Broadband impedance spectroscopy of Li4Ti5O12: from nearly constant loss effects to long-range ion dynamics

Li4Ti5O12 (LTO) is known as one of the most robust and long-lasting anode materials in lithium-ion batteries. As yet, the Li-ion transport properties of LTO are, however, not completely understood. Here, we used broadband impedance spectroscopy spanning a wide temperature range to investigate the full electrical response of LTO over a wide frequency range. It turned out that the isotherms recorded entail information about two relaxation processes. While at high temperatures the isotherms show a frequency independent plateau that corresponds to poor long-range ion transport (<10−11 S cm−1 (298 K), 0.79 eV), they reveal a second region, seen at lower temperatures and higher frequencies, which we attribute to short-range ion dynamics (10−8 S cm−1) with a significantly reduced activation energy of ca. 0.51 eV. At even lower temperatures, the isotherms are fully governed by nearly constant loss behavior, which has frequently been explained by cage-like dynamics. The present results agree with those earlier presented by 7Li NMR spin-lattice relaxation measurements being sensitive to dynamic processes taking place on quite different length scales. Our findings unveil complex Li+ ion dynamics in LTO and help understand its superior electrochemical properties

Direct Assessment of Ultralow Li+ Jump Rates in Single Crystalline Li3N by Evolution-Time-Resolved 7Li Spin-Alignment Echo NMR

Diffusion processes of small cations and anions play important roles in many applications such as batteries and sensors. Despite the enormous progress we have witnessed over the past years in characterizing the irregular movement of ions such as Li+, new methods able to sharpen our view and understanding of fast and slow diffusion phenomena are steadily developed. Still, very few techniques are, however, available to directly sense extremely slow Li+ diffusion processes. Here, we took advantage of 1D evolution-time resolved 7Li spin-alignment echo NMR that is able to probe the extremely slow interlayer Li+ hopping process in layer-structured Li3N, which served as a model substance for our purposes. The use of single crystals enabled us to study this translational process without being interfered by the fast intralayer Li+ motions. At 318 K the corresponding jump rate of interlayer dynamics turned out to be in the order of 2500(200) s−1 resulting in a diffusion coefficient as low as 1×10−17 m2 s−1, which is in excellent agreement with results from literature. The method, comparable to 1D and 2D NMR exchange spectroscopy, relies on temporal fluctuations of electric interactions the jumping ions are subjected to. 7Li single crystal 1D SAE NMR offers new opportunities to precisely quantify slow Li+ diffusion processes needed to validate theoretical models and to develop design principles for new solid electrolytes

High Li+ and Na+ Conductivity in New Hybrid Solid Electrolytes based on the Porous MIL-121 Metal Organic Framework

Solid-state electrolytes (SSEs) can leapfrog the development of all-solid-state batteries (ASSBs), enabling them to power electric vehicles and to store renewable energy from intermittent sources. Here, a new hybrid Li+ and Na+ conducting SSE based on the MIL-121 metal-organic framework (MOF) structure is reported. Following synthesis and activation of the MOF, the free carboxylic units along the 1D pores are functionalized with Li+ or Na+ ions by ion exchange. Ion dynamics are investigated by broadband impedance spectroscopy and by Li-7 and Na-23 NMR spin-lattice relaxation. A crossover at 50 degrees C (Li+) and at 10 degrees C (Na+) from correlated to almost uncorrelated motion at higher temperature is observed, which is in line with Ngai\u27s coupling model. Alternatively, in accordance to the jump relaxation model of Funke, at low temperature only a fraction of the jump processes are successful as lattice rearrangement in the direct vicinity of Li+ (Na+) is slow. H-1 NMR unambiguously shows that Li+ is the main charge carrier. Conductivities reach 0.1 mS cm(-1) (298 K, Na+) while the activation energies are 0.28 eV (Li+) and 0.36 eV (Na+). The findings pave the way towards development of easily tunable and rationally adjustable high-performance MOF-based hybrid SSEs for ASSBs

Ion dynamics in Al-Stabilized Li7La3Zr2O12 single crystals – Macroscopic transport and the elementary steps of ion hopping

Li7La3Zr2O12 (LLZO) garnet-type ceramics are considered as very promising candidates for solid electrolytes and have been extensively studied in the past few years. Several studies report on an increase in ionic conductivity by doping with ions, such as Al3+ and Ga3+, to stabilize the cubic modification of LLZO. Unfortunately, so far ion dynamics have mainly been studied using powdered samples. Such studies may suffer from chemical heterogeneities concerning Al distribution. Here, we took advantage of Al-stabilized LLZO single crystals to throw light on the elementary steps of ion hopping. We used 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements and conductivity spectroscopy to probe dynamic parameters from both a microscopic and macroscopic point of view. At 293 K the total conductivity turned out to be 0.082 mS cm−1, which is remarkably good for LLZO exhibiting an Al-content of only 0.37 wt%. Most importantly, 7Li NMR spin-lock transients revealed two overlapping diffusion-induced processes. Overall, activation energies from spin-lock NMR excellently agree with that from conductivity measurements; both techniques yield values around 0.36 eV. The corresponding diffusion coefficients deduced from NMR and conductivity measurements almost coincide. The magnetic spin fluctuations sensed by NMR provide an in-depth look at the elementary jump processes, which can barely be revealed by macroscopic impedance spectroscopy providing average values. In particular, we were able to precisely measure the local hopping barrier (0.20 eV) characterizing forward-backward jumps between the sites 24d and 96h. © 2019 The Author(s

