Aqueous Zn−organic batteries: Electrochemistry and design strategies

Organic electroactive materials are increasingly recognized as promising cathode materials for aqueous zinc–ion batteries (AZIBs), owing to their structural diversity and renewable nature. Despite this, the electrochemistry of these organic cathodes in AZIBs is still less than optimal, particularly in aspects such as output voltage, cyclability, and rate performance. In this review, we provide an overview of the evolutionary history of organic cathodes in AZIBs and elucidate their charge‐storage mechanisms. We then delve into the strategies to overcome the prevailing challenges faced by aqueous Zn−organic batteries, including low achievable capacity and output voltage, poor cycling stability, and rate performance. Design strategies to enhance cell performance include tailoring molecular structure, engineering electrode microstructure, and modulation of electrolyte composition. Finally, we highlight that future research directions should cover performance evaluation under practical conditions and the recycling and reuse of organic electrode materials.


| INTRODUCTION
2][3] LIBs are well-suited for powering portable devices and electric vehicles on a single charge.5][6][7] Among various "beyond lithium-ion" battery technologies, aqueous Zn-ion batteries (AZIBs) have attracted increasing attention due to the high theoretical specific capacity (820 mAh g −1 ), appropriate redox potential (−0.76 V vs. standard hydrogen electrode), cheaper price of Zn metal, and facile manufacturing process.][17] However, the Zn 2+ cations, with its high charge density and small ionic radius, often induce a high electrostatic repulsion.This typically results in the slow diffusion of Zn 2+ within the crystal lattice of the inorganic hosts.Additionally, the repeated insertion and extraction of Zn 2+ within the crystal lattice could cause the inorganic host structure to collapse and dissolve, resulting in capacity decay over prolonged use. 180][21][22][23][24] OEMs use a unique coordination reaction to store energy, which involves the interaction between the active functional groups and the cations or anions present within the aqueous electrolytes.This reaction mechanism effectively avoids the large structural changes that often occur in inorganic materials. 25[28][29][30][31]  OEMs in AZIBs can be traced back to 1985 when conductive polyaniline (PANI) was electrochemically deposited on a platinum electrode. 32During charging processes, the reaction involved the protonic acid doping of PNAI in a ZnCl 2 /NH 4 Cl aqueous electrolyte, while counter anions participated in the reaction between the ═NH + − and −NH− moieties.However, their specific capacity often falls below 200 mAh g −1 due to the limited doping levels of conductive polymers.In 2009, poly(2,2,6,6tetramethylpiperidinyloxy-4-yl vinyl ether) (PTVE), a nitronyl nitroxide, was developed as a p-type cathode. 33The PTVE cell showed high rate performance, achieving a specific capacity of 131 mAh g −1 even at 60 C, which can be attributed to the rapid electron-transfer process of the radicals during the redox process.The PTVE electrode exhibited an output voltage plateau of 1.7 V, maintaining its performance over 500 cycles.In 2016, 9,10-di(1,3-dithiol-2ylidene)-9,10-dihydroanthracene (exTTF), an organosulfur polymer, was developed as a p-type cathode. 34The exTTF cathode featured an output voltage of 1.1 V, a specific capacity of 128 mAh g −1 , and maintained over 1000 cycles at 120 C (30 s).

| DEVELOPMENT HISTORY OF OEMS AND THEIR CHARGE STORAGE MECHANISMS
In 2018, calix [4]quinone (C4Q), a carbonyl compound, was developed as a high-capacity n-type cathode. 35The oxygen redox centers within C4Q engage in a reversible enolization reaction to gain or release electrons, which leads to an impressive capacity of 335 mAh g −1 .C4Q displayed an output voltage of 1.0 V and a lifespan of 1000 cycles at 500 mA g −1 .In 2019, 1,4bis(diphenylamino)-benzene (BDB), a triphenylamine F I G U R E 1 The representative organic electroactive materials (OEMs) for AZIBs and their reaction mechanisms.The years correspond to the first published development featuring each redox motif.The red, gray, and green colors represent the n-type, p-type, and bipolar-type OEMs, respectively.][34][35][36][37][38][39][40][41] derivative, was developed as a p-type cathode. 36The dual tertiary nitrogen atoms in BDB can be successively oxidized, accompanied by reversible anion insertion.BDB displayed a specific capacity of 125 mAh g −1 at 26 mA g −1 and an output voltage of 1.25 V.In 2020, tetracyanoanthraquinodimethane (TCNAQ), a nitrile compound, was introduced as an n-type cathode with a capacity of 169 mA g −1 . 37Notably, the electronwithdrawing cyano group significantly elevated the working potential up to 1.1 V.In the same year, diquinoxalino [2,3-a:2′,3′-c] phenazine (HATN), an imine compound, was developed as an n-type cathode, delivering a high capacity of 405 mAh g −1 . 38Contrary to the conventional Zn 2+ insertion/extraction charge storage mechanism, HATN cathode displayed a unique H + uptake/removal mechanism due to the proton chelation ability of imino moiety (−C═N−) through a coordination reaction.In 2021, cross-linked CLPy, belonging to the polyprene family, was developed as a p-type cathode. 39uring the charging process, a positive CLPy radical is formed by the loss of the delocalized π-electrons, which is then compensated by anion insertion.CLPy delivers a capacity of 180 mAh g −1 and an output voltage of 0.94 V.In 2022, para-dinitrobenzene (p-DB), a nitroaromatic compound, was developed as an n-type cathode, harvesting a high capacity of 402 mAh g −1 . 40Each nitro group (−NO 2 ) in p-DB is capable to accept two electrons during the discharge process.In 2023, azo benzene (AZOB), an azo compound, was developed as an n-type cathode, delivering a high capacity of 284 mAh g −1 and exhibiting rapid redox kinetics. 41Notably, the azo group is capable of reversibly accepting two electrons based on N═N/N−N redox reaction, a process that is accompanied by proton storage.
In general, the OEMs are classified as n-type, p-type, and bipolar-type, according to their ability to either release or accept electrons during the electrochemical reaction.n-type materials are typically known to accept electrons and undergo reduction first, transitioning from a neutral state to a negatively charged state.In contrast, p-type materials are inclined to donate electrons and undergo oxidation first, shifting from a neutral state to a positively charged state.Meanwhile, bipolar-type materials have the unique ability to either accept or donate electrons, contingent on the reaction potentials.The charge-storage mechanisms of OEMs are largely determined by their molecular structure and the composition of the electrolytes involved.For n-type materials, they typically involve a Zn 2+ storage, H + storage, or a combination of the two in a co-storage mechanism.The traditional Zn 2+ -storage mechanism is kinetically slow, due to the high desolvation and intercalation energies needed for the absorption of hydrated Zn 2+ into the cathode host.In contrast, H + carries a smaller charge and has a weaker electrostatic interaction with the cathode host, contributing to a more rapid kinetic behavior.During the discharge process, a continuous hydrolysis reaction (H 2 O ↔ H + + OH − ) occurs, accompanied by the insertion of H + .This process raises the concentration of OH − ions around the cathode, resulting in the formation of Zn 4 (OH) 6 SO 4 •nH 2 O byproduct through the following chemical reaction: 4Zn 2+ + SO 4 2− + 6OH − + nH 2 O = Zn 4 (OH) 6 SO 4 •nH 2 O. 30,31 Interestingly, the byproduct can precipitate and dissolve reversibly on the cathode surface during the discharge/charge cycle, thereby evidencing H + storage behavior.However, it is worth noting that some studies indicate that this precipitation can escalate the battery impedance and consequently impair the battery performance. 30In the actual discharge process, relying solely on the uptake and removal of H + ions despite their fast kinetics proves to be challenging.This is primarily due to the concentration of Zn 2+ in the aqueous electrolyte being significantly higher than that of H + .Therefore, the Zn 2+ /H + co-storage mechanism has become the subject of extensive research.This highlights an urgent need for further studies to decipher the competitive dynamics between Zn 2+ and H + insertions.When it comes to p-type materials, they usually depend on the reversible absorption and release of anions, such as SO .p-type materials exhibit negligible bond rearrangement during the redox process.In general, p-type cathodes may exhibit superior rate capability due to the fast process of anion absorption and desorption.However, it is important to note that other factors, such as the type of active material, its synthesis method, the nature of charge carriers, and the electrode microstructure, could all have an influence on the kinetics.
Upon reviewing the documented OEMs, the majority exhibit n-type redox behavior.Figure 2 illustrates the molecular structures of the reported n-type OEMs in AZIBs, including a variety of small molecules, macrocycles, oligomers, and polymers.

