Towards High‐Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species

Nonaqueous redox flow batteries are emerging flow‐based energy storage technologies that have the potential for higher energy densities than their aqueous counterparts because of their wider voltage windows. However, their performance has lagged far behind their inherent capability due to one major limitation of low solubility of the redox species. Here, a molecular structure engineering strategy towards high performance nonaqueous electrolyte is reported with significantly increased solubility. Its performance outweighs that of the state‐of‐the‐art nonaqueous redox flow batteries. In particular, an ionic‐derivatized ferrocene compound is designed and synthesized that has more than 20 times increased solubility in the supporting electrolyte. The solvation chemistry of the modified ferrocene compound. Electrochemical cycling testing in a hybrid lithium–organic redox flow battery using the as‐synthesized ionic‐derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic‐charged pendant significantly improves the system energy density. When coupled with a lithium‐graphite hybrid anode, the hybrid flow battery exhibits a cell voltage of 3.49 V, energy density about 50 Wh L^?1, and energy efficiency over 75%. These results reveal a generic design route towards high performance nonaqueous electrolyte by rational functionalization of the organic redox species with selective ligand.     

Na3V2(PO4)3 as the Sole Solid Energy Storage Material for Redox Flow Sodium‐Ion Battery

Redox flow batteries have considerable advantages of system scalability and operation flexibility over other battery technologies, which makes them promising for large‐scale energy storage application. However, they suffer from low energy density and consequently relatively high cost for a nominal energy output. Redox targeting–based flow batteries are employed by incorporating solid energy storage materials in the tank and present energy density far beyond the solubility limit of the electrolytes. The success of this concept relies on paring suitable redox mediators with solid materials for facilitated reaction kinetics and lean electrolyte composition. Here, a redox targeting‐based flow battery system using the NASICON‐type Na₃V₂(PO₄)₃ as a capacity booster for both the catholyte and anolyte is reported. With 10‐methylphenothiazine as the cathodic redox mediator and 9‐fluorenone as anodic redox mediator, an all‐organic single molecule redox targeting–based flow battery is developed. The anodic and cathodic capacity are 3 and 17 times higher than the solubility limit of respective electrolyte, with which a full cell can achieve an energy density up to 88 Wh L^?1. The reaction mechanism is scrutinized by operando and in‐situ X‐ray and UV–vis absorption spectroscopy. The reaction kinetics are analysed in terms of Butler–Volmer formalism.     

A High Energy Density Aqueous Battery Achieved by Dual Dissolution/Deposition Reactions Separated in Acid‐Alkaline Electrolyte

Aqueous batteries are facing big challenges in the context of low working voltages and energy density, which are dictated by the narrow electrochemical window of aqueous electrolytes and low specific capacities of traditional intercalation‐type electrodes, even though they usually represent high safety, low cost, and simple maintenance. For the first time, this work demonstrates a record high‐energy‐density (1503 Wh kg^?1 calculated from the cathode active material) aqueous battery system that derives from a novel electrolyte design to expand the electrochemical window of electrolyte to 3 V and two high‐specific‐capacity electrode reactions. An acid‐alkaline dual electrolyte separated by an ion‐selective membrane enables two dissolution/deposition electrode redox reactions of MnO₂/Mn²⁺ and Zn/Zn(OH)₄^2? with theoretical specific capacities of 616 and 820 mAh g^?1, respectively. The newly proposed Zn–Mn²⁺ aqueous battery shows a high Coulombic efficiency of 98.4% and cycling stability of 97.5% of discharge capacity retention for 1500 cycles. Furthermore, in the flow battery based on Zn–Mn²⁺ pairs, more excellent stability of 99.5% of discharge capacity retention for 6000 cycles is achieved due to greatly improved reversibility of the Zn anode. This work provides a new path for the development of novel aqueous batteries with high voltage and energy density.     

