Suppressing a battery fire is no easy task, and a critical condition of success is how fast a technology managed to detect a fault and trigger its responses. A team of researchers led by Oleg Levin, Professor of the Department of Electrochemistry at St. Petersburg University, has developed a new approach that promises to have a competitive edge in early detection and response – polymers that change conductivity as a function of heat or voltage. The researchers claim their method is cheap and scalable. Convinced of the applicability of the technology, the researchers have already received a patent for their invention.
There are multiple reasons for a battery fire, but most come down to overcharging or short-circuiting. For example, in the case of the battery fires in Arizona, local utility Arizona Public Service concluded in a report jointly developed with technical auditor DNV GL that dendrites must have been the culprit. Dendrites in lithium-ion batteries are lithium deposits on the anode. If they grow large enough, they can short-circuit a battery, resulting in a fire.
Batteries have monitoring systems in the form of microcircuits attached to them. They are supposed to track all the battery parameters and shut the circuits down should there be a problem. But Levin claims that in most incidents of the big battery fires, these monitoring systems had failed due to manufacturing defects.
“This is why it was particularly important to develop a safety strategy of the battery based on the chemical reactions to block the flow of electric current inside the battery pack,” said Professor Levin. “To this end, we propose to use a special polymer. Its electrical conductivity can adjust to the voltage fluctuations in the battery. If the battery works normally, the polymer does not prevent the electric current from flowing. If the battery is overcharged, there is a short circuit, or battery voltage drops below normal operating levels, the polymer goes into a so called isolator, circuit breaker, mode.”
When a battery fails due to overcharging or short-circuiting, the temperature rises to above 70°C or 90°C chemical reactions inside the battery are triggered, causing a self-exciting chain reaction. This chain reaction, the thermal runaway, needs to be prevented before a point-of-no-return has been reached. According to the team, this has made much of the research. When the polymer starts acting as an isolator, the damage could already be so significant that the fire will still propagate. In its current development stage, the polymer reacts to voltage rather than heat and is applied over the inner current collector's entire surface.
“The most difficult part in developing the ‘chemical fuse ‘was to find an active polymer,” said Oleg Levin. “We knew a great variety of polymers of this class. Yet choosing the one that would be suitable to create a prototype was a hard nut to crack. Moreover, we had to advance the technology by developing an industrial version to show that we had come up with an idea of effective battery safety strategy.”
Levin reports that researchers at this institute had been dealing with such polymers for the past eight years, but a bespoke project gave new impetus to the project over the last two. That is a little surprising, given the number of battery fires that occurred. According to Levin, between 1999 and 2012 that were 1,013 reported cases, according to Levin, but that number exploded to 25,000 between 2012 and 2018.
And the problem is still far from being solved. Just this week the Australian Competition and Consumer Commission has formalized a recall of a home storage product line by LG Chem, due to the risk of overheating and fire.
For the time being, the polymer only works for lithium-iron-phosphate (LFP) batteries. The reason is that different cathode compositions work at different voltage levels. For LFP batteries, this is at 3.2 V. Their rival nickel-manganese-cobalt (NMC) cathodes operate at voltages between 3.7 and 4.2 V depending on the type of NMC-cell.
“Changing the structure of the polymer might result in changing its conductivity to make it suitable for other types of cathodes that are on the market today. We have some thoughts as to how to make this safety strategy more universal by adding a safety component into the polymer to adjust to changes in temperature levels in the battery. This is expected to eliminate all fire risks associated with the batteries,” said Levin.
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