Sodium-ion Batteries: Overview of the electrolyte

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries due to the abundance and affordability of sodium. The electrolyte, a key component of SIBs, significantly influences their performance, safety, and cost. This article delves into the electrolyte requirements of SIBs, focusing on common salts and organic solvents used in their composition.

In the context of sodium-ion batteries, an electrolyte is a substance that allows for the flow of electrical charge between the positive and negative electrodes of the battery. The electrolyte is typically a liquid or a gel-like material that contains ions, which are atoms or molecules with a positive or negative electrical charge.

In a sodium-ion battery, the electrolyte plays a critical role in the battery's performance, stability, and safety. It must be able to conduct sodium ions (Na+) between the electrodes, but it must also be able to prevent the flow of electrons (which would cause a short circuit) and the unwanted side reactions that can degrade the battery over time.

Here are some of the key properties that are needed in an electrolyte for sodium-ion batteries, along with an indication of whether the salt or solvent is primarily responsible for that property:

1. Ionic conductivity: The electrolyte should have a high ionic conductivity, which is a measure of how well it conducts electricity. The salt is primarily responsible for this property, as it provides the ions that carry the electrical charge. However, the solvent can also affect the ionic conductivity by influencing the mobility of the ions.
2. Electrochemical stability: The electrolyte should have a high electrochemical stability, which is a measure of how well it withstands the high voltages and currents that occur during battery charging and discharging. The salt and solvent both contribute to this property, as they must be able to resist oxidation and reduction at the electrodes.
3. Thermal stability: The electrolyte should have a high thermal stability, which is a measure of how well it withstands high temperatures. The salt and solvent both contribute to this property, as they must be able to resist decomposition and evaporation at elevated temperatures.
4. Compatibility with other battery components: The electrolyte should be compatible with the other components of the battery, such as the electrodes, separators, and current collectors. The salt and solvent both contribute to this property, as they must not react with or corrode these components.
5. Cost and availability: The electrolyte should be cost-effective and widely available, as this can be an important factor in the commercialization and widespread adoption of sodium-ion batteries. The salt and solvent both contribute to this property, as they must be able to be produced and purified at a reasonable cost and in sufficient quantities.

Common Salts in Sodium-Ion Batteries

The choice of salt in the electrolyte formulation is crucial for the ionic conductivity, electrochemical stability, and overall performance of SIBs. Common salts include:

- Sodium perchlorate (NaClO4): Known for its high solubility and good ionic conductivity, NaClO4 is a popular choice in SIB electrolytes. Sodium perchlorate is also a highly conductive salt that is stable over a wide voltage range.

It has been used in sodium-ion battery electrolytes due to its high solubility in organic solvents and its ability to form a stable passivation layer on the surface of the sodium anode. However, it is an oxidizing agent and can be hazardous to handle and transport. Additionally, it is not as environmentally friendly as some of the other sodium salts.

- Sodium hexafluorophosphate (NaPF6): This is a highly conductive salt that is stable over a wide voltage range. It is commonly used in sodium-ion battery electrolytes due to its high ionic conductivity and good electrochemical stability.

However, it is sensitive to moisture and can react with water to form hydrogen fluoride, which is a highly toxic and corrosive gas. Therefore, special care must be taken to ensure that the electrolyte solution is prepared and stored in a dry environment.

- Sodium tetrafluoroborate (NaBF4): It is a highly conductive salt that is stable over a wide voltage range. It is less sensitive to moisture than sodium hexafluorophosphate and is less hazardous to handle and transport than sodium perchlorate. This salt mostly found as white/colourless crystals with pH value around 3.0

It has been used in sodium-ion battery electrolytes due to its high solubility in organic solvents and its ability to form a stable passivation layer on the surface of the sodium anode. However, it can react with trace amounts of water to form hydrogen fluoride, which is a highly toxic and corrosive gas. This salt is selected for its thermal stability and compatibility with various electrode materials.

- Sodium trifluoromethanesulfonate (NaCF3SO3): This is a highly conductive salt that is stable over a wide voltage range. It is less sensitive to moisture than sodium hexafluorophosphate and is less hazardous to handle and transport than sodium perchlorate.

It has been used in sodium-ion battery electrolytes due to its high solubility in organic solvents and its ability to form a stable passivation layer on the surface of the sodium anode. Additionally, it is more environmentally friendly than some of the other sodium salts. It offers a balance between conductivity and stability, making it suitable for moderate temperature applications.

- Sodium bis(fluorosulfonyl)imide (NaFSI) : These FSI salts are noted for their wide electrochemical windows and low lattice energies, which contribute to high ionic conductivity and thermal stability.

