. In these embodiments, mixing of the positive and negative electrode electrolytes through an ion exchange membrane or separator 112 does not cause contamination of the respective half-cells. The positive and negative electrode electrolytes may also be prepared from the same starting solvent and solute. For example, in the case of a sealed V redox battery according to some embodiments, both the positive and negative electrode electrolytes contain sulfuric acid. The electrolyte is prepared, for example, by dissolving 0.1M to 2.5M VOSO4 (vanadyl sulfate) in a 0.1M to 6M aqueous solution of MH2SO4, and the tetravalent vanadium ions (V 4+ ) and / or trivalent vanadium ions (V 3+ ) can be formed. The tetravalent and trivalent vanadium ions can be electrochemically oxidized to form pentavalent vanadium ions (V 5+ Conversely, the tetravalent / trivalent vanadium ions can be electrochemically reduced to divalent vanadium ions (V 2+ A negative electrode electrolyte (cathode solution) can be formed containing a solution of

[0055] Referring now to FIG. 2a, in various embodiments, the positive and negative electrodes disposed in the positive and negative electrolyte reservoirs 106A and 106B, respectively, comprise carbon-based materials such as carbon or graphite felt, carbon cloth, carbon black, graphite powder, and graphene. Carbon-based materials preferably provide a relatively high operating range, excellent stability, and high reversibility. The electrodes are optimized for relatively high electrochemical activity, low bulk resistance, and large specific area. Improving the electrochemical activity of the electrodes increases the energy efficiency of the sealed redox battery 200A. To improve the performance of the sealed redox battery 200A, the electrode surfaces can be modified, for example, by metal coating, increasing surface roughness, or doping with additives.

[0056] The positive and negative electrode electrolyte reservoirs 106A, 106B, which define the reaction spaces, are filled partially or completely with the respective electrodes between the first and second separator plates 208A, 208B, respectively, or between the ion exchange membrane or separator 112 and the positive and negative electrode current collectors 108A, 108B, respectively, if an ion exchange membrane or separator 112 is present. After filling the respective electrodes, the remaining space in the positive and negative electrode electrolyte reservoirs 106A, 106B is filled partially or completely with the respective electrolyte between the first and second separator plates 208A, 208B, respectively, if an ion exchange membrane or separator 112 is present, or between the ion exchange membrane or separator 112 and the positive and negative electrode current collectors 108A, 108B. In various embodiments, unless intentionally perforated or made porous, the ion exchange or separation membrane 112 substantially separates the two half-cells, substantially preventing mixing of the two electrolytes and redox couples and allowing H + ion exchange membrane 112. The ion exchange membrane or separation membrane 112 may be an anion exchange membrane or a cation exchange membrane.

[0057] Although various embodiments shown include an ion exchange or separation membrane 112 that may be selective for a particular type of ion, e.g., cations or anions, embodiments are not limited thereto. For example, in various embodiments, the ion exchange or separation membrane 112 may be a non-selective membrane, e.g., a porous membrane.

[0058] Referring now to FIG. 2A, in some embodiments, output power can be scaled by connecting multiple single redox battery cells, for example, in series, to form a cell stack. In these configurations, first and second separators 208A, 208B can facilitate the series connection of the single cells, and the current collectors 108A, 108B between adjacent separators can be eliminated. The first and second separators 208A, 208B can be formed of a suitable material, such as graphite, carbon, carbon plastic, etc., to provide high electrical conductivity and low internal resistance to the cell stack. The first and second separators 208A, 208B also support contact pressure when pressed against the electrodes, increasing electrical conductivity. The first and second separators 208A, 208B are also designed to have high acid resistance to prevent corrosion or oxidation of the current collectors 108A, 108B.

[0059] The positive and negative electrode current collectors 108A and 108B contain a metal with high electrical conductivity, such as copper or aluminum, and serve to pass current during charge and discharge processes.

[0060] The single sealed redox battery 200A described above has an output voltage characteristic of the electrochemical reaction, and additional cells, for example, up to about 1.65 V, can be connected electrically in series or electrically in parallel as described herein to obtain higher voltages and currents, respectively.

