Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing applications and quality of NMR as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal ions based DNP, the polarization agents are distributed in the bulk sample and act as both source of relaxation and of polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted and the entire sample can be directly polarized. We exploit this finding to measure high quality NMR spectra of 17O in the electrode material Li4Ti5O12 doped with Fe(III) despite its low abundance and long relaxation time.
Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li 4Ti 5O 12. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.
In recent years dynamic nuclear polarization (DNP) has greatly expanded the range of materials systems that can be studied by solid state NMR spectroscopy. To date, the majority of systems studied by DNP were insulating materials including organic and inorganic solids. However, many technologically-relevant materials used in energy conversion and storage systems are electrically conductive to some extent or are employed as composites containing conductive additives. Such materials introduce challenges in their study by DNP-NMR which include microwave absorption and sample heating that were not thoroughly investigated so far.Here we examine several commercial carbon allotropes, commonly employed as electrodes or conductive additives, and consider their effect on the extent of solvent polarization achieved in DNP from nitroxide biradicals. We then address the effect of sample conductivity systematically by studying a series of carbons with increasing electrical conductivity prepared via glucose carbonization. THz spectroscopy measurements are used to determine the extent of vw absorption. Our results show that while the DNP performance significantly drops in samples containing the highly conductive carbons, sufficient signal enhancement can still be achieved with some compromise on conductivity. Furthermore, we show that the deleterious effect of conductive additives on DNP enhancements can be partially overcome through pulse-DNP experiments.
Surface modification of electrode materials using chemical treatments and atomic layer deposition is documented as an efficient method to stabilize the lattice structure as well as to reinforce the electrode/electrolyte interface. Nevertheless, expensive instrumentation and intrinsic deterioration of the material under high-temperature conditions and aggressive chemical treatments limit their practical application. Here, we report enhanced electrochemical stability and performances by simple atomic surface reduction (ASR) treatment of Li- and Mn-rich 0.35Li(2)MnO(3). 0.65LiNi(0.35)Mn(0.45)Co(0.20)O(2) (HE-NMC). We provide mechanistic indications showing that ASR altered the electronic structure of surface Mn and Ni, leading to higher stability and reduced parasitic reactions. We demonstrate significant improvement in the battery performance with the proposed surface reduction, which is reflected by the enhanced capacity (290 mA h g(-1)), rate capabilities (similar to 15% enhancement at rates of 1 and 2 C), 50-60 mV narrow voltage hysteresis, and faster (twice) Li+ diffusion. Utilizing online electrochemical mass spectrometry (OEMS), we show in-operando that the reduced surface layer results in suppressed side reactions. We further characterized the surface coating with high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and solid-state NMR before and after cycling. The results presented herein address all the critical challenges associated with the complex HE-NMC material and thus provide a promising research direction for choosing relevant methodology for surface treatment.
In the present work, a simple and agile methodology for atomic surface reduction of interfaces is introduced. Using a surface directed vapor phase reaction, at relatively low temperature, we show that a highly reactive and volatile molecule can be used to selectively reduce the interface, without changing the bulk of the treated material, and without the need of alternating sequence of multiple precursors, normally involved in ALD. The model system we use to demonstrate the efficacy, and potential of our approach is trimethyl aluminum, and high energy Li and Mn rich cathode (HE-NCM) as the functional material of interest. We demonstrate that with the proposed method, the particles of HE-NMC were conformally coated with similar to 3 nm amorphous layer of the reduced surface in less than 1 h (including the cooling time), as witnessed using HR-TEM. XPS and solid-state NMR, further confirmed that surface treatment was successfully achieved using the proposed method and is well explained by DFT calculations. Utilizing online electrochemical mass spectrometry (OEMS), we show in-operando that this amorphous layer helps to suppress parasitic reactions under extreme electrochemical conditions as indicated by the significant reduction in oxygen and CO2 evolution. The surface treatment further resulted in enhancement in specific capacity during the first cycle. This methodology provides a non-conventional path to achieve thin layer surface modification under facile conditions, and opens a new way to meet the requirements of surface modification strategies for improving the performance of electrode materials without utilizing expensive instrumentation and high temperature processes.
