Improved electrochemical reversibility of Zn plating/stripping: a promising approach to suppress water-induced issues through the formation of H-bonding

Highlights

• The H-bonding is constructed by adding an ether-based additive.

• The hydrogen evolution reaction can be restrained through H-bonding formation.

• The suppressed zinc corrosion can be achieved with the help of H-bonding.

• The electrode/electrolyte interface can be tuned by increasing the wettability.

Abstract

Rechargeable aqueous zinc ion batteries (ZIBs) are emerging as impressive candidates for renewable energy storage, benefiting from their cost-effectiveness, environmental benignancy, and intrinsic safety. Unfortunately, the further development of aqueous ZIBs is plagued by the free water–induced parasitic reactions, including inevitable hydrogen evolution reaction (HER) and corrosion issues. Herein, with the aim to reduce the reactivity of solvent water, an effective strategy is proposed to construct the H-bonding with free water by adding an ether-based additive. It is found that the suppressed HER and zinc corrosion can be achieved through the participation of the H-bonding. Meanwhile, the electrode/electrolyte interface is tuned by increasing the wettability as well, which contributes to enlarge the effective area of electrode reaction. Furthermore, the results demonstrate that the performance of ZIBs can be improved through enhancing the stability of Zn anode.

Introduction

Lithium-based batteries, known as the ‘holy grail’ of batteries, are the state-of-the-art landmark technology in the realms of mobile electronic devices and electrical vehicles due to their large capacity, high energy density and long cycling lifespan [[1], [2], [3], [4]]. However, the inherent safety remains an austere challenge stemming from the use of highly flammable and environmentally hazardous organic electrolytes. As the sustainable alternative for non-aqueous batteries, aqueous batteries open a viable route to enhance the safety of sustainable energy storage, which possess the merits of cost-effectiveness, eco-friendliness, and intrinsic safety [[5], [6], [7], [8], [9]]. Among various anode materials, metallic zinc, taking distinct properties of high chemical stability against oxygen and moisture, is an ideal anode material for aqueous battery systems. Additionally, metal Zn anode holds some intrinsic features including high theoretical volume capacity (5854 mAh cm⁻³), low equilibrium potential (−0.762 V vs. standard hydrogen electrode (SHE)), highly abundant and non-poisonous properties [[10], [11], [12], [13]]. In recent years, numerous researches have been carried out on Zn-host cathode materials with high specific energy, including manganese-based oxides [[14], [15], [16]], vanadium-based oxides [[17], [18], [19]], and organic compounds [[20], [21], [22]], etc. Nevertheless, the understanding of the reversibility of zinc anodes, especially in neutral or mild acidic electrolyte is insufficient, which is crucial for the practical development of zinc ion batteries (ZIBs).

According to the involved reactions (Eqns. (1), (2)), Zn plating process is inevitably interfered by the hydrogen evolution reaction (HER) in aqueous electrolytes, which leads to the depletion of zinc electrode and electrolyte during the charge/discharge processes. The above unfavorable parasitic reactions would be more detrimental when the free water remains the main ingredient in the electrolytes [23,24]. The formation of H₂ may increase internal pressure of cell, which could induce the rupture of the cell itself. This irreversible side-reaction could influence the following electrochemical reactions at the Zn anode/electrolyte interface. In addition, Zn is prone to chemical corrosion in the mild acidic electrolyte, which simultaneously consumes the metallic Zn anode and electrolyte. Overall, these problems could result in the decreased utilization, low coulombic efficiency (CE) and poor cycling stability of Zn anode.

HER: 2H⁺ + 2e⁻ → H₂ (0 V vs. SHE)

Zn reduction: Zn²⁺ + 2e^– → Zn (−0.762 V vs. SHE)

To tackle these issues, large efforts have been devoted over the past decades. Recently, a novel ‘water-in-deep eutectic solvent’ (water-in-DES) electrolyte is proposed by Zhao and co-workers, where all water molecules can involve the DES's internal interaction network [25]. This strategy significantly suppresses the reactivity of water. Generally, as shown in Fig. 1, Zn²⁺ can form solvation-sheath structure with the surrounding water molecules in the aqueous electrolyte, that is, [Zn(H₂O)₆]²⁺. To circumvent the problems involved by water, a ligand-assisted eutectic system is demonstrated to form hydration-deficient complexes, which enable highly reversible Zn plating/stripping processes with favorable CE [26]. Besides, another emerging approach using the highly concentrated electrolytes, termed ‘water in salt’ system, has been put forward to address H₂ evolution issues and regulate the solvation-sheath structure of Zn²⁺, leading to favorable zinc reversibility [27]. However, the high cost of highly concentrated systems may restrict the practical viability. Therefore, the development of competitive strategies to suppress HER and improve the reversibility of Zn anode through the achievement of decreased reactivity of water is highly significant [28,29].

As a proof-of-concept demonstration, an available electrolyte additive, 1,2-dimethoxyethane (DME), is proposed to improve the performance of ZIB in an aqueous medium. As a versatile solvent, DME has been used in the research realm of rechargeable batteries such as lithium-sulfur batteries and sodium ion batteries, showing a good electrochemical stability [30,31]. Inspirationally, DME shows the merits of unlimited mutual solubility in water. Thus, amphiphilic DME holds great potential in constructing H-bonding with H₂O to improve aqueous battery performance. In this work, the H-bonding, formed between hydrophilic hydration of the ether oxygen of DME and water molecules, alleviates the free water–induced corrosion reaction. In addition, it is found that the participation of DME can impede chemical corrosion in the mild acidic electrolyte. Meanwhile, the even distribution of Zn²⁺ ions at the Zn metal/electrolyte interface can be manipulated by the improved wettability. In virtue of an optimal concentration of 1 vol% DME into the aqueous electrolyte, the enhanced electrochemical performance can be obtained by stabilizing zinc anode. This article reveals that the reduced reactivity of water solvent is advantageous to inhibit the occurrence of HER, which results in the improved reversibility of zinc anode and longer cycle stability of zinc battery. Hence, this study offers a rational basis to gauge Zn-based rechargeable batteries.