| DESIGN STRATEGIES OF ORGANIC CATHODES FOR HIGH-PERFORMANCE AZIBS
An aqueous Zn−organic battery is composed of a metallic Zn as anode, an OEM-based composite as cathode, and a water-based solution containing Zn 2+ ions as electrolyte.Figure 3 outlines the challenges associated with aqueous Zn−organic batteries, including inadequate output voltage, low attainable capacity, F I G U R E 2 Chemical structures and abbreviations of n-type organic cathode materials for AZIBs.References. 18,35,37,38, slowinetics, poor reversibility, and susceptibility of electrolyte freezing at low temperatures.These intricately connected challenges restrain the performance of AZIBs, especially with respect to energy density, power density, and cycle performance.Design strategies to mitigate these challenges will be discussed in the following sections.

| Specific capacity
The theoretical capacity of OEMs can be expressed as , where n represents the number of transferred electrons per structure unit, F denotes the Faraday constant, and M w signifies the molecular mass of the structure unit.The enhancement of specific capacity requires facilitating multielectron reactions and simplifying structure units as shown in Figure 4.

| Facilitating multielectron reactions per active group
Most studied OEMs exhibit a traditional single-electron reaction per active group (e.g., C═O and C═N).By devising a multielectron reaction mechanism per active site, a higher capacity could be reached.Song et al. 40 reported para-dinitrobenzene (p-DB), a nitroaromatic compound for AZIBs.Each nitro group (−NO 2 ) in the p-DB molecule can accept two electrons during the discharge process, leading to a high capacity of 402 mAh g −1 .Remarkably, Chen et al. 88 reported a six-electron reaction chemistry per nitro group for 1,5-dinitronaphthalene (1,5-DNN) cathode, which demonstrates an ultrahigh capacity of 1338 mAh g −1 and specific energy of 3273 Wh kg −1 in lithium batteries.Chen et al. 41 developed azobenzene (AZOB) for high-capacity AZIBs.The azo group of AZOB is able to reversibly transfer two electrons based on the N═N/N-N reaction mechanism.Research of OEMs with multielectron reaction mechanisms is still in its infancy, and many challenges await resolution.For example, AZOB displays a low melting point, a characteristic that poses a challenge to the electrode fabrication process.On the other hand, p-DB experiences severe dissolution in aqueous electrolytes.We are confident that by modifying their molecular structures and regulating electrolyte composition, these drawbacks could potentially be surmounted.Thus, these multielectron reaction mechanisms may pave a new pathway for the development of highenergy OEMs.
Of note, the full potential of the high theoretical capacity may not always be realized, as the practical capacity often falls short of expectations.This discrepancy can be attributed to several factors.First, the inherent poor electrical conductivity and sluggish ionic diffusion of OEMs.Second, the use of specific redox sites in OEMs may be challenging due to the significant charge repulsion in their reduced state, particularly as the depth of discharge increases.Finally, the reduction potential of some redox centers might fall below the voltage threshold of hydrogen evolution or Zn plating, rendering their capacity unusable.To overcome these hurdles, a rational design of the type, number, and

| Output voltage
Another factor that affects the energy density of AZIBs is the output voltage, which is determined by the redox potential of organic cathodes.The redox potential (E) can be calculated using the formula: E = G nF Δ , where ΔG denotes the change of Gibbs free energy, n represents the number of transferred electrons, and F is the Faraday constant.Since a given molecule possesses a known n value, its voltage can be modulated by adjusting ΔG.Typically, ΔG can be calculated by ΔG = ΔG 1 + ΔG 2 -ΔG 3 , where ΔG 1 represents the electron loss or gain process of organic molecules, ΔG 2 signifies the desolvation process of ion carriers in the electrolytes, and ΔG 3 denotes the coordination process between the desolvated ion carriers and the organic molecules.Therefore, the E can be enhanced either by increasing ΔG 1 and ΔG 2 or by decreasing ΔG 3 .ΔG 1 can be influenced by modulating the electronic environment surrounding the redox centers.Meanwhile, ΔG 2 and ΔG 3 can be adjusted by the strategic selection of ion carriers and solvents. 95igure 5 illustrates molecular design strategies to improve ΔG 1 and redox potential.

