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 . Scroll down for video . 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 . 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 . Li-ion batteries have been known to overheat and are vulnerable to thermal runaway . This occurs when the rate of heat generated in the cell is greater than the rate of heat lost . Lithium-ion cells with cobalt cathodes , for example , are not designed to rise above 130°C ( 265°F ) . This is because , at 150°C ( 302°F ) the cell becomes 'thermally unstable ' . Once it reaches this state it can lead to a thermal runaway in which flaming gases are released . 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 failing cells from affecting nearby cells . Source : Battery University . 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 .
Researchers subjected two Lithium-ion batteries to external heat . They used thermal imaging to see what happened inside each cell . Copper inside one cell reached temperatures of at least 1,085°C ( 1,985°F ) , while copper in the other was only able to reach a temperature of about 1,100°C.