In Situ Electrochemical Synthesis of MXenes without Acid/Alkali Usage in/for an Aqueous Zinc Ion Battery

The traditional method to fabricate a MXene based energy storage device starts from etching MAX phase particles with dangerous acid/alkali etchants to MXenes, followed by device assembly. This is a multistep protocol and is not environmentally friendly. Herein, an all‐in‐one protocol is proposed to integrate synthesis and battery fabrication of MXene. By choosing a special F‐rich electrolyte, MAX V₂AlC is directly exfoliated inside a battery and the obtained V₂CT_X MXene is in situ used to achieve an excellent battery performance. This is a one‐step process with all reactions inside the cell, avoiding any contamination to external environments. Through the lifetime, the device experiences three stages of exfoliation, electrode oxidation, and redox of V₂O₅. While the electrode is changing, the device can always be used as a battery and the performance is continuously enhanced. The resulting aqueous zinc ion battery achieves outstanding cycling stability (4000 cycles) and rate performance (97.5 mAh g^?1 at 64 A g^?1), distinct from all reported aqueous MXene‐based counterparts with pseudo‐capacitive properties, and outperforming most vanadium‐based zinc ion batteries with high capacity. This work sheds light on the green synthesis of MXenes, provides an all‐in‐one protocol for MXene devices, and extends MXenes’ application in the aqueous energy storage field.     

Membrane‐Free Zn/MnO2 Flow Battery for Large‐Scale Energy Storage

The traditional Zn/MnO₂ battery has attracted great interest due to its low cost, high safety, high output voltage, and environmental friendliness. However, it remains a big challenge to achieve long‐term stability, mainly owing to the poor reversibility of the cathode reaction. Different from previous studies where the cathode redox reaction of MnO₂/MnOOH is in solid state with limited reversibility, here a new aqueous rechargeable Zn/MnO₂ flow battery is constructed with dissolution–precipitation reactions in both cathodes (Mn²⁺/MnO₂) and anodes (Zn²⁺/Zn), which allow mixing of anolyte and catholyte into only one electrolyte and remove the requirement for an ion selective membrane for cost reduction. Impressively, this new battery exhibits a high discharge voltage of ≈1.78 V, good rate capability (10C discharge), and excellent cycling stability (1000 cycles without decay) at the areal capacity ranging from 0.5 to 2 mAh cm^‐2. More importantly, this battery can be readily enlarged to a bench scale flow cell of 1.2 Ah with good capacity retention of 89.7% at the 500th cycle, displaying great potential for large‐scale energy storage.     

A Low‐Cost Durable Na‐FeCl2 Battery with Ultrahigh Rate Capability

Na‐based batteries have long been regarded as an inexpensive, sustainable candidate for large‐scale stationary energy storage applications. Unfortunately, the market penetration of conventional Na‐NiCl₂ batteries is approaching its limit for several reasons, including limited rate capability and high Ni cost. Herein, a Na‐FeCl₂ battery operating at 190 °C is reported that allows a capacity output of 116 mAh g^?1 at an extremely high current density of 33.3 mA cm^?2 (≈0.6C). The superior rate performance is rooted in the intrinsically fast kinetics of the Fe/Fe²⁺ redox reaction. Furthermore, it is demonstrated that a small amount of Ni additive (10 mol%) effectively mitigates capacity fading of the Fe/NaCl cathode caused by Fe particle pulverization during long‐term cycling. The modified Fe/Ni cathode exhibits excellent cycling stability, maintaining a discharge energy density of over 295 Wh kg^?1 for 200 cycles at 10 mA cm^?2 (≈C/5).     

High‐Performance Aqueous Rechargeable Li‐Ni Battery Based on Ni(OH)2/NiOOH Redox Couple with High Voltage

New energy storage and conversion systems require large‐scale, cost‐effective, good safety, high reliability, and high energy density. This study demonstrates a low‐cost and safe aqueous rechargeable lithium‐nickel (Li‐Ni) battery with solid state Ni(OH)₂/NiOOH redox couple as cathode and hybrid electrolytes separated by a Li‐ion‐conductive solid electrolyte layer. The proposed aqueous rechargeable Li‐Ni battery exhibits an approximately open‐circuit potential of 3.5 V, outperforming the theoretic stable window of water 1.23 V, and its energy density can be 912.6 W h kg^‐1, which is much higher than that of state‐of‐the‐art lithium ion batteries. The use of a solid‐state redox couple as cathode with a metallic lithium anode provides another postlithium chemistry for practical energy storage and conversion.     