| Introducing electron-withdrawing groups
The electron-withdrawing groups, such as −CN, −Cl, and −CF 3 , can attract electrons from the redox centers via an induction effect, thereby resulting in a reduction in the energy level of the lowest unoccupied molecular orbital (LUMO) and an increase in the redox potential of OEMs.For instance, An et al. 73 synthesized HATN-P6CN by modifying HATN with six electron-withdrawing cyano groups.The average voltage of HATN-P6CN can be increased to 0.73 V, compared with 0.48 V of the unmodified HATN.Other examples with elevated voltages include tetrachlorobenzoquinone (TCBQ) 46 and poly (2-chloro-3,5,6-trisulfide-1,4-benzoquinone) (PCTB) 67 modified with chlorine groups, and tetracyanoanthraquinodimethane (TCNAQ) 37 and (HATN-3CN) 74 modified with cyano groups.

| Arrangement of redox centers
The arrangement of active centers can also have an influence on the redox potential of OEMs.For instance, the discharge potential of quinone derivatives with 9,10phenanthrenequinone (9,10-PQ) is much higher than that of 9,10-AQ, which is due to the lower LUMO energy levels of the ortho-carbonyls (9,10-PQ) compared with the para-carbonyls (9,10-AQ).Moreover, the 9,10-PQ exhibits higher capacities and smaller overpotentials, attributed to the chelation effect of adjacent carbonyls with Zn 2+ cations; they not only enhance the stability of discharge products but also facilitate their reaction kinetics. 35

| p-type organic materials
p-type organic materials inherently exhibit a high output voltage ranging from 1.1 to 1.7 V, in contrast to n-type materials, whose voltage often falls below 1.0 V. Figure 6 presents the molecular structures of p-type materials used in ZIBs, some of which utilize organic-based electrolytes to obtain a wider stale potential window.These p-type molecules typically contain arylamine groups, 87,[89][90][91][92] nitroxide free radicals, 93,94 conjugated electron-rich groups, 39 or thioether groups. 34,95,96However, it is worth noting that this high voltage could potentially result in the decomposition of aqueous electrolytes.An additional challenge associated with p-type compounds is their relatively low specific capacity, which stems from the need to store each positive charge within more than one aromatic ring.

| Modulating electrolyte composition
The composition of electrolytes, particularly the solvents and ion species, plays a crucial role in determining the F I G U R E 5 Molecular design strategies to improve the output voltage and some representative structures of organic electroactive materials (OEMs).Reference. 35,37,44,46,48,67,73utput voltage.As mentioned above, the reaction of p-type materials involves an electron loss process (ΔG 1 ), accompanied by a desolvation process of anions (ΔG 2 ) and a coordination process between anions and the oxidized molecules (ΔG 3 ) (Figure 7A).The relationship between Gibbs free energy change and cathode potential can be expressed as: 3 .One strategy to enhance the cathode potential is to increase the anionsolvent interaction (ΔG 2 ).Cui et al. 95 reported a high output voltage of 1.7 V for the thianthrene (TT) cathode.As shown in Figure 7B,C, the reduction peaks of TT cathode in electrolytes with different solvents, polyethylene glycol (PEG)+H 2 O, ethylmethyl carbonate (EMC), and acetonitrile (ACN) are 1.3, 1.5, and 1.7 V, respectively.This result is due to the stronger interaction between ACN and anion, which forms a more compact solvation structure.Another strategy is to establish a robust coordination interaction between the anions and the oxidized organic host (ΔG 3 ), which can be accomplished by modulating the anion species. 100For example, Zn-PTVE batteries exhibit a higher operating voltage in ZnSO 4 electrolyte compared with Zn(CF 3 SO 3 ) 2 and Zn (ClO 4 ) 2 electrolytes (Figure 7D-F).This relationship positively correlates with the DFT-calculated binding energies between the PTVE organic radicals and the anions, with the highest binding energy observed between SO 4 2− anion and PTVE radical.To sum up, in terms of electrolyte solution, increasing the desolvation energy barrier (ΔG 2 ) and reducing the coordination energy barrier (ΔG 3 ) can both contribute to a higher redox potential.It is important to acknowledge that the intensification of binding energy may result in sluggish reaction kinetics, potentially undermining the cycling stability of organic electrodes.

| Cycling stability
The cycling lifespan of aqueous Zn−organic batteries is often constrained by multiple factors.The first obstacle is the chronic dissolution of OEMs and their discharged F I G U R E 6 Chemical structures of the reported p-type organic electroactive materials (OEMs) for zinc-ion batteries (ZIBs).88][89][90][91][92][93][94][97][98][99]114 products.Dissolved OEMs migrate toward the Zn anode, forming irreversible byproducts, leading to Zn anode corrosion and a degradation of battery performance.Second, dendrite growth at the Zn anode side poses a significant concern.Dendrites may detach from electrodes, causing a reduced C.E. and simultaneously pose safety hazards, including the potential for causing shortcircuits if they pierce separators.The third challenge stems from the freezing of the electrolyte under low operational temperatures.As temperature decreases, a typical aqueous electrolyte transitions from disordered water to ordered ice due to the formation of H-bonds among water molecules.This ice crystallization hampers ion mobility and reduces the wettability of the electrolyte toward electrodes, leading to a degradation of the electrode-electrolyte interphase.The final one is the deactivation or deprotonation of certain specific electrodes, such as PANI.The charge transport of PANI is propelled by protonation in an acidic environment.However, when used with mild aqueous electrolytes, such as ZnSO 4 and Zn(CF 3 SO 3 ) 2 solutions, the PANI electrode tends to exhibit deprotonation throughout the cycling process.This deprotonation process in nearneutral solutions could detrimentally impact battery performance, resulting in a fast capacity decay and sluggish reaction kinetics.To address these issues, Figure 8 provides an overview of the strategies employed to extend the cycling lifespan, including molecular engineering, electrolyte optimization, the combination with carbon materials, separator modification, and the innovation of cell configurations.

