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Manganese (Mn), possessing ample reserves on the earth, exhibits various oxidation states and garners significant attentions within the realm of battery technology. Mn-based flow batteries (MFBs) are recognized as viable contenders for energy storage owing to their environmentally sustainable nature, economic feasibility, and enhanced safety features. Nevertheless, the advancement of MFBs is hindered by contentious reaction mechanisms, suboptimal energy density, and inadequate cycling stability. This review offers a comprehensive analysis of various MFBs based on the specific redox couples utilized in the catholyte, including Mn3+/Mn2+, MnO2/Mn2+, and MnO4−/MnO42−. Moreover, recent advancements and concerns encountered by each type of MFBs are subsequently addressed and discussed in detail. Additionally, the current understanding of the mechanisms for different Mn-based pairs and their potentials for energy storage applications are introduced. Finally, challenges for the future development of MFBs, along with suggested improvement strategies are outlined. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long‐term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ((PEG3/PerF)‐TTF) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene (TTF) was applied, with the incorporation of polyethylene glycol (PEG) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro (PerF) group to increase its redox potential. When paired with a lithium metal anode, the two‐electron‐active (PEG3/PerF)‐TTF catholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li|(PEG3/PerF)‐TTF batteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm2. Furthermore, owing to the high redox potential and solubility of (PEG3/PerF)‐TTF, the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical). [ABSTRACT FROM AUTHOR]

Nonaqueous flow batteries hold promise given their high cell voltage and energy density, but their performance is often plagued by the crossover of redox compounds. In this study, we used permselective lithium superionic conducting (LiSICON) ceramic membranes to enable reliable long-term use of organic redox molecules in nonaqueous flow cells. With different solvents on each side, enhanced cell voltages were obtained for a flow battery using viologen-based negolyte and TEMPO-based posolyte molecules. The thermoplastic assembly of the LiSICON membrane realized leakless cell sealing, thus overcoming the mechanical brittleness challenge. As a result, stable cycling was achieved in the flow cells, which showed good capacity retention over an extended test time. [ABSTRACT FROM AUTHOR]

In the pursuit of sustainable energy solutions, zinc‐based flow batteries stand out for their potential in large‐scale energy storage, offering a blend of cost efficiency and safety. Although the porous electrode provides an increased specific surface area for reaction, the non‐uniform deposition of zinc, attributed to uneven concentration distribution within the porous electrode, has been a pivotal issue in accelerating the formation of zinc dendrites, which hinders the enhancement of energy density. A composite electrode with a strategic hierarchical pore structure has been developed with aligned nitrogen‐doped carbon fibers and traditional carbon felt. This structure takes advantage of the large pores of the carbon felt for efficient through‐flow paths, ensuring higher flow rates, while the dual‐scale pores within the electrospun film enhance mass transfer and increase the specific surface area. At 320 mA cm−2, it achieved an ≈11.4% improvement in the battery's energy efficiency. Moreover, the nitrogen doping and the optimized reaction uniformity within the composite electrode have been instrumental in reducing the generation of zinc dendrites. The battery, integrated with the innovative composite electrode, maintained an energy efficiency of 59.2% at 320 mA cm−2 after 300 cycles, which is a substantial improvement in operational longevity. [ABSTRACT FROM AUTHOR]

Development and application of large‐scale energy storage systems are surging due to the increasing proportion of intermittent renewable energy sources in the global energy mix. Redox flow batteries are prime candidates for large‐scale energy storage due to their modular design and scalability, flexible operation, and ability to decouple energy and power. To date, several different redox couples are exploited in redox‐flow batteries; some are already commercialized. This battery technology is facing a lot of challenges in the science, engineering, and economic front. Issues plaguing flow batteries are low energy density, high overall cost, poor stability of electrolytes, shifting of solvent from anolyte to catholyte while using cation exchange membrane, reverse flow with anion exchange membrane, and corrosion of graphite felt in the catholyte side. Significant research efforts are ongoing to address these challenges. This comprehensive and critical review summarizes the recent progress in electrolyte technologies, including electrochemical performance and stability, strategies to enhance the energy and power densities and, in the end, the levelized and life‐cycle cost of these batteries analyzed. A comprehensive outlook on this technology with respect to practical energy storage applications is also provided. [ABSTRACT FROM AUTHOR]

Among multivalent redox flow batteries, the Zn‐based redox flow battery (RFB) has the advantages of high energy density, nontoxicity, and low cost. However, the severe dendrite issue hinders its deployment. Therefore, the Sn‐based RFB is reported as a prospective alternative, particularly in alkaline systems, due to its lower redox potential and four electrons transfer in redox reactions. However, the continuous growth of Sn during charge forms large Sn bulk, which increases the risk of “dead Sn” formation. By combining experiments and theoretical calculations, urea is proposed as an electrolyte additive to regulate Sn deposition on graphite felt. This is achieved through the enhanced adsorption and robust reaction of Sn(OH)62− with graphite felt. Consequently, a more uniform morphology ensues, reducing “dead Sn” formation and thus improving the average discharge voltage and areal capacity. Moreover, the Columbic efficiency becomes more stable, extending the cycle life to over 420 h (>200 cycles) without deep discharge during cycling. This study demonstrates the role of urea in achieving high‐performance Sn‐I flow battery with long cycle life, paving the way for further development of metal‐based hybrid RFBs. [ABSTRACT FROM AUTHOR]

Alkaline zinc‐ferricyanide flow batteries are efficiency and economical as energy storage solutions. However, they suffer from low energy density and short calendar life. The strongly alkaline conditions (3 mol L−1 OH−) reduce the solubility of ferri/ferro‐cyanide (normally only 0.4 mol L−1 at 25 °C) and induce the formation of zinc dendrites at the anode. Here, we report a new zinc‐ferricyanide flow battery based on a mild alkalescent (pH 12) electrolyte. Using a chelating agent to rearrange ferri/ferro‐cyanide ion‐solvent interactions and improve salt dissociation, we increased the solubility of ferri/ferro‐cyanide to 1.7 mol L−1 and prevented zinc dendrites. Our battery has an energy density of ~74 Wh L−1catholyte at 60 °C and remains stable for 1800 cycles (1800 hours) at 0 °C and for >1400 cycles (2300 hours) at 25 °C. An alkalescent zinc‐ferricyanide cell stack built using this alkalescent electrolyte stably delivers 608 W of power for ~40 days. [ABSTRACT FROM AUTHOR]

