Academia – LRCS LAb (CNRS/University of Amiens Jules Verne) and RS2E ( the French network for Research and Technology on the Electrochemical Storage of Energy) have joined forces to unveil the best 3D structure for electrodes in lithium-ion batteries in electric vehicles, using synchrotron X-ray holotomography on ID16B. The results are published in Advanced Energy Materials.
The market for electric cars is starting to take off. In 2019, they registered a 40% year-on-year increase, and they account for 2.6% of global car sales in 2019, despite only being introduced to the market a decade ago.
A team from CNRS at Amiens, LRCS lab & RS2E network, together with the automobile manufacturer Renault and the ESRF went on a quest to find the best electrodes for the new generation of electric cars of the brand. Renault developed different new electrodes to study for best performance.
The electrode microstructure with electronic and ionic percolation networks plays a crucial role in the performance of lithium-ion battery. They consist of three different phases. The active material part is called NMC, for nickel, manganese and cobalt (LiNi0.5Mn0.3Co0.2O2). Then there is the carbon binder domain (CBD), where the binder is a polymer and the carbon enables a good electronic connectivity inside the electrode. The third phase is porosity, which enables the liquid electrolyte to travel through the electrode.
“In order to find a correlation between the performance of the electrodes with their 3D configuration, we needed to quantify in 3D the three different phases”, explains Arnaud Demortière, CNRS scientist from LRCS laboratory and corresponding author of the paper. “So we went to ID16B, a beamline at the ESRF, with the capability to do exactly what we needed in a very short time of acquisition”, he adds. The team in beamline ID16B used X-ray holography nanotomography to study the three phases of different electrodes with a variable amount of CBD. The technique is based on phase contrast, which gives the ability to properly distinguish heavy (NMC) and light elements (CBD) in the composite electrode.
A lot of current technology is based on a ‘trial-and-error’ approach. In this case, the characterisation of the materials done at the ESRF induced a change in some well-established beliefs. “It was thought that when you have lots of CBD, your battery is supposed to have a better performance. However, what we observe is the complete opposite: we observe a better performance when the amount of carbon binder is lower”, explains Demortière. The data led the team to come up with a new hypothesis: when there is not much carbon/polymer (CBD), the electrolyte liquid can circulate better and easily reach the active material. “Without our synchrotron experiments, we could not have reached this conclusion”, concludes Demortière.
From high quality holotomography 3D volumes, the team has correlated the 3 phases of 500 individual particles with a spatial resolution of 50 nanometres on a volume of 50 microns. From each particle they made statistics about the surface contact with CBD, the porosity and even the particle-to-particle connectivity. With this statistic approach, researchers can monitor the evolution of the performance as a function of the interfacial areas.
The next step for the team is to monitor in 3D the lithiation/delithiation process inside electrode during the electrochemical cycling using home-made operando cell. The ultimate goal is to be able to correlate in 3D the lithiation mechanism of several individual NMC particles with the 3D local configuration of carbon/polymer (CBD) and porosity networks inside the cathode electrode. Using this new strategy scientists will have a complete picture of what is going on in battery electrode during the charge/discharge process and therefore, new solutions improving their performance will emerge. Julie Villanova, scientist on ID16B, adds the value of the new EBS in this research: “EBS will improve the spatial and temporal resolution for nano-tomography, opening new opportunities for operando experiments”.
Featured image: Visualisation of the individual NMC particle (in red), along with the interfacial area with the other phases. Credits: A. Demortière.
Provided by ESRF