Summary

Ionic conductivity remains useful for screening fluorine-free sodium-ion electrolytes, but it cannot determine whether a formulation will remain stable in a working cell. Recent studies show that solvation structure, sodium-ion transport, hard-carbon compatibility, interphase formation, voltage limits, temperature exposure and formation conditions can reverse the ranking suggested by conductivity alone. The most common error is to compare conductivity values measured under different temperatures, concentrations or formulations and treat the highest number as evidence of better cell performance. The more defensible qualification unit is the complete electrolyte formulation matched with a defined anode, cathode, voltage range, temperature range and cell design. This shift is expanding the validation matrix for fluorine-free systems rather than simplifying electrolyte selection.

The Research Direction Is Moving from Bulk Transport to Interface Control

The early technical question surrounding fluorine-free sodium-ion electrolytes was relatively narrow: can a non-fluorinated salt and solvent system provide sufficient ionic transport?

That question has not disappeared, but it is no longer enough.

Recent research is increasingly focused on whether a formulation can:

• maintain reductive stability on hard carbon;
• tolerate the intended cathode cutoff voltage;
• form stable electrode–electrolyte interfaces;
• control continuous solvent or salt decomposition;
• retain performance over a wider temperature range;
• maintain its advantage beyond simplified laboratory cells.

This changes the meaning of electrolyte qualification. A high conductivity value may identify a promising transport medium, but it does not establish a usable cell operating window.

“Fluorine-free” also remains a compositional description rather than a complete safety or performance conclusion. Solvent flammability, salt reactivity, gas formation, impurity sensitivity and process compatibility still require separate evidence.

What the 2026 Nature Study Demonstrated

A Nature Communications study published on July 20, 2026 examined fluorine-free acetonitrile-based electrolytes for hard-carbon||Prussian-blue sodium-ion full cells.

The optimized formulation contained:

• 1.0 M sodium perchlorate, NaClO₄;
• acetonitrile and ethyl methyl carbonate at a 1:1 volume ratio;
• 3 wt% vinylene carbonate.

Acetonitrile can provide high ionic conductivity and favorable oxidation resistance, but its limited reductive stability can create poor compatibility with hard-carbon anodes.

The researchers used ethyl methyl carbonate to alter ion–dipole interactions. The modified solvation environment increased anion participation in the inner solvation sheath and supported the formation of a more stable electrode interface.

The study reported stable operation from 25°C to 100°C, 62.5% capacity retention after 600 cycles at 55°C and 1.0 C, and electrochemical testing of ampere-hour-level pouch cells at 25°C and 55°C. At the time of writing, Nature identifies the article as an unedited early-access manuscript that may still undergo editorial correction.

The main qualification lesson is not that adding one cosolvent automatically creates a commercially ready electrolyte.

The result shows that improving solvation and interfacial chemistry can matter more than maximizing the original bulk conductivity value.

Why Conductivity Can Produce the Wrong Ranking

Ionic conductivity measures the collective movement of charged species under a defined condition. It does not measure only useful Na⁺ transport.

A conductivity result cannot independently determine:

• the sodium-ion contribution to total current;
• the degree of ion pairing or aggregation;
• desolvation behavior at the electrode surface;
• reductive decomposition on hard carbon;
• oxidative decomposition at the cathode;
• interphase stability during repeated cycling;
• gas generation or continuous electrolyte consumption.

Two formulations with similar conductivity can therefore produce very different impedance growth, initial Coulombic efficiency and cycle retention.

A formulation with lower conductivity may perform better when it forms a more stable interface. A formulation with higher conductivity may lose its advantage when the cathode voltage, temperature or electrode surface accelerates decomposition.

Total conductivity is not the same as sodium-ion transport

Both cations and anions contribute to measured ionic conductivity.

A high total value does not prove that Na⁺ carries a proportionally high share of the current. This distinction becomes relevant when two formulations show similar conductivity but different polarization, rate capability or concentration gradients.

Conductivity should therefore be interpreted with the available evidence on:

• sodium-ion transference;
• viscosity across the target temperature range;
• ion association or aggregation;
• concentration-dependent transport;
• cell impedance under relevant conditions.

Not every development project requires every advanced characterization method. The purpose is to avoid using a bulk measurement to support an interfacial or cell-level conclusion.

A reported stability window is not an electrode qualification

Electrochemical-window measurements on stainless steel, platinum, glassy carbon or another simplified surface can support initial screening.

They do not fully reproduce reactions on porous hard carbon, Prussian blue analogues, layered oxides, conductive carbon and binder-containing commercial electrodes.

The observed stability boundary can change with:

• electrode surface chemistry;
• porosity and active surface area;
• residual water;
• active-material impurities;
• conductive additives;
• current density;
• formation conditions.

The relevant question is not only when oxidation or reduction current first appears. It is whether the complete formulation remains sufficiently passivated on the intended electrode pair throughout the required operating range.

A Second Study Shows Why the Highest Conductivity May Still Lose

A 2026 Energy Advances study evaluated sodium tetraphenylborate, NaBPh₄, as a fluorine-free electrolyte salt in ethylene carbonate and diethyl carbonate.

Among the tested concentrations, 0.5 M NaBPh₄ showed the highest bulk conductivity at 25°C. That result supported its selection for further testing, but it did not predict the final cell operating boundary.

Prussian-white||hard-carbon cells showed poor capacity retention when cycled to an upper cutoff voltage of 3.6 V. Cycling became more stable when the upper limit was reduced to 3.0 V, but the usable capacity also decreased.

Post-cycling nuclear magnetic resonance analysis indicated electrolyte degradation, with triphenylborane and biphenyl proposed as likely oxidation products. The study therefore provides a direct example of why the formulation with the strongest bulk transport result may still require a restricted voltage window.

