Regeneration of active species from decomposed species in batteries

Abstract

The invention provides batteries, e.g., flow batteries, and methods of cycling the batteries that reduce loss of capacity. The loss of capacity of may be mitigated by employing a mixture of redox active species.

Description

[0001] REGENERATION OF ACTIVE SPECIES FROM DECOMPOSED SPECIES IN BATTERIES

[0002] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0003] This invention was made with support under grant number DE-AC05-76RL01830 from the U.S. Department of Energy. The U.S. Government has certain rights to the invention.

[0004] BACKGROUND OF THE INVENTION

[0005] Aqueous organic and metalorganic redox flow batteries have shown promise for storing electricity generated from intermittent renewable sources. However, a significant challenge in commercializing these batteries lies in the decomposition of redox active species within the battery system. While structural modifications have been explored to improve stability, they often result in increased synthetic costs and molecular weight. Thus, there is a need for new methodologies to improve stability.

[0006] SUMMARY OF THE INVENTION

[0007] The invention features batteries, e.g., flow batteries, exhibiting reduced loss of capacity over time and methods related thereto.

[0008] In a first aspect, the invention provides a battery, e.g., a flow battery, including a negolyte in contact with a first electrode, a posolyte in contact with a second electrode, and a barrier separating the negolyte and the posolyte, wherein the negolyte and / or posolyte includes a mixture of at least two different redox active species, each of which undergoes electrochemical cycling during charge and discharge of the battery, and wherein a first of the redox active species undergoes decomposition by oxidation or reduction, forming a decomposed species, and a second of the redox active species facilitates the recovery of the first redox active species from its decomposed state. This can be achieved by either reversing the decomposition of the first redox active species or by facilitating electrochemical recovery during cell cycling by lowering the energy barrier of the recovery process.

[0009] In some embodiments, the first redox active species or the second redox active species includes an organic redox active species. In some embodiments, first organic redox active species or the second organic redox active species includes an anthraquinone, a viologen, a fluorenone, a naphthoquinone, a phenoxazine, a phenothiazine, a phenazine, or a reduced form thereof. In some embodiments, the anthraquinone is of formula (I) wherein each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa; -SRa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); - P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group.

[0010] In some embodiments, the first redox active species or the second redox active species includes an inorganic redox active species. In some embodiments, the first redox active species or the second redox active species includes a metalorganic redox active species. In some embodiments, the negolyte includes the mixture. In some embodiments, the posolyte includes the mixture.

[0011] In some embodiments, the first of the redox active species undergoes decomposition to the decomposed species by reduction and the second of the redox active species oxidizes the decomposed species to recover the first redox active species.

[0012] In some embodiments, the first of the redox active species undergoes decomposition to the decomposed species by oxidation and the second of the redox active species reduces the decomposed species to recover the first redox active species.

[0013] In a second aspect, the invention provides a method of regenerating a redox active species in a battery, the method including: a) providing a battery of the first aspect; and b) cycling the battery, wherein after charge or discharge the first of the redox active species decomposes by oxidation or reduction forming a decomposed species, and the second of the redox active species either reverses the oxidation or reduction process or lowers the energy barrier of the recovery process, thereby directly recovering the first redox active species or enabling its recovery during cell cycling, respectively.

[0014] In some embodiments, the first of the redox active species is dissolved in solution, and the decomposed species is removed from solution as a solid. In some embodiments, the first of the redox active species is dissolved in solution, and the decomposed species is dissolved in solution. In some embodiments, the first of the redox active species is a solid, and the decomposed species is a solid. In some embodiments, the first of the redox active species is a solid, and the decomposed species is dissolved in solution.

[0015] Definitions

[0016] By “alkyl” is meant straight chain or branched saturated groups from 1 to 6 carbons. Alkyl groups are exemplified by methyl, ethyl, n- and iso-propyl, n-, sec-, iso- and tert-butyl, neopentyl, and the like, and may be optionally substituted with one or more, substituents. By “alkoxy” is meant a group of formula -OR, wherein R is an alkyl group, as defined herein.

[0017] By “alkyl thio” is meant -S-R, where R is an alkyl group, as defined herein.

[0018] By “alkyl ester” is meant -COOR, where R is an alkyl group, as defined herein.

[0019] By “aryl” is meant an aromatic cyclic group in which the ring atoms are all carbon. Exemplary aryl groups include phenyl, naphthyl, and anthracenyl. Aryl groups may be optionally substituted with one or more substituents.

[0020] By “carbocyclyl” is meant a non-aromatic cyclic group in which the ring atoms are all carbon. Exemplary carbocyclyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. Carbocyclyl groups may be optionally substituted with one or more substituents.

[0021] By “halo” is meant, fluoro, chloro, bromo, or iodo.

[0022] By “hydroxyl” is meant -OH. An exemplary ion of hydroxyl is -O'.

[0023] By “amino” is meant -NH2. An exemplary ion of amino is -NHs+.

[0024] By “nitro” is meant -NO2.

[0025] By “carboxyl” is meant -COOH. An exemplary ion of carboxyl is -COO-.

[0026] By “phosphoryl” is meant -PO3H2. Exemplary ions of phosphoryl are -POsH- and -POs2-.

[0027] By “phosphonyl” is meant -PO3R2, wherein each R is H or alkyl, provided at least one R is alkyl, as defined herein. An exemplary ion of phosphoryl is -POsR-.

[0028] By “oxo” is meant =0.

[0029] By “sulfonyl” is meant -SO3H. An exemplary ion of sulfonyl is -SOs-.

[0030] By “thiol” is meant -SH.

[0031] By “heteroaryl” is meant an aromatic cyclic group in which the ring atoms include at least one carbon and at least one O, N, or S atom, provided that at least three ring atoms are present. Exemplary heteroaryl groups include oxazolyl, isoxazolyl, tetrazolyl, pyridyl, thienyl, furyl, pyrrolyl, imidazolyl, pyrimidinyl, thiazolyl, indolyl, quinolinyl, isoquinolinyl, benzofuryl, benzothienyl, pyrazolyl, pyrazinyl, pyridazinyl, isothiazolyl, benzimidazolyl, benzothiazolyl, benzoxazolyl, oxadiazolyl, thiadiazolyl, and triazolyl. Heteroaryl groups may be optionally substituted with one or more substituents.

