Fundamentals and perspectives of electrolyte additives for non-aqueous Na-ion batteries

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.^[39], it was discussed that the FEC addition does not affect the capacity and 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.^[63], who stated that 0.5 wt.% FEC in electrolytes hardly improves the cyclability of HC symmetric cells. However, they found that it has a positive effect on HC/Na cells.

Table 3

Fluorine-containing additives

Chemical name of the additive	Fluoroethylene carbonate (FEC)	Difluoroethylene carbonate (DFEC)
Chemical structure of additive

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 Na₂CO₃ [binding energy (BE) ≈ 290 eV for CO₃], R-COONa (BE ≈ 286.6 eV for CO), and NaF (BE ≈ 687 eV) [Figure 4A-C]. In addition, the 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^[72]. Here, the reaction process on the surface of HC is provided below^[73]:

Figure 4. XPS spectra of HC electrodes after 135 cycles in half-cells with NaPF₆/EC:DMC electrolyte. The effect of 3% FEC: (A), (B) C1s and (C) F1s; the effect of 3% DMCF: (D), (E) C1s and (F) F1s. Reproduced from ref^[24], copyright 2020 IOP Publishing Limited. DMC: Dimethyl carbonate; DMCF: fluorinated dimethyl carbonate; EC: ethylene carbonate; FEC: fluoroethylene carbonate; HC: hard carbon; XPS: X-ray photoelectron spectroscopy.

Difluoroethylene carbonate (DFEC, Table 3) was considered as a possible additive for the HC anode, and it is widely used for improving the characteristics of LIBs^[74,75]. However, Komaba et al.^[39] concluded that this organic solvent does not improve the electrochemical characteristics of SIBs. A small quantity of DFEC (0.2%-2% vol.) in 1M NaClO₄ + propylene carbonate (PC) decreases both initial CE and capacity retention [Table 1].

Fluorinated dimethyl carbonate (DMCF) was also proposed as an alternative electrolyte additive for SIBs. Fondard et al.^[24] concluded that CE [Table 1] decreased with an increase in the amount of DMCF (70% in additive-free electrolyte and 48% and 27% in electrolytes with 1.5% and 3% of DMCF, respectively). The presence of DMCF in the electrolyte results in loss of capacity due to the low voltage pseudo plateau; however, cyclability is higher than in additive-free electrolytes. Chemical characterization of the surface of the HC electrode by XPS at different BE after cycling in NaPF₆ + EC:DMC (1:1) electrolyte with and without 3% vol. DMCF shows the appearance of similar compounds [Figure 4D-F] as it was mentioned for electrolytes with FEC additive [Figure 4A-C]. However, the increase of the CO₃ peak (BE ≈ 290 eV) suggests a larger quantity of Na₂CO₃ on the HC surface which could be the reason for the cell capacity decay. It is unfortunate that Fondard et al.^[24] did not clearly report the fluorination degree of the DMCF used in this work. According to Yu et al.^[76], such a factor could determine the cell performance through additive decomposition routes.

Sulfur-containing Additives

Table 4 listed a few sulfur-containing additives that have been reported for protecting HC anode materials. 1,3-Propane sultone (PS, Table 4) is known as an MIB additive, which reacts with radicals in electrolytes, forming sulfite or sulfate inorganic species^[69]. This additive can also be used to decrease the gas evolution in LiNi_0.5Mn_1.5O₄/graphite full cells at the state of full charge^[77]. Moreover, it was mentioned that PS can repair broken SEI layers at high temperatures^[78]. Yan et al.^[69] found that electrolytes with 3% PS create a SEI layer with a lower impedance and good capacity retention. In recent studies, authors^[59] stated that the reduction peak for PS is observed at 2.02 V vs. Na⁺/Na, which means that PS can prevent decomposition of other solvents in the electrolyte. Zhang et al.^[59] tested PS as an additive in the electrolyte [1M NaPF₆ + EC/DMC(1/1) + 2% wt. FEC], and the results showed that 2% wt. PS increased initial CE from 72.3% to 76.7% and capacity retention from 58.7% to 82.0% after 50 cycles in full-cells Na(Ni_0.4Mn_0.4Cu_0.1Ti_0.1)_0.999La_0.001O₂/HC full-cells. XPS characterization showed that the electrolytes with PS form ROSO₂Na, RSO₃Na, Na₂SO₃, and Na₂SO₄ on the surface of HC.

