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Halide-based solid electrolytes are currently growing in interest in solid-state batteries due to their high electrochemical stability window compared to sulfide electrolytes. However, often a bilayer separator of a sulfide and a halide is used and it is unclear why such setup is necessary, besides the instability of the halides against lithium metal. It is shown that an electrolyte bilayer improves the capacity retention as it suppresses interfacial resistance growth monitored by impedance spectroscopy. By using in-depth analytical characterization of buried interphases by time-of-flight secondary ion mass spectrometry and focused ion beam scanning electron microscopy analyses, an indium-sulfide rich region is detected at the halide and sulfide contact area, visualizing the chemical incompatibility of these two electrolytes. The results highlight the need to consider more than just the electrochemical stability of electrolyte materials, showing that chemical compatibility of all components may be paramount when using halide-based solid electrolytes in solid-state batteries.

A diode requires the combination of p- and n-type semiconductors or at least the defined formation of such areas within a given compound. This is a prerequisite for any IT application, energy conversion technology, and electronic semiconductor devices. Since the discovery of the pnp-switchable compound Ag

The exploitation of high-capacity conversion-type materials such as sulfur in solid-state secondary batteries is a dream combination for achieving improved battery safety and high energy density in the push towards a sustainable future. Yet, the exact rate-limiting step, bottlenecking further development of solid-state lithium-sulfur batteries, has not been determined. Here, we directly visualize the spatial distribution of lithium via neutron imaging during operation and show that sluggish macroscopic ion transport within the composite cathode is rate-limiting. Observing a reaction front propagating from the separator side towards the current collector confirms detrimental influences of a low effective ionic conductivity. Furthermore, irreversibly concentrated lithium in the vicinity of the current collector, revealed via state-of-charge-dependent tomography, highlights a hitherto-overlooked loss mechanism triggered by sluggish effective ionic transport within a composite cathode. This discovery will be a cornerstone for future research on solid-state batteries, irrespective of the type of active material.

The exploitation of high-capacity conversion-type materials such as sulfur in solid-state secondary batteries is a dream combination for achieving improved battery safety and high energy density in the push towards a sustainable future. Yet, the exact rate-limiting step, bottlenecking further development of solid-state lithium-sulfur batteries, has not been determined. Here, we directly visualize the spatial distribution of lithium via neutron imaging during operation and show that sluggish macroscopic ion transport within the composite cathode is rate-limiting. Observing a reaction front propagating from the separator side towards the current collector confirms detrimental influences of a low effective ionic conductivity. Furthermore, irreversibly concentrated lithium in the vicinity of the current collector, revealed via state-of-charge-dependent tomography, highlights a hitherto-overlooked loss mechanism triggered by sluggish effective ionic transport within a composite cathode. This discovery will be a cornerstone for future research on solid-state batteries, irrespective of the type of active material.

Ultra-low lattice thermal conductivity as often found in superionic compounds is greatly beneficial for thermoelectric performance, however, a high ionic conductivity can lead to device degradation. Conversely, high ionic conductivities are searched for materials in solid-state battery applications. It is commonly thought that ionic transport induces low thermal conductivity and that ion and thermal transport are not completely independent properties of a material. However, no direct comparison or underlying physical relationship has been shown between the two. Here we establish that ionic transport can be varied independent of thermal transport in Ag+ superionic conductors, even though both phenomena arise from atomic vibrations. Thermal conductivity measurements, in conjunction with two-channel lattice dynamics modeling, reveals that the vast majority of Ag+ vibrations have non-propagating diffuson-like character, which provides a rational for how these two transport properties can be independent. Our results provide conceptually novel lattice dynamical insights to ionic transport and confirm that ion transport is not a requirement for ultra-low thermal conductivity. Consequently, this work bridges the fields of solid state ionics and thermal transport, thus providing design strategies for functional ionic conducting materials from a vibrational perspective.

The lithium argyrodites Li6PS5X (X = Cl, Br, and I) have been gaining momentum as candidates for electrolytes in all-solid-state batteries. While these materials have been well-characterized struct...

Summary Li7La3Zr2O12 (LLZO)-based garnet materials are recently being investigated as suitable electrolytes for solid-state batteries with lithium-metal electrodes. Unfortunately, lithium-metal penetration through polycrystalline garnet-type electrolytes limits the electric current density during cell charging. In this study, we introduce an electrochemical operando approach that is well suited to get insights into the early stage of lithium-metal penetration that was yet only accessible with very elaborate neutron measurements. Combined with in situ as well as ex situ electron microscopic techniques, we investigate the morphological instability of the lithium-metal anode on garnet-type solid electrolytes under cathodic load and demonstrate the inter-relationship between microkinetic aspects and lithium-penetration susceptibility.

