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Page 6 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

Oxygen vacancy creation in active nanostructures often can improve capacity and charge transfer kinetics.
+
It has been observed that in SnO , these vacancies could enhance Na storage capacity and increase electrical
2
conductivity. Oxygen vacancies can also cope well with volume changes in the process of (de)sodiation,
improve the cyclic stability of SnO anodes, and extend the lifespan of batteries. Ma et al. have utilized these
2
oxygen vacancies effectively to present oxygen vacancy-bearing SnO and combine it with porous CNFs
2-x
(PCNF) to construct a homogeneously confined nanoparticles (SnO /C) composite . The prepared
[77]
2-x
composite was directly used as an anode without a binder or other conductive additives for SIBs. It
displayed superb electrochemical properties including high reversible capacity, sustaining rate capability,
-1
and ultra-long cyclic stability even after thousands of cycles. The discharge capacity of 565 mAh g over
-1
2,000 cycles at 1A g was attained, whereas the bare SnO showed a meager capacity retention of 57 mAh g
-1
2
after 800 cycles. Heterostructure methodology along with C compositing is a promising approach to
enhancing the cyclic capacity of electrode materials in SIBs. One such approach has been adopted by
Fan et al. by preparing a SnO /N-doped graphene (SnO /NG) composite . Strong coupling between N-
[78]
2
2
doped graphene and Sn is developed because additional active sites are created due to doping. Optimal
4+
nitrogen doping level also enhanced reversible capacities and rate capabilities over un-doped anodes. SnO /
2
NG composite with a high level of pyridinic-N (1.97%) gave an outstanding reversible capacity of
409.6 mAh g at 50 mA g over 100 cycles. It also presented superior rate capabilities of 416.5, 366.9, and
-1
-1
318.7 mAh g at 200, 400, and 800 mA g , respectively. Such outstanding electrochemical performance of
-1
-1
the obtained composite is due to low electrochemical effects, polarization effects, and entrusting high Na
+
diffusion.
A unique composite structure of 1D ultrafine SnO nanorods and 3D graphene aerogel (SnO NRs/GA) has
2
2
been fabricated by a reduction-induced self-assembly method. Vitamin C was used to facilitate the
reduction of graphene oxide (GO). The combination of 3D aerogel and 1D SnO nanorods resulted in a
2
synergistic effect that improved electrochemical performance of the material . As an SIB anode, the
[79]
-1
material delivered an initial discharge capacity of 232 mAh g at 0.02 A g in 100 cycles. The composite
-1
(SnO NRs/GA) also demonstrated excellent cycling stability as an SIB anode, with a high reversible capacity
2
-1
-1
of 96 mAh g at a high current density of 1 A g for 500 cycles. Demir et al. have demonstrated that an
in-situ formulated SnO @ hard carbon obtained by hydrothermal carbonization methodology using bio-
2
waste of apricot shells can be utilized for high-performance anodic material in SIBs . It was observed that
[80]
the SnO @hard carbon anode derived at 1,000 °C could effectively uptake more Na ions and sustain a
+
2
capacity of 184 mAh g over 250 cycles than the corresponding anode obtained by mechanically mixing
-1
SnO and hard carbon.
2
An amorphous tin oxide (a-SnO ) with a nano-helical structure containing extended defects has been
x
prepared via a solution and surfactant-free oblique angle deposition method (as shown in
[81]
Figure 1A and B) . The challenging task is that such morphologies are not achievable by conventional
methods that require heating materials to remove any remaining solvent or other additives that cause subtle
structural damage to the amorphous phase. The low oxidation state of tin oxide in this amorphous structure
has been dually verified by the X-ray absorption near-edge structure (XANES) where the anisotropy of
a-SnO structure points towards lower local symmetry. These microstructural arrangements in the a-SnO
x
x
phase cannot be achieved in the crystalline phase. They render it to store more Na with synergistic
+
influence of vertically aligned nano-helices, providing high porosity and surface areas. This uniquely crafted
material has delivered an excellent performance as an SIB anode [Figure 1C and D], with a reversible
capacity as high as 915 mAh g after 50 cycles and a retention capacity of 48.1% at 2 A g . These qualities
-1
-1
are clearly much better than electrodes made up of crystalline nanoparticles.