Table 4

Sulfur-containing additives

Chemical name of the additive	1,3-Propane sultone (PS)	1,3-Propylene sulfite (1,3-PS)	Prop-1-ene-1,3-Sultone (PES or PST)	1,3,2-Dioxathiolane-2,2-Dioxide (DTD)	Ethylene Sulfite (ES)
Chemical structure of additive

Using density functional theory (DFT) calculations, Liu et al.^[72] calculated free energies during decomposition of 1,3-Propylene sulfite (1,3-PS, Table 4) through the formation of a 1,3-PS-Na complex, ring opening, and the production of simpler species. Here, reduction reactions are provided:

After ring opening, there are two paths. The first goes through SO₂ gas evolution:

The second path leads to the formation of CH₃CH=CH₂ gas and inorganic Na₂SO₃:

Based on the calculations, the authors^[72,79] concluded that the two-electron reduction of 1,3-PS takes more effort to form Na₂SO₃ compared to the reduction of carbonate esters, such as EC, PC, and vinylene carbonate (VC). But it is easier for a one-electron reduction of 1,3-PS to produce an organic layer on the HC anode compared to EC, PC, and VC.

Che et al.^[70] showed that the additive of 1% wt. prop-1-ene-1,3-sultone (PES or PST, Table 4) in the electrolyte [1M NaPF₆ + EC/EMC(1/1) + 2% wt. FEC] increased capacity retention after 1,000 cycles from 76.6% to 84.4% in pouch cells (NaNi_1/3Fe_1/3Mn_1/3O₂/HC). However, in recent studies, Zhang et al.^[59] concluded that increasing the PES amount in the electrolyte [1M NaPF₆ + EC/DMC(1/1) + 2% wt. FEC] leads to the formation of a high-resistance SEI layer, so the amount of the additive should not exceed 0.2% wt. In any case, even the small amount of PES in the electrolyte worsens the electrochemical performance because the magnitude of SEI impedance (R_SEI) and charge transfer impedance (R_ct) after 50 cycles is three times greater than that after the first cycle. According to XPS data, PES decomposes both at the cathode and at the anode into several sulfur-containing species (ROSO₂Na, PSO₃Na, and predominantly Na₂SO₃), which consume too many active Na-ions in full-cells. Thus, the PES additive adversely affects the electrochemical performance of the full-cells and their initial CE and capacity retention.

Che et al.^[70] claimed that the addition of 1% wt. ethylene sulfate or 1,3,2-dioxathiolane-2,2-dioxide (DTD, Table 4) to the electrolyte [1M NaPF₆ + EC/EMC(1/1) + 2% wt. FEC + 1% wt. PES] increased capacity retention after 1,000 cycles from 84.4% to 92.2% in NaNi_1/3Fe_1/3Mn_1/3O₂/HC pouch cells. In another research group, Zhang et al.^[59] proved that 2% wt. DTD additive in the electrolyte [1M NaPF₆ + EC/DMC(1/1) + 2% wt. FEC] improves electrochemical performance of cells [Table 1]: the initial CE increased from 72.3% to 79.1%, and capacity retention after 50 cycles increased from 58.7% to 79.4% in Na(Ni_0.4Mn_0.4Cu_0.1Ti_0.1)_0.999La_0.001O₂/HC full cells. XPS surface characterization showed that the sample has a strong Na−O−(CO)O−C₂H−R and weak C−C bonds, which means that the DTD additive formed a thick SEI layer, so that is why the signal of the C−C bond of the HC is weak. In addition, DTD formed ROSO₂Na, RSO₂Na, ROSO₃Na, and SO₄^2-on the surface of electrodes. It was also observed that 2 wt.% FEC additive with 2 wt.% DTD synergistically produced more NaF in the SEI film than that with PS or PES.

Ethylene sulfite (ES, Table 4) is a well-known effective additive for LIBs; however, Komaba et al.^[39] stated that ES shows a detrimental effect in the case of SIBs. Liu et al.^[72], using DFT, calculated Gibbs-free energy for decomposition of ES:

Phosphorus-containing Additives

Some phosphorus (P)-containing additives [Table 5] are also listed due to their functionality on the HC anode. Ethoxy(pentafluoro)cyclotriphosphazene (EFPN, Table 5) was first used as a novel flame-retarding additive for LIBs by Xia et al.^[80]. In recent years, Feng et al.^[61] proposed a non-flammable electrolyte based on EFPN for SIBs. Due to the low dielectric constant of EFPN, the ionic conductivity of the electrolyte with high content of EFPN is low. According to the self-extinguishing time (SET) test, the flammability of the electrolyte decreases with an increase in the content of EFPN [Figure 5A]^[61]. Thus, the most appropriate content of EFPN in the electrolyte is 5% vol.