Solid state NMR spectroscopy is an extremely versatile and powerful method for determining the chemical composition and the structural and dynamic properties of solids. Here we provide an overview of the rich arsenal of experimental approaches offered by this spectroscopy for materials characterization with selected examples of recent applications.
In recent years magic angle spinning-dynamic nuclear polarization (MAS-DNP) has developed as an excellent approach for boosting the sensitivity of solid-state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials' surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Recently, an alternative in the form of paramagnetic metal ions has emerged. Here we demonstrate the remarkable efficacy of Mn(II) dopants, used as endogenous polarization agents for MAS-DNP, in enabling the detection of O-17 at a natural abundance of only 0.038%. Distinct oxygen sites are identified in the bulk of micron-sized crystals, including battery anode materials Li4Ti5O12 (LTO) and Li2ZnTi3O8, as well as the phosphor materials NaCaPO4 and MgAl2O4, all doped with Mn(II) ions. Density functional theory calculations are used to assign the resonances to specific oxygen environments in these phases. Depending on the Mn(II) dopant concentration, we obtain significant signal enhancement factors, 142 and 24, for Li-6 and Li-7 nuclei in LTO, respectively. We furthermore follow the changes in the Li-6,Li-7 LTO resonances and determine their enhancement factors as a function of Mn(II) concentration. The results presented show that MAS-DNP from paramagnetic metal ion dopants provides an efficient approach for probing informative nuclei such as O-17, despite their low gyromagnetic ratio and negligible abundance, without isotope enrichment.
Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of x-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at. %) Mn-doped lithium titanate (Li4/3Ti5/3O4, or LTO) the dopant Mn2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s -> 4p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. This dopant location can explain the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result, the observed remarkable 20% decrease in electrochemical capacity of the 1% Mn-doped LTO compared to pristine LTO.
Rechargeable battery cells are composed of two electrodes separated by an ion-conducting electrolyte. While the energy density of the cell is mostly determined by the redox potential of the electrodes and amount of charge they can store, the processes at the electrode-electrolyte interface govern the battery's lifetime and performance. Viable battery cells rely on unimpeded ion transport across this interface, which depends on its composition and structure. These properties are challenging to determine as interfacial phases are thin, disordered, heterogeneous, and can be very reactive. The recent developments and applications of solid state NMR spectroscopy in the study of interfacial phenomena in rechargeable batteries based on lithium and sodium chemistries are reviewed. The different NMR interactions are surveyed and how these are used to shed light on the chemical composition and architecture of interfacial phases as well as directly probe ion transport across them is described. By combining new methods in solid state NMR spectroscopy with other analytical tools, a holistic description of the electrode-electrolyte interface can be obtained. This will enable the design of improved interfaces for developing battery cells with high energy, high power, and longer lifetime.
Out of the many challenges in the realization of lithium-O-2 batteries (LOB), the major is to deal with the instability of the electrolyte and the cathode interface under the stringent environment of both oxygen reduction and evolution reactions. Lithium nitrate was recently proposed as a promising salt for LOB because of its capability to stabilize the lithium anode by the formation of a solid electrolyte interphase, its low level of dissociation in aprotic solvents, and its catalytic effect toward oxygen evolution reaction (OER) in rechargeable LOB. Nevertheless, a deeper understanding of the influence of nitrate on the stability and electrochemical response of the cathode in LOB is yet to be realized. Additionally, it is well accepted that carbon instability toward oxidation therefore, it is essential to investigate the effect of electrolyte components on this side of the battery. In the present work, we show that nitrate leads to interfacial changes, which result in the formation of a surface protection domain on the carbon scaffold of LOB cathode, which helps in suppressing the oxidative damage of the carbon. This effect is conjugated with an additional electrocatalytic effect of the nitrate ion on the OER Using in operando online electrochemical mass spectroscopy, we herein deconvolute these two positive effects and show how they are dependent on nitrate concentration and the potential of cell operation. We show that a low amount of nitrate can exhibit the catalytic behavior; however, in order to harness its ability to suppress the oxidative damage and passivate the carbon surface, an excess of LiNO3 is required.
