Active Materials For Non-aqueous Redox Flow Battery

Redox flow battery is one of the most promising large-scale energy storage technologies.Nowadays,aqueous redox flow batteries have made great strides.However,their large-scale deployment is hindered by the low voltage and low energy density.High-voltage and energy-dense storage systems are expected to be achieved through the design of non-aqueous electrolytes.The current research on non-aqueous redox flow batteries is still in its infancy,and the low solubility,low stability,and crossover of the active materials during charge/discharge process are the main challenges.This paper focuses on the construction of high-performance non-aqueous redox flow batteries through the design and optimization of electrolyte system and active materials.The main research results are as follows:（1）An all-iron non-aqueous redox flow battery based on iron acetylacetonate（Fe（acac）₃）anolyte and ionic Ferrocene derivative（Fc1N112-TFSI）catholyte was designed.The effects of different supporting electrolytes and membranes on battery performance were investigated.Using an anion exchange membrane FAP-375-PP as the separator and TEATFSI as the supporting electrolyte,the resultant battery shows a Coulombic efficiency of 98.7%,voltage efficiency of 84.5%,and energy efficiency of83.4%during 100 cycles at the current density of 10 m A cm^-2.（2）Designing bi-redox active species to form symmetric batteries is an effective way to mitigate the crossover of the electrolyte.By covalently linking two active components,ferrocene and anthraquinone,a new bi-redox active material,Fc Me AAQ,was designed,which can be employed as both positive and negative active materials to construct a symmetric battery.The assembled symmetric battery with a Daramic 250porous membrane delivers a Coulombic efficiency of 90.8%,voltage efficiency of 90.1%and energy efficiency of 81.8%over 100 cycles at the current density of 2 m A cm^-2.（3）Extended conjugation and ionic side chain modification are effective ways to improve the stability and solubility of active species,respectively.By incorporating acetamide group and tetraalkylammonium ionic group into the anthraquinone structure and pairing with hydrophobic counter anions,three ionic active species,AQNBF₄,AQNPF₆ and AQNTFSI,were designed,and their solubility and two-electron redox reaction stability were significantly improved.Using the most soluble AQNTFSI（0.91M）as the negative active species,and the ionic ferrocene derivative Fc NTFSI as the positive active species,the resultant two-electron transfer redox flow battery delivers excellent cycling performance with Coulombic efficiency of 96.8%,energy efficiency of 82.4%,and overall discharge capacity retention of 86.0%over 200 cycles（99.93%capacity retention per cycle）at 10 m A cm^-2.（4）Ionic side chain and oligoether side chain modification are two general strategies to improve the solubility of active materials.Through the side chain coupling strategy,a series of anthraquinone-based ionic active species were designed by coupling the tetraalkylammonium ion side chain and the flexible ethylene glycol ether side chain in the anthraquinone structure.By adjusting the length of the ethylene glycol ether chain,the obtained AQEG2TFSI and AQEG3TFSI are miscible with acetonitrile at room temperature,which greatly improves the solubility.The cells assembled with 0.1 M and0.4 M AQEG2TFSI exhibited discharge capacity retentions of 99.96%and 99.74%per cycle over 100 charge/discharge cycles,respectively.