Solvent is one of the important components of organic liquid electrolytes. Electrolyte solvents need to satisfy the most basic conditions such as stability, non-toxicity and cheapness. Besides, the electrolyte solvent should have a wide electrochemical stability window, sufficient sodium salt solubility, high dielectric constant, low viscosity and wide liquid range (i.e., low melting point and high boiling point). The solvent should also maintain electrochemical stability or promote the formation of a high-quality passivation layer during cell operation. However, these different and sometimes conflicting requirements for the same solvent are difficult to be met by a single solvent, and therefore multiple solvents are often used in combination. The main solvents currently used in sodium ion batteries are ester solvents and ether solvents. These two types of organic solvents have provided excellent performance in battery applications (Fenton, 1973; Armand et al., 2009; Monti et al., 2014; Hong et al., 2015; Ponrouch et al., 2015; Niu et al., 2019; Wang et al., 2020).

Ester solvents are a more commonly used class of solvents, especially cyclic [propylene carbonate (PC) and ethylene carbonate (EC)] and chain [dimethyl carbonate (DMC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC)] carbonates are most commonly used, and electrolytes of carbonate solvents tend to have the advantages of high ionic conductivity and good oxidation resistance (Ponrouch et al., 2012). The relevant physical and chemical properties of carbonate solvents are shown in Table 1. The dielectric constant of ether solvents is much lower than that of cyclic carbonates, but higher than that of chain carbonates. The resistance to oxidation is relatively poor, and they tend to decompose at high voltages. However, ether solvents generate thinner SEI on anode with higher initial Coulomb efficiency, and ether solvents are more compatible with anode such as metallic sodium and can co-embed graphite with sodium ions and show good reversibility, making graphite that cannot be embedded with sodium in ester solvents can be used as anode in this solvent system (Dubois et al., 1997). In the actual application process, the use of two or even a variety of solvent mix is a more common method (Komaba et al., 2011b), the ratio of different solvents is controlled, and the advantages of multiple solvents are integrated to maximize the performance of electrolytes.

Sodium salts are another important component of organic liquid electrolytes and play a vital role in the performance of electrolytes (Che et al., 2017). For the selection of sodium salt. Firstly, the sodium salt should have sufficient solubility and dissociation ability in the solvent, and the dissociated cations should be free to move without obstacles to provide sufficient charge carriers. Secondly, the sodium salt should remain electrochemically stable within a certain electrochemical window without oxidation or reduction. The sodium salt and the solvent together determine the redox potential of the electrolyte, and the salt anion and the solvent are coupled by electrostatic interaction, which affects the oxidative stability of the electrolyte. In addition, the sodium salt should have good chemical stability as well as safety, remaining chemically inert to the diaphragm, solvent, electrode and collector fluid. If the sodium salt can effectively promote the formation of SEI film at the interface between electrode and electrolyte, it can better enhance the electrochemical performance of electrolyte, such as cycling stability (Strauss, 1993; Borodin and Jow, 2010; Ponrouch et al., 2015; Eshetu et al., 2019; Fadel et al., 2019). The commonly used sodium salts are sodium perchlorate (NaClO₄), sodium tetrafluoroborate (NaBF₄), sodium hexafluoroborate (NaPF₆), sodium trifluoromethanesulfonate (NaCF₃SO₃, abbreviated as NaOTf), sodium bis(fluorosulfonyl)imide [Na(FSO₂)₂N, abbreviated as NaFSI] and sodium bis(trifluoromethanesulfonyl)imide [Na(CF₃SO₂)₂N, abbreviated as NaTFSI] (Hu et al., 2020). The advantages and disadvantages of these sodium salts are shown in Table 2.

Based on the fact that each of the commonly used sodium salts has its own advantages and disadvantages, which are difficult to overcome. Hybrid systems combining two or more sodium salts have also been investigated, and the aim of avoiding disadvantages is expected to be realized, but the results are not significant. Therefore, it is necessary to develop new sodium salts (Hu et al., 2020). Some new salts that have been synthesized and reported are sodium difluorooxalate borate (NaODFB), sodium 4,5-dicyano-2-(trifluoromethyl)imidazolate (NaTDI), sodium 4,5-dicyano-2-(pentafluoroethyl)imidazolate (NaPDI), sodium bisoxalate borate (NaBOB), sodium bis [salicylato (2-)]-borate (NaBSB), sodium salicylic benzylic acid borate (NaBDSB), sodium tetraphenyl borate (NaBPh₄), etc.

Additives are the third main component of organic liquid electrolytes. Additives are components that are present in small amounts (less than 5%) in the electrolyte and are characterized by high specificity and small dosage. By adding a small amount of additives, it is possible to make up for the deficiencies of the original electrolyte and significantly optimize the specific performance of the battery without increasing the production cost or changing the production process (Xu, 2014; Wang et al., 2020). According to the function of additives, they are divided into film-forming additives, flame retardant additives, overcharge protection additives and other types of additives (Zhang, 2006).

