The Nature of d 0 Ion Effect on the Electrochemical Activity of the O 2– /O – -Redox-Couple in Oxyfluorides with the Disordered Rock-Salt Structure

The effect of the nature of the transition metal ion (electron configuration 3 d 0 and 4 d 0 ) on the local structure and electrochemical properties of lithium-rich oxyfluorides with disordered rock-salt structure Li 1 + x (MеMn 3+ ) 1 – x O 2 – y F y , where Mе = Ti 4+ , Nb 5+ , 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: Mn 3+ /Mn 4+ 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 Mn 3+ –O–Mn 4+ whose number increased with the increasing of Mn content. The largest clusterization is observed for the sample Li 1.266 Nb 0.217 Mn 0.55 O 1.85 F 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 Mn 3+ ions, which hinders the Li + ion macrodiffusion and, as a consequence, deteriorates the kinetics of the process.