Redox Mediator Chemistry Regulated Aqueous Batteries: Insights into Mechanisms and Prospects

Open AccessCCS ChemistryMINI REVIEW5 Sep 2022Redox Mediator Chemistry Regulated Aqueous Batteries: Insights into Mechanisms and Prospects Tengsheng Zhang, Qianru Chen, Xinran Li, Jiahao Liu, Wanhai Zhou, Boya Wang, Zaiwang Zhao, Wei Li, Dongliang Chao and Dongyuan Zhao Tengsheng Zhang Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Qianru Chen Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Xinran Li Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Jiahao Liu Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Wanhai Zhou Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Boya Wang Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Zaiwang Zhao Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Wei Li Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author , Dongliang Chao *Corresponding author: E-mail Address: email protected Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author and Dongyuan Zhao Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, School of Chemistry and Materials, Fudan University, Shanghai 200433 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202125 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Redox mediators (RMs), serving as intermediate electron carriers or reservoirs, play vital roles in developing new charge transfer energy storage systems with high voltage or capacity in aqueous batteries. However, the underlying mechanism and selection criteria of RMs remain unclear in aqueous batteries, which hinders the further exploitation of new RMs and aqueous battery chemistries. Herein, rather than simply compiling recent progress, we critically appraise the potential correlation between the electrochemical charge storage mechanisms and the selection criteria of RMs. For practical applications, current challenges and promising strategies are synergistically proposed, corresponding to representative applications in holistic deposition–dissolution aqueous batteries, sulfur-based aqueous batteries, and redox flow aqueous batteries. Last, this perspective provides synergistic concerns on and a roadmap for RM chemistry to render the future development of RM-assisted high-efficiency and high-energy aqueous batteries. Download figure Download PowerPoint Introduction Given the looming consequences of climate change, the development of affordable energy harvesting and storage technology has emerged in recent times as a grand challenge in lowering humanity’s dependence on fossil fuels.1 Therefore, rechargeable battery technologies are deemed as one of the most promising candidates for energy storage. Specially, aqueous batteries have become a focus in the last few years due to their low-cost, nontoxic, and high-safety merits, showing an unprecedented potential for large-scale energy storage systems. So far, great efforts have been devoted toward designing and preparing advanced electrode materials with a tailored structure, composition, and morphology to improve the electrochemical performance of aqueous batteries. However, the intrinsic deficiency in energy density still hardly meets the practical demands.2,3 These factors have prompted significant interest in the development of new charge transfer energy storage systems with high voltage or capacity redox couples like sulfur-based aqueous batteries, metal–air aqueous batteries, redox flow aqueous batteries, and some holistic deposition–dissolution aqueous batteries.4–6 In comparison to mature Li-ion battery (LIB) technologies, these approaches are still in their infancy. Poor cycling performance, less-active material utilization, low coulombic efficiency, slow electrochemical kinetics, and high overpotential severely impede their development. Therefore, exploring effective strategies to overcome the above blemishes is highly demanded. A redox mediator (RM)-strategy is promising in regulating reversibility and kinetics of the redox couples. They can be considered as soluble catalysts with electrochemical activities, serving as intermediate electron carriers or reservoirs to react chemically and spontaneously with the active materials (AMs), without changing the original and final products of the redox reactions.7 The chemically reduced or oxidized RMs can be electro-oxidized or electro-reduced back with high reversibility and fast kinetics. So the electrochemical and chemical reactions occur concomitantly or intermittently in the cyclic utilization of RMs. Specifically, RMs are a series of molecules, polymers, ions, or compounds that can be reversibly oxidized and reduced upon electrochemical cycles.8 The RMs strategy was first proposed in 1990 to achieve overcharge protection by adding n-butylferrocene into the electrolyte. This reagent was unreactive until the existence of an overcharge.9 The RMs strategy has been since applied to address the conductivity problem of materials in the LIBs,10 catalyze oxygen reduction in lithium-oxygen battery (LOB),11 enable