Allergy / Prevention of allergy by viruslike nanoparticles (VNP) delivering shielded versions of major allergens in a humanized murine allergy model

Background: In highrisk populations, allergenspecific prophylaxis could protect from sensitization and subsequent development of allergic disease. However, such treatment might itself induce sensitization and allergies, thus requiring hypoallergenic vaccine formulations. We here characterized the preventive potential of viruslike nanoparticles (VNP) expressing surfaceexposed or shielded allergens. Methods: Fulllength major mugwort pollen allergen Art v 1 was selectively targeted either to the surface or to the inner side of the lipid bilayer envelope of VNP. Upon biochemical and immunological analysis, their preventive potential was determined in a humanized mouse model of mugwort pollen allergy. Results: Viruslike nanoparticles expressing shielded version of Art v 1, in contrast to those expressing surfaceexposed Art v 1, were hypoallergenic as they hardly induced degranulation of rat basophil leukemia cells sensitized with Art v 1specific mouse or human IgE. Both VNP versions induced proliferation and cytokine production of allergenspecific T cells in vitro. Upon intranasal application in mice, VNP expressing surfaceexposed but not shielded allergen induced allergenspecific antibodies, including IgE. Notably, preventive treatment with VNP expressing shielded allergenprotected mice from subsequent sensitization with mugwort pollen extract. Protection was associated with a Th1/Tregdominated cytokine response, increased Foxp3+ Treg numbers in lungs, and reduced lung resistance when compared to mice treated with empty particles. Conclusion: Viruslike nanoparticles represent a novel and versatile platform for the in vivo delivery of allergens to selectively target T cells and prevent allergies without inducing allergic reactions or allergic sensitization.DKW1248SFB F4605SFB F4609(VLID)313247

The small molecule inhibitor BX-795 uncouples IL-2 production from inhibition of Th2 inflammation and induces CD4+ T cells resembling iTreg

BackgroundTreg cells have been shown to be an important part of immune-homeostasis and IL-2 which is produced upon T cell receptor (TCR)-dependent activation of T lymphocytes has been demonstrated to critically participate in Treg development.ObjectiveTo evaluate small molecule inhibitors (SMI) for the identification of novel IL-2/Treg enhancing compounds.Materials and methodsWe used TCR-dependent and allergen-specific cytokine secretion of human and mouse T cells, next generation messenger ribonucleic acid sequencing (RNA-Seq) and two different models of allergic airway inflammation to examine lead SMI-compounds.ResultsWe show here that the reported 3-phosphoinositide dependent kinase-1 (PDK1) SMI BX-795 increased IL-2 in culture supernatants of Jurkat E6-1 T cells, human peripheral blood mononuclear cells (hPBMC) and allergen-specific mouse T cells upon TCR-dependent and allergen-specific stimulation while concomitantly inhibiting Th2 cytokine secretion. RNA-Seq revealed that the presence of BX-795 during allergen-specific activation of T cells induces a bona fide Treg cell type highly similar to iTreg but lacking Foxp3 expression. When applied in mugwort pollen and house dust mite extract-based models of airway inflammation, BX-795 significantly inhibited Th2 inflammation including expression of Th2 signature transcription factors and cytokines and influx into the lungs of type 2-associated inflammatory cells such as eosinophils.ConclusionsBX-795 potently uncouples IL-2 production from Th2 inflammation and induces Th-IL-2 cells, which highly resemble induced (i)Tregs. Thus, BX-795 may be a useful new compound for the treatment of allergic diseases