Non-graphitic carbons (NGCs) represent the most abundant class of sp2-hybridized carbon, materials (coal char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks pos...

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1-xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe2.

A nondegrading, low-impedance interface between a solid electrolyte and cathode active materials remains a key challenge for the development of functional all-solid-state batteries (ASSBs). The widely employed thiophosphate-based solid electrolytes are not stable toward oxidation and suffer from growing interface resistance and thus rapid fading of capacity in a solid-state battery. In contrast, NASICON-type phosphates such as Li1+ xAl xTi2- x(PO4)3 and Li1+ xAl xGe2- x(PO4)3 are stable at high potentials, but their mechanical rigidity and high grain boundary resistance are thought to impede their application in bulk-type solid-state batteries. In this work, we present a comparative study of a LiNi0.8Co0.1Mn0.1O2 (NCM-811) cathode composite employing either β-Li3PS4 (LPS) or Li1.5Al0.5Ti1.5(PO4)3 (LATP) as a solid electrolyte. LPS is employed as a separator in both cases to assemble a functional ASSB. To avoid high-temperature processing of LATP, along with subsequent detrimental interfacial reactions with NCM materials, the ASSBs are constructed and operated in a hot-press setup at 150 °C. The cathode interfaces are investigated using in situ electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy, which reveals that the interface resistance is strongly suppressed and the chemical state of the composite is unchanged during cycling when employed with LATP. The cell using LATP is reversibly charged and discharged for multiple cycles and outperforms a comparable cell using a thiophosphate composite electrode. The results indicate that LATP in the cathode composite represents an excellent candidate to overcome interfacial challenges in bulk-type solid-state batteries.

All-solid-state batteries are promising candidates for next-generation energy-storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there ar...

All-solid-state lithium-ion batteries (ASSBs) are expected to represent a future alternative compared to conventional lithium-ion batteries with liquid electrolytes (LIBs). The excellent performanc...

Inorganic lithium-ion conductors have garnered considerable attention as separators for all-solid-state lithium-ion battery applications, given their potential to solve the safety issues and improve the energy and power densities of conventional devices possessing liquid electrolytes. However, achieving this transition requires the optimization of solid electrolyte materials and thus, developing a better understanding of the structure–property relationships, that govern ionic transport, is of crucial importance. Herein, inspired by the growing technological interest, a systematic study on the correlations between structural modifications and transport properties in the thio-LISICON family resulting from the substitution of Ge4+ by Sn4+ within Li4Ge1–xSnxS4 has been conducted. Using Rietveld refinements against neutron diffraction data coupled with maximum-entropy method analyses of nuclear densities, a rigorous investigation into the Li+ diffusion pathways was performed. The substitution of Ge4+ by Sn4+ i...

Because of a remarkably high theoretical energy density, the lithium–sulfur (Li–S) battery has attracted significant attention as a candidate for next-generation batteries. While employing solid el...

For the development of next-generation lithium batteries, major research effort is made to enable a reversible lithium metal anode by the use of solid electrolytes. However, the fundamentals of the solid-solid interface and especially the processes that take place under current load are still not well characterized. By measuring pressure-dependent electrode kinetics, we explore the electrochemo-mechanical behavior of the lithium metal anode on the garnet electrolyte Li6.25Al0.25La3Zr2O12. Because of the stability against reduction in contact with the lithium metal, this serves as an optimal model system for kinetic studies without electrolyte degradation. We show that the interfacial resistance becomes negligibly small and converges to practically 0 Ω·cm2 at high external pressures of several 100 MPa. To the best of our knowledge, this is the smallest reported interfacial resistance in the literature without the need for any interlayer. We interpret this observation by the concept of constriction resistance and show that the contact geometry in combination with the ionic transport in the solid electrolyte dominates the interfacial contributions for a clean interface in equilibrium. Furthermore, we show that-under anodic operating conditions-the vacancy diffusion limitation in the lithium metal restricts the rate capability of the lithium metal anode because of contact loss caused by vacancy accumulation and the resulting pore formation near the interface. Results of a kinetic model show that the interface remains morphologically stable only when the anodic load does not exceed a critical value of approximately 100 μA·cm-2, which is not high enough for practical cell setups employing a planar geometry. We highlight that future research on lithium metal anodes on solid electrolytes needs to focus on the transport within and the morphological instability of the metal electrode. Overall, the results help to develop a deeper understanding of the lithium metal anode on solid electrolytes, and the major conclusions are not limited to the Li|Li6.25Al0.25La3Zr2O12 interface.

Crystalline materials with ultralow thermal conductivity are essential for thermal barrier coating and thermoelectric energy conversion. Nontoxic n-type bulk cubic AgBiS2 exhibits exceptionally low...

Inspired by the recent interest in fast ionic conducting solids for electrolytes, the ionic conductivity of a novel ionic conductor Na1+x Ti2-x Gax (PS4 )3 has been investigated. Using X-ray diffraction and impedance spectroscopy the sodium ionic conductivity in this compound was demonstrated, in which bond valence sum analysis suggests a tunnel diffusion for Na+ . Substitution with Ga3+ leads to an increasing Na+ content, an expansion of the lattice and an increasing conductivity with increasing x in Na1+x Ti2-x Gax (PS4 )3 . Given the relation to the NASICON family, upon replacement of the phosphate by a thiophosphate group, a rich structural chemistry can be expected in this class of materials. This work demonstrates the potential for making NaTi2 (PS4 )3 an ideal system to study structure-property relationships in ionic conductors.

All-solid-state batteries require solid electrolytes that exhibit both high mechanical and chemical stability, as well as a high ionic conductivity. Despite the many decades of research that have led to significant breakthroughs in this field, the synthesis of high-performance ionic conductors is still quite costly with limited scalability, i.e. it is highly energy and time intensive. To this end, the development of cheap and scalable solution-based approaches for fabricating state-of-the-art solid electrolytes is of great interest; however, a deeper understanding over the fundamental solution chemistry and reaction mechanisms governing these synthetic approaches is either absent or not broadly conveyed in the literature. Herein, we review some of the more recent works on solution-based syntheses of alkali thiophosphates contextualized within a broader scope of the literature in an attempt to provide some additional chemical insights and underline the areas where specific knowledge is lacking. Focusing primarily on Li+ containing electrolytes, we provide a deeper look into both prototypical reagents and possible alternatives, highlight the importance and potential influences of polysulfide/S–S bonding, and discuss the significance of precursor stoichiometry. We hope that this review provides a unique outlook on solution-based syntheses of alkali thiophosphates leading to a better understanding over the critical parameters that govern the optimization of this class of superionic conductors.

Advanced lithium-ion batteries are of great interest for consumer electronics and electric vehicle applications; however, they still suffer from drawbacks stemming from cathode active material limitations (e.g., insufficient capacities and capacity fading). One approach for alleviating such limitations and stabilizing the active material structure may be anion doping. In this work, fluorine and nitrogen are investigated as potential dopants in Li1.02(Ni0.8Co0.1Mn0.1)0.98O2 (NCM) as a prototypical nickel-rich cathode active material. Nitrogen doping is achieved by ammonia treatment of NCM in the presence of oxygen, which serves as an unconventional and new approach. The crystal structure was investigated by means of Rietveld and pair distribution function analysis of X-ray diffraction data, which provide very precise information regarding both the average and local structure, respectively. Meanwhile, time-of-flight secondary-ion mass spectroscopy was used to assess the efficacy of dopant incorporation with...

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-the-art lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li6+xP1-xGexS5I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm-1 in a cold-pressed state and 18.4 ± 2.7 mS cm-1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

In recent years, ternary halides Li3MX6 (M = Y, Er, In; X = Cl, Br, I) have garnered attention as solid electrolytes due to their wide electrochemical stability window and favorable room-temperature conductivities. In this material class, the influences of iso- or aliovalent substitutions are so far rarely studied in-depth, despite this being a common tool for correlating structure and transport properties. In this work, we investigate the impact of Zr substitution on the structure and ionic conductivity of Li3InCl6 (Li3-xIn1-xZrxCl6 with 0 ≤ x ≤ 0.5) using a combination of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. Analysis of high-resolution diffraction data shows the presence of an additional tetrahedrally coordinated lithium position together with cation site-disorder, both of which have not been reported previously for Li3InCl6. This Li+ position and cation disorder lead to the formation of a three-dimensional lithium ion diffusion channel, instead of the expected two-dimensional diffusion. Upon Zr4+ substitution, the structure exhibits non-uniform volume changes along with an increasing number of vacancies, all of which lead to an increasing ionic conductivity in this series of solid solutions.

