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Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 3 of 64

There is no doubt that carbon materials have become the foremost choices as SIB anode materials. The
+
incompatibility of the interlayer spacing of graphite for intercalation of large-sized Na has given room for
expanded graphitic carbon and hard carbon for opting as SIB anode materials. Interestingly, they are proven
to be viable as SIB anode materials. In fact, hard carbon from various sources, including biomass-derived
hard carbon, has been widely searched for an optimum capacity for delivering SIB anodes [3,31-37] . The high
+
porosity of carbonaceous materials helps sustain a high capacity by introducing more Na in pores [34,38,39] .
Although diverse efforts have been devoted to the incorporation of defects and functionalities in
carbonaceous materials for improved performances of SIB anodes as their effects on capacity and overall
performance enhancement are evident in many instances, their participation in capacity-fading pathways
+
hinders their benefits [40,41] . For example, the trapping and reaction of Na with defects in functionalities often
lead to capacity fading, particularly in initial cycles, contributing to trap effect and some irreversible
capacity loss over long cycling. Still, their role in capacity fading has not yet been unveiled [42-44] . Although
there are diverse approaches of doping in carbon to generate heterogenous surfaces and compositing these
carbonaceous materials to alloying anode materials that bear both intrinsic and extrinsic defects, their
participation in side reactions demands close monitoring to have an optimized performance. Also, in some
instances, these defects in-situ generated during (dis)charging cycles remain undetected due to non-
utilization of conclusive mechanistic approaches [3,17,45] . Although many modifications have been made for
carbonaceous anodes, none has fully satisfied the standard of a commercial SIB anode [34,46-49] . This has led to
a recent shift towards alloying and conversion/alloying anode materials that show more promise for high-
energy-density SIBs.

While conversion-type anodes, including transition metal compounds such as oxides , sulfides ,
[51]
[50]
selenides , tellurides , phosphides , and so on, have been widely focused, they have not achieved
[52]
[53]
[54]
performance targets. Amongst all, alloying-type materials that mainly include Sn, Sb, Si, Ge, and P are the
most explored SIB anode materials [24,48] . Although these anode materials have high theoretical sodiation
capacity, the alloying reaction (xNa + xe + M Na M) often results in high accompanying volume
+
-
x
expansions, leading to severe pulverization and low capacity. Fully alloyed compositions with theoretical
capacity, volume expansion, and average voltage are summarized in Table 1.
Although breakthrough has not been achieved yet, their extreme performance potentials have been
extensively focused on as they can deliver very high and stable capacities . Many developments have
[55]
recently been made to improve capacities of alloying SIBs, including electrode materials, morphologies,
electrolytes, and binder modulations [17,29,56-59] . Although hundreds of research papers have been added to the
literature on alloying SIB materials in the past few years, unfortunately, no recent review has solely covered
recent progress on alloying SIB anodes. Therefore, a comprehensive overview of research developments,
particularly during the last six years, which are uniquely devoted to alloying anodes, namely Sn, Sb, P, Ge,
and Si, is needed.

This review paper addresses alloy materials in five main sections: (1) Alloy-based Anodes for Sodium-Ion
Batteries; (2) Materials Design Strategies; (3) Challenges Associated with Sodium-Ion Batteries; (4)
Optimizations of Sodium-Ion Batteries; and (5) Summary and Future Prospects. In detail, the current
review focuses on recent trends in material design strategies employed to have efficient SIB alloying anodes
with details of unresolved challenges, such as initial huge capacity fading accompanied by multifold volume
variations. Lastly, various modifications adapted to cope with challenges faced by alloying SIB anodes are
briefly detailed, followed by concrete recommendations for future research. In this regard, using advanced
characterization tools to unveil the mechanism through spatial and temporal tracing of species evolved
during the de(alloying) process is highly encouraged to sustain alloying SIB materials.