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Page 6 of 37 Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22

challenges by regulating SEI formation due to their higher reduction potentials compared to electrolyte
solvents and salts. Thus, we will discuss the additives according to different anode materials in the following
sections.

Carbonaceous anode
Carbonaceous materials, including both “soft carbon” and “hard carbon” (HC), are the most dominant
candidates for commercial metal ion batteries (MIBs) due to their high specific capacity, relative ease of
production, and low cost. Unlike LIBs, attempts to use graphite as an anode material in SIBs were not
-1
[35]
successful, with a trivial reversible capacity of ~12 mAh g during the first cycles . Instead of graphite,
non-graphitizing carbon material with a disordered structure (so-called “HC”) is used in sodium systems,
which will be the focus of this work.

The electrochemical performance of HC is highly sensitive to its annealing temperature during synthesis,
which was proven by many studies [36,37] . However, only the influence of electrolyte composition on
electrochemical characteristics will be discussed in this review. The issue of a high irreversible capacity is
very typical for HC material and can be attributed to the SEI formation due to the low intercalation voltage
+
around ~0 V vs. Na /Na. A robust SEI is very crucial for extending the lifetime of a cell, so it is important to
realize what additives and how they affect the SEI formation. As an overview, Tables 1 and 2 summarize the
effect of selected electrolyte additives on the irreversible capacity and capacity retention of HC/Na half-cells
and HC-based full-cells, respectively. Detailed functional mechanisms classified by additive chemistries will
be discussed in the following sections.

Fluorine-containing additives
Certain fluorine-containing compounds have a lower LUMO compared to carbonate solvents, which makes
them suitable candidates to modify the SEI and improve the anode reversibility. Fluoroethylene carbonate
(FEC, Table 3) is the most effective additive for the formation of reliable SEI. There was a conclusion in
some papers that the addition of FEC (2%-3% vol.) as an additive decreases initial Coulombic efficiency
(ICE, Table 1) and increases the overpotential between charge and discharge [46,71] . However, in an earlier
report by Komaba et al. , it was discussed that the FEC addition does not affect the capacity and
[39]
Coulombic Efficiency (CE) at the first cycle and improves electrochemical performance in all cases.
Komaba et al. noticed that the addition of 10% vol. of FEC impairs cell performance, from which it was
concluded that an addition of 2% of FEC was the most acceptable amount. A similar conclusion was
made by Kim et al. , who stated that 0.5 wt.% FEC in electrolytes hardly improves the cyclability of HC
[63]
symmetric cells. However, they found that it has a positive effect on HC/Na cells.

To understand the working principle of FEC additives, XPS was employed to reveal the chemical
information of the surface of HC after cycling. Species are present with Na CO [binding energy (BE) ≈
2
3
290 eV for CO ], R-COONa (BE ≈ 286.6 eV for CO), and NaF (BE ≈ 687 eV) [Figure 4A-C]. In addition, the
3
peak, representing NaF species, increases with a higher FEC concentration, which validates the statement
that FEC enhances the formation of the inorganic compound NaF. The same contribution of FEC in the SEI
formation was noticed in the electrolyte sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) + EC:DMC
(1:1). In many studies, NaF is assumed as an efficient passivation agent, which forms through the
decomposition of FEC . Here, the reaction process on the surface of HC is provided below :
[72]
[73]
FEC + Na + 1ē → NaF + CO + C H O (1)
+
2
3
2