Tin‐based hybrid flow batteries have demonstrated dendrite‐free morphology and superior performance in terms of cycle life and energy density. However, the quick accumulation of electrodeposits near the electrode/membrane interface blocks the ion transport pathway during the charging of the battery, resulting to a very limited areal capacity (especially at high current density) that significantly hinders its deployment in long‐duration storage applications. Herein, a conductivity‐activity dual‐gradient design is disclosed by electrically passivating the carbon felt near the membrane/electrode interface and chemically activating the carbon felt near the electrode/current collector interface. In consequence, the tin metals are preferentially plated at the region near electrode/current collector, preventing the ion transport pathway from being easily blocked. The resultant gradient electrode demonstrated an unprecedentedly high areal capacity of 268 mAh cm−2 at a current density of as high as 80 mA cm−2. Numerical modeling and experimental characterizations show that the dual‐gradient electrode differs from conventional electrodes with regard to their reaction current density distribution and electrodeposit distribution during charging. This work demonstrates a new design strategy of 3D electrodes for hybrid flow batteries to induce a desirable distribution of electrodeposits and achieve a high areal capacity at commercially relevant current densities. [ABSTRACT FROM AUTHOR]

The 6-electron electrochemical reduction of IO3− to I− represents a breakthrough for the development of next-generation redox flow batteries, offering substantially higher energy densities for oxidizer storage. Our study reveals that on a glassy carbon (GC) electrode in acidic electrolytes, HIO3 undergoes an autocatalyzed electrochemical reduction to I−. This process is mediated by the formation of a thin iodine layer on the electrode, acting as an intermediate and a catalyst. Under steady-state conditions, the iodine layer forms via a comproportionation reaction (HIO3 + I− + 5H+ = I2 (s) + 3H2O). Initially, the iodine layer is generated through the slow direct electrochemical reduction of HIO3 on pristine GC. Once established, this layer significantly enhances the rate of iodate reduction. On voltammetry curves, it is clearly observable as a step-wise current surge to reach a plateau. The limiting current density on the GC seemingly aligns with the Levich equation, varying with the RDE rotation rate. Earlier, we demonstrated the electrochemical oxidation of I− back to HIO3 using an H2/HIO3 flow cell, showcasing a full cycle that underpins the feasibility of this approach for energy storage. This study advances the understanding of iodate electroreduction and underscores its role in enhancing the capacity of next-generation energy storage systems. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries (VRFBs) hold great promise for large‐scale energy storage, but their performance requires further improvement. Herein, a design is proposed for vanadium colloid flow batteries (VCFBs) that integrates the redox chemistry of polyvalent vanadium‐based colloid suspensions with dispersed conductive agents into traditional vanadium electrolytes. The redox‐active colloids combine the advantages of nanoparticle suspensions and dissolved electrolytes, exhibiting good dispersibility, fluidity, conductivity, redox reversibility, and electrochemical kinetics. By leveraging a reversible dissolution/suspension process of high‐concentration vanadium‐based colloids, the VCFBs achieved an energy density of 48 Wh L−1, nearly double that of conventional VRFBs. This work presents a rational design for homologous active material colloids to enhance the energy density of aqueous redox flow batteries, thereby advancing the potential for grid‐scale and renewable energy storage. [ABSTRACT FROM AUTHOR]

Zinc bromine flow batteries (ZBFBs) are well suited for stationary energy storage due to their attractive features of high energy density and low cost. Nevertheless, the ZBFBs suffer from low power density and limited efficiency owing to the relatively severe polarization of the Br2/Br− redox couple. Herein, a three‐dimensional (3D) hierarchical composite electrode based on core‐shell Ni/NiO heterostructures anchored on graphite felt (Ni/NiO@GF) is designed to promote the kinetics of the Br2/Br− couple, so as to improve the power density and efficiency of the ZBFB. In this design, the highly conductive carbon felt and Ni cores provide a composite electrode with a 3D electron transporting framework to guarantee excellent electronic conductivity, while the NiO shells possess great absorption ability to Br2 and brilliant catalytic activity for the Br2/Br− redox reaction to reduce the electrochemical polarization. As a result, an enhanced ZBFB with Ni/NiO@GF electrode shows an outstanding energy efficiency of 86% at 20 mA cm−2 and can be operated at a current density of up to 160 mA cm−2 with a respectable energy efficiency of 67%. These results exhibit a promising strategy to fabricate catalytic electrodes for high‐performance ZBFBs. [ABSTRACT FROM AUTHOR]

Flexible, scalable, and low‐cost energy storage solutions are required for the widespread use of renewable energy and the mitigation of climate change. State‐of‐the‐art lithium‐ion batteries provide high specific energy density; however, designing a safe and cost‐effective grid‐scale lithium‐ion battery is still a major challenge. Redox flow batteries are scalable due to their ability to decouple power and energy; however, the commercial applications of these batteries are limited because of expensive ion‐selective membranes. In this paper, we report a modified battery design approach in which Bi/BiOCl and V4+/V5+ reaction‐based redox couples are utilized while employing a gel‐based architecture. We show, for the first time, that Bi/BiOCl conversion reaction based redox couple can reversibly work against traditional vanadium‐based redox pair in an aqueous electrolyte. Redox active materials in this cell design are in the gel form, and a traditional membrane or a separator is not required. This proof‐of‐concept battery delivers 0.9 V with a volumetric energy density of 22.14 Wh/L. [ABSTRACT FROM AUTHOR]