[0032] By “heterocyclyl” is meant a non-aromatic cyclic group in which the ring atoms include at least one carbon and at least one O, N, or S atom, provided that at least three ring atoms are present. Exemplary heterocyclyl groups include epoxide, thiiranyl, aziridinyl, azetidinyl, thietanyl, dioxetanyl, morpholinyl, thiomorpholinyl, piperazinyl, piperidinyl, pyrrolidinyl, tetrahydropyranyl, tetra hydrofuranyl, di hydrofuranyl, tetrahydrothienyl, dihydrothienyl, dihydroindolyl, tetrahydroquinolyl, tetrahydroisoquinolyl, pyranyl, pyrazolinyl, pyrazolidinyl, dihydropyranyl, tetrahydroquinolyl, imidazolinyl, imidazolidinyl, pyrrolinyl, oxazolidinyl, isoxazolidinyl, thiazolidinyl, isothiazolidinyl, dithiazolyl, and 1 ,3-dioxanyl. Heterocyclyl groups may be optionally substituted with one or more substituents. By an “oxygen protecting group” is meant those groups intended to protect an oxygen containing (e.g., phenol, hydroxyl, or carbonyl) group against undesirable reactions during synthetic procedures. Commonly used oxygen protecting groups are disclosed in Greene, “Protective Groups in Organic Synthesis,” 3rd Edition (John Wiley & Sons, New York, 1999), which is incorporated herein by reference. Exemplary oxygen protecting groups include acyl, aryloyl, or carbamyl groups, such as formyl, acetyl, propionyl, pivaloyl, t-butylacetyl, 2-chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl, o- nitrophenoxyacetyl, a-chlorobutyryl, benzoyl, 4-chlorobenzoyl, 4-bromobenzoyl, t-butyldimethy Isilyl, tri- iso-propylsilyloxymethyl, 4,4'-dimethoxytrityl, isobutyryl, phenoxyacetyl, 4-isopropylpehenoxyacetyl, dimethylformamidino, and 4-nitrobenzoyl; alkylcarbonyl groups, such as acyl, acetyl, propionyl, and pivaloyl; optionally substituted arylcarbonyl groups, such as benzoyl; silyl groups, such as trimethylsilyl (TMS), tert-butyldimethy Isilyl (TBDMS), tri-iso-propylsilyloxy methyl (TOM), and triisopropylsilyl (TIPS); ether-forming groups with the hydroxyl, such methyl, methoxymethyl, tetrahydropyranyl, benzyl, p- methoxybenzyl, and trityl; alkoxycarbonyls, such as methoxycarbonyl, ethoxycarbonyl, isopropoxycarbonyl, n-isopropoxycarbonyl, n-butyloxycarbonyl, isobutyloxycarbonyl, secbutyloxycarbonyl, t-butyloxycarbonyl, 2-ethylhexyloxycarbonyl, cyclohexyloxycarbonyl, and methyloxycarbonyl; alkoxyalkoxycarbonyl groups, such as methoxymethoxycarbonyl, ethoxymethoxycarbonyl, 2-methoxyethoxycarbonyl, 2-ethoxyethoxycarbonyl, 2-butoxyethoxycarbonyl, 2- methoxyethoxymethoxycarbonyl, allyloxycarbonyl, propargyloxycarbonyl, 2-butenoxycarbonyl, and 3- methyl-2-butenoxycarbonyl; haloalkoxycarbonyls, such as 2-chloroethoxycarbonyl, 2- chloroethoxycarbonyl, and 2,2,2-trichloroethoxycarbonyl; optionally substituted arylalkoxycarbonyl groups, such as benzyloxycarbonyl, p-methylbenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p- nitrobenzyloxycarbonyl, 2,4-dinitrobenzyloxycarbonyl, 3,5-dimethylbenzyloxycarbonyl, p- chlorobenzyloxycarbonyl, p-bromobenzyloxy-carbonyl, and fluorenylmethyloxycarbonyl; and optionally substituted aryloxycarbonyl groups, such as phenoxycarbonyl, p-nitrophenoxycarbonyl, o- nitrophenoxycarbonyl, 2,4-dinitrophenoxycarbonyl, p-methyl-phenoxycarbonyl, m- methylphenoxycarbonyl, o-bromophenoxycarbonyl, 3,5-dimethylphenoxycarbonyl, p- chlorophenoxycarbonyl, and 2-chloro-4-nitrophenoxy-carbonyl); substituted alkyl, aryl, and alkaryl ethers (e.g., trityl; methylthiomethyl; methoxymethyl; benzyloxymethyl; siloxymethyl; 2,2,2,- trichloroethoxymethyl; tetrahydropyranyl; tetra hydrofuranyl; ethoxyethyl; 1 -[2-(trimethylsilyl)ethoxy]ethyl; 2-trimethylsilylethyl; t-butyl ether; p-chlorophenyl, p-methoxyphenyl, p-nitrophenyl, benzyl, p- methoxybenzyl, and nitrobenzyl); silyl ethers (e.g., trimethylsilyl; triethylsilyl; triisopropylsilyl; dimethylisopropylsilyl; t-butyldimethy Isilyl ; t-butyldipheny Isilyl; tribenzylsilyl; triphenylsilyl; and diphenymethylsilyl); carbonates (e.g., methyl, methoxy methyl, 9-fluorenylmethyl; ethyl; 2,2,2- trichloroethy I; 2-(trimethy Isily l)ethy I; vinyl, allyl, nitrophenyl; benzyl; methoxybenzyl; 3,4-dimethoxybenzyl; and nitrobenzyl); carbonyl-protecting groups (e.g., acetal and ketal groups, such as dimethyl acetal, and 1 ,3-dioxolane; acylal groups; and dithiane groups, such as 1 ,3-dithianes, and 1 ,3-dithiolane); carboxylic acid-protecting groups (e.g., ester groups, such as methyl ester, benzyl ester, t-butyl ester, and orthoesters; and oxazoline groups.

