Making batteries safer by understanding the important thing thermodynamic variations between the preferred cell chemistries
Whereas NMC cells dominate the present technology of US and European BEVs, rising price pressures are forcing OEMs to take a more durable have a look at LFPs. Regardless of their decrease power density, LFP cells are nonetheless susceptible to single-cell thermal runaway, and controlling its propagation to adjoining cells stays a high precedence. To correctly handle thermal runaway propagation (TRP), pack designers should first perceive the three key thermodynamic variations between the 2 cell sorts.
First, NMC cells set off extra simply. The chart under reveals two equally sized prismatic cells triggered into thermal runaway with an exterior heating pad. The NMC cell goes unstable at a temperature of 160°C, whereas the LFP holds regular as much as 230°C. This deeper thermal nicely, mixed with LFPs’ larger measurement and weight for a similar vary, supplies an LFP pack with extra thermal mass than its NMC counterpart. The thermal mass of adjoining cells is likely one of the few benign pathways that stray power can take throughout a runaway occasion (the opposite being rejection by the pack’s cooling system). Whereas cell-to-cell conduction by itself can’t soak up the total brunt of a runaway occasion, thermal mass can present further time for occupants to drag over and get away from the automobile.
Second, NMC cells burn hotter than LFPs. As soon as pushed into thermal runaway, the NMC cell-face temperature peaks at 800°C, whereas the LFP solely spikes to 620°C. That is considerably paradoxical since, regardless of their decrease power density, calorimetry reveals that LFP cells usually have the next gas content material per amp-hour of storage capability. Nevertheless, as a result of NMCs smuggle extra elemental oxygen into the cell, their combustion effectivity and, subsequently, warmth launch is larger.
Lastly, the ejection phenomena related to every cell chemistry are very completely different. When an NMC cell goes into thermal runaway, there tends to be a 10- to 30-second interval wherein liquid, gasoline, and stable supplies are violently ejected by the cell vent. These stable supplies are usually bits of aluminum, carbon, and burning plastic. So NMC cells deliver all three parts of the hearth triangle – gas, oxygen, and an ignition supply – to a thermal runaway occasion. The ensuing torch-and-grit blast can burn by all however the sturdiest enclosure supplies and blankets the encircling cells in flaming gases and particles. Managing these flows is likely one of the most troublesome facets of designing a TRP-resistant NMC pack.
In distinction, LFP cells are inclined to emit principally smoke and gasoline which, though scorching, is usually not actively combusting. Whereas subsequent combustion and even explosions are attainable, the inside of an LFP pack is usually oxygen-starved throughout a thermal runaway occasion, so these dangers exist primarily exterior the automobile. Additional, the whole mass ejected from an LFP thermal runaway is barely 20-25% of the unique cell mass, versus 40-50% for an NMC. So, each the hazard stage and amount of LFP ejecta are decrease than that of NMC designs.
Briefly, controlling thermal propagation in an NMC pack is primarily about gasoline administration and secondarily about direct cell-to-cell warmth switch. In an LFP pack, that dynamic is reversed. That is lucky: with the arrival of aerogel thermal obstacles resembling PyroThin cell obstacles, cell-to-cell warmth switch is now a solved downside. Managing combustion gases and flaming ejecta stays stubbornly onerous to deal with.
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