[0061] 2b is a schematic diagram of a sealed redox battery including multiple sealed redox battery cells in a stacked configuration, according to some embodiments. The illustrated sealed redox battery 200B includes multiple stackable redox battery cells 200B-1, 200B-2,..., 200B-n, where each cell is configured in a manner similar to sealed redox battery 200A (FIG. 2a). Each of the multiple redox battery cells 200B-1, 200B-2,..., 200B-n includes a positive electrode electrolyte reservoir 106A, a negative electrode electrolyte reservoir 106B, and an ion exchange or separation membrane 112. In the illustrated embodiment, each of the multiple redox battery cells 200B-1, 200B-2,..., 200B-n is separately sealed in a casing 212. A plurality of redox battery cells 200B-1, 200B-2, . . . , 200B-n can be electrically connected in series to increase the output voltage.

[0062] 2c is a schematic diagram of a sealed redox battery including multiple stacked sealed redox battery cells according to some other embodiments. The illustrated sealed redox battery 200C includes multiple stackable redox battery cells 200C-1, 200C-2,..., 200C-n, where each of the multiple redox battery cells 200C-1, 200C-2,..., 200C-n is configured in a manner similar to sealed redox battery 200A (FIG. 2a), including a positive electrode electrolyte reservoir 106A, a negative electrode electrolyte reservoir 106B, and an ion exchange or separation membrane 112. However, unlike sealed redox battery 200B (FIG. 2b), in the illustrated embodiment, the multiple redox battery cells 200C-1, 200C-2,..., 200C-n are sealed within a common casing 222. In a manner similar to the sealed redox battery 200B (FIG. 2b), multiple redox battery cells 200C-1, 200C-2, ..., 200C-n can be electrically connected in series to increase the output voltage. In some embodiments, the positive electrode electrolyte reservoirs 106A of the multiple redox battery cells 200C-1, 200C-2, ..., 200C-n can be fluidly connected to each other, and the negative electrode electrolyte reservoirs 106B of the multiple redox battery cells 200C-1, 200C-2, ..., 200C-n can be fluidly connected to each other. The sealed redox battery 200C can be configured as a pouch-type redox battery or a rigid case-type redox battery.

[0063] 2d is a schematic diagram of a sealed redox battery including multiple sealed redox battery cells in a cylindrically stacked configuration according to an embodiment. The illustrated sealed redox battery 200D includes multiple cylindrically stackable redox battery cells 200D-1, 200D-2,..., 200D-n, where each of the multiple redox battery cells 200D-1, 200D-2,..., 200D-n is configured in a manner similar to sealed redox battery 200A (FIG. 2A), including a positive electrode electrolyte reservoir 106A, a negative electrode electrolyte reservoir 106B, and an ion exchange or separation membrane 112. The multiple redox battery cells 200D-1, 200D-2,..., 200D-n may be individually sealed within a casing in a manner similar to that described above for sealed redox battery 200B (FIG. 2b). Alternatively, the multiple redox battery cells 200D-1, 200D-2, ..., 200D-n may be enclosed by a common casing 222 in a manner similar to that described above for the sealed redox battery 200C (FIG. 2c). The multiple redox battery cells 200D-1, 200D-2, ..., 200D-n may be electrically connected in series to increase the output voltage in a manner similar to that for the sealed redox battery 200B (FIG. 2b). Additionally, in some embodiments, the positive electrode electrolyte reservoirs 106A of the multiple redox battery cells 200D-1, 200D-2, ..., 200D-n may be fluidly connected to one another, and the negative electrode electrolyte reservoirs 106B of the multiple redox battery cells 200D-1, 200D-2, ..., 200D-n may be fluidly connected to one another.

[0064] It will be appreciated that some or all of the multiple redox battery cells in each of the stack configurations described above with respect to Figures 2b-2c can be electrically connected in series by appropriately electrically connecting the opposite polarity current collectors of some or all of the cells, or can be electrically connected in parallel by appropriately electrically connecting the same polarity current collectors of some or all of the cells.

