The LixNi1-yMgyO2 (y=0.05, 0.10) system: structural modifications observedupon cycling

A detailed structural characterization of deintercalated LixNi1-yMgyO2 (y = 0.05 and 0.10) phases was performed in order to determine the effect of ma gnesium substitution for nickel on the structural evolution upon cycling. F or both systems, a solid solution exists over the entire lithium-compositio n domain, which shows that substitution of 5% of magnesium for nickel is hi gh enough to suppress the phase transitions observed for LixNiO2 during the cycling process. Rietveld refinement of the XRD patterns of the materials recovered after one cycle with an intermediate floating at high potential ( 4.35 V) or after 50 cycles (between 2.7 and 4.15 V), gave evidence for the migration of all the Mg2+ ions from the slab to the interslab space during the electrochemical process. This cationic displacement is responsible for the existence of a small irreversible capacity at the end of the first disc harge. However, Mg2+ cations do not induce any local collapse of the inters lab space as is observed for the LixNi1+zO2 systems when the Ni2+ ions pres ent in the interslab space are oxidized. It is assumed that the presence of Mg2+ ions in the lithium sites reduces the cell parameters changes upon cy cling and, therefore, is at the origin of the improvement of the cycling pr operties observed for the magnesium substituted positive electrode material s. (C) 2000 Elsevier Science B.V. All rights reserved.