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Understanding fast-ion conduction in solid electrolytes - POSTPONED

Scientific meeting

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

This meeting is postponed. More details to follow. 

Theo Murphy international scientific meeting organised by Dr Benjamin Morgan, Dr Wolfgang Zeier and Professor Serena Corr.

Fast-ion conduction in solid electrolytes

'Superionic' solid electrolytes exhibit exceptional room-temperature ionic conductivities, giving them the potential to enable next-generation battery and fuel-cell technologies. Developing new high-performance electrolytes requires understanding how chemistry can control fast ionic conduction. This meeting will bring together experts on solid electrolytes, superionic conduction, and complex diffusion mechanisms in solids, to discuss the mechanistic origins of fast-ion conduction in solid electrolytes.

The schedule of talks and speaker biographies are available below. Speaker abstracts will be available closer to the meeting date. Recorded audio of the presentations will be available on this page after the meeting has taken place. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

Attending this event

This meeting is postponed. More details to follow.  

Enquiries: contact the Scientific Programmes team.

Schedule of talks

16 March

09:00-12:30

Session 1

4 talks Show detail Hide detail

Chairs

Dr Ainara Aguadero, Imperial College London,UK

09:00-09:30 Annealing-induced strong Li+ conductivity enhancement in the amorphous solid electrolyte 0.33 LiI + 0.67 Li3PS4

Professor Berhard Roling, Philipps-Universität Marburg, Germany

Abstract

Compared to conventional lithium-ion batteries containing liquid electrolytes, all-solid-state batteries (ASSBs) exhibit potentially a higher safety and a higher energy density. The research in this field was boosted by the discovery of sulfide-based crystalline solid electrolytes with ionic conductivity in the range of 10-25 mS/cm. However, in the case of crystalline solid electrolytes, much effort has to be put in finding optimized annealing protocols at elevated temperatures in order to reduce grain boundary resistances. Amorphous solid electrolytes, like Li2S-P2S5-based glasses, which can be easily synthesized by mechanical milling, do not exhibit grain boundary resistances, however, the bulk ionic conductivity is often lower than for the crystalline counterparts. The addition of LiI increases the bulk ionic conductivity and lowers the interfacial resistances in contact to metallic lithium.

In this talk, it is shown that the ionic conductivity of the amorphous solid electrolyte 0.33 LiI + 0.67 Li3PS4 can be increased by a factor of about 7 by means of a short annealing step, leading to conductivities of about 6.5 mS/cm. The combination of electrochemical impedance spectroscopy, powder X-ray diffractometry, 7Li NMR spectroscopy and positron annihilation lifetime spectroscopy gives strong indication that the conductivity enhancement is caused by an annealing-induced formation of monovacancies in the bulk amorphous phase.

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09:30-09:45 Discussion

09:45-10:15 Understanding lithium ion dynamics in thiophosphate and sulphide solid electrolytes

Professor Bettina Lotsch, Max Planck Institute for Solid State Research, Stuttgart and University of Munich, Germany

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Diffusion in solids 101: reviewing the basics

Dr Nella Vargas-Barbosa, Max Planck Institute for Solid State Research, Germany

Abstract

The fundamental understanding of the elements that drive ion diffusion in the solid-state is crucial for the development of next-generation rechargeable all-solid-state batteries. In these systems, the solid electrolyte acts as both a physical separator between the anode and cathode and a medium for the charge carriers (Li, Na, etc.) to move from one electrode to another. As such, the efficient transport of ions across the electrolyte phase is an important parameter to optimize the performance of such batteries. Solid electrolytes are highly concentrated electrolytes, where dilute theory models no longer apply and interactions between the ions are present. Therefore, the ion dynamics in these electrolytes show pronounced directional correlations between successive ion movements, which can exert a strong influence on charge and mass transport.

This talk will review the basics of ion diffusion in solids by defining all relevant transport quantities (diffusion coefficients and correlation factors) based on Onsager’s linear irreversible thermodynamics (in the laboratory frame of reference). This will enable the establishment of the relationship between these parameters and correlation functions of the equilibrium ion dynamics by means of linear response theory. The final part of the presentation will showcase the results of an interlaboratory study on the reproducibility of ionic conductivity values obtained from impedance spectroscopy measurements for a single class of solid electrolytes. The latter results underscore the challenges in connecting experimental results with theoretically-predicted ones.

