Breaking the trade-off between capacity and stability in vanadium-based zinc-ion batteries

Water in electrolytes is a double-edged sword in zinc-ion batteries (ZIBs). While it allows for proton insertion in the cathode, resulting in a significant increase in capacity compared to that of organic ZIBs, it also causes damage to electrodes, leading to performance degradation. To overcome the capacity-stability trade-off, organic solvents containing a small amount of water are proposed to mitigate the harmful effects of water while ensuring sufficient proton insertion. Remarkably, in a Zn(OTf)₂ electrolyte using 8% H₂O in acetonitrile as the solvent, Zn‖(NH₄)_0.5V₂O₅·0.5H₂O exhibited a capacity as high as 490 mA h g^-1 at a low current (0.3 A g^-1), with a capacity retention of 80% even after 9000 cycles at high current (6 A g^-1), simultaneously achieving the high capacity as in pure aqueous electrolytes and excellent stability as in organic electrolytes. We also found that the water content strongly impacts the kinetics and reversibility of ion insertion/extraction and zinc stripping/plating. Furthermore, compared to electrolytes with pure acetonitrile or H₂O solvents, electrolytes with only 8% H₂O in acetonitrile provide higher capacities at temperatures ranging from 0 to -50 °C. These discoveries enhance our understanding of the mechanisms involved in ZIBs and present a promising path toward enhancing electrolyte solutions for the creation of high-performance ZIBs.