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some alternative energy storage battery systems with lower cost, such as sodium-ion batteries (SIBs) and
[4,5]
potassium-ion batteries (PIBs), are put on the agenda for replacing LIBs . At the same time, some metal-
based batteries with high theoretical energy density, such as lithium-oxygen batteries and lithium-sulfur
[6,7]
batteries, have also been proposed to cope with the increasing demand for high-energy-density batteries .
Nonetheless, the electrolytes used in those metal-based batteries are usually both water and air sensitive,
2+
3+
posing safety and environmental concerns. In contrast, aqueous batteries (Zn , Fe , Mg , Al , and so
2+
2+
forth) are considered promising next-generation battery systems due to their safety and environmental
friendliness . Among them, rechargeable Zn-based batteries are gaining increasing attention for replacing
[8,9]
Li-ion batteries due to their high theoretical energy density, good stability, low cost and environmental
[10]
friendliness .
2+
Aqueous Zn-based batteries are built on reversible Zn /Zn dissolution/deposition reactions with a redox
potential of 0.76 V vs. standard hydrogen electrode (SHE) [11-16] . Depending on the battery system, the
electrolyte can be neutral, acidic or alkaline solution. According to the electrochemistry of cathode
materials, Zn-based batteries mainly consist of the following battery systems: Zn-ion batteries, Zn-based
redox flow batteries and Zn-air batteries. Besides, some modified batteries, such as flexible devices, have also
been proposed [Figure 1]. Despite the rapid development of Zn-based batteries in recent years, more efforts
are still needed to drive them toward commercialization. In this focused review, recent progress on aqueous
Zn-based battery systems is outlined. The operating mechanisms of each battery system are briefly
introduced, followed by their existing challenges and research directions. Perspectives are also provided for
the future development of Zn-based battery systems.

ZN BASED BATTERIES
Zn anode
As an important part of Zn-based battery systems, Zn anodes usually exist in the form of Zn foils in the
battery system. When discharging, Zn loses two electrons to form Zn and dissolves into the electrolyte;
2+
when charging, it regains two electrons and is plated onto the Zn flakes [17,18] . Zn anodes undergo side
reactions during electrochemical processes, especially the hydrogen evolution reaction (HER) due to its
poor thermodynamic stability in aqueous electrolytes, leading to severe self-discharge reactions [19-23] . There
are two main solutions to this problem: surface engineering and electrolyte additives. Surface engineering
strategies usually employ a layer of artificial SEI, such as inorganic passivation layers, carbon coating layers
or polymer membranes, to prevent the electrolyte from directly contacting the Zn metal anode [24,25] . For
example, Li et al. adopted graphite-modified Zn anode, a mitigated corrosion reaction was obtained and a
[26]
high Coulombic efficiency was achieved . Additionally, some organic additives, such as thiourea, diethyl
ether (Et O), sodium dodecyl sulfate (SDS), or cetyltrimethylammonium bromide (CTAB), contribute to
2
alleviating the corrosion of Zn metal anode .
[27]
Another major problem is the growth of dendrites in developing alkaline rechargeable batteries. The growth
of dendrites will not only lead to the growth of “dead Zn” which degrades the coulombic efficiency of the
battery, but also may pierce the separator and cause the battery to short circuit. One of the most direct
solutions is to modify the surface of the Zn anode to form a stable solid-electrolyte interface (SEI).
Higashi et al. used polypropylene to modify the surface of the Zn anode to ensure the stable operation of the
Zn-Ni battery 800 times [Figure 2A and B] . Besides surface engineering, an epitaxial electrodeposition
[28]
technique was also developed by Yu et al. to suppress dendrite growth . Using graphene with a low lattice
[29]
mismatch with metallic Zn as the substrate, highly reversible Zn metal reversible deposition was
achieved . In addition, salt concentration electrolytes are also employed to regulate the deposition of Zn
[29]
metal anode. A high-concentration electrolyte containing 1 M Zn(TFSI) and 20 M LiTFSI was proposed by
2