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Abstract Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm²and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm²and at a temperature of 27 °C. 

Non-aqueous organic material-based redox flow batteries (NAORFBs) possess the advantage of using organic solvents to achieve high electrochemical potential. However, regardless of the great progress made in this regard in the past decade, further development has been restricted by the lack of stable electroactive organic materials and highly selective separators. Here, we present a NAORFB with all-PEGylated, metal-free, organic compounds as electroactive materials. PEGylated phenothiazine and PEGylated viologen are utilized as the catholyte and anolyte, respectively. Combined with a composite nanoporous aramid nanofiber separator, the all-PEGylated NAORFB presents outstanding cyclability, with a capacity retention of 99.90% per cycle and average coulombic efficiency of 99.7%. By contrast, NAORFBs using half-PEGylated and non-PEGylated electrolytes display inferior cyclability owing to the crossover of non-PEGylated materials. An extended investigation was also performed on the batteries using non-PEGylated or half-PEGylated materials for mechanistic elucidation. This work validates the PEGylation strategy in NAORFBs for enhanced overall performance with respect to solubility, cyclability, and alleviated crossover.