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Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 33 of 64

Figure 16. (A) Material characterization. (a) Illustration of Bi-NTs synthesis. (b) TEM image and (c) SEM image of Cu-NWs. (d) XRD
powder patterns of Cu-NWs and Bi-NTs. (e and f) TEM images, (g) SEM image, and (h) HRTEM image of Bi-NTs. (i) Elemental
mapping and EDS spectrum of Bi-NTs. (B) (a) Rate performance of Bi-NTs, Bi-NPs, and commercial bulk Bi. (b) Comparison of rate
performance of Bi-NTs with many previously reported Bi-based anodes for SIBs. Long-term cycling performance of Bi-NTs at (c)
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20 A g and (d) 50 A g . (C) In-situ TEM analysis of a single Bi-NT. (a) Schematic representation of the in-situ nano-battery used and
(b) morphological evolution of Bi-NT during cycling. Time-lapse TEM imaging of a single Bi-NT during the first (c) sodiation and (d)
desodiation steps. (D) In-situ XRD study of the Bi-NT electrode during (dis)charge. (a) In-situ XRD contour plot, (b) In-situ XRD
patterns and (c) Charge/discharge profiles of the Bi-NT electrode during initial and second (dis)charge steps. Reproduced with
permission from [204] . Copyright © 2022 American Chemical Society.
at different (de)sodiation stages. Initially, the sodiation induced a high volume-change in the tubular
structure, expanding the diameter from 85 to 124 nm. However, hollow structured NTs successfully
accommodated stress strains during (de)sodiation without any visible cracks together. The in-situ XRD
showed the formation of a NaBi (tetragonal) sodiated state that further moved to Na Bi (hexagonal) with a
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reverse conversion pattern during desodiation.

The compromised conductivity and volume enlargements during sodiation/desodiation of Bi anodes
necessitates modification at an atomic level to leverage the conductivity along with the addition of some
volume buffering media. A Bi-C-based composite has been fabricated in a multistep process that could serve
as an ultrafast SIB anode with superb high-temperature utility along with optimum performance at high
[189]
current densities . The multi-layered design helped mitigate volumetric stresses and particle aggregation
during sodiation/desodiation. It also improved ionic mobility, electrolyte wetting, and conductivity
[Figure 17A]. The material was tested both in half-cell and full-cell with excellent stability and capacity
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retention [Figure 17B]. The initial cycle capacity was 301 mAh g at high temperatures. It was diminished to
245 mAh g at high temperatures and a current density of 200 A g with a good reversibility (capacity
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retention of 78%). The full cell with NVP sustained a capacity of 93 mAh g at 1 A g over 700 cycles and a
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capacity of 66 mAh g at a current density of 2 A g , manifesting an ultrafast charging/discharging in just
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75 S. The stability of the anode was reflected in a post cycling morphological study whereby the modified