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Rehman et al. Energy Mater 2024;4:400068 https://dx.doi.org/10.20517/energymater.2024.06 Page 41 of 64

Commonly employed electrolytes in SIBs include NaClO , NaPF , sodium bis(trifluoromethane)
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sulfonamide (NaTFSI) in conjunction with organic solvents such as ethylene carbonate (EC), propylene
carbonate (PC), dimethyl carbonate, and so on [66,240-242] . However, each electrolyte system inherently involves
some capacity fading phenomena to a different degree. NaClO is less moisture adsorbing than NaPF .
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However, when they are combined with carbonate-based solvents, both could not minimize ICE loss in a
wide variety of SIBs, although ICE losses are much diminished in ether-based solvent systems (diglyme,
triglyme, DME, tetrahydrofuran (THF), and so on) . Other than ether-based additives, the most
[243]
concerned and effective electrolyte additive in SIBs is fluoroethylene carbonate (FEC), which, in
combination with other additives, has a pivotal ICE-sustaining role . Electrolyte additives not only play a
[26]
vital role in stabilizing the SEI and electrochemical capacity retention, but also have diverse effects on the
electrolyte itself. Electrolyte additives are believed to participate in SEI formation and stabilization. They can
[65]
also inhibit side reactions that deteriorate the SEI .
The recent shift toward the utilization of ionic liquid [244-246] and solid [247,248] electrolytes has ample success
stories. A wide potential window, low flammability, and high temperature workability are some of the
features that have received much interest. However, both ionic liquid [244-246] and solid [247,248] electrolytes need
optimizations in terms of electrochemical and techno-economic limitations for their commercialization.
Although various models for SEI have been proposed, there is very little knowledge of SEI formation
mechanism and its in-situ analysis in alloy-based anodes [249-251] . The composition of the SEI film can give
some clues about the species’ involvement along with possible underlying reaction mechanisms. Recently,
[201]
Zhang et al. have designed 3D carbon-coated Bi nanospheres as SIB anodes . These anodes showed a
stable performance of up to 10,000 cycles at 2 A g with a rate capability performance of 94% under
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100 A g . They compared SEI compositions using linear and cyclic ether-based and ester electrolytes to
confirm the presence of an inorganic-rich composition of the SEI along with organic species. In both ether
and ester-based solvents, the formation of Na CO , NaF, and other organic species was detected.
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Particularly, the presence of NaF facilitated Na diffusion. Whereas in an ester-based solvent, the increased
+
F-content at depth rendered more organic species at the electrode surface, contributing to low ion
conduction with thick and unstable SEI formation. Meanwhile, in ether-based solvents, a homogeneous
distribution of NaF resulted in stable SEI that improved the performance of the Bi nanosphere-based anode.
However, such studies are not very common to elucidate the exact mechanism or involvement of different
species in the charge/discharge reaction.

Lately, Yang et al. have also given details about the origin of SEI components by performing in-depth XPS
studies . They also validated the presence of organic and inorganic species evolving from electrolyte
[75]
decomposition in different electrolyte systems with their effects on SEI stabilization. In the DEGDME
solvent, a stable fluorinated SEI evolved by decomposition of the NaPF electrolyte optimized the SIB
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performance. Also, the DEGDME and its complex [NaPF -DEGDME] presented a higher HOMO energy
+
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and helped form sustaining cathode electrolyte interphase, as shown in Figure 19A. In fact, electrolyte has
various interactive roles, among which solvation of Na is very critical as it can affect SEI growth and its
+
stability. The DEGDME solvent has recently been shown to be able to create a hybrid organic-inorganic SEI
that synergistically involves carbonaceous materials to impart stability to SEI without much ionic hindrance.
The DEGDME solvent also interacts with the C in heterostructured composites to maintain structural
variations upon cycling. The cyclic performance in DEGDME and EC/PC solvent [Figure 19B] highlighted
improved capacity and cyclability. The flair of DEGDME solvent for better compatibility with NaPF than
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the commonly used EC/PC solvent was proved by DFT calculations that showed a higher energy difference
for electron promotion from HOMO to LUMO than in commonly employed carbonate-based
electrolytes .
[241]