Lithia/(Ir, Li2IrO3) nanocomposites for new cathode materials based on pure anionic redox reaction

Abstract

Anionic redox reactions attributed to oxygen have attracted much attention as a new approach to overcoming the energy-density limits of cathode materials. Several oxides have been suggested as new cathode materials with high capacities based on anionic (oxygen) redox reactions. Although most still have a large portion of their capacity based on the cationic redox reaction, lithia-based cathodes present high capacities that are purely dependent upon oxygen redox. Contrary to Li-air batteries, other systems using pure oxygen redox reactions, lithia-based cathodes charge and discharge without a phase transition between gas and condensed forms. This leads to a more stable cyclic performance and lower overpotential compared with those of Li-air systems. However, to activate nanolithia and stabilize reaction products such as Li2O2 during cycling, lithia-based cathodes demand efficient catalysts (dopants). In this study, Ir based materials (Ir and Li2IrO3) were introduced as catalysts (dopants) for nanolithia composites. Oxide types (Li2IrO3) were used as source materials of catalyst because ductile metal (Ir) can hardly be pulverized during the milling process. Two types of Li2IrO3 were prepared and used for catalyst-sources. They were named ‘1-step Li2IrO3’ and ‘2-step Li2IrO3’, respectively, since they were prepared by ‘1-step’ or ‘2-step’ heat treatment. The nanocomposites prepared using lithia & 2-step Li2IrO3 presented a higher capacity, more stable cyclic performance, and lower overpotential than those of the nanocomposites prepared using lithia & 1-step Li2IrO3. The voltage profiles of the nanocomposites prepared using lithia & 2-step Li2IrO3 were stable up to a limited capacity of 600 mAh·g−1, and the capacity was maintained during 100 cycles. XPS analysis confirmed that the capacity of our lithia-based compounds is attributable to the oxygen redox reaction, whereas the cationic redox related to the Ir barely contributes to their discharge capacity.