(Invited) Entropy-Enhanced Ion Transport in Solid Electrolytes

Although solid-state batteries had been suffering from low rate capability due to low ionic conductivities of solid electrolytes, conductivities comparable to non-aqueous liquid electrolytes used in current lithium-ion batteries have been achieved in some recently-developed solid electrolytes. Especially among sulfide systems, the highest conductivity has reached 10−2 S cm−1 [1], which is even higher that of liquid electrolytes when taking the transport number of unity into account. Consequently, ion transport in solid electrolytes can cease from rate-determining; however, it has been replaced by that across interfaces. Ionic conductors often exhibit anomalous ionic conduction at their surface or interface, which is categorized as “nanoionics” taking place in space-charge layers formed with thickness around 10 nm at the interface or surface. It appears in solid-state batteries with sulfide electrolytes as high interfacial resistances to high voltage cathodes, which result in the unimproved power densities of the solid-state batteries in spite of the high ionic conductivities of the employed solid electrolytes. Classical Nernst equation infers that the high potential of cathode makes the space-charge layers lithium-depleted and thus highly-resistive due to the depletion of charge carriers. The high interfacial resistance has been successfully reduced by an interfacial design based on the space-charge model, where an ion-conducting and electron-insulating oxide is interposed at the interface in order to shield the sulfide electrolyte from the high cathode potential and thus act as a buffer against the lithium depletion [2]. The changes in the electrode properties upon the interposition are well explainable by the space-charge layer model [3]. However, because the space-charge layer that lies between solids with only several nanometers in thickness is difficult to access, it has not revealed its presence even before advanced analyses; whereas it has emerged in computations. Electronic structure of LiFePO4 and Li3PS4, which are cathode and solid electrolyte materials, respectively, obtained by the first principles calculation suggests preferential formation of lithium vacancies on the electrolyte side of the LiFePO4/Li3PS4 interface. In addition, when FePO4/Li3PS4 interface is relaxed in order to simulate the interface structure during the battery operation, lithium ions move from the electrolyte to electrode side to bring about lithium depletion on the electrolyte side.

Objective: Despite recent global efforts to establish all-solid-state batteries (ASSB) as potentially safe and stable high-energy and high-speed storage technology, long-term performance, specific power, and economic viability remain challenging. However, efficient and short ionic and electronic transport pathways and optimized interfacial contacts between the cell materials and compounds are essential. This poses a significant challenge for the microstructure of cathodes and a careful design and tailoring of anodes and separators in ASSB, which rely solely on solid-state contacts. Here, we present our recent work on ASSB with thiophosphate electrolytes conducted within the German Competence Cluster for Solid-State Batteries (FestBatt). Our work aims to investigate relevant parameters in different cell components on material, electrode, and cell level, such as the importance of particle size distributions, microstructures, and kinetic factors to optimize ASSB cathodes, separators, and anodes. Methodology: The work will be presented in four sections, which showcase key challenges as well as optimization strategies mainly focusing on cathode composites, but also discussing selected examples for separators and anodes in ASSB with thiophosphate electrolytes. Further, an outlook for relevant production steps of ASSB electrodes and cells will be given. For these studies, we mainly conducted focused ion beam scanning electron microscopy tomography, electrochemical impedance spectroscopy, cycling experiments on ASSB cells with thiophosphate solid electrolytes. Results: Fast conducting solid electrolytes are necessary to ensure sufficient transport in composite cathodes but does not represent the only key factor for high-quality cathode composites in ASSB. In our studies, we aimed to optimize the microstructure of composite cathodes consisting of intercalation-type active material and a thiophosphate-based solid electrolyte. We varied the particle size distribution of the solid electrolyte to create various cathode microstructures and assessed the effectiveness of electronic and ionic conduction pathways using partial conductivities. We used both electronically and ionically blocking electrodes for electrochemical impedance spectroscopy measurements to quantify the respective partial conductivities. Our analysis revealed that the particle size of the solid electrolyte significantly influences the charge transport and electrochemical performance of the cathodes. Additionally, using FIB-SEM tomography, we created detailed 3D reconstructions of the cathode microstructure and correlated the obtained partial conductivities with microstructural descriptors such as tortuosity factors, identifying possible kinetic bottlenecks. We found that minimizing residual porosity, which blocks ion and electron transport, is necessary to optimize cathode microstructures and enhance ASSB performance. Moreover, high-performance anodes, along with protective concepts, are crucial for establishing dense, high-energy ASSB cells. Lithium metal anodes may not be the ultimate solution, and silicon anodes can pose advantages, e. g. in terms of electrochemical stability for thiophosphate separators. Discussion: Our studies demonstrate the importance of optimizing the microstructure of cathodes and carefully designing Si anodes for improved ASSB cells. By manipulating the particle size distribution of the solid electrolyte, we achieved more efficient electronic and ionic transport pathways, leading to improved battery performance. The presented work is primarily published in references 1-5, detailing key challenges and showcasing optimization strategies for these topics. Beyond that, diversity in materials, research teams, and approaches is crucial for long-term successful development of ASSB. Acknowledgement: Scientific and administrative coordination of the BMBF Competence Cluster for Solid-State Batteries (FestBatt), FB2-Koord (03XP0431), and cell platform Thiophosphates, FB2-Thio (03XP0430A).

