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

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.