To establish highly soluble single redox couple for non-aqueous flow batteries, N-(alpha-ferrocenyl)ethylphthalimide (alpha-FcEtPI) is derived from a promising single redox couple, N-ferrocenylphthalimide (FcPI), by inserting an amethyl group between the ferrocenyl and phthalimide moieties. The resulting material (alpha-FcEtPI) shows the maximum solubility of 0.81 M in 1.0 M tetrabutylammonium tetrafluoroborate (TBABF(4)) in 1,3-dioxolane at 25 degrees C. This notable increase in solubility (versus 0.3M for FcPI) is attributed to the disruptive effect of the amethyl group on molecular packing in a solid state. alpha-FcEtPI exhibits two stable redox reactions at E-1/2 = 0.01 V and -1.97 V (vs. Fc/Fc(+)), thus functioning as a single redox couple with working voltage (E-wk) of 1.98 V. Considering the solubility and working voltage, the theoretical energy density of alpha-FcEtPI is calculated as 21.06 W h L-1 (versus 7.8 W h L-1 for FcPI). The cell data of 0.6M alpha-FcEtPI in 1 M TBABF(4) in 1,3-dioxolane electrolytes, indicates remarkably high Coulombic efficiency (97.8%) and excellent cycle retention.