23Na Magic-angle Spinning Research Articles

A New Class of Oxyhalide Solid Electrolytes NaNbCl6-2xOx for Solid-state Sodium Batteries.

Sodium-based batteries are gaining momentum due to the abundance and lower cost of sodium compared to lithium. Solid-state sodium batteries can also provide further safety advantages. However, sodium-based solid-state electrolytes (SSEs) that meet all the rigorous requirements, such as high ionic conductivity, oxidative stability with the cathode, and ease of processability, are lacking. We present here a new class of sodium-based oxyhalide electrolytes NaNbCl6-2xOx with a facile mechanochemical synthesis. The oxyhalide NaNbCl4O exhibits close to two orders of magnitude higher ambient-temperature sodium-ion conductivity (1.03 x 10-4 S cm-1) compared to the halide counterpart NaNbCl6 (3 x 10-6 S cm-1). Structural motifs unique to the oxygen content in NaNbCl6-2xOx are identified with 23Na and 93Nb magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and x-ray diffraction (XRD). Solid-state sodium batteries assembled with NaNbCl4O electrolyte and the cobalt- and nickel-free layered Na0.70Fe0.3Mn0.65Al0.05O2 cathode exhibit a maximum discharge capacity of 155 mA h g-1 with good cycle life at ambient temperature.

A New Class of Oxyhalide Solid Electrolytes NaNbCl6‐2xOx for Solid‐state Sodium Batteries

Sodium‐based batteries are gaining momentum due to the abundance and lower cost of sodium compared to lithium. Solid‐state sodium batteries can also provide further safety advantages. However, sodium‐based solid‐state electrolytes (SSEs) that meet all the rigorous requirements, such as high ionic conductivity, oxidative stability with the cathode, and ease of processability, are lacking. We present here a new class of sodium‐based oxyhalide electrolytes NaNbCl6‐2xOx with a facile mechanochemical synthesis. The oxyhalide NaNbCl4O exhibits close to two orders of magnitude higher ambient‐temperature sodium‐ion conductivity (1.03 x 10‐4 S cm‐1) compared to the halide counterpart NaNbCl6 (3 x 10‐6 S cm‐1). Structural motifs unique to the oxygen content in NaNbCl6‐2xOx are identified with 23Na and 93Nb magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and x‐ray diffraction (XRD). Solid‐state sodium batteries assembled with NaNbCl4O electrolyte and the cobalt‐ and nickel‐free layered Na0.70Fe0.3Mn0.65Al0.05O2 cathode exhibit a maximum discharge capacity of 155 mA h g‐1 with good cycle life at ambient temperature.

Densification of sodium and magnesium aluminosilicate glasses at ambient temperature: structural investigations by solid-state nuclear magnetic resonance and molecular dynamics simulations.

Sodium and magnesium aluminosilicate glasses with compositions 20Na2O-20Al2O3-60SiO2 (NAS) and 20MgO-20Al2O3-60SiO2 (MAS) were subjected to a 12 and 25 GPa compression and decompression at room temperature, resulting in density increases from 3.7% to 5.3% (NAS) and from 8.2 to 8.4% (MAS), respectively. The pressurization at 25 GPa was done on 17O-enriched glasses, to facilitate characterization by 17O NMR. The structural changes associated with this process have been investigated by solid state 29Si, 27Al, 23Na, 25Mg, and 17O magic-angle spinning NMR and compared with the situation in thermally relaxed glasses and/or glasses prepared at ambient pressure. While in the Na aluminosilicate glass only subtle structural changes were observed in a sample densified at 12 GPa, the average coordination number of Al 〈CN(Al)〉 increases moderately from 4.00 to 4.26 by pressurization at 25 GPa. In the Mg-based system, 〈CN(Al)〉 increases from 4.34 to 4.57 to 4.83 in the sequence 10-4 GPa → 12 GPa → 25 GPa. The experimental result at 25 GPa was qualitatively confirmed by molecular dynamics (MD) simulations. Overall, pressurization results in more positive 29Si and 17O chemical shifts, most likely reflecting a reduction in the Si-O-Si and Si-O-Al bonding angles in the pressurized glasses. Furthermore, the results are also consistent with either an increased number of non-bridging O-atoms upon pressurization, or a larger number of Si-O-Al or Al-O-Al linkages. The significantly higher sensitivity of MAS, compared to NAS glass, to an increase in 〈CN(Al)〉 upon pressurization, provides a good structural rationale for its significantly higher crack initiation resistance.