| Molecular engineering
Extending monomers to oligomers, macrocycles, and polymers Organic compounds with higher molecular weight typically exhibit lower solubility and improved cycling stability.For instance, PQ-Δ, a triangular phenanthrenequinone-based macrocycle synthesized using phenanthrenequinone (PQ) molecule as the monomer compound, displays negligible capacity decay even after 500 cycles. 51The reduced solubility is attributed to the layered superstructure of PQ-Δ.Following this strategy, various fused-ring polymers, such as poly(triazine-5,7,12,14-tetraaza-6,13-pentacenequinone) (TTPQ) 72 and triazine-linked triquinoxalinylene polymer (P3Q-t), 79 have been developed.Their extensive conjugated systems effectively curtailed the occurrence of dissolution.Additionally, advanced structures such as covalent organic frameworks (COFs) 82 and metal-organic frameworks (MOFs) 101 can also inhibit dissolution.These polymeric materials feature a unique topological architecture, boasting high porosity and large surface area, which facilitates charge transfer.In sum, the strategic design and deployment of oligomers, macrocycles, and polymers have proven to be effective in inhibiting the dissolution of OEMs.

Forming intermolecular interactions
Unlike the strong covalent or ionic bonds in inorganic materials, organic materials primarily exhibit relatively weak intermolecular interactions, such as hydrogen bonds (H-bonds) and π-π interactions.These interactions affect the physicochemical properties of organic materials, including the melting/boiling point, density, solubility, morphology, charge transport, and ionic diffusion behavior.For example, Sieuw et al. 102 proposed using H-bonding to stabilize the molecular crystal structure and reduce the solubility of small quinone materials.Compared with benzoquinone (BQ), 2,5diamino-1,4-benzoquinone (DABQ) exhibits remarkably higher thermal stability and lower solubility, ascribed to the H-bonds formed between carbonyl and amino groups in DABQ (Figure 9A).In addition, the formation of H-bonding can also prevent the sublimation of small molecule quinones, such as tetraamino-p-benzoquinone (TABQ), as demonstrated in Figure 9B. 47Recently, Song et al. 56 9C).The self-assembly of the planar structure generates well-defined 3D superstructures via out-of-plane π-π stacking.The HBOSs exhibit reduced dissolution in the electrolyte due to the in-plane H-bonding and out-of-plane π-π stacking, as evidenced by the absence of absorption signals in UV/vis spectra (Figure 9D,E).HBOSs/Zn batteries demonstrate exceptional high-rate performance (135 mAh g -1 at 150 A g -1 ) and ultralong cycle life (50,000 cycles at 10 A g -1 ) (Figure 9F,G).

Self-doping of PANI
To combat the deprotonation issue of PANI, one approach is to synthesize self-doped PANI electrodes.Shi et al. 109 reported a sulfo-self-doped PANI electrode, produced through the copolymerization of aniline and metanilic acid.The −SO 3 2− dopant functions as an internal proton reservoir, creating a highly acidic local environment.This environment promotes the redox process of the PANI cathode, even in high-pH solutions.
As a result, the PANI cathode delivers a capacity of An overview of strategies employed to extend the cycling lifespan of aqueous Zn−organic batteries.
180 mAh g −1 and exhibits an extended cycle life surpassing 2000 cycles.

| Electrolyte optimization
High-concentration electrolytes Utilizing high-concentration salts in aqueous electrolytes presents an efficient method to suppress the dissolution of OEMs, mitigate water splitting, and effectively prevent the freezing of the aqueous electrolytes.Lee et al. 92 utilized a hybrid high-concentration aqueous electrolyte (17 M NaClO 4 ) for 5,10-dihydro-5,10-dimethylphenazine stability window of the aqueous electrolyte by mitigating water splitting.Recently, Zhang et al. 107 developed a 7.5 M ZnCl 2 aqueous electrolyte, remarkable for its exceptionally low freezing point of −114°C.The solid-liquid transition temperature (T t ) of electrolytes has been found to be closely associated with the concentration of ZnCl 2 (C ZnCl2 ).In pure water, the existence of only H-bonds allows for the water network to easily transition into an ice network at 0°C.However, the introduction of ZnCl 2 disrupts the initial Hbond network.This is a result of the strong ion interactions between Zn 2+ ions and water molecules, which subsequently reduces the electrolyte's T t .However, above a critical C ZnCl2 concentration (7.5 M), the T t begins to increase due to the enhancement of these ion interactions, leading to a V-shaped relationship between the T t and C ZnCl2 (Figure 10E).By balancing the strength of H-bonds and ion interactions in the solution, a minimum T t of −114°C can be realized in a 7.5 M ZnCl 2 electrolyte.The PANI||Zn battery shows superior lowtemperature tolerance, retaining 64.7% capacity at −50°C (Figure 10F).4M Zn(BF 4 ) 2 aqueous electrolyte was also reported to improve the low-temperature performance of Zn//TCBQ batteries. 108Wu et al. 114 reported the use of ferrocene compound as an anode material for a reverse dualion battery.Despite the reversible one-electron transfer reaction between the neutral state of ferrocene and the cation state of ferrocenium is straightforward, the high solubility of ferrocenium in water presents a challenge.To minimize this dissolution, a 30 M ZnCl 2 was used as a waterin-salt electrolyte, replacing the diluted ZnCl 2 electrolyte (5 M).This use of high-concentration electrolytes has another benefit of widening the full cell voltage by as much as 0.35 V.This enhancement is achieved by concurrently raising the cathode potential and lowering the anode potential.
Nonetheless, when considering the broader application of highly concentrated electrolytes, several important factors need to be considered.These include the reduced ionic conductivity and the energy density, as well as the increased production costs.For example, the high viscosity can hinder ion transport, especially under conditions of high-rate and low-temperature.

