(Invited) The Scalability of Accelerating Rate Calorimetry (ARC) with State of Charge and Capacity

Abstract

Sandia National Laboratories has used Accelerating Rate Calorimetry (ARC) as a means to evaluate and compare the total energies of thermal runaway. This has been an invaluable tool for evaluating the behavior of new chemistries as they become available in commercial and prototype cells. However, this testing has typically been performed in 18650 cells of ~1-1.5 AH and on cells at 100% state of charge. This has been invaluable for the evaluation and comparison of the thermal runaway of different lithium ion battery chemistries, however it leaves open questions as how changes to energy density, cell size and state of charge impact the results from calorimetry testing. This work examines the results of accelerating rate calorimetry testing on different lithium ion cathode chemistries (LCO, NCA, NMC, LFP) different formats (18650, pouch, large format cylindrical) and states of charge to understand how ARC results are impacted by both energy density and total energy. Also studied are how thermal runaway is impacted by energy in general, looking at both the total energy release during thermal runaway as well as the peak heating rates observed. While the total energy release is expected to correlate with the stored energy of the cell, the peak heating rates will impact how cells might behave in a variety of conditions. This has implications for battery design, as data suggests that even cells at relatively low states of charge may be hazardous if sufficiently insulation allows self-heating to occur even at relatively low rates; figure 1 shows data collected on 14 AH pouch cells summarizing the observed runaway energies and peak heating rates. This shows a relatively linear response as the stored energy increases, however an exponential response is observed with respect to state of charge. While this affirms the conventional wisdom that low state of charge cells are unlikely to go into thermal runaway in most environments, we can still see significant stored energy even at relatively low states of charge. The right conditions may still make these cells hazardous, making it important to fully understand the thermal decomposition of cells under a wide variety of conditions. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Figure 1 Comparison of total enthalpy observed during runaway of a 14 AH pouch cell to the peak heating rates at various states of charge. Figure 1