As the world moves toward electromobility and a concomitant decarbonization of its electrical supply, modern society is also entering a so-called fourth industrial revolution marked by a boom of electronic devices and digital technologies. Consequently, battery demand has exploded along with the need for ores and metals to fabricate them. Starting from such a critical analysis and integrating robust structural data, this review aims at pointing out there is room to promote organic-based electrochemical energy storage. Combined with recycling solutions, redox-active organic species could decrease the pressure on inorganic compounds and offer valid options in terms of environmental footprint and possible disruptive chemistries to meet the energy storage needs of both today and tomorrow. We review state-of-the-art developments in organic batteries, current challenges, and prospects, and we discuss the fundamental principles that govern the reversible chemistry of organic structures. We provide a comprehensive overview of all reported cell configurations that involve electroactive organic compounds working either in the solid state or in solution for aqueous or nonaqueous electrolytes. These configurations include alkali (Li/Na/K) and multivalent (Mg, Zn)-based electrolytes for conventional "sealed" batteries and redox-flow systems. We also highlight the most promising systems based on such various chemistries relying on appropriate metrics such as operation voltage, specific capacity, specific energy, or cycle life to assess the performances of electrodes.

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Opportunities and Challenges for Organic Electrodes in Electrochemical Energy Storage.

With high scalability and independent control over energy and power, redox flow batteries (RFBs) stand out as an important large-scale energy storage system. However, the widespread application of conventional RFBs is limited by the uncompetitive performance, as well as the high cost and environmental concerns associated with the use of metal-based redox species. In consideration of advantageous features such as potentially low cost, vast molecular diversity, and highly tailorable properties, organic and organometallic molecules emerge as promising alternative electroactive species for building sustainable RFBs. This review presents a systematic molecular engineering scheme for designing these novel redox species. We provide detailed synthetic strategies for modifying the organic and organometallic redox species in terms of solubility, redox potential, and molecular size. Recent advances are then introduced covering the reaction mechanisms, specific functionalization methods, and electrochemical performances of redox species classified by their molecular structures. Finally, we conclude with an analysis of the current challenges and perspectives on future directions in this emerging research field.

Molecular engineering of organic electroactive materials for redox flow batteries

Recent developments in organic redox flow batteries: A critical review

Secondary batteries have received huge attention due to their attractive features in applications of large-scale energy storage and portable electronic devices, as well as electrical vehicles. In a secondary battery, a membrane plays the role of separating the anode and cathode to prevent the occurrence of a short circuit, while allowing the transport of charge carriers to achieve a complete circuit. The properties of a membrane will largely determine the performance of a battery. In this article, we review the research and development progress of porous membranes in secondary battery technologies, such as lithium-based batteries together with flow batteries. The preparation methods as well as the required properties of porous membranes in different secondary battery technologies will be elucidated thoroughly and deeply. Most importantly, this review will mainly focus on the optimization and modification of porous membranes in different secondary battery systems. And various modifications on commercial porous membranes along with novel membrane materials are widely discussed and summarized. This review will help to optimize the membrane material for different secondary batteries, and favor the understanding of the preparation–structure–performance relationship of porous membranes in different secondary batteries. Therefore, this review will provide an extensive, comprehensive and professional reference to design and construct high-performance porous membranes.

Porous membranes in secondary battery technologies

Redox flow batteries (RFBs) are propitious stationary energy storage technologies with exceptional scalability and flexibility to improve the stability, efficiency, and sustainability of our power grid. The redox-active materials are the key component for RFBs with which to achieve high energy density and good cyclability. Traditional inorganic-based materials encounter critical technical and economic limitations such as low solubility, inferior electrochemical activity, and high cost. Redox-active organic materials (ROMs) are promising alternative “green” candidates to push the boundaries of energy storage because of the significant advantages of molecular diversity, structural tailorability, and natural abundance. Here, the recent development of a variety of ROMs and associated battery designs in both aqueous and nonaqueous electrolytes are reviewed. The critical challenges and potential research opportunities for developing practically relevant organic flow batteries are discussed.

Materials and Systems for Organic Redox Flow Batteries: Status and Challenges

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cell's low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

The substitution of sterically bulky groups at precise locations along the periphery of fused-ring aromatic systems is demonstrated to increase electrochemical oxidation potentials by preventing relaxation events in the oxidized state. Phenothiazines, which undergo significant geometric relaxation upon oxidation, are used as fused-ring models to showcase that electron-donating methyl groups, which would generally be expected to lower oxidation potential, can lead to increased oxidation potentials when used as the steric drivers. Reduction events remain inaccessible through this molecular design route, a critical characteristic for electrochemical systems where high oxidation potentials are required and in which reductive decomposition must be prevented, as in high-voltage lithium-ion batteries. This study reveals a new avenue to alter the redox characteristics of fused-ring systems that find wide use as electroactive elements across a number of developing technologies.

Peter L. Zhang

Papers

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

Beyond the Hammett Effect: Using Strain to Alter the Landscape of Electrochemical Potentials