Understanding fast-ion conduction in solid electrolytes

Ion mobility in solids is important for the development of solid-state batteries that employ solid electrolytes. Early studies have argued that the rotational dynamics of complex (or cluster) anions play a role in fostering cation motion, typically at temperatures above a volume-increasing polymorphic phase transformation. This talk will discuss the impact of anion dynamics on cation mobility across several ion-conducting solids. Detailed atomic-scale simulations are performed to map out the nature of anion motion and to quantify correlations with cation mobility in space and in time. Analyses are conducted to identify the circumstances under which anion rotations may occur. With this knowledge it may be possible to design solid electrolytes whose cation mobility is intentionally enhanced by anion dynamics.

Professor Donald Siegel, University of Michigan, USA

Professor Donald Siegel, University of Michigan, USA

Don Siegel is Professor and Chair of the Walker Department of Mechanical Engineering at the University of Texas at Austin (UT). At UT he is a Temple Foundation Endowed Professor and holds a Cockrell Family Chair for Departmental Leadership. He is a member of the Texas Materials Institute and the Oden Institute for Computational Engineering and Sciences. Prior to joining UT in 2021, Professor Siegel spent 12 years as a faculty member at the University of Michigan.

Siegel is a computational materials scientist whose research targets the development of energy storage materials and lightweight alloys. He is a recipient of the NSF Career Award and a Gilbreth Lectureship from the National Academy of Engineering.

Professor Siegel received a Ph.D. in physics from the University of Illinois at Urbana-Champaign. His postdoctoral training was performed at Sandia National Laboratories and at the U.S. Naval Research Lab. In 2015-2016 he was a Visiting Professor at the Technical University of Denmark.

Superionic conductors are solid-state materials that exhibit liquid-like ionic conductivities, which that makes them promising candidates for applications in energy harvesting and storage. In a subclass of these materials, the logarithm of the ion conductivity exhibits a change in its slope with respect to inverse temperature. Interestingly, these temperature-induced changes in the apparent energy barrier for ion transport occur without the materials undergoing any structural changes in their framework. In this work, the authors use classical molecular dynamics (MD) simulations to elucidate the atomistic mechanisms underlying such non-Arrhenius behaviour in the superionic conductor AgCrSe₂. They first derive interatomic potential energy functions for mobile and framework ions based on trajectories collected from ab-initio MD (AIMD) simulations. Through a multistate iterative Boltzmann inversion approach, the authors develop minimal force field that reproduces the interatomic radial distribution functions, phonon density of states, and short-time Ag+ ion dynamics calculated from AIMD. The force field is then used within long timescale MD simulations to predict ionic conductivity and illuminate on the mechanism of ion diffusion. Apart from reproducing the experimentally measured conductivity of Ag+ ions in AgCrSe₂, the simulations reveal how the non-Arrhenius behaviour in ion conductivity emerges from temperature-induced variations in the fraction of 'active' ions available for migration induced by an order-disorder transition in the Ag+ sublattice. The simulations also reveal the crucial role played by the repulsion between ions occupying adjacent sublattice sites in producing the order-disorder transition, intriguing temperature-dependent spatiotemporal correlations exhibited by ions, and the role of lattice flexibility in both lowering the energy barriers for ion migration and mitigating ion repulsion effects to lower the order-disorder temperature. The transport mechanisms revealed here along with the modified activated-transport model of ion diffusion proposed here should be relevant to a broad range of superionic materials.

Professor Gaurav Arya, Duke University, USA

Professor Gaurav Arya, Duke University, USA

Gaurav Arya is a Professor of Mechanical Engineering and Material Science at Duke University. Prior to joining Duke in Fall 2017, he was an Assistant Professor and then Associate Professor in the Department of NanoEngineering at UC San Diego. He obtained his BTech degree in Chemical Engineering from IIT Bombay in 1998, and PhD degree, also in Chemical Engineering, from the University of Notre Dame in 2003. He carried out his postdoctoral research at Princeton University and New York University. Professor Arya’s research focuses on the development and application of molecular simulations to provide a fundamental, molecular-level understanding of various biomolecular and nanoscale systems of interest, including protein motors, chromatin, ion-transport materials, DNA nanostructures, and nanoparticle-polymer composites.

