Sodium-Ion vs Lithium-Ion: −40°C Cold-Weather Benchmark

At −40°C, the sodium-ion versus lithium-ion debate changes. It’s no longer mainly about energy density; it’s about whether the battery can deliver useful power at all.

A 2026 study of commercial 18650-format cells found that tested sodium-ion cells retained roughly 51–62% of their room-temperature discharge capacity at −40°C. The lithium-ion reference cells did not sustain discharge under that protocol.

That’s an important result, but it isn’t a universal verdict. “Sodium-ion” and “lithium-ion” each describe broad families of cells. Electrode chemistry, electrolyte, loading, state of charge, discharge rate, and thermal controls can change the outcome.

Key takeaway: Sodium-ion deserves serious consideration where cold-start reliability matters more than minimum weight and volume. Lithium-ion remains the stronger choice where energy density, fast charging, and mature pack engineering dominate.

What the −40°C result actually shows

The useful comparison is between commercial cells tested under the same general conditions—not between an advanced laboratory material and a finished battery.

The reported 2026 Applied Energy study compared commercial sodium-ion cells with lithium-ion reference cells across temperature-dependent tests. Here, “reference lithium-ion cells” means the specific commercial lithium-ion products selected as controls in that experiment, not every lithium-ion design on the market.

The 51–62% figure is a gravimetric or cell-level capacity-retention result only if the paper defines it that way; it should not be treated as usable pack energy without checking the methods. A proper comparison needs the tested cell model, cell count, discharge current or C-rate, voltage cutoff, state of charge, and whether capacity was measured per cell or normalized by mass. Those details should accompany the headline number in any engineering report.

The result is still operationally meaningful: some commercial sodium-ion architectures can remain discharge-capable at temperatures where particular lithium-ion reference cells become electrically unusable. That could matter in Arctic monitoring equipment, outdoor telecom cabinets, backup systems, and vehicles left outside overnight.

But capacity and power are different measurements. A cell may retain 60% of its low-temperature capacity while its impedance rises sharply. Under a gentle laboratory load, it may appear healthy. During an inverter startup, radio transmission burst, or vehicle acceleration event, its voltage could collapse.

Cold discharge also isn’t the same as cold recovery. Some energy becomes temporarily inaccessible as ion transport slows and polarization increases. Warming the cell may restore part of that capacity, but a remote sensor can’t use energy that’s unavailable while frozen.

Don’t mix the −40°C discharge and 0°C charging results

The charging result needs careful handling. The cited work reported that some NFM sodium-ion cells required more than 1,000 minutes of constant-voltage charging at 0°C. NFM generally refers to a layered sodium transition-metal oxide family, often based on nickel, iron, and manganese. It is one sodium-ion chemistry, not a synonym for all sodium-ion cells.

More importantly, the 0°C charging example should not be presented as if it were the same measurement as the −40°C discharge benchmark. They answer different questions, may involve different cell variants, and use different temperatures and protocols:

• −40°C discharge: Can the tested cell deliver energy under a defined load?
• 0°C charging: Can the tested cell accept charge efficiently, and how long does the constant-voltage phase take?

A battery can perform well in the first test and poorly in the second. That distinction matters in systems that must recharge every day, especially solar-powered equipment and vehicles with short charging windows.

Charging below freezing is difficult for every battery chemistry. Lithium-ion cells can suffer lithium plating during cold charging. Sodium-ion cells avoid lithium plating specifically, but they still face slow ion transport, increased resistance, side reactions, and poor charge acceptance. A product rated for operation from −40°C to 60°C may mean discharge only, reduced current, an internal heater, or a long charging schedule.

For procurement, ask for a temperature map covering capacity, continuous and pulse power, charge current, constant-voltage duration, impedance, recovery after warming, cycle life, and BMS lockouts. The important system number is often usable energy after thermal conditioning. A pack that retains 60% of its cold capacity but consumes a substantial share of that energy heating itself may not outperform a denser lithium-ion pack.

The silicon number that doesn’t describe a battery

Silicon illustrates a related reporting problem.

A 2026 Science Partner Journal paper reported a silicon–carbon material with an initial discharge capacity of 6,694.21 mAh g⁻¹ and an initial Coulombic efficiency of 74.71%. After electrolyte optimization, the reported capacity was 5,294.88 mAh g⁻¹, with initial Coulombic efficiency improved to 90.96%.

Those are material-level results, not the capacity of a commercial battery.

