E.R.H. van Eck

Li-Ion Diffusion in the Equilibrium Nanomorphology of Spinel Li4+xTi5O12

Li(4)Ti(5)O(12) spinel as Li-ion electrode material combines good capacity, excellent cycleability with a high rate capability. Although the potential of about 1.56 V vs Li is relatively high, these features make it the anode of choice for state of the art high power Li-ion batteries. Although the flat voltage profile reflects a two-phase reaction during lithiation, the small change in lattice parameters upon lithiation (zero-strain property) leads to a solid solution in equilibrium, as recently demonstrated with diffraction. In this study, the morphology and Li-ion mobility is studied by NMR spectroscopy leading to a more detailed picture, showing that the solid solution in Li(4+x)Ti(5)O(12) spinel should actually be described as domains with sizes less than 9 nm having either tetrahedral (8a) Li occupation or octahedral (16c) Li occupation. The abundant domain boundaries and the associated disorder appear to be responsible for the facile diffusion through the lattice, and hence these nm-sized domains are most likely the origin of the relative high rate capability of this material as electrode for Li-ion batteries. The small domain size, smaller than typical Debye lengths, makes that the material electrochemically behaves as a solid solution. As such, the results give insight in the fundamental properties of the zero-strain Li(4)Ti(5)O(12) spinel material explaining the favorable Li-ion battery electrode properties on an atomic level.

Lithium Storage in Amorphous TiO2 Nanoparticles

Amorphous titanium oxide nanoparticles were prepared from titanium isopropoxide. In situ measurements reveal an extraordinary high capacity of 810 mAh/g on the first discharge. Upon cycling at a charge/discharge rate of 33.5 mA/g, this capacity gradually decreases to 200 mAh/g after 50 cycles. The origin of this fading was investigated using X-ray absorption spectroscopy and solid-state nuclear magnetic resonance. These measurements reveal that a large fraction of the total amount of the consumed Li atoms is due to the reaction of H2O/OH species adsorbed at the surface to Li2O, explaining the irreversible capacity loss. The reversible capacity of the bulk, leading to the Li0.5TiO2 composition, does not explain the relatively large reversible capacity, implying that part of Li2O at the TiO2 surface may be reversible. The high reversible capacity, also at large (dis)charge rates up to 3.35 A/g (10C), makes this amorphous titanium oxide material suitable as a low cost electrode material in a high power battery.

Complete resolution of SiOSi and SiOAl fragments in an aluminosilicate glass by 17O multiple quantum magic angle spinning NMR spectroscopy

Abstract Complete NMR resolution of Siue5f8Oue5f8Si and Siue5f8Oue5f8Al species in an aluminosilicate glass (NaAlSi3O8) is shown using 17O multiple quantum magic angle spinning (MAS) NMR and indicates that no Alue5f8Oue5f8Al is present at the detection limit of 0.5%.

A novel application of nuclear spin-echo double-resonance to aluminophosphates and aluminosilicates

Abstract It is shown that by applying a spin-echo double-resonance technique to 31 P and 27 Al in AlPO 4 -5, one can detect that 31 P is coupled to 27 Al. Although the pulse sequence used resembles that of the SEDOR experiment, it actually differs and also the mechanism of the phenomena we observe seems to differ from the mechanism of the original SEDOR experiment. The experiment described in this paper is also tried on 29 Si and 27 Al in the zeolites NaA and NaY. For NaA, there is a small but definite effect of aluminum irradiation on the 29 Si spin-echo intensity. For NaY, the signal-to-noise ratio is too small to observe an effect.

