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

Abstract

© 2016 American Chemical Society.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

A New Sodium Thioborate Fast Ion Conductor: Na3B5S9

“…As a result, a direct bi-exponential ("modelfree") fit of the total integrated intensity was performed (Figure 4e), split into "large" and "small" signal components, to simply extract the temperature dependence and resulting activation energies, with the two signals giving respective values of 0.192(1) eV (large signal) and 0.17 (1) eV (small signal). These compare favorably with other Na-ion conductors which have been previously investigated by 23 Na relaxation measurements (0.19 eV for NASICONtype Na 3.4 Sc 2 (SiO 4 ) 0.4 (PO 4 ) 2.6 ; [58] 0.19 eV and 0.21 eV for cubic and tetragonal Na 3 PS 4 (respectively); [59] 0.29 eV for sodium strontium silicate [60] ). This is a strong metric for facile local motion, but it is again challenging to link it directly with dynamics of individual Na site populations as extracted from the NMR line shapes, without invoking additional assumptions regarding how the slice-by-slice signal attribution is performed.…”

4 V Na Solid State Batteries Enabled by a Scalable Sodium Metal Oxyhalide Solid Electrolyte

“…To further elucidate the Na + local motional dynamics, static 23 Na saturation recovery measurements of the longitudinal relaxation rate, T 1 , were performed as a function of temperature. The resulting Arrhenius plot (Figure d) demonstrates a low activation energy of 0.110 eV over the temperature range extending from 273 K to approximately 353 K, which indicates fast Na + local motion comparable with what has been measured using this technique for Ga-doped layered Na 2 Zn 2 TeO 6 and Na-doped SrSiO 3 , , and half of what has been reported for Na 3 PS 4 , Na 3 B 5 S 9 and NASICON-type Na 3.4 Sc 2 (SiO 4 ) 0.4 (PO 4 ) 2.6 . − Interestingly, above 353 K and to the maximum temperature that the magnet shim can withstand (398 K), there is a marked inflection in the Arrhenius plot, and a significantly lower-activation relaxation process begins to dominate. Correspondingly, there is a considerable shift in the 7.33 ppm peak position toward the primary (−10 ppm) signal and concomitant broadening of both signals (Figure S8) indicative of exchange approaching a coalescence point .…”

Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via 17O Solid-State NMR Spectroscopy

“…Despite the very low inherent receptivity of 17 O SSNMR spectroscopy, there is a sizable body of high-quality work; the reader is directed toward several comprehensive reviews of the field. − There has recently been significant progress in 17 O SSNMR, including the development of 17 O dynamic nuclear polarization (DNP) SSNMR spectroscopy. − 17 O SSNMR has been used to investigate the structure and disorder in variety of glasses, − along with water, ice, hydrated compounds, and clathrate hydrates. − This technique has been applied across very diverse chemical fields, ,− including organic and biological molecules, ,,,,, drugs and pharmaceuticals, , and battery systems. , …”