Amorphous and Poorly Crystalline Ferrofluoro-Phosphate Frameworks for Divalent-Ion Storage

Details

Supervisors

Vlad, Alexandru

Faculty

Faculté des sciences

Degree label

Master [120] en sciences chimiques, à finalité approfondie

Abstract

enThe growing limitations associated with lithium-based batteries, including resource availability, cost, and sustainability concerns, have stimulated interest in alternative energy-storage chemistries based on more abundant elements such as sodium and calcium. Among emerging systems, calcium-ion batteries (CIB) are particularly attractive due to the high theoretical capacity of Ca2+. However, the strong electrostatic interactions and sluggish diffusion kinetics of divalent calcium ions remain major challenges for the development of suitable cathode materials. In this context, this work investigates the fluorophosphate system Na1.2Fe1.2PO4F0.6 (NFPF) as a potential host structure for divalent-ion storage and as a precursor toward calcium-containing fluorophosphate cathodes. The first and principal objective focused on the synthesis optimization and electrochemical investigation of amorphous and poorly crystalline NFPF. The material was synthesized through a combination of mechanochemical activation by ball-milling and thermal treatment, while the influence of annealing temperature, annealing duration, and ball-milling frequency was systematically studied. Structural characterization by powder X-ray diffraction (PXRD) and Fourier-transform infrared spectroscopy (FTIR) demonstrated that mechanochemical treatment already induces the formation of a new phase prior to annealing. Increasing annealing temperature and ball-milling frequency improved reaction completion and reduced residual precursor species and organic impurities. Electrochemical investigations revealed that poorly crystalline NFPF exhibits reversible Ca2+ storage through electrochemical Na+/Ca2+ exchange. The best electrochemical performances were generally obtained for poorly crystalline materials, although reaction completion also appeared to strongly influence the observed behavior. All samples displayed an important initial irreversible capacity loss followed by stable cycling and high coulombic efficiencies. The second objective consisted in extending the study to other divalent ions. NFPF demonstrated reversible storage capabilities for Mg2+, Sr2+, and Ba2+ in addition to Ca2+, highlighting the versatility of this disordered fluorophosphate framework for multivalent-ion storage. The electrochemical behavior appeared to depend on ionic size, with Ba2+ generally exhibiting the highest insertion levels. These observations suggest the possible presence of insertion sites with different size requirements within the poorly crystalline structure, although additional characterization is required to fully understand the storage mechanism. Finally, preliminary attempts toward the direct synthesis of Ca0.6Fe1.2PO4F0.6 (CFPF) were carried out using calcium hydroxide and calcium acetate precursors. Although phase-pure CFPF was not successfully obtained, both synthesized materials exhibited reversible calcium storage, demonstrating the feasibility of direct calcium-containing fluorophosphate systems. Future work will focus on optimizing the thermal profile to further promote amorphization and on exploring alternative calcium and fluorine precursors to improve CFPF synthesis.