Donald J. Siegel

Multivalent batteries offer potential advantages over their lithium counterparts, but their development is currently hindered by a scarcity of solid electrolyte and electrode materials with high multivalent ionic conductivity. Multivalent ions often migrate via a vacancy-mediated mechanism, but the intrinsic concentration of vacancies in many materials is too low to support fast ion conduction. The vacancy concentration, and potentially the conductivity, can be increased via aliovalent substitution of ions with those of a different charge. Such strategies have been applied previously to monovalent ion conductors but remain underexplored in the context of their multivalent counterparts. This study leverages density functional theory and molecular dynamics simulations to assess the thermodynamic stability and transport properties of multivalent ion conductors containing vacancies and aliovalent substitutions. We consider ten prototypical antiperovskite compounds with the initial formula M 3 AB, where M is Mg or Ca and A and B are pnictogens, and replace pnictogens with either halogens or chalcogens to introduce vacancies. The energy above the convex hull, defect formation energy, and various reaction energies representing possible synthetic pathways are used to assess stability. This analysis reveals substituted Ca 3 PSb, Ca 3 NBi, and Mg 3 NBi as stable compositions. In all compounds, substitutions with smaller ions, such as F and O, are generally found to be more energetically favorable than those with larger ions. Molecular dynamics simulations reveal that ion migration events only involve a single ion, in contrast to many monovalent superionic conductors, in which concerted mechanisms are often observed. These findings suggest possible synthetic approaches to improving multivalent ionic conductivity for future experimental efforts.

The confinement of Li-based salts in metal−organic frameworks (MOFs) has attracted attention as a pathway for developing Li-ion conducting solid electrolytes. Nevertheless, optimization of these systems remains a challenge due to the complex environment that Li-ions encounter as they migrate through the salt-filled pores of the MOF. The present study aims to clarify the atomistic mechanisms governing the migration of a Li-based salt, LiTFSI, within the benchmark MOF UiO-66. The impact of LiTFSI loading, electric-field direction, and temperature on the transport behavior of Li-ions and TFSI anions is investigated by calculating the ions’ drift velocities and transference numbers. The Li + drift velocity increases with increasing LiTFSI loading and varies by up to three orders of magnitude across low to high-loading scenarios. Enhanced mobility at high loadings arises in part from the higher density of TFSI anions, which provide shorter Li + hopping distances between favorable anion/MOF sites. Compared to Li +, the drift velocities of the TFSI anions are slower overall and less sensitive to salt loading. At low salt loadings, TFSI migration is responsible for the majority of the (limited) ionic current. This trend is reversed at medium-to-high salt loadings, where Li + migration dominates. Differences in drift velocities between Li + and TFSI − are rationalized in terms of ion size effects. A Helmholtz free energy analysis reveals that at low loadings, Li + is trapped at the MOF nodes. At higher loadings, favorable Li + sites are distributed uniformly throughout the MOF pore space, mimicking the distribution of TFSI − anions. The migration of Li + and TFSI − occurs via repeated trapping and escape events, where occasional long-range hops are interrupted by quiescent periods where the ions are immobile. At high loadings, migration occurs via the simultaneous (correlated) motion of Li + and TFSI − . In contrast, at low loadings, the limited migration activity that exists does not involve correlated motion.

K2CO3 is a promising thermochemical energy storage material due to its ability to absorb and release heat through reversible hydration reactions. Nevertheless, the implementation of K2CO3 and other salts that undergo hydration reactions is constrained by their slow reaction kinetics. Experimentally, these sluggish kinetics are reflected in the existence of a metastable zone near the hydration/dehydration equilibrium line. However, the atomistic mechanisms that limit reaction rates are incompletely understood. Here, first-principles calculations are employed to investigate the thermodynamics and kinetics of hydration and dehydration in bulk K2CO3 and its hydrate, K2CO3 · 1.5 H2O. The energetics and site preferences for water insertion into K2CO3 at the early stages of hydration are predicted, as are the formation energies for water vacancies at the outset of K2CO3 · 1.5 H2O dehydration. In the hydrate, the energetics for water desorption are consistent with experimental observations of a two-step dehydration process. Several water diffusion pathways are explored in the bulk for both the anhydrous and hydrated phases. The calculated free-energy migration barriers reveal that in anhydrous K2CO3 diffusion occurs preferentially along [120] and [120] directions, parallel to the carbonate planes. In the hydrate, vacancymediated water migration is also anisotropic, with the lowest barriers observed for migration within water planes in the [001] direction. The calculated diffusion coefficients suggest that water diffusion in the hydrate is slightly faster than in the salt. In sum, this study provides insights into the atomic scale energetics and mechanisms underlying hydration reactions for thermal energy storage.

