New Samsung Galaxy Note7 phones were available in U.S. stores Wednesday, September 21, after exploding lithium-ion (Li-ion) batteries forced the company to recall about a million units.

Lithium-ion batteries have been making this kind of news for years—they’ve caused fires in hoverboards, laptops, in other phones, and even in the electrical system of a Boeing 787 Dreamliner jumbo jet.

So why, 25 years after the batteries hit the market, are lithium-ion batteries still prone to these problems? And when will engineers finally have a solution?

First a reality check: Despite these high-profile incidents, if you look at the numbers, Li-ion-powered devices are relatively safe. K.M. Abraham, one of the pioneers of the Li-ion battery and a professor at Northeastern University, says, “There are more than a billion cell phones and computers used in the world every day,” and by comparison, the number of accidents has been small.

“It’s a very, very low probability of your phone catching fire,” said Abraham. "Lithium-ion batteries have a failure rate that’s less than one in a million.” By comparison, the National Oceanic and Atmospheric Administration says your chance of being struck by lightning in the course of a lifetime is about 1 in 13,000. 

Of course, even one fire or explosion is one too many. Here’s everything you need to know about what causes Li-ion batteries to occasionally catch fire, and what improvements to mobile phone battery tech could be on the way.

Why Li-ion Batteries Explode

Normally, it’s a manufacturing defect, and apparently that was the situation with the Note7 phones. But the underlying issue is that Li-ion batteries contain a lot of energy in a compact package—which, of course, is why they are used in everything from phones to Tesla electric cars.

A Li-ion battery has an energy density of up to around 160 watt hours per kilogram (Wh/kg), roughly twice that of a fresh alkaline battery or a NiCad rechargeable battery. To produce that power it relies on three main components: the positively charged cathode, which is made of metal oxide, the negatively charged anode, which is made of graphite, and the liquid electrolyte—a solvent containing lithium salts—that enables the electric charge to flow between the two poles.

Like two troublemakers in a grammar school classroom, the cathode and the anode need to be physically separated. Lithium-ion batteries accomplish that with a permeable polyethylene separator, which can be as little as 10 microns thick. As batteries improve and engineers try to pack more power into a smaller package, that thin plastic separator is taxed to its limit.

“The separator has really gotten thin,” says Isidor Buchmann, founder and CEO of Cadex, a battery equipment manufacturer that also runs the educational website Battery University. “And when that happens, the battery becomes more delicate.”

When the separator is breached, it causes a short circuit, which starts a process called thermal runaway. According to Abraham, this is one of the major ways that fires begin. The chemicals inside the battery begin to heat up, which causes further degradation of the separator. The battery can eventually hit temperatures of more than 1,000° F. At that point the flammable electrolyte can ignite or even explode when exposed to the oxygen in the air.

Will these catastrophic failures, rare though they may be, spell the end of the Li-ion battery? Not likely, says Buchmann. While safety is a concern, it’s just one of a number of factors in battery design. Most of the others are related to energy density, battery life and charging performance.

Li-ion vs. the Contenders

“It’s very difficult to produce something better than lithium-ion,” says Buchmann. But that may not be necessary, because Li-ion batteries themselves are getting better.

“There is a lot of effort going on in using better materials that will improve safety even if there is a flaw in the battery,” Abraham says. The next generation of Li-ion batteries will feature more-rugged polymer separators that have a much higher melting point, in case thermal runaway does begin. And that’s not the only advance we can expect in the next few years. “There are new electrolytes that won’t catch fire when they come in contact with the air,” he adds.

While Li-ion represents the state of the art in battery design and will continue to do so in the near future, there’s a huge market waiting for the company that introduces a battery that can top Li-ion in safety and performance.

The most promising contender is the so-called solid-state battery. In these batteries, that problematical liquid electrolyte would be replaced by a solid electrolyte, which is far less flammable. The result could be a super-stable battery with better energy density that’s far less likely to ignite or explode.

A Michigan-based company, Sakti3, which has been working on a solid-state battery, was bought by Dyson, the vacuum cleaner giant, in 2015 for $90 million.

That valuation, substantial though it is, seems to represent a glass that’s only half full. The company’s solid-state technology shows promise, but if the company were ready to introduce a Li-ion replacement that could drop into the next generation of smartphones, an electronics giant like Apple or Samsung would have offered 10 times as much. At least, that's the way George Crabtree see it. He's the director of the Joint Center for Energy Strorage Research, which is run by the Department of Energy. 

“Everyone would agree, if you could make a battery with a solid-state electrolyte, you’d do it,“ Crabtree says. “It’s safer, it’s not as flammable, it doesn’t degrade.”

For all its potential, the inherent problem with a solid-state battery is that lithium ions move much more slowly in solids than in the liquid electrolytes currently in use. For that reason, the charging speed of solid-state batteries tends to be glacial.

A variation on the solid-state theme, a lithium-sulfur battery, is already being marketed for use in high-altitude drones where its light weight makes it ideal for the application. But that battery has its own drawback. A severely limited number of use cycles—currently around 20—means that much more progress would be needed before lithium-sulphur batteries could power mobile phones, laptops, and other consumer electronics.

There are still more battery technologies being developed, each with its own promises and problems. For instance, lithium-air batteries could potentially offer as much as 10 times the energy density of Li-ion batteries—but lithium-air batteries need to use oxygen from the air in the cathode, but must be protected from moisture, carbon dioxide, and other impurities.

Magnesium-ion batteries could pack more power than today’s cellphone batteries at a lower cost, and potentially could be more stable. But none of the cathodes or anodes currently used by Li-ion cells would work, and revamping those components could take another decade of work.

Still, while no replacement for Li-ion batteries will be appearing in phones in the next year or two, the field is so crowded with contenders that experts think we’re not far from devices powered by batteries that are both safer and more powerful.

“I think it’s a big moment for storage. Over the next five to 10 years we’ll see big changes in storage technology,” Crabtree says. “And the reason is that so many people want it.”