Advances in Sodium Ion Batteries Based on Mixed Electrolytes of ILs and Organic Solvents

Abstract

Sodium-ion batteries (SIBs) represent a topic of extreme interest in the research field, especially because the materials used are cheaper than those in lithium-ion batteries (LIBs). In SIBs, the choice of cathodes and electrolytes is very important because they will affect the energy density, cycling stability, and safety of the battery. This work focuses on the prospect of hybrid electrolyte cells that incorporate ionic liquids (ILs) into organic liquids in order to improve the safety and performance of SIBs. Organic solutes make ionic conductivity higher due to larger IL electrochemical windows, good thermal stability and low volatility. They have some issues like flammability, dissolution, and transport limitations, but these aspects could be solved by using hybrid electrolyte systems. In this study, we investigate the effect of using different salts and solvents on the characteristics of the SIBs. We analyze ionic conductivity, electrochemical stability, and the development of stable solid electrolyte interphase (SEI) in the SIBs by using hybrid electrolytes. Additionally, we demonstrate that the addition of ILs to organic electrolytes can improve their thermal stability, so as a result, the safety and lifecycle of the battery will be increased. In conclusion, this research shows how hybrid electrolytes could have great potential for SIB battery technology in high-performance and large-scale energy storage applications.

1. Introduction

The growing global demand for energy requires the development of sustainable and highly efficient energy storage technologies [1]. Although LIBs currently hold a majority of the market, concerns about the cost and geographical concentration of lithium resources limit their viability for large-scale energy storage and future electric vehicles [2,3]. Consequently, SIBs have emerged as a promising, economical, and environmentally friendly alternative. Sodium is abundant and widely distributed, offering a significant advantage in terms of resource security [4,5,6].

Table 1. Summary of key hybrid electrolyte systems for sodium-ion batteries.

Electrolyte System	Ionic Conductivity (mS·cm−1)	Specific Capacity (mAh·g−1)	ESW (V vs. Na/Na+)	Key Innovation for SIBs	Ref
NaPF6 in EC:PC			2.5–4.2	Baseline for organic Na-ion electrolytes.	[7]
0.8 m NaTFSI in EC0.45:PC0.45:Pyr13TFSI0.1		182 @ C/10		First systematic study of hybrid IL/organic electrolytes.	[8]
1 M NaFSI in EC:PC:C3mpyrTFSI 50:50 v/v	3.2	115 @ 0.2 C		Identified optimal IL/organic ratio for high-rate SIBs.	[9]
EC:PC + EMI-TFSI + FEC		410	5.5	Safe high-performance hybrid (high flash point).	[10]
1 M Na[FSA] + NaClO4 in PC:[C3C1pyrr][FSA]				Long-cycle-life full-cell operation (>700 cycles) with IL/organic mixture.	[11]
EC:PC + Py14TFSI	15.1 to 11.6 (20% IL)		0 to 6.1	Mapped performance vs. IL fraction, showing safety gains (reduced flammability).	[12]
Mixed-cation IL (P111i4 + C3mpyrFSI)	4.4 @ 50 °C			Tuning properties (conductivity, stability) by mixing IL cations.	[13]

provides a summary of the key performance metrics for various mixed electrolyte systems for SIBs, highlighting the evolution from early organic systems to hybrid ionic-liquid systems.

The electrolyte is an important component that impacts the performance, safety, and lifecycle of SIBs [14,15]. The electrolyte’s function is to facilitate the transport of sodium ions between the anode and cathode during charge and discharge processes, which is important for the battery’s performance. Additionally, the electrolyte must be electrochemically stable within the operating voltage range because if the electrolyte is unstable, it can decompose or react with the electrode materials and generate gas, heat, or solid products that can cause capacity loss, impedance increase, self-discharge, or even fires or explosions [16].

Electrolytes can be divided into two parts, aqueous and non-aqueous, which can conduct an electric current by the transportation of sodium ions. Because of their small electrochemical stability window, aqueous electrolytes have lower voltages and energy densities. Regardless of pH, water has a small electrochemical stability window, which is 1.229 V. If the applied voltage exceeds this threshold, water is electrolyzed and split into hydrogen and oxygen. This can lead to some problems, like limited energy density, self-discharge, and gas evolution [17,18]. On the other hand, non-aqueous electrolytes have a larger voltage range, but they require more attention for the selection of solvents, salts, and additives to ensure a good electrode compatibility and stable SEI [19]. Organic electrolytes are a specific type of non-aqueous electrolytes that are commonly employed in SIBs because they can provide high ionic conductivity, a wide electrochemical stability window, low toxicity, and good compatibility with organic electrode materials [20,21,22]. Usually, one or more sodium salts dissolved in one or more electrolytes form electrolytes [23]. However, organic electrolytes have certain disadvantages; their flammability is the first factor that may cause thermal runaway and possible explosions in battery systems [24]. Another issue is the dissolution of electrode materials [25], especially small molecules that can be decomposed or dissolved in traditional electrolytes, reducing battery efficiency and causing the loss of active materials [26]. Researchers have explored a number of approaches to address this issue, including modifying electrode materials [27,28], adding functional additives [29,30], and using different electrolytes such as ILs [25]. Due to their unique physicochemical properties, such as low volatility, high thermal stability, and wide electrochemical windows, ILs are promising in enhancing battery safety and performance. Moreover, their distinct polarity and weaker solvating power compared to conventional organic solvents may reduce the dissolution of active materials, helping to improve the long term stability of the battery [31,32].

IL-based electrolytes can be considered from the group of non-aqueous electrolytes that can use ILs as solvents instead of conventional IL solvents and provide improved safety and performance characteristics in SIBs [33,34]. Compared to conventional organic electrolytes, IL-based electrolytes provide many benefits, like a wide electrochemical window, low volatility, thermal stability, and low flammability [35,36,37,38,39]. These properties make IL-based electrolytes attractive for the safe and nonflammable SIBs, especially for high-temperature applications [10]. For example, by mixing the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) with sodium salt (NaTFSI), the resulting sodium-ion conducting electrolyte exhibited excellent ionic conductivity up to 5.5 mS·cm⁻¹ and a wide thermal window of −86 °C to 150 °C, making it suitable for sodium-ion battery applications [40]. In Table 1, we have a history of mixed electrolytes for SIBs.

Despite their high cost, ILs retain their economic value in battery systems due to their limited use, which has little impact on the overall cost of the battery system. By improving battery life and economics, the use of ILs for electrolytes can address financial concerns. However, ILs have low transport properties, such as high viscosity. To solve this issue, researchers have searched for the possibility of combining ILs with organic solvents to improve the negative properties. In this review article, we will provide a detailed analysis of current research on electrolytes based on IL compounds and IL solvents for SIBs. In these hybrids, some properties such as viscosity, thermal stability, flammability, electrochemical compatibility, and their performance with different electrode materials are studied. This review will investigate the strategies to overcome the ionic transportation limitations of ILs and investigate the conductivity of the electrolyte mixtures. It also works to find future research directions and opportunities to develop more efficiency, safety, and high-performance of SIBs.

2. Background and Theory

In the batteries, chemical energies convert to electrical energies. The battery structure consists of the anode, cathode and electrolyte; by connecting the batteries to a load, the current will flow in the circuit. The reason for the current flow is the transfer of ions in the electrolyte and electrons in the external circuits. In charging mode, electrons move from the cathode to the anodes in the external circuits; for discharging, this process is reversed [41,42].

2.1. Electrolytes

Electrolytes are an important part of the battery. Electrolytes act with their chemical functions like a bridge in the battery between the anode and cathode. They affect the practical capacity, rate capability, chemical/thermal stress, and lifetime of the battery [19]. Below are some key characteristics of electrolytes in SIBs:

• Ionic conductivity, essential for the performance of SIBs. Faster ion transportation between the electrodes and higher power output could be reachable with higher ionic conductivity [14].
• The electrochemical window of the electrolyte in a rechargeable battery should be larger than the voltage ranges at which the cathode or anode are subjected to charge-transfer reactions, which can prevent unwanted reduction or oxidation reactions in the electrolyte [43].
• To minimize the short circuits and reduce energy losses within the battery, electrolytes should have high electronic resistance (i.e., low electronic conductivity) [44].
• Safe and stable electrolytes will mitigate risks of thermal runaway and fire [45].

SIB electrolytes are classified into two main categories: aqueous and non-aqueous electrolytes. Non-aqueous electrolytes include organic liquids, solid electrolytes and ILs.

2.1.1. Solid Electrolytes

Solid electrolytes are materials that conduct sodium ions without liquid solvents. They have several advantages over liquid electrolytes: They are safer, less flammable, more durable, and offer better electrochemical stability [46]. They have larger electrochemical windows than liquid electrolytes, so they can withstand high voltage differences between the electrodes, or so-called voltage gaps, and thus increase the energy density and efficiency of the battery. For instance, Na_3−xY_1−xZr_xCl₆ (NYZC) has a wide electrochemical window of up to 3.8 V vs. Na/Na⁺, which means it is compatible with high voltage cathodes like NaCrO₂ with a voltage of about 3.5 V vs. Na/Na⁺ [46]. However, there are some issues with solid electrolytes as well, including low ionic conductivity, high interfacial impedance and poor electrode compatibility [47,48,49].

The SIBs are divided into gel polymer electrolytes (GPEs) and ceramic electrolytes (CEs) based on their chemical composition and physical structure of solid electrolytes. The gel-like structure is formed by combining the advantages of both liquid and solid electrolytes in GPE, where a liquid electrolyte is encapsulated in a polymer matrix. The GPEs also employ a number of polymer hosts such as poly(ethylene oxide), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), and poly(vinylidene fluoride-hexafluoro propylene), and these serve as the gel electrolyte’s structural back bone though these polymer hosts contribute to the overall stability and integrity of the GPE and facilitate ion transport within the electrolyte system [19,50]. Besides the polymer matrix, gel-polymer electrolytes for SIBs also contain a liquid electrolyte and an ion-conductive salt. The polymer matrix forms a gel-like structure which holds the liquid electrolyte and can include compounds such as carbonate esters, ethers, glycols, selected sulfones and ILs. The source of moving ions (in this case Na⁺) in the gel matrix is the ion-conducting salt. The most commonly used Na⁺ conductive salts in sodium ion batteries are sodium hexafluorophosphate (NaPF₆), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), sodium trifluoromethane sulfonate (NaCF₃SO₃), sodium tetrafluoroborate (NaBF₄), and sodium perchlorate (NaClO₄) [51,52]. Ceramic electrolytes are solid electrolytes with a crystalline or amorphous structure composed of metal oxides, sulfides, halides or phosphates. These electrolytes have excellent thermal and chemical stability, but low ionic conductivity and limited flexibility [45,53].

Z. Deng et al. illustrated that Na_3.4Zr₂Si_2.4P_0.6O₁₂ had an ionic conductivity of ~0.165 S·cm⁻¹ at 473 K [54]. S. Song et al. reported another NASICON structural ceramic-based electrolyte, Na_3.1Zr_1.95Mg_0.05Si₂PO₁₂, showing high ionic conductivity at room temperature, 3.5  mS·cm⁻¹ [55]. J. Yang et al. prepared the NASICON-structured solid electrolyte Na_3.1Zr_1.95Mg_0.05Si₂PO₁₂ using a simple solid-state reaction method. The resultant product was indeed in solid electrolyte form and displayed an excellent sodium ionic conductivity of 1.33 mS·cm⁻¹ at room temperature. The cell utilizing this electrolyte and a Na_0.9Cu_0.22Fe_0.3Mn_0.48O₂ cathode exhibited a discharge capacity of 57.9 mAh·g⁻¹ at the end of 100 cycles under a current density of 0.5 C rate at room temperature [56].

