Ion Transport In Solids Research Articles

Solid‐State Revolution: Assessing the Potential of Solid Polymer Electrolytes in Lithium‐Ion Batteries

AbstractLithium‐ion batteries (LIBs) are crucial for achieving sustainable energy goals due to their high energy density and long cycle life. They dominate markets like consumer electronics, electric vehicles, and stationary energy storage systems. However, current LIBs use liquid electrolytes, which are toxic, flammable, and their liquid state does not resist dendrite growth, causing battery capacity decline and failure. Additionally, the limited availability of lithium and other metals makes liquid‐based LIBs less sustainable. On the other hand, solid polymer electrolytes (SPEs) offer a safer alternative as they are non‐volatile and can resist dendrite growth. However, ion transport in solids is much more restricted than in liquids, while imperfect solid‐solid interfaces contribute to interfacial resistance leading to lower ionic conductivity and increasing Ohmic losses or requiring battery operation at elevated temperatures. Chemical and mechanical degradation of these interfaces can also result in battery capacity fade, and poorer cyclic performance compared to liquid electrolytes. Understanding the ionic transport mechanisms in SPEs is critical for designing and optimizing the nanostructure of polymers and polymer/electrode interfaces to overcome these limitations. In this review, the fundamental mechanisms of ion transport in SPEs will first be explored. Various state‐of‐the‐art approaches for addressing the key challenges in SPEs and their solutions are then discussed. Furthermore, the current status of SPEs is analyzed to determine their potential for replacing liquid electrolytes in the future.

Diffusion-facilitated transport of self-driven particles in polycrystalline structures

We study particle transport in a polycrystalline system with mesoscale regular structures, which was inspired by ionic transport in solids. We found a novel type of congestion dynamics in which the density of particles spontaneously became strongly inhomogeneous with increasing driving force field and inverse temperature, leaving strongly localized pathways that allowed for substantial particle current. Consequently, particle current was maximized at a certain driving force strength and inverse temperature, which effectively avoided overcrowding of particles via a moderate level of diffusion. Our findings not only extend the theory of collective transport of self-propelled particles but also provide a baseline for understanding the transport properties of real polycrystalline systems, including superionic materials. Published by the American Physical Society 2024

Can large lattice volume always facilitate ion diffusion in solids?

Large lattice volume is usually expected to expand the bottleneck size and enlarge free volume for fast Li ion transport in solids. But the universality of lattice expansion decreasing activation energy barrier for Li ion migration is still debatable. Herein, our molecular dynamics simulations and density functional theory calculations detect the unconventional phenomenon of lattice expansion increasing activation energy barrier for Li ion migration in β-Li3PS4, Li4SnS4 and Li4GeS4 sulfides with the hcp-type anion sublattice, and discover the underlying mechanism. On one hand, lattice expansion enlarges the relative energies between the octahedral and tetrahedral Li sites, and correspondingly increase activation energy barrier for tetrahedral Li ions migration along the Tet-Oct-Tet migration paths in β-Li3PS4, Li4SnS4 and Li4GeS4. On the other hand, the lattice dynamics calculations show Li energy landscapes of β-Li3PS4, Li4SnS4 and Li4GeS4 are anharmonic and enlarging lattice volume make the local minimum of Li potential energy surface flatter, simultaneously reduce the relative energy of Li occupation site, eventually resulting in the higher activation energy barrier for Li ion migration. Thus, understanding the effect of lattice volume on ion migration in solids should not only consider the factor of bottleneck size and free volume, but also combine the aspects of Li ion occupation type, Li ion migration path and anion sublattice type. These new understandings of the lattice expansion increasing activation energy barrier for ion migration would further promote the further optimization and rational design of superionic conductors by element doping or strain engineering.

Diameter-dependent ultrafast lithium-ion transport in carbon nanotubes.

Ion transport in solids is a key topic in solid-state ionics. It is critical but challenging to understand the relationship between material structures and ion transport. Nanochannels in crystals provide ion transport pathways, which are responsible for the fast ion transport in fast lithium (Li)-ion conductors. The controlled synthesis of carbon nanotubes (CNTs) provides a promising approach to artificially regulating nanochannels. Herein, the CNTs with a diameter of 5.5 Å are predicted to exhibit an ultralow Li-ion diffusion barrier of about 10meV, much lower than those in routine solid electrolyte materials. Such a characteristic is attributed to the similar chemical environment of a Li ion during its diffusion based on atomic and electronic structure analyses. The concerted diffusion of Li ions ensures high ionic conductivities of CNTs. These results not only reveal the immense potential of CNTs for fast Li-ion transport but also provide a new understanding for rationally designing solid materials with high ionic conductivities.

