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Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 39 of 64

Figure 18. Major challenges faced by alloying SIB anodes.

expansions in the most promising anodes such as Sn, Sb, Bi, and P. Besides universal methodologies of C
compositing, various structural engineering methodologies to introduce a volume buffering medium have
been vastly reported [17,29] .

Voltage hysteresis
Voltage hysteresis is another challenge that both LIBs and SIBs alloying anodes suffer. Despite various
prepositions made for the origin of voltage hysteresis, the root cause is not very well understood yet.
Voltage hysteresis reflects capacity loss. It has both kinetic and thermodynamic origins. Unstable SEI, low
ionic motilities, structural transformations, and other barriers in ionic/electronic transportation have been
mostly nominated as causes of voltage hysteresis.

Voltage hysteresis represents a potential gap between discharge and charge cycles. It highlights an unsteady
performance of the selected electrode material in SIBs. Electrodes showing large voltage hysteresis have
impractical cyclic efficiency. Large voltage hysteresis is associated with other capacity deprivation problems.
Reaction overpotential, ohmic polarization, and component inhomogeneity between charged and
discharged stages are labeled as culprits for overpotential. They can be determined using galvanostatic and
potentiostatic intermittent titrimetric analysis. Among these three factors, composition inhomogeneity
+
+
+
arises from diffusion of Na in the solid state with different Na rich and Na deficient layers being formed at
the material's surface during sodiation and desodiation, respectively [17,41] .
Unstable solid electrolyte interphase
The most addressed and key issue in LIBs and SIBs is unstable SEI formation that is always formed at an
electrode-electrolyte interface. Anode materials having high fermi levels can induce reduction of
electrolytes, whose LUMO lies below that of anode material. The SEI layer is ion-conducting in nature. It is
an electron insulator. However, it does offer much resistance to Na diffusion in a thick form. While a
+
stable, suitably thin SEI is vital for suppressing electrolyte side reactions although, in most cases, it is highly
difficult to have a stable and non-degrading thin SEI. One of the major capacity-fading roles is displayed by
this SEI, which often constantly forms and degrades, causing a substantial loss of Na . Particularly for alloy
+
anodes, stabilizing SEI is quite difficult. When stress generated at the interface during sodiation/desodiation