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The Carr–Purcell–Meiboom–Gill (CPMG) experiment has gained popularity in solid-state NMR as a method for enhancing sensitivity for anisotropically broadened spectra of both spin 1/2 and half integer quadrupolar nuclei. Most commonly, the train of CPMG echoes is Fourier transformed directly, which causes the NMR powder pattern to break up into a series of sidebands, sometimes called “spikelets.” Larger sensitivity enhancements are observed as the delay between the π pulses is shortened. As the duration between the π pulses is shortened, however, the echoes become truncated and information about the nuclear spin interactions is lost. We explored the relationship between enhanced sensitivity and loss of information as a function of the product Ω 2τ, where Ω is the span of the anisotropic lineshape and 2τ is the π pulse spacing. For a lineshape dominated by the nuclear shielding anisotropy, we found that the minimum uncertainty in the tensor values is obtained using Ω 2τ values in the range Ω 2τ≈12-1+6 and Ω 2τ≈9-3+3 for ηs=0 and ηs=1, respectively. For an anisotropic second-order quadrupolar central transition lineshape under magic-angle spinning (MAS), the optimum range of Ω 2τ≈9-2+3 was found. Additionally, we show how the Two-dimensional One Pulse (TOP) like processing approach can be used to eliminate the cumbersome sideband pattern lineshape and recover a more familiar lineshape that is easily analyzed with conventional lineshape simulation algorithms. [ABSTRACT FROM AUTHOR]

Recent methodological advances have made it possible to measure fine structure on the order of a few hertz in the nuclear magnetic resonance (NMR) spectra of quadrupolar nuclei in polycrystalline samples. Since quadrupolar couplings are often a significant fraction of the Zeeman coupling, a complete analysis of such experimental spectra requires a theoretical treatment beyond first-order. For multiple pulse NMR experiments, which may include sample rotation, the traditional density matrix approaches for treating higher-order effects suffer from the constraint that undesired fast oscillations (i.e., multiples of the Zeeman frequency), which arise from allowed overtone transitions, can only be eliminated in numerical simulations by employing sampling rates greater than 2I times the Zeeman frequency. Here, we present a general theoretical approach for arbitrary spin I that implements an analytical “filtering” of undesired fast oscillations in the rotating tilted frame, while still performing an exact diagonalization. Alternatively, this approach can be applied using a perturbation expansion for the eigenvalues and eigenstates, such that arbitrary levels of theory can be explored. The only constraint in this approach is that the Zeeman interaction remains the dominant interaction. Using this theoretical framework, numerical simulations can be implemented without the need for a high sampling rate of observables and with significantly reduced computation times. Additionally, this approach provides a general procedure for focusing on the excitation and detection of both fundamental and overtone transitions. Using this approach we explore higher-order effects on a number of sensitivity and resolution issues with NMR of quadrupolar nuclei. [ABSTRACT FROM AUTHOR]

The article focuses on in situ nuclear magnetic resonance (NMR) approach for studying lithium (Li) ion batteries (LIB). It states that NMR is a way to study how the dendrites were formed in the lithium metal as well as allows to monitor their growth to further investigate how the radio frequency (rf) field penetrates into the lithium metal. It adds that the approach is a means for knowing the changes that occur in LIB which is accompanied with the distinct NMR signals.

Abstract: Central transition (CT) sensitivity enhancement schemes that transfer polarization from satellites to the CT through selective saturation or inversion of neighboring satellite transitions have provided a welcome improvement for magic-angle spinning spectra of half-integer quadrupole nuclei. While many researchers have investigated and developed different methods of creating enhanced CT populations, here we investigate the conversion of these enhanced CT populations into observable CT coherence. We show a somewhat unexpected result that a conversion pulse length optimized for maximum sensitivity on equilibrium populations may not be optimum for an enhanced (non-equilibrium) polarization. Furthermore, CT enhancements can be lost if excessive rf field strength is used to convert this enhanced polarization into CT coherence. While a maximally enhanced CT signal is expected when using a perfectly selective CT conversion pulse, we have found that significant sensitivity loss can occur when using surprisingly low rf field strengths, even for sites with relatively large quadrupole coupling constants. We have systematically investigated these issues, and present some general guidelines and expectations when optimizing the conversion of enhanced (non-equilibrium) CT populations into observable CT coherence. [Copyright &y& Elsevier]

Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of 17O NMR in the same materials has not yet been reported. In this work, we present a combined 17O NMR and hybrid density functional theory study of the local O environments in Li2MnO3, a model compound for layered Li-ion batteries. After a simple 17O enrichment procedure, we observed five resonances with large 17O shifts ascribed to the Fermi contact interaction with directly bonded Mn4+ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the 17O shifts of environments similar to the 4i and 8j sites in pristine Li2MnO3, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li2MnO3 structure. From the ratio of the intensities of the different 17O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying 17O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides. [ABSTRACT FROM AUTHOR]

Highlights: [•] Challenges of in situ NMR of Li-ion batteries containing paramagnetic electrodes are described. [•] The orientation dependence of BMS effects is evaluated using a dipolar coupling model. [•] The shape and packing density dependence of BMS effect of paramagnetic electrodes are discussed. [•] The BMS induced shift can be minimized by orienting the battery at a rotation angle of 54.7°. [ABSTRACT FROM AUTHOR]