23Na Nuclear Magnetic Resonance Research Articles

In-cell dehydration of sodium manganese hexacyanoferrate cathode revealed by solid-state NMR

The hard-to-remove lattice water has been regarded as a significant obstacle impeding the practical use of Prussian blue analogue cathodes for sodium-ion batteries. This work monitored the electrochemical evolution of a hydrated monoclinic sodium manganese hexacyanoferrate cathode by solid-state nuclear magnetic resonance (NMR). For the first time, we established a correlation between the chemical shifts of 23Na NMR signals and the presence or absence of lattice water within this cathode. Through this method, we verified the electrochemical dehydration process that coincides with the merging of two redox platforms and a phase transformation in the initial cycles. Furthermore, we discovered that the lattice water is completely removed after several-day cell rest following a single activation cycle.

The effect of iodine on the local environment of network‐forming elements in aluminoborosilicate glasses: An NMR study

AbstractKnowledge of the structural organization of nuclear waste glasses at the atomic scale is the basis for further studies of macroscopic properties (e.g., fracture toughness, chemical durability, and leaching rate). Recent studies have focused on the iodine dissolution mechanism in oxide‐based glasses synthesized under high‐pressure conditions, but there is still a lack of key information on the extent of the modifications caused by iodine entering the glass network. We therefore decided to conduct an in‐depth study by nuclear magnetic resonance on two series of aluminoborosilicate glasses synthetized under high pressure conditions and doped with either I2 or I2O5. The obtained 11B, 23Na, and 27Al nuclear magnetic resonance (NMR) spectra were fitted and correlated with the iodine content in order to find possible relationships with the local environment change for the probed cations. These data showed that iodine has no effect on the coordination of aluminum and very little effects on that of boron. In contrast, we found an expected relationship of the I content with the 23Na NMR parameters. In comparison with previous studies on the non‐bridging oxygen (NBO) distribution in aluminoborosilicate, we suggest that Na+ is mainly scavenged from silica tetrahedra with one or two NBO ( or ) as no boron planar triangles with one NBO are expected for such compositions. From those information, we establish a model for iodine dissolution in nuclear waste glass that is consistent with previous studies.

Drop That Activation Energy: Tetragonal to Cubic Transformations in Na3PS4−xSex for Solid State Sodium Ion Battery Materials

AbstractSodium‐containing chalcogenide materials are emerging as a class of solid electrolytes for application in inexpensive all‐solid‐state sodium‐ion batteries due to their high ionic conductivity, abundance, and degree of synthetic and structural variability. Members of the solid solution Na3PS4−xSex, which are promising solid electrolytes for sodium‐ion batteries, are prepared by reaction at high temperature. With increasing substitution of S by Se, the structure transforms from tetragonal (space group P21c for x = 0, 1) to cubic (space group I3m for x = 2, 3, 4). Within the solid solution, the S and Se atoms are completely disordered in the environments around the P atoms, in accordance with a binomial distribution, as inferred by the 31P nuclear magnetic resonance (NMR) spectra. In 23Na NMR experiments conducted at different magnetic fields and temperatures, quadrupolar lineshapes are observed that are influenced by sodium ion dynamics; the activation energies decrease from 0.21 to 0.15 eV on progressing from the S‐ to the Se‐rich members. A dynamic model is proposed to account for the changes in the 23Na quadrupolar lineshapes by switching the orientations of the electric field gradient and chemical shift anisotropy tensors when Na+ ions hop to the four nearest Na sites.

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy.

This work provides insight into the local structure of Na in MgO-based CO2 sorbents that are promoted with NaNO3. To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO2 capture (MgO + CO2 ↔ MgCO3). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO3 in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO3/MgCO3 interfaces and/or in the vicinity of [Mg2+···CO32-] ionic pairs that are trapped in the cooled NaNO3 melt. The changes observed are reversible, i.e., the local environment of NaNO3 was restored after a regeneration treatment to decompose MgCO3 to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning 23Na nuclear magnetic resonance (MAS 23Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO3 is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO3 open new opportunities to investigate the atomic scale structure of CO2 sorbents modified with Na-based molten salts by using XAS.

