  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.
