Influence of Defects and Surfaces on the Electrochemical Performance of MnO<sub>2</sub> Cathodes in Rechargeable Alkaline Zn/MnO<sub>2</sub> Batteries: A First-Principles Study
Nirajan Paudel, Birendra Ale Magar, Krishna Prasad Acharya, Timothy N. Lambert, Igor Vasiliev
Abstract
Manganese dioxide is a promising cathode material for energy storage applications because of its high redox potential, large theoretical energy density, abundance, and low cost. It has been shown that the performance of MnO 2 electrodes in rechargeable alkaline Zn/MnO 2 batteries could be improved by nanostructuring and by increasing the concentration of defects in MnO 2 . However, the underlying mechanism of this improvement is not completely clear. We used an ab initio density functional computational approach to investigate the influence of nanostructuring and crystal defects on the electrochemical properties of the MnO 2 cathode material. The mechanism of electrochemical discharge of MnO 2 in Zn/MnO 2 batteries was studied by modeling the process of H ion insertion into the structures of pyrolusite, ramsdellite, and nsutite polymorphs containing oxygen vacancies, cation vacancies, and open surfaces. Our calculations showed that the binding energies of H ions inserted into the structures of MnO 2 polymorphs were strongly affected by the presence of surfaces and bulk defects. In particular, we found that the energies of H ions inserted under the surfaces and attached to the surfaces of MnO 2 crystals were significantly lower than those for bulk MnO 2 . The results of our study provide an explanation for the influence of crystal defects and nanostructuring on the electrochemical reactivity of MnO 2 cathodes in rechargeable alkaline Zn/MnO 2 batteries.