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Page 16 of 31 Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89

(which is an electrolyte composition) and the Zn anode allows for the demonstration of the high Zn affinity
that the adsorbate exhibits in electrolytes. By using species density profiles from MD simulation, it is
possible to determine the distribution of electrolyte compositions perpendicular to Zn anodes, visualizing
the Zn/electrolyte interfacial structure. The local current density and the electric field distribution at the Zn
electrode surface can be simulated by multi-physical simulations, which contributes to explaining where Zn
is initially deposited and further extended during cycling.

Electrochemical properties
The evolution of working ions and electrons during the charge/discharge process can be demonstrated by
electrochemical testing [Figure 6D]. Methods for electrochemical measuring of the Zn/electrode interface
include linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry (CA), electric
double-layer capacitance (EDLC), and electrochemical impedance spectroscopy (EIS). Among them, LSV is
conducted to investigate the activity of HER that happens at the Zn/electrode interface. The potential
-2
[93]
corresponding to the current density at 10 mA cm is usually regarded as the onset potential of HER . The
lower the onset potential, the better the electrolyte used inhibits HER. Not only can the reversibility of Zn be
assessed by CV testing, but it can also roughly estimate the Zn nucleation overpotential. Testing with CA
can reveal the Zn diffusion modes. An abrupt decrease in the current density represents the 2D diffusion of
Zn . This process can easily lead to the formation of dendrites. When the CA curve becomes flat, the Zn
2+
2+
transport switches to the 3D diffusion mode, which promotes uniform Zn deposition . The results of
[67]
EDLC in Zn||Zn symmetrical cells can determine whether the additives/cosolvents have priority over water
to adsorb on the Zn surface . In general, when the EDLC value decreases, more additives/cosolvents are
[83]
absorbed on the Zn surface. Interfacial conductivity can be observed in the EIS spectrum. Moreover, in-situ
EIS testing is capable of assessing the stability of SEI and Zn anodes.

In short, the Zn/electrolyte interfacial structure can be characterized using a variety of ways depending on
the requirements and goals. An in-depth understanding of the Zn/electrolyte interfacial structure and
reaction requires a combination use of the aforementioned methodologies. The Zn/electrolyte interfacial
structure is actually very complicated and only can be deduced or indirectly detected by the existing theories
and methods so far. In order to visually examine the Zn/electrolyte interfacial structure and its response, it
may call for the employment of in-situ techniques and their combinations, or the development of more
exact and new characterization instruments.

Characterization challenges for analyzing the structures of bulk electrolytes and Zn/electrolyte
interfaces
Although electrolyte modification is an effective strategy to optimize battery performance, there are still
some long-standing challenges that hinder the design of high-performance aqueous electrolytes, starting
with inadequate characterization techniques [Figure 7]. The traditional FTIR, Raman, and NMR spectra
characterization can obtain bulk electrolyte information. However, Raman and FTIR are not sensitive to
weak interactions between electrolyte microstructures, leading to incomplete insights into the electrolyte
mechanisms. In addition, Raman and FTIR also struggle to decipher the local electrolyte properties, such as
the electrolyte compositions at the Zn surface . While NMR can sensitively detect structural variations at
[57]
the atomic scale, researches still focus on strong interactions between cation and solvent molecules. Anion-
solvent and solvent-solvent interactions are often overlooked.

DFT calculation provides insight into the interactions between molecules/ions in the electrolyte. But limited
by the power of computers, it is not capable of accurately describing electrolyte structures. Although MD
simulation can characterize electrolyte microstructures, the simulation time and the choice of the force field
have an impact on the computational accuracy; that is, the simulated electrolyte structure is significantly