Eutectic electrolytes
Hydrated eutectic electrolytes, also referred to "water-ineutectic" electrolytes, have attracted interest due to their unique ability to facilitate uniform Zn nucleation.These electrolytes can alter the solvation structure of Zn 2+ ions, thereby preventing the formation of unstable [Zn(OH 2 ) 6 ] 2+ complexes that typically initiate parasitic reactions at the Zn anode.Yang et al. 63 reported a hydrated eutectic electrolyte fabricated by combining Zn (ClO 4 ) 2 •6H 2 O with a neutral ligand, succinonitrile (SN).The Lewis basic SN participates in the primary solvation shell of Zn 2+ , forming a more stable [Zn(OH 2 ) x (SN) y ] 2+ configuration compared to the conventional [Zn(OH 2 ) 6 ] 2+ structure (Figure 11A).In Zn/Zn symmetric cells, traditional aqueous electrolyte exhibits a sudden rise in polarization voltage, whereas Zn(ClO 4 ) 2 •6H 2 O/SN electrolyte displays stable cycling (Figure 11B).Figure 11C displayed smoother Zn deposition.The Zn−poly(2,3dithiino-1,4-benzoquinone (PDB) full cell exhibits a minimal capacity decay rate of 0.004% per cycle over 3500 cycles (Figure 11D).Similarly, Shi et al. 76 developed a "water-in-eutectic" electrolyte, ZnCl 2 :acetamide:H 2 O = 1:3:1 (ZES-1), for Zn//PNZ battery.It should be noted that the hydrated eutectic electrolytes tend to absorb water during the fabrication process.This process could lead to the disruption of the Zn coordination structure, leading to a conversion of the unique "water-in-eutectic" structure into the ordinary "aqueous solution" structure.Therefore, the precise control of water content is crucial to leverage the benefits of hydrated eutectic electrolytes.

Addition of organic solvent
The addition of organic anti-solvent can improve the reversibility of the Zn plating/stripping process in aqueous electrolyte.For example, methanal, which is miscible with water but incapable of dissolving ZnSO 4 , can serve as a low-cost antisolvent for aqueous electrolytes. 104On the initial addition of methanol to the electrolyte, a homogeneous phase was formed, but beyond a certain threshold (55 vol.%),ZnSO 4 recrystallization occurred, suggesting the disruption of the original solvation structure (Figure 12A).Given the small size and high dielectric constant of methanol, it has the potential to easily insert into the primary Zn 2+ solvation sheath (Figure 12B).This interaction with the coordinated water molecules can disrupt the solvation balance, consequently diminishing water activity and weakening the solvation of Zn 2+ ions (Figure 12C).The antisolvent electrolyte, composed of 50 vol.%methanol (Anti-M-50%), demonstrated a dual advantage: it not only suppressed side reactions at the Zn anode but also improved the electrolyte's freezing tolerance at temperatures as low as −10°C (Figure 12D).The PANI/Zn full cell maintained capacity retention of 89.3% after 2000 cycles at −10°C, indicating its potential for real-world applications (Figure 12E).Other organic solvents, such as N, N-dimethylformamide (DMF) 105 and ethylene glycol (EG), 106 have also been reported as an additive to aqueous electrolyte.These additives can alleviate the issues of dendrite formation and electrolyte solidification under low-temperature conditions.However, it should be emphasized that the introduction of an organic cosolvent can lead to a reduction of the electrolyte's ionic conductivity and impede the dissolution of Zn salts.Moreover, the volatile and combustible nature of organic solvents could pose a challenge to the development of sustainable and eco-friendly battery technologies.

Acid-doped gel electrolytes
To address the deprotonation issue of PANI, acid-doped gel electrolytes were developed.Feng et al. 110 synthesized a polymeric acid gel electrolyte (PAGE) with 3 M Zn (ClO 4 ) 2 for use in PANI batteries.Unlike the conventional small molecule doping acid, the PAGE functioned with SO 3 − groups is a polymeric doping acid, which offers superior stability and consistently supplies protons to the PANI cathode during prolonged operation.Furthermore, the negative SO 3 − groups serve as nucleophilic centers to coordinate with Zn 2+ ions.This interaction aids in regulating the solvation structure of Zn 2+ and facilitates their smooth deposition.Mathanesulfonic acid (MSA) was also reported to fabricate acid-doped gel electrolytes for Zn-PANI batteries. 111

| Hybridization with carbon materials
To overcome the dissolution of OEMs, nanostructured carbon materials have been employed for encapsulating OEMs.For example, para-dinitrobenzene (pDB) material has been hosted within carbon nanoflower.pDB cathode has demonstrated superior stability, enduring up to 25,000 cycles at a rate of 5 A g −1 . 40Naphthoquinone (NQ)-based organic materials have been effectively contained within the nanoporous structure of a carbon nanotube (CNT) network, significantly minimizing dissolution. 42

| Ion selective membrane
Ion selective membrane has been employed as a separator to minimize the dissolution of the organic cathode.Zhao et al. 103 used a Nafion membrane for the C4Q cathode.The membrane's surface is abundant with actively charged groups, which form a double electric layer in association with the counter-charged ions present in the electrolyte.This membrane can exclude negative charges (e.g., C4Q 2x− species as discharged product) while promoting the absorption and migration of positive charges (e.g., Zn 2+ ).Thereby, the discharged products can be successfully confined to the cathode side and the cell achieves a capacity retention of 93% after 100 cycles.It should be noted that the cost of ion-selective membranes is significantly higher than that of the common separators, such as filters and glass fiber.

| Innovation of cell configuration
Self-stratified battery structure Meng et al. 103 proposed a novel self-stratified battery design, which prevents the organic catholyte from diffusing and subsequently contaminating the Zn anode.Unlike most types of battery configurations, the battery structure has no membrane separating the electrodes.Instead, they take the natural miscibility of the electrolyte fluids to keep the components separated, just like oil and water.This design is composed of a Zn anode at the bottom, an aqueous electrolyte in the middle, and an organic catholyte on the top.This unique arrangement is created by gravitational force and differences in solubility (Figure 13A).To prevent mutual miscibility between the organic and aqueous phases, MgSO 4 salt was added to the mixture, facilitating the salting out of the organic phase and resulting in the creation of an organic-aqueous biphasic system (Figure 13B).The redox couple selected for this design is TEMPO/TEMPO + .To prevent the oxidized TEMPO + species from diffusing to the aqueous phase, TFSI − anion was introduced to the system.TFSI − anions possess a strong hydrophobic nature and can form stable ion pairs with the TEMPO + cations, which effectively confines TEMPO + species to the organic phase (Figure 13C,D).As a result, the electrolyte containing TFSI − effectively eliminated the self-discharge of the TEMPO/Zn battery, enabling it to achieve a high C.E. (Figure 13E).

