Site discrimination in mixed-alkali glasses studied by cross-polarization NMR

Cation-cation interactions are thought to play a significant role in shaping the nonlinear compositional dependence of ionic conductivity, known as the mixed-alkali effect (MAE) in glassy solid electrolytes. For providing a structural rationale of this effect, the discrimination of various cation sites in mixed-alkali glasses is of interest. In the present study, cross-polarization (CP) experiments have been applied to glasses in the system [(Li(2)O)x(Na(2)O)(1-x)](0.3)[B(2)O(3)]0.7 to discriminate between alkali ions by virtue of different heteronuclear (7)Li-(23)Na dipole-dipole coupling strengths. Cross-polarization studies involving two types of quadrupolar nuclei (both (7)Li and (23)Na have a spin-quantum number I = 3/2) are complicated by spin state mixing under radio frequency irradiation and magic-angle spinning (MAS). Therefore careful validation and optimization protocols are reported for the model compound LiNaSO(4) prior to conducting the measurements on the glassy samples. (23)Na -->( 7)Li CP/MAS NMR spectra have been obtained on glasses containing the Na(+) ions as the dilute species. They reveal that those lithium species interacting particularly strongly with sodium ions have the same average (7)Li chemical shift as the entire lithium population; the symmetrical situation applies to the (23)Na nuclei at the sodium rich end of the composition range. On the other hand, a clear site discrimination is afforded by temperature-dependent static (23)Na -->( 7)Li CP experiments, indicating that the Li ions that are most strongly interacting with sodium ions are strongly immobilized. This finding provides the first direct experimental evidence for the proposed secondary mismatch concept invoked for explaining the strong MAE in the dilute foreign ion limit.     

Studies of lithium and sodium complexation by silicon podand solvents

The complex formation of lithium and sodium ions with silicon podand solvents: phenyl-tris(1,4-dioxapentyl) silane (PhSi23) and ethyl-tris(1,4-dioxapentyl) silane (EtSi23) has been studied by FTIR, ¹H-, ¹³C-, ⁷Li- and ²³Na NMR. The far FTIR spectra show that the Li⁺ cations fluctuate very fast whereas Na⁺ cations are still localised between the oxygen atoms of the oxaalkyl chains. The ⁷Li NMR spectra prove that one Li⁺ cation can be coordinated not only by one but also two silicon podand molecules. The concentration dependence of the molar conductivity of LiClO₄ in the podand solvents indicates charge transfer between ion clusters.     

Investigation of sodium cations in dehydrated zeolites LSX, X, and Y by 23Na off-resonance RIACT triple-quantum and high-speed MAS NMR spectroscopy.

We investigated by two-dimensional 23Na ORIACT MQMAS NMR and one-dimensional 23Na high-speed MAS NMR spectroscopy a homologous series of dehydrated zeolites with faujasite structure. The framework silicon to aluminum ratios varied between 1.06 and 2.60. In the case of zeolites Y (nSi/nAl = 2.60), we studied materials with sodium exchange degrees between 0 and 95%. The recently introduced ORIACT method (Caldarelli, S.; Ziarelli, F. J. Am. Chem. Soc. 2000, 122, 12015) significantly improved the resolution of the MQMAS spectra, in comparison with earlier studies. It was thus possible to extract meaningful quadrupole parameters by MQMAS NMR, which were used as a starting point for the simulation of 1D MAS NMR spectra to obtain more accurate values of the NMR parameters and site occupancy. We were able to show by this approach that in zeolite NaLSX the SI positions in the hexagonal prisms are occupied by sodium cations. For the homologous series of zeolites Y, it was found that sodium cations located at SII positions are the easiest to be substituted by ammonium ions through the exchange process.     