High‐Performance Flexible Solid‐State Ni/Fe Battery Consisting of Metal Oxides Coated Carbon Cloth/Carbon Nanofiber Electrodes

Aqueous Ni/Fe batteries have great potential as flexible energy storage devices, owing to their low cost, low toxicity, high safety, and high energy density. However, the poor cycling stability has limited the widely expected application of Ni/Fe batteries, while the use of heavy metal substrates cannot meet the basic requirement for flexible devices. In this work, a flexible type of solid‐state Ni/Fe batteries with high energy and power densities is rationally developed using needle‐like Fe₃O₄ and flake‐like NiO directly grown on carbon cloth/carbon nanofiber (CC–CF) matrix as the anode and cathode, respectively. The hierarchical CC–CF substrate with high electric conductivity and good flexibility serves as an ideal support for guest active materials of nanocrystalline Fe₃O₄ and NiO, which can effectively buffer the volume change giving rise to good cycling ability. By utilizing a gel electrolyte, a robust and mechanically flexible quasi‐solid‐state Ni/Fe full cell can be assembled. It demonstrates optimal electrochemical performance, such as high energy density (5.2 mWh cm^?3 and 94.5 Wh Kg^?1), high power density (0.64 W cm^?3 and 11.8 KW Kg^?1), together with excellent cycling ability. This work provides an example of solid‐state alkaline battery with high electrochemical performance and mechanical flexibility, holding great potential for future flexible electronic devices.     

A High Voltage Aqueous Zinc–Organic Hybrid Flow Battery

Water‐soluble redox‐active organic molecules have attracted extensive attention as electrical energy storage alternatives to redox‐active metals that are low in abundance and high in cost. Here an aqueous zinc–organic hybrid redox flow battery (RFB) is reported with a positive electrolyte comprising a functionalized 1,4‐hydroquinone bearing four (dimethylamino)methyl groups dissolved in sulfuric acid. By utilizing a three‐electrolyte, two‐membrane configuration this acidic positive electrolyte is effectively paired with an alkaline negative electrolyte comprising a Zn/[Zn(OH)₄]^2? redox couple and a hybrid RFB is operated at a high operating voltage of 2.0 V. It is shown that the electrochemical reversibility and kinetics of the organic redox species can be enhanced by an electrocatalyst, leading to a cyclic voltammetry peak separation as low as 35 mV and enabling an enhanced rate capability.     

A High‐Energy‐Density Multiple Redox Semi‐Solid‐Liquid Flow Battery

A new concept of multiple redox semi‐solid‐liquid (MRSSL) flow battery that takes advantage of active materials in both liquid and solid phases, is proposed and demonstrated. Liquid lithium iodide (LiI) electrolyte and solid sulfur/carbon (S/C) composite, forming LiI‐S/C MRSSL catholyte, are employed to demonstrate this concept. Record volumetric capacity (550 Ah L^?1_catholyte) is achieved using highly concentrated and synergistic multiple redox reactions of LiI and sulfur. The liquid LiI electrolyte is found to increase the reversible volumetric capacity of the catholyte, improve the electrochemical utilization of the S/C composite, and reduce the viscosity of catholyte. A continuous flow test is demonstrated and the influence of the flow rate on the flow battery performance is discussed. The MRSSL flow battery concept transforms inactive component into bi‐functional active species and creates synergistic interactions between multiple redox couples, offering a new direction and wide‐open opportunities to develop high‐energy‐density flow batteries.     

