Structure and Sodium Ion Dynamics in Sodium Strontium Silicate Investigated by Multinuclear Solid-State NMR

Kenneth K Inglis,Ryan D Bayliss,John P Corley,Jordi Cabana,Frédéric Blanc,Pierre Florian

doi:10.1021/acs.chemmater.6b00941

Abstract

The high oxide ion conductivity of the proposed sodium strontium silicate ion conductors Sr0.55Na0.45SiO2.775 (>10–2 S·cm–1 at 525 °C) and its unusual alkali metal substitution strategy have been extensively questioned in the literature. Here, we present a comprehensive understanding of the structure of this material using a combination of XRD and multinuclear 17O, 23Na, and 29Si solid-state NMR spectroscopy data and a detailed investigation of the Na ion dynamics by high temperature 23Na NMR line shape analysis and relaxation rates measurements. Both 23Na and 29Si NMR spectra demonstrate the absence of Na doping in strontium silicate SrSiO3 and the presence of an amorphous phase identified as Na2O·2SiO2 glass as the Na-containing product. Devitrification at 800 °C yields crystallization of the Na2O·2SiO2 glass into the known crystalline α-Na2Si2O5 phase which was positively identified by its XRD pattern and the extensive and clear 17O, 23Na, and 29Si NMR fingerprints. High temperature 23Na NMR reveals that the Na ions are mobile in the Na2O·2SiO2 amorphous component below its glass transition temperature (∼450 °C). In contrast, 23Na NMR data obtained on the crystalline α-Na2Si2O5 shows limited Na dynamics below ∼650 °C, and this result explains the large discrepancy in the conductivity observed in the literature which strongly depends on the thermal history of the Sr0.55Na0.45SiO2.775 material. These insights demonstrate that the high conductivity observed in Sr0.55Na0.45SiO2.775 is due to Na conduction in the Na2O·2SiO2 glass, and this motivates the quest for the discovery of low temperature fast ion conductors in noncrystalline solids.

Highlights

Strontium silicate SrSiO3 and alkali metal (A site aliovalent substituents) solid solutions were recently investigated as potential intermediate temperature solid oxide fuel cell (SOFC) electrolytes
These newly proposed superior oxide ion conductors have been reported to possess oxide ion conductivities as high as 1.04 × 10−2 S·cm−1 at 625 °C in the Sr0.8K0.2Si0.5Ge0.5O2.9 material which initiated a surge of interest in these new phases.[1]
Alkali metals are typically avoided in high temperature oxide ion conductors due to their ability to be highly mobile at low temperatures in solid state materials such as the well-known Garnets.[3]

Summary

■ INTRODUCTION

Strontium silicate SrSiO3 and alkali metal (A site aliovalent substituents) solid solutions were recently investigated as potential intermediate temperature solid oxide fuel cell (SOFC) electrolytes. Article two normal oxygen sites which could be part of the mechanism for rapid oxide ion conductivity This mechanism was supported by a 29Si solid state nuclear magnetic resonance (NMR) study by Kuang and Wu et al.[5] confirming the existence of a Si signal indicating breaking of the Si3O9 rings into Si3O8 chains, proposing the formation of linked tetrahedral Si in the rings. Through detailed analysis of the structural, electrochemical, and mass transport properties obtained, it was proposed that both Sr0.8K0.2Si0.5Ge0.5O2.9 and by implication of the results the Sr0.55Na0.45SiO2.775 materials were not single phase materials but rather biphasic mixes consisting of crystalline SrSiO3 and some form of amorphous glass, postulated to be potassium digerminate K2O·2GeO2 or sodium disilicate Na2O·2SiO2 glass, with the latter being responsible for the high total conductivity previously reported and the mobile defect most likely to be the alkali ion K+ or Na+, respectively. Na2O·2SiO2 glass and its absence in α-Na2Si2O5, respectively, and highlight potential for the design of noncrystalline solid state fast ion conductors

■ MATERIALS AND METHODS

■ RESULTS AND DISCUSSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES