Development of first principles paramagnetic NMR methodologies to probe the complex local structural properties of Li-ion battery materials

NMR spectroscopy of paramagnetic solids provides detailed information about the local configuration and the chemical environment of the NMR observed center, as well as about the structural, magnetic and electronic properties of the coordianted paramagnetic centres. In the case of complex paramagnetic solids such as cathode materials for (rechargeable) batteries, NMR represents an invaluable tool to provide insight into the structural and electronic properties of the systems, which are at the base of the electrochemical performance of these materials. However, the paramagnetism makes the interpretation of the NMR data very challenging. This is primarily due to the interactions of the unpaired electrons with the NMR observed nucleus, and the interpretation of the NMR spectra often requires the aid of reliable theoretical and computational methods. Often the dominant interaction contributing to the measured isotropic shifts is the hyperfine interaction between the unpaired electrons and the observed nucleus, which results from the transfer of unpaired electrons from the paramagnetic centre(s) to the NMR observed site. In systems such as the ones studied here, in which the paramagnetic ions are a major constituent of the lattice, the multitide of different local environments results in a complex distribution of resonances. As in the case of the Li$_x$V$_6$O$_{13}$ cathode material, a methodical investigation of the configurational stability from first principles gives insight into the preferred site configurations. The combination of experimental $^7$Li NMR spectra and hyperfine shift DFT calculations of the so-found stable Li environments allows to unravel the complex lithiation mechanism of this material. In the other case of the LiTi$_x$Mn$_{2-x}$O$_4$ cathode materials, the $^7$Li hyperfine shifts calculated from first principles for a variety of Li environments are combined in a lattice model which allows to assign the isotropic regions of the experimental $^7$Li NMR spectra, helping to resolve the complex cation ordering as a function of Mn/Ti content in the series. For paramagnetic centres with an unquenched orbital component of the electron magnetic moment(s), the spin-orbit coupling effects also contribute to the paramagnetic NMR shift and shift anisotropy. A first principles model is derived, which describes how spin-orbit coupling and the single-ion $g$-tensor are defined and calculated in periodic paramagnetic solids, and how they can be coupled with the hyperfine interaction to model their effects on the NMR spectrum. The method is applied to a series of olivine-type LiTMPO$_4$ cathode materials (with TM = Mn, Fe, Co, and Ni) and the respective $^7$Li and $^{31}$P NMR spectra are simulated and compared with the experiments. The other paramagnetic effect considered in this thesis involves the bulk magnetic susceptibility (BMS), which is particularly important for paramagnetic single crystals and solids of complex shape. The BMS effect results from the discontinuity of the bulk susceptibility at the surface of the crystal, inducing a demagnetizing field throughout the sample which changes the measured NMR shift and shift anisotropy. A method to analytically calculate the demagnetising field and the BMS shift in crystals of different shapes is derived, and it is applied to a series of LiFePO$_4$ single crystals for which the $^7$Li NMR spectra are also measured experimentally. The study confirms that, particularly for $^7$Li NMR, the macroscopic shape-dependent BMS shift can indeed be a significant contribution to the measured resonances, determining the large variation in shift measured for the crystals of different shapes

Ultrafast inside‐out NMR assessment of rechargeable cells

Rechargeable battery cells are notoriously difficult to analyze. Conductive casings and the close spacing between electrode layers prevent the penetration of radiofrequency into the active compartment, and thus preclude direct nuclear magnetic resonance studies of cells unless they are specifically designed for such studies. Recently, an inside‐out magnetic resonance imaging (MRI) method was developed that allowed measuring the magnetic field distributions in the volume surrounding the cells, and inferring internal parameters, such as the state of charge and current distributions. While the imaging approach provides a potentially very detailed picture of internal mechanisms, it can often be sensitive to background gradients and can be slow. In this work, an alternative approach is presented, which is based on the acquisition of free induction decays in the sample volume surrounding the cells. The signals encode intrinsic battery properties via the induced magnetic fields from the battery materials. A large range of cells were studied with different cathode materials, electrolyte amounts and cycle numbers (age). The spectroscopic signatures from these studies are shown to provide strong classification power for cathode materials. In addition, the derived principal components follow distinct pathways as a function of state of charge. The method is simple and fast (completes in less than a second), and requires only minimal hardware.Fil: Pigliapochi, Roberta. University of New York; Estados UnidosFil: Benders, Stefan. University of New York; Estados UnidosFil: Silletta, Emilia Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Glazier, Stephen L.. Dalhousie University Halifax; CanadáFil: Lee, Elizabeth. Dalhousie University Halifax; CanadáFil: Dahn, Jeff. Dalhousie University Halifax; CanadáFil: Jerschow, Alexej. University of New York; Estados Unido

When do Anisotropic Magnetic Susceptibilities Lead to Large NMR Shifts? Exploring Particle Shape Effects in the Battery Electrode Material LiFePO4.