The most studied are film-forming additives, which are usually easily consumed. During the initial activation cycle they participate and contribute to the formation of the interface between the electrode and the electrolyte, leaving a chemical signal only at the interface and not in the electrolyte itself. The ideal film-forming additives should have higher Fermi energy (E_g) located in the highest occupied molecular orbital-lowest unoccupied molecular orbital gap than solvents, electrolyte salts, etc., so that they preferentially undergo oxidation or reduction, which in turn improves the film-forming quality and efficiency of the SEI film and effectively enhances the electrochemical performance of the cell (Goodenough and Kim, 2010; Zhu et al., 2017; Niu et al., 2019). The mechanism of action of various additives is shown in Table 3. Other types of additives including acidity enhancers, impurity scavengers, viscosity reducers, free radical scavengers, etc., Also have potential applications in sodium ion batteries (Ponrouch et al., 2015).

NaClO₄-based organic liquid electrolyte is a widely used electrolyte for sodium ion batteries with good compatibility with common cathode materials (layered oxides, polyanionic compounds, and Prussian blue-like compounds). The battery system consisting of layered oxide and sodium perchlorate-based organic liquid electrolytes exhibited suitable reversible capacity, rate capability, and cycle life. The binary layered oxides P2-Na_xCo_0.7Mn_0.3O₂ (x ≈ 1) perform well in the electrolyte of NaClO₄ dissolved in PC with FEC, which is easier to coordinate with the ClO₄ ⁻ on the cathode surface than the PC solvent and contributes to the formation of the NaF protective layer on the cathode surface (Cheng et al., 2019). Ternary layered oxides Na_0.67Ni_0.15Fe_0.2Mn_0.65O₂ (N-NFM) CEI membranes formed in NaClO₄ based electrolytes in EC/DEC contain more organic compounds but less inorganic compounds, leading to increased impedance. In addition, CEI membranes are sensitive to perchlorate, which has strong oxidizing properties. A small fraction of the CEI film peels off from the cathode surface, accelerating the dissolution of transition metal (TM) ions and leading to reactivation of electrolyte decomposition (Wang S. et al., 2021). The mechanism of action and subsequent solubilization of TM ions by CEI films is shown in Figure 3A. Therefore, further optimization of electrolyte replenishment is required. Polyanionic compounds have a very solid framework matched to NaClO₄ electrolytes, allowing for higher cyclability and safety, and have been extensively studied by researchers. For example, Na₃V₂(PO₄)₃(NVP) showed high discharge capacity and coulomb efficiency with good cycling and rate capability, cycle life in 0.9 M NaClO₄ dissolved in triethyl phosphate (TEP) electrolyte solution (Liu et al., 2019), and 1 M NaClO₄ in PC added with 5 wt% [C₃mpyr] [NTf2] IL (Manohar et al., 2018a). TEP electrolyte has non-flammable and high safety features. The IL additive makes the passivation layer organic and IL-containing and with sulfur in the surface film, and the surface film is more stable (Figures 3B,C). Na₂VM(PO₄)₃ (M = Ga or Al) behaves differently in an electrolyte of 1 M NaClO₄ with EC/PC = 1:1 v/v, FEC of 5 vol%, which is attributed to the fact that Na₃VAl(PO₄)₃ possesses a larger diffusion bottleneck to transfer more electrons than Na₃VGa(PO₄)₃ and exhibits a higher redox reaction potential (Wang Q. et al., 2021). Therefore, the V⁵⁺/V⁴⁺ redox reaction can be induced by the substitution of smaller-sized low-valent (≤+3) cations, which improves the utilization efficiency of the V⁵⁺/V⁴⁺ redox reaction. The Na₄Fe₃(PO₄)₂(P₂O₇) cathode formed a protective surface film in the EC/PC/DEC (5/3/2, v/v/v) electrolyte with 0.5 M NaClO₄ added with FEC and prevented undesirable decomposition of the linear carbonate, leading to excellent cycling performance of the cathode (Figures 3D–F) (Lee et al., 2016). The discharge capacity, impedance, multiplicity performance, cycling stability, and capacity retention of Prussian blue-like cathode materials sodium hexacyanoferric (Fe-HCF) composites coated with polypyrrole (PPy) were greatly improved in the EC/PC electrolyte of NaClO₄ (Figures 3G–I) (Tang et al., 2016). The insertion of Na⁺ in a series of Prussian blue compounds in organic carbonate electrolytes of NaClO₄ was investigated. KFe [Fe(CN)₆] provides the highest reversible capacity at ∼3.6 V for both the high-spin and low-spin Fe³⁺/Fe²⁺ couples (Lu et al., 2012; Wang Q. et al., 2022).

