Effects of partial acid delithiation on the electrochemical lithium insertion properties of nickel-stabilized LiMn2O4 spinel oxides

Use of nickel-containing LiMn2O4 up to 20 at.% as an insertion electrode in lithium cells combined with preliminary acid treatment results in improved cycling performance. An increase in the average oxidation state of manganese, a decrease in the Li/Mn ratio, partial H+/Li+ exchange, and a decrease in the unit cell parameters of the solid products take place during treatment with aqueous HNO3. X-Ray diffraction line broadening analysis shows that for a sample with a Ni/Mn ratio of 0.25 and a temperature of preparation of 800 degrees C, crystallinity increases slightly with acid treatment, as a result of the dissolution of the external layers of the solid in which defects concentrate. For samples prepared below 800 degrees C an increase in line broadening associated with a larger extension of the cation-exchange process is observed. The use of acid-treated solids in lithium cells results in higher capacities and better capacity retention as compared with pristine spinels. Larger capacities are always observed for samples prepared at 550 degrees C and a Ni/Mn ratio of 0.11. The role of nickel is of particular interest in stabilizing the spinel structure and allowing high average oxidation states of manganese in the solid. (C) 2000 Academic Press.