2.1.2. Organic Liquid Electrolytes

Organic liquid electrolytes in sodium-ion batteries are the solutions of sodium salts and organic solvents, which provide the medium for the transportation of sodium ions across the electrodes. The preferred solvent should be polar, having a high dielectric constant (ε > 15) to improve salt solubility and enhance conductivity; low viscosity to enhance ionic mobility [57] and ensure electrochemical stability during the operation of the battery, without reacting on either the cathode or anode surface; a wide liquid range, low melting point and high boiling point; safety; non-toxicity; and last but not least, reasonable cost. Due to the capability of the solvent as an electron acceptor/donor, the Lewis acidity/basicity concept is also very important in the electrochemical stability window (ESW). The strong Lewis basicity of solvent molecules acts as an enhanced way in the solvation process of sodium salts, with the coordination of solvent molecules to the Na⁺ ion. On the other hand, too strong interaction between the Na⁺ ion and the solvent molecules will impede the desolvation process and lead to co-intercalation of the Na⁺ ion with the solvent molecules.

Families of organic solvents for LIBs include carbonate esters ethylene carbonate (EC), ropylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), gamma-butyrolactone (GBL), mentioned by their full names in the abbreviation table, ethers, dimethyl ether (DME), diglyme, triglyme, and tetrahydrofuran (THF), among others based on heteroatoms, such as dimethyl sulfoxide (DMSO) and trimethyl phosphate (TMP). Among them, EC is one of the most common carbonate ester-based solvents with a high dielectric constant, showing a strong dissolving power. However, it solidifies at room temperature, which results in a high viscosity and can strongly limit ion mobility. In contrast, lower viscosities but lower dielectric constants are obtained by DEC and ethyl methyl carbonate (EMC) [58]. In this context, proper solvent selection is also crucial to prevent graphite anode exfoliation. Graphite, which is widely used in LIBs, has limited compatibility with sodium-ion storage due to the larger size and weaker interaction of Na⁺ with graphite layers. Under conventional conditions, sodium ions cannot be effectively and reversibly intercalated into graphite. However, some intercalation has been observed in specific systems, such as with co-intercalation of solvent molecules or using special electrolytes like ether-based systems. In carbonate ester-based solvents such as EC, the formation of a stable SEI and strong sodium solvation may help mitigate electrode degradation, although alternative anode materials such as hard carbon are typically preferred for sodium-ion batteries due to their superior compatibility and performance [58].

Among the ester electrolytes, EC is the most important solvent, with a very high dielectric constant of 89.78. It exhibits a melting point in the range of ca. 34 to 37 °C; hence, pure EC cannot be used at ambient temperature [59]. The handling of PC is also more convenient since, unlike EC, it is liquid at room temperature, but the dielectric constant is lower than EC (64.92) [7]. The most widely used strategy for enhancing the performance of the electrolyte involves mixing solvents. Most research involving organic solvents focuses on combinations such as EC/DEC, EC/PC, PC, and EC/DMC [60]. For example, EC and PC can be mixed together in a weight ratio of 0.5:0.5 to optimize both the electrochemical stability and the thermal stability of the electrolyte. Adding a low-viscosity dimethyl carbonate (DMC) into the above binary EC/PC solvent into a ternary EC_0.45/PC_0.45/DMC_0.1 solvent can improve ionic conductivity from 6.2 up to 10 mS·cm⁻¹ [61]. The phosphorus-based TMP solvent is an electrolyte currently used in most SIBs due to the widened liquid temperature range of such a solvent [62].

DME, diglyme, triglyme, and THF are important ether-based solvents used in SIBs. In the presence of diglyme-based electrolytes, anodes such as highly crystalline few-layered graphene demonstrated an ultrafast Na-ion insertion with remarkable cycle stability. Additionally, the use of diglyme-based electrolytes resulted in a first-cycle reversible capacity of 294 mAh·g⁻¹ and a Coulombic efficiency of 87.2% when using layered FeSe as the anode material [63]. When compared to carbonate-based electrolytes, diglyme-based electrolytes exhibited better reversible capacity and long-term cycle life in cells utilizing reduced graphene oxide as the anode material [64]. Moreover, TiO₂ anodes coupled with diglyme-based electrolytes showed an unprecedented reversible capacity at various current densities thanks to a thinner and more uniform SEI formation, leading to faster charge transfer dynamics and enhanced Na⁺ intercalation pseudocapacitive behavior [65]. DME, characterized by its relatively short chain length, exhibits low viscosity and a moderate dielectric constant, facilitating. Ion transportation, a relatively high conductivity of about 2 × 10⁻³ S·cm⁻¹ at room temperature [66].

The commercial hard carbon (HC) anode for sodium-ion batteries works differently with different electrolytes. It works very well with tetrahydrofuran (THF)-based electrolytes. In fact, THF helps sodium ions move faster and more easily at the interface between the electrolyte and SEI on the anode. It also helps form a uniform and stable SEI that contains NaF and organic compounds [67]. The best electrolyte for the HC anode is NaPF₆/THF, resulting in a high discharge capacity of 212 mAh·g⁻¹ at a very high current density of 5 A·g⁻¹, doing better than DME (193 mAh·g⁻¹), PC (24 mAh·g⁻¹), and EC/DEC (5 mAh·g⁻¹) solvents. The THF-based electrolyte also helps the HC anode perform well at low temperatures, staying stable at −20 °C, and keeping a high specific capacity of 175 mAh·g⁻¹ (74% of its room-temperature capacity) at 2 A·g⁻¹. In addition, the HC anode in NaPF₆/THF electrolyte has superior cycling stability, with a high specific capacity of 206 mAh·g⁻¹ after 1000 cycles at 2 A·g⁻¹, about four times the value seen in EC/DEC and PC-based electrolytes [67].

The salts of the organic liquid electrolytes for SIBs are sodium compounds that provide the source of sodium ions for the battery operation. They should have high solubility, high ionic conductivity, chemical stability against the electrode, a wide electrochemical stability window, thermal stability, and low cost [14]. Commonly adopted salts for SIBs include NaTFSI, NaPF₆, NaClO₄, NaBF₄, NaTf, and NaFSI [68,69]. Of these, salts like NaClO₄ and NaPF₆ were conventionally employed, though both have certain setbacks. Thus, NaClO₄ is thermally stable [70], but its usage is restricted, as it is prone to explosion. Besides this, it is incompatible with moisture. For NaPF₆ [71], even though it is electrochemically stable, it produces corrosive hydrogen fluoride when exposed to water and is hence problematic for the SEI.

Among the different parameters that determine the choice of sodium salts, conductivity and the highest occupied molecular orbital (HOMO) level are two important parameters. The conductivity of the crystal, which is related to lattice energy, plays a vital role in determining the ionic conductivity of the electrolyte. Sodium salts with low lattice energy show higher conductivity, such as NaPF₆, and therefore are preferred choices: NaPF₆ > NaClO₄ > NaTFSI/NaOTf > NaBF₄ [72]. The ESW is the difference between the energy level of the lowest unoccupied molecular orbital (LUMO) and that of the HOMO. The wider the ESW, the more voltage range the electrolyte can endure in operation without taking part in any undesired reaction or decomposition. Thus, the PF₆⁻ ion, with the lowest HOMO energy level of −11.67 eV, is not easily oxidized and decomposed, and it is more electrochemically stable. In contrast, for the ClO₄⁻ ion, the corresponding HOMO level is −7.89 eV, reflecting poorer chemical stability in the case of oxidation processes [58]. In the series of NaOTf > NaClO₄ > NaTFSI > NaBF₄ > NaPF₆, it can be noticed that those sodium salts with high HOMO levels, such as NaOTf and NaClO₄, are more easily oxidized and then limit the voltage window of SIBs.

Ionic conductivity of different electrolytes based on NaPF₆, NaOTf, and NaClO₄ salts in a mixture of EC : DMC (30:70 wt%) was measured at ambient temperature [73]. The variation of ionic conductivity with salt concentration was more pronounced for NaOTf and NaClO₄ electrolytes compared to the NaPF6-based electrolytes. Among the electrolytes tested, the maximum ionic conductivity values were obtained at a salt concentration of 0.6 M NaPF₆ (6.8 mS·cm⁻¹), 1 M NaClO₄ (5.0 mS·cm⁻¹), and 0.8 M NaOTf (3.7 mS·cm⁻¹), respectively [73].

Among the most popular sodium salts, NaTFSI has been widely used in the electrolyte solutions for SIBs and was studied with different solvents, such as alkyl carbonates like EC, PC, and DMC, as well as ether-based solvents like monoglyme, tetraglyme, and poly(ethylene oxide) [74].

Compared to NaPF6-based electrolytes, NaTFSI-based electrolytes exhibit lower dissociation due to the stronger ionic association of the TFSI anion [14]. While NaTFSI and its counterpart NaFSI are not ideal as standalone salts in SIB electrolytes—primarily because they tend to corrode aluminum current collectors—they are gaining popularity due to their ability to form effective IL mixtures. These salts offer several advantages over alternatives like NaPF₆ and NaBF₄, including lower toxicity, greater thermal stability, and higher ionic conductivity than sodium triflate (NaTf).

2.1.3. IL Electrolytes

ILs are a group of salts that are liquid at room temperature. They consist of ions, that is, molecules or atoms equipped with an electrical charge, exclusively. This unique property of ILs opens many doors for their use as solvents, electrolytes and catalysts. ILs are highly customizable, i.e., their characteristics are able to be modified based on specific cation and anion selection; therefore, viscosity, ionic conductivity, temperature of degradation, and melting point are all modifiable. They are also stable against heat, chemicals, and electrochemical stress, and possess low volatility and non-flammability. As a result of these traits, ILs have become extremely popular in recent years as a potential substitute for organic solvents and traditional electrolytes to be used in a broad spectrum of applications.

ILs can be classified based on their chemical constitution. They can be zwitterionic, protic, or aprotic [75]. Zwitterionic ILs consist of both negative and positive charges within a single molecule and thus consist of a net zero charge. These liquids find greatest application in the formation of membranes that make use of ILs. Protic ILs consist of proton-donating functionality, e.g., hydroxyl (-OH) functionality. They can be used as electrolytes for fuel cells because the proton-donating group can promote proton transfer reactions. They are not suitable for use in other electrochemical devices, such as lithium batteries and supercapacitors, because they are less stable than aprotic ILs. Aprotic ILs without a proton donor group are more stable for electrical use and can be used in lithium batteries, as well as in other devices such as dye-sensitized solar cells and electrochromic devices [76,77,78].

In IL electrolytes, all ionic components contribute to ionic conductivity; however, only sodium ions can be electrochemically active and participate in charge transfer reactions. The battery’s capacity is dependent on the properties of the electrode materials, specifically the amount of sodium that can be stored reversibly, rather than the IL’s conductivity. However, the efficiency of sodium ion transport significantly impacts the system’s rate capability and power density. In contrast to common perceptions, increasing the salt concentration usually results in decreased ion mobility because of the simultaneous increase in electrolyte viscosity and the formation of neutral ion pairs or larger ionic groups that do not contribute to charge transport [79]. Nevertheless, IL electrolytes often exhibit a lower activation energy for ion transport through the SEI compared to organic carbonates, which can reduce interfacial resistance and help suppress dendrite formation [80].

ILs typically comprise a bulky organic cation (the positively charged component) and a smaller inorganic or organic anion (the negatively charged component). Anion and cation pair to form a liquid salt at or near room temperature in ILs. To this end, various anions including chloride (Cl⁻), tetrafluoroborate (BF₄⁻), hexafluorophosphate (PF₆⁻), bis(trifluoromethylsulfonyl)imide (TFSI⁻) and perchlorate (ClO₄⁻) are explored in IL sodium-ion battery systems. Considering the thermal stability of these anions is very important, especially when wanting to use ILs for high-temperature applications. Anions such as TFSI⁻ and FSI⁻ have long-term stability in high temperatures. These anions show a great stability in experiments that were carried out over 96 h at temperatures of 200 °C, 250 °C, and 300 °C, which is essential for the reliable operation of sodium-ion batteries in related conditions [5,81]. Understanding these anions helps us to characterize their function for using them in IL electrolytes.

Table 2. Comparative anion properties.