Cell design with decoupled alkali-acid electrolyte
To reconcile the conflicting pH requirements of the PANI cathode and Zn anode in aqueous electrolytes, Liu et al. 112 proposed a decoupling cell design with two separate chambers-an acidic chamber for the PANI cathode and an alkaline chamber for the Zn anode (Figure 14A).The chambers are separated by a bipolar membrane (BPM) composed of a cation-selective membrane (CEM) and an anion-selective exchange membrane (AEM).The CV curves of the PANI electrode reveal that the H 2 SO 4 electrolyte presents a reduced potential gap between the reduction and oxidation peaks compared with that observed in the mild ZnSO 4 electrolyte.This implies a more facilitated coordination/incorporation reaction of H + over Zn 2+ (Figure 14B,C).Remarkably, the acid-alkaline electrolyte-decoupling design displays a capacity retention ratio of 64%, significantly surpassing the 30% observed when employing the ZnSO 4 electrolyte alone (Figure 14D).

| Rate performance
The rate performance of Zn−organic batteries depends on the swift transport of both ions and electrons within the battery system.To improve the rate performance, multiple strategies have been proposed, as summarized in Figure 15.These strategies include the expansion of the π-conjugated structure within OEMs, the hybridization of OEMs with nanocarbon materials, the design of advanced structures with unique topological architectures, and the careful selection of charge carriers.

| Extension of π-conjugated structure
For small molecules, extending their π-electron conjugation system would promote electron delocalization, which, in turn, narrows the energy gap between the highest occupied molecular orbital (HOMO) and LUMO.This approach could potentially pave the way for boosting the electrical conductivity of OEMs.For example, Li et al. 78 developed a novel N-heteroaromatic material, hexaazatrinaphthalene-phenazine (HATN-PNZ), for aqueous Zn-organic batteries with ultrahigh rate (Figure 16A).The extended conjugation length of the phenazine and aromatic ring creates a large charge delocalization area, thereby promoting fast kinetic reactions.The HATN-PNZ cathode displays a high capacity of 257 mAh g −1 at 5 A g −1 , and a respectable 144 mAh g −1 even at 100 A g −1 (Figure 16B).The extended aromatic molecular structure can also effectively mitigate dissolution.Therefore, the HATN-PNZ cathode displays robust cycling stability, enduring more than 45,000 cycles at 50 A g −1 .In a similar context, dipyrido[3ʹ,2ʹ:5,6;2″,3″:7,8]quinoxalino[2,3-i]dipyrido[3,2a:2ʹ,3ʹ-c]phenazine-10,21-dione (DQDPD) 68 and 2,3,7,8tetraamino-5,10-dihydrophenazine-1,4,6,9-tetraone (TDT) 45 were also developed to facilitate fast charge transfer.For redox polymers, the primary cause of slow reaction kinetics at high rates is their lack of molecular planarity.To address this, Wang et al. 79 enhanced the conjugated planarity of redox polymers by using triquinoxalinylene (3Q) as a basic unit to design two distinct redox polymers.One is a 3Q-based homopolymer (P3Q) and the other is a triazine-linked 3Q polymer (P3Q-t).Compared with P3Q, the inclusion of triazine cores in P3Q-t promotes high planarity and reduces steric hindrance, which accelerates the kinetics of the Zn 2+ coordination reaction (Figure 16C).Moreover, the presence of highly electronegative fused rings aids fast intramolecular charge transfer.This synergy of intermolecular and intramolecular effects accelerates the reaction dynamics and enhances P3Q-t's performance.The P3Q-t cathode achieved a capacity of 237 mAh g −1 at 0.3 A g −1 , maintaining over 45% of this capacity even at a high current density of 15 A g −1 (Figure 16D).Ye et al. 65  designed a novel linear π-conjugated quinone-based polymer, poly(phenazine-alt-pyromellitic anhydride) (PPPA).The expansive π-conjugated plane of PPPA accelerates a rapid intramolecular electron transfer during the charge/discharge process.The PPPA cathode displayed a high capacity of 210.2 mAh g −1 at 50 mA g −1 and 139.7 mAh g −1 at 5000 mA g −1 , significantly surpassing that of the phenazine monomer at equivalent current densities (Figure 16E).Overall, the design of redox polymers with extended π-conjugated structures fosters dense π-conjugated stacking.This arrangement generates an abundance of delocalized π-electrons and creates plentiful open 2D channels, enabling fast Zn 2+ transport.These properties are beneficial in promoting both ion diffusion and electron transport during redox reactions.

