Redox flow batteries with non-aqueous catholytes

Abstract

An example redox flow battery (100) can include a catholyte reservoir (110) containing a non-aqueous catholyte (112). The catholyte can include an ether-based organic solvent and an organic redoxmer. The redoxmer can include a carbonyl group, an amine group, or a combination thereof. The amine group can include a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof. The redox flow battery (100) can also include a flow cell (120) in fluid communication with the catholyte reservoir (110) to circulate the catholyte (112) through the flow cell (120). A separator (140) in the flow cell (120) can be in contact with the catholyte (112) in the flow cell (120). A cathode (150) can be in contact with the catholyte (112) in the flow cell (120). An anode (160) can be separated from the catholyte (112) by the separator (140).

Description

[0001] PATENT APPLICATION Attorney Docket No. 00846-U8663. PCT

[0002] REDOX FLOW BATTERIES WITH NON-AQUEOUS CATHOLYTES

[0003] CROSS REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority to U. S. Provisional Application No. 63 / 741.686. filed January 3, 2025 which is incorporated herein by reference.

[0005] STATEMENT REGARDING F EDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0006] This invention was made with government support under 2247407 awarded by the National Science Foundation. The government has certain rights in the invention.

[0007] BACKGROUND

[0008] Owing to limited resources and negative environmental impacts of fossil fuel, various renewable energy resources have been explored for electricity production. Because of the intermitent nature of many renewable electricity generation technologies, efficient energy¬ storage solutions have become more important. Among different storage technologies, electrochemical energy storage (e.g. batteries) has gained broad attention. According to the International Energy Agency (IEA), energy storage must grow six-fold to triple global renewable energy capacity by 2030, where batteries will drive 90% of this expansion. This increase reflects the heightened social awareness of the importance of electrochemical storage. Li-ion batteries (LIBs) are one of the most studied and deployed energy storage technologies. However, these batteries can suffer from supply chain challenges, poor safety, and high cost.

[0009] Redox flow batteries (RFBs) have potential to be used for energy storage to provide grid resilience. In these batteries, redox-active materials (i.e. redoxmers) are dissolved into liquid electrolytes and pumped from external tanks to electrodes for electrochemical reactions. This structure releases the redoxmers from ‘lie electrodes, allowing for readily and independently scalable energy and power, as well as oilier advantages such as fast solution reaction kinetics, minimal phase transformation, long cycle life and safety.SUMMARY

[0010] This disclosure relates to redox flow batteries. An example redox flow battery’ can include a catholyte reservoir containing a non-aqueous catholyte. The catholyte can include an ether-based organic solvent and an organic redoxmer. The redoxmer can include a carbonyl group, an amine group, or a combination thereof. The amine group can include a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof. The redox flow battery can also include a flow cell m fluid communication with the catholyte reservoir to circulate the catholyte through the flow cell. A separator in the flow cell can be in contact with the catholyte in the flow' cell, A cathode can also be in contact with the catholyte in the flow' cell. An anode can be separated from the catholyte by tire separator.

[0011] An example electrochemical energy storage method can include providing a catholyte reservoir containing a non-aqueous catholyte including an ether-based organic solvent and an organic redoxmer. The redoxmer can include a carbonyl group, an amine group, or a combination thereof. The amine group can include a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof. The method can also include circulating the catholyte through a flow7cell. The catholyte in the flow7cell can be in contact with a cathode and a separator. An anode can be separated from the catholyte by the separator The method can also include oxidizing or reducing the redoxmer by applying an electric current between the anode and the cathode.

[0012] An example non-aqueous catholyte for a redox flow7battery can include an organic solvent comprising an ether, and an organic redoxmer dissolved in the solvent at a concentration of at least 0.5 M. The redoxmer can include a carbonyl group, an amine group, or a combination thereof. The amine group can include a nitrogen atom bonded to an alkyl group, an ether group, or a combination thereof.

[0013] There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken w7ith the accompanying drawings and claims, or may be learned by7the practice of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a schematic view of an example redox flow battery in accordance with an example of the present technology.

[0015] FIG. 2 is a schematic view of another example redox flow battery' in accordance with an example of the present technology.

[0016] FIG.3 shows molecular structures of several example amine molecules in accordance with an example of the present technology.

[0017] FIG. 4 is a flowchart illustrating an example electrochemical energy storage method in accordance with an example of the present technology.

[0018] FIG. 5 show s molecular structures of several example organic molecules screened for use m redox flow batteries in accordance with an example of the present technology.

[0019] FIG 6 show's cyclic voltammetry' profiles of the organic molecules from FIG. 5 FIG. 7 is a graph ofpeak-to-peak separation of tire organic molecules from FIG. 5. FIG. 8 is a graph of percent current retention after 300 cycles for the organic molecules from FIG. 5.

[0020] FIG. 9 shows molecular structures of several amine molecules that were investigated for use in redox flow batteries in accordance with an example of the present technology.

[0021] FIG. 10 show's cyclic voltammetry' profiles of the molecules from FIG. 9.

[0022] FIG. 11 is a plot of degree of conjugation vs. redox potential for tire molecules from FIG. 9.

[0023] FIG. 12 is a graph solubility in DME of the molecules from FIG. 9.

[0024] FIG. 13 is a graph of solubility vs. redox potential for the molecules from FIG. 9. FIG. 14 shows an example reaction mechanism of TDPA m accordance with an example of the present technology.

[0025] FIG. 15 plo ts diffusion coefficients of the several example amine molecules.

[0026] FIG. 16 plots the heterogeneous reduction rate constants of the example amine molecules.

[0027] FIG 17 shows a cyclic voltammetry' profile of a redox flow' battery’ including a magnesium anode and TDPA redoxmer, in accordance with an example of the present technology.FIG. 18 is a graph of capacity vs. voltage charge-discharge curve for a redox flow battery' including a magnesium anode and TDPA redoxmer with a porous membrane and with an anion exchange membrane, in accordance with an example ofthc present technology.

[0028] FIG. 19 shows cyclic voltammetry profiles of TDPA in an electrolyte containing either NaTFSI or NaClOr, in accordance with an example of the present technology.

[0029] FIG. 20 shows an FTIR profile of DME, TDPA in DME, TDPA in DME with NaTFSI, and TDPA in DME with NaClCfi, in accordance with several examples of the present technology.

[0030] These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.

[0031] DETAILED DESCRIPTION

[0032] While these exemplary' embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics ofthe present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.

[0033] Definitions

[0034] In describing and claiming the present invention, the following terminology will be used.

[0035] Hie singular forms “a,” “an,’’ and ‘"die"’ include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a solvent” includes reference to one or more of such materials and reference to "‘the terminal” refers to one or more of such features.

[0036]

[0037] As used herein with respect to an identified property or circumstance, ’'substantially” refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified roperty or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.

[0038] As used herein, ‘’adjacent” refers to the proximity of two structures or elements. Particularly, elements that are identified as being ‘‘adjacent” may be either abutting or connected. Such elements may also be near or close to each other without necessarily contacting each other. The exact degree of proximity may in some cases depend on the specific context.

