High-Pressure Synthesis of Cation-Disordered Rock-Salt Oxyfluorides with High Crystallinity

Lithium-excess transition metal (M) oxyfluorides, LixMO1+x−yFy, have received considerable attention as positive electrode materials for lithium-ion batteries. Little is known about the relationship between the crystallinity and electrochemical reactivities although this offers further understanding and improvement of LixMO1+x−yFy, because conventionally ball-milled LixMO1+x−yFy exhibits limited crystallinity, i.e., amorphous-like nanoparticles. We herein adapted a high-pressure/high-temperature method at 12 GPa and 1000 °C to synthesize high-crystallinity LixMO1+x−yFy (M = Fe, Mn, V, Nb, Mo, and W) and investigated the electrochemical properties of this series. Rietveld analyses based on X-ray diffraction (XRD) and cross-sectional elemental mapping clarified that Li3VO3F and Li4WO4F crystalized as an almost-single-phase rock-salt structure with homogenous cation/anion distributions and formed well-faceted particles with sizes of 1–20 µm. Their rechargeable capacities over 1.8–5.0 V vs. Li+/Li were ∼40 mAh g−1 and ∼10 mAh g−1, respectively. According to ex situ XRD measurements of the cycled Li3VO3F electrodes, these partial rechargeable capacities were caused by a decomposition reaction during the initial charge, which differed from the topotactic reaction proposed for the low-crystallinity phases. This information is helpful for designing the microstructure of LixMO1+x−yFy to improve its performance now that both crystal and amorphous LixMO1+x−yFy phases are attainable.