Structure Effect on Pseudocapacitive Properties of A2LaMn2O7 (A = Ca, Sr)

Herein, the effect of structure on pseudocapacitive properties in alkaline conditions is demonstrated through the investigation of isoelectronic oxides Ca2LaMn2O7 and Sr2LaMn2O7, where the difference in ionic radii of Ca2+ and Sr2+ leads to a change in structure and lattice symmetry, resulting in an orthorhombic Cmcm structure for the former and a tetragonal I4/mmm structure for the latter. While calcium and strontium do not make a direct contribution to the near-surface faradaic processes that are essential to the pseudocapacitive properties, their effect on the structure leads to a change in the oxygen intercalation process and the associated pseudocapacitive energy storage. It is shown that Sr2LaMn2O7 has a significantly greater specific capacitance than Ca2LaMn2O7. In addition, the former shows a considerably higher-energy density compared to the latter. Furthermore, these materials show highly stable energy-storage properties, and retain their specific capacitance over 10 000 cycles of charge-discharge in a symmetric pseudocapacitive cell. Importantly, these findings show the structure-property relationships, where a change in the structure and lattice symmetry can result in a significant change in pseudocapacitive charge-discharge properties in isoelectronic systems.