[0033] By a “nitrogen protecting group” is meant those groups intended to protect an amino group against undesirable reactions during synthetic procedures. Commonly used nitrogen protecting groups are disclosed in Greene, “Protective Groups in Organic Synthesis,” 3rdEdition (John Wiley & Sons, New York, 1999), which is incorporated herein by reference. Nitrogen protecting groups include acyl, aryloyl, or carbamyl groups such as formyl, acetyl, propionyl, pivaloyl, t-butylacetyl, 2-chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl, o-nitrophenoxyacetyl, a-chlorobutyryl, benzoyl, 4-chlorobenzoyl, 4-bromobenzoyl, 4-nitrobenzoyl, and amino acids such as alanine, leucine, and phenylalanine; sulfonyl- containing groups such as benzenesulfonyl, and p-toluenesulfonyl; carbamate forming groups such as benzyloxycarbonyl, p-chlorobenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p-nitrobenzyloxycarbonyl, 2-nitrobenzyloxycarbonyl, p-bromobenzyloxycarbonyl, 3,4-dimethoxybenzyloxycarbonyl, 3,5-dimethoxybenzyloxycarbonyl, 2,4-dimethoxybenzyloxycarbonyl, 4-methoxybenzyloxycarbonyl, 2-nitro-4,5-dimethoxybenzyloxycarbonyl, 3,4,5-trimethoxybenzyloxycarbonyl, 1 -(p-bi ph e ny ly I)- 1 - methylethoxycarbonyl, a,a-dimethyl-3,5-dimethoxybenzyloxycarbonyl, benzhydryloxy carbonyl, t-butyloxycarbonyl, diisopropylmethoxycarbonyl, isopropyloxycarbonyl, ethoxycarbonyl, methoxycarbonyl, allyloxycarbonyl, 2, 2, 2, -trichloroethoxycarbonyl, phenoxycarbonyl, 4-nitrophenoxy carbonyl, fluorenyl-9- methoxycarbonyl, cyclopentyloxycarbonyl, adamantyloxycarbonyl, cyclohexyloxycarbonyl, and phenylthiocarbonyl, alkaryl groups such as benzyl, triphenylmethyl, and benzyloxymethyl, and silyl groups, such as trimethylsilyl. Preferred nitrogen protecting groups are alloc, formyl, acetyl, benzoyl, pivaloyl, t-butylacetyl, alanyl, phenylsulfonyl, benzyl, t-butyloxycarbonyl (Boc), and benzyloxycarbonyl (Cbz).

[0034] As noted, substituents may be optionally substituted with halo, optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; - S(=O)2Ra; -S(=O)2ORa; -P(=O)Ra2; -O-P(=O)(ORa)2, or -P(=O)(ORa)2, or an ion thereof; wherein each Rais independently H, C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group. Cyclic substituents may also be substituted with C1-6 alkyl. In specific embodiments of alloxazines, substituents may include optionally substituted with halo, optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -NO2; -ORa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; -S(=O)2Ra; -S(=O)2ORa; - P(=O)Ra2; -O-P(=O)(ORa)2, or -P(=O)(ORa)2, or an ion thereof; wherein each Rais independently H, C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group, and cyclic substituents may also be substituted with C1-6 alkyl. In specific embodiments of quinones, alkyl groups may be optionally substituted with one, two, three, or, in the case of alkyl groups of two carbons or more, four substituents independently selected from the group consisting of halo, hydroxyl, C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, thiol, C1-6 alkyl ester, optionally substituted C1-6 alkyl thio, and oxo, or an ion thereof. Exemplary ions of substituent groups are as follows: an exemplary ion of hydroxyl is -O-; an exemplary ion of -COOH is -COO-; exemplary ions of -PO3H2 are -POsH- and -POs2-; an exemplary ion of -POsHRa is — POsRa-, where Rais not H; exemplary ions of -PO H2 are -PO4I+ and -PO42-; and an exemplary ion of — SO3H is — SO3-.

[0035] By “organic redox active species” is meant a species capable of undergoing reversible electrochemical oxidation and reduction and including at least one bond between carbon and an element other than carbon and oxygen, e.g., a carbon-hydrogen bond, and not including a metal atom.

[0036] By “inorganic redox active species” is meant a species capable of undergoing reversible electrochemical oxidation and reduction and not including at least one bond between carbon and an element other than carbon and oxygen, e.g., a carbon-hydrogen bond.

[0037] By “metalorganic redox active species” is meant a species capable of undergoing reversible electrochemical oxidation and reduction and including at least one bond between carbon and an element other than carbon and oxygen, e.g., a carbon-hydrogen bond, and a metal atom.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIGs. 1A-1 B show the long-term cell cycling of a battery operated with a constant current (40 mA cm-2) followed by constant potentials of 1 .4 V (charging) and 0.5 V (discharging) during 12 days of operation at room temperature. The cell includes a mixture of 0.1 M 2,6-N-TSAQ (3,3’,3”,3”’-((9,10-anthraquinone-2,6- diyl)bis(azanetriyl))tetrakis(propane-1 -sulfonate)) + 0.1 M 2,6-DHAQ (2,6-dihydroxy-anthraquinone) in 1 M KOH in the negolyte, paired with 50 mL of 0.2 M ferrocyanide / 0.06 M ferricyanide in 1 M KOH in the posolyte, separated by a Nation 212 cation exchange membrane. The cell cycling tests were conducted in a N2-filled glove box. The capacity is presented in units of ampere-hours per liter of negolyte. FIG. 1A shows the discharge capacity and coulombic efficiency of the cell. FIG. 1 B shows a fit to the discharge capacity curve.

[0040] FIGs. 2A-2B show the long-term cell cycling of a battery operated with a constant current (100 mA cm-2) followed by constant potentials of 1 .4 V (charging) and 0.6 V (discharging) during 5 days of operation at room temperature. The cell includes a mixture of 0.1 M 2,6-DBEAQ+ 0.1 M 2,6-DHAQ in 1 M KOH in the negolyte, paired with 50 mL of 0.2 M ferrocyanide / 0.06 M ferricyanide in 1 M KOH in the posolyte, separated by a Nation 212 cation exchange membrane. The cell cycling tests are conducted in a N2-filled glove box. The capacity is presented in units of ampere-hours per liter of negolyte. FIG. 2A shows the discharge capacity and coulombic efficiency of the cell. FIG. 2B shows a fit to the discharge capacity curve.

