Mechanism of Na-Ion Conduction in the Highly 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, acconductivity, 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. With the 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 νC =10 4 Hz at 353 K) is mainly controlled by the critical frequency (νC) that increases with the increasing temperature. Our results reveal that the 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 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. The present comprehensive study facilitates the understanding of the microscopic ionic conduction mechanism in the highly efficient 2D layered battery material Na2Mn3O7 having high energy storage capacity and high structural stability, paving way for the discovery of 2D materials for functional battery applications.

Role of Crystal Structure on the Ionic Conduction and Electrical Properties of Germanate Compounds A2Cu3Ge4O12 (A = Na, K)

“…The hopping is a physical process which means a sudden displacement of the charge carrier from one site to another either by jump over the potential or by quantum mechanical tunneling, 38,39 as reported for many compounds. 10,40,41 The derived values of s(T) and α(T) for both compounds show strong temperature dependence (Figures 3c and d). The frequency exponent s(T) describes the slope of power law, and provides information about the degree of interaction among the charge carriers.…”

“…Here, we would like to mention that the CBH conduction mechanism was reported for most of the ionic conductors. 10,59 The behavior of the imaginary part of impedance (Figures 1b,e) as well as electrical modulus (Figures 9b,d), indicates a spread of relaxation time for both the ACGO compounds. Therefore, they are non-Debye types.…”

“…Therefore, understanding the microscopic mechanism of ionic conduction in Na-and K-based ionic conductors and the role of the underlying crystal structure on ionic conductor properties is essential for fundamental interest and also to control and improve ionic transport behaviors of new battery materials. 9,10 Currently, the research is focused on the solid-state battery due to intrinsic safety, high density, good thermal stability, and environmental friendliness. The inorganic solid-state ionic conductors are believed to be promising candidates for the applications as well as for the fundamental understanding of the microscopic mechanism of ionic conduction.…”

Effects of Nickel/Manganese Variation on Na2Mn3–zNizO7 for Sodium-Ion Battery Cathodes

“…54,55 Lowden charges were investigated to verify the change of total charge in the system upon nickel substitution. 56,57 The nudged elastic band (NEB) method 58,59 was adopted to find the transition state between two stationary states, thus explaining sodium ion's movement through the chosen paths. Ordinary NEB calculations were performed before the climbing image NEB calculations, namely, to optimize the initial and final positions of the stationary points.…”

“…Ordinary NEB calculations were performed before the climbing image NEB calculations, namely, to optimize the initial and final positions of the stationary points. 40,41,59 The deintercalation order corresponds to the value of the potential cathode. This process was carried out by removing y ions from the Na x−y Mn 3−z O 7 Ni z lattice, followed by the geometry optimization procedure.…”