Lithium-containing thiophosphates represent promising ceramic electrolytes for all-solid-state batteries. The underlying principles that cause high Li+ diffusivity are, however, still incompletely understood. Here, beta-Li3PS4 served as a model compound to test the recently presented hypothesis that a channel-like Li+ diffusion pathway influences ionic transport in its 3D network of the LiS4, LiS6, and PS4 polyhedra. We looked at the temperature dependence of diffusion-induced Li-7 nuclear spin lattice relaxation rates to check whether they reveal any diagnostic differences as compared to the nuclear spin response frequently found for isotropic (3D) diffusion. Indeed, distinct anomalies show up that can be understood if we consider the influence of low-dimensional diffusion. Hence, even for isotropic materials without clearly recognizable 1D or 2D diffusion pathways, such as layered or channel-structured materials, structurally hidden dimensionality effects might help explain high ionic conductivities and refine the design principles currently discussed. In the present case, such rapid pathways assist the ions to move through the crystal structure.

Abstract 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 LixNbS2. 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.

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 electro-chemical 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 TiO2 in the form of nanorods, nanotubes or being present as mesoporous material. Further examples deal with nanocrystalline peroxides such as Li2O2 or nanostructured oxides (Li2TiO3, LiAlO2, LiTaO3, Li2CO3 and Li2B4O7). These materials served as model systems to explore the influence of ball-milling on overall ionic transport.

All-solid-state batteries with ceramic electrolytes and lithium metal anodes represent an attractive alternative to conventional ion battery systems. Conventional batteries still rely on flammable liquids as electronic insulators. Despite the great efforts reported over the last years, the optimum solid electrolyte has, however, not been found yet. One of the most important properties which decides whether a ceramic is useful to work as electrolyte is ionic transport. The various time-domain nuclear magnetic resonance (NMR) techniques might help characterize and select the most suitable candidates. Together with conductivity measurements it is possible to analyze ion dynamics on different length-scales, i.e., to differentiate between local, within-site hopping processes from long-range ion transport. The latter needs to be sufficiently fast in the ceramic, in the best case competing with that of liquid electrolytes. In addition to conductivity spectroscopy, NMR can help understand the relationship between local structure and dynamic parameters. Besides information on activation energies and jump rates the data also contain suggestions about the relevant elementary steps of ion hopping and, thus, diffusion pathways through the crystal lattice. Recent progress in characterizing ion dynamics in ceramic electrolytes by NMR relaxometry will be briefly reviewed. Focus is put on presently discussed solid electrolytes such as garnets, phosphates and sulfides, which have so far been studied in our lab.

Various time-domain nuclear magnetic resonance (NMR) techniques are highly useful to study Li-ion dynamics in solid electrolytes and electrode materials from an atomic-scale point of view. In particular, by combining lithium NMR techniques being sensitive to Li jump processes on different length scales and time scales detailed information on activation energies, jump rates and diffusion pathways can be collected. This helps understand the relationship between local structure and dynamic parameters and assists in developing new materials for solid-state energy storage. In this chapter, we will review recent examples where NMR relaxation has been successfully applied to both highly conducting solid electrolytes and selected anode materials.

Lithium aluminium titanium phosphate (LATP) belongs to the class of materials exhibiting very high Li+ ionic conductivities. While many impedance studies can be found in literature that report on overall ion conductivities, a discrimination of bulk and grain boundary electrical responses via conductivity spectroscopy has rarely been reported so far. Here, we took advantage of impedance measurements that were carried out at low temperatures to separate bulk contributions from the grain boundary response. It turned out that bulk ion conductivity is by at least three orders of magnitude higher than ion transport across the grain boundary regions. At temperatures well below ambient long-range Li ion dynamics is governed by activation energies ranging from 0.26 to 0.29 eV, depending on the sintering conditions. As an example, at temperatures as low as 173 K, the bulk ion conductivity, measured in N2 inert gas atmosphere, is in the order of 8.1 x 10−6 S cm−1. Extrapolating this value to room temperature yields ca. 3.4 x 10−3 S cm−1 at 293 K. Interestingly, exposing the dense impedance pellets to air atmosphere over a long period of time causes a significant decrease of bulk ion transport. The effect can be reversed if the phosphate is calcined at elevated temperatures again.