bulk solid-phase charge storage in the redox flow battery (RFB),12 enhance the utilization of Li2S cathodes in Li-S battery,13 and so on. Given the phenomenal success in the application of the aforementioned organic–electrolyte-based systems, it is sensible to exploit the great prospects of RMs in aqueous batteries. Until 2017, the RM strategy was first employed in a redox flow aqueous battery, which played a similar role as in the nonaqueous RFB.14 Recently, the utilization of RMs in aqueous Zn-S and Zn-MnO2/Mn2+ batteries has also aroused research interest in high-energy aqueous batteries (Figure 1a).15,16 In summary, the RM strategy offers an additional charge transfer route beyond the localized interface, which enables homogeneous and complete oxidation of the electrode. As shown in Figure 1b, the addition of RMs can lower the charge potential of the electrochemical reaction because the electrochemical oxidation process is dominated by the RMs. Subsequently, the AMs can be chemically oxidized by the RMox. Moreover, the RMs usually feature fast kinetics and high reversibility. Thus, the original slow kinetics and high polarization in the electrochemical processes can be improved via the addition of RMs. It can be concluded that the RMs strategy provides new perspectives for the development of high-energy aqueous battery systems. Indeed, different aqueous battery systems may pose various challenges, while their mechanistic interactions with RMs could be exploited. The successful strategies established for a specific system could be applied to others. To the best of our knowledge, the existing reviews of aqueous batteries mostly specialize in active electrode materials, electrolytes, or different charge carriers. It is desirable to provide an overview of the integrated strategies of RMs in regulating the redox couples in the broader context of different aqueous batteries. Figure 1 | Timeline and significance of RMs in aqueous batteries. (a) Development of RM-strategy in batteries. (b) Operational principle of RM-strategy in regulating the energy efficiency, power capability, and coulombic efficiency. Download figure Download PowerPoint Very recently, several RM strategies have been carried out in aqueous batteries. However, a comprehensive understanding of the reaction mechanism and selection criteria has not been available. In particular, a timely and critical perspective of the RM strategy in developing high-performance aqueous batteries is lacking. Herein, this perspective first reveals a current understanding of the fundamentals in developing RMs, corresponding with recent applications in aqueous batteries. Furthermore, future perspectives on the development and applications of RMs in aqueous batteries are rendered collectively for the future development of high-energy aqueous batteries. Fundamentals in Developing RMs in Aqueous Batteries Electrochemical and chemical reaction mechanism of RMs To advance the development of RMs in aqueous batteries, a better understanding of the electrochemical and chemical reactions mechanism of RMs is prerequisite. An RM is an electronic shuttle between the electrode current collector and the reactants (i.e., AMs), allowing the solid reactants to undergo spontaneous chemical reactions without physical contact with the electrode. Usually, as illustrated in Figure 2, a typical RM participates in the operation of a redox couple through the following mechanism: Figure 2 | Fundamentals in developing redox mediators in aqueous batteries. (a–c) Schematic diagram of RMs where chemical and electrochemical processes occur concomitantly at the same reactive site (a), concomitantly while at different reactive sites (b), and intermittently at different reactive sites (c). (d) Reaction mechanism of RMs with concomitant chemical and electrochemical processes at the same reactive site. (e) Proper redox potential of RMs for different scenarios. (f) RMs in lowering the energy barrier of electrochemical reaction. Download figure Download PowerPoint Diffusion process RM solution → RM electrode surface (1) Electrochemical process RM electrode surface ± e − → RM electrode surface ± (2) Diffusion process RM electrode surface ± → RM solution ± (3) Spontaneous chemical process RM solution ± + AM → RM solution + AM ± (4) These reactions indicate that the RMs do not change the final products of the whole electrochemical process but alter the specific reaction pathway. Upon charge or discharge, RMs are electrochemically oxidized or reduced prior to the AMs, followed by chemical reduction or oxidation of AM by RMox or RMred (RMox and RMred refer to oxidized and reduced states of RMs, respectively) in the electrolyte. Based on the differences in when and where the electrochemical and chemical behaviors of RMs take place, the RMs can be mainly divided into three categories, that is, (1) the chemical and electrochemical processes occur concomitantly at the same reactive site (Figure 2a); (2) the chemical and electrochemical processes take place