With growing interest in solution‐based processing of electrolytes for all‐solid‐state batteries comes the need to more deeply understand potential detrimental effects of the solvent on electrolyte materials, as well as effects on the cell performance that may not have been evident by structural characterization alone. Herein, the superionic solid electrolyte Li6PS5Cl is treated with five organic solvents selected for a range of different physical and chemical properties. The electrolytes treated with solvents that do not lead to obvious degradation are used in cathode composites of solid‐state batteries In/LiIn│Li6PS5Cl│NCM‐622:Li6PS5Cl. After treatment in some solvents, the solid electrolyte remains seemingly unaffected, but a strong influence on the solid‐state battery performance is observed, revealing underlying effects that warrant deeper study.

Exploration of sulfidic sodium solid electrolytes and their design contributes to advances insolid state sodium batteries. Such design is guided by a better understanding of fast sodiumtransport, for instance in the herein studied Na11Sn2PS12-type materials. By using Rietveldrefinements against synchrotron X-ray diffraction and electrochemical impedancespectroscopy, the influence of aliovalent substitution onto the structure and transport inNa11+xSn2P1−xMxS12 with M = Ge and Sn is investigated. Whereas Sn induces strongerstructural changes than Ge, the found influence on the sodium sublattice and the ionic transportproperties are comparable. Overall, a reduced in-grain activation energy of Na+ transport canbe found with the reducing Na+ vacancy concentration. This work explores previouslyunreported phases in the Na11Sn2PS12 structure type that, based on their determined propertiesreveal Na+ vacancy concentrations to be an important factor guiding further understandingwithin Na11Sn2PS12-type materials

Commercialization of solid-state batteries requires the upscaling of the material syntheses as well asthe mixing of electrode composites containing the solid electrolyte, cathode active materials, bindersand conductive additives. Inspired by recent literature about the tremendous influence of the employedmilling and dispersing procedure on the resulting ionic transport properties of solid ionic conductorsand the general performance of all solid-state batteries, in this review, we want to shed light on theunderlying physical and mechanochemical processes that influence this processing. By discussing andcombining the theoretical backgrounds of mechanical milling with regard to mechanochemicalsynthesis and dispersing of particles together with a wide range of examples, we aim to provide thefield with a better understanding of the critical parameters attached to mechanical milling of solidelectrolytes and solid-state battery components.

As the world transitions away from fossil energy to green and renewable energy,electrochemical energy storage increasingly becomes a vital component of the mix to conduct thistransition. The central goal in developing next-generation batteries is to maximize gravimetric andvolumetric energy density, battery cycle life and improve safety. All solid-state batteries using asolid electrolyte and a lithium metal anode represent one of the most promising technologies thatcan achieve this goal. Highly conductive solid electrolytes (>10 mS·cm-1) are the key componentto remove the safety concerns inherent with flammable organic liquid electrolyte and achieve highenergy density by enabling high active material loading. Considering a range of inorganic solidelectrolytes that haven been developed to date, sulfide solid electrolytes exhibit highest ionicconductivity that even surpasses that of conventional organic liquid electrolytes. Argyrodite-structured sulfide solid electrolytes are one of the most promising materials in this class and arecurrently the dominantly used solid electrolyte for all solid-state battery fabrication. Argyroditesolid electrolytes are particularly appealing, owing to their ultra-high Li-ion conductivity, quasistablesolid electrolyte interphase (SEI) formed with Li metal and their ability to be prepared viascalable solution-assisted synthesis approaches. These factors are all vital for commercialapplications.In this Account, we afford an overview of our recent development of several argyroditesuperionic conductors, including Li6.6Si0.6Sb0.5S5I (24 mS·cm-1), Li6.6Ge0.6P0.4S5I (18 mS·cm-1) andLi5.5PS4.5Cl1.5 (12 mS·cm-1), and a comprehensive understanding of the origin of the underlyinghigh conductivity: namely sulfide/halide anion site-disorder and Li cation site disorder. A highdegree of sulfide/halide anion site-disorder (changes in anion distribution) modifies the anioniccharge that in turn strongly influences the lithium distribution. A more inhomogeneous chargedistribution in anion-disordered systems generates a spatially diffuse and delocalized lithiumdensity resulting in faster ionic transport. Lithium cation site-disorder generated by increasing Licarrier concentration through aliovalent substitution, creates high-energy interstitial sites for Liion diffusion which activate concerted ion migration and flatten the energy landscape for Li iondiffusion. This enables high conductivity in Li-rich argyrodite superionic conductors. Theseconcepts are also expected to promote rational new solid electrolyte design and fundamentalunderstanding of the structure - ion transport relationships in inorganic ionic conductors.Collectively, a comprehensive and deep understanding of the interphase formation betweenargyrodite solid electrolytes and cathode active materials/Li metal, and the failure mechanism ofall solid-state batteries with argyrodite solid electrolytes, will lead to the bottom-up engineering of the cathode/anode-solid electrolyte interfaces, which will accelerate the development of safe, highenergy density all solid-state lithium batteries.

