Liquid-Like Li-Ion Conduction in Oxides Enabling Anomalously Stable Charge Transport across the Li/Electrolyte Interface in All-Solid-State Batteries.

The softness of sulfur sublattice and rotational PS4 tetrahedra in thiophosphates result in liquid-like ionic conduction, leading to enhanced ionic conductivities and stable electrode/thiophosphate interfacial ionic transport. However, the existence of liquid-like ionic conduction in rigid oxides remains unclear, and modifications are deemed necessary to achieve stable Li/oxide solid electrolyte interfacial charge transport. In this study, by combining the neutron diffraction survey, geometrical analysis, bond valence site energy analysis, and ab initio molecular dynamics simulation, 1D liquid-like Li-ion conduction is discovered in LiTa2 PO8 and its derivatives, wherein Li-ion migration channels are connected by four- or five-fold oxygen-coordinated interstitial sites. This conduction features a low activation energy (0.2 eV) and short mean residence time (<1 ps) of Li ions on the interstitial sites, originating from the Li-O polyhedral distortion and Li-ion correlation, which are controlled by doping strategies. The liquid-like conduction enables a high ionic conductivity (1.2 mS cm-1 at 30 °C), and a 700 h anomalously stable cycling under 0.2 mA cm-2 for Li/LiTa2 PO8 /Li cells without interfacial modifications. These findings provide principles for the future discovery and design of improved solid electrolytes that do not require modifications to the Li/solid electrolyte interface to achieve stable ionic transport.

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

Solids with extraordinarily high Li+ dynamics are key for high performance all-solid-state batteries. The thiophosphate Li10GeP2S12 (LGPS) belongs to the best Li-ion conductors with an ionic conductivity exceeding 10 mS cm-1 at ambient temperature. Recent molecular dynamics simulations performed by Dawson and Islam predict that the ionic conductivity of LGPS can be further enhanced by a factor of 3 if local disorder is introduced. As yet, no experimental evidence exists supporting this fascinating prediction. Here, we synthesized nanocrystalline LGPS by high-energy ball-milling and probed the Li+ ion transport parameters. Broadband conductivity spectroscopy in combination with electric modulus measurements allowed us to precisely follow the changes in Li+ dynamics. Surprisingly and against the behavior of other electrolytes, bulk ionic conductivity turned out to decrease with increasing milling time, finally leading to a reduction of σ20°C by a factor of 10. 31P, 6Li NMR, and X-ray diffraction showed that ball-milling forms a structurally heterogeneous sample with nm-sized LGPS crystallites and amorphous material. At -135 °C, electrical relaxation in the amorphous regions is by 2 to 3 orders of magnitude slower. Careful separation of the amorphous and (nano)crystalline contributions to overall ion transport revealed that in both regions, Li+ ion dynamics is slowed down compared to untreated LGPS. Hence, introducing defects into the LGPS bulk structure via ball-milling has a negative impact on ionic transport. We postulate that such a kind of structural disorder is detrimental to fast ion transport in materials whose transport properties rely on crystallographically well-defined diffusion pathways.

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1-xGexS5I as Probed by Neutron Diffraction and NMR.

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li+ ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li6+xP1-xGexS5I served as an application-oriented model system to study the effect of cation substitution (P5+ vs Ge4+) on Li+ ion dynamics. While Li6PS5I is a rather poor ionic conductor (10-6 S cm-1, 298 K), the Ge-containing samples show specific conductivities on the order of 10-2 S cm-1 (330 K). Replacing P5+ with Ge4+ not only causes S2-/I- anion site disorder but also reveals via neutron diffraction that the Li+ ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used 7Li and 31P NMR to show that this Li+ site disorder has a tremendous effect on both local ion dynamics and long-range Li+ transport. For the Ge-rich samples, NMR revealed several new Li+ exchange processes, which are to be characterized by rather low activation barriers (0.1-0.3 eV). Consequently, in samples with high Ge-contents, the Li+ ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Extremely Fast Interfacial Li Ion Dynamics in Crystalline LiTFSI Combined with EMIM-TFSI.

Materials providing fast transport pathways for ionic charge carriers are at the heart of future all-solid state batteries that completely rely on sustainable, nonflammable solid electrolytes. The mobile ions in fast ion conductors may take benefit from structural disorder, cation and anion substitution, or dimensionality effects. While these effects concern the bulk regions of a given material, one may also manipulate the surface or interfacial regions of a polycrystalline poorly conducting electrolyte to enhance its transport properties. Here, we used 7Li NMR to characterize interfacial effects in crystalline lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) to which a small amount of ionic liquid EMIM-TFSI (EMIM: 1-ethyl-3-methylimidazolium cation, C6H11N2 +) was added. We recorded longitudinal spin-lattice relaxation (SLR) curves M z (t d) that directly mirror the 7Li spin-fluctuations controlled by motional processes in such ionic-liquids-in-salt composites. Already at room temperature we observe strongly bimodal buildup curves M z (t d) leading to two distinct SLR rates differing by a factor of 100. While the slower rate does exactly reflect the temperature behavior expected for poorly conducting LiTFSI, the faster rate mirrors rapid motional processes that are governed by an activation energy as low as 73 meV. We attribute these fast processes to highly mobile Li+ ions in or near the contact area of crystalline LiTFSI and EMIM-TFSI. By using a method that characterizes motional processes from the atomic-scale point of view, we emphasize the importance of interfacial regions as reservoirs for fast Li+ ions in such solid composite electrolytes.