Figure 5. Influence of phosphorus-containing additives: (A) flammability and ionic conductivity of 1M NaPF₆ + EC/DEC (1/1) electrolyte with different amounts of EFPN additive; (B) the 1st_, 2nd, and 200th cycles of AB/Na half-cell with 1M NaPF₆ + EC/DEC (1/1) + 5% vol. EFPN electrolyte; Reproduced from ref^[61], Copyright 2015 RSC; (C) The cycling performances of Na₃V₂(PO₄)₂F₃/C cells with various electrolytes at 55 °C; (D) the percentage of capacity retention and recovery after the self-discharge measurements of Na₃V₂(PO₄)₂F₃/C cells at 100% state of charge for one week. Reproduced from ref^[68], Copyright 2019 IOP Publishing Limited. DEC: Diethyl carbonate; DMC: dimethyl carbonate; EC: ethylene carbonate; EFPN: ethoxy(pentafluoro)cyclotriphosphazene; NaODFB: sodium (oxalate) difluoro borate; SET: self-extinguishing time; TMSPi: tris(trimethylsilyl) phosphite; VC: vinylene carbonate.

Table 5

Phosphorus-containing additives

Chemical name of the additive	Ethoxy(pentafluoro)cyclotriphosphazene (EFPN)	Tris(trimethylsilyl) phosphite (TMSPi)
Chemical structure of additive

The electrochemical testing of acetylene black/Na half-cells shows a low initial CE [Figure 5B], and the authors claimed that this is due to the decomposition of the electrolyte, which participates in the SEI formation. However, the CE increased in a few cycles to ~100% and remained within 200 cycles [Table 2]. Moreover, the use of EFPN additives decreases the cell resistance and improves cycling performance: capacity retention increased from 78% in an additive-free electrolyte to 97% in an electrolyte with 5% vol. EFPN.

Tris(trimethylsilyl) phosphite (TMSPi, Table 5) has been used as an additive due to its ability to react with an excess of HF, H₂O, O₂, and PF₅ and suppress parasitic reactions on the surface of both the cathode and anode. Leveraging Na₃V₂(PO₄)₂F₃/HC full-cells testing. Cometto et al.^[68] concluded that 1% TMSPi, acting as an F^- scavenger, produces fluorotrimethyl silane (CH₃)₃SiF in the presence of 0.5% sodium (oxalate) difluoro 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 performance of HC. Further studies^[64] 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^-1 after 2,000 cycles with an initial CE of 83.0% and a capacity of 221 mAh g^-1 during the first cycle in the electrolyte of 1M NaPF₆ + 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^[39] showed that VC-added electrolytes have a decreased reversible capacity and initial CE [Table 1]; however, other studies showed that a lower amount of VC improved the characteristics of full-cells, which made VC more useful in practical applications^[64].

Table 6

Other unsaturated chemical compounds as additives

Chemical name of the additive	Vinylene carbonate (VC)	Succinic Anhydride (SA)	Succinonitrile (SN)
Chemical structure of additive

Bai et al.^[64] analyzed the surface of cycled HC electrodes by XPS. The results showed that the carbonate group peak (Na₂CO₃ and/or RCO₃Na) is much stronger in VC-containing electrolytes. Moreover, it was noticed that VC could be reduced to polymeric species, such as −(OCO₂CH=CH)_n− and −(CHOCO₂CH)_n−, during the process of SEI formation. Liu et al.^[72] carried out DFT calculations to understand various reduction routes of VC as below:

Succinic anhydride (SA, Table 6) is an organic compound with a ring and a formula of (CH₂CO)₂O. Kim et al.^[63] assembled HC/HC symmetric cells to estimate their electrochemical behavior in the electrolyte of 1M NaPF₆ + EC/ diethyl carbonate (DEC) (1/1) with 0.5 wt.% SA. It was stated that 0.5 wt.% is the optimum amount for SA additives. It also has been reported^[63] that the addition of SA can improve the cell 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^[63], the addition of SA leads to the formation of a higher amount of Na₂CO₃ and 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.^[69], using the SN additive (1 wt.%) alone within the electrolyte of 1M NaPF₆ + EC/PC (1/1) increases the resistance of Na₃V₂(PO₄)₂F₃/HC full-cells and causes a high irreversible capacity during the first cycle (40 mAh g^-1vs. 27 mAh g^-1 for the additive-free electrolyte). 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].

Ionic additives

In addition to molecular compounds, ionic compounds (salts, Table 7) were reported to participate in the SEI formation, with either cation or anion. Trisaminocyclopropenium perchlorate (TAC·ClO₄) is an organic salt that is used for SIB systems as an additive for overcharge protection. This additive for Na₃V₂(PO₄)₃/HC full cells was described for the first time by Ji et al.^[81] This group showed that the addition of 0.1M TAC·ClO₄ to 1M NaClO₄ + EC/DMC (1/1) does not affect the initial CE of HC (71.2% in the base electrolyte and 72.4% in TAC-added electrolyte). Moreover, the Na₃V₂(PO₄)₃/HC full-cell showed a strong anti-overcharging ability during 176 cycles at a 0.5C rate with 100% overcharge.

