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Page 40 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

exceeds the stability limit of SEI, it causes its degradation and makes a fresh surface prone to generation of
+
new SEI that eventually gets peeled off after a certain thickness, making a portion of Na unavailable. Many
studies have detailed the origin, morphology, and mechanism of SEI formation, along with its detection and
evidence during electrochemical charging/discharging. SEI formation and its stabilization have been
extensively presented in the literature and various modulating methods such as the use of electrolytes and
additives have been proposed [17,66,235-237] . A detailed account is presented in the optimization section of
electrolyte.

Inadequate mechanistic understanding
Multicomponent battery systems need detailed mechanistic understandings that vary with the type and
morphologies of materials used as anodes, cathodes, electrolyte systems, and many other factors involved in
the charging and discharging processes. Unfortunately, for alloying SIB anodes, there is little consensus on
the mechanism of charge storage as the compact and sealed battery environment presents challenges in
ex-situ characterization. There is a need for applications of operando characterization techniques for the
[235]
detailed origin of species in different states of (de)sodation . Although there is a slow but increasing trend
in utilizing operando advanced characterization tools, they pose many difficulties in terms of specific
assemblies to truly track the origin of species, capacity failure, the role of materials and intermediates
evolved in capacity degradation. Due to the complex nature of species that evolve with different chemical
states, such as the presence of variant oxidation states and crystalline/amorphous phases in different levels
of (de)sodiated states, a single in-situ characterization tool often proves less useful for a concrete
understanding of the mechanism. Similarly, the first sodiation cycle often shows different characteristics
than subsequent cycles due to diverse mechanisms/species involved in SEI formation and other side
reactions. For binary and ternary alloy composites, further complications can occur due to multiple phases
that arise during cycling [26,31] .

OPTIMIZATIONS OF SIBS
The development of SIBs started alongside LIBs. However, their low energy density, coupled with other
+
effects partly due to large Na size, has not ascended at a commercial scale to replace costly LIBs. Although
many commercial-scale SIBs have been presented by companies such as Novasis, Faradion, Natron Energy,
AGM, TIAMAT, Altris, and others, they have proven renaissance for improvisations in SIBs
performance . Recently, many companies (including China’s BYD and Swedish NORTHVOLT) have
[238]
claimed breakthroughs in SIBs, nominating them as “Rising stars” for commercial-scale applications .
[239]
Despite that, it seems realistic that the current decade will be dominated by high-performing LIBs because
most research studies on SIBs put forth hitherto suffer from major capacity degradation processes that need
to be addressed immediately. In this regard, many optimizations in electrolyte, binder, structural, and
surface engineering are necessary to cope with challenges alloying SIB anodes face.

Efficient electrolyte system
A major concern in SIB alloy anodes is their low ICE with a major contributor being the electrolyte, which
participates in formation of SEI, which, in turn, determines capacity retention, cyclic stability, and
performance. Particularly, the stability of an SEI and the overall capacity and efficiency highly depend on
the electrolyte system. Few electrolyte systems have proven their compatibility when coupled with alloying
anode materials in SIBs. A relatively thin and sustaining SEI formed from a well-matched electrolyte and
+
additive combination consumes electrolyte molecules and Na into the active passivation layer. The
electrolyte in SIBs should have the following characteristics: (i) a low difference between its LUMO and the
electrode's Fermi level; (ii) a good ionic conductivity along with lower viscosity; (iii) anode and cathode
material’s compatibility with the electrolyte; (vi) thermal stability; and (v) cost effectiveness .
[31]