There have also been a number of studies on the compatibility of sodium perchlorate-based organic liquid electrolytes with anode. The most commonly used anode materials are intercalation type materials (e.g., hard carbon). Hard carbon as the anode of sodium ion batteries showed good compatibility with NaClO₄ electrolytes with solvent of EC, PC, DMC and DEC exhibiting high specific capacity and good capacity retention. The electrochemical properties of hard carbon in electrolyte are further enhanced by the action of some additives (e.g., 1-ethyl-3-methylimidazolium bis(fluoromethanesulfonyl)imide (EMImFSI) (Benchakar et al., 2020), N,N-diethyl-N-methoxyethylammonium bis(trifluoromethanesulfonyl)imide (DEMETFSI) (Egashira et al., 2012), FEC (Liu X. et al., 2018; Pan et al., 2018; Rangom et al., 2019)). For example, The reversibility of sodium insertion became evident at a volume content of 70% of DEMETFSI (Egashira et al., 2012). FEC promotes the formation of the initial sodiuming process SEI and improves the cycle life. By using added FEC and high salt-to-solvent molar ratio TMP electrolytes, it is possible to achieve both low R_SEI and very small R_ct on HC electrodes, thereby simultaneously inhibiting TMP decomposition and building thin and dense SEI membranes (Figure 4A). The electrolyte with a 5 vol% FEC ratio of 1:3 NaClO₄/trimethyl phosphate (TMP) exhibited considerable reversible capacity (238 mAh g^-1 at 20 mA g^-1), long-term cycle life up to 1,500 cycles, 84% capacity retention at 200 mA g^-1, and high rate capability (Liu X. et al., 2018). Graphite-based intercalated anode perform well in electrolytes with solvent combinations of EC/DEC (Luo et al., 2015), EC/DMC (Wang S.-W. et al., 2017), and PC (Wang et al., 2013). In addition to carbon materials, other inserted anodes have been investigated, such as TiO₂, Na₂Ti₃O₇. TiO₂ in 1M NaClO₄ in EC/PC electrolyte exhibits the best high-magnification performance of all titanium-based sodium ion anode materials reported so far (Wu et al., 2014). The SEI film generated in Na₂Ti₃O₇@C anode material with sodium perchlorate (NaClO₄)-based EC/DEC electrolyte contains more Na₂CO₃ and NaF, which is caused by the continuous decomposition of NaClO₄ salt in carbonate solvent. This makes the SEI film thicker and the interfacial impedance greater, leading to a decrease in cell cycling performance, this is in contrast to the NaOTf electrolyte (Figure 4B) (Wen et al., 2023). Metal oxides/sulfides/phosphides are typical conversion electrodes in SIBs, which typically suffer from poor conversion reaction reversibility and shuttle effects. Similar to other electrode materials, stable cycling of conversion electrodes relies on dense SEIs to provide stability and suppress losses of high mechanical strength actives. Hollow γ-Fe₂O₃ nanoparticles in PC electrolytes of NaClO₄ showed superior performance in terms of capacity retention, Coulomb efficiency, multiplicative performance, and cycle life (Figure 4C) (Koo et al., 2013).

In general, NaClO₄-based organic liquid electrolyte has been widely used as a relatively mature electrolyte for sodium ion batteries. NaClO₄ electrolyte has contributed to the improvement of energy density of sodium ion batteries with its own stability and oxidation resistance along with some high-voltage cathode materials. However, the compatibility with some positive and negative electrodes is not very good, and it will promote the dissolution of transition metal ions as well as the generation of thick SEI films, which may require a combination of additives and different solvent combinations to optimize the electrolyte in the future.

NaPF₆-based organic liquid electrolyte is also one of the commonly used electrolytes for sodium ion batteries. Studies on related cathode materials are often paired with NaPF₆-based organic liquid electrolytes for a series of electrochemical tests. First is the layered oxide cathode material. The layered oxide exhibits excellent retention and outstanding multiplicative performance, specific capacity, capacity retention and long-term cycling stability in NaPF₆-based electrolytes. Among them, The CEI film formed by Na_0.67Ni_0.15Fe_0.2Mn_0.65O₂ (N-NFM) in NaPF6-based electrolyte is dense and homogeneous, which effectively inhibits the dissolution of transition metal ions and provides a low-energy barrier for Na⁺ transport (Wang S. et al., 2021). The NaNi_1/3Fe_1/3Mn_1/3O₂ cathode material, paired with a hard carbon anode, maintains up to 92.2% capacity after 1,000 cycles at 1C between 2.0 V and 3.8 V using an optimized electrolyte of 1 M NaPF₆ dissolved in 1:1 (v/v) PC-EMC +2 wt% FEC, 1 wt% PST, and 1 wt% DTD. The PST and DTD additives promote the formation of a robust SEI on the anode and prevent the dissolution of transition metal ions by inducing the formation of a dense and dense electrolyte (Figure 5A) (Che et al., 2018). The second type of cathode material that is often adapted to NaPF₆ electrolytes is the polyanionic compound cathode material. Polyanionic compounds matching NaPF₆ ester and ether electrolytes have been reported. Na₃(VOPO₄)₂F (NVOPF)/rGO 3D sub-microspheres and Na₂Ti₂O₅ nanosheet anode electrode and NaPF₆ diglyme electrolyte, the full cell was further designed with high initial Coulombic efficiency (90%), excellent multiplicative performance (40°C) and ultra-stable cycling performance (>4,000 cycles without degradation) (Figures 5C,D). The electrolyte defines a robust fluorine-rich inorganic-organic interface, which effectively improves the interface and promotes ultra-fast charge transfer (Figure 5B) (Ba et al., 2022). The SEI layer between Na₃V₂(PO₄)₃ and NaPF₆/1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide (BMITFSI) ionic liquid electrolyte consists of NaOH, Na₂SO₄, Na₂S₂O₇ and NaF (Figure 5E). This is the reason for its good electrochemical properties (Wu et al., 2018). Half-cell tests of Na₄Co₃(PO₄)₂P₂O₇ in an electrolyte solution of EC/DEC with 1 M NaPF₆ showed the formation of a double layer in the fully Na⁺ extracted state of charge, with semi-organic-rich material found in the subsurface region near the electrode and more organic material in the outermost surface region near the electrolyte. At the same time, an additional outermost inorganic cover layer consisting of sodium carbonate and sodium fluorophosphate was formed after complete Na⁺ insertion (Figure 5F). Therefore, the Na₄Co₃(PO₄)₂P₂O₇ cathode provided excellent cycling performance (Zarrabeitia et al., 2021). The third cathode material used to match the NaPF₆-based electrolyte is a Prussian blue-like compound. The Prussian blue cathode electrode exhibits enhanced capacity retention in a volume ratio of 7: 3 of di-(2,2,2 trifluoroethyl) carbonate (TFEC)/fluoroethylene carbonate (FEC) consisting of 0.9 mol L^-1 NaPF₆. The electrolyte has excellent flame retardancy and good compatibility with sodium electrodes. The polycarbonate formed on the cathode surface plays an important role in the studied electrolyte system by enhancing the ionic conductivity and reducing the impedance of the solid electrolyte interphase (SEI) layer (Zeng et al., 2020).