Anion	Thermal Stability	ESW (V)	Ionic Conductivity	Cost/ Availability	Key Advantage	Key Drawback	Ref.
DFOB−	High	Wide	Moderate	Moderate	SEI formation (low LUMO), high-T performance	Limited solubility, high cost	[83]
BOB−	High	Wide	Low	Moderate	SEI/CEI stabilization, halide-free	Low conductivity (high viscosity), limited solubility	[84]
TFSI− FSI−	High	Wide	High	Moderate	High stability, good conductivity, less corrosive	Synthesis complexity, potential environmental persistence	[36]
PF6−	Moderate	Wide	High	Low	High conductivity, low cost, common in commercial use	Hydrolytic instability (forms corrosive HF)	[36]
ClO4−	High	Narrow	Moderate	Low	High thermal stability	Strong oxidant, potential explosion risk, narrow ESW	[85]
Cl−	Low	Narrow	High	Very Low	High conductivity, low cost	Highly corrosive nature	[36]

and

Table 3. Comparative cation properties.

Example Ion	Thermal Stability	ESW (V)	Ionic Conductivity	Cost/ Availability	Key Advantage	Key Drawback	Ref.
C3mpyr+	Moderate	Very wide (~5 V)	High	Moderate	Excellent ESW high conductivity	Decomposes in highly anodic regions	[36]
EMI+	Moderate	Narrow	Very High	Low	High ionic conductivity, good for low-viscosity mixtures	Lower cathodic stability, limiting ESW	[36]
P111i4+	Very High	Very wide (~5.5–6 V)	Low	High	Excellent thermal and electrochemical stability	Low ionic conductivity (high viscosity)	[86]
Quaternary N+	High	Wide (~5.0–5.2 V)	Low	Moderate	Good cathodic stability	Low ion mobility (large size)	[87]

show the properties of anions and cations for the IL electrolytes. These anions are often paired with a variety of cations to form different types of ILs. Common examples of cations that are used in ILs include imidazolium, pyridinium, ammonium, and phosphonium cations; moreover, one can freely choose combinations of cations and anions to align with the ideal performance specifications of the IL [82].

Tetrafluoroborate (BF₄⁻) anions are one class of fluoro complex anions that have been widely used to prepare low-melting-point salts. However, these anions easily undergo hydrolysis and usually lead to high viscosity, hindering their effective use as electrolytes in battery systems. The strong hydrophilicity of BF₄⁻ has motivated research into methods to improve the hydrolytic stability of BF₄⁻ in SIBs; however, their synthesis is mostly not cost-effective or practical. In contrast, sulfonamide-based ILs, especially those with the bis(trifluoromethylsulfonyl)imide anion (TFSI⁻), have gained popularity. Their attractiveness is due to their ease of synthesis and high ionic conductivity. These ILs are a promising alternative for application in battery systems [36].

The difluoro(oxalate)borate (DFOB⁻) anion is being increasingly recognized as a dual-functional component that can be used as a main salt or a high-performance additive. It has a low lowest unoccupied molecular orbital (LUMO) level, allowing it to easily undergo reductive decomposition at a higher potential than common organic solvents. This reduction results in the formation of a dense, boron-rich SEI layer that passivates the electrode surface, preventing further electrolyte degradation. Recent studies have demonstrated that NaDFOB-based electrolytes significantly improve the high-temperature performance of SIBs. For example, NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon pouch cells using NaDFOB achieved a capacity retention of 92.4% at elevated temperatures compared to 88.2% for standard NaPF₆-based systems. Furthermore, DFOB⁻ has a lower fluorine content than NaPF₆, reducing its toxicity and environmental impact and making it more sustainable for large-scale energy storage applications [83].

The bis(oxalate)borate (BOB⁻) anion is known for its halide-free nature and excellent thermal stability. Due to the presence of four carbonyl groups that facilitate the formation of a stable passivation film, it is particularly effective at stabilizing both the SEI on the anode and the cathode electrolyte interphase (CEI). Although BOB⁻ is highly effective as an interface stabilizer, its use as a salt is often limited by its low solubility in common carbonate solvents and the high viscosity it imparts to the electrolyte, which reduces ionic conductivity. However, recent studies have shown that using NaBOB in non-flammable solvents, such as TMP, or in dual-salt configurations can leverage its stabilizing properties while maintaining good transport kinetics [88].

The cations for ILs are usually imidazolium, pyrrolidinium, ammonium, and phosphonium. Cations are vital in the development of state-of-the-art electrolytes for SIBs. Among these tested cations, pyrrolidinium-based cations have been investigated the most. They present high ionic conductivity closer to imidazolium but show higher electrochemical stability with a possibility of reaching 5 V electrolyte systems [89]. Despite their electrochemical stability, the operating window was not extended beyond 5 V as they easily decompose in a more anodic region. Quaternary ammonium cations, on the other hand, depicted cathodic stability and higher operational voltage of up to 5.1 V; however, due to their large size, the ion mobility was found to be lower ionic conductivity with respect to pyrrolidinium [90]. Lastly, quaternary phosphonium cations have very good thermal stability above 600 K [91] and very good electrochemical stability up to 6.0 V; though they generally have low ionic conductivity, functionalization is able to improve their performance and may render them promising electrolyte materials for electrochemical applications. In one study, they could reach 0.94 mS·cm⁻¹ at 50 °C and better cyclic stability than conventional organic solvents [92]. Another attractive group of cations for high-voltage sodium batteries is piperidinium-based cations. While they are the least investigated among the cations, studies have shown their potential in creating 5 V electrolyte systems for both sodium-ion and lithium-ion batteries [93]. In a study, eight types of piperidinium cations with TFSI anion were investigated, all of which showed an ESW range from 4 to 5 V. Piperidinium-based ionic cations are suitable for high-voltage sodium battery applications [94].

In general, all these cations offer a number of advantages and drawbacks. Among them, pyrrolidinium, quaternary ammonium, and quaternary phosphonium cations are the most studied and have their own peculiarities for electrolyte development in SIBs. Imidazolium-based cations, even showing good ionic conductivity, present some limitations due to their lower cathodic stability. For this reason, in order to improve their electrochemical window, their combination with highly electronegative anions has been proposed [95]. Among those, the pyrrolidinium-based cations come out to be very promising due to their very high ionic conductivity and very excellent electrochemical stability, especially within 5 V electrolyte systems. Quaternary ammonium cations show good cathodic stability with a wide operating window and are highly valued in applications involving alkaline media. Finally, quaternary phosphonium-based cations show excellent thermal and electrochemical stability; however, the higher viscosity impacts ion mobility.

Each draft of these cations offers interesting aspects on the one hand; however, on the other hand, they pose several difficulties, highlighting the need to carefully define the cation-anion pairs to meet certain requirements in their applications, such as high-voltage IL electrolytes in the area of energy storage cells, supporting those of batteries. Continuing to investigate and study in the directions provided will be of crucial importance in order to be able to make progress in areas such as electrochemistry and battery technology.

As shown in Figure 1, hybrid electrolytes combine some of the favorable properties of ILs and organic liquids for sodium-ion batteries. ILs possess a number of characteristic properties that provide safety and efficiency at an elevated temperature: high thermal stability, low volatility, wide electrochemical window, and high ionic conductivity. Organic liquids, on the other hand, contribute to low viscosity, a high dielectric constant, compatibility with various electrode materials, and low cost, which enhance ion mobility and overall conductivity. By incorporating the two components together, the hybrid electrolyte inherits IL thermal stability and low volatility, adding low viscosity due to organic solvents, which enhances ionic conductivity; this condition maintains wide electrochemical window conditions for a chance toward high energy density and optimization for battery performance. The hybrid electrolyte thus creates a stabler, highly efficient, safe environment for SIBs.

3. Synergistic Effects and Interfacial Stabilization Mechanisms

The safety and long-term performance of SIBs depend critically on the properties of the electrode/electrolyte interfaces. While traditional organic liquid electrolytes offer high ionic conductivity, they suffer from high flammability and limited thermal stability. Hybrid electrolytes, which combine ILs with organic solvents, have become a promising solution that allows us to take advantage of the benefits of both components. A key aspect of their performance is the formation of stable passive layers on the anode and cathode. Recent research suggests that the formation mechanisms and roles of SEI and CEI are different enough and require separate discussion [96,97].

3.1. SEI on the Anode

The SEI is a passivation layer formed on the anode surface (typically hard carbon in SIBs) due to the irreversible reductive decomposition of the electrolyte during the initial charging cycles. Its primary role is to act as an ionic conductor for Na⁺ ions while remaining an electronic insulator to prevent further electrolyte reduction [60].

In hybrid electrolyte systems, the presence of ILs significantly influences the SEI’s composition and morphology. IL cations and anions can participate in the solvation shell of Na⁺ ions, leading to a more robust and thermally stable SEI compared to pure organic electrolytes. For instance, the use of TFSI⁻ or FSI⁻ anions in hybrid systems often results in an inorganic-rich SEI (containing NaF, Na₂O, and Na₂CO₃), which is less prone to decomposition at elevated temperatures [98]. This stabilization is crucial for preventing continuous electrolyte consumption and the loss of active sodium, thereby enhancing the battery’s cycle life and safety [99].

3.2. CEI on the Cathode

Unlike the SEI, the CEI forms on the cathode surface through the oxidative decomposition of the electrolyte at high operating potentials. The CEI’s formation mechanism is more complex and involves multiple side reactions, including the chemical reaction of the electrolyte with the natural passivation layer (like carbonates) present on oxide cathode materials [96].

The role of the CEI is to passivate the highly active cathode surface, prevent the dissolution of transition metals, and bear the potential drop between the electrode and the electrolyte. In SIBs, the CEI is often subjected to continuous attack by free radicals and reactive oxygen species escaping from the cathode lattice during cycling [96,100]. Hybrid electrolytes can enhance CEI stability by providing a wider electrochemical stability window and forming a more compact, electrically insulating layer. This prevents the oxidative degradation of carbonate esters and maintains the structural integrity of the cathode material, especially in high-voltage applications [96]. Also, in Figure 2, a schematic comparison of SEI and CEI formation mechanisms is reported.

3.3. Synergistic Effects on Thermal Stability

SIBs are gaining significant attention as a promising alternative to LIBs due to the abundance and low cost of sodium resources. However, the safety and long-term performance of SIBs are critically dependent on the properties of their electrolytes. Traditional organic liquid electrolytes, while offering high ionic conductivity, suffer from inherent flammability and limited thermal stability, posing significant safety risks, particularly under abnormal operating conditions such as overcharging or elevated temperatures. To address these challenges, hybrid electrolytes, which combine ILs with organic solvents, have emerged as a compelling solution. ILs are non-volatile, non-flammable, and possess high thermal and electrochemical stability, making them ideal candidates for enhancing battery safety. However, their high viscosity often leads to lower ionic conductivity compared to organic solvents, which can impede battery performance. The synergistic combination of ILs and organic solvents in hybrid electrolytes aims to leverage the advantages of both components while mitigating their individual drawbacks. This section will elaborate on the mechanisms behind the enhanced thermal stability observed in these hybrid electrolyte systems, focusing on the synergistic effects between ILs and organic solvents at a molecular level.

The enhanced thermal stability of hybrid electrolytes, compared to pure organic electrolytes, is a result of complex synergistic interactions between the IL and organic solvent components. This synergy primarily manifests through several key mechanisms:

3.3.1. Reduction of Organic Solvent Volatility and Flammability

One of the primary contributions of ILs to the enhanced thermal stability of hybrid electrolytes is their ability to significantly reduce the volatility and flammability of the organic solvent component. ILs possess negligible vapor pressure, meaning they do not readily evaporate even at elevated temperatures. When mixed with organic solvents, ILs can form strong intermolecular interactions, such as hydrogen bonding, van der Waals forces, and electrostatic interactions, with the organic solvent molecules. These interactions effectively “trap” the volatile organic molecules within the less volatile IL matrix, thereby reducing their escape into the gas phase. This phenomenon is akin to increasing the effective boiling point of the organic solvent within the mixture. A lower vapor pressure directly translates to reduced flammability, as the concentration of flammable organic vapors above the electrolyte surface is significantly diminished. Studies have shown that increasing the IL content in hybrid electrolytes leads to a higher flash point and ignition temperature, along with a longer ignition time and shorter self-extinguishing time, indicating a substantial improvement in safety [8,14]. For instance, a 20% IL composition in an EC:PC electrolyte might have a flash point of around 140 °C, which can increase to as high as 180 °C at 80% IL content [14]. This reduction in volatility and flammability is a critical factor in preventing thermal runaway and enhancing the overall safety of SIBs, especially under abusive conditions.