The solid solution in the system NaMgAl(SO4)3–KMgAl(SO4)3

We synthesized six samples in the compositional field NaMgAl(SO4)3–KMgAl(SO4)3 in 20 mol% increments from pure Na to pure K compounds. We investigated them by Powder X-Ray diffraction, 23Na, and 27Al Nuclear Magnetic Resonance spectroscopy. The results confirm NaMgAl(SO4)3 as a unique phase identical to a presumed new mineral found in the fumaroles of Eldfell and Hekla volcanoes in Iceland. It tolerates less than 10 mol% K substitution for Na. There exists a compositional gap to approximately Na0.65K0.35MgAl(SO4)3 from where a solid solution extends to KMgAl(SO4)3. The mineral koryakite [NaKMg2Al2(SO4)6] is a member of the latter solid solution series. The crystal structures of all (Na,K)MgAl(SO4)3 phases are akin to NASICON (NA Super Ionic CONductor). NaMgAl(SO4)3 has R3¯c\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R\\overline{3}c$$\\end{document} symmetry and a disordered distribution of Mg and Al among the octahedral sites with only one unique site for the alkali atom. The members of the solid solution have R3¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R\\overline{3}$$\\end{document} symmetry with ordered Mg–Al distribution and two unique alkali sites with different preferences for Na and K. In the crystal structure, the coordination of Na and/or K is trigonal antiprismatic, and these share bases with two octahedral Mg (Na) or Al (K) coordinations. These polyhedra are arranged in columns parallel to [001] and interconnected by SO4 tetrahedral groups. The alkali atoms from a column lie in the same (001) layers as the octahedrally coordinated atoms from the three neighboring rows. On the same level, parallel to (001), there are gaps in the other three neighboring columns forming channels containing Na+ or K+ ions.

Na4-xSn2-xSbxGe5O16, an Air-Stable Solid-State Na-Ion Conductor.

The structure of a Na4Sn2Ge5O16 phase was established via single-crystal X-ray diffraction. Unusually large displacement parameters of Na atoms suggested the possibility of Na+ ionic conductivity. To create Na deficiencies and thus increase the Na+ mobility in Na4Sn2Ge5O16, Sn4+ cations were partially substituted with Sb5+. A series of Na4-xSn2-xSbxGe5O16 samples (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or 0.35) were prepared by solid-state reactions and characterized with electrical impedance spectroscopy in the range of 25-200 °C. The highest ionic conductivity value was achieved in the Na3.8Sn1.8Sb0.2Ge5O16 sample (1.6 mS cm-1 at 200 °C). Na+ migration pathways were calculated using the bond-valence energy landscape approach, and two-dimensional conductivity channels with low energy barriers (≈0.4 eV) were found in the structure. Three-dimensional conductivity can also be achieved in the structure; however, it has a much higher energy barrier. The pristine phase and Na3.8Sn1.8Sb0.2Ge5O16 sample were studied via 23Na and 119Sn solid-state nuclear magnetic resonance. A faster exchange between the Na sites was observed in the doped sample.

Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2-xTeO6 Solid Electrolytes with Solid-State NMR.

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (23Na, 125Te, 25Mg, and 67Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na2MgxZn2-xTeO6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10-4 S cm-1 for Na2MgZnTeO6 at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the P6322 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25Mg and 67Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τNMR = 0.83 × 109 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23Na NMR, which are sensitive to different Mg/Zn ratios. The Na2MgZnTeO6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.

(Invited) Investigating Polymorphism in Na2Fe2F7 and Its Effect on Electrochemical Properties

