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Page 44 of 64 Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06

additives, and counter electrodes have been highlighted in recent studies to diminish voltage hysteresis, the
detailed significant role contributing to hysteresis needs to be mechanistically probed for each material in a
specific set of cell assembly and operating conditions [41,263,264] .

Structural defects and surface engineering
One of the essential and perhaps overemphasized strategies of tuning the performance of the battery
materials is by performing structural modifications that often can achieve suitably sustaining performances.
Various methodologies have been reported, including nanosizing, designing functional surfaces by
introducing functionalities, coatings, elemental doping, multicomponent design, and compositing to
construct 1D, 2D, and 3D structures to introduce interconnected channels, pores, and other short pathways
to achieve high conductivity with efficient ion/electron transfer and manage volume expansions. Although
many of these optimizations for high-capacity SIB electrodes are difficult to achieve without compromising
on one or more performance aspects, they always show some negative traits. For example, nanosized alloy
+
particles have a high surface area that can effectively shuttle more Na along with high electrode/electrolyte
interfacial contacts. However, when the surface area is beyond a certain size, it can enhance mechanical
cracking of electrode materials similar to in alloying nanostructures, which bear high volume expansions
during charge/discharge. An exceptionally high surface area also offers highly reactive surfaces for
undesired reaction pathways. Alongside smaller nanoparticles, incomplete alloying can occur because
smaller equilibrium ion concentrations can lessen the overall capacity. Many nano morphological
optimizations, including porous architectures, nanorods, nanowires, nanosheets, hollow structures, and
yolk-shell morphologies, have effectively yielded high capacity and managed other shortcomings in alloying
SIBs [24,228,230,265] .

Structural resilience of nanostructured alloying anodes during the charge process is critically important as it
is directly linked to the performance. However, in some instances, it has been reported that structural
evolutions also have a cohesive impact on capacity retention, as crystalline to amorphous phase conversions
are often detected during in-situ characterization [81,173] . Similarly, dual active/inactive phases have also been
shown to take part alternatively during charge/discharge phenomena. Versatile and multi-modal stability is
often ensured by introducing some carbonaceous material such as CNTs, graphene, N-doped C, C NF, hard
+
C, and others in bulk or using some coating methodology. Besides improving conductivity and Na
transport kinetics, it offers functional surfaces for sustaining the capacity. Nanosizing effects have also been
imparted in C-derived alloy anodes. Volume buffering effects are primarily of interest when carbonaceous
hybrid alloying anodes with low ICE are targeted [201,228,266] .

Various intermetallic alloys have been tuned with carbonaceous materials for stable SIB anode performance.
For example, when ultrasmall 3D-SnSb/NPC (nanoporous carbon) was utilized as an SIB anode, it
furnished an optimal capacity of 266.6 mAh g over 15,000 cycles at 2 A g with rate performance of
-1
-1
-1
[267]
-1
566.1 mAh g at 10 A g . Thus, it is highly appreciated among various SnSb C-coated materials reported .
Other than the superior ultrasmall-coated nanoparticle assembly, multiple optimizations were applied,
including electrolyte and binder modifications, all of which improved the ICE and long-term performance
of the derived SIB anode.
A recent exciting electrode design has been presented by Kang et al. . They compared anode
[268]
performances of synthesized AlSb, AlSb@C, and oxide terminated Al/AlSb/Sb O multicomponent anode
x
y
with N-doped C. The superb oxide terminated array (Al/AlSb/Sb O ) enabled the anode to deliver a stable
x
y
258 mAh g capacity at 10 C, much higher than previously reported micro-sized Sb SIB anodes. The specific
-1
nanoparticle structural assembly helps buffering volumetric strains and stable SEI creation, ensuring