Abusive battery testing has most typically dealt with the behavior of single cell failure. However, large and complex battery systems require the consideration of how a single cell failure may lead to a more serious failure that consumes a large portion of a pack or module. Initial failure that leads to the thermal runaway of other cells within the system creates a much more serious condition than the failure of a single cell. Figure 1 shows how a single cell failure may not be indicative of multi-cell performance. Figure 1 (left) shows the results of the nail penetration of a single LiFePO_4­-graphite 26650 cell. This failure resulted in a temperature increase to a peak of 100 °C along with venting of the cell. Figure 1 (right) shows a single cell failure within a 10 cell parallel string of the same cells. The initially observed failure is first more severe, with a peak observed temperature of ~250 °C immediately after the single cell failure. This then develops into a failure of the entire string 10 minutes later. This is significantly more severe than the single cell failure as well, with all cells peaking above 200 °C and the central cell seeing 370 °C.

This work examines the behavior of small modules of cylindrical and stacked pouch cells after thermal runaway is induced in a single cell. The limits of cell-to-cell failure propagation are explored by initiating failure on cells with reduced states of charge, packs with air gaps between cells, and packs with physical separation between cells. This data shows both how thermal failure propagation may be mitigated as well as demonstrating the behavior of a propagating cell failure under these conditions. Some data shows how a propagating failure of even a small pack may stretch over several minutes as the latent heat available causes the cells impacted to slowly reach critical points where thermal runaway occurs. This work also provides a better understanding of how cell failure propagates through a system, providing a fundamental basis for modeling work presented elsewhere. Establishing this basic understanding is crucial for the informed design of battery packs as well as predicting the effectiveness of failure mitigation strategies.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and 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.