Magic Angle Spinning Nuclear Magnetic Resonance Measurements Research Articles

Structural Insight into Protective Alumina Coatings for Layered Li-Ion Cathode Materials by Solid-State NMR Spectroscopy.

Layered transition metal oxide cathode materials can exhibit high energy densities in Li-ion batteries, in particular, those with high Ni contents such as LiNiO2. However, the stability of these Ni-rich materials often decreases with increased nickel content, leading to capacity fade and a decrease in the resulting electrochemical performance. Thin alumina coatings have the potential to improve the longevity of LiNiO2 cathodes by providing a protective interface to stabilize the cathode surface. The structures of alumina coatings and the chemistry of the coating-cathode interface are not fully understood and remain the subject of investigation. Greater structural understanding could help to minimize excess coating, maximize conductive pathways, and maintain high capacity and rate capability while improving capacity retention. Here, solid-state nuclear magnetic resonance (NMR) spectroscopy, paired with powder X-ray diffraction and electron microscopy, is used to provide insight into the structures of the Al2O3 coatings on LiNiO2. To do this, we performed a systematic study as a function of coating thickness and used LiCoO2, a diamagnetic model, and the material of interest, LiNiO2. 27Al magic-angle spinning (MAS) NMR spectra acquired for thick 10 wt % coatings on LiCoO2 and LiNiO2 suggest that in both cases, the coatings consist of disordered four- and six-coordinate Al-O environments. However, 27Al MAS NMR spectra acquired for thinner 0.2 wt % coatings on LiCoO2 identify additional phases believed to be LiCo1-xAlxO2 and LiAlO2 at the coating-cathode interface. 6,7Li MAS NMR and T1 measurements suggest that similar mixing takes place near the interface for Al2O3 on LiNiO2. Furthermore, reproducibility studies have been undertaken to investigate the effect of the coating method on the local structure, as well as the role of the substrate.

Rechargeable Aluminum-chalcogen Batteries: Molecular-Level Mechanistic Insights & Scientific Perspectives

Rechargeable aluminum metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric and gravimetric capacities. Commercialization of rechargeable aluminum battery systems has been limited due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal and (ii) positive electrode materials that are (electro)chemically within those electrolytes while providing high capacity and cycle life. Metal-chalcogen (e.g., sulfur, selenium) batteries are among the most promising candidates for low-cost, high-energy-density electrochemical energy storage systems; however, to date, rechargeable aluminum-chalcogen batteries have seldom been explored scientifically or technologically.Here, rechargeable aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were prepared with a chloroaluminate ionic liquid electrolyte (AlCl3:[EMIm][Cl], molar ratio of 1.5:1) and their electrochemical reaction mechanisms and products were studied at a molecular-level with solid-state nuclear magnetic resonance (NMR) spectroscopy. Galvanostatic cycling of both Al-S and Al-Se cells showed similar initial capacities and overpotentials to previously reported systems. For Al-S systems, the higher specific capacity upon charge, compared to discharge, points towards either an unwanted side reaction or deleterious polysulfide-shuttle-like behavior. This phenomenon is heavily prevalent at low current densities and affects reversibility. For Al-Se systems, two different reaction mechanisms that have been previously reported were found to be dependent on the applied current density and the crystallinity of the selenium. We were able to consolidate both reaction mechanisms during a single charge/discharge step, significantly increasing specific capacity and energy density. For both Al-S and Al-Se systems, significant capacity fade was also observed over multiple galvanostatic cycles. Solid-state 27Al single-pulse magic-angle-spinning (MAS) NMR spectra acquired on sulfur electrodes cycled to different states-of-charge reveal the presence of both solid and liquid discharge products. Solid-state 2D 27Al{27Al} multiple-quantum (MQ-)MAS measurements reveal the amorphous nature of the solid discharge product, while 2D 27Al{27Al} exchange spectroscopy (EXSY) NMR spectra reveal chemical exchange between electrolyte-soluble sulfur species coordinating with AlCl4 - and Al2Cl7 - after discharge. For Al-Se systems, solid-state 27Al and 77Se single-pulse MAS NMR measurements also reveal liquid-state reaction products in the electrolyte, leading to loss of active material from the cathode, while revealing compositional changes in the selenium electrode. Scientific perspectives on these aluminum-chalcogen battery chemistries will be discussed. This work also highlights the importance that molecular-level analytical tools, such as solid-state NMR spectroscopy, can have in clarifying electrochemical mechanisms in emerging electrochemical systems.