Solid-state electrolytes are an alternative to conventional liquid systems for safer and more efficient batteries, whereas a shift from Li to Na would unlock systems with increased resource availability and simplified supply chains. Antiperovskite Na2[NH2][BH4] recently emerged as a candidate that showcases that internal polyanion dynamics can facilitate Na transport. However, experimental validation of the predicted mechanisms of atomic motion remains scarce. To this end, we investigate the intricate relationship between local atomic disorder and dynamic motion. Maps of atomic disorder from total scattering data reproduce the predictions of polyanion rotation and Na translation by ab initio molecular dynamics (AIMD). The comparison also reveals the concurrence of BD4- translations, which were overlooked in previous analyses, and the existence of static disorder due to the different degrees of freedom of each anion, even while maintaining their ideal shape. This complex interplay refines mechanisms governing ion dynamics that determine solid-state electrolyte functionality. Realistic design strategies for rotor-based electrolytes must explicitly account for polyanion translation and static disorder, rather than optimizing rotational freedom in isolation. The combination of total scattering experiments with AIMD provides a route to screen potential polyanion-based candidates for favorable multi-modal disorder, thus steering the discovery of new phases with transformational ionic conductivity.

, heat - would improve the efficiencies of numerous processes throughout multiple sectors of the global economy. Nevertheless, the development of these thermal storage devices remains at a relatively early stage. To engage more researchers in the development of these devices and to accelerate their commercialization, this review presents an introduction to the properties of thermal storage materials that absorb and release heat through thermochemical reactions. Thermochemical materials typically exhibit the largest energy densities among all approaches to material-based heat storage. Nevertheless, they suffer from limited reaction rates and poor cycle life. An additional challenge is the multiscale nature of the energy storage process, which ranges from atomistic interactions that govern the storage of heat through alteration of chemical bonds, to mesoscale processes that control the transport of mass and heat. Following an overview of general concepts related to thermal energy storage, emphasis is placed on describing properties relevant for low-temperature applications. These applications include domestic heat storage/amplification (hot water heating), adsorptive cooling (air conditioning), and heat-moisture recuperation. Subsequently, detailed introductions are provided to the mechanisms and materials relevant for the three primary approaches to low-temperature thermochemical storage, including: (i) absorption in solids (hydrates, ammoniates, and methanolates); (ii) adsorption in porous hosts (zeolites, metal-organic frameworks); and (iii) dilution in liquids. For each category, advantages and shortcomings of benchmark and emerging materials are discussed. Finally, challenges and opportunities are highlighted for research aimed at developing optimal materials for thermochemical energy storage.

The rate of Li transport in the Li metal anode is important for the operation of Li metal-solid state batteries. Here, transport rates due to diffusion and creep are predicted in Li using atomistic simulations. First, molecular dynamics is used to estimate the rate of Li diffusion along dislocations and in grain boundary triple junctions. By combining this data with that from a prior study of grain boundary diffusion the dominant mechanisms and rates of self-diffusion in Li polycrystals are predicted as a function of grain size, grain shape, dislocation density, and temperature. Second, the dominant creep mechanisms are predicted and used to estimate critical current densities and void annihilation times. Grain boundary sliding and coble creep are the dominant mechanisms for micron-sized grains. Finally, a continuum model for interfacial contact loss reveals that high dislocation densities of ∼10 12 /cm 2 enable achieving battery performance targets for Li grain sizes of ∼10 μm.

The functioning of Li metal-solid state batteries (LMSSB) requires that interfacial contact between the Li metal anode and the solid electrolyte (SE) be maintained during cycling. A reduction in the Li/SE contact area during stripping will increase the local current density during subsequent plating, fostering dendrite nucleation. This contact area is influenced by the rates of Li transport within the anode towards the interface. Relevant transport mechanisms include diffusion and creep, with faster rates for these processes resulting in improved performance. Given the importance of these transport modes, predicting them as a function of the anode’s microstructure, stress state, and temperature will be helpful in the design of LMSSB. Towards this goal, the present study predicts the rates of diffusion and creep in Li using atomic scale simulations. First, molecular dynamics simulations are used to estimate the rate of Li diffusion along dislocation cores and in grain boundary triple junctions. By combining this data with that from a prior study of grain boundary diffusion, the dominant diffusion mechanisms and overall rates of self-diffusion in Li polycrystals are predicted as a function of grain size, grain shape, dislocation density, and temperature. Secondly, the dominant creep deformation mechanisms are predicted as a function of applied stress, grain size, and temperature using the computed atomistic data as input to constitutive equations from the literature. Grain boundary sliding and coble creep are predicted to be the dominant mechanisms for micron-sized grains. Finally, a 1D model for Li interfacial contact loss is parameterized with atomistic diffusion data for dislocations, triple junctions, grain boundaries, and the bulk. The model predicts that high dislocation densities of ~1012/cm2 enable achieving battery performance targets for Li grain sizes on the order of 10 μm.