NMR spectroscopy of bismuth fluoride glasses with alkali cations

Results of the ⁷Li, ¹⁹F, and ²³Na NMR studies of ionic mobility in bismuth fluoride glasses in the systems BiF₃-LiF and BiF₃-MF-ZrF₄ (M = Li, Na, K, Cs) are summarized. Analysis of the ⁷Li, ¹⁹F, and ²³Na NMR spectra made it possible to reveal changes in the nature of ion motions in the fluoride, lithium and sodium sublattices of glasses upon temperature variation and to determine their types. The temperature ranges were found where main types of ion motions in the tested glasses are represented by diffusion of lithium ions, reorientations of fluorine-containing groups constituting the glass network, and diffusion of fluorine ions. The role of alkali cations in the formation of ionic mobility in bismuth fluorozirconate glasses is considered.     

Montmorillonite-poly(ethylene oxide) nanocomposites: interlayer alkali metal behavior

Clay-PEO nanocomposites can have large electrical conductivities that make them potential electrolyte materials for rechargeable lithium batteries, but the origin of these large conductivities, especially for Li-containing materials, is poorly understood. This paper presents X-ray diffraction (XRD), TGA-DTA, and (7)Li and (23)Na NMR data for PEO nanocomposites made with natural (SWy-1) and synthetic (MNTS) montmorillonite clays that provide new insight into interlayer structure. An increase in basal d(001)-spacings demonstrates successful intercalation of PEO in all samples, and X-ray line narrowing shows that this intercalation improves the layer stacking order. The basal spacings of 17.9-19.4 A are consistent with a helical or bilayer structure of PEO in the interlayer. TGA-DTA provides quantitative results for the hydration state of the nanocomposites, demonstrates PEO intercalation, and shows that the composites prepared from the synthetic montmorillonite are less stable than those made with SWy-1. (7)Li NMR shows that the nearest neighbor hydration state of Li(+) is unaffected by PEO intercalation and suggests weak interaction of Li(+) with PEO. (23)Na NMR shows that PEO intercalation results in the conversion of the multiple Na(+) hydration states observed for the pristine clay into inner sphere sites most likely formed through coordination with the basal oxygens of the clay. These differences between lithium and sodium suggested that tighter binding of the Na to the clay may be the origin of the conductivity of Li-montmorillonite-PEO nanocomposites being as much as 2 orders of magnitude larger than those of Na-montmorillonite-PEO nanocomposites. The results confirm the idea that polymer oxygen atoms do not participate in sequestering the exchangeable cations and agree with the jump process for cation migration advanced by Kuppa and Manias (Kuppa, V.; Manias, E. Chem. Mater. 2002, 14, 2171).     

Synthesis, characterization, and catalytic performance of copper-containing SBA-15 in the phenol hydroxylation

A series of copper-containing SBA-15 samples were successfully synthesized via evaporation-induced self-assembly route. The resulting materials were characterized by X-ray diffraction (XRD), (29)Si MAS NMR spectroscopy, transmission electron microscopy (TEM), N(2) sorption, inductively coupling plasma-atomic emission spectrometer (ICP-AES), thermogravimetry, and differential thermal analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectra (UV-vis) and X-ray photoelectron spectroscopy (XPS). The results indicated that: (1) all the samples exhibited typical hexagonal arrangement of mesoporous structure; (2) copper ions could be incorporated into the framework of SBA-15; (3) the addition of urea in the hydrothermal stage efficiently reduced the leaching of copper and improved the thermal stability of the mesoporous materials. Catalytic performances of the obtained materials were evaluated in the hydroxylation of phenol with H(2)O(2). The catalytic tests showed that the synthesized materials exhibited high activity for this reaction and copper ions in the framework were more active than copper species in the extra-framework position. The nitric acid treatment on the samples removed the bulk CuO species, which resulted in a dramatic increase in the catalytic activity.     

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.     

Local environment and distribution of alkali ions in polyelectrolyte complexes studied by solid-state NMR