Microscopic theory of ionic motion in solids

Drag and diffusion of mobile ions in solids are of interest for both purely theoretical and applied scientific communities. This article proposes a theoretical description of ion drag in solids that can be used to estimate ionic conductivities in crystals, and forms a basis for the rational design of solid electrolyte materials. Starting with a general solid-state Hamiltonian, we employ the nonequilibrium path integral formalism to develop a microscopic theory of ionic transport in solids in the presence of thermal fluctuations. As required by the fluctuation-dissipation theorem, we obtain a relation between the variance of the random force and friction. Because of the crystalline nature of the system, however, the two quantities are tensorial. We use the drag tensor to write down the formula for ionic mobility, determined by the potential profile generated by the crystal's ions.

Insights of the Ionic Transport in Intercalated Two-Dimensional Materials Leveraging Lattice Dynamics

Ionic transport in solids is a key factor in affecting battery performance. Inspired by the recent findings of the correlation between lattice dynamics and ionic transport, we conduct a more in-depth analysis of phonon spectra of ionic intercalation structures LixMoS2. In this paper, phonon dispersion curves and projected density of states of intercalation structures at different Li concentrations are calculated to compare with the corresponding migration barrier. The result shows that Li-dominated phonon modes are mainly distributed at the phonon gap of MoS2, and these modes tend to fill the gap at higher Li concentration. At the frequency range of these Li-dominated modes, the energy occupation of the migration ion is put forward as an appropriate descriptor that is found to be logarithmically related to the migration barrier. On the basis of the relationship, the Arrhenius formula for diffusion coefficient is modified and some parameters are estimated. Our study represents the power of the lattice dynamics in analyzing ionic transport, which can be used to distinguish the migration pattern and predict migration paths. Lattice dynamics provide the methodology to regulate ionic transport by phonon engineering and screen for better 2D materials.

Argyrodite-type advanced lithium conductors and transport mechanisms beyond paddle-wheel effect

Development of next-generation solid-state Li-ion batteries requires not only electrolytes with high room-temperature (RT) ionic conductivities but also a fundamental understanding of the ionic transport in solids. In spite of considerable work, only a few lithium conductors are known with the highest RT ionic conductivities ~ 0.01 S/cm and the lowest activation energies ~0.2 eV. New design strategy and novel ionic conduction mechanism are needed to expand the pool of high-performance lithium conductors as well as achieve even higher RT ionic conductivities. Here, we theoretically show that lithium conductors with RT ionic conductivity over 0.1 S/cm and low activation energies ~ 0.1 eV can be achieved by incorporating cluster-dynamics into an argyrodite structure. The extraordinary superionic metrics are supported by conduction mechanism characterized as a relay between local and long-range ionic diffusions, as well as correlational dynamics beyond the paddle-wheel effect.

Solid-state ionics: The key to the discovery and domination of lithium batteries: some learnings from β-alumina and titanium disulfide

Solid-state ionics, the study of fast ion transport in solids, expanded explosively after the discovery of sodium ion transport in β-alumina 50 years ago and has revolutionized energy storage. Lithium-ion batteries have come from a dream with titanium disulfide to enabling the communications revolution and are enabling renewable energy. Much can be learned from these early studies that will allow the attainment of a fossil-free society and a sustainable environment.

Recent Developments of Nanomaterials and Nanostructures for High-Rate Lithium Ion Batteries.

Lithium ion batteries have been considered as a promising energy-storage solution, the performance of which depends on the electrochemical properties of each component, including cathode, anode, electrolyte and separator. Currently, fast charging is becoming an attractive research field due to the widespread application of batteries in electric vehicles, which are designated to replace conventional diesel automobiles in the future. In these batteries, rate capability, which is closely linked to the topology and morphology of electrode materials, is one of the determining parameters of interest. It has been revealed that nanotechnology is an exceptional tool in designing and preparing cathodes and anodes with outstanding electrochemical kinetics due to the well-known nanosizing effect. Nevertheless, the negative effects of applying nanomaterials in electrodes sometimes outweigh the benefits. To better understand the exact function of nanostructures in solid-state electrodes, herein, a comprehensive review is provided beginning with the fundamental theory of lithium ion transport in solids, which is then followed by a detailed analysis of several major factors affecting the migration of lithium ions in solid-state electrodes. The latest developments in characterisation techniques, based on either electrochemical or radiology methodologies, are covered as well. In addition, state-of-the-art research findings are provided to illustrate the effect of nanomaterials and nanostructures in promoting the rate performance of lithium ion batteries. Finally, several challenges and shortcomings of applying nanotechnology in fabricating high-rate lithium ion batteries are summarised.