Superionic lithium argyrodites are attractive as solid electrolytes for all-solid-state-batteries. These materials of composition Li6PS5X (X = Cl, Br, and I) exhibit structural disorder between the X−/S2− positions, with higher disorder realizing better Li+ transport. Further replacement of the sulfide by chloride anions (for the series Li7−xPS6−xClx) has been shown to increase the ionic conductivity. However, the underlying changes to the lithium substructure are still relatively unknown. Here we explore a larger range of nominal halide compositions in this material from x = 0.25 to x = 1.5 and explore the changes with neutron diffraction and impedance spectroscopy. The replacement of S2− by Cl−causes a lowered average charge in the center of the prevalent Li+ “cages”, which in turn causes weaker interactions with Li+ ions. Analysis of neutron diffraction data reveals that the increased Cl− content causes these clustered Li+ “cages” to become more interconnected, thereby increasing Li+ conductivity through the structure. This study explores the understanding of the fundamental structure–transport correlations in the argyrodites, specifically structural changes withinthe Li+ ion substructure upon changing anionic charge distribution.

The energy density of the ubiquitous lithium-ion batteries is rapidly approaching its theoretical limit. To go beyond, a promising strategy is the replacement of conventional intercalation-type materials with conversion-type materials possessing substantially higher capacities. Among the conversion-type cathode materials, sulfur constitutes a cost-effective and earth-abundant element with a high theoretical capacity that has a potential to be game-changing, especially within an emerging solid-state battery configuration. Employment of nonflammable solid electrolytes that improves battery safety and boosts the energy density, as lithium metal anodes are also viable. The long-standing inherent problem of conventional lithium–sulfur batteries, arising from the reaction intermediates dissolved in liquid electrolytes, can be eliminated with inorganic solid ion conductors. In particular, the highly conducting and easily processable lithium-thiophosphates have successfully enabled the lab-scale solid-state lithium–sulfur cells to achieve close-to-theoretical capacities. For applications requiring safe, energy-dense, lightweight batteries, solid-state lithium–sulfur batteries are an ideal choice that could surpass conventional lithium-ion batteries.Nevertheless, there are challenges specific to practical solid-state lithium–sulfur batteries, beyond the typical challenges inherent to solid-state batteries in general. While the conversion reaction of sulfur realizes a large specific capacity, the associated significant total volume changes of the active material results in contact losses among the cathode components and, consequently, decreases reversible capacity. Additionally, the ionically and electronically insulating active material requires composite formation with solid electrolytes and electron-conductive additives to secure sufficient ion and electron supply at a triple-phase boundary. However, the compositing process itself makes the carrier transport pathways very tortuous and requires the balancing of carrier transport and optimization of the attainable energy density. Lastly, the requirement of a high interfacial area to establish sufficient triple-phase boundaries promotes the degradation of the solid electrolytes, and the formation of less-conductive interphases further deteriorates the transport in the composites.This Account focuses on the challenges associated with developing practical solid-state lithium–sulfur batteries and provides an overview over recently developed concepts to tackle these critical challenges: (1) Introduction of the conversion efficiency to enable quantitative assessments of the impact of chemo-mechanical failure. (2) For long-term cycling, the electrolyte degradation at the interface and the electrochemical activity of the formed interphases come into play. Practical stability tests with increased interfacial areas and subsequently altered reversal potentials can quantify the magnitude of the electrolyte degradation and confirm influences of reversible redox activity of the interphases. (3) Monitoring the effective conductivity in the composites clarifies correlations between transport and cyclability, further highlighting the need of quantitative measurements to address the composite carrier transport. (4) Impedance spectroscopy combined with transmission-line model analysis as a function of applied potentials can visualize the stability window of good effective ion transport to utilize both the capacity contributions from redox-active interphases and the high ionic conductivity. In the end, a roadmap toward the practical solid-state lithium–sulfur batteries will be presented.