Table 7

Ionic additives

Chemical name of the additive	Trisaminocyclopropenium perchlorate (TAC·ClO4)	RbPF6	CsPF6
Chemical structure of additive

RbPF₆ and CsPF₆ [Table 7] are used as a source of Rb⁺ and Cs⁺ ions, which were proposed as electrolyte additives for HC anodes in SIBs. Che et al.^[65] compared electrochemical performance of Na/HC cells in an electrolyte of 0.8M NaPF₆ + EC/PC (1/1) + 2% FEC with and without 0.05M RbPF₆ or CsPF₆. It was noticed that the presence of the Rb⁺ and Cs⁺ ions improves the cell lifetime [Table 2] by showing a higher initial CE, decreasing the cell impedance, and maintaining a higher capacity retention. XPS characterization of the SEI layer showed that Rb⁺ and Cs⁺ ions increase the content of P−F or C−F components and decrease the number of organic species of C=O, C−F, and C−O−C(R₁) [Figure 6].

Figure 6. XPS spectra of HC electrodes after three cycles in different electrolytes (A) O 1s and (B) C 1S. Reproduced from ref^[65], Copyright 2017 Elsevier. HC: Hard carbon; XPS: X-ray photoelectron spectroscopy.

Ionic additives

Many ionic additives have been reported to improve the sodium metal reversibility by tuning the SEI composition and even forming an alloy layer to homogenize the Na deposition behavior. Tin chloride (SnCl₂, Table 8) is an inorganic compound that is used as an additive for SIBs. Zheng et al.^[82] reported that SnCl₂ can react with sodium metal to form a Na-Sn alloy. At the same time, Cl^- anion participates in the formation of NaCl in the SEI. XPS data showed that the SEI consisted of NaOH, Na₂CO₃, ROCO₂Na, NaCl, and Na-Sn alloy layers on the surface of metallic sodium. Thus, 50 mM SnCl₂ can maintain a stable overpotential for a Na/Na symmetric cell during more than 500 h of cycling. Sodium polysulfide (Na₂S₆, Table 8) was reported by Wang et al.^[83] as a pre-passivation additive for SIBs with sodium metal anodes. Due to the decomposition of Na₂S₆, the SEI layer consists of several inorganic compounds (Na₂O, Na₂S₂, and Na₂S), which results in a stable operation of SIBs. Antimony trifluoride (SbF₃, Table 8) is an inorganic compound, which was introduced as an additive by Fang et al.^[84] They tested Na₃V₂(PO₄)₃/Na cells in a high concentration electrolyte of 4M sodium bis-(fluorosulfonyl)imide (NaFSI)/DME with 1% SbF₃. They stated that a bilayer-structure SEI appears with a Na-Sb alloy and NaF-rich inorganic compounds during cell cycling, which was confirmed by energy dispersive X-ray spectroscopy (EDS) mappings [Figure 7A]. Moreover, according to XPS spectra, there are different products due to the decomposition of carbonate electrolytes (Na₂CO₃, ROCO₂Na, etc.). SbF₃ additives also improve the stability of metallic sodium without increasing the polarization of symmetric cells [Figure 7B]. With the addition of SbF₃, the impedance of the cell was decreased from 643.1 Ω to 9.3 Ω [Figure 7C and D]. Thus, electrolytes with SbF₃ additives exhibit an outstanding effect in stabilizing the surface of metallic sodium and improving the electrochemical characteristics of cells.

Figure 7. Electrochemical performance of cells with SbF₃ additive. (A) Cross-sectional SEM and EDS mappings of cycled Na metal in the electrolyte with 1% SbF₃ additive; (B) Testing in symmetric Na||Na cells. Nyquist plots for Na₃V₂(PO₄)₃/Na cells with different electrolytes after (C) three cycles and (D) 20 cycles. Reproduced from ref^[84], Copyright 2020 Elsevier. EDS: Energy dispersive X-ray spectroscopy; SEM: scanning electron microscopy.

Table 8

Ionic additives

Chemical name of the additive	Tin Chloride (SnCl2)	Sodium Polysulfide (Na2S6)	Antimony trifluoride (SbF3)	Sodium Hexafluoroarsenate (NaAsF6)
Chemical structure of additive

Sodium hexafluoroarsenate (NaAsF₆, Table 8) was found by Wang et al.^[85] as an effective SEI-film forming additive for SIBs with metallic Na anodes. The addition of 0.75 wt.% NaAsF₆ to 1M NaTFSI with FEC can stabilize the Na/Al cell over 400 cycles and Na/Na symmetric cells for more than 350 h. XPS data showed that NaAsF₆ additives helped to form a NaF-rich SEI with O-As-O polymer species. Thus, a trace amount of NaAsF₆ can stabilize the surface of sodium metal anodes.

