Maxwell Technologies, a San Diego company that specializes in ultracapacitor energy storage, announced Monday that it is being bought by Tesla in a stock swap valued at $218 million.

Tesla CEO Elon Musk has said in the past that he expects ultracapacitors could provide a more likely bet for a breakthrough in energy storage than batteries.

Maxwell has developed new ultracapacitors with solid, rather than liquid, electrodes. It says the technology can improve durability and offer double the life-span of existing batteries at 10 to 20 percent less cost.

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Tesla is known for producing the largest number and most reliable and efficient lithium-ion batteries for electric cars, but like all electric-car-makers it is still looking for ways to make batteries store more energy in less space and for less money.

The ultracapacitors can hold up to 300 watt-hours of electricity per kilogram, about 20 percent more than Tesla's latest battery cells, and Maxwell says they have the potential to store more than double the energy by mass versus Tesla's current cells.

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Looking farther out on the horizon, Musk may be interested in Maxwell's ultracapacitors because they use solid-state technology, with no flammable electrolytes, Maxwell says, and they don't require cobalt, a rare metal required in lithium batteries whose price has been on the rise, and for which mining techniques have raised concerns about child labor and human-rights violations.

Tesla's current battery supplier, Panasonic, has said it expects solid-state batteries are at least 10 years in the future, even though upcoming Tesla competitors such as Volkswagen, Mercedes-Benz, Fisker, and Dyson say they expect to have solid-state batteries in production between 2022 and 2025.

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Maxwell has also supplied ultracapacitors to Geely, Volvo's parent company, to use in Volvo's plug-in hybrids.

Besides batteries for electric cars, Tesla also builds stationary battery packs for homes, businesses, and utilities that could benefit from any breakthrough in storage technology. Musk said in the company's most recent earnings call with investors that he thinks Tesla's energy storage business is poised for the biggest growth this year.