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11:30-11:45 Discussion

11:45-12:15 Influence of lattice dynamics on ionic mobility in solid-state electrolytes

Dr Sokseiha Muy, EPFL, France

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12:15-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 2

4 talks Show detail Hide detail

Chairs

Dr Karen E Johnston, Durham University, UK

13:30-14:00 Insights into correlated Li+ diffusion from molecular dynamics of promising solid electrolytes

Professor Nicole Adelstein, San Francisco State University, USA

Abstract

Engineering or discovery of better solid electrolytes for Li-ion batteries is especially limited by the lack of understanding of diffusion mechanisms, demanding novel analysis of diffusion using molecular dynamics (MD). The Adelstein Research Group’s in-depth analyses have revealed the importance of correlated motion on diffusion in many of the most promising solid electrolytes, such as lithium borohydrides, lithium germanium thiophosphate (LGPS), lithium thiophosphate, lithium lanthanum zirconium oxide (garnet), and the lithium rich anti-perovskites.

Correlated motion between mobile species is often described quantitatively with the Haven ratio or qualitatively with the distinct Van Hove analysis. These methods do not directly describe correlated diffusion mechanisms. Insight into these correlated diffusion mechanisms is gained by identifying each diffusion event (Li jump) and quantifying its relationship to nearby jumps (in space and time) or movements of the host-lattice, such as hydrogen rotations in Li2HOCl. 

The results presented will focus on correlated motion in a couple of the promising electrolyte materials, including comparison of crystalline versus amorphous diffusion mechanisms. In addition, a survey of the effect of correlation on diffusion will be presented for the various promising solid electrolytes listed above. 

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14:00-14:15 Discussion

14:15-14:45 Fast ion dynamics in solids as probed by Li NMR spin-lattice relaxation: disorder, dimensionality and correlation effects

Professor Martin Wilkening, Graz University of Technology, Austria

Abstract

Nuclear magnetic resonance offers a wide variety of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain 1H(2H), 6,7Li, 19F, 23Na NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. 

It is well accepted that in materials with strong site preferences, such as LiAlO2, the Li+ ions are subjected to extremely slow exchange processes. The loss of this site preference, as it is well known for glasses and some nanocrystalline phases, may, however, lead to rapid cation diffusion. Examples that benefit from this effect include cation-mixed, high-entropy fluorides ((Ba,Ca)F2) and LiTi2(PS4)3. In general, in non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of local defects will affect both the migration activation energy and the corresponding Arrhenius pre-factor. Whereas in (Me,Ca)F2 (Me = Ba, Pb) cation mixing influences F anion dynamics, in Li6PS5X (X = Br, Cl, I) the potential landscape can be manipulated by anion site disorder and the introduction of extrinsic defects. N the other hand, in the mixed conductor Li4+xTi5O12 cation-cation repulsions immediately lead to a boost in Li+ diffusivity at the early stages of chemical lithiation. For Na-bearing closo-borates the influence of rotational anion dynamics is usually considered to explain long-range cation transport.

Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined dimensions, as it is the case in layer-structured fluorides such as RbSn2F5 or MeSnF4 with their buried interfaces. 

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14:45-15:00 Discussion

15:00-15:30 Coffee

15:30-16:00 String-like relaxation in superionic conductors

Professor Jacob Eapen, North Carolina State University, USA

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16:00-16:15 Discussion

16:15-16:45 Professor Olivier Delaire, Duke University, USA

Abstract

 

16:45-17:00 Discussion

17 March

09:00-12:30

Session 3

4 talks Show detail Hide detail

Chairs

Professor Neil Allan, University of Bristol, UK

09:00-09:30 Characterising co-operative and heterogeneous molecular motion: lessons from glassy systems

Dr Robert Jack, University of Cambridge, UK

Abstract

Glasses are mechanically-stable materials where the molecular packing is amorphous. They are usually formed by cooling a liquid phase through its glass transition. Since the 1990s, it has been understood that on cooling the liquid, the molecular motion becomes increasingly heterogeneous and correlated - this can be rationalised intuitively as a result of molecular crowding.

In order to characterise this correlated motion, a number of theoretical tools have been developed. These were originally applied in glasses, but they are applicable much more generally.

