Synthesis and characterization of new LiNi1-yMgyO2 positive electrode materials for lithium-ion batteries

New LiNi1-yMgyO2 (0 less than or equal to y less than or equal to 0.20) lay ered oxides were synthesized by a coprecipitation method followed by a high -temperature thermal treatment. Rietveld refinements of their X-ray diffrac tion patterns showed that they exhibit a quasi-two-dimensional structure, i sostructural to LiNiO2, for small substitution amounts (y less than or equa l to 0.10). For larger amounts (y = 0.15, 0.20), the Li/(Ni + Mg) ratio is significantly lower than unity. In all cases, the extra ions located in the inter-slab space for lithium deficiency compensation are preferentially Mg 2+ ions. A magnetic study confirmed the cationic distributions which result from the size difference between Ni3+ and Mg2+ ions. An electrochemical st udy showed reversible behavior for all materials. A high capacity (greater than or equal to 150 Ah kg(-1)) was found for LiNi1-yMgyO2 phases (y less t han or equal to 0.02), which decreased when y increased. The presence of Mg 2+ cations in the inter-slab space, which cannot be oxidized and have a siz e close to Li+, prevents the local collapses of the structure which occurs for the Li1-zNi1+zO2 System; therefore good cycling stability is observed. (C) 2000 The Electrochemical Society. S0013-4651(99)10-069-7. All rights re served.