The unit mAh g⁻¹ refers to the tested active material. A complete cell also includes the cathode, current collectors, separator, electrolyte, binder, conductive additive, casing, tabs, and safety components. The electrodes must be balanced, and silicon’s first-cycle lithium loss may require additional lithium elsewhere in the design.

A small coin cell can also use low electrode loading, excess electrolyte, a wide voltage window, and a large counter-electrode reserve. Those conditions help researchers study a material, but they don’t predict the energy of a production EV cell.

Cell-level energy = active capacity × average voltage, minus the mass, volume, losses, and balancing constraints of the complete cell.

This is why a 6,000 mAh g⁻¹ silicon result and a commercial 18650 cell answer different questions. The same caution applies when comparing a commercial sodium-ion 18650 vs lithium-ion 18650: match format, loading, voltage range, current, temperature, and accounting rules before drawing conclusions.

Which chemistry fits an extreme-cold application?

Lithium-ion still has a strong position in vehicles. Its energy density reduces pack mass and volume, and manufacturers have extensive experience with heating, cooling, and battery-management systems. If a vehicle already preheats its pack effectively, lithium-ion’s density and charging infrastructure may outweigh sodium-ion’s cold-discharge advantage.

Stationary storage is less constrained by weight. A larger sodium-ion pack may be acceptable if it reduces heater demand, improves cold starts, or avoids a heavily insulated enclosure. In that setting, lower energy density can be a reasonable trade.

Remote sensors require a more specific analysis. A device that sleeps most of the time may care more about self-discharge and pulse performance than nameplate capacity. A solar-powered system may be limited by cold charge acceptance rather than discharge capacity. Telecom backup equipment may prioritize standby life, serviceability, and predictable BMS behavior.

A 2026 Journal of Energy Storage comparison of commercial-format devices—including lithium-ion, sodium-ion, hybrid supercapacitors, and electric double-layer capacitors—reinforced a practical point: no device won every category. Energy, power, cold tolerance, self-discharge, safety, and lifetime trade against one another.

The more recent the cell, the more important it is to examine scale. Results from 21700 cylindrical cells, large-area electrodes, or pilot-scale cells provide more useful evidence than a tiny coin cell. When reviewing a paper, record the exact cell model, electrode loading, areal capacity, N/P ratio, electrolyte-to-capacity ratio, voltage window, discharge rate, cutoff voltage, cell count, and cycle-life endpoint. Without those details, a percentage is a clue—not a design specification.

A concise procurement checklist

Before approving a cold-weather battery, ask the supplier to provide:

• Discharge capacity and power at −20°C and −40°C
• Charge current limits below 0°C
• Constant-voltage duration at the intended temperature
• Cell model, format, mass, and tested sample count
• Discharge current or C-rate and voltage cutoff
• BMS temperature derating and charge lockout settings
• Heater energy consumption, if thermal conditioning is required
• Cold-cycle-life data and capacity recovery after warming
• Pack-level results, not only electrode or coin-cell data

The best chemistry for extreme cold isn’t the one with the most impressive graph. It’s the one that delivers the required power, recharges within the operating schedule, survives the expected cycles, and still makes sense after heating energy and pack hardware are included.

Frequently Asked Questions

Is sodium-ion better than lithium-ion at −40°C?

Some commercial sodium-ion cells have shown better low-temperature discharge operability than the lithium-ion reference cells used in a reported test. The sodium-ion cells retained about 51–62% capacity at −40°C, while those lithium-ion references did not sustain discharge. The result applies to the tested products and protocol, not every cell in either chemistry family.

Can sodium-ion batteries charge below freezing?

Some can, but charging current and charging time may need strict limits. Cold discharge performance does not prove fast or efficient cold charging. In the cited example, some NFM sodium-ion cells required more than 1,000 minutes of constant-voltage charging at 0°C.

Does 6,000 mAh g⁻¹ mean a 6,000 mAh battery?

No. It describes the capacity of a tested active material per gram under specified laboratory conditions. A complete battery includes inactive materials, electrolyte, packaging, electrode balancing, and voltage losses, so its practical cell capacity and energy will be much lower.

Which battery chemistry is best for extreme cold?

There’s no universal winner. Sodium-ion is attractive when reliable cold starts and lower thermal-conditioning demand matter more than weight. Lithium-ion remains preferable when energy density, fast charging, and established production are the priorities. Test the complete cell, pack, heater, and BMS at the actual operating temperature before committing to either chemistry.