Polyelectrolyte complexes (PECs) formed by the addition of substoichiometric amounts of (poly(diallyldimethyl ammonium chloride)) (PDADMAC) solutions to sodium or lithium poly(styrene sulfonate) (Na- or Li-PSS) solution contain adjustable amounts of charge balancing Li(+) or Na(+) cations, which possess ionic mobility of interest for solid electrolyte applications. Very little is known regarding the local environments and the spatial distributions of these cations and their interactions with the polyelectrolyte chains in these amorphous materials. To address such issues, the present work develops a comprehensive solid state NMR strategy based on complementary high-resolution magic-angle spinning (MAS) NMR and various dipolar spectroscopic techniques. (6,7)Li and (23)Na chemical shifts measured on a series of PECs with general composition described by B((2x-1))PSS(x)PDADMA((1-x)) (B = Li or Na and 0.53 ≤x≤ 1) reveal composition-independent local cation environments. In contrast, (7)Li{(6)Li} spin echo double resonance (SEDOR) experiments measured on (6)Li enriched materials and (7)Li{(1)H} rotational echo double resonance (REDOR) experiments are consistent with an approximately random ion distribution. The same conclusion is suggested by (23)Na{(1)H} REDOR measurements on the analogous sodium containing system indicating a non-segregated PEC structure. In apparent contrast to this conclusion, (23)Na spin echo decay spectroscopy yields nearly constant dipolar second moments over a wide composition range. This can be explained by considering that the (23)Na spin echo decays are affected by both (23)Na-(23)Na homonuclear dipolar couplings and (23)Na-(1)H heteronuclear dipolar interactions in the presence of strong homonuclear (1)H-(1)H spin exchange. In protonated Na-PSS both contributions are of comparable magnitude. In the PECs the contribution from (23)Na-(23)Na interactions decreases, while that from (23)Na-(1)H dipolar couplings with the protons from the PDADMA chains increases with decreasing Na content, resulting in superimposed opposite dependences on the ion concentration. All results for Li and Na containing PECs point at a non-phase separated polymer network with uniform ionic sites of very similar environment. The cations can be viewed as randomly distributed and located close to the polyion sulfate groups.     

The reactions of phosphorus(III) halides with β-diketiminato ligands

Reactions of the lithium or sodium salt, [HC(CMeNDipp)₂]M (Dipp = 2,6-diisopropylbenzene, M = Li or Na), with PBr₃ or PCl₃ affords a series of phosphenium cations that were characterized in the solid state by X-ray crystallography. Solid-state analysis reveals that for compounds 1 and 2, C–H activation of the ligand occurs and the phosphorus is bound to a nitrogen and carbon atom of the ligand. Compound 3 exhibits the more traditional nitrogen chelation of the ligand to the phosphorus atom.     

Distinguishing surface versus buried cation sites in aluminosilicate mesoporous materials

Mesoporous MCM-41 aluminosilicates were prepared through direct synthesis and surface grafting resulting in the incorporation of aluminum into the pore walls and onto the wall surface, respectively. 7Li and 23Na NMR studies of ion-exchanged Li and Na-Al-MCM-41 were able to distinguish between cations in the surface region and those buried deeper in the pore walls. Thus it was demonstrated that most of the cations in the grafted Al-MCM-41 locate in the surface region, whereas the cations in the synthesized Al-MCM-41 are distributed throughout the pore walls. The NMR spectra of dehydrated Li- and Na-MCM-41 resemble those of glassy materials, reflecting the amorphous nature of this class of mesoporous materials. 7Li NMR studies of dehydrated Li-Al-MCM-41 prepared from direct synthesis in the presence of oxygen showed that most of the Li+ cations are not accessible to O2, while the Li+ cations in Al-grafted Li-Al-MCM-41 are accessible, which also confirms their locations. This study provides valuable insights for the understanding of the structure and properties of aluminosilicate mesoporous materials.     

Solid-state Na and C NMR characterization of Na3C60

Two separate samples of Na₃C₆₀ were prepared by direct reaction of C₆₀ with sodium metal vapor, and subjected to different annealing times of 10 days and 16 days. Solid-state ¹³C and ²³Na NMR, along with elemental analysis, powder X-ray diffraction (XRD) and Raman spectroscopy, were used to characterize both samples. The Raman spectra of both materials have a single peak at 1447 cm⁻¹ which correspond to the A_g peak of C₆₀³⁻, consistent with the stoichiometry of Na_xC₆₀ with x=3. The powder XRD patterns are also virtually identical for both samples. However, solid-state ²³Na and ¹³C NMR spectra of the two samples are significantly different, suggesting a relationship between annealing times and the final structure of the alkali fulleride. Variable-temperature ²³Na magic-angle spinning (MAS) NMR experiments reveal the existence of two or three distinct sodium species and reversible temperature-dependent diffusion of sodium ions between octahedral and tetrahedral interstitial sites. ¹³C MAS NMR experiments are used to identify resonances corresponding to free C₆₀ and fulleride species, implying that the samples are segregated-phase materials composed of C₆₀ and non-stoichiometric Na₃C₆₀. Variable-temperature ¹³C MAS NMR experiments reveal temperature-dependent motion of the fullerides.     