NaFSI is a highly conductive salt that is stable over a wide voltage range. It has a high thermal stability and a low melting point, which makes it suitable for use in high-temperature and low-temperature applications. Additionally, it is less sensitive to moisture than sodium hexafluorophosphate and is less hazardous to handle and transport than sodium perchlorate. It has been used in sodium-ion battery electrolytes due to its high solubility in organic solvents and its ability to form a stable passivation layer on the surface of the sodium anode.

- Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI): NaTFSI has a high ionic conductivity and a wide electrochemical stability window, which makes it suitable for use in high-voltage SIBs. It is also less sensitive to moisture than sodium hexafluorophosphate and is less hazardous to handle and transport than sodium perchlorate. Additionally, it is more environmentally friendly than some of the other sodium salts.

The properties of these salts, such as solubility, thermal stability, and lattice energy, play a pivotal role in their suitability for SIBs. For instance, NaTFSI is preferred for its lower tendency to form ion pairs, resulting in higher ionic dissociation and conductivity.

Selection of a salt

The salts used in sodium-ion battery electrolytes can be classified based on several factors, including their chemical structure, conductivity, stability, and cost. Here are some common parameters for selection of salts for battery production:

1. Anion type: The salts used in sodium-ion battery electrolytes can be classified based on the type of anion they contain. For example, sodium hexafluorophosphate (NaPF6) contains a hexafluorophosphate anion (PF6-), while sodium bis(fluorosulfonyl)imide (NaFSI) contains a bis(fluorosulfonyl)imide anion (FSI-). The anion type can affect the salt's conductivity, stability, and compatibility with other battery components.
2. Conductivity: The salts used in sodium-ion battery electrolytes can be classified based on their ionic conductivity, which is a measure of how well they conduct electricity. A higher ionic conductivity is generally desirable, as it can improve the battery's performance and efficiency. Sodium bis(fluorosulfonyl)imide (NaFSI) and sodium trifluoromethanesulfonate (NaCF3SO3) are examples of salts with high ionic conductivity.
3. Stability: The salts used in sodium-ion battery electrolytes can be classified based on their stability, both thermal and electrochemical. Thermal stability refers to how well the salt withstands high temperatures, while electrochemical stability refers to how well the salt withstands the high voltages and currents that occur during battery charging and discharging. Sodium bis(fluorosulfonyl)imide (NaFSI) and sodium trifluoromethanesulfonate (NaCF3SO3) are examples of salts with high thermal and electrochemical stability.
4. Cost: The salts used in sodium-ion battery electrolytes can be classified based on their cost, which can be an important factor in battery production. Sodium chloride (NaCl) and sodium sulfate (Na2SO4) are examples of salts that are relatively inexpensive, while sodium hexafluorophosphate (NaPF6) and sodium bis(fluorosulfonyl)imide (NaFSI) are examples of salts that are more expensive.

Organic Solvents for Sodium-Ion Batteries

Organic solvents are used to dissolve the aforementioned salts to create liquid electrolytes. The most commonly used solvents in SIBs are:

- Propylene carbonate (PC): It is a cyclic organic carbonate that is commonly used as a solvent in electrolytes for sodium-ion batteries and other types of electrochemical devices. It is a colorless, odorless liquid with a relatively high boiling point (242°C) and a high dielectric constant (64.9), which makes it a good solvent for ionic compounds.

PC has several properties that make it well-suited for use in sodium-ion battery electrolytes. For example, it has a high electrochemical stability, which means that it can withstand the high voltages and currents that occur during battery charging and discharging without decomposing or reacting with other components of the battery. It also has a relatively high ionic conductivity, which means that it can facilitate the movement of ions through the electrolyte and improve the overall performance of the battery.

In addition to its use as a solvent, PC can also be used as a co-solvent or additive in electrolyte formulations to improve their properties. For example, it can be used to enhance the thermal stability of the electrolyte, reduce the viscosity of the electrolyte, or improve the compatibility of the electrolyte with other components of the battery.

- Ethylene carbonate (EC): It is a colorless, odorless liquid with a relatively higher boiling point (248°C) and a higher dielectric constant (89.8) than Propylene carbonate (PC). EC's high boiling point and low volatility make it a stable solvent choice for SIBs. EC also has high electrochemical stability and it high ionic conductivity.

A typical sodium intercalation type battery would use an electrolyte consisting of fluoroethylene carbonate (FEC) (99%), metallic Na (99.9%), and 1.0 M sodium perchlorate (NaClO4) solutions in ethylene carbonate and diethyl carbonate (EC/DEC), 1:1 v/v% battery-grade, mixed with FEC (10% by weight).