[0039] As used herein, the term “about” is used to provide flexibility and imprecision associated with a given term, metric or value. The degree of flexibility for a particular vanable can be readily determined by one skilled in the art. However, unless otherwise enunciated, the term “about” generally connotes flexibility’ of less than 2%, and most often less than 1 %, and in some cases less than 0.01%.

[0040] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0041] As used herein, the term “at least one of’ is intended to be synonymous with “one or more of.” For example, “at least one of A. B and C” explicitly includes only A, only B, only C, or combinations of each.

[0042] Numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and subranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only onenumerical value, such as ‘'less than about 4 5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.

[0043] Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) "'means for” or "step fbr” is expressly- recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus-function are expressly recited in the description herein Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein. Example Embodiments

[0044] This disclosure describes redox flow batteries that include certain non-aqueous catholytes. The non-aqueous catholytes described herein can include an organic solvent and an organic redoxmer. The redoxmer can be an organic compound drat is redox-active and which can be dissolved in tire solvent. In some examples, the redoxmer can include a carbonyl group, an amine group, or a combination of both of these groups. When the redoxmer includes an amine group, the amine group can include a nitrogen atom that is bonded to an alkyl group, or to a phenyl group, or to a combination of both ofthese groups In certain examples, the organic solvent can be ether-based. These non-aqueous catholytes can be used with an anode. The anode material can be reduced while the redoxmer of the catholyte is oxidized, and vice versa.

[0045] A variety of anode materials can be used in the batteries described herein, hi some examples, the anode material can be a liquid anolyte which can flow through a flow cell similar to tire catholyte. The liquid anolyte can include an anolyte redoxmer that can be redox-active, similar to the catholyte redoxmer. In alternative examples, a metallic anode can be used, and the metal of the anode can be oxidized or reduced when the battery is used. Anode metals can include, but are not limited to. lithium, sodium, or magnesium, in some examples.

[0046] The redox flow batteries described herein can have higher voltage and power density than many previous types of flow batteries. Many commercially available flow batteries arevanadium or zinc -bromine batteries. However, the operation voltage of these aqueous flow batteries is below 2.0 V due to electrochemical water splitting. The low' voltage limits the power density and energy density of aqueous redox flow batteries. Moreover, the use ofhigh-cost and toxic materials such as vanadium and bromine in these aqueous redox flow batteries poses challenges to the environment and energy sustainability, hi contrast, the redox flow batteries described herein can use a non-aqueous catholyte, which allows for higher voltages because no water is present to undergo electrochemical splitting Thus, the redox flow batteries can be substantially or complete!}’ free of water.

[0047] These non-aqueous catholytes can be particularly’ useful when utilized with a magnesium anode. Magnesium has several useful features as a battery / anode, including high voltage and energy density. Magnesium anodes have low redox potential (about -2.37 V vs. standard hydrogen electrode) and a high volumetric capacity (about 3833 mAh / cm3). In addition, magnesium metal is denser than lithium, offering twice the capacity at the same volume. Magnesium is also a highly reversible and stable anode metal. Side reactions between the magnesium metal anode and the electrolyte are minimal in ethereal organic electrolytes, which can lead to reversible magnesium deposition and stripping for thousands of cycles. Thus, batteries that include a magnesium metal anode and the non-aqueous catholyte can have a stable anode / electrolyte interface with a long cycle life In addition to magnesium metal anodes, other metal anodes can also be used such as sodium metal or lithium m some examples.

[0048] Redox-active organic molecules can be particularly usefill as charge carriers in redox flow batteries because of their natural abundance, molecular diversity, and structural tunability. The physical and electrochemical properties of these molecules can depend on the presence and arrangement of various functional groups. For example, the functional groups can affect solubility, chemical stability, redox potential, and kinetics. Some organic redoxactive molecules have been formulated in acetonitrile-based electrolytes or propylene carbonate -based electrolytes because such electrolytes can provide high conductivity and wide potential window's. However, acetonitrile and propylene carbonate are not compatible with magnesium metal anodes due to high reactivity of the magnesium with these polar solvents. Therefore, many previously formulated organic electrolytes are not suitable for use with magnesium anodes. Additionally, some cathode redox-active organic molecules are notstable when used with a magnesium anode, again due to reactivity. However, the nonaqueous catholyte described herein can provide high stability and compatibility with magnesium anodes, because the catholyte can include both a redoxmer and a solvent that arc compatible with magnesium. In certain examples, the solvent can be an ether-based solvent, which can have low reactivity with magnesium metal.

[0049] Energy density can be calculated as

[0050]

[0051] where n is the number of transferred electrons, C is the redoxmer concentration, is Faraday’s constant, and V is the cell voltage. The redoxmers used in the flow batteries described herein can provide high energy density because they' can have one ormore ofthe following properties: multiple electrons transferred per molecule in a redox reaction; high solubility in the catholyte solvent; and high voltage.

[0052] FIG. 1 shows a schematic view' of an example redox flow battery' 100. This battery includes a catholyte reservoir 110 that contains a non-aqueous catholyte 112. A flow cell 120 is in fluid communication with the catholyte reservoir so that the catholyte can be circulated through the flow cell. In this example, a pump 130 is used to circulate the catholyte. A separator 140 is in the flow cell and in contact with the catholyte. A cathode 150 is also in contact with the catholyte in tlie flow cell. An anode 160 is separated from the catholyte by the separator. The anode can be a metal anode such as a magnesium anode. The cathode is electrically connected to a positive terminal 152, and the anode is electrically' connected to a negative terminal 162. The terminals ofthe battery can be used to charge and discharge the battery'. In particular, when the battery is discharged, electrons can flow from the negative terminal of tire battery, through any type of electrical circuit / load, to the positive tenninal of the batery. As the electrons leave tire negative tenninal, tire anode can be oxidized. In some examples, the anode can release oxidized metal ions, such as Mg2+ions. As electrons flow' to the positive terminal, the electrons can be conducted through the catholyte toward the cathode. These electrons can be absorbed by the redoxmer molecules in the catholyte, reducing tire redoxmer molecules. In further examples, charging the battery' can be accomplished when electrons flow in the opposite direction, reducing the anode and oxidizing the catholyte redoxmer.

[0053] FIG. 2 shows a schematic view of a specific example battery 100 being charged. During the charging process, an electric current is applied between the anode 160 and the cathode 150, in which electrons 102 flow' from the cathode to the anode. Inside tire flow' cell120, molecules of the redoxmer 114 are oxidized at the cathode. In this example, the redoxmer used is tris[4-(diethylamino)phenyl]amine (TDPA). This molecule gives up two electrons to adopt a charge of 2+ The anode used in this example is magnesium metal. Magnesium ions (Mg2+) 164 are present in the catholyte. These ions can pass through the separator 140 to be reduced at the anode, which causes magnesium metal to be deposited on the anode.