3.3.2. Formation of Thermally Stable Solvation Structures

The interaction between the ILs, organic solvent, and the dissolved sodium salt plays a crucial role in forming thermally stable solvation structures. In pure organic electrolytes, sodium ions are typically solvated by the organic solvent molecules. Under elevated temperatures, these solvation shells can become unstable, leading to the decomposition of the solvent and potential side reactions. In hybrid electrolytes, the presence of IL ions (cations and anions) can alter the solvation environment of the sodium ions. The strong coordination ability of certain IL anions (e.g., TFSI⁻) and cations can lead to the formation of more robust and thermally stable solvation complexes around the Na⁺ ions. This can involve the co-solvation of Na⁺ by both organic solvent molecules and IL ions, or even the preferential solvation by IL ions. The formation of these stable complexes can hinder the decomposition of the organic solvent at higher temperatures, thereby contributing to improved thermal stability. Raman spectroscopy studies have indicated that Na⁺ coordinated organic solvent molecules, e.g., EC become more stable and less ignitable in the presence of ILs, suggesting a stronger interaction between Na⁺ and EC when ILs are added [14]. This enhanced stability of the solvation shell effectively raises the decomposition temperature of the electrolyte system.

3.3.3. Suppression of Side Reactions and SEI Stabilization

Thermal stability concerns are not only the intrinsic stability of the electrolyte components but also their reactivity with the electrode materials, particularly at elevated temperatures. In SIBs, the formation of a stable SEI layer on the anode surface is crucial for long-term cycling stability and safety. Unstable SEI layers can lead to continuous electrolyte decomposition, gas evolution, and thermal runaway. ILs, with their wide electrochemical windows and inherent stability, can contribute to the formation of a more robust and thermally stable SEI layer in hybrid electrolytes. The presence of ILs can influence the composition and morphology of the SEI, making it less prone to decomposition at higher temperatures. This suppression of undesirable side reactions between the electrolyte and electrode surfaces at elevated temperatures further contributes to the overall thermal stability of the battery system. Cyclic voltammetry results have shown that hybrid electrolytes can lead to the formation of a stable passivation film on the cathode, indicating improved interfacial stability [9]. The reduced reactivity and enhanced SEI stability provided by ILs help to maintain the integrity of the battery components and prevent catastrophic failures under thermal stress.

3.3.4. Dilution Effect and Heat Dissipation

While not a direct molecular interaction, the dilution effect of ILs on organic solvents also contributes to enhanced thermal stability. By replacing a portion of the flammable organic solvent with a non-flammable IL, the overall concentration of combustible material in the electrolyte is reduced. This dilution inherently lowers the energy content of the electrolyte, making it less prone to rapid exothermic reactions during thermal events. Furthermore, the higher thermal conductivity of some ILs compared to organic solvents can facilitate better heat dissipation within the battery. Efficient heat dissipation can prevent localized hot spots and mitigate the propagation of thermal runaway, thereby improving the overall thermal safety of the battery. The Thermal Stability Limit (TGA) assessments in the original article demonstrated that hybrid electrolytes exhibited significantly lower mass losses at elevated temperatures compared to pure organic electrolytes, indicating enhanced thermal stability due to reduced solvent evaporation and decomposition [9].

In summary, the enhanced thermal stability of hybrid electrolytes in sodium-ion batteries is a multifaceted phenomenon arising from the synergistic interplay between ILs and organic solvents. This includes the reduction of organic solvent volatility and flammability, the formation of thermally stable solvation structures around sodium ions, the suppression of detrimental side reactions and stabilization of the SEI layer, and the beneficial dilution and heat dissipation effects. These combined mechanisms contribute to a safer and more reliable electrolyte system for next-generation SIBs.

3.3.5. Thermal Stability of Hybrid Electrolytes

The thermal stability of hybrid electrolytes is a phenomenon originating from the interaction between ILs and organic solvents. This stability is crucial for preventing thermal runaway and ensuring the safety of SIBs under extreme conditions, as summarized in

Table 4. Thermal stability of hybrid electrolytes.

Mechanism	Description	Impact on Safety
Volatility Suppression	ILs reduce the vapor pressure of organic solvents.	Higher flash point; reduced ignition risk.
Solvation Stabilization	Na+ solvent coordination is strengthened by IL presence.	Higher decomposition temperature.
Interfacial Robustness	Formation of inorganic-rich SEI/CEI layers.	Prevention of thermal runaway propagation.
Flame Retardancy	ILs do not support combustion.	Self-extinguishing behavior; reduced fire hazard.

. ILs directly contribute to thermal stability by reducing the vapor pressure of organic co-solvents. Organic carbonates like EC and PC are highly volatile and flammable. When mixed with ILs, which have negligible vapor pressure, the overall vapor pressure of the electrolyte system decreases significantly. This dilution effect raises the flash point of the mixture and reduces the concentration of flammable vapors in the battery space, which increases the self-extinguishing time (SET) [101,102]. Recent research highlights the role of the solvation structure in enhancing thermal stability. In hybrid electrolytes, Na⁺ ions are preferentially solvated by organic solvent molecules due to their high dielectric constants. The presence of bulky IL cations and anions like TFSI⁻ or FSI⁻ can strengthen the coordination between Na⁺ and the organic molecules, like the lone pairs on the oxygen atoms of EC. This anchoring effect stabilizes the solvent molecules, making them less prone to oxidative or thermal decomposition at elevated temperatures [103]. Thermal runaway often begins with the exothermic decomposition of the SEI on the anode or the CEI on the cathode. Hybrid electrolytes promote the formation of inorganic-rich interphases, containing Na₂CO₃, that are more thermally robust than the organic-rich layers formed in pure carbonate electrolytes. These stable interphases act as a physical barrier, preventing direct contact between the highly reactive electrode surfaces and the bulk electrolyte, thus suppressing the parasitic side reactions that lead to heat generation [104]. The non-flammable nature of ILs provides a flame-resistant effect. In the event of an internal short circuit or external heating, the IL components do not support combustion. Studies have shown that adding 20–40 wt.% of IL to an organic electrolyte can transform a highly flammable system into a non-flammable or self-extinguishing one [105]. This is often quantified by the SET, where hybrid systems show a dramatic decrease in burning time compared to pure organic counterparts.

4. Advanced Characterization and Computational Modeling for Deeper Analysis

The field of SIBs is rapidly evolving, with hybrid electrolytes emerging as a key area of innovation to overcome the limitations of traditional liquid and solid electrolytes. While initial research often focused on empirical observations of performance, recent advancements necessitate a deeper, more analytical approach to understanding the fundamental mechanisms governing these complex systems. This section delves into novel strategies employed in the design and characterization of hybrid electrolytes, moving beyond mere enumeration of research findings to provide a more profound analysis of their underlying principles and performance enhancements. The emphasis is on integrating advanced analytical techniques and theoretical insights to elucidate the intricate interplay between ILs and organic solvents, and their impact on overall battery performance and safety. Figure 3 shows experimental techniques and computational modeling and highlights the roles of molecular dynamics (MD) in predicting transport properties and density functional theory (DFT) in calculating electrochemical stability and binding energies.

4.1. Advanced Characterization Techniques for Mechanistic Understanding

To move beyond descriptive summaries, a deeper analysis of hybrid electrolytes requires the application of advanced characterization techniques that can provide molecular-level insights into their structure, dynamics, and interfacial behavior. These techniques are crucial for understanding the synergistic effects that lead to improved thermal stability, enhanced ionic conductivity, and stable electrode–electrolyte interfaces.

4.1.1. Spectroscopic Probes of Solvation Structure and Interactions

Traditional electrochemical measurements provide macroscopic performance data, but they often lack the resolution to reveal the molecular-level interactions within hybrid electrolytes. The following advanced spectroscopic techniques are indispensable for this purpose:

• Raman Spectroscopy: This technique is highly effective in probing the local solvation environment of Na⁺ ions and the interactions between ILs and organic solvents. By analyzing shifts in vibrational modes of solvent molecules and IL ions, researchers can identify the formation of stable solvation complexes, ion-pairing phenomena, and the degree of solvent coordination around Na⁺ ions. For instance, studies have used Raman spectroscopy to show that the presence of ILs can strengthen the coordination of organic solvent molecules (e.g., ethylene carbonate, EC) to Na⁺, leading to more robust solvation shells and contributing to enhanced thermal stability [14]. This provides direct evidence of the synergistic effects at a molecular level.
• Nuclear Magnetic Resonance (NMR) Spectroscopy: Solid-state and liquid-state NMR can provide detailed information on ion transport mechanisms, molecular dynamics, and the chemical environment of various species within the electrolyte. Diffusion coefficients of Na⁺ ions, IL ions, and solvent molecules can be measured, offering insights into the factors limiting ionic conductivity. Furthermore, changes in chemical shifts can indicate specific interactions between components, such as hydrogen bonding or preferential solvation. For example, ²³Na NMR can directly probe the local environment of sodium ions, revealing how their coordination changes with varying IL and organic solvent ratios, and how this impacts ion mobility [106].
• Fourier Transform Infrared (FTIR) Spectroscopy: FTIR can complement Raman spectroscopy by identifying functional groups and their interactions. It is particularly useful for detecting hydrogen bonding and other intermolecular associations between ILs and organic solvents, which are critical for understanding the reduced volatility and enhanced thermal stability of hybrid systems.

4.1.2. X-Ray Techniques for Interfacial Analysis

The electrode–electrolyte interface, particularly the SEI on the anode, is paramount for battery performance and safety. The following X-ray techniques offer powerful tools for its characterization:

• X-Ray Photoelectron Spectroscopy (XPS): XPS is widely used to analyze the chemical composition and electronic states of elements within the SEI layer. By examining the binding energies of core-level electrons, researchers can identify the various inorganic (e.g., NaF, Na₂CO₃) and organic (e.g., R-ONa, R-OCO₂Na) components of the SEI. This helps in understanding how ILs influence the formation and stability of the SEI, leading to improved interfacial properties and reduced side reactions at elevated temperatures [107].
• Synchrotron-based X-Ray Absorption Spectroscopy (XAS) and X-Ray Diffraction (XRD): These techniques can provide insights into the atomic and electronic structure of electrolyte components and interfacial layers. XAS can reveal the local coordination environment of specific elements (e.g., Na, F), while XRD can identify crystalline phases within the SEI or bulk electrolyte. These techniques are particularly useful for understanding the long-term stability and degradation mechanisms of hybrid electrolytes.

4.2. Computational Modeling and Simulation for Predictive Design

Complementing experimental characterization, computational modeling and simulation play an increasingly vital role in the deeper analysis and predictive design of hybrid electrolytes. These tools allow researchers to explore molecular interactions and transport phenomena that are difficult to observe experimentally.

4.2.1. MD Simulations

MD simulations can provide dynamic insights into ion transport, solvation structures, and intermolecular interactions within hybrid electrolytes. By simulating the movement of atoms and molecules over time, MD can:

• Predict Ionic Conductivity: Calculate diffusion coefficients of ions and correlate them with experimental ionic conductivity values, identifying the rate-limiting steps in ion transport.
• Visualize Solvation Shells: Provide a detailed picture of how Na⁺ ions are solvated by both organic solvent molecules and IL ions, revealing the competition between different species for coordination. This can help explain the formation of thermally stable solvation structures [108].
• Analyze Intermolecular Interactions: Quantify the strength and nature of interactions (e.g., hydrogen bonding, electrostatic) between ILs and organic solvents, which directly relates to the reduced volatility and enhanced thermal stability.

4.2.2. DFT Calculations

DFT calculations are powerful for understanding the electronic structure, stability, and reactivity of electrolyte components and their interactions. DFT can be used to:

• Calculate Binding Energies: Determine the strength of interactions between Na⁺ ions and various solvent/IL molecules, providing a theoretical basis for preferential solvation and solvation shell stability.
• Predict ESW: Calculate the theoretical oxidation and reduction potentials of electrolyte components, helping to design electrolytes with wider ESWs for higher voltage applications.
• Investigate SEI Formation Mechanisms: Model the initial decomposition reactions of electrolyte components on electrode surfaces, offering insights into the formation pathways and composition of the SEI layer [109].

