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

Abstract

Lithium-excess transition metal (M) oxyfluorides, Li_xMO_1+x−yF_y, 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 Li_xMO_1+x−yF_y, because conventionally ball-milled Li_xMO_1+x−yF_y 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 Li_xMO_1+x−yF_y (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 Li_xMO_1+x−yF_y to improve its performance now that both crystal and amorphous Li_xMO_1+x−yF_y phases are attainable.