Dynamic nuclear polarization (DNP), a technique in which the high electron spin polarization is transferred to surrounding nuclei via microwave irradiation, equips solid-state NMR spectroscopy with unprecedented sensitivity. The most commonly used polarization agents for DNP are nitroxide radicals. However, their applicability to inorganic materials is mostly limited to surface detection. Paramagnetic metal ions were recently introduced as alternatives for nitroxides. Doping inorganic solids with paramagnetic ions can be used to tune material properties and introduces endogenous DNP agents that can potentially provide sensitivity in the particles' bulk and surface. Here we demonstrate the approach by doping Li4Ti5O12 (LTO), an anode material for lithium ion batteries, with paramagnetic ions. By incorporating Gd(III) and Mn(II) in LTO we gain up to 14 fold increase in signal intensity in static Li-7 DNP-NMR experiments. These results suggest that doping with paramagnetic ions provides an efficient route for sensitivity enhancement in the bulk of micron size particles.
Liquid-free solid polymer electrolyte (SPE) Li-O-2 batteries are considered advantageous power sources for multiple applications, albeit their cycle performance is far from being acceptable. A most challenging SPE stability in Li-O-2 battery operating at 80 degrees C is described here, presenting possible directions for this battery type future development. Hereby, we investigated poly(ethylene oxide) (PEO) stability in Li-O-2 batteries after cycling and determined that the polymer instability is originated from an accumulation of formate-based species, which required high decomposition potential and showed low decomposition efficiency. This poses a key challenging issue of unfavorable round-trip efficiency, dictating a poor cycle performance.
Spinels (AB(2)O(4)) form a niche class of ceramics, which is rich in structural (dis)ordering due to the inherent mixing between the constituent tetrahedral and octahedral sites. The cations (A and B) can form antisite defects under the influence of external parameters like pressure, temperature and nuclear irradiation. The current study reports the formation and evolution of disorder-order structural transition in hydrothermally prepared zinc aluminate spinel ZnAl2O4. The effect of final calcination temperature (300-900 degrees C for 9 h) on the degree of cation ordering has been investigated with powder X-ray diffraction, Raman and Al-27 solid-state NMR spectroscopy. Rietveld refinement revealed a gradual disorder to order structural transition accompanied by lower inversion parameter (i(s)) and smaller lattice parameter (a) with higher calcination temperature. It was further affirmed by Raman analysis and solid-state NMR spectroscopy probing the ZnO4 and AlO4 tetrahedra in spinel. Independent of the degree of cation ordering, nanometric particle size with high surface area was observed in ZnAl2O4 spinel.
Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.
High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the UPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction.
Symmetry plays an important role in the retention or annihilation of a desired interaction Hamiltonian in NMR experiments. Here, we explore the role of symmetry in the radio-frequency interaction frame Hamiltonian of the refocused-continuous-wave (rCW) pulse scheme that leads to efficient H-1 heteronuclear decoupling in solid-state NMR. It is demonstrated that anti-periodic symmetry of single-spin operators (I-x, I-y, I-z) in the interaction frame can lead to complete annihilation of the H-1-H-1 homonuclear dipolar coupling effects that induce line broadening in solid-state NMR experiments. This symmetry also plays a critical role in cancelling or minimizing the effect of H-1 chemical-shift anisotropy in the effective Hamiltonian. An analytical description based on Floquet theory is presented here along with experimental evidences to understand the decoupling efficiency of supercycled (concatenated) rCW scheme.
