Description du sujet de thèse

Over the past two decades, computational chemistry has become a cornerstone in battery research. However, theoretical investigations of alkali-ion insertion and extraction in electrode materials have relied almost exclusively on adiabatic quantum mechanical approaches. These methods inherently assume thermodynamic reversibility and therefore fall short in describing partially or fully irreversible processes, for which access to diabatic electronic states is critical. Understanding these irreversible mechanisms is essential, particularly in systems where redox reactions induce significant structural reorganization or even material degradation during charge/discharge cycles.

This limitation is especially relevant in the context of anionic redox processes, first experimentally observed in 2013 within the family of lithium-rich transition metal oxides (A[AxM1–x]O2). [1,2] These systems continue to raise fundamental questions regarding the nature of the (O–O)^n– species formed during oxidation and the extent to which the process is reversible upon discharge. [3–6] While the identification of charged-state species is generally tractable using adiabatic methods—owing to their focus on equilibrium configurations—the characterization of the excited electronic states involved in their formation is significantly more challenging.

On the experimental front, X-ray spectroscopies, such as X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS), provide unique access to these excited states with elemental and chemical specificity. These high-resolution techniques exploit ultrafast radiation sources—enabling femtosecond and attosecond time scales—to probe the local electronic structure of materials in a highly sensitive manner. X-ray spectroscopies are now well established in the study of energy materials [7], offering precise insights into the electronic configurations of ordered systems, where the response of chemically equivalent atoms is nearly homogeneous.

However, interpretation becomes markedly more complex in disordered or compositionally heterogeneous materials, where the same element may exist in multiple oxidation states, bonding environments, or coordination geometries. This complexity is characteristic of alkali-rich transition metal oxides of the A[AxM1–x]O2 type, which exhibit statistical disorder on A and M cationic sites, variable alkali content, multiple cationic redox centers, and dynamic cation migration during cycling. These factors contribute to a highly intricate structural and electronic landscape.

In such materials, the oxygen sublattice is best described as a composite of distinct oxygen sub-networks, each with different reactivities and electronic structures that are strongly influenced by their local environments and bonding characteristics—ionic or covalent—with neighboring cations. These local environments can evolve over repeated electrochemical cycles, further complicating the behavior of the system.

The central objective of this PhD project is to harness the atomic specificity of X-ray spectroscopies to deconvolute the electrochemical reactivity of these various oxygen sub-networks. By identifying the excited states involved in redox activity at a local scale, the project aims to provide a mechanistic understanding of anionic redox phenomena. This research will be conducted in collaboration with Eleonora Luppi (LCT, Sorbonne Université) and Valérie Pralong (CRISMAT, Caen), with funding from the RS2E (French Network on Electrochemical Energy Storage).

References

[1] H. Koga, et al. Reversible Oxygen Participation to the Redox Processes Revealed for Li1.20Mn0.54Co0.13Ni0.13O2 J. Electrochem. Soc. (2013) 160, A786.
[2] M. Sathiya, et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes, Nature Materials (2013) 12 827.
[3] E. McCalla et al. Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries, Science (2016) 350, 1516.
[4] K. Luo, et al. Charge-compensation in 3d-transition metal oxide intercalation cathodes through the generation of electron holes on oxygen, Nature Chemistry (2016), 8, 684.
[5] W. E. Gent et al., Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxide, Nature Communications (2017), 8:2091.
[6] J.-J. Marie et al. Trapped O2 and the origin of voltage fade in layered Li-rich cathodes Nature Materials, (2024) 23, 818.
[7] W. Yang and T. P. Devereaux, Anionic and cationic redox and interfaces in batteries: Advances from soft X-ray absorption spectroscopy to resonant inelastic scattering, J. Power Sources (2018), 15, 188.

Contexte de travail

The PhD work will take place in the Department of Theoretical Chemistry & Physics of the Charles Gerhardt Institute in Montpellier.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

No risk - Computational work.