Magnesium-ion batteries hold the potential to outperform the energy density of lithium-ion batteries, given the divalent charge carried by each Mg 2+ cation, but remain in an early stage of development. Here, 25 Mg solid-state nuclear magnetic resonance (ssNMR) is used to gain insight into the local structure and Mg-ion dynamics of candidate Mg-ion solid electrolytes, the antiperovskites Mg 3 SbN and Mg 3 AsN. Using the highest available magnetic field (35.2 T) for high-resolution solid-state NMR, the largest 25 Mg quadrupole coupling constants ( C Q ) yet measured of up to 22 MHz are reported and corroborated by first-principles calculations. Predicted C Q values are shown to correlate with the antiperovskite’s tolerance factor; thus, 25 Mg NMR linewidths can report on lattice distortions and phase stability of these antiperovskites. Variable-temperature 25 Mg NMR spectra demonstrate changes at elevated temperatures, ascribed to Mg-ion motional effects. 25 Mg T 1 relaxometry measurements at ultrahigh field reveal a lower activation energy for the more distorted Mg 3 AsN phase, matching computational predictions of a lower energy barrier for Mg 2+ ion migration and suggesting that additional scrutiny of antiperovskites as Mg-ion conductors is warranted. Given the inherent challenges of 25 Mg NMR, this work demonstrates the benefits of combining ultrahigh field NMR spectroscopy, advanced pulse sequences, modern signal processing, and first-principles calculations to facilitate NMR of quadrupolar nuclei as a tool to probe the local structure and ion dynamics in beyond-Li battery materials.

The functioning of Li metal-solid state batteries (LMSSB) requires that interfacial contact between the Li metal anode and the solid electrolyte (SE) be maintained during cycling. A reduction in the Li/SE contact area during stripping will increase the local current density during subsequent plating, fostering dendrite nucleation. This contact area is influenced by the rates of Li transport within the anode towards the interface. Relevant transport mechanisms include diffusion and creep, with faster rates for these processes resulting in improved performance. Given the importance of these transport modes, predicting them as a function of the anode’s microstructure, stress state, and temperature will be helpful in the design of LMSSB. Towards this goal, the present study predicts the rates of diffusion and creep in Li using atomic scale simulations. First, molecular dynamics simulations are used to estimate the rate of Li diffusion along dislocation cores and in grain boundary triple junctions. By combining this data with that from a prior study of grain boundary diffusion, the dominant diffusion mechanisms and overall rates of self-diffusion in Li polycrystals are predicted as a function of grain size, grain shape, dislocation density, and temperature. Secondly, the dominant creep deformation mechanisms are predicted as a function of applied stress, grain size, and temperature using the computed atomistic data as input to constitutive equations from the literature. Grain boundary sliding and coble creep are predicted to be the dominant mechanisms for micron-sized grains. Finally, a 1D model for Li interfacial contact loss is parameterized with atomistic diffusion data for dislocations, triple junctions, grain boundaries, and the bulk. The model predicts that high dislocation densities of ~1012/cm2 enable achieving battery performance targets for Li grain sizes on the order of 10 μm.

Anode-free solid-state batteries contain no active material at the negative electrode in the as-manufactured state, yielding high energy densities for use in long-range electric vehicles. The mechanisms governing charge–discharge cycling of anode-free batteries are largely controlled by electro-chemo-mechanical phenomena at solid–solid interfaces, and there are important mechanistic differences when compared with conventional lithium-excess batteries. This Perspective provides an overview of the factors governing lithium nucleation, growth, stripping and cycling in anode-free solid-state batteries, including mechanical deformation of lithium, the chemical and mechanical properties of the current collector, microstructural effects, and stripping dynamics. Pathways for engineering interfaces to maximize performance and extend battery lifetime are discussed. We end with critical research questions to pursue, including understanding behaviour at low stack pressure, tailoring interphase growth, and engineering current collectors and interlayers. Anode-free batteries contain no active material at the negative electrode when manufactured, and this can enable them to have high energy density. This Perspective presents a critical overview of the mechanisms governing the behaviour of anode-free solid-state batteries and provides guidance to improve this type of battery.