Bromine‐based flow batteries (Br‐FBs) are highly competitive for stationary energy storage due to their high energy density and cost‐effectiveness. However, adding bromine complexing agents (BCAs) to electrolytes slows down Br2/Br− reaction kinetics, causing higher polarization and lower power density of Br‐FBs. Herein, in situ vertically aligned MoS2 nanosheet arrays on traditional carbon felt substrates as electrodes to construct high power–density BCA‐free Br‐FBs are proposed. MoS2 arrays exhibit strong adsorption capacity to bromine, which helps the electrodes capture and retain bromine species. Even without BCAs, the battery self‐discharge caused by bromine diffusion is also inhibited. Moreover, the rate‐determining step of Br2/Br− reactions is boosted and the vertically aligned array structure provides sufficient sites, motivating Br2/Br− reaction kinetics and decreasing the battery polarization. The capacity retention rate of the BCA‐free Br‐FB based on MoS2 arrays‐based electrodes reaches 46.34% after the 24‐h standing test at 80 mA cm−2, meeting the requirements of practical applications. Most importantly, this BCA‐free Br‐FB exhibits a high Coulombic efficiency of 97.00% and an ultralong cycle life of 1000 cycles at a high current density of 200 mA cm−2. This work provides an available approach to developing advanced electrode materials for high power–density and long‐lifespan Br‐FBs. [ABSTRACT FROM AUTHOR]

In recent years, aqueous organic redox flow batteries (AORFBs) have attracted considerable attention due to advancements in grid‐level energy storage capacity research. These batteries offer remarkable benefits, including outstanding capacity retention, excellent cell performance, high energy density, and cost‐effectiveness. The organic electrolytes in AORFBs exhibit adjustable redox potentials and tunable solubilities in water. Previously, various types of organic electrolytes, such as quinones, organometallic complexes, viologens, redox‐active polymers, and organic salts, were extensively investigated for their electrochemical performance and stability. This study presents an overview of recently published novel organic electrolytes for AORFBs in acidic, alkaline, and neutral environments. Furthermore, it delves into the current status, challenges, and prospects of AORFBs, highlighting different strategies to overcome these challenges, with special emphasis placed on their design, composition, functionalities, and cost. A brief techno‐economic analysis of various aqueous RFBs is also outlined, considering their potential scalability and integration with renewable energy systems. [ABSTRACT FROM AUTHOR]

The use of graphene oxide (GO) has shown potential in improving the performance of redox flow lithium‐ion batteries (RFLIBs). These types of batteries use a liquid electrolyte containing redox‐active species to store and release energy. Despite being scalable, RFLIBs face limitations, namely, low energy density of the electrolyte and reduced cycling stability of the electrodes. However, GO's unique properties, such as its high conductivity as well as large surface area, create an attractive option for enhancing the electrochemical properties of both the electrolyte and electrodes in RFLIBs. When used as an electrode, GO improves the kinetic reversibility reactions, leading to increased electrochemical activity towards redox couples. Charge transfer resistances of positive and negative reactions are reduced, leading to increased voltage energy and efficiency of lithium batteries in terms of energy usage. As redox flow batteries made of lithium ions are an established subsystem and a growing research and development field, there is potential to enhance their performance and reduce costs through the use of GO. The objective of this review is to provide an overview of the chemistry of GO as it pertains to RFLIBs use, covering topics such as its surface chemistry, functionalization, and interactions with redox‐active species, as well as its potential for enhancing high‐density storage of electricity in batteries. Specifically, it will discuss the impact of GO on redox reactions in the electrolyte, including its ability to raise the redox‐active species concentration as well as enhance their stability. The review will also examine how GO impacts the electrodes, including its potential to increase their surface area and conductivity and promote cycling stability. Additionally, the review will address the importance of optimizing the quantity and distribution of GO in both the electrolyte and electrodes of RFLIBs. [ABSTRACT FROM AUTHOR]

Non-aqueous redox flow batteries (NARFBs) have been attracting much attention because they can significantly increase power and energy density compared to conventional RFBs. In this study, novel pore-filled anion-exchange membranes (PFAEMs) for application to a NAPFB employing metal polypyridyl complexes (i.e., Fe(bpy)32+/Fe(bpy)33+ and Co(bpy)32+/Co(bpy)33+) as the redox species are successfully developed. A porous polyethylene support with excellent solvent resistance and mechanical strength is used for membrane fabrication. The PFAEMs are prepared by filling an ionic liquid monomer containing an imidazolium group and a crosslinking agent into the pores of the support film and then performing in situ photopolymerization. As a result, the prepared membranes exhibit excellent mechanical strength and stability in a non-aqueous medium as well as high ion conductivity. In addition, a low crossover rate for redox ion species is observed for the prepared membranes because they have relatively low swelling characteristics in non-aqueous electrolyte solutions and low affinity for the metal-complex redox species compared to a commercial membrane. Consequently, the PFAEM is revealed to possess superior battery performance than a commercial membrane in the NARFB tests, showing high energy efficiency of about 85% and stable operation for 100 cycles. [ABSTRACT FROM AUTHOR]

Electrolytes containing multiple redox couples are promising for improving the energy density of flow batteries. Here, two chelated chromium complexes that are structural isomers are characterized and combined to generate electrolytes containing up to 2 M of active species, corresponding to 53.6 Ah L−1. The mixed isomer approach enables a significantly higher active material content than the individual materials would allow, affording energy dense cells with Coulombic efficiencies of ≥99.6% at 100 mA cm−2 and an open circuit voltage of 1.65 V at 50% state-of-charge. This high concentration, however, comes with a caveat; at a given concentration, an equimolar mixed electrolyte leads to lower voltage efficiency compared to using the individual isomers, while Coulombic efficiency remains constant. Our work demonstrates that exploiting structural isomerism is an efficient approach to improve capacity, but active materials must be selected carefully in mixed systems as differences in operating potentials negatively affect energy efficiency. [ABSTRACT FROM AUTHOR]

