From ultraslow to fast lithium diffusion in the 2D ion conductor Li0.7TiS2 probed directly by stimulated-echo NMR and nuclear magnetic relaxation

LaOMS2 (M = Ti, V, and Cr): novel crystal spin valves without contact

2D materials are widely investigated for application in nanodevices and spintronics. Here, we demonstrate that a family of structures, LaOMS2 (M = Ti, V, and Cr), where a La2O2 layer is sandwiched between two magnetic MS2 layers, are suitable to be used as spin valves without contact. We show that they are stable and exhibit unique magnetic and electronic properties. We find that (1) each MS2 layer is ferromagnetic with the magnetic moment mainly contributed by the M ion and the coupling between the two MS2 layers in one structure is negligible due to the blocking of the La2O2 layer; (2) LaOMS2 can be a half-metal in the ferromagnetic (FM) state and a conductor in the interlayer antiferromagnetic (inter-AFM) state, and the total energies of the two states are almost identical; (3) the magnetic properties and exchange energy can be effectively controlled by contacting with other materials; and (4) a 100% spin current is achieved in FM LaOMS2. Our results provide not only novel structures for practical application in spintronics, but also strategies for designing new devices.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that [Formula: see text] ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides [Formula: see text], Li-bearing garnets ([Formula: see text]) and thiophosphates such as [Formula: see text]. In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in [Formula: see text] ([Formula: see text]) cation mixing influences F anion dynamics, in [Formula: see text] ([Formula: see text]) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor [Formula: see text] cation-cation repulsions immediately lead to a boost in [Formula: see text] diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured [Formula: see text] or [Formula: see text]. Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Independent component analysis combined with Laplace inversion of spectrally resolved spin-alignment echo/T 1 3D 7Li NMR of superionic Li10GeP2S12

The use of independent component analysis (ICA) for the analysis of two-dimensional (2D) spin-alignment echo–T 1 7Li NMR correlation data with transient echo detection as a third dimension is demonstrated for the superionic conductor Li10GeP2S12 (LGPS). ICA was combined with Laplace inversion, or discrete inverse Laplace transform (ILT), to obtain spectrally resolved 2D correlation maps. Robust results were obtained with the spectra as well as the vectorized correlation maps as independent components. It was also shown that the order of ICA and ILT steps can be swapped. While performing the ILT step before ICA provided better contrast, a substantial data compression can be achieved if ICA is executed first. Thereby the overall computation time could be reduced by one to two orders of magnitude, since the number of computationally expensive ILT steps is limited to the number of retained independent components. For LGPS, it was demonstrated that physically meaningful independent components and mixing matrices are obtained, which could be correlated with previously investigated material properties yet provided a clearer, better separation of features in the data. LGPS from two different batches was investigated, which showed substantial differences in their spectral and relaxation behavior. While in both cases this could be attributed to ionic mobility, the presented analysis may also clear the way for a more in-depth theoretical analysis based on numerical simulations. The presented method appears to be particularly suitable for samples with at least partially resolved static quadrupolar spectra, such as alkali metal ions in superionic conductors. The good stability of the ICA analysis makes this a prospect algorithm for preprocessing of data for a subsequent automatized analysis using machine learning concepts.

Nuclear magnetic resonance (NMR) studies of sintering effects on the lithium ion dynamics in Li1.5Al0.5Ti1.5(PO4)3

Various nuclear magnetic resonance (NMR) methods are combined to study the structure and dynamics of Li1.5Al0.5Ti1.5(PO4)3 (LATP) samples, which were obtained from sintering at various temperatures between 650 and 900 °C. 6Li, 27Al, and 31P magic angle spinning (MAS) NMR spectra show that LATP crystallites are better defined for higher calcination temperatures. Analysis of 7Li spin-lattice relaxation and line-shape changes indicates the existence of two species of lithium ions with clearly distinguishable jump dynamics, which can be attributed to crystalline and amorphous sample regions, respectively. An increase of the sintering temperature leads to higher fractions of the fast lithium species with respect to the slow one, but hardly affects the jump dynamics in either of the phases. Specifically, the fast and slow lithium ions show jumps in the nanoseconds regime near 300 and 700 K, respectively. The activation energy of the hopping motion in the LATP crystallites amounts to ca. 0.26 eV. 7Li field-gradient diffusometry reveals that the long-range ion migration is limited by the sample regions featuring slow transport. The high spatial resolution available from the high static field gradients of our setup allows the observation of the lithium ion diffusion inside the small (<100 nm) LATP crystallites, yielding a high self-diffusion coefficient of D = 2 × 10−12 m2/s at room temperature.

CNSi/MXene/CNSi: Unique Structure with Specific Electronic Properties for Nanodevices

2D materials have been interesting for applications into nanodevices due to their intriguing physical properties. In this work, four types of unique structures are designed that are composed of MXenes and C/N-Si layers (CNSi), where MXene is sandwiched by the CNSi layers with different thicknesses, for their practical applications into integrated devices. The systematic calculations on their elastic constants, phonon dispersions, and thermodynamic properties show that these structures are stable, depending on the composition of MXene. It is found: 1) different from MXene or N-functionalized MXene (M₂ CN₂ ), SiN₂ /M₂ X/SiN₂ possess new electronic properties with free carriers only in the middle, leading to 2D free electron gas; 2) CNSi/MXene/CNSi shows an intrinsic Ohmic semiconductor-metal-semiconductor (S-M-S) contact, which is potential for applications into nanodevices; and 3) O/M₂ C/SiN₂ and N/M₂ C/OSiN are also stable and show different electronic properties, which can be semiconductor or metal as a whole depending on the interface. A method is further proposed to fabricate the 2D structures based on the industrial availability. The findings may provide a novel strategy to design and fabricate the 2D structures for their application into nanodevices and integrated circuits.