This presentation will include a description of some of these tools, with a view to characterisation of correlated motion in electrolytes. The theoretical toolbox includes one-particle measurements (for example, intermittency and non-Gaussian parameters for diffusion), two-particle measurements (for example, the dynamical susceptibility), and many-particle correlations (for example, large deviations of the dynamical activity).

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09:30-09:45 Discussion

09:45-10:15 Role of local structure and kinetics on ionic conductivity in A2B2O7 pyrochlores

Dr Chris Mohn, University of Oslo, Norway

Abstract

Many compounds with the A2B2O7 stoichiometry possess high values of both oxide ion and hydrogen conductivity and are therefore candidate materials as components in fuel cells. Previous attempts to classify many of these, such as La2Ce2O7, as either fully disordered fluorites or defective pyrochlores has led to some confusion due to the lack of agreement within the literature which hamper further progress. Such "average'' structural models are also unsuitable to capture the changes in local structure an ion experiences when travelling through the lattice. Nature of ion conductivity is explored in Ln2Ce2O7 pyrochlores, and links between local structure and collective transport mechanisms are investigated from ab initio molecular dynamics calculations. On the time-scale of the molecular dynamics simulations, it is possible at high temperature to fully quilibrate the oxygen sublattice for a given configurations of cations. Calculating oxygen ion diffusion in a range of different cation configurations representing different possible “frozen in” disorder, can help constraining the ionic conductivity to directly compare with conductivity measurement. This, in turn can give important input in lab synthesis strategies when developing new materials for the use within fuel cells technologies.

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 NMR techniques to measure Li ion diffusivity in battery materials

Professor Lauren Marbella, Columbia University, USA

Abstract

Li ion diffusivity in battery materials is intimately related to Li ion conductivity and ion transport phenomena. However, direct measurement of Li ion diffusion in solid materials is challenging and is often performed indirectly with electrochemical titration techniques. NMR exchange spectroscopy, relaxometry, and diffusivity offer potential routes to capture molecular-level information on ion dynamics in a wide variety of battery materials and simultaneously provide high chemical resolution of individual charge carriers. The unique challenges and opportunities of NMR spectroscopy to measure Li ion diffusivity are presented for a range of electrode materials and solid electrolytes.

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11:30-11:45 Discussion

11:45-12:15 Probing oxide ionic diffusion by quantitative fitting of quasielastic neutron scattering data

Professor Christopher Ling, The University of Sydney, Australia

Abstract

Inelastic neutron scattering is the only experimental technique that simultaneously probes ionic diffusion (as quasielastic neutron scattering, QENS) and lattice dynamics (as a generalised density of states, GDOS). In solid-state ionic conductors (SSICs) where the diffusing species has a predominantly incoherent neutron scattering cross section – the exemplar of which is hydrogen – key parameters describing the atomistic nature of diffusion, such as jump lengths and residence times, can be extracted directly by modelling the form of the QENS. However, this is a far more challenging problem when the diffusing species have significant coherent cross-sections, such as oxygen and lithium. We show here that it is possible to quantitatively model coherent QENS from a SSIC, by starting with the ideal case – high-temperature cubic δ-Bi2O3 – for which the diffusing species (oxygen) is an almost purely coherent scatterer, the structure is simple, and the conductivity (hence the QENS signal) is high. The results show that oxide-ionic diffusion in δ-Bi2O3 is isotropic (liquid-like), even though some directions present shorter oxygen-vacancy distances, an insight corroborated by computational dynamics simulations. More broadly, they demonstrate the power of QENS for studying functional energy materials, notably for solid-oxide fuel cells and, potentially, lithium-ion batteries.

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12:15-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 4

4 talks Show detail Hide detail

Chairs

Dr Stephen Hull, The ISIS Facility, UK

13:30-14:00 Ion migration mechanisms in glassy solid electrolytes at low temperatures