[0065] Differences between sealed redox batteries and conventional secondary batteries The differences and advantages of the sealed redox batteries according to the embodiments over conventional RFBs include the elimination of the electrolyte tank, pumping system, and conduit network that contributed to the slow commercialization of conventional RFBs, as previously described. While the separate electrolyte tank is not required, the sealed redox batteries 200A-200D (FIGS. 2a-2d) maintain some of the inherent design flexibility found in conventional RFBs. For example, due to the inherent flexibility of liquids, cell geometry design is substantially more flexible than conventional secondary batteries. Furthermore, power and energy storage capacity can be independently decoupled and scaled within a limited range, for example, by adjusting the ratio of electrolyte volume to electrode surface area. This ratio can be adjusted, for example, by adjusting the thickness of the positive and negative electrode electrolyte reservoirs 106A, 106B, as previously described. On the other hand, the sealed redox batteries according to the embodiments also share the key advantages of conventional batteries, such as being fully sealed and modularized. It will be understood that although the sealed redox battery according to the embodiment and a conventional secondary battery, e.g., a LIB, may have components designated using similar terms, the components and operating principles of the sealed redox battery according to the embodiment can be distinguished from the conventional secondary battery described herein.

[0066] Although the following comparison is made between a sealed redox battery according to an embodiment and a LIB, it will be understood that the comparison can also be applied to other conventional secondary batteries.

[0067] First, the structure, functional role, and operating principle of the electrolyte in the sealed redox battery according to the embodiment may be distinguished from those of conventional secondary batteries, such as LIBs. During operation, the electrolyte in a LIB does not store energy itself and does not participate in electrochemical reactions during charge and discharge. Instead, the electrolyte in a LIB primarily serves to provide a path for lithium ions to be transferred between the positive and negative electrodes during charge and discharge. Thus, electrolyte movement is not substantially restricted by a separator. In contrast, electrochemical energy in the sealed redox battery 200A according to the embodiment is stored in the electrolyte in the form of dissolved active materials, e.g., redox couples dissolved in the positive and negative electrode electrolytes that undergo electrochemical reactions during charge and discharge. Thus, the electrolyte can be considered an energy storage medium in the sealed redox battery according to the embodiment. As described above, in the example of a V redox battery, the oxidation states of V ion species dissolved in the positive and negative electrode electrolytes change depending on their respective half-reactions. Therefore, the chemical compositions of the cathode and anode electrolytes in a sealed redox battery are different from those of a LIB. Also, unlike a LIB, in a sealed redox battery according to embodiments, the electromotive force resulting from the difference in the chemical compositions of the cathode and anode electrolytes leads to energy storage, and therefore, mixing of the cathode and anode electrolytes results in a loss of stored energy.

[0068] Second, the structure, functional role, and operating principle of the electrodes in a sealed redox battery according to embodiments may be distinguished from those of conventional secondary batteries, such as LIBs. In LIBs, the active materials contained in the electrodes directly participate in electrochemical reactions. During operation, lithium ions in a LIB transfer between the active materials of the positive electrode and the active materials of the negative electrode, achieving electrochemical equilibrium, with the electrodes themselves acting as the primary mediators for energy storage. In contrast, the electrodes in a sealed redox battery according to embodiments play very different roles. The positive electrode of a sealed redox battery does not participate in the first redox half reaction, and the negative electrode of a sealed redox battery does not participate in the second redox half reaction. As described herein, an electrode that does not participate in the redox half reaction does not eliminate the electrode's function of providing physical sites for the electrochemical reaction in a manner similar to a catalyst. However, the electrode itself does not participate in the electrochemical reaction, and redox ions do not transfer between the positive and negative electrodes during charging and discharging of the redox battery. Depending on the composition, functional groups may be present on the surface that act as catalysts, but this is distinct from electrodes that actively participate in electrochemical reactions, as in LIBs. Instead, electrodes essentially passively transfer electrons generated by the electrochemical reactions.

[0069] Third, the structure, functional role, and operating principle of the ion exchange membrane in a sealed redox battery according to an embodiment may be distinguished from the separator in a conventional secondary battery, such as a LIB. In a LIB, the active material of the electrode where the electrochemical reaction occurs is generally in a solid state, and the separator disposed between the positive and negative electrodes primarily serves to prevent an electrical short between them. Therefore, while the separator serves to prevent electrical contact between the positive and negative electrodes, the separator in a LIB is not specifically designed to restrict the transfer of lithium ions or the electrochemical reaction therebetween. That is, the separator in a LIB primarily serves to allow ion movement as part of the electrochemical reaction for charge and discharge, and to electrically insulate the positive and negative electrodes from each other. Therefore, the separator for a LIB is designed to allow the free transfer of lithium ions between the electrodes. In contrast, in a sealed redox battery according to an embodiment, the redox active species are dissolved in the electrolyte, and an ion exchange membrane or separator 112 (FIG. 2a) serves to electrically separate the positive and negative electrode electrolytes and prevent them from mixing with each other.