Fundamental understanding of the lithium-ion transport mechanism in polymer-inorganic composite electrolyte is crucially important for the rational design of composite electrolytes for solid-state batteries. In this work, the Li+ ion transport pathway in a model composite electrolyte of PEO containing sparsely dispersed LLZO (PEO-LLZO) was studied by an advanced characterization technique, i.e., 6Li-tracer NMR spectroscopy. By analyzing the 6Li distribution within the PEO-LLZO composite at the end of the discharge of an electrochemical cell of 6Li | PEO-LLZO | stainless steel with a fixed capacity (less than the total amount of the Li+ in the composite) at various current densities, it is found that the interfacial barrier between LLZO and PEO could cause a reduced Li+ flux through LLZO, particularly at high current densities, and therefore plays a critical role in determining the Li+ transport pathway in the composite electrolyte. This work provides an intuitive picture of Li+ ion transport in a polymer-inorganic composite electrolyte that is helpful to optimize and design better composite electrolytes.

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

On the road to highly performing solid electrolytes for solid state batteries, aliovalent substitution is a powerful strategy to improve the ionic conductivity. While the substitution allows optimization of the charge carrier concentration, effects on the local structure are often overlooked. Here, by pair distribution function analyses is shown that partial substitution of PS43− by SnS44− polyanion in the fast sodium ionic conductor Na11+xSn2+xP1−xS12 results in discrepancies between the local and average structure. The significantly larger SnS44− polyanions lead to inhomogeneities in the local environments of sodium ions and induce micro strain in the material. The combination of nuclear magnetic resonance spectroscopy and quasi‐elastic neutron scattering reveals a decrease in the activation energy of fast local ionic jumps. The substitution widens the bottleneck size of some diffusion pathways, and a correlation between the increased strain and improved local ionic transport is observed. Local frustrations caused by the induced inhomogeneities may flatten the energy landscape and lead to the detected decrease in the activation barrier. Understanding these effects of cationic substitution on the local structure, induced crystallographic strain and ionic transport can open up new possibilities to design fast conducting solid electrolytes.

Probing disorder and dynamics in composite electrolytes of an organic ionic plastic crystal and lithium functionalised acrylic polymer nanoparticles

Small molecule ion transporters have shown promise as potential therapeutics for microbial infections, cancer and channelopathies. However, there are still gaps in our understanding of how ion transport function in model vesicle membranes translates to cell membranes of interest. The lipid composition of the membranes of bacterial and cancer cells differs markedly from normal human cells, yet the influence of the lipid composition on membrane function is rarely investigated - in part because of the low throughput and high cost of ion transport experiments. Here, we report an increased throughput Workflow to identify biologically relevant, pH-driven ion transport and membrane lysis pathways in vesicle membranes. We developed a set of four assays designed to report on different transport and lysis processes. We validated our assays against a panel of known transporters and produced a stepwise Workflow for the evaluation of libraries of compounds. We applied our Workflow to screen a library (Library 1) of 31 supramolecular, self-associating amphiphiles (SSAs) for transport and lysis activity in a range of vesicles with different lipid compositions, designed to mimic different types of cells, and consequently identified seven promising transporters. Antimicrobial experiments found that six of these promising transporters showed good antimicrobial activity against clinically relevant Staphylococcus aureus strains, highlighting the promise of our Workflow in identifying potential antimicrobial agents. We then applied our Workflow to investigate the ion transport and lysis properties of a second library (Library 2) of SSAs with established antimicrobial and anticancer properties, aiming to provide insight into the biological modes of action.

