High‐Performance Lithium‐Iodine Flow Battery

A cathode‐flow lithium‐iodine (Li–I) battery is proposed operating by the triiodide/iodide (I₃^?/I^?) redox couple in aqueous solution. The aqueous Li–I battery has noticeably high energy density (≈0.28 kWh kg^?1_cell) because of the considerable solubility of LiI in aqueous solution (≈8.2 m ) and reasonably high power density (≈130 mW cm^?2 at a current rate of 60 mA cm^?2, 328 K). In the operation of cathode‐flow mode, the Li–I battery attains high storage capacity (≈90% of the theoretical capacity), Coulombic efficiency (100% ± 1% in 2–20 cycles) and cyclic performance (>99% capacity retention for 20 cycles) up to total capacity of 100 mAh.     

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.     

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.     

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.     

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).     

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.     

Fast and Reversible Four‐Electron Storage Enabled by Ethyl Viologen for Rechargeable Magnesium Batteries

Magnesium (Mg) batteries are the most promising “post‐lithium‐ion” energy storage technologies owing to their high theoretical energy density, low cost, and intrinsic safety with air and moisture. However, the development of Mg batteries has been limited to cathode materials leading to low power, low reversible energy density, and poor cycle life. Here, a new Mg cathode is reported based on ethyl viologen (EV), which not only has a fast redox couple EV²⁺/EV⁰ but also is capable of coupling with redox‐active anions, such as iodide (I^?), achieving a total four‐electron storage. The EV²⁺/EV⁰ redox couple demonstrates a superior rate performance (10 C) and stable cycle life (500 cycles) owing to intrinsic fast electrode kinetics. A high material utilization (>80%) can be achieved at 1.0 C under a high areal loading of 5 mg cm^?2. When coupling with iodide I^?, a reversible four‐electron storage is achieved with a high energy density (304.2 Wh kg^?1) and a stable cycle life (>100 cycles). This study provides effective strategies for designing reversible multielectron storage for high‐rate and high‐energy rechargeable Mg batteries.     

High‐Voltage Oxygen‐Redox‐Based Cathode for Rechargeable Sodium‐Ion Batteries

Recently, anionic‐redox‐based materials have shown promising electrochemical performance as cathode materials for sodium‐ion batteries. However, one of the limiting factors in the development of oxygen‐redox‐based electrodes is their low operating voltage. In this study, the operating voltage of oxygen‐redox‐based electrodes is raised by incorporating nickel into P2‐type Na_2/3[Zn_0.3Mn_0.7]O₂ in such a way that the zinc is partially substituted by nickel. As designed, the resulting P2‐type Na_2/3[(Ni_0.5Zn_0.5)_0.3Mn_0.7]O₂ electrode exhibits an average operating voltage of 3.5 V and retains 95% of its initial capacity after 200 cycles in the voltage range of 2.3–4.6 V at 0.1C (26 mA g^?1). Operando X‐ray diffraction analysis reveals the reversible phase transition: P2 to OP4 phase on charge and recovery to the P2 phase on discharge. Moreover, ex situ X‐ray absorption near edge structure and X‐ray photoelectron spectroscopy studies reveal that the capacity is generated by the combination of Ni²⁺/Ni⁴⁺ and O^2?/O^1? redox pairs, which is supported by first‐principles calculations. It is thought that this kind of high voltage redox species combined with oxygen redox could be an interesting approach to further increase energy density of cathode materials for not only sodium‐based rechargeable batteries, but other alkali‐ion battery systems.     

A High‐Energy NASICON‐Type Cathode Material for Na‐Ion Batteries

Over the last decade, Na‐ion batteries have been extensively studied as low‐cost alternatives to Li‐ion batteries for large‐scale grid storage applications; however, the development of high‐energy positive electrodes remains a major challenge. Materials with a polyanionic framework, such as Na superionic conductor (NASICON)‐structured cathodes with formula Na_xM₂(PO₄)₃, have attracted considerable attention because of their stable 3D crystal structure and high operating potential. Herein, a novel NASICON‐type compound, Na₄MnCr(PO₄)₃, is reported as a promising cathode material for Na‐ion batteries that deliver a high specific capacity of 130 mAh g^?1 during discharge utilizing high‐voltage Mn^2+/3+ (3.5 V), Mn^3+/4+ (4.0 V), and Cr^3+/4+ (4.35 V) transition metal redox. In addition, Na₄MnCr(PO₄)₃ exhibits a high rate capability (97 mAh g^?1 at 5 C) and excellent all‐temperature performance. In situ X‐ray diffraction and synchrotron X‐ray diffraction analyses reveal reversible structural evolution for both charge and discharge.     

