Low-temperature performance

As temperature falls, electrolyte viscosity usually rises and conductivity falls. Sodium desolvation, transport through SEI and CEI, solid-state diffusion, and charge transfer all become slower. At the terminals this appears as higher resistance, lower discharge voltage, stronger polarization, and less usable capacity, but the dominant step differs across cathodes, hard-carbon structures, salt-solvent systems, and electrode thicknesses. Low-temperature capacity of one material in a half-cell cannot establish complete-cell performance. Both electrodes, interfaces, electrolyte, and electrode transport need evaluation with stated rest, temperature, and rate.

The same cell may release substantial capacity at a low cold-discharge rate yet reach its lower voltage limit early during a power pulse because of voltage drop. A cold “capacity retention” figure should state the reference temperature, rate, and voltage window. Vehicle starting, tools, and storage discharge also use different power durations. Practical pouch work in 2025 and commercial cylindrical studies in 2026 provide full-cell data at very low or multiple temperatures, but chemistry, insulation, soak, and test equipment differ. One percentage cannot be generalized to every sodium-ion product.

During charging, sodium leaves the cathode, crosses electrolyte and interfaces, and enters hard carbon. As cold polarization rises, local anode potential can approach conditions for metallic sodium deposition, producing inactive sodium, resistance, and possible internal-short risk. Lower current, a narrower SOC window, preheating, or tailored electrolyte and interphase design can help, but permitted charge power needs calibration against cell temperature, SOC, ageing, and heat rejection. Passing a cold-discharge test does not authorize cold fast charging. Post-charge rest, self-discharge, capacity recovery, and later safety also need follow-up.

A system can raise cell temperature through external preheating, reversible thermal loops, heating films, or controlled self-heating. The strategy should consider uniformity, heating rate, auxiliary energy, local overheating, and continued heat loss to the cold environment. Heating the enclosure surface to a target does not prove the center and edges of the electrode assembly are uniform. The BMS needs multiple measurements, models, and waiting rules before allowing charge. Insulation reduces standby heat loss but may obstruct cooling after high-power operation, so a wide-temperature system usually needs switchable heating, insulation, and cooling strategies.

A useful cold specification is not one minimum temperature but a map of permitted charge and discharge power, usable energy, duration, and recovery across temperature and SOC. Testing should cover representative cells, sufficient cold soak, sensor placement, both current directions, and repeats and record whether preheating was used. A manufacturer’s minus-40-degree capacity or power claim indicates product direction, while procurement and system design still need the cell revisions and protocol. Operation can derate dynamically from temperature, SOC, SOH, and expected duty rather than demand room-temperature peak performance from the coldest, low-SOC, or aged condition.