Solid Polymer and Lean-water Hydrogel Electrolytes for Highly Stable Zinc Batteries

Abstract

Aqueous zinc batteries (AZBs) are promising candidates for large-scale energy storage considering their intrinsically safe features, competitive cost, and environmental friendliness. However, owing to the existence of water molecules, the fascinating metallic zinc anode is subjected to several issues, including dendrite formation, corrosion, passivation as well as parasitic hydrogen evolution reaction (HER). These adverse reactions will eventually lead to irreversible electrolyte loss, battery swell, decaying performances and even short-circuit of the battery. As an essential alternative to traditional aqueous electrolytes, liquid/solvent-free solid polymer electrolytes (SPEs) are anticipated to effectively address these unpleasant challenges due to their high chemical stability by merely adding metal salts in polymer host materials.

Considering the intrinsic multivalence of Zn²⁺, Zn-based SPEs suffer from the poor ability to dissociate Zn²⁺ from anionic traps and inferior kinetics in the polymer matrix, resulting in an extremely low ionic conductivities and transference number of Zn²⁺ at room temperature. Here, we first report a novel SPE for promoting zinc salt dissociation and constructing fast ion transport channels by utilizing high dielectric polycarbonate-based polymer and unique structural montmorillonite (MMT), which can optimize the solvation structures that dissociate Zn²⁺ from zinc salts to generate more free ions. Moreover, the interlayer of the MMT nanosheets can accelerate Zn²⁺ transport under the electric field. As a result, a high room-temperature ionic conductivity (~1.45 mS cm^-1), large zinc-ions transference number (~0.72), and low activation energy can be delivered. The fabricated solid-state full battery demonstrates extraordinary rate capability and a long lifespan.

Hydrogels can retain water molecules and possess high ionic conductivities (10^-2 S cm^-1); however, they contain plenty of free water molecules, inevitably causing dendrites, corrosion, and gas evolution reactions on the Zn anode. Then, we develop a lean-water hydrogel electrolyte (water content of 20 wt%), targeting a new balance between ion transfer, stability of anode, electrochemical stability window, and interfacial resistance. The lean-water content hydrogel is equipped with a molecular lubrication mechanism, which leads to an impressive ionic conductivity (10^-3 S cm^-1) compared with SPEs (10^-4-10^-6 S cm^-1) and other hydrogels with the same water content (10^-4 S cm^-1). In addition, lean-water design leads to a widen electrochemical stability window and effectively suppressed gas evolution. Dendrite-free Zn plating/stripping with high reversibility is also achieved. Subsequently, the prepared full cells show excellent cycling stability of 4000 and 1500 cycles and superior capacity retentions of 91% and 94% at rates of 5 C and 1 C, respectively. Moreover, the superior adhesion ability of the lean-water hydrogel to electrodes can ensure adequate interface force to withstand various deformations, meeting the need for flexible devices.

In addition, the high reactivity of water molecules remains a fundamental barrier in aqueous electrolytes, especially when operating in aggressive environments (over 60℃). Herein, we design a lean-water hydrogel electrolyte via elaborate molecular engineering to optimize ion transport and electrochemical stability. Especially, with the water lubrication and assisted polymer backbones, the Zn²⁺ transport can be efficiently expressed even under a lean water state. Moreover, the abundant bound water caused by the lean water strategy and the formation of strong hydrogen bonds between the polymer backbone and water molecules makes it possible to reduce water reactivity, endowing enhanced electrochemical stability and generating highly reversible zinc plating/stripping. Even performing at a high temperature of 90℃, no clear gas evolution and side reactions can be observed. The Zn||Zn and Zn||Ti batteries can stably and reversibly cycle over 6000h at room temperature and over 1400h at 90℃. The full batteries show remarkable cycling stability at room temperature and even at a challenging temperature of 90℃ (>90% capacity retention over 750 cycles).

In summary, SPE with enhanced ionic conductivity and cation transfer number was subtly designed. To further improve the ion transport and interface, the lean-water hydrogel electrolyte was explored, in which the water molecules can act as lubricants to weaken the intermolecular interactions between polymer-polymer and polymer-ions. This will result in reduced crystallinity, intensive chain segmental motions of the polymer matrix, and improved dissociation of ions and mobility. Therefore, it is promising to take advantage of lubrication effects to develop lean-water hydrogel electrolytes equipped with the merits of both hydrogel electrolytes and SPEs. More importantly, the lean-water hydrogel electrolyte can provide valuable insights for the manufacture of highly reversible, and environment-adaptable ZBs.

Date of Award	7 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)