The attractive long-term stability and safety of so-called all-solid-state batteries has given new impetus to developing powerful solid Li+ electrolytes. Post Li-ion batteries equipped with solid electrolytes may also be very attractive for hybrid electric vehicles as these systems are expected to withstand high operation temperatures. Moreover, they may serve as attractive components in Li-oxygen and Li-sulfur batteries. In addition to the various oxides and phosphates studied so far, see, e.g., refs. [1,2], sulfide-based materials have attracted particular attention due to their very high ionic conductivities [3,4]. Li3PS4 represents such a promising candidate, since its achievable Li-ion conductivity is comparable to that of liquid electrolytes. Phase-pure Li3PS4 was prepared by joint high-energy ball-milling of Li2S and P2S5 for 35 hours and subsequent heat treatment at 260 °C for 17 hours. X-ray diffraction analysis corroborated the phase purity of the sample prepared. To study the elementary steps of ion hopping in Li3PS4 7Li solid-state nuclear magnetic resonance (NMR) spectroscopy was employed. In particular, we used temperature-variable relaxometry measurements carried out in both the laboratory and rotating frame of reference to shed light on local as well as long-range ion dynamics. Line shape measurements revealed fast Li-ion exchange able to completely average homonuclear dipole-dipole interactions at temperatures much below ambient. This perfectly agrees with a recent NMR study focusing on field gradient experiments [5]. Moreover, it is in line with our spin-lattice relaxation (SLR) measurements performed at low temperatures: From the low temperature flank of the diffusion-induced SLR rates an activation energy as low as 0.1 eV was determined. Such a low value indicates fast, localized Li jumps present in Li3PS4. The high Li+ diffusivity also manifests itself in the appearance of an SLR spin-lock NMR rate peak that shows up well below room temperature. [1] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lottnyk, V. Duppel, L. Kienle, J. Janek, Phys. Chem. Chem. Phys. 13 (2011) 19378 [2] V. Epp, Q. Ma, E.-M. Hammer, F. Tietz, M. Wilkening, Phys. Chem. Chem. Phys., 17 (2015) 32115 [3] M. Tatsumisago, M. Nagao, A. Hayashi, J. Asian Ceram. Soc., 1 (2013) 17 [4] V. Epp, O. Gün, H.-J. Deiseroth, M. Wilkening, J. Phys. Chem. Lett., 4 (2013) 2118 [5] K. Hayamizu, Y. Aihara, T. Watanabe, T. Yamada, S. Ito, N. Machida, Solid State Ion., DOI: 10.1016/j.ssi.2015.06.016

Lithium aluminium titanium phosphate (LATP) belongs to one of the most promising solid electrolytes. Besides sufficiently high electrochemical stability, its use in lithium-based all-solid-state batteries crucially depends on the ionic transport properties. While many impedance studies can be found in literature that report on overall ion conductivities, a discrimination of bulk and grain boundary electrical responses via conductivity spectroscopy has rarely been reported so far. Here, we took advantage of impedance measurements that were carried out at low temperatures to separate bulk contributions from the grain boundary responses. It turned out that bulk ion conductivity is by at least three orders of magnitude higher than ion transport across the grain boundary regions. At temperatures well below ambient long-range Li ion dynamics is governed by activation energies ranging from 0.26 to 0.29 eV depending on the sintering conditions. As an example, at temperatures as low as 173 K, the bulk ion conductivity, measured in N2 inert gas atmosphere, is in the order of 8.1 × 10−6 S cm−1. Extrapolating this value to room temperature yields ca. 3.4 × 10−3 S cm−1 at 293 K. Interestingly, exposing the dense pellets to air atmosphere over a long period of time causes a significant decrease of bulk ion transport. This process can be reversed if the phosphate is calcined at elevated temperatures again.

