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

Li-excess transition-metal-disordered rocksalt oxyfluorides continue to attract attention as next-generation cathode materials for Li-ion batteries that offer exceptional capacity while avoiding costly transition metals like Co and Ni. The best-performing rocksalts contain high fluorine contents (>10%), usually only accessible via high-energy ball-milling synthesis. However, mechanochemical synthesis routes are difficult to scale and produce high-surface-area particles that suffer from parasitic side reactions with the electrolyte. A lack of understanding of mechanochemical reactions hinders the development of alternate synthetic approaches that enable scalable production of these best-in-class disordered rocksalt oxyfluorides. Here, we use in situ powder X-ray diffraction, performed during mechanochemical synthesis, to provide insight into the formation mechanisms of four different Mn-based disordered rocksalt oxyfluorides. Regardless of the targeted composition, we see that all reactions pass through a similar poorly crystalline intermediate structure that reacts slowly with the Mn precursor, indicating that this is a common node to rocksalt formation. Ex situ neutron and X-ray scattering, alongside solid-state nuclear magnetic resonance (NMR) spectroscopy, suggest that the intermediate is thermodynamically unstable, decomposing into a collection of crystalline compounds. While diffraction indicates that precursors like LiF and Li₂O are expelled from the intermediate during milling breaks, NMR relaxometry suggests that both of these phases contain small quantities of Mn impurities. These data point to an unreported intermediate accessible only in the ball mill critical for rocksalt formation. This study highlights both the unique chemistry of mechanochemical reactions as well as parallels with high temperature synthesis routes, which may ultimately direct synthetic approaches that enable a wider range of fluorination for disordered rocksalt cathodes.