Materials used as electrodes in energy storage devices have been extensively studied with solid-state NMR spectroscopy. Due to the almost ubiquitous presence of transition metals, these systems are also often magnetic. While it is well known that the presence of anisotropic bulk magnetic susceptibility (ABMS) leads to broadening of resonances under MAS, we show that for mono-disperse and non-spherical particle morphologies, the ABMS can also lead to considerable shifts, which vary substantially as a function of particle shape. This, on one hand, complicates the interpretation of the NMR spectrum and the ability to compare the measured shift of different samples of the same system. On the other hand the ABMS shift provides a mechanism with which to derive the particle shape from the NMR spectrum. In this work, we present a methodology to model the ABMS shift, and relate it to the shape of the studied particles. The approach is tested on the $^7$Li NMR spectra of single crystals and powders of LiFePO$_4$. The results show that the ABMS shift can be a major contribution to the total NMR shift in systems with large magnetic anisotropies and small hyperfine shifts, $^7$Li shifts for typical LiFePO$_4$ morphologies varying by as much as 100 ppm. The results are generalised to demonstrate that the approach can be used as a means with which to probe the aspect ratio of particles. The work has implications for the analysis of NMR spectra of all materials with anisotropic magnetic susceptibilities, including diamagnetic materials such as graphite

Dynamic-nuclear-polarization-weighted spectroscopy of multi-spin electronic-nuclear clusters

Nuclear spins and paramagnetic centers in a solid randomly group to form clusters featuring nearly-degenerate, hybrid states whose dynamics are central to processes involving nuclear spin-lattice relaxation and diffusion. Their characterization, however, has proven notoriously difficult mostly due to their relative isolation and comparatively low concentration. Here, we combine field-cycling experiments, optical spin pumping, and variable radio-frequency (RF) excitation to probe transitions between hybrid multi-spin states formed by strongly coupled electronic and nuclear spins in diamond. Leveraging bulk nuclei as a collective time-integrating sensor, we probe the response of these spin clusters as we simultaneously vary the applied magnetic field and RF excitation to reconstruct multi-dimensional spectra. We uncover complex nuclear polarization patterns of alternating sign that we qualitatively capture through analytical and numerical modeling. Our results unambiguously expose the impact that strongly-hyperfine-coupled nuclei can have on the spin dynamics of the crystal, and inform future routes to spin cluster control and detection

Sodium Intercalation Mechanism of 3.8 v Class Alluaudite Sodium Iron Sulfate

Alluaudite sodium iron sulfate Na$_{2+2x}$Fe$_{2−x}$(SO$_4$)$_3$ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe$^{3+}$/Fe$^{2+}$ redox reactions (3.8 V vs Na/Na$^+$ ), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), $^{23}$Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe$^{3+}$/Fe$^{2+}$ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. The synchrotron XRD experiments were performed under KEK-PF User Program (No. 2013G670). Crystal structures and the Fourier difference maps were drawn by VESTA.65 G.O. acknowledges financial support from JSPS Research Fellowships under “Materials Education Program for the Future Leaders in Research, Industry, and Technology” (MERIT) project. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 655444 (O.P.). R.P. gratefully acknowledges financial support through the Marie Curie Actions People Program of the EU’s Seventh Frame work Program (FP7/2007-2013), under the grant agreement n.317127, the ‘pNMR project’. K.J.G. gratefully acknowledges funding from The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. This work made use of the facilities of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.6b0109

Molecular Structure and Dynamics of a Nonionic Surfactant (C12E5) in Lamellar Phase : 13C Solid State NMR and MD Simulations

Popular Science Description Surfactants are compounds broadly used in daily life products as cosmetics, soaps or detergents. A better understanding of the processes occurring at a molecular scale leads to an improvement of their applications. The following work presents a 13C NMR procedure that allows to study the dynamics of these systems: an important insight into the motional mechanisms is hence obtained thorugh common experimental analysis.The following work presents a theoretical model for evaluating the eective correlation time, e, in regards to surfactants in lamellar phase, by focusing on nuclear spin relaxation processes. The dynamics of the C-H bonds is studied through a specic form of the autocorrelation function, g( ), which undergoes a plateau region at intermediate time-scales equals to the square of the order parameter, S2 CH. Thus, the area between the curve and its plateau is dened to be equal to (1 - S2CH)e. The presented experimental scheme shows how to measure e through 13C Solid State NMR analysis, basing on the dependence of R1 and R1 rate constants on the spectral density j(w), Fourier Transform of the correlation function, at particoular frequencies. The results are compared with measurements of the eective correlation time through MD simulations. Furthemore, regarding the latter method, a better insight into the structure of the lamellar phase system is shown

Model-free estimation of the effective correlation time for C-H bond reorientation in amphiphilic bilayers: (1)H-(13)C solid-state NMR and MD simulations.

Molecular dynamics (MD) simulations give atomically detailed information on structure and dynamics in amphiphilic bilayer systems on timescales up to about 1 μs. The reorientational dynamics of the C-H bonds is conventionally verified by measurements of (13)C or (2)H nuclear magnetic resonance (NMR) longitudinal relaxation rates R1, which are more sensitive to motional processes with correlation times close to the inverse Larmor frequency, typically around 1-10 ns on standard NMR instrumentation, and are thus less sensitive to the 10-1000 ns timescale motion that can be observed in the MD simulations. We propose an experimental procedure for atomically resolved model-free estimation of the C-H bond effective reorientational correlation time τe, which includes contributions from the entire range of all-atom MD timescales and that can be calculated directly from the MD trajectories. The approach is based on measurements of (13)C R1 and R1ρ relaxation rates, as well as (1)H-(13)C dipolar couplings, and is applicable to anisotropic liquid crystalline lipid or surfactant systems using a conventional solid-state NMR spectrometer and samples with natural isotopic composition. The procedure is demonstrated on a fully hydrated lamellar phase of 1-palmitoyl-2-oleoyl-phosphatidylcholine, yielding values of τe from 0.1 ns for the methyl groups in the choline moiety and at the end of the acyl chains to 3 ns for the g1 methylene group of the glycerol backbone. MD simulations performed with a widely used united-atom force-field reproduce the τe-profile of the major part of the acyl chains but underestimate the dynamics of the glycerol backbone and adjacent molecular segments. The measurement of experimental τe-profiles can be used to study subtle effects on C-H bond reorientational motions in anisotropic liquid crystals, as well as to validate the C-H bond reorientation dynamics predicted in MD simulations of amphiphilic bilayers such as lipid membranes