[0054] The redoxmers used in the batteries described herein can hav e a variety of molecular structures, hi various examples, the redoxmers can include functional groups that provide usefill properties such as high voltage, transfer of multiple electrons, solubility in the catholyte solvent, and stability. In some examples, the redoxmer can include a carbonyl group or an amine group. In further examples, the redoxmer can include both a carbonyl group and an amine group, hi still further examples, the redoxmer can include multiple carbonyl groups, multiple amine groups, or a combination thereof. When the redoxmer includes an amine group, the nitrogen atom of the amine group can be bonded to an alkyl group, or a phenyl group, or the nitrogen atom can be bonded to both an alkyl group and a phenyl group, or any combination of one or more alkyl groups and one or more phenyl groups.

[0055] Some example redoxmers can include an amine group that includes a nitrogen atom bonded to at least one phenyl group. In certain examples, the redoxmer can include a nitrogen atom bonded to one phenyl group, or to two phenyl groups, or to three phenyl groups. In fiirtlier examples, the redoxmer can include an amine group that includes a nitrogen atom bonded to at least one alkyl group. In fiirtlier examples, the nitrogen atom can be bonded to one alkyl group, to two alkyl groups, or to three alkyl groups. In still further examples, the redoxmer can include an amine group that includes a nitrogen atom bonded to at least one phenyl group and to at least one alkyl group, hr certain examples, the nitrogen atom can be bonded to one phenyl group and to one alkyl group, or to one phenyl group and to two alkyl groups, or to two phenyl groups and to one alkyl group. Additional example redoxmers can include multiple amine groups. The different amine can be bonded to different functional groups, include different combinations of phenyl groups and alkyl groups. In further examples, tire redoxmer can include an amine group that include includes a nitrogen atomthat is not a member of a ring (i.e., either an aromatic or non-aromatic ring), in certain examples, tire redoxmer can be devoid of nitrogen atoms that are members of a ring.

[0056] Alkyl groups that can be bonded to the nitrogen atom in an amine group can include methyl groups, ethyl groups, propyl groups, isopropyl groups, butyl groups, isobutyl groups, tert-butyl groups, and others, In further examples, the alkyl groups can be a straight chain alkyl group from C1 to C20.

[0057] The nitrogen atom can also be bonded to an ether group The ether group can be a single ether (i.e., a group having a single oxygen atom bonded to two carbon atoms) or a polyether that has multiple oxygen atoms. In certain examples, the polyether group can be (CH2CH2O)XCH3- where X can be any integer from 1 to 20.

[0058] In some examples, the redoxmer can have a π-conjugated structure, including fully π-conjugated and partially π-conjugated structures. In certain examples, π-conjugated structures can include a nitrogen atom bonded to a phenyl group. In further examples, non-conjugated structures can include a nitrogen atom bonded to an alkyl group. Various combinations of phenyl groups and alkyl groups can be present in molecules that are partially π-conjugated. FIG. 3 shows several examples of molecules along a spectrum from fully non-conjugated to fully π-conjugated. Redoxmers that have a higher degree of conjugation can have a higher redox potential. This can be explained by the electron delocalization in n-conjugated structures, which stabilizes electrons in foe molecule and helps the redoxmer achieve high potential before being oxidized.

[0059] Certain redoxmers can be capable of being reversibly oxidized to a 2+ charge and reduced to a neutral charge. These redoxmers can be characterized by two pairs of redox peaks in cyclic voltammetry profiles (i.e., two oxidation peaks and two reduction peaks). The redoxmer can undergo two redox reactions: one reaction for a first electron and a second reaction for the second electron. In some examples, the second redox reaction can have a higher redox potential than the first redox reaction. Tire first and second redox reactions can have a redox potential from about 2.0 V to about 3.5 V in some examples.

[0060] The functional groups of the redoxmer can also influence the solubility of the redoxmer in the catholyte solvent. When the solvent is an ether-based solvent, then non-conjugated structures can often have a higher solubility than conjugated structures. In certain examples, the redoxmers that have a mixed structure with some conjugation and some non-conjugation can have a good combination of redox potential and solubility. In some examples, tire redoxmer can have a solubility of at least about 0.5 M in the catholyte solvent. In further examples, the redoxmer can have a solubility from about 0.5 M to about 1.5 M, or from about 0.5 M to about 1.0 M m the catholyte solvent.

[0061] The redoxmer can include any of the following non-limiting example compounds in some examples: phenothiazine, phenazine, anthraquinone, perylene diimide, ferrocene derivative. methyl viologen, PEGylate, azobenzene, oxoverdazyl radical, tetracyanoethylene, nitronyl nitroxide radical, N-butylphthalimide, and combinations thereof. These molecules can be used in their standalone form as the redoxmer in some examples, while in other examples these molecules or derivatives thereof can make up a portion of the redoxmer.

[0062] Several more example redoxmer molecules are represented by the following molecular structures labelled (I) to (XIV):

[0063] (CH2)XCH3

[0064] G O

[0065] X = 0-20 (1);

[0066] (CH2)XCH3

[0067]

[0068] (CH2)YCH:

[0069] X = 0-20; Y = 0-20 (II):(CH2)XCH3

[0070] X = 0~20; Y = 0~20; Z = 0~20 (Ill);

[0071] X = 0~20 (IV);

[0072] (CH2)XCH3(CH2)XCH3

[0073] " N

[0074] X = 0~20; Y = 0~20

[0075]

[0076] (V);(CH2)XCH3 (CH2)XCH3

[0077] " N

[0078] (CH2)ZCH3{ y li X (C^)YCH3 I \ (CH2)ZCH3(CH2)YCH3X = 0~20; Y = 0~20; Z = 0~20 (VI);

[0079] (CH2CH2O)XCH; i.(cH2CH2O)xCH3

[0080] "'N

[0081] X = 0~20 (VII);

[0082] (CH2CH2O)XCH3Z(CH2CH2O)XCH3

[0083] '' N''

[0084] \ (CH2CH2O)YCH3

[0085]

[0086] X = 0~20; Y = 0~20 (VIII):(CH2CH2O)XCH3 / (CH2CH2O)XCH3X'NZ(CH2CH2O)ZCH^ J Il J. (CH2CH2O)YCH3

[0087] ~''N'ZI

[0088] (CHOCHQO)7CH3(CH2CH2O)YCH X = 0~20; Y = 0~20; Z = 0~20 (IX);

[0089] (CH2)XCH3(CH2)YCH, I

[0090]

[0091] X = 0~20; Y= 0~20; Z = 0~20; I = 0~20 QQ.(CH2CH?O)XCH3(CH, CH2O)YCH, i I.f\L „t< H3Cx(OH2CH2Cr Y 'll p" (CH2CH2O)YCH3H3Ci(OH2CH2C) (CH2CH2O)ZCH;

[0092] (CH2CH2O)ICH3(CH2CH2O)ZCH3X = 0~20; Y= 0~2Q; Z = 0~20; I = 0~20 (XI);

[0093]

[0094]