Elucidating Reversible Electrochemical Anionic Redox in Intercalation Electrodes for Rechargeable Multivalent-Ion Batteries

Battery electrodes that enable electrochemical intercalation of multivalent ions result in multiple electron transfers per ion, respectively, yielding significant enhancements in electric charge storage capacity. When multivalent cathodes are paired with their corresponding metal anodes, potentially transformative gains in energy density are possible. However, few positive electrode materials exist that intercalate multivalent ions, in part due to the slow solid-state diffusion of highly charged multivalent ions. The chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and electron charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries.Here, we couple electrochemical and solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) measurements to reveal the concurrent aluminum-ion intercalation and electron charge transfer mechanisms in chevrel electrodes Mo6S8 and Mo6Se8 up from the molecular level. Solid-state 27Al single-pulse MAS NMR measurements were performed on the thio-chevrel Mo6S8 electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Solid-state 77Se and 95Mo MAS NMR measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that electrons are transferred preferentially to the anionic chalcogen framework upon aluminum ion intercalation, as opposed to the transition metals (Mo). Anionic redox was further confirmed by density functional theory (DFT) calculations and X-ray absorption near-edge structure (XANES) measurements. Lastly, we generalize the results to other ion valencies, demonstrating reversible electrochemical anionic redox in the Chevrel phase upon divalent Zn2+ cations in aqueous Zn batteries.Overall, for the first time, we demonstrate reversible electrochemical anionic redox as a charge storage mechanism for rechargeable batteries, which preserves the crystalline framework structure and does not involve the breaking and forming of chemical bonds. This charge storage mechanism is fundamentally different than that observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored within the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Evolution mechanism of surface hydroxyl groups of silica during heat treatment

The surface hydroxyl groups of nano-silica have a great influence on its application properties. In this paper, the changes of the number of various hydroxyl groups after dehydroxylation and rehydration progress of silica were quantitatively analyzed by thermogravimetric (TG) analysis and 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. According to the different stability of Si-O-Si bond formed after dehydroxylation, we propose two types of hydroxyl groups depending on the relative positions of them: chain adjacent hydroxyl groups (C-OHs) and spatially adjacent hydroxyl groups (S-OHs). DFT calculations indicate that two C-OHs dehydroxylation form a four-atoms ring structure, and the resulting OSiO bond angle deviation from optimal value is up to 19.56°, and the average bond length of the SiO bond increase more than 4.66%. Further more, its rehydration reaction activation energy (Ear) is very low, even only 0.21 kJ/mol, so it is easy to re-open and return to hydroxyl groups under environmental conditions. Conversely, the S-OHs dehydroxylation form a stable multi-atoms ring. The DFT calculations are a good illustration of the phenomenon in which some unstable siloxane bonds in a dehydroxylated sample are restored to hydroxyl groups in TG analysis and 29Si MAS NMR measurements.

The structure of model glasses of the amorphous phase of glass-ceramic glazes from the SiO2[sbnd]Al2O3[sbnd]CaO[sbnd]MgO[sbnd]Na2O[sbnd]K2O[sbnd]ZnO system

In this work, particular attention was paid to the structural characteristics of the amorphous phase in glass-ceramic glazes from the SiO2Al2O3CaOMgONa2OK2OZnO system. The study was carried out on model glasses whose chemical composition corresponds to the chemical composition of the vitreous phase of the glazes determined on the basis of electron probe micro-analyzer (EMPA). Structural investigations of the obtained model glasses were carried out using the mid-infrared (MIR) and solid-state magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The solid-state MAS-NMR study carried out on 27Al and 29Si nuclei showed that all aluminum and silicon ions have tetrahedral coordination and thus form the glass network. The results of the 29Si solid-state MAS-NMR measurements also showed that with the increase of ZnO content in the glass chemical composition, the number of Q3 and Q2 silicon-oxygen units increases at the expense of Q4 (where n is the number of bridging oxygens), thus indicating an increase in the degree of depolymerization of the alumino-silico-oxygen network of studied of glasses. Analysis of the MIR spectra of the obtained glasses confirmed the conclusions from the solid-state MAS-NMR research, i.e. that Zn2+ ions cause depolymerization of the glass network of glazes. The weakening of the network of glasses resulted in lowering of the transformation and dilatometric softenings temperature. The tests carried out on model glasses allow drawing conclusions about the structure and properties of the vitreous phase of real glazes.