An advance understanding of the alkali activation of cover layers waste rocks from phosphate mines: Mechanical, structure and microstructure studies

This study details the effect of silica modulus variation (SiO2/Na2O 1.25, 1.50 and 1.75) on the mechanical properties and the microstructure evolution of Alkali Activated Materials (AAM) based on phosphate mining waste rocks and issued from the cover layer. The mechanical behavior of samples was assessed by compression and flexural strength tests at 7 and 28 days of curing. A resulting highly compact microstructure was formed by decreasing the silica modulus (Ms), leading to an enhancement of the mechanical strength. The microstructure of hardened specimens was studied using X-ray Diffraction (XRD), Fourier Transform InfraRed (FTIR), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX) and 29Si, 27Al, 23Na Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The results show the formation of CSH gels as main reaction products, C–(A)–SH and N–(A)–SH gels, with the possible formation of a hybrid C–(N–A)–SH gels. The structure of these formed gels appears to depend on the raw material’s chemical composition, as confirmed by MAS NMR and SEM/EDX analysis. These results provide insight in alkali activation of phosphate mining waste from the cover layer, which can promote their valorization in several construction applications, and consequently, contribute to sustainable phosphate mining..

Synthesis and ionic conductivity of novel high charged tetrasilisic type micas

The objectives of this study were the synthesis of novel high charged tetrasilisic type micas with the chemical compositions of Na4(MgxLi2(4-x))Si8O20F4 (x = 2, 3 and 4) and the estimation of ionic conductivity of the novel micas. The mica with x = 2 has no vacancies in the octahedral sheets and the ratio Mg2+/Li+ in the octahedral sheets is 2/4 in the chemical formula. The micas with x = 3 and 4 have vacancies in the octahedral sheets and the ratio Mg2+/Li+/vacancy is 3/2/1 and 4/0/2, respectively. The synthesis was tried by firing the mixtures of chemical reagents in sealed container. The obtained materials were characterized using X-ray diffraction analyzer, scanning electron microscopy, Fourier transform infrared spectrometer and 23Na magic angle spinning nuclear magnetic resonance equipment. The novel micas with x = 2, 3 and 4 were formed during cooling after melting the mixtures of the reagents. However, a part of Na+ ions which should be interlayer cations originally entered the vacant sites in the octahedral sheets of the obtained mica with x = 4. The mica with x = 2 showed the highest ionic conductivity and the conductivity was 2.4 × 10−2 S/cm at 600 °C. This resulted from the facts that the mica with x = 2 was actually high charged tetrasilisic type mica of which Na+ ions in the interlayers did not transfer to the octahedral sheets and had many mobile Na+ ions in the interlayers.

Structural Role and Spatial Distribution of Carbonate Ions in Amorphous Calcium Phosphate

The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2–14 wt %) were studied by multinuclear 1H, 13C, 23Na, and 31P magic-angle spinning ...

Original Layered OP4-(Li,Na)xCoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR.

The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, 7Li and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. 7Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. 23Na MAS NMR showed that some Na+ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li0.42Na0.05]Na0.32CoO2 formula for the material. In addition, 7Li and 23Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for 7Li up to 450 K, the 23Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.

Nanostructure of CaO-(Na2O)-Al2O3-SiO2-H2O Gels Revealed by Multinuclear Solid-State Magic Angle Spinning and Multiple Quantum Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

(Calcium,alkali)-aluminosilicate gel frameworks are the basis of modern cements and alkali-activated materials for sustainable infrastructure and radioactive waste immobilization and also find application in glass alteration, mineral weathering, and zeolite synthesis. Here, we resolve the nanostructure of these gels that dictates mass transport, solubility, and mechanical properties. The key structural motifs comprising hydrous (calcium,alkali)-aluminosilicate gels are identified via 17O, 23Na, and 27Al triple-quantum magic angle spinning and 29Si magic angle spinning nuclear magnetic resonance spectroscopy of a novel class of stoichiometrically controlled 17O-enriched multiphase gels. Increased Ca content promotes low-Al, high-Ca chain-structured “C-S-H-type” products exhibiting significant nanostructural ordering, low levels of chain cross-linking, predominant Ca coordination of nonbridging oxygen atoms, and an increase in proton association with CaO layers to form Ca–OH sites. Al substitution is identified in multiple sites in the silicate chains, including cross-linking, bridging, and pairing tetrahedra. Increased Al content increases the proportion of cross-linking sites and gel disorder. The large increase in SiIV–O–AlIV sites increases the relative amounts of Na+ and AlV species charge-balancing AlO4– tetrahedra and results in the formation of an additional disordered low-calcium, framework-structured alkali aluminosilicate (“N-A-S-H-type”) gel, with high Al and Na contents. Changes in bulk composition significantly alter the nanostructures of the C-S-H-type and N-A-S-H-type gels. Mean SiIV–O–AlIV bond angles for each type of AlIV site environment are highly consistent, with compositional changes dictating the relative proportions of individual AlIV species but not altering the local structure of each AlIV site. These findings reveal the molecular interactions governing the (calcium,alkali)-aluminosilicate gel nanostructure, which are crucial in controlling material properties and durability.

