Embedded sensor pinpoints stress inside batteries

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay.  

Imagine you have a completely sealed box filled with chemicals that are causing an electrochemical reaction. You can detect the effects of this reaction outside of the box, but you can’t see what’s happening inside. This is the current state of commercial battery technology, but now, PhD candidate Laura Albero Blanquer and Professor Jean-Marie Tarascon at the Collège de France in Paris have taken a leap forward to find out what happens inside the cells of both liquid and solid-state batteries with startling clarity.

LAURA ALBERO BLANQUER: For solid-state batteries, that is, the pellets; we are able to place the sensor within the electrode and also in the interface between the electrode and the solid electrolyte. And that allows us to monitor stress evolutions locally […] Up to now in the field, that was only done by external force sensors which monitor the stress changes at the cell level.

JEAN-MARIE TARASCON: The battery was a black box with respect to pressure because they put the sensor outside. So they measure strain, but they don’t know where the strain was coming from.

LAURA LEAY: Laura and Jean-Marie are using optical Fiber Bragg grating sensors. These sensors have been used to monitor composites in large-scale engineering projects for decades and their new research shows that they can also be used to monitor localized changes in batteries as they are in use.

LAURA ALBERO BLANQUER: Batteries, in another way, can be also seen as a mixture of composite materials as they are a mixture of the active material, binder, carbon, and in the case of solid-state batteries also the solid electrolyte.

LAURA LEAY: Pressure of between 2-8 MPa is applied to the composite battery materials to ensure that the different layers remain in good contact. During charging, the electrode expands and the pressure increases by around 4 MPa. 

The increased pressure could cause enough strain in a solid-state battery that the different layers delaminate and so prevent the battery from charging and discharging. 

LAURA ALBERO BLANQUER: They have some chemo-mechanical issues as, due to the stiff nature of the solid electrolyte, the volume changes cannot be any more be accommodated as it happens in liquid batteries. There is a close relationship between the chemo-mechanics and the battery performance.

LAURA LEAY: The pressure also causes the optical fiber, which has a circular profile, to compress, squashing it into an elliptical shape. 

Fiber Bragg grating sensors reflect monochromatic light and a shift in the peak wavelength will be seen when there is a change in temperature, pressure, or stress. The sensors were carefully calibrated so that only changes in stress could be detected. 

When the optical fiber compresses into the elliptical shape, the peak also splits in two. This effect allows stress to be detected in both the axial direction, that is, the axis between the electrodes, and the transverse or radial direction. This meant that Laura could figure out where the strain developed within the battery, and with more accuracy than using external sensors.

LAURA ALBERO BLANQUER: The first time that we were applying the transverse pressure to the fiber we started seeing peak splitting, and that’s actually a phenomenon well known in optics that is dynamic birefringence, and by tracking the difference between the two peaks you can get directional stress. 

LAURA LEAY: This research will lead to new opportunities to look at commercial liquid cells, and to greater insight into the chemo-mechanical processes in electrodes and in all solid-state batteries which could lead to enhanced performance. 

Future work will also focus on large cells and working with battery manufacturers to make commercial batteries with embedded sensors. This could also be relevant to hydrogen fuel cells which will also use composite materials. For Jean-Marie, this is all about bringing different fields together.

JEAN-MARIE TARASCON: We are at the beginning of, kind of, you know, bridging different communities in order to cross-fertilize our expertise and knowledge on different types of topics.

LAURA LEAY: This work was published in a recent issue of Nature Communications. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.