Researchers often choose NaPF₆-based organic liquid electrolytes for the study of anode materials (intercalation-type, conversion-type materials, and non-metallic materials). Hard carbon HC performs well in NaPF₆ electrolytes with ester and ether solvents. Among them, The TEGDME-based NaPF₆ electrolyte can exhibit excellent rate capability, capacity retention and cyclic Coulombic efficiency at the HC anode. This is because the stable layer-by-layer SEI in the TEGDME-based electrolyte combined with the solvent layer “pseudo-SEI” on the HC facilitates high-performance Na⁺ ion storage in the HC and extends the cycle life of the HC anode material (Figure 6A) (Ma et al., 2021). In the EC/DMC electrolyte, the use of 3% FEC significantly increased the total capacity and capacity retention, while the use of DMCF additive had a negative impact on capacity but provided better cycling performance than the additive-free electrolyte (Figure 6B) (Fondard et al., 2020). The good performance can be attributed to the SEI composition consisting mainly of sodium ethylene dicarbonate NaO₂CO-C₂H₄-OCO₂Na (NEDC) and NaF. FEC as an additive promotes the production of NaF, which enhances the NEDC-rich SEI and results in a significant increase in capacity retention during cycling. Graphite-based materials have good reversible capacity and good cycling performance in NaPF₆/diethylene glycol dimethyl ether (DEGDME) electrolytes (Han et al., 2015; Cabello et al., 2017; Nacimiento et al., 2019; Li Z. et al., 2021; Zheng et al., 2022). Transformation-based electrode materials (e.g., copper phosphorothioate (Cu₃PS₄) (Brehm et al., 2020), dandelion-shaped manganese sulfide (DS-MnS) (Duong Tung et al., 2018), tin phosphide (Sn₄P₃/C) electrodes (Luis Gomez-Camer et al., 2019), TiS₂ (Tao et al., 2018), etc.) are also often studied matching NaPF₆-based organic liquid electrolytes. The choice of solvent is often ether-based solvents (diethylene glycol dimethyl ether, DME), and to a lesser extent, esters. In these electrolytes the converted electrode materials exhibit high capacity, cycling performance, multiplicative performance, first turn Coulomb efficiency, etc. It indicates that the conversion-type electrodes have high compatibility with ether-based NaPF₆ electrolytes. Non-metallic elemental anode materials [Micron Pb particles (Darwiche et al., 2016), and Bi electrodes (Wang C. et al., 2017; Li Y. et al., 2021)] exhibit good electrochemical performance in NaPF₆-based diethylene glycol dimethyl ether electrolytes and good compatibility with NVP and NVPF cathode materials, and the full-cell test solution exhibited good performance. For the study of sodium metal electrodes in NaPF₆-based electrolytes, EC/DMC is often chosen as the solvent. When FEC is added, the Na metal electrode forms a multilayer SEI structure, including an external NaF-rich amorphous phase and an internal Na₃PO₄ phase. This layered structure stabilizes the SEI and prevents further reactions between the electrolyte and the Na metal. Without FEC, the carbonate-based electrolyte containing NaPF₆ reacts with the metal electrode to produce an unstable SEI, rich in Na₂CO₃ and Na₃PO₄, which continuously depletes the cell’s sodium reserves during cycling (Figures 6C,D) (Han et al., 2021). The Na metal deposition/dissolution efficiency increased with increasing NaDFP concentration when sodium difluorophosphate NaDFP additive was added. NaDFP suppressed the overpotential and interfacial resistance. A high multiplicative capacity and long cycle life of 76.3% capacity retention after 500 cycles were achieved with 1 wt% NaDFP. The NaDFP-containing electrolyte formed a more stable SEI layer than the pure electrolyte, thus mitigating further degradation of the electrolyte (Yang et al., 2021). In specific glyme (chain ether) electrolytes, the sodium-metal interface produces a thin, homogeneous inorganic SEI composed primarily of Na₂O and NaF that may not support extensive or extreme cycling conditions, but the addition of FEC provides a more robust SEI to facilitate a large number of consistent sodium plating and stripping cycles (Seh et al., 2015; Sarkar et al., 2023).

In general, NaPF₆-based organic electrolytes, as a commercially available electrolyte, have good compatibility with common cathode and anode materials. NaPF₆ shows superior performance in ether solvents compared to other sodium salts. However, the study of NaPF₆-based electrolyte body solutions (e.g., solvation structure, sodium ion transport kinetics) is still at the beginning stage and will be further enhanced to reveal the reasons for their superior electrochemical performance in the future.

