Exposed! Why those Samsung batteries exploded

Lithium ion batteries show up in all sort of tech these days, from your phone and laptop to airplanes and electric vehicles. But a voluntary recall of some 2.5 million Samsung Galaxy Note 7 smartphones after reports of battery explosions is raising new concerns about their safety.
Last week the Federal Aviation Authority warned passengers not to turn on or charge the devices during flights -- or even put them in checked baggage. Some international airlines have placed similar restrictions on traveling with the smartphone. And the U.S. Consumer Product Safety Commission, advised consumers "to stop charging or using the device."
Here’s what you need to know about the lithium batteries that probably power a lot of your tech -- and why they sometimes catch fire.
What are lithium ion batteries, and why are they so popular?
Lithium ion batteries are a bit different from the basic AA's you use to power your TV remote. They're rechargeable, often built directly into devices and rely on the chemical lithium as their primary fuel.
Lithium ion batteries are especially popular in devices such as laptops and phones because they store energy super efficiently and are slow to lose their charge.
“You get a lot of oomph relative to older technologies that were the same size -- often two to four times the voltage,” explained Stephen Hackney, a professor of materials science and engineering at Michigan Tech University.

Lithium ion batteries help power many of our devices.

How do they work?
Like pretty much all batteries, lithium ion batteries work by storing energy and releasing it through controlled chemical reactions. A lithium ion battery has two electrodes -- places where electricity can enter or leave the battery -- on opposite sides. One electrode, called the anode, is filled with negatively charged ions. The other electrode, called a cathode, contains positively charged ions and lithium. You can think of the anode and the cathodes like the plus and minus signs you often see on batteries.
When you use a battery, the lithium moves over from the cathode to the anode -- and when you charge it, the lithium moves back over to the cathode. There’s a separator inside that keeps the anode and the cathode from touching because that can trigger mishaps such as fires and explosions.
So, what can cause the explosions?
The reason you can shove so much power into lithium ion batteries is that lithium basically “wants to react to almost anything” -- which can lead to explosive results, Hackney said.
But one of the most common reasons the batteries can explode is because of mistakes in the charging process, he said. Inside the devices that rely on  the batteries there is software that tells them exactly how much the batteries should be charged and how fast. If those protocols are set incorrectly, it can destabilize some chemicals inside the battery and cause a chain reaction that researchers call a “thermal runaway” that may lead to fire or explosions.
Overheating can also cause explosions,- which is why your phone probably pops up with an alert about needing to cool down when it gets too hot.
Another reason could be shoddy manufacturing or rough user treatment. If unwanted materials, like scraps of metal, accidentally end up inside the battery when it’s being made, they can short a cell of the battery and set off a thermal runaway. So could dropping a device if the impact causes a break in the separator between the anode and cathode.
What happened with the Galaxy Note 7?
It seems like a manufacturing problem. The company reports at least 35 cases where the batteries combusted due to “a very rare manufacturing process error” in which the anode and cathode touched, the company said in a statement on its U.K. website.
The company decided to temporarily pull the phone off the market just two weeks after it was released and is offering replacements to people who already purchased the Device