Insights into the Redox Chemistry of Organosulfides Towards Stable Molecule Design in Nonaqueous Energy Storage Systems

Sulfide-Bridged Covalent Quinoxaline Frameworks for Lithium-Organosulfide Batteries

The chelating ability of quinoxaline cores and the redox activity of organosulfide bridges in layered covalent organic frameworks (COFs) offer dual active sites for reversible lithium (Li)-storage. The designed COFs combining these properties feature disulfide and polysulfide-bridged networks showcasing an intriguing Li-storage mechanism, which can be considered as a lithium-organosulfide (Li-OrS) battery. The experimental-computational elucidation of three quinoxaline COFs containing systematically enhanced sulfur atoms in sulfide bridging demonstrates fast kinetics during Li interactions with the quinoxaline core. Meanwhile, bilateral covalent bonding of sulfide bridges to the quinoxaline core enables a redox-mediated reversible cleavage of the sulfursulfur bond and the formation of covalently anchored lithium-sulfide chains or clusters during Li-interactions, accompanied by a marked reduction of Li-polysulfide (Li-PS) dissolution into the electrolyte, a frequent drawback of lithium-sulfur (Li-S) batteries. The electrochemical behavior of model compounds mimicking the sulfide linkages of the COFs and operando Raman studies on the framework structure unravels the reversibility of the profound Li-ion-organosulfide interactions. Thus, integrating redox-active organic-framework materials with covalently anchored sulfides enables a stable Li-OrS battery mechanism which shows benefits over a typical Li-S battery.

Organosulfur Materials for Rechargeable Batteries: Structure, Mechanism, and Application

Lithium-ion batteries have received significant attention over the last decades due to the wide application of portable electronics and increasing deployment of electric vehicles. In order to further enhance the performance of the batteries and overcome the capacity limitations of inorganic electrode materials, it is imperative to explore new cathode and functional materials for rechargeable lithium batteries. Organosulfur materials containing sulfur-sulfur bonds as a kind of promising organic electrode materials have the advantages of high capacities, abundant resources, tunable structures, and environmental benignity. In addition, organosulfur materials have been widely used in almost every aspect of rechargeable batteries because of their multiple functionalities. This review aims to provide a comprehensive overview on the development of organosulfur materials including the synthesis and application as cathode materials, electrolyte additives, electrolytes, binders, active materials in lithium redox flow batteries, and other metal battery systems. We also give an in-depth analysis of structure-property-performance relationship of organosulfur materials, and guidance for the future development of organosulfur materials for next generation rechargeable lithium batteries and beyond.

High Energy Density, Asymmetric, Nonaqueous Redox Flow Batteries without a Supporting Electrolyte

Energy density in nonaqueous redox flow batteries (RFBs) is often limited by the modest solubility of the redox-active organic molecules (ROMs). In addition, the lack of a separator that prevents ROMs from crossing between anolyte and catholyte solutions necessitates the use of 1:1 mixtures of two ROMs in both the anolyte and catholyte solutions in symmetric RFBs, further limiting concentrations. We show that permanently cationic oligomers of viologen, tris(dialkylamino)cyclopropenium, and phenothiazine molecules have high solubility in acetonitrile and cross over an anion exchange membrane at slow to undetectable rates, enabling the creation of asymmetric RFBs with low crossover. No added supporting electrolyte is necessary, with only the PF6- counteranions of the ROMs crossing the membrane during charge/discharge. An oligomeric viologen + oligomeric cyclopropenium RFB at 1.0 M (redox equivalents) has a voltage of 1.66 V and a theoretical energy density of 22.2 Wh/L, one of the highest reported for nonaqueous RFBs.

Unraveling the Role of Aromatic Ring Size in Tuning the Electrochemical Performance of Small-Molecule Imide Cathodes for Lithium-Ion Batteries

Organic electrode materials have the typical advantages of flexibility, low cost, abundant resources, and recyclability. However, it is challenging to simultaneously optimize the specific capacity, rate capability, and cycling stability. Radicals are inevitable intermediates that critically determine the redox activity and stability during the electrochemical reaction of organic electrodes. Herein, we select a series of aromatic imides, including pyromellitic diimide (PMDI), 1,4,5,8-naphthalenediimide (NDI), and 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which contain different extending π-conjugated aromatic rings, to study the relationship between their electrochemical performance and the size of the aromatic ring. The results show that regulating the aromatic ring size of imide molecules could finely tune the energies of the lowest unoccupied molecular orbital (LUMO), thus optimizing the redox potential. The rate performance of PMDI, NDI, and PTCDI increases with the aromatic ring size, which is consistent with the decrease in the LUMO-HOMO gap of these imide molecules. DFT calculations and experiments reveal that the redox of imide radicals dominates the charge/discharge processes. Also, extending the aromatic rings could more effectively disperse the spin electron density and improve the stability of imide radicals, contributing to the enhanced cycling stability of these imide electrodes. Hence, aromatic ring size regulation is a simple and novel approach to simultaneously enhance the capacity, rate capability, and cycling stability of organic electrodes for high-performance lithium-ion batteries.

Emerging chemistries and molecular designs for flow batteries

Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.

Advances of Organosulfur Materials for Rechargeable Metal Batteries

Battery materials have become a hotspot in the academic research. Organosulfur compounds are considered as a promising class of cathode materials for rechargeable metal batteries. They have attracted increasing attention in recent years after a long-term stagnancy since 1980s. Recent studies have focused on the understanding of redox mechanism of linear organosulfur molecules R-Sn -R with defined structures. In addition, some new organosulfur compounds are developed. The reversible sulfursulfur (SS) bond breakage/formation of organosulfur in batteries makes them applicable as functional materials in batteries. In this review, new organosulfur materials including molecules, polymers, and composites are introduced. In the following, organosulfur-inorganic hybrid materials are discussed, which have shown unique redox process and enhanced battery performance. In the third part, organosulfur additives are used in Li-S batteries, which can improve the formation of solid-electrolyte interphase (SEI) and alter the redox pathways of sulfur cathodes. In the fourth part, organosulfur materials used in other metal batteries are introduced. Lastly, a summary and some perspectives are given. This review presents an overview of the recent advances of organosulfur materials in batteries and provides guidance for the future development of these materials.

Size Effect of Organosulfur and In Situ Formed Oligomers Enables High-Utilization Na-Organosulfur Batteries

Organosulfurs are promising cathode materials for rechargeable metal batteries due to their high capacities, diverse structures, and electrochemical properties. Herein, the electrochemical behavior of three organosulfur compounds, i.e., 4,4'-thiobisbenzenethiol (TBBT), 1,4-benzenedithiol (1,4-BDT), and diphenyl disulfide (DPDS), is revealed in room-temperature rechargeable sodium (Na) batteries, which show significantly improved performances when sodiated Nafion membranes are used. Large oligomers of organosulfur can be formed during charging, and they are readily blocked by the nanosized ion-conducting clusters in the Nafion membrane. In addition, large organosulfur monomers can also be blocked. Only 5.4% of TBBT diffuses through the Nafion membrane after 800 h. The Na|TBBT cell sustains 77% of the theoretical capacity after 300 cycles (2420 h). Moreover, the Na|TBBT redox flow cell shows promising rechargeability. Due to the medium molecular size, the organosulfur oligomers are expected to provide a new avenue to develop high-capacity chalcogen cathodes, besides inorganic S and S-containing polymers.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.