Inside the 3D electrode architecture: towards better performing batteries for electric cars

29-01-2021

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.

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In 2019, electric car sales registered a 40% year-on-year increase, and account for 2.6% of global automobile 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 batteries for the new generation of electric cars of the brand. To that end Renault developed new electrodes to be studied for best performance.

The electrode microstructure with electronic and ionic percolation networks plays a crucial role in the performance of lithium-ion batteries. The electrodes consist of three different components: The active material 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 carbon facilitates good electronic connectivity inside the electrode. The third structural component is a network of pores, which enables the liquid electrolyte to fill 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 components”, 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 need in a very short time of acquisition”, he adds. The team at beamline ID16B used X-ray holotomography with nanometric resolution to study the three structural components of different electrodes with a variable amount of CBD. The technique exploits phase contrast, which allows one 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 particular case, the characterisation of the materials performed at the ESRF induced a change in some well-established beliefs. “It was thought that when you have lots of CBD, your battery has 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 liquid electrolyte can circulate better and easily reach the active material. “Without our synchrotron experiments, we could not have reached this conclusion”, concludes Demortière.

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Remi Tucoulou, beamline operation manager of ID16B and part of the team, with beamline technician Benjamin Holligerin in the experimental hutch of the beamline. Credits: S. Candé.

 

Using the high quality holotomography 3D volumes, the team could quantitatively analyse the spatial relation of the three components of the electrode in an ensemble of 500 individual active particles with a spatial resolution of 50 nanometres in a volume of 50x60x60 microns. From each particle they derived statistics about the surface contact with CBD, the porosity and even the particle-to-particle connectivity. With this statistical approach, researchers can now 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 an electrode during the electrochemical cycling using a custom-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 porous network inside the cathode. Using this new strategy a complete picture will emerge of what is going on in battery electrodes during the charging/discharging process, leading to new solutions improving their performance. Julie Villanova, scientist on ID16B, highlights the value of the new EBS for this research: "EBS will improve the spatial and temporal resolution for nano-tomography, opening new opportunities for operando experiments".

Reference:

Nguyen T-T, et al, Advanced Energy Materials, doi.org/10.1002/aenm.202003529.

Text by Montserrat Capellas Espuny

Top image: Visualisation of the individual NMC particle (in red), along with the interfacial area with the other components. Credits: A. Demortière.