With the present materials supply crunch caused by the burgeoning Li-ion battery-based electric vehicle market, more sustainable battery chemistries are needed. When considering possible chemistries, sodium (Na) and iron (Fe) stand out as promising candidates for the intercalant ions and redox centers, respectively. Recently, weberite-type sodium metal fluorides (Na2 M 2+ Mʹ 3+F7) have emerged as potential high performance Na cathode materials1-4 with predicted energy densities in the 600 to 800 Wh/kg range (higher than any known Na cathode material) and high ionic conductivity and structural stability. One of the few weberites that has been electrochemically tested so far is Na2Fe2F7 and while one study has shown it to have great, high-rate performance,2 there have been inconsistent reports regarding the structure and electrochemical properties of Na2Fe2F7.In this study, we endeavor to reconcile these characteristics using a combined experimental-computational approach. Density functional theory calculations reveal the inherent metastability of the weberite phase and the close energetics of several Na2Fe2F7 polymorphic forms, as well as the predicted (de)intercalation behavior for each Na2Fe2F7 polymorph. Experimentally, we find that Na2Fe2F7 samples inevitably contain a mixture of weberite polymorphs as characterized using a broad suite of long-range and local probes sensitive to both amorphous and crystalline phases. Notably, we present the first ever 23Na nuclear magnetic resonance (NMR) investigation into weberites and showcase NMR’s ability to parse out different local environments despite the presence of polymorphism. Electrochemically, our Na2Fe2F7 material shows respectable early capacities yet exhibits steady capacity fade, which ex situ synchrotron X-ray diffraction and 23Na NMR reveal to be associated with transformation of the weberite phase upon cycling. However, fine tuning of electrode composition and design appears to be effective at reducing this effect.Overall, this study presents an in-depth characterization of Na2Fe2F7 and provides key insights that pave the way to unlocking weberites as high energy density Na-ion cathode materials.This work made use of the shared facilities of the UC Santa Barbara MRSEC (DMR 1720256) and Center for Scientific Computing (CNS 1725797, DMR 1720256). E. E. Foley and V. C. Wu were supported by the NSF Graduate Research Fellowship Program under Grant No. DGE 1650114. This work is supported by an NSF CAREER award under Grant No. DMR 2141754. Euchner, O. Clemens, and M.A. Reddy, Npt Comput. Mater. 5 31 (2019).Park, Y. Lee, M.-K. Cho, J. Kang, W. Ko, Y.H. Jung, T.-Y. Jeon, J. Hong, H. Kim, S.-T. Myung, and J. Kim, Energy Environ. Sci. 14 1469-1479 (2021).K. Dey, N. Barman, S. Ghosh, S. Sarkar, S.C. Peter, P. Senguttuvan, Chem. Mater. 31 295-299 (2019).Kang, J. Ahn, H. Park, W. Ko, Y. Lee, S. Lee, S. Lee, S.-K. Jung, J. Kim, J. Kang, J. Ahn, H. Park, W. Ko, Y. Lee, S. Lee, J. Kim and S.-K. Jung, Adv. Funct. Mater. 32 2201816 (2022). Fig. 1 , Charge- discharge curve for Na2Fe2F7 and the weberite structure. Figure 1

Structural organization of phase-separated bioactive glasses and the clustering of Si, P, B, Na and F atoms investigated by solid-state NMR and Monte Carlo simulations

In this paper we studied the effect of NaBO2 addition to a phase-separated alkali-free bioactive glass with a composition of 38.49 SiO2 • 36.07 CaO • 19.24 MgO • 5.61 P2O5 • 0.59 CaF2. Microscopy reveals binodal phase separation involving two Si-containing microphases with a droplet size of ∼200 μm, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system were studied, using 29Si, 31P, 11B, 19F, 25Mg, and 23Na nuclear magnetic resonance (NMR) and infrared spectroscopy. The silicate units are dominantly of the metasilicate (Si2) type. The phosphate units exist mostly as orthophosphate (P0) while the borate is present in the form of pyroborate (B1). Multinuclear dipolar re-coupling experiments indicate that the minority components F, P, B and Na all occur within a common phase. Thus, atomic distribution scenarios involving the separation of these components into separate phases can be excluded. The 31P spin echo decay (SED) method was used along with Monte Carlo simulations to characterize the spatial distribution of the phosphate component. Based on the analysis, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate, having a random distribution. While 19F SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed, dipolar recoupling of 31P suggests a local environment resembling that of fluorapatite.

Multiple quantum filtered nuclear magnetic resonance of 23Na+ in uniformly stretched and compressed hydrogels.

Stretching or compressing hydrogels creates anisotropic environments that lead to motionally averaged alignment of embedded guest quadrupolar nuclear spins such as 23Na+. These distorted hydrogels can elicit a residual quadrupolar coupling that gives an oscillation in the trajectories of single quantum coherences (SQCs) as a function of the evolution time during a spin-echo experiment. We present solutions to equations of motion derived with a Liouvillian superoperator approach, which encompass the coherent quadrupolar interaction in conjunction with relaxation, to give a full analytical description of the evolution trajectories of rank-1 (T^1±1), rank-2 (T^2±1), and rank-3 (T^3±1) SQCs. We performed simultaneous numerical fitting of the experimental 23Na nuclear magnetic resonance (NMR) spectra and rank-2 (T^2±1) and rank-3 (T^3±1) SQC evolution trajectories measured in double and triple quantum filtered experiments, respectively. We estimated values of the quadrupolar coupling constant CQ, rotational correlation time τC, and 3 × 3 Saupe order matrix. We performed simultaneous fitting of the analytical expressions to the experimental data to estimate values of the quadrupolar coupling frequency ωQ/2π, residual quadrupolar coupling ωQ/2π, and corresponding spherical order parameter S0*, which showed a linear dependence on the extent of uniform hydrogel stretching and compression. The analytical expressions were completely concordant with the numerical approach. The insights gained here can be extended to more complicated (biological) systems such as 23Na+ bound to proteins or located inside and outside living cells in high-field NMR experiments and, by extension, to the anisotropic environments found in vivo with 23Na magnetic resonance imaging.