Within information theory, heat-activated diffusion and transport can be treated as the erasure of enthalpically stabilized memory of an initial state over time. First, this conceptual framework is tested using molecular dynamics simulations of the historic model superionic conductors, the beta- and beta"-aluminas [1]. The simulated distributions of displacements approach maximum-entropy Gaussian distributions only slowly, and ion transport is sub-diffusive to macroscopic timescales. From the features and descriptors of sub-diffusion, the full production of information entropy and elimination of path-dependent effects emerge as necessary criteria for the establishment of detailed balance and ergodic behaviour. The time dependence of correlations in ion transport is tested experimentally using a non-linear optical measurement [2]. Single-cycle terahertz pulses near-resonant with phononic vibrations excite ionic hopping in solid electrolytes, and transient birefringence probes the pump-induced anisotropy of subsequent hops. The picosecond-timescale relaxation of birefringence measures the decay of orientational memory as a time-dependence of the correlation factor in ion diffusion. Finally, computational extension of the optical experiments allows for a determination of vibrational attempt frequencies for ion hopping.

[1] Poletayev, Dawson, Islam, Lindenberg, Nature Materials (2022) 21, 1066-1073.
[2] Poletayev, Hoffmann, Dawson, Teitelbaum, Trigo, Islam, Lindenberg, arXiv:2110.06522.

Dr Andrey Poletayev, University of Oxford, UK

Dr Andrey Poletayev, University of Oxford, UK

Dr Andrey Poletayev is a postdoctoral scholar at the University of Oxford. He has completed his PhD at Stanford University. His work has focused on triggering and probing ionic conduction at picosecond timescales, and bridging atomistic to macroscopic models of ion conduction using stochastic and non-equilibrium thermodynamics. He continues to contribute to a carbon-free economy by studying energy-storage materials with atomistic simulations, electrochemistry, and ultrafast spectroscopy.

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. The author shows 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.

Professor Christopher Ling, The University of Sydney, Australia

Professor Christopher Ling, The University of Sydney, Australia

Chris Ling is a Professor in the School of Chemistry at the University of Sydney. He has particular interests in solid-state ionic conduction at high temperatures, and strongly-correlated electronic behaviour at low temperatures. His research in inorganic solid-state chemistry makes particular use of neutron and synchrotron X-ray scattering methods to characterise the structures and dynamics of new materials. Chris is currently a Theme Leader for Functional Energy Materials in the School of Chemistry, Deputy Director of the Sydney Analytical Core Research Facility, and Chair of the IUCr Commission on Inorganic and Mineral Structures.

The garnet Li₇La₃Zr₂O₁₂ (LLZO) is one of the most promising solid electrolyte candidates due to its large electrochemical stability window against a lithium anode, reasonable ionic conductivity, from 10^-4 to 10^-3 S cm^-1, and mechanical strength. However, the effect of the polycrystalline microstructure is not completely solved. The author compares the atomic-scale diffusion mechanisms in the amorphous grain-boundaries versus the bulk grains and calculate the effect of the microstructure on the effective diffusivity. The Li-ion diffusion is simulated with molecular dynamics and the author shows that using an accurate potential is important to accurately understand the diffusion mechanism.

Professor Nicole Adelstein, San Francisco State University, USA

Professor Nicole Adelstein, San Francisco State University, USA

Nicole Adelstein is an Assistant Professor in the Department of Chemistry and Biochemistry at San Francisco State University. Together with her group of undergraduate and master students, she studies ionic diffusion in solid-state materials using molecular dynamics, both quantum and classical simulations. Before arriving at SFSU in 2015, Nicole was a post-doc at Lawrence Livermore National Laboratory for three years. She received her PhD from the University of California, Berkeley in Materials Science and Engineering and her BA from Reed College in Chemistry and Physics. 

