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

TLDR

Nasicon (Na1+xZr2SixP3–xO12) is a promising Na‑ion solid electrolyte for high‑temperature molten‑Na batteries, offering a 3D Na diffusion network while remaining electronically insulating. This study synthesizes standard and 10 at % Na‑excess Nasicon to investigate how excess Na influences grain‑boundary versus grain‑diffusion mechanisms and conductivity across low and high temperatures. The authors performed XRD, DSC, SEM, and EIS measurements and ab initio atomistic modeling to characterize structural, thermal, and ionic transport properties and to support experimental observations. They found that grain‑boundary diffusion dominates at low temperatures while grain diffusion dominates at high temperatures, and that 10 at % excess Na increases conductivity to ~0.12 S cm⁻¹ at 300 °C by enlarging bottleneck areas in Na‑diffusion channels.

Abstract

The Na superionic conductor (aka Nasicon, Na1+xZr2SixP3–xO12, 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 (Na3Zr2Si2PO12, bare sample) and 10 at% Na-excess Nasicon (Na3.3Zr2Si2PO12, 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 × 10–1 S/cm at 300 °C) of Nasicon electrolytes resulting from the enlargement of the bottleneck areas in the Na diffusion channels of polycrystalline grains.