Insights into Structural Transformations in the Local Structure of Li₂VO₂F Using Operando X-ray Diffraction and Total Scattering: Amorphization and Recrystallization

Abstract

Disordered rock salt Li 2 VO 2 F cathode material for lithium-ion batteries was investigated using operando X-ray diffraction and total scattering to gain insight into the structural changes of the short-range and long-range orders during electrochemical cycling. The X-ray powder diffraction data show the well-known pattern of the disordered rock salt cubic structure, whereas the pair distribution function (PDF) analysis reveals significant deviations from the ideal cubic structure. During battery operation, a reversible rock salt-toamorphous phase transformation is observed, upon Li extraction and reinsertion. The X-ray total scattering data show strong indications of the formation of tetrahedrally coordinated V in a nondisordered rock salt phase of the charged electrode material. The results show that the disordered rock salt Li 2 VO 2 F material undergoes a hidden structural rearrangement during battery operation.

Short‐range ordering in the Li‐rich disordered rock salt cathode material Li2VO2F revealed by Raman spectroscopy

“…Rietveld refinement of powder neutron and X‐ray diffraction patterns is a commonly used tool to determine the crystal structure of the target electrode material, including the Li‐rich DRS materials. Li 2 VO 2 F diffraction patterns have been published before [ 3,9,27 ] and provide evidence that it is a face‐centered cubic rock salt structure (space group Fm‐3 m ), with Li + and V 3+ occupying 4a , and O 2− and F − occupying 4b Wyckoff sites. It was originally assumed that both anions and cations are distributed completely randomly, as depicted in Figure 1a.…”

Redox Chemistry and the Role of Trapped Molecular O2 in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes

“…Recently, there has been growing interest in disordered Li-rich intercalation materials, especially disordered rocksalt structures, − including early work on systems based on Li 3 NbO 4 and Li 2 VO 2 F [refs − ]. House et al .…”

Formation of LiF Surface Layer During Direct Fluorination of High-Capacity Co-Free Disordered Rocksalt Cathodes

“…18 The role of electron localization on electrochemical performance was recently discussed in detail by Wang et al 19 To obtain high capacity and good rate performance, DRX cathodes require percolating Li + transport pathways. 11,12 The formation of such networks requires careful tuning of the composition (e.g., Li content and TM selection) to optimize Li-TM and Li−Li interactions, where excess Li is typically required to form percolating 0-TM diffusion channels. 8 Due to the tendency of Li incorporation of more redox-active TM ions and thereby improving stability at high states of charge (i.e., by reducing reliance on anionic charge compensation).…”

“…To obtain high capacity and good rate performance, DRX cathodes require percolating Li + transport pathways. , The formation of such networks requires careful tuning of the composition (e.g., Li content and TM selection) to optimize Li-TM and Li–Li interactions, where excess Li is typically required to form percolating 0-TM diffusion channels . Due to the tendency of Li + and F – species to cluster, F – substitution is predicted to facilitate the formation of percolating 0-TM pathways in compounds without Li excess, enabling the incorporation of more redox-active TM ions and thereby improving stability at high states of charge (i.e., by reducing reliance on anionic charge compensation). , Furthermore, the F – solubility of DRX cathodes depends on the material’s cation short-range ordering …”