Molecularly engineered three-dimensional covalent organic framework protection films for highly stable zinc anodes in aqueous electrolyte
Introduction
The development of grid energy storage as sparked an ever-increasing interest in investigating alternative energy storage technologies to Li-ion batteries owing to their safety issues and high cost [1,2]. Aqueous rechargeable batteries, which are based on safe and low-cost water-based electrolytes, have been regarded as one of the most promising candidates for grid energy storage. As for the anode choice of aqueous batteries, metallic zinc (Zn) has attracted tremendous attention as anode owing to its low cost, high theoretical capacity (820 mA h g−1), and low potential (−0.762 V versus the standard hydrogen electrodes) [3], [4], [5], [6], [7], [8]. However, Zn anodes suffer from a few challenging issues such as the poor reversibility and the growth of Zn dendrites in neutral aqueous solutions which not only shorten the lifespan of batteries but also bring the safety concerns [5], [6], [7], [8], [9], [10].
Recently, a variety of strategies have been developed to stabilize the Zn anode, such as the constructing the nucleation layer or skeletons [10,11], optimizing the electrolytes or additives [12], [13], [14], [15], [16], and creating the surface protective layers [17], [18], [19], [20], [21], [22], [23]. Among these strategies, the creation of protective layer offers a facile and promising route to guide the uniform Zn deposition. Although the reported protective layers can modify the electrochemical performance of Zn, there are still some problems in the design of the protective film. First, most of the reported films are constructed by coating functional inorganic/organic fillers mixed with the polymer binder on the Zn foil [24,25]. These fillers were coated on the Zn foil by the polymer binder, and the distribution of fillers is easy to be inhomogeous. Additionally, the size and distribution of pores between fillers are also nonuniform, which affect Zn2+ to uniformly pass through the protective layer homogeneously. Finally, the reported polymer composite film generally shows a single function and cannot block the anions to pass through the protective film, which cannot suppress the side reactions that anoions take part in on the Zn surface [4]. Therefore, rational design and construction of protective films on the Zn anode is essential to overcome the above problems, which remains a challenge.
As an emerging versatile material, covalent organic frameworks (COFs) have received tremendous attention due to their designable structures and extremely ordered mass transfer channels. By deeply exerting these distinctive advantages, COFs have shown great potentials in a variety of advanced applications, such as sensing, separation, energy storage and conversion [26], [27], [28]. Owing to the nano-sized aperture sizes and prominent porosities, COFs potentially allow the fast and exclusive transport of metal ions, benefiting the performance improvement during charge and discharge cycles. In addition, the post-synthetic modification promises the precise engineering of COF pore wall environments for on-demand purposes, in favor of devising specific functions without compromising structural stability and uniformity. Moreover, as a subfamily of COFs, three-dimensional (3D) COFs constructed by reticular chemistry can provide interconnected channels, which potentially benefit for the promotion of ion transport. Thus, the molecular engineering of COFs has huge potential to construct novel protective layers to obtain high-performance and stable Zn anode.
Here, we have designed and in-situ synthesized a ultrathin and uniform three-dimensional (3D) COOH-functionalized covalent organic frameworks (COF) film (denoted as 3D-COOH-COF) with high mechanical strength to protect the Zn anodes. This unique and novel 3D-COOH-COF protective layer provides a variety of advantages. First, a thin thickness and uniform nanochannels of COF layer facilitate the fast and homogeneously transport of Zn2+ around the surface of Zn anode. Second, abundant negative COOH-functionalized groups of 3D skeleton and nanochannels of the COF film impede sulfate ions to pass through the protective film, resulting a high transfer number of Zn2+ and dendrite-free deposition of Zn metal. Third, the 3D-COOH-COF film can uniformly and fully cover the current collectors and Zn anodes without any gaps, effectively prevent the direct contact between Zn anode and aqueous electrolyte, and significantly suppress the corrosion reactions. Such advantages of 3D-COOH-COF film can not only reduce side reaction through selectively accelerating Zn2+ and inhibiting anions transport, but also suppress dendrite growth by ensuring even Zn2+ plating/stripping (Fig. 1). Consequently, Zn-ion batteries based on Zn anodes with 3D-COOH-COF layers exhibit excellent electrochemical performance.
Section snippets
Results and discussions
The fabrication process of 3D-COOHCOF film coated on the Zn foil is illustrated in Fig. 2a, which involves two major steps. First, -OH functionalized 3D-OHCOF precursor film was synthesized on the Zn foil by in-situ growth method [26,27]. The thickness and surface flatness are dependent on the concentration of tetra(4-formylphenyl) methane (TFPM) and 3,3′-dihydroxybenzidine (DHBD), as shown in Fig. S1. Scanning electron microscopy (SEM) images reveal that the prepared OHCOF precursor film is
Conclusions
In summary, we have demonstrated that the rationally designed 3D-COOHCOF films on Zn plates realize the fast and stable Zn electrodeposition. The thin 3D-COOHCOF film has uniform distribution, homogenesous nanochannels, and abundent negative COOH groups, facilitating the Zn2+ transport and impeding the pass through of anions. Moreover, the in-situ generated COF film significantly reduces the corrosions between metallic Zn and aqueous electrolyte. This novel COF protective film enables the
CRediT authorship contribution statement
Kuan Wu: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing – original draft, Visualization. Xiansong Shi: Conceptualization, Methodology. Fangfang Yu: Validation, Formal analysis, Investigation. Haoxuan Liu: Data curation, Visualization. Yuanjun Zhang: Data curation. Minghong Wu: Project administration, Funding acquisition, Formal analysis. Hua-Kun Liu: Project administration, Writing – review & editing. Shi-Xue Dou: Project administration, Funding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by China Postdoctoral Science Foundation (2020M681260), Science and Technology Commission of Shanghai Municipality (No.20010500400), and Australian Research Council (DP200100365).
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These authors contributed equally to this work.