"Anions-in-Colloid" Hydrated Deep Eutectic Electrolyte for High Reversible Zinc Metal Anodes.

ABSTRACT

Zn metal suffers from severe zinc dendrites, anion-related side reactions, hydrogen evolution reaction (HER) and narrow electrochemical stable window (ESW). Herein, an"anions-in-colloid" hydrated deep eutectic electrolyte (ACDE-3) is designed to improve the stability of zinc anode. The ACDE-3 reconfigures the hydrogen-bond (HB) network and regulates the solvation shell. More importantly, the hydroxyl-rich ß-cyclodextrins (ß-CDs) in ACDE-3 self-assemble into micelles, in which the steric effect between the adjacent ß-CDs restricts the movement of anions. This unique "anions-in-colloid" structure enables the eutectic system with a high Zn2+ transference number (tZn2+) of 0.84.  Thus, ACDE-3 inhibits the formation of dendrite, prevents the anion-involved side reactions, suppresses the HER, and enlarges the ESW to 2.32 V. The Zn//Zn symmetric cell delivers a long lifespan of 900 hours at 0.5 mAh cm-2, and the Zn//Cu half cells have a high average columbic efficiency (ACE) of 97.9% at 0.5 mAh cm-2 with a uniform and compact zinc deposition. When matched with a poly(1,5-naphthalenediamine) cathode, the full battery with a low negative/positive capacity ratio of 2 can still cycle steadily for 200 cycles at a current density of 1.0 A g-1. Additionally, this electrolyte can operative over a wide temperature range from -40 °C to 40 °C.