Redox　flow　batteries　(RFBs)　are　highly　promising　electrochemical　systems　for　large-scale　energy　storage.　To　develop　high　energy　density　non-aqueous　catholyte　for　RFBs,　herein,　two　polar　and　bulky　polyethylene　glycol　tails　have　been　attached　to　the　tetrathiafulvalene　(TTF)　core　by　ester　exchange　reactions.　Such　modification　turns　scarcely　soluble　TTF　particles　into　viscous　oils,　which　are　totally　miscible　with　the　conventional　organic　electrolytes.　The　4,4′(5′)-bis(2-(2-methoxyethoxy)ethyl)carboxylate　-tetrathiafulvalene　(TTF-BMEEC)　with　a　mass-based　specific　capacity　of　107.84　mAh　g−1　is　systematically　evaluated　in　a　hybrid　RFB.　When　paired　with　a　Li　foil　electrode　and　a　permselective　separator,　the　catholyte　consisting　of　1.0　M　of　TTF-BMEEC　delivers　two　high　discharge　voltage　plateaus　of　3.55　and　3.88　V,　a　large　volumetric　capacity　of　49.6　Ah　L−1,　an　outstanding　energy　density　of　178.0　Wh　L−1,　and　a　high　cycling　capacity　retention　of　91.5%　after　100　cycles　at　the　current　density　of　10　mA　cm−2.　The　excellent　electrochemical　stability　of　TTF-BMEEC　is　confirmed　by　UV–vis　and　1H　NMR　investigations　and　supported　by　density　functional　theory　(DFT)　calculations.　These　results　demonstrate　that　molecular　engineering　has　great　potential　for　the　development　of　high　performance　organic　redox-active　materials　for　RFBs.　©　2023　Elsevier　B.V.