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FIRE! … the tiny triggers that can send lithium-ion batteries up in flames

As engineers and investors place their hope in enormous batteries to calm the entrance of more renewable energy into the grid, news photos of Australia’s latest Tesla “big battery” gushing flames only days after testing began in late July will have brought on instant nausea.

At least two Megapack battery packs at the 300MW project were damaged in the blaze at Neoen’s Victorian Big Battery at Moorabool, near Geelong, with roads closed due to toxic smoke.

With the National Electricity Market effectively on the drawing board as the states and the Australian Energy Market Operator slowly work towards the best combination of new transmission and generation that will deliver a clean and stable grid, a flare up like the one at Moorabool will fuel the doubters who make the energy transition such hard work.

It’s not yet known how the fire started, but any time is a good time to understand more about risks in lithium-ion storage technology.

Lithium-ion batteries contain materials that are inherently flammable, says Australian National University associate professor Alexey Glushenkov. Liquid electrolytes, the medium through which lithium ions travel during battery operation, are flammable, and graphite used inside batteries will also burn at high temperatures, as will plastic used in insulation. “There are quite a few flammable bits inside batteries,” Glushenkov tells EcoGeneration.

Looking for a spark

The blaze in Moorabool was limited to two Tesla Megapacks. Photo: Fire Rescue Victoria.

One of the unintended ways energy can be released from a battery is via what Glushenkov calls “chemical means”, or fire.

When fire happens, it occurs initially at the individual cell level. A containerised battery solution may contain thousands of cells, and the remote chance of cell failure – no matter how small – is multiplied by the number of cells in a unit.

Understanding that the failure of a single cell might endanger a battery that houses many thousands of cells, manufacturers dedicate a chunk of R&D to making sure ideal operating conditions are not violated. It is not good for cells to overheat and it is not good for them to be overcharged, or exceed the voltage they were designed to tolerate.

If operating conditions veer off course gases can build up inside cells (known as “gas evolution”). Hydrogen, carbon monoxide, carbon dioxide and hydrocarbons including methane and propane can be produced. Depending on the chemistry used in the cell, some of these gases can be flammable.

A cell that is heating up and swollen by a build-up of gas can open, allowing oxygen to fuel fire inside.

Cells are rated to be charged within a voltage range. To exceed voltage on the upside by overcharging is to invite trouble. Battery management systems should stop this happening, unless something goes wrong. Overcharging may cause overheating, unexpected chemical reactions and gas evolution.

Short circuits

Photo: Fire Rescue Victoria.

The most common cause of fire in batteries is a short circuit, when electrodes are joined and the battery instantly discharges. This shouldn’t happen, because the electrodes in a cell are kept apart by electrolyte and a membrane separator.

When electrodes are separated, electrons can only travel outside a battery. For it to discharge, a battery must be connected to external circuits.

“A separator doesn’t allow those two electrodes to touch,” says Glushenkov, who specialises in battery materials at the ANU’s Battery Storage and Grid Integration Program.

A short circuit can be the result of a manufacturing defect or straight-out bad design, he says. The reason it’s not always easy to determine the cause of a short circuit is that they can also start at a molecular scale, with overcharging leading to a build-up of metallic lithium on the negative electrode (the anode).

A deposit will grow like a fractal tree, branching and rebranching. If a deposit grows to push against and rupture a separator membrane, it can touch the positive electrode (the cathode).

“As the process happens again and again those structures, called dendrites, can penetrate the separator,” he says. “A short circuit will result and the battery discharges instantaneously, causing a lot of heat to be generated.

“If there is the possibility of explosion, extreme heat makes it possible.”

The process is reversible, so that a deposit will dissolve into the electrolyte on discharge.

Dendrites are not supposed to occur in a lithium-ion battery naturally, Glushenkov says. “If it happens, it’s because of some oversight in the electronics or the safety measures they deploy.” Researchers around the world are working on new electrolytes and different structures of raw materials to suppress the way dendrites may form.

If a fire starts in one cell within a battery that contains thousands of them, it is likely to spread sequentially as neighbouring cells overheat, deform and start to burn. These are not conventional conflagrations and fire brigades will usually work to contain them rather than extinguish them.

It will be a long time before the cause of the Moorabool fire is determined.


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