Length-scale-dependent ion dynamics in Ca-doped Na₃PS₄

The sodium ion conductor Na3PS4 is a promising electrolyte for future all-solid-state batteries using Na+ ions as ionic charge carriers. Its readily available components make it a compelling and more sustainable alternative to recent Li-ion technologies. At ambient temperature, the ionic conductivity is in the order of 10–4 S cm–1, which can be optimized by adjusting doping and processing parameters. Even though several studies have focused on explaining the dynamic properties of doped and undoped Na3PS4, the driving forces that lead to fast Na+ exchange are not yet completely understood. Here, we synthesized nanocrystalline, defect-rich cubic Na3PS4 via a solid-state synthesis route and compared its properties with those of highly crystalline Ca-doped Na3–2xCaxPS4. The interconnected effects of doping and synthesis procedure on both structure and dynamic properties are investigated. X-ray diffraction reveals that the undoped samples show clear cubic and tetragonal symmetry, while for the doped samples, a phase mixture of both polymorphs is seen. High-resolution 23Na magic angle spinning NMR spectra acquired at temperatures as low as −60 °C clearly reveal two different Na sites when ionic motion is partially frozen out. Ion dynamics of the powder samples were analyzed using high-precision broadband impedance spectroscopy and variable-temperature, time-domain 23Na NMR spin–lattice relaxation rate measurements. Localized Na+ jumps detected by NMR showed higher energy barriers but faster Na+ dynamics for the Ca-doped samples. A similar trend was observed in conductivity spectroscopy with lowest activation energy for Na-ion transport in tetragonal Na3PS4 but highest attempt frequencies for the hopping motion in Ca-doped Na3PS4 with x = 0.135, making the doped sample the superior ion conductor at elevated temperatures. Our study highlights the importance of breaking down ionic transport in its elemental steps to understand the complex interplay of intrinsic and extrinsic parameters in solid electrolyte materials

Ionic Conductivity of Nanocrystalline and Amorphous Li10GeP2S12: The Detrimental Impact of Local Disorder on Ion Transport

Solid electrolytes with extraordinarily high Li-ionic conductivities are key for high performance all-solid-state batteries. So far, the thiophosphate Li10GeP2S12 (LGPS) belongs to the best Li ion conductors with an ionic conductivity exceeding 10 mS cm–1 at ambient. Recent molecular dynamics simulations performed by Dawson and Islam predict that the ionic conductivity of LGPS can be further enhanced by a factor of three if the crystallite size is reduced to the nanometer regime. A change in local ion coordination, hence local disorder, has been assumed to facilitate Li diffusion in the ab-plane of LGPS. As yet, no experimental evidence exists supporting this fascinating prediction. Here, we synthesized nanocrystalline LGPS by high-energy ball milling, characterized the material structurally and probed the Li+ ion transport parameters. Whereas X-ray powder diffraction and high-resolution 31P and 6Li magic angle spinning nuclear magnetic resonance (NMR) spectroscopy helped us to determine morphological changes and local structures upon milling, broadband conductivity spectroscopy in combination with electric modulus measurements allowed us to precisely follow the changes in Li+ ion dynamics. Surprisingly and against the behavior of other electrolytes, ionic conductivity turned out to decrease with increasing milling time, finally leading to a reduction of σ20°C by almost a factor of 10. This decrease affects both, bulk ion dynamics and total conductivity, which also comprises Li+ transport across grain boundary regions in LGPS. As could be shown by NMR, ball-milling leads to a structurally heterogeneous sample with the nm-sized LGPS crystallites being embedded in an amorphous matrix. This amorphous phase is responsible for the reduced performance of the milled electrolyte. Importantly, careful separation of the amorphous and (nano)crystalline contributions to the overall ionic conductivity revealed that even in the nanocrystalline regions Li+ ion dynamics is slowed down compared to untreated, coarse-grained LGPS. We conclude that defects introduced into the LGPS bulk structure via ball milling have a negative impact on ionic transport. We postulate that such kind of structural disorder is detrimental to fast ion transport in materials whose transport properties rely on crystallographically well-defined diffusion pathways

The Electronic Conductivity of Single Crystalline Ga-Stabilized Cubic Li7La3Zr2O12 : A Technologically Relevant Parameter for All-Solid-State Batteries

The next‐generation of all‐solid‐state lithium batteries need ceramic electrolytes with very high ionic conductivities. At the same time a negligible electronic conductivity σeon is required to eliminate self‐discharge in such systems. A non‐negligible electronic conductivity may also promote the unintentional formation of Li dendrites, being currently one of the key issues hindering the development of long‐lasting all‐solid‐state batteries. This interplay is suggested recently for garnet‐type Li7La3Zr2O12 (LLZO). It is, however, well known that the overall macroscopic electronic conductivity may be governed by a range of extrinsic factors such as impurities, chemical inhomogeneities, grain boundaries, morphology, and size effects. Here, advantage of Czochralski‐grown single crystals, which offer the unique opportunity to evaluate intrinsic properties of a chemically homogeneous matrix, is taken to measure the electronic conductivity σeon. Via long‐time, high‐precision potentiostatic polarization experiments an upper limit of σeon in the order of 5 × 10−10 S cm−1 (293 K) is estimated. This value is by six orders of magnitude lower than the corresponding total conductivity σtotal = 10−3 S cm−1 of Ga‐LZO. Thus, it is concluded that the high values of σeon recently reported for similar systems do not necessarily mirror intragrain bulk properties of chemically homogenous systems but may originate from chemically inhomogeneous interfacial areas