Peeking at pillowing in batteries
The intense, non-invasive light radiation generated by the synchrotron operated by the Canadian Light Source CLS in Saskatoon has provided one a glimpse of what can go wrong inside the powerful lithium ion batteries that so many of us now carry around in our pockets and purses. More specifically, this imaging strategy has revealed the gas dynamics responsible for “pillowing”, which causes a cell’s internal assembly to swell and lead to potential problems.
Such problems became all too real for tech giant Samsung last year, when it had to abandon an entire line of its smart phones because their batteries were spontaneously catching fire. In spite of this financial and technological catastrophe, however, some observers marvelled that this kind of problem was comparatively rare, given the huge number of batteries in use.
“The industry does go through a lot of safety tests — puncture tests, drop tests, vibration, heating, pressure and all sorts of different kinds of abuse — before they go to market,” says CLS investigator Toby Bond, who specializes in composite materials and energy storage systems. “None the less, with all the billions of cells that get manufactured, occasionally it happens.”
As he explains, looking for the causes of battery failure is why he spends a great deal of his time placing them in front of beam lines at the CLS synchrotron. “It’s a non-destructive technique and you can collect data much faster than you can with a typical x-ray source.”
Bond and his colleagues Jigang Zhou and Jeffrey Cutler recently published their findings in the Journal of the Electrochemical Society, where they described their computed tomographic technique for analysing the geometry of the jellyroll-like arrangement of electrodes within a battery. The resulting images of these structures are usually characterized by distinctive ripples, which represent material imperfections that can play a key role in determining how the battery responds to damage.
“When you abuse the cell, the internal components react by producing gas,” says Bond. “These imperfections grow and change more than other parts of the cell, which affects the distribution of pressure and current and the overall electrochemical performance.”
He adds that the findings do not answer crucial question of whether these changes are directly linked to dramatic failures, but they do show how to explore that possibility.
“For now we’ve identified that these small defects, which seem to be ubiquitous in the batteries that we’ve observed, may have a role to play in safety,” says Bond. “And it may have a role to play in understanding how the generation of gas and the physical change in the battery might affect its safety characteristics.”
Such research is warranted by the lithium ion battery’s place as the indispensable workhorse leading high tech innovations like the smart phone. This role is only going to become more pervasive as growing numbers of these energy sources find their way into our vehicles, as well as household and industrial power grids.
That also means these cells are being employed under ever more diverse conditions, including wide swings in temperature, pressure, and mechanical stress. In this context, Bond insists that it will be necessary to augment battery manufacturing practices with more detailed scientific insight into how these devices are functioning.
“Ultimately this could lead to a more holistic approach to understanding batteries,” he concludes.