concomitantly while at different reactive sites (Figure 2b); and (3) the chemical and electrochemical processes happen intermittently at different reactive sites (Figure 2c). Most of the RMs follow the regulation of the first two categories (see illustration in Figure 2a,b), which only need to shuttle between the current collector and corresponding AMs at the same electrode side. As shown in Figure 3, the introduction of RMs has been identified as an effective protocol for enhancing the energy efficiency, power capability, coulombic efficiency, and so on, of the batteries.17–19 Such RMs usually possess fast electrochemical reaction kinetics and dominate the electrochemical reaction. For instance, the corresponding product of electrochemical reaction (RMox) should react spontaneously with the AMs, bypassing the original slow-electrochemical reaction pathway of AMred → AMox (AMox and AMred refer to oxidized and reduced states of AM, respectively, see Figure 2d); thus, the power capability and coulombic efficiency could be significantly improved. Furthermore, the electrochemical redox potential of RMs could tune the charge and discharge voltages as the RMs participate in the electrochemical reaction of the AM, and therefore, the polarization of the battery can be optimized concomitantly, that is, the energy efficiency is elevated. Figure 3 | Schematic of mechanism, applications, selection criteria, and functions of RMs. Download figure Download PowerPoint As for RMs shuttling between the anode and cathode (see illustration in Figure 2c), it has been employed as and overcharge and over-discharge protector and dead metallic anode rejuvenator.9,20 When acting as the overcharge protector, the redox molecules always have higher oxidation potential than the cathodic materials. Once the potential above the normal charge cutoff voltage of the cell is reached, the RMs are electro-oxidized at the positive electrode. Then the oxidized products diffuse to the negative electrode, where they are chemically reduced by the anode to reform the starting material. When serving as a rejuvenator for dead metallic anode, the electro-oxidized RMox enriched at the cathode side gradually diffuses back to the anode, and continues to attack the dead metal debris spontaneously, driving the recycling of the metallic source into the cathode. Selection criteria of redox mediators A rational basis for the selection of appropriate RMs is elusive as designing or choosing efficient RMs requires a serious tradeoff among many conflicting and stringent requirements. By carefully analyzing their different functions, the essential selection criteria of RMs and corresponding reasons can be summarized as follows (see illustration in Figure 3). Proper redox potential From the thermodynamic analysis, the equilibrium potential of the RMs is a key parameter for choosing the proper RMs. The oxidation or reduction of the RMs should be thermodynamics favorable so that the charge or discharge potential can be tuned to the redox potential of the RMs. Specifically, the redox potential of the RMs should meet the requirement of the as-designed redox couples of RMs. For instance, the redox potential of RMs is slightly lower than the oxidation potential of the pure active material or higher than its reduction potential for most scenarios, as in the requirement of higher energy efficiency, power capability, and coulombic efficiency (Figure 2e). Spontaneous chemical reaction The oxidized or reduced RMs can undergo a spontaneous chemical reaction with the targeted charge or discharge products of AMs, which means that the chemical steps would be downhill in free energy and have a ΔG < 0. This criterion is the core of the RM strategy. Fast electrochemical kinetics The electrochemical kinetics of the RMs should also be fast with a low reaction barrier to ensure adequate material transportation for the following chemical process (Figure 2f). Full solubility The RMs in the electrolytes are required to maintain excellent wettability with the electrode AMs and current collectors. Super stability without side reactions In all scenarios, the RMs should be nonconsumable and fully regenerated after each charge or discharge cycle. The redox reaction of the RMs should have high stability without side reactions beyond the aforementioned chemical and electrochemical processes. Status of Redox Mediators in Aqueous Batteries To date, based on the disparate energy storage behavior, three charge storage mechanisms are involved in the reported various electrode materials: intercalation-type AMs (TiS2, Zn2(OH)VO4, MXene, etc.),21–24 conversion-type AMs (Bi2O3, FeOx, S, Te, etc.),5,25–29 and stripping/plating-type AMs (electrolytic MnO2, ZnMn2O4, Zn, etc.).6,30,31 With the former intercalation-type electrochemistry, it is difficult to achieve high energy due to the limited amount of ions and redox voltage during intercalation, while the latter ones featuring high energy suffer from inferior cycling stability and low coulombic