Tracking Ions the Direct Way: Long-Range Li+ Dynamics in the Thio-LISICON Family Li4MCh4 (M = Sn, Ge; Ch = S, Se) as Probed by 7Li NMR Relaxometry and 7Li Spin-Alignment Echo NMR

Solid electrolytes are key elements for next-generation energy storage systems. To design powerful electrolytes with high ionic conductivity, we need to improve our understanding of the mechanisms that are at the heart of the rapid ion exchange processes in solids. Such an understanding also requires evaluation and testing of methods not routinely used to characterize ion conductors. Here, the ternary Li₄MCh₄ system (M = Ge, Sn; Ch = Se, S) provides model compounds to study the applicability of ⁷Li nuclear magnetic resonance (NMR) spin-alignment echo (SAE) spectroscopy to probe slow Li⁺ exchange processes. Whereas the exact interpretation of conventional spin-lattice relaxation data depends on models, SAE NMR offers a model-independent, direct access to motional correlation rates. Indeed, the jump rates and activation energies deduced from time-domain relaxometry data perfectly agree with results from ⁷Li SAE NMR. In particular, long-range Li⁺ diffusion in polycrystalline Li₄SnS₄ as seen by NMR in a dynamic range covering 6 orders of magnitude is determined by an activation energy of E _a = 0.55 eV and a pre-exponential factor of 3 × 10¹³ s^-1. The variation in E _a and 1/τ₀ is related to the LiCh₄ volume that changes within the four Li₄MCh₄ compounds studied. The corresponding volume of Li₄SnS₄ seems to be close to optimum for Li⁺ diffusivity.

Homo–hetero/core–shell structure design strategy of NaYF4 nanocrystals for superior upconversion luminescence

A comprehensive strategy has been developed to construct nano-sized homogeneous and heterogeneous core/shell structures of NaYF₄ host. Synthesis conditions of cubic phase/α-NaYF₄ and hexagonal phase/β-NaYF₄ are discussed. Pure cubic NaYF₄:Yb,Er nanocrystals were synthesized with different average sizes extending from 7 nm to 15 nm by varying the reaction time. Temperature and time thresholds of hexagonal nucleation were determined and utilized for controlled core/shell structures of different phases. α-NaYF₄:Yb,Er@α-NaYF₄, α-NaYF₄:Yb,Er@β-NaYF₄, β-NaYF₄:Yb,Er@α-NaYF₄, and β-NaYF₄:Yb,Er@β-NaYF₄ core/shell structures were prepared by adopting the required conditions to achieve the desired phase. Excess sodium was used to grow hexagonal shell over metastable cubic core under controlled conditions of reaction time and temperature to prevent the structural transition of the core. Upconversion emission spectra have also been obtained. UCL integrated intensities demonstrated about 5-fold enhancement for α-shell over α-core as compared to the core alone and 22-fold enhancement with β-shell. On the other hand, α-shell over β-core exhibited 5-fold enhancement and β-shell over β-core exhibited 6-fold enhancement.

Establishment of essential conditions of different phases of NaYF₄ and their utilization for the synthesis of core/shell structures to achieve the enhancement of UCL intensities.

Rapid Low-Dimensional Li+ Ion Hopping Processes in Synthetic Hectorite-Type Li0.5[Mg2.5Li0.5]Si4O10F2

Understanding the origins of fast ion transport in solids is important to develop new ionic conductors for batteries and sensors. Nature offers a rich assortment of rather inspiring structures to elucidate these origins. In particular, layer-structured materials are prone to show facile Li+ transport along their inner surfaces. Here, synthetic hectorite-type Li0.5[Mg2.5Li0.5]Si4O10F2, being a phyllosilicate, served as a model substance to investigate Li+ translational ion dynamics by both broadband conductivity spectroscopy and diffusion-induced 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation experiments. It turned out that conductivity spectroscopy, electric modulus data, and NMR are indeed able to detect a rapid 2D Li+ exchange process governed by an activation energy as low as 0.35 eV. At room temperature, the bulk conductivity turned out to be in the order of 0.1 mS cm-1. Thus, the silicate represents a promising starting point for further improvements by crystal chemical engineering. To the best of our knowledge, such a high Li+ ionic conductivity has not been observed for any silicate yet.

Dynamics and Glass Transition of Supercooled Water Confined in Amphiphilic Polymer Films

The glass transition of supercooled water is not well understood yet. We have observed a clear glass transition of the supercooled water confined in channel of amphiphilic polymer films at 145 K. Using NMR, we probe two types of relaxations occurred in the glass former, e.g., a rapid local β-process and a slow α-process (most likely). It is found that slow α-relaxation follows the Arrhenius relationship, indicating the glass former is a strong liquid. We also find a dynamic crossover from low-temperature Arrhenius α-process to high-temperature VFT process at 198-208 K, accompanying with simultaneous disappearing of local β-relaxation.

Investigation of ionic and anomalous magnetic behavior in CrSe2 using 8Li β-NMR

We have studied a mosaic of 1T-CrSe₂ single crystals using β-detected nuclear magnetic resonance of ⁸Li from 4 to 300 K. We identify two broad resonances that show no evidence of quadrupolar splitting, indicating two magnetically distinct environments for the implanted ion. We observe stretched exponential spin lattice relaxation and a corresponding rate (1/T₁) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in 1/T₁ is observed at the low temperature magnetic transition near 20 K. Between these limits, 1/T₁ exhibits a broad minimum with an anomalous absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest ⁸Li⁺ site occupation within the van der Waals gap between CrSe₂ trilayers. Possible origins of the two environments are discussed.

We report the first nuclear magnetic resonance measurements of an atypical transition metal dichalcogenide, CrSe₂.

Tracking sodium migration in TiS2 using in situ TEM

For alkali-metal ion batteries, revealing the phase transformation and the ion migration dynamics in the electrodes is vital for understanding how the electrodes work and thereby how we can improve them. Here, using in situ transmission electron microscopy, we track the structural evolution and migration dynamics during sodium insertion into TiS2 nanostructures with the lattice fringe resolution. We find that the sodiation process of TiS2 is initiated by an intercalation reaction and followed by a conversion reaction. From the same reaction event, the velocity of intercalation/conversion phase boundary migration is found to be ∼1.0-1.7 nm s-1, while the pristine/intercalation phase boundary migrates at a velocity of ∼2.5 nm s-1. The sodium migration leads to structural fracture to form nanometer-sized domains (∼3 nm) with volume expansion. During migration, Na prefers to transport along specific directions. Furthermore, a superstructured Na0.25TiS2 intermediate phase with ordered Na ions occupied within the (0001) plane is formed at the reaction front, which is different from the common staging phase. These findings help us understand the working principle and the failure mechanism of the sodium ion battery and also provide useful insights into the general ionic doping of transition metal dichalcogenides.