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.

Collective dipole effects in ionic transport under electric fields

In the context of ionic transport in solids, the variation of a migration barrier height under electric fields is traditionally assumed to be equal to the classical electric work of a point charge that carries the transport charge. However, how reliable is this phenomenological model and how does it fare with respect to Modern Theory of Polarization? In this work, we show that such a classical picture does not hold in general as collective dipole effects may be critical. Such effects are unraveled by an appropriate polarization decomposition and by an expression that we derive, which defines the equivalent polarization-work charge. The equivalent polarization-work charge is not equal neither to the transported charge, nor to the Born effective charge of the migrating atom alone, but it is defined by the total polarization change at the transition state. Our findings are illustrated by oxygen charged defects in MgO and in SiO2.

Classical and Emerging Characterization Techniques for Investigation of Ion Transport Mechanisms in Crystalline Fast Ionic Conductors.

Ion transport in crystalline fast ionic conductors is a complex physical phenomenon. Certain ionic species (e.g., Ag+, Cu+, Li+, F-, O2-, H+) in a solid crystalline framework can move as fast as in liquids. This property, although only observed in a limited number of materials, is a key enabler for a broad range of technologies, including batteries, fuel cells, and sensors. However, the mechanisms of ion transport in the crystal lattice of fast ionic conductors are still not fully understood despite the substantial progress achieved in the last 40 years, partly because of the wide range of length and time scales involved in the complex migration processes of ions in solids. Without a comprehensive understanding of these ion transport mechanisms, the rational design of new fast ionic conductors is not possible. In this review, we cover classical and emerging characterization techniques (both experimental and computational) that can be used to investigate ion transport processes in bulk crystalline inorganic materials which exhibit predominant ion conduction (i.e., negligible electronic conductivity) with a primary focus on literature published after 2000 and critically assess their strengths and limitations. Together with an overview of recent understanding, we highlight the need for a combined experimental and computational approach to study ion transport in solids of desired time and length scales and for precise measurements of physical parameters related to ion transport.

Numerical comparison of usual Arrhenius-type equations for modeling ionic transport in solids

The primary objective of this work is to draw attention to the inherent differences between Arrhenius-type equations used for modeling ionic conductivity in solids. The used methodology comprises a comparison between the two most used Arrhenius-type equations by generating data applying one equation and fitting them by using the other equation. Data are generated in different conditions by varying input parameters such as the extent of the temperature range and the average temperature of this range, as well as the pre-exponential term and activation energy of the Arrhenius-type equation. The fitting procedure is performed by plotting the generated data based on the linearized form of those equations followed by the linear fitting. The results show a substantial difference between the activation energy value of those equations, which is mainly dependent on the average temperature of the range. In contrast with the widely held perception that differences in the activation energy of both equations usually can be neglected, this work reveals that relative differences between activation energy of these equations are usually significant. These relative differences are often around 10% and may reach over 15% depending on a combination of intrinsic and extrinsic input parameters such as low activation energy and high-temperature of measurement, respectively. An analytical equation that expresses the relative difference between activation energy calculated from both equations is also presented and discussed.

Measurement of ion mobility based on a reversible migration process in solids

Ion mobility, which facilitates ion migration in lattices, not only is important for selection of solid electrolytes from a large number of candidate materials but also plays an essential role in the study of the fundamentals of ion transport in solids. In the present work, a simple approach is developed for measuring the mobility of oxygen ions in a typical solid electrolyte, lanthanum-doped ceria. Changes in the concentration of oxygen vacancies, which are equivalent to variations in oxygen ions, are recorded using integrated peak intensity ratios of Raman spectra under a reversible electric field. A model of the migration process of oxygen ions in a reversing electric field is proposed based on the experimental results. The mobility of oxygen ions is determined by analyzing the time interval between two steady states based on a simplified model. By altering the electric field, this approach is applicable to the measurement of the mobilities of different ion species.

