Structure, Shift in Redox Potential and Li-Ion Diffusion Behavior in Favorite LiFe_1–xV_xPO₄F Solid-Solution Cathodes

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.

Keywords

• LiFePO4F–LiVPO4F
• solid solution
• single-phase reaction
• redox potential
• diffusion coefficient