Spatial confinement – rapid 2D F− diffusion in micro- and nanocrystalline RbSn2F5

NMR and conductivity spectroscopy reveal 2D diffusion in both microcrystalline and nanocrystalline RbSn₂F₅.

Substitutional disorder: structure and ion dynamics of the argyrodites Li6PS5Cl, Li6PS5Br and Li6PS5I

Li NMR spectroscopy reveals rapid Li ion dynamics in the poor Li ion conductor Li₆PS₅I; long-range motion is, however, only possible for Li₆PS₅Br and Li₆PS₅Cl with anion site disorder.

Fast Na ion transport triggered by rapid ion exchange on local length scales

The realization of green and economically friendly energy storage systems needs materials with outstanding properties. Future batteries based on Na as an abundant element take advantage of non-flammable ceramic electrolytes with very high conductivities. Na₃Zr₂(SiO₄)₂PO₄-type superionic conductors are expected to pave the way for inherently safe and sustainable all-solid-state batteries. So far, only little information has been extracted from spectroscopic measurements to clarify the origins of fast ionic hopping on the atomic length scale. Here we combined broadband conductivity spectroscopy and nuclear magnetic resonance (NMR) relaxation to study Na ion dynamics from the µm to the angstrom length scale. Spin-lattice relaxation NMR revealed a very fast Na ion exchange process in Na_3.4Sc_0.4Zr_1.6(SiO₄)₂PO₄ that is characterized by an unprecedentedly high self-diffusion coefficient of 9 × 10⁻¹² m²s⁻¹ at −10 °C. Thus, well below ambient temperature the Na ions have access to elementary diffusion processes with a mean residence time τ_NMR of only 2 ns. The underlying asymmetric diffusion-induced NMR rate peak and the corresponding conductivity isotherms measured in the MHz range reveal correlated ionic motion. Obviously, local but extremely rapid Na⁺ jumps, involving especially the transition sites in Sc-NZSP, trigger long-range ion transport and push ionic conductivity up to 2 mS/cm at room temperature.

Lattice-geometry effects in garnet solid electrolytes: a lattice-gas Monte Carlo simulation study

Ionic transport in solid electrolytes can often be approximated as ions performing a sequence of hops between distinct lattice sites. If these hops are uncorrelated, quantitative relationships can be derived that connect microscopic hopping rates to macroscopic transport coefficients; i.e. tracer diffusion coefficients and ionic conductivities. In real materials, hops are uncorrelated only in the dilute limit. At non-dilute concentrations, the relationships between hopping frequency, diffusion coefficient and ionic conductivity deviate from the random walk case, with this deviation quantified by single-particle and collective correlation factors, f and f_I, respectively. These factors vary between materials, and depend on the concentration of mobile particles, the nature of the interactions, and the host lattice geometry. Here, we study these correlation effects for the garnet lattice using lattice-gas Monte Carlo simulations. We find that, for non-interacting particles (volume exclusion only), single-particle correlation effects are more significant than for any previously studied three-dimensional lattice. This is attributed to the presence of two-coordinate lattice sites, which causes correlation effects intermediate between typical three-dimensional and one-dimensional lattices. Including nearest-neighbour repulsion and on-site energies produces more complex single-particle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_Li=3 (from site energies) and x_Li=6 (from nearest-neighbour repulsion), where x_Li=9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile-ion stoichiometry to maximize the ionic conductivity, and show that the 'optimal' composition is highly sensitive to the precise nature and strength of the microscopic interactions. Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.

Lattice-geometry effects in garnet solid electrolytes: a lattice-gas Monte Carlo simulation study

Ionic transport in solid electrolytes can often be approximated as ions performing a sequence of hops between distinct lattice sites. If these hops are uncorrelated, quantitative relationships can be derived that connect microscopic hopping rates to macroscopic transport coefficients; i.e. tracer diffusion coefficients and ionic conductivities. In real materials, hops are uncorrelated only in the dilute limit. At non-dilute concentrations, the relationships between hopping frequency, diffusion coefficient and ionic conductivity deviate from the random walk case, with this deviation quantified by single-particle and collective correlation factors, f and f_I, respectively. These factors vary between materials, and depend on the concentration of mobile particles, the nature of the interactions, and the host lattice geometry. Here, we study these correlation effects for the garnet lattice using lattice-gas Monte Carlo simulations. We find that, for non-interacting particles (volume exclusion only), single-particle correlation effects are more significant than for any previously studied three-dimensional lattice. This is attributed to the presence of two-coordinate lattice sites, which causes correlation effects intermediate between typical three-dimensional and one-dimensional lattices. Including nearest-neighbour repulsion and on-site energies produces more complex single-particle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_Li=3 (from site energies) and x_Li=6 (from nearest-neighbour repulsion), where x_Li=9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile-ion stoichiometry to maximize the ionic conductivity, and show that the 'optimal' composition is highly sensitive to the precise nature and strength of the microscopic interactions. Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.