Solar Rechargeable Batteries Based on Lead–Organohalide Electrolyte

A dye‐sensitized solar cell (DSC) with in situ energy storage capacity is demonstrated using a lead–organohalide electrolyte CH₃NH₃I·PbCl₂ (LOC) to replace the conventional I^?/I₃^? electrolyte. The coupling of lead and iodine in one electrolyte creates a dual‐function rechargeable solar battery that combines the working processes of photoelectrochemical cells with electrochemical batteries. Optimization of the H⁺ concentration in the electrolyte leads to increased photocharging efficiency and storage. The power conversion efficiency of the LOC–DSC is 8.6% under one sun illumination (AM 1.5, 100 mW cm^?2) as a DSC. When operating as a battery, Faraday efficiency can be achieved as high as 81.5% using a bromide‐based CH₃NH₃Br·PbBr₂ (LOB) electrolyte in a DSC configuration. This new cell design suggests a means of combining photovoltaic energy conversion and electrical energy storage.     

Achieving High‐Voltage and High‐Capacity Aqueous Rechargeable Zinc Ion Battery by Incorporating Two‐Species Redox Reaction

Herein, a two‐species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) incorporated in cobalt hexacyanoferrate (CoFe(CN)6) is proposed as a breakthrough to achieve jointly high‐capacity and high‐voltage aqueous Zn‐ion battery. The Zn/CoFe(CN)₆ battery provides a highly operational voltage plateau of 1.75 V (vs metallic Zn) and a high capacity of 173.4 mAh g^?1 at current density of 0.3 A g^?1, taking advantage of the two‐species redox reaction of Co(II)/Co(III) and Fe(II)/Fe(III) couples. Even under extremely fast charge/discharge rate of 6 A g^?1, the battery delivers a sufficiently high discharge capacity of 109.5 mAh g^?1 with its 3D opened structure framework. This is the highest capacity delivered among all the batteries using Prussian blue analogs (PBAs) cathode up to now. Furthermore, Zn/CoFe(CN)₆ battery achieves an excellent cycling performance of 2200 cycles without any capacity decay at coulombic efficiency of nearly 100%. One further step, a sol–gel transition strategy for hydrogel electrolyte is developed to construct high‐performance flexible cable‐type battery. With the strategy, the active materials can adequately contact with electrolyte, resulting in improved electrochemical performance (≈18.73% capacity increase) and mechanical robustness of the solid‐state device. It is believed that this study optimizes electrodes by incorporating multi redox reaction species for high‐voltage and high‐capacity batteries.     

A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4‐HO‐TEMPO Catholyte

Increasing worldwide energy demands and rising CO₂ emissions have motivated a search for new technologies to take advantage of renewables such as solar and wind energies. Redox flow batteries (RFBs) with their high power density, high energy efficiency, scalability (up to MW and MWh), and safety features are one suitable option for integrating such energy sources and overcoming their intermittency. However, resource limitation and high system costs of current RFB technologies impede wide implementation. Here, a total organic aqueous redox flow battery (OARFB) is reported, using low‐cost and sustainable methyl viologen (MV, anolyte) and 4‐hydroxy‐2,2,6,6‐tetramethylpiperidin‐1‐oxyl (4‐HO‐TEMPO, catholyte), and benign NaCl supporting electrolyte. The electrochemical properties of the organic redox active materials are studied using cyclic voltammetry and rotating disk electrode voltammetry. The MV/4‐HO‐TEMPO ARFB has an exceptionally high cell voltage, 1.25 V. Prototypes of the organic ARFB can be operated at high current densities ranging from 20 to 100 mA cm², and deliver stable capacity for 100 cycles with nearly 100% Coulombic efficiency. The MV/4‐HO‐TEMPO ARFB displays attractive technical merits and thus represents a major advance in ARFBs.     

