Work

In Situ X-ray Diffraction during Ball Milling Reveals Poorly Crystalline Metastable Intermediates during the Formation of Disordered Rocksalt Oxyfluorides

Year: 2026

Type: article

Abstract: Li-excess transition metal disordered rocksalt oxyfluorides continue to attract attention as next-generation cathode materials for Li-ion batteries. To this end, high-energy ball milling is the most versatile synthesis method for disordered rocksalt cathode materials, and many of the best-performing compounds with high fluorine content have only been synthesised mechanochemically. A lack of mechanistic understanding of mechanochemistry hinders the development of different synthesis approaches, as it is not clear which part of mechanochemistry enables high fluorination. In this report, in situ powder X-ray diffraction performed during the mechanochemical synthesis of Mn-based disordered rocksalt oxyfluorides reveals a previously unknown, Mn-poor amorphous intermediate prior to forming the final rocksalt compound. We find that the intermediate forms regardless of transition metal composition and continues to react slowly with the remaining Mn precursors and thus, appears to be a common node to rocksalt formation for at least four compounds studied here. Attempts to characterise the intermediate with ex situ neutron and X-ray scattering, alongside solid-state nuclear magnetic resonance spectroscopy, suggest that the intermediate is thermodynamically unstable, decomposing into a collection of crystalline compounds when removed from the mill. This study highlights the unique chemistry of mechanochemical reactions and pathways accessible at moderate temperatures, ultimately pointing synthetic chemists to the intermediates that enable a wider range of fluorination for disordered rocksalt cathodes.

Source: Journal of the American Chemical Society

DOI: 10.1021/jacs.6c01486