Home Batteries, Supercapacitors & Fuel CellsRole of Advanced Electrolytes in Sodium Ion Batteries

From Research to Application: The Role of Advanced Electrolytes in Sodium-Ion Batteries

Section Overview

Introduction
Sodium-Ion Battery Electrolyte
Ionic Conductivity of Sodium Ion Battery Electrolyte
Electrochemical Stability Window
SEI Formation
Materials for Sodium-Ion Battery Electrolytes
Conclusion
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Introduction

Sodium-ion batteries (SIBs) are emerging as a compelling alternative to lithium-ion batteries (LIBs), primarily due to the abundant availability and low cost of sodium resources. As the demand for energy storage solutions continues to grow,¹ SIBs offer a promising pathway to meet energy storage needs while addressing concerns related to lithium supply and environmental impacts.² With advancements in materials science, SIB technology is rapidly evolving, particularly through the development of high-performance electrolytes that enhance the efficiency and longevity of these systems. In this article, we will discuss the fundamental science of electrolytes and then dive into the advantages and tradeoffs of key materials.

Sodium-Ion Battery Electrolyte

Electrolytes play a pivotal role in the performance of sodium-ion batteries (SIBs). They serve as the medium through which sodium ions travel between the cathode and anode during the charge and discharge cycles. A sodium-ion battery electrolyte is composed of a sodium salt, a solvent or solvent mixture, and additives. The choice of electrolyte significantly impacts material properties such as ionic conductivity, electrochemical stability window, and formation of a solid-electrolyte interphase (SEI), which in turn affects critical performance metrics like cyclability, performance during fast-charging, operable temperature window, and energy density.³

Ionic Conductivity of Sodium Ion Battery Electrolyte

A line graph depicting the ionic conductivity of sodium-ion electrolytes as a function of salt concentration. The X-axis represents salt concentration in moles per liter (M), ranging from 0 to 2.0, while the Y-axis represents ionic conductivity in millisiemens per centimeter (mS/cm), ranging from 0 to 15. Three distinct curves are shown, each representing a different electrolyte. The first curve, in green, corresponds to NaPF₆ in EC/DMC 1:1, starting near zero, rising steeply, and plateauing just above 12 mS/cm at higher concentrations. The second curve, in purple, represents NaTFSI in EC/DMC 1:1, with a similar trend but a slightly lower maximum conductivity of around 8 mS/cm. The third curve, in pink, corresponds to NaOTf in diglyme, which rises more gradually and peaks around 3 mS/cm before declining slightly. Circular markers along each curve indicate data points. The graph includes a clean white background, labeled axes, and a legend positioned to the right of the graph, identifying the electrolytes.

Figure 1.Ionic conductivity of sodium-ion electrolytes, showing that ionic conductivity is related to salt concentration.

Ionic conductivity is a measure of how easily ions can move through the electrolyte. High ionic conductivity is essential for efficient ion transport, with typical values around 3-20 mS/cm.⁴ Allowing more ions to move between electrodes in a set period enables faster charging and discharging rates. Conversely, poor ionic conductivity can degrade many battery parameters, such as increasing electrochemical polarization and higher internal resistance.

There are a few critical factors that influence ionic conductivity:

Three stacked bar charts showing the viscosity, melting point, and dielectric constants of various common battery solvents used in sodium-ion electrolytes. The top chart, labeled "Viscosity (cP)" on the Y-axis, displays the viscosity values of different solvents. The bars, filled with a purple dotted pattern, show that most solvents have viscosities below 2 cP, except for one labeled "solid," which exceeds 2 cP. The middle chart, labeled "Melting Point (°C)" on the Y-axis, shows melting points with pink bars. Most solvents have negative melting points, ranging from approximately -100°C to just above 0°C, except for one solvent with a positive melting point near 50°C. The bottom chart, labeled "Dielectric Constant" on the Y-axis, features cyan-striped bars. It indicates a wide range of dielectric constants, with EC, PC, and EC:DMC 1:1 showing high values around 100, 70, and 50 while others, such as DMC, DEC, EMC, and Diglyme, have much lower values below 10. The solvents are listed on the X-axis for all three charts, including EC, PC, DMC, DEC, EMC, Diglyme, TFTFE, EC:DMC 1:1, and EC:DMC:EMC 1:1:1.

Figure 2.Viscosity, melting point, and dielectric constants of common battery solvents used in sodium-ion electrolytes.

