Phenothiazine‐Based Organic Catholyte for High‐Capacity and Long‐Life Aqueous Redox Flow Batteries

Redox‐active organic materials have been considered as one of the most promising “green” candidates for aqueous redox flow batteries (RFBs) due to the natural abundance, structural diversity, and high tailorability. However, many reported organic molecules are employed in the anode, and molecules with highly reversible capacity for the cathode are limited. Here, a class of heteroaromatic phenothiazine derivatives is reported as promising positive materials for aqueous RFBs. Among these derivatives, methylene blue (MB) possesses high reversibility with extremely fast redox kinetics (electron‐transfer rate constant of 0.32 cm s^?1), excellent stability in both neutral and reduced states, and high solubility in an acetic‐acid–water solvent, leading to a high reversible capacity of ≈71 Ah L^?1. Symmetric RFBs based on MB electrolyte demonstrate remarkable stability with no capacity decay over 1200 cycles. Even concentrated MB catholyte (1.5 m ) is still able to deliver stable capacity over hundreds of cycles in a full cell system. The impressive cell performance validates the practicability of MB for large‐scale electrical energy storage.     

Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the commercialization of ARFB energy-storage systems are highlighted.     

Liquid Quinones for Solvent‐Free Redox Flow Batteries

Liquid benzoquinone and naphthoquinone having diethylene glycol monomethyl ether groups are designed and synthesized as redox active materials that dissolve supporting electrolytes. The Li‐ion batteries based on the liquid quinones using LiBF₄/PC show good performance in terms of voltage, capacity, energy efficiency, and cyclability in both static and flow modes. A battery is constructed without using intentionally added organic solvent, and its high energy density (264 W h L^?1) demonstrates the potential of solvent‐free organic redox flow batteries using liquid active materials.     

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions (Adv. Mater. 35/2010)

Despite the imminent commercial introduction of Li‐ion batteries in electric drive vehicles and their proposed use as enablers of smart grids based on renewable energy technologies, an intensive quest for new electrode materials that bring about improvements in energy density, cycle life, cost, and safety is still underway. This Progress Report highlights the recent developments and the future prospects of the use of phases that react through conversion reactions as both positive and negative electrode materials in Li‐ion batteries. By moving beyond classical intercalation reactions, a variety of low cost compounds with gravimetric specific capacities that are two‐to‐five times larger than those attained with currently used materials, such as graphite and LiCoO₂, can be achieved. Nonetheless, several factors currently handicap the applicability of electrode materials entailing conversion reactions. These factors, together with the scientific breakthroughs that are necessary to fully assess the practicality of this concept, are reviewed in this report.     

Water‐Lubricated Intercalation in V2O5·nH2O for High‐Capacity and High‐Rate Aqueous Rechargeable Zinc Batteries

Low‐cost, environment‐friendly aqueous Zn batteries have great potential for large‐scale energy storage, but the intercalation of zinc ions in the cathode materials is challenging and complex. Herein, the critical role of structural H₂O on Zn²⁺ intercalation into bilayer V₂O₅·nH₂O is demonstrated. The results suggest that the H₂O‐solvated Zn²⁺ possesses largely reduced effective charge and thus reduced electrostatic interactions with the V₂O₅ framework, effectively promoting its diffusion. Benefited from the “lubricating” effect, the aqueous Zn battery shows a specific energy of ≈144 Wh kg^?1 at 0.3 A g^?1. Meanwhile, it can maintain an energy density of 90 Wh kg^?1 at a high power density of 6.4 kW kg^?1 (based on the cathode and 200% Zn anode), making it a promising candidate for high‐performance, low‐cost, safe, and environment‐friendly energy‐storage devices.