Bonding character and ionic conduction in solid electrolytes

Abstract The properties of the materials are intimately related to the nature of the chemical bond. Research to explain the peculiarities of superionic materials by focusing on the bonding character of the materials is presented. In particular, a brief review of some fundamental aspects of superionic conductors is given based on the talk presented at “Solid State Chemistry 2018, Pardubice” in addition to some new results related to the subject. Specifically, the topics on bond fluctuation model of ionic conductors, the role of medium range structure in the ionic conductivity, bonding aspects of non-Arrhenius ionic conductivity and elastic properties of ionic conductors are discussed. Key concepts that are gained from these studies is stressed, such as the importance of the coexistence of different types of bonding, and the role of medium range structure in glasses for efficient ionic transport in solids. These concepts could help the development of new materials.

Field-enhanced ion transport in solids: Reexamination with molecular dynamics simulations

Classical molecular-dynamics simulations were used to examine the effect of an electric field on the mobility of oxygen ions in the model crystalline oxide ${\mathrm{CeO}}_{2}$. Simulation cells containing oxygen vacancies were subjected at temperatures $1000\ensuremath{\le}T/\mathrm{K}\ensuremath{\le}1600$ to electric field strengths $0.1\ensuremath{\le}E/\mathrm{MV}{\mathrm{cm}}^{\ensuremath{-}1}\ensuremath{\le}40$ to obtain the oxygen-ion mobility ${u}_{\mathrm{i}}(E,T)$. In addition, static nudged-elastic-band calculations were performed to obtain directly the forward/reverse barriers for oxygen-ion migration, $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{mig}}^{\mathrm{f}/\mathrm{r}}$. Qualitatively, ${u}_{\mathrm{i}}$ behaves as expected: independent of $E$ at low values of $E$ and exponentially dependent on $E$ at high values. The quantitative (standard) Mott-Gurney treatment, however, underestimates $\mathrm{\ensuremath{\Delta}}{H}_{\mathrm{mig}}^{\mathrm{f}}$ at high $E$ and thus overestimates ${u}_{\mathrm{i}}$. A new, superior analytical expression for ${u}_{\mathrm{i}}(E,T)$ is consequently derived.

Novel Solid Electrolytes for Li-Ion Batteries: A Perspective from Electron Microscopy Studies

Solid electrolytes can simultaneously overcome two of the most formidable challenges facing current state-of-art Li-ion batteries: the severe safety issues and insufficient energy densities. On the other hand, two major limitations of solid electrolytes must be overcome before they can be implemented for practical use: the relatively slow ionic conductivity and their limited interfacial stability and conductivity with electrodes. Since ion transport in solids and their interfaces is based on a hopping mechanism from atomic sites to nearby vacancy sites or suitable interstitial sites, a prerequisite to overcome these challenges is a thorough understanding of the material’s behavior at the microscopic and/or the atomic level. Transmission electron microscopy is a powerful tool that provides the spatial resolution required. Here, we review recent electron microscopy investigations towards understanding the ion transport behavior in solid electrolyte materials and their interfaces. Specifically, three aspects of recent microscopy research will be highlighted: the influence of individual grain microstructure on ion conductivity, the contribution of grain boundaries, and the behavior of solid electrolyte/electrode interfaces. Based on these results, the perspectives for future research will be discussed.

Rational Design of Cathode Materials for High Rate Performance Sodium-Ion Batteries