Fullerene-containing additives

Fullerene (C60) cages have been reported to preferentially anchor on the sodium surface to induce uniform Na deposition^[86,87]. With the addition of special functional groups on the C60 cages, those additives are able to further improve the reversibility of sodium metal anodes. Nitrofullerene [C₆₀(NO₂)₆, Table 9] was first proposed as an additive for LIBs by Jiang et al.^[86] Recently, this additive was reported as a highly electrolyte compatible additive for SIBs. Li et al.^[87] showed that C₆₀(NO₂)₆ is compatible with different solvents (PC, FEC, DEC, DMC, EMC, DME, triglyme, and diglyme). Moreover, the additive improves the stability of sodium metal by maintaining a constant overpotential [Figure 8A] and reducing the interface resistance and charge transfer resistance [Figure 8B] of symmetrical cells. XPS spectra [Figure 8C] showed that the addition of C₆₀(NO₂)₆ forms three new species on the surface of the electrode: C₆₀, NaN_xO_y, and Na₃N, which can protect the electrolyte from further decomposition on the electrode surface. C₆₀(NO₂)₆ also can improve the Na₃V₂(PO₄)₃/Na cell lifetime [Figure 8D].

Figure 8. The effect of C₆₀(NO₂)₆ additives on sodium metal anodes. (A) cycling performance of and (B) Nyquist plots for the Na/Na symmetric cells; (C) XPS spectra of cycled Na; (D) cycling performance of Na₃V₂(PO₄)₃/Na cells. Reproduced from ref^[87], Copyright 2021 Elsevier. EC: Ethylene carbonate; PC: propylene carbonate; XPS: X-ray photoelectron spectroscopy.

Table 9

Fullerene-containing additives

Chemical name of the additive	Nitrofullerene [C60(NO2)6]	Trifluormethylfullerene [C60(CF3)6]
Chemical structure of additive

Trifluormethylfullerene [C₆₀(CF₃)₆, Table 9] was described as a new additive for SIBs by Li et al.^[88]. They concluded that C₆₀(CF₃)₆ can enhance Na⁺ migration in the electrolyte, form a NaF-rich SEI, and improve the cycling performance and rate capabilities. Overpotential maintained a constant value within over 1,200 h in Na/Na symmetric cells. According to XPS spectra, the SEI layer has a higher NaF and C₆₀ content in the electrolyte with C₆₀(CF₃)₆ additives because it is more favorable for C₆₀(CF₃)₆ to decompose compared to NaPF₆ or DME solvent. Thus, C60 and CF₃ groups can significantly improve the electrochemical performance of cells with sodium metal anodes.

Nitrogen-containing additives

EFPN [Table 10] is an organic compound that was proposed^[61] as an additive for non-flammable electrolytes for SIBs. This compound and its characteristics were described in this review previously as an additive for carbonaceous anode additives. Compatibility testing of the EFPN with metallic sodium showed no reactions. Oxidation current starts at 4.8V vs. Na/Na⁺ for 5% EFPN-added electrolyte in coin cells with Na/stainless steel electrode.

Table 10

Nitrogen-containing additives

Chemical name of the additive	Ethoxy(pentafluoro)cyclotriphosphazene (EFPN)	Acetamide (N, O-bis(trimethylsilyl) trifluoroacetamide (BSTFA)	Diamondoid (bis-N,N’-propyl-4,9-dicarboxamidediamantane) (DCAD)
Chemical structure of additive

Acetamide (N, O-bis(trimethylsilyl) trifluoroacetamide (BSTFA, Table 10) was found by Jiang et al.^[89] as a promising additive for SIBs with a sodium metal anode. The main purpose of this additive is to scavenge HF and H₂O. This additive was introduced into an ultra-low concentrated electrolyte of 0.3M NaPF₆ + EC/PC (1/1) to improve Na₃V₂(PO₄)₃/Na cells. 2 wt.% BSTFA decreases the impedance of Na/Na symmetric cells and keeps it constant even after 100 h of cycling. BSTFA helps to protect the sodium metal anode from dendrite growth by creating a uniform and dense SEI. Moreover, this additive improves the electrochemical performance by decreasing impedance and increasing capacity retention and CE of the Na₃V₂(PO₄)₃/Na cells.

Bis-N,N’-propyl-4,9-dicarboxamidediamantane (DCAD, Table 10) was used as an additive to avoid the dendrite formation in SIBs with a sodium metal anode. Kreissl et al.^[90] showed that 2.5 mg/mL of DCAD improves the performance of Na/Na symmetric cells and Na/O₂ cells by stabilizing the surface of metallic sodium.