Sodium Dynamics in the Cellular Environment.

Sodium ions are essential for the functions of biological cells, and they are maintained at the balance between intra- and extracellular environments. The quantitative assessment of intra- and extracellular sodium as well as its dynamics can provide crucial physiological information on a living system. 23Na nuclear magnetic resonance (NMR) is a powerful and noninvasive technique to probe the local environment and dynamics of sodium ions. However, due to the complex relaxation behavior of the quadrupolar nucleus in the intermediate-motion regime and because of the heterogeneous compartments and diverse molecular interactions in the cellular environment, the understanding of the 23Na NMR signal in biological systems is still at the early stage. In this work, we characterize the relaxation and diffusion of sodium ions in the solutions of proteins and polysaccharides, as well as in the in vitro samples of living cells. The multi-exponential behavior of 23Na transverse relaxation has been analyzed according to the relaxation theory to derive the crucial information related to the ionic dynamics and molecular binding in the solutions. The bi-compartment model of transverse relaxation and diffusion measurements can corroborate each other to quantify the fractions of intra- and extracellular sodium. We show that 23Na relaxation and diffusion can be used to monitor the viability of human cells, which offers versatile NMR metrics for in vivo studies.

Operando NMR electrochemical gating studies of ion dynamics in PEDOT:PSS

Although organic mixed ionic-electronic conductors are widely proposed for use in bioelectronics, energy generation/storage and neuromorphic computing, our fundamental understanding of the charge-compensating interactions between the ionic and electronic carriers and the dynamics of ions remains poor, particularly for hydrated devices and on electrochemical cycling. Here we show that operando 23Na and 1H nuclear magnetic resonance (NMR) spectroscopy can quantify cation and water movement during the doping/dedoping of films comprising the widely used mixed conductor poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS). A distinct 23Na quadrupolar splitting is observed due to the partial ordering of the PSS chains within the PEDOT:PSS-rich domains, with respect to the substrate. Operando 23Na NMR studies reveal a close-to-linear correlation between the quadrupolar splitting and the charge stored, which is quantitatively explained by a model in which the holes on the PEDOT backbone are bound to the PSS SO3- groups; an increase in hole concentration during doping inversely correlates with the number of Na+ ions bound to the PSS chains within the PEDOT-rich ordered domains, leading to a decrease in ions within the ordered regions and a decrease in quadrupolar splitting. The Na+-to-electron coupling efficiency, measured via 23Na NMR intensity changes, is close to 100% when using a 1 M NaCl electrolyte. Operando 1H NMR spectroscopy confirms that the Na+ ions injected into/extracted from the wet films are hydrated. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.

Sodium is a negative allosteric regulator of the ghrelin receptor

The functional properties of G protein-coupled receptors (GPCRs) are intimately associated with the different components in their cellular environment. Among them, sodium ions have been proposed to play a substantial role as endogenous allosteric modulators of GPCR-mediated signaling. However, this sodium effect and the underlying mechanisms are still unclear for most GPCRs. Here, we identified sodium as a negative allosteric modulator of the ghrelin receptor GHSR (growth hormone secretagogue receptor). Combining 23Na-nuclear magnetic resonance (NMR), molecular dynamics, and mutagenesis, we provide evidence that, in GHSR, sodium binds to the allosteric site conserved in class A GPCRs. We further leveraged spectroscopic and functional assays to show that sodium binding shifts the conformational equilibrium toward the GHSR-inactive ensemble, thereby decreasing basal and agonist-induced receptor-catalyzed G protein activation. All together, these data point to sodium as an allosteric modulator of GHSR, making this ion an integral component of the ghrelin signaling machinery.

On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43- by WS42- in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm-1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.