Forming a stable solid electrolyte interphase (SEI) is critical for rechargeable batteries' performance and lifetime. Understanding its formation requires analytical techniques that provide molecular-level insight. Here, dynamic nuclear polarization (DNP) is utilized for the first time to enhance the sensitivity of solid-state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced graphene oxide (rGO) cycled in Li-ion cells in natural abundance and C-13-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance C-13 spectra from this component of the SEI on reasonable time frames. Furthermore, C-13-enriched electrolyte measurements at 100 K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiation of the inner and outer SEI layer composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the necessary sensitivity to probe the SEI efficiently.
We have synthesized the products of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) via lithium naphthalenide reduction. By analyzing the resulting solid precipitates and gas evolution, our results confirm that both FEC and VC decomposition products include HCO2Li, Li2C2O4, Li2CO3, and polymerized VC. For FEC, our experimental data supports a reduction mechanism where FEC reduces to form VC and LiF, followed by subsequent VC reduction. In the FEC reduction product, HCO2Li, Li2C2O4, and Li2CO3 were found in smaller quantities than in the VC reduction product, with no additional fluorine environments being detected by solid-state nuclear magnetic resonance or X-ray photoelectron spectroscopy analysis. With these additives being practically used in higher (FEC) and lower (VC) concentrations in the base electrolytes of lithium-ion batteries, our results suggest that the different relative ratios of the inorganic and organic reduction products formed by their decomposition may be relevant to the chemical composition and morphology of the solid electrolyte interphase formed in their presence.
The delithiation mechanisms occurring within the olivine-type class of cathode materials for Li-ion batteries have received considerable attention because of the good capacity retention at high rates for LiFePO4. A comprehensive mechanistic study of the (de)lithiation reactions that occur when the substituted olivine-type cathode materials LiFexCo1-xPO4 (x = 0, 0.05, 0.125, 0.25, 0.5, 0.75, 0.875, 0.95, 1) are electro-chemically cycled is reported here using in situ X-ray diffraction (XRD) data and supporting ex situ P-31 NMR spectra. On the first charge, two intermediate phases are observed and identified: Li1-x(Fe3+)(x)(Co2+)(1-x)PO4 for 0 <x <1 (i.e., after oxidation of Fe2+ to Fe3+) and Li2/3FexCo1-xPO4 for 0
The solid electrolyte interphase (SEI) of the high capacity anode material Si is monitored over multiple electrochemical cycles by Li-7, F-19, and C-13 solid-state nuclear magnetic resonance spectroscopies, with the organics dominating the SEI. Homonuclear correlation experiments are used to identify the organic fragments -OCH2CH2O-, -OCH2CH2-, -OCH2CH3, and -CH2CH3 contained in both oligomeric species and lithium semicarbonates ROCO2Li, RCO2Li. The SEI growth is correlated with increasing electrode tortuosity by using focused ion beam and scanning electron microscopy. A two-stage model for lithiation capacity loss is developed: initially, the lithiation capacity steadily decreases, Li+ is irreversibly consumed at a steady rate, and pronounced SEI growth is seen. Later, below 50% of the initial lithiation capacity, less Si is (de)lithiated resulting in less volume expansion and contraction; the rate of Li+ being irreversibly consumed declines, and the Si SEI thickness stabilizes. The decreasing lithiation capacity is primarily attributed to kinetics, the increased electrode tortuousity severely limiting Li+ ion diffusion through the bulk of the electrode. The resulting changes in the lithiation processes seen in the electrochemical capacity curves are ascribed to non-uniform lithiation, the reaction commencing near the separator/on the surface of the particles.
Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.
We present a bimodal Floquet analysis of the recently introduced refocused continuous wave (rCW) solid-state NMR heteronuclear dipolar decoupling method and compare it with the similar looking X-inverse X (XiX) scheme. The description is formulated in the rf interaction frame and is valid for both finite and ideal pi pulse rCW irradiation that forms the refocusing element in the rCW scheme. The effective heteronuclear dipolar coupling Hamiltonian up to first order is described. The analysis delineates the difference between the two sequences to different orders of their Hamiltonians for both diagonal and off-diagonal parts. All the resonance conditions observed in experiments and simulations have been characterised and their influence on residual line broadening is highlighted. The theoretical comparison substantiates the numerical simulations and experimental results to a large extent. (C) 2016 Elsevier Inc. All rights reserved.
