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Page 2 of 37 Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22

INTRODUCTION
Regulating atmospheric concentrations of greenhouse gas is a critical step toward curbing the potentially
catastrophic consequences of climate change, including unprecedented wildfires, extreme weather, and
acidification of the oceans. One of the key priorities in this effort is the transition from fossil fuels to
renewable energy sources. As an electrochemical energy storage technology, the lithium-ion battery (LIB)
has been predominantly deployed among grid-scale energy storage and electric vehicles (EVs) to support
such a carbon-neutral energy transition by storing intermittent renewable energy sources with reliable
durability, compelling energy density, and declining costs. However, rapidly growing demands in many
other energy sectors (e.g., energy grid storage systems and electric bicycles) require reliable, affordable, and
complementary electrochemical energy storage systems to circumvent the key resource crisis of lithium.
The sodium-ion battery (SIB) has been regarded as one of the promising routes to complement LIB
technology by its integration into those applications that do not demand requirements on the cell energy
density (e.g., grid energy storage system). This is ascribed to the abundant availability of sodium (Na) and
the discovery of electrode materials with cheap and abundant elements, such as carbon, copper, manganese,
[1-4]
and iron .
Despite the surging research interest and achievements in the development of SIBs over the past few years,
the insufficient lifetime of SIBs, especially under harsh operation conditions, greatly impedes their large-
scale deployment. Similar to a LIB, a typical SIB is composed of a cathode, anode, electrolyte (with sodium
salts dissolved in non-aqueous solvents), separator, and current collector (Al-foil for both cathode and
anode materials). To extend the cell lifetime and improve cell safety, significant efforts have been made by
fabricating artificial interphase, performing pre-sodiation, and regulating electrolyte components, especially
electrolyte additives, because additives enable efficient modifications on the interphases where side reactions
occur between electrodes and electrolytes. To realize the importance of electrolyte additive studies, more
detailed discussions on the interphase will be illustrated in the next section.

Currently, there are many comprehensive reviews in the field of SIBs covering various aspects, including a
[6-8]
general overview of SIBs [1,4,5] , the development and prospects of cathode materials , research progress of
non-aqueous liquid electrolytes and relevant interphases [9-11] , the progress and strategies for stabilizing
anode in SIBs [12-14] , etc. However, there have been few prospective reviews of electrolyte additives in non-
aqueous SIBs. Considering that the use of electrolyte additives is closely related to the performance of the
cathode, anode, and electrolyte, a timely review with an academic perspective in this area is urgently needed.
Such a review would summarize our current understanding of Na-ion-containing liquid electrolytes and
provide significant assistance for the further development of SIBs. Herein, a systematic and comprehensive
summary of the functions of electrolyte additives used in non-aqueous SIBs was introduced in this work.
We particularly highlighted the fundamental scientific understanding of the effects of different electrolyte
additives on different anode and cathode materials, respectively. Moreover, the outlook on the development
of Na-ion electrolyte additives regarding tolerance on extreme conditions (i.e., fast charging, wide
temperature range), development of electrolyte additive combinations, understanding the effect of additives
on cathode-anode interaction, understanding the characteristics of electrolyte additives, and designing of
novel electrolyte additives for improving cell safety was proposed.

INTERPHASE FORMATION MECHANISMS AND CHARACTERIZATIONS
In principle, electrode materials should be operated within the electrochemical stability window of a certain
electrolyte system. The operating voltage of an electrolyte is determined by the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). However, the Fermi energy of
many electrode materials surpasses the HOMO or LUMO of the electrolyte , which leads to the electrolyte
[15]