Sulfur-containing additives

Sulfur-containing electrolytes were found to improve the sodium metal anode reversibility by forming sulfur-containing SEI species (e.g., Na₂S, Na₂SO₃). Diphenyl disulfide (DPDS, Table 11) is an organic compound with a structure of di-Ph-S, which was tested by Zhu et al.^[91] as an additive to form a stable SEI film and suppress the dendrite growth. They showed that an addition of 1% DPDS additive can maintain a stable overpotential of Na/Na symmetric cells for more than 200 h of cycling. SEM data showed that DPDS decomposes to Ph-S-Na and participates in the formation of a uniform and smooth SEI layer. Moreover, Prussian blue/Na cells show a stable cycling performance with a 1% DPDS additive.

Table 11

Sulfur-containing additives

Chemical name of the additive	Diphenyl disulfide (DPDS)	Tetramethylthiuram disulfide (TMTD)	Ethylene sulfate or 1,3,2-dioxathiolane-2,2-dioxide (DTD)
Chemical structure of additive

Tetramethylthiuram disulfide (TMTD, Table 11) was investigated by Zhu et al.^[92] as a film-forming additive with organic sulfide salts. It was stated that the overpotential of Na/Na symmetric cells remains stable for 600 h of cycling in an electrolyte with 2 wt.% TMTD. Moreover, the electrolyte with 2 wt.% TMTD improved the electrochemical performance of Prussian blue/Na full-cells with a high CE of 94.25% and capacity retention of 80% after 600 cycles at 4C-rate.

DTD [Table 11] was used by Zhu et al.^[93] as an additive to form an SEI with the function of preventing sodium dendrite growth. Na/Na symmetric cells keep a stable performance for > 1,350 h of cycling in an electrolyte with a 5% DTD additive. They stated that the synergistic effect of two additives, FEC and DTD, in trimethyl phosphate (TMP)-containing electrolytes can form a SEI layer consisting of Na₂S, Na₂SO₃, NaF, and Na₃PO₄, which helps to avoid further electrolyte and electrode degradation. In addition, TMP-FEC-DTD electrolytes can increase capacity retention and CE of Prussian blue/Na cells.

Other unsaturated chemical compounds as additives

A few other common unsaturated additives have also been reported to modify the SEI of sodium metal anodes, thus improving its lifetime. In the electrolytes with FEC [Table 12], the passivation of metallic sodium can be improved, which suppresses side reactions between Na and other electrolyte components. Komaba et al.^[39] assumed that almost all polar organic solvents are not thermodynamically stable at ~0 V vs. Na⁺/Na, but FEC as an additive can help to achieve the highly reversible Na plating. Rodriguez et al.^[94] tested 1M NaPF₆ + PC/FEC (98/2) and claimed that FEC improves the electrochemical performance and helps to form a NaF-rich SEI but leads to gas evolution and dendrite formation, which, in turn, leads to poor contact between electrodes and more safety concern. However, according to many other reports^[63,94-98], the presence of FEC in electrolytes showed improvements of electrochemical performance with a stabilized SEI, which helps to avoid permanent electrolyte degradation on the electrode surface.

Table 12

Other unsaturated chemical compounds as additives

Chemical name of the additive	Fluoroethylene carbonate (FEC)	Succinic anhydride (SA)	Biphenyl (BP)
Chemical structure of additive

SA [Table 12] has already been described previously in the section of additives for carbonaceous anodes. Fan et al.^[99] concluded that SA participates in the formation of CEI/SEI in half-cells and improves the electrochemical behavior in electrolytes with FEC. Kim et al.^[63] stated that SA forms a more resistive SEI but suppresses the dendritic growth on the sodium metal anode.

Biphenyl (BP, Table 12) is an organic aromatic compound consisting of two benzene rings connected by a covalent bond. BP is a well-known additive for the overcharge protection in LIBs, so Feng et al.^[100] tested 1M NaPF₆ + EC/DEC (1/1) electrolyte with and without BP. They claimed that a BP-added electrolyte starts to oxidize at 4.3V vs. Na/Na⁺, during which the BP molecules are electropolymerized on the electrode surface with H₂ production. This can be seen from the experiment on the cell overcharging process [Figure 9A and B]. It was also claimed that products of BP decomposition can increase the impedance of a cell [Figure 9C]. Moreover, the presence of 3% BP in electrolytes does not affect the electrochemical performance of the cell [Figure 9D].

Figure 9. Electrochemical testing for Na_0.44MnO₂/Na cells in electrolyte (A) without BP and (B) with 3% BP; (C) the impedance before and after cell overcharging with 3% BP; (D) cycling performance with different BP amount in the electrolyte at 50 mA g^-1 between 2 and 4 V. Reproduced from ref^[100], Copyright 2015 RSC publication. BP: Biphenyl.

