High-resolution imaging of Li-ion electrodes by micro-LIBS
Imagerie haute résolution d'électrodes de batteries Li-ion par micro-LIBS
Résumé
The improvement of Li-ion batteries is a major challenge for the decarbonization of individual means of transport. Their behavior and performance mainly differs by the nature of their electrode materials, some of which are still in development. In order to fit with electrochemical models, the elemental distribution of lithium in electrodes needs to be measured at micrometric resolution. Nowadays, few techniques (e.g. MRI (Magnetic Resonance Imaging)) are able to characterize lithium. These techniques are generally time consuming and are difficult to apply in routine due to technical constraints. Furthermore, other approaches such as SEM (Scanning Electron Microscopy) or TEM (Transmission Electron Microscopy) combined to EDS (Energy Dispersive Spectroscopy) can image most of the elements with a high spatial resolution (~ μm) [1, 2] but cannot detect lithium with enough sensitivity due to its low atomic number. In this work we propose to apply the micro LIBS-based imaging approach for the characterization of the lithium distribution in various types of electrodes. Thanks to its high sensitivity for light elements and the possibility to focus the laser beam on very small surfaces, LIBS is very promising for this application [3-5]. However, there are several major obstacles to overcome. Firstly, the measurement resolution must be comparable to that of SEM – EDS (~ 1 μm). Achieving this resolution is indeed essential to observe properly lithium grains of a few micrometers in size. Secondly, given the small amount of material ablated per shot (~ pg), it is essential to work with a high sensitivity and optimized detection system. In this presentation, we will describe an original LIBS microscope using a high numerical aperture objective (×50 magnification), a UV laser beam (266 nm) shaped to achieve an M² close to 1, and a detection system optimized for measurement in the visible range. With this instrument, the lithium distribution has been imaged with a resolution of 3 μm (cf. figure 1), as well as other elements of interest, such as C, Co, Ni, Cu and Al. Charged and discharged NMC type electrodes with different lithiation states will be shown. To the best of our knowledge, this is the first time that such type of LIBS analysis is performed. Figure
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