The visualization of atomic or ionic jump processes on the Ångström length scale is important to identify the preferred diffusion pathways in solid electrolytes for energy storage devices. 2D high-resolution ⁶Li NMR exchange spectroscopy (EXSY) is highly suited to yield site-specific insights into local Li⁺ hopping processes within a single measurement. Here, the beta-modification of Li₃VF₆ is used as a model system for such an investigation as it provides a range of important Li⁺ geometric environments in one and the same crystal structure that are useful to elucidate qualitatively a ranking of energetic preferences of the Li⁺ exchange processes. It turned out that Li⁺ ion jumps between face-shared polyhedra are preferred, followed by Li⁺ exchange between edge-shared configurations for which interstitial sites are needed to jump from site to site. Surprisingly, Li⁺ exchange between corner-shared polyhedra and Li⁺ hopping involving almost isolated LiF₄ polyhedra do contribute to overall Li⁺ self-diffusion as well. In this sense, the current study experimentally verifies current predictions by theory but also extends our understanding of ion dynamics between corner-shared Li-bearing polyhedra. 

Professor Martin Wilkening, Graz University of Technology, Austria

Professor Martin Wilkening, Graz University of Technology, Austria

Martin Wilkening has been a Full Professor for Physical Solid state Chemistry at the Graz University of Technology since 2011. He is Director of the Christian Doppler Laboratory of Lithium Batteries - a collaboration between TU Graz and industrial companies such as AVL, Infineon and TDK Electronics. He studied Chemistry at the Leibniz University Hannover and obtained his doctoral degree in 2005 with a dissertation on ‘Ultraslow Li Motions in Solids’. His PhD thesis was awarded the Starck-Promotionspreis of the German Chemical Society (GDCh) and the Wissenschaftspreis Hannover. In 2009 he received the ADUC annual award of the GDCh for his contributions to spin-alignment echo NMR to directly probe the shape of single-spin motional correlation functions in solids.

In his workgroup at the TU Graz, cation and anion dynamics in solids are studied with a broad range of NMR techniques and broadband conductivity spectroscopy. Specifically, his research team focusses on self-diffusion processes in nanostructured and amorphous energy materials including mechanosynthesized non-equilibrium compounds and nanoglasses. One overarching aim is to identify the origins of rapid ion exchange in model substances. His work aims to learn how the dimensionality of diffusion pathways and correlation effects influence the elementary steps of ion hopping.

Chemical and mechanical changes local to the electrode/electrolyte interface critically impact performance in all-solid-state batteries. Unfortunately, the dynamics at electrochemical interfaces are exceptionally challenging to probe in all-solid-state batteries because these changes take place across multiple length scales (from the nano- to meso-scale) and are buried within the system (at the solid/solid electrode/electrolyte interface). Here, the author will show their efforts to couple operando acoustic transmission measurements with nuclear magnetic resonance spectroscopy and imaging to correlate changes in interfacial mechanics with the growth of Li microstructures and the solid electrolyte interphase (SEI) in a non-invasive, multimodal fashion. Specifically, the research group studies chemo-mechanical changes at the interface between Li metal anodes and Li₇La₃Zr₂O₁₂ solid electrolytes as a function of stack pressure and current density.

Professor Lauren Marbella, Assistant Professor in the Department of Chemical Engineering, Columbia University, USA

Professor Lauren Marbella, Assistant Professor in the Department of Chemical Engineering, Columbia University, USA

Lauren Marbella is an Assistant Professor in the Department of Chemical Engineering at Columbia University. Her research group focuses on understanding the relationship between electrochemical performance and interfacial chemistry in devices for energy storage and conversion. Her research relies heavily on the use of nuclear magnetic resonance imaging (MRI) and spectroscopy to evaluate changes in material properties in real time to elucidate the chemical mechanisms underpinning degradation in Li and beyond Li ion battery systems. Marbella’s research has received numerous awards including the Cottrell Scholar Award (2022), the National Science Foundation (NSF) Faculty Early Career Development (CAREER) Award (2021), and the Scialog Collaborative Innovation Award for Advanced Energy Storage (Sloan Foundation, 2019).