Structural and spectroscopic properties of phosphate–tungsten glasses doped with Nd3+ and Tb3+

Glasses of composition 90NaPO3-10WO3-xNd2O3, (0 ≤ x ≤ 1.7 mol%) and 90NaPO3-10WO3-yTbF3, (0 ≤ y ≤ 5.0 mol%) have been prepared and characterized. 31P and 23Na magic-angle spinning (MAS) NMR and 23Na{31P} rotational echo double resonance results are consistent with previously published data, indicating the presence of Q(2) and Q(1)1 W units. In addition, linear dependences of 31P MAS-NMR linewidths and spin-lattice relaxation rates and the electron spin-spin relaxation behavior (in the case of Nd2O3 doping) are consistent with a random dopant distribution, For Nd2O3- doped glass samples, absorption spectra in the visible and infrared regions were collected and analysed, and photoluminescence excitation and emission spectra were obtained under 575 nm excitation. Using the Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and by using them, the radiative lifetime (τr) were calculated for the 4F3/2 excited state. The fluorescence quantum efficiency value for this level (QE = τf/τr) of 13% was determined for glass with x = 1.0. The photophysical properties of Tb- doped glasses were also studied and upon UV excitation (372 nm) the glasses show emissions in the green (540 nm, 5D4 → 7F5 transition) and blue (434 nm, 5D3 → 7F4). The relative luminescence intensity ratio 5D3 → 7F5/5D4 → 7F5 is higher for lower Tb3+ content (0.5 Tb glass sample), because for higher Tb3+ contents, a cross-relaxation energy transfer process takes place favoring the green emission.

Density Functional Theory-Assisted 31P and 23Na Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Na3V2(PO4)2F3–Na3V2(PO4)2FO2 Solid Solution: Unraveling Its Local and Electronic Structures

The local and electronic structures of Na3V2(PO4)2F3–Na3V2(PO4)2FO2 electrode materials have been investigated by a combination of 23Na and 31P magic-angle spinning NMR spectroscopy and density fun...

Structure and Stability of High CaO- and P2O5-Containing Silicate and Borosilicate Bioactive Glasses.

The present work elucidates about the structure of bioactive glasses having chemical compositions expressed as (mol %) (50.0 - x)SiO2-xB2O3-9.3Na2O-37CaO-3.7P2O5, where x = 0.0, 12.5, 25, and 37.5, and establishes a correlation between the structure and thermal stability. The structural modifications in the parent boron-free glass (B0) with the gradual substitutions of B2O3 for SiO2 are assessed by Raman and 29Si, 31P, 11B, and 23Na magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopies. The structural studies reveal the presence of QSi2 and QSi3 structural units in both silicate and borosilicate glasses. However, QSi4(3B) units additionally form upon incorporating B2O3 in B0 glass. B-containing silicate glasses exhibit both three-coordinated boron (BIII) and four-coordinated boron (BIV) units. The 31P MAS-NMR studies reveal that the majority of phosphate species exist as isolated orthophosphate (QP0) units. The incorporation of B2O3 in B0 glass increases the cross-linking between the SiO4 and BO4 structural units. However, incorporation of B2O3 lowers the glass thermal stability (ΔT), as shown by differential scanning calorimetry. Although both silicate and borosilicate glasses exhibit good in vitro apatite-forming ability and cell compatibility, the bactericidal action against Escherichia coli bacteria is more evident in borosilicate glass in comparison to silicate base glass. The controlled release of (BO3)3- ions from boron-modified bioactive glasses improves both the cell proliferation and the antibacterial properties, making them promising for hard tissue engineering applications.

Capping Structure of Ligand-Cysteine on CdSe Magic-Sized Clusters.