The solid electrolyte interphase (SEI) passivating layer that grows on all battery electrodes during cycling is critical to the long-term capacity retention of lithium-ion batteries. Yet, it is inherently difficult to study because of its nanoscale thickness, amorphous composite structure, and air sensitivity. Here, we employ an experimental strategy using H-1, Li-7, F-19, and C-13 solid-state nuclear magnetic resonance (ssNMR) to gain insight into the decomposition products in the SET formed on silicon electrodes, the uncontrolled growth of the SET representing a major failure mechanism that prevents the practical use of silicon in lithium-ion batteries. The voltage dependent formation of the SET is confirmed, with the SEI growth correlating with irreversible capacity. By studying both conductive carbon and mixed Si/C composite electrodes separately, a correlation with increased capacity loss of the composite system and the low-voltage silicon plateau is demonstrated. Using selective C-13 labeling, we detect decomposition products of the electrolyte solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) independently. EC decomposition products are present in higher concentrations and are dominated by oligomer species. Lithium semicarbonates, lithium fluoride, and lithium carbonate products are also seen. Ab initio calculations have been carried out to aid in the assignment of NMR shifts. ssNMR applied to both rinsed and unrinsed electrodes show that the organics are easily rinsed away, suggesting that they are located on the outer layer of the SEI.
The pyrolysis of 1,2-diphosphinobenzene at 800 degrees C gives a phosphorus-doped graphite (P-DG) with an unprecedented high phosphorus content, ca. 20 at%. In contrast with previously studied boron and nitrogen doped graphite materials, thorough characterisation and analysis of this material demonstrates that it is extensively disordered and contains substitutional P-atoms along with P=O units in the host graphitic lattice, as well as P-4 molecules trapped between the graphitic sheets. This represents a stabilised form of P-4, which has been shown to covalently bind to lithium as Li3P in this material.
The hydroboration 1,3- and 1,4-cyclic dienes has been systematically investigated. The behavior of such dienes towards mono and dihydroboration was monitored directly by B-11 NMR to identify the actual boron species formed, as opposed to the most common analysis of the resultant oxidation products. Quantitative dihydroboration was achieved for the full range of cyclic dienes investigated including dienes, which were previously reported to be resistant to dihydroboration, leading to the formation of new boron-containing polymeric materials. The conditions favoring dihydroboration are reported as well as full characterisation of the materials. Furthermore, a hydroboration cascade mechanism is proposed for the formation of such boron-containing polymers, supported by both experimental and theoretical data.
The rechargeable aprotic lithium-air (Li-O-2) battery is a promising potential technology for next-generation energy storage, but its practical realization still faces many challenges. In contrast to the standard Li-O-2 cells, which cycle via the formation of Li-2 O-2, we used a reduced graphene oxide electrode, the additive LiI, and the solvent dimethoxyethane to reversibly form and remove crystalline LiOH with particle sizes larger than 15micrometers during discharge and charge. This leads to high specific capacities, excellent energy efficiency (93.2%) with a voltage gap of only 0.2 volt, and impressive rechargeability. The cells tolerate high concentrations of water, water being the dominant proton source for the LiOH; together with LiI, it has a decisive impact on the chemical nature of the discharge product and on battery performance.
Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO2 looping, where the material reacts reversibly with CO2 to form Li2CO3, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li2CO3 has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li2CO3 over the temperature range of 293 973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li2CO3. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations.
