For stable sodium–air battery electrolytes, start with ether-based solvents—especially diglyme/DEGME or DME—and pair them with NaPF₆. These formulations generally produce fewer side reactions than carbonate-based electrolytes and can support a more stable, passivating SEI on the sodium-metal anode. Low-concentration NaI may be added when the primary objective is improved oxygen-electrode reversibility and cycling.
Core takeaway: A practical starting point is NaPF₆ in DME or diglyme (DEGME), with careful control of water and oxygen exposure. Avoid assuming that every ether or salt is interchangeable: TFSI-based salts can be unstable in DME, while perchlorate and triflate systems may promote continued SEI growth.
Why Ether-Based Solvents Are Usually Preferred
Diglyme/DEGME as a leading candidate
Diethylene glycol dimethyl ether, commonly called diglyme or DEGME, is a strong candidate for sodium–air electrolyte development. The primary reference identifies it as having lower side-reaction rates and better stability than TEGDME and carbonate solutions.
Its relatively favorable behavior makes it useful when sodium-metal compatibility and suppression of continuous electrolyte decomposition are priorities.
DME for NaPF₆ formulations
1,2-Dimethoxyethane (DME) is another important ether solvent, particularly for NaPF₆-based electrolytes. NaPF₆ dissolved in DME is specifically recommended as a practical formulation for sodium–air research.
DME can support good ionic transport, but its compatibility with the selected salt and additives must be verified experimentally because some salt chemistries are unstable in it.
TEGDME and carbonate solvents
TEGDME can be used in sodium–air research, but the reference indicates that it generally exhibits more side reactions than shorter-chain glymes such as diglyme.
Carbonate-based systems, including EC/DMC mixtures, are also possible, but they are generally less attractive than ether systems for sodium–air cells because carbonate decomposition and oxygen-related reactions can compromise the sodium-metal interface.
EC/DMC as a secondary comparison formulation
A NaPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC) mixture can serve as a comparison or control electrolyte. It is not the primary recommendation for sodium–air operation, but it may be useful for benchmarking salt and solvent effects under controlled conditions.
Which Sodium Salts Are Recommended?
NaPF₆ as the primary salt candidate
Sodium hexafluorophosphate (NaPF₆) is the leading recommendation from the supplied references. In DME or EC/DMC, it is associated with formation of a more stable, passivating SEI on sodium metal than the alternatives identified.
A stable SEI is essential because it limits direct contact between sodium, solvent, and dissolved oxygen species while still allowing sodium-ion transport.
NaI as a functional additive
Sodium iodide (NaI) is best considered a low-concentration additive rather than the main supporting electrolyte. Approximately 0.05 M NaI is identified as a useful experimental concentration for improving discharge/charge reversibility and extending cycle life in sodium–air test cells.
Its role is primarily connected to oxygen-electrode reaction chemistry and reversibility, so its benefit should be evaluated separately from the baseline SEI-forming capability of NaPF₆.
FEC as an SEI-engineering additive
Fluoroethylene carbonate (FEC) may be considered when additional sodium-metal protection is required. It preferentially reduces at the sodium surface and can promote a NaF-rich SEI, which is comparatively resistant to solvent and dissolved-oxygen crossover.
However, FEC should not automatically be transferred from conventional sodium-ion systems into sodium–air cells. Its effect depends on concentration, solvent, oxygen chemistry, and the cathode environment, so it requires dedicated compatibility testing.
Why Some Salt Choices Are Less Attractive
TFSI-based salts in DME
TFSI-based sodium salts are not the preferred choice for DME formulations in the supplied guidance. They tend to be unstable in DME, making them a poor first option when the objective is a durable sodium-metal SEI.
Perchlorate and triflate salts
Perchlorate and triflate salts may lead to continuous SEI growth rather than a stable, self-limiting interphase. This can increase impedance, consume electrolyte, and progressively reduce sodium-metal reversibility.
NaClO₄ can have favorable properties in some conventional sodium-ion formulations, but that does not make it the preferred salt for sodium–air cells, where oxygen crossover and sodium-metal passivation are especially demanding.