The performance of metal-ion batteries at low temperatures and their fast charge/discharge rates are determined mainly by the electrolyte (ion) transport. Accurate transport properties must be evaluated for designing and/or optimization of lithium-ion and other metal-ion batteries. In this review, we report and discuss experimental and atomistic computational studies on ion transport, in particular, ion diffusion/dynamics, transference number, and ionic conductivity. Although a large number of studies focusing on lithium-ion transport in organic liquids have been performed, only a few experimental studies have been conducted in the organic liquid electrolyte phase for other alkali metals that are used in batteries (such as sodium, potassium, magnesium, etc.). Atomistic computer simulations can play a primary role and predict ion transport in organic liquids. However, to date, atomistic force fields and models have not been explored and developed exhaustively to simulate such organic liquids in quantitative agreement to experimental measurements.

The oxygen ionic conductivity of apatite-type and increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for and and vary in the range 0.96-0.99 for other compositions. Doping of with iron results in an increasing fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric apatites stabilize the state. Among the studied materials, the highest ionic and electronic transport is observed for where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are K−1 at 300-1250 K. © 2004 The Electrochemical Society. All rights reserved.

Ac conductivity and ion transport in K +-for-Na + ion-exchanged glasses: exchange experiments below and above the glass transition temperature

Polymerized ionic liquids (PolyILs) are a novel class of functional polymers that combine the unique physicochemical properties of molecular ionic liquids with the outstanding mechanical characteristics of polymers [1,2]. This special mix of features might help to circumvent the key limitations of low molecular weight ionic liquids, namely, leakage and poor mechanical properties while utilizing their outstanding characteristics such as low vapor pressures, wide liquidus ranges, high thermal stability, high ionic conductivity, and wide electrochemical windows. PolyILs have shown remarkable advantages when employed in electrochemical devices such as dye-sensitized solar cells, lithium batteries, actuators, field-effect transistors, light emitting electrochemical cells, and electrochromic devices, among others. Despite their promising prospects as ideal polymer electrolytes, the role of molecular structure, morphology, and polymer dynamics on charge transport in PolyILs remains poorly understood. According to classical theories, the self-diffusion and ion transport in electrolytes are controlled by structural relaxation. These approaches predict similar temperature dependence for the dc conductivity and structural dynamics. Although this prediction has been shown to hold reasonably well for low molecular weight aprotic ionic liquids, it fails for PolyILs [2,3]. In addition, the impact of morphology charge transport is not considered within the framework of these theories. The first part of this talk will focus on new insights obtained from experimental studies employing broadband dielectric spectroscopy, temperature-modulated differential scanning calorimetry, rheology, and scattering techniques to elucidate charge transport and structural dynamics in a systematic series of bulk polymerized ammonium- and imidazolium- based ionic liquids. Detailed analyses reveal strong decoupling of these processes in the PolyILs, implying the limitation of the classical theories in describing charge transport and molecular dynamics in these materials, in contrast to low molecular weight systems (4,5). In addition, the strong correlation observed between ionic conductivity from dielectric experiments and morphologies from scattering studies will be reviewed. In the second part of this talk, our recent results from a new approach to dielectric experiments yielding details about ion dynamics in ultrathin films of polymerized ionic liquids down to sub-5nm film thicknesses will be discussed. While the characteristic charge transport rates remain bulk-like down to about 5 nm film thicknesses, ionic-liquid/substrate interactions play an increasingly significant role in determining ion dynamics in confined polymerized ionic liquids (5). These results will be considered within the context of recent models of charge transport and polymer dynamics under confinement. References 1) Mecerreyes, D. Progress in Polymer Science 2011, 36, 1629. 2) Sangoro, J. R. and Kremer, F. Accounts of Chemical Research 2012, 45, 525 3) Sangoro, J. R. et al., Soft Matter 2014, 10, 3536. 4) Wojnarowska, Z. et al., Macromolecules 2015, 48 (23), 8660-8666. 5) Heres, M. F. et al., Submitted.