The cathode materials matched to NaFSI electrolytes mainly include layered oxides, polyanionic compounds, and organics. For the compatibility study of layered oxide cathode materials with NaFSI-based organic liquid electrolytes, the solvents used with NaFSI are esters, ethers, ionic liquids. Among them, NaNi_0.68Mn_0.22Co_0.10O₂ (NaNMC) exhibited good long-cycle performance and capacity retention in the phosphate electrolyte NaFSI-TEP (Figure 7A). The stable cycling can be attributed to the formation of a stable CEI layer on the NaNMC cathode, which suppresses the surface reconstruction of the cathode, the dissolution of transition metals at the cathode, and the persistent side reactions at the electrolyte/electrode interface (Jin et al., 2022). NaFe_0.4Ni_0.3Ti_0.3O₂ matches well in electrolytes using IL solvents (e.g., C₃C₁pyrrFSI). This ionic liquid electrolyte enhances the formation of passivation layer on the surface of Al current collectors, stabilizes the surface to 5 V, and prevents Al corrosion even at 55°C (Otaegui et al., 2015). For the compatibility study of polyanionic compounds with NaFSI electrolytes, NVP in NaFSI electrolytes with ester solvents and ionic liquids has high Coulombic efficiency, fast charging capability, stable cycling, high reversible capacity and capacity retention (Jian et al., 2013; Manohar et al., 2018b; Zheng et al., 2018; Li et al., 2022). Among them, the ionic liquid can promote the formation of a stable and thin SEI layer on the surface, which improves the discharge capacity and cycling performance of NVP@C cathode materials (Figures 7C,D) (Manohar et al., 2018b).

The anode materials matched to NaFSI electrolyte mainly include intercalation type materials, alloy type materials, conversion type materials and Na metal electrodes. The NaFSI electrolytes suitable for the study of intercalation-type materials include ester electrolytes, ether electrolytes, and ionic liquid electrolytes. Among them, in 3 mol dm^-3 NaFSI/PC + EC electrolyte, an organic-inorganic equilibrium SEI (mainly composed of (CH₂)_n and NaF) was formed on the surface of the hard carbon electrode. This SEI not only enables easy charge transfer and fast Na⁺ transport, but also exhibits strong passivation ability and excellent durability (Patra et al., 2019). CMK half-cells exhibit extraordinary cycling stability and high reversible capacity in a 3.8 M NaFSI IL electrolyte in C₃mpyrFSI (Figure 8A). This is due to the contribution of anionic decomposition species in ILs leading to inorganic SEI on mesoporous carbon CMK electrodes with high ionic conductivity, which promotes Na⁺ desolvation and diffusion kinetics. This rapid Na + migration facilitates improved reaction rates and cycling stability (Sun et al., 2021). Graphite materials do not perform well in the ether electrolyte of NaFSI, where side reactions occur between the electrolyte and the graphite electrode, and the formation of SEI films, which consist mainly of salt decomposition products and hydrocarbons, as shown in Figure 8B, leading to a low Coulomb efficiency of the studied cell system (Maibach et al., 2017; Goktas et al., 2019). For the study of Na metal electrodes in NaFSI-based organic electrolytes, electrolytes that have been reported are high concentration electrolytes with NaFSI and ester and ether electrolytes with additives. Among them, The Na|| Na₃V₂(PO₄)₃ cell is stable for nearly 1,400 cycles at 2 C in a highly concentrated electrolyte of DME with the addition of a small amount of SbF₃ at 4 mol L^-1 NaFSI and also exhibits excellent multiplicative performance of 80 mAh g^-1 at 40°C. This is because the SbF₃ additive forms a hard Na-Sb alloy layer, while the high concentration contributes to the formation of a dense NaF-rich SEI layer on the Na metal surface. This bilayer structure of the SEI layer effectively prevents dendrite growth and provides fast interfacial ion transport (Fang et al., 2020). The addition of NaFSI to methyl propylpyrrole dicyandiamide ([C₃mpyr]DCA) ionic liquid produces a more stable SEI layer (Figure 8C) (Forsyth et al., 2019). The addition of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) to 3.8 M NaFSI/DME electrolyte forms a localized high concentration electrolyte (LHCE), which helps to construct a stable SEI for SMBs. TTE also decomposes on Na metal anodes, synergistically forming dense SEI with low surface resistance and good mechanical properties, rich in NaF, which facilitates the transport of Na⁺ ions and inhibits the growth of Na dendrites (Figure 8D) (Wang Y. et al., 2021). Transformation-based anode materials (e.g., Cu_1.8S/C (Li H. et al., 2021), SnP nanocrystals (NCs) (Liu J. et al., 2018), tin phosphide (Sn₄P₃) (Mogensen et al., 2017; Mogensen et al., 2018)) in FSI-based organic electrolytes exhibited stable cycling ability and high capacity. Both NaFSI and FEC additives contribute to the formation of a stable NaF-rich SEI on the anode surface.

In general, NaFSI electrolytes dissolved in carbonate ester and ionic liquids exhibit better electrochemical performance and have better compatibility for hard carbon electrodes and sodium metal electrodes compared to other sodium salt electrolytes. In the future, combining different types of cathode and anode electrodes to assemble a complete battery for electrochemical testing, as well as contributing to the design and implementation of flame-retardant batteries, may be the trend for this electrolyte.

