Dataset of publication DOI: 10.1002/aenm.202301929 published in Advanced Energy Materials journal Despite the excellent electrochemical properties of non-functionalized 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), its use in aqueous organic redox flow battery (AORFB) is hindered to date due to its insolubility in water. However, in this study, an unprecedented solubility of 5.6 M is demonstrated in an aqueous solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which is 80 times higher than in water (0.07 M). A computational study reveals that the unique interaction between TEMPO and TFSI is essential to achieve this record solubility. TEMPO catholytes are tested in symmetric flow cells, demonstrating high capacity (23.85 AhL-1), high material utilization (89%), and robust reversible performance with long-term stability (low capacity fading of 0.082%/day). When paired with sulfonated viologen anolyte ((SPr2)V), an AORFB with low capacity fading over cycling (0.60%/day, 0.048%/cycle) is achieved, constituting the first example of a non-functionalized TEMPO catholyte for AORFB. Notably, this solubilization strategy could be applied to other unexplored chemistries in aqueous electrolytes, leading to the development of new AORFBs

Spanish Government (PID2021-124974OB-C21, PID2021-124974OB-C22 and TED2021-129378B-C22), Comunidad de Madrid (TALENTO grant 2017-T1/AMB-5264 and 20215A/AMB-20946), European Union through the MeBattery, Light-cap and MFreeB projects. MeBattery and Light-cap received funding from the European Innovation Council (grant Agreement no. 101046742 and No 101017821, respectively) and MFreeB project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 (grant agreement No. 726217).