[0041] FIGs. 3A-3B illustrate schematics of a mixed electrolyte recovery system using DHAQ as an example. In this system, the first redox active species is 2,6-DHAQ, while the second redox active species can be any molecule with a higher redox potential than the first. FIG. 3A details the decomposition and regeneration mechanism of 2,6-DHAQ. During normal cell cycling, DHAQ can be over-reduced to form dihydroxyanthrone (DHA), which can further oxidize into a dimer, leading to capacity loss and otherwise irreversible decomposition. Adding a second redox active molecule to this system can facilitate recovery. FIG. 3B shows a detailed recovery mechanism, indicating two pathways: (1) chemical recovery (dashed lines), where DHA is directly oxidized back to DHAQ through a chemical reaction, and (2) lowering the energy barrier for recovery (solid lines). In this second pathway, DHA turns into an intermediate state, the dimer, which enables its recovery to DHAQ during the discharge stage.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] Batteries, e.g., flow batteries, have emerged as promising systems for energy storage from intermittent renewable sources. The lifetime of these batteries is limited by electrolyte stability. Under ideal conditions, discharging a battery involves the reversible oxidation and concurrent reduction of the low potential (negolyte) and high potential (posolyte) active species, respectively. However, many negolytes and posolytes are subject to various degradation mechanisms, which lead to capacity loss.

[0044] The present invention employs mixed redox active species to address the issue of regenerating a redox active species (e.g., organic redox active species, metalorganic redox active species, or inorganic redox active species) from a decomposed species (e.g., a species which does not participate in the capacity of a battery) in a battery, e.g., a flow battery. The method does not cause capacity imbalance and does not involve electrochemical regeneration at voltages outside the normal cycling range. The method can be applied to any mixture of redox active species, as long as the decomposition product of a first redox active species can be retrieved through the oxidation or reduction of a second redox active species in the mixture (e.g., via the second redox active species acting as an oxidizing agent or reducing agent to the first redox active species).

[0045] In this method, the battery includes a mixture of multiple redox active species (>2, e.g., 2, 3, 4, 5, 6, 7, 8, 9, or 10) in the negolyte, posolyte, or both sides of the battery. For example, in the specific scenario involving two redox active species in the negolyte: ref s ref

[0046] If f ™ is further reduced to a new redox active compound, for example, by gaining n electrons, we have: Volts versus ref which results in capacity loss. However, in presence of compound A with E?> the active species can be recovered from the decomposed species by either of the following two pathways: (1) homogenous chemical reaction between A and B:

[0047] Note that in the above example the decomposed compound (Bm-n) needed to be oxidized to regenerate the redox active species (Bm); hence the condition E > (i.e., Am+nacts as an oxidizing agent to Bm n). In the case where reduction of the decomposed species is needed, the condition would be E < 0e->aredox form of compound A acting as a reducing agent to a redox form of compound B). or (2) lowering the energy barrier for recovery by converting a decomposed molecule to an intermediate state for subsequent electrochemical recovery.

[0048] During this pathway, the decomposed species can be converted to an intermediate state by a chemical reaction between A and B:

[0049] Bintermediate can be recovered to redox active species (Bm) during cell cycling.

[0050] The mixture can include organic, inorganic, and / or metalorganic species.

[0051] Additionally, the redox active reaction of one or more of the species in the mixture may involve deposition and / or dissolution reactions. For example, the methods of the present invention can be used to recover the first redox active species from a decomposed species, wherein the first redox active species is dissolved in solution and the decomposed species is dissolved in the solution. Alternative, the methods can be used to recover the first redox active species from a decomposed species, wherein the first redox active species is a solid, and the decomposed species is dissolved in solution. Alternatively, the methods of the present invention can be used to recover a first redox active species from a decomposed species, wherein the first redox active species is dissolved in solution and the decomposed species is a solid. Alternatively, the methods of the present invention can be used to recover a first redox active species from a decomposed species, wherein the first redox active species is a solid phase, and the decomposed species is a solid phase. Examples of solid phases include deposition on a surface of the battery or as a precipitant.

[0052] Notably, the number of electrons transferred between the decomposed species and the second redox active species (e.g., acting as an oxidizing agent or reducing agent to the decomposed species) during the recovery of the first redox active species from the decomposed species does not need to equal the number of electrons transferred during ordinary use, e.g., battery cycling. By employing this method, the battery system can efficiently recover the first redox active species of interest without causing an imbalance or requiring additional electrical energy, as it avoids the need to oxidize or reduce at voltages outside the normal cycling range. Additionally, preparing a mixture of redox active species can bring additional advantages such as higher volumetric energy density.

[0053] In this method the second redox active species (e.g., the oxidizing agent or reducing agent) is not added to the system after the decomposition compound is formed; rather the oxidizing / reducing agent is dissolved in the electrolyte from the beginning and is participating in the charge and discharge of the battery during operation. The second redox active species is not a sacrificial component and is not used as an additive in the mixture; rather it is an active participant in the charge / discharge process. The distinction is important because, with mixed redox species, the oxidizing or reducing agent remains available because the electrode will oxidize or reduce it back to the required oxidation or reduction state during the charging or discharging process. For example, in the reactions shown above, compound Am+n(i.e., an oxidized form of a compound A) is needed for oxidation of the decomposed compound Bm n. This reaction results in Am(i.e., a reduced form of A, having been reduced by n electrons relative to Am+n). Given that A is an active species participating in the charge and the discharge process, the following charge step in the battery will result in oxidation of Amback to Am+n, which is again capable of recovering Bmfrom its decomposed compound Bm n. In contrast, when utilizing a sacrificial reducing / oxidizing species, each conversion diminishes the concentration of the oxidizing / reducing agent without its subsequent return to the initial state.

[0054] This method can effectively address several scenarios, such as the case of metal complexes used in the electrolyte of a battery, e.g., flow battery. For example, in the tris(4,4'-bis(hydroxymethyl)-2,2'-bipyridine) iron complex [1] and tris(2,2'-bipyridine) iron complex [2], it has been observed that a redox active dimer forms during battery operation. However, this dimer has low solubility, leading to its accumulation and subsequent precipitation over time. To prolong the battery's lifespan and enable operation at high concentrations, a viable approach is to provide a mixed redox active electrolyte with an additional compound capable of oxidizing the dimer. By introducing this compound, the accumulation of the undesired dimer can be prevented or minimized.