Concentration of Cations (Na⁺) and Viscosity: At low concentrations, ionic conductivity increases linearly with the concentration of sodium ions (Na⁺). However, as more salt is added, the conductivity eventually plateaus and then begins to decline. This decline occurs because higher salt concentrations increase the viscosity of the electrolyte, making it more difficult for ions to move and increasing the solution's resistance to flow.⁵ The optimal concentration for achieving the highest ionic conductivity is typically around 1.0 M, where the balance between ion availability and solution viscosity is ideal.

Solvation Structure: When ions are strongly solvated, meaning they are tightly surrounded by solvent molecules, their mobility is reduced, which lowers ionic conductivity.⁶ It is important to have enough solvation energy to dissolve the salt without having so much solvation energy that desolvation impedes the release of the cation (Na⁺) at the electrode interfaces. Therefore, the selection of both the salt and the solvent plays a significant role in determining the electrolyte's ionic conductivity.⁷

Electrochemical Stability Window

A horizontal bar chart illustrating the electrochemical window of various 1.0 M sodium-ion electrolytes, as derived from cyclic voltammograms. The chart features six purple-colored bars aligned vertically, each corresponding to a different electrolyte composition. The x-axis is labeled with voltage values ranging from 0 to 5 volts versus Na+/Na. The y-axis lists the electrolyte compositions from top to bottom in the order: NaPF6 in PC, NaPF6 in EC/DMC 1:1, NaPF6 in EC/DMC/EMC 1:1:1, NaTFSI in EC/DMC 1:1, NaTFSI in EC/DMC/EMC 1:1:1, and NaOTf in diglyme. Each bar's length represents the electrochemical window of the corresponding electrolyte. The first three from the top show longest bar, corresponding to 5 V. Then with a 4.5 V, the electrolyte NaOTf in diglyme, shows the second widest. electrochemical window. Third widest window is shown by the electrolyte NaTFSI in EC/DMC 1:1, followed by NaTFSI in EC/DMC/EMC 1:1:1, each showing values close to 4. The chart provides a clear visual comparison of the electrochemical stability of these sodium-ion electrolytes.

Figure 3.Electrochemical window of 1.0 M sodium-ion electrolytes taken from cyclic voltammograms.

The electrochemical stability window of an electrolyte is the voltage range within which the electrolyte can operate without decomposing.⁸ A wide stability window is preferred because it supports higher operating voltages, thereby increasing the battery's energy density. Generally, NaPF₆-based and NaClO₄-based electrolytes have stability windows of at least 0-4 V vs. Na⁺/Na, making them useful for benchmarking cathode and anode materials.⁹

However, the stability window is not solely a property of the electrolyte; it also depends on the materials used for the current collectors and electrodes. For instance, NaTFSI and NaFSI in carbonate solvents tend to corrode aluminum current collectors at voltages beyond 3.7 V vs. Na⁺/Na, thus limiting their stability window.

SEI Formation

A stable solid-electrolyte interphase (SEI) is crucial for the longevity and safety of sodium-ion batteries (SIBs). The protective layer forms on the anode, preventing further electrolyte decomposition while allowing sodium ions to pass through.¹⁰ This stable SEI layer formation significantly impacts battery performance by retaining energy density over many cycles and facilitating fast charging. Additives play a critical role in enhancing the SEI. Sodium salt additives like NaBF₄, NaDFOB, and NaPO₂F₂ are effective at forming a fluorine-rich SEI, which improves battery performance. Similarly, liquid additives such as FEC, DFDEC, and DTD oxidize on the anode to create SEI films rich in fluorine or sulfur moieties, facilitating better sodium-ion conduction.^11,12

Chemical structures of four different sodium salt additives, each represented as molecular diagrams, commonly used in SEI layer formation in sodium ion batteries. The first structure on the left represents sodium tetrafluoroborate (NaBF4), with a central boron atom bonded to four fluorine atoms, surrounded by a sodium cation. The second structure shows sodium hexafluorophosphate (NaPF6), with a phosphorus atom bonded to two fluorine atoms with single bonds, one oxygen atom by a single bond, and one oxygen atom by a single bond in a tetrahedral arrangement, also paired with a sodium cation. The third structure depicts Bis(2,2,2-trifluoroethyl) carbonate (DFDEC), with a central carbonate group (-O-C(=O)-O-) bonded to two identical 2,2,2-trifluoroethyl groups (-CH2-CF3). The fourth structure represents ethylene sulfate, a cyclic organosulfur compound with a five-membered ring structure consisting of an ethylene group (-CH2-CH2-) bonded to a sulfate group (-SO4) via two oxygen atoms.

