This study examined state of charge (SOC) effects on active material elemental composition changes between pre-thermal-runaway and post-failure states for 8-1-1 nickel-manganese-cobalt 18650 cells.
This paper introduces a practical approach to determine a Nickel-Manganese-Cobalt (NMC) (8,1,1) Li-ion battery's state-of-charge (SOC) according to the elemental composition collected from its debris after a failure event. The approach is based on the correlation between SOC and Ni-Mn-Co ratios and supported by the theoretical reaction kinetics of NMC cathode materials during thermal runaway, as evidenced by the identified Ni-containing products. A battery's ability to initiate a fire depends on its SOC, which influences a failed cell's energy output and capacity to ignite fuel contents. Knowing a failed battery's pre-failure SOC is thus of great interest in battery failure forensics in helping analyze whether it is the cause or a victim of a fire. To determine the pre-failure SOC of burned batteries, a simple and repeatable technique is needed to analyze burned batteries and provide insight on the possible pre-failure SOC. The composition-SOC correlation was determined by (1) acquiring a baseline composition from the NMC cathode material in a pristine 18650 cell, (2) triggering thermal runaway of 18650 cells (by overheating) at different SOCs and collecting burned electrodes, (3) performing chemical analysis using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) to obtain post-failure composition, and (4) examining elemental composition variation with respect to SOC. In addition to characterizing the burned contents inside, the ejected (gas and solid) contents during thermal runaway were also collected in a sealed pressure vessel chamber and analyzed to explain the mechanistic background behind the observed variation. The ejected solids were analyzed using SEM-EDS, and the ejected gases were analyzed using gas chromatography (GC) and gas detection tubes. The results show that as the SOC increases, the Ni/Mn and Co/Mn ratios after thermal runaway decrease from the atomic ratios in the pristine condition. (Published Abstract Provided)
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