Thoughts on EV Fires — Part 2

By Apoorv Shaligram

Apoorv Shaligram
5 min readApr 23, 2022
source: Twitter

To add to the previous blog on battery fires, a few more thoughts on the recent spate of fire incidents in EVs, mainly provoked by some of the conversations I have had over the past week.

  1. The flammability of LIBs has got nothing to do with the electrode chemistries, but rather the electrolytes that are used in these batteries. Since all LIBs use flammable organic electrolytes, every LIB can catch fire, irrespective of whether it utilizes LCO, NMC, NCA, LFP, LMO or LTO chemistries. The impact of chemistry is merely on the probability of “fire starters” and its extinguishability. Thus, a LTO anode based battery can reduce chances of lithium-plating related short circuits, whereas LFP or LMO cathode based battery allows us to be able to extinguish battery fires by reducing the chances of oxygen evolution at cathodes.
  2. A flammable component does not make LIBs unsafe. After all, we have been safely using ICE vehicles that have run on petrol, which is extremely flammable. If LIBs are unsafe, it is because of improper construction or incorrect usage. The same would be the case for ICE vehicles if the fuel tanks/engines were not designed properly or if people handled vehicles incorrectly. There is a reason why we have no smoking signs at petrol pumps.
  3. Battery fires are a result of the flammable electrolyte facing conditions of sudden heating that result in vaporizing, overpressurizing, venting and then ignition in presence of oxygen. This sudden heating is known as thermal runaway in battery terminology and is a result of sudden release of all stored energy (stored electrochemical energy + chemical energy released due to combustion of flammable electrolyte). Thermal runaway occurs in case the battery cell sees an external short circuit or an internal short circuit. Both cases lead to degradation of the separator layer insulating the electrodes from each other in the cell, and result in a hard short circuit between the electrode areas directly. This is the trigger point for the ignition of almost all LIB fires.

Does thermal management ensure complete safety?

Some blame has been laid recently on the higher ambient temperature in India as well as on vehicles drawing too much power from batteries, and has led to a belief that thermal management can prevent all battery fires. That is not entirely correct. Even if the EVs draw current at full power continuously, the battery cells won’t reach high enough temperature to trigger the degradation of separators. If we think about it, not only does the separator have to degrade, but also at a point when the battery has enough energy stored to cause a thermal runaway. So, discharging should be out of the picture anyway, since the amount of energy stored is dropping as the temperature is increasing! The concern can only be charging, where the rate is lower than peak discharge power, thus causing a lower temperature rise. So, by itself, charging or discharging at high rates is not responsible for thermal runaway. Improved thermal stability of separators over the past few years makes it even less likely.

The cause of these separator degradations is most likely some freak incidents such as short circuits due to variety of reasons that I had covered in the earlier blog here. Thermal management systems can control uniform temperature increase, but is unlikely to even pick up the onset of these thermal events as they are too fast and may be too localized. Remember that batteries use only a few thermal sensors and it is not possible to know the temperature at every point in a battery; and that it takes a single very fast thermal event to trigger the chain of events snowballing into a battery fire. However, thermal management can play a role in reducing the probability of battery fires. It all boils down to the probability of thermal events and the temperature rise that they can cause.

If I were asked how I see thermal events, I would respond with the above representative animation that shows the probability of thermal events causing temperature rise of a particular value or above, to help us visualize what thermal events do. The orange wall denotes the temperature rise required to trigger battery fires under given conditions. Therefore, this wall moves as per change in conditions e.g., say the separator is stable up to 140 C. If the starting temperature of the battery is 30 C, the wall would stand at a temperature rise of 110 C. In simple terms, any thermal event causing a rise of 110 C, even locally, will lead to thermal runaway and a battery fire. If the starting temperature is 60 C, the wall would move leftwards to 80 C. For better safety, we want the wall to move as far right as possible. Thermal management helps with one of them i.e., keeping the starting temperature of the battery on the lower side. Even then, there is variations in ambient temperatures and summer temperatures can get very high in parts of India. So, unless we are using active thermal management with refrigeration, and intend to keep it on full-time, thermal management can play only a minor role in minimizing the probability of battery fires. The other way is is to improve the thermal stability of separators and is inherently better as it increases the threshold of fire resistance. However, this works only till the temperature where the electrolyte self ignites and where the cathodes decompose to release oxygen (in case of layered oxide type cathodes).

Since battery fires are probabilistic events, it is sensible to add safety features which bring down the chance of a fire and add redundancies. Thus, multiple fire safety features will work better to contain fires as compared to any single feature.

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Apoorv Shaligram

Co-founder & CEO, e-TRNL Energy Working on next-gen battery technology to kickstart the EV revolution…