[0070] Typically, the ion exchange membrane or separator 112 includes a selectively permeable membrane through which cations or anions are transferred to maintain charge balance between the two half-cells. For example, the ion exchange membrane may be configured to selectively allow cations or anions to pass through. Therefore, since the electrolyte that stores energy in the sealed redox battery according to the embodiment is a liquid, without the ion exchange membrane or separator 112, an electrical short circuit would occur due to mixing of the positive and negative electrode electrolytes, regardless of whether the positive and negative electrodes are in contact with each other.

[0071] Thus, in the sealed redox battery according to the embodiment, the first and second redox half reactions occur without substantial ionic transport of the first or second redox couple across the ion exchange or separation membrane 112 separating the positive electrode electrolyte reservoir 106A and the negative electrode electrolyte reservoir 106B.

[0072] As explained herein, an ion exchange membrane or separator 112 that does not substantially transfer ions of a redox couple refers to an ion exchange membrane or separator 112 that substantially prevents electrolyte crossover between the positive and negative electrode electrolyte reservoirs 106A, 106B (FIG. 2a). Thus, the base material of the ion exchange membrane or separator 112 preferably blocks the migration of redox species in the electrolyte, e.g., V ions in a V redox battery, while allowing other ions, e.g., H ions in a V redox battery, to pass through for charge balance between half-cells. + It may be a membrane that selectively permits the movement of ions, but an ion exchange or separation membrane 112 that does not substantially transmit ions of the redox pair can still allow unintended crossover or limited intended mixing to alleviate internal pressure buildup.

[0073] Polybenzimidazole-based separation membrane and its manufacturing method As previously mentioned, ion exchange membranes or separators perform, among other functions, the function of conducting ions of a supporting electrolyte between the positive and negative electrode electrolyte reservoirs while substantially inhibiting the passage of redox-active ions, e.g., vanadium ions. To further improve the performance of redox battery cells, further improvements in membranes or separators and the development of efficient manufacturing methods are needed to achieve, among other improvements, one or more of: lowering ionic resistance to enable operation at higher current densities; improving barrier properties; balancing the transport of pure electrolyte to minimize capacity imbalance; and improving the chemical stability of the membrane or separator material.

[0074] To address these and other needs, one embodiment of the present invention uses a polybenzimidazole-based separator made of polybenzimidazole (PBI) material. PBI has high chemical stability in the acidic electrolyte used in redox battery cells, and upon contact with the aqueous sulfuric acid used in the electrolyte, the imidazole groups of PBI are protonated, giving the PBI a positive charge and exhibiting ion exchange properties.

[0075] Polybenzimidazole (PBI)-based separators may have superior chemical resistance, heat resistance, and mechanical strength compared to other conventional separators, and can be applied to various secondary batteries.

[0076] In particular, various embodiments of the ion exchange or separation membranes disclosed herein may prove particularly effective when integrated as part of a sealed redox battery (e.g., ion exchange or separation membrane 112 of FIG. 2a), particularly because sealed redox batteries may be exposed to more severe conditions, including higher internal pressures, that existing membranes may not be able to effectively and stably withstand.

[0077] However, it will be appreciated that embodiments of the separator are not limited to use in sealed redox batteries, and that the ion exchange membranes or separators disclosed herein can be preferably integrated for use in any suitable secondary battery, including redox flow batteries, such as redox flow battery 100 described above with respect to FIG. 1, as well as lithium ion batteries.

[0078] One method for producing a polybenzimidazole-based separator is to coat a solution of polybenzimidazole in an organic solvent onto a backing film (or substrate film) made of a hard material to form a membrane, and then peel and remove the backing film and heat treat the membrane to obtain a polybenzimidazole-based separator.

[0079] However, such conventionally produced polybenzimidazole-based films may not satisfy the extremely high mechanical strength required for use as separators in secondary batteries.

[0080] On the other hand, backing films, such as polyethylene terephthalate (PET)-based backing films, used to improve mechanical strength during separator fabrication can be a major factor in increasing process costs and reducing manufacturing efficiency. Furthermore, during intensive research, the present inventors discovered that fine air layers are formed when a polybenzimidazole-based solution is coated on a backing film to form a membrane. These air layers significantly reduce voltage efficiency and also reduce charge / discharge efficiency when the secondary battery is operated. Therefore, there is a need for an efficient method for manufacturing polybenzimidazole-based separators that can improve the mechanical strength of the film and improve secondary battery performance without using a backing film.

