Fig. 4: Operando monitoring internal temperature and pressure during 18650 cells thermal runaway.

Thermal runaway characteristics of 18650 cells with 100% SOC (a, b), 50% SOC (c, d) and 0% SOC (e, f). a, c, e In situ internal temperature (dark blue line) and internal pressure (orange) of the cells during thermal runaway captured by implanted optical fiber sensor (gray circles highlight their peaks) and surface temperature read from thermocouples attached to the cell (light blue line). The voltage output (dark gray line) sharply drops at the occurrence of an internal short circuit, causing about 20 °C of temperature rise lasting for tens of seconds (light gray squares and their zoomed insets for all the SOC cases). The mass (red line) also suddenly drops with safety venting. b, d, f Photographs at moments of the cells critical reactions during thermal runaway, including safety venting (I), incubation period (II), thermal runaway (III), and cooling down (IV). The cell with 0% SOC does not contain Stage III (blue shading) because of insufficient energy to initiate thermal runaway. g characterization of all correlated steps, in which the blue-labelled ones relate to temperature and the orange ones corresponding to pressure (“TR” indicates thermal runaway).