Analysis of the impact of simulated seawater immersion on the electrochemical performance and thermal hazards of 26700 sodium-ion and lithium-ion batteries

During natural disasters like heavy rain and typhoons, batteries frequently undergo water immersion, yet the impacts of such exposure on their electrochemical performance and thermal hazards remain unclear. This study systematically evaluates how simulated seawater immersion duration influences the electrochemical stability and thermal safety of 100 %SOC SIB and LIB. Corrosion analysis revealed preferential aluminum corroded over iron. Metal chlorides formed prior to metal oxides in SIB, contrasting with LIB where stable oxides emerged in last. Severe corrosion occurred in both batteries after 6h of immersion. The evolution mechanisms and changes in electrochemical performance and thermal hazards were revealed. At 12h, SIB's AC impedance exceeded measurable limits, while LIB increased by only 57.6 %. Concurrently, LIB exhibited a larger voltage drop than SIB (17.71 % > 11.57 %). Thermal hazard evolution was quantified through characteristic temperatures, mass loss, and gas emissions. After 6h, SIB showed dramatically elevated thermal hazard: safety valve opening temperature, maximum temperature, and maximum temperature rise rate surged by 12.4 %, 162.39 % and 8505.56 %. Conversely, LIB demonstrated reduced thermal hazard during this stage. Finally, a quantitative assessment model for thermal hazard was established through linear normalization. As immersion time increased, the thermal hazard for both battery types initially rose and then fell. While the peak thermal hazard occurring after 6h of immersion in sodium-ion batteries, LIB demonstrated higher overall hazard levels. To operationalize these findings, a predictive model was developed to forecast thermal hazard variations across 0–12h of immersion. This research provides theoretical guidance for battery safety and material optimization in water-exposed environments.