[0081] Therefore, according to one aspect of the present invention, a method for producing a polybenzimidazole-based film includes dissolving a polybenzimidazole-based compound in an amide-based organic solvent to form a polybenzimidazole solution; impregnating a porous membrane with the polybenzimidazole solution; and drying the porous membrane impregnated with the polybenzimidazole solution at a temperature of 80°C or less. Unlike a backing film, the porous membrane has the advantage of not needing to be removed separately. For example, the amide-based organic solvent according to the present invention may be N,N-dimethylacetamide (DMAc), dimethylformamide (DMF), or the like, with N,N-dimethylacetamide being preferred.

[0082] The polybenzimidazole compound of the present invention is a polymer of polybenzimidazole, and refers to a mixture or copolymer of polybenzimidazole precursors (repeating units or monomers for forming polybenzimidazole polymers). The polybenzimidazole precursor may be, but is not limited to, ab-PBI (Poly(2,5-benzimidazole)), oPBI (Poly[2,2'-(4,4'-oxybis(1,4-phenylene))-5,5'-bibenzimidazole), m-PBI (meta-polybenzimidazole), pPBI (para-polybenzimidazole), s-PBI (sulfonated polybenzimidazole), f-PBI (fluorine-containing polybenzimidazole), 2OH-PBI (Dihydroxy polybenzimidazole), PIPBI (Phenylindane-polybenzimidazole), PBI-OO (poly[(1-(4,4'-diphenylether)-5-oxybenzimidazole)-benzimidazole]), and combinations thereof.

[0083] Meanwhile, it has been known that the solubility of PBI in 100% by weight of organic solvent is 2-6% by weight. Therefore, according to one embodiment of the present invention, in order to improve the solubility of polybenzimidazole-based compounds in organic solvents, the step of dissolving a PBI precursor in an amide-based organic solvent is carried out at a temperature of 130°C or higher and / or a pressure of 0.1 MPa or higher, thereby increasing the solubility of polybenzimidazole-based compounds in 100% by weight of organic solvent to 8-20% by weight.

[0084] According to one embodiment of the present invention, the porous membrane may be made of one or more polyolefin-based materials. For example, the porous membrane of the present invention may be made of a material including, but not limited to, polypropylene (PP), polyethylene (PE), or a combination thereof. For example, if porous nylon film, Teflon film, or cloth film is used, problems that reduce the quality of the separator, such as rolling or wrinkling caused by thermal shrinkage during the process of forming and drying the separator, may occur. Therefore, in the present invention, a porous membrane made of PP (polypropylene) or PE (polyethylene) is used, which does not cause these problems.

[0085] The thickness of the porous membrane according to the present invention may be, for example, 1 to 30 μm, for example, 3 to 20 μm, for example, 5 to 15 μm, or for example, 8 to 10 μm, but is not necessarily limited thereto and can be selected depending on the thickness of the separation membrane to be finally produced.

[0086] Furthermore, in accordance with the present invention, the drying process after impregnating the porous membrane with the polybenzimidazole solution is carried out at a temperature of 80°C or less, which is lower than the conventional temperature of about 100°C, thereby preventing deterioration of the separator quality, such as rolling and wrinkling due to thermal shrinkage, and further enabling the use of equipment such as a pinch roll for the membrane formation and drying processes. However, since a drying temperature that is too low increases the drying time and can result in process inefficiency, it is advantageous for the drying process temperature to be 40°C or higher, and the drying process temperature may be, for example, 40 to 80°C, for example, 40 to 70°C, or for example, 50 to 60°C.

[0087] Meanwhile, when a drying process is carried out after forming a membrane from a polybenzimidazole solution on a backing film as in the conventional method, if the drying process is carried out at a low drying temperature range as in the present invention, the drying time increases further and the process efficiency decreases. This is presumably because the hot air, IR, and UV energy used for drying are not easily transmitted to one side of the separator due to the hard material.

[0088] If the porous membrane used in the polybenzimidazole-based separator according to the present invention is hydrophobic and the electrolyte used in the electrode assembly is hydrophilic, there may be portions where the hydrophilic electrolyte does not come into contact with the hydrophobic separator, which may result in charging / discharging defects in the secondary battery.

