Investigations of the Structural and Electrochemical Properties of Overheated Li-Ion Batteries and Its Effects in Single Cells Vs. Multi-Cells Packs
Authors: Torres-Castro Loraine , Lamb Joshua , Karulkar Mohan 
Published: February 2020

Abuse of electrochemical cells and batteries can have a wide range of effects depending on the properties of the battery as well as the nature of the damage caused. From a safety perspective, simple passive monitoring is often unable to identify the onset of failure until it is too late to intervene. Once a significant voltage change or self-heating of a cell is identified, a catastrophic runaway is often inevitable. The ultimate weakness of traditional monitoring techniques, typically based on voltage and temperature, is that they detect only the symptoms of cell failure. Once a significant change has been registered, irreversible internal changes have likely already occurred within the cell. Compounding the issue is the fact that cells in a battery are comprised of all the elements needed to release their stored energy, meaning that once a catastrophic failure has initiated no amount of intervention can keep the reaction from proceeding to its conclusion. The prevention of catastrophic failure requires detection of internal faults well before they have developed to the point of no return. This makes techniques that can monitor the internal health of a cell essential to provide dependable protection from catastrophic failure. Safety issues are exacerbated by the use of multi-cell modules. Monitoring a pack of multiple cells with traditional techniques can be misleading, a battery pack that otherwise appears healthy could have a single problematic cell within the pack. Abusive conditions may also be unintentionally created, as voltage imbalance can create overcharge/overdischarge conditions, while the poor thermal design of a module can generate thermally abusive conditions. However, monitoring the changes to the impedance would aid in identifying ailing cells before a dangerous and costly failure.

This work examines the application of Electrochemical Impedance Spectroscopy (EIS) and Differential Capacity calculations (dQ/dV) as tools for determining the state of stability (SOS) of an electrochemical cell and multi-cell battery pack. The cells used for this study were commercial 10 Ah NMC cells subjected to thermal abuse coupled with EIS monitoring. This aims to not only provide a deeper understanding of how abused cells and batteries fail but also form the technical basis of a tool that could ultimately be used to interrogate cells of unknown stability and even monitor active cells for early signs of damage or failure. Fast impedance monitoring hardware previously developed at Idaho National Laboratory is used to provide not only monitoring after an abusive battery test but also look for changes in the cell while abusive conditions are applied. Differential capacity calculations are explored both before tests and after exposing single cells to different temperature ranges to examine any noticeable changes that may be monitored during charge and discharge operations. Lastly, X-ray diffraction measurements are performed on the cathode/anode before and after the abuse test to understand the effect of overheat in the structural changes. Figure 1 and Figure 2 presents the EIS and dQ/dV measurements for the NMC single cells heated from 50°C-150°C. Markers within the data can be identified and applied to cross-examine batteries of unknown stability as well as provide the basis for an active diagnostic method as part of a battery management system.

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.

Figure 1