Low-Cost Titanium-Bromine Flow Battery with Ultrahigh Cycle Stability for Grid-Scale Energy Storage

Synergistic ionic modification strategy enhances the stability of naphthalene diimide zwitterions for cost-effective aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold significant promise for energy storage due to their unique advantages and characteristics. However, their development is hindered by the lack of decomposition resistance and cycle stability over long periods. In this study, we synthesized naphthalene diimide (NDI) derivatives with zwitterions in their side chains via the atmospheric pressure method, namely (CBu)2NDI and (SPr)2NDI. The electrostatic repulsion between (CBu)2NDI precisely regulates π-π stacking into a parallel-staggered pattern. The synergistic zwitterions strategy effectively mitigates the positive charge of N+ in (CBu)2NDI compared with (NPr)2NDI and dex-NDI; this not only enhances the aromaticity of the naphthalene diimide core but also inhibits the side chain decomposition caused by the SN2 nucleophilic attack of hydroxyl ions (OH-) on the C=O. The calculation of the single point energy proves that during the charging processes of (CBu)2NDI, the K+ will be close to the naphthalene core to form dimers or monomers with lower energy configurations under electrostatic attraction. (CBu)2NDI achieved a water solubility up to 1.49 M, which can be paired with K4Fe(CN)6 under two-electron transfer with total electrolyte costs as low as $6.58 Ah-1. The 0.1 M battery maintains full capacity after 5070 cycles. Furthermore, the battery delivers an impressive 100% capacity retention under 2 M e- during 220 cycles.

A Highly Reversible Aqueous Sulfur-Dual-Halogen Battery Enabled by a Water-in-Bisalt Electrolyte

The chlorine-based redox reaction applied in aqueous rechargeable batteries (ARBs) has attracted extensive attention owing to the high theoretical capacity and redox potential. However, it generally suffers from low reversibility and poor Coulombic efficiency due to the evolution of toxic Cl2 gas and the decomposition of aqueous electrolytes. Herein, an aqueous sulfur-dual halogen chemistry is demonstrated by employing highly-concentrated water-in-bisalt (WiBS) electrolyte, sulfur anode, and iodine composite electrodes. The freestanding iodine/carbon cloth cathode and Cl--containing WiBS electrolyte not only enable the continuous I+/I0 reaction by forming [IClx]1-x interhalogens but also achieve the oxidation of Cl- in [IClx]1-x at higher redox potential and immobilize Cl0 species via I+─Cl0 chemical bonds. Therefore, the as-assembled aqueous sulfur-dual halogen batteries (ASHBs) based on the dual-halogen conversion on the cathode and the S/Sx 2- redox reaction on the anode deliver a high energy density of 304 Wh kg-1 with an average output voltage of 1.32 V. These key findings open an avenue for the development of low-cost and high-performance ARBs for energy storage applications.

New Redox Chemistries of Halogens in Aqueous Batteries

Halogen-based redox-active materials represent an important class of materials in aqueous electrochemistry. The existence of versatile halogen species and their rich bonding coordination create great flexibility in designing new redox couples. Novel redox reaction mechanisms and electrochemical reversibility can be unlocked in specifically configurated electrolyte environments and electrodes. In this review, the halogen-based redox couples and their appealing redox chemistries in aqueous batteries, including redox flow batteries and traditional static batteries that have been studied in recent years, are discussed. New aqueous electrochemistry provides hope to outperform the state-of-the-art materials and systems that are facing resources and performance limitation, and to enrich the existing battery chemistries.

Zincophilic CuO as electron sponge to facilitate dendrite-free zinc-based flow battery

Zinc (Zn)-based batteries have been persistently challenged by the critical issue of inhomogeneous zinc deposition/stripping process on substrate surface. Herein, we reveal that zinc electrodeposition behaviors dramatically improved through the introduction of highly zincophilic copper oxide nanoparticles (CuO NPs). Strong electronic redistribution between Zn and CuO explains the high Zn affinity on CuO, with negligible nucleation overpotential. Additionally, CuO exhibits remarkable electron-accepting and -donating capabilities in electron-rich and electron-deficient environments, resembling a sponge. This 'Electron Sponge' effect emerges from stable Zn-O bonding in CuO, enhancing electron duality in the Zn-O bond region. This unique strategy is pivotal in mitigating dendritic growth, fostering dendrite-free zinc-based flow batteries with enhanced rate performance and cyclability. It presents significant performance with not only high energy density (180 Wh L-1) but also the long cycle stability (> 2500 cycles) at high current density (140 mA cm-2).

Bismuth Single Atoms Regulated Graphite Felt Electrode Boosting High Power Density Vanadium Flow Batteries

Vanadium flow batteries (VFBs) are considered one of the most promising candidates for large-scale energy storage. However, VFBs suffer from relatively low power density due to severe electrochemical polarization. Herein, we report Bi single atoms supported by an N-doped carbon-regulated graphite felt electrode (Bi SAs/NC@GF) with high electrocatalytic activity and stability, owing to the greatly improved active sites and optimized Bi-N4 configuration. Electrochemical in situ characterization and theoretical calculations elucidate the desolvation process and specific inner sphere reaction mechanism of [V(H2O)6]3+/[V(H2O)6]2+. As a result, a VFB single cell assembled with Bi SAs/NC@GF achieves a much higher energy efficiency of 81.1% at 240 mA cm-2 than NC@GF (70.5%). Moreover, a 5 kW VFB stack equipped with Bi SAs/NC@GF is assembled for the first time and ran stably for over 400 cycles. This work confirms that a single-atom catalyst is efficient for scalable VFBs with high power density and low cost.