The cathode materials adapted to NaTFSI-based organic liquid electrolytes for electrochemical testing mainly include layered oxides and polyanionic compounds. Layered oxides (P2-type Na_2/3Ni_1/3Mn_2/3O₂ (Risthaus et al., 2018), Na_0.45Ni_0.22Co_0.11Mn_0.66O₂ (Chagas et al., 2014), and Na_0.44MnO₂ (Stigliano et al., 2022)) exhibited high discharge capacity and capacity retention in the ionic liquid electrolyte with NaTFSI. For the polyanionic compound, NaFePO₄/Na half-cells in sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-bonded butylmethylpyrrolidine (BMP)-TFSI ionic liquid (IL) electrolyte operate at 3 V. This IL electrolyte shows high thermal stability and non-flammability. NaFePO₄ has the best capacity at 50 °C in 0.5 M NaTFSI mixed IL electrolyte (Wongittharom et al., 2014).

The most commonly used anode material for NaTFSI organic electrolyte is hard carbon. NaTFSI organic electrolytes used for hard carbon electrodes include ester electrolytes and ionic liquid electrolytes. Among them, hard carbon electrodes in 2 M NaTFSI/EC:DMC electrolyte provided the best initial reversible capacity, high electrochemical stability, and good cycling stability. In addition, a sharp capacity decay was observed after cycling in an ultra-high concentration electrolyte (5 M NaTFSI/EC: DMC) (Figures 9A,B) (Chen et al., 2020). In NaTFSI/EC: DMC electrolyte, the addition of FEC or DMCF was found to be beneficial for overall capacity and capacity retention during cycling (Figure 9C) (Fondard et al., 2020).

In addition, some applications of NaTFSI-based electrolytes in full batteries have been reported. For example, the full cell consisting of Na₃V₂(PO₄)₂F₃ cathode and (Na_2+xTi₄O₉/C) anode exhibited high capacity retention in a nonflammable low eutectic solvent (DES) including sodium bis(trifluoromethane) sulfonate (NaTFSI) dissolved in N-methylacetamide (NMA). The improved electrochemical stability was associated with a stronger surface film formed at the electrode/electrolyte interface (De Sloovere et al., 2022). P2-Na_0.6Ni_0.22Fe_0.11Mn_0.66O₂ cathode and Nanostructured Sb-C composite anode cells in a 0.2M NaTFSIPyr₁₄TFSI ionic liquid-based electrolyte exhibited high specific capacity for the full cell. The electrolyte has a high ionic conductivity and high thermal stability. The anodic stability of this electrolyte was up to 4.7 V vs. Na⁺/Na (Hasa et al., 2016).

In general, NaTFSI ionic liquid electrolytes show good electrochemical performance. However, there are some problems, such as corrosion of the aluminum foil, capacity decay and some disadvantages of the ionic liquid electrolyte itself (e.g., high viscosity, poor wettability to the electrode, etc.). Efforts are still needed to improve these shortcomings in the future.

Sodium difluoro (oxalato)borate (NaODFB) is a new chelated sodium salt discovered by researchers in recent years. Only a few articles have been published to study this electrolyte. Chen et al. (Chen et al., 2015) found that Na/Na_0.44MnO₂ half-cells combined with NaDFOB-based electrolytes exhibited greatly enhanced multiplicative capacity and cycling performance. Sun et al. (Sun et al., 2020) developed a high-capacity nanoconstrained FeF₃ SIB-based cathode and found that the best cycling performance was achieved using NaDFOB salts in a ternary electrolyte (EC:DEC:DMC), with much higher cycling performance compared to the conventionally used NaClO₄, which was associated with the formation of a thin and conformal CEI protective film on the cathode. They further predicted that the DFOB anion reduction-mediated radical oligomer/polymer pathway may be an important part of the formation of CEI films. As shown in Figure 10A. Wang et al. (2023) investigated the compatibility of NaODFB electrolyte with NVP cathode.1 M NaODFB-DME electrolyte contributed to the formation of thinner CEI film on the surface of NVP material with low content of B₂O₃, which resulted in high specific capacity and capacity retention of the cell. Zhao et al. (2022) investigated the compatibility of NaODFB ether electrolyte with HC anode at high temperature. The Na/HC half-cell with 1 M NaODFB in DME has a high reversible capacity of 249.9 mAh/g at 100 mA/g and 55°C, exhibiting excellent cycling stability attributed to the SEI membrane groups B-F and B-O containing inorganic substances. Gao et al. (2022) found that there was a dense and smooth SEI film on the surface of sodium sheets after cycling in NaODFB-based carbonate electrolyte, and the SEI film could effectively inhibit the growth of sodium dendrites. They provide insight into the underlying mechanism of the protective effect provided by SEI derived from sodium difluoro (oxalate)borate (NaDFOB) (Figure 10B). The pre-reduction of the DFOB⁻ contributes to the formation of SEI and inhibits the decomposition of the carbonate solvent, and the DFOB⁻ is gradually transformed into a borate- and fluoride-rich SEI with cycling. The protective effect of SEI is optimized at 50 cycles, resulting in a threefold increase in the lifetime of the sodium metal batteries.

