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Page 28 of 37 Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22

A combination of four additives (NaODFB, PS, VC, SN) were also utilized to improve the performance of
[69]
SIBs with the Na V (PO ) F cathode at a high temperature . The capacity retention of 99% after 60 cycles
4 2 3
2
3
was achieved at a temperature of 55 °C, and it was discovered that the combination of VC and SN additive
is useful in creating a stable CEI layer. Various additive combinations of FEC, VC, PES, DTD, and TTSPi
were also assessed using 0.35 M NaBOB in TEP electrolyte with Prussian white material . Cells containing
[138]
combinations of PES + DTD as well as PES + TTSPi demonstrated improvements in the initial CE and
capacity retention compared to cells without additives due to the formation of benign CEIs.
While the use of multiple electrolyte additives can prove beneficial to battery functionality, certain
combinations can have negative effects on cell electrochemical performance. For instance, Nimkar et al.
[139]
noted in their finding that a mixture of PC and FEC additives performed better than a combination of
PC, FEC, and DFEC with the Na MnO cathode. This was attributed to a thick CEI generated by the
2
0.44
double-substitution of FEC and DFEC.
Function on water scavenging
Even short exposures of layered oxide cathodes to moisture can alter their surface chemistry and lead to
detrimental effects on battery performance, primarily regarding battery capacity retention and CE. H O
2
molecules could occupy the position of Na-ions in cathodes, thus hindering Na cation diffusion , in
+
[140]
-
[141]
addition to its side reactions with PF anion and a generation of HF within the electrolytes. Chen et al.
6
combated this issue using N, N-diethyltrimethylsilylamine (DETMSA) additive within 1 M NaPF in
6
EC:DEC (1:1, by vol.) electrolyte in the cells containing the Na [Ni Mn Mg Ti ]O cathode. DETMSA acted
x
a
w
y
z
2
as a water scavenging additive and contributed to the creation of more robust CEIs [Figure 12D]. The
additive significantly improved the cyclability by increasing the capacity retention from 39% to 79% after
500 cycles. Functionally, silicon atoms detached from the DETMSA molecule stabilized the SEI. The
multifunctional additive, BSTFA, similar to DETMSA, was used within the ultralow-concentration
electrolyte of 0.3 M NaPF in EC:PC (1:1, by vol.) in the cells containing a Na V (PO ) cathode, as described
2
6
4 3
3
in the report of Jiang et al. . BSTFA was able to suppress the NaPF decomposition by reacting with trace
[89]
6
amounts of moisture and removing H O and HF from the cell. Additionally, CEI stability was improved via
2
the formation of a NaF-rich, organic species-dominated interphase.
Additives for improving safety
The thermal stability of SIBs often declines significantly when deviating from room temperature, and in
extreme cases, fires and complete battery failures can occur . The electrolytes used in state-of-the-art SIBs
[143]
are usually composed of a mixture of flammable carbonate-based solvents (e.g., EC, DMC, PC), which are
one of the primary safety hazards associated with SIBs. To enhance the inherent safety of SIBs, tremendous
efforts have been devoted to suppressing electrolyte flammability. Feng et al. tested the performance of
tri(2,2,2-trifluoroethyl) phosphite (TFEP), dimethyl methylphosphonate (DMMP), methyl nonafluorobuyl
ether (MFE), and TMP as non-flammable co-solvents with Prussian blue cathodes. They found MFE to be
the most promising, as it showed an insignificant effect on sodium insertion reactions at electrodes and
[145]
good compatibility with the cathode . Similar work of Zeng et al. showed that electrochemical
[144]
performance of Prussian blue cathodes can be improved in non-flammable fluorinated electrolyte 0.9M
NaPF /FEC-TFEC (3:7 by vol.), where TFEC is di-(2,2,2-trifluoroethyl carbonate). As reported by Jin
6
[31]
et al. , NaFSI/TEP/TTE (1/1.5/2 in mole) electrolyte is non-flammable and highly efficient in SIBs with
NaCu Ni Fe Mn O cathode materials. The SIBs with this electrolyte exhibited excellent cycling stability
2/9
1/9
2
1/3
1/3
and capacity retention of 94.8% after 500 cycles. This was, in part, due to the presence of a stable and
uniform CEI layer (confirmed by XPS and TEM) with a high inorganic composition and increased amounts
of S- and F-based compounds, which suppressed cathode surface reconstruction and electrolyte dissolution.