The Carr-Purcell-Meiboom-Gill (CPMG) sequence is commonly used in high resolution NMR spectroscopy and in magnetic resonance imaging for the measurement of transverse relaxation in systems that are subject to diffusion in internal or external gradients and is superior to the Hahn echo measurement, which is more sensitive to diffusion effects. Similarly, it can potentially be used to study dynamic processes in electrode materials for lithium ion batteries. Here we compare the Li-7 signal decay curves obtained with the CPMG and Hahn echo sequences under static conditions (i.e., in the absence of magic angle spinning) in paramagnetic materials with varying transition metal ion concentrations. Our results indicate that under CPMG pulse trains the lifetime of the Li-7 signal is substantially extended and is correlated with the strength of the electron-nuclear interaction. Numerical simulations and analytical calculations using Floquet theory suggest that the combination of large interactions and a train of finite pulses, results in a spin locking effect which significantly slows the signal's decay. While these effects complicate the interpretation of CPMG-based investigations of diffusion and chemical exchange in paramagnetic materials, they may provide a useful approach to extend the signal's lifetime in these often fast relaxing systems, enabling the use of correlation experiments. Furthermore, these results highlight the importance of developing a deeper understanding of the effects of the large paramagnetic interactions during multiple pulse experiments in order to extend the experimental arsenal available for static and in situ NMR investigations of paramagnetic materials.
Viscoelastic, gel-like, polymeric dispersions (HVPDs) can be prepared by crosslinking polyols with borax or boric acid in water under alkaline conditions. Rheologically similar HVPDs have been prepared in organic liquids containing no water or hydroxylic groups through crosslinking partially or fully hydrolysed poly(vinyl acetate)s with trimethyl borate, boric acid, or borax. The organo-HVPDs are water-sensitive and rheoreversible on exposure to water. They were characterised rheologically and by solution and solid-state 11B NMR spectroscopy. Spectroscopic analyses show the presence of mono- and di-diol crosslinks, as well as non-crosslinked boron species in HVPDs prepared with trimethyl borate or boric acid. The number of crosslinks in organo-HVPDs prepared with borax increased over the course of several days. Results from solution and solid-state 11B NMR spectroscopy are comparable; no solid-like component was detectable. We demonstrate that hydro, organo, or organo-aqueous HVPDs can be obtained from partially hydrolysed poly(vinyl acetate)s by 'tuning' the structure of the boron-based crosslinker.
Vanadium sulfide VS4 in the patronite mineral structure is a linear chain compound comprising vanadium atoms coordinated by disulfide anions [S-2](2-). V-51 NMR shows that the material, despite having V formally in the d(1) configuration, is diamagnetic, suggesting potential dimerization through metal metal bonding associated with a Peierls distortion of the linear chains. This is supported by density functional calculations, and is also consistent with the observed alternation in V V distances of 2.8 and 3.2 angstrom along the chains. Partial lithiation results in reduction of the disulfide ions to sulfide S2-, via an internal redox process whereby an electron from V4+ is transferred to [S-2](2-) in oxidation of V4+ to V5+ and reduction of the [S-2](2-) to S2- to form Li3VS4 containing tetrahedral [VS4](3-) anions. On further lithiation this is followed by reduction of the V5+ in Li3VS4 to form Li3+xVS4 (x = 0.5-1), a mixed valent V4+/V5+ compound. Eventually reduction to Li2S plus elemental V occurs. Despite the complex redox processes involving both the cation and the anion occurring in this material, the system is found to be partially reversible between 0 and 3 V. The unusual redox processes in this system are elucidated using a suite of short-range characterization tools including V-51 nuclear magnetic resonance spectroscopy (NMR), S K-edge X-ray absorption near edge spectroscopy (XANES), and pair distribution function (PDF) analysis of X-ray data.
