Introduction
Zinc metal is a highly promising anode material for aqueous electrochemical energy storage systems due to its high theoretical capacity (820 mAh g-1 and 5855 mAh cm-3) [1], natural abundance, and excellent safety profile [2]. However, the rampant zinc dendrite growth and parasitic side reactions lead to the short service time of the aqueous zinc batteries, which has significantly limited their practical application[3]. During the zinc electrodeposition, non-uniform zinc ion transport and accumulation of the ”tip effect” lead to the formation of highly porous flake-like zinc dendrites, which can penetrate the separator and result in an internal short circuit[4]. Meanwhile, solvated Zn2+ions, typically in the form of [Zn(H2O)6]2+, undergo desolvation in the electric double layer before reaching the zinc surface and receiving electrons to be reduced to Zn. The desolvated water molecule is more prone to decomposition, resulting in parasitic hydrogen evolution reactions (HER) and zinc corrosion[5]. Therefore, it is urgently needed to search for effective strategies for addressing these abovementioned issues. Various strategies have been proposed to circumvent challenges encountered by zinc anodes, including electrode design[6], interfacial modification[7], and electrolyte design[8]. Interfacial modification is regarded as one of the most promising ways to improve the Zn anode performance, as the electrode-electrolyte interface plays a crucial role in ion transport and electrochemical reactions.
Various electron-conductive or insulating materials with different functions and mechanisms have been successfully adopted to form protective layers on the Zn surface. Electron-conductive materials include carbon material [9], which can easily incorporate different functional groups and interact with Zn2+, and metal [10] or alloys[11], which can provide abundant and uniform nucleation sites and exhibit low nucleation barriers. However, zinc deposits on top of the electron-conducting layer may eventually develop into dendrites and penetrate the membrane. Electron-insulating materials, including a series of insulating polymers[12] that exhibit excellent mechanical properties and functional groups, can constrain the zinc dendrite growth and guide the diffusion of Zn2+ ions, and inorganic materials (e.g., CaCO3 [13], TiO2 [14], ZrO2[15], etc.), which are typically synthesizedex situ and coated on the zinc surface with binders to form a porous layer with binders can avoid the direct contact of zinc and aqueous electrolyte and uniformize the Zn2+ flux. However, the characteristics of zinc deposition with various coating layers vary due to large deviations in the particle size of inorganic materials.
Among various coating materials, metal-organic frameworks (MOFs) representing a class of electron-insulating materials with ordered and tunable pore sizes have been ex situ or in situ coated on the zinc surface to suppress dendrite growth [16]. For example, ZIF-8 [17], UiO-66[18], and functionalized MOFs such as UiO-66-(COOH)2 have been ex situ coated and serve as ion-conductive layers that promote the zinc ion desolvation and Zn2+ diffusion [19]. Zhou et al. constructed a super-saturated electrolyte front surface by coating pre-synthesized ZIF-7 particles with polyvinylidene fluoride (PVDF) binder on the zinc surface. The channel size of ZIF-7 is smaller than that of water-solvated Zn2+ ions, which enables the MOF layer to reject large-size solvated ion complexes and promote the desolvation of Zn2+ ions[20]. Despite the demonstrated effects in extending the lifespan of zinc anode, the ex situ coating inevitably involves polymer binders, which leads to a large thickness, sacrificed energy density, and unclear underlying mechanisms of MOF functions.
To overcome this limitation, there have been works to in situsynthesize binder-free MOF layers and provide critical discussions on the size effect of the MOF channels and the crystallization of the MOF materials. To be specific, binder-free layers composed of MOFs such as ZIF-L [21], ZIF-8 [22], ZSB (Zn-stp-by) [23], Zn-TCPP[24], and MOFs derived by the coordination between Zn2+ and [Fe(CN)6]3-[25] are in situ grown on zinc anode surface. These layers with functional groups can interact with Zn2+ and their ordered nanochannels can homogenize Zn2+ flux. In addition to these in situ grown crystalline MOF layers, Xiang et al. constructed a continuous amorphous ZIF-8 MOF layer, which can eliminate the dendrite growth at the grain boundaries in crystalline MOF layer and make the protective functions more extraordinary[26]. Furthermore, Zhou et al. applied a fast current-driven synthesis method to in situ grow a crack-free hydrophobic ZIF-7x-8 layer to promote zinc ion desolvation[27]. Based on the above review, it can be concluded that the in situ growth of seamless MOF layers with rationally selected pores and functional groups effectively boosts zinc ion desolvation and suppresses dendrite growth.
Herein, after screening various MOFs, Zn2(bim)4 [28] was selected as a promising coating material for zinc anodes, which has pore sizes of only ~2.1 Å. In addition, taking advantage of the 2D structures of Zn2(bim)4, we innovatively apply the gel vapor deposition (GVD) method to grow the 2D sheet on the zinc surface layer by layer and achieve a dense and connected MOF layer. The in situ stacked layer of nanosheet with a fine pore structure can effectively reject the large size of [Zn(H2O)6]2+association (~8.6Å), and only allow the transport of compact Zn2+ ion pairs, since the diameter of zinc ion is around 1.48 Å [29]. The uniform channels with small pore sizes lead to a homogenized flux of partially desolvated Zn2+, as shown in Figure 1, enabling the Zn@Zn2(bim)4//Zn@Zn2(bim)4symmetric cell to be stably operated for over 1000 h at 0.5 mA cm-2 and 0.5 mAh cm-2, and 700 h at 1 mA cm-2 and 1 mAh cm-2. In addition, the full cell assembled with Zn@Zn2(bim)4 anode aand MnO2 cathode can be stably cycle for 1200 cycles at 1 A g-1.