Sodium Ion Diffusion in Nasicon (Na₃Zr₂Si₂PO₁₂) Solid Electrolytes: Effects of Excess Sodium

Abstract

The Na superionic conductor (aka Nasicon, NaZrSiPO, where 0 ≤ x ≤ 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 (NaZrSiPO, bare sample) and 10 at% Na-excess Nasicon (NaZrSiPO, 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 S/cm at 300 °C) of Nasicon electrolytes resulting from the enlargement of the bottleneck areas in the Na diffusion channels of polycrystalline grains.

Fast Na ion transport triggered by rapid ion exchange on local length scales

“…In the monoclinic form of NZSP the Na + ions occupy three crystallographically inequivalent sites 4 d , 4 e , and 8  f , which are partially occupied. The large fraction of vacant Na sites and the favourable connection of their polyhedra ensure rapid exchange of the Na + ions as discussed in literature 22,40 . The preparation and characterisation of the samples studied here (NZSP and Sc-NZSP) has already been described elsewhere 18,22 .…”

“…This landscape contains stable as well as transitions states with shorter residence times for the Na + ions. We anticipate that forward-backward jumps involving interjacent bottleneck sites 22,40,44 result in highly correlated jump processes that manifests as dispersive regions in the conductivity isotherms. These dispersive regions are seen in Fig.…”

Electrolytes and Interphases in Sodium‐Based Rechargeable Batteries: Recent Advances and Perspectives

“…Ma et al [ 232 ] evidenced that the partial substitution of zirconium with scandium can improve the ionic conductivity of the solid electrolyte, with an optimal composition of Na 3 Sc 0.4 Zr 1.6 Si 2 PO 12 . Park et al [ 233 ] evidenced improved ionic conductivity value for Na 3.3 Zr 2 Si 2 PO 12 with a 10% Na excess. Song et al [ 234 ] reported a Na 3.1 Zr 1.95 Mg 0.05 Si 2 PO 12 NASICON electrolyte exhibiting a RT conductivity of 3.5 × 10 −3 S cm −1 , that was successfully employed for the fabrication of a RT solid‐state Na–S cell.…”

A P2‐Type Layered Superionic Conductor Ga‐Doped Na2Zn2TeO6 for All‐Solid‐State Sodium‐Ion Batteries

“…Thus, large Na + ions gain a large migration space in layered compounds more easily than in close‐packed oxide ceramic hosts that are bonded strongly in three dimensions . Larger migration paths induce longer Na−O bonds to decrease the attractive force between Na + and the O 2− , facilitating Na + ions’ migration . Therefore, considerable attention has been given to exploring the intercalation and diffusion of Na in lamellar oxides .…”