[0095] (XIV) In any of the above structures, X, Y, Z. I. and n can each represent integers from 0 to 20, in some cases 1 to 20, and in other cases 5 to 10, and which may be the same or different from each other.The solvent used in the catholyte can be an ether-based solvent. The ether-based solvent can include a glycol ether (glyme) in some examples. Some example ethers that can be included in the solvent include dimethoxyethane (DME), bis(2-methoxyethyl) ether, tetraethylene glycol dimethyl ether, tetrahydrofuran (THF), or a combination thereof. As mentioned above, the redoxmer can include functional groups that allow the redoxmer to be soluble in such ether-based solvent. In further examples, the solvent can be free of compounds that have high reactivity with magnesium metal, hi particular, the solvent can be free of acetonitrile and propylene carbonate in certain examples. Although acetonitrile and propylene carbonate have been used previously in non-aqueous electrolytes, the amine redoxmers described above do not have high solubility in these polar solvents. Indeed, the solubility of the redoxmers can decrease with increasing polarity of the solvent. As an example, the solubility of TDPA is 16 times higher in DME than in acetonitrile, and 25 times higher in DME than in propylene carbonate. Similar solubility differences are observed for DPPD and TPPD. Additionally, as mentioned above, acetonitrile and propylene carbonate can be reactive with magnesium metal anodes. Therefore, despite the fact that acetonitrile and propylene carbonate have been used in other electrolytes, the redox flow batteries described herein can include a catholyte solvent that is devoid of acetonitrile and propylene carbonate in some examples.

[0096] The catholyte can also include a supporting salt. The supporting salt can be an ionic compound that can dissociate in the catholyte solvent. Some example supporting salts that can be used in the catholyte include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiFTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), sodium perchlorate (NaClO4), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), Sodium trifluoromethanesulfonate (NaOTf), sodium hexafluorophosphate (NaPF6), magnesium chloride (MgCl2), magnesium bis(trifluoromethanesulfonyl)imide (MgTFSI2), and combinations thereof. The supporting salt can be present in the catholyte at a concentration from about 0.1 M to about 1.5 M, or from about 0.2 M to about 1.0 M, or from about 0.3 M to about 0.7 M, or from about 0.5 M to about 1.0 M, or from about 05 M to about 1,5 M in some examples.

[0097] The catholyte can also include magnesium ions, metallic magnesium, or a combination thereof. Magnesium ions can be introduced into the catholyte by dissolving amagnesium salt such as magnesium chloride (MgCl2). Magnesium ions can be included in the catholyte at a concentration from about 0.1 M to about 1.5 M, or from about 0.2 M to about 1.0 M, or from about 0.3 M to about 0.7 M, or from about 0.5 M to about 1.0 M, or from about 0.5 M to about 1.5 M in some examples. Metallic magnesium can be added by mixing magnesium metal powder in the catholyte, tor example. In some examples, the metallic magnesium powder can be added in an amount from about 0.1 M to about 0.5 M, or from about 0.1 M to about 0.3 M. Additionally, in some cases the catholyte can be filtered after mixing the ingredients, which can remove at least a portion of the metallic magnesium powder. In certain examples, the magnesium powder can be fairly coarse, such as mesh 50 size or larger.

[0098] A separator can be placed in the flow cell of the redox flow battery, separating the flowing catholyte from the anode. In some examples, the separator can be a porous membrane that can separate materials by size exclusion. The membrane can be capable of allowing small ions such as Mg2+to transfer across the membrane, while preventing or reducing transfer of the redoxmer across the membrane, hi certain examples, the porous membrane can have an average pore size from about 0.01 pm to about 0.2. and in some cases to 0.1 pm. Similarly, the membrane can have a thickness which is sufficient to provide mechanical integrity and desired separation. As a general guideline, the thickness can range from about 10 pm to 0.8 mm, and in some cases 20 pm to 50 pm. In some examples, the separator can be a membrane made from polymers such as polyethylene, polypropylene, dense ceramic membranes, cation exchange membranes, or a combination thereof. In other examples, the separator can be an anion exchange membrane (AEM). The AEM can block crossover of the redoxmer in its neutral or positively charged state.

[0099] As mentioned above, the anode used in the redox flow battery can be a metal anode in some examples, or alternatively a flowing anolyde containing an anode redoxmer in other examples. Metal anodes can include magnesium, lithium, or sodium in some examples. In particular examples, the anode can be magnesium metal. Tlie anode can be formed as a ribbon, plate, or other fonn factor. In some examples, the anode can be in contact with the separator on the opposite side of the separator from the catholyte. In examples using a flowing anolyte can include an anolyte redoxmer and an electrolyte (i.e. a salt and a solvent). In some examples, the anolyte salt and anolyte solvent can include those listed above assuitable for the catholyte. Similarly, the anolyte redoxmer can include, but is not limited to, viologen and its derivatives, quinones and its derivatives, fluorenone and its derivatives, ferrocene and its derivatives, and the like.

[0100] The redox flow battery can also include a cathode, which can be a conductive material that is in contact with the catholyte in the flow cell and separated from the anode by the separator. In the redox flow batteries described herein, the cathode itself may not undergo a redox reaction when the battery is in operation. The redox reaction can be undergone by the redoxmer dissolved in the catholyte, as explained above. The cathode can be used to conduct the electrons given off or absorbed by the redoxmer molecules. The cathode can also be called a current collector in these batteries.

[0101] Battery terminals can be electrically connected to the anode and the cathode. When the battery is discharging, the voltage between the terminals can be from about 2.0 V to about 3.5 V, or from about 2.0 V to about 3.0 V, or from about 2.0 V to about 2.5 V, or from about 2.5 V to about 3.5 V, or from about 2.5 V to about 3.0 V. or from about 3.0 V to about 3.5 V, or from about 3.0 V to about 3.3 V, or from about 2.7 V to about 3.3 V. The redox flow battery can have a discharge capacity from about 75 mAh / g to about 150 mAh / g, or from about 90 mAh / g to about 150 mAh / g, or from about 100 mAh / g to about 150 mAh / g, or from about 90 mAh / g to about 120 mAh / g in some examples (i e per gram of organic molecules) FIG. 4 is a flowchart illustrating an example electrochemical energy storage method 400. Uris method includes: providing a catholyte reservoir containing a non-aqueous catholyte comprising an ether-based organic solvent and an organic redoxmer, wherein the redoxmer comprises a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof 410; circulating the catholyte through a flow cell, wherein the catholyte in the flow cell is in contact with a cathode and a separator, and wherein an anode is separated from the catholyte by the separator 420; and oxidizing or reducing the redoxmer by applying an electric current between the anode and the cathode 430. This method can be performed using the redox flow batteries described herein. When the battery is charged, the redoxmer can be oxidized at the cathode. As explained above, in some examples the redoxmer can be oxidized to a charge of 2+ when the batery is charged The anode can be a metal anode in some examples, the metal ions can be reduced at the anodewhen the batten is charged. For example, the anode can be magnesium metal, and magnesium ions can be reduced at the anode, which can cause magnesium metal to be plated on the anode. When the battery is discharged, magnesium can be oxidized at the anode, causing magnesium ions to be released into the catholyte solution. At the same time, the redoxmer in the catholyte can be reduced back to its neutral state when the battery is discharged.

