In order to obtain information on the structural changes during electrochemical cycling such as the change of local environments or the deformation of the sites, or to reveal the appearance of new phases, precise structural studies and if possible in situ or operando are necessary. Thanks to the close collaboration with the SOLEIL synchrotron, significant advances have been made in designing measuring devices to study the materials during their operation.
Unfortunately, due to the often limited resolution of diffraction techniques on our materials (nanocrystallized or disordered character), multitechnical approaches combining diffraction with XAS, NMR or Mössbauer are necessary. Such an analysis approach can be supplemented by modeling. For several years, we have been using DFT tools to improve the structural description of the phases we are studying and to extract relevant crystallochemical parameters.
The strong coupling between X-ray diffraction, NMR studies and DFT modeling can be highlighted in the work we have done in collaboration with colleagues from the University of Pierre and Marie Curie on the study of the Cs4[H2V10O28].4H2O compound during the PhD thesis of Lionel Truflandier.[Truflandier 2010] As shown in the figure below, the optimization of the structure of cesium decavanate allowed us to position specifically the water molecules, to define the hydrogen bond network and to clearly identify the oxygen active sites of the structure that may be involved in the various synthesis methods when using these decavanate anions.
This work was successful thanks to tight coupling between NMR measurements and the modeling of NMR parameters. The approach has greatly been inspired by the advances made in this field few years ago in our laboratory (see the section on the modeling of NMR parameters). [Truflandier 2007]
Figure showing the 51V NMR chemical shifts assignment of the five vanadium sites for the cesium decavanate. It highlights the octahedral VO6 geometry, the existence of hydroxyl bonds, and the proximity of water molecules.
The works we do on the FePO4/NaxFePO4 system are also an excellent example of the contribution of modeling to characterize the structures. In 2010, the NaFePO4 phase was synthesized by our group (olivine type, orthorhombic space group Pnma ) and has been fully characterized by combining geometry optimization (DFT methods, GGA + U calculations) with the crystallographic data extracted from the X-ray powder diffraction pattern. [Moreau, 2010] From total energy calculations, an excellent agreement was obtained between the calculated and experimental values of the electrochemical potential. The study of the phase with the Na2/3FePO4 intermediate composition has required the contribution of several techniques: X-ray diffraction at SOLEIL synchrotron, Mossbauer measurements, NMR measurements, but it is only through modeling (optimization geometry and calculation of Mossbauer parameters) that we were able to validate the structural model for this phase: monoclinic lattice with three fold superstructure, space group P21/n. [Moreau 2011]
Figure: Structural model of the Na2/3FePO4 composition obtained by coupling the DFT modeling with X-ray diffraction (synchrotron), Mossbauer, and NMR experiments.
[Truflandier 2007] "Density functional theory investigation of 3d transition metal NMR shielding tensors in diamagnetic systems using the gauge-including projector augmented-wave method"
L. Truflandier, M. Paris, and F. Boucher, Phys. Rev. B: Condens. Matter Mater. Phys. 76 (2007) 035102. doi : 10.1103/PhysRevB.76.035102
[Truflandier 2010] "DFT-NMR Investigation and V-51 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]. 4H2O"
L. Truflandier, F. Boucher, C. Payen, R. Hajja, Y. Millot, C. Bonhomme, and N. Steunou, J. Am. Chem. Soc. 132 (2010) 4653-4668. doi : 10.1021/ja908973y
[Moreau 2010] "Structure and Stability of Sodium Intercalated Phases in Olivine FePO4"
P. Moreau, D. Guyomard, J. Gaubicher, and F. Boucher, Chem. Mater. 22 (2010) 4126-4128. doi : 10.1021/cm101377h
[Moreau 2011] "Elucidation of the Na2/3FePO4 intermediate phase: clues to the insertion process in olivine"
P. Moreau, M. Cuisinier, B. Turpin, J. Gaubicher, F. Boucher, and D. Guyomard, 7th International Symposium on Inorganic Phosphate Materials (ISIPM7) (2011)