Ion diffusion is important in a variety of applications, yet fundamental understanding of the diffusive process in solids is still missing, especially considering the interaction of lattice vibrations (phonons) and the mobile species. In this work, we introduce two formalisms that determine the individual contributions of normal modes of vibration (phonons) to the diffusion of ions through a solid, based on (i) Nudged Elastic Band (NEB) calculations and (ii) molecular dynamics (MD) simulations. The results for a model ion conductor of $\rm{Ge}$-substituted $\rm{Li_3PO_4}$ ($\rm{Li_{3.042}Ge_{0.042}P_{0.958}O_4}$) revealed that more than 87% of the $\rm{Li^+}$ ion diffusion in the lattice originated from a subset of less than 10% of the vibrational modes with frequencies between 8 and 20 THz. By deliberately exciting a small targeted subset of these contributing modes (less than 1%) to a higher temperature and still keeping the lattice at low temperature, we observed an increase in diffusivity by several orders of magnitude, consistent with what would be observed if the entire material (i.e., all modes) were excited to the same high temperature. This observation suggests that an entire material need not be heated to elevated temperatures to increase diffusivity, but instead only the modes that contribute to diffusion, or more generally a reaction/transition pathway, need to be excited to elevated temperatures. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material, and/or increasing diffusivity by external stimuli/excitation of phonons (e.g., via photons or other interactions) without necessarily changing the compound chemistry., 50 pages, 17 figures

The recent interest in the halide-based solid electrolytes Li3MX6 (M = Y, Er, In; X = Cl, Br, I) shows these materials to be promising candidates for solid-state battery application, due to high ionic conductivity and large electrochemical stability window. However, almost nothing is known about the underlying lithium sub-structure within those compounds. Here, we investigate the lithium sub-structure of Li3YCl6 and Li3YBr6 using temperature-dependent neutron diffraction. We compare compounds prepared by classic solid-state syntheses with a mechanochemical synthesis to shed light on the influence of the synthetic approach on the reported yttrium disorder and the resulting surrounding lithium sub-structure. This work provides a better understanding of the strong differences in ionic transport depending on the synthesis procedure of Li3MX6.

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na

Identifying and optimizing highly-conducting lithium-ion solid electrolytes is a critical step towards the realization of commercial all–solid-state lithium-ion batteries. Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical-bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect was proposed, whereby changes in bonding within the solid-electrolyte host-framework modify the potential-energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. This concept has since been invoked to explain anomalous conductivity trends in a number of solid electrolytes. Direct evidence for a solid-electrolyte inductive effect, however, is lacking—in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host-framework. Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li10Ge1−xSnxP2S12, using Rietveld refinements against high-resolution temperature-dependent neutron-diffraction data, Raman spectroscopy, and density functional theory calculations. Substituting Ge for Sn weakens the {Ge,Sn}–S bonding interactions and increases the charge-density associated with the S2- ions. This charge redistribution modifies the Li+ substructure causing Li+ ions to bind more strongly to the host-framework S anions; which in turn modulates the Li-ion potential-energy surface, increasing local barriers for Li-ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations further predict that this inductive effect occurs even in the absence of changes to the host-framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid-electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.

Developing reversible lithium metal anodes with high rate capability is one of the central aims of current battery research. Lithium metal anodes are not only required for the development of innovative cell concepts such as lithium-air or lithium-sulfur batteries, they can also increase the energy density of batteries with intercalation-type cathodes. The use of solid electrolyte separators is especially promising to develop well-performing lithium metal anodes, because they can act as a mechanical barrier to avoid unwanted dendritic growth of lithium through the cell. However, inhomogeneous electrodeposition and contact loss often hinder the application of a lithium metal anode in solid-state batteries. In this review, we assess the physicochemical concepts that describe the fundamental mechanisms governing lithium metal anode performance in combination with inorganic solid electrolytes. In particular, our discussion of kinetic rate limitations and morphological stability intends to stimulate further progress in the field of lithium metal anodes.

The lithium argyrodites Li

The lithium-argyrodites Li6PS5X (X = Cl, Br, I) exhibit high lithium-ion conductivities, making them promising candidates for use in solid-state batteries. These solid electrolytes can show considerable substitutional X−/S2− anion-disorder, with greater disorder typically correlated with higher lithium-ion conductivities. The atomic-scale effects of this anion site-disorder within the host lattice—in particular how lattice disorder modulates the lithium substructure—are not well understood. Here, we characterize the lithium substructure in Li6PS5X (X = Cl, Br, I) as a function of temperature and anion site-disorder, using Rietveld refinements against temperature-dependent neutron diffraction data. Analysis of these high-resolution diffraction data reveals an additional lithium position previously unreported for Li6PS5Xargyrodites, suggesting that the lithium conduction pathway in these materials differs from the most common model proposed in earlier studies. Analysis of the Li+ positions and their radial distributions reveals that greater inhomogeneityof the local anionic charge, due to X−/S2− site-disorder, is associated with more spatially-diffuse lithium distributions. This observed coupling of site-disorder and lithium distribution provides a possible explanation for the enhanced lithium transport in anion-disordered lithium argyrodites, and highlights the complex interplay between anion configuration and lithium substructure in this family of superionic conductors.