Professor Donald Siegel, University of Michigan, USA

Abstract

Sulphur-based glasses are promising candidates for use as solid electrolytes in Li-based batteries. Nevertheless, due to their amorphous structure, the atomic-scale mechanisms that underlie Li-ion conductivity in these systems are challenging to characterize. The present study employs ab initio molecular dynamics to predict the local structure and migration processes in the prototype Li-ion conducting glass, 75Li2S–25P2S5. A model of the amorphous structure was generated and shown to closely match the measured neutron pair distribution function. Lithium migration is observed to occur via a complex mechanism that combines concerted motion of lithium ions with large, quasi-permanent rotational displacements of the PS43- tetrahedra. This latter effect, commonly referred to as the ‘paddlewheel’ mechanism, is most commonly observed in lower-density crystalline phases that are stable only at elevated-temperatures. Unlike these crystalline analogues, in the glass, the present calculations indicate that paddlewheel dynamics contribute to Li-ion mobility at temperatures as low as 300 K.  Paddlewheel contributions are confirmed through analyses of spatial, temporal, vibrational, and energetic correlations with Li motion.  Furthermore, the dynamics in the glass are shown to differ from those in the stable crystalline phase (-Li3PS4), where contributions from anion reorientations are negligible and the conductivity is much smaller. These data imply that glasses based on complex anions, and in which covalent network formation is minimized, have the potential to exhibit paddlewheel dynamics at low temperature. Glasses that satisfy these requirements may be fertile ground in the search for new solid electrolytes.

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14:00-14:15 Discussion

14:15-14:45 Interplay of site-disorder, interplay and ionic conductivity of superionic conductors: insights from atomistic computer simulations

Professor Karsten Albe, Technische Universität Darmstadt, Germany

Abstract

Glassy, glass–ceramic, and crystalline lithium thiophosphates have attracted interest in their use as solid electrolytes in all-solid-state batteries. Despite similar structural motifs, including PS43–, P2S64–, and P2S74– polyhedra, these materials exhibit a wide range of possible compositions, crystal and amorphous structures, as well as ionic conductivities. Calculations based on density functional theory can be a helpful tool for understanding diffusion pathways and Li+ ionic conductivity and interface stabilities.

This contribution will include a discussion of recent results on the defect chemistry and conductivity of the solid electrolyte Li4P2S6 as well as its interfacial instability with respect to Li. Then, molecular dynamics simulations of crystalline and amorphous Li4PS4I, will be shown, which unravel the diffusion mechanism and can be explained by a rate-equation model based on superbasins. Finally, results on the Lithium argyrodites of the type Li6PS5X (X = Cl, Br, I) are presented, where the influence of S2-/Br- site-disorder was studied. The simulations reveal that local “Li cages” trap Li ions in the ordered material. At higher degrees of site-disorder the cage structures dissolve and long-range low energy pathways are established. The analysis of pair distribution functions (PDF) and Li-density maps elucidates the correlation between structural disorder and ionic conductivity.

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14:45-15:00 Discussion

15:00-15:30 Coffee

15:30-16:00 Paradigms of structural, chemical, and dynamical frustration in superionic conductors

Dr Brandon Wood, Lawrence Livermore National Laboratory, USA

Abstract

Rationally motivated computational discovery and optimization of solid electrolytes require the development of reliable descriptors for fast solid-state ionic conductivity. However, many of the fundamental motivations for superionic behaviour in solids remain enigmatic, which has generally slowed progress in screening new candidates or tuning existing materials to maximize ionic conductivity. Dr Wood will discuss the use of high-performance computer simulations and advanced analytical techniques to unravel various mechanisms of ionic conductivity in model classes of solid electrolytes. Using computational “experiments”, the simulations systematically isolate factors such as stoichiometry, strain, composition, crystal structure, and local environment in the determination of ionic conductivity. Collectively, the results point to the importance of a frustrated energy landscape in promoting ultrafast diffusion. Different types of frustration in model superionic conductors will be discussed, arising from factors such as off-stoichiometry, competition between interstitial site occupancies, symmetry incompatibilities between local bonding character and lattice geometry, and dynamical frustration coupled to anharmonic lattice motion. Dr Wood will explore the physicochemical relevance of these factors for understanding and promoting cation mobility, with a view towards developing design rules for engineering faster ionic conductors. Among the topics to be discussed is the dependence of the different frustration paradigms on the fundamental nature of the lattice-forming ions, which suggests there may be no single universal descriptor for ionic conductivity, but rather classes of superionic conductors with similar underlying motivations. Specific examples will be drawn from recent results on superionic materials based on oxides, halides, and polyatomic anions. 

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16:00-16:15 Discussion

16:15-17:00 Panel discussion

Understanding fast-ion conduction in solid electrolytes - POSTPONED Theo Murphy international scientific meeting organised by Dr Benjamin Morgan, Dr Wolfgang Zeier and Professor Serena Corr. Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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