Zwitterion-mediated interface chemistry for practical Zn-iodine batteries

Introduction

Aqueous zinc metal-based batteries provide promising solutions to the next-generation energy storage systems, among which zinc-iodine (Zn||I₂) batteries stand out due to their potential low cost, high safety, and environmental friendliness^{1, 2–3}. Typically, Zn||I₂ batteries consist of a metal Zn negative electrode, an I₂ positive electrode supported by a conductive porous host, and a mild aqueous electrolyte containing Zn salt^4,5. In Zn||I₂ batteries, the I₂ positive electrode primarily undergoes a conversion reaction between I₂ and iodide (I⁻) ions while the Zn negative electrode follows a typical plating/striping reaction, rendering a promising theoretical specific energy of approximately 220 Wh kg⁻¹ (based on the mass of the active materials of the positive electrode and negative electrode)^4,5. Despite their advantages, Zn||I₂ batteries encounter significant challenges for their practical implementations, primarily due to notorious parasitic reactions occurring at both the negative electrode and positive electrode^{6, 7, 8–9}. Specifically, the Zn metal in aqueous electrolytes is prone to dendrite formation (the root of short circuits), hydrogen evolution reactions (HER) that compete with Zn electrodeposition, and persistent corrosion induced by water or polyiodides (Fig. 1a)^6,7. At the positive electrode, the I⁻ ions existing in a hydrated state promote the undesired dissolution of I₂, leading to the formation of polyiodides (Fig. 1a)⁸. The polyiodide shuttle contributes to self-discharge, causing continuous depletion of active materials, which ultimately results in capacity degradation and low Coulombic efficiency (CE)^9,10.

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Fig. 1

Interface Chemistry for Zn||I₂ batteries.

Schematic illustration of the interface reactions at typical aqueous electrolytes (a) and zwitterion-containing electrolytes (b).

To stabilize the Zn metal electrode, surface modification with protective coatings is viewed as the most straightforward and efficient approach, proficiently mitigating detrimental processes such as dendrite formation, HER, and electrochemical corrosion^11,12. Unfortunately, these pre-formed artificial interfaces often suffer from high interface impedance and poor mechanical stability due to their low ionic conductivity, uncontrollable thickness, and poor adhesion^13,14. Even worse, they are prone to delamination from the Zn metal electrode during repeated deposition/dissolution cycles or mechanical deformations^15,16. To mitigate the shuttle of polyiodide,...