Influence of saltwater immersion on the electrical degradation and thermal runaway behavior of lithium-ion batteries under different states of charge

This study investigates the effects of saltwater immersion on the electrochemical performance and thermal stability of lithium-ion batteries at different states of charge (25%, 50%, 75%, 100%). Commercial 18650-type lithium-ion batteries were immersed in a 3.5 wt% NaCl solution for 2 h, and their electrochemical performance and thermal stability were evaluated through cycling tests and controlled heating experiments. The results reveal a pronounced state of charge–dependent immersion damage mechanism. Higher state of charge leads to significantly increased initial capacity loss and a markedly accelerated capacity fading rate. Scanning Electron Microscopy observations reveal severe electrode cracking, particle detachment, and morphological degradation induced by saltwater immersion. Corrosion-induced structural damage, particularly cap corrosion, fundamentally alters thermal runaway evolution by promoting premature gas release, suppressing internal pressure buildup, and modifying venting behavior, thereby affecting key thermal runaway time characteristics and external failure manifestations. Moreover, this study proposes the first voltage drop ( t d ) as a robust, corrosion-insensitive early warning signal for thermal runaway. The t d – t tr interval consistently shortens with increasing state of charge, providing a more stable and earlier indicator than conventional vent- or temperature-based criteria. These findings offer practical guidance for post-immersion safety assessment and emergency handling of batteries in flood or marine environments.