[0055] Moreover, this method is applicable in cases where redox active species undergo multiple electron transfers with separated redox potentials, and the avoidance of specific electron transfer events is necessary. Consider viologens as an example, where it has been observed that the reduced form after a single electron transfer exhibits good stability, while the compound formed after the second electron transfer is unstable [3]. In situations where the potential separation between these two redox events is not significantly large, effectively preventing the undesired second electron transfer during battery operation becomes challenging. In such cases, a mixture of redox active species can be designed to selectively recover the desirable first compound from the undesirable second compound. This approach ensures control over the desired electron transfer events and enhances the stability and performance of the system.

[0056] Previously, the recovery of the anthrone of anthraquinone-based redox flow batteries was achieved by an electrochemical regeneration method to address the decomposition issue [4]. Alternatively, oxygen exposure or aeration [5] or chemical treatment can be used to recover the decomposed compound. While the electrochemical regeneration method effectively recovers most of the decomposed compound, it has not been shown to recover 100%, and it necessitates an electrochemical energy input every few cycles. In contrast, the present invention offers reduced energy intensity and complexity. Some other methods of recovery, e.g., re-oxidation by oxygen exposure or a chemical treatment with an oxidative agent, would cause an imbalance in the battery.

[0057] In this regard, the present invention proposes a novel method for regenerating the first redox species from the decomposed species, such as anthrone. For anthrones, the proposed method involves introducing a redox active species with a less negative redox potential than the decomposed anthrone as an additional, actively participating redox active species into the battery system. During the charging or discharging process, the redox active species with the lower potential undergoes a chemical reduction. Simultaneously, the anthrone can be oxidized either into anthraquinone or into a dimer. In the first case, this results in its regeneration to anthraquinone in the desired reduced or oxidized state. In the second case, where oxidation leads to the formation of a dimer, this dimer can be regenerated into anthraquinone during cell cycling by utilizing additional electrochemical energy from the electrode. In other systems, either of the two pathways may recover the decomposition product through introducing a redox active species with a more negative redox potential than the decomposed material as an additional actively participating redox active species into the battery system.

[0058] By utilizing a mixed redox active species, this method offers a potentially less energy-intensive alternative to electrochemical regeneration. Additionally, it does not cause an imbalance in the battery associated with oxygen aeration or chemical treatment. Furthermore, considering that the oxidizing agent itself functions as an active redox species within the battery, the concentration of the oxidized form is recovered as the electrode oxidizes its reduced form — a distinction from scenarios involving sacrificial electroactive species. Additionally, the use of a mixed redox active species may enhance the overall volumetric energy density of a battery, e.g., flow battery.

[0059] Batteries

[0060] Batteries of the invention, e.g., flow batteries, include a negolyte, a posolyte, and a barrier separating the two. The battery further includes at least two electrodes, one in contract with the negolyte and one in contact with the posolyte. At least one of the negolyte and posolyte includes a mixture of redox active species, e.g., that are not tautomers, positional isomers, or redox products of one another.

[0061] Batteries described herein may also include at least one electrocatalyst, e.g., graphene, carbon nanotubes, carbon nanoparticles, metal nanoparticles, or metal oxide nanoparticles in contact with the negolyte. The battery may include a source of hydronium or hydroxide ions, e.g., an acid or base, to, e.g., control the pH of the negolyte.

[0062] A battery of the invention may include additional components as is known in the art. Negolytes and posolytes may be housed in a suitable reservoir. A battery may further include one or more pumps to pump aqueous solutions or suspensions past one or both electrodes. Alternatively, the electrodes may be placed in a reservoir that is stirred or in which the solution or suspension is recirculated by any other method, e.g., convection, sonication, etc. Batteries may also include graphite flow plates and corrosionresistant metal current collectors.

[0063] The balance of the system around the cell includes fluid handling and storage, and voltage and round-trip energy efficiency measurements can be made. Systems configured for measurement of negolyte and posolyte flows and pH, pressure, temperature, current density and cell voltage may be included and used to evaluate cells. Fluid sample ports can be provided to permit sampling of both electrolytes, which will allow for the evaluation of parasitic losses due to reactant crossover or side reactions. Electrolytes can be sampled and analyzed with standard techniques. Suitable cells, electrodes, barriers / membranes, and pumps for batteries, e.g., flow batteries, are known in the art, e.g., WO 2014 / 052682, WO 2015 / 048550, WO 2016 / 144909, and WO 2020 / 072406, the battery components of which are hereby incorporated by reference.

[0064] Negolyte Redox active Species

[0065] A negolyte of the present invention may include any suitable redox active species. Suitable redox active species include organic redox active species (e.g., anthraquinones, phenazines, phenoxazines, diquaternized bipyridines, naphthoquinones, fluorenones, and redox states thereof), inorganic redox active species such as vanadium, e.g., V2+, V3+, VO2+, and VO2+, and metalorganic redox active species such as ferrocene / ferrocenium or ferricyanide / ferrocyanide. In some embodiments, the negolyte may be a mixture of redox active species (e.g., more than one organic redox active species, more than one inorganic redox active species, more than one metalorganic redox active species, or combinations thereof).

[0066] Suitable organic redox active species includes an anthraquinone or a redox state thereof. In some embodiments, the first state of the anthraquinone is of formula (la). (la), or a salt, protonated form, or tautomer thereof.

[0067] In some embodiments, the second state of the anthraquinone is an anthrahydroquinone, e.g., an anthrahydroquinone of formula (lb) salt, protonated form, or tautomer thereof.

[0068] In some embodiments, the third state of the anthraquinone is an anthrahydroquinone, e.g., an anthrahydroquinone of formula (Ic) salt, protonated form, or tautomer thereof.

[0069] In any of formulas (la), (lb), or (lc), each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group. An anthraquinone of the invention is a source of electrons during discharge and not merely a charge transfer agent. In embodiments, the anthraquinone is water soluble.

[0070] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H.

[0071] In other embodiments, the anthraquinone, such as a 9,10-anthraquinone, is substituted with at least one hydroxyl group and optionally further substituted with a C1-4 alkyl, such as methyl. Exemplary quinones include 2,6-dihydroxy-9,10-anthraquinone (2,6-DHAQ), 1 ,5-dimethyl-2,6-dihydroxy-9,10-anthraquinone, 2,3,6,7-tetrahydroxy-9,10-anthraquinone, 1 ,3,5,7-tetrahydroxy-2,4,6,8-tetramethyl-9,10-anthraquinone, and 2,7-dihydroxy-1 ,8-dimethyl-9,10-anthraquinone. Ions and reduced species thereof are also contemplated.

