Aluminium ion doping mechanism of lithium thiophosphate based solid electrolytes revealed with solid-state NMR

We investigate the impact of Al incorporation on the structure and dynamics of Al-doped lithium thiophosphates (Li_3-3xAl_xPS₄) based on β-Li₃PS₄. ²⁷Al and ⁶Li magic-angle spinning NMR spectra confirm that Al³⁺ ions occupy octahedral sites in the structure. Quantitative analyses of ²⁷Al NMR spectra show that the maximum Al incorporation is x = 0.06 in Li_3-3xAl_xPS₄. The ionic conductivity of β-Li₃PS₄ is enhanced by over a factor 3 due to Al incorporation. Further increase of the Al doping level leads to the formation of a more complicated material consisting of multiple crystalline and distorted phases as indicated by ³¹P NMR spectra and powder X-ray diffraction. Consequently, novel Li ion diffusion pathways develop leading to a very high ionic conductivity at room temperature. NMR relaxometry shows that the activation barrier for long-range Li ion diffusion in β-Li₃PS₄ hardly changes upon Al incorporation, but the onset temperature for motional narrowing comes down significantly due to Al doping. The activation barrier in the subsequently formed multiphase material decreases significantly, however, indicating a different more efficient Li ion conduction pathway.

Lithium confinement and dynamics in hexagonal and monoclinic tungsten oxide nanocrystals: a 7Li solid state NMR study

Mixed-valence tungsten bronzes A_xWO₃ (A = alkali metal, NH₄⁺, etc.) are a series of compounds with adaptive structural and compositional features that make them a hot research topic in thermoelectrics, electrochromics, catalysis or energy applications in battery electrodes. The mixed hexagonal lithium ammonium bronze Li_x(NH₄)_yWO₃ is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH₄)_xWO₃. Surface and structural (nanoconfined) Li⁺ cations were characterized by ⁷Li single pulse excitation and ¹H-⁷Li cross-polarization (CP) NMR experiments. CP build-up curves and two-dimensional heteronuclear correlation solid-state NMR techniques provide information about the spatial connectivity between different proton and Li⁺ species. At 500 °C the bronze structurally transforms from the hexagonal to a monoclinic phase, and defects are formed that are characterized through the Li⁺ environment. ⁷Li exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured ⁷Li T₁ and T₂ relaxation time constants and the T₁/T₂ ratio allow characterizing the local strength of Li⁺ binding.

Operando spectroelectrochemistry of bulk-exfoliated 2D SnS2 for anodes within alkali metal ion batteries reveals unusual tin (III) states

In this study we report an affordable synthesis and preparation of an electrochemically exfoliated few-layer 2-dimensional (2D) SnS2 anode material of high cycling durability and demonstrate its performance on the example of alkali metal batteries. The metalation mechanism consists of highly unusual and previously only speculated Sn (III)-state grasped by operando Raman spectroelectrochemistry aided by symmetry analysis. The prepared 2D material flakes were characterized by high resolution transmission electron microscopy, X-ray photoelectron and Raman spectroscopies. The operando Raman spectroelectrochemistry was chosen as a dedicated tool for the investigation of alkali-metal-ion intercalation (Li, Na, K), whereby the distortion of the A1g Raman active mode (out-of-plane S-Sn-S vibration) during battery charging exhibited a substantial dependence on the electrochemically applied potential. As a result of the structural dynamics a considerable Raman red-shift of 17.6 cm−1 was observed during metalation. Linewidth changes were used to evaluate the expansion caused by metalation, which in case of sodium and potassium were found to be minimal compared to lithium. Based on the spectroscopic and electrochemical results, a mechanism for the de-/intercalation of lithium, sodium and potassium is proposed which includes alloying in few-layer 2D SnS2 materials and the generation of point-defects.

Ionic transport in K2Ti6O13

The increasing demand for batteries forced the development of energy storage systems that rely on materials consisting of abundant elements in the Earth’s crust. Switching from Li+ to K+ as the main ionic charge carrier needs highly conducting potassium-bearing electrolytes to realize K+ ion batteries (PIBs). The knowledge gained from the design of Li-ion batteries (LIBs) and Na-ion batteries (NIBs) may conceptually inspire also the establishment of PIBs. Considering, for instance, the hexatitanates Na2Ti6O13, Li2Ti6O13, and H2Ti6O13, which were previously investigated as components for LIBs and NIBs, here we investigated ion dynamics in the K-analog K2Ti6O13. Ionic transport in polycrystalline samples of K2Ti6O13 was studied in a moisture-free atmosphere by broadband impedance spectroscopy in a temperature range from 20 °C to 450 °C. The current study aims at establishing a correlation between structural features of K2Ti6O13 and long-range ionic transport. As expected for K+ transport in K2Ti6O13 with its geometrically obstructed structure, the overall activation energy of ion transport in the ternary oxide takes a rather high value of 0.97(2) eV. Almost the same result (0.95(3) eV) is obtained for the migration activation energy, which we extracted from the analysis of crossover frequencies of the corresponding conductivity isotherms. By comparing our results with those of Na2Ti6O13 (0.82 eV), Li2Ti6O13 (0.65 eV), and H2Ti6O13, we clearly see how the size of the mobile cation correlates with both specific conductivities and activation energies. This comparison points to K+ being the main charge carrier in K2Ti6O13. It also helps in laying the foundations to derive the relevant structure-property relationships in this class of materials.

