Fig. 5: Establishing a safety warning range by detection of the switch from reversible and irreversible reaction through temperature and pressure time derivatives.

a Internal temperature and pressure responses of the cells with 100%, 50% and 0% SOC during thermal runaway. b Rates of increase of internal temperature and pressure with time. Insets magnify the vertical scale during the two response periods ➀ and ➁ prior to the internal short circuit. c, d and e The combination of temperature and pressure rise rates forming a rhombus-shaped characteristic response in each of the 100%, 50% and 0% SOC cell cases. ➀ shows a rapid temperature rate increase while the pressure rate remains constant, and ➁ shows the opposite tendency. Between ➀ and ➁ the turning section identifies the parts of the derivative curves used to generate onset of a safety warning range (red squares), regardless of the SOCs. f Unveils the inherent physical-chemical mechanism of reversible and irreversible reactions between stages ➀ and ➁. A warning can be issued as the cell suffers from healthy reversible variations (electrolyte evaporation) but before harmful irreversible reactions occur (SEI decomposition).