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Page 38 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

interfacial contacts, and ionic and electronic conductivities. In many cases, a porous composite structure
has dual benefit of involvement of multiple active species at different stages of the sodiation/desodiation,
which helps in overall improved kinetics along with shuttering effects of volumetric stresses, which ensures
stable capacities over extended cycling [17,28] .

Heterostructure and composites design
Recently, compositing methodologies have been much in the limelight for their adaptable surface and bulk
properties, matching with desired SIB anode characteristics. In this regard, fabrication of metallic,
polymeric, and other hybrid composites often involving a carbonaceous material comes as the foremost
choice primarily due to enhanced conductivity and multiple short diffusion pathways with intermittent
pores in 3D materials that can afford volume expansions with other added advantages. Often, binary or
ternary phase composites have some surface functionalities and defects that garner their effectiveness.
Nonetheless, these hybrid phases contribute to side reactions that, in many instances, have been directly
[26]
linked to initial high-capacity losses .
Ahead of others, carbon-based materials always positively contribute to overall performance and long-cyclic
capacity retention. It is worth mentioning that capacity boosting, conductivity enhancement, and volume
shuttling effect of graphitic carbons in the form of graphene, CNTs, CF, and others often afford 3D
interconnected channels that cascade ion shuttling kinetics with improved stability, particularly buffering
volumetric expansion and particle aggregation drawbacks [33,227] . Delocalized electrons in the defect/vacancies
+
of graphitic frameworks are grave concerns and entailed as Na ion binding sites. Thus, defect/vacancy
creation using single and dual heteroatom doping at a certain level is vitally benign in improving SIB anode
performance [17,145,228-231] .

Tailoring intermetallic alloy composition
Many recent studies have determined the potential of definite intermetallic alloy compositing for SIBs
anode. In these advanced heterostructure cum compositing-alloying methodologies, binary, ternary, and
even quaternary phases can synergistically and coherently support sodiation/desodiation
processes [29,164,232-234] . These synthetic strategies have allowed notable improvisations in the material’s
performance as SIB anodes. They have been discussed in detail in optimization strategies.

CHALLENGES ASSOCIATED WITH SIBS
SIBs are on the way to improved performance, particularly aimed at grid-level applications. However, there
are still several challenges that need immense attention to dominate in the lithium era. Despite the
aforementioned material design strategies in Section “MATERIAL DESIGN STRATEGIES”, there are still
main bottlenecks in the vast applicability of alloying SIB anodes. The challenges facing the SIB system
include Volume Variations, Voltage Hysteresis, Inadequate Mechanistic Understanding, and Unstable Solid
Electrolyte Interphase, as presented in Figure 18. In the case of carbon-composited SIB alloying anodes,
scanning the role of defects/functionalities in high initial capacity fading needs to be fixed before presenting
SIBs commercially. In the case of intermetallic and conversion-alloying anodes, evolution of intermediate
phases and the volume variations need to be probed to deliver sustaining capacities as SIB anodes.

Volume variations
Large-sized Na insertion with subsequent alloying during the sodiation causes drastic volume variations
+
that can lead to irreparable structural damage to the alloying host. Accompanying issues include
pulverization of the uniquely designed active material to the point of losing electrical contact with current
collectors and rendering the SEI unstable, thereby imparting huge initial capacitive losses. Typical volume
expansions for alloying anodes vary. They are often reported to range from 200% to 400%, with the highest