5. Novel Strategies in Hybrid Electrolyte Design

Beyond fundamental understanding, recent research has focused on novel strategies to further optimize hybrid electrolytes for SIBs, addressing remaining challenges such as dendrite formation, low-temperature performance, and long-term cycling stability.

5.1. Localized High-Concentration Electrolytes (LHCEs)

LHCEs represent a promising strategy to combine the advantages of highly concentrated electrolytes (which suppress side reactions and dendrite growth) with the lower viscosity of dilute electrolytes. In LHCEs, a small amount of a highly polar diluent is added to a concentrated salt-in-solvent electrolyte, leading to a localized solvation structure around the ions while maintaining overall low viscosity. This approach can significantly improve the electrochemical performance and safety of hybrid electrolytes by:

• Suppressing Side Reactions: The reduced free solvent content minimizes parasitic reactions with electrode surfaces.
• Enhancing SEI Stability: The unique solvation structure promotes the formation of a more robust and uniform SEI layer, which is critical for long-term cycling stability and dendrite suppression [110].
• Improving Thermal Stability: By reducing the amount of highly volatile free solvent, LHCEs inherently possess improved thermal stability compared to conventional dilute electrolytes.

5.2. Functional Additives and Co-Solvents

The judicious selection of functional additives and co-solvents can further tailor the properties of hybrid electrolytes. These can include:

• Flame Retardants: Non-flammable additives can further enhance the safety of hybrid electrolytes, particularly those with higher organic solvent content. TMP and Triethyl Phosphate (TEP) are common organophosphorus compounds used as flame retardants. They can be incorporated into electrolytes to suppress combustion. For instance, studies have shown that adding TMP can make organic electrolytes non-flammable [21].
• SEI-Forming Additives: Specific additives can be designed to decompose preferentially on the electrode surface, forming a stable and protective SEI layer that prevents further electrolyte decomposition and dendrite growth. Fluoroethylene Carbonate (FEC) is one of the most widely used and effective SEI-forming additives for both lithium-ion and sodium-ion batteries. FEC decomposes to form a robust, fluorine-rich SEI layer that improves cycling stability and Coulombic efficiency [107].
• Redox Shuttles: These additives can improve overcharge protection by reversibly oxidizing and reducing at the electrodes, preventing thermal runaway. Trisaminocyclopropenium perchlorate (TAC.ClO₄) is an organic salt that has been demonstrated as an effective redox shuttle for overcharge protection in Na-ion battery systems [111]. Benzophenone (BP) is more commonly studied in Li-ion batteries; the concept of using organic molecules with reversible redox potentials applies to SIBs as well [112].
• Plasticizers: For hybrid quasi-solid or solid electrolytes, plasticizers can improve ionic conductivity and flexibility at lower temperatures. EC/DMC are primary solvents that can also act as plasticizers in polymer-based electrolytes, improving chain mobility and ion transport [113]. Succinonitrile (SN) is an organic compound used in quasi-solid-state electrolytes due to its high dielectric constant and ability to dissolve salts, contributing to good ionic conductivity [114].

5.3. Polymer-in-Salt and IL-in-Polymer Electrolytes

These novel electrolyte designs leverage the properties of polymers to create quasi-solid or solid-like hybrid systems with enhanced safety and mechanical integrity. In polymer-in-salt electrolytes, the salt concentration is very high, and the polymer acts as a matrix. In IL-in-polymer electrolytes, ILs are incorporated into a polymer matrix. These systems aim to:

• Improve Mechanical Stability: They suppress dendrite growth and improve overall battery safety by providing a more rigid electrolyte structure.
• Enhance Interfacial Contact: They ensure good contact with electrode materials, reducing interfacial resistance.
• Maintain Ionic Conductivity: They optimize the polymer and IL content to achieve a balance between mechanical stability and sufficient ionic conductivity, even at lower temperatures [115].

5.4. Conclusion on Novel Strategies and Deeper Analysis

Moving forward, the development of hybrid electrolytes for SIBs will increasingly rely on a synergistic combination of advanced experimental characterization and computational modeling. Techniques such as Raman and NMR spectroscopy, coupled with MD simulations and DFT calculations, provide the necessary tools for a deeper, molecular-level understanding of the complex interactions within these systems. Furthermore, novel design strategies like localized high-concentration electrolytes, functional additives, and polymer-based hybrid systems offer promising avenues for overcoming existing challenges and pushing the boundaries of SIB performance and safety. By embracing these advanced approaches, researchers can accelerate the rational design of next-generation hybrid electrolytes, paving the way for the widespread adoption of SIBs in various energy storage applications.

Figure 4 summarizes advanced design strategies, showing how LHCEs utilize diluents to achieve stable SEI formation while maintaining low viscosity. The illustration highlights the specific roles of functional additives, such as FEC for interface protective and TMP/TEP for flame retardant properties, which actively tailor electrolyte properties. Additionally, the diagram illustrates polymer-based hybrid systems, including polymer-in-salt structures, which increase mechanical stability and prevent dendrite growth, making SIBs safer and more powerful.

6. Comparative Analysis

Following a thorough examination of novel design techniques and cooperative effects in hybrid electrolytes, this section provides a comprehensive comparative analysis of performance indicators for various IL/organic solvent systems. The objective is to evaluate the impact of combining components, specifically ionic liquid, organic solvent, and sodium salt, on improvements in battery performance, safety, and longevity. This analysis will establish a benchmark for the current state of the art and identify the most promising formulations for future commercialization.

6.1. Physical Properties of Hybrid Electrolytes for SIBs

6.1.1. Viscosity and Density

The works of Monti et al. introduce hybrid electrolytes obtained by mixing 0.8 M sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) with two kinds of ILs. The first IL combination consists of ethylene carbonate (EC), propylene carbonate (PC), and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide represented as EC_x:PC_x:BMImTFSI_(1−2x); the second one is the combination of EC, PC, and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide represented as EC_x:PC_x:Pyr₁₃TFSI_(1−2x). In the above, “x” shows the variable proportions of EC to PC in these mixtures [8].

It is observed that an increase in the IL concentration in the electrolyte increases its density. On the other hand, the rise in temperature decreases its density linearly. The temperature and IL content that give the maximum value of the density are 10 °C and 80%, respectively. Pure ILs and hybrid electrolytes have different densities due to different concentrations of sodium salt. It observes a decrease in viscosity with an increase in temperature and an increase in viscosity with higher IL content. Pure IL electrolytes have higher viscosities in spite of having lower sodium salt content [8].

This observed increase in viscosity with higher IL content, as noted by Monti et al. [14] is consistent with the formation of more complex ion-ion and ion-solvent aggregates, as suggested by molecular dynamics simulations and spectroscopic studies on similar hybrid systems. These stronger intermolecular interactions, while contributing to thermal stability, inherently restrict molecular mobility, leading to higher macroscopic viscosity.

Similar trends in viscosity increase and transport limitation with rising IL content were previously reported for mixed IL/organic electrolytes, where a clear trade-off between thermal stability and ionic mobility was identified [116].

6.1.2. Ionic Conductivity

The significance of Na⁺ (or Li⁺) charge carriers’ transport in IL-based electrolytes was highlighted [8]. Ionic conductivity primarily stems from the IL cations and anions, rather than the added Na-salt [8]. The addition of Li-salt or Na-salt to IL-based electrolytes leads to a progressive decrease in ionic conductivity [40,117]. However, electrolytes such as pure Pyr₁₃TFSI and 1 M LiTFSI in BMImTFSI exhibit comparable ionic conductivities, emphasizing the IL matrices’ substantial role.

Adding 10–20% IL to hybrid electrolytes (0.8 M NaTFSI in EC_x:PC_x:CatTFSI_(1−2x)) slightly increases ionic conductivity and viscosity. Yet, beyond 50% IL content, ionic conductivity diminishes, likely due to heightened viscosities. Monti et al. further point out the temperature-dependent ionic conductivity in both organic and IL-based electrolytes, driven by viscosity variation at different temperatures. A very high viscosity can lead to over-potential, probably delaying redox reaction limits and influencing sodium plating in an IL-based electrolyte due to future concerns and applications.

The addition of NaFSI significantly reduces the ionic conductivity, particularly at room temperature, with a gradual decline at high temperatures [118]. For instance, at 298 K, the conductivity of pure C₃mpyrFSI at 5.7 mS·cm⁻¹ decreases to 1.75 mS·cm⁻¹ and 0.62 mS·cm⁻¹ for Na_0.20[C₃mpyr]_0.80[FSI] and Na_0.55[C₃mpyr]_0.45[FSI], respectively. This is attributed to enhanced viscosity due to ion-ion interactions and cluster formation at higher salt concentrations. Nevertheless, the ionic conductivity of Na_0.20[C₃mpyr]_0.80[FSI] increases with temperature, reaching 9.2 mS·cm⁻¹ at 80 °C. The addition of EC to Na_0.55[C₃mpyr]_0.45[FSI] electrolytes significantly enhances conductivity, with 30 wt.% EC exceeding the conductivity of pure IL. EC promotes ion dissociation and increases the number of mobile carriers, while its oxygen’s lone pair facilitates coordination with metal cations [118]. The significant enhancement in conductivity upon EC addition (e.g., 30 wt.% EC exceeding pure IL conductivity) highlights the role of organic solvents in promoting ion dissociation and increasing mobile carrier concentration.

Manohar et al. [9] studied four kinds of electrolytes: Organic, Hybrid-1, Hybrid-2, and Hybrid-3. Their composition is given in

Table 5. Ionic conductivity and electrolyte compositions.

Electrolyte	Composition	Ionic Conductivity	References
Organic	1 M NaFSI in EC:PC (1:1) v/v	5.5 mS·cm−1	[9]
Hybrid-1	1 M NaFSI in (EC:PC):C3mpyrTFSI (75:25) v/v	4.2 mS·cm−1	[9]
Hybrid-2	1 M NaFSI in (EC:PC):C3mpyrTFSI (50:50) v/v	3.2 mS·cm−1	[9]
Hybrid-3	1 M NaFSI in (EC:PC):C3mpyrTFSI (25:75) v/v	2.9 mS·cm−1	[9]
Ref 1	1 M Na[FSA]-PC	6.3 mS·cm−1	[11]
Ref 2	1 M Na [ClO4 + FSA]-PC	7 mS·cm−1	[11]
ILOL 0	1 M Na[ClO4]-PC	6.9 mS·cm−1	[11]
ILOL 50	1 M Na [FSA]-[C3C1pyrr] [FSA] + 1 M Na [ClO4]-PC (5:5, v/v)	9.7 mS·cm−1	[11]
ILOL 80	1 M Na [FSA]-[C3C1pyrr] [FSA] + 1 M Na [ClO4]-PC (8:2, v/v)	6.6 mS·cm−1	[11]
ILOL 100	1 M Na[FSA]-[C3C1pyrr][FSA]	5 mS·cm−1	[11]

.

The organic electrolyte’s ionic conductivity at room temperature is 5.5 mS·cm⁻¹ (Organic: 1 M NaFSI in EC:PC). As IL concentration increases, hybrid electrolytes’ conductivities decline (4.2, 3.2, and 2.9 mS·cm⁻¹ for Hybrid-1, Hybrid-2, and Hybrid-3, respectively), correlating with rising viscosity and stronger ion interactions.

Another study [11], assesses three organic electrolytes’ ionic conductivities at 303 K: ILOL 0 (1 M Na[ClO₄]-PC), Ref 1 (1 M Na[FSA]-PC), and Ref 2 (1 M Na[ClO₄ + FSA]-PC). These electrolytes exhibit similar conductivities (6.9, 6.3, and 7.0 mS·cm⁻¹, respectively), suggesting minimal anion type impact. Increasing IL concentration initially boosts conductivity, peaking at 50% IL (9.7 mS·cm⁻¹ for ILOL 50), but further increases cause a decline (6.6 mS·cm⁻¹ at 80% IL, 5 mS·cm⁻¹ at 100% IL), aligning with the cube-root law and indicating the need for a more complex theoretical approach at high concentrations (Table 5).