High solubility of active materials is crucial for achieving a high‐energy‐density catholyte/anolyte in redox flow batteries. However, solubility largely depends on the compatibility with electrolyte, limiting the types of redox‐active materials that can be used in aqueous electrolytes. Herein, a universal strategy is introduced to attain a high solubility of active materials regardless of the compatibility with aqueous electrolytes while preserving their intrinsic redox activity via micellar solubilization. Leveraging the amphiphilic nature of surfactant molecules, insoluble redox‐active materials are encapsulated by surfactants to be dissolvable with significant solubility. As a demonstration, it is showed that an order‐of‐magnitude solubility enhancement can be achieved for (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) in aqueous catholyte (≈0.8 m). Consequently, the catholyte performance of TEMPO is fully harnessed, leading to an energy density enhancement of more than ten times compared to that in bare electrolyte. It is also observed that micellar solubilization unexpectedly improves the cycle stability, attributed to the mitigation of intermolecular side reactions and reduced crossover. Finally, the fundamental electrochemical reaction mechanism of micelle‐encapsulated TEMPO is discussed. This strategy offers a new insight regarding the solubility and stability of the catholyte/anolyte, and is expected to be applicable to other redox‐active molecules, opening up an unexplored micellar chemistry in redox flow batteries. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries (LIBs) are widely used as power sources for electric vehicles due to their various advantages, including high energy density and low self-discharge rate. However, the safety challenges associated with LIB thermal runaway (TR) still need to be addressed. In the present study, the effects of the battery SOC value and coolant flow rate on the TR behavior in a LIB pack are comprehensively investigated. The battery pack consists of 10 18650-type LIBs applied with the serpentine channel liquid-cooling thermal management system (TMS). The TR tests for various SOC values (50%, 75% and 100%) and coolant flow rates (0 L/h, 32 L/h, 64 L/h and 96 L/h) are analyzed. The retarding effect of the TMS on TR propagation is found to be correlated with both the coolant flow rate and the battery SOC value, and a larger coolant flow rate and lower SOC generally result in fewer TR batteries. Furthermore, the TR propagation rate, evaluated by the time interval of TR occurrence between the adjacent batteries, increases with the battery SOC. The battery pack with 100% SOC shows more rapid TR propagation, which can be completed in just a few seconds, in contrast to several minutes for 50% and 75% SOC cases. In addition, the impact of the battery SOC and coolant flow rate on the maximum temperature of the TR battery is also examined, and no determined association is observed between them. However, it is found that the upstream batteries (closer to the external heater) show a slightly higher maximum temperature than the downstream ones, indicating a weak association between the TR battery maximum temperature and the external heating duration or the battery temperature at which the TR starts to take place. [ABSTRACT FROM AUTHOR]

Neutral aqueous flow batteries with common traits of the redox flow batteries, such as the independence of energy and power, scalability and operational flexibility, and additional merits of outstanding safety and low corrosivity show great promise for storing massive electrical energy from solar and wind energy. Particularly, the ferricyanide/ferrocyanide ([Fe(CN)6]3−/4−) couple has been intensively employed as redox mediator to store energy in the catholyte ascribed to its abundance, low corrosivity, remarkable redox reversibility and stability. However, the low energy density arising from poor solubility of [Fe(CN)6]3−/4− restricts their commercial applications for energy storage systems. In this study, the practical energy density of a [Fe(CN)6]3−/4−‐based catholyte is significantly boosted from 10.5 to 92.8 Wh L−1 by combining the counter‐ion effect and the single‐molecule redox‐targeting (SMRT) reactions between [Fe(CN)6]3−/4− and Prussian blue (Fe4[Fe(CN)6]3, PB)/Prussian white (PW). Paired with concentrated K2S anolyte, we demonstrate a neutral aqueous SMRT‐based PB−Fe/S flow battery with ultra‐long lifespan over 7000 cycles (4500 h) and ultra‐low chemical cost of electrolytes in the cell as 19.26 $ kWh−1. Remarkably, under the influences of SMRT reactions in the presence of PB granules in the catholyte, the capacity after 7000 cycles of the PB−Fe/S flow battery is 181.8 % of the initial capacity without PB. [ABSTRACT FROM AUTHOR]

Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm2 and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm2 and at a temperature of 27 °C. Redox flow batteries are promising energy storage systems but are limited in part due to high cost and low availability of membrane separators. Here, authors develop a membrane-free, nonaqueous 3.5 V all-organic lithium-based battery and demonstrate its operation in both static and flow conditions. [ABSTRACT FROM AUTHOR]

Due to the high solubility, high reversibility, and low cost of iodide, iodine-based redox flow batteries (RFBs) are considered to have great potential for upscaling energy storage. However, their further development has been limited by the low capacity of I- as one-third of the I- is used to form I3- (I2I-) during the charging process. Herein, we have demonstrated that the pseudohalide ion, thiocyanate (SCN-), is a promising complexing agent for catholyte of iodinebased RFBs to free up the I- by forming iodine-thiocyanate ions ([I2SCN]-) instead of I3-, unlocking the capacity of iodide. Applying this strategy, we have demonstrated iodine-based RFBs with full utilization of iodide to achieve high capacity and high energy density. Both the zinc/iodine RFB and polysulfide/iodine RFB with SCN- complex agent achieve their theoretical capacity of around 160 A h Lposolyte-1 (6.0MI- in catholyte). Therefore, the zinc/iodine RFB delivers a high energy density of 221.34Wh Lposolyte-1, and the polysulfide/iodine RFB achieves a highenergy density of 165.62Wh Lposolyte-1. It is believed that this effective catholyte engineering can be further generalized to other iodine-based RFBs, offering new opportunities to unlock the capacity of iodide and achieve high energy density for energy storage. [ABSTRACT FROM AUTHOR]

Redox flow batteries have been discussed as scalable and simple stationary energy storage devices. However, currently developed systems encounter less competitive energy density and high costs, restricting their wider application. There is a lack of appropriate redox chemistry, preferably based on active materials that are abundant in nature and show high solubility in aqueous electrolytes. A nitrogen‐centered redox cycle operating between the limiting species ammonia and nitrate via an eight‐electron redox reaction stayed practically unnoticed, albeit its ubiquity in biological processes. Ammonia or nitrate are world‐scale chemicals with high aqueous solubility, and are then comparably safe. We demonstrate here the successful implementation of such a nitrogen‐based redox cycle between ammonia and nitrate with eight‐electron transfer as a catholyte for Zn‐based flow batteries, which continuously worked for 12.9 days with 930 charging‐discharging cycles. A very competitive energy density of 577 Wh L−1 can be reached, which is well above most reported flow batteries (e.g. 8 times the standard Zn‐bromide battery), demonstrating that the nitrogen cycle with eight‐electron transfer can offer promising cathodic redox chemistry for safe, affordable, and scalable high‐energy‐density storage devices. [ABSTRACT FROM AUTHOR]

