Sodium batteries at low temperatures: why they still deliver 95% at −20 °C

There is a defect in home batteries that catalogues rarely mention: in the cold they perform less well. This is not a marginal detail, because the time when they perform less coincides with the time when they are needed most. In January, photovoltaic production is a fraction of what it produces in July, days are short, evening energy consumption is high — and it is precisely then that the storage system delivers only a fraction of the capacity you paid for.

On this point the difference between the chemistries is clear and measurable. This article reports the qualification data for polyanionic NFPP cells at low temperatures and explains where the gap comes from.

The data

The test protocol is simple: the cell is charged to 3.5 V at 0.5P power, left to rest for ten minutes, and then discharged at the test temperature. A measurement is taken of how much of the nominal capacity it is actually able to deliver.

In the direct test at −20 °C, the value measured on the NFPP cell is 95,04% of the nominal capacity. The reference LFP cell, in the same test, stops at 67,8%.

The −40 °C row deserves careful reading, because the zero is not a rounding off: lead-acid, lithium iron phosphate and nickel-manganese-cobalt at that temperature are not simply inefficient, they are unusable. The sodium polyanionic chemistry remains above 85%.

Why sodium gets off more lightly

There are two reasons, and they act on different parts of the cell.

Desolvation at the interface

An ion moving within the electrolyte does not travel naked: it is surrounded by a shell of solvent molecules that attach themselves around it. To enter the electrode, it must get rid of them, and this costs energy. This is the process known as desolvation, and it is the main bottleneck at low temperatures: when the cell is cold, the energy available to make that transition is reduced and the reaction slows down.

The sodium ion is larger than the lithium one, and for this very reason it retains its solvent shell less firmlythe charge density on its surface is lower. Paradoxically, being bulkier — which is why sodium stores less energy per kilogram — is also why it detaches from the solvent more easily in the cold.

Diffusion pathways in the cathode

The second reason relates to the structure of the active material. In lithium iron phosphate, which has an olivine structure, the ions move along one-dimensional channelsa single possible path, and if that path becomes obstructed — due to a defect in the lattice or the slowdown induced by temperature — there are no alternatives.

The NFPP polyanionic framework instead offers a network of three-dimensional diffusion pathways. The ion has multiple pathways to reach its site, and the slowdown caused by the cold weighs much less on the overall result.

What changes to the system

The number on its own says little. It's worth translating.

Let's take a 10 kWh battery storage system in an unheated technical room — a garage, a cellar, an outdoor meter cupboard — on a January morning in Northern Italy, at −5 °C. An LFP battery delivers a reduced fraction of its nominal capacity; the owner sees a “10 kWh” battery that delivers noticeably less, and they see this precisely during the days of the highest energy bills. The phenomenon is reversible — when mild temperatures return, the capacity comes back — but in the meantime, the storage system has operated below expectations for the entire cold season.

With a cell that retains over 90% at −20 °C, the loss at −5 °C is negligible. The battery performs just as well in winter as it does in summer.

The distinction becomes decisive in three situations:

• High-level installations, where winter minimum temperatures remain steadily below freezing and a heated technical room is rarely available.
• External installations or in non-air-conditioned spaces, which are the norm in the commercial and industrial sectors.
• Northern and Eastern European markets — Germany, Poland, the Czech Republic, Sweden — where the cold season is longer and harsher than the Italian one, and where the yield gap builds up over many more days a year.

The other side of the coin

For the sake of completeness: the cold-weather advantage is not free. It stems from the same properties of the sodium ion that limit its energy density, as explained in another in-depth article. There is no single chemistry that wins on all parameters: there is the chemistry suited to the application.

For a stationary storage system, which stays where it was installed and has to operate predictably for twenty years in an unheated room, low-temperature performance matters more than watt-hours per kilogram.

Frequently asked questions

Does the battery room need heating?

Given that the battery retains over 90% of its capacity at −20 °C, no. Rather, it is a precaution that some lithium-ion battery installations adopt precisely to compensate for the drop in performance during winter.

Is the capacity drop in the cold permanent?

No, it is reversible: as the temperature rises, the capacity becomes available again. The problem is not battery damage, it is that the energy is not there when it is needed.

Why does the sodium ion, which is larger, perform better in the cold?

Because its charge is distributed over a larger surface area, so it holds less firmly the shell of solvent molecules that it must leave behind to enter the electrode. That step is the main obstacle at low temperatures.

Does this apply while charging, or only when discharging?

The data reported here refer to the controlled-temperature discharge test, which is the critical condition for a photovoltaic storage system: the discharge takes place in the evening and at night, when the temperature is at its lowest.