| Designing advanced structures
Conductive MOFs (c-MOFs) are assembled by coordinating metal ions (such as Ni 2+ and Cu 2+ ) with π-conjugated organic ligands.These structures are especially suited for high-rate ZIBs for three main reasons.First, c-MOFs feature a tunable pore structure and a high specific surface area, both attributes facilitating efficient ion transfer.Second, c-MOFs are rich in π-π/π-d conjugated structure and delocalized electrons, resulting in high electrical conductivity.Finally, the stable coordination bonds formed between metal ions and organic ligands within c-MOFs help to resolve the dissolution issue typically associated with OEMs.Nam et al. 101 developed a two-dimensional (2D) c-MOF, Cu 3 (HHTP) 2 (where HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) to be used as cathode material for AZIBs.Its large pores (~2 nm) facilitate the insertion of hydrated Zn 2+ ions into the host structure (Figure 17A).Cu 3 (HHTP) 2 exhibits a high capacity of 228 mAh g −1 at 50 mA g −1 and retains 57.9% of the initial capacity at 4000 mA g −1 (Figure 17B).Recently, another Cu-based c-MOF was synthesized by coordinating ultrasmall 1,2,4,5-benzenetetramine (BTA) linkers with copper ions. 113D porous crystalline COFs are constructed from predesigned symmetric organic building units.This assembly forms an extensive, ordered π-conjugated backbone, characteristic of inherent porosity.Yu et al. 84 developed a 2D polyacrylamide COF (PI-COF) for Zn 2+storage anode.The PI-COF anode was intended to replace the traditional Zn metal anode, given that Zn metal often encounters challenges, such as dendrite growth, side reactions, and excessive usage.PI-COF was synthesized through a condensation reaction that combined 1,4,5,8naphthalene tetracarboxylic dianhydride (NTCDA) and tris (4-aminophenyl) amine (TAPA).This process resulted in the formation of a parallel-stacked topology with 15 An overview of strategies to improve the rate performance of organic cathodes.hexagonal 1D nanochannels (Figure 17C).More specifically, the PI-COF electrode was fabricated by synthesizing PI-COF on a CNT array, which was grown on carbon cloth (Figure 17D).The PI-COF electrode delivered a capacity of 92 mAh g −1 at 0.7 A g −1 (Figure 17E).Moreover, the PI-COF//MnO 2 cell exhibits a capacity retention of 64% under 10 mA cm −2 , substantially surpassing the Zn//MnO 2 cell's 23%, thereby affirming the PI-COF anode's superiority over the Zn metal anode (Figure 17F).Other COFs, such as the orthoquinone-based COF (BT-PTO COF) with multiple carbonyl active sites, 80 the phenanthroline COF (PA-COF) with abundant nitrogen active sites, 85 the Tp-PTO-COF 83 and the hydroquinone-based COF (HqTp) 81 featuring carbonyl groups as active sites, have also demonstrated great potential as cathode materials for AZIBs.
Compared with the standard Super P, CMK-3 offers shorter path lengths for efficient diffusion.Consequently, the PT cathode displayed a superior capacity and reduced overpotential (Figure 18B).The PT electrode maintained over 95% capacity retention up to 5000 cycles and held onto 46% of its capacity at rates of 20 A g PT −1 (63 C).Recently, Sun et al. 18 developed a hybrid organic cathode for AZIBs, achieved by compositing benzo[b]naphtho[2′,3′:5,6][1,4]dithiino[2,3-i] thianthrene-5,7,9,14,16,18-hexone (BNDTH) with RGO.This BNDTH/RGO hybrid material was fabricated by a unique solvent exchange composition process.During this process, the dissolved BNDTH underwent a secondary aggregation process and precipitated onto the surface of RGO.This led to the creation of strong π-π interactions within BNDTH/ RGO hybrid material (Figure 18C).In addition, the BNDTH itself possesses an extended π-conjugated structure and abundant electroactive groups.These properties allow the BNDTH/RGO//Zn cell to display a high capacity of 296 mAh g −1 and a remarkable rate capability of 115 mAh g −1 at 6 A g −1 (Figure 18D,E).

| Selection of charge carriers
Charge carriers, featured with different ionic radii, valence states, and atomic masses, exhibit diverse kinetic behaviors.The variations of charge carriers can significantly affect the battery's charging and discharging process, thereby potentially influencing the battery's rate capability. .In theory, H + possesses rapid kinetics due to its remarkably small ionic radius and low atomic mass.For some organic electrodes, H + dominates the energy storage process and exhibits a pseudocapacitive behavior, which can contribute to a faster charging/discharging process, thereby improving the battery's rate performance.Despite this, there is still a lack of in-depth research on designing electrolyte systems that can significantly enhance pseudocapacitive behavior in AZIBs, a topic that certainly warrants further exploration.Copyright 2020, American Chemical Society.

| Design strategies at the molecular, electrode, and electrolyte levels
Aqueous Zn−organic batteries offer a compelling substitute for LIBs, particularly in stationary energy storage systems, where environmental sustainability and costefficiency take precedence.Figure 19 presents an overview of the design strategies aimed at enhancing the performance of aqueous Zn−organic batteries, and encompasses the optimization at molecular, electrode microstructure, and electrolyte levels.Molecular engineering entails the introduction of functional groups, redox-active sites, and π-conjugated systems.This integration enables a comprehensive modulation of cathode properties, including solubility, output voltage, kinetics, and specific capacity.For example, the incorporation of specific groups, like −NH 2 and −CN, promotes the formation of intermolecular interactions like H-bonding, which can address the dissolution issue.The introduction of electronwithdrawing groups, like −CN and −Cl, lowers the LUMO energy level, thereby enhancing the discharge voltage.Notably, the type, number, and spatial arrangement of redox-active sites may all have an influence on the specific capacity, voltage, and rate capability of organic electrodes.In addition, the extension of the π-conjugated system within organic molecules enhances electron mobility by promoting π-electron delocalization across the molecular plane.This specific arrangement also facilitates the π-π stacking assembly of organic molecules, which, in turn, stabilizes the structure and expedites the redox reactions.The elaborate design of 2D conductive COFs or MOFs offers well-organized nanochannels for rapid ion migration.
Strategic engineering of the electrode's microstructure can profoundly impact the charge transport systems.These systems, analogous to the "traffic system" in a metropolis, hold a pivotal role in dictating the performance of a composite electrode.Engineering of electrode microstructure includes adjusting the particle size and morphology of OEMs, the conductive carbon matrix, and the porous electrode architecture.For example, reducing particle size and regulating particle morphology can minimize the diffusion distance of charges within the electrode and also stabilize the electrode structure during volume changes.Introducing a conductive carbon matrix, such as CNTs, rGO, and the like, enables efficient electron transfer.Additionally, constructing a well-defined porous carbon host enhances electrolyte wettability and also helps alleviate dissolution.
The composition of the electrolyte affects numerous aspects of battery performance, including the plating/ stripping behavior of the Zn metal anode, the electrochemical window of electrolytes, as well as the charge storage mechanism, output voltage, and reaction kinetics of the cathode.Recent studies indicate that the solvation structure of Zn 2+ can be manipulated using eutectic electrolytes, highly concentrated electrolytes, organic cosolvents, and selected charge carriers.These techniques are effective in preventing dendrite growth, suppressing hydrogen evolution reactions, and expanding the operational voltage window.

| Practical application
To meet the industrial criteria for practical applications, OEMs should be assessed under conditions that feature high mass loadings, minimal electrolyte usage, and optimized cathode/anode mass ratios.The areal capacity of most existing organic electrodes falls below 1 mAh cm −2 , typically accompanied by low mass loading (less than 5 mg cm −2 ) and high carbon content (exceeding 20−30 wt.%).Blindly increasing mass loading could lead to substantial polarization, diminished rate performance, and pose manufacturing challenges.Besides, the volume of electrolyte used can directly influence battery performance.For example, p-type organic materials often require a large amount of electrolyte salt (highconcentration electrolytes) to facilitate a smooth charging process, given that their dual-ion storage mechanism requires an adequate concentration of both anions and cations.Achieving stable cycling of p-type materials becomes problematic under lean electrolyte conditions in practical applications, even though this is not a concern for lab-scale cells where excessive amounts of electrolyte are commonly used.

| Recycling and reusing of organic electrode materials
The recycling process of OEMs is currently in its infant stage, with only a handful of examples reported so far.Many OEMs, particularly those based on small organic molecules, demonstrate air stability in both charged and discharge states.Moreover, they display high solubility in inexpensive organic solvents.These attributes enable their recycling at any state of charge, with a high potential for recovery.A straightforward dissolving and extracting process can recover OEMs with remarkable yield and exceptional purity, rivaling that of commercially available raw materials.In comparison, traditional recycling processes for inorganic materials require the complete decomposition or destruction of the electrode materials, a process that is more complex and time-consuming.
In summary, this review offers a timely evaluation of aqueous Zn−organic batteries.We firmly believe that they will soon earn their place in green and sustainable energy storage applications in the near future.