Redoxmers are organic active molecules storing electrochemical energy in nonaqueous redox flow batteries (NRFBs). Increasing the solubility of redoxmers is an important approach for increasing energy density of NRFBs as effective redoxmer concentration determines how much electricity can be stored in a given volume. Molecular engineering redoxmers towards liquid forms is regarded as one promising strategy as liquid redoxmers represent an extreme scenario where fluidity is maintained at maximum concentration using a minimum amount of supporting solvents. In this Perspective, recent examples of liquid redoxmers as well as their development strategy will be discussed. [ABSTRACT FROM AUTHOR]

High energy density and cost‐effective zinc‐iodide flow battery (ZIFB) offers great promise for future grid‐scale energy storage. However, its practical performance is hindered by poor cyclability, because of irreversible zinc plating/stripping, slow kinetics of redox reactions, and solid I2 precipitation. Herein, we report NaCl‐supported electrolyte chemistry to address these issues simultaneously. The formation of soluble chloride anions by coordination interactions between Zn2+ and Cl−, are key factor for the improvement of Zn/Zn2+ redox reversibility. While formation of soluble I2Cl− complex stabilizes ZIFB from I2 precipitation, participation of Na+ and K+ as dominant migrating carriers passing through Nafion, restricts Zn2+ migration, thus, blocking electrolyte crossover. A ZIFB with this improved electrolyte stably cycled 100 times with excellent capacity retention and energy efficiency at 20 mA cm−2, while conventional ZIFB shows the trend of capacity fade from 10th cycle onwards. These encouraging results of NaCl‐added electrolyte chemistry enlighten great prospects for high‐performance ZIFB applications. [ABSTRACT FROM AUTHOR]

Semi-solid lithium slurry battery is an important development direction of lithium battery. It combines the advantages of traditional lithium-ion battery with high energy density and the flexibility and expandability of liquid flow battery, and has unique application advantages in the field of energy storage. In this study, the thermal stability of semi-solid lithium slurry battery material system was investigated for the first time employing C80 micro-calorimeter. In this new electrode material system, the heat generation of the electrolyte is the decisive factor for its thermal stability. Then, the measurement of in-situ dynamic cycle heat generation of semi-solid lithium slurry battery indicated a lower heat generation than traditional lithium-ion battery. What's more commendable is that its electrochemical performance is basically the same as traditional lithium-ion battery. In addition, through the HPPC test, the mechanism of semi-solid lithium slurry battery cycle heat generation was explained. So, in this work, a preliminary evaluation on the safety and cycling stability of semi-solid lithium slurry battery is carried out. The electrochemical performance test affirms the application prospects of semi-solid lithium slurry battery, and the evaluation on the fire safety provides a reference for the future industrial applications of semi-solid lithium slurry battery. [ABSTRACT FROM AUTHOR]

Biphasic self-stratified batteries (BSBs) provide a new direction in battery philosophy for large-scale energy storage, which successfully reduces the cost and simplifies the architecture of redox flow batteries. However, current aqueous BSBs have intrinsic limits on the selection range of electrode materials and energy density due to the narrow electrochemical window of water. Thus, herein, we develop nonaqueous BSBs based on Li-S chemistry, which deliver an almost quadruple increase in energy density of 88.5 Wh L−1 as compared with the existing aqueous BSBs systems. In situ spectral characterization and molecular dynamics simulations jointly elucidate that while ensuring the mass transfer of Li+, the positive redox species are strictly confined to the bottom-phase electrolyte. This proof-of-concept of Li-S BSBs pushes the energy densities of BSBs and provides an idea to realize massive-scale energy storage with large capacitance. The use of energy-dense materials is inherently limited in biphasic self-stratified batteries due to the aqueous electrolyte environment. Here, the authors extended the concept of biphasic self-stratified batteries to non-aqueous systems, resulting in increased energy density and output voltage. [ABSTRACT FROM AUTHOR]

Redox targeting reaction is an emerging idea for boosting the energy density of redox-flow batteries: mobile redox mediators transport electrical charges in the cells, whereas large-density electrode-active materials are fixed in tanks. This study reports 4 V-class organic polymer mediators using thianthrene derivatives as redox units. The higher potentials than conventional organic mediators (up to 3.8 V) enable charging LiMn2O4 as an inorganic cathode offering a large theoretical volumetric capacity of 500 Ah/L. Soluble or nanoparticle polymer design is beneficial for suppressing crossover reactions (ca. 3% after 300 h), simultaneously contributing to mediation reactions. The successful mediation cycles observed by repeated charging/discharging steps indicate the future capability of designing particle-based redox targeting systems with porous separators, benefiting from higher energy density and lower cost. [ABSTRACT FROM AUTHOR]

Zinc–bromine redox flow battery (ZBFB) is one of the most promising candidates for large-scale energy storage due to its high energy density, low cost, and long cycle life. However, numerical simulation studies on ZBFB are limited. The effects of operational parameters on battery performance and battery design strategy remain unclear. Herein, a 2D transient model of ZBFB is developed to reveal the effects of electrolyte flow rate, electrode thickness, and electrode porosity on battery performance. The results show that higher positive electrolyte flow rates can improve battery performance; however, increasing electrode thickness or porosity causes a larger overpotential, thus deteriorating battery performance. On the basis of these findings, a genetic algorithm was performed to optimize the batter performance considering all the operational parameters. It is found that the battery energy efficiency can reach 79.42% at a current density of 20   mA   cm − 2 . This work is helpful to understand the energy storage characteristics and high-performance design of ZBFB operating at various conditions. [ABSTRACT FROM AUTHOR]

