SOLEIL is the French national synchrotron facility, located on the Saclay Plateau near Paris. It is a multi-disciplinary instrument and research laboratory whose mission is to conduct research using synchrotron radiation, to develop cutting edge instrumentation on the beamlines, and to make these developments available to the scientific community. SOLEIL synchrotron, a unique tool for both academic research and industrial applications across a wide range of disciplines including physics, biology, chemistry etc., opened in 2008. It is used annually by thousands researchers from France and abroad. SOLEIL is based on a synchrotron source that is state-of-the-art both in terms of brilliance and stability. This large scale facility, a partner of the Université Paris Saclay, is a “publically owned” private company, founded by the CNRS and the CEA.

The development of specific liquid-electrochemical cells is required to carry out Operando X-ray microscopy studies of Li-ion battery materials during their cycling, in order to capture the electrochemical mechanisms occurring at the nanoscale in individual active grains. For that purpose, we propose here a Master 2 internship on the development and commissioning of an existing Operando liquid-electrochemical cell (Norcada) available at the HERMES beamline.

Since last 20 years, due to sustainability issues dictated by societal demands, more importance has been given to the rechargeable batteries as a viable solution to reinforce the use renewable energies. Among the storage devices, Li-ion batteries, because of their high energy density, have conquered most of today’s portable electronic and EV markets, and they stand as serious contenders for grid applications. On the other hands, Na-ion batteries have recently attracted interest of the energy storage material community because of the huge availability and low cost of Na compounds compared to Li.  To push the frontier of both technologies, the investigation at the micro- and nano-scale is vital to monitor the electrochemical reaction paths inside a single active particle, and therefore to prevent serious side effects limiting the energy density or the long life cyclability.

Scanning transmission X-ray microscopy (STXM) technique is based on the use of a focused X-ray beam (order of tens of nm) to map a sample using a very accurate motorized stage.  Since the incident energy can be tuned close to the absorption edge of the selected elements (i.e. transition metal, oxygen, sodium), it is possible to monitor the evolution of the oxidation state of the redox centers as well as the Na distribution in a single particle upon cycling, making operando STXM the suitable technique to unveil the electrochemical insertion/extraction mechanisms in a large class of electrodes at the nanometric scale. Recently, Chue et al. showed the possibility to follow electrochemical reactions within LiFePO₄ individual grains using Operando STXM experiments (XANES on Fe-L₃ edges) with specific liquid-electrochemical cell. They observed, in real-time, the evolution of the spatial localization of Fe³⁺ and Fe²⁺, which is related to the state of charge and also demonstrated that the Li composition and surface reaction rates, at nanoscale, control the kinetics and uniformity in solid-solution domains during electrochemical cycling¹.  Another STXM study on the working mechanism of the a-Fe₂O₃ anode have underlined how it is important the knowledge of detailed composition and electronic structure during cycling process in order to deep understand their effect on capacity².

Thanks to the high spatial resolution and the energy range available at the HERMES beamline³, we can track the evolution of the different phases during cycling in a single particle (~200 nm).

We propose to investigate the nanoscale spatial variations of Vanadium oxidation states into Na₃V₂(PO₄)₂F₃ (NVPF)⁴ individual particles (1-0.5µm large, 100-50 nm thickness) that control the Na-ion pathway during electrochemical cycling. The crystal structure of pristine Na₃V₂(PO₄)₂F₃ has been studied by X-ray diffraction⁵: the structure exhibits P4₂/mnm symmetry and is consisted of pairs of corner shared (via one F atom) VO₄F₂ octahedra, which are equatorially connected to the PO₄ tetrahedra via O atoms. Na ions are located in the channel sites along the a/b-axes providing good diffusion pathways for Na ions. There are two different Na sites with an occupancy ratio of 2:1, and both are surrounded by four O ions and three F ions. The fully occupied Na site is denoted as Na1, with Na ions being slightly off center due to the repulsions between two neighboring Na1 ions. The other Na site (denoted as Na2) is partially occupied due to the short distance (1.87 Å) between two nearby Na2 sites. This study will provide important information on the kinetics and uniformity of ion insertion reactions at the solid-liquid interface governing the rate capability and lifetime of NVPF cathode materials for Na-ion batteries.

The student will have several missions:

(1) Testing the electrochemical (NVPF/NP30 electrolyte) and liquid electrolyte compatibility using the Norcada cell equipped with different chips (with and without beam).

(2) Estimating the X-ray beam damage on liquid cell with the appropriate chip configuration.

(3) Participating in the operando experiments on the HERMES beamline.

References:

[1] Lim, J. et al., Science 353, 566 (2016) (courtesy for images).

[2] Lv, X. et al., J. Mater. Chem. A 3, 5183 (2015).

[3] R. Belkhou, S. Stanescu, S. Swaraj, A. Besson, M. Ledoux, M. Hajlaoui, D. Dalle, J. Synchrotron Radiat., 2015, 22 (4): 968-979, 10.1107/S1600577515007778

[4] T. Broux et al. Chemistry of Materials 28.21 (2016): 7683-7692.
[5] Z. Liu et al. Chemistry of Materials 26.8 (2014): 2513-2521.