Bad wiring or bad chemistry? What's behind Samsung's exploding smartphone batteries?

Samsung has been forced to recall thousands of its Galaxy Note 7 smartphones because some of the batteries in the devices overheated and exploded.

The company hasn't yet explained the cause. And it's curious, because lithium-ion batteries — the kinds used in the phones — are safely used in millions of wireless devices around the world.

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"Lithium-ion manufacturing is a very competitive business," said Ulrich von Sacken, a B.C.-based lithium-ion battery consultant with a PhD in physics. "The manufacturers are all pushing the limits of the technology to produce cells at higher capacity. So in this case, Samsung apparently tried to go too fast."

So where exactly did Samsung go wrong?

How lithium-ion batteries work

Lithium-ion cells, put together in a kind of sandwich, make up a battery.

There are many variations, but all cells contain these core parts:

• Cathode: the positive side, made of conductive material like aluminum.
• Anode: the negative side, often made of graphite.
• Electrolyte: flammable liquid, often made of lithium salt and an organic solvent.
• Separator: a thin piece of material, usually plastic, that separates the positive side from the negative.

To charge a battery, an electrical current has to reach the cells via the conductive material. That's why we plug phones into electrical sockets.

When charging, von Sacken said in an interview, positively charged lithium-ions floating in the electrolyte pass from the positive side to the negative side of the cell, flowing through the separator, which is permeable to the ions. As a battery dies, the ions travel back to the positive side, waiting to be charged up again.

The separator, which von Sacken said can be thinner than a human hair, is a physical barrier to keep the positive and negative sides apart so that the battery doesn't short-circuit and overheat.

He said certain spots are more vulnerable than others.

"The manufacturers go to considerable lengths to make sure there aren't shorts there, but evidently something went wrong with Samsung's manufacturing, and they're having a lot of spectacular shorts," said von Sacken.

Every company has its own battery formula that it guards.

And "everything matters," said Jeff Dahn, a physics professor at Dalhousie University who studies lithium-ion batteries. "It's a very complicated interconnected system."

He said that what may work in one company's battery may not work in another.

Number of potential problems

In Samsung's case, Dahn said the company may have changed the electrolyte to something less stable, it could have increased the voltage going to the cells, or it could have thinned the separator too much.

He said there could also be a "weird interaction" between the charging method and the cell.

"Maybe they've done this and that and whatever — there's a lot of things that could have an impact on this particular lithium-ion cell in this particular piece of equipment."

It could also come down to the testing.

Before a device goes out to the market, it undergoes an "enormous number of safety tests," said Dahn.

No guarantee

"These tests push a lithium-ion battery hard, so I suspect people were very surprised when this started happening," he said.

The companies test the devices under a variety of conditions, but no test can ever completely guarantee a device's safety. 

"No testing protocol can ever guarantee zero failures. The number zero is not scientifically credible — the question is how small can you make it," said Gary Rubloff, a professor at the University of Maryland's Laboratory for Advanced Materials Processing.

For example, Rubloff said a company can't test to see whether the batteries will explode in 20 years. If they did that, no product would ever get out the door.

Dendrite formation

Another theory is that something called dendrites formed, well-known to wreak havoc.

Rubloff said dendrites are microscopic fibres made of lithium. They aren't supposed to form in batteries, but they sometimes do.

The separator is meant to keep these conductive filaments from connecting the anode and cathode.

A short-circuit can be dangerous because batteries are designed to last for a long time, so they store a lot of energy.

"If something fails, that's a big problem, because you dissipate all the energy [stored in the cells] very, very quickly and that can heat up the battery and ignite the organic electrolyte," he said.

'Minuscule' failure rate

This is clearly bad for Samsung, but this problem has plagued other companies too, from the makers of so-called hoverboards, electronic cigarettes and even backup power systems on airplanes.

Dahn said the failure rate is very small. Ten years ago, there were 200 documented safety incidents.

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"Today, even though the production rates of lithium-ion have probably gone up by a factor or 10 compared to 10 years ago, it's still 200 [safety incidents] a year," he said.

Rubloff agreed the record is good, but said it may be irrelevant.

"The failure rate is probably minuscule but it doesn't matter because some of those failures could have been dangerous, for example on an airplane," he said.

Dahn said he hopes to get a closer look at the Samsung batteries to "do the CSI on them" to find out their exact chemical and structural makeup.

He and Rubloff both said it's impossible to know for sure what may have gone wrong unless Samsung releases that information.