Flow batteries (FBs) have been demonstrated in several large‐scale energy storage projects, and are considered to be the preferred technique for large‐scale long‐term energy storage in terms of their high safety, environmental friendliness, and long life, including all‐vanadium flow batteries (VFBs) and Fe‐Cr flow batteries (ICFBs). As the electrochemical reaction site, the electrode parameters, such as the specific surface area, active site, and so on, have a significant impact on the flow battery performance and reliability. Extensive research has been carried out on electrode modification to improve the current density and energy efficiency of the FBs. In this review, the reaction mechanisms of VFBs and ICFBs are discussed in detail firstly, and then the electrodes modification methods are overviewed and summarized from four aspects: self‐modification, carbon‐based electrocatalysts, metal‐based electrocatalysts and composite electrocatalysts. Finally, the recent catalytic mechanism, in situ characterization technology, and future research directions are presented. [ABSTRACT FROM AUTHOR]

There is a strong interest in finding highly soluble redox compounds to improve the energy density of redox flow batteries (RFBs). However, the performance of electrolytes is often negatively influenced by high solute concentration. Herein, we designed a high‐potential (0.5 V vs. Ag/Ag+) catholyte for RFBs, where the charged and discharged species are both gaseous nitrogen oxides (NOx). These species can be liberated from the liquid electrolyte and stored in a separate gas container, allowing scale‐up of storage capacity without increasing the concentration and volume of the electrolyte. The oxidation of NO in the presence of NO3− affords N2O3, and the reduction of N2O3 regenerates NO and NO3−, together affording the electrochemical reaction: NO3−+3 NO⇌2 N2O3+e− with a low mass/charge ratio of 152 grams per mole of stored electron. A proof‐of‐concept NOx symmetric H‐cell shows 200 stable cycles over 400 hours with >97 % Coulombic efficiency and negligible capacity decay. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium, (CBu)2V, as a highly stable, high capacity anolyte material under near pH neutral conditions. (CBu)2V can achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of (CBu)2V is attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)2V/(NH)4Fe(CN)6 AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L−1 for 1000 cycles, power densities of up to 85 mW cm−2, and an energy efficiency of 70 % at 60 mA cm−2. (CBu)2V not only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date. [ABSTRACT FROM AUTHOR]

Bromine polyhalide (BP) based redox flow batteries are considered as specific potential nominees for extreme energy level storage due to their high energy density as well as cost‐effectiveness. However, the BP system encountered with slow kinetics of Br2/Br− redox couple due to high positive electrode polarization results in poor power density. Herein, the heteroatom (nitrogen) doped reduced graphene oxide (N‐rGO) composite electrode is designed and used as a positive electrode to improve the electro‐catalytic activity of Br2/Br− redox couple. The cellulose (Rayon) based graphite felt incorporated with N‐rGO catalyst offers high conductivity and reduces the electrode polarization which in turn provides more catalytic sites for Br2/Br− redox reaction. N‐linked rGO preparation is carried out by two different methods namely, pyrolysis (thermally treated) and hydrothermally treated process called as HT‐N‐rGO@GF and Hy‐N‐rGO@GF, respectively. More impressively, the N‐rGO adopted graphite felt (N‐rGO@GF) exhibits remarkable rate performance even at 120 mA cm−2. Moreover, no detectable degradation is observed for both N‐rGO adopted felt over 100 cycles. The HT‐N‐rGO@GF and Hy‐N‐rGO@GF electrode shows 91.1% and 88.8% of Coulombic efficiency at 100th cycle respectively. On the other hand, the power density region of 1472 (HT‐N‐rGO@GF) and 1456 mW cm−2 (Hy‐N‐rGO@GF) are observed, which seems to be much better than that of pristine felt (1275 mW cm−2). [ABSTRACT FROM AUTHOR]

The energy density of redox flow batteries (RFBs) is generally affected by the standard electrode potential and the solubility of the redox active species. These crucial factors are closely related to the solvent in which the active materials are dissolved. Aqueous RFBs have been widely studied due to their excellent reaction kinetics and high solubility of the redox couple in aqueous media. However, the low voltage of conventional aqueous RFBs has hindered them from being candidates for practical applications. Recently, high‐voltage aqueous RFBs are implemented based on the low negative potential of the Zn/[Zn(OH)4]2− reaction in an alkaline solution. Here, we review recent progress in the design of high energy density RFBs in both aqueous and non‐aqueous electrolytes, notably focusing on the Zn/MnO2 hybrid RFBs in detail. Furthermore, strategies for inhibiting zinc dendritic growth and stabilizing manganese redox couple in the RFBs system are discussed. [ABSTRACT FROM AUTHOR]

Flow batteries are a promising energy storage solution. However, the footprint and capital cost need further reduction for flow batteries to be commercially viable. The flow cell, where electron exchange takes place, is a central component of flow batteries. Improving the volumetric power density of the flow cell (W/Lcell) can reduce the size and cost of flow batteries. While significant progress has been made on flow battery redox, electrode, and membrane materials to improve energy density and durability, conventional flow batteries based on the planar cell configuration exhibit a large cell size with multiple bulky accessories such as flow distributors, resulting in low volumetric power density. Here, we introduce a submillimeter bundled microtubular (SBMT) flow battery cell configuration that significantly improves volumetric power density by reducing the membrane-to-membrane distance by almost 100 times and eliminating the bulky flow distributors completely. Using zinc–iodide chemistry as a demonstration, our SBMT cell shows peak charge and discharge power densities of 1,322 W/Lcell and 306.1 W/Lcell, respectively, compared with average charge and discharge power densities of <60 W/Lcell and 45 W/Lcell, respectively, of conventional planar flow battery cells. The battery cycled for more than 220 h corresponding to >2,500 cycles at off-peak conditions. Furthermore, the SBMT cell has been demonstrated to be compatible with zinc–bromide, quinone–bromide, and all-vanadium chemistries. The SBMT flow cell represents a device-level innovation to enhance the volumetric power of flow batteries and potentially reduce the size and cost of the cells and the entire flow battery. [ABSTRACT FROM AUTHOR]