Function on CEI formation

Single additive

FEC is one of the most commonly used functional electrolyte additives in extending the lifetime of SIBs, largely due to its ability to compositionally and structurally improve the CEI^[126]. 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 cathode structural transformation in the NaFeO₂ cathode [Figure 10A]^[20]. Such behavior has been observed with 5 wt.% FEC and Na₂FeP₂O₇ cathode material, in which a specific capacity of 81 mAh g^-1 after 500 cycles was achieved, compared to only 31 mAh g^-1 without FEC^[127]. In the report of Lee et al.^[128], several linear carbonate-containing electrolytes were compared with 5 wt.% FEC additives using the Na₄Fe₃(PO₄)₂(P₂O₇) cathode. One such combination included 0.5 M NaClO₄ in EC:PC:DEC (5:3:2, by vol.) with 5 wt.% FEC and 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_xCo_0.7Mn_0.3O₂ cathodes in the electrolyte of 1M NaClO₄ in PC [Figure 10B], which was confirmed with XPS^[129] In addition to fluorinating the CEI, FEC has also been shown to structurally change the interphase. TEM and XPS analysis by Wu et al.^[130] showed that the use of FEC with Na_2/3Ni_1/3Mn_2/3O₂ cathodes fluorinated the CEI and 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].

Figure 10. The effect of FEC on SIB cathodes. (A) Interfacial degradation mechanism of the NaFeO₂ cathode, Reproduced from ref^[20], copyright 2022 IOP Publishing; (B) Na 1s XPS profiles of P2-Na_xCo_0.7Mn_0.3O₂ cathodes after 20 cycles in electrolyte with and without FEC, Reproduced from ref^[129], Copyright 2019 Acta Physico-Chimica Sinica publication; (C) TEM images of Na_2/3Ni_1/3Mn_2/3O₂ cathodes cycled in FEC electrolytes and PC electrolytes characterizing the CEI layer, Reproduced from ref^[130], copyright 2021 Wiley. CEI: Cathode electrolyte interphase; FEC: fluoroethylene carbonate; PC: propylene carbonate; SEI: solid electrolyte interphase; SIB: sodium-ion battery; TEM: transmission electron microscopy; XPS: X-ray photoelectron spectroscopy.

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.^[131][Figure 11A and B], the addition of VC additives into 1 M NaCF₃SO₃^- diglyme (DGM) electrolyte with a Na₃V₂(PO₄)₃@C cathode allowed the VC to 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 Na₃V₂(PO₄)₃ cathode in 1.2 M NaTFSI-TMP/bis(trifluoromethanesulfonyl)imide (BTFE) electrolyte to help create a stable, fire-retardant SIB [Figure 11C]^[132]. XPS and SEM results indicated 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 Na₂MnSiO₄ cathode material generated a “meta-stable passivation film” on the cathode that suppressed Mn dissolution and lowered the cell impedance^[133]. 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.

Figure 11. The effect of VC on SIB cathodes. (A) Illustration of the function of VC additives on the surface of Na₃V₂(PO₄)₃@C cathode; (B) XPS spectra of CEI layer covered on Na₃V₂(PO₄)₃@C cathode cycled in different electrolytes (0% vol. vs. 5% vol. VC) after the first cycle, Reproduced from ref^[131], copyright 2021 Elsevier; (C) Ex situ HRTEM images of NVP and PB electrodes cycled in 1M NaPF₆-EC/DEC and 1.2M NaTFSI-TMP/BTFE/VC electrolyte, Reproduced from ref^[132], copyright 2021 Wiley; (D) Amount of manganese dissolution detected by immersing Na₂MnSiO₄ electrodes in the respective electrolytes (with 0% vol. vs. 5% vol. VC) at room temperature for 30 days, Reproduced from ref^[133], copyright 2017 Elsevier. BTFE: Bis(trifluoromethanesulfonyl)imide; CEI: cathode electrolyte interphase; DEC: diethyl carbonate; NaTFSI: sodium bis(trifluoromethanesulfonyl)imide; SIB: sodium-ion battery; TMP: trimethyl phosphate; VC: vinylene carbonate; XPS: X-ray photoelectron spectroscopy.

Dinitriles have been proposed as electrolyte additives and solvents due to their decent thermal and electrochemical stability^[134]. Song et al.^[135] assessed the effectiveness of adiponitrile (ADN) as an additive for SIBs using the Na_0.76Ni_0.3Fe_0.4Mn_0.3O₂ cathode at different temperatures and with various percentages of 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 inhibited side reactions and ensured a charge balance at the cathode surface. With a more effective CEI, the ADN-containing SIB showed increased discharge capacities of 10.5%, 8%, and 13% at operating temperatures of 45 °C, -10 °C, and -20 °C, respectively. Additionally, these cells delivered capacity retention of 78% after 220 cycles compared to a cell without ADN additive, which dropped to 75% capacity after 40 cycles.

