The Nature of d0 Ion Effect on the Electrochemical Activity of the O2–/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 Li1 + x(MеMn3+)1 – xO2 – yFy, 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 O2–/O–. In the case of Ti-containing oxyfluorides with disordered rock-salt structure, with the increasing of fluorine content the contribution from O2–/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 Li1.266Nb0.217Mn0.55O1.85F0.15. 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.