Metal-Free, Solid-State, Paperlike Rechargeable Batteries Consisting of Redox-Active Polyethers

Crafting Core-Shell Heterostructures with Enriched Active Centers for High-Energy-Density Symmetric Lithium-Ion Batteries

Current research strives to create sustainable and ecofriendly organic electrode materials (OEMs) due to the rising concerns about traditional inorganic electrode materials that call for substantial resource consumption in battery manufacturing. However, OEMs often exhibit unbalanced performance, with high capacity conflicting with a long lifespan. Herein, a 2D fully conjugated covalent organic framework featuring abundant C═O and C═N groups (HTPT-COF) was strategically synthesized by coupling 2,3,7,8-tetraamino-1,4,6,9-tetraketone with hexaketocyclohexane octahydrate. It stabilizes the enriched active centers by an extended π-conjugated skeleton, thereby affording a high theoretical capacity in conjunction with potential structure stability. To further unlock the barriers of fast charge, the HTPT-COF was interwoven around highly conductive carbon nanotubes, creating a robust core-sheath heterostructure (HTPT-COF@CNT). Consequently, the crafted HTPT-COF@CNT achieves large reversible capacities of 507.7 mA h g-1, high-rate performance (247.8 mA h g-1 at 20.0 A g-1), and long-term durability (1000 cycles). Aiming to streamline the process and cut the cost of battery manufacturing, all-organic symmetric batteries were well fabricated using HTPT-COF@CNT as both cathode and anode, demonstrating high energy/power density (up to 191.7 W h kg-1 and 3800.3 W kg-1, respectively) and long-term stability over 1000 cycles. Such HTPT-COF@CNT represents a promising sustainable electrode that effectively addresses irreconcilable contradictions encountered by OEMs, boosting the development of advanced organic batteries with high capacity and cycling stability.

Molecule and Microstructure Modulations of Cyano-Containing Electrodes for High-Performance Fully Organic Batteries

Cyano-containing electrodes usually promise high theoretical potentials while suffering from uncontrollable self-dissolution and sluggish reaction kinetics. Herein, to remedy their limitations, an unprecedented core-shell heterostructured electrode of carbon nanotubes encapsulated in poly(1,4-dicyanoperfluorobenzene sulfide) (CNT@PFDCB) is rationally crafted via molecule and microstructure modulations. Specifically, the linkage of sulfide bridges of PFDCB prevents the active cyano groups from dissolving, resulting in a robust structure. The fluorinations modulate the electronic configurations in frontier orbitals, allowing higher electrical conductivity and elevated output voltage. Combined with the core-shell architecture to unlock the sluggish diffusion kinetics for both electrons and guest ions, the CNT@PFDCB exhibits an impressive capacity (203.5 mAh g-1), remarkable rate ability (127.6 mAh g-1 at 3.0 A g-1), and exceptional cycling stability (retaining 81.1 % capacity after 3000 cycles at 1.0 A g-1). Additionally, the Li-storage mechanisms regarding PFDCB are thoroughly revealed by in situ attenuated total reflection infrared spectroscopy, in situ Raman spectroscopy, and theoretical simulations, which involve the coordination interaction between Li ions and cyano groups and the electron delocalization along the conjugated skeleton. More importantly, a practical fully organic cell based on the CNT@PFDCB is well-validated that demonstrates a tremendous potential of cyanopolymer as the cathode to replace its inorganic counterparts.

From Squaric Acid Amides (SQAs) to Quinoxaline-Based SQAs─Evolution of a Redox-Active Cathode Material for Organic Polymer Batteries

The search for new redox-active organic materials (ROMs) is essential for the development of sustainable energy-storage solutions. In this study, we present a new class of cyclobuta[b]quinoxaline-1,2-diones or squaric acid quinoxalines (SQXs) as highly promising candidates for ROMs featuring exceptional stability and high redox potentials. While simple 1,2- and 1,3-squaric acid amides (SQAs), initially reported by Hünig and coworkers decades ago, turned out to exhibit low stability in their radical cation oxidation states, we demonstrate that embedding the nitrogen atoms into a quinoxaline heterocycle leads to robust two-electron SQX redox systems. A series of SQX compounds, as well as their corresponding radical cations, were prepared and fully characterized, including EPR spectroscopy, UV-vis spectroscopy, and X-ray diffraction. Based on the promising electrochemical properties and high stability of the new ROM, we developed SQX-functionalized polymers and investigated their physical and electrochemical properties for energy-storage applications. These polymers showed remarkable thermal stability well above 200 °C with reversible redox properties and potentials of about 3.6 V vs Li+/Li. By testing the galvanostatic cycling performance in half-cells with lithium-metal counter electrodes, a styrene-based polymer with SQX redox side groups showed stable cycling for single-electron oxidation for more than 100 cycles. These findings render this new class of redox-active polymers as highly promising materials for future energy-storage applications.

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices

Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.

TEMPO-Substituted Poly(ethylene sulfide) for Solid-State Electro-Chemical Charge Storage

A poly(ethylene sulfide) backbone is introduced as the main chain of a radical polymer. Anionic ring-opening polymerization of an episulfide monomer substituted with 2,2,6,6tetramethylpiperidin1oxyl (TEMPO), a robust nitroxide radical, yields the corresponding polythioether. Compared to the traditional poly(ethylene oxide) backbone, the new polymer shows a lower glass transition temperature (-10 °C), and about threefold higher solid-state ionic conductivity. The polythioether is also shown to improve the charge/discharge properties of a cathode in solid-state lithium-ion batteries.