Adjusting Hirshfeld charge of TEMPO catholytes for stable all-organic aqueous redox flow batteries.

2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives are typical catholytes in aqueous organic redox flow batteries (AORFBs), but reported lifetime of them is limited. We find that the increase of Hirshfeld charge decreases the Gibbs free energy change (ΔG) values of side reactions of TEMPO, a near-linear relationship, and then exacerbates their degradation. Here we predict and synthesize a TEMPO derivative, namely TPP-TEMPO, by analyzing the Hirshfeld charge. TPP-TEMPO, with the smallest Hirshfeld charge and highest ΔG, is an order of magnitude more stable than TMA-TEMPO (N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride, a control with the largest Hirshfeld charge and lowest ΔG). We further elaborate on their decomposition pathways, identify byproducts, and mitigate degradation by supporting electrolyte engineering. Finally, a TPP-TEMPO/BTMAP-Vi (1,1'-bis[3-(trimethylammonio)propyl]-4,4'-bipyridinium tetrachloride) cell achieves a capacity density of ~12 Ah L and a low capacity fade rate of 0.0018% per cycle (or 0.0067% per hour).