Ligand molecules capping on clusters largely affect the formation and stabilization mechanism and the property of clusters. In semiconductor CdSe clusters, cysteine is used as one of the ligands and allows the formation of ultrastable (CdSe)34 magic-sized clusters. Cysteine has sulfhydryl, amine, and carboxylate groups, all of which have coordination ability to the CdSe surface, and the bonding states of the three functional groups of ligand–cysteine on the CdSe core have not been determined. In this work, the capping structure of ligand–cysteine is examined by performing Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. FT-IR, XPS, and 1H, 13C, and 23Na magic-angle spinning NMR show that the sulfhydryl group of ligand–cysteine forms a sulfur–cadmium bond with a cadmium atom at the CdSe surface, while the carboxylate group does not contribute to the protection of the CdSe core and binds to a sodium ion contained as a counterion. 15N–{77Se} through-bond J-single quantum filtered NMR experiment reveals that the amine group of ligand–cysteine has no coordination to selenium atoms. By considering the N–Cd bond forming ratio (∼43%) revealed in our previous work, which is confirmed in this work by analyzing 13Cα signal intensity (∼42%), we concluded that cysteine capping on (CdSe)34 occurs in two ways: one involves both the sulfur–cadmium and nitrogen–cadmium bonds, and the other bears only the sulfur–cadmium bond.

States of thermochemically or electrochemically synthesized NaxPy compounds analyzed by solid state 23Na and 31P nuclear magnetic resonance with theoretical calculation

Phosphorus is a promising material for the electrode in sodium ion batteries (NIBs). In this study, the states of NaxPy compounds synthesized by thermochemical reaction and electrochemical sodiation were compared using solid state 23Na magic angle spinning (MAS) NMR, 23Na multiple quantum (MQ) MAS NMR, and 31P MAS NMR. The NMR signals in thermochemically synthesized NaxPy compounds (Na3P, NaP, Na3P7, Na3P11, NaP7) are assigned in reference to theoretical chemical shifts, based on first-principles calculations. Furthermore, the NMR signals in electrochemically prepared NaxPy compounds after two sodiation/desodiation cycles are ascribed to Na3P compounds and three amorphous compositions. The amorphous compounds ascribed to Na1−αP (0 < α < 1), Na2−βP (0 < β < 1), and Na3−γP (0 < γ < 1) are formed below 0.58 V in the charge and discharge process. These Na3P and the amorphous phases overlapped in the range from 0.20 V (sodiation process) to 0.58 V (desodiation process). 23Na and 31P NMR spectra reveal reversible sodiation and desodiation processes in the third cycle.

Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way To Improve the Na Ion Electrode Activity of Layered Titanate.

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.

Alkaline-Earth Rhodium Hydroxides: Synthesis, Structures, and Thermal Decomposition to Complex Oxides.

The rhodium(III) hydrogarnets Ca3Rh2(OH)12 and Sr3Rh2(OH)12 crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl3·3H2O and either Ca(OH)2 or Sr(OH)2 in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO2 is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH)6, are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. 23Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. Ca3Rh2(OH)12 yields CaRh2O4 and CaO above 650 °C, from which phase-pure CaRh2O4 is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. Sr3Rh2(OH)12 decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite Sr6Rh5O15, which contains mixed-valent Rh3+/4+, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH)6, a complex set of decomposition events takes place via transient phases.

Sodiation and Desodiation via Helical Phosphorus Intermediates in High-Capacity Anodes for Sodium-Ion Batteries

Na-ion batteries are promising alternatives to Li-ion systems for electrochemical energy storage because of the higher natural abundance and widespread distribution of Na compared to Li. High capacity anode materials, such as phosphorus, have been explored to realize Na-ion battery technologies that offer comparable performances to their Li-ion counterparts. While P anodes provide unparalleled capacities, the mechanism of sodiation and desodiation is not well-understood, limiting further optimization. Here, we use a combined experimental and theoretical approach to provide molecular-level insight into the (de)sodiation pathways in black P anodes for sodium-ion batteries. A determination of the P binding in these materials was achieved by comparing to structure models created via species swapping, ab initio random structure searching, and a genetic algorithm. During sodiation, analysis of 31P chemical shift anisotropies in NMR data reveals P helices and P at the end of chains as the primary structural components in amorphous Na xP phases. X-ray diffraction data in conjunction with variable field 23Na magic-angle spinning NMR support the formation of a new Na3P crystal structure (predicted using density-functional theory) on sodiation. During desodiation, P helices are re-formed in the amorphous intermediates, albeit with increased disorder, yet emphasizing the pervasive nature of this motif. The pristine material is not re-formed at the end of desodiation and may be linked to the irreversibility observed in the Na-P system.

