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Page 14 of 37 Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22

borate (NaODFB), which can improve the cell performance [Figure 5].

Other unsaturated chemical compounds as additives
Partially inspired by the success of vinylene carbonate (VC, Table 6), other unsaturated chemical
compounds as additives have also drawn substantial attention due to their unique functionalities double or
triple bonds, cyclic structures, etc., which could provide a site for polymerization under the reductive
condition to modify the SEI. VC is one of the most well-known electrolyte additives for modification of the
interphase of the electrodes of LIBs. Komaba et al. showed a negative effect of the 2% vol. VC on the
[64]
performance of HC. Further studies on VC show that a small amount of VC (no more than 0.5% vol.) in
the electrolyte is enough to produce a robust SEI layer on the electrodes and stabilize them. Bai et al.
[64]
compared the performance of SIBs with different VC-containing electrolytes (0.5, 1, 5, 10, and 100% by vol.)
and concluded that the initial CE decreases significantly with an increase of VC concentration [Table 1].
They concluded that HC can retain a reversible capacity of 211 mAh g after 2,000 cycles with an initial CE
-1
-1
of 83.0% and a capacity of 221 mAh g during the first cycle in the electrolyte of 1M NaPF +
6
Dimethoxyethane (DME) + 0.5% vol. VC. This shows that a lower amount of VC suppresses the continuous
decomposition of the electrolyte, which ensures a stable long cycling life of half cells with HC anodes.
Earlier full-cell studies showed that VC-added electrolytes have a decreased reversible capacity and initial
[39]
CE [Table 1]; however, other studies showed that a lower amount of VC improved the characteristics of
[64]
full-cells, which made VC more useful in practical applications .
[64]
Bai et al. analyzed the surface of cycled HC electrodes by XPS. The results showed that the carbonate
group peak (Na CO and/or RCO Na) is much stronger in VC-containing electrolytes. Moreover, it was
3
3
2
noticed that VC could be reduced to polymeric species, such as −(OCO CH=CH) − and −(CHOCO CH) −,
n
n
2
2
during the process of SEI formation. Liu et al. carried out DFT calculations to understand various
[72]
reduction routes of VC as below:
VC + 2Na + 2ē → Na CO + C H ↑ ΔG = -321.67 kcal/mol (8)
+
2
2
3
2
+
2VC + 2Na + 2ē →(CHCHCO Na) 2 ΔG = -360.84 kcal/mol (9)
3
FEC → VC + HF ΔG = 9.31 kcal/mol (10)
Succinic anhydride (SA, Table 6) is an organic compound with a ring and a formula of (CH CO) O. Kim
2
2
et al. assembled HC/HC symmetric cells to estimate their electrochemical behavior in the electrolyte of
[63]
1M NaPF + EC/ diethyl carbonate (DEC) (1/1) with 0.5 wt.% SA. It was stated that 0.5 wt.% is the optimum
6
amount for SA additives. It also has been reported that the addition of SA can improve the cell
[63]
performance at both 25 °C [Table 1] and 60 °C with a lower resistance, which is caused by the modified SEI.
According to the XPS data , the addition of SA leads to the formation of a higher amount of Na CO and
[63]
3
2
Na alkyl carbonates in the SEI of the HC anode.
Succinonitrile (SN, Table 6) is an organic solvent with a strong triple bond (−C≡N), which is known as a
SEI-forming additive for MIBs. As reported by Yan et al. , using the SN additive (1 wt.%) alone within the
[69]
electrolyte of 1M NaPF + EC/PC (1/1) increases the resistance of Na V (PO ) F /HC full-cells and causes a
3
6
2
4 2 3
high irreversible capacity during the first cycle (40 mAh g vs. 27 mAh g for the additive-free electrolyte).
-1
-1
However, this SN additive can contribute to the cell performance improvement at a high temperature by
decreasing the self-discharge and retaining the capacity retention when it is used as a part of blended
electrolyte additives (i.e., with NaODFB, PS, and VC additives) .
[69]