Imaging used to capture exactly what goes on in a lithium-ion battery when a runaway chemical reaction occurs
Researchers in Europe have tracked the thermal runaway of lithium-ion batteries, using high-speed computed tomography and radiography together with thermal imaging to work out with unprecedented precision what goes on inside the batteries as they overheat. This knowledge should enable lithium-ion battery designers to devise safer batteries, reducing the risk of fire and allowing them to be used more safely in situations such as on aircraft.
Owing to their high charge capacity and voltage output, lithium-ion batteries are becoming more and more common in an ever-increasing range of electrical equipment, from portable electronics to electric vehicles. However, they have been dogged by rare instances in which the batteries have ignited or exploded because overheating or mechanical damage has triggered runaway exothermic reactions between the components.
A notable example occurred on the new Boeing 787 Dreamliner aircraft, which led to the temporary grounding of the fleet. Manufacturers have since installed various safety features such as pressure relief vents to reduce both the risk and severity of such incidents. But designing safety features is difficult as researchers' understanding of the rapid structural changes that occur during a battery's thermal runaway is extremely limited.
Paul Shearing of University College London and colleagues placed two standard commercial batteries in the x-ray beam-line of the European Synchrotron Radiation Facility in Grenoble, rotating them rapidly on a turntable while recording images at 1250 frames per second. They heated the batteries from the outside, recording thermal images until a runaway reaction was triggered. The first battery contained an internal steel cylindrical support: when thermal runaway began, the cell retained its mechanical integrity, although a jet of hot gas and liquid – produced by the battery's internal components melting – shot out of a pressure release vent in the top. In the second cell, which lacked the internal support, the battery's cap blew off, exposing the battery's interior. The test was conducted in an inert atmosphere for safety, but the researchers note that, in the real world, this would have increased the danger by allowing in oxygen and accelerating the thermal runaway. ‘It was quite surprising how that very simple addition had such a large influence on the thermal runaway process,’ says Shearing.
Daniel Doughty of Battery Safety Consulting in the US is impressed: ‘This is the very first time that some of the dynamics of the internal structure of the cell during runaway have been visualised,’ he says. ‘It is a most interesting, very fine piece of experimental work.’