Solid-State NMR to Study Translational Li Ion Dynamics in Solids with Low-Dimensional Diffusion Pathways

Fundamental research on lithium ion dynamics in solids is important to develop functional materials for, e.g. sensors or energy storage systems. In many cases a comprehensive understanding is only possible if experimental data are compared with predictions from diffusion models. Nuclear magnetic resonance (NMR), besides other techniques such as mass tracer or conductivity measurements, is known as a versatile tool to investigate ion dynamics. Among the various time-domain NMR techniques, NMR relaxometry, in particular, serves not only to measure diffusion parameters, such as jump rates and activation energies, it is also useful to collect information on the dimensionality of the underlying diffusion process. The latter is possible if both the temperature and, even more important, the frequency dependence of the diffusion-induced relaxation rates of actually polycrystalline materials is analyzed. Here we present some recent systematic relaxometry case studies using model systems that exhibit spatially restricted Li ion diffusion. Whenever possible we compare our results with data from other techniques as well as current relaxation models developed for 2D and 1D diffusion. As an example, 2D ionic motion has been verified for the hexagonal form of LiBH4; in the high-temperature limit the diffusion-induced 7Li NMR spin-lattice relaxation rates follow a logarithmic frequency dependence as is expected from models introduced for 2D diffusion. A similar behavior has been found for Li x NbS2. In Li12Si7 a quasi-1D diffusion process seems to be present that is characterized by a square root frequency dependence and a temperature behavior of the 7Li NMR spin-lattice relaxation rates as predicted. Most likely, parts of the Li ions diffuse along the Si5 rings that form chains in the Zintl phase.

Density Functional Theory Evaluated for Structural and Electronic Properties of 1T-Li x TiS2 and Lithium Ion Migration in 1T-Li0.94TiS2

In many applications it has been found that the standard generalized gradient approximation (GGA) does not accurately describe weak chemical bond and electronic properties of solids containing transition metals. In this work, we have considered the intercalation material 1T-Li x TiS2 (0≤x≤1) as a model system for the evaluation of the accuracy of GGA and corrected GGA with reference to the availabile experimental data. The influence of two different dispersion corrections (D3 and D-TS) and an on-site Coulomb repulsion term (GGA+U) on the calculated structural and electronic properties is tested. All calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional. An effective U value of 3.5 eV is used for titanium. The deviation of the calculated lattice parameter c for TiS2 from experiment is reduced from 14 % with standard PBE to −2 % with PBE+U and Grimme’s D3 dispersion correction. 1T-TiS2 has a metallic ground state at PBE level whereas PBE+U predicts an indirect gap of 0.19 eV in agreement with experiment. The 7Li chemical shift and quadrupole coupling constants are in reasonable agreement with the experimental data only for PBE+U-D3. An activation energy of 0.4 eV is calculated with PBE+U-D3 for lithium migration via a tetrahedral interstitial site. This result is closer to experimental values than the migration barriers previously obtained at LDA level. The proposed method PBE+U-D3 gives a reasonable description of structural and electronic properties of 1T-Li x TiS2 in the whole range 0≤x≤1.

Local Ion Dynamics in Polycrystalline β-LiGaO2: A Solid-State NMR Study

Solid-state nuclear magnetic resonance spectroscopy is an efficient technique to characterize dynamics and structure of materials. It has been widely used to elucidate ion dynamics in lithium ion conductors. Fast moving lithium ions are needed in energy storage devices, whereas slow ion motion is exploited in some materials used, for example, as blankets in fusion reactors. β-lithium gallium oxide (LiGaO2) is a slow Li+ ionic conductor similar to γ-lithium aluminum oxide (LiAlO2). In an ion conductor, in addition to the main diffusion process, localized motions (to-and-fro jumps) may be present. In the present work, with the help of solid-state NMR experiments, we report on the localized movements of Li+ ionic species in β-LiGaO2 in the temperature range between 300 K and 450 K. In this work, we have mainly extracted the peculiarities of ion dynamics from 7Li spin-alignment echo NMR measurements and the observation of the motional narrowing of the central transition signal of 7Li.

Diffusion Pathways and Activation Energies in Crystalline Lithium-Ion Conductors

Geometric information about ion migration (diffusion pathways) and knowledge about the associated energy landscape (migration activation barriers) are essential cornerstones for a comprehensive understanding of lithium transport in solids. Although many lithium-ion conductors are discussed, developed, and already used as energy-storage materials, fundamental knowledge is often still lacking. In this microreview, we give an introduction to the experimental and computational methods used in our subproject within the research unit FOR 1277, “Mobility of Lithium Ions in Solids (molife)”. These comprise, amongst others, neutron diffraction, topological analyses (procrystal-void analysis and Voronoi–Dirichlet partitioning), examination of scattering-length density maps reconstructed via maximum-entropy methods (MEM), analysis of probability-density functions (PDFs) and one-particle potentials (OPPs), as well as climbing-image nudged-elastic-band (cNEB) computations at density-functional theory (DFT) level. The results of our studies using these approaches on ternary lithium oxides and sulfides with different conduction characteristics (fast/slow) and dimensionalities (one-/two-/three-dimensional) are summarized, focusing on the close orbit of the research unit. Not only did the investigations elucidate the lithium-diffusion pathways and migration activation energies in the studied compounds, but we also established a versatile set of methods for the evaluation of data of differing quality.

On the Mechanisms of Chemical Intercalation of Lithium in Electrode Materials

Reaction with n-butyl lithium (BuLi) is commonly used to study the kinetics of intercalation of lithium in electrode materials for batteries. We performed lithium isotope exchange experiments on TiS2 single crystals as model system to determine the irreversible step in the intercalation process. Single crystals of TiS2 were prepared by chemical vapor transport and intercalated by lithium with natural isotopic signature (7% 6Li 93% 7Li) using a 2.5 molar solution of BuLi in hexane. Crystals were homogenized at 313 K in a dry argon atmosphere for 1–2 months and then exposed for several days to ~1 molar BuLi solution enriched in the light lithium isotope (95% 6Li, 5% 7Li). After the isotopic exchange experiments concentration profiles of the isotopes were measured parallel to the a/b-plane of the crystal using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Profiles show that already intercalated lithium isotopes can be completely exchanged without significant increase in total lithium content. This observation implies that, after adsorption of BuLi at the crystal surface, the butyl radical can freely jump from one lithium atom to a neighbouring one. The jump sequence is either finished by desorption of a BuLi molecule or by combination of two adjacent butyl radicals forming an octane molecule or other kinds of deactivation of butyl radicals. The latter step is irreversible. Self diffusion coefficients determined by fitting of the lithium isotope profiles are in good agreement with chemical diffusivities determined by lithium intercalation experiments.