efficiency due to the uncontrollable accompanying side reactions and inferior electrochemical kinetics during the phase transition, thereby seriously impeding further development of aqueous batteries. So far, we have witnessed the significant potential of RMs. They not only boost the overall electrochemical performances of aqueous batteries but also make the redox reactions more feasible and reliable within the electrochemical-stability window of the electrolytes. In the following section, we comprehensively appraise the mechanism of RMs in constructing high-efficiency and high-energy Mn-based aqueous batteries with the deposition–dissolution-type mechanism, S-based aqueous batteries with conversion-type mechanism, redox flow aqueous batteries, and other potential applications in aqueous batteries, and elaborate the state-of-the-art strategies toward energetic aqueous batteries. Redox mediators for deposition–dissolution-type-mechanism-based aqueous batteries Deposition–dissolution electrochemistry is universal in material synthesis and anti-corrosion coatings by the in situ synthesis of insoluble deposits at the heterogeneous solid-liquid interface.32 Deposition–dissolution electrochemistry is perceived to have huge potential in delivering high capacity and gravimetrical specific energy. Still, it is quite challenging to develop new battery chemistry to promote the areal energy density and fulfill the high utilization ratio of AMs. As a proof of concept, the electrolytic MnO2/Mn2+ redox couple as a typical deposition–dissolution chemistry possesses a superior theoretical capacity (∼616 mAh g−1) due to its two-electron transfer process. However, the deposited porous MnO2 is prone to form exfoliated dead MnO2 or aggregated residual MnO2 due to the MnO2 accumulation (Figure 4a), especially under high areal capacities, which deactivates interfacial contact of the electrode after cycles. Subsequently, severe overpotential and capacity decay inevitably emerge, restricting its reversibility and efficiency for high-energy aqueous batteries (Figure 4b). Figure 4 | RMs for deposition–dissolution-type-mechanism-based aqueous batteries. (a) Accumulation of dead MnO2 during cycling, and (b) corresponding capacity decline. (c) Cyclic voltammetry curves of 50 mM Mn(Ac)2 and 5 mM KI at 10 mV s−1. (d) CVs of the MnO2 /Mn2+ and Br3−/Br− reactions. (e) The representative discharge profiles of Zn∣Zn2+∥MnO2∣Mn2+ cells with and without 0.1 M KI at the 10th cycle. (f) The representative discharge profiles of the Zn∣Zn2+∥MnO2∣Mn2+ batteries with and without Br2. (g) The schematic representation of the manganese cathode with X− (X = Br or I) during the cycles. Reproduced with permission from ref 15. Copyright 2021 Royal Society of Chemistry. (h) Energy efficiency and capacity differences with and without RMs at discharge currents from 10 to 200 mA. Reproduced with permission from ref 33. Copyright 2021 American Chemical Society. Download figure Download PowerPoint Fortunately, the RM strategy, such as halide RMs X3−/X− (X = Br or I), provides a feasible solution to accelerate the electrolysis process and dissolution of the aggregated residual MnO2, which eliminates the exfoliated dead MnO2.15,33 As shown in Figure 4c–f, the X3−/X− redox couple possesses slightly lower potential than active electrolytic MnO2/Mn2+. The X3−/X− RMs work as electron transport carriers when added to the electrolyte, which facilitates the dissolution of MnO2/Mn2+. During the discharge process, partial MnO2 is preferentially dissolved in the electrolyte to form Mn2+ with priority due to higher potential, and then X3− is reduced into X−. The exfoliated dead MnO2 or aggregated residual MnO2 continue to be reduced by X− through a spontaneous chemical reaction as below: MnO 2 + 3 X − + 4 H + → Mn 2 + + X 3 − + 2 H 2 O (5) Subsequently, oxidized X3− becomes X− concomitantly through the electrochemical reaction, forming a cyclic utilization of RMs (Figure 4g). The specific chemical reaction between RMs and the deposition–dissolution redox couple compensates for the incomplete electrochemical reaction of the latter couple. As an electron transport carrier, RMs effectively expand the participation of residual and exfoliated deposits, reducing the MnO2 accumulation phenomenon, thereby improving the utilization of AMs and minimizing the overpotentials. Because of the fast electrochemical kinetics and highly reversible X3−/X− redox couples, the rate capability and cycle stability of the whole cells are further promoted (Figure 4h). As a consequence, the introduction of RMs is a simple but effective method to develop new battery chemistry based on the deposition–dissolution mechanism. Note that the crossover of soluble RMs is also important because of the undesired attack on the metallic anode. Moreover, some facile and powerful RMs need to be developed for high-performance aqueous batteries. For instance, it is anticipated that the deposition–dissolution of cobalt oxides and other Mn-based oxides could also be