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high performing electrolytes in terms of temperature-dependent ionic transport is based upon the fundamental understanding of structure – transport relationships within the given structural phase boundaries inherent to the investigated materials class. In this work, the Na+ superionic structural family of Na11Sn2PS12 is explored by using the systematic antimony substitution with phosphorous in Na11+xSn2+x(Sb1-yPy)1-xS12. A combination of Rietveld refinements against X-ray synchrotron diffraction data with electrochemical impedance spectroscopy is used to monitor the changes in the anionic framework, the Na+ substructure and the ionic transport. A new simplified descriptor for the average Na+ diffusion pathways, the average Na+ polyhedral volume is introduced, which is used to correlate the contraction of the overall lattice and the found activation barriers in the system. This study exemplifies how substitution affects diffusion pathways in ionic conductors and widens the knowledge about the related structural motifs and their influence on the ionic transport in this novel class of ionic conductors.

Driven by the increasing attention that the superionic conductors Li3MX6 (M = Y, Er, In, La; X = Cl, Br, I) have gained recently for the use of solid-state batteries, and the idea that a softer, more polarizable anion sublattice is beneficial for ionic transport, here we report Li3ErI6, the first experimentally-obtained iodine-based compound within this material system of ionic conductors. Using a combination of synchrotron and neutron diffraction, we elucidate the structure, the lithium positions and possible diffusion pathways of Li3ErI6. Temperature-dependent impedance spectroscopy shows low activation energies of 0.37 and 0.38 eV alongside promising ionic conductivities of 0.65 mS·cm-1 and 0.39 mS·cm-1directly after ball milling and the subsequently annealed Li3ErI6, respectively. Speed of sound measurements are used to determine the Debye frequency of the lattice as a descriptor of the lattice dynamics and overall lattice softness, and Li3ErI6 is compared to the known material Li3ErCl6. The softer, more polarizable framework from the iodide anion leads to improved ionic transport, showing that the idea of softer lattices holds up in this class of materials. This work provides Li3ErI6 as an interesting novel framework for optimization in the class of halide-based ionic conductors.

Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure–property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li6PS5–xSexBr (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li6PS5–xSexBr transport behavior was found to be largely unaffected by the incorporation of Se2– due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimiz...

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of “the softer the lattice, the better” does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na3PS4–xSex by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, ...

To utilize all-solid-state batteries as high power and energy density energy storage devices it is necessary to improve the current electrolyte materials and the cell architecture. In this work, we present α- and β-Li8GeP4 as potential compounds for the use in cathode composites of solid-state batteries because of their ability to conduct both Li+ ion and electrons. Each polymorph was synthesized via mechanical alloying of the elements and subsequent annealing. Structural analysis of α- and β-Li8GeP4 via X-ray diffraction reveals isolated [GeP4]8– tetrahedra. α-Li8GeP4 (Pa3) and β-Li8GeP4 (P43n) are isotypic with Li8SiP4 and Li8SnP4, respectively. The lithium ion mobility indicated by partially filled octahedral voids was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy and reveals low activation energies for lithium hopping in the range from 34 and 42 kJ mol–1 as well as high ionic conductivities of up to 8 × 10–5 S cm–1 and electronic conducti...

The ionic conductivity and activation energy of lithium in the Li3PO4-Li3VO4-Li4GeO4 system was systematically investigated. The sharp decrease in activation energy upon Ge substitution in Li3PO4 and Li3VO4 was attributed to the reduction in the defect formation energy while the variation in activation energy upon increasing Ge content was rationalized in term of the inductive effect. We also found a correlation between the pre-exponential factors and the activation energies in agreement with the well-known Meyer-Neldel rule. The series of compound with and without partial lithium occupancy were shown to fall into two distinct lines. The slope of the line was found to be related to the inverse of the energy scale associated with phonons in the system, which agrees with the multiexcitation entropy theory. The intercept of the line was found to be related to the Gibbs free energy of defect formation. Compiled data of pre-exponential factor and activation energy for commonly studied lithium-ion conductors shows that this correlation is very general, implying an unfavorable trade-off between high pre-exponential factor and low activation energy needed to achieve high ionic conductivity. Understanding the circumstances under which this correlation can be violated might provide a new opportunity to further increase the ionic conductivity in lithium-ion conductors.

Lithium titanate Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages, such as the sluggish lithium-ion diffusion and poor electronic conductivity. To modify the performance of LTO as an anode material, nanosizing constitutes a promising approach and the impact is studied here by a systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies, including high-resolution transmission electron microscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry, and electrochemical impedance spectroscopy, a crystallite...

