“Anions‐in‐Colloid” Hydrated Deep Eutectic Electrolyte for High Reversible Zinc Metal Anodes

Abstract

Zn metal as a promising anode for aqueous batteries 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 consisting of Zn(ClO₄)₂ ⋅ 6H₂O, β-cyclodextrin (β-CD), and H₂O with mass ratio of 7 : 4.5 : 3 (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 β-cyclodextrins (β-CDs) in ACDE-3 self-assemble into micelles, in which the steric effect between adjacent β-CDs in micelles restricts the movement of anions. This unique "anions-in-colloid" structure enables the eutectic system with a high Zn²⁺ transference number (t_Zn ²⁺) 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 mA cm^-2, and the Zn//Cu half cells have a high average columbic efficiency (ACE) of 97.9 % at 0.5 mA cm^-2 from cycle 15 to 200 with a uniform and compact zinc deposition. When matched with a poly(1,5-naphthalenediamine) (poly(1, 5-NAPD)) cathode, the full battery with a low negative/positive capacity (N/P) ratio of 2 can still cycle steadily for 200 cycles at a current density of 1.0 A g^-1. Additionally, this electrolyte has been proven to be operative over a wide temperature range from -40 °C to 40 °C.