Structural and electrochemical study of fast Li diffusion in Li₃V₂(PO₄)₃-based electrode material

Highlights: • Asymmetry of (de)lithiation is due to non-equivalence of Li sites in Li₃V₂(PO₄)₃. • Ultrafast Li transport in Li₃V₂(PO₄)₃ was confirmed by PITT, GITT and EIS methods. • Sharp decline of Li diffusion coefficient is related to the V₂(PO₄)₃ phase formation. - Abstract: NASICON-type lithium-vanadium phosphate (Li₃V₂(PO₄)₃) based electrodes capable to provide extremely fast lithium transport properties were studied by a combination of structure, morphology and surface characterization methods: X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), laser diffraction particle size distribution (PSD), as well as electrochemical methods: potentiostatic (PITT) and galvanostatic (GITT) intermittent titration techniques, electrochemical impedance spectroscopy (EIS), and constant current chronopotentiometry. Significant differences in the kinetics of reversible lithium intercalation depending on the lithiation stage were found: lithium diffusion coefficients, found from the PITT, GITT and EIS data, demonstrate an abrupt drop by 2–3 orders of magnitude (from 10⁻⁹ to 10⁻¹² cm² s⁻¹) in the 4.3–4.4 V potential range vs Li/Li⁺, which is attributed to the LiV₂(PO₄)₃ ↔ V₂(PO₄)₃ phase transition. The electrochemical extraction/insertion of two lithium equivalents can occur at ultra-high rates (up to 320C) from/into structurally more accessible Li2 and Li3 sites, while the de/intercalation of the third lithium equivalent from/into the Li1 position is supposedly hindered kinetically. To analyze the electrochemical data, specially developed theoretical models were used, which take into account geometry and phase configuration of the diffusion space, as well as the properties of the phase boundary interfaces. Morphology and size distribution parameters, essential for mathematical processing of the electrochemical data, were obtained by SEM, PSD and BET methods.