Annulated Dialkoxybenzenes as Catholyte Materials for Non‐aqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution

Abstract

Abstract1,4‐Dimethoxybenzene derivatives are materials of choice for use as catholytes in non‐aqueous redox flow batteries, as they exhibit high open‐circuit potentials and excellent electrochemical reversibility. However, chemical stability of these materials in their oxidized form needs to be improved. Disubstitution in the arene ring is used to suppress parasitic reactions of their radical cations, but this does not fully prevent ring‐addition reactions. By incorporating bicyclic substitutions and ether chains into the dialkoxybenzenes, a novel catholyte molecule, 9,10‐bis(2‐methoxyethoxy)‐1,2,3,4,5,6,7,8‐octahydro‐1,4:5,8‐dimethanenoanthracene (BODMA), is obtained and exhibits greater solubility and superior chemical stability in the charged state. A hybrid flow cell containing BODMA is operated for 150 charge–discharge cycles with a minimal loss of capacity.

Bio-inspired Molecular Redesign of a Multi-redox Catholyte for High-Energy Non-aqueous Organic Redox Flow Batteries

“…It was expected that the flexibility and bulkiness of the alkyl ether chains would not only improve the solubility but also enhance the stability of the dication form by spatially hindering the redox-active N atoms from undergoing side reactions. [48][49][50] Synthesis of BMEPZ was performed starting with the reduction of phenazine using Na 2 S 2 O 4 , followed by N-substitution of both amino groups with 2-chloroethylmethyl ether. The molecular structure of the synthesized BMEPZ was characterized using nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectroscopy (HRMS), and elemental analysis (see the Experimental Procedures and Figure S2 for the synthetic details and reaction schemes).…”

Stability of molecular radicals in organic non-aqueous redox flow batteries: A mini review

“…Substituted dialkoxybenzenes such as DBBB [39,41], DBMMB [19,36,40] or other derivatives [43][44][45] have been employed as lithium-ion redox shuttles and NA RFB catholytes because their radical cations are stable due to delocalisation, electron-donating substituents and steric protection [18]. While DBBB displays poor solubility (~0.4 M in PC) [46,47], DBMMB is a highly miscible liquid at room temperature [19,43].…”

Toward High‐Voltage, Energy‐Dense, and Durable Aqueous Organic Redox Flow Batteries: Role of the Supporting Electrolytes

“…For instance, the chemical instability of the 2‐phenyl‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (PTIO) led to continuous capacity loss of the cell during cycling . Except for the inherent structural features of organic compounds, it was found that supporting solvent and salt may affect their chemical stability and lifetime ,. Dialkoxybenzene radical cations, formed upon electrochemical oxidation, are instable in water .…”

“…TEA‐TFSI in 1,2‐dimethoxyethane supporting electrolyte shows the best compatibility with these radicals, compared to other different combinations of Li + , TEA + , TFSI − , BF 4 − , 1,2‐dimethoxyethane and acetonitrile. As another example, dialkoxybenzene radical cations with a bicyclic substitution showed a stability tendency in different polar solvents: dichloromethane>carbonate>acetonitrile, and are instable in water …”