[0072] Other organic species amenable to use in batteries of the invention include, but are not limited to, naphthoquinones (e.g., hydronaphthoquinones), reduced forms of phenazines (e.g., the reduced form of 7,8-dihydroxyphenazine-2-sulfonic acid), reduced monoquaternized or N,N'-diquaternized phenazines, reduced phenoxazines, reduced phenothiazines, reduced fluorenones, or reduced forms of diquaternized bipyridines (e.g., alkyl viologen radical monocations). Exemplary phenazines, N,N'-disubstituted phenazines, monoquaternized phenazines, or N,N'- diquaternized phenazines are, e.g., of formula (II): where X and Y are both N, or where X is NRx and Y is N, or where X is NRXand Y is NRY; where Rxand RYare independently selected from H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; or a nitrogen protecting group; where each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group.

[0073] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H. In some embodiments, at least one of Ri-Ra is a substituted alky or substituted alkoxy. Exemplary phenazines include, e.g., 7,8-dihydroxyphenazine-2-sulfonic acid. Ions and reduced species thereof are also contemplated.

[0074] Exemplary phenoxazines and phenothiazines are of, e.g., formula (III): reduced form thereof, or a salt thereof, where dashed bonds are single or double bonds; where X is N or NRX, Y is O or S, and Z is CR6, C=O, C=S, C=NRZ, or C=NH+Rz; where Rxis selected from H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; or a nitrogen protecting group, where Rzis selected from H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; or a nitrogen protecting group, where each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group.

[0075] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H. In some embodiments, at least one of R1-R8is a substituted alky or substituted alkoxy.

[0076] Exemplary reduced diquaternized bipyridines are of, e.g., formula (IV):

[0077] Yl-Xl~N\\ \ / / N+~X2“Y2

[0078] ' - ' ' - ' (IV), a reduced form thereof (e.g., singly reduced radical monocations or doubly reduced 4,4'-bipyridinylidenes), or a salt thereof, where Xi and X2 are independently optionally substituted C1-20 hydrocarbyl (e.g., C1-10 alkylene) or heteroalkylene, and Y1 and Y2 are independently an optionally substituted water solubilizing group, e.g., a quaternary ammonium (e.g., trimethyl ammonium), ammonium, nitrogen-containing heterocyclyl, sulfonate, or sulfate. In certain embodiments, Xi and X2 are independently C1-10 alkylene, e.g., C3-6 alkylene. Exemplary groups for Y1 and Y2 are quaternary ammonium independently substituted with three C1-6 hydrocarbyl groups, e.g., trimethyl ammonium. An exemplary diquaternized bipyridine is

[0079] In particular embodiments, the water-solubilizing group is charged at a pH between 6-8. Further embodiments of diquaternized bipyridines may have the above formula, except that the two pyridines are linked 2-2’ instead of 4-4’. Ions and reduced species thereof are also contemplated.

[0080] In some embodiments, the negolyte includes an organic species that is a naphthoquinone. Exemplary naphthoquinones are of e.g., formula (V): reduced form thereof (e.g., a naphthohydroquinone), or a salt thereof. wherein the dashed bonds are single or double bonds; where either W and X, W and Z, or Z and Y are C=O, and where the two of W, X, Y, or Z that are not C=O are independently selected from C-R, where R is H; halo; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); - P(=O)Ra2; and -P(=0)(0Ra)2 (e.g., phosphonyl or phosphoryl); or any two adjacent R groups are joined to form an optionally substituted non-aromatic 3-6 membered ring, or an ion thereof, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group; where each of R1, R2, R3, and R4is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2(e.g., amino); - C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2(e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group. In certain embodiments, W and Z are C=O.

[0081] In certain embodiments, each of R1, R2, R3, and R4is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, and R4is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, and R4being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, and R4is not H. In some embodiments, at least one of R1-R4is a substituted alky or substituted alkoxy. Ions and reduced species thereof are also contemplated.

[0082] Exemplary fluorenones are of formula (VI): , reduced forms thereof, and salts thereof. where each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -0Ra(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group. In embodiments, the fluorenone is water soluble.

[0083] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H.

[0084] In some embodiments, the mixture does not include DBEAQ (4,4'-([9,10-anthraquinone-2,6-diyl]dioxy)di- butyric acid), DPPEAQ ([9,10-dioxo-9,10-dihydroanthracene-2,6-diyl]bis[oxy]bis[propane-3,1- diyl])bis(phosphonic acid)), DPivOHAQ (3,3'-(9,10-anthraquinone-diyl)bis(3-methyl- butanoic acid)), DBAQ (4,4'-(9,10-anthraquinone-diyl)dibutanoic acid), DPAQ (anthraquinone-2,6-dipropionic acid), a benzoquinone, or a naphthoquinone.

[0085] Posolyte Redox active Species

[0086] A posolyte of the present invention may include any suitable redox active species. Examples of redox active species for the posolyte include halogen redox active species (e.g., bromine, chlorine, iodine), molecular oxygen, organic redox active species (e.g., benzoquinone), and inorganic redox active species or metalorganic redox active species (e.g., vanadium (e.g., VOSO4), chromium (e.g., a Cr(bipyridine)3 salt), cobalt (e.g., a Co(bipyridine) salt), iron (e.g., ferricyanide / ferrocyanide or a ferrocene derivative, e.g., as described in WO 2018 / 032003, Fe(CIO )2, etc.), aluminum (e.g., aluminum(lll) biscitrate monocatecholate), manganese, cobalt, nickel, copper, lead, or an oxide thereof (e.g., a manganese oxide, a vanadium oxide, a cobalt oxide, or a lead oxide). Other redox active species suitable for use in batteries of the invention are described in WO 2014 / 052682, WO 2015 / 048550, WO 2016 / 144909, and WO 2020 / 072406, the redox active species of which are incorporated by reference. The posolyte redox active species may be dissolved or suspended in solution (such as aqueous solution), be in the solid state, or be gaseous, e.g., molecular oxygen in air.

