Vincent MAISONNEUVE

Abstract:

Fluoride materials attract much interest as cathode materials for secondary batteries because of the high electronegativity of the fluorine atom, affording higher potentials than oxide analogues.[1] In this context, iron trifluoride FeF₃ has been intensively used due to its straightforward elaboration in relatively mild synthesis conditions; iron is considered as environmentally friendly. In the case of an intercalation mechanism, the theoretical capacity for FeF₃ (1Li⁺ per Fe) reaches 237 mAh.g^-1, a value higher than that obtained for the LiFePO₄ commercial material (170 mAh.g^-1). With the conversion reaction, this capacity can even reach 712 mAh.g^-1, implying a reduction of the trivalent iron. Despite good electrochemical performances in capacity and redox potential, the high ionicity of M-F bonds induces a large band-gap resulting in poor electronic conductivity. A recent study shows that the dehydration of HTB-FeF_2.2(OH)_0.8.0.33H₂O (Hexagonal Tungsten Bronze) leads to a lacunar oxyfluoride with anionic vacancies, thus having a positive effect on electrochemical performances (cyclability and capacity).[3]
In this work, mixed fluorides M^IIM^III₂F₈(H₂O)₂, M^IIM^IIIF₅(H₂O)₂ weberites, and their corresponding dehydrated intermediate phases were considered for their electrochemical activity (M = V, Mn, Fe, Co, Ni, Cu).[4] The hydrated phases were synthesized by a solvothermal route, eventually assisted by microwave heating and characterized by X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer spectrometry. The formulations and structural features of the intermediates stabilized after heating treatments under different atmospheres were determined by combining thermal analysis, powder XRD, MET, pair distribution function, IRTF and Mössbauer spectrometry. Finally, the electrochemical performances of all-synthesized fluoride materials demonstrate that several dehydrated phases could be a good alternative as positive electrode materials with capacities at the first discharge up to 306 mAh.g^-1.[4]

[1] C.X. Zu, H. Li, "Thermodynamic analysis on energy densities of batteries". Energy & Environmental Science, 2011, 4(8), 2614.
[2] D.E. Conte, N. Pinna "A review on the application of iron(III) fluorides as positive electrodes for secondary cells". Mater. Renew. Sustain. Energy, 2014, 3:37.
[3] M. Duttine, D. Dambournet, N. Penin, D. Carlier, L. Bourgeois, A. Wattiaux, K.W. Chapman, P. J. Chupas, H. Groult, E. Durand, A. Demourgues. "Tailoring the Composition of a Mixed Anion Iron-Based Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical Performance in Lithium Cells". Chem. Mater., 2014, 26, 4190.
[4] K. Lemoine, L. Zhang, D. Dambournet, J.-M. Greneche, A. Hömon-Ribaud, M. Leblanc, O. Borkiewicz, J.-M. Tarascon, V. Maisonneuve, J. Lhoste."Pyrochlore and HTB type Iron Hydroxyfluorides: from Synthesis to Li Insertion Properties". Chem. Mater., 2019, DOI: 10.1021/acs.chemmater.9b01252.