To establish highly soluble single redox couple for non-aqueous flow batteries, N-(α-ferrocenyl)ethylphthalimide (α-FcEtPI) is derived from a promising single redox couple, N-ferrocenylphthalimide (FcPI), by inserting an α-methyl group between the ferrocenyl and phthalimide moieties. The resulting material (α-FcEtPI) shows the maximum solubility of 0.81 M in 1.0 M tetrabutylammonium tetrafluoroborate (TBABF4) in 1,3-dioxolane at 25 °C. This notable increase in solubility (versus 0.3 M for FcPI) is attributed to the disruptive effect of the α-methyl group on molecular packing in a solid state. α-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 α-FcEtPI is calculated as 21.06 W h L−1 (versus 7.8 W h L−1 for FcPI). The cell data of 0.6 M α-FcEtPI in 1 M TBABF4 in 1,3-dioxolane electrolytes, indicates remarkably high Coulombic efficiency (97.8%) and excellent cycle retention.