[0089] According to one embodiment of the present invention, a surfactant may be mixed into the polybenzimidazole solution during preparation. When a porous membrane is impregnated with a solution containing a polybenzimidazole-based compound, a solvent, and a surfactant, the surfactant renders the porous membrane hydrophilic, thereby eliminating or reducing the charge / discharge failure problem. The surfactant may be present in an amount greater than 0.1 wt % and less than 5.0 wt %, for example, 2.0 wt % to 4.0 wt %, for example, 0.5 wt % to 2.0 wt %, or 0.5 wt % to 10.0 wt %, based on the total solution (100 wt %), but is not limited thereto. The surfactant may be one or more of an ionic surfactant, a nonionic surfactant, and an organic surfactant, such as a silicon-based organic surfactant.

[0090] According to one embodiment of the present invention, the viscosity of the polybenzomidazole solution can be reduced by further adding a viscosity adjusting solvent. In a film-forming process using a solution, as in the present invention, if the solution has a high viscosity, the fluidity during film formation decreases, making it difficult to form a thin film and making it difficult to control the precision. Therefore, by adding a viscosity adjusting solvent in addition to the organic solvent for dissolving the ion-conductive resin, the performance and precision of the film formation can be improved.

[0091] Therefore, the viscosity adjusting solvent may be contained in an amount of, for example, 10% by weight to 25% by weight, for example, 12% by weight to 20% by weight, or for example, 15% by weight to 18% by weight relative to 100% by weight of the polybenzomidazole solution, but is not necessarily limited to these.

[0092] According to an embodiment of the present invention, the viscosity adjusting solvent may be one or more of a ketone solvent or an alcohol solvent. More specifically, the ketone solvent may be acetone, methyl ethyl ketone, methyl isobutyl ketone, etc., and the alcohol solvent may be methanol, ethanol, isopropanol, butanol, isobutanol, etc.

[0093] The method for manufacturing a polybenzimidazole-based separator according to the present invention allows for the formation of a uniform separator, and realizes a low-cost, highly efficient process without the need for a separate backing film. Furthermore, by impregnating one or both sides of a porous membrane with a polybenzimidazole solution as needed, a single-sided or double-sided film can be advantageously manufactured.

[0094] In particular, polybenzimidazole-based separators prepared by the polybenzimidazole-based separator manufacturing method according to the present invention have excellent tensile strength and maintain high tensile strength even after heat treatment or exposure to an acidic electrolyte. This is superior to conventional polybenzimidazole-based separators, which exhibit very low tensile strength after heat treatment or exposure to an electrolyte. The polybenzimidazole-based separator according to the present invention has an average tensile strength of, for example, 100 MPa or more, for example, 120 MPa or more, or even 150 MPa or more, and thus has excellent mechanical strength and uniform coating properties. According to an embodiment, a secondary battery separator prepared using a PBI film can contribute to improved performance of the produced redox battery.

[0095] On the other hand, increasing the thickness of a polybenzimidazole-based separator increases resistance to the flow of protons, resulting in a decrease in the voltage efficiency of the secondary battery. On the other hand, decreasing the thickness decreases mechanical strength or decreases Coulomb efficiency due to increased crossover and permeation. In view of this, the thickness of the polybenzimidazole-based separator according to the present invention may be, for example, in the range of 2 to 40 μm, for example, in the range of 5 to 30 μm, or in the range of 10 to 20 μm, but is not necessarily limited thereto and may be varied as needed. The polybenzimidazole-based separator according to the present invention is prepared by impregnating a porous membrane with the polybenzimidazole and drying it to remove the solvent. Therefore, the thickness of the polybenzimidazole-based separator can be adjusted based on the content and amount of polybenzimidazole used, and can be selected taking into account the operating conditions of the battery and the state of manufacture. [Example]

[0096] The following examples of the present invention will be described. However, the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention.

[0097] <Production example> Example 1 m-PBI, a polybenzimidazole precursor, was added to dimethylacetamide (DMAC) and dissolved under stirring at 160°C and atmospheric pressure for 24 hours to produce a PBI solution with a maximum dissolution rate of 12 wt%. A PE film (thickness: 20 μm) was then immersed in the PBI solution and dried with hot air at 50°C for 2 minutes to obtain a 27 μm thick separator. The separator thus produced exists in the form of a PE film impregnated with PBI.