Nanostructured Ceramics: Ionic Transport and Electrochemical Activity

Ceramics with nm-sized dimensions are widely used in various applications such as batteries, fuel cells or sensors. Their oftentimes superior electrochemical properties as well as their capabilities to easily conduct ions are, however, not completely understood. Depending on the method chosen to prepare the materials, nanostructured ceramics may be equipped with a large area fraction of interfacial regions that exhibit structural disorder. Elucidating the relationship between microscopic disorder and ion dynamics as well as electrochemical performance is necessary to develop new functionalized materials. Here, we highlight some of the very recent studies on ion transport and electrochemical properties of nanostructured ceramics. Emphasis is put on TiO

High-Resolution Tracking Asymmetric Lithium Insertion and Extraction and Local Structure Ordering in SnS2

In the rechargeable lithium ion batteries, the rate capability and energy efficiency are largely governed by the lithium ion transport dynamics and phase transition pathways in electrodes. Real-time and atomic-scale tracking of fully reversible lithium insertion and extraction processes in electrodes, which would ultimately lead to mechanistic understanding of how the electrodes function and why they fail, is highly desirable but very challenging. Here, we track lithium insertion and extraction in the van der Waals interactions dominated SnS2 by in situ high-resolution TEM method. We find that the lithium insertion occurs via a fast two-phase reaction to form expanded and defective LiSnS2, while the lithium extraction initially involves heterogeneous nucleation of intermediate superstructure Li0.5SnS2 domains with a 1-4 nm size. Density functional theory calculations indicate that the Li0.5SnS2 is kinetically favored and structurally stable. The asymmetric reaction pathways may supply enlightening insights into the mechanistic understanding of the underlying electrochemistry in the layered electrode materials and also suggest possible alternatives to the accepted explanation of the origins of voltage hysteresis in the intercalation electrode materials.

High-Rate Intercalation without Nanostructuring in Metastable Nb2O5 Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb2O5 are inherent properties of the bulk crystal structure. Their unique "room-and-pillar" NbO6/NbO7 framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g(-1)), while the higher temperature (Wadsley-Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g(-1)) but only at moderate rates. High-resolution (6/7)Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb2O5 revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)-98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb2O5 synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs.

Correlated fluorine diffusion and ionic conduction in the nanocrystalline F(-) solid electrolyte Ba(0.6)La(0.4)F(2.4)-(19)F T1(ρ) NMR relaxation vs. conductivity measurements

Chemical reactions induced by mechanical treatment may give access to new compounds whose properties are governed by chemical metastability, defects introduced and the size effects present. Their interplay may lead to nanocrystalline ceramics with enhanced transport properties being useful to act as solid electrolytes. Here, the introduction of large amounts of La into the cubic structure of BaF2 served as such an example. The ion transport properties in terms of dc-conductivity values of the F(-) anion conductor Ba1-xLaxF2+x (here with x = 0.4) considerably exceed those of pure, nanocrystalline BaF2. So far, there is only little knowledge about activation energies and jump rates of the elementary hopping processes. Here, we took advantage of both impedance spectroscopy and (19)F NMR relaxometry to get to the bottom of ion jump diffusion proceeding on short-range and long-range length scales in Ba0.6La0.4F2.4. While macroscopic transport is governed by an activation energy of 0.55 to 0.59 eV, the elementary steps of hopping seen by NMR are characterised by much smaller activation energies. Fortunately, we were able to deduce an F(-) self-diffusion coefficient by the application of spin-locking NMR relaxometry.

Li ion dynamics along the inner surfaces of layer-structured 2H-LixNbS2

Layer-structured materials, such as graphite (LiCy) or Lix(Co,Ni,Mn)O2, are important electrode materials in current battery research that still relies on insertion materials. This is due to their excellent ability to reversibly accommodate small alkali ions such as Li(+) and Na(+). Despite of these applications, microscopic information on Li ion self-diffusion in transition metal sulfides are relatively rare. Here, we used (7)Li nuclear magnetic resonance (NMR) spectroscopy to study translational Li ion diffusion in hexagonal (2H) LixNbS2 (x = 0.3, 0.7, and 1) by means of variable-temperature NMR relaxometry. (7)Li spin-lattice relaxation rates and (7)Li NMR spectra were used to determine Li jump rates and activation barriers as a function of Li content. Hereby, NMR spin-lattice relaxation rates recorded with the spin-lock technique offered the possibility to study Li ion dynamics on both the short-range and long-range length scale. Information was extracted from complete diffusion-induced rate peaks that are obtained when the relaxation rate is plotted vs inverse temperature. The peak maximum of the three samples studied shifts toward higher temperatures with increasing Li content x in 2H-LixNbS2. Information on the dimensionality of the diffusion process was experimentally obtained by frequency dependent Rρ measurements carried out at T = 444 K, that is in the high-temperature regime of the rate peaks. A slight, but measurable frequency-dependence within this limit is found for all samples; it is in good agreement with predictions from relaxation models developed to approximate low-dimensional (2D) jump diffusion.

Interpretation of 1H and 2H spin-lattice relaxation dispersions: insights from molecular dynamics simulations of polymer melts

We demonstrate that molecular dynamics simulations are a versatile tool to ascertain the interpretation of spin-lattice relaxation data. For (1)H, our simulation approach allows us to separate and to compare intra- and inter-molecular contributions to spin-lattice relaxation dispersions. Dealing with the important example of polymer melts, we show that the intramolecular parts of (1)H spectral densities and correlation functions are governed by rotational motion, while their inter-molecular counterparts provide access to translational motion, in particular, to mean-squared displacements and self-diffusion coefficients. Exploiting that the full microscopic information is available from molecular dynamics simulations, we determine the range of validity of experimental approaches, which often assume Gaussian dynamics, and we provide guidelines for the determination of free parameters required in experimental analyses. For (2)H, we examine the traditional methodology to extract correlation times of complex dynamics from relaxation data. Furthermore, based on knowledge from our computational study, it is shown that measurement of (2)H spin-lattice relaxation dispersions allows one to disentangle the intra- and inter-molecular contributions to the corresponding (1)H data in experimental work. Altogether, our simulation results yield a solid basis for future (1)H and (2)H spin-lattice relaxation analysis.

