The Nature of d⁰ Ion Effect on the Electrochemical Activity of the O^2–/O^–-Redox-Couple in Oxyfluorides with the Disordered Rock-Salt Structure

Abstract

The effect of the nature of the transition metal ion (electron configuration 3d⁰ and 4d⁰) on the local structure and electrochemical properties of lithium-rich oxyfluorides with disordered rock-salt structure Li_{1 + x}(MеMn³⁺)_{1 – x}O_{2 – y}F_y, where Mе = Ti⁴⁺, Nb⁵⁺, 0.2 ≤ x ≤ 0.288 and 0.05 ≤ y ≤ 0.15 is studied. The compounds are thoroughly investigated by the methods of X-ray diffraction analysis, scanning electron microscopy, granulometry, electron spin resonance spectroscopy, and galvanostatic cycling. The galvanostatic-cycling curves of the compounds have two plateaus in the voltage regions of 3.3–3.4 and 4.1–4.3 V. They can be attributed to redox-processes involving two couples: Mn³⁺/Mn⁴⁺ and O^2–/O^–. In the case of Ti-containing oxyfluorides with disordered rock-salt structure, with the increasing of fluorine content the contribution from O^2–/O^–-couple during the electrochemical process decreases. In both systems of the oxyfluorides with disordered rock-salt structure we observed formation of paramagnetic clusters Mn³⁺–O–Mn⁴⁺ whose number increased with the increasing of Mn content. The largest clusterization is observed for the sample Li_1.266Nb_0.217Mn_0.55O_1.85F_0.15. At the same time, the diffusion coefficient for Nb-containing oxyfluorides with disordered rock-salt structure is lower by order of magnitude than for the Ti-containing ones. This may be connected with the strongest clustering of Mn³⁺ ions, which hinders the Li⁺ ion macrodiffusion and, as a consequence, deteriorates the kinetics of the process.