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Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22 Page 29 of 37

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According to Jia et al. , the flammability of an electrolyte is not necessarily the determining factor in
superior cell safety. The reactivity between charged electrodes and electrolytes at elevated temperatures
often outweighs the flammability of the bulk electrolyte in terms of effects on battery safety. Therefore, the
use of suitable electrolyte additives could affect cell safety by modifying the interphase, which has been
demonstrated with LIBs [147,148] . In terms of SIBs, Yu et al. reported that the use of FEC additives in the
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solvent of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE):TMP (1:2 by vol.) shows
improvements in both safety and capacity retention over a traditional EC:DEC (1:2 by vol.) electrolyte
[Figure 12E]. This is due to the generation of a dense and robust CEI layer. However, it is difficult to
distinguish the effect of the CEI on cell safety due to the non-flammability of F-EPE and TMP. Future
studies on the effect of CEI chemistries and structures, tuned by different electrolyte additives, on the
thermal stability of SIBs are highly encouraged.

CONCLUSION AND OUTLOOK
Conclusion
Substantial research efforts have been devoted to the wide commercialization of SIBs as complementary
energy storage systems of LIBs in several sectors, mainly including transportation (e.g., low-speed EVs, e-
scooters, e-bikes) and large-scale stationary energy storage. Achieving an extremely long lifetime is essential
to accelerate the widespread adoption of SIBs in these applications. Although limited success has been
achieved, much effort is aimed at upgrading the performance of SIBs by developing new materials chemistry
to replace those currently in use. The development of electrolyte additives is an equivalently important
research direction aimed at improving the performance of SIBs by extending cell lifetime and decreasing
safety concerns. This review has summarized various types of electrolyte additives [Figure 13] used in
current SIBs. The anode additives introduced here are classified based on their applications in specific
electrode materials, while the cathode additives are discussed based on their functions. To summarize this
review, Figure 14 clearly divides all additives into SEI-forming, CEI-forming, and additives that contribute
to both SEI- and CEI-layers with an optimal amount. Although significant efforts have been devoted to
improving the performance of SIBs using electrolyte additives, many challenges still need to be overcome in
this area, some of which are discussed below.

Outlook
· Enhance the tolerance to extreme conditions: batteries are required to maintain excellent performance at
extreme conditions (i.e., fast charging, high and low temperature) due to the operational environment or
conditions of applications. Proper modifications of SEI and CEI by using electrolyte additives are the key to
regulating interphase impedance, preventing side reactions between electrolytes and electrodes, and
improving cell performance under extreme conditions. In addition, the balance of cell lifetime and rate
capability at a wide temperature range must be carefully considered in the study of electrolyte additives.
Many of the earlier approaches focus only on improving performance at one temperature while
compromising the performance of cells at other temperatures. It is important to design electrolyte additives
for fast charging and operating within a wide temperature range, long-term cycling, and calendar lifetime.

· Developing and understanding a mixture of additives: researchers are likely to focus on understanding
the fundamental mechanisms and interactions between additives, electrolytes, and electrode materials. The
function of one electrolyte additive on one electrode is relatively simple to understand. However, its
beneficial effect may not ameliorate every property of the electrode and could even have a negative impact
on another property. With this in mind, the development of electrolyte additive combinations may be a
viable solution. For example, prop-1-ene-1,3-sultone, TMSPi, and ethylene sulfate can produce a synergistic
effect to inhibit gas evolution, control impedance growth, and extend cell lifetime in carbonate-based
electrolytes for LIBs . Therefore, investigating the interaction discipline between different electrolyte
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additives for the development of SIB electrolyte additives is of great significance.