Insights into the Effects of Organic Electrolyte Additives on Proton Insertion for Low-Temperature Aqueous Zinc Batteries

The further development of aqueous zinc batteries is restricted by their low-temperature performance. At present, introducing organic additives into aqueous electrolytes is a universal strategy to reduce the freezing point and improve the low-temperature performance. However, the effect of organic additives on charge carriers is neglected, and the corresponding impact on low-temperature performance is not clear. Herein, a common organic additive (propylene carbonate, PC) was introduced into the Zn(CF₃SO₃)₂ electrolyte to explore the effect of organic additives on charge carriers. PC optimizes the coordination environment of water in the electrolyte, which reduces the freezing point. However, this changed coordination environment of water decreases the adsorption energy of water on the surface of the oxide cathode, which restrains the proton insertion behavior. With the increase of PC, the proton insertion behavior is restrained gradually. At -40 °C, a Zn||CaV₈O₂₀·<i>n</i>H₂O battery with pure Zn(CF₃SO₃)₂ electrolyte fails because of the freezing of the electrolyte. However, the Zn||CaV₈O₂₀·<i>n</i>H₂O battery based on the PC-optimized electrolyte with proton insertion still displays a capacity of 177 mAh g^-1 at 0.5 A g^-1 at -40 °C, which is obviously higher than that (111 mAh g^-1) based on the PC-optimized electrolyte without proton insertion. This work provides a theoretical basis for selecting suitable organic additives to develop low-temperature aqueous zinc batteries.