Effect of magnesium substitution on the cycling behavior of lithium nickelcobalt oxide

LiNi0.91Co0.009O2 and LiNi0.86Co0.09Mg0.05O2 phases were comparatively cycl ed in 4/5 'A'-size lithium-ion cells (500 cycles, 60 degreesC, C rate, 2 h floating at 4.1V at each charge). The Mg-Co substituted positive electrodes exhibit a very small capacity fading versus the Co substituted one. Compar ative Rietveld refinements of the XRD patterns before and after cycling sho w unambiguously that an irreversible migration of Mg2+ ions from the slabs to the interslab spaces occurs upon cycling. The variation of the cell para meters of the LixNi0.86Co0.09Mg0.05O2 deintercalated phases, obtained durin g the 1st and the 501st charges, were determined. The smaller changes obser ved after the Mg-migration suggest that the mechanical constraints typicall y observed during lithium intercalation-deintercalation are reduced by the simultaneous presence of magnesium ions in the lithium sites and of vacanci es in the slabs, leading to a smaller capacity fading. (C) 2001 Elsevier Sc ience B.V. All rights reserved.