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.     

Vanadate‐Based Materials for Li‐Ion Batteries: The Search for Anodes for Practical Applications

While the practical application of electrode materials depends intensively on the Li⁺ ion storage mechanisms correlating ultimately with the coulombic efficiency, reversible capacity, and morphology variation of electrode material upon cycling, only intercalation‐type electrode materials have proven viable for commercialization up to now. This paper reviews the promising anode materials of metal vanadates (M_xV_yO_z, M = Co, Cu, Mn, Fe, Zn, Ni, Li) that have high capacity, low cost, and abundant resource, and also discusses the related Li⁺ ion storage mechanism. It is concluded that most of these (M_xV_yO_z, M = Co, Cu, Mn, Fe, Zn, Ni) exhibit irreversible redox reactions upon lithiation/delithiation accompanied by large volume expansion, which is not favorable for industrial applications. In particular, Li₃VO₄ with specific intercalation Li⁺ ion storage mechanism and compatible merits of safety and energy density exhibits great potential for practical application. This review systematically summarizes the latest progress in Li₃VO₄ research, including the representative fabrication approaches for advanced morphology and state‐of‐the‐art technologies to boost performance and the morphology variation associated with Li⁺ ion storage mechanisms. Furthermore, an outlook on where breakthroughs for Li₃VO₄ may be most likely achieved will be provided.     

High‐Performance Reversible Aqueous Zn‐Ion Battery Based on Porous MnOx Nanorods Coated by MOF‐Derived N‐Doped Carbon

Rechargeable aqueous zinc‐ion batteries (ZIBs) have been emerging as potential large‐scale energy storage devices due to their high energy density, low cost, high safety, and environmental friendliness. However, the commonly used cathode materials in ZIBs exhibit poor electrochemical performance, such as significant capacity fading during long‐term cycling and poor performance at high current rates, which significantly hinder the further development of ZIBs. Herein, a new and highly reversible Mn‐based cathode material with porous framework and N‐doping (MnO_x@N‐C) is prepared through a metal–organic framework template strategy. Benefiting from the unique porous structure, conductive carbon network, and the synergetic effect of Zn²⁺ and Mn²⁺ in electrolyte, the MnO_x@N‐C shows excellent cycling stability, good rate performance, and high reversibility for aqueous ZIBs. Specifically, it exhibits high capacity of 305 mAh g^?1 after 600 cycles at 500 mA g^?1 and maintains achievable capacity of 100 mAh g^?1 at a quite high rate of 2000 mA g^?1 with long‐term cycling of up to 1600 cycles, which are superior to most reported ZIB cathode materials. Furthermore, insight into the Zn‐storage mechanism in MnO_x@N‐C is systematically studied and discussed via multiple analytical methods. This study opens new opportunities for designing low‐cost and high‐performance rechargeable aqueous ZIBs.     

Sulfiphilic Few‐Layered MoSe2 Nanoflakes Decorated rGO as a Highly Efficient Sulfur Host for Lithium‐Sulfur Batteries

Lithium‐sulfur batteries (LSBs) have been regarded as a competitive candidate for next‐generation electrochemical energy‐storage technologies due to their merits in energy density. The sluggish redox kinetics of the electrochemistry and the high solubility of polysulfides during cycling result in insufficient sulfur utilization, severe polarization, and poor cyclic stability. Herein, sulfiphilic few‐layered MoSe₂ nanoflakes decorated rGO (MoSe₂@rGO) hybrid has been synthesized through a facile hydrothermal method and for the first time, is used as a conceptually new‐style sulfur host for LSBs. Specifically, MoSe₂@rGO not only strongly interacts with polysulfides but also dynamically strengthens polysulfide redox reactions. The polarization problem is effectively alleviated by relying on the sulfiphilic MoSe₂. Moreover, MoSe₂@rGO is demonstrated to be beneficial for the fast nucleation and uniform deposition of Li₂S, contributing to the high discharge capacity and good cyclic stability. A high initial capacity of 1608 mAh g^?1 at 0.1 C, a slow decay rate of 0.042% per loop at 0.25 C, and a high reversible capacity of 870 mAh g^?1 with areal sulfur loading of 4.2 mg cm^?2 at 0.3 C are obtained. The concept of introducing sulfiphilic transition‐metal selenides into the LSBs system can stimulate engineering of novel architectures with enhanced properties for various energy‐storage devices.     