Figure 4. Sodium electrolyte salt additives.
a) NaBF₄ (940135), b) NaPO₂F₂ (939773), c) DFDEC (934011), d) DTD (935646)

Materials for Sodium-Ion Battery Electrolytes

The most commonly used electrolytes in sodium-ion batteries include NaClO₄ and NaPF₆, while NaTFSI, NaFSI, NaTfO, and NaDFOB are gaining attention as alternatives. Each salt has its advantages and tradeoffs.

Chemical structure of sodium perchlorate (NaClO4). On the left, the sodium ion (Na) is depicted, accompanied by a positive charge symbol. To the right, there is perchlorate ion, consisting of a central chlorine atom bonded to four oxygen atoms in a tetrahedral geometry. Three of these oxygen atoms are connected to the chlorine atom with double bonds, while the fourth oxygen atom is linked with a single bond and carries a negative charge.

Sodium Perchlorate (NaClO₄)
(931950)

Sodium Perchlorate (NaClO₄): NaClO₄-based liquid electrolytes are commonly used in SIB research due to their high ionic conductivity and wide electrochemical stability window. These properties make them ideal for fundamental studies of high-voltage oxide and polyanionic cathode materials. However, sodium perchlorate poses significant safety risks for industrial adoption and commercialization. It can explosively decompose at moderately high temperatures (~130°C) and is prone to explosion in its dry state when subjected to friction or shock.¹³

Chemical structure of sodium hexafluorophosphate (NaPF6). On the left side, the sodium ion (Na) is shown with a positive charge symbol. To the right, the hexafluorophosphate ion is illustrated, featuring a central phosphorus atom surrounded by six fluorine atoms. Each fluorine atom is connected to the phosphorus atom with a single bond, forming an octahedral geometry around the phosphorus.

Sodium Hexafluorophosphate (NaPF₆)
(940135)

Sodium Hexafluorophosphate (NaPF₆): NaPF₆-based liquid electrolytes are highly attractive for industrial and commercial applications because they do not carry the same explosion risk as sodium perchlorate. Common commercial formulations include 1.0 M NaPF₆ in PC, EC/DMC 1:1, and EC/DMC/EMC 1:1:1. These electrolytes offer high ionic conductivity (>5 mS/cm) and do not corrode aluminum foil. They also have wide electrochemical stability windows of 0-4 V vs. Na⁺/Na.¹⁴ While NaPF₆-based electrolytes are compatible with most electrode materials, they generally require additives like FEC, NaDFOB, NaBF₄, and NaPO₂F₂ to efficiently plate and strip sodium-metal anodes. The main drawbacks of NaPF6-based electrolytes are their thermal stability and sensitivity to moisture. Therefore, battery-grade NaPF₆ and its electrolytes must be kept rigorously dry,¹⁵ and additives may need to be developed to facilitate their use at higher operating temperatures.

Chemical structure of sodium trifluoromethanesulfonimide. On the left, a sodium ion (Na) is shown with a positive charge symbol. To the right, the trifluoromethanesulfonimide ion is depicted, featuring a central nitrogen atom bonded to two sulfonyl groups. Each sulfonyl group consists of a sulfur atom double-bonded to two oxygen atoms and single-bonded to a trifluoromethyl group (CF3). The nitrogen atom is linked to each sulfur atom, forming a bridging structure.

Sodium Bis(trifluoromethanesulfonyl)imide (NaTFSI) (936049)

Sodium Bis(trifluoromethanesulfonyl)imide (NaTFSI): NaTFSI-based electrolytes offer better thermal stability and are less sensitive to moisture compared to NaPF₆-based electrolytes.¹⁶ They are considered safer to handle because they do not hydrolyze to form HF when exposed to trace moisture. Two excellent formulations are 1.0 M NaTFSI in EC/DMC 1:1 and 1.0 M NaTFSI in EC/DMC/EMC 1:1:1. These electrolytes achieve similarly high ionic conductivities (>5 mS/cm) as NaPF₆-based or NaClO₄-based electrolytes and are compatible with many common electrode materials, including hard carbon, layered oxides, sodium vanadium phosphate, and polyanionic cathode materials. The main drawback of NaTFSI-based electrolytes is their tendency to corrode aluminum foil at potentials around 3.7 V vs. Na⁺/Na, which limits their use in very high-voltage batteries. This corrosion can be mitigated by adding 0.05 M NaPF₆ to the formulation, which helps passivate the aluminum current collector. This dual-salt approach is a promising avenue for NaTFSI-electrolyte design.¹⁶

Chemical structure of sodium bis(fluorosulfonyl)imide (NaFSI). On the left, there is a sodium ion (Na+) with a positive charge symbol. To the right, the bis(fluorosulfonyl)imide ion is illustrated, featuring a central nitrogen atom bonded to two sulfonyl groups. Each sulfonyl group consists of a sulfur atom double-bonded to two oxygen atoms and single-bonded to a fluorine atom. The nitrogen atom connects to each sulfur atom, forming a bridging structure.

