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°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°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°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.