Conductivity remained useful as a screening measurement. It did not define whether the electrolyte could support the intended energy, voltage and cycle-life targets.

The First Validation Gaps Appear at the Electrodes

Hard carbon is often one of the first boundaries exposed by fluorine-free electrolyte development.

Its surface area, pore structure, functional groups, residual species and binder system can influence initial electrolyte decomposition and solid electrolyte interphase formation.

For a hard-carbon system, a conductivity advantage should be reconsidered when accompanied by:

• low initial Coulombic efficiency;
• continuing impedance growth;
• abnormal voltage-profile changes;
• pressure or gas development;
• rapid decline after elevated-temperature storage.

The cathode introduces a separate boundary.

A formulation that performs adequately with one Prussian blue analogue may not retain the same oxidative stability with another cathode family, upper cutoff voltage, surface treatment or residual water level.

This is why the emerging validation unit is not simply “fluorine-free electrolyte.” It is a defined formulation matched with a defined electrode pair and operating window.

Development Stages Expose Different Electrolyte Risks

The same conductivity result carries a different meaning at the laboratory, pilot and commercial stages.

Laboratory screening tests chemical feasibility

An early formulation can be considered promising when it combines adequate transport with acceptable formation behavior on the intended electrodes.

At this stage, researchers should determine whether:

• the complete salt, solvent and additive composition is known;
• conductivity and viscosity remain acceptable across the target temperatures;
• the intended voltage range is plausible;
• early impedance, gas and cycling behavior justify further testing.

A laboratory sample may receive additional drying, filtration or purification. Its performance should not automatically be attributed to the routine material grade.

Pilot cells expose process-dependent behavior

Moving into pouch cells or representative pilot conditions introduces variables that coin-cell screening may hide:

• electrode loading;
• electrolyte-to-capacity ratio;
• separator wetting;
• filling and rest time;
• solvent volatility;
• formation protocol;
• cell pressure or swelling.

A formulation that performs only with excess electrolyte, unusually low loading or a restricted voltage range has not demonstrated the same qualification status as one tested under the intended process conditions.

This wider gap between laboratory availability and application readiness is also visible across the broader battery material qualification process.

Commercial material introduces composition consistency

Bulk approval should focus on electrolyte-specific attributes that can change transport or interface behavior.

Depending on the formulation, relevant controls may include:

• water and other protic impurities;
• salt assay and ionic impurities;
• residual organic compounds;
• acidity or decomposition indicators;
• additive concentration;
• insoluble matter or particulate contamination;
• packaging exposure during storage and delivery.

The control list should follow the failure modes of the specific electrolyte. It should not be copied from a generic battery-grade specification.

Decision Framework: What Each Result Can Actually Support

The table also highlights a frequent comparison error: supplier or literature conductivity data are not directly comparable unless their measurement basis is aligned.

At minimum, the comparison should identify:

• salt concentration;
• solvent and additive composition;
• test temperature;
• measurement method;
• equilibration conditions;
• water basis;
• sample storage history.

A value measured at elevated temperature should not be ranked directly against a room-temperature result. A complete electrolyte containing additives should not be compared as if it were equivalent to a salt–solvent solution.

The Emerging Trend Is a Larger Qualification Matrix

Current discussion often presents fluorine-free electrolyte development as a substitution task: replace a fluorinated component with a non-fluorinated alternative and compare conductivity.

The research evidence points to a more complex transition.

Each salt–solvent–additive combination changes several linked variables:

• ion association;
• desolvation;
• hard-carbon passivation;
• cathode oxidation;
• temperature behavior;
• formation requirements;
• impurity sensitivity.

The near-term result is likely to be a larger qualification matrix rather than a simpler material list.

This means one broad description cannot support interchangeability. A more useful technical identity is:

Salt + solvent ratio + additive package + electrode pair + voltage window + temperature range + formation protocol

For R&D teams, conductivity remains a first filter, followed by interface and full-cell evidence.

For production teams, wetting, volatility, filling, formation and gas behavior may determine whether the chemistry is usable.

For quality teams, the priority is identifying composition changes that could alter solvation or interphase formation.

For procurement teams, two products should not be treated as equivalent merely because they share a fluorine-free claim and a similar conductivity value.

The central commercial implication is specific:

A conductivity result has purchasing value only when the tested formulation, measurement conditions, sample identity and proposed commercial material are traceably connected.

What Is Still Uncertain

Current research does not establish one universal fluorine-free electrolyte for sodium-ion batteries.

It also does not demonstrate that every fluorine-free formulation is automatically:

• safer;
• nonflammable;
• less corrosive;
• easier to manufacture;
• lower in cost;
• commercially available at scale.

Those conclusions require separate evidence.

The most useful next signals will be:

• multi-batch full-cell results;
• higher active-material loading;
• leaner electrolyte conditions;
• calendar-aging performance;
• gas and pressure measurements;
• low- and high-temperature cycling;
• representative filling and formation trials;
• commercial purification and storage controls.

Until those boundaries are demonstrated, conductivity should remain a candidate-screening parameter rather than a final qualification result.

Qualification Should Start with the Required Cell Window

The first action is to define the intended electrode pair, voltage range, temperature range, cell format, formation process and unacceptable failure mode.

Conductivity can then eliminate candidates with inadequate bulk transport. A formulation should progress only when electrode-specific evidence shows that its transport advantage is not offset by continuous decomposition, restricted voltage, impedance growth, gas generation or unstable temperature performance.

For a specification-focused discussion, provide the target salt, solvent and additive scope, electrode pair, operating window, water requirement, sample quantity and required technical documents. Availability and custom-development feasibility for related battery chemicals and energy-storage materials should be confirmed for the specific project with ChemicalCell.