Safety assessment of overcharged batteries and a novel passive warning method based on relaxation expansion force

Abstract

Due to batteries inconsistencies and potential faults in battery management systems, slight overcharging remains a common yet insufficiently understood safety risk, lacking effective warning methods. To illuminate the degradation behavior and failure mechanism of various overcharged states (100% SOC, 105% SOC, 110% SOC, and 115% SOC), multiple advanced in-situ characterization techniques (accelerating rate calorimeter, electrochemical impedance spectroscopy, ultrasonic scanning, and expansion instrument) were utilized. Additionally, re-overcharge-induced thermal runaway (TR) tests were conducted, with a specific emphasis on the evolution of the expansion force signal. Results indicated significant degradation at 110% SOC, including conductivity loss, loss of lithium inventory, and loss of active material accompanied by internal gas generation. These failure behaviors slow down the expansion force rate during re-overcharging, reducing the efficacy of active warnings that depend on rate thresholds of expansion force. Specifically, the warning time for 115% SOC battery is only 144 s, which is 740 s shorter than that for fresh battery, and the time to TR is advanced by 9 min. Moreover, the initial self-heating temperature (T₁) is reduced by 62.4 °C compared to that of fresh battery, reaching only 70.8 °C. To address the low safety of overcharged batteries, a passive overcharge warning method utilizing relaxation expansion force was proposed, based on the continued gas generation after stopping charging, leading to a sustained increase in force. Compared to active methods that rely on thresholds of expansion force rate, the passive method can issue warnings 115 s earlier. By combining the passive and active warning methods, guaranteed effective overcharge warning can be issued 863–884 s before TR. This study introduces a novel perspective for enhancing the safety of batteries.