Identifying the redox activity of cation-disordered Li–Fe–V–Ti oxide cathodes for Li-ion batteries

Abstract

Cation-disordered oxides have recently shown promising properties on the way to explore high-performance intercalation cathode materials for rechargeable Li-ion batteries. Here, stoichiometric cation-disordered Li₂FeV_yTi_1−yO₄ (y = 0, 0.2, 0.5) nanoparticles are studied. The substitution of V for Ti in Li₂FeV_yTi_1−yO₄ increases the content of active transition metals (Fe and V) and accordingly the amount of Li⁺ (about (1 + y)Li⁺ capacity per formula unit) that can be reversibly intercalated. It is found that Fe³⁺/Fe²⁺ and V⁴⁺/V³⁺ redox couples contribute to the overall capacity performance, whereas Ti⁴⁺ remains mainly inert. There is no evidence for the presence of Fe⁴⁺ species after charging to 4.8 V, as confirmed from the ex situ⁵⁷Fe Mössbauer spectroscopy and the Fe K-edge absorption spectra. The redox couple reactions for iron and vanadium are examined by performing in situ synchrotron X-ray absorption spectroscopy. During charging/discharging, the spectral evolution of the K-edges for Fe and V confirms the reversible Fe³⁺/Fe²⁺ and V⁴⁺/V³⁺ redox reactions during cycling between 1.5 and 4.8 V.