Role of Salts in Phase Transformation of Clathrate Hydrates under Brine Environments

Although ion exclusion is a naturally occurring and commonly observed phenomenon in clathrate hydrates, an understanding for the effect of salt ions on the stability of clathrate hydrates is still unclear. Here we report the first observation of phase transformation of structure I and structure II clathrate hydrates using solid-state 13C, 19F, and 23Na magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, combined with X-ray diffraction and Raman spectroscopy. The phase transformation of clathrate hydrates in salt environments is found to be closely associated with the quadruple point of clathrate hydrate/hydrated salts and the eutectic point of ice/hydrated salts. The formation of the quasi-brine layer (QBL) is triggered at temperatures a little lower than the eutectic point, where an increasing salinity and QBL does not affect the stability of clathrate hydrates. However, at temperatures above the eutectic point, all hydrated salts and the QBL melt completely to form brine solutions, desta...

Novel method for the preparation of Cs-containing FAU(Y) catalysts for aniline methylation

Cs-containing FAU(Y)-type zeolite catalysts were prepared by conventional and novel ion exchange procedures followed by incipient wetness impregnation with CsOH. The novel ion exchange procedure involved hydrothermal treatment of NaY zeolite in aqueous solution of CsCl at 140–200 °C for 6–24 h. The samples were characterized by low-temperature nitrogen adsorption, X-ray fluorescence analysis, X-ray powder diffraction, scanning electron microscopy, 23Na, 27Al and 133Cs magic angle spinning nuclear magnetic resonance, CO2 and NH3-Temperature programmed desorption. The results show that hydrothermal treatment at 200 °C allows to obtain higher degrees of ion-exchange (up to 83%) with respect to conventional method giving maximum 66%–69%. Catalytic properties of Cs-containing FAU(Y) were studied in aniline methylation. The yield of N-methylaniline is shown to correlate with catalyst’s basicity. The best catalyst performance was achieved over the catalyst with the highest ion-exchange degree impregnated with CsOH. The selectivity to N-methylaniline over this catalyst reached 96.4%.

Glass-to-crystal transition in the NASICON glass-ceramic system Na1+xAlxM2−x(PO4)3 (M=Ge, Ti)

The glass-to-crystal transition of Na1+xAlxGe2−x(PO4)3 (NAGP), and Na1+xAlxTi2−x(PO4)3 (NATP), both crystallizing in variants of the Na-superionic conducting (NASICON) structure, has been investigated by solid-state NMR. The ceramic materials produced by annealing the precursor glasses above the glass transition temperature are candidate materials for solid-state separator membranes in sodium ion batteries. The different local structural environments involving both network former and network modifier species have been characterized by comprehensive 23Na, 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) experiments. In crystalline Na1+xAlxGe2−x(PO4)3 samples multiple phosphate environments are observed, corresponding to n Al and 4-n Ge species in their second coordination spheres. In contrast, no site resolution is observed in the analogous Na1+xAlxTi2−x(PO4)3 (NATP) system. This can be understood on the basis of X-ray powder diffraction (XRD) data, which reveal a significant lattice expansion in the former, but no lattice expansion in the latter material. 27Al MAS-NMR data reveal that in the glassy state, Al occurs with coordination numbers four, five and six, with the fraction of four-coordinated Al being substantially higher in the NATP glasses than in the NAGP glasses. 23Na MAS-NMR and spin echo decay measurements reveal distinct differences between glassy and crystallized materials with regard to the local environments and the spatial distributions of the sodium ions.

Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide.

The composite formation with a conductive metal sulfide domain can provide an effective methodology to improve the Na-ion electrode functionality of metal oxide. The heat treatment of TiO2(B) under CS2 flow yields an intimately coupled TiO2(B)-TiS2 nanocomposite with intervened TiS2 domain, since the reaction between metal oxide and CS2 leads to the formation of metal sulfide and CO2. The negligible change in lattice parameters and significant enhancement of visible light absorption upon the reaction with CS2 underscore the formation of conductive metal sulfide domains. The resulting TiO2(B)-TiS2 nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiation-desodiation process than does the pristine TiO2(B). The 23Na magic angle spinning nuclear magnetic resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na+ ions, and the TiS2 domains in TiO2(B)-TiS2 nanocomposites play a role as mediators for Na+ ions to and from TiO2(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS2 leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance. The present study provides clear evidence for the usefulness in composite formation between the semiconducting metal oxide and metal sulfide in exploring new efficient NIB electrode materials.