Biphasic, Membrane-Free Zn/Phenothiazine Battery: Effects of Hydrophobicity of Redox Materials on Cyclability

Abstract

The lack of a suitable ionic exchange membrane has retarded the development of organic nonaqueous redox flow batteries (RFBs). Membrane-free redox stratified batteries, wherein electroactive materials in immiscible nonaqueous and aqueous solvents as anolyte and catholyte, have emerged as a promising strategy to mitigate the high dependence of RFBs on battery separators. Here we report the exploration of the application of immiscible electrolytes, water and dichloromethane, in membrane-free redox stratified batteries. With 0.5 M phenothiazines in dichloromethane as the catholytes and zinc metal in aqueous electrolyte as the anolyte, the aqueous/ nonaqueous stratified battery presents stable long cycling with a capacity retention of 79.1% over 202 cycles under ambient testing conditions. Study of phenothiazines with varying lengths of alkyl chains (C0, C3, C8, and C18) reveals that the hydrophobicity of the phenothiazine molecules greatly affects the solubility in dichloromethane and battery cyclability. Computation on free energy of solvation and molecular dynamics is also performed to elucidate the hydrophobicity effects. The results presented in this work lay a solid foundation for potential development of the membrane-free RFBs.

Demixing the miscible liquids: toward biphasic battery electrolytes based on the kosmotropic effect

“…9 Unfortunately, most of these electrolytes reported to date have been based on singlephase characteristics, resulting in difficulties for simultaneously fulfilling the ever-intensifying heterogeneous requirements of anodes and cathodes. To resolve this challenge, biphasic electrolytes composed of two different electrolytes have been proposed, [10][11][12][13][14][15][16][17] including the use of solvent pairs with opposite polarities [10][11][12][13][14][15] and the introduction of thick ion-conducting barriers (e.g., lyophobic polymeric membranes 16 and solid-state inorganic electrolytes 17 ) to physically isolate two miscible electrolytes. However, a biphasic electrolyte incorporating a nonpolar solvent shows low ionic conductivity because of its insufficient dissociation ability.…”

Coupling Tetraalkylammonium and Ethylene Glycol Ether Side Chain To Enable Highly Soluble Anthraquinone-Based Ionic Species for Nonaqueous Redox Flow Battery

“…The kinetic rate constants of the corresponding four materials are 2.47 × 10 –2 , 2.36 × 10 –2 , 2.59 × 10 –2 , and 2.10 × 10 –2 cm s –1 , respectively. It should be noted that although AQEG1TFSI, AQEG2TFSI, and AQEG3TFSI have longer side chains due to the attached EG group, the D and k 0 (i.e., mass- and charge-transfer kinetics) values are still high, which are comparable to or greater than that of state-of-the-art active materials ,− and, more importantly, superior to those of neutral materials modified with a long oligoether chain. ,− This is mainly attributed to their ionic structure, where the redox-active sites mounted on the ionic groups are electroadsorbed on the electrode surface, leading to a facile Faraday reaction …”

Development and Challenges of Biphasic Membrane‐Less Redox Batteries

“…In 2021, dichloromethane (DCM, CH 2 Cl 2 ) and water were used as immiscible solvents by Jianbing Jiang, [ 65 ] while Zn/ZnSO 4 as the negative active molecule to dissolve in water phase and Phenothiazine (PTZ) and its derivatives as the positive one in DCM to form a battery ( Figure 15 a ). In addition, PF 6 ‐ served as an ion exchange between the two phases.…”

“…Figure 15. a) Schematic of a stratified, membrane-less battery; b) Battery performances of stratified Zn/PTZ batteries with 0.5 m C8-PTZ. Reproduced with permission [65]. Copyright 2021, ACS.…”