High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (Cq) for 23Na. Off-resonance nutation experiments gave a mean Cq = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δiso, and to provide an independent measurement of the values of Cq. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean Cq is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

The electronic structure and ionic diffusion of nanoscale LiTiO2 anatase

Upon lithium insertion in the pristine TiO2 anatase phase the theoretical maximum of LiTiO2 can be reached in crystallite sizes less than approximately 10 nm, whereas bulk compositions appear limited to Li(x) approximately 0.6TiO2 at room temperature. Both X-ray absorption spectroscopy (XAS) and ab initio calculations have been applied to probe the electronic structure of the newly formed LiTiO2 phase. These results indicate that a large majority of the Li-2s electrons reside at the Ti-3d(t2g)/4s hybridized site. About 10% of these electrons are transferred to non-localized states which makes this compound a good electronic conductor. Ionic conductivity is probed by nuclear magnetic resonance (NMR) relaxation experiments indicating relatively small hopping rates between the Li-ion sites in LiTiO2. Formation of the poor ionic-conducting LiTiO2 at the surface of the particles explains why micro-anatase Li(x)TiO2 is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximum capacity reached in nano-sized materials cannot be (dis)charged at high rates.

Humin based by-products from biomass processing as a potential carbonaceous source for synthesis gas production

Lignocellulosic biomass is addressed as potential sustainable feedstock for green fuels and chemicals. (Hemi)cellulose is the largest constituent of the material. Conversion of these polysaccharides to bio-based platform chemicals is important in green chemical/fuel production and biorefinery. Hydroxymethyl furfural, furfural and levulinic acid are substantial building blocks from (poly)saccharides. Synthesis of these molecules involves acid catalysed hydrolysis/dehydration reactions which leads large formation of insoluble by-products, called humins. Humin obtained from dehydration of glucose is used in this study. Fractionisation of humin was investigated using various solvents (e.g., acetone, H2O, and NaOH 1%). Characterisation of humin using various techniques including ATR-IR, HR-SEM, solid state NMR, elemental analysis, Raman spectroscopy, pyrolysis, etc. confirms its furan rich structure with aliphatic oxygenate linkages. The influence of thermal treatment on humin was investigated. Humin undergoes a lot of changes both in morphology and structure. Humin loses ca. 45 wt% when preheated to 700 °C (prior to the gasification temperature) and contains above 92 wt% C in mainly aromatic/graphitic structures. Valorisation of humin via dry reforming was studied. Non-catalytic dry reforming of humin is very difficult; however, alkali catalysts (e.g. Na2CO3) can enhance the reaction rate tremendously.

Observation of hydroxyl groups by 17O solid-state multiple quantum MAS NMR in sol–gel-produced silica

17O NMR parameters (CQ, eta, delta(iso) and T1) are reported for both Si-O-Si and Si-OH fragments within a silica gel. The Si-OH units have a wide spread of parameters but are typically characterised by a very short T1 (approximately 0.1 ms) and CQ < 200 kHz. These observations have extremely important implications for the quantification of such units in these gels and related glassy materials by 17O NMR. In light of these observations, the 17O NMR experiments have been optimised and a distinct resonance from the OH group is observed in 1D static and magic angle spinning (MAS) NMR measurements as well in the multiple quantum (MQ) experiment.

Orientation of the quadrupole and dipole tensors of hydroxyl groups by 17O quadrupole separated local field NMR

The static 17O NMR spectra of Mg(OH)2 and amorphous Mg(OH)x(OCH3)2−x were measured. Simulation of these spectra gave e2qQ/h=6.8u2009MHz, η=0, and δiso=20u2009ppm, and e2qQ/h=7.25u2009MHz, η=0, and δiso=−25u2009ppm for the hydroxyl oxygen in, respectively, Mg(OH)2 and Mg(OH)x(OCH3)2−x. An OH distance in Mg(OH)2 of 1.001 A was obtained using Lee–Goldberg decoupling to obtain the OH dipolar coupling constant. Dipolar oscillations in the 1H–17O cross-polarization curve provided an OH distance of 0.995 A in Mg(OH)x(OCH3)2−x; these oscillations were not observed in Mg(OH)2. Based on differences in the OH distance and in the 17O quadrupole coupling constant it was concluded that the OH bond in Mg(OH)x(OCH3)2−x was more covalent. 17O 2D quadrupole separated local field experiments were performed on both samples in order to obtain the relative orientation of the 17O quadrupole and OH dipole tensors. In both cases the interaction tensors were found to be collinear. Lee–Goldberg decoupling during the dipolar evolution time t1 impro...