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'.

The ionic conductivity of alkali aluminum germanium phosphate glasses – comparison of Plasma CAIT with two electrode DC measurements

The ionic conductivity of alkali aluminum germanium phosphates (MAGP) has been investigated by two different techniques, i.) a fs-Plasma-Charge Attachment Induced Transport (CAIT) approach and ii.) a classical two electrode DC approach. Amorphous MAGP samples of the composition M1.5Al0.5Ge1.5(PO4)3 M=(Li–Cs) have been synthesized by the melt-quenching technique. Comparison of fs-Plasma-CAIT and DC data reveal that the ionic conductivities as well as the activation energies for ion transport agree within the error margins of the experiment. While conventional expectation suggests that a DC approach should fail because of spontaneous charge carrier blocking, this work demonstrates that DC measurements are a simple tool for quantifying ionic conductivities provided that only a small amount of charge has been transported in total.

Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH4/Al2O3

Synthesizing Li-ion-conducting solid electrolytes with application-relevant properties for new energy storage devices is a challenging task that relies on a few design principles to tune ionic conductivity. When starting with originally poor ionic compounds, in many cases, a combination of several strategies, such as doping or substitution, is needed to achieve sufficiently high ionic conductivities. For nanostructured materials, the introduction of conductor-insulator interfacial regions represents another important design strategy. Unfortunately, for most of the two-phase nanostructured ceramics studied so far, the lower limiting conductivity values needed for applications could not be reached. Here, we show that in nanoconfined LiBH₄/Al₂O₃ prepared by melt infiltration, a percolating network of fast conductor-insulator Li⁺ diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid ⁷Li NMR spin fluctuations giving direct evidence for very fast Li⁺ jump processes in both nanoconfined LiBH₄/Al₂O₃ and LiBH₄-LiI/Al₂O₃. Compared to the nanocrystalline, Al₂O₃-free reference system LiBH₄-LiI, nanoconfinement leads to a strongly enhanced recovery of the ⁷Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH₄-LiI/Al₂O₃ and LiBH₄/Al₂O₃ unequivocally reveals that the overall ⁷Li NMR spin-lattice relaxation rates are solely controlled by the spin fluctuations near or in the conductor-insulator interfacial regions. Thus, the conductor-insulator nanoeffect, which in the ideal case relies on a percolation network of space charge regions, is independent of the choice of the bulk crystal structure of LiBH₄, either being orthorhombic (LiBH₄/Al₂O₃) or hexagonal (LiBH₄-LiI/Al₂O₃). ⁷Li (and ¹H) NMR shows that rapid local interfacial Li-ion dynamics is corroborated by rather small activation energies on the order of only 0.1 eV. In addition, the LiI-stabilized layer-structured form of LiBH₄ guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport.

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.

The Effect of Crystalline Microstructure of PVDF Binder on Mechanical and Electrochemical Performance of Lithium-Ion Batteries Cathode

In this paper, the effect of the crystalline microstructures of polyvinylidene fluoride (PVDF), as cathode binder, on mechanical and electrochemical properties of the cathode, and on the cell performance is investigated. The crystalline phases of the PVDF films prepared at different temperatures are determined by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) and also mechanical strength of PVDF films evaluated by a tensile test. The cathodes were prepared at altered temperatures to achieve different PVDF phases. The effect of various crystalline phases on the cathode performance was studied. The obtained cathodes were analyzed by scanning electron microscope (SEM), contact angle measurement, and adhesion test. The electrochemical performance of the cathodes was evaluated by charge-discharge cycling test and AC impedance spectroscopy. Mechanical tests results showed that the cathode which is prepared at 60 °C has the best adhesion and mechanical stability. In addition, the charge-discharge cycling studies showed that this cathode has the highest capacity efficiency. AC impedance spectroscopy illustrated that this electrode has the lowest charge transfer resistance and SEI resistance.

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

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

Chemical Synthesis and Characterization of Nano Alumina, Nano Composite of Carbon–Alumina and Their Comparative Studies

Aluminium oxide (Al2O3) nano particles were synthesized by using both the sol gel technique and solid state reaction (SSR) method. Different proportion of nano carbon cones from 0.5% to 3.5% is doped with aluminium nitrate nano hydrate and annealed subsequently at 1000°C for 3 h to synthesize the nano composite of carbon–alumina. The synthesized samples were characterized by X-ray diffraction to identify the presence of different phases and transitions during this process. The average crystallite size of the nano alumina is found to be 45 nm by sol gel and 43 nm by SSR method respectively by Debye–Scherrer method. Average crystallite size and lattice strain of nano alumina are also estimated from Williamson Hall (WH) plot analysis. It is found to be 69 nm with the strain of 3.3×10−3 in sol gel, and in SSR method, it is 72 nm with the strain is 3.9×10−3. The interplanar distance of various planes of alumina are estimated and compared with JCPDS values. Similar analysis has also been extended for the nano composite of carbon–alumina. The surface morphology of the samples are analyzed using scanning electron microscopy and rough estimate of the crystallites is also given. From the Raman analysis, the presence of alpha phase of alumina has been confirmed. The presence of carbon in the composite has been established through diffuse reflectance spectroscopy. The FTIR spectra of the composite samples ensured the presence of Al–O–Al, O–H and C=O bonds.

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.