Redox flow batteries are receiving wide attention for electrochemical energy storage due to their unique architecture and advantages, but progress has so far been limited by their low energy density (~25 Wh l−1). Here we report a high-energy density aqueous zinc-polyiodide flow battery. Using the highly soluble iodide/triiodide redox couple, a discharge energy density of 167 Wh l−1 is demonstrated with a near-neutral 5.0 M ZnI2 electrolyte. Nuclear magnetic resonance study and density functional theory-based simulation along with flow test data indicate that the addition of an alcohol (ethanol) induces ligand formation between oxygen on the hydroxyl group and the zinc ions, which expands the stable electrolyte temperature window to from −20 to 50 °C, while ameliorating the zinc dendrite. With the high-energy density and its benign nature free from strong acids and corrosive components, zinc-polyiodide flow battery is a promising candidate for various energy storage applications.

Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

storage, sodium-ion batteries (NIBs) are attracting more attention because of the abundant sodium resource. [7][8][9] In comparison to the natural abundance of lithium (20 ppm) in the Earth's crust, the abundances of Na (23 000 ppm) and K (17 000 ppm) seem infinite. [10][11][12] Unlike NIBs, really few researches are focused on potassium-ion batteries (KIBs) in a long-trem period. Till last two years, the new concept of KIBs has begun to gain much more attention. [13][14][15][16][17] The advantages of KIBs are obvious: the abundant resource and the closer redox potential of K/K + (−2.93 V vs standard hydrogen electrode) to that of Li/Li + (−3.04 V) than that of Na/Na + (−2.71 V), implying their higher voltage plateau and energy density.Different K ion anode materials such as graphite, [13,18,19] nitrogen-doped graphene, [14,20] Prussian Blue, [21][22][23] and transition metal compound [24,25] have been