Hollow NiCo2S4 Nanospheres Hybridized with 3D Hierarchical Porous rGO/Fe2O3 Composites toward High‐Performance Energy Storage Device

Hierarchical hollow NiCo₂S₄ microspheres with a tunable interior architecture are synthesized by a facile and cost‐effective hydrothermal method, and used as a cathode material. A three‐dimensional (3D) porous reduced graphene oxide/Fe₂O₃ composite (rGO/Fe₂O₃) with precisely controlled particle size and morphology is successfully prepared through a scalable facile approach, with well‐dispersed Fe₂O₃ nanoparticles decorating the surface of rGO sheets. The fixed Fe₂O₃ nanoparticles in graphene efficiently prevent the intermediates during the redox reaction from dissolving into the electrolyte, resulting in long cycle life. KOH activation of the rGO/Fe₂O₃ composite is conducted for the preparation of an activated carbon material–based hybrid to transform into a 3D porous carbon material–based hybrid. An energy storage device consisting of hollow NiCo₂S₄ microspheres as the positive electrode, the 3D porous rGO/Fe₂O₃ composite as the negative electrode, and KOH solution as the electrolyte with a maximum energy density of 61.7 W h kg^?1 is achieved owing to its wide operating voltage range of 0–1.75 V and the designed 3D structure. Moreover, the device exhibits a high power density of 22 kW kg^?1 and a long cycle life with 90% retention after 1000 cycles at the current density of 1 A g^?1.     

A Stable Vanadium Redox‐Flow Battery with High Energy Density for Large‐Scale Energy Storage

The all‐vanadium redox flow battery is a promising technology for large‐scale renewable and grid energy storage, but is limited by the low energy density and poor stability of the vanadium electrolyte solutions. A new vanadium redox flow battery with a significant improvement over the current technology is reported in this paper. This battery uses sulfate‐chloride mixed electrolytes, which are capable of dissolving 2.5 M vanadium, representing about a 70% increase in energy capacity over the current sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of ?5 to 50 °C, potentially eliminating the need for electrolyte temperature control in practical applications. This development would lead to a significant reduction in the cost of energy storage, thus accelerating its market penetration.     

A Novel Aluminum‐Ion Battery: Al/AlCl3‐[EMIm]Cl/Ni3S2@Graphene

Due to an ever‐increasing demand for electronic devices, rechargeable batteries are attractive for energy storage systems. A novel rechargeable aluminum‐ion battery based on Al³⁺ intercalation and deintercalation is fabricated with Ni₃S₂/graphene microflakes composite as cathode material and high‐purity Al foil as anode. This kind of aluminum‐ion battery comprises of an electrolyte containing AlCl₃ in an ionic liquid of 1‐ethyl‐3‐methylimidazolium chloride ([EMIm]Cl). Galvanostatic charge/discharge measurements have been performed in a voltage range of 0.1–2.0 V versus Al/AlCl₄ ^?. An initial discharge specific capacity of 350 mA h g^?1 at a current density of 100 mA g^?1 is achieved, and the discharge capacity remains over 60 mA h g^?1 and coulombic efficiency of 99% after 100 cycles. Typically, for the current density at 200 mA g^?1, the initial charge and discharge capacities are 300 and 235 mA h g^?1, respectively. More importantly, it should be emphasized that the battery has a high discharge voltage plateau (≈1.0 V vs Al/AlCl₄ ^?). These meaningful results represent a significant step forward in the development of aluminum‐ion batteries.     

A High‐Performance Composite Electrode for Vanadium Redox Flow Batteries

A composite electrode composed of reduced graphene oxide‐graphite felt (rGO‐GF) with excellent electrocatalytic redox reversibility toward V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples in vanadium batteries was fabricated by a facile hydrothermal method. Compared with the pristine graphite felt (GF) electrode, the rGO‐GF composite electrode possesses abundant oxygen functional groups, high electron conductivity, and outstanding stability. Its corresponding energy efficiency and discharge capacity are significantly increased by 20% and 300%, respectively, at a high current density of 150 mA cm^?2. Moreover, a discharge capacity of 20 A h L^?1 is obtained with a higher voltage efficiency (74.5%) and energy efficiency (72.0%), even at a large current density of 200 mA cm^?2. The prepared rGO‐GF composite electrode holds great promise as a high‐performance electrode for vanadium redox flow battery (VRFB).     

