Solid-state electrolytes are crucial for the realization of safe and high capacity all-solid-state batteries. Lithium-containing complex hydrides represent a promising class of solid-state electrolytes, but they exhibit low ionic conductivities at room temperature. Ion substitution and nanoconfinement are the main strategies to overcome this challenge. Here, we report on the synthesis of nanoconfined anion-substituted complex hydrides in which the two strategies are effectively combined to achieve a profound increase in the ionic conductivities at ambient temperature. We show that the nanoconfinement of anion substituted LiBH 4 (LiBH 4 −LiI and LiBH 4 −LiNH 2 ) leads to an enhancement of the room temperature conductivity by a factor of 4 to 10 compared to nanoconfined LiBH 4 and nonconfined LiBH 4 −LiI and LiBH 4 -LiNH 2 , concomitant with a lowered activation energy of 0.44 eV for Li-ion transport. Our work demonstrates that a combination of partial ion substitution and nanoconfinement is an effective strategy to boost the ionic conductivity of complex hydrides. The strategy could be applicable to other classes of solid-state electrolytes.

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Solid electrolytes based on LiBH 4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH 4 can be stabilized via the incorporation of LiI. If the resulting LiBH 4 -LiI is confined to the nanopores of an oxide, such as Al 2 O 3 , interface-engineered LiBH 4 -LiI/Al 2 O 3 is obtained that revealed promising properties as a solid electrolyte. The underlying principles of Li + conduction in such a nanocomposite are, however, far from being understood completely. Here, we used broadband conductivity spectroscopy and 1 H, 6 Li, 7 Li, 11 B, and 27 Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH 4 -LiI/Al 2 O 3 . In particular, diffusion-induced 1 H, 7 Li, and 11 B NMR spin–lattice relaxation measurements and 7 Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27 Al magic angle spinning NMR revealed surface interactions of LiBH 4 -LiI with pentacoordinated Al sites, and two-component 1 H NMR line shapes clearly revealed heterogeneous dynamic processes. These results show that interfacial regions have a determining influence on overall ionic transport (0.1 mS cm –1 at 293 K). Importantly, electrical relaxation in the LiBH 4 -LiI regions turned out to be fully homogenous. This view is supported by 7 Li NMR results, which can be interpreted with an overall (averaged) spin ensemble subjected to uniform dipolar magnetic and quadrupolar electric interactions. Finally, broadband conductivity spectroscopy gives strong evidence for 2D ionic transport in the LiBH 4 -LiI bulk regions which we observed over a dynamic range of 8 orders of magnitude. Macroscopic diffusion coefficients from PFG NMR agree with those estimated from measurements of ionic conductivity and nuclear spin relaxation. The resulting 3D ionic transport in nanoconfined LiBH 4 -LiI/Al 2 O 3 is characterized by an activation energy of 0.43 eV.

The Y-halides Li3YBr6 and Li3YCl6 have recently gained considerable attention as they might be used as ceramic electrolytes in all-solid-state batteries. Such materials need to show sufficiently high ionic conductivities at room temperature. A thorough investigation of the relationship between ion dynamics and morphology, defect structure, and size effects is, however, indispensable if we want to understand the driving forces behind Li ion hopping processes in these ternary compounds. Li3YBr6 can be prepared by conventional solid-state synthesis routes. Nanostructured Li3YBr6 is, on the other hand, directly available by mechanosynthesis under ambient conditions. The present study is aimed at shedding light on the question of whether (metastable) mechanosynthesized Li3YBr6 might serve as a sustainable alternative to annealed Li3YBr6. For this purpose, we studied the impact of structural disorder on ionic transport by combining mechanosynthesis with soft-annealing steps to prepare Li3YBr6 in two different morphologies. While structural details were revealed by X-ray powder diffraction and by high-resolution 6Li and 79Br magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, broadband impedance measurements in conjunction with time-domain 7Li NMR relaxation measurements helped us to characterize Li+ dynamics over a wide temperature range. Interestingly, for Li3YBr6, annealed at 823 K, we observed a discontinuity in conductivity at temperatures slightly below 273 K, which is almost missing for nano-Li3YBr6. This feature is, however, prominently seen in NMR spectroscopy for both samples and is attributed to a change of the Li sublattice in Li3YBr6 Although a bit lower in ionic conductivity, the nonannealed samples, even if obtained after a short milling period of only 1 h, shows encouraging dynamic parameters (0.44 mS cm–1, E a = 0.34 eV) that are comparable to those of the sample annealed at high temperatures (1.52 mS cm–1, E a = 0.28 eV). 7Li nuclear magnetic relaxation, being solely sensitive to Li+ hopping processes on shorter length scales, revealed highly comparable Li+ self-diffusion coefficients on the order of 10–12 m2 s–1, which we extracted directly from purely diffusion-controlled 7Li NMR rate peaks. Spin-lock 7Li NMR reveals a change from uncorrelated to correlated dynamics at temperatures as low as 220 K.

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

Over the past years, ceramic fluorine ion conductors with high ionic conductivity have stepped into the limelight of materials research, as they may act as solid-state electrolytes in fluorine-ion batteries (FIBs). A factor of utmost importance, which has been left aside so far, is the electrochemical stability of these conductors with respect to both the voltage window and the active materials used. The compatibility with different current collector materials is important as well. In the course of this study, tysonite-type La0.9Ba0.1F2.9, which is one of the most important electrolyte in first-generation FIBs, was chosen as model substance to study its electrochemical stability against a series of metal electrodes viz. Pt, Au, Ni, Cu and Ag. To test anodic or cathodic degradation processes we carried out cyclic voltammetry (CV) measurements using a two-electrode set-up. We covered a voltage window ranging from −1 to 4 V, which is typical for FIBs, and investigated the change of the response of the CVs as a function of scan rate (2 mV/s to 0.1 V/s). It turned out that Cu is unstable in combination with La0.9Ba0.1F2.9, even before voltage was applied. The cells with Au and Pt electrodes show reactions during the CV scans; in the case of Au the irreversible changes seen in CV are accompanied by a change in color of the electrode as investigated by light microscopy. Ag and Ni electrodes seem to suffer from contact issues which, most likely, also originate from side reactions with the electrode material. The experiments show that the choice of current collectors in future FIBs will become an important topic if we are to develop long-lasting FIBs. Most likely, protecting layers between the composite electrode material and the metal current collector have to be developed to prevent any interdiffusion or electrochemical degradation processes.

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 Li 0.5 [Mg 2.5 Li 0.5 ]Si 4 O 10 F 2 , being a phyllosilicate, served as a model substance to investigate Li + translational ion dynamics by both broadband conductivity spectroscopy and diffusion-induced 7 Li 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.

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   1 , 2 H ,   6 , 7 Li ,   19 F ,   23 Na NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that Li + 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 ( Ba, Ca) F 2 , Li-bearing garnets ( Li 7 La 3 Zr 2 O 12 ) and thiophosphates such as LiTi 2 ( PS 4 ) 3 . 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 ( Me, Ca ) F 2 ( Me = Ba ,   Pb ) cation mixing influences F anion dynamics, in Li 6 PS 5 X ( X = Br ,   Cl ,   I ) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor Li 4 + x Ti 5 O 12 cation-cation repulsions immediately lead to a boost in Li + 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 RbSn 2 F 5 or MeSnF 4 . 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’.