Fast Mg-ion insertion kinetics in V2Se9

Abstract

V₂Se₉ displays facile electrochemical insertion of up to 1.6 Mg²⁺ per unit formula with fast diffusion (coefficients of 10⁻¹⁰–10⁻¹² cm² s⁻¹) surpassing best-in-class materials like Mo₆S₈. Detailed structural characterization of synchrotron X-ray diffraction data with ab initio Maximum Entropy Method analysis reveals Mg²⁺ insertion into octahedral sites within the large vdW space between [V₄Se₁₈]_∞ chains. Fast rate performance is attributed to low structural perturbation and low diffusion barriers, calculated by bond valence pathway analysis, resulting from the low charge-per-size of anionic selenium. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy reveal that reversible insertion of Mg²⁺ is facilitated by V⁵⁺/V³⁺ redox. V₂Se₉ demonstrates that selenides, despite their larger molecular weight, offer potential as fast-rate positive electrode materials for magnesium batteries over well-explored oxides and sulfides.