Rechargeable Iron–Sulfur Battery without Polysulfide Shuttling

Sulfur represents one of the most promising cathode materials for next‐generation batteries; however, the widely observed polysulfide dissolution/shuttling phenomenon in metal–sulfur redox chemistries has severely restricted their applications. Here it is demonstrated that when pairing the sulfur electrode with the iron metal anode, the inherent insolubility of iron sulfides renders the shuttling‐free nature of the Fe–S electrochemical reactions. Consequently, the sulfur electrode exhibits promising performance for Fe²⁺ storage, where a high capacity of ≈1050 mAh g^?1, low polarization of ≈0.16 V as well as stable cycling of 150 cycles are realized. The Fe–S redox mechanism is further revealed as an intriguing stepwise conversion of S₈ ? FeS₂ ? Fe₃S₄ ? FeS, where a low volume expansion of ≈32.6% and all‐solid‐state phase transitions facilitate the reaction reversibility. This study suggests an alternative direction to exploit sulfur electrodes in rechargeable transition metal–sulfur batteries.     

Additional Sodium Insertion into Polyanionic Cathodes for Higher‐Energy Na‐Ion Batteries

Na‐ion technology is increasingly studied as a low‐cost solution for grid storage applications. Many positive electrode materials have been reported, mainly among layered oxides and polyanionic compounds. The vanadium oxy/flurophosphate solid solution Na₃V₂(PO₄)₂F_3‐y O_2y (0 ≤ y ≤ 1), in particular, has proven the ability to deliver ≈500 Wh kg^‐1, operating on the V³⁺/V⁴⁺ (y = 0) or V⁴⁺/V⁵⁺ redox couples (y = 1). This paper reports here on a significant increase in specific energy by enabling sodium insertion into Na₃V₂(PO₄)₂FO₂ to reach Na₄V₂(PO₄)₂FO₂ upon discharge. This occurs at ≈1.6 V and increases the theoretical specific energy to 600 Wh kg^?1, rivaling that of several Li‐ion battery cathodes. This improvement is achieved by the judicious modification of the composition either as O for F substitution, or Al for V substitution, both of which disrupt Na‐ion ordering and thereby enable insertion of the 4th Na. This paper furthermore shows from operando X‐Ray Diffraction (XRD) that this energy is obtained in the cycling range Na₄V₂(PO₄)₂FO₂–NaV₂(PO₄)₂FO₂ with a very small overall volume change of 1.7%, which is one of the smallest volume changes for Na‐ion cathodes and which is a crucial requisite for stable long‐term cycling.     

Metal‐Organic Framework Cathodes Based on a Vanadium Hexacyanoferrate Prussian Blue Analogue for High‐Performance Aqueous Rechargeable Batteries

Despite the unique advantages of the metal‐organic framework of Prussian blue analogues (PBAs), including a favorable crystallographic structure and facile diffusion kinetics, the capacity of PBAs delivered in aqueous systems has been limited to ≈60 mA h g^?1 because only single species of transition metal ions incorporated into the PBAs are electrochemically activated. Herein, vanadium hexacyanoferrate (V/Fe PBA) is proposed as a breakthrough to this limitation, and its electrochemical performance as a cathode for aqueous rechargeable batteries (ARBs) is investigated for the first time. V/Fe PBAs are synthesized by a simple co‐precipitation method with optimization of the acidity and molar ratios of precursor solutions. The V/Fe PBAs provide an improved capacity of 91 mA h^?1 under a current density of 110 mA g^?1 (C‐rate of ≈1.2 C), taking advantage of the multiple‐electron redox reactions of V and Fe ions. Under an extremely fast charge/discharge rate of 3520 mA g^?1, the V/Fe PBA exhibits a sufficiently high discharge capacity of 54 mA h g^?1 due to its opened structure and 3D hydrogen bonding networks. V/Fe PBA‐based ARBs are the most promising candidates for large‐scale stationary energy storage systems due to their high electrochemical performance, reasonable cost, and high efficiency.