Empirical analysis of contributing factors to heating in lithium-ion cells: Anode entropy versus internal resistance

Charging a battery beyond its maximum capacity can lead both to cell overheating and to the venting of gasses. A fundamental understanding of cell heating could lead to the development of real-time sensors that anticipate and avert catastrophic battery failure. Joule heating (also called ohmic or resistive heating) from cell internal resistance (Rint) dominates the overall thermal energy (ΔQ) generated during charging. Contrary to prior hypotheses, though, Joule heating does not appear to contribute to venting observed during overcharging. In this manuscript, we examine an alternate hypothesis, that heat released by the entropy change in the anode (ΔSanode) and the concomitant increase in the anode temperature (Tanode) triggers the venting. Using our recently developed non-invasive battery internal temperature (BIT) sensor to monitor Tanode, we separated the contributions of ΔSanode, Rint and the anode resistance (Ranode) to ΔQ. These quantities were tracked during constant current charging of a 18650 Lithium-ion cell, from zero state of charge (SoC) to overcharge. The resulting analysis suggests that anode entropy change is more important than resistive heating resulting from Ranode to the overall thermal energy. Anode entropy measurements, enabled by the BIT sensor, might serve as an alternative or adjunct method for anticipating and avoiding cell venting events.