Ion transport in high temperature rotator phase solid electrolytes

The transport of the lithium ions is the basis of lithium ion conductivity of currently used electrolytes. Understanding how the transport of lithium ions within the matrix is influenced by the interactions with solvating moieties is needed to improve their performance. Along these lines well-defined model compounds based on cyclotriphosphazene (CTP) and hexaphenylbenzene (HPB) cores, bearing side groups of ethylene carbonate, a common solvent for lithium salts used as electrolytes in Li-ion batteries (Thielen et al. Chem. Mater, 23, 2120, 2011) and blended with different amounts of Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) have been studied by multinuclear nuclear magnetic resonance (NMR) spectroscopy. The local dynamics of the matrix was probed by 1H and 31P NMR, while the local dynamics of the Li+ cations was unraveled by 7Li and 13C NMR. Transport of both ions was studied by pulsed-field gradient (PFG) NMR. Based on the different temperature dependences of the dynamics the bulk ion transport is not attributed to local dynamics, but to translational diffusion best characterized by PFG NMR. Although the glass transition temperatures of the blends are low, their conductivities are only in the range of typical polymer electrolytes. The results of NMR spectroscopy are in accord with the conjecture that the coordination between the cyclic carbonate functionality and the Li+-ion is too tight to allow for fast ion dynamics.

The operation of batteries, fuel cells, and other energy storage and conversion devices is underpinned by the intricate synergy of ion transport, phase transformations, and electrochemical reactions. While electrochemical processes in well-defined molecular systems are amenable to classical chemical techniques including in situ optical spectroscopies and nuclear magnetic resonance (NMR), phase transformations and reactivity in solids and at solid interfaces remain largely an enigma. The presence of multiple types of structural defects and imperfections, competing electron and ion transport pathways, and multiple possible surface reconstruction types, combined with the directionality of ion flows and state of charge transformation fronts render these phenomena inaccessible to macroscopic interrogation techniques. At the same time, knowledge of the individual mechanisms involved in electrochemical reactivity of solids at the level of individual, atomic size structural elements is required for understanding the fundamental mechanisms of energy storage, factors controlling charge/discharge rates, degradation and failure processes, and ultimately knowledge-driven prediction and optimization of energy materials. 1-3

Ten shale samples of Frio and Vicksburg (Tertiary) overpressured shales from Brazoria and Hidalgo Counties, Texas, were examined by transmission electron microscopy (TEM) techniques. Initially, illite and smectite (with some mixed-layer component?) are present in subparallel domains, with smectite packets dominating and providing abundant dislocations for ion transport, supplying avenues that contribute to local permeability. As the transition proceeds, illite packets grow within a shrinking matrix of smectite, dislocations decrease, and pathways for ion transport are restricted. We suggest this development of illite packets and diminished ion transport causes loss of local permeability and a rise in the fluid pressure gradient. As illite packets coalesce, local permeability is increasingly lost; hydraulic continuity with the surface is further restricted and a more efficient geopressure seal is formed. In both Hidalgo and Brazoria Counties, the base of normal hydropressure (0.465 psi/ft or 10.5 kPa/m) coincides with the depth of onset of the smectite-to-illite transition. Geopressure greater than 0.7 psi/ft (15.8 kPa/m) occurs at depths corresponding to 50% illite packets in Brazoria County and 70% in Hidalgo County; in both cases, 0.7 psi/ft (15.8 kPa/m) geopressure occurs prior to the depth at which the transition ceases. These relationships suggest that if the smectite-to-illite transition begins and proceeds to completion, the development of coalescing illite packets during diagenesis will lead to increasing geopressure, with the actual values of illite proportion and corresponding fluid pressure gradient determined by the local geologic setting.