New Alkalescent Electrolyte Chemistry for Zinc-Ferricyanide Flow Battery

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-1 catholyte 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.

A Choline-Based Antifreezing Complexing Agent with Selective Compatibility for Zn-Br2 Flow Batteries

The high freezing point of polybromides, charging products, is a significant obstacle to the rapid development of zinc-bromine flow batteries (Zn-Br2 FBs). Here, a choline-based complexing agent (CCA) is constructed to liquefy the polybromides at low temperatures. Depending on quaternary ammonium group, choline can effectively complex with polybromide anions and form dense oil-phase that has excellent antifreezing property. Benefiting from indispensable strong ion-ion interaction, the highly selectively compatible CCA, consisting of choline and N-methyl-N-ethyl-morpholinium salts (CCA-M), can be achieved to further enhance bromine fixing ability. Interestingly, the formed polybromides with CCA-M are able to keep liquid even at -40 °C. The CCA-M endows Zn-Br2 FBs at 40 mA cm-2 with unprecedented long cycle life (over 150 cycles) and high Coulombic efficiency (CE, average ≈98.8%) at -20 °C, but also at room temperature (over 1200 cycles, average CE: ≈94.7%). The CCA shows a promising prospect of application and should be extended to other antifreezing bromine-based energy storage systems.

Carbon nanofibers confined polyoxometalate derivatives as flexible self-supporting electrodes for robust sodium storage

Flexible self-supporting film electrodes, which eliminate the need for additional adhesive, conductive agents, or current collectors, offer significant advantages in terms of mechanical properties, specific capacity, and energy density for energy storage applications. In this study, we successfully developed a flexible film electrode by incorporating derivatives of Mo and Fe-based polyoxometalates (POMs-D) into carbon nanofibers (CNFs). The integration of CNFs significantly enhanced the structural stability of POMs-D, while the internally formed electrical field facilitated efficient electron transfer, resulting in good performance in sodium storage. The film electrode demonstrated a high capacitive contribution of 90.0 % for sodium uptake/release at a scan rate of 1.0 mV s-1. It maintained a capacity of approximately 170 mA h g-1 even after 8000 cycles at a current density of 3.0 A g-1. Moreover, the film electrode exhibited a decent capacity with a 40.0-fold increase in current density, along with high power capability and energy density in sodium-ion hybrid supercapacitors, showcasing the versatility. These findings unveil the structure-functionality relationship and offer an advanced approach for developing high-performance film electrode materials, opening new possibilities in the fields of material science and energy storage.

Development of flow battery technologies using the principles of sustainable chemistry

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

Scalable Alkaline Zinc-Iron/Nickel Hybrid Flow Battery with Energy Density up to 200 Wh L-1

Achieving net-zero emissions requires low-cost and reliable energy storage devices that are essential to deploy renewables. Alkaline zinc-based flow batteries such as alkaline zinc-iron (or nickel) flow batteries are well suited for energy storage because of their high safety, high efficiency, and low cost. Nevertheless, their energy density is limited by the low solubility of ferro/ferricyanide and the limited areal capacity of sintered nickel electrodes. Here, combining the electrochemical reaction with the chemical reaction of ferro/ferricyanide couple in a homemade nickel electrode, an alkaline zinc-iron/nickel hybrid flow battery with a high energy density of 208.9 Wh L^-1 and an energy efficiency of 84.7% at a high current density of 80 mA cm^-2 is reported. The reversible chemical reactions between dual couples are proven to stabilize the nickel electrode by promoting the activation of the nickel electrode and further preventing the formation of γ-NiOOH. A kW-scale stack is demonstrated by the integration of ferro/ferricyanide couple with nickel electrode, delivering a coulombic efficiency of 98% and an energy efficiency of 89% at 40 mA cm^-2 . This work demonstrates a promising pathway for constructing and upscaling flow batteries with high energy density and low cost.

Bromine Assisted MnO2 Dissolution Chemistry: Toward a Hybrid Flow Battery with Energy Density of over 300 Wh L-1

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.

NIR-II Responsive Molybdenum Dioxide Nanosystem Manipulating Cellular Immunogenicity for Enhanced Tumor Photoimmunotherapy

Photothermal therapy (PTT) in the second near-infrared (NIR-II) window has emerged as a better candidate for deep-tissue tumor elimination. More interestingly, the photothermal ablated tumor cells also manifest somewhat immunostimulation potency to elicit antitumor immunity, although most dying cells are undergoing apoptosis that is commonly considered as immunologically silent. Here, a NIR-II responsive nanosystem is established for tumor photoimmunotherapy using molybdenum dioxide (MoO₂) nanodumbbells as the nanoconverter. Meanwhile, an apoptosis-blocking strategy is proposed to regulate the cell death pattern under NIR-II laser irradiation in order to improve the immunogenic cell death. The nanoformulation can efficiently block caspase 8-dependent apoptotic pathway in photothermal ablated tumor cells and transform into more immunogenic death patterns, thereby activating systemic immunity to inhibit tumor growth and metastasis. In addition, this strategy also helps enhance the body's responses to α-PD-1 immune checkpoint inhibitor, which implies a potential optimal combination for cancer immunotherapy.

Rechargeable aqueous zinc-bromine batteries: an overview and future perspectives

Zinc-bromine batteries (ZBBs) receive wide attention in distributed energy storage because of the advantages of high theoretical energy density and low cost. However, their large-scale application is still confronted with some obstacles. Therefore, in-depth research and advancement on the structure, electrolyte, anode, cathode and membrane are of great significance and impendency. Herein, we review the past and present investigations on ZBBs, discuss the key problems and technical challenges, and propose perspectives for the future, with the focus on materials and chemistry. This perspective would provide valuable information on further development of ZBBs.