How do ADN and FEC enhance SEI layer stability and cycle life in sodium-ion battery research? Boosting performance.

Adiponitrile (ADN) and fluoroethylene carbonate (FEC) improve sodium-ion battery cycle life by forming more stable, protective SEI layers at the electrode–electrolyte interface. ADN promotes a compact, uniform film that suppresses ongoing electrolyte decomposition, while FEC preferentially reduces during early cycling to produce a dense, mechanically robust, NaF-rich SEI. Together, these effects can improve Coulombic efficiency, reduce interfacial resistance, limit parasitic reactions, and preserve capacity over extended cycling.

The central principle is controlled interfacial chemistry: because sodium-based SEI species can be relatively soluble and sodium-ion transport is kinetically slower than lithium-ion transport, functional additives help replace unstable, continuously reforming interfaces with thinner, stronger, and more protective passivation layers.

Why Sodium-Ion Batteries Need SEI Stabilization

Sodium creates a more difficult interface

Sodium ions have a larger ionic radius and lower Lewis acidity than lithium ions. These characteristics can contribute to slower interfacial kinetics and less uniform ion transport through the SEI.

Some sodium-containing SEI products, including sodium carbonate, are also more soluble than comparable lithium species. The resulting film may partially dissolve or repeatedly reform during cycling.

Unstable SEI consumes electrolyte and sodium

An unstable SEI allows continued electrolyte reduction at the anode. This consumes electrolyte and cyclable sodium, increases irreversible capacity loss, and lowers first-cycle Coulombic efficiency.

Repeated film formation also raises interfacial resistance. Over time, this can appear as increasing polarization, declining usable capacity, and accelerated cell failure.

How Adiponitrile Improves the Interface

ADN forms a compact passivation film

Adiponitrile is a functional nitrile additive that participates in interfacial reactions during early battery operation. At concentrations around 3 wt.%, it promotes formation of a compact and highly effective SEI.

This film reduces the electrode’s direct exposure to the bulk electrolyte. As a result, continuous solvent decomposition and uncontrolled SEI growth are substantially suppressed.

The film improves structural uniformity

A compact SEI can provide a more consistent path for sodium-ion transport across the electrode surface. More uniform coverage helps reduce localized reactions, which are particularly damaging on rough, heterogeneous, or volume-changing electrode materials.

The practical result is a more stable electrode–electrolyte interface during repeated charge and discharge.

ADN supports wide-temperature operation

The ADN-modified interface can remain more effective across temperature extremes. Reported improvements include higher discharge capacity at 45°C, −10°C, and −20°C compared with electrolyte formulations without ADN.

This does not mean ADN removes all low- or high-temperature limitations. Rather, it reduces the additional interfacial degradation that becomes more severe when reaction kinetics or electrolyte stability change with temperature.

ADN extends capacity retention

In the referenced sodium-ion cell study, an ADN-containing electrolyte maintained approximately 78% capacity after 220 cycles, whereas the cell without ADN degraded much more rapidly.

Related results report about 75% retention after only 40 cycles without ADN, illustrating the scale of the improvement under the stated test conditions. These values should be treated as formulation- and protocol-specific rather than universal performance guarantees.

How FEC Improves the Interface

FEC reduces preferentially during initial cycling

Fluoroethylene carbonate is an electron-withdrawing, halogenated cyclic carbonate. Its chemical structure makes it more readily reduced than many of the base electrolyte components during initial electrode polarization.

This preferential reduction directs early decomposition toward formation of a protective interphase instead of allowing uncontrolled solvent decomposition to dominate.

FEC produces a NaF-rich SEI

A key product of FEC decomposition is a sodium fluoride-rich SEI. NaF is attractive for interfacial stabilization because it is mechanically robust and has very low solubility in typical organic electrolyte environments.

The resulting dense film can reduce parasitic reactions, prevent solvent co-intercalation, and protect hard-carbon or alloy-type anodes.

FEC lowers interfacial resistance

When the FEC-derived layer is sufficiently thin, continuous, and well distributed, it supports more uniform sodium-ion transport. This can reduce SEI resistance and limit the increase in impedance during cycling.

Lower impedance generally translates into lower polarization, improved rate behavior, and better preservation of capacity.

