The intermittent characteristic of renewable energy sources leads to a significant challenge in energy generation. Considerable efforts have been made to explore viable solutions, with rechargeable batteries standing out among various energy storage systems. Recently, lithium-ion batteries (LIBs) are widely used in various energy storage devices. However, the high cost, inherent safety risks, and limited lithium resources seem to restrict their large-scale application. Therefore, different new rechargeable battery systems such as aluminum-ion, magnesium-ion, and zinc-ion batteries (ZIBs) have been developed. Among these, rechargeable ZIBs with high safety, abundant electrode materials, high theoretical capacity, environmentally benignity, and low-cost, have received extensive attention.

Typically, a ZIB system consists of a host material capable of storing zinc ions as the cathode, zinc metal as the anode. The cathode material becomes the primary limitation for battery performance and manganese dioxide (MnO₂) is considered the most promising candidate due to its natural abundance, environmental benignity and low-cost. Generally, mild aqueous electrolyte made of sodium sulfate (Na₂SO₄) solution are implemented. However, the narrow electrochemical stability window of aqueous electrolytes has limited the operating voltage of the host materials. Thus, deep eutectic solvents are introduced to address this issue. Nevertheless, the mechanism of zinc ion storage and the reason for the capacity fading in ZIB system in deep eutectic solvents have not been thoroughly clarified.

In this work, we present a rechargeable ZIB that is composed of a zinc anode prepared from nickel foam supported zinc, a cathode made of stainless steel mesh supported MnO₂/carbon film and an electrolyte made of a non-flammable deep eutectic solvent (ethylene glycol/(2-hydroxyethyl) trimethylammonium chloride). ZnCl₂ additive in the electrolyte was examined, and the optimal ratio of ZnCl₂ was determined. The fabricated battery operated through the electrochemical deposition and dissolution of zinc at the anode, and intercalation/de-intercalation of zinc ions in the MnO₂ structure. The fabricated battery exhibited a large capacity of about 200 mAhg^-1 and a charge efficiency of approximately 98%. The cathode was found to withstand more than 2,000 cycles without significant capacity decay. The results demonstrated a deep eutectic solvent-based ZIB and offered a fundamental basis for understanding the electrochemical intercalation chemistry of zinc ions in a deep eutectic solvent.