It turns out Li ion batteries (LIBs) vent TDLAS friendly gases. So I’m going to quickly break down which gases are vented and in what concentrations. This summary is focused on LiFePO4 (LFP) batteries since it seems these are the safest of the LIBs currently available.
First, a little context on what causes the gas venting in LFPs. Thermal runaway can be triggered by overcharging, internal shorts, or external heating. This can lead to decomposition of the cell materials and the release of flammable gases. While LFPs are more stable than other LIBs, they can undergo thermal runaway around 200 °C (392 °F). To avoid this, batteries are typically thermally managed to operate below 60 °C (140 °F).
In an MDPI Energies Journal article, Qian et al. (link here) placed a 50 Ah LFP pouch cell in a nitrogen filled pressure vessel. They triggered thermal runaway via external heating. They monitored temperature and pressure and identified two venting events. They used gas chromatography and identified the major constituents, CO, H2, CO2, and CH4.
Figure 1 contains plots of the three battery conditions the researchers ran, 0%, 50%, and 100% State of Charge (SOC). In all three cases, as the battery temperature increases, at approximately 5000 seconds, there is a release of gas from the battery. This corresponds to the initial opening of the battery valve. For the 50% and 100% SOC runs, there is a more dramatic increase in pressure near 6500 seconds. This second event corresponds with the failure of the batteries.
Figure 1 – Battery voltage and chamber pressure for three states of charge.
Table 1 contains the gas chromatography results. The table shows gas composition by volume for the primary gases measured. In both cases hydrogen gas is present in levels above the lower flammability limit of 4%. From a spectroscopy and TDLAS perspective, CO, CO2, and CH4 appear in high enough levels that a TDLAS sensor could be developed to make highly precise and time resolved measurements. While it would not be a direct measure of the more dangerous hydrogen gas, methane for example could still be monitored and by its existence infer that hydrogen is also present.
Table 1 – Gas composition by volume.
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