Crystal Structures, Local Atomic Environments, and Ion Diffusion Mechanisms of Scandium-Substituted Sodium Superionic Conductor (NASICON) Solid Electrolytes

Yue Deng,Kent J Griffith,Matthieu Courty,Jean-Noël Chotard,Christian Masquelier,Clare P Grey,Oliver Pecher,Christopher Eames,Benoit Fleutot,Long H B Nguyen,M Saiful Islam

doi:10.1021/acs.chemmater.7b05237

Abstract

The importance of exploring new solid electrolytes for all-solid-state batteries has led to significant interest in NASICON-type materials. Here, the Sc3+-substituted NASICON compositions Na3ScxZr2–x(SiO4)2–x(PO4)1+x (termed N3) and Na2ScyZr2–y(SiO4)1–y(PO4)2+y (termed N2) (x, y = 0–1) are studied as model Na+-ion conducting electrolytes for solid-state batteries. The influence of Sc3+ substitution on the crystal structures and local atomic environments has been characterized by powder X-ray diffraction (XRD) and neutron powder diffraction (NPD), as well as solid-state 23Na, 31P, and 29Si nuclear magnetic resonance (NMR) spectroscopy. A phase transition between 295 and 473 K from monoclinic C2/c to rhombohedral R3̅c is observed for the N3 compositions, while N2 compositions crystallize in a rhombohedral R3̅c unit cell in this temperature range. Alternating current (AC) impedance spectroscopy, molecular dynamics (MD), and high temperature 23Na NMR studies are in good agreement, showing that, with a higher Sc3+ concentration, the ionic conductivity (of about 10–4 S/cm at 473 K) decreases and the activation energy for ion diffusion increases. 23Na NMR experiments indicate that the nature of the Na+-ion motion is two-dimensional on the local atomic scale of NMR although the long-range diffusion pathways are three-dimensional. In addition, a combination of MD, bond valence, maximum entropy/Rietveld, and van Hove correlation methods has been used to reveal that the Na+-ion diffusion in these NASICON materials is three-dimensional and that there is a continuous exchange of sodium ions between Na(1) and Na(2) sites.

Highlights

Rechargeable lithium ion batteries (LIBs) have come to dominate portable electronics over the past two decades, primarily due to their light weight and high energy density.[1,2] there are concerns over the safety of LIBs and the abundance and cost of lithium
Alternating current (AC) impedance spectroscopy, molecular dynamics (MD), and high temperature 23Na nuclear magnetic resonance (NMR) studies are in good agreement, showing that, with a higher Sc3+ concentration, the ionic conductivity decreases and the activation energy for ion diffusion increases. 23Na NMR experiments indicate that the nature of the Na+-ion motion is two-dimensional on the local atomic scale of NMR the long-range diffusion pathways are three-dimensional
A combination of MD, bond valence, maximum entropy/Rietveld, and van Hove correlation methods has been used to reveal that the Na+-ion diffusion in these NASICON materials is three-dimensional and that there is a continuous exchange of sodium ions between Na(1) and Na(2) sites

Summary

Introduction

Rechargeable lithium ion batteries (LIBs) have come to dominate portable electronics over the past two decades, primarily due to their light weight and high energy density.[1,2] there are concerns over the safety of LIBs and the abundance and cost of lithium. Sodium ion batteries (NIBs) are being actively investigated as an alternative, since sodium is more abundant and lower in cost while offering a similar intercalation chemistry to lithium.[3−5] safety issues related to flammable liquid electrolytes remain a serious concern. All solid-state batteries (ASSBs), which use nonflammable ion-conducting solid electrolytes, have been considered as potential candidates for alternative energy storage devices.[6−8] One such family of candidate solid electrolytes is the NASICON (sodium (NA) SuperIonic CONductor) class of materials, which offer both high chemical stability and high Na+ mobility within a three-dimensional (3D) framework.[9−17].

Methods

Results

Conclusion