In summary, NaDFOB has high compatibility with various common solvents used for NIBs, which means that NaDFOB may be very effective for various electrode materials for other NIBs. However, the preliminary work mainly focused on its application as an additive. We should optimize the electrolyte of NaODFB as the main salt and apply it to the full battery. Further studies on the complex interactions of NaDFOB electrolytes with different solvents with various electrode materials are also necessary. The goal of exploring its full potential as a emerging and high-performance electrolyte for sodium ion batteries will be realized in the future.

In 1979, Peled (1979) found that alkali and alkaline earth metals in non-aqueous batteries form a surface film in contact with the electrolyte, which is an intermediate phase between the metal and the electrolyte and has the characteristics of an electrolyte, and hence the concept of “solid electrolyte interphase (SEI)” was introduced. At this time he considers the SEI model to be a simple two-dimensional passivation film structure. In 1997, Peled et al. (1997) suggested the mosaic model by arguing that insolubles generated from all types of reduction reactions of the electrolyte occurring simultaneously are deposited randomly mixed on the anode and stacked on each other to form a mosaic-like structure. In this model, grain boundaries and interfaces in SEI may act as electron conduction paths to promote the growth of dendrites and electron leakage. In 1999, Aurbach et al. (1999) proposed a multilayer structure of SEI films in lithium-ion battery systems using various means such as infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy, arguing that the passivation film formed at the beginning of the metallic Li surface is unstable and changes during the electrochemical process, with various types of substances forming one by one, and that traces of water in the electrolyte, solute Anion decomposition products also continue to influence the generated SEI film to form a multilayer film structure. This dynamic concept has also been applied to sodium ion batteries to derive bilayer and even multilayer structure models.

It is generally believed that the electrode-electrolyte interfacial film consists of the inorganic layer located on the inside connected to the electrode material and the organic layer located on the outside extending into the electrolyte. The inorganic layer is mostly inorganic with some sodium, and the organic layer is mostly organic with sodium formed by the reaction of solvent molecules with sodium. The formation of such a bilayer structure can be divided into two stages, namely, the formation of a bilayer on the surface when electrons flow into the anode and the participation of electrons in the reaction process. When the anode is filled with electrons, Na⁺ will be enriched on the electrode surface to form a bilayer. At the beginning the passivation film is thin and electron transfer is easy, so the double electron reaction occurs preferentially. The solvent molecules of the solubilized coordinated Na⁺ get electrons to be reduced and are more likely to produce inorganic products such as Na₂CO₃ and Na₂O, which are precipitated on the electrode surface, while at the same time, sodium salt anions or additives may also participate in the reaction to produce NaF, NaCl, NaS, and Na₂SO₄, etc (Hu et al., 2020). The positive effect of NaF on dense SEI formation and unstable interphase growth control. Content increases appropriately to suppress the solubility of organic sodium carbonate (NaO₂CO-C₂H₄-OCO₂Na) and promote the conductivity of Na⁺ through the SEI layer, thus improving the electrochemical properties, whereas an increase in Na₂CO₃ content does not (Fondard et al., 2020). F-S or S=O species were also detected in the case of FSI⁻ or bis(trifluoromethane)sulfonylimine (TFSI⁻) anions (Ding et al., 2019). However, as the thickness of the membrane increases, electron transfer is blocked and single-electron reactions begin to dominate, with organic species such as ROCO₂Na (R is an organic group) organically accumulating on the inorganic layer to form an organic layer. The specific species of the organic and inorganic components depend on the reaction between the electrode surface and the electrolyte. Different electrode materials have different SEI components in different electrolyte systems. The thickness of the SEI film is usually between a few nanometers and tens of nanometers, which is mainly related to the electron tunneling distance, and if there is no surface damage or decomposition, after reaching the longest distance of electron tunneling, the solvent will not be able to continue to get electrons to be reduced and thus stop decomposing, and the thickness change of the SEI film will decrease and become an electron insulator and ion conductor, and stabilized (Hu et al., 2020). Recently, Cui et al. (Zhang et al., 2022) revealed the original structure and redefined the composition of SEI by using advanced cryo-electron microscopy to characterize the swelling state of SEI in various electrolytes, and showed that the swelling behavior depends on the electrolyte type and profoundly affects the ion transport in SEI. In the inorganic-rich SEI, the swelling rate is lower, resulting in a more stable electrochemical cycle of the cell.

The ideal SEI film should have the following characteristics: (i) good electronic insulator, preventing the electrolyte from being oxidized or reduced by charge transfer on the surface; (ii) good sodium ion conductivity, selectively allowing the passage of Na⁺ and preventing the solvent from entering the electrode material or directly contacting the electrode; (iii) good chemical and electrochemical stability, with no side reactions in the cell system; (iv) good thermal stability, stably adhering to the surface of the electrode material even at high temperatures; (v) homogeneous, dense and thin, possessing good mechanical properties and not easily flaking and dissolving (Hu et al., 2020).

The formation of the interfacial film is mainly the result of a combination of three factors: the energy polarization difference between the electrode and the electrolyte, the specific adsorption behavior and the ion solvation behavior, and the formation is accompanied by the interfacial growth and evolution. The specific formation mechanism is shown in Figure 11. The details of each part are developed below.