A tetracationic viologen based on dimethylaminoethanol is reported as a novel negolyte for aqueous‐organic flow batteries. The compound (1,1′‐bis(3‐((2‐hydroxyethyl)dimethylammonio)propyl)‐[4,4′‐bipyridine]‐1,1′‐diium tetrachloride), termed [(DMAE‐Pr)2‐Vi]Cl4, was prepared by replacing the trimethylammonium groups of a former analogue for dimethylaminoethanol derivatives as permanently charged moieties, improving its solubility and modifying its electronic structure, as revealed by DFT calculations. Its electrochemical properties and diffusion coefficient were investigated by voltammetry and DOSY. The battery concept was demonstrated by pairing [(DMAE‐Pr)2‐Vi]Cl4 with a TEMPOL posolyte using a low‐cost anion exchange membrane and a neutral‐pH supporting electrolyte. [(DMAE‐Pr)2‐Vi]Cl4 has great potential for enhancing energy density in viologen flow batteries. [ABSTRACT FROM AUTHOR]

In redox flow batteries (RFBs), energy density mainly depends on the volume and concentration of active species in electrolyte. However, the poor solubility of organic molecules limits the further development of aqueous organic redox flow batteries (AORFBs). Herein, the viologen core structure is modified with asymmetric functional groups such as propyl and triethyl ammonium propyl for enhancing the solubility up to 1.82 m in aqueous medium. Further, the modified viologen structure lowers the redox potential (−0.58 V vs Ag/AgCl) and significantly reduces radical dimerization. Interestingly, synthesized [1‐propyl‐1'‐(3‐triethylammonio)propyl‐4,4'‐bipyridinium tribromide] (PV3+) coupled with bromide/bromine (2Br−/Br2) redox species shows cell voltage (1.66 V). The tested PV3+/PV2+•||2Br−/Br2 RFB having Nafion‐117 membrane exhibits a round‐trip efficiency of 99% over 100 cycles. The self‐discharge behavior of PV3+/PV2+•||2Br−/Br2 RFB system retains 85% of the cell voltage over 20 h, indicating the lowest permeability of bromine in RFB. Thus, the present approach of viologen modification with asymmetric functionalities drastically improves the RFB performance in terms of energy density and cycle life of the system. [ABSTRACT FROM AUTHOR]

Mn2+/Mn3+ redox pair has been considered as a promising cathode for high energy density batteries, due to its attractive features of high redox potential, solubility and outstanding kinetics. However, the disproportionation side reaction of Mn3+, which results in accumulation of "dead" MnO2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn2+ and Br− was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO2 can be fully discharged via Br− by a chemical‐electrochemical reaction. Coupled with Cd/Cd2+ as anode, the assembled Bromine‐Manganese flow battery (BMFB) demonstrates a high energy efficiency of 76 % at 80 mA cm−2 with energy density of 360 Wh L−1. The battery assembled with silicotungstic acid as anode could continuously run for over 2000 cycles at 80 mA cm−2. With high power density, energy density and durability, the BMFB shows great potential for large‐scale energy storage. [ABSTRACT FROM AUTHOR]

Mn2+/Mn3+ redox pair has been considered as a promising cathode for high energy density batteries, due to its attractive features of high redox potential, solubility and outstanding kinetics. However, the disproportionation side reaction of Mn3+, which results in accumulation of "dead" MnO2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn2+ and Br− was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO2 can be fully discharged via Br− by a chemical‐electrochemical reaction. Coupled with Cd/Cd2+ as anode, the assembled Bromine‐Manganese flow battery (BMFB) demonstrates a high energy efficiency of 76 % at 80 mA cm−2 with energy density of 360 Wh L−1. The battery assembled with silicotungstic acid as anode could continuously run for over 2000 cycles at 80 mA cm−2. With high power density, energy density and durability, the BMFB shows great potential for large‐scale energy storage. [ABSTRACT FROM AUTHOR]

Mn2+/Mn3+ redox pair has been considered as a promising cathode for high energy density batteries, due to its attractive features of high redox potential, solubility and outstanding kinetics. However, the disproportionation side reaction of Mn3+, which results in accumulation of "dead" MnO2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn2+ and Br− was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO2 can be fully discharged via Br− by a chemical‐electrochemical reaction. Coupled with Cd/Cd2+ as anode, the assembled Bromine‐Manganese flow battery (BMFB) demonstrates a high energy efficiency of 76 % at 80 mA cm−2 with energy density of 360 Wh L−1. The battery assembled with silicotungstic acid as anode could continuously run for over 2000 cycles at 80 mA cm−2. With high power density, energy density and durability, the BMFB shows great potential for large‐scale energy storage. [ABSTRACT FROM AUTHOR]

Mn2+/Mn3+ redox pair has been considered as a promising cathode for high energy density batteries, due to its attractive features of high redox potential, solubility and outstanding kinetics. However, the disproportionation side reaction of Mn3+, which results in accumulation of "dead" MnO2 limits its reversibility and further energy density. Herein, a novel catholyte based on mixture of Mn2+ and Br− was proposed for flow batteries with high energy density and long cycle life. In the design, the "dead" MnO2 can be fully discharged via Br− by a chemical‐electrochemical reaction. Coupled with Cd/Cd2+ as anode, the assembled Bromine‐Manganese flow battery (BMFB) demonstrates a high energy efficiency of 76 % at 80 mA cm−2 with energy density of 360 Wh L−1. The battery assembled with silicotungstic acid as anode could continuously run for over 2000 cycles at 80 mA cm−2. With high power density, energy density and durability, the BMFB shows great potential for large‐scale energy storage. [ABSTRACT FROM AUTHOR]

Non‐aqueous redox flow batteries (RFBs) are emerging electrochemical technologies for grid energy storage. Non‐aqueous Mg RFBs that use Mg metal as the anode are especially promising due to various benefits of the Mg metal anode, including its low potential, high volumetric capacity, SEI‐free, highly reversible operation and low cost. Despite the potential, there are rarely any studies on developing non‐aqueous Mg RFBs. Herein, a non‐aqueous Mg redox flow battery using a polymer catholyte is reported. Through rational molecular engineering, a carbonyl‐based moiety is combined with a polyethylene glycol moiety to achieve a polymer with high voltage and high solubility in the ether‐based electrolyte. A series of polymers with different polyethylene glycol chain lengths are synthesized and their performances are measured first at the molecular level, and then at the device level in a Mg redox flow battery using a Mg foil as the anode, the polymer solution as the catholyte and a porous membrane as the separator. The flow battery delivers a voltage of 1.8 V, a maximum capacity of 475 mAh/L, an average Coulombic efficiency of 90.5%, an average voltage efficiency of 67.4%, an energy efficiency of 61.0%, and an energy density of 0.855 Wh/L. Systematic mechanistic studies are performed to understand the performance decay mechanism and possible strategies for future improvement are discussed. This work opens a new avenue for the development of energy storage technologies for grid electricity storage. [ABSTRACT FROM AUTHOR]