With a Little Help from 31P NMR: The Complete Picture on Localized and Long-Range Li+ Diffusion in Li6PS5I.

Li6PS5I acts as a perfect model substance to study length scale-dependent diffusion parameters in an ordered matrix. It provides Li-rich cages which offer rapid but localized Li+ translational jump processes. As jumps between these cages are assumed to be much less frequent, long-range ion transport is sluggish, resulting in ionic conductivities in the order of 10-6 S cm-1 at room temperature. In contrast, the site disordered analogues Li6PS5X (X = Br, Cl) are known as fast ion conductors because structural disorder facilities intercage dynamics. As yet, the two extremely distinct jump processes in Li6PS5I have not been visualized separately. Here, we used a combination of 31P and 7Li NMR relaxation measurements to probe this bimodal dynamic behavior, that is, ultrafast intracage Li+ hopping and the much slower Li+ intercage exchange process. While the first is to be characterized by an activation energy of ca. 0.2 eV as directly measured by 7Li NMR, the latter is best observed by 31P NMR and follows the Arrhenius law determined by 0.44 eV. This activation energy perfectly agrees with that seen by direct current conductivity spectroscopy being sensitive to long-range ion transport for which the intercage jumps are the rate limiting step. Moreover, quantitative agreement in terms of diffusion coefficients is also observed. The solid-state diffusion coefficient D σ obtained from conductivity spectroscopy agrees very well with that from 31P NMR (D NMR ≈ 4.6 × 10-15 cm2 s-1). D NMR was directly extracted from the pronounced diffusion-controlled 31P NMR spin-lock spin-lattice relaxation peak appearing at 366 K.

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'.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry.

We have synthesized the first isolable geminal bisenolates L2 K2 Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

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.

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.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry.

We have synthesized the first isolable geminal bisenolates L2K2Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

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.

Anomalies in Bulk Ion Transport in the Solid Solutions of Li7La3M2O12 (M = Hf, Sn) and Li5La3Ta2O12.

Cubic Li7La3Zr2O12(LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ'bulk. Here, we synthesized solid solutions of Li7-x La3M2-x Ta x O12 with M = Hf, Sn over the full compositional range (x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li7-x La3M2-x Ta x O12. The maximum in total conductivity for LLHTO (2 × 10-4 S cm-1) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ'bulk turned out to be 7.94 × 10-5 S cm-1 (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer-Neldel's compensation behavior showing that a decrease in E a is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li7La3Zr2O12 as LLHTO is also anticipated to be highly stable against Li metal.

Li-Ion Diffusion in Nanoconfined LiBH4-LiI/Al2O3: From 2D Bulk Transport to 3D Long-Range Interfacial Dynamics.

Solid electrolytes based on LiBH4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH4 can be stabilized via the incorporation of LiI. If the resulting LiBH4-LiI is confined to the nanopores of an oxide, such as Al2O3, interface-engineered LiBH4-LiI/Al2O3 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 1H, 6Li, 7Li, 11B, and 27Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH4-LiI/Al2O3. In particular, diffusion-induced 1H, 7Li, and 11B NMR spin-lattice relaxation measurements and 7Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27Al magic angle spinning NMR revealed surface interactions of LiBH4-LiI with pentacoordinated Al sites, and two-component 1H 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 LiBH4-LiI regions turned out to be fully homogenous. This view is supported by 7Li 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 LiBH4-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 LiBH4-LiI/Al2O3 is characterized by an activation energy of 0.43 eV.

Understanding the Origin of Enhanced Li-Ion Transport in Nanocrystalline Argyrodite-Type Li6PS5I.

Argyrodite-type Li6PS5X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation all-solid-state lithium batteries. In contrast to Li6PS5Br and Li6PS5Cl compounds showing ionic conductivities on the order of several mS cm-1, the iodine compound Li6PS5I turned out to be a poor ionic conductor. This difference has been explained by anion site disorder in Li6PS5Br and Li6PS5Cl leading to facile through-going, that is, long-range ion transport. In the structurally ordered compound, Li6PS5I, long-range ion transport is, however, interrupted because the important intercage Li jump-diffusion pathway, enabling the ions to diffuse over long distances, is characterized by higher activation energy than that in the sibling compounds. Here, we introduced structural disorder in the iodide by soft mechanical treatment and took advantage of a high-energy planetary mill to prepare nanocrystalline Li6PS5I. A milling time of only 120 min turned out to be sufficient to boost ionic conductivity by 2 orders of magnitude, reaching σtotal = 0.5 × 10-3 S cm-1. We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and 7Li nuclear magnetic relaxation. X-ray powder diffraction and high-resolution 6Li, 31P MAS NMR helped characterize structural changes and the extent of disorder introduced. Changes in attempt frequency, activation entropy, and charge carrier concentration seem to be responsible for this increase.