A rechargeable battery that employs a Li metal anode requires that Li be plated and stripped in a uniform fashion during cycling. In “anode-free” configurations, plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Under these circumstances phenomena at the Li/CC interface play an important role. On the other hand, at intermediate states of charge, maintaining interfacial contact between the anode and the solid electrolyte is a key factor in the operation of solid state batteries. This contact is controlled by chemical interactions at the interface (DOI: 10.1021/acs.chemmater.7b03002), but is also influenced by phenomena in the bulk regions of the anode, such as mass transport and plastic deformation. This presentation will provide an overview of atomic scale simulations that characterize several of these phenomena: interfacial interactions with the CC and with the solid electrolyte, mass transport within the anode, and deformation mechanisms of bulk Li. As one example, a multi-scale model is developed for predicting transport properties within the anode. The model predicts that a Li microstructure consisting of columnar, micron-scale grains can improve cycling performance due to fast grain boundary diffusion that can minimize void formation at the solid electrolyte interface (DOI: 10.1039/D3TA03814A). As another example, a combination of first-principles calculations and sessile drop experiments were used characterize the thermodynamics and adhesive ( i.e. , wetting) properties of interfaces involving Li and other phases present on or near the CC. It is demonstrated that Cu CCs can exhibit both lithiophillic and lithiophobic interactions with Li. These heterogeneities impact the performance of anode-free Li metal batteries (DOI: 10.1021/acsami.3c19034).

Solid-state batteries (SSBs) have the potential to bring about dramatic increases in safety and performance compared to current liquid electrolyte systems, allowing for electrification of crucial sectors such as transportation as well as development of novel device architectures for consumer electronics. Finding a suitable solid-state electrolyte remains a key challenge in the path to realization of SSBs, and finding candidate materials that exhibit sufficient ionic conductivity alongside the many other materials properties required of a solid-state electrolyte is the crux of this challenge. Within the past two decades, historic improvements in the conductivities of solid-state electrolytes have been made by understanding and exploiting the effects of lattice volume, vacancy concentration, and concerted migration mechanisms (amongst others), to the point at which the conductivities of some solid-state electrolytes have surpassed those of commercially used liquid electrolytes. [1] Recently, the influence of the dynamics of the anions, specifically the rotation of “cluster anions” (also known as molecular anions or polyanions such as PS 4 3- or BH 4 - ), on cation migration mechanisms has garnered attention as a potential avenue for designing high conductivity materials. [2] Typical experimental signatures of cluster anion rotation include rotor atom spatial probability density calculated from diffraction techniques showing a different, more spherically symmetric geometry, such as a tetrahedral anion showing an octahedral signal, or a disorder-increasing phase transformation accompanied by a sharp jump in ionic conductivity. Because the cluster anion rotations occur at picosecond and Angstrom time and length scales, understanding the rotational behavior of the cluster anions and determining whether this behavior has an impact on the migration mechanism anion is difficult to do experimentally. Molecular dynamics simulations have been used to probe the rotational behavior of cluster anions, with typical characterizations including vibration/libration spectra, rotational autocorrelation functions, spatial probability densities of rotor atoms, and 2D histograms of θ-ϕ spherical coordinates of rotor bond vectors. However, most of these techniques average the signals from the rotor atoms across clusters and across simulation times in a way that obfuscates important time dependent behavior (see Figure 1A). For example, they are unable to determine the specific set of orientations accessed by a cluster anion species that creates the time and space averaged signal seen by diffraction techniques. In this work, we present a novel framework for MD trajectory analysis which uses a 3D parameterization of the orientation of each cluster in each frame to (1) determine the stable orientations (an orientation-based analog of the position-based concept of a lattice site) of a rotationally active cluster anion, (2) characterize the rotations between stable orientations, and (3) probe the interaction between cluster anion rotations and cation migrations. We apply this framework to the dual cluster anion containing electrolyte sodium amide borohydride (Na 2 NH 2 BH 4 ), for which both the tetrahedral borohydride (BH 4 - ) anion and the bent amide (NH 2 - ) anion show octahedral signals by X-ray and neutron diffraction. We determine that the borohydride anions achieve this averaged spatial density by pointing one hydrogen atom along the +/- a, b, or c axis and rotating about that axis, while the amide anions achieve this by pointing both hydrogen atoms roughly along two adjacent +/- a, b, or c axes (see Figure 1B). We also corroborate previous results showing that the migration of the sodium cations happens in close concert with the rotation of the amide anions, while the borohydride anions spin more freely. This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. References [1] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, A lithium superionic conductor. Nature Materials 10, 682–686 (2011). [2] Z. Zhang, L. F. Nazar, Exploiting the paddle-wheel mechanism for the design of fast ion conductors. Nat Rev Mater 7, 389–405 (2022). Figure 1. (A) Traditional MD cluster anion rotation analyses applied to the amide anion in Na 2 NH 2 BH 4 . (B) 100 sampled amide anion positions from each stable orientation within the trajectory. Na: Purple (frontmost Na omitted for clarity), B: pink (hydrogens on BH 4 - anions omitted for clarity) N: blue, H: white. Figure 1

