The range is too limited, charging is too slow when it’s cold . . . the list of prejudices against electric cars is long. Even though progress is rapid, batteries remain the critical component for electromobility – as well as for many other applications, from smartphones to large storage devices designed to stabilise the power grid. The problem: Battery developers still lack a full understanding of what is happening, chemically and physically, during charging and discharging, especially in liquid electrolytes between the two electrodes through which charge carriers are exchanged.
Now Eric Ricardo Carreon Ruiz of PSI is bringing light into this darkness. A doctoral researcher in Pierre Boillat’s group at PSI, he is using neutrons from the Swiss spallation neutron source SINQ to investigate different electrolytes, studying for example their behaviour at fluctuating temperatures. His results provide important insights that could help in the development of new electrolytes and higher-performance batteries.
Insight with neutrons
Measurements made by the team, which includes PSI researchers Jong Min Lee, Natalie Stalder, and Lorenz Gubler as well as researchers from the United Kingdom and Denmark, rely on a technique called time-of-flight. This is well established in neutron imaging but has, until now, come a disadvantage: Exposure times were too long to make rapid changes visible. By combining the principles of spectroscopic neutron imaging and neutron attenuation, the team enhanced the time-of-flight technique in such a way that now even rapid changes – for example, when the liquid electrolyte in a battery solidifies at low temperatures – can now be seen.
During the analysis, the continuous neutron beam from SINQ is broken up into small packages. Since the neutrons in the beam have different energies and therefore different speeds, they pull away from each other as they travel through a tube several metres long. The fast neutrons reach the end of the tube first, with the slow ones following shortly afterwards. Because in physics the energy of an elementary particle is linked to its wavelength, neutrons fly out of the tube with different wavelengths, first those with short and then those with increasingly long wavelengths. The sample with the electrolyte behind it is scanned by neutrons with different wavelengths within a pulse of a few milliseconds. The neutrons interact with hydrogen atoms in the organic molecules that make up the electrolyte. By altering this interaction over time, depending on the wavelength, the researchers can identify which physical and chemical processes are taking place in the electrolyte.
How do batteries react at different temperatures?
Every battery manufacturer uses its own recipe for the electrolytes. The most common formula is a one-to-one mixture of the two organic substances ethylene carbonate and dimethyl carbonate, with a pinch of lithium salt. But other mixtures are also common, and the PSI experiment can reliably distinguish between them. The actual goal, however, is to track physical and chemical changes in the electrolyte at different temperatures. To this end, the temperature is adjusted up and down multiple times between minus 20 and plus 50 degrees Celsius. One thing this shows is that the liquid becomes solid at low temperatures. This has been known for a long time, which is why batteries in electric vehicles are warmed up before charging in the winter. But now, the PSI experiment makes it possible to understand exactly how, and at which sites in the battery, this process takes place. The spectroscopy images even show how the two organic components separate at low temperatures and how the ethylene carbonate sinks downwards. The measurements also provide, for the first time, spatial information about which electrolytes undergo this phase transition and at which temperatures.
Movies of processes inside a battery
«With time-of-flight neutron imaging, we can follow physical and chemical changes in an electrolyte as if in a movie, so that we can visually assess when and where these changes occur,» says Carreon Ruiz. «This is only the first step, as we are pushing the limitations of the technique to understand and improve batteries.» The method functions not only ex situ – that is, when the electrolyte is placed in the neutron beam in a sample container – but also in situ in complete battery cells like those installed in electric cars. And even operando, when these batteries are in operation and are being charged and discharged.
The method is suitable for studying not only electrolytes, but also many other materials as long as they contain a high proportion of hydrogen, stresses Pierre Boillat. He invites other researchers to further develop the method for additional applications and to combine it with other measurement methods. «No measurement technique, including time-of-flight neutron imaging, gives the answer to everything. It is only collaboration using techniques from both fundamental and applied research that can provide a sufficiently comprehensive picture.» In principle, battery manufacturers would also be welcome, in conjunction with interesting research projects. The method is well suited to the study of individual batteries – including with metallic casings –, the main limitation being the thickness of the sample.The results of this research project were recently published in the journal Science Advances.