A new device allows researchers to make lithium ion (Li-ion) batteries fail on demand, and that’s a crucial step in making them safer.
Modern life relies on lithium ion batteries, which are found in all kinds of consumer electronics as well as hybrid and electric vehicles. We’re demanding more and more from them, in terms of the energy they store, the speed at which they recharge, and the conditions in which we expect them to work. Yet under certain electrical, thermal, or mechanical conditions, the active materials within Li-ion batteries can break down exothermically, generating large amounts of heat that can lead to a positive feedback loop called thermal runaway. Whilst these failures are rare, where cells are combined into battery packs, the failure of one cell can lead to cell-to-cell propagation of thermal runaway, resulting in fires, and even explosions.
Understandably, these catastrophic battery failures make the news. For example, Boeing 787 Dreamliner aircraft were grounded by the US Federal Aviation Administration in 2013, after two battery incidents. An updated battery system offering better containment of battery fires allowed the aircraft to return to the skies three months later.
Samsung ceased production of their Galaxy Note 7 phones in October 2016, after batteries from two different batches (and suppliers) caught fire. And earlier this year, Amazon recalled six versions of the AmazonBasics portable lithium-ion battery charger/power bank after more than 50 incidents of the units overheating, leading to chemical burns and property damage.
Thankfully, thermal runaway is rare in batteries under normal conditions, but batteries do need to be designed to be as safe as possible. One factor hampering designers is a limited understanding of what happens inside batteries when they fail.
The 18650 cell
One of the most widely used Li-ion cells has a cylindrical geometry with a diameter of 18mm and a length of 65mm; 18650 cells have even been used in space. These cells are designed with numerous safety features to prevent thermal runaway or minimise its effects. Safety features include pressure relief vents, current interrupt devices (CIDs) and positive temperature coefficient (PTC) switches. Outside of the cell, devices such as battery management systems (BMSs) and temperature controls are also used to prevent hazardous conditions arising. However, these features cannot prevent internal short circuits (ISCs), which can arise from manufacturing defects that are difficult to detect.
Investigating what happens when batteries fail is very challenging, due in part to the unpredictability of where thermal runaway starts, and the rapid changes it causes.
The ISC device
The need for a reliable test method that replicates the behaviour of a real commercial cell design undergoing an ISC led Eric Darcy of NASA and Matthew Keyser of the National Renewable Energy Laboratory (NREL) to develop a device to generate an electrical short between two electrically conducting layers. The ISC device is a metallic ‘puck’ encased in low melting point wax between electrodes, making it possible to activate an ISC on-demand and at a pre-determined location within the cell, to recreate ‘worst-case’ failure scenarios.
Professor Paul Shearing and his group from UCL Chemical Engineering have been collaborating with NASA, NREL, the UK’s National Physical Laboratory (NPL), Diamond Light Source (DLS) and the European Synchrotron Radiation Facility (ESRF) to investigate how batteries fail, a task that requires the use of multi-scale, multi-speed imaging techniques to observe thermal runaway during ISC tests.
Using the ISC device means researchers know the location at which a short circuit will occur, but that’s just the first stage in capturing the initiation and propagation of thermal runaway. A time resolution above 1000 images per second is needed to capture sufficient detail to describe the dynamic failure mechanisms, as thermal runaway can propagate throughout 18650 cells in fractions of a second. Cell rupture can occur in under 0.01 seconds, requiring an even higher time resolution and imaging rate. If you want to see inside a battery cell as it fails, you need to use an X-ray source with an extremely high photon flux – a synchrotron.
Prof. Shearing and his team have used the UK’s national synchrotron (DLS) and ESRF in Grenoble. The proximity of these world-leading synchrotrons allows researchers to carry out more complicated experiments, involving extreme conditions associated with battery failure. As Prof. Shearing explains, “We originally developed these techniques at the Advanced Proton Source in America. Having the UK’s national synchrotron on the doorstep, and the European Synchrotron Radiation Facility in Grenoble, has completely changed the way we work. We’re able to do far more complicated experiments now we don’t have to ship everything over to the US.”
High-speed synchrotron imaging techniques
The team use high-speed synchrotron X-ray computed tomography (CT) and radiography, combined with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during the initiation and propagation of thermal runaway in 18650 cells. This new approach gives unparalleled insights into the structural and thermal dynamics associated with thermal runaway and failure of commercial Li-ion batteries.
X-ray CT is an effective tool for diagnosing battery failure mechanisms post-mortem, and rapidly advancing capabilities make it possible to capture failure events and degradation mechanisms across multiple length scales and over very short time periods. It provides high-resolution 3D images of the battery cells.
High-speed radiography allows the team to identify and characterise the mechanisms that lead to the most catastrophic types of failure, rupture, and explosion - events that progress too quickly to be captured by high-speed tomography. The combination of both techniques provides a powerful tool for linking external risks with internal phenomena during thermal runaway.
At DLS, researchers are able to make use of beamlines (experimental stations) optimised with different capabilities. Using the high-speed radiographic imaging capability of the I12 beamline allowed them to image the nucleation of failure within a working battery, and see at very high speed how that failure can propagate through an individual cell and from one cell to another. They were able to see the exact mechanism by which failure starts and how it spreads, and to design systems to prevent the spread of that failure.
The higher spatial resolution of a neighboring beam line, I13, allowed them to probe the microstructure of a battery electrode and explore how it changes over time and a number of charge cycles. High-resolution diffraction studies carried out on I11 allowed them to see very small changes to the crystallography of the electrodes as a function of state of charge of the battery. These can be linked to the changes in the electrode morphology, which together provide a signature of battery degradation. I11 is also home to a unique Long Duration Experiment (LDE) capability that allows electrodes to be monitored over weeks and months, probing key questions over battery lifetimes.
Using their synchrotron X-ray studies, the research team were able to characterise four types of failure: controlled ejection of contents, cell bursting, puncture of the top button, and escape of the internal mandrel. They captured the lead-up to cell rupture in unprecedented detail, and showed that the bursting process occurs in distinct stages, but can be prevented by the inclusion of a second vent at the base of the cell. The inclusion of a base vent improves the ability to safely manage the rapid generation of gas during thermal runaway, and greatly reduces the risk of cell rupture. Coupled with an improved understanding of the battery electrode morphology and crystallography, the team are gaining new insights into the performance and safety of these devices. Being able to investigate battery cells at different length-scales, from the atom to the system, provides a wealth of information that will inform the design of the next generation of lithium ion battery cells.