Figure 1
Figure 1 presents the development chronology of OEMs in AZIBs, with the years corresponding to the first published OEM representing each redox motif.The exploration of

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I G U R E 3 Schematic of the challenges faced by aqueous Zn−organic batteries.spatial arrangement of redox centers is imperative.Moreover, other factors such as electrode microstructure and electrolyte composition can also influence the attainable capacity.
developed supramolecular H-bonded organic superstructures (HBOSs) through the self-assembly of cyanuric acid (CA) and 1,3,5-triazine-2,4,6-triamine (TT) molecules.The formation of the supramolecular plane is driven by multiple H-bonds in the form of N-H•••O and N-H•••N, resulting in a 2D arrangement of TT and CA molecules (Figure

(
DMPZ) cathode.NaClO 4 was chosen as the salt due to its low cost, high solubility, and ability to modify the water-coordinating structure.Dissolution tests indicated that the charged states of DMPZ + and DMPZ 2+ were more prone to dissolve in conventional aqueous (1 M NaClO 4 , H 2 O) or nonaqueous electrolyte (1 M NaClO 4 , TEGDME) than in the high-concentration electrolyte (Figure10A,B).The cycle stability of the DMPZ cathode improved owing to the suppressed dissolution (Figure10C,D).The cause of this suppressed dissolution can be attributed to the reduced free water activity in the high-concentration system.Beyond inhibiting dissolution, this electrolyte can also extend the electrochemical F I G U R E 9 (A) A view of the 2,5-diamino-1,4-benzoquinone (DABQ) crystal structure.Yellow dotted line: intramolecular H-bonds; Green dotted line: intermolecular H-bonds.Reproduced with permission.102Copyright 2019, Royal Society of Chemistry.(B) Sublimation test of BQ and tetraamino-p-benzoquinone (TABQ) under different heating temperatures.Reproduced with permission.47Copyright 2021, Springer Nature.(C) 2D arrangement of thianthrene (TT) and cyanuric acid (CA) molecules by forming intermolecular H-bonding networks of H-bonded organic superstructures (HBOSs).(D) Voltage-capacity profiles of HBOS cathode at 1 A g -1 .(E) UV/vis spectra of the electrolyte after immersing HBOS cathode at different voltages for a month.(F) Rate performance and (G) cycling stability of HBOSs/Zn batteries.Inset are the scanning electron microscope (SEM) images of HBOS cathode before and after cycling.Reproduced with permission.56Copyright 2023, Wiley-VCH.

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I G U R E 10 (A) Dissolution tests of 5,10-dihydro-5,10-dimethylphenazine (DMPZ) cathode in its pristine, half-charged, and fully charged states in different electrolytes.Inset photos illustrate the color variation of electrolytes at different charging levels.(B) Relative dissolution amounts of DMPZ electrode in different electrolytes.Charge/discharge profiles of DMPZ in (C) high-concentration aqueous electrolyte and (D) 1 M NaClO 4 /TEGDME electrolyte.Reproduced with permission. 92Copyright 2022, American Chemical Society.(E) Photographic comparison of ZnCl 2 electrolytes with various concentrations at 25 and −70°C.(F) Rate performance of batteries in various electrolytes under a range of temperature conditions.Reproduced with permission. 107Copyright 2020, Springer Nature.

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I G U R E 11 (A) Comparison of solvation shells between a traditional aqueous electrolyte and succinonitrile (SN)-contained electrolyte.(B) Voltage profiles of symmetric Zn/Zn cells.(C) SEM images of Zn metal after plating.(D) Long-term cycling of poly(2,3-dithiino-1,4benzoquinone (PDB) cathode in different electrolytes.Reproduced with permission. 63Copyright 2020, Cell Press.

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I G U R E 12 (A) Optical images of ZnSO 4 aqueous electrolytes with methanol as anti-solvent.Inset is the recrystallization of ZnSO 4 salt in an antisolvent electrolyte.(B) Comparison of dielectric constant and molecular diameter among different solvents.(C) Schematic of the changes in Zn 2+ solvent sheath along with methanol addition.(D) Zn reversibility of Zn/Cu cells in different electrolytes at −10°C.(E) Cycling stability of Zn/PANI full cell under −10°C.Reproduced with permission. 105Copyright 2021, Wiley-VCH.

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I G U R E 13 (A) Schematic of the self-stratified battery design.(B) Phase separation images of TEGDME-H 2 O system with different concentrations of MgSO 4 additive.The orange color indicates the dissolving of TEMPO molecules in TEGDME.(C) The influence of hydrophobic anion on the distribution of TEMPO + cation.(D) Simulated hydration shell structures of anions, and digital images displaying the phase separation effect.(E) First cycle charge/discharge profiles of TEMPO/Zn battery with different anions.The blue bands represent the irreversible capacity.Reproduced with permission. 103Copyright 2020, Cell Press.

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I G U R E 14 (A) Working mechanisms of the electrolyte decoupling Zn-polyaniline (PANI) hybrid battery.CV curves of PANI electrode in (B) H 2 SO 4 and (C) ZnSO 4 electrolytes.(D) Cycling performance of Zn-PANI battery in different electrolytes.Reproduced with permission. 112Copyright 2022, Elsevier.

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I G U R E 19 Summary of design strategies aimed at optimizing aqueous Zn−organic batteries at the molecular, electrode, and electrolyte levels, respectively.