Understanding Li diffusion in Li-intercalation compounds

Intercalation compounds, used as electrodes in Li-ion batteries, are a fascinating class of materials that exhibit a wide variety of electronic, crystallographic, thermodynamic, and kinetic properties. With open structures that allow for the easy insertion and removal of Li ions, the properties of these materials strongly depend on the interplay of the host chemistry and crystal structure, the Li concentration, and electrode particle morphology. The large variations in Li concentration within electrodes during each charge and discharge cycle of a Li battery are often accompanied by phase transformations. These transformations include order-disorder transitions, two-phase reactions that require the passage of an interface through the electrode particles, and structural phase transitions, in which the host undergoes a crystallographic change. Although the chemistry of an electrode material determines the voltage range in which it is electrochemically active, the crystal structure of the compound often plays a crucial role in determining the shape of the voltage profile as a function of Li concentration. While the relationship between the voltage profile and crystal structure of transition metal oxide and sulfide intercalation compounds is well characterized, far less is known about the kinetic behavior of these materials. For example, because these processes are especially difficult to isolate experimentally, solid-state Li diffusion, phase transformation mechanisms, and interface reactions remain poorly understood. In this respect, first-principles statistical mechanical approaches can elucidate the effect of chemistry and crystal structure on kinetic properties. In this Account, we review the key factors that govern Li diffusion in intercalation compounds and illustrate how the complexity of Li diffusion mechanisms correlates with the crystal structure of the compound. A variety of important diffusion mechanisms and associated migration barriers are sensitive to the overall Li concentration, resulting in diffusion coefficients that can vary by several orders of magnitude with changes in the lithium content. Vacancy clusters, groupings of vacancies within the crystal lattice, provide a common mechanism that mediates Li diffusion in important intercalation compounds. This mechanism emerges from specific crystallographic features of the host and results in a strong decrease of the Li diffusion coefficient as Li is added to an already Li rich host. Other crystallographic and electronic factors, such as the proximity of transition metal ions to activated states of hops and the occurrence of electronically induced distortions, can result in a strong dependence of the Li mobility on the overall Li concentration. The insights obtained from fundamental studies of ionic diffusion in electrode materials will be instrumental for physical chemists, chemical engineers, synthetic chemists, and materials and device designers who are developing these technologies.

Two-dimensional diffusion in Li0.7NbS2 as directly probed by frequency-dependent 7Li NMR

Li ion diffusion in layer-structured Li0.7NbS2 has been complementary investigated by nuclear magnetic resonance (NMR) spectroscopy from an atomic scale point of view. In the present case, (7)Li NMR spin-lattice relaxation (SLR) rates R1ρ probed in the rotating frame of reference proved very informative in characterizing the Li self-diffusion process in the van der Waals gap between the NbS2 layers. While temperature-variable SLRρ measurements were used to determine dynamic parameters such as jump rates (τ(-1)) and the activation energy (Ea), frequency-dependent measurements were used to specify the dimensionality of the diffusion process. In particular, the effect of annealing, i.e., the distribution of Li ions between the layers, on overall Li dynamics has been studied. When plotted in an Arrhenius diagram, the R1ρ rates of an annealed sample, which were recorded at a locking frequency of 20 kHz, pass through a diffusion-induced relaxation peak whose maximum shows up at 320 K. Employing an appropriate diffusion model and appropriately accounting for a non-diffusive background relaxation, a Li jump rate τ(-1)(300 K) ≈ 1.3 × 10(5) s(-1) and an activation energy Ea of 0.43(2) eV can be deduced. Most importantly, in the high-T limit of the diffusion-induced rate peak, i.e., when ω1τ << 1 holds, the rates follow a logarithmic frequency dependence. This points to a diffusion process of low dimensionality and is in good agreement with predictions of relaxation models developed for 2D diffusion.

Combining 7Li NMR field-cycling relaxometry and stimulated-echo experiments: a powerful approach to lithium ion dynamics in solid-state electrolytes

We use (7)Li NMR to study lithium ion dynamics in a (Li2S)-(P2S5) glass. In particular, it is shown that a combination of (7)Li field-cycling relaxometry and (7)Li stimulated-echo experiments allows us to cover a time window extending over 10 orders of magnitude without any gaps. While the (7)Li stimulated-echo method proved suitable to measure correlation functions F2(t) of lithium ion dynamics in solids in recent years, we establish the (7)Li field-cycling technique as a versatile tool to ascertain the spectral density J2(ω) of the lithium ionic motion in this contribution. It is found that the dynamic range of (7)Li field-cycling relaxometry is 10(-9)-10(-5)s and, hence, it complements in an ideal way that of (7)Li stimulated-echo experiments, which amounts to 10(-5)-10(1)s. Transformations between time and frequency domains reveal that the field-cycling and stimulated-echo approaches yield results for the translational motion of the lithium ions that are consistent both with each other and with findings for the motional narrowing of (7)Li NMR spectra of the studied (Li2S)-(P2S5) glass. In the (7)Li field-cycling studies of the (Li2S)-(P2S5) glass, we observe the translational ionic motion at higher temperatures and the nearly constant loss at lower temperatures. For the former motion, the frequency dependence of the measured spectral density is well described by a Cole-Davidson function. For the latter phenomenon, which was considered as an universal phenomenon of disordered solids in the literature, we find an exponential temperature dependence.

Li Diffusion in (110) Oriented LiNbO3 Single Crystals

Li diffusion is investigated in Li2O-deficient, (110) oriented LiNbO3single crystals in the temperature range between 523 and 673 K by secondary ion mass spectrometry. A thin layer of ion-beam sputtered isotope enriched6LiNbO3was used as a tracer source, which allows one to study pure isotope interdiffusion. The diffusivities coincide with those of (001) oriented single crystals and follow the Arrhenius law with an activation enthalpy of 1.33 eV. The results prove the existence of a three-dimensional diffusion mechanism.