Complex electrolytes, including ILs in a wide temperature range from 20 °C to 80 °C, have been studied [10]. Underlined that ILs exhibit high viscosity, further increased by Na-salt addition, which reduces the ionic conductivity. In concentrated IL solutions, the strong ion-ion interactions affect conductivity significantly; therefore, a compromise should be found between IL concentration and electrolyte performance. Temperature changes become pivotal: viscosity decreases upon increasing temperature, and ionic mobility is enhanced according to Vogel–Tammann–Fulcher (VTF) behavior typical of supercooled liquids. The higher the IL content, the higher the viscosity and the more pronounced ion interactions, increasing the activation energy for conductivity. Nevertheless, it is still lower than that for pure ILs or IL-rich solutions, testifying to the complexity and optimization necessity of electrolyte composition for diverse applications.

6.1.3. The Significance of the Sodium-Ion Transference Number

The sodium-ion transference number t_Na⁺ is a fundamental transport property that defines the fraction of the total electrical current carried by sodium ions in an electrolyte. While ionic conductivity measures the overall movement of all ions, cations and anions, the transference number specifically quantifies the efficiency of the working ion transport. Its significance in the development of high-performance SIBs is multifaceted. In a typical liquid electrolyte, both the sodium cations and the corresponding anions, like TFSI⁻, FSI⁻, and PF₆⁻, are mobile. During discharge, Na⁺ ions move toward the cathode while anions move toward the anode. If t_Na⁺ is low, typically <0.4 for most liquid electrolytes, the anions carry the majority of the current, leading to a buildup of salt concentration at one electrode and depletion at the other. This concentration polarization creates an internal potential gradient that opposes the battery’s operation, resulting in increased overpotential and reduced voltage efficiency [119]. A high t_Na⁺ is essential for fast-charging and high-power applications. By ensuring that Na⁺ ions are the primary charge carriers, the electrolyte can maintain a more uniform salt concentration profile even at high current densities. This minimizes the transport-related bottlenecks, allowing the battery to maintain its capacity and deliver high power without premature voltage cutoff [98]. In sodium-metal batteries, the transference number plays a critical role in interfacial stability. A low t_Na⁺ leads to a large concentration gradient near the metal surface, which can trigger the onset of Sande’s time, the point at which the ion concentration at the electrode surface drops to zero, leading to unstable, dendritic sodium growth. Increasing t_Na⁺ helps maintain a steady supply of Na⁺ ions at the interface, promoting uniform plating and extending the cycle life of the battery [120].

In hybrid electrolytes, the interaction between ILs (ILs) and organic solvents can be tuned to optimize t_Na⁺. While pure ILs often exhibit low t_Na⁺ due to the high mobility of the IL anions, the addition of organic co-solvents or the use of superconcentrated (salt-in-IL) strategies can significantly enhance t_Na⁺. For instance, increasing the sodium salt concentration in certain IL systems has been shown to increase t_Na⁺ from ~0.1 to over 0.3 by promoting the formation of Na⁺ anion clusters that move more efficiently than free anions [121]. Similarly, incorporating EC into mixed-solvent electrolytes is known to increase the cation transference number because EC strongly coordinates with Na⁺ and simultaneously reduces anion mobility. Its high dielectric constant stabilizes Na⁺ in a tight solvation shell, while EC anion interactions slow anion diffusion, decreasing the anionic contribution to total current. As a result, a larger fraction of the ionic current is carried by Na⁺, leading to an overall increase in t_Na⁺ [122].

6.1.4. Chemical Stability

Raman spectroscopy is important in identifying the species of electrolytes and studying the stability and quantity of Na⁺ complexes in hybrid electrolytes. The Raman spectra of electrolytes with sodium bis(trifluoromethane)sulfonimide (NaTFSI) and mixed with increasing concentrations of EC, PC, and two varieties of ILs, such as BMImTFSI and Pyr₁₃TFSI, were presented by Monti et al. [8], also showing the effect of ignition. Raman spectroscopy after burning showed that the EC and PC solvents were partly consumed, while the IL cations were stable.

The high thermal stability of TFSI⁻ and FSI⁻ anions, as noted by Domingues et al. [5], is a key contributor to the overall enhanced thermal performance of hybrid electrolytes. This stability is not only intrinsic to the anion but also plays a crucial role in the formation of a robust and thermally stable SEI layer, as detailed in Section 3.3.3.

The research also conducts a detailed study of the chemical interactions occurring within ILs and IL-gel systems by Fourier Transform Infra-Red spectroscopy. Changes in the FTIR bands assigned to the FSI anion and the ether groups from ethylene carbonate are considered. Pronounced shifts and broadening of FTIR bands were observed upon the addition of NaFSI to the IL:EC mix, indicating the complexation of Na⁺, FSI⁻, and EC. A spectral comparison of pure C3mpyrFSI, pure EC, IL:EC, Na_0.55[C₃mpyr]_0.45[FSI], Na_0.55[C₃mpyr]_0.45[FSI] + 30 wt.% EC, and Na_0.55[C₃mpyr]_0.45[FSI] + 30 wt.% EC gel presented different spectral patterns that indicate the dynamical nature of these interactions [118]. Interestingly, the addition of SiO₂ to the gelled electrolytes did not alter the FTIR bands, suggesting the formation of a unique physical gel state. This thorough investigation sheds light on specific shifts and interactions, reinforcing the robustness of these phenomena and offering valuable insights for the development and comprehension of IL-based electrolytes and gels.

6.1.5. Mathematical Correlations Between Ionic Conductivity, Viscosity, and Temperature

The transport behavior of liquid and hybrid electrolytes is defined by physicochemical relationships that connect ionic conductivity, viscosity, and temperature. For simple liquid electrolytes, the temperature dependence of ionic conductivity σ(T) is typically described by an Arrhenius-type expression.

$$\mathrm{\sigma }={\mathrm{\sigma }}_0\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}})$$

where E_a represents the activation energy for ion migration. This model applies when ion motion is dominated by thermally activated hopping. In systems where ion transport is strongly coupled to the structural relaxation of the medium, such as polymer electrolytes, ILs, and highly concentrated solutions, the Vogel–Tammann–Fulcher (VTF) equation provides a more accurate description:

$$\mathrm{\sigma }={\mathrm{\sigma }}_0{\mathrm{T}}^{1/2}\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{\mathrm{B}}{\mathrm{T}-{\mathrm{T}}_0}})$$

With B reflecting the pseudo activation energy and T₀ corresponding to the ideal glass-transition temperature. Viscosity $\mathrm{\eta }\left(\mathrm{T}\right)$ follows analogous Arrhenius or VTF behavior, reflecting the thermally activated nature of viscous flow and segmental dynamics.

The inverse relationship between ionic conductivity and viscosity is classically captured by the Walden rule,

$$\mathrm{\Lambda }{\mathrm{\eta }}^{\mathrm{\alpha }}=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t},$$

where $\mathrm{\Lambda }$ is the molar conductivity and $\mathrm{\alpha }\approx 1$ for ideal, fully dissociated electrolytes. Deviations from the ideal Walden line provide insight into ion pairing, aggregation, or structural organization within the electrolyte. Together, these mathematical correlations offer a robust framework for interpreting how temperature and fluidity govern ion mobility and overall electrolyte performance [59,123].

6.2. Electrochemical Tests

6.2.1. Cyclic Voltammetry

In this section, we underline the importance of cationic transport number evaluation for electrolyte performance. Current variations with time, as observed for the Na_0.55[C₃mpyr]_0.45[FSI] electrolyte by the DC polarization technique at ambient temperature, stabilized after 1.5 h [118]. The Na⁺ transference number at room temperature (∼298 K) was 0.11, lower than the values reported at higher temperatures [124,125]. Indeed, even for such high salt concentration electrolytes at 323 K, the addition of ethylene carbonate increased the transference number up to 0.32, indicative of important cationic mobility.

In such a context, the Na_x[C₃mpyr]_1−x[FSI] system was investigated on symmetrical cells of sodium metal electrodes. Accordingly, in stable deposition and stripping currents to 250 μA·cm⁻², the failure due to dendrite formation occurred only in the 16th cycle at the highest current density [118].

The cyclic voltammograms of Na_0.55[C₃mpyr]_0.45[FSI] + 30 wt.% EC presented the reversible deposition of sodium at an overpotential as low as −0.03 V vs. Na/Na⁺. The asymmetric cells using sodium and Na₃V₂(PO₄)₃ electrodes were further used to evaluate the potential of the electrolyte in sodium electrochemical devices. First, irreversible capacity was observed, which could be attributed to the formation of a stable SEI layer on the electrode of sodium metal. This suggests the suitability of the electrolyte for application in sodium-ion batteries, with the potential for enhanced cycle life and energy density [118].

Cyclic voltammetry was utilized to explore the electrochemical stability window and to evaluate the viability of sodium metal plating and stripping within a range of electrolytes [8]. The investigation focused on the reduction stability of pure IL-based electrolytes.

The observed low current densities at lower potentials indicate high reductive stability and the successful formation of a passivating SEI layer, which effectively suppresses further electrolyte decomposition. SEI is a crucial part of the diffusion of Na⁺ ions, which is the first step for sodium deposition and plating to occur. Furthermore, ILs containing the TFSI anion were found to improve the corrosion resistance of aluminum electrodes, thereby increasing the durability and safety of the electrolytes.

For hybrid electrolytes with a high IL concentration, the CV outcomes mirrored those of pure ILs, both in terms of ESW and the absence of sodium plating/stripping. The lack of reversible sodium deposition is postulated to be influenced by the TFSI anion, as inferred from prior studies [126] that demonstrated the facilitation of reversible Li⁺ and Na⁺ deposition by FSI-based IL electrolytes, suggesting that the TFSI anion’s characteristics may impede this process. Hybrid electrolytes that include organic solvents show superior performance in sodium plating/stripping, probably related to the distinct SEI composition that promotes efficient sodium ion transit. Among all of the hybrid electrolytes examined, the combination containing 0.8 M NaTFSI in EC_0.45:PC_0.45:Pyr₁₃TFSI_0.10 came out as most promising, displaying a positive onset potential and Coulombic efficiency. This composition deserves further investigation for its potential integration into sodium metal electrode systems [8].

In their work [9], Manohar et al. conducted a comparative study using four different types of electrolytes: Organic, Hybrid-1, Hybrid-2, and Hybrid-3 (as shown in Table 3).

The composition of the different electrolytes varied from 1 M NaFSI in EC:PC (1:1) v/v for the organic electrolyte to several hybrid versions with ratios of EC:PC and C3mpyrTFSI. Employing CV, the team assessed the electrochemical oxidative stability of these electrolytes in contact with sodium metal. The results indicated that the hybrid electrolytes outperformed the organic electrolyte in terms of oxidative stability, with a higher concentration of IL within the hybrids correlating to greater stability. Among them, Hybrid-3 exhibited the best stability, up to 5.1 V versus Na⁺/Na, followed by Hybrid-2 and Hybrid-1. Sodium vanadium phosphate (NVP) nanoparticles embedded in a carbon matrix (NVP@C) were also investigated, a composite material used in the anodes of SIB. The behavior of hybrid and organic electrolytes was different in the presence of NVP@C. For those electrolytes, the oxidation and reduction reactions occurred at about 3.44 ± 0.02 V and 3.22 ± 0.02 V versus Na/Na⁺, respectively. In particular, a sharp peak of the Hybrid-2 electrolyte emerged with NVP@C, indicating a more stable passivation film formed on the cathode. Subsequently, such an electrolyte provided a relatively stable cycling performance with discharge capacity in the wide current rate range from 0.2 C to 5 C between 115 and 100 mAh·g⁻¹. Besides, after cycling, it regained its initial capacity of 115 mAh·g⁻¹ at 0.2 C, showing great retention in energy storage capacity. In the case of NVP@C, using the organic electrolyte, its capacity significantly decreased with the increase in discharge rates to 95 mAh·g⁻¹. This represents the superiority of the Hybrid-2 electrolyte in maintaining operability under high-speed conditions without degradation, which is not evidenced by its organic counterpart. The results underline the potential of hybrid electrolytes, especially Hybrid-2, toward SIBs with better performance and cycle life [9].