Effect of the size of the aliphatic chain on the NMR spectra and the electrical conductivity of melts of alkali metal alkanoates in alkanoic acids

The dependence of NMR spectra of ⁷Li, ²³Na, ¹³³Cs and of the conductivity of melts of lithium, sodium, and cesium alkanoates in melts of alkanoic acids on the length of the aliphatic chain of the anion has been studied. As the anion size increases, chemical shifts of ²³Na and ¹³³Cs nuclei toward weak fields and ⁷Li nuclei toward strong fields are observed. The nature of the chemical shifts of alkali metal nuclei and of the electricity transport characteristic in the studied systems is discussed in connection with the nature of cation-anion interactions and the size and degree of free rotation of the anion.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 1, pp. 81–87, January–February, 1993.     

Probing the location and distribution of paramagnetic centers in alkali metal-loaded zeolites through (7)Li MAS NMR

The nature and surroundings of lithium cations in lithium-exchanged X and A zeolites following loading with the alkali metals Na, K, Rb, and Cs have been studied through (7)Li solid-state NMR spectroscopy. It is demonstrated that the lithium in these zeolites is stable with respect to reduction by the other alkali metals. Even though the lithium cations are not directly involved in chemical interactions with the excess electrons introduced in the doping process, the corresponding (7)Li NMR spectra are extremely sensitive to paramagnetic species that are located inside the zeolite cavities. This sensitivity makes (7)Li NMR a useful probe to study the formation, distribution, and transformation of such species.     

Mesoporous electrode material from alumina-stabilized anatase TiO2 for lithium ion batteries

A mesoporous electrode material whose structure is composed of anatase nanocrystals stabilized by alumina is reported. Powder X-ray diffraction shows the anatase phase only, but micro-Raman spectroscopy shows that the materials have a core-shell morphology with grains of bulk anatase covered by a thin rutile layer on the surface. This structure is unique when compared to analogous materials stabilized by zirconia (PNNL-1). Nitrogen adsorption isotherms demonstrate a monotonous increase in surface area and mesopore volume with increasing Al content. Thin film electrodes from these materials were characterized by lithium insertion electrochemistry. Cyclic voltammograms exhibit significant differences in Li accommodation in Al-free and Al-stabilized materials.     

Charge distribution and mobility of lithium ions in Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> from <Superscript>6,7</Superscript>Li NMR data

A comparative analysis of ^6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the ⁷Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies v _Q and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (v _Q ～ 27 kHz); Li2 (v _Q ～ 59 kHz); Li3 (v _Q ～ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of ⁷Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li₂TiO₃ sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of ⁶Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.     

Influence of vacancy ordering on the percolative behavior of (Li(1)(-x)Na(x))(3y)La(2/3-y)TiO(3) perovskites

Influence of the vacancy concentration on the Li conductivity of the (Li(1-x)Na(x))(0.2)La(0.6)TiO(3) and (Li(1-x)Na(x)(0.5)La(0.5)TiO(3) perovskite series, with 0 < or = x < 1, has been investigated by neutron diffraction (ND), impedance spectroscopy (IS), nuclear magnetic resonance (NMR), and Monte Carlo simulations. In both series, Li(+) ions occupy unit cell faces, but Na(+) ions are located at A sites of the perovskite. From this fact, the amount of vacant A sites that participate in Li conductivity is given by the expression n(v) = [Li] + square, where square is the nominal vacancy concentration. Substitution of Li by Na decreases the amount of vacancies, reducing drastically the Li conductivity when n(v) approaches the percolation threshold of the perovskite conduction network. In disordered (Li(1-x)Na(x))(0.5)La(0.5)TiO(3) perovskites, the percolation threshold is 0.31; however, in ordered (Li(1-x)Na(x))(0.2)La(0.6)TiO(3) perovskites, this parameter changes to 0.26. Near the percolation threshold, the amount of mobile Li species deduced by (7)Li NMR spectroscopy is lower than that derived from structural formulas but higher than deduced from dc conductivity measurements. Conductivity values have been explained by Monte Carlo simulations, which assume a random walk for Li ions in the conduction network of the perovskite. In these simulations, distribution of vacancies conforms to structural models deduced from ND experiments.     

