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.