Salt selection cannot rely on one property
Solubility, ionic dissociation, electrochemical stability, chemical compatibility, thermal stability, toxicity, and cost all matter. A salt that performs well in a sodium-ion cell may still be unsuitable for a sodium–air configuration because the oxygen cathode and sodium-metal anode create a more reactive environment.
Understanding the Trade-offs
Ether stability versus oxygen and moisture sensitivity
Ethers generally offer better sodium-metal compatibility than carbonates, but they remain sensitive to water, oxygen, and reactive discharge products. Strict drying and controlled atmosphere handling are therefore essential for meaningful comparisons.
Trace contaminants can alter SEI composition and make an apparently stable formulation appear inconsistent between laboratories.
DME transport versus formulation stability
DME is attractive for ion transport and NaPF₆ formulations, but its stability must be confirmed under the specific oxygen partial pressure, current density, electrode composition, and cycling protocol being used.
Diglyme may offer a more stable baseline in some experiments, but solvent choice should ultimately be based on measured sodium-plating efficiency, impedance growth, and full-cell cycling.
Additives can improve one interface while harming another
NaI or FEC can improve a targeted process, such as oxygen reduction/oxidation or sodium-metal passivation. However, additives may also change cathode reaction pathways, increase parasitic reactions, or alter gas-generation behavior.
Additives should therefore be screened individually before combining them in a final formulation.
A Practical Screening Sequence
Establish a solvent baseline
Begin with an ether-based baseline, preferably NaPF₆ in diglyme/DEGME or DME. Use carbonate-based NaPF₆ in EC/DMC as a comparison rather than assuming it will be the best sodium–air electrolyte.
Characterize the sodium-metal interface
Measure sodium plating/stripping efficiency, interfacial impedance, polarization, and SEI evolution. These measurements are more informative than initial conductivity alone because sodium–air performance is often limited by interfacial instability.
Add oxygen-chemistry modifiers cautiously
Evaluate approximately 0.05 M NaI as a separate additive condition if discharge/charge reversibility is poor. Test FEC independently when the dominant failure mode is unstable sodium-metal passivation.
Control the test environment
Use rigorously dried electrolyte and controlled oxygen exposure. Record solvent, salt concentration, additive concentration, water content, electrode loading, current density, and cutoff potentials so that SEI stability can be compared reproducibly.
Making the Right Choice for Your Goal
- If your primary focus is sodium-metal SEI stability: Start with NaPF₆ in diglyme/DEGME, and compare it with NaPF₆ in DME under rigorously dry conditions.
- If your primary focus is oxygen discharge/charge reversibility: Use a stable NaPF₆ ether baseline and evaluate approximately 0.05 M NaI as a separate additive condition.
- If your primary focus is stronger NaF-rich passivation: Screen FEC with the NaPF₆ ether formulation, while checking for adverse effects at the oxygen electrode.
- If your primary focus is comparative electrolyte screening: Include NaPF₆ in EC/DMC as a carbonate reference, but do not treat it as the default sodium–air formulation.
- If your primary focus is minimizing continuous SEI growth: Avoid making TFSI-, perchlorate-, or triflate-based salts the first choice without direct stability data in the selected solvent.
For most sodium–air research programs, the most defensible starting point is NaPF₆ in diglyme/DEGME or DME, with NaI or FEC introduced only after the baseline interfacial behavior is established.
Summary Table:
| Component | Recommended Option | Key Benefits | Considerations |
|---|---|---|---|
| Solvent | Diglyme/DEGME | Low side reactions, better stability than TEGDME | Sensitive to water/oxygen |
| Solvent | DME | Good ion transport, works with NaPF₆ | Verify compatibility with salt |
| Salt | NaPF₆ | Forms stable, passivating SEI | Primary choice |
| Additive | NaI (0.05 M) | Improves oxygen reversibility | Use as additive, not main salt |
| Additive | FEC | Promotes NaF-rich SEI | Test compatibility; may affect cathode |
| Avoid | TFSI salts in DME | Unstable | Not recommended |
| Avoid | Perchlorate/triflate | Continuous SEI growth | Not ideal for Na-air |
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