Zirconium phosphate dispersed on a cellulose fiber surface: preparation, characterization, and selective adsorption of Li+, Na+, and K+ from aqueous solution

Highly dispersed zirconium phosphate was prepared by reacting Cel/ZrO(2) (ZrO(2)=6.7 wt%; 0.56 mmol g(-1) of zirconium atom per gram of the material) with phosphoric acid. High power decoupling magic angle spinning (HPDEC-MAS)(31)P NMR and X-ray photoelectron spectroscopy data indicated that HPO(2-)(4) is the species present on the fiber surface. The X-ray diffraction patterns showed that zirconium hydrogen phosphate particles were amorphous and had an ion-exchange capacity, determined by ammonia gas adsorption, of 0.30 mmol g(-1). The ion-exchange capacities for Li(+), Na(+), and K(+) ions were determined from ion-exchange isotherms at 298 K and showed the following values (in mmol g(-1)): Li(+)=0.01, Na(+)=0.23, and K(+)=0.30. The higher affinity of the surface hydrogen phosphate particles for Na(+) and K(+) is due to its lamelar structure which permits easier diffusion of these two ions whose hydrated radii are smaller than that of Li(+).     

Organometallic ionophore for alkali metal cations

Complexes of a novel synthetic organometallic ionophore with lithium and sodium cations have been characterized by single-crystal X-ray diffraction. The crystal structure of the lithium complex consists of cation-liganddimers with a tetrahedral coordination around Li. The sodium complex reveals a different structure type consisting of cation-ligandtrimers, with water molecules being included between the trimeric entities. The coordination sphere around the Na ions has a distorted octahedral symmetry. It is anticipated that the observed structures of dinuclear Li and trinuclear Na complexes represent possible modes of aggregation of the cation-ligand entities in lipophilic media.     

A highly selective bicyclic fluoroionophore for the detection of lithium ions

The macrobicyclic molecule, 21-(9-anthrylmethyl)-4,17,13,16-tetraoxa-1,10,21-triazabicyclo [8.8.5]tricosane-19,23-dione, I, was designed, synthesized and characterized as a fluoroionophore for the selective, optical detection of lithium ions. Compound I is based on a bridged diazacrown structure, which provides a semirigid binding framework. Binding takes place by electrostatic interactions between the oxygen atoms of the crown and the cation and is transduced to fluorescence emission from an attached anthracene fluorophore. In a 75:25 dichloromethane/tetrahydrofuran solvent mixture, I acts as an intramolecular electron transfer "off-on" fluorescence switch, exhibiting a greater than 190-fold enhancement in fluorescence emission intensity in the presence of lithium ions. The relative selectivity of I for lithium ions over sodium, potassium and ammonium ions was found to be log K(Li+,Na+) approximately -3.36, log K(Li+,K+) approximately -1.77 and log K(Li+,NH4+) approximately -2.78.     

锂离子电池电极材料固体核磁共振研究进展

对于研究材料的结构变化和考察原子所处的化学环境，固体核磁共振技术是一种有效的手段。通过⁶Li和⁷Li核磁共振谱的变化，可以清楚地了解锂离子电池电极材料中Li与邻近金属或碳原子的配位情况及在充放电过程中对应于锂离子嵌/脱过程中材料的结构变化，对于研究电极材料的电化学性能有重要的意义。本文综述了固体NMR技术在研究锂离子电池电极材料的结构及嵌锂机理方面的一些进展。