[0087] Electrolyte Solution

[0088] In some embodiments, the electrolytes are both present in a solution (e.g., an aqueous solution), where the negolyte and posolyte are dissolved or suspended in the aqueous solution. In addition, an electrolyte may include other solutes, e.g., acids (e.g., HCI) or bases (e.g., LiOH, NH4OH, NaOH, or KOH) or alcohols (e.g., methyl, ethyl, or propyl) and other co-solvents to increase the solubility of a particular species, e.g., quinone / hydroquinone. Counter ions, such as cations, e.g., NH4+, Li+, Na+, K+, or a mixture thereof, may also be present. In certain embodiments, the pH of the electrolyte may be >7, e.g., at least 8, 9, 10, 11 , 12, 13, or 14, 8-14, 9-14, 10-14, 11-14, 12-14, 13-14, or about 14. The electrolyte may or may not be buffered to maintain a specified pH. In some embodiments, the solution is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass.

[0089] The concentration of the redox active species may be any suitable amount. Ranges include, for example, from 0.1 M to liquid species, e.g., 0.1 -15 M. Redox active species in mixtures may be present in molar ratios between 1 :10 and 10:1 , e.g., 40:60 to 60:40, 45:55 to 55:45, or equimolar. In addition to water, solutions or suspensions may include alcohols (e.g., methyl, ethyl, or propyl) and other co-solvents to increase the solubility of a particular species. In some embodiments, the solution or suspension is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass. Alcohol or other co-solvents may be present in an amount required to result in a particular concentration of species. The pH of the aqueous solution or suspension may also be adjusted by addition of acid or base, e.g., to aid in solubilizing a species.

[0090] Methods

[0091] The invention provides methods for reducing the loss of capacity in a battery, e.g., a flow battery. In the methods of the present invention, the negolyte and / or posolyte includes a mixture of redox active species. The first redox active species undergoes a decomposition by oxidization or reduction during cycling of the battery, and a second redox active species has a redox potential sufficient to reverse the decomposition, e.g., in a spontaneous redox reaction. The decomposition may occur in either or both of the posolyte or negolyte and may occur after charge or discharge of the battery.

[0092] The methods of the invention may be employed to reduce loss of capacity as a function of time (independent of the number of cycles). In some embodiments, the methods reduce the loss of capacity to a rate of less than 5% per day, e.g., less than 4, 3, 2, 1 , 0.5, 0.1 , 0.05, or 0.001 . For example, the loss of capacity may be between 0.0001 -5% per day, e.g., 0.0001-1 %, 0.0001 -0.1 %, 0.0001-0.05%, 0.001-1 %, 0.001-0.1 %, 0.001-0.05%, 0.01 -1 %, 0.01-0.5%, or 0.01-0.1 %. The methods may be practiced for a period of at least one week, one month, six months, or one year. The method may be applied to any organic, inorganic, or metalorganic redox active species, such as an anthraquinone as described herein. In embodiments of the methods described herein, the battery is cycled at least 100 times.

[0093] EXAMPLES

[0094] The invention will be further described by the following non-limiting example.

[0095] We operated a flow battery with 0.1 M 2,6-N-TSAQ (anthraquinone) + 0.1 M 2,6-DHAQ in 1 M KOH paired with 0.2 M ferrocyanide / 0.06 M ferricyanide in 1 M KOH for over 12 days with constant-current followed by constant voltage (CCCV) protocol at 40 mA cm-2with voltage cut-off values of 1 .4 V and 0.6 V. 2,6-N-TSAQ and 2,6-DHAQ are reported to separately demonstrate fade rates of 0.025% / day [6] and 4-8% / day [7], respectively. As such, in a one-to-one mixture of these two compounds and in the absence of any interaction between them, a fade rate between 2.0% / day and 4% / day was expected to be observed. As shown in FIGs. 1A-1B, an overall fade rate of 1 .4% / day was found from the operation of the mixed solution of 2,6-N-TSAQ and 2,6-DHAQ, which was lower than the expected fade rate of a noninteracting mixture of the two compounds. This result was consistent with the expectation that mixing these redox active molecules can result in the oxidation of the anthrone of 2,6-DHAQ back to its reduced form via the reduction of 2,6-N-TSAQ.

[0096] The potential of 2,6-N-TSAQ is -0.63 V versus SHE. By using a compound with less negative redox potential, we expected to see larger improvements in the stability. Consequently, we operated a flow battery with 0.1 M 4,4'-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate 2,6-DBEAQ + 0.1 M 2,6-DHAQ in 1 M KOH paired with 0.2 M ferrocyanide / 0.06 M ferricyanide in 1 M KOH for over 5 days with CCCV protocol at 100 mA cm-2with voltage cut-off values of 1 .4 V and 0.6 V. 2,6-DBEAQ and 2,6-DHAQ are reported to separately demonstrate fade rates of 0.027% / day [8] and 4-8% / day [7], respectively. As such, in a one-to-one mixture of these two compounds and in the absence of any interaction between them, a fade rate between 2.0% / day and 4% / day is expected to be observed. As shown in FIGs. 2A-2B, an overall fade rate of 0.9% / day was found from the operation of the mixed solution of 2,6-DBEAQ and 2,6- DHAQ, which was lower than the predicated fade rate of the two compounds if they are non-interacting and was lower than the case of 0.1 M 2,6-N-TSAQ + 0.1 M 2,6-DHAQ, further demonstrating the applicability of mixed redox active species in recovering anthrone and enhancing the stability of the battery.

[0097] Additionally, detailed mechanisms for the recovery system were suggested, as shown in FIGs 3A-B. 2,6- DBEAQ was utilized as a representative ‘second redox active compound,’ referred to as “A” in the figure, to demonstrate this system. Two mechanisms were proposed and validated, as shown in FIG 3B. The first route is a chemical recovery route (dashed lines). After chemically mixing 2,6-DBEAQ and DHA, we detected some amount of 2,6-DHAQ and the reduced form of 2,6-DBEAQ, indicating that 2,6-DBEAQ chemically oxidized DHA to 2,6-DHAQ. The second route involves two steps (solid lines). First, 2,6- DBEAQ oxidizes DHA to a dimer through a chemical reaction. Afterward, this dimer can be electrochemically recovered to 2,6-DHAQ during cell cycling. Notably, the presence of 2,6-DBEAQ allowed for electrochemical dimer recovery within the normal cycling range, without the need for an additional oxidation step as suggested in [5].