Aqueous organic redox flow batteries (AORFBs) have been recognized as a promising technology for large‐scale, long‐duration energy storage of renewables (e.g., solar and wind) by overcoming their intermittence and fluctuation. However, simultaneous demonstration of high energy densities and stable cycling are still challenging for AORFBs. Herein, asymmetrically substituted sulfonate viologen molecular designs, e.g. (1‐[3‐sulfonatopropyl]‐1′‐[4‐sulfonatobutane]‐4,4′‐bipyridinium (3,4‐S2V), as capacity dense, chemically stable anolytes for cation exchange AORFBs are presented. The robust cycling performance of 3,4‐S2V is confirmed using half‐cell and full‐cell flow battery studies at pH neutral conditions. The 3,4‐S2V based AORFB is demonstrated with a discharge capacity of 23.2 Ah L−1 for 1700 cycles or 100 days without observing chemical degradation. Furthermore, a 3,4‐S2V/(NH4)4[Fe(CN)6] AORFB with a discharge capacity of 259.9 mAh is demonstrated for 50 days of authentic energy storage for the first time with a total capacity retention of 97.77% or a temporal capacity retention rate of 99.955% per day, representing the most stable, longest cycled AORFB to date. [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFB) have emerged as one of the most promising technologies for large‐scale energy storage owing to their high safety, long operation life, and decoupled design of energy and power. However, the problems of high cost and low energy density restrict their further development. The cost merit and tunable structure of organic redox‐active materials have prompted the development of organic RFBs. The solubility of the redoxmer is recognized as a parameter that contributes directly to the energy density. Herein, we focus on strategies for enhancing the solubility of organic redoxmers in aqueous RFBs. The effects of incorporating different hydrophilic functional groups on the solubility of the redoxmer and its effect on the performance of other batteries are systematically and exhaustively described. Other strategies, such as molecular symmetry tuning and employing more soluble counterions and cosolvents, are also summarized. The development trends and prospects for organic RFBs are also discussed. [ABSTRACT FROM AUTHOR]

Zinc-bromine flow batteries (ZBFBs) offer great potential for large-scale energy storage owing to the inherent high energy density and low cost. However, practical applications of this technology are hindered by low power density and short cycle life, mainly due to large polarization and non-uniform zinc deposition. In this work, a systematic study is presented to decode the sources of voltage loss and the performance of ZBFBs is demonstrated to be significantly boosted by tailoring the key components (electrolyte, electrodes, and membranes) and operating conditions (flow rate and temperature). Results show that the optimized battery exhibits an energy efficiency of 74.14 % at a high current density of 400 mA cm−2 and is capable of delivering a current density up to 700 mA cm−2. Furthermore, a peak power density of 1.363 W cm−2 and a notable limiting discharge current density of ∼1.5 A cm−2 are achieved at room temperature. More remarkably, the battery is stably operated for over 1200 cycles (∼710 h) at 200 mA cm−2 and 60 mAh cm−2, which sheds light on the development of high-rate and long-life ZBFBs for next-generation energy storage. • A systematic study is presented to decode the sources of voltage losses of ZBFBs. • The key components and operating conditions are judiciously tailored. • The ZBFB shows an energy efficiency of 74.14 % at 400 mA cm−2. • The ZBFB delivers a peak power density of 1.363 W cm−2 at room temperature. • The ZBFB stably runs over 1200 cycles (∼710 h) at 200 mA cm−2 and 60 mAh cm−2. [ABSTRACT FROM AUTHOR]

Absorption thermal battery (ATB) has been regarded as a promising solution to increase the penetration of renewable energy in the energy supply. Currently, one of the biggest challenges for the ATB application is to overcome the attenuation problem in the discharging process caused by solution dilution. This study proposes a novel multi-level ATB to achieve adjustable cooling capacities. A three-level ATB is performed as an example and compared with the conventional ATB. Moreover, the solution flow rate regulation is further applied to reach more accurate supply-demand matching. Then, the effects of various parameters on the discharging performance are investigated. Results indicate that instead of gradually attenuating from 17.2 kW to 4.3 kW of the conventional ATB, three stable and switchable cooling capacities (i.e., 13.7 kW, 8.1 kW, and 5.1 kW) are achieved by a three-level ATB. The energy storage density of the three-level ATB (123.2 kWh/m3) is also higher than that of the conventional ATB (114.5 kWh/m3). Besides, three stable loads distributed in an arithmetic series have been satisfied accurately by the three-level absorption thermal battery with solution flow rate regulation. This work aims to provide theoretical references and suggestions for the development and applications of ATBs. • A novel multi-level absorption thermal battery is proposed for output adjustment. • Multiple levels of stable and switchable cooling capacities are achieved. • Multi-level cooling capacities are accurately remained with flow rate regulation. • The energy storage density of multi-level absorption thermal battery is improved. [ABSTRACT FROM AUTHOR]

High‐concentration operation of redox flow batteries (RFBs) is essential for increasing their energy‐storage capacity, but non‐acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy‐dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3‐propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L−1, than previously reported for non‐additive‐utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E‐620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm−2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies >99.7% and energy efficiencies >87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window. [ABSTRACT FROM AUTHOR]

This review aims to highlight the current advances in hybrid redox flow battery (HRFB) technology, encompassing one of the best combinations of efficiency, cost and flexibility due to its module construction, which offers independent scaling of power density and energy capacity. This work emphasizes the interest of the scientific community both in (i) studying the properties and principles of HRFB operation in order to improve commonly proposed systems, and in (ii) the development of energy storage devices with new reagent types or RFB concepts. The data provided enhances the reader to conclude whether novel concepts in halogen oxidizers utilization could help to overcome the problem of insufficient power and energy densities of common RFB. [ABSTRACT FROM AUTHOR]