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.     

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.     

Cover Picture: Nanostructured Electrodes and the Low‐Temperature Performance of Li‐Ion Batteries (Adv. Mater. 1/2005)

The cover image shows a scanning electron micrograph of a commercially available track‐etch polycarbonate filter. This porous membrane serves as the host for the template‐synthesis of V₂O₅ nanowires of various diameters. Nanowires that are 70 nm in diameter are shown in the inset. Because V₂O₅ reversibly intercalates Li‐ions, it has potential for use as a cathode material in Li‐ion batteries. On p. 125, Sides and Martin report the use of these V₂O₅ nanowires as tools to investigate the poor low‐temperature performance of Li‐ion batteries.     

Bioinspired,Spine‐Like,Flexible, Rechargeable Lithium‐Ion Batteries with High Energy Density

The rapid development of flexible and wearable electronics proposes the persistent requirements of high‐performance flexible batteries. Much progress has been achieved recently, but how to obtain remarkable flexibility and high energy density simultaneously remains a great challenge. Here, a facile and scalable approach to fabricate spine‐like flexible lithium‐ion batteries is reported. A thick, rigid segment to store energy through winding the electrodes corresponds to the vertebra of animals, while a thin, unwound, and flexible part acts as marrow to interconnect all vertebra‐like stacks together, providing excellent flexibility for the whole battery. As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery can be over 85% of that in conventional packing. A nonoptimized flexible cell with an energy density of 242 Wh L^?1 is demonstrated with packaging considered, which is 86.1% of a standard prismatic cell using the same components. The cell also successfully survives a harsh dynamic mechanical load test due to this rational bioinspired design. Mechanical simulation results uncover the underlying mechanism: the maximum strain in the reported design (≈0.08%) is markedly smaller than traditional stacked cells (≈1.1%). This new approach offers great promise for applications in flexible devices.