Mechanism of Na-ionic conduction in the high efficient layered battery material Na2Mn3O7

Abstract: The ionic conduction properties of the technologically important two-dimensional (2D) layered battery material Na2Mn3O7, with exceptional small-voltage hysteresis between charge and discharge curves, have been investigated as a function of temperature and frequency by an impedance spectroscopy. The detailed analyses of the impedance data in the form of dc-conductivity, ac-conductivity, electrical modulus, dielectric constant and complex polarizability reveal a long-range Na-ionic conductivity with negligible contribution from a local dipole relaxation. A significant enhancement (~10^4 times) of the Na-ion conductivity has been found with the increasing temperature from 353 K to 713 K. The temperature dependent conductivity reveals thermally activated conduction process with activation energies of 0.161 and 0.377 eV over the two temperature regions of 383-518 K and 518-713 K, respectively. AC conductivity study reveals a long-range hopping process for the conduction of charge carriers with a sharp increase of the hopping range at 518 K. The scaling study of the ac-conductivity reveals that the frequency-activated conductivity (above 10^4 Hz at 353 K) is mainly controlled by the critical frequency that increases with the increasing temperature. The Na-ion conduction in Na2Mn3O7 occurs predominantly by a correlated barrier hopping process. Besides, a correlation between ionic conduction and crystal structure has been established by x-ray and neutron diffraction study. We have further shown that the conductivity of Na2Mn3O7 can be enhanced by reduction of the stacking faults in the crystal structure. Our study facilitates the understanding of the microscopic ionic conduction mechanism in the highly efficient 2D battery material Na2Mn3O7 having high energy storage capacity and high structural stability, paving way for the discovery of materials for battery applications.