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Smartphones often overheat if they're held close to the body or are used for long periods of time. 

And while the dangers of overheating lithium ion (Li-ion) batteries have been known for some time, researchers have now revealed exactly what happens inside the 'burning' cells. 

Thermal images show copper inside the battery reaching temperatures of at least 1,085°C (1,985°F) causing jets of molten material to burst from its vent.

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Researchers subjected Lithium-ion batteries to heat and used thermal imaging and non-invasive high speed imaging techniques to observe changes in their internal structure. Copper inside melted, indicating temperatures of at least 1,085°C (1,985°F), and it caused molten material to stream from the vent (pictured)

The footage is the first time the failure of Li-ion batteries due to overheating has been recorded. 

It was filmed by Paul Shearing from University College London and the findings are published in the journal Nature Communications.

Li-ion batteries are vulnerable to a condition known as thermal runaway, which occurs when the rate of heat generation is greater than the rate of heat lost. 

Although battery failure is rare, prevention of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries.

Dr Shearing and his colleagues subjected two commercial Li-ion batteries, known as Cell 1 and Cell 2, to external heat. 

They then used thermal imaging and non-invasive high speed imaging techniques to observe the internal structure of each.

Watch as a battery melts when it is subjected to heat abuse

Li-ion batteries are vulnerable to a condition known as thermal runaway, which occurs when the rate of heat generation is greater than the rate of heat lost. A Li-ion battery before being subjected to high temperatures is shown left, the same battery after heat abuse is shown right

During tests, one cell remained intact during battery failure and as the heat-generating reactions continued, hot gas followed by molten material jetted out through the battery's vent (pictured left to right). By comparison, the rapid pressure rise in a second cell caused the entire cap of the cell to detach

Cell 1 remained intact during battery failure and as the heat-generating reactions continued, hot gas followed by molten material jetted out through the battery's vent.

The copper material inside Cell 1 melted, indicating internal temperatures of at least 1,085°C (1,985°F).

By comparison, the rapid pressure rise in Cell 2 caused the entire cap of the cell to detach. 

In a real-world situation this could have increased thermal runaway because it allows oxygen into the cell. 

During a thermal runaway, the high heat in the failing cell can travel to the next cell, causing it to become thermally unstable as well.

In some cases, a chain reaction occurs in which each cell disintegrates one-by-one.

This means a battery pack can be destroyed within seconds or the venting of gases and heat can last for hours. 

As a safety measure, packs are fitted with dividers to protect the failing cell form affecting nearby cells. 

The authors found that thermal and electrochemical reactions inside both cells produced gas pockets that deformed the spiral wound layers of the cells. 

Cell 2 showed severe internal distortion of its architecture, which the authors suggest compromised its safety. 

Unlike Cell 2, Cell 1 was designed with a central cylindrical support, which seemed to help maintain its structural integrity. 

The authors hope their observations will lead to improvements in the design of Li-ion batteries and improve their safety features.