[0102] While the battery is being charged or discharged, the catholyte can be circulated through the flow cell. This can allow constant replacement of the redoxmer in the flow cell, so that fresh redoxmer molecules can be made available as redoxmer molecules are oxidized or reduced at the cathode. When the catholyte is circulated in this way, the overall proportion of redoxmer molecules that are oxidized and reduced can gradually change in the catholyte reservoir while the battery is being charged or discharged. In an alternative example, the redox flow battery can include two catholyte reservoirs, where one reservoir is used to store catholyte with oxidized redoxmer molecules and the other reservoir is used to store reduced redoxmer molecules. Instead of circulating the catholyte in a loop back to the same reservoir, the catholyte can be circulated back and forth between the two reservoirs when tire battery is charged and discharged.

[0103] In some examples, when the battery is charged, the amount of oxidized redoxmer molecules in the catholyte reservoir can be from about 70% to about 100% with respect to the total amount of redoxmer in the catholyte reservoir. In further examples, the amount of oxidized redoxmer molecules can be from about 80% to about 100%, or from about 90% to about 100%, or from about 95% to about 100%. After the battery has been discharged, the amount of oxidized redoxmer molecules can be from about 0% to about 30%. or from about 0% to about 20%, or from about 0% to about 10%, or from about 0% to about 5%.

[0104] Examples

[0105] Redox flow batteries with a magnesium anode were investigated. These batteries were found to be useful in low-cost, high-energy-density and long-cycle-life stationary-energy storage applications. However, many available cathode redoxmers suffer from low-solubility in non-aqueous electrolytes and low redox potentials. In this example, a variety of cathode redoxmers were investigated to identify molecules that can be used as redoxmers compatible with magnesium anodes. The properties of several amine derivatives and theirperformance m an example flow cell were collected to establish the correlation between molecular structures and electrochemical performance. The results confirm that the redox potential and solubility of these amine molecules arc influenced by ^-conjugated and non¬ conjugated structures of the amine derivatives. Density functional theory (DFT) simulations and inverse aromatic fluctuation index (FLU'1) verified that the conjugation plays an important role in stabilizing the molecule and increasing its redox potential. Tris [4-(diethylamino)phenyl jamine (TDPA) with mixed 7t-conjugated and non-conjugated structure shows the highest theoretical energy density (-120 Wh / L) due to its high solubility (-0.9M) in the electrolyte and high voltage (-2.7V versus Mg / Mg2+). The effect of solvents and ions in supporting electrolyte was also explored, indicating that the ether solvent can be used to achieve a high-stability and high-solubility amine-based catholyte, while bulk anions do not influence the redox peak potential of these p-type molecules. The electrochemical performance was evaluated using a Mg-amine redox flow battery configuration, which delivers a voltage of 2.50 V, a discharge capacity of 106.5 mAh / g, a CE of 90.74%, and 93.88% capacity retention after 150 cycles.

[0106] Several organic molecules were screened for use as compatible redoxmers in nonaqueous Mg RFBs. Several amine-based molecules stand out due to their high voltage and high compatibility with Mg anode. Multiple catholyte parameters were investigated, including the influence of redox-active moiety of the redoxmer, non-conjugated and rr-conjugated structures, and supporting electrolytes. By combining electrochemical tests with spectroscopic characterizations, performance was evaluated for selected amine-based catholytes in both a three-electrode beaker cell and in Mg RFBs and tire performance of the catholytes w'as correlated with redoxmer structures and properties (solubilities, transports, kinetics and stabilities). Density functional theory (DFT) simulations and the average inverse aromatic fluctuation index (FLU"1) w7ere conducted to assist in verifying the impact of conjugation degree on the oxidation potential.

[0107] Several organic molecules were initially screened using cyclic voltammetry. These molecules included azobenzene (AB), anthraquinone (AQ), tetrachloro- 1.4-benzoquinone (TCBQ), N, N, N'. N'-tetraphenyl-l,4-phenylenediamme (TPPD), (2, 2.6.6-tetramethylpiperidin-l-yl)oxyl (TEMPO). 1,10-phenanthroline (Phen), diphenyl sulfide (DPS) and 1,4-di-tert-buty Ibenzene (DTBB). The structures of these molecules are shown inFIG. 5. According to the different electric charges carried by the redox centers during the electrochemical reactions, these organic molecules can be divided into p-type and n-type active materials. AB, AQ, and TCBQ arc n-type active materials, carrying negative charges during the charge / discharge processes, while the other molecules are p-type active materials, carrying positive charges during the charge / discharge processes.

[0108] Cyclic voltammetry (CV) was conducted to test the redox activities and potentials of these molecules in Mg electrolyte. The cyclic voltammetry profiles are shown in FIG. 6. FIG.

[0109] 7 graphs the peak-to-peak separation of each molecule, and FIG. 8 graphs the percent of current retention after 300 cycles. Tire p-type molecules show higher redox potential (above 2.6 V vs Mg / Mg2+) than n-type molecules. Among these high-potential molecules, TPPD shows the smallest peak-to-peak separation (AE), indicating that its redox reaction is most reversible. All the organic molecules, upon oxidation, form radicals or charged species that are stabilized by their conjugated structures (phenyl or N-0 bond), resulting in enhanced stability ofthe charged molecules. The electrochemical stabilities of these organic molecules were obtained from the retention ratio (calculated as Ip^' oi Iy,i) of the peak current during 300-cycle CV scan. TPPD shows the highest stability (100%) compared with the other molecules, demonstrating the stable electron-transfer reaction ofthe amine group.

[0110] In general, amine redoxmer showcases its potential in Mg RFBs owing to its high redox potential, highest reversibility and highest stability. Therefore, additional amine molecules were studied to explore the influence of chemical structure on properties (thermodynamics, solubilities, kinetics, transports and stabilities) to determine additional structures for cathode design in Mg RFBs.

[0111] Five amine molecules were chosen to understand how the ^-conjugated and / or nonconjugated structures influence the electrochemical performances and other properties of amine-type molecules. FIG. 9 shows the structures of these molecules. N, N, N', N'- Tetramethylethylenediamine (TMED) is anon-conjugated amine molecule, where the twoN atoms are connected to alkyl chains. N, N, N', M '-Tetraphenyl- 1,4-phenylenediamine (TPPD) is a fully n-conjugated molecule where the two N atoms are connected to phenyl groups. Tris[4-(diethylamino)pheiiyl]amine (TDPA). N. N, N', N'-Tetramethyl-p-phenylenediamine (TMPD), and N, N'-Diphenyl-p-phenylenediamine (DPPD) are mixed structures where the two N atoms are both connected with alkyl chains (or hydrogen atoms) and phenyl groups.simultaneously. Chemical intuition suggests that TPPD has the highest delocalization owing to its highly conjugated structure. However, for the mixed conjugated systems, the situation is more nuanccd owing to the presence of delocalizing phenyl and electron withdrawing methyl groups. To quantify this, the average inverse aromatic fluctuation index (FLU'!) of these molecules was calculated in their neutral state. A higher FLU’1value corresponds to stronger aromaticity, indicating a higher electron delocalization. The degree of conjugation follows the sequence: TMED < TDPA < TMPD < DPPD < TPPD.