Sodium Bis(Fluorosulfonyl)imide (NaFSI) (939900)

Sodium Bis(Fluorosulfonyl)imide (NaFSI): NaFSI-based electrolytes behave similarly to NaTFSI-based electrolytes. They are also considered safer to handle and can achieve high ionic conductivities. However, like NaTFSI, NaFSI-based electrolytes tend to corrode aluminum current collectors and do not offer the same improvements in thermal stability as NaTFSI.¹⁷

Chemical structure of sodium triflate (NaTfO). On the left, a sodium ion (Na) is shown with a positive charge symbol. To the right, there is triflate ion, consisting of a central sulfur atom bonded to three oxygen atoms and one trifluoromethyl group (CF3). The sulfur atom is double-bonded to two oxygen atoms and single-bonded to the third oxygen atom, which carries a negative charge and forms an ionic bond with the sodium ion.

Sodium-triflate (NaTfO)
(936030)

Sodium Triflate (NaTfO): Sodium-triflate-based electrolytes are emerging as a safer and more stable alternative in the realm of sodium-ion batteries. The triflate anion is stable and does not hydrolyze or form hydrofluoric acid, making NaOTf electrolytes safer to handle than NaPF₆-based ones. Additionally, NaOTf offers better thermal stability and does not corrode aluminum current collectors, unlike NaTFSI and NaFSI. The main challenge with NaOTf electrolytes is their ionic conductivity. NaOTf tends to form ion pairs in solution, leading to strong solvation that impedes ionic conductivity, especially in carbonate solvents. Therefore, selecting the appropriate solvent is more challenging for sodium-triflate-based electrolytes. One effective formulation is 1.0 M NaOTf in diglyme, which achieves an ionic conductivity of ~4 mS/cm at room temperature.¹⁸ This formulation is gaining attention for sodium metal batteries due to its efficient sodium plating and stripping capabilities.

Chemical structure of sodium difluoro(oxalato)borate (NaDFOB). At the center is a boron atom bonded to two fluorine atoms and an oxalate group. The oxalate group is depicted as a four-membered ring with two double-bonded oxygen atoms, each attached to a carbon atom, and two single-bonded oxygen atoms connecting the carbon atoms to the boron. Below the boron, a sodium ion is shown with a positive charge, indicating its ionic interaction with the borate complex.

Sodium Difluoro(oxalato)borate (NaDFOB)
(933953)

Sodium Difluoro(oxalato)borate (NaDFOB): NaDFOB-based electrolytes are highly promising but less studied. With surprising solubility (>1.0 M in carbonates), NaDFOB can serve as the primary electrolytic salt in a sodium-ion electrolyte and achieve high ionic conductivity (>5 mS/cm in EC/DMC, EC/DEC, and EC/PC). It exhibits excellent electrochemical stability from 0-5.5V vs. Na⁺/Na, surpassing even NaClO₄. Additionally, NaDFOB-based electrolytes and electrolytes using NaDFOB as an additive perform well with sodium metal anodes. Researchers have proposed that NaDFOB forms a protective SEI layer on sodium metal anodes that is made of sodium borates and sodium fluorides and prevents dendrite formation. Moreover, NaDFOB is stable and does not produce any toxic or hazardous substances upon contact with air and water. These observations make NaDFOB-based electrolytes a promising direction for SIB electrolytes.¹⁹

Conclusion

Sodium-ion battery research is on the brink of significant technological breakthroughs. The development of advanced electrolytes, including innovative electrolytic salts, additives, and formulated electrolytes, is crucial for enhancing the efficiency, safety, and longevity of these batteries. By focusing on critical aspects of electrolyte performance—such as ionic conductivity, electrochemical stability, and SEI formation—researchers can drive forward the capabilities of sodium-ion batteries and contribute to a more sustainable and energy-efficient future.

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