Mechanism of Na-Ion Conduction in the Highly Efficient Layered Battery Material Na2Mn3O7

Ionic conduction properties of the technologically important two-dimensional (2D) layered battery material Na2Mn3O7, with exceptionally small-voltage hysteresis between charge and discharge curves, have been investigated as a function of temperature and frequency using impedance spectroscopy. The detailed analyses of the impedance data in terms of dc conductivity, ac conductivity, electrical modulus, dielectric constant, and complex polarizability reveal a long-range Na-ionic conductivity with negligible contribution from local dipole relaxation. A significant enhancement (similar to 10(4) times) of the Na-ion conductivity has been found on increasing the temperature from 353 to 713 K. The temperature-dependent conductivity reveals a thermally activated conduction process with activation energies of 0.161 and 0.377 eV over the two temperature regions of 383-518 and 518-713 K, respectively. An AC conductivity study reveals a long-range hopping process for conduction of charge carriers with a sharp increase of the hopping range at 518 K. With increasing frequency, the activation energies decrease for both the temperature regions. The scaling study of the ac conductivity reveals that the frequency-activated conductivity (above nu(C) = 10(4) Hz at 353 K) is mainly controlled by the critical frequency (nu(C)) that increases with increasing temperature. Our results reveal that thermally activated Na-ion conduction in the present 2D layered compound Na2Mn3O7 occurs predominantly by a correlated barrier hopping process. Besides, a correlation between ionic conduction and the crystal structure has been established by an 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. The present comprehensive study facilitates the understanding of the microscopic ionic conduction mechanism in the highly efficient 2D layered battery material Na2Mn3O7 having a high energy storage capacity and structural stability, thereby paving the way for the discovery of 2D materials for functional battery applications.