The photoemission spectroscopy (X-ray, XPS, or ultraviolet, UPS) is one of the direct probes of the electronic structure of materials change during redox processes involved in lithium ions-batteries at the atomic scale. However, it is limited by the extreme surface sensitivity, with a typical photoelectron path length of a few nanometers to the energies usually available in the laboratory , . Moreover, the spectra interpretation requires the ability to accurately model the electronic structure, which is particularly delicate in the case of transition metal based electrode materials. Upon lithium insertion and de-insertion, the charge transfer toward cations and anions induces local electronic structure changes requiring an adapted model that takes in account the electronic correlations between atoms.
In this thesis, we propose to use these limitations to our advantage to explore the electronic surface structure including the solid electrolyte interphase (SEI), and the bulk of the active cathode particle.
Thanks to the lab-based hard X-ray photoemission spectrometer (HAXPES), the electronic structure of the bulk of the electrodes (LiCoO2 and LiNiO2) materials have been studied up to about 30 nanometers , . To widen our picture on the role of cation and anion from surface to bulk in the lamellar metal oxide electrode for lithium-ion battery, this thesis will focus on mixed lamellar metal oxide Li(Ni1-x-yMnxCoy)O2 (NMC).
The comparison between the Soft-XPS and HAXPES spectra, during battery operation (operando) and post-mortem, will allow decoupling of the surface and core spectra for different NMC compositions and at different stages of the battery life cycle. The interpretation of the photoemission spectra will be done by direct comparison with ab-initio calculations combining density functional theory (DFT) with dynamical mean field theory (DMFT) , . This coupled approach will allow to go beyond the usual techniques based on cluster models, which do not take into account long-range screening, and to validate the quality of theoretical predictions on the effects of electronic correlations (effective mass, potential transfer of spectral weight to Hubbard bands) .
The thesis will include an instrumental (in particular, calibration of Scofield factor on model systems) and theoretical (prediction of core photoemission spectra based on DFT+DMFT calculations) development. The performance of electrochemical systems based on different cathode materials (NMC with different compositions) in combination with liquid and solid electrolytes and a Li metal anode will be studied in the frame of combined experimental and theoretical soft and hard X-ray photoemission spectroscopy.
The candidate will be hosted at the PFNC in the Laboratory of Characterization for the Energy of CEA Grenoble under the direction of Dr. Anass BENAYAD (department of Material) and LMP (Department of Electricity and Hydrogen for Transport) under the supervision of Dr. Ambroise Van Roekeghem.
Contact : anass.benayad@cea.fr et ambroise.vanroekeghem@cea.fr