Modulation of hydrogel electrolyte enabling stable zinc metal anode

Highlights

• Modulation of hydrogel electrolytes is realized by crosslinking of proper polymers and grafting of polymetric anions.

• The obtained PSX gel electrolyte displays excellent performance for Zn metal anode that overcomes the drawbacks of normal liquid electrolytes.

• The mechanism discussion and clear insight on how the structural features of hydrogels affect the electrochemical/mechanical performance are provided.

• The PSX gel electrolyte shows a bright application prospect in a variety of energy storage devices, including flexible pouch batteries even under harsh conditions.

Abstract

The uneven flux and strong solvation of Zn²⁺ ions in aqueous electrolytes bring undesirable dendritic deposition and side reactions to zinc metal anodes (ZMAs). Hydrogel electrolytes have considerable mechanical strength and limited water content, which can suppress dendrite growth and alleviate side reactions. Nevertheless, the design of hydrogel electrolytes with desired electrochemical and mechanical properties is still challenged due to the lack of understanding of how structural features affect performances. Herein, we systematically investigate the hydrogel electrolytes for ZMAs by regulating their crosslinking and grafting structures. Experiments and theoretical simulations emphasize the importance of the network structure and the polymetric anions bonded on the hydrogel. The PSX gel crosslinked by carboxyl-grafted polyvinyl alcohol and xanthan gum shows superior performance, contributed by its abundant -COO⁻ groups and stable three-dimensional structure. The PSX electrolyte shows high ionic conductivity (18.86 mS cm⁻¹) and a considerable Zn²⁺ transference number (0.8). Zn//Zn cells with PSX electrolytes deliver long cycle lives accompanied by uniform zinc deposition, few by-products, and suppressed hydrogen evolution. When paired with V₂O₅ or active carbon cathodes, the PSX electrolyte also shows good compatibility and excellent performance. Flexible pouch cells are further demonstrated to work normally under different deformation and stress conditions.

Introduction

As a kind of emerging, clean and efficient energy storage device, batteries have attracted extensive attention in recent years. Compared with the batteries based on the chemistry of alkali-metal ion and organic electrolyte, zinc metal battery (ZMB) with aqueous electrolyte stands out because of its safety, low cost, and the high volumetric capacity of the Zn metal anode (5855 mAh cm⁻³) [1], [2], [3]. Nevertheless, ZMB is faced with many challenges [4,5]. The aqueous electrolyte is a double-edged sword, which promises safety but also brings unfavorable side reactions, including the corrosion of Zn metal and the hydrogen evolution reaction (HER) at the anode side, as well as the dissolution of cathode material and the oxygen evolution reaction (OER) at the cathode side [6], [7], [8], [9], [10]. The side reactions on the Zn anode are particularly serious, which is closely related to the strong solvation of Zn²⁺ with water molecules and the non-uniform Zn²⁺ flux in electrolytes (Fig. 1a, left). The solvated [Zn(OH₂)₆]²⁺ structure can easily bond with anions and produce insoluble hydroxides. The uneven Zn²⁺ flux results in dendritic deposition, where the side reactions will be aggravated due to the high activity at the tip of the dendrites. As a result, the reversibility of Zn deposition is greatly obstructed [11].

Hydrogel electrolytes are made from polymers such as poly (ethylene oxide) (PEO) [12,13], polyvinyl alcohol (PVA) [14], [15], [16], [17], polyacrylamide (PAM) [18], [19], [20], and xanthan gum (XG) [21,22]. The water content in the hydrogel electrolyte is limited, which can effectively suppress the side reactions in ZMB. The hydrogel electrolyte can also be used as the separator, enabling a simpler assembly of the battery. Moreover, the gels usually have unique flexibility, which provides considerable application prospects in wearable devices [23], [24], [25]. The structure and composition of the hydrogel greatly affect the property of the electrolyte [26]. The desired hydrogel electrolyte should have high ionic conductivity and good mechanical strength. The design and modification of the gel are usually realized by crosslinking and grafting different organic molecules [27], [28], [29], [30], [31], [32]. Despite the important progress recently made in developing hydrogel electrolytes for ZMB, there is a lack of in-depth study correlating the fabrication and property of hydrogel electrolytes, especially in terms of the intricate chemistry in ZMB.

Herein, we fabricate a series of hydrogels with different crosslinking and grafting structures. PVA and XG are chosen as the crosslinking main chains. The function of grafting carboxyl groups on PVA is also explored. PVA has low cost, environmental friendliness and good mechanical property, yet its ionic conductivity is low. XG has high ionic conductivity and good biocompatibility as a biological polysaccharide, while its mechanical strength is poor. The gels crosslinked by pure PVA, grafted PVA, XG, or their combinations show distinct performance as the electrolytes for Zn metal anode. The grafted PVA and XG are crosslinked to produce the PSX gel, which shows the best performance in terms of dendrite inhibition and anti-corrosion. The advantages of the PSX electrolyte are systematically studied through experiments and theoretical simulations. The superior performance is benefiting from the abundant carboxylate groups and the stable three-dimensional structure (Fig. 1a, right). The -COO⁻ groups as the bonded polymetric anions can direct the transport of Zn²⁺ ions and weaken their coordination with water or anions. The network structure can provide fast ion transport channels and constrain free water. Thanks to the stability of the PSX electrolyte at the cathode, Zn//V₂O₅ full batteries and hybrid Zn//AC capacitors also deliver steady cycling and good rate performance. The excellent mechanical properties of PSX further enable flexible pouch batteries to operate normally even under bending, pressing and hammering conditions.