Figure 12. The effect of additives beyond FEC and VC. (A) TEM images of Na_0.76Ni_0.3Fe_0.4Mn_0.3O₂ particles after working in batteries with electrolytes containing various concentrations of ADN (0%, 1%, 3%, 5%). Reproduced from ref^[135], copyright 2018 Elsevier; (B) The high-resolution C 1s, O 1s, F 1s, and S 2p XPS spectra of NMCT-La electrodes cycled in base electrolytes with and without 2% 1,3-PS additive after the first cycle. Reproduced from ref^[59], Copyright 2022 Elsevier; (C) The cyclic performance of Na/NLNMC cell cycled at a high temperature of 45 °C. Reproduced from ref^[99], copyright 2021 Wiley; (D) The molecule structure of DETMSA and the reactions with HF and with H₂O by breaking the Si-N bond. Reproduced from ref^[141], copyright 2022 Springer Science + Business Media; (E) Digital photographs of flammability tests for EC-DEC-FEC electrolytes and the concentrated HT12 and HT12-F electrolytes. Reproduced from ref^[142], copyright 2022 Elsevier. DEC: Diethyl carbonate; DETMSA: N, N-diethyltrimethylsilylamine; EC: ethylene carbonate; FEC: fluoroethylene carbonate; SA: succinic anhydride; TEM: transmission electron microscopy; VC: vinylene carbonate; XPS: X-ray photoelectron spectroscopy; 1,3-PS: 1,3-propylene sulfite.

Sulfur-containing additives have been pursued, similar to other singular additives such as FEC, due to their ability to form stable CEIs and prevent the decay of cells^[136]. PS was used as an additive along with FEC as the co-additive in an attempt to create stable interphases with multifunctional gel polymer electrolyte^[137]. A thinner and more stable CEI layer with PS decomposition products was observed on the surface of the graphite cathode, which prevented electrolyte decomposition. In another experiment that combined several electrolyte additives, PS led to the formation of a CEI enriched with sulfates^[69]. The effects of PS, PES, and DTD on a Na(Ni_0.4Mn_0.4Cu_0.1Ti_0.1)_0.999La_0.001O₂ cathode with 1 M NaPF6 in EC/DMC (1/1) electrolyte were compared in the research of Zhang et al.^[59]. Both cells containing the PS and DTD additive showed increased cell capacity retention of 82.0% and 79.4%, respectively, compared with the one of 58.7%. DTD and PS contributed to the formation of a CEI with sulfates and sulfites [Figure 12B], which passivated the cathode surface and prevented electrolyte oxidation. PES, however, consumed a large amount of Na-ions during its decomposition and caused cracking in the Na(Ni_0.4Mn_0.4Cu_0.1Ti_0.1)_0.999La_0.001O₂ cathode.

Synergistic additive combinations

By combining the effects of multiple additives, it is possible to create highly efficient and electrochemically performing SIBs. Che et al. employed three additives simultaneously, including FEC, DTD, and PST, with NaNi_1/3Fe_1/3Mn_1/3O₂ as the cathode. The cell containing the three additives achieved capacity retention of 92.2% after 1,000 cycles^[70]. The extended lifetime was due to the formation of a dense CEI that prevented the dissolution of TMs. In Fan et al.’s research, FEC and SA were utilized in conjunction to create a higher-performance SIB with the Na_0.6Li_0.15Ni_0.15Mn_0.55Cu_0.15O₂ cathode [Figure 12C]^[99]. After 150 cycles, it was shown that a cell with both SA and FEC had capacity retention of 89.9%, while a cell with FEC alone had a retention of 54.6%, which was attributed to a more intact and homogenous CEI layer.

A combination of four additives (NaODFB, PS, VC, SN) were also utilized to improve the performance of SIBs with the Na₃V₂(PO₄)₂F₃ cathode at a high temperature^[69]. The capacity retention of 99% after 60 cycles 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^[138]. Cells containing 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. noted in their finding^[139] that a mixture of PC and FEC additives performed better than a combination of PC, FEC, and DFEC with the Na_0.44MnO₂ cathode. This was attributed to a thick CEI generated by the 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. H₂O molecules could occupy the position of Na-ions in cathodes, thus hindering Na⁺ cation diffusion^[140], in addition to its side reactions with PF₆^- anion and a generation of HF within the electrolytes. Chen et al.^[141] combated this issue using N, N-diethyltrimethylsilylamine (DETMSA) additive within 1 M NaPF₆ in EC:DEC (1:1, by vol.) electrolyte in the cells containing the Na_a[Ni_wMn_xMg_yTi_z]O₂ cathode. DETMSA acted 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 NaPF₆ in EC:PC (1:1, by vol.) in the cells containing a Na₃V₂(PO₄)₃ cathode, as described in the report of Jiang et al.^[89]. BSTFA was able to suppress the NaPF₆ decomposition by reacting with trace amounts of moisture and removing H₂O and HF from the cell. Additionally, CEI stability was improved via the formation of a NaF-rich, organic species-dominated interphase.