In many cases fast solid ion conductors are characterized by a large number fraction of defects and vacant positions that enable the ions to move over long distances in a facile way. The introduction of structural disorder via high‐energy mechanical impact represents a very promising possibility to improve and to tune the transport properties of otherwise poorly conducting solids. Lithium tetraborate, Li2B4O7, in its single crystalline form or with an average crystallite size in the μm range, is known as a very poor Li ion conductor and can serve as a model compound to study the influence of structural disorder on ion dynamics. In the present study, we used high‐energy ball milling to prepare nanocrystalline defect‐rich Li2B4O7 characterized by a mean crystallite diameter of ca. 20 nm. With increasing milling time the sample became partly amorphous. Polycrystalline Li2B4O7 with crystallite sizes in the order of 100 nm served as starting material. The nanostructured samples obtained show dc conductivities σdc in the order of 2.5 × 10−7 S/cm at 490 K which represents an increase by more than four orders of magnitude compared to the source material. While conductivity spectroscopy was applied to study the effect of different milling times on ionic conductivity in detail; Li ion self‐diffusion in nanostructured Li2B4O7 as well as in the starting material was investigated by variable‐temperature solid‐state 7Li nuclear magnetic resonance (NMR) relaxometry. While the first is sensitive to long‐range ion transport, lithium NMR is able to access also short‐ranged ion motions.

Silicon is one of the most promising anode materials for lithium-based rechargeable batteries. Provided the volume changes during Li uptake can be brought under control, Li ion diffusivity is expected to crucially determine the performance of such energy storage systems. Therefore, studying diffusion properties in amorphous Li-Si underpins applied research that is being directed towards the development of powerful storage devices. So far, only little information is available on Li self-diffusion in amorphous Si. Here, we used 7 Li NMR spectroscopy [1] to precisely quantify microscopic activation energies and Li jump rates in amorphous Li-Si which is primarily formed if monocrystalline Si is lithiated electrochemically. Our results reveal relatively fast Li diffusivity with an average activation energy for long-range ion transport as high as ca. 0.65 eV; jump rates turn out to be in the order of 2.5 × 10 5 1/s at 246 K, see also ref. [2]. Comparisons with data from laboratory frame NMR relaxometry, which is sensitive to more localized ion hopping, points to complex dynamics that is most likely governed by non-exponential motional correlation functions originating from a large distribution of activation energies. Noteworthy, a second sample, which is a mixture of amorphous Li-Si and metastable, crystalline Li 15 Si 4 that forms at lower discharge potentials, points to slightly enhanced Li diffusivity (0.51 eV). Interestingly, first high-resolution 6 Li MAS NMR experiments on X-ray amorphous Li-Si, which have been carried out using isotope enriched samples, indicate the presence of structural motifs that resemble those of Li 12 Si 7 . The data obtained might help optimizing Li-based silicon batteries whose performance critically depend on fast Li-ion transport. Acknowledgement Financial support by the Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged. References [1] V. Epp, M. Wilkening, Phys. Rev. B 82 (2010) 020301. [2] A. Kuhn, P. Sreeraj, R. Pöttgen, H.-D. Wiemhöfer, M. Wilkening, P. Heitjans, J. Am. Chem. Soc. 113 (2011) 11018.