All‐Organic Battery Composed of Thianthrene‐ and TCAQ‐Based Polymers

An all‐organic battery consisting of two redox‐polymers, namely poly(2‐vinylthianthrene) and poly(2‐methacrylamide‐TCAQ) is assembled. This all‐organic battery shows excellent performance characteristics, namely flat discharge plateaus, an output voltage exceeding 1.3 V, and theoretical capacities of both electrodes higher than 100 mA h g^?1. Both organic electrode materials are synthesized in two respective three synthetic steps using the free‐radical polymerization technique. Li‐organic batteries manufactured from these polymers prove their suitability as organic electrode materials. The cathode material poly(2‐vinylthianthrene) (3) displays a discharging plateau at 3.95 V versus Li⁺/Li and a discharge capacity of 105 mA h g^?1, corresponding to a specific energy of about 415 mW h g^?1. The anode material poly(2‐methacrylamide‐TCAQ) (7) exhibits an initial discharge capacity of 130 mA h g^?1, corresponding to 94% material activity. The combination of both materials results in an all‐organic battery with a discharge voltage of 1.35 V and an initial discharge capacity of 105 mA h g^?1 (95% material activity).     

Conductive and Catalytic Triple‐Phase Interfaces Enabling Uniform Nucleation in High‐Rate Lithium–Sulfur Batteries

Rechargeable lithium–sulfur batteries have attracted tremendous scientific attention owing to their superior energy density. However, the sulfur electrochemistry involves multielectron redox reactions and complicated phase transformations, while the final morphology of solid‐phase Li₂S precipitates largely dominate the battery's performance. Herein, a triple‐phase interface among electrolyte/CoSe₂/G is proposed to afford strong chemisorption, high electrical conductivity, and superb electrocatalysis of polysulfide redox reactions in a working lithium–sulfur battery. The triple‐phase interface effectively enhances the kinetic behaviors of soluble lithium polysulfides and regulates the uniform nucleation and controllable growth of solid Li₂S precipitates at large current density. Therefore, the cell with the CoSe₂/G functional separator delivers an ultrahigh rate cycle at 6.0 C with an initial capacity of 916 mAh g^?1 and a capacity retention of 459 mAh g^?1 after 500 cycles, and a stable operation of high sulfur loading electrode (2.69–4.35 mg cm^?2). This work opens up a new insight into the energy chemistry at interfaces to rationally regulate the electrochemical redox reactions, and also inspires the exploration of related energy storage and conversion systems based on multielectron redox reactions.     

Confined Fe2VO4⊂Nitrogen‐Doped Carbon Nanowires with Internal Void Space for High‐Rate and Ultrastable Potassium‐Ion Storage

Developing low‐cost, high‐capacity, high‐rate, and robust earth‐abundant electrode materials for energy storage is critical for the practical and scalable application of advanced battery technologies. Herein, the first example of synthesizing 1D peapod‐like bimetallic Fe₂VO₄ nanorods confined in N‐doped carbon porous nanowires with internal void space (Fe₂VO₄?NC nanopeapods) as a high‐capacity and stable anode material for potassium‐ion batteries (KIBs) is reported. The peapod‐like Fe₂VO₄?NC nanopeapod heterostructures with interior void space and external carbon shell efficiently prevent the aggregation of the active materials, facilitate fast transportation of electrons and ions, and accommodate volume variation during the cycling process, which substantially boosts the rate and cycling performance of Fe₂VO₄. The Fe₂VO₄?NC electrode exhibits high reversible specific depotassiation capacity of 380 mAh g^?1 at 100 mA g^?1 after 60 cycles and remarkable rate capability as well as long cycling stability with a high capacity of 196 mAh g^?1 at 4 A g^?1 after 2300 cycles. The first‐principles calculations reveal that Fe₂VO₄?NC nanopeapods have high ionic/electronic conductivity characteristics and low diffusion barriers for K⁺‐intercalation. This study opens up new way for investigating high‐capacity metal oxide as high‐rate and robust electrode materials for KIBs.     