CV was performed with a three-electrode configuration employing Pt and GC, which were used separately as working electrodes, while Na metal was used both as counter and reference electrodes [11]. For the ILOL 50 system, anodic limits estimated by threshold current density of 0.1 mA·cm⁻² were 5.7 V for the GC electrode and 5.5 V for the Pt electrode. This trend in anodic limits was consistent with the GC electrode in both ILOL 0 and ILOL 100 systems. Notably, in the ILOL 50 experiments, the copper (Cu) electrode showed efficient sodium metal deposition and dissolution, achieving a high Coulombic efficiency of 83% for these processes. These findings underscore the electrolyte system’s potential for stable electrochemical performance and effective sodium management within the specified experimental framework.

The electrochemical stability of electrolytes is also important to secure the performance and cycle life of sodium-ion batteries. Mixtures containing IL EMI-TFSI and an organic solvent blend of EC-PC resulted in a wider electrochemical stability window that was able to allow the application of various electrode materials. High IL content, however, created serious irreversible capacity in the first discharge cycle due to SEI layer formation on the anode [10]. The study underscores the nuanced balance needed in electrolyte composition, as evidenced by the decrease in oxidation current and the increase in oxidation potential by over 50 mV at 0.02 mA·cm⁻² with rising IL content. Due to high IL concentrations, stabilization of the SEI layer in sodium-ion batteries is more difficult in comparison with lithium systems, and such IL results in low cathodic stability and increased viscosity, impeding diffusion of sodium ions. Consequently, the optimum electrolyte composition identified was a composition of EC-PC, 20 wt.% IL, 2 wt.% FEC, and 1 M NaTFSI; about 410 mAh·g⁻¹ capacity was achieved with it. Hard carbon half-cells in the same composition showed stable discharge capacities of about 300 mAh·g⁻¹ under similar conditions [10]. In cycling tests, half-cells with Na_0.44MnO₂ cathode material using EMI-TFSI-based electrolytes displayed a performance comparable to those with carbonate-based electrolytes at the peak discharge capacity of 92 mAh·g⁻¹. These results have shed light on the puzzling relationship between electrolyte composition and electrochemical stability while providing a road map for refining sodium-ion battery formulations toward high performance and a long cycle life [10].

The wide electrochemical stability window observed in hybrid electrolytes, e.g., 4–5 V for piperidinium-based ILs [94], is a direct consequence of the inherent stability of the IL components and their ability to form a protective SEI layer on the electrode surfaces. This stable SEI prevents further electrolyte decomposition, thereby extending the operational voltage range. The specific composition of the SEI, which can be probed by techniques like XPS, is critical in determining the ultimate electrochemical stability. As we can see in

Table 6. Electrochemical performance metrics of key hybrid systems.

Electrolyte System	Composition Focus	Ionic Conductivity (mS·cm−1)	Stability Window (V vs. Na/Na+)	Capacity Metric (Cathode/Anode)	Key Performance Feature
Pyrrolidinium Hybrid	EC:PC + 20% Py14TFSI	15.1	0–6.1	-	Ultra-high conductivity matching aqueous systems; wide voltage window.
Pyrrolidinium Hybrid	EC:PC + 50% C3mpyrTFSI	3.2	~5.2	115 mAh·g−1 (NVP@C)	Excellent capacity retention (95% after 100 cycles); stable CEI formation.
Imidazolium Hybrid	EC:PC + 20% EMIm-TFSI + FEC	-	~5.5	410 mAh·g−1 (Anode)	High capacity enabled by FEC-derived SEI; improved safety over pure organic.
Standard Organic	NaPF6 in EC:PC	6–12	2.5–4.2	-	High conductivity but limited voltage and poor thermal safety.

, the electrochemical performance of hybrid electrolytes has shown.

6.2.2. Galvanostatic Cycling with Potential Limitation (GCPL)

In this regard, the electrochemical behavior of a hybrid electrolyte (0.8 M NaTFSI in EC_0.45:PC_0.45:Pyr₁₃TFSI_0.10) coupled with a hard carbon (HC) electrode within a C-rate window from C/10 to 2 C was investigated [8], and the potential vs. capacity profiles showed a linear decay followed by a pseudo-plateau at approximately 50 mV versus Na⁺/Na, indicating Na⁺ insertion between graphene layers and adsorption in HC pores. The first cycles present a large irreversible capacity, as the Coulombic efficiency amounts to 40%, which rises to 96% and reaches a stable value of 182 mAh·g⁻¹ at C/10 after prolonged SEI growth. However, hybrid electrolytes exhibit lower values of CEs, pointing out less effective HC passivation with respect to pure organic solvent-based electrolytes, which encourages further SEI studies. Increasing C-rates lead to lower capacities because of IR drop effects [127]. In spite of all these challenges, the hybrid electrolyte still retains considerable capacity: a specific capacity of 182 mAh·g⁻¹ at C/10 after 40 cycles, which is in agreement with literature values for organic electrolytes [7,128].

The enhanced cycling stability and high coulombic efficiency observed in hybrid electrolytes are directly linked to the formation of a stable and robust SEI layer. This stable SEI minimizes electrolyte consumption during cycling and prevents dendrite growth, leading to improved long-term performance. Furthermore, the optimized solvation structure in hybrid electrolytes can facilitate more efficient Na⁺ transport kinetics, contributing to better rate capability and overall cycling performance.

The cyclic performance of NVP@C, a battery material, has been investigated with organic and hybrid electrolytes at 0.5 C up to 100 cycles [9]. In such cases, a discharge capacity of 112 ± 3 mAh·g⁻¹ was retained for hybrid electrolytes, whereas only 87% was retained for the organic electrolyte. Thus, hybrid electrolytes with higher IL content delivered higher capacities and 99% Coulombic efficiency that guaranteed more stable battery cycling than organic electrolytes.

Galvanostatic tests on a HC/NaCrO₂ (NCO) full cell at 298 K were recently performed [11]. Charge-discharge curves of the third cycle showed Cr (III)/Cr (IV) redox activities within NCO and reversible Na⁺ insertion/extraction in HC/NCO cells, with an average operational voltage of 2.8 V. The hybrid ILOL systems (combination shown in Table 5) outperformed ILOL 0, with a capacity of 66.3 mAh·g⁻¹ but failed to reach the highest reversible capacity of ILOL 100, at 78.2 mAh·g⁻¹. Capacities decreased with the increase of IL concentrations: ILOL 20, ILOL 50, and ILOL 80 reached 74.0, 73.8, and 71.1 mAh·g⁻¹, respectively. Cyclic performance of ILOL systems in an HC/NCO full cell was performed at 100 mA·g⁻¹ and 298 K. The capacity retention and Coulombic efficiency of ILOL 20 and ILOL 50 were better than those of nonhybrid systems. Even after 700 cycles, they remained at a capacity retention of about 55%, with Coulombic efficiencies higher than 99%. Correspondingly, the energy efficiency at the 700th cycle was 95.4% for ILOL 50. Among those, ILOL 50 delivered the starting capacity of 79.5 mA·g⁻¹ at 363 K, with fast decay, and maintained capacities of 62.5% after 200 cycles and 48.4% after 500 cycles, hence indicating well the performance at an elevated temperature with a fast drop as the cycles progressed over time [11].

6.2.3. EIS

Electrochemical impedance spectroscopy (EIS) was used to evaluate the performance of sodium-based batteries with varying electrolyte compositions [9]. The findings indicate that electrolytes with higher percentages of ILs facilitate the creation of stable SEI layers. This results in a reduction of charge transfer resistance, which is crucial for enhancing cell longevity and maintaining battery stability. In contrast, batteries with organic electrolytes exhibited increased charge transfer resistance, making them more susceptible to capacity loss over time.

To complement the EIS data, thermogravimetric (TG) measurements were conducted to assess the thermal stability of the electrolytes, and a marked improvement in thermal stability was observed with increasing IL content. The organic liquid electrolyte, denoted as ILOL 0, began to lose weight at 333 K, primarily due to the evaporation of the PC solvent. Conversely, the IL-rich electrolyte (ILOL 100) displayed minimal weight loss below 550 K, with thermal decomposition initiating at approximately 600 K. These results suggest that higher IL ratios not only suppress PC evaporation but also contribute to reduced flammability, enhancing the safety profile of the electrolytes.

The integration of EIS and TG analyses provides a comprehensive understanding of how IL content influences both the electrochemical and thermal behaviors of sodium-based batteries. This knowledge is instrumental in designing hybrid electrolytes that offer improved performance and safety for advanced energy storage applications.

The low and stable interfacial resistance observed in the EIS measurements in Figure 5d provides further evidence for the formation of a stable and ionically conductive SEI layer in the presence of hybrid electrolytes. The composition and morphology of this SEI, which can be further investigated using techniques like XPS, are critical for maintaining low interfacial impedance and ensuring long-term cycling stability.

6.3. DSC–TGA

The thermal behavior of electrolytes composed of ILs and sodium salts was meticulously examined in study [118]. The research utilized differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to analyze two electrolyte formulations: Na_0.55[C₃mpyr]_0.45[FSI] and Na_0.55[C₃mpyr]_0.45[FSI] + 30 wt.% EC.

The DSC results revealed two solid–solid transitions and a melting point for pure C₃mpyrFSI, an IL with a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion. Notably, the liquid phase underwent significant supercooling, resulting in a crystallization temperature lower than the melting point. The addition of 55 mol% NaFSI prevented C₃mpyrFSI crystallization, forming a glassy solid at low temperatures with a glass transition temperature (Tg) of −80 °C, indicating strong sodium ion–anion interactions that altered the thermal behavior.

The presence of silica nanoparticles did not influence the thermal properties of the Na_0.55[C₃mpyr]_0.45[FSI] gel electrolyte, suggesting that silica served solely as a structural component. The incorporation of EC as a plasticizer reduced the Tg from −80 °C to −84 °C, modestly broadening the operational temperature range and enhancing ionic conductivity.

TGA assessments demonstrated that both electrolyte types were thermally stable up to 317 °C, surpassing the decomposition temperature of NaFSI (144 °C). This stability suggests that the IL and the salt form a complex resistance to thermal degradation, contributing to a safer battery design. The thermal stability of organic and hybrid electrolytes was compared through TGA [9], and the organic electrolyte experienced significant mass loss (>75%) at 250 °C due to solvent evaporation. In contrast, hybrid electrolytes () exhibited lower mass losses (55%, 27%, and 12% for Hybrid-1, -2, and -3, respectively), indicating enhanced thermal stability.

Electrolyte mixtures containing EMI-TFSI and EC-PC were evaluated for their thermal properties. EMI-TFSI, known for its low melting point (−18.5 °C) and high thermal stability (decomposition at 358 °C), improved the thermal stability and oxidation potential of EC-PC mixtures without compromising ionic conductivity. The TGA analysis identified two decomposition stages, with EMI-TFSI-EC mixtures starting to evaporate above 170 °C, which is higher than pure EC or EC-PC mixtures (120–130 °C). Mixtures with 50 wt.% EMI-TFSI exhibited the highest evaporation temperature and lowest weight loss, maintaining stability up to 350 °C. These findings affirm the role of EC in enhancing EMI-TFSI’s thermal stability, underscoring its potential to improve battery safety [10].

The DSC-TGA results presented by Noor et al. (Figure 6) clearly demonstrate the superior thermal stability of the hybrid electrolytes compared to the pure organic electrolyte [118].

6.4. Safety Tests

The flammability characteristics of electrolytes in energy storage applications are related to the safety considerations of such systems. In this section, the flammability point (FT), ignition time (IT), and self-extinguishing time (SET) of the different IL content mixed electrolytes have been studied.

With increasing amounts of ILs in the mix, the FP increases, which refers to the temperature a chemical must have to produce enough vapor to ignite in air. For instance, a 20% IL composition like BMImTFSI and Pyr₁₃TFSI in an EC:PC electrolyte has a rather low FP of about 140 °C but increases to as high as 180 °C at 80%. This therefore suggests that with increasing ILs, it bears a higher contribution towards lowering vaporization and decomposition of organic solvents and hence lowers the risk of ignition.

IT, or the time taken for ignition of a substance, was apparently much longer in the case of electrolytes with higher IL content. In fact, no ignition was possible in the case of an electrolyte that contained more than 80% IL, indicating its protective effect against ignition. The SET, or the time it takes for a material to stop burning after being ignited, decreased with an increase in the IL content. Interestingly, the cation type of the IL had no significant impact on the IT, which can be interpreted as the overall IL percentage being more critical to prevent ignition rather than the cation type itself [8].