Composition dependence of the short range order structures in 0.2Na2O + 0.8[xBO3/2 + (1-x)GeO2] mixed glass formers

The atomic level structures of ternary 0.2Na2O + 0.8[xBO3/2 + (1-x)GeO2] glasses have been carefully examined for the first time using a combination of Raman, infrared (IR) and 11B magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. It was found that, like in other mixed glass former (MGF) systems, there is an unequal sharing of the sodium oxide between the two glass formers, B and Ge, across this series 0 < x < 1. It is found that for all x > 0, the Na2O preferentially reacts with boron to create a larger number of tetrahedral boron, BO4/2−1 ≡ B4, units than would be found for a binary sodium borate glass of the same mole fraction, 0.20, of Na2O. In addition to this higher than expected fraction of B4 units as determined by the 11B MAS-NMR measurements, the Raman spectra further suggest the presence of high concentrations of octahedrally coordinated GeO6/2−2, Ge6, units in the GeO2 rich end of this compositional series, x ≤ 0.2. Across this series, it is found that there are at most five short range order (SRO) structural units present in these glasses, BO3/2 (B3), B4, GeO6/2−2 (Ge6), GeO3/2O−1 (Ge3), and GeO4/2 (Ge4), where the superscript gives the number of bridging oxygens (BOs) bonded to the central atom. The composition, x, dependence of all of these SRO structural units were determined by combining the results all of these spectral techniques and requiring that the total sum of all the negatively charged SRO units balance the positive (+0.4) charges from the Na+ ions and that the total amount of B balance to 0.8×, the total amount of Ge balance to 0.8*(1-x), and the total amount of oxygen balance to1.8–0.4× as required by the stoichiometry of this compositional series.

Structural study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses by 11B and 27Al MAS NMR spectroscopy and thermal analysis

Glasses of bismuth borosilicates, aluminoborates and aluminoborosilicates were prepared by melt quenching and characterized by density measurements, 11B and 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy, and Differential Scanning Calorimetry (DSC). Increase in silica concentration increases density without any change in the fraction of tetrahedral borons (N4) in the borosilicate glass network, whereas increase in alumina concentration in aluminoborate and aluminoborosilicate glasses drastically reduces density and N4. 27Al MAS NMR measurements revealed that Al ions are in four, five and six fold coordination with oxygen and about 47% of these are in [5]Al and [6]Al structural units, the concentration of the latter species decreases by adding silica in the glass network. Higher cation field strength of Bi3+ generates very large concentration of non-bridging oxygens, which act as charge balancing centers in the glass network. DSC measurements found that alumina and silica incorporation in bismuth borate glasses do not produce any systematic changes in the glass transition temperature but the tendency towards crystallization decreases steadily.

A nuclear magnetic resonance spectrometer concept for hermetically sealed magic angle spinning investigations on highly toxic, radiotoxic, or air sensitive materials

A concept to integrate a commercial high-resolution, magic angle spinning nuclear magnetic resonance (MAS-NMR) probe capable of very rapid rotation rates (70 kHz) in a hermetically sealed enclosure for the study of highly radiotoxic materials has been developed and successfully demonstrated. The concept centres on a conventional wide bore (89 mm) solid-state NMR magnet operating with industry standard 54 mm diameter probes designed for narrow bore magnets. Rotor insertion and probe tuning take place within a hermetically enclosed glovebox, which extends into the bore of the magnet, in the space between the probe and the magnet shim system. Oxygen-17 MAS-NMR measurements demonstrate the possibility of obtaining high quality spectra from small sample masses (~10 mg) of highly radiotoxic material and the need for high spinning speeds to improve the spectral resolution when working with actinides. The large paramagnetic susceptibility arising from actinide paramagnetism in (Th(1-x)U(x))O2 solid solutions gives rise to extensive spinning sidebands and poor resolution at 15 kHz, which is dramatically improved at 55 kHz. The first (17)O MAS-NMR measurements on NpO(2+x) samples spinning at 55 kHz are also reported. The glovebox approach developed here for radiotoxic materials can be easily adapted to work with other hazardous or even air sensitive materials.

