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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 13 of 31

To control Zn dendrite growth, constructing coating functional layers on Zn metal anodes is also an
2+
efficient strategy since it is able to guide homogeneous Zn diffusion and shield water and anion to
suppress side reactions. The surface coating prevents the Zn dendrite formation can be categorized as
electronic regulators, ion regulators (including organic and inorganic ion regulators), and electronic-ionic
mixed conductive coatings. Coating electronic materials on the Zn surface is an effective approach to evenly
[10]
[11]
distribute charge to redirect uniform electrodeposition, such as metal nanoparticles (e.g., Au and Cu )
and carbon-based materials (epitaxial graphene , reduced graphene oxide , etc.). Zn deposition on these
[12]
[80]
high conductive coatings is more uniform, but the inhibitory effect of Zn dendrite growth is not significant
because electronic regulators tend to nucleate on the coating surface as the cycle process. Therefore, some
electrically insulating but ionically conductive materials, including organic ion regulators (such as zeolitic
[6]
[88]
imidazolate framework-8 (ZIF-8) and metal-organic framework (MOF) ) and inorganic ion regulators
(such as CaCO and TiO ) as alternatives to electronic regulators have been coated on the Zn surface.
[7]
[8]
3
2
Among them, organic ion regulators usually have highly reversible shape changes, which can alleviate the
volume change caused by metal expansion. However, pure organic components are easily punctured by Zn
dendrite during the cycle process due to the brittle nature, thus impeding their application in Zn
batteries [6,88] . The inorganic ion regulators can provide space shielding to control the growth direction of Zn.
But this kind of material has poor plasticity, which inevitably results in serious structural pulverization
during the long cycling . To alleviate the disadvantages of electron and ion regulators, the strategy of
[7-9]
constructing electronic-ionic mixed conductive coating is reported. For instance, Fan et al. introduced a
mixed electronic-ionic conductive coating layer which consists of Zn alginate gel (Alg-Zn) and acidified
conductive carbon black (AB) (Alg-Zn + AB@Zn) on the Zn surface . This coating effectively prevents Zn
[5]
dendrite formation and enables Zn||Zn symmetric cells to exhibit a cycling life of up to 500 h with low
reversible deposition potential. Overall, introducing interfacial film between Zn anode and electrolyte is
indeed one of the most efficient strategies to stabilize Zn metal anodes. However, in contrast, electrolyte
regulation with the features of facile preparation and cost-effectiveness seems easier to facilitate the
commercialization of Zn batteries. Meanwhile, some modified electrolytes are decomposed to generate SEI
layers during cycling. Unlike the artificial coating layer, which is not self-repairable, the in-situ generated
2+
SEI layer can continuously regulate the behaviors of electrons and Zn on the Zn surface, thus addressing
the Zn dendrite issue.

Construction of SEI
The interfacial reactions between the electrolyte and the Zn anode trigger the formation of the SEI layer.
The SEI layer is electrically insulating but ionically conductive. It is highly permeable to Zn while limiting
2+
the penetration of electrolytes. Most SEI layer is made up of an inorganic layer close to the anode from the
decomposition of ions and an organic layer exposed to the electrolyte side from the decomposition of
organics. The SEI layer plays an important role in stabilizing the electrode/electrolyte interface and
determines the battery performance. However, in initial aqueous electrolytes, the products of the reactions
between water and Zn metal are ZnO, Zn(OH) , and alkaline Zn salt, which have discrete structures and
2
cannot be regarded as the SEI. An effective SEI can be formed by optimizing the electrolyte composition,
such as by adding organic additives/cosolvents or inorganic additives that comprise C, F, and P [89-92] into the
electrolyte. Unlike the artificial SEI layer, which is not self-repairable and gradually loses its protective
function, the SEI layer generated by in-situ electrolyte decomposition can continuously regulate the
behaviors of electrons and Zn at the Zn/electrolyte interface, thus promoting the uniform Zn
2+
[90]
deposition . Moreover, the SEI layer can prevent the electrolyte from directly contacting with Zn metal,
[63]
thus inhibiting interfacial side reactions [Figure 5E] .