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

Abstract

Lithium-excess transition metal (M) oxyfluorides, Li x MO 1+x−y F 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 x MO 1+x−y F y , because conventionally ball-milled Li x MO 1+x−y F 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 highcrystallinity Li x MO 1+x−y F 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 Li 3 VO 3 F and Li 4 WO 4 F 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 Li 3 VO 3 F 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 x MO 1+x−y F y to improve its performance now that both crystal and amorphous Li x MO 1+x−y F y phases are attainable.

Chemical Pressure Effect on the Stabilization of Rock-Salt ZnO—Lin−2MeOn−1 Solid Solutions Synthesized at High Pressure

“…We believe that some solid solutions with a rock-salt crystal structure, synthesized for the first time in the present work, may be of interest as promising Li-containing materials [50,51] or as materials with high dielectric characteristics [52,53].…”

Durable Manganese-Based Li-Excess Electrode Material without Voltage Decay: Metastable and Nanosized Li2MnO1.5F1.5

“…Another strategy for material design is found in fluorination of materials, and a Li-excess Mn oxyfluoride, Li 2 MnO 2 F, has been synthesized and tested as an electrode material. , The addition and substitution of F – ions for O 2– ions decrease the oxidation state of Mn ions, and thus Mn 3+ /Mn 4+ cationic redox is also used coupled with partial anionic redox. Now, several Li-excess oxyfluorides with different transition-metal ions have been reported, and these materials are metastable phases. − Li-excess metal oxyfluorides are synthesized by high-energy mechanical milling, and nanosized compounds with a higher symmetry, generally a cation-/anion-disordered rocksalt phase, are obtained. Although inferior electrode kinetics are anticipated for the disordered phase, percolative Li migration in nanosized compounds effectively solves the kinetics problem.…”