Effect of the ZnNiyMn2-yO4 (0≤y≤1) spinel composition on electrochemical lithium insertion

The electrochemical insertion of lithium in the spinel-type manganite with the formula ZnNiyMn2-yO4 has been studied. The galvanostatic discharge curves show that the best performance is obtained for y = 0.25, where a tetragonal to cubic structural transformation occurs. The thermodynamics and kinetics of the process of insertion of the lithium into the tetragonal spinel LixZnNi0.25Mn1.75O4 (x = 0.05-1.3) have been studied. The molar thermodynamic quantities, such as enthalpy, entropy and free energy determined by EMF-T measurements, varied with the lithium concentration in the oxide structure, and a major variation was observed around x = 0.8. The chemical diffusion coefficient of lithium in these spinels was also determined. Structural analysis, degree of oxidation and magnetic susceptibility measurements were carried out for the lithiated oxides in order to obtain the cationic distribution as a function of x. It has been possible to demonstrate that, upon lithium insertion, Mn4+ ions on B sites are reduced to Mn3+ and then to Mn2+. A cooperative Jahn-Teller effect is present in these spinel manganese-nickel oxides.