Resumen de Difracció de pols de radiació sincrotró en bateries operando a temperatura no ambient
Raphaëlle Gaétane L. Maria Houdeville
The aim of this thesis is to develop and test new thermalized in situ cells for X-ray diffraction in operando batteries to facilitate studies on electrode materials. The general reason for developing such cells is to study systems with slow kinetics, materials with temperature dependent phase transitions and/or temperature accelerated ageing processes. To assess the cells designed by ALBA’s engineering team and LRCS, experiments were carried out with three electrochemical systems.
Solvent cointercalation upon reduction in TiS2 was studied first in lithium cells, allowing the identification of propylene carbonate as solvent promoting this phenomenon. It was found to be electrochemically driven and to result in the formation of a phase indexed within the P3̅m1 space group with parameters a,b = 3.514(3) Å and c = 17.931(2) Å in early stages of reduction. These cell parameters are independent of the electrolyte salt used, but the amount of cointercalated phase formed is determined by the amount of propylene carbonate solvent in the electrolyte. Its formation induces a loss in the cell capacity as it does not evolve upon oxidation.
A study at non-ambient temperature was carried out in calcium cells where development and optimization of the system and protocols was crucial. At 65°C, the reduction of a cell with TiS2 as positive electrode, an activated carbon negative electrode and Ca(TFSI)2 in propylene carbonate as electrolyte promoted the formation of a cointercalated phase at early stages of reduction, prior to the observation of another more reduced phase. The cointercalated phase has an expanded c parameter estimated to be 18.41(4) Å while the more reduced phase would have a c parameter estimated at 19.23(5) Å, both phases evolving in the course of reduction and being indexed in the P3̅m1 space group. Despite the reaction taking also place at room temperature, the amount of reduced phases is significantly smaller in agreement with the sluggish kinetics of the process which is also consistent with a significant amount of TiS2 remaining unreacted even at 65°C. Yet, and in contrast to what was observed in lithium cells, these phases are electrochemically active upon oxidation to yield TiS2.
Finally, experiments were carried out on both α- and β- polimorphs of Na3V2(PO4)3 (respectively at 2 and 40 °C) to check if the thermally driven phase transition could affect the material’s redox behavior. During that study, three thermalized systems were used. The intermediary Na2V2(PO4)3 phase was observed clearly and indexed in the P2/m space group with parameters: a = 14.847(2) Å, b = 8.617(7) Å, c = 21.554(2) Å and β = 90.435(1)°. Additional measurements proved that the thermalization was necessary to test the α-NVP phase or the material would revert back to the β-NVP if kept at room temperature (~20°C) for too long. Yet, even in the case of thermalized cells, the characteristic peak of the α-NVP phase (-111) was lost upon oxidation and did not reappear upon reduction, which would yield β- Na3V2(PO4)3, although additional experiments would be needed to confirm this aspect.
