Selective Compatibility of High-Entropy Electrolytes for Low-Temperature Aqueous Zinc–Iodine Batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) have attracted massive interest on account of their environmental friendliness, low price, and high security. Nevertheless, the application of AZIBs is seriously constrained by the high liquid–solid transition temperature of aqueous electrolytes, which is strongly related to the water network connected through hydrogen bonds (HBs). Another critical technical issue is to explore the appropriate electrode material compatible with a low-temperature aqueous electrolyte. In order to ensure the battery works properly at low-temperature conditions, a high-entropy electrolyte (HEE) with multicomponent perchlorate salts, (Zn, Ca, Mg, Li)ClO₄, is developed. The calorimetric analysis indicates that the HEE exhibits an extremely low liquid–glass transition temperature (−114 °C). Structural characterizations using Raman, FTIR, and NMR spectroscopy indicate that the introduction of multicomponent perchlorate salts into the aqueous electrolyte breaks the initial water network by the formation of M···(H₂O)_n···ClO₄^– (M is Zn²⁺, Ca²⁺, Mg²⁺ or Li⁺) configurations, and the HEE therefore remains unfrozen even at −70 °C. The in situ viscosity measurement indicates the HEE has a viscosity of 13.8 mPa S at −70 °C. The electrochemical measurements indicate that the ionic conductivity of the HEE is 22.6 mS cm^–1 at 25 °C and 2.7 mS cm^–1 at −70 °C, and it has excellent electrochemical compatibility with Zn metal upon cycling Zn||Zn symmetric cells. The compatibility of the HEE and different electrode materials, particularly vanadate oxide with preinserted cations (KVO) and a carbon composite material with iodine (CCM/I₂) in this study, is systematically investigated, and the results of electrochemical measurements indicate the HEE shows the selectivity of battery systems. The KVO|HEE|Zn battery exhibits poor cycling stability at room temperature (only 33 mA h g^–1 after 5,000 cycles at 5.0 A g^–1), while the CCM-I₂|HEE|Zn battery displays a capacity of 182 mA h g^–1 at 100 mA g^–1 in the first cycle and superior cycling performances (102 mA h g^–1 after 5,000 cycles at 5.0 A g^–1). Low-temperature electrochemical measurements demonstrate that the battery system with the HEE exhibits enhanced electrochemical performances at −70 °C when compared with the binary electrolyte system (Zn, 3Ca)ClO₄. This work reveals the significance of electrode/electrolyte adaptability on the electrochemical performances of AZIBs and provides valuable insights for constructing low-temperature electrolytes using a multicomponent high-entropy strategy.