Zinc-iodine (Zn/I2) batteries are recognized as a kind of leading candidate for large-scale energy storage systems, owing to the high-capacity dissolution-deposition reactions on both electrodes. Nevertheless, the lifespan of Zn/I2 batteries is severely limited by the uncontrolled shuttling of triiodide ions (I3-) and unfavorable side reactions on Zn anodes. Herein, an alginate-based polyanionic hydrogel electrolyte is designed and synthesized by ion exchange and Zn2+-induced cross-linking. The immobile, negatively charged polyanionic chains on the hydrogel skeleton effectively block I3- from shuttling, while simultaneously transporting cations that are indispensable for battery chemistry. Moreover, this hydrogel can also enhance the cycling durability of Zn anodes by alleviating Zn's dendritic growth and corrosion reactions, due to the homogenized Zn2+ flux and reduced interfacial contact between free water and metallic Zn. Consequently, this alginate-based hydrogel electrolyte enables stable Zn plating/stripping for over 600 h at 2 mA cm-2 and 2 mAh cm-2 (corresponding to 10% depth of discharge). Serving as an electrolyte for Zn/I2 full batteries, this hydrogel helps the battery to achieve a high capacity of 183.4 mAh g-1 (capacity retention = 97.6%) after even 200 cycles at 0.2 A g-1, 77.4% higher than that of the traditional ZnSO4 aqueous counterpart (residual capacity = 41.5 mAh g-1). This work indicates the promising potential of electrolyte design on the performance improvement of aqueous Zn/I2 batteries.
Keywords: hydrogel electrolyte; iodine cathode; self-discharge; shuttle effect; sodium alginate; zinc ion batteries.