Previous theoretical studies of C3B have suggested that boron-doped graphite is a promising H-2- and Li-storage material, with large maximum capacities. These characteristics could lead to exciting applications as a lightweight H-2-storage material for automotive engines and as an anode in a new generation of batteries. However, for these applications to be realized a synthetic route to bulk C3B must be developed. Here we show the thermolysis of a single-source precursor (1,3-BBr2)(2)C6H4) to produce graphitic C3B, thus allowing the characteristics of this elusive material to be tested for the first time. C3B was found to be compositionally uniform but turbostratically disordered. Contrary to theoretical expectations, the H-2- and Li-storage capacities are lower than anticipated, results that can partially be explained by the disordered nature of the material. This work suggests that to model the properties of graphitic materials more realistically, the possibility of disorder must be considered.
We present a Floquet theory approach for the analysis of homonuclear recoupling assisted by radio frequency (RF) irradiation of surrounding heteronuclear spins. This description covers a broad range of systems from fully protonated to deuterated proteins, focusing in detail on recoupling via protons and deuterons separately as well as simultaneously by the double nucleus enhanced recoupling (DONER) scheme. The theoretical description, supported by numerical simulations and compared to experimental results from a partially deuterated model compound, indicates that in perdeuterated systems setting the RF amplitude equal to the magic angle spinning (MAS) frequency is not necessarily optimal for recoupling via (1)H and/or (2)H nuclei and modified recoupling conditions are identified. (C) 2011 Elsevier Inc. All rights reserved.
Schemes such as phase-modulated Lee-Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution H-1 spectra at high magic-angle spinning (MAS) frequencies of 50-70 kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here. a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be Successful at high MAS frequencies. (C) 2009 Elsevier Inc. All rights reserved.
We present a rf scheme designed to excite triple quantum (TQ) coherences for proton solid state NMR. This recoupling scheme is based on the phase modulated Lee Goldburg sequence combined with echo pulses and applied nonsynchronous with the magic angle spinning period. Based on the effective bimodal Floquet Hamiltonian we optimize the conditions for TQ coherence excitation. Numerical simulations are used to further adjust the recoupling conditions as well as define the sequence limitations. Experimental TQ filtered one-dimensional spectra and two-dimensional correlations of TQ to single quantum coherences are presented for standard amino acids. These results are compared with the crystal structures showing that this scheme can aid in resonance assignments and in resolving local spin topologies.
We report here high-resolution H-1 solid-state nuclear magnetic resonance spectra acquired by a combination of magic-angle spinning (MAS) and radiofrequency pulse methods up to MAS frequencies of 65 kHz. The details of the pulse methods and experimental conditions are outlined together with spectra from model compounds. (C) 2008 Elsevier B.V. All rights reserved.
A homonuclear dipolar decoupling scheme based on windowed phase-modulated Lee-Goldburg (wPMLG) pulse sequences that causes a"z-rotation" of the spins for high-resolution proton NMR spectroscopy of solids is described and analyzed. This supercycled scheme suppresses the effect of pulse imperfections on the spectra and significantly relaxes the off-resonance dependence of the line-narrowing efficiency and scale factor. This leads to a broad spectral window that is free of artifacts such as zero lines, image peaks, and localized rotor-radio-frequency resonances. High-resolution (1)H spectra and two-dimensional homonuclear (1)H-(1)H correlation spectra of standard amino acids, obtained by a combination of this supercycled scheme with magic angle spinning frequencies up to 25 kHz, are demonstrated. (c) 2008 American Institute of Physics.
High-resolution H-1 spectroscopy in solid-state NMR, rendered difficult due to the strong H-1-H-1 homonuclear dipolar coupling, has been made possible under magic-angle spinning with homonuclear dipolar decoupling schemes, such as windowed phase-modulated Lee-Goldburg. Here, we outline the theory and implementation of a modification of this scheme with which an effective z-rotation for the magnetisation is obtained over a wide range of spectral window. Experimental results are presented for samples, such as glycine, histidine, and tryosine. (C) 2007 Elsevier B.V. All rights reserved.
A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear H-1-H-1 dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SWf-TPPM) sequence. It is shown that the experimental robustness of SWf-TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments. (C) 2007 American Institute of Physics.
We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the H-1 spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional H-1-H-1 homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences. (c) 2006 American Institute of Physics.