Li Ion Diffusion in Nanocrystalline and Nanoglassy LiAlSi2O6 and LiBO2 - Structure-Dynamics Relations in Two Glass Forming Compounds

Abstract In the present study the Li diffusivity in nanostructured samples of two glass forming model systems, spodumene (LiAlSi2O6) and lithium metaborate (LiBO2), was examined using 7Li nuclear magnetic resonance (NMR) spin-lattice relaxometry and dc conductivity measurements. The nanostructured samples were prepared by high-energy ball milling of the respective crystalline starting material on the one hand and the corresponding glass on the other hand. The diffusivity of the glass exceeds that of the crystalline sample for both systems. However, when the crystalline samples are mechanically treated by ball milling the diffusivity is enhanced. Nevertheless, the diffusivity of these nanocrystalline samples remains lower than that of the corresponding glass. Surprisingly, when the glassy samples are treated in the same way the diffusivity decreases. After sufficiently long milling times the diffusivity of these nanoglassy samples approaches that of the nanocrystalline samples. This convergence effect seems to be due to structural relaxation processes as is suggested by supplementary infrared spectroscopy and 27Al, 12B magic angle spinning NMR measurements.

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100− x)B 2O 3 ( x = 30, 40, 50 and 60 mol%), 50PbO– yAl 2O 3–(50− y)B 2O 3 ( y = 2, 4, 6, 8 mol%), 50PbO– ySiO 2–(50− y)B 2O 3 ( y = 5, 10, 20, 30 mol%) and 50PbO–5SiO 2– yAl 2O 3–(45− y)B 2O 3 ( y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and 11B and 27Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B 2O 3 glass. 11B MAS NMR measurements determine that the fraction of tetrahedral borons (N 4) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B 2O 3 glass series and that N 4 is sharply reduced upon adding small amounts of Al 2O 3 into lead borate and lead borosilicate systems. 27Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al 3+ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al 3+ over charge-balancing Pb 2+ cations. [5]Al and [6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.

Stability of antiferroelectricity and molecular reorientation in the hexatic smectic IA* phase as studied by X-ray diffraction and NMR spectroscopy

We have investigated molecular orientation in the hexatic antiferroelectric SIA* phase by X-ray diffraction and nuclear magnetic resonance (NMR) spectroscopy. In the phase transition from SCA* to SIA*, the layer thickness increases and becomes larger than that in SA. The third-order diffraction peak intensity becomes stronger than the second-order one at this transition and increases with decreasing temperature in SIA*. According to the magic angle spinning NMR measurements, on the other hand, the isotropic chemical shift scarcely moves at this transition. These results suggest that the layer structure is reconstructed by the molecular reorientation but not by a molecular conformation change. We have also discussed the stability of antiferroelectricity and the origin of its appearance in hexatic ordering.

The speciation of water in hydrated alkali silicate glasses

A combination of infrared, Raman, and magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy is used to study the speciation of water (molecular versus OH) within synthetic alkali silicate glasses with the following compositions: 30% Li 2O70%SiO 2, 45% Na 2O55% SiO 2, and 35% K 2O65%SiO 2. These glasses were hydrated over a period of about 6 years at ambient conditions. The lithium glass was found to be the least hydrated. The sodium glass contained ∼ 5.2 wt% H 2O whereas the potassium glass was essentially a gel with approximately 32 wt% H 2O. The lithium glass was estimated to contain less than 0.01 wt% H 2O. Near-IR spectra show strong combination bands at ∼ 5200 cm −1, which are attributed to the presence of molecular water in the sodium and potassium glasses. The absence of combination bands at ∼ 4500 cm −1 indicates that hydroxyl groups are not present in these glasses. Both sodium and potassium silicate glasses show very strong fundamental absorption in the OH stretching region at ∼ 3550 cm −1. The lithium glass has a very weak fundamental water band at ∼ 3550 cm −1. The Raman spectra for the K 2O− and Na 2OSiO 2 glasses are consistent with the infrared data and have relatively strong bands in the 3000–4000 cm −1 region, similar to the spectrum of pure water. Bands observed at ∼ 1630 cm −1 correspond to the bending mode of molecular water. No Raman band that could be attributed to a SiOH stretch was observed in the spectra of the Na and K glasses. MAS-NMR measurements were carried out on the hydrated Na and K glasses and samples of the same glasses dehydrated by remelting. The 29Si spectra indicate increasing polymerization in the order Na<K<Li, as might be expected from their compositions. The only difference in the spectra of the Li glass is a slight shift to high field in the 29Si spectrum, which is attributed to water associating with the SiO 4 tetrahedra.