Extremely slow Li ion dynamics in monoclinic Li2TiO3--probing macroscopic jump diffusion via 7Li NMR stimulated echoes

A thorough understanding of ion dynamics in solids, which is a vital topic in modern materials and energy research, requires the investigation of diffusion properties on a preferably large dynamic range by complementary techniques. Here, a polycrystalline sample of Li(2)TiO(3) was used as a model substance to study Li motion by both (7)Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) and ac-conductivity measurements. Although the two methods do probe Li dynamics in quite different ways, good agreement was found so that the Li diffusion parameters, such as jump rates and the activation energy, could be precisely determined over a dynamic range of approximately eleven decades. For example, Li solid-state diffusion coefficients D(σ) deduced from impedance spectroscopy range from 10(-23) m(2) s(-1) to 10(-12) m(2) s(-1) (240-835 K). These values are in perfect agreement with the coefficients D(SAE) deduced from SAE NMR spectroscopy. As an example, D(SAE) = 2 × 10(-17) m(2) s(-1) at 433 K and the corresponding activation energy determined by NMR amounts to 0.77(2) eV (400-600 K). At room temperature D(σ) takes a value of 3 × 10(-21) m(2) s(-1).

Kinetics of Lithium Intercalation in Titanium Disulfide Single Crystals

Single crystals of titanium disulfide TiS2 were synthesized by chemical vapor synthesis and subsequently intercalated with n-butyl lithium (BuLi) in n-hexane. Experiments were carried out using a butyl lithium concentration between 0.8 and 10 mol L-1 and the temperature range was from 248 K to 328 K. The duration of the intercalation was varied from 2 h to 30 d. After the intercalation experiments concentration profiles of lithium, titanium and sulfur were measured parallel to the ab-plane of the crystal by LA-ICP-OES (LASER Ablation — Inductively Coupled Plasma - Optical Emission Spectroscopy). Chemical diffusion coefficients (D) were determined by fitting of the profiles to the specific solution of Fick's 2 nd law for the given boundary conditions.

The measured diffusivity in the ab-plane (D|| a/b ) varies between 10-13 and 10-15 m2 s-1 at room temperature. These variations of D cannot be correlated to the reaction time. On the other hand a systematic increase of D|| a/b with the concentration of butyl lithium (cBuLi ) was observed, with a corresponding decrease in activation energy from 59.6 ± 7.6 kJ mol-1 (cBuLi=1.6 mol L-1 ) to 42.6 ± 11.7 kJ mol-1 (cBuLi=10 mol L-1 ). Furthermore, profiles measured on the same crystals reveal D|| a/b values differing by up to a factor of 3. ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectromety) images gave evidence of inhomogenous insertion of lithium along crystal edges.

These findings indicate that stress induced by widening of the crystal layers plays a crucial role in the intercalation kinetics. SIMS profiling perpendicular to the ab-plane gives evidence that D⊥ a/b is at least four orders of magnitude lower than D|| a/b .

Theoretical Investigation of Migration Pathways for Li Diffusion in h-LiTiS 2

Lithium diffusion in highly lithiated hexagonal titanium disulphide (h-LixTiS2, x=0.88, 1.0) is investigated theoretically with periodic quantum-chemical methods. The calculated lithiation energies confirm that Li preferentially occupies the octahedral site rather than the tetrahedral site. Surprisingly, uncorrected density-functional theory (DFT) methods give better agreement with experiment for the structural parameters than the dispersion-corrected DFT-D approaches. Among the considered point defects, VLi , VTi , and Tii, Li point defects are thermodynamically preferred in h-LixTiS2. A moderate relaxation is observed for the atoms surrounding the Li defect or Ti defect site, whereas a pronounced relaxation of the nearest neighboring atoms of a Ti Frenkel defect occurs. Competing pathways for Li diffusion in h-LixTiS2 are investigated using the climbing-image Nudged-Elastic-Band (cNEB) approach. Li + ions can migrate within the crystallographic ab plane either in a direct pathway through shared edges of neighboring octahedra or via vacant tetrahedral sites. The possibility of three-dimensional Li + diffusion along the c direction is investigated via inclusion of Ti point defects and Ti Frenkel defects.

Studying Li Dynamics in a Gas-Phase Synthesized Amorphous Oxide by NMR and Impedance Spectroscopy

Li diffusion parameters of a structurally disordered Li-Al-Si-oxide prepared by gas-phase synthesis were complementarily investigated by both time-domain NMR techniques and impedance spectroscopy. The first include 7Li NMR spin-lattice relaxation (SLR) measurements in the laboratory as well as in the rotating frame of reference. An analysis of variable-temperature NMR line widths point to an activation energy Ea of approximately 0.6 eV. The value is confirmed by rotating-frame SLR NMR data recorded at approximately 11 kHz. Above room temperature the low-temperature flank of a diffusion-induced rate peak shows up which can be approximated by an Arrhenius law yielding Ea=0.56(1) eV. This is in very good agreement with the result obtained from 7Li spin-alignment echo (SAE) NMR being sensitive to even slower Li dynamics. For comparison, dc-conductivity measurements, probing long-range motions, yield Ea=0.8 eV. Interestingly, low-temperature SAE NMR decay rates point to localized Li motions being characterized with a very small activation energy of only 0.09 eV.

NMR relaxometry as a versatile tool to study Li ion dynamics in potential battery materials

NMR spin relaxometry is known to be a powerful tool for the investigation of Li(+) dynamics in (non-paramagnetic) crystalline and amorphous solids. As long as significant structural changes are absent in a relatively wide temperature range, with NMR spin-lattice (as well as spin-spin) relaxation measurements information on Li self-diffusion parameters such as jump rates and activation energies are accessible. Diffusion-induced NMR relaxation rates are governed by a motional correlation function describing the ion dynamics present. Besides the mean correlation rate of the dynamic process, the motional correlation function (i) reflects deviations from random motion (so-called correlation effects) and (ii) gives insights into the dimensionality of the hopping process. In favorable cases, i.e., when temperature- and frequency-dependent NMR relaxation rates are available over a large dynamic range, NMR spin relaxometry is able to provide a comprehensive picture of the relevant Li dynamic processes. In the present contribution, we exemplarily present two recent variable-temperature (7)Li NMR spin-lattice relaxation studies focussing on Li(+) dynamics in crystalline ion conductors which are of relevance for battery applications, viz. Li(7) La(3)Zr(2)O(12) and Li(12)Si(7).

