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Page 24 of 37 Shipitsyn et al. Energy Mater 2023;3:300038 https://dx.doi.org/10.20517/energymater.2023.22

especially when side reactions are caused by trace amounts of water. To extend the lifetime of SIBs, a robust
CEI with minimum moisture is necessary to prevent parasitic reactions between cathode materials and
electrolytes. This work discusses cathode additive studies aimed at improving the lifetime of SIBs from the
perspective of CEI formation and water scavenging, respectively.

Function on CEI formation
Single additive
FEC is one of the most commonly used functional electrolyte additives in extending the lifetime of SIBs,
[126]
largely due to its ability to compositionally and structurally improve the CEI . Several case studies
demonstrate the positive role of FEC with various cathode chemistries. For instance, FEC contributes to the
increased formation of NaF as the inorganic passivation layer, which suppresses the TM dissolution and
[20]
cathode structural transformation in the NaFeO cathode [Figure 10A] . Such behavior has been observed
2
with 5 wt.% FEC and Na FeP O cathode material, in which a specific capacity of 81 mAh g after 500 cycles
-1
7
2
2
was achieved, compared to only 31 mAh g without FEC . In the report of Lee et al. , several linear
-1
[127]
[128]
carbonate-containing electrolytes were compared with 5 wt.% FEC additives using the Na Fe (PO ) (P O )
3
4
2
7
4 2
cathode. One such combination included 0.5 M NaClO in EC:PC:DEC (5:3:2, by vol.) with 5 wt.% FEC and
4
showed an improvement in both cyclability and specific capacity compared to a reference electrolyte. XPS
results confirmed the presence of a NaF-containing protective layer, which is believed to have contributed
to cell stability and the suppression of DEC decomposition. On a more mechanistic level, Cheng et al.
demonstrated that 3% vol. FEC promotes the formation of NaF-enriched species on P2-Na Co Mn O
x
0.7
0.3
2
[129]
cathodes in the electrolyte of 1M NaClO in PC [Figure 10B], which was confirmed with XPS In addition
4
to fluorinating the CEI, FEC has also been shown to structurally change the interphase. TEM and XPS
[130]
analysis by Wu et al. showed that the use of FEC with Na Ni Mn O cathodes fluorinated the CEI and
2
2/3
2/3
1/3
decreased its thickness from approximately 43 nm to 8 nm, thereby generating a more compact and stable
CEI to prevent an excessive electrolyte decomposition [Figure 10C].
Another commonly used electrolyte additive, VC, can enhance the performance of a SIB by modifying the
structure and composition of CEI. As reported by Shi et al. [Figure 11A and B], the addition of VC
[131]
additives into 1 M NaCF SO diglyme (DGM) electrolyte with a Na V (PO ) @C cathode allowed the VC to
-
2
3
3
3
4 3
oxidize with DGM to form a more consecutive and uniform CEI layer that prevented electrolyte
degradation. With FeS as an anode, the full-cell showed a promising capacity retention of 67% after 1,000
cycles. VC was used with a Na V (PO ) cathode in 1.2 M NaTFSI-TMP/bis(trifluoromethanesulfonyl)imide
2
3
4 3
(BTFE) electrolyte to help create a stable, fire-retardant SIB [Figure 11C] . XPS and SEM results indicated
[132]
that VC played a role in forming a more stable and homogenous organic-inorganic CEI rather than one
saturated with organic compounds. Adding the proper amount of an additive is critical to optimize the
electrochemical performance. It was reported by Law et al. [Figure 11D] that utilizing 5% vol. VC with a
Na MnSiO cathode material generated a “meta-stable passivation film” on the cathode that suppressed Mn
4
2
[133]
dissolution and lowered the cell impedance . VC, with the addition of lower than 5% vol., did not provide
a thick or stable enough passivation layer, while percentages higher than 5% vol. created a resistant and
thick interphase.
Dinitriles have been proposed as electrolyte additives and solvents due to their decent thermal and
electrochemical stability . Song et al. assessed the effectiveness of adiponitrile (ADN) as an additive for
[135]
[134]
SIBs using the Na Ni Fe Mn O cathode at different temperatures and with various percentages of
0.76
0.3
0.4
2
0.3
ADN. TEM and XPS analyses demonstrated the addition of 3 wt.% ADN helped form a compact and
uniform CEI layer [Figure 12A] containing NaF- and NaCN-rich complexes. These inorganic compounds