Now having talked about a wholistic approach to battery fires, and also a way to estimate the size of the risk, it’s time to discuss the actual strategies to mitigate the risk.

To summarize the last two newsletters in a single sentence each:

1. Safety is an important problem which will not have a single solution, rather, many solutions will be stacked on top of each other, as they have been with seat belts, airbags, crumple zones, anti-lock brakes, etc.
2. Lithium-ion battery hazard scales like the size of the battery squared (one factor for the size of the fire and one for the likelihood of a fire), which means that an e-bike battery fire hazard is 10,000 times greater than that of a cellphone, and an electric vehicle battery fire hazard is 100,000,000 times greater than that of a cellphone.

This last point, when you consider it, is a jaw dropper. But if you’ve seen videos of a cellphone fire and of an electric vehicle fire, that gets you partway there.  (Kelsey-can we insert a link to one of each here?)

One solution that was popular a few years ago was the concept of a solid-state battery.  After all, the liquid electrolyte was eliminated, and that was most of the fuel, right?  Since then, some solid-state batteries have come to full production, and the industry has had a chance to dig in, and while it is clear that solid electrolytes can contribute to battery safety, they are by no means a silver bullet to the problem.  Rather than answer this myself, I’ll provide a few significant references:

• Video of an electric bus powered by solid-state batteries burning in Paris.
• Video of a solid-state battery factory burning.
• “Are solid-state batteries safer than lithium-ion batteries,” a paper by scientists at several US DOE national labs.

We have since realized that the electrolyte is actually only a small fraction of the fuel—with high energy density cells, most of it is in the anode an cathode, which can’t really be eliminated.  But eliminating the burning electrolyte is a step in the right direction.

So if solid-state batteries are not the final solution, what is?  It’s a good question—and the answer is that there isn’t one. 

It comes down to this: while there is a significant effort in the industry to improve battery safety, there is a much more significant effort to increase the energy stored, reduce the size and weight of batteries, reduce the cost and eliminate excess complexity.  These work against each other—putting more and more energy into a smaller package will always reduce the safety. 

The solution, rather, is not going to lie in a single solution, but in using a strategic portfolio of tactics that are appropriate for the specific application.  Below are several large categories of tactics that are available:

Protection uses external means to protect the cell from the environment.  This could be both mechanical protections, as well as thermal protection by controlling the temperature. 

Examples include the titanium plates that Tesla added to the bottom of the Model S when fires were started by road debris, or the phase change materials such as those offered by KULR or Latent Heat Solutions.

Mitigation protects the external environment from the cell, if the cell goes into thermal runaway. These can also be both mechanical and thermal and may be redundant with Protection tactics.

Examples include the box placed around the battery in the Boeing 787, and also intumescent materials.  

Detection uses equipment or electronics to detect TR early and takes measures to make it less likely to occur.  These sensors will feed into a battery management system (BMS) that can then take action to mitigate any potential thermal runaway.

Examples can be as simple as a thermocouple or voltage sensor, or as complex as an ultrasonic detection system that constantly probes the inside of the battery.

Reduction involves replacing internal parts of the battery with materials that are not flammable, have reduced flammability, or are difficult to ignite.  Of course, solid-state batteries are the first example of this, but there are others.

Examples include, in addition to solid-state batteries, non-burning electrolytes, a reduction in the amount of aluminum in the battery, using less energetic electrode materials like lithium iron phosphate or a reduction in the flammability of the materials that are used in the battery pack.

Perfection involves the improbable process of producing millions or billions of cells that are perfect, without defect.  This was the strategy, along with correcting some design flaws, taken by Samsung after the recall of the Galaxy Note 7 in 2016. 

The route to perfection involves thousands of quality controls, measurements and checks.  For Samsung, they implemented a 100% inspection by CT scan of the final battery, which was expensive but has so far eliminated a repeat of the issue.

Control the flow of energy inside the cell and stopping it if it goes above certain limits.  These can be either devices to disconnect the cell in certain circumstances, or materials inside the jelly roll that can do the same thing.

Examples include a positive thermal coefficient (PTC) device and current interrupt device (CID), as well as a shutdown separator and metallized polymer current collector.

How to put it all together

While it’s clear each of these will reduce the hazard of a lithium-ion battery pack fire, it’s not clear how it all comes together.  It’s my belief that its multiplicative.  If you use an electrode like LFP that has half the energy, and is half as likely to ignite, then that would reduce the fire size by 2 and the probability of ignition by 2 as well, taking the hazard for a 100 kWh EV battery fire down from 100,000,000 times worse than a cell phone to only 25,000,000 times worse.  It may not look like much, but it’s a great start.  And it’s possible those numbers are underestimations.  The point is that this framework gives us a way to calculate the hazard, and set about systematically reducing it in a way that we can quantify.  And with a way to quantify it and all the creative and talented engineers in the industry working on it, the risk will be reduced to a hazard level that will be acceptable without further recalls.

In the next newsletters, we’ll apply this framework to both the e-bike battery pandemic in New York City, and to recent electric vehicle battery recalls, so see how effective the industry is right now.