Pseudo‐Zn–Air and Zn‐Ion Intercalation Dual Mechanisms to Realize High‐Areal Capacitance and Long‐Life Energy Storage in Aqueous Zn Battery

Aqueous zinc batteries are considered as promising alternatives to lithium ion batteries owing to their low cost and high safety. However, the developments of state‐of‐the‐art zinc‐ion batteries (ZIB) and zinc–air batteries (ZAB) are limited by the unsatisfied capacities and poor cycling stabilities, respectively. It is of significance in utilizing the long‐cycle life of ZIB and high capacity of ZAB to exploit advanced energy storage systems. Herein, a bulk composite of graphene oxide and vanadium oxide (V₅O₁₂·6H₂O) as cathode material for aqueous Zn batteries in a mild electrolyte is employed. The battery performance is demonstrated to arise from a combination of the reversible cations insertion/extraction in vanadium oxide and especially the electrochemical redox reactions on the surface functional groups of graphene oxide (named as pseudo‐Zn–air mechanism). Along with adjusting the hydroxyl content on the surface of graphene oxide, the specific capacity is significantly increased from 342 mAh g^?1 to a maximum of 496 mAh g^?1 at 100 mA g^?1. The surface‐controlled kinetics occurring in the bulk composite ensure a high areal capacity of 10.6 mAh cm^?2 at a mass loading of 26.5 mg cm^?2, and a capacity retention of 84.7% over 10 000 cycles at a high current density of 10 A g^?1.     

Realizing Three‐Electron Redox Reactions in NASICON‐Structured Na3MnTi(PO4)3 for Sodium‐Ion Batteries

Developing multielectron reaction electrode materials is essential for achieving high specific capacity and high energy density in secondary batteries; however, it remains a great challenge. Herein, Na₃MnTi(PO₄)₃/C hollow microspheres with an open and stable NASICON framework are synthesized by a spray‐drying‐assisted process. When applied as a cathode material for sodium‐ion batteries, the resultant Na₃MnTi(PO₄)₃/C microspheres demonstrate fully reversible three‐electron redox reactions, corresponding to the Ti^3+/4+ (≈2.1 V), Mn^2+/3+ (≈3.5 V), and Mn^3+/4+ (≈4.0 V vs Na⁺/Na) redox couples. In situ X‐ray diffraction results reveals that both solid‐solution and two‐phase electrochemical reactions are involved in the sodiation/desodiation processes. The high specific capacity (160 mAh g^?1 at 0.2 C), outstanding cyclability (≈92% capacity retention after 500 cycles at 2 C), and the facile synthesis make the Na₃MnTi(PO₄)₃/C a prospective cathode material for sodium‐ion batteries.     

A High‐Power Na3V2(PO4)3‐Bi Sodium‐Ion Full Battery in a Wide Temperature Range

Sodium‐ion batteries (SIBs) that operate in a wide temperature range are in high demand for practical large‐scale electric energy storage. Herein, a novel full SIB is composed of a bulk Bi anode, a Na₃V₂(PO₄)₃/carbon nanotubes composite (NVP‐CNTs) cathode and a NaPF₆‐diglyme electrolyte. The Bi anode gradually evolves into a porous network in the ether‐based electrolyte during initial cycles, and in the NVP‐CNTs cathode the NVP is cross linked by CNTs to show large exchange current density. These unique features merit the full SIB of Bi//NVP‐CNTs with high Na⁺ diffusion coefficient and low reaction activation energy, and hence fast Na⁺ transport and facile redox reaction kinetics. The resultant full SIB presents high power density of 2354.6 W kg^?1 and energy density of 150 Wh kg^?1 and superior cycling stability in a wide temperature range from ?15 to 45 °C. This will shed light on battery design, and promote their development for practical applications in various weather conditions.     

A New Fe/V Redox Flow Battery Using a Sulfuric/Chloric Mixed‐Acid Supporting Electrolyte

A redox flow battery using Fe²⁺/Fe³⁺ and V²⁺/V³⁺ redox couples in chloric/sulfuric mixed‐acid supporting electrolyte is investigated for potential stationary energy storage applications. The Fe/V redox flow cell using mixed reactant solutions operates within a voltage window of 0.5–1.35 V with a nearly 100% utilization ratio and demonstrates stable cycling over 100 cycles with energy efficiency >80% and no capacity fading at room temperature. A 25% improvement in the discharge energy density of the Fe/V cell is achieved compared with a previously reported Fe/V cell using a pure chloride acid supporting electrolyte. Stable performance is achieved in the temperature range between 0 and 50 °C as well as when using a microporous separator as the membrane. The improved electrochemical performance makes the Fe/V redox flow battery a promising option as a stationary energy storage device to enable renewable integration and stabilization of the electric grid.