Page 74 - Read Online

Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 5 of 64

[75]
commonly exploited SIB anodes . Effective surface tuning of this anode enabled it to deliver a good
capacity for 10 k cycles. When this anode was employed in full cell (taking NVP as the cathode), the cell
-1
delivered an energy density of 215 Wh Kg over a broad temperature range of -20 to 50 °C. The floret-like
3D morphology with improved Na diffusion kinetics and high sodium affinity has dually been validated by
+
density functional theory (DFT) calculations. Moreover, their findings also support the formation of a stable
hybrid SEI of diethylene glycol dimethyl ether (DEGDME) with inorganic fluoride-rich ionic conductive
-1
-1
polyether electrolyte to achieve a capacity of 347 mAh g at 2 A g retained for 10,000 cycles. Such
marvelous performance has left behind many other strategies and opened a new door for exploring new
electrolyte formulations. The flair of DEGDME for better compatibility with NaPF has been proven by
6
DFT calculations that show a higher energy difference for electron promotion from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) than in commonly
employed carbonate-based electrolytes.

Tin-based oxides
Tin oxide is a promising class of anodic materials for SIBs owing to its intriguing features such as
abundance, low toxicity, and high theoretical capacity. Tetragonal tin dioxide (SnO ) and orthorhombic tin
2
monoxide (SnO) are two main types of tin-based oxides. SnO has a layered structure with a Sn-O-Sn
sequence. Its large spacing between layers allows easy insertion and extraction of Na . The reaction
+
mechanism of conversion and alloying is given as follows:

Conversion reactions:

SnO + 2Na + 2e ↔ SnO + Na O
+
-
2
2
-
+
SnO + 2Na + 2e ↔ SnO + Na O
2
Alloying reaction:
Sn + 3.75Na + 3.75e ↔ Na Sn
-
+
3.75
Sn + xNa + xe ↔ Na Sn (0 ≤ x ≤ 3.75)
+
-
x
Due to their dual conversion and alloying reactions with Na , high theoretical specific capacities of 1,375
+
and 1,150 mAh g ¹ are achieved for SnO and SnO, respectively. However, the low reversibility of the
-
2
conversion reaction can lead to rapid capacity declination during initial cycles that additionally suffer from
large volume change and the low electrical conductivity of tin-based oxides hamper their practical
applications. One effective strategy is by forming a hybrid, often involving designing a structure with
nanosized tin oxides and combining it with nanostructures such as nanofibers (NFs), nanosheets, nan-
flowers, yolk shells, and so on . Zhang et al. have prepared a flexible fibrous composite of CNFs decorated
[24]
with N-doped carbon nanotubes (CNTs) and SnO nanoparticles as bare nanostructured SnO is prone to
2
2
fracture and detachment from the conductive matrix, which loses electronic contact during an
[76]
electrochemical reaction . Processed anodes can deliver a highly stable cycling performance and
impressive rate capabilities for SIBs. The optimized composite showed a discharge capacity of 460 mAh g
-1
after 200 cycles and 222.2 mAh g at a high current density of 3.2 A g . The continuous fibrous structure
-1
-1
gave additional stability to the anode, thus retaining its fibrous morphology during sodiation/desodiation.