Rust: The Electric Vehicle Battery’s Worst Enemy

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The electrolyte in batteries corrodes the electrodes, degrading them much like rust eats away at iron and steel.Photo courtesy of MitsubishiThe electrolyte in batteries corrodes the electrodes, degrading them much like rust eats away at iron and steel

“Rust Never Sleeps.”

For fans of Neil Young, the words signify a great album from 1979.

They’re bad news, though, to those folks developing batteries for electric vehicles.

Corrosion, a form of rust, is a big reason it’s so tough to increase range and longevity, and cut costs.

I got an explanation from Erik Spek, chief engineer at TUV SUD Canada, in Newmarket. The business — which tests battery cells under normal and extreme condition, trying to mimic a decade of use in a year or two — is part of a global corporation based in Munich, Germany.

Batteries are simple in theory but fiendishly complex in reality, so anyone hoping EVs can duplicate the range, refuelling speed and cost of gasoline engines needs patience. Such performance is at least a decade away.

“Breakthroughs don’t really happen,” Spek says. “Every time you change something, it may improve one aspect but will likely hurt another, so you must retest. It takes time and money — lots of both.”

Batteries are comprised of cells, each containing two electrodes. One, the cathode, is positive. The other, the anode, is negative. Between them is a liquid called the electrolyte, as well as a separator — a bit of plastic material that prevents the electrodes from touching so they can’t short circuit.

When a typical EV battery is fully charged, each anode is full of lithium ions. As the driver hits the accelerator, ions flow through the electrolyte to the cathode, creating an electric current. When most have made that journey, the battery must be recharged, which pulls the ions back to the anode, ready to move again when power is demanded.

For sufficient power, the ions must move quickly and in large numbers. For range, the electrodes must be able to hold a lot of them.

Ideally, the anode and cathode contain the same number of spaces for ions. If the cathode holds fewer than the anode, power and range are reduced. Excess cathode capacity wastes materials and adds weight, the enemy of EVs.

Equalizing the number of spaces isn’t easy, because the electrodes differ in the materials they’re made from, the jobs they perform and how they change as the cell is used.

A previous column described how electrodes expand and contract as ions arrive and depart and, gradually, crumble under the stress. One solution is to break the material down into microscopic nanoparticles, too tiny to change size.

But, says Spek, there’s worse: The electrolyte corrodes the electrodes, degrading them like rust eats away at iron and steel. Echoing Young, he says: “Corrosion never goes away.”

The damage happens faster on the anode, generally made of graphite, than on the cathode, usually a metallic oxide. It’s most intense when the battery cell is nearly depleted or fully charged.

That’s one reason many EV batteries are engineered to operate within the mid-range of their capacity: The restriction is intended to help them to last longer, allowing carmakers to offer warranties consumers find acceptable — at least eight years or 160,000 kilometres. But that, unfortunately, means the vehicle carries unusable capacity, and weight, which cuts its range.

Researchers are trying different materials; in particular, how to replace graphite, which despite its poor corrosion performance is the best we have to date. One possibility is a form of silicon, which appears to not only resist corrosion better but also hold more ions.

It’s at an early research stage, Spek says. “Not many people are talking about it yet.”

Another approach is a less corrosive electrolyte. A lithium salt known as LiBOB, “looks really attractive,” he says.

But the reality of battery development is that to gain improvements, “you give up something in almost every case. Most possibilities look promising until you start testing; then, it’s not so rosy.”

Rust never sleeps. Neither, it seems, do the researchers.

 

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