Analysis of Transient Current and Heat Flow during Voltage Holds: For Relaxed and Unrelaxed G/NMC811 Cells

Abstract

This study investigates the transient heat flow during voltage holds for multi-layer pouch G/NMC811 lithium-ion cells. When applying a voltage hold to estimate the amount of parasitic side reactions, there is first a transient phase followed by a steady state. According to the literature, reversible processes mainly influence the transient phase, while the steady state presents the side reaction current. This work investigates the contributions to the transient behavior, evaluating the thermal and electrical signals and deconvoluting the different parasitic side reactions. In addition, a comparison between relaxed and unrelaxed states is performed, analyzing different temperatures and voltages. The results show that the relaxed graphite cells quickly reached a constant parasitic heat flow value with minimal transient effects. In contrast, the unrelaxed cells presented increased transient effects in the parasitic heat flow signal that can last more than 300 h. The electric transient originates mainly from anode overhang effects and SEI regrowth, while the thermal transient consists predominantly of SEI regrowth. Overall, the thermal signals stabilize faster than the electrical signals, giving calorimetry an advantage for faster estimation of parasitic side reactions. The thermal signal tends to stabilize faster than the electrical, providing accelerated prediction of side reactions. Equalization effects with anode overhang do not seem to impact the thermal signal. The different processes dominating the thermal and electrical signals could be resolved.