[0098] Comparative Example 1 The polybenzimidazole precursor m-PBI was added to DMAc and dissolved at 160°C under normal pressure for 24 hours with stirring to produce a PBI solution with a maximum dissolution rate of 12 wt%. The PBI solution was then sprayed onto one side of the PET film substrate using a slot die coater to produce a polybenzimidazole separation membrane. After drying with hot air at 80°C for 2 minutes, the PBI film was peeled off from the PET film to produce a 22μm-thick separation membrane.

[0099] When the amount of PBI solution used to produce the 27 μm thick separator in Example 1 is taken as 10, the amount of PBI solution used to produce the 22 μm thick separator in Comparative Example 1 was 22. Therefore, under the conditions of producing a separator of the same thickness, the amount of PBI used in the production method according to Example 1 of the present invention is further reduced, resulting in higher efficiency in terms of cost and process. When the amount of PBI used is reduced, the drying temperature and drying time can also be reduced, resulting in lower energy consumption during production and a shorter process time.

[0100] <Battery performance evaluation> Two current collectors were fabricated by laminating a carbon current collector (graphite composite, thickness: 0.2 mm) and a metal current collector (aluminum foil, thickness: 0.2 mm). These were used as the positive and negative current collectors, respectively. Unit cells were fabricated including a positive electrode electrolyte reservoir and a negative electrode electrolyte reservoir formed using the separators fabricated in Example 1 or Comparative Example 1. A 1.7M V solution was placed in each of the positive electrode electrolyte reservoir and the negative electrode electrolyte reservoir. 3.5+ An electrolyte (manufactured by Standard Energy) was supplied, and the battery was charged at a constant current of 1C until the voltage reached 1.55V, and then discharged at a constant current of 1C until the voltage reached 1.10V. The energy efficiency (VE, CE, and EE) (%) was measured and is shown in Table 1 below.

[0101] <Mechanical strength measurement> The average tensile strength (MPa) of each of the separators prepared in Example 1 and Comparative Example 1 was measured according to ASTM D882. The measurements were also carried out before and after heat treatment at 80°C, and after soaking in 50 ml of electrolyte for 1 hour and 24 hours. The results are shown in Table 1 below. The electrolyte used was a 1.7M V 3.5+ An electrolyte containing 1.7M aqueous sulfuric acid solution and 1.4V vanadium ions was used.

[0102] [Table 1]

[0103] As can be seen from Table 1 above, when the separator of Example 1 according to the present invention was used, the battery efficiency increased slightly and the average tensile strength was significantly improved compared to when the separator of Comparative Example 1 was used. In particular, the separator of Example 1 showed almost no change in average tensile strength even after heat treatment, unlike Comparative Example 1. Furthermore, after being loaded in the electrolyte, the decrease in tensile strength of the separator of Example 1 was much lower than that of the separator of Comparative Example 1, confirming that the separator has high mechanical strength, durability, and acid resistance.

[0104] <Experiment on the Temperature Conditions of the Drying Process> Although a PBI separation membrane was produced in the same manner as in Example 1 or Comparative Example 1, as shown in Table 2 below, the temperature of the drying process was varied to produce the PBI separation membrane.

[0105] Although a PBI separation membrane was produced in the same manner as in Example 1, the case where only the drying temperature was different was designated as Experimental Group A, and although it was carried out in the same manner as in Comparative Example 1, the case where only the drying temperature was different was designated as Experimental Group B. For each separation membrane, the drying temperature, drying time, and the occurrence or non-occurrence of shrinkage of the PE film were confirmed and shown in Table 2 below. When shrinkage did not occur, it was described as "-", and when shrinkage occurred, it was indicated.

[0106]

Table 2

[0107] As can be seen from Table 2 above, as the drying temperature increased, the drying time became shorter and the process efficiency increased. However, it can be seen that there is a problem that shrinkage of the PE film, which is a porous membrane, occurs when the temperature becomes too high. Also, different from the present invention, it can be confirmed that in Experimental Group B using a conventional backing film, even when dried at the same temperature as Experimental Group A, the drying time further increased and the process efficiency decreased.

