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Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 47 of 64

Bhar et al. have recently reported a freestanding carbon fiber-based composite electrode composed of
[278]
Sn-rich Ni-Sn/CF with leaf-like morphology . The electrode was tested as an SIB anode without a metallic
-1
-1
current collector. It delivered a capacity of 220 mAh g after 300 cycles at 150 mAh g . The unique
framework was able to buffer volume expansion/contraction changes accompanying the charge/discharge
process. This is appealing as it can avoid the use of a current collector that can lower the cost. A CuS/Sn Sb
2
3
hybrid alloying-based anode that furnishes an initial discharge capacity of 659 mAh g in the initial cycle at
-1
0.1 A g while the composite sustains almost total (266 mAh g ) capacity in 100 cycles has been
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-1
proposed .
[279]
Recently, Sb porous hollow microspheres presented by Hou et al. have shown a good SIB alloying anode
-1
potential, delivering an ICE of 64.6% (a charging capacity of 634.6 mAh g at 100 mA g ) with a reasonable
-1
rate performance of about 313 mAh g-a over 100 cycles at 3,200 mA g -1[280] . Sn nanorods modified by
N-doped C proposed by Yang et al. have been presented as SIB anodes that could deliver stable extra-long
cycle performance with a wide temperature window (-20-50 C) . Performances of the anode in various
[75]
electrolytes and full-cell configurations have been presented along with optimized morphology. In
DEGDME-based electrolytes, an ICE of 78.4% has been achieved with a rate performance as high as
437 mAh g at 5.0 A g . Element doping is essential for attenuating the crystal structure, band gap, and
-1
-1
oxidation states and creating vacancies. Thus, an overall increase of conductive properties can enhance the
SIB alloy behavior for higher Na uptake.
+
Another common strategy is to create defects and oxygen vacancies with potential to enhance the intrinsic
electronic conductivity and sodiation potential of a material [61,229,281,282] . Amorphous-crystalline defect-bearing
hetero-conjunctions formed between the amorphous SeP and crystalline graphene conductive framework
have been proposed to facilitate the SIB performance of SeP@HCG (high conductive crystalline
graphene) . The material effectively sodiated into an amorphous Na SeP phase, leading to nanocrystalline
[283]
x
Na Se and Na P to attain a high capacity of 855 mAh g at 0.2 A g with an extended cyclability. After 500
-1
-1
3
2
cycles, it maintained a capacity of 732 mAh g .
-1
Recently, Liu et al. have reported a high-capacity 3D Bi-derived electrode that undergoes a dual alloying-
stripping mechanism, unlike conventional alloying anodes . The bulk Bi material was converted to a 3D
[284]
framework after activation and alloying, followed by subsequent plating, resulting in a sodiophilic Na@3D
Na Bi framework that dually exploited SIBs and sodium-metal batteries (SMBs)’s characteristic ion
3
exchange process. Above all, the ultimate capacity delivered by the anode was about 7.7 times higher than
that of the alloying Bi anode, retaining a capacity of 2,000 mAh g for 800 h at a current density of 1 A g .
-1
-1
The hybrid 3D-designed anode could shuttle Na for alloying in combination with Na plating in the inner
+
+
space of the Na Bi network, which suppressed dendrite formation, unlike in SNBs. Similarly, MoS /SnS
2
3
showed a superior capacity retention behavior to store 634 mA g after 100 cycles at 2 A g . It also sustained
-1
-1
-1
a capacity of 745 mAh g at 10 A g during rate capacity cycling .
[285]
-1
Suitable cathode materials
There are a variety of cathode materials in SIBs, including layered sodium transition metal oxide-based
materials, polyanionic compounds (sulfate, oxalate, phosphates, etc.), Prussian blue analogs, and
others [238,286,287] . Although many of these cathodes have demonstrated their commercial viability, such as
oxides, Prussian blue, Prussian white analogs, and Na V (PO ) F (NVPF), unfortunately, all of them are
3
4 2 3
2
coupled with hard carbon anode materials without any major research breakthrough using alloying-based
anodes [238,287] . Noteworthy, each category poses specific shortfalls that need to be overcome. For example,
oxide-based materials always have stability issues despite their high capacity. Polyanionic cathodes have a