The interfacial film arises from the difference in energy states between the two main parts of the electrode and the electrolyte (Gauthier et al., 2015). If the electrochemical potential μ_A of the anode is higher than the lowest unoccupied molecular orbital LUMO of the electrolyte, electrons are spontaneously transferred from the anode to the electrolyte, which leads to the reduction of the electrolyte and the formation of the interfacial film. Similarly, when the electrochemical potential μ_C of the cathode is lower than the highest occupied molecular orbital level HOMO of the electrolyte, electrons are transferred from the electrolyte to the anode and the solvent molecules of the electrolyte lose electrons leading to oxidation of the solvent, while the anode gains electrons and the transition metal cations (such as Mn⁴⁺, Ni⁴⁺, Co⁴⁺, etc.) in the material are reduced (Wang et al., 2020). The interfacial film on the surface of the anode, called SEI, is distinguished from the anode, and the products of electrolyte oxidation decomposition remain on the surface of the cathode, defined as the CEI passivation layer. To ensure a higher energy density, one chooses to initiate the redox reaction of the electrode at a voltage that exceeds the stability of the electrolyte. It is well known that non-aqueous electrolytes typically used in Li-ion batteries are oxidized if the operating voltage is higher than about 4.5 V with respect to Li⁺/Li. Given that the equilibrium potential of Na⁺/Na is 0.3 V higher than that of Li⁺/Li, some commonly used carbonate electrolytes will become thermodynamically unstable if the voltage in SIBs is higher than about 4.2 V (vs. Na⁺/Na) (You and Manthiram, 2018).

Specific adsorption behavior and ion solvation behavior are also the main factors affecting the formation of interfacial layers. The occurrence of specific adsorption behavior precedes the ion solvation behavior, i.e., the strong interaction of some substances with the electrode surface promotes the formation of interfacial films. The interfacial film model includes the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP). In general, the specific absorption behavior of unsolvated molecules is mainly present in IHP, while ionic solvated structures are mainly present in OHP (Zhang and Zhao, 2009). The enrichment of specific substances initially adsorbed on the electrode surface determines the initial interfacial composition and structure, while the ion solvation structure subsequently acts to promote the growth of the interfacial layer (Yan et al., 2020). For ionic solventization behavior, the solventized structure is mainly related to the coordination of alkali metal cations (e.g., Na⁺) to electronegative atoms of the solvent molecule (e.g., carbonyl/ether oxygen) or anions (e.g., fluorine in NaPF₆ salts). Their binding energy depends strongly on the type of cation, anion and solvent. Differences in the solvation structure and diffusion kinetics in different electrolytes subsequently lead to differences in the electrochemical properties of their organic and inorganic interfacial products. In addition, differences in ion solvation structures can lead to different degrees of changes in the electrolyte LUMO energy levels. The solventized Na⁺ structure at the molecular level in the electrolyte can change the preferential decomposition order of the solvent and the anion. As the salt concentration increases, the anions become more involved in the solventized shell layer, driving the transfer of LUMO from the solvent to the anion and forming an inorganic-rich interface (Xing et al., 2018). Thus, changes in the solventization environment will alter the previous order of solvent molecule or anion consumption, determining the initial composition formation of the internal interfacial layer, further affecting the organic/inorganic composition arrangement, structural evolution and overall ion transport capacity. The ionic solventization behavior is the main induction of surface interfacial phase formation and depends on the reduction/oxidation order of solvent molecules or anions (Li et al., 2020). The solvent-induced interfacial layer is dominated by the predominant organic matter in dilute electrolytes. However, the anion-induced interfacial layer in highly concentrated electrolytes consists of more inorganic species, such as NaF, NaCl and Na₂CO₃ (Zeng et al., 2018; Zheng et al., 2018; Yamada et al., 2019).

In addition to the regulation of specific adsorbed species during electrochemistry, the subsequent interfacial evolution is extremely important. The successive interfacial reactions are mainly driven by electron transfer based on a radical reaction mechanism. Usually, the outer organic layer is vulnerable to attack due to the preferential propagation of free radicals at the interface between the outer interfacial phase layer and the electrolyte, leading to organic polymerization (Soto et al., 2015). The evolutionary origin of the interfacial layer is therefore largely dependent on the electrochemical stability of the outer organic components. It determines which component is polymerized first and to what extent the dissolution and growth of the interfacial layer can occur. In particular, for some carbon anode materials with special microstructures (e.g., mesopores or nanopores), inward growth may occur inside the material (Bommier et al., 2016). The interfacial phase growth is related to the electrochemical reactivity of the components in the matrix electrolyte in addition to the interfacial phase components. In addition, the transport of sodium ions in the interfacial layer is a key factor affecting the evolution of the interfacial layer growth. The migration of Na⁺ ions in the interfacial layer is related to the desolvation process associated with the solventization behavior, the migration of Na⁺ ions through the interfacial reaction products, and the crystallinity and composition distribution of the interfacial layer. Among them, the desolvation behavior of Na⁺ at the electrode/electrolyte interface is a key step in determining the reaction rate. The ion desolvation energy potential depends on a combination of factors such as the strength of ion-solvent or ion-ion interactions, the choice of electrode, the presence of interfacial membrane, and the composition or structural condition of the interfacial membrane (Yamada and Yamada, 2015; Yan et al., 2020).

At present, the regulation strategy for interfacial film mainly includes four parts: electrolyte body regulation, concentration regulation, addition of functional additives and construction of artificial interfacial film (Figure 12). They are described as follows.