[0112] Cyclic voltammetry profiles were measured for these amine compounds over the range of 0.5 V to 3 5 V vs. Mg / Mg2+. The CV profiles are shown in FIG. 10, TMED only' shows one visible oxidation peak, indicating that molecules with non-conjugated structure have no reversible redox reactivity in this condition. The DFT calculation results shows that while the neutral TMED molecule is stable m solution, divalent TMED disintegrates during the optimization in both the gas and solvent phases, which indicates that TMED is not stable in its charged state, and it undergoes complete irreversible dissociation after oxidation. The remaining molecules with ir-conjugated structures show two pairs of redox peaks, demonstrating great promise to deliver twice as much capacity as that from one-electron redox couples, such as TEMPO. As their degrees of conjugation increase, their redox potential also increases. FIG. 11 shows a plot of the degree of conjugation of each of these molecules vs. the redox potential of each molecule. Tliis demonstrates that the electron delocalization in n-conj ugated structures is beneficial to achieve high potential. The maximum potential was realized by TPPD at 2.74 V vs Mg / Mg2+for 1stredox reaction and 3.14V vs Mg / Mg2+for 2ndredox reaction. To understand the origins of the experimental trends, the redox potential and highest occupied molecular orbital (HOMO) were calculated by DFT from the neutral to charged state for each molecule. The qualitative trends mirror the experimental results. This shows that the conjugation (or delocalization of electrons) plays a role in stabilizing the molecule and increasing the HOMO and redox potential. Tire higher the stabilization, the higher energy (or redox potential) is involved to oxidize the molecule.

[0113] The solubilities of these molecules were tested in DME solutions. The solubility of each molecule is graphed in FIG. 12. The non-conjugated molecule, TMED, is miscible with DME, while the fully rc-conjugated molecule, TPPD, shows the lowest solubility (-0.12 M).Molecules with mixed structures exhibit moderate solubilities, ranging from approximately 0.5 to 0.89 M. Theoretical energy densities were calculated for these molecules. A graph of solubility vs. redox potential of each molecule is shown in FIG. 13, which also shows energy density levels. Molecules with mixed structures show energy densities of 50-120W11 / L, while the energy densities of non-conjugated molecule and ^-conjugated molecules are 0 and — 1 OWh / L due to either low reversible reactivity or low' solubility. Overall, mixed structures offer a balance of optimized potential and solubility. Among all the molecules, TDPA demonstrates promising performance, achieving an energy density of approximately 120 Wh / L.

[0114] The reaction mechanism of TDPA is shown in FIG. 14 as an example. The TDPA molecule undergoes a 2 -electron transfer reaction, and 2.05 V and 2.49 V were obtained for the TDPA / TDPA+and TDPA+ / TDPA2+couples, respectively. The mass transport, kinetics and stabilities of these molecules were also evaluated. The diffusion coefficient Do and heterogeneous reduction rate constants k° were calculated from cyclic voltammetry' data at different scan rates. FIG. 15 plots tire diffusion coefficient Do of the molecules, and FIG. 16 plots the reduction rate constants k°. The observed diffusion coefficients values are in tire range of 3-18×10-6cm2 / s, w'hich is in the same order as the most, used organic catholyte molecule in flow batteries. The reaction rates of these amines are lower than that of organic cathode molecules but higher than those of other common inorganic redox couples such as V37V4’ and Fe2+ / Fe3+. The CV curves were measured for TDPA, TMPD, and DPPD at the 2ndand 100thcycles at the scan rate of 100 mV / s. The CV curves almost completely overlap with each other, demonstrating the high stabilities of these amine redox couples even after 100 cycles. The second redox peak of TPPD decayed during cycling which may be caused by the electrolyte decomposition at beyond 3.00 V vs Mg / Mg2’. When the up-limit scan range was set to 3.00 V vs Mg / Mg2+, TDPA showed completely overlapped profiles during 100 cycles. When the scan range includes both anode (Mg↔Mg2+) and cathode reactions (TDPA↔TDPA2+, for example), there was still no decay during 100 cycles in the CV profile, indicating the TDPA reaction and Mg reaction are not affecting each other. Generally, amine molecules show' the high diffusivities, moderate kinetics and high stabilities in Mg electrolytes system, implying that they are favorable for low-polarization, high-efficiency and high-stability' Mg RFBs.To confirm the exceptional performance for practical applications, a flow cell device was constructed to examine the electrochemical performance of TDPA at the device level The typical CV curves ofthc electrolyte and TDPA arc shown in FIG. 17, which demonstrate the feasibility of realizing a flow cell with a high voltage of 2.70 V. RFBs using Mg ribbons as the anode, a porous membrane as the separator, and TDPA as catholytes redoxmer were assembled and galvanostatically discharged / charged. As shown in FIG. 18, the Mg-TDPA cell can deliver an average voltage of 2.50 V, a discharge capacity of 95.3 mAli / g with a Coulombic efficiency (CE) of 69.07%. The unsatisfied discharge capacity and CE were caused by crossover of TDPA molecule through porous membrane Since amine molecules belong to p-type molecules, their states are neutral or positively charged during reaction. Anion exchange membrane (AEM) was used to block the cross-over of neutral or positively charged TDPA. and such a Mg-TDPA cell can deliver a discharge capacity of 106.5 mAh / g with a CE of 90,74% and achieve 93.88% capacity retention after 150 cycles. After disassembling the cell, cycled Mg ribbon was washed with DME to remove any soluble precipitate and examined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The Mg ribbon surface showed obvious holes, indicating depositiondissolution of Mg metal. The EDX mapping also shows the signals for Mg. The signal for N in TDPA molecule is missing in EDX, demonstrating that AEM effectively inhibited the crossover of TDPA. The supporting electrolyte and catholyte before and after test were examined by Fourier Transform Infrared Spectroscopy (FTIR). Compared with the supporting electrolyte, the catholyte before the test showed multiple peaks around 685-720 cm’1attributed to C-II bending of TDPA's phenyl group. After cycling, the peak at 696 cm’1shows a little shift to higher wavenumbers, indicating structure changes of TDPA during cycling. Together with an average CE of 86.59%, these results indicate that Mg-TDPA still shows somewhat irreversibility during flow cell cycling, which may be caused by TDPA’s insufficient kinetics and the side reaction such as dimenzation of TDPA at oxidized states.

