Constant-rate heating-induced thermal runaway in 18650-type Li-ion cells charged/discharged at 1 °C: Effect of undischargeable Li at anode

Charge/discharge cycling tests of lithium-ion batteries were carried out at 1 °C. During cycling, the delivered capacity decreased substantially, just 30% of the capacity at 25 °C is retained after 45 cycles. The exothermic behavior of fresh and aged cells is analyzed by an accelerated rate calorimeter (ARC) via two different heating protocols. In the heat-wait-search (HWS) experiments, aged cells show an unstable self-heating process from the start of the test, which is aggravated by the depth of discharge (DOD). This is not observed in fresh cells. In the constant-rate heating method, the aged cells show different thermal behavior at different cutoff temperatures, with a high risk of thermal runaway (TR) above 160 °C. Most importantly, in aged cells heated to 140 °C, TR could be averted by external cooling. Analyses of the differential capacity profiles reveal that thermal stability degradation of aged cells is closely related to the undischargeable Li accumulated in the graphite anode during cycling at 1 °C.