FEC improves early-cycle efficiency

By suppressing continued electrolyte reduction, FEC can reduce irreversible capacity loss during formation and improve early-cycle Coulombic efficiency.

Monitoring the first several cycles is important because an apparently stable long-term capacity can still conceal poor initial sodium inventory utilization.

How ADN and FEC Differ

ADN primarily stabilizes the overall passivation process

ADN is best understood as an additive that helps generate a compact, uniform, and persistent protective film while suppressing electrolyte decomposition.

Its reported benefit is especially notable in wide-temperature testing and long-term capacity retention.

FEC provides a fluoride-rich protective chemistry

FEC more directly changes the chemical composition of the SEI by promoting formation of a NaF-rich interphase.

This is particularly relevant for hard-carbon and alloy anodes, where solvent co-intercalation, surface reactions, and electrode volume changes can destabilize the interface.

Both additives target the same failure pathway

ADN and FEC use different chemical routes, but both address the same underlying problem: an SEI that is too soluble, mechanically weak, resistive, or continuously reforming.

The appropriate additive depends on the electrode chemistry, base solvent, concentration, temperature range, and desired balance between initial efficiency and long-term stability.

Understanding the Trade-offs

More additive is not automatically better

FEC is commonly investigated in the approximate range of 2–5%, while ADN has been evaluated near 3 wt.% in the referenced work. These are useful starting points, not universal optima.

Excess additive can produce an overly thick or resistive film, consume electrolyte, reduce conductivity, or increase formation losses.

FEC can degrade during extended cycling

Although FEC-derived SEI layers can be highly effective initially, FEC is gradually consumed. Continued film degradation may lead to rising overpotential, increasing impedance, sudden performance loss, or gas generation, particularly at elevated temperature.

Long-term cycling is therefore essential; early-cycle improvement alone is insufficient evidence of a successful formulation.

Additive effects depend on the electrode

An additive that benefits hard carbon may behave differently on layered oxide cathodes, tin-phosphide anodes, or other alloy-type materials.

The electrode’s surface chemistry, volume change, operating potential, and catalytic activity all influence how the additive decomposes and what type of SEI forms.

Cell fabrication can obscure the chemical effect

Inconsistent slurry mixing, electrode density, electrolyte volume, cell pressure, or sealing can produce capacity and impedance differences larger than the additive effect itself.

Reliable additive research requires controlled electrode preparation, consistent pressing and wetting, inert-atmosphere assembly, and reproducible cell sealing.

Cycling data must be interpreted with interface diagnostics

Capacity retention should be evaluated alongside Coulombic efficiency, voltage polarization, impedance, and post-cycling morphology.

Electrochemical impedance spectroscopy and early-cycle efficiency measurements can help distinguish a genuinely stable SEI from temporary performance improvements caused by excess electrolyte or inconsistent cell assembly.

How to Apply This to Sodium-Ion Battery Research

A disciplined screening program should compare additive-free, ADN-containing, and FEC-containing electrolytes under identical fabrication and testing conditions.

• If your primary focus is long-term capacity retention: Start by evaluating ADN near the reported 3 wt.% level and track capacity, Coulombic efficiency, and impedance over hundreds of cycles.
• If your primary focus is hard-carbon anode stabilization: Screen FEC in the approximate 2–5% range and monitor irreversible capacity loss, early-cycle efficiency, and SEI resistance.
• If your primary focus is low-temperature performance: Test ADN-containing formulations at sub-zero temperatures, while controlling electrolyte wetting and cell pressure because transport limitations can mask interfacial improvements.
• If your primary focus is high-temperature operation: Evaluate both capacity retention and gas generation, since FEC consumption and interphase degradation can become more pronounced at elevated temperature.
• If your primary focus is formulation optimization: Use concentration matrices and identical cell-assembly protocols rather than assuming that a single additive concentration transfers across electrode chemistries.
• If your primary focus is mechanism verification: Combine cycling data with impedance measurements and post-mortem surface analysis to confirm that performance gains arise from a stable SEI.

Stable SEI design is the essential link between electrolyte formulation and durable sodium-ion battery performance.

Summary Table:

Both additives stabilize the electrode–electrolyte interface, but via different chemical routes.

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