1 research outputs found
Sodium Ion Diffusion in Nasicon (Na<sub>3</sub>Zr<sub>2</sub>Si<sub>2</sub>PO<sub>12</sub>) Solid Electrolytes: Effects of Excess Sodium
The
Na superionic conductor (aka Nasicon, Na<sub>1+<i>x</i></sub>Zr<sub>2</sub>Si<sub><i>x</i></sub>P<sub>3–<i>x</i></sub>O<sub>12</sub>, where 0 ≤ <i>x</i> ≤ 3) is one of the promising solid electrolyte materials
used in advanced molten Na-based secondary batteries that typically
operate at high temperature (over ∼270 °C). Nasicon provides
a 3D diffusion network allowing the transport of the active Na-ion
species (i.e., ionic conductor) while blocking the conduction of electrons
(i.e., electronic insulator) between the anode and cathode compartments
of cells. In this work, the standard Nasicon (Na<sub>3</sub>Zr<sub>2</sub>Si<sub>2</sub>PO<sub>12</sub>, bare sample) and 10 at% Na-excess
Nasicon (Na<sub>3.3</sub>Zr<sub>2</sub>Si<sub>2</sub>PO<sub>12</sub>, Na-excess sample) solid electrolytes were synthesized using a solid-state
sintering technique to elucidate the Na diffusion mechanism (i.e.,
grain diffusion or grain boundary diffusion) and the impacts of adding
excess Na at relatively low and high temperatures. The structural,
thermal, and ionic transport characterizations were conducted using
various experimental tools including X-ray diffraction (XRD), differential
scanning calorimetry (DSC), scanning electron microscopy (SEM), and
electrochemical impedance spectroscopy (EIS). In addition, an ab initio
atomistic modeling study was carried out to computationally examine
the detailed microstructures of Nasicon materials, as well as to support
the experimental observations. Through this combination work comprising
experimental and computational investigations, we show that the predominant
mechanisms of Na-ion transport in the Nasicon structure are the grain
boundary and the grain diffusion at low and high temperatures, respectively.
Also, it was found that adding 10 at% excess Na could give rise to
a substantial increase in the total conductivity (e.g., ∼1.2
× 10<sup>–1</sup> S/cm at 300 °C) of Nasicon electrolytes
resulting from the enlargement of the bottleneck areas in the Na diffusion
channels of polycrystalline grains