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A Trick for Making Batteries Safer Could Also Make EVs Affordable

SolidEnergy says its new battery materials store more energy and won’t catch fire.

Bulky and expensive batteries are the bane of electric vehicles. A new MIT spinoff company, SolidEnergy, says it has a solution: materials that can increase the amount of energy that lithium-ion batteries store by 30 percent or more and lower costs enough to make electric vehicles affordable.

Power prototype: One of SolidEnergy’s batteries (left). The foil on the right is one of the company’s lithium-metal electrodes.

The startup recently raised $4.5 million in its first round of venture funding. It is working with A123 Venture Technologies, part of the battery maker A123 Systems, to scale up the technology and bring it to market.

SolidEnergy replaces the graphite electrode used in conventional lithium-ion batteries with a high-energy lithium-metal one. That’s been tried before, but the metal tends to cause short circuits and fires. So the company has also developed improved electrolytes to make them safer. It plans to sell materials to battery manufacturers, rather than making batteries itself.

So far, SolidEnergy has made small, hand-built battery cells, similar to what you would find in a cell phone, using equipment and experts at an A123 Systems lab near Boston. (A123 Systems went bankrupt last year, and was acquired by the Chinese company Wanxiang.) These experimental cells store 30 percent more energy than conventional lithium-ion batteries, but the company calculates that the approach could eventually lead to a 40 percent improvement.

The first application of the technology will likely be in portable electronics, says cofounder and chief technology officer Qichao Hu. Electric vehicle batteries take longer to develop, in part because they need to last a decade, whereas batteries for powering electronics need only last a few years (see “How Tesla Is Driving Electric Car Innovation” and “Will Electric Vehicles Finally Succeed?”).

Lithium-metal electrodes are used in some specialty batteries now, but the measures that battery makers use to prevent short circuits weaken the batteries’ performance and increase the cost of making them. Typically researchers replace the liquid electrolytes used in conventional lithium-ion batteries with solid polymer ones, which are poor conductors of lithium ions, and have to be heated up to work properly.

SolidEnergy uses a two-part electrolyte. First, it coats the lithium metal with a thin polymer, much like the solid electrolyte others have used. The key difference is that it’s very thin, so it doesn’t slow down lithium ions, and the battery doesn’t need to be heated. The thin polymer can be applied using conventional electrode-coating equipment, Hu says. On its own, the thin polymer isn’t enough to prevent short circuits, so he supplements it with a liquid electrolyte.

Unlike conventional liquid electrolytes, the ones SolidEnergy is using—they’re a type of what’s known as an ionic liquid—are not flammable, which improves safety. In some recent, highly publicized battery fires, the part that was burning was primarily the electrolyte (see “What the Tesla Battery Fire Means for Electric Vehicles” and “Musk is Wrong to Say Tesla’s Model S is Less Likely to Catch Fire than Conventional Cars”).

SolidEnergy calculates that its materials could be used to make battery packs that cost $130 per kilowatt-hour, in line with U.S. Department of Energy goals for making electric vehicles affordable. Battery pack costs are typically kept secret, but estimates range from $250 to $500 per kilowatt-hour for packs in commercial electric vehicles.

It’s difficult to judge SolidEnergy’s claims, because it isn’t making any data about its cells public, says Jeff Dahn, professor of physics and chemistry at Dalhousie University. He says a major question is how many times the batteries can be recharged. Another significant challenge will be reducing the cost of the ionic liquid electrolytes, which are expensive in part because they’re made in low volume. 

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