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Page 52 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

Other than morphology, material selection, and electrolyte systems, many other recently introduced
+
strategies such as ion incorporation including (K ), sacrificial cathode additives, and presodiation of anodes
have a good impact on capacity enhancement. However, current SIBs have much less energy density than
prevailing LIBs. Most of the capacity destructive processes arise in the initial cycle, causing a low ICE,
particularly in the case with alloying SIB anodes. For alloying anodes to ensure high ICE and stable long-
term performance, remodeling of different constituents, especially the electrolyte system and its
compatibility with the nanostructured alloying material, needs more focus. In this regard, the formation
process of SEI needs to be closely observed to ensure thin, stable, non-destructive SEI that can further assist
in fast kinetics.

Although promising results of alloying SIB anodes have been reported, giving some glimpse into
commercial-scale achievement, they are usually limited to lab-scale breakthroughs. To sort the right
combination of alloying materials and other constituents, the fundamental aspect of capacity fading must
first be sorted. Notably, a complete mechanistic understanding of phases evolved during (de)sodiation
needs operando characterization tools that can trace the formation and reconversion of species during
continuous cycling. Unfortunately, most reports lack this vital information. The multistep alloying and
alloying-conversion mechanism must be fully tracked using different operando tools. Mainly, in-situ TEM
analysis is capable of giving atomistic details using HAADF-STEM (with corresponding SAED and FFT
patterns) and EDS mapping that can ensure intermediate species while the charging-discharging proceeds.
However, the current level of in-situ TEM has certain limitations, such as using the Na O electrolyte in a
2
nano-battery setup, which could not match or withstand the same as real-world batteries. Moreover, in-situ
TEM of the nano-battery could not offer evidence of SEI formation. Nonetheless, coupling these in-situ
findings with other in-situ and theoretical studies can decipher the mechanism involved in (de)sodiation,
capacity fading, and these anode materials to a greater extent. In this regard, the continually varying
interphase behavior can be tracked using in-situ XRD under (dis)charging conditions. Ex-situ XRD, in-situ
Raman, impedance, and the less employed Mossbauer, solid-state NMR spectroscopic evidence can
sometimes validate the detailed mechanism for the capacity failure. However, these methods are not
frequently adapted in most of the ongoing research. Operando synchrotron XPS studies are also capable of
tracking solid-liquid interphases and the exact mechanism of SEI constitution in different alloying-based
SIB cells that can help mitigate randomized search going on for efficient alloying anodes in SIB. Various
other non-destructive synchrotron operando imaging techniques that use X-ray photons instead of e beam
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for elemental, spatial, and local electronic structures probing with the most excellent sensitivity have been
reported. Remarkably, the power of tomographic techniques has allowed scanning and 3D real imaging with
many highly selective techniques such as scanning transmission X-ray microscopy, scanning transmission
X-ray microscopy, micro X-ray fluorescence, and transmission X-ray tomography. When coupled with X-
ray absorption, they can enable detailed structural evolutions, particularly for performing morpho-chemical
analysis of SEI in unique and temporal domains [320,321] . It is worth mentioning that the ultimate alloying
anodes must provide wide temperature and voltage operative windows with simple and scalable synthetic
procedures at a marginal cost with the least toxicity.

DECLARATIONS
Authors’ contributions
Writing-original draft: Rehman Au, Saleem S
Data curation: Ali S
Investigation: Rehman Au, Abbas SM, Choi M
Visualization: Choi M