[0115] It is noted that tire amine-based redoxmer molecules are p-type active material. When the amines are oxidized, the positively charged molecules can bond with anions present in the catholyte by electrostatic force, and such binding may influence the peak potential. TFSI’ and ClOf were selected as example anions to investigate the bulk anion influence on TDPA performance. The CV profiles were measured for (1) TDPA with NaTFSI and (2) TDPAwith NaCIC These CV profiles are shown in FIG 19.. These CV profiles show' similar redox peaks. FTIR was conducted to study the chemical bonding environment of TDPA. The FTIR profile is shown in FIG. 20. Compared with pure DME solvent, the profile of TDPA-DME shows a characteristic peak at 1507.121 cm-1, indicating the presence of TDPA. When 0.5M TFST or C1O4’ are added to the TDPA-DME solution, there is no change of peak position, which means the chemical environment of TDPA was not changed by the anions. Based on the combined electrochemistry' and spectroscopy results, it can be inferred that TFSI or CIO4 ions are only weakly associated with the TDPA molecule and do not significantly influence its redox potential. These amine-based catholytes are also applicable to Na and Li RFBs due to the compatibility of ether with metal anode. In Li-based and Na- based electrolytes, TDPA catholyte also enables two electron transfer reactions (2.57 and 2.96 V vs L1 / L1+, 2.78 and 3.20 V vs Na / Na+).

[0116] Example Clauses

[0117] The following clauses are representative of iterations consistent with the invention and the preceding description, although they are not exclusive of such iterations which are supported by the corresponding detailed description.

[0118] Clause 1. A redox flow' battery', comprising:

[0119] a catholyte reservoir containing a non-aqueous catholyte comprising an ether-based organic solvent and an organic redoxmer, wherein the redoxmer comprises a carbonyl group, an amine group, or a combination thereof wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof;

[0120] a flow cell in fluid communication with the catholyte reservoir to circulate the catholyte through the flow' cell;

[0121] a separator m the flow' cell in contact with the catholyte in the flow cell;

[0122] a cathode in contact with the catholyte in the flow cell; and

[0123] an anode separated from the catholyte by the separator.

[0124] Clause 2. The redox flow battery of any one of Clauses 1, 3-22, wherein the redoxmer is nonpolymeric.

[0125] Clause 3, The redox flow battery' of any one of Clauses 1-2, 4-22, wherein the redoxmer comprises the amine group.Clause 4. The redox flow battery of Clause 3, wherein the nitrogen atom is bonded to both a phenyl group and an alkyl group.

[0126] Clause 5. The redox flow battery of Clause 3, wherein the nitrogen atom is not a member of a ring.

[0127] Clause 6. The redox flow battery1of any one of Clauses 1-5, 7-22, wherein the redoxmer comprises the carbonyl group.

[0128] Clause 7. The redox flow battery of any one of Clauses 1-6, 8-22, wherein the redoxmer is capable of being reversibly oxidized to a 2+ charge and reduced to a neutral charge.

[0129] Clause 8. The redox flow battery’ of any one of Clauses 1-7, 9-22, wherein the redoxmer comprises a phenothiazine, a phenazine, an anthraquinone, a peiylene diimide, a ferrocene derivative, a methyl viologen, a PEGylate. an azobenzene, an oxoverdazyl radical, a tetracyanoethylene, a nitronyl nitroxide radical, a N-butylphthalimide, or a combination thereof.

[0130] Clause 9. The redox flow battery of any one of Clauses 1-8, 10-22, wherein the redoxmer has a molecular structure according to any of structures (I) to (XIV):

[0131] (CH2)XCH3

[0132] (D-

[0133] (CH2)XCH3

[0134]

[0135] (CH2)XCH3

[0136] (CH2)YCH3

[0137] (V); (CH2)XCH3 / (CH2)XCH3'" N

[0138] (CH2)ZCH3(CH2)YCH3

[0139] (VI);

[0140]

[0141] (CH2CH2O)XCH3 (CH2CH2O)XCH3

[0142] (VII);

[0143] (CH2CH2O)XCH3(CH2CH2O)XCH3

[0144] (CH2CH2O)YCH3(Vlll).

[0145] (CH2CH2O)XCH3 / (CH2CH2O)XCH3'">.1

[0146] (CH2CH2O)ZCH3(CH2CH2O)YCH3

[0147] (IX):

[0148]

[0149] (CH2)XCH3(CH2)YCH3

[0150] I

[0151] (X);

[0152] (CH2CH2O)XCH3(CH2CH, O)YCH3I I

[0153] H3CX(H2C) (CH2CH2O)YCH3

[0154] H3CI(H2C) / (CH2CH2O)ZCH3

[0155]

[0156] (CH2CH2O)ICH3(CH2CH2O)ZCH3

[0157] (XI);

[0158]

[0159]

[0160] W (XIV); wherein X. Y. Z, I. and n are independently integers from 0 to 20.

[0161] Clause 10. The redox flow battery of any one of Clauses 1 -9, 11-22, wherein the redoxmer is dissolved in the solvent at a concentration greater than about 0.5 M.

[0162] Clause 11. The redox flow battery of any one of Clauses 1-10, 12-22, wherein the organic solvent is substantially free of acetonitrile and propylene carbonate.

[0163] Clause 12. The redox flow batter}'- of any one of Clauses 1-11. 13-22, wherein the organic solvent comprises dimethoxyethane, bis(2-methoxyethyl) ether, tetraethylene glycol dimethyl ether, tetrahydrofuran (THF), or a combination thereof.

[0164] Clause 13. The redox flow battery of any one of Clauses 1-12, 14-22, wherein the catholyte further comprises a supporting salt.

[0165] Clause 14. The redox flow battery of any one of Clauses 1-13, 15-22, wherein the catholyte further comprises magnesium ions.

[0166] Clause 15. The redox flow battery of any one of Clauses 1-14, 16-22, wherein the separator comprises a size exclusion membrane or an anion exchange membrane.

[0167] Clause 16. The redox flow battery of any one of Clauses 1-15, 17-22, wherein the anode is a metal anode.Clause 17. The redox flow battery of Clause 16, wherein the catholyte further comprises an ion of the metal of the metal anode.

[0168] Clause 18. The redox flow battery of Clause 16, wherein the anode comprises magnesium, sodium, or lithium.

[0169] Clause 19. The redox flow battery of Clause 16. wherein the anode is a magnesium anode. Clause 20. The redox flow battery of any one of Clauses 1-19, 20-22, wherein the battery does not comprise a flowing anolyte.

[0170] Clause 21. The redox flow battery of any one of Clauses 1-20, 22, wherein a voltage between the anode and the cathode is from about 20 V to about 3,5 V during a discharge cycle ofthe battery.

[0171] Clause 22. The redox flow battery of claim 1, wherein the battery has a discharge capacity from about 75 mAh / g to about 150 mAh / g.

[0172] Clause 23. An electrochemical energy storage method, comprising:

[0173] providing a catholyte reservoir containing a non-aqueous catholyte comprising an ether-based organic solvent and an organic redoxmer, wherein the redoxmer comprises a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof:

[0174] circulating the catholyte through a flow cell, wherein the catholyte in the flow cell is in contact with a cathode and a separator, and wherein an anode is separated from the catholyte by the separator; and

[0175] oxidizing or reducing the redoxmer by applying an electric current between the anode and the cathode.

[0176] Clause 24. The method of Clause 23 using the flow battery of any one of Clauses 1-22, wherein the redoxmer is oxidized during a charging cycle.

