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Page 10 of 31 Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89

provide details on the geometry of the coordinating atom and the valence state of the absorbing atom. For
instance, in the Zn K-edge XANES spectroscopy, the larger white-line peak energy corresponds to the
[39]
2+
higher Zn valence state ; The strength of the white-line peak reflects the N(r) of Zn , where the N(r)
[60]
gradually declines as the white-line peak intensity increases from large to tiny . The bond length
information of the coordination atom can be determined by examining the R space generated by EXAFS .
[61]
Wavelet transform is a new technique for identifying and visualizing EXAFS atomic contributions. It has
the advantage of simultaneously exhibiting the length of the coordination bond and the type of
[61]
coordination atom, with the peak’s position to the right indicating the bigger atomic number . Small-angle
neutron scattering (SANS) measurements are another effective method to understand the electrolyte
[23]
structure . It has the ability to evaluate different RDFs in solution and validate the intermediate-range
electrolyte structures predicted by MD simulations.

In a nutshell, theoretical computation and experimental characterization complement one another by
allowing the former to explain the experimental data and the latter to verify computational accuracy. Each
calculational or experimental method has specific and irreplaceable strengths, and meanwhile, it has its own
limitations. Therefore, multi-method research is indispensable to achieving a full understanding of the
electrolyte properties in Zn batteries. In fact, we have not been able to actually observe the electrolyte
structure so far. The dynamic evolution of the electrolyte structure has not been directly examined yet.
Thus, there are still a lot of challenges and room for advancement in characterization techniques for viewing
and understanding the electrolyte structure.

UNDERSTANDING OF ZN/ELECTROLYTE INTERFACES
Interfacial structures and reactions
The Zn/electrolyte interfacial structure is another critical aspect related to the performance of the Zn anode.
The composition and behavior of the electrolyte at the Zn surface are completely different from that of the
bulk electrolyte . Through physical electrostatic interactions or chemical adsorptions, the Zn electrode
[62]
captures molecules or ions from the bulk electrolyte to produce an electric double layer (EDL). The
structure of EDL determines the ionic solvation and desolvation process, the compositions and properties of
solid electrolyte interphase (SEI), Zn transports crossing SEI, and the depositional mode of Zn . Figure 5A
2+
2+
illustrates the electrochemical deposition process of Zn in the EDL, approximately divided into five
2+
steps : (i) During the charging process, solvated Zn moves in the bulk electrolyte towards the Zn
[63]
2+
electrode under an electric field; (ii) After crossing the outer Helmholtz layer (OHL), it gradually desolvates
to bare Zn at the inner Helmholtz layer (IHL); Then, bare Zn is adsorbed to the Zn anode surface (iii)
2+
2+
and acquires electrons to be reduced to Zn metal (iv); and (v) In order to lower the system energy, the
deposited Zn on the electrode may be self-diffuse at the same time. The surface morphology of the Zn
anode affected by the Zn deposition behavior depends on the chemical environment of the Zn surface,
[64]
which is sensitive to the electrolyte characteristics . The strategy of regulating Zn electrodeposition
morphology by electrolyte modification is thus proposed. Based on recent reports [64-66] , the mechanisms of
electrolyte regulation on Zn anodes include controlling the Zn nucleation process, adjusting the Zn surface
texture, and restricting the site of Zn deposition.

Regulation of nucleation process
The electrochemical deposition of Zn begins with the Zn nucleation process. The free energy diagram for
the Zn reduction reaction [left of Figure 5B] shows that the formation of Zn metal needs to overcome a
nucleation energy barrier . The nucleation barrier can be changed by adjusting the reduction reaction
[67]
overpotential. The nucleation overpotential is known as the difference between the tip potential and the
subsequent stable potential, which represents the thermodynamic cost of creating the initial Zn atomic