Thermal characterization and modeling of sodium-ion batteries at medium and low temperatures

Sodium-ion batteries show promise for energy storage due to their low cost and excellent low-temperature performance; however, their thermal behavior over a wide temperature range, especially under sub-zero conditions, remains insufficiently understood. In this study, thermophysical parameters and electrothermal characteristics of a 26,700 cylindrical sodium-ion battery are measured from −35 °C to 55 °C, and a thermal model is established. Key findings reveal strong thermal anisotropy (axial/radial conductivity ratio of 11.5) and a specific heat capacity comparable to lithium-ion batteries. At −35 °C, the discharge capacity remains 92.5% of that at 25 °C, outperforming lithium-ion batteries. The entropy thermal coefficient is negative over the entire SOC range and becomes more negative at low temperatures when SOC < 60%, enhancing self-heating during discharge. A thermal model based on the Bernardi equation accurately predicts temperature rise under both medium and low temperatures, with end-of-discharge errors below 0.8 °C. Analysis of heat source contributions reveals that polarization heat increases dramatically and becomes a major heat source at low temperatures. This work not only provides fundamental thermophysical data and mechanistic insights into the heat generation processes in SIBs, but also delivers a reliable engineering tool for thermal management design, particularly for low-temperature applications.