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Page 30 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

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interconnected PCNFs framework, facilitated Na transfer kinetics and buffered volume changes to
maximize Na storage.

Silicon-based anodes for SIBs
Silicon (Si), a cheap, abundant, and environmentally friendly material, is an ideal choice for alloying SIBs
+
whereby a single mol of Na in the NaSi can result in a theoretical capacity of 960 mAh g . However, a very
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low practically obtained capacity (less than 40 mAh g ) has been shown by the crystalline Si. Hence, Si was
declared electrochemically inactive previously (with +0.6 eV binding energy of Na) . However, many
[175]
theoretical studies have verified the potential of Si for SIB anodes, although this needs to be experimentally
verified [176,177] . Compared to crystalline Si with limited Si uptake, the performance of amorphous Si as an SIB
anode is practically more optimistic. The conversion of crystalline Si to amorphous Si after initial cycles also
reincarnates the capacity of Si anodes. Various other prepositions for improved anode characters in Si have
[178]
[179]
been proposed, such as C coating , porous structuring , and other composite modifications .
[180]
Regarding composite modifications, Kempf et al. have studied the temperature-dependent performance of
[181]
ion carbon-rich Sn-SiOC composites for SIB anodes . The best performance was shown by the composite
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(SnO ) pyrolyzed at 900 C, which could deliver a steady capacity of 234 mAh g at 37.2 mA g . At a current
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density of 2,380 mAh g , it could sustain a capacity of 131 mAh g . Their findings have opened a door for
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temperature-dependent material synthesis as temperature-performance correlations have been widely
addressed to control the morphology and thereby the corresponding capacity, although the alloying
mechanism in Si is unclear and somewhat controversial. Very recently, an electrochemical derivatized
varyingly ordered Si has been proposed by Li et al. . The combination of short, medium, and long-range
[175]
ordering induced strong Na-Si interactions well matched for fast ion/-transfer phenomena. The anode in
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bare form delivered a capacity of 352.7 mAh g at 50 mA g (95.2% CE), while its C composite Si/C
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supplied a capacity of 449.5 mAh g [Figure 15]. Interestingly, they proved an adsorption-interaction
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mechanism instead of the commonly evidenced alloying mechanism.
Amorphous Si comprising nanocubes or boxes with hollow structures has been composited with rGO to
[182]
exploit the SIB anode potential of this novel architecture material . The composite imparted stable
performance over 2,000 cycles with rate capacities of 261.2 and 73.3 mAh g at 0.1 A and 3 A g ,
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respectively. An ordered mesoporous C with Si/SiO hybrid constituting 2D mesochannels was found to be
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highly effective in enhancing Na and e transfer kinetics, suppressing volume variation effects, and
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providing highly conductive Na diffusion pathways. It showed a daring storage capacity of 423 mAh g at
0.02 A g after 100 cycles with an extended longevity (capacity of 190 mAh g at 1 A g after 500 cycles) .
[183]
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The mechanistic-driven phase conversions have been presented in C-composited SiP SIB anodes using
2
[184]
in-situ/ex-situ XRD and TEM . The proposed anode showed an ICE of 76% with a 100-cycle capability of
410 mAh g together with a rate performance of 198 mAh g at 1 A g . In-situ XRD and ex-situ XRD with
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2D contour plots have outlined their real-time performance and conversion behavior. Sodiation resulted in
diminished peak intensities of SiP resulting from amorphization, while partial recovery of the SiP peak
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(along with features of P and S peaks) was observed in charging. During discharge, new peaks related to
Na P and NaSi appeared, with some peaks from some unreacted SiP . Furthermore, these observations were
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counter-verified by fabricating a nano-battery for in-situ TEM and SAED patterns. In the charging process,
the clear amorphous behavior with dual crystalline/amorphous SiP phase was well confronted in the initial
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cycle, with complete amorphization in the third cycle as observed in the in-situ TEM/SAED.
SiP → Na P + NaSi (Discharging)
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2
Na P + NaSi → (SiP ) + P (Charging)
2 crystalline/amorphous
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