Stabilizing four-electron zinc-iodine chemistry via glycine hydrochloride additives in eutectic electrolytes

Zn-I₂ batteries are conventionally restricted to the two-electron I₂/2I^- redox reaction, resulting in a limited voltage window and modest energy density. In this work, we demonstrate that introducing glycine hydrochloride (GH) into a bare eutectic electrolyte (BEE) transforms it into a BEE-GH system that enables reversible four-electron Zn-I₂ chemistry. The GH additive modulates the electrolyte coordination environment, suppresses parasitic reactions at the zinc anode, and stabilizes iodine intermediates, thereby broadening the operational voltage window with the high-voltage 2I⁺/I₂ redox couple. Electrochemical evaluations reveal that BEE-GH delivers a high specific capacity of 288 mAh g^-1 at 0.5 A g^-1, which is much higher than the two-electron BEE electrolyte (92 mAh g^-1 at 0.5 A g^-1). The BEE-GH electrolyte also maintains excellent capacity retention over 1500 cycles. These findings highlight additive-driven electrolyte engineering as an effective strategy to stabilize both electrodes and unlock high-energy multi-electron iodine chemistry, providing a promising route for next-generation Zn-I₂ batteries.