SGLT2 inhibitor ertugliflozin decreases elevated intracellular sodium, and improves energetics and contractile function in diabetic cardiomyopathy

BackgroundElevated myocardial intracellular sodium ([Na+]i) was shown to decrease mitochondrial calcium ([Ca2+]MITO) via mitochondrial sodium/calcium exchanger (NCXMITO), resulting in decreased mitochondrial ATP synthesis. The sodium-glucose co-transporter 2 inhibitor (SGLT2i) ertugliflozin (ERTU) improved energetic deficit and contractile dysfunction in a mouse model of high fat, high sucrose (HFHS) diet-induced diabetic cardiomyopathy (DCMP). As SGLT2is were shown to lower [Na+]i in isolated cardiomyocytes, we hypothesized that energetic improvement in DCMP is at least partially mediated by a decrease in abnormally elevated myocardial [Na+]i. MethodsForty-two eight-week-old male C57BL/6J mice were fed a control or HFHS diet for six months. In the last month, a subgroup of HFHS-fed mice was treated with ERTU. At the end of the study, left ventricular contractile function and energetics were measured simultaneously in isolated beating hearts by 31P NMR (Nuclear Magnetic Resonance) spectroscopy. A subset of untreated HFHS hearts was perfused with vehicle vs. CGP 37157, an NCXMITO inhibitor. Myocardial [Na+]i was measured by 23Na NMR spectroscopy. ResultsHFHS hearts showed diastolic dysfunction, decreased contractile reserve, and impaired energetics as reflected by decreased phosphocreatine (PCr) and PCr/ATP ratio. Myocardial [Na+]i was elevated > 2-fold in HFHS (vs. control diet). ERTU reversed the impairments in HFHS hearts to levels similar to or better than control diet and decreased myocardial [Na+]i to control levels. CGP 37157 normalized the PCr/ATP ratio in HFHS hearts. ConclusionsElevated myocardial [Na+]i contributes to mitochondrial and contractile dysfunction in DCMP. Targeting myocardial [Na+]i and/or NCXMITO may be an effective strategy in DCMP and other forms of heart disease associated with elevated myocardial [Na+]i.

Preparation, characterization, and structural studies of new sodium gallium tungstate phosphate glasses

A new glass forming region in the quaternary system Na2O-P2O5-WO3-Ga2O3 along the composition line (NaPO3)80-x(Ga2O3)20(Na2WO4)x (x = 0, 5, 10, 15, 20 and 25 mol%) has been prepared and characterized by X-ray diffraction, Differential Scanning Calorimetry (DSC), Raman scattering and 31P, 23Na and 71Ga nuclear magnetic single and double resonance NMR techniques. The partial substitution of the NaPO3 component by Na2WO4 produces a significant increase of molar volume (decreased oxygen packing density), whereas the glass transition temperatures Tg pass through a shallow maximum near x = 10 to 15. While in the x = 0 sample the structure is dominated by Q1 units linked to gallium in four-, five- and six-coordination, the gradual substitution of phosphate by tungstate significantly affects network reconstruction, eliminating P-O-P linkages, the concentration of which is found to be greatly diminished above the x = 10 level. While the structure of these glasses is dominated by Q0 units, connected to both gallium and tungstate species, 71Ga{31P} REDOR data suggest significant decreases in the extent of gallium-phosphorus connectivity with increasing Na2WO4 content. Finally, monotonic changes in both 23Na chemical shifts and 23Na-31P second moment values, M2(Na-P), indicate that the anionic surroundings of the sodium ions evolve from a pure phosphate to a mixed phosphate/tungstate environment.

Solid state NMR characterization of NaNbO3 and LiNbO3 nanoparticles obtained by microwave-assisted combustion

Nanoparticles of LiNbO3 and NaNbO3 were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO3, there are two polimorphs which are stable at room temperature, space groups P21ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state 23Na and 93Nb Nuclear Magnetic Resonance (NMR) techniques. 23Na results reveal the presence of both phases, Pbma and P21ma, for samples prepared using NaNO3 precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P21ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO3 nanoparticles display the usual rhombohedral structure R3c. 7Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li+ ions are found at the surface of the nanoparticles, while bulk Li+ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.

