C2I 2017: Understanding failure - a path towards safer lithium ion batteries

To forge a path towards safer lithium ion batteries, a team at UCL set about studying the mechanisms that cause rapid and catastrophic failure in li-ion batteries

The year 2013 was one to forget for a pair of high-profile OEMs whose use of lithium-ion batteries in big-ticket transportation technologies resulted in high-profile failure. In the spring of that year, Boeing’s fleet of 50 787 Dreamliners was grounded by regulators after batteries smouldered during a flight over a Japan, and others caught fire on a different aircraft parked at a US airport.

By the autumn, Tesla’s shares plummeted by six per cent after one of its vehicles struck an obstacle and burst into flames. The batteries hadn’t failed, but the collision was said to have set off a chain of events that caused the inferno. In 2015, three airlines halted bulk shipments of lithium-ion batteries in their cargo aircraft after tests by the US Federal Aviation Administration found overheating batteries could cause major fires.

By 2016 the highly competitive world of smartphones was brought into sharp focus by Samsung, which issued a global recall of all Galaxy Note 7 models due to well-publicised anomalies in the handset’s battery that caused phones to smoulder and burn.

Despite sometimes making headlines for the wrong reasons, lithium-ion batteries are a serious player in the energy-generation game. According to Variant Market Research, the global market for li-ion batteries is expected to reach $56bn by 2024 from $25bn in 2016; growing at a compound annual growth rate of 10.6 per cent.

Li-ion batteries are feted for their light weight and high-energy density, which can act against the ubiquitous energy source.

For this reason, a team at UCL set about studying the mechanisms that cause rapid and catastrophic failure in li-ion batteries. It was a journey that saw them work with partners in the UK, France and the US to share knowledge and develop an understanding of the batteries in order to mitigate the risks associated with them.

A key factor of li-ion battery failure is a phenomenon called thermal runaway, which occurs when the rate of heat generated inside a battery exceeds the rate of heat dissipation. When thermal runaway occurs, the battery can transform from being entirely intact to being completely destroyed within a fraction of a second.

Understanding what happens at initiation and during propagation is key to the design of safer li-ion batteries and the team at UCL was the first to hit upon an idea that would capture these events in real time.

One of the project’s leaders, Dr Donal Finegan, who now works at the National Renewable Energy Laboratory (NREL) in the US, said the UCL team combined the European Synchrotron Radiation Facility’s (ESRF) high-energy synchrotron X-rays plus thermal imaging to map changes to the internal structure and external temperature of two types of li-ion batteries that were exposed to extreme levels of heat.

The work built upon observations made previously with X-ray computed tomography (CT), a technique limited to capturing static images after thermal runaway had occurred. The new method, developed in collaboration with ESRF, led to an experimental design that allowed simultaneous high-speed X-ray imaging, thermal imaging and electrochemical spectroscopy while safely inducing catastrophic failure of commercial Li-ion batteries.

Despite this, Finegan said the team were, to an extent, ‘feeling in the dark’ because there was no way of inducing thermal runaway on demand and from a known location within a battery. What the UCL team needed was a device that could induce ‘worse-case scenarios’ on their terms.

A magazine article alerted Finegan to developments in the US, where in October 2015 NREL announced it had patented the internal short circuit (ISC) device, a tiny piece of equipment designed to induce the very conditions that UCL was looking for.

In doing so, the NREL team had provided a marked improvement on other methods of inducing failure that included nail and rod penetration, crushing the battery, applying voltage, or increasing the battery’s temperature.

Dr Eric Darcy, battery technical discipline lead at NASA’s Johnson Space Centre, teamed up with colleagues at NREL to co-develop ISC. Matthew Keyser, NREL senior engineer and one of ISC’s inventors, estimates that the risk of thermal runaway is extremely small: one to 10 in a million cells (one to 10 parts per million) for electronics. Despite the low risks, Finegan explained that they needed a device that would help inform the design and manufacture of li-ion batteries in safety-critical applications such as spacesuits.

Finegan was put into contact with NASA via NREL and was subsequently sent six samples to analyse in the synchrotron. Having the ISC in the batteries meant that the UCL team were able to observe runaway initiate and propagate on their terms. The resulting experiment was the first of its kind, using a custom-made multi-functional battery abuse system, the ISC device, and

X-ray imaging facilities.

The results were well received in the US and the resulting partnership saw NASA and NREL focus on developing and testing an approach that induces ‘worst-case’ failure within commercial and prototype battery designs. UCL and NPL prepare and carry out experiments, while ESRF continues to fine-tune and monitor the imaging conditions and experimental set-ups required for optimal results.

In practice, NREL manufactures the ISC device, carries out quality control and inserts them into prototype batteries with integrated safety devices that are representative of commercial designs. Finegan said the batteries are transported to the NASA Johnson Space Centre where thermal runaway is induced in a small number of ISC-loaded batteries to identify appropriate operating conditions and features of greatest interest to observe using high-speed X-ray imaging at the ESRF. The results and progress from NREL and NASA are conveyed to UCL during weekly conference calls, and the remaining ISC-loaded batteries, along with other samples of interest, are then shipped to UCL for lab-based X-ray CT inspection and preparation for the experiments at the ESRF. The team at UCL and ESRF then prepares and carries out the high-speed imaging experiments at the ESRF in France.

Finegan, who now works at NREL, concluded that UCL’s work with NPL, NASA and NREL has led to the identification of the safest commercial battery designs for aerospace and automotive applications, and that battery manufacturers are now supplying their battery and safety designs for testing.