[0108] <Experiment on the Thickness of the PBI Separation Membrane> Although a PBI separation membrane was produced in the same manner as in Example 1 above, by adjusting the amount of PBI solution used, the thickness of the PBI separation membrane was varied to produce each PBI separation membrane. The battery efficiency was evaluated for each in the same manner as in <Battery Performance Evaluation> and shown in Table 3 below.

[0109]

Table 3

[0110] As can be seen from Table 3, increasing the amount of PBI used and increasing the thickness of the separator increases the resistance to hydrogen ion flow, reducing the voltage efficiency (VE) of the battery. On the other hand, decreasing the amount of PBI used and decreasing the thickness of the separator increases the probability of vanadium electrolyte crossover, thereby reducing the coulombic efficiency (CE).

[0111] Since the thickness of the PBI separation membrane is affected by the thickness of the porous membrane (PE), to produce a thinner separation membrane, a thinner porous membrane can be selected and the amount of PBI solution used can be adjusted to produce a thinner separation membrane.

[0112] Although specific embodiments have been described above, these embodiments are presented merely as examples and are not intended to limit the scope of the disclosure. Indeed, the novel devices, methods, and systems described herein may be embodied in a variety of different forms. Furthermore, various omissions, substitutions, and modifications may be made to the methods and systems described herein without departing from the spirit of the disclosure. Any suitable combination of elements and acts of the above-described embodiments may be combined to arrive at further embodiments. The various features and processes described above may be embodied independently of one another or combined in various ways. All possible combinations and subcombinations of features of the disclosure are intended to fall within the scope of the disclosure.

Claims

1. dissolving a polybenzimidazole-based compound in an amide-based organic solvent to form a polybenzimidazole solution; impregnating a porous membrane with the polybenzimidazole solution; and drying the porous membrane impregnated with the polybenzimidazole solution at a temperature of 80°C or less to obtain a polybenzimidazole-based separator; Including, A method for producing a polybenzimidazole-based separation membrane.

2. No backing film is used to form the polybenzimidazole-based separator; A method for producing the polybenzimidazole separation membrane according to claim 1.

3. The step of impregnating the porous membrane includes impregnating a cross section or both sides of the porous membrane. A method for producing the polybenzimidazole separation membrane according to claim 1.

4. The porous membrane is made of a material including polypropylene, polyethylene, or a combination thereof. A method for producing the polybenzimidazole separation membrane according to claim 1.

5. The thickness of the porous membrane is 1 to 30 μm, The thickness of the polybenzimidazole-based separation membrane is 2 to 40 μm. A method for producing the polybenzimidazole separation membrane according to claim 1.

6. forming the polybenzimidazole solution, and mixing together a surfactant; A method for producing the polybenzimidazole separation membrane according to claim 1.

7. The surfactant is contained in an amount of more than 0.1 wt % and less than 5.0 wt % based on 100 wt % of the polybenzimidazole solution. A method for producing the polybenzimidazole separation membrane according to claim 6.

8. The surfactant includes one or more of an ionic surfactant, a nonionic surfactant, and an organic surfactant. A method for producing the polybenzimidazole separation membrane according to claim 6.

9. The step of preparing the polybenzimidazole solution comprises dissolving the polybenzimidazole in an amide-based organic solvent under a temperature condition of 130° C. or more and / or a pressure condition of 0.1 MPa or more. A method for producing the polybenzimidazole separation membrane according to claim 1.

10. When the amide-based organic solvent is taken as 100% by weight, the maximum solubility of the polybenzimidazole-based compound is 8 to 20% by weight. A method for producing the polybenzimidazole separation membrane according to claim 1.

11. The viscosity adjusting solvent is contained in an amount of 10% by weight to 25% by weight based on 100% by weight of the polybenzimidazole solution. A method for producing the polybenzimidazole separation membrane according to claim 1.

12. The viscosity adjusting solvent includes one or more of acetone, methyl ethyl ketone, methyl isobutyl ketone, methanol, ethanol, isopropanol, butanol, and isobutanol; A method for producing the polybenzimidazole separation membrane according to claim 11.

13. Produced by the method for producing a polybenzimidazole-based separation membrane according to any one of claims 1 to 12, Polybenzimidazole separation membrane.

14. Produced by the method for producing a polybenzimidazole-based separation membrane according to any one of claims 1 to 12, A secondary battery comprising a polybenzimidazole-based separator.

15. a redox battery comprising an oxidation and reduction reaction of a vanadium redox couple; The secondary battery according to claim 14.

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