feasible via the introduction of RM chemistry. Redox mediators for conversion-type-mechanism-based aqueous batteries AMs featuring conversion-type reaction mechanisms can generally permit high capacity and energy density, which is crucial for aqueous batteries owing to the limited output voltage. For example, S-based AMs with a typical conversion-type charge-storage mechanism, of which the theoretical capacity is higher than 1000 mAh g−1, can fulfill the requirement due to their multielectron electrochemical reaction mechanism.5,34,35 Unfortunately, S-based materials in aqueous charge storage suffer from an uncontrollable polysulfide anion shuttle effect, which is caused by diverse phase transformation (solid-to-liquid or solid1-to-solid2). Unfortunately, the sluggish kinetics caused by the high energy barrier of the redox process lead to large overpotential.5,36 Actually, a high round-trip efficiency is critical, especially for grid-scale applications; otherwise, a significant fraction of the energy generated is wasted instead of being stored in the battery. In addition, the wasted energy should release as heat, which in turn causes further degradation issues. Therefore, enhancing the kinetics and efficiency of the intrinsic conversion-type redox process remains a critical challenge for the development of S-based aqueous batteries. Impressively, RM chemistry provides an effective strategy to address the kinetics and efficiency concerns in S-based aqueous batteries. This approach depends on the electrochemical redox of RMs in solution, which can in turn chemically oxidize polysulfides to S8 on the surface of the electrodes. The additional charge transfer route beyond the localized interface enables homogeneous and complete oxidation with a reduced overpotential.37 Specifically, for aqueous Zn∥S cells, the large overpotential should result in the low energy efficiency of the battery. Since a solid1-to-solid2 (S ↔ S) electrochemical conversion reaction is indispensable for the energy storage of some polyvalent-metal-based sulfides (i.e., ZnS, Cu2S, PbS, etc.) (Figure 5a), and the reaction kinetics of S ↔ S is much slower than the liquid-to-solid (L ↔ S) ones (Figure 5b).16,38–40 To understand these concerns, the application of RM (i.e., I2) can convert the electrochemical reaction from the S2−/S (S ↔ S) to I3−/I2 (L ↔ S) conversion, which bypasses the electrochemical sulfur oxidation process via spontaneous chemical oxidation of S2− and feasible electrochemical reaction of I2 (Figure 5c). As a consequence, the charging polarization of the Zn∥S battery can be effectively reduced (Figure 5d,e). Because of the fast electrochemical reaction of I3−/I2, the electrochemical reaction kinetics of the whole battery can also be (Figure Figure 5 | RMs for conversion-type-mechanism-based aqueous batteries. (a) of charge-storage mechanism for S-based materials through an S ↔ S electrochemical conversion reaction. (b) Electrochemical process of RM with ↔ S reaction. (c) Spontaneous chemical oxidation of S2− with RMs. (d) curves of Zn∥S cell with and without Reproduced with permission from ref Copyright (e) curves of cell with and without (f) of at discharge currents from to 5 Reproduced with permission from ref Copyright 2021 Download figure Download PowerPoint the RM strategy has been in nonaqueous S-based the introduction of RMs to accelerate the kinetics of the aqueous S-based materials is still in its infancy. As the reaction mechanism of aqueous S-based batteries S ↔ S, S ↔ and ↔ reactions due to different charge For different reaction it is to various for RMs to the specific applications of RMs in nonaqueous Li-S batteries, a and may as RMs in S-based aqueous the introduction of RM chemistry can significantly enhance the electrochemical of a conversion-type redox reaction under challenging approaches that usually and interface which inevitably the RMs can be simply added to the electrolyte and thereby are for Redox mediators for redox flow aqueous batteries RMs have played essential roles in the process of redox flow aqueous batteries. redox flow aqueous batteries soluble dissolved in electrolytes and between the cell and the However, the cell voltage and the energy density are with the of is than that of solid materials. For instance, the redox flow aqueous batteries feature only and to a practical energy An solution is to flow batteries with electrolytes to M to the solubility However, this approach requires a large of which in high and To the above concerns, redox flow aqueous batteries that solid materials in to expand the energy density have been In this the dissolved are the in energy but play a role as RMs. Specifically, during the charging process, RMs are oxidized → on the (i.e., current and then flow to the to further oxidize the solid AMs (Figure the RMred back to the and a new and the discharge process in a similar The strategy was first employed to solid in the two molecules, and as charging and However, the introduction of and a mV voltage when with