Understanding fast ion dynamics in sodiated Li4Na x Ti5O12: from interfacial to extended Li+ and Na+ dynamics in its mixed-conducting solid solutions

Climate change and energy crises require the development of new sustainable materials to realise reliable electrochemical energy storage devices. Spinel-type Li4Ti5O12 (LTO) is one of the most promising anode materials not only for Li-based batteries, but also for those relying on sodium. While Li+ ion dynamics at the early stages of lithiation has been studied already previously, almost no data on the diffusion properties of Na+ ions can be found in the literature. Here, we used nucleus-specific 7Li and 23Na nuclear magnetic resonance (NMR) spectroscopy to quantify the motional processes in mixed-conducting Li4Na x Ti5O12 with x = 0.1, 0.5 and 1.5 on the angstrom length scale. Most importantly, our results reveal a strong increase in Li+ diffusivity in the early stages of chemical sodiation that is accompanied by a sharp decrease in activation energy when x reaches 0.5. The two-component 7Li NMR spectra point to the evolution of an interfacial solid solution at very low sodiation levels (x = 0.1). At x = 0.5, these regions emerge over almost the entire crystallite area, enabling rapid 8a-16c-8a Li+ exchange (0.4 eV), which leads to facile long-range ion transport. We direct the attention of the reader towards the initial formation of solid solutions in LTO-based anode materials and their capital impact on overall ion dynamics. In contrast to macroscopic electrochemical testing, NMR is uniquely positioned to detect and to resolve these exceptionally fast ion dynamics during the initial stages of sodiation. As these processes crucially determine the fast-charging performance of LTO-type batteries, our study lays the atomistic foundations to establish a general understanding of why two-phase materials such as LTO can act as an impressive insertion host for both Li and Na ions.

Mechanochemical Synthesis and Study of the Local Structure of NaGaS2 Glass and Glass-Ceramics.

NaGaS2 is a newly discovered compound that has already shown great promise for a variety of applications because of its layered structure and ion exchange properties. In this work, crystalline NaGaS2 has been synthesized by an alternative method to what has been previously published, namely, by mechanochemistry, either by a direct one-step process or by a two-step process. In the one-step process, crystalline NaGaS2 is directly formed by milling sodium sulfide Na2S and gallium(III) sulfide Ga2S3. However, an amorphous material is present in majority together with the crystalline phase. In the two-step process, amorphous NaGaS2 is first obtained by mechanical milling and then heated above its glass transition temperature to obtain a glass-ceramic mainly composed of crystalline NaGaS2. For the two-step process, changes of the local atomic-level structure in amorphous NaGaS2 and after crystallization were analyzed by high-field solid-state nuclear magnetic resonance (NMR) spectroscopy as well as by X-ray total scattering and pair distribution function (PDF) analysis. Based on quantitative analysis on the 23Na NMR spectra, modifying the annealing treatment can promote the formation of the crystalline phase up to a molar fraction of 83.8%.

Mechanical, thermal, and structural investigations of chemically strengthened Na2O–CaO–Al2O3–SiO2 glasses

For a series of conventional soda-lime-silicate glasses with increasing Al2O3content, we investigated the thermal, mechanical, and structural properties before and after K+-for-Na+ion-exchange strengthening by exposure to molten KNO3. The Al-for-Si replacement resulted in increased glass network polymerization and lowered compactness. The glass transition temperature (Tg), hardness (H) and reduced elastic modulus (Er), of the pristine glasses enhanced monotonically for increasing Al2O3content.HandErincreased linearly up to a glass composition with roughly equal stoichiometric amounts of Na2O and Al2O3where a nonlinear dependence on Al2O3was observed, whereasHandErof the chemically strengthened (CS) glasses revealed a strictly linear dependence.Tg, on the other hand, showed linear increase with Al-for-Si for pristine glasses while for the CS glasses a linear to nonlinear trend was observed. Solid-state27Al nuclear magnetic resonance (NMR) revealed the sole presence of AlO4groups in both the pristine and CS glasses.23Na NMR and wet-chemical analysis manifested that all Al-bearing glasses had a lower and near-constant K+-for-Na+ion exchange ratio than the soda-lime-silicate glass. Differential thermal analysis of CS glasses revealed a “blurred” glass transition temperature (Tg) and an exothermic step belowTg; the latter stems from the relaxation of residual compressive stresses. The nanoindentation-derived hardness at low loads and <5 mol% Al2O3showed evidence of stress relaxation for prolonged ion exchange treatment. The crack resistance is maximized for molar ratiosn(M(2)O)/n(Al2O3)≈1for the CS glasses, which is attributed to an increased elastic energy recovery that is linked to the glass compactness.