Recent work has shown an increased interest in lithium titanium oxide phosphates as possible anode materials for lithium ion batteries. However, the intrinsic transport properties of these materials have not been studied yet. In this work, we employ X-ray diffraction and electrical impedance spectroscopy to characterize lithium titanium oxide phosphate LiTiOPO 4 . In order to better understand the transport of lithium ions, substitution with metal cations of different charges and different radii are used to elucidate changes in the crystal structure and their effects on the lithium ion conductivity. We show that substitution of the transition metal has a small influence on the ionic conductivity only, but comes with a noticeable impact on the activation energy. In addition to the experimental work, density functional theory calculations are employed to provide possible diffusion pathways and mechanism, and explain the decrease in the activation barriers for ionic motion.

We report on the first quaternary selenide-based Na+ superionic solid electrolyte, Na11.1Sn2.1P0.9Se12 (further denoted as NaSnPSe), which shows virtually the same room temperature Na+ ion conductivity (3.0 mS/cm) as the current record holder for sulfide-based systems, Na11Sn2PS12 (denoted as NaSnPS), but with a considerably lower activation energy of 0.30 eV. Both electrolytes belong to the currently highly topical class of solids comprising group I, IV, V, and VI atoms, which we summarize as 1–4–5–6 electrolytes. Herein, they are compared to each other with regard to their structural characteristics and the resulting ion transport properties. The lower activation energy of Na+ ion transport in NaSnPSe is well in line with the results of speed of sound measurements, Raman spectroscopy, bond-valence site energy calculations, and molecular dynamics simulations, all of which point to a lower lattice rigidity and to weaker Na–chalcogen interactions as compared to NaSnPS.

Elucidating the underlying structural principles that govern ionic transport in thiophosphate solid electrolytes will enable the discovery of novel ionic conductors. Additionally, improving the properties of ionic conductors and exacting control over interfacial reactions and interphase stabilities are critical to the advancement of solid-state batteries. In this perspective, we focus on two major aspects at the foundation of solid-state battery development. First, we address the typical static structural requirements for achieving high ionic conductivities within thiophosphates, which is then extended to how a dynamic lattice and local structural effects can influence ionic transport. Furthermore, we provide an overview of some of the challenges that are currently hindering the progress of solid-state battery research, with particular attention being paid to interfacial instabilities and mechanochemical effects. We hope that this perspective provides a unique outlook on ionic conduction in thiophosphates...

Lithium superionic conductors with the argyrodite structure Li 6 PS 5 X (X = Cl, Br, I) are considered as suitable candidates for the fabrication of all-solid-state batteries (SSB) facilitating Li metal anodes. The use of metal anodes is required to achieve SSB with high energy densities, however, the thermodynamic stability of the different argyrodites in contact with Li metal has not been systematically investigated yet. The stability against lithium metal is of practical interest for long-term stability of SSB utilizing argyrodites. Here, data on the stability of Li 6 PS 5 X (X = Cl, Br, I) in contact with Li metal are reported, obtained from an in situ X-ray photoemission technique in combination with time-resolved impedance spectroscopy. In contact with Li metal, Li 6 PS 5 X decomposes into an interphase composed of Li 3 P, Li 2 S and LiX, which serves as an SEI and results in an increasing interfacial resistance. The growth of the SEI and the resulting resistance evolution is further analyzed in terms of its kinetics and is compared to other thiophosphate superionic conductors. Li 6 PS 5 I is found to show particularly strong SEI formation with severe resistance growth.

The sodium superionic conductor Na3PS4 is known to crystallize in one of two different structural polymorphs at room temperature (i.e., cubic or tetragonal, depending on the synthetic conditions). Experimentally, the cubic structure is known to exhibit a higher ionic conductivity than the tetragonal structure, despite theoretical investigations suggesting that there should be no difference at all. Employing a combination of Rietveld and pair distribution function (PDF) analyses, as well as electrochemical impedance spectroscopy, we investigate the open question of how the crystal structure influences the ionic transport in Na3PS4. Despite the average structures of Na3PS4 prepared via ball-milling and high-temperature routes being cubic and tetragonal, respectively, the structural analysis by PDF indicates that both compounds are best described by the structural motifs of the tetragonal polymorph on the local scale. Ultimately, the high ionic conductivity of Na3PS4 prepared by the ball-milling approach is ...

Li1+xAlxGe2–x(PO4)3 (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe2(PO4)3 and AlPO4. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li+ incorporation in the material and the actual Li+ content in the sample increases with the nominal Li+ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing car...