The hybrid electrolyte with the highest IL ratio studied hereinafter, termed ILOL 100 (Table 5), showed completely inhibited flammability, whereas pure organic liquid electrolyte ILOL 0 was highly flammable [11]. Systems with 30% or less IL ignited in a very short time after fire exposure, while systems with 40% or more IL did not ignite even after preheating to 353 K. These results underline the role of IL concentration in improving ignition resistance under various thermal conditions, which is a critical aspect for ensuring safe operation of batteries [11]. The observations are further supported by Vo et al. [10], who stated that IL may combust if EC-PC vapors are ignited. No carbon residue was observed on glass fiber mats when only the combustion of EC-PC vapors occurred, showing complete combustion. However, the mats covered by IL-based electrolytes showed carbon residues, which increased with IL concentration. The surprising phenomenon is that the organic electrolyte vapors continuously burned even after turning off the burner at elevated temperatures, supporting the combustion process.

The improved safety performance observed in our flammability tests, with longer ignition time, showed that self-extinguishing behavior is a direct consequence of the reduced flammability of the hybrid electrolyte, as explained in Section 3.3.3. The dilution effect of the non-flammable IL in Section 3.3.4 also plays a significant role in mitigating the fire risk [11]. In

Table 7. Thermal and safety characteristics of hybrid formulations.

Electrolyte System	IL Content (vol. %)	Flash Point (°C)	Thermal Stability Limit (TGA)	Flammability/SET
Standard Organic (EC:PC)	0%	~35 (mixture)	~150 °C (evaporation)	Highly flammable
Low-Conc Hybrid	10–20%	~140	>200 °C	Reduced flammability; measurable SET.
Balanced Hybrid	40–50%	>160	>250 °C	Self-extinguishing (SET ≈ 0 s); no sustained flame.
High-Conc Hybrid	>80%	None (Non-flammable)	>350 °C	Non-flammable; essentially zero vapor pressure.

, we show the thermal and safety characteristics of hybrid electrolytes.

In summary, a comprehensive analysis of hybrid electrolytes shows that their physical, chemical, and electrochemical properties are interconnected. Adding ILs to organic solvents generally improves thermal stability and safety. Systems with more than 40–50% IL show significant resistance to ignition and reduced flammability. However, this safety improvement is often accompanied by increased viscosity and decreased ionic conductivity, particularly at room temperature. Spectroscopic studies show that organic solvents, such as EC, play an important role in enhancing ion dissociation and increasing the concentration of mobile Na⁺ carriers. This can partially mitigate the limitations on mobility caused by viscosity. Furthermore, switching from pure ILs to hybrid systems improves the sodium-ion transference number. This mitigates polarization and enables more stable cycling at higher current densities. Ultimately, the optimal hybrid electrolyte composition needs a balance of providing enough IL to ensure nonflammability and a stable SEI while maintaining sufficient organic solvent to ensure the high ionic mobility required for practical power densities.

7. Perspectives, Limitations, and Future Pathways

7.1. Opinions on the Current State of Hybrid Electrolytes

Based on the extensive literature review and analysis of recent advancements, it is our considered opinion that hybrid electrolytes represent a pivotal breakthrough in the development of high-performance and safe SIBs. The synergistic combination of ILs and organic solvents offers a unique pathway to overcome the inherent limitations of individual electrolyte components. While organic liquid electrolytes provide high ionic conductivity and good wettability, their flammability and narrow electrochemical stability windows pose significant safety concerns. Conversely, ILs offer excellent thermal stability, non-flammability, and wide electrochemical windows, but often suffer from high viscosity and relatively lower ionic conductivity at room temperature. Hybridizing these two classes of electrolytes allows for a judicious balance of these properties, leading to systems that exhibit enhanced safety without significantly compromising electrochemical performance.

We believe that the true potential of hybrid electrolytes lies in their tunability. By carefully controlling the ratio of ILs to organic solvents, and by selecting appropriate chemistries for both components, it is possible to tailor the electrolyte properties to meet the specific demands of various SIB applications, ranging from grid-scale energy storage to electric vehicles. The ongoing research into novel IL chemistries, functional additives, and advanced electrolyte designs further underscores the dynamic and promising nature of this field. However, realizing the full commercial potential of hybrid electrolytes necessitates a concerted effort to address several critical challenges.

7.2. Key Challenges in Hybrid Electrolyte Development

Despite significant progress, several challenges must be overcome to facilitate the widespread adoption of hybrid electrolytes in SIBs.

7.2.1. Optimization of IL/Organic Solvent Ratio and Component Selection

Determining the optimal ratio of ILs to organic solvents is crucial for balancing performance and safety. A higher IL content generally improves thermal stability and safety but can lead to increased viscosity and reduced ionic conductivity, especially at lower temperatures. Conversely, a higher organic solvent content enhances ionic conductivity but compromises safety. The selection of specific ILs and organic solvents is also critical, as their chemical compatibility and interaction mechanisms directly influence the overall electrolyte properties. Developing predictive models and high-throughput screening methods to rapidly identify optimal combinations remains a significant challenge.

7.2.2. Interfacial Stability and SEI Formation

The stability of the electrode–electrolyte interface, particularly the formation of a robust and stable SEI on the anode, is paramount for long-term cycling performance and safety. While ILs can contribute to a more stable SEI, the complex interplay between ILs, organic solvents, and electrode materials can lead to heterogeneous SEI formation, which may degrade over extended cycling. Understanding and controlling the SEI formation mechanism in hybrid electrolytes, especially under various operating conditions (e.g., high voltage, low temperature), is a persistent challenge. In situ characterization techniques are vital for unraveling these complex interfacial phenomena.

7.2.3. Dendrite Formation and Suppression

Sodium dendrite growth on the anode during plating/stripping cycles remains a critical safety concern, leading to short circuits and thermal runaway. While some hybrid electrolytes have shown promise in suppressing dendrite formation due to their unique solvation structures and improved SEI, a universal solution is yet to be found. Strategies involving localized high-concentration electrolytes, functional additives, and solid-state components within hybrid systems are being explored, but a more fundamental understanding of dendrite nucleation and growth in these complex media is required.

7.2.4. Cost-Effectiveness and Scalability

The relatively high cost of some ILs compared to conventional organic solvents can be a barrier to large-scale commercialization. While the enhanced safety and performance benefits may offset some of these costs, developing more cost-effective IL synthesis routes and efficient recycling methods is essential. Furthermore, scaling up the production of hybrid electrolytes while maintaining consistent quality and performance presents manufacturing challenges that need to be addressed.

7.2.5. Low-Temperature Performance

Many hybrid electrolytes, particularly those with high IL content, exhibit increased viscosity and reduced ionic conductivity at low temperatures, limiting their practical application in cold climates. Developing hybrid systems that maintain high performance across a wide temperature range without compromising safety or stability is a crucial area for future research.

7.3. Future Research Directions and Outlook

To overcome the aforementioned challenges and fully unlock the potential of hybrid electrolytes for SIBs, we propose several promising future research directions:

7.3.1. Rational Design Through Advanced Modeling and AI

The future of hybrid electrolyte development will increasingly rely on rational design principles guided by advanced computational modeling and artificial intelligence (AI). Machine learning algorithms can be trained on vast datasets of electrolyte properties to predict optimal compositions and identify novel IL/solvent combinations. MD simulations and DFT calculations will continue to provide fundamental insights into molecular interactions, ion transport, and interfacial reactions, enabling the design of electrolytes with tailored properties before extensive experimental synthesis.

7.3.2. Development of Novel IL Chemistries and Functional Additives

Continued exploration of new IL chemistries with improved properties (e.g., lower viscosity, higher ionic conductivity, wider electrochemical windows, lower cost) is essential. This includes designing ILs with specific functional groups that can participate in favorable interactions with organic solvents or contribute to a more stable SEI. The development of multi-functional additives that can simultaneously address issues like dendrite suppression, gas evolution, and interfacial stability will also be critical.

7.3.3. In Situ and Operando Characterization

To gain a real-time understanding of the dynamic processes occurring within hybrid electrolytes and at the electrode–electrolyte interface, increased emphasis should be placed on in-situ and operando characterization techniques. Techniques such as in situ Raman, FTIR, XPS, and EIS can provide invaluable insights into SEI evolution, ion solvation changes, and degradation mechanisms during battery operation, guiding the rational design of more stable and efficient systems.

7.3.4. Sustainable and Environmentally Friendly Electrolytes

As the demand for energy storage grows, the environmental impact of battery components becomes increasingly important. Future research should focus on developing hybrid electrolytes from sustainable and renewable resources, minimizing the use of toxic or hazardous materials, and exploring efficient recycling strategies for both ILs and organic solvents. This aligns with the broader goal of developing a circular economy for battery technologies.

7.3.5. Integration with Advanced Electrode Materials

The performance of hybrid electrolytes is intrinsically linked to the electrode materials. Future research should focus on the co-design of hybrid electrolytes with advanced anode and cathode materials for SIBs. This includes optimizing electrolyte formulations for specific electrode chemistries (e.g., hard carbon, Prussian blue analogs, layered oxides) to ensure optimal interfacial compatibility, reduce parasitic reactions, and maximize overall battery performance and lifespan.

To summarize, hybrid electrolytes provide an appropriate choice for the upcoming generation of SIBs. However, a comprehensive strategy using fundamental expertise, advanced characterization, computational modeling, and creative material design is needed for their future growth. We will be able to understand the potential of hybrid electrolytes and develop safer, more effective, and sustainable SIB technologies by solving the problems and investigating these research opportunities. This strategic roadmap for SIB development in the future is shown in Figure 7. It links present hybrid electrolyte research to cutting-edge technologies like artificial intelligence (AI), machine learning, and in situ characterization, which lead to sustainable solutions and widespread uses in consumer electronics, grid storage, and electric vehicles.

8. Conclusions

Hybrid electrolytes, formed by the synergistic combination of ILs and organic solvents, have emerged as a highly promising strategy for addressing the critical challenges of safety and performance in sodium-ion batteries. This review highlighted how the unique properties of hybrid electrolytes, including enhanced thermal stability, wider electrochemical windows, and improved SEI formation, can effectively mitigate the flammability and dissolution issues associated with conventional organic electrolytes, while simultaneously improving ionic conductivity and cycling performance.

While the initial cost of ILs has been a significant consideration, their strategic integration into hybrid electrolytes presents a compelling case for cost-effectiveness. As discussed in this review, enhanced safety features can reduce the need for expensive battery management and cooling systems, while the extended cycle life and improved performance can lower the overall cost of ownership by reducing the frequency of battery replacement.

The successful commercialization of hybrid electrolytes hinges on addressing key scalability challenges, including the large-scale production of ILs from sustainable resources and the optimization of manufacturing processes to ensure consistent quality. However, significant opportunities exist in developing continuous flow synthesis methods and establishing efficient recycling protocols, which will be crucial for the long-term economic and environmental sustainability of SIBs.

The enhanced safety and thermal stability of hybrid electrolytes make them particularly well-suited for large-scale applications such as grid energy storage, where safety is paramount. Furthermore, as technology matures, SIBs with hybrid electrolytes could offer a viable and more sustainable alternative to LIBs in electric vehicles and consumer electronics, contributing to a more diversified and resilient energy landscape.

Looking ahead, the future development of hybrid electrolytes will require a multi-faceted approach. As detailed in “Perspectives, Limitations, and Future Pathways”, continued research is needed in several key areas, including the rational design of novel ILs through advanced computational modeling, the development of functional additives and polymer-based systems, and the use of in-situ/operando characterization techniques to gain a deeper understanding of the complex interfacial phenomena at play.

In conclusion, hybrid electrolytes represent a critical enabling technology for the next generation of safe, high-performance, and sustainable sodium-ion batteries. By bridging the gap between the high performance of organic electrolytes and the exceptional safety of ILs, they offer a clear pathway towards the widespread adoption of SIBs in a variety of energy storage applications, from grid-scale systems to electric vehicles. The continued exploration of the rich design space of hybrid electrolytes, guided by advanced characterization and computational modeling, will undoubtedly unlock their full potential and contribute to a more sustainable energy future.