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Miao et al. Energy Mater 2023;3:300014 https://dx.doi.org/10.20517/energymater.2022.89 Page 9 of 31

[55]
to screen through a large chemical database represents the idea of HTVS . The “computational funnel
approach” is the sequential scheme that HTVS commonly uses. It can be interpreted as follows: An
individual phase of the screening is assigned to each quantity of interest, and unsuitable electrolytes from
the candidate pool are eliminated at each step according to a certain criterion, leaving behind a substantially
reduced set of electrolytes as candidates for the following step. A sizable and effective database is required
prior to the execution of high-throughput screening. Despite the high accuracy of DFT and MD algorithms
in predicting material properties, these techniques are computationally expensive and fail to build material
databases with millions of molecules. The screening process can be sped up by using ML. By training a
reliable model using the current data, the low-cost computational prediction of specific properties of new
[56]
materials can be made. Combined with HTVS, it enables quick electrolyte design . To the best of our
knowledge, however, HTVS and ML have not been performed in developing aqueous electrolytes in Zn
batteries, which provides more room for further research in this field.

Experimental characterizations
In addition to theoretical calculations, spectroscopic techniques like nuclear magnetic resonance (NMR),
FTIR, and Raman spectroscopy can reveal the structural property of electrolytes [bottom of Figure 4]. NMR
is a significant and useful technique for examining the electrolyte structure, which can reveal details about
the geometry, chemical environment, and the number of nuclei. Especially for light elements (such as H, O,
and C), the strength of the interaction between atoms can be quantitatively determined. For instance, the
1
[40]
disruption of the H-bond network of water can be identified by H NMR spectra . When the H-bond
1
interaction in water weakens, it will lower the electron density of H atoms in water, leading to the H peak
shifting to a lower field with a larger chemical shift value. In addition, the evolution of the water H-bond
network can be illustrated by O NMR . The O chemical shift in water will experience a downshift (high
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17
[50]
field) if the H-bond network of water is broken. This is because breaking the water H-bond network leads to
an increase in the electron density on O atoms in water, resulting in a stronger shielding effect to external
electric fields.
The aggregation states of water can be illustrated via FTIR as well. In liquid water, water molecules associate
with each other by H bonds. In order to reduce the system energy, water molecules tend to form large
clusters through strong H bonds, which is represented in FTIR as a redshift of the O-H stretching peak in
water. Otherwise, the FTIR peak gradually blueshifts as the H-bond interaction wanes . By analyzing the
[57]
evolution of the O-H stretching peak in water, the H-bond network structure of water can be identified.
-1
[50]
According to Gaussian fitting , the broad O-H stretching band (3,200-3,600 cm ) for water is usually
divided into three states: (i) “Network water” (~3,205 cm ) is the water molecules with about four strong H
-1
-1
bonds; (ii) “Intermediate water” (~3,410 cm ) corresponds to the water molecules with distorted H bonds
but unable to form a fully connected network; and (iii) “Multimer water” (~3,560 cm ) refers to the water
-1
molecules that are poorly connected to their surroundings and exist as a free monomer, dimer, or trimer.
Raman spectroscopy is a complementary partner to FTIR in studying electrolyte structures. FTIR is
responsible for the detection of polar bonds, while Raman spectroscopy detects the vibration of non-polar
bonds. Raman spectroscopy can identify the aggregation states of ions by performing a deconvolution
analysis. In general, the Raman peak of anions gradually shifts to a higher frequency as ion aggregate states
from SSIP, CIP to AGG .
[58]

X-ray absorption fine structure (XAFS) spectroscopy including X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS), as a potent technique for characterizing
the coordination environment and the atomic site structures of the absorbing center with high sensitivity,
2+
[59]
can be used to reveal the solvation and electronic structure of Zn in electrolytes . In general, XANES can