State of charge effects on active material elemental composition changes between pre-thermal-runaway and post-failure states for 8-1-1 nickel-manganese-cobalt 18650 cells

Abstract

Lithium-ion battery failures have presented challenges to fire forensic investigators in determining in what scenarios they may have been able to initiate a fire, and what post-failure signatures exist to determine if the battery was more likely the cause or a victim of the fire. A battery's ability to initiate a fire depends on its state-of-charge (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. This paper introduces a technique that deduces the pre-failure SOC based on the elemental composition of 8-1-1 NMC Li-ion batteries before and after thermal runaway at different SOCs. The composition-SOC correlation was determined by (1) acquiring a baseline composition from the Nickel-Manganese-Cobalt (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. Test repetition at each SOC produced an uncertainty in the Ni/Mn ratios of less than 10 % in the 30–70 % SOC tests and less than 20 % in the 100 % SOC tests for externally heated cells, and less than 10 % in the 30 % SOC tests and less than 25 % in the 50–100 % SOC tests for internally heated cells. Ni/Co ratio does not vary significantly with respect to the SOC, suggesting that nickel and cobalt are lost from the electrode at a faster rate than manganese, with nickel being lost slightly faster than cobalt. The pressure vessel test conducted with 100 % SOC cells has also been found to contain nickel carbonyl in the inert test chamber after thermal runaway, which may be a product of nickel reactions in the electrode. Overall, this paper introduces a practical approach to determine a NMC (8,1,1) Li-ion battery's 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.