Marbella received her PhD in chemistry from the University of Pittsburgh in 2016, under the direction of Professor Jill Millstone. In 2017, she was named a Marie Curie Postdoctoral Fellow at the University of Cambridge in the group of Professor Clare Grey. There, she was also named the Charles and Katharine Darwin Research Fellow, which recognizes the top junior fellow at Darwin College at the University of Cambridge. She joined the chemical engineering faculty at Columbia University in 2018.

Li ions transport is the key sought physical property and, in this contribution, we will reveal several efficient NMR methods to directly probe the Li ions dynamics in a range of recently discovered sulphide^2,4–6- and oxide^7–9-containing materials. We exploit a range of variable temperature multinuclear (⁶Li and ⁷Li) and multidimensional NMR approaches, such as line shape analysis, exchange phenomena and relaxometry measurements, to determine the Li ion mobility pathways, including the dimensionality of the diffusion processes, and quantify Li ions jump rates. For example, these approaches deployed on Li₃AlS₃² identify that Li ion diffusion is fast within the tetrahedral and tetrahedral/octahedral layers but slow between these layers limiting long range translational Li ion mobility.¹⁰ These provide a framework for the further development of more highly conductive Li solid-state electrolytes such as Li_4.3AlS_3.3C_l0.7.⁵ 

References: (1) Collins, C. et al. Nature 2017, 280. (2) Gamon, J. et al. Chem. Mater. 2019, 9699. (3) Vasylenko, A. et al. Nat. Commun. 2021, 5561. (4) Leube, B. T. et al. Chem. Mater. 2018, 7183. (5) Gamon, J. et al. Chem. Mater. 2021, 8733. (6) Morscher, A. et al. Chem. Mater. 2021, 2206. (7) Enciso-Maldonado, L. et al. Chem. Mater. 2015, 2074. (8) Santibáñez-Mendieta, A. B. et al. Chem. Mater. 2016, 7833. (9) Han, G. et al. J. Am. Chem. Soc. 2021, 18216. (10) Duff, B. B. et al. submitted. 

Professor Frédéric Blanc, University of Liverpool, UK

Professor Frédéric Blanc, University of Liverpool, UK

Frédéric obtained his PhD in chemistry in 2008 from the Université de Lyon and the Centre for High Field NMR at the Ecole Normale Supérieure de Lyon, under the supervision of Professor Christophe Copéret and Professor Lyndon Emsley. He studied the structure-dynamics relationship in heterogeneous catalysis and developed new solid state NMR methods to understand the chemistry of catalysts on surfaces. He then received a Lavoisier fellowship and underwent postdoctoral work at the State University of New York in Stony Brook, NY in 2008-2010 with Professor Clare P Grey, where he got interested in the understanding of the structure elucidation of energy materials with NMR spectroscopy. He then moved to the University of Cambridge as a Marie Curie fellow in 2011 and looked at the dynamics of oxygen and protons in electrolytes by high temperature NMR spectroscopy. In December 2012, he was appointed to a lectureship in the Department of Chemistry and the Stephenson Institute for Renewable Energy at the University of Liverpool and was promoted to Senior Lecturer in 2016, Reader in 2018 and to a Personal Chair in March 2022, where he is focusing on the development and application of NMR spectroscopy in materials chemistry. Recent work has focused on enhancing the NMR signal of unreceptive nuclei such as 17O at natural abundance and on exploiting the ability of NMR to probe dynamics to investigate a range of solid state Li ion materials as Li conductors.