Comparison of equilibrium electrochemical behavior of PdH_x and Li_xMn₂O₄ intercalation electrodes in terms of sorption isotherms

This paper is devoted to measurements of hydrogen sorption into Pd and Li-ion insertion into the transition metal oxides (λ-MnO₂, in particular) performed under strict equilibrium and quasi-equilibrium conditions, respectively. The related charging-discharging curves show common features to the two systems such as potential plateaus, which can be ascribed to highly attractive interactions between the intercalation sites, leading, eventually, to first-order phase transitions. Fitting an equilibrium charging curve to a simple Frumkin-type isotherm for a highly defective Pd lattice demonstrates a rather good agreement between the curves within the whole region of compositions 0<x<1. In the case of Li_xMn₂O₄, we found a clearly expressed hysteresis, which does not disappear even at the lowest rate of charge and discharge used. We believe that this kind of behavior is due to the combined effect of slow Butler-Volmer kinetics for Li-ion transfer and the true interaction parameter g, which may be more negative than the critical value -4. It was found that treating averaged charging and discharging curves of the Li_xMn₂O₄ electrode, the modified thermodynamic relations and the corresponding equation for the sorption isotherm should be used with the term x²/(1-x) substituted for x/(1-x) in the simple isotherm under discussion.