EC is also a key component in the formation of the solid electrolyte interphase (SEI) layer on the surface of the anode in sodium-ion batteries. The SEI layer is a thin, protective film that forms during the initial charging of the battery and helps to prevent unwanted side reactions and the degradation of the anode. EC has been shown to play a critical role in the formation of a stable and effective SEI layer in sodium-ion batteries.

- Dimethyl carbonate (DMC) and Diethyl carbonate (DEC): These solvents are valued for their low viscosity, which enhances ionic mobility and battery performance. Dimethyl carbonate (DMC) and diethyl carbonate (DEC) are two linear alkyl carbonates that are commonly used as solvents in electrolytes for sodium-ion batteries and other types of electrochemical devices.

DMC is a colorless, odorless liquid with a relatively low boiling point (90°C) and a moderate dielectric constant (3.1), which makes it a good solvent for ionic compounds. It has a relatively high ionic conductivity, which means that it can facilitate the movement of ions through the electrolyte and improve the overall performance of the battery. DMC is also relatively inexpensive and widely available, which makes it an attractive option for use in sodium-ion battery electrolytes.

DEC is a colorless, odorless liquid with a slightly higher boiling point (126°C) and a slightly higher dielectric constant (3.8) than DMC. It has a similar ionic conductivity to DMC and is also relatively inexpensive and widely available. DEC is sometimes used as a co-solvent or additive in electrolyte formulations to improve their properties, such as by reducing the viscosity of the electrolyte or enhancing its thermal stability.

Both DMC and DEC are relatively stable and safe to handle, although they can react with strong acids or bases and should be stored and used in a well-ventilated area. They are also biodegradable and have low toxicity, which makes them environmentally friendly options for use in sodium-ion battery electrolytes.

Both DMC and DEC are commonly used as co-solvents or additives in electrolyte formulations to improve their properties.

Why Carbonate based solvents?

These carbonate-based solvents are chosen for their ability to dissolve the sodium salts and form a conductive electrolyte solution. They also have a wide electrochemical stability window, which means they can withstand the high voltages and currents that occur during battery charging and discharging.

Sodium hexafluorophosphate, sodium tetrafluoroborate, and sodium trifluoromethanesulfonate are all highly soluble in carbonate-based solvents, making them suitable for use in sodium-ion battery electrolytes.

The combination of these solvents with the salts forms an electrolyte that determines the battery's operational window, safety, and compatibility with electrode materials. For example, a mixture of EC and DMC provides a balance between high dielectric constant and low viscosity, optimizing the solvation and transport of sodium ions.

Making the electrolyte

Creating electrolytes for sodium-ion batteries involves a meticulous process that ensures the right combination of salts and solvents to achieve optimal performance. Here’s a simplified step-by-step guide:

1. Selection of Sodium Salts: Choose the appropriate sodium salt based on the desired properties of the electrolyte. Each salt affects the electrolyte’s conductivity, stability, and safety.
2. Solvent Selection: Select a suitable solvent that can dissolve the chosen sodium salts and facilitate ion transport.
3. Dissolving the Salt: Mix the sodium salt with the chosen solvent. The concentration of the salt in the solvent will depend on the required ionic conductivity and viscosity of the electrolyte.
4. Analyzing Physico-Chemical Properties: Test the macroscopic physico-chemical properties such as ionic conductivity, viscosity, and thermal stability of the electrolyte solution.
5. Molecular Level Properties: Study the molecular level properties like ion-pairing and solvation. This involves understanding how the sodium ions interact with the solvent molecules and how this affects the overall performance of the electrolyte.
6. Optimization: Based on the findings from the physico-chemical and molecular level analysis, adjust the salt concentration, solvent mixture, or add different additives to optimize the electrolyte for better performance.
7. Quality Control: Conduct thorough testing to ensure the electrolyte meets the necessary standards for safety, performance, and longevity.
8. Scaling Up: Once the electrolyte formulation is finalized and tested, scale up the production to meet the demands for battery manufacturing.

Remember, this is a high-level overview, and the actual process may involve more detailed steps and considerations, especially when it comes to ensuring the electrolyte’s compatibility with the battery’s other components and its performance under various conditions.

Conclusion

The development of efficient and stable electrolytes for SIBs is a dynamic field of research. The interplay between the chosen salts and solvents defines the battery's characteristics and potential applications. As the demand for sustainable and cost-effective energy storage solutions grows, the optimization of electrolyte compositions for SIBs remains a critical area of focus for researchers and industry alike.

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