Spin-alignment echo NMR: probing Li+ hopping motion in the solid electrolyte Li7La3Zr2O12 with garnet-type tetragonal structure

(7)Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) spectroscopy has been used to measure single-spin hopping correlation functions of polycrystalline Li(7)La(3)Zr(2)O(12). Damping of the echo amplitude S(2)(t(m),t(p)), recorded at variable mixing time t(m) but fixed preparation time t(p), turns out to be solely controlled by slow Li jump processes taking place in the garnet-like structure. The decay rates τ(SAE)(-1) directly obtained by parametrizing the curves S(2)(t(m),t(p)) with stretched exponential functions show Arrhenius behaviour pointing to an activation energy of approximately 0.5 eV. This value, probed by employing an atomic-scale NMR method, is in very good agreement with that deduced from impedance spectroscopy used to measure macroscopic Li transport parameters. Most likely, the two methods are sensitive to the same hopping correlation function although Li dynamics are probed in a quite different manner.

Ultra-slow Li ion dynamics in Li(2)C(2)--on the similarities of results from (7)Li spin-alignment echo NMR and impedance spectroscopy

Li diffusion and transport parameters of binary lithium carbide Li(2)C(2) were complementarily investigated by (7)Li (nuclear magnetic resonance) NMR and impedance spectroscopy. Long-range Li diffusion parameters were measured by using mixing-time-dependent and temperature-variable stimulated echo NMR spectroscopy. The method is sensitive to ultra-slow Li hopping processes which were probed from an atomic-scale point of view. Two-time phase correlation functions S(2) obtained can be parameterized by stretched exponentials only. The corresponding echo decay rates τ(-1), which were recorded at a resonance frequency of e.g. 155.5 MHz, show Arrhenius behaviour revealing an activation energy of 0.80(2) eV. This value is in very good agreement with that deduced from dc conductivity measurements (0.79(2) eV) probing Li transport processes on a macroscopic length scale. The comparison of impedance data with the measured NMR echo decay functions showed that both methods reflect diffusion processes being characterized by very similar motional correlation functions.

Lithium ionic jump motion in the fast solid ion conductor Li(5)La(3)Nb(2)O(12)

Using (7)Li NMR line-shape analysis, spin-lattice relaxation measurements and stimulated-echo spectroscopy, we investigate the lithium ionic jump motion in the garnet Li(5)La(3)Nb(2)O(12). Results for two samples are compared, which were annealed at 850( composite function)C (GR-850) and at 900( composite function)C (GR-900), respectively. All (7)Li NMR data consistently show that two lithium species with distinguishable dynamical behaviors coexist in each of the samples. While the less mobile species is the majority component in GR-850, the more mobile species is the majority component in GR-900. (7)Li NMR stimulated-echo spectroscopy provides straightforward access to the correlation functions describing the jumps of the respective majority component in both samples. From the temperature-dependent correlation times, we obtain activation energies of 56 and 32kJmol(-1) for GR-850 and GR-900, respectively. For both samples, the correlation functions substantially deviate from simple exponential behavior, indicating a high complexity of the lithium ionic motion in Li(5)La(3)Nb(2)O(12).

Microscopic Access to Long-Range Diffusion Parameters of the Fast Lithium Ion Conductor Li7BiO6 by Solid State 7Li Stimulated Echo NMR

Li self-diffusion in rhombohedral Li7BiO6, being a promising basic material for cathodes of rechargeable ion batteries, is studied by means of 7Li stimulated echo NMR. Using the pulse sequence introduced by Jeener and Broekaert, a spin-alignment echo is created whose amplitude decay is recorded as a function of mixing time. The so-obtained two-time correlation functions follow stretched exponential behavior and lead to decay rates which can be identified directly with microscopic Li motional correlation rates (tau(-1)). Using a jump distance of about 0.2 nm, this results in a diffusion coefficient (D) of about 0.5 x 10(-16) m(2) s(-1) at 294 K. The activation energy turned out to be 0.53(3) eV which is in very good agreement with recently obtained results by means of dc-conductivity measurements probing long-range diffusion parameters. This shows that stimulated echo NMR, due to its inherent time scale, gives microscopic access to long-range transport. The prefactor tau(0)(-1) of the corresponding Arrhenius law lies in the typical range of phonon frequencies, tau(0)(-1) = 3 x 10(12) s(-1).

Ultraslow Li diffusion in spinel-type structured Li4Ti5O12 - a comparison of results from solid state NMR and impedance spectroscopy

The cubic spinel oxides Li(1+x)Ti(2-x)O(4) (0 < or =x< or = 1/3) are promising anode materials for lithium-ion rechargeable batteries. The end member of the Li-Ti-O series, Li(4)Ti(5)O(12), can accommodate Li ions up to the composition Li(7)Ti(5)O(12). Whereas a number of studies focus on the electrochemical behaviour of Li insertion into and Li diffusion in the Li intercalated material, only few investigations about low-temperature Li dynamics in the non-intercalated host material Li(4)Ti(5)O(12) have been reported so far. In the present paper, Li diffusion in pure-phase microcrystalline Li(4)Ti(5)O(12) with an average particle size in the microm range was probed by (7)Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin-lattice relaxation (SLR) measurements. Between T = 295 K and 400 K extremely slow Li jump rates tau(-1) ranging from 1 s(-1) to about 2200 s(-1) were directly measured by recording the decay of spin-alignment echoes as a function of mixing time and constant evolution time. The results point out the slow Li diffusion in non-intercalated Li(4)Ti(5)O(12) x tau(-1) (1/T) follows Arrhenius behaviour with an activation energy E(ASAE) of about 0.86 eV. Interestingly, E(ASAE) is comparable to activation energies deduced from conductivity measurements (0.94(1) eV) and from SLR measurements in the rotating frame (0.74(2) eV) rather than from those performed in the laboratory frame, E(A)(low-T) = 0.26(1) eV at low T.