Fast ion conductors play one of the most important roles in solid state ionics as there is a great demand for their application in safe and powerful electrochemical energy storage systems. For such materials, it is known that the synthesis conditions may have significant impact on the final properties of the materials prepared. In this contribution, we made use of mechanosynthesis, carried out via high-energy ball milling, to influence the ionic transport parameters of tetragonal, i.e., layer-structured, BaSnF4. X-ray powder diffraction (XRD) revealed that mechanical treatment of the binary fluorides BaF2 and SnF2 leads to a powder pointing to a nanocrystalline fluoride with (distorted) cubic symmetry. Differential scanning calorimetry (DSC) as well as preliminary in situ XRD measurements were used to follow the transformation towards the tetragonal modification with the composition BaSnF4. Broadband impedance spectroscopy was used to measure the overall electrical conductivity of the ternary fluoride. Remarkably, the layered form shows a room temperature conductivity of 7 x 10(-4) S cm(-1). Further emphasis was put on the characterization of the dielectric properties of the material, which was investigated by using different electrode materials to distinguish artefacts from intrinsic properties. Since we found a strong dependence of the real part of the permittivity on the electrode materials applied (carbon paste or sputtered Pt), we tend to assign the huge increase in permittivity, which was recently interpreted as giant dielectric constant, to interfacial polarization effects rather than to intrinsic properties. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7Li NMR relaxation has been used to study lithium-ion diffusion in layer-structured SnS2. Keeping the Li intercalation degree in LixSnS2 below x=0.49, the Li ions preferentially occupy sites in the van der Waals gap between the SnS2 sheets. In contrast to conventional NMR spin-lattice relaxation (SLR) rate measurements in the laboratory frame of reference, which are sensitive to rather fast Li exchange processes, with the help of spin-locking SLR NMR slower Li motions were extracted from characteristic diffusion-induced rate peaks. The latter contain information on both Li+ activation energies Ea and Li ion jump rates τ−1 characterizing the elementary steps of Li+ hopping. Our results point to two different diffusion processes (Ea(I)=0.38eV; Ea(II)=0.28eV), a slower and a faster one, observable directly after chemical Li insertion. Interestingly, the diffusion behaviour irreversibly changes when the sample has been exposed to temperatures as high as 573K. Diffusion-induced NMR rates and corresponding line shapes are discussed with respect to an inhomogenous distribution of Li ions in SnS2, which seems to be present directly after Li intercalation.

Silicon is one of the most promising anode materials for lithium-based rechargeable batteries. Provided the volume changes during Li uptake can be brought under control, Li kin diffusivity is expected to crucially determine the performance of such next-generation energy storage systems. Therefore, studying diffusion properties in e.g. amorphous Li-Si underpins applied research that is being directed toward the development of powerful storage devices. So far, only little information is available on Li+ self-diffusion in amorphous Si. Here, we used Li-7 NMR spectroscopy to precisely quantify microscopic activation energies and Li jump rates in amorphous Li-Si which is primarily formed if monocrystalline Si is lithiated electrochemically. Surprisingly, our results reveal relatively fast Li ion diffusivity with low activation energies for localized Li+ motions being in agreement with results from theory. The average activation energy for long-range ion transport is as high as ca. 0.65 eV; jump rates turn out to be in the order of 2.5 x 10(5) s(-1) at 246 K Our results point to complex dynamics that is most likely governed by nonexponential motional correlation functions originating from a distribution of activation energies. The data obtained might help optimizing Li-based silicon batteries whose performance critically depend on fast Li ion transport.

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.

The realization of large powerful all-solid-state batteries is still hampered by the availability of environmentally friendly and low-cost Li ion conductors that can easily be produced on a large scale and with high reproducibility. Advanced solid electrolytes benefit from fast ion-selective transport and non-flammability, but they may have low electrochemical stability with respect to Li metal. Sol-gel-synthesized lithium titanium aluminum phosphate Li(1.5)Al(0.5)Ti(1.5)(PO4)3 (LATP), which was prepared via a new synthesis route taking advantage of an annealing step at relatively low temperatures, has the potential to become one of the major players in this field although it may suffer from reduction upon direct contact with metallic lithium. Its ion dynamics is, however, as yet poorly understood. In the present study, (7)Li nuclear magnetic resonance (NMR) spectroscopy was used to monitor the key Li jump processes on the atomic scale. NMR relaxation clearly reveals heterogeneous dynamics comprising distinct ultra-fast and slower diffusion processes. The high Li ion self-diffusion coefficients deduced originate from a rapid Li exchange with activation energies as low as 0.16 eV which means that sol-gel synthesized LATP is superior to other solid electrolytes. Our NMR results fully support recent theoretical investigations on the underlying diffusion mechanism, indicating that to rapidly jump from site to site, the ions use interstitial sites connected by low-energy barriers in LATP.

Impedance spectroscopy measurements down to very low temperatures allowed for resolving bulk ion transport properties in highly conducting ceramic electrolytes.