Millions of lithium-ion batteries are in use worldwide every minute of the day.

But they can do go wrong, catching fire or even exploding. 

The situation is now so worrying that many airlines – including American giant Delta – now have AvSax fire containment bags on board to deal with burning electronic devices safely. 

The bags means the fire can be sorted there and then so the plane does not have to make an emergency landing and passengers are not put at risk.

But why do the batteries suddenly explode? 

A new video by the American Chemical Society and PBS Digital Studios explains why this can happen. 

It’s complicated but here goes. 

According to the video, electricity is generated by the flow of electrons through a conductive material and a battery is a way of storing and controlling this flow.  

The four basic parts of a battery include a cathode, anode, electrolyte and circuit. 

The electrons flow from the negative anode around to the positively charged cathode, and the electrolyte is usually a fluid between the two.  

Lithium metal is the current standard for cathodes - it allows for the easy transfer of electrons and it's lightweight, making for a more portable battery pack.   

Lithium batteries usually have a lithium cobalt oxide as a cathode, graphite as an anode and a lithium salt bathed in a mixture of one or more alkyl carbonates as the electrolyte.  

However, these materials come with problems.  

For example, lithium is an alkali metal which means it's highly reactive, and organic electrolytes like dimethly carbonate are very combustible. 

Commonly used liquid electrolytes can be flammable, and they're also prone to forming dendrites - thin, finger-like protrusions of metal that build up from one electrode and, if they reach all the way across to the other electrode, can create a short-circuit that can damage the battery.  

The lithium and electrolyte form what's called 'thermal runaway.' 

Thermal runaway is what happens when a battery cell spontaneously explodes, and there are different reasons why this can happen, including overcharging a battery, overheating, physical damage, or faulty manufacturing, leading to an electrical short. 

“To keep it simple, anything that can lead to a rapid increase in heat can cause a problem,” the narrator of the video said.  

When multiple chemical reactions cause super-heating, the lithium cobalt oxide begins to release oxygen which can react with the alkyl carbonate electrolyte and even the leftover cobalt oxide. 

And during overcharging, the alkyl carbonate electrolyte can break down, creating carbon dioxide gas that will expand outward and burst the electrolyte open, exposing its flammable contents.  

According to the video, in laboratory studies, two flammable gases, hydrogen and methane, have also been detected as well as temperatures a high as 850°C (1562°F). 

This kind of heat is dangerous. The issue has become serious enough that the Federal Aviation Administration has recently banned certain smartphone models from planes.