[0177] Clause 25. The method of Clause 24, wherein the redoxmer is oxidized to a charge of 2+ during the charging cycle.

[0178] Clause 26. The method of Clause 24, wherein the anode is a metal anode, wherein the catholyte further comprises ions of the metal of the metal anode, and wherein the ions are reduced at the metal anode during the charging cycle.Clause 27. The method of Clause 23 using the flow battery of any one of Clauses 1-22, wherein a voltage between the anode and the cathode is from about 2.0 V to about 3.5 V during a discharge cycle.

[0179] Clause 28. A non-aqueous catholyte for a redox flow battery, comprising:

[0180] an organic solvent comprising an ether; and

[0181] an organic redoxmer dissolved in the solvent at a concentration of at least 0.5 M, the redoxmer comprising a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof.

[0182] Clause 29. The non-aqueous catholyte for a redox flow battery of claim 28. further comprising a supporting salt and magnesium ions.

[0183] Clause 30. The non-aqueous catholyte for a redox flow battery using the materials of any one or more of Clauses 2-14.

[0184] Reference was made to the examples illustrated in the drawings and specific language was used herein to describe tire same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Alterations and further modifications of the features illustrated herein and additional applications of the examples as illustrated herein are to be considered within the scope of the description.

[0185] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description, numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. It will be recognized, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.

[0186] Although the subject matter has been described in language specific to structural features and / or operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features and operations described above. Rather, the specific features and acts described above are disclosed as example formsof implementing the claims. Numerous modifications and alternative arrangements may be devised without departing from the spirit and scope of the described technology.

Claims

CLAIMSWhat is claimed is:

1. A redox flow battery, comprising:a catholyte reservoir containing a non-aqueous catholyte comprising an ether-based organic solvent and an organic redoxmer, wherein the redoxmer comprises a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof;a flow cell in fluid communication with the catholyte reservoir to circulate the catholyte through the flow cell;a separator in the flow cell in contact with the catholyte in the flow cell;a cathode in contact with the catholyte in the flow cell; andan anode separated from the catholyte by the separator.

2. The redox flow battery of claim 1, wherein the redoxmer is nonpolymeric.

3. The redox flow battery of claim 1, wherein the redoxmer comprises the amine group.

4. The redox flow battery of claim 3, wherein the nitrogen atom is bonded to both a phenyl group and an alkyl group.

5. The redox flow battery of claim 3, wherein the nitrogen atom is not a member of a ring.

6. The redox flow battery' of claim 1, wherein the redoxmer comprises the carbonyl group.

7. The redox flow battery of claim 1, wherein tire redoxmer is capable of being reversibly oxidized to a 2+ charge and reduced to a neutral charge.

8. Tire redox flow battery' of claim 1, wherein the redoxmer comprises a phenothiazine, a phenazine, an anthraquinone, a perylene diimide, a ferrocene derivative, a methyl viologen,a PEGylate, an azobenzene, an oxoverdazyl radical, a tetracyanoethylene, a nitronyl nitroxide radical, a N-butylphthalimide, or a combination thereof.

9. The redox flow battery of claim 1, wherein the redoxmer has a molecular structure according to any of structures (1) to (XIV):(CH2)XCH3(CH2)XCH3(CH2)XCH3(CH2)XCH3(CH2)YCH3(yy (CH2)XCH3(CH2)XCH3'■'N (CH2)2CH3(CH2)yCH3(VI); (CH2CH2O)XCH3 (CH2CH2O)XCH3(VII);(CH2CH2O)XCH3(CH2CH2O)XCH3(CH2CH2O)YCH3( V 111).(CH2CH2O)XCH3 / (CH2CH2O)XCH3(CH2CH2O)ZCH3(CH2CH2O)YCH3(CH2)YCH3I^(CH2)YCH3(CH2CH2O)XCH3(CH2CH2O)YCH3(CH2CH2O)ICH3(CH2CH2O)ZCH3(XI);(XIV): wherein X, Y, Z, I, and n are independently integers from 0 to 20.

10. The redox flow battery- of claim 1, wherein the redoxmer is dissolved in the solvent at a concentration greater than about 0.5 M.

11. The redox flow battery' of claim 1, wherein the organic solvent is substantially free of acetonitrile and propylene carbonate.

12. The redox flow battery' of claim 1. wherein the organic solvent comprises dimethoxyethane, bis(2-methoxyethyl) ether, tetraethylene glycol dimethy l ether, tetrahydrofuran (THF), or a combination thereof.

13. lire redox flow battery' of claim 1, wherein the catholyte further comprises a supporting salt.

14. Hie redox flow battery' of claim 1, wherein the catholyte further comprises magnesium ions.

15. The redox flow battery of claim 1, wherein the separator comprises a size exclusion membrane or an anion exchange membrane.

16. 'Hie redox flow battery of claim 1, wherein tire anode is a metal anode.

17. The redox flow battery' of claim 16, wherein the catholyte further comprises an ion of the metal of the metal anode.

18. The redox flow battery- of claim 16, wherein the anode comprises magnesium, sodium, or lithium.

19. The redox flow battery' of claim 16. wherein the anode is a magnesium anode.

20. The redox flow battery of claim 1, wherein the battery does not comprise a flowing ano hie.

21. The redox flow battery of claim 1, wherein a voltage between the anode and the cathode is from about 2.0 V to about 3.5 V during a discharge cycle of the battery'.

22. The redox flow battery' of claim 1. wherein the battery has a discharge capacity from about 75 mAh / g to about 150 mAh / g.

23. An electrochemical energy storage method, comprising:pro viding a catholyte reservoir containing a non-aqueous catholyte comprising an ether-based organic solvent and an organic redoxmer. wherein the redoxmer comprises a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, ora combination thereof;circulating the catholyte through a flow cell, wherein the catholyte in tire flow cell is in contact with a cathode and a separator, and wherein an anode is separated from the catholyte by' the separator; andoxidizing or reducing the redoxmer by applying an electric current between the anode and the cathode.

24. The method of claim 23, wherein the redoxmer is oxidized during a charging cycle.

25. The method of claim 24, wherein the redoxmer is oxidized to a charge of 2+ during the charging cycle.

26. Hie method of claim 24, wherein the anode is a metal anode, wherein the catholyte further comprises ions of the metal of the metal anode, and wherein the ions are reduced at the metal anode during the charging cycle.

27. The method of claim 23, wherein a voltage between the anode and the cathode is from about 2.0 V to about 3.5 V during adischarge cycle.

28. A non-aqueous catholyte for a redox flow battery, comprising:an organic solvent comprising an ether; andan organic redoxmer dissolved in the solvent at a concentration of at least 0.5 M, the redoxmer comprising a carbonyl group, an amine group, or a combination thereof, wherein the amine group comprises a nitrogen atom bonded to an alkyl group, an ether group, a phenyl group, or a combination thereof29. The non-aqueous catholyte for a redox flow' battery of claim 28. further comprising a supporting salt and magnesium ions.

Images