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Guo et al. Energy Mater. 2025, 5, 500041 https://dx.doi.org/10.20517/energymater.2024.214 Page 11 of 21

[76]
matrices . The fabricated battery cell was affixed to a plate equipped with electrical contacts, facilitating
electrochemical cycling and temperature regulation [Figure 7D-F]. To mitigate potential radiation-induced
damage to the polymer electrolyte, rigorous testing and precise control of both the X-ray flux and beam
dimensions were executed utilizing a specialized refocusing mirror system. While the primary motivation
behind utilizing polymer electrolytes was to streamline technical design, their findings reveal intriguing
charge transport characteristics associated with these electrolytes, suggesting that this cell configuration
holds promise for exploring solid-state electrolyte research avenues as well.

In-situ/operando neutron technique
Neutron depth profiling technique
In neutron depth profiling (NDP), a cold or thermal neutron beam interacts with the isotopes through a
sample, which emit a proton and a recoil nucleus. As a near-surface analysis technique, NDP is often used
to track the concentration profiles of light elements as a function of depth, since it is directly related to the
residue energy of recoil nuclei after they penetrate the sample [77-79] . Upon interaction with a cold neutron
beam (within the energy spectrum of 0.1-10 meV), specific nuclides within the material emit charged
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particles, including He, Li, B, N, O, S, Cl, and K. Leveraging its exceptional sensitivity towards Li
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detection, NDP is ideally suited for non-invasive quantification of the lithium concentration gradient
throughout the electrode’s depth [80-82] . Liu et al. leveraged the unique features of NDP-selectivity, sensitivity,
and non-destructive testing to monitor the lithium plating and stripping processes in LiNO -gel polymer
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electrolytes within a SSB [Figure 8A] . Firstly, they established the in-operando NDP setup. Subsequently,
[83]
the detection data were collected as the assembled pouch cell underwent ten plating and stripping cycles at a
current density of 1 mA cm [Figure 8B]. When compared with the control pouch cells assembled with
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either a single unit of LiNO or gel polymer electrolyte, the pouch cell combining both LiNO and gel
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polymer electrolyte showed a limited generation of inactive Li-species. Additionally, a thin and
homogeneous SEI was formed, and Li N was observed within the SEI, which promoted conductivity.
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Small angle neutron scattering technique
The small angle neutron scattering (SANS) technique is highly sensitive to light elements, enabling the
acquisition of real-space structural information on various microstructural elements within materials [84-86] . In
the field of SSBs, in-situ SANS technology has the capability to monitor interfacial reactions between SE and
electrode materials in real-time, thereby unveiling the microscopic mechanisms underlying ion transport
and charge transfer within SSBs [87-89] . Aqueous LIBs are popular for their safety, low cost, and environmental
friendliness. Inevitably, lithium batteries based on aqueous electrolytes face many tough challenges,
especially including low practical voltage and limited electrode materials. Extensive efforts have been made
to construct a SEI to improve the electrochemical stability window. To form a more stable SEI, Hou et al.
developed an aqueous polymeric electrolyte consisting of polyacrylamide (PAM) and LiTFSI, whose design
[90]
was inspired by the “water-in-salt” concept . They investigated the formation and evolution mechanism of
PAM-assisted SEI with the operando SANS technique [Figure 8C]. The morphological dimensions of SEI
were obtained by collecting the scattering data [Figure 8D] fitting with Porod’ law [Figure 8E]. According to
the results, as the voltage is scanned towards lower values, the evolution behaviors of two electrolytes with
PAM or without PAM diverge. Due to the addition of PAM into the electrolytes, SANS revealed that the SEI
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morphological changes were confined to a narrow voltage range of 2.0 to 1.5 V vs. Li|Li , above which the
SEI stabilized and parasitic reactions were minimized.

SIMULATION METHODS FOR INTERFACE BETWEEN SPES AND ELECTRODE
Advancements in simulation methodologies for SSBs span a wide range of technical approaches, offering
vital support for the design, optimization, and commercialization of these batteries. Molecular dynamics