ABSTRACT
The SiS2 rods exhibit a significant anisotropy property applied in a special field such as in the one of all-solid-state lithium-ion batteries and so on. In this work, the orthorhombic SiS2 rods with high chemical/phase purity were prepared by an elemental method, either through a boiling or a steaming process, at 1023-1073 K for 3 h and under the saturated S-vapor pressure (2.57-3.83 MPa) in a closed sealed-tube system. The composition, crystal structure, morphology, and growth mechanism were investigated. Results showed that the growth orientation of SiS2 along the <0 0 1> is intrinsically governed by the crystal structure motif. It could exist in both processes and the latter tends to show in macroscopic morphology. Using the pressure-temperature diagram, structure refinement, pole figures, image analyses, and so forth, factors influencing the purity and growth of SiS2 rods were concluded from the thermodynamics and kinetics viewpoints.
ABSTRACT
Solid-solution Li-ion cathode materials transform through a single-phase reaction thus leading to a long-term structural stability and improved cyclability. In this work, a two- to single-phase Li+-extraction/insertion mechanism is studied through tuning the stoichiometry of transition-metal Fe/V cations to trigger a transition in the chemical reactivity path. Tavorite triclinic-structured LiFe1-xVxPO4F (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) solid-solution powders were prepared by a facile one-step solid-state method from hydrothermal-synthesized and commercial raw materials. The broad shape of cyclic voltammetry (CV) peaks, sloping charge/discharge profiles and sloping open-circuit voltage (OCV) profiles were observed in LiFe1-xVxPO4F solid-solution cathodes while 0 < x < 1. These confirm strongly a single-phase behavior which is different from the two-phase behavior in the end-members (x = 0 or 1). The electronegativity of M (M = Fe1-xVx) for the redox potential of Fe2+/3+ couple or the M-O4F2 bond length for the V3+/4+ couple plays respectively a dominant role in LiFe1-xVxPO4F solid-solution cathodes.