Sodium ion batteries attracted a large research community because of earth-abundance, low cost and possibility of high energy density batteries. Despite the cost motivation of the research a considerable fraction of this activity still focuses on electrode materials analogous to those in Li-ion batteries often containing costly transition metals such as Co, which appears problematic for various reasons: differences in reaction mechanisms of the Na analogues to the Li transition metal oxides, different structural requirements for Na+ mobility, the fact that a cost advantage of Na over Li will only translate into low cost batteries if the concept of earth-abundance is applied consistently. Understanding of transport in solids both electrode and electrolyte is pivotal in designing such batteries. The analysis and prediction of ion transport in solids from static and dynamic structure models has become an interesting application for the bond valence (BV) approach [1]. Here we present a rational and quantitative characterization of rate performances of insertion-type cathode materials based on the diffusion relaxation model. Bond-valence site energy modelling and DFT simulations were employed to clarify Na+ ion migration barriers in more than 20 sodium-ion battery cathode materials based on earth-abundant elements. Migration energy barriers calculated from our bond valence site energy models closely agree with the DFT models and with the help of the diffusion relaxation model allow for a direct semi-quantitative prediction of the rate performance of half-cells based on local structure models. The demonstrated method is computationally cheap enough to be applied for the fast screening of candidate structure databases and to allow for the testing of a series of local structure models (e.g. to explore the role of antisite dopants or stoichiometry deviations) for each structure type. Rate performance predictions are tested with molecular dynamics and Kinetic Monte Carlo simulations and yield specific guidelines for the design of novel sodium-ion battery cathode materials in terms of pathway dimensionality, migration barriers the effect of low-lying unoccupied sites, dopants or antisite defects as well as particle sizes. As an example of the materials identified by the computational study as promising for high rate performance the alluaudite-type non-stoichiometric Na2+xFe2-x/2(SO4)3, (wherein Fe vacancies and NaFe / antisite defects cross-link the otherwise one-dimensional Na+ pathways [2]) was synthesized by mechanical milling of Na2SO4 and FeSO4 followed by annealing at 400ºC under argon environment and its favourable rate performance is demonstrated in a room-temperature all-solid state sodium-ion battery using Na3PS4 as the solid electrolyte.

Solid Polymer Electrolytes Based on Chitosan and Europium Triflate

The solid polymer electrolytes (SPEs) have been extensively studied since the pioneering work of Wright and Armand et al[1, 2]. Previous studies were focused on poly(ethylene oxide) (PEO) and lithium salts [3]. However recent research aimed at developing SPE based on natural polymers, which are interesting due to their natural abundance, low price and environmentally friendly nature [4]. Chitosan is a natural polymer obtained from chitin and it has been largely investigated due to its biodegradability, non-toxicity, absorption properties and biocompatibility. In this presentation are described the preparation and characterization of novel systems from chitosan and europium (III) trifluoromethanesulfonate salt. The transparent samples were characterized by thermal analysis (DSC and TGA), impedance spectroscopy and electron paramagnetic resonance (EPR). The Chitosan55.04%Eu(Trif)3 sample showed the best ionic conductivity of 1.52×10-6 and 7.66×10-5 S∙cm-1 at 25ºC and 80ºC, respectively. The thermal analysis revealed that the degradation started at around 120 - 145ºC and the weight loss ranged from 15 to 40%. The EPR analysis showed a broadening of the EPR resonance lines with increasing europium contents in the chitosan membranes due to the magnetic dipole–dipole coupling and spin-spin exchange between the Eu2+ions. ACKNOWLEDGEMENTS We are grateful to the Fundação para a Ciência e Tecnologia (PestC/QUI/UI0686/2013, POPH/FSE for a grant SFRH/BD/97232/2013 (R. Alves)) for financial support of this work. The authors are also grateful to, FAPESP, CNPq and CAPES for financial support. M. M. Silva acknowledges CNPq, for the mobility grant provided by this institution. REFERENCES [1] P. V. Wright, British Polymer Journal 7 (1975) 319. [2] M. B. Armand. J. M. Chabagno, M. J. Duclot, In Fast Ion Transport in Solids; Vashishta, P., Mundy, J. N., Shenoy, G. K., Eds.; North-Holland: Amsterdam, The Netherlands, 1979, p 131. [3] P. K. Varshney, S. Gupta, Ionics 17 (2011) 479-483. [4] I. S. M. Noor, S. R. Majid, A. K. Arof, D. Djurado, S. C. Neto, A. Pawlicka, Solid State Ionics 225 (2012) 649-653.

Recent Progress in Interfacial Nanoarchitectonics in Solid-State Batteries

Employing solid electrolytes in lithium-ion batteries is anticipated to be a solution to some issues originated from their organic electrolytes. However, solid-state lithium batteries had not been practicable, because ionic conductivities of lithium-ion conductive solid electrolytes had been very low. Therefore, many studies aiming at the development of solid-state lithium batteries have been focused on enhancement of ionic transport in solids. They have succeeded in enhancing the conductivity to be comparable to or even higher than that of liquids. At this stage, rate-determining step of the battery reactions is sometimes ionic transport at interface rather than bulk in the battery components. Since anomalous ionic conduction at the interface takes place in space-charge layers with ca. 10 nm in thickness, it will be controlled by “nanoarchitectonics”. This paper reviews some interfacial nanoarchitectures that control the interfacial ionic conduction to enhance the performance of solid-state lithium batteries.