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The conventional dialogues on the reasons for thermal runaway, venting and fire of Li-ion cells point to temperature increase and gas formation. There is, however, no consensus on which of these events occurs first, thereby enabling research to correctly target the root cause. The recent work conducted at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) demonstrates that neither the temperature increase nor gas formation may be the primary reason for the venting or fire. Instead, the primary reason could potentially be a thermodynamic parameter that is often associated with both anode and cathode, namely entropy. Typically, a decrease in entropy, and a concomitant increase in the electrode temperature have been observed in the anode during charging, and in the cathode during discharging. We find that under ambient operating conditions (0 to 40 °C), entropy-driven thermal energy accounts for more than 2/3rd of the heat. More importantly, sudden changes in the entropy increases the electrode temperature by an order-of-magnitude that if left unchecked could drive the electrode temperatures sufficiently high to disrupt the SEI layer, bringing the active materials in the electrodes in direct contact with the electrolyte, enabling exothermic reactions. Battery internal temperature (BIT) sensor, a technique that we recently developed at JHU/APL enables one to follow the anode and cathode temperatures in real time, while the cell is under charge and discharge. We will discuss the application of BIT sensor in estimating the entropy changes that define the limits of safety in Li-ion cells.
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