Lithium-Ion Transport in Nanocrystalline Spinel-Type Li[InxLiy]Br4 as Seen by Conductivity Spectroscopy and NMR.

Currently, a variety of solid Li+ conductors are being discussed that could potentially serve as electrolytes in all-solid-state Li-ion batteries and batteries using metallic Li as the anode. Besides oxides, sulfides and thioposphates, and also halogenides, such as Li3YBr6, belong to the group of such promising materials. Here, we report on the mechanosynthesis of ternary, nanocrystalline (defect-rich) Li[In x Li y ]Br4, which crystallizes with a spinel structure. We took advantage of a soft mechanochemical synthesis route that overcomes the limitations of classical solid-state routes, which usually require high temperatures to prepare the product. X-ray powder diffraction, combined with Rietveld analysis, was used to collect initial information about the crystal structure; it turned out that the lithium indium bromide prepared adopts cubic symmetry ( Fd 3 ¯ m ). The overall and electronic conductivity were examined via broadband conductivity spectroscopy and electrical polarization measurements. While electric modulus spectroscopy yielded information on long-range ion transport, 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements revealed rapid, localized ionic hopping processes in the ternary bromide. Finally, we studied the influence of thermal treatment on overall conductivity, as the indium bromide might find applications in cells that are operated at high temperatures (330 K and above).

Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides.

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 LiBH4 (LiBH4-LiI and LiBH4-LiNH2) leads to an enhancement of the room temperature conductivity by a factor of 4 to 10 compared to nanoconfined LiBH4 and nonconfined LiBH4-LiI and LiBH4-LiNH2, 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.

Fluoride-Ion Batteries: On the Electrochemical Stability of Nanocrystalline La0.9Ba0.1F2.9 against Metal Electrodes.

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 La 0.9 Ba 0.1 F 2.9 , 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 La 0.9 Ba 0.1 F 2.9 , 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.

Lithium ion dynamics in LiZr2(PO4)3 and Li1.4Ca0.2Zr1.8(PO4)3.

High ionic conductivity, electrochemical stability and small interfacial resistances against Li metal anodes are the main requirements to be fulfilled in powerful, next-generation all-solid-state batteries. Understanding ion transport in materials with sufficiently high chemical and electrochemical stability, such as rhombohedral LiZr2(PO4)3, is important to further improve their properties with respect to translational Li ion dynamics. Here, we used broadband impedance spectroscopy to analyze the electrical responses of LiZr2(PO4)3 and Ca-stabilized Li1.4Ca0.2Zr1.8(PO4)3 that were prepared following a solid-state synthesis route. We investigated the influence of the starting materials, either ZrO2 and Zr(CH3COO)4, on the final properties of the products and studied Li ion dynamics in the crystalline grains and across grain boundary (g.b.) regions. The Ca2+ content has only little effect on bulk properties (4.2 × 10-5 S cm-1 at 298 K, 0.41 eV), but, fortunately, the g.b. resistance decreased by 2 orders of magnitude. Whereas, 7Li spin-alignment echo nuclear magnetic resonance (NMR) confirmed long-range ion transport as seen by conductivity spectroscopy, 7Li NMR spin-lattice relaxation revealed much smaller activation energies (0.18 eV) and points to rapid localized Li jump processes. The diffusion-induced rate peak, appearing at T = 282 K, shows Li+ exchange processes with rates of ca. 109 s-1 corresponding, formally, to ionic conductivities in the order of 10-3 S cm-1 to 10-2 S cm-1.

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

Diffusion of small ions in materials with confined space for translational dynamics can be quite different to isotropic (3D) diffusion, which is found in the majority of solids. Finding credible indications for 2D diffusion is not as easy as it looks at first glance, especially if only powder samples are available. Here we chose the ternary fluoride RbSn2F5 as a new model system to seek out low-dimensional anion diffusion in a nanocrystalline material. We prepared RbSn2F5via mechanochemically-assisted solid state synthesis and used both ac conductivity spectroscopy and spin-lock NMR relaxation measurements to find evidence that the fluorine ions preferably diffuse between the Rb-rich layers. In both cases the diffusion induced spin-lock NMR rates are only consistent with conductivity data if they are analyzed with the semi-empirical spectral density function for 2D jump diffusion as introduced by P. M. Richards [Solid State Commun., 1978, 25, 1019].