[0098] References

[0099] [1] Gao, J., Amini, K., George, T.Y., Jing, Y., Tsukamoto, T., Xi, D., Gordon, R.G. and Aziz, M.J., 2022. A High Potential, Low Capacity Fade Rate Iron Complex Posolyte for Aqueous Organic Flow

[0100] Batteries. Advanced Energy Materials, 12(44), p.2202444.

[0101] [2] Ruan, W., Mao, J., Yang, S. and Chen, Q., 2020. Communication — Tris (bipyridyl) iron Complexes for High-Voltage Aqueous Redox Flow Batteries. Journal of the Electrochemical Society, 167(10), p.100543. [3] Jin, S., Fell, E.M., Vina-Lopez, L., Jing, Y., Michalak, P.W., Gordon, R.G. and Aziz, M.J., 2020. Near neutral pH redox flow battery with low permeability and long-lifetime phosphonated viologen active species. Advanced Energy Materials, 10(20), p.2000100.

[0102] [4] Goulet, Marc-Antoni, Liuchuan Tong, Daniel A. Pollack, Daniel P. Tabor, Susan A. Odom, Alan Aspuru-Guzik, Eugene E. Kwan, Roy G. Gordon, and Michael J. Aziz. "Extending the lifetime of organic flow batteries via redox state management." Journal of the American Chemical Society 141 , no. 20 (2019): 8014-8019.

[0103] [5] Jing, Yan, Evan Wenbo Zhao, Marc-Antoni Goulet, Meisam Bahari, Eric M. Fell, Shijian Jin, Ali Davoodi et al. "In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries." Nature chemistry 14, no. 10 (2022): 1 103-1109.

[0104] [6] Wu, Min, Meisam Bahari, Yan Jing, Kiana Amini, Eric M. Fell, Thomas Y. George, Roy G. Gordon, and Michael J. Aziz. "Highly stable, low redox potential quinone for aqueous flow batteries." Batteries & Supercaps 5, no. 6 (2022): e202200009.

[0105] [7] Lin, Kaixiang, Qing Chen, Michael R. Gerhardt, Liuchuan Tong, Sang Bok Kim, Louise Eisenach, Alvaro W. Valle et al. "Alkaline quinone flow battery." Science 349, no. 6255 (2015): 1529-1532.

[0106] [8] Fell, Eric Michael, and Michael J. Aziz. "High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Battery Active Material." In Electrochemical Society Meeting Abstracts 239, no. 3, pp. 207-207. The Electrochemical Society, Inc., 2021 .

Claims

What is claimed is:CLAIMS1 . A battery comprising a negolyte in contact with a first electrode, a posolyte in contact with a second electrode, and a barrier separating the negolyte and the posolyte, wherein the negolyte and / or posolyte comprises a mixture of at least two different redox active species, each of which undergoes electrochemical cycling during charge and discharge of the battery, and wherein a first of the redox active species undergoes decomposition by oxidation or reduction, forming a decomposed species, and a second of the redox active species facilitates the recovery of the first redox active species from the decomposed species.

2. The battery of claim 1 , wherein the first redox active species is recovered by: a chemical recovery process, wherein the second redox active species reverses the decomposition of the first redox active species; or a process of lowering the energy barrier for electrochemical recovery, wherein the second redox active species converts the decomposed species into an intermediate state, facilitating its electrochemical recovery during battery cycling.

3. The battery of claim 1 or 2, wherein: i) the second redox active species has a lower redox potential than the first redox active species; or ii) the second redox active species has a higher redox potential than the first redox active species.

4. The battery of claims 1-3, wherein: i) the first redox active species is first further reduced by the second redox active species, prior to being oxidized during battery cycling; or ii) the first redox active species is first further oxidized by the second redox active species, prior to being reduced during battery cycling.

5. The battery of claims 1-4, wherein the first redox active species or the second redox active species comprises an organic redox active species.

6. The battery of claim 5, wherein first organic redox active species or the second organic redox active species comprises an anthraquinone, a viologen, a fluorenone, a naphthoquinone, a phenoxazine, a phenothiazine, a phenazine, or a reduced form thereof.

7. The battery of claim 6, wherein the anthraquinone is of formula (I)wherein each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa; -SRa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); - P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group.

8. The battery of claims 1-4, wherein the first redox active species or the second redox active species comprises an inorganic redox active species.

9. The battery of claims 1-4, wherein the first redox active species or the second redox active species comprises a metalorganic redox active species.

10. The battery of any of claims 1 -9, wherein the negolyte comprises the mixture.

11. The battery of any of claims 1-9, wherein the posolyte comprises the mixture.

12. The battery of any of claims 1-11 , wherein the first of the redox active species undergoes decomposition to the decomposed species by reduction and the second of the redox active species oxidizes the decomposed species to recover the first redox active species.

13. The battery of any of claims 1-11 , wherein the first of the redox active species undergoes decomposition to the decomposed species by oxidation and the second of the redox active species reduces the decomposed species to recover the first redox active species.

14. The battery of any of claims 1-13, wherein the battery is a flow battery.

15. A method of regenerating a first redox active species in a battery, the method comprising: a) providing a battery of any one of claims 1-14; and b) cycling the battery; wherein after charge or discharge the first of the redox active species decomposes by oxidation or reduction forming a decomposed species, and the second of the redox active species either reverses the oxidation or reduction to recover the first redox active species or lowers the energy barrier for electrochemical recovery.

16. The method of claim 15, wherein the first of the redox active species is dissolved in solution, and the decomposed species is removed from solution as a solid.

17. The method of claim 15, wherein the first of the redox active species is dissolved in solution, and the decomposed species is dissolved in solution.

18. The method of claim 15, wherein the first of the redox active species is a solid, and the decomposed species is a solid.

19. The method of claim 15, wherein the first of the redox active species is a solid, and the decomposed species is dissolved in solution.

20. The method of claim 15, wherein: i) the second redox active species has a lower redox potential than the first redox active species;or ii) the second redox active species has a higher redox potential than the first redox active species.21 . The method of claim 15 or 20, wherein: i) the first redox active species is first further reduced by the second redox active species, prior to being oxidized during battery cycling; or ii) the first redox active species is first further oxidized by the second redox active species, prior to being reduced during battery cycling.

Images