A significant challenge hindering the development of batteries based on the redox of multivalent ions is the sluggish mobility of such ions in most solids. Computational methods for efficiently predicting conductivity can accelerate the discovery of faster ion conductors. Direct first-principles calculations of conductivity are expensive and difficult to automate, which has prompted a search for other properties related to conductivity that are easier to calculate or measure. Previous studies have identified features related to the electronic charge density and phonon spectrum that are correlated with energy barriers for ion migration in monovalent conductors. Results from our first-principles simulations demonstrate that these features are not well correlated with energy barriers for multivalent ion migration. I will discuss potential reasons for this lack of correlation and propose modifications that are found to improve correlations. These findings quantify the promise of using such features to efficiently screen for better multivalent ion conductors. Figure 1

A machine learning (ML) model is developed for predicting useable methane (CH 4 ) capacities in metal–organic frameworks (MOFs). The model applies to a wide variety of MOFs, including those with and without open metal sites, and predicts capacities for multiple pressure swing conditions. Despite its wider applicability, the model requires only 5 measurable structural features as input, yet achieves accuracies that surpass less-general models. Application of the model to a database of more than a million hypothetical MOFs identified several hundred whose capacities surpass that of the benchmark MOF, UMCM-152. Guided by the computational predictions, one of the promising candidates, UMCM-153, was synthesized and demonstrated to achieve superior volumetric capacity for CH 4 . Feature importance analyses reveal that pore volume and gravimetric surface area are the most important features for predicting CH 4 capacity in MOFs. Finally, a reverse ML model is demonstrated. This model predicts the set of elementary MOF structural properties needed to achieve a desired CH 4 capacity for a prescribed operating condition.

Abstract 3D‐framework Prussian blue analogues (PBAs) are appealing as a cost‐effective, sustainable cathodes for Na‐ion batteries. However, the aqueous‐based synthesis of PBAs inherently introduces three different forms of water molecules (surface, interstitial and crystal) into the structure. Removal of water molecules causes phase transformation from monoclinic (M) to rhombohedral (R). This work presents the effects of water molecules on the structure before the phase transformation temperature, employing two promising PBA cathodes, Na 2 Fe[Fe(CN) 6 ]·1.69H 2 O and Na 2 Mn[Fe(CN) 6 ]·1.76H 2 O. Specifically, the water molecules impact the molecular interactions at the local structure and the electrochemical properties. This work has performed calculations on low‐vacancy Na 2 M[Fe(CN) 6 ] PBAs (where M = Mn, Fe, Co, Ni and Cu) to understand the dehydration energy. Employing in situ high‐temperature X‐ray diffraction and Raman spectroscopy, this work observes that water removal induces negative thermal expansion and stronger interactions between C≡N and Na ions, resulting in biphasic reactions with sluggish kinetics. Additionally, water molecules play a role in maintaining the open 3D tunnels and facilitating a solid‐solution like insertion of Na ions. Calculated phonon‐Raman spectra provide insights into cyanide group deformations, revealing the interactions between water molecules, alkali‐ions, and transition‐metal ions. This study enhances the understanding of the relationship among electronic, vibrational, and electrochemical properties.

Batteries based on the redox of multivalent cations (Mg 2+, Ca 2+, Zn 2+, Al 3+, etc.) offer potential advantages over today’s lithium-ion batteries, but their development is hindered by the sluggish migration of such ions in solid electrodes and electrolytes. Computational screening can accelerate the discovery of more conductive materials, provided that ionic conductivity can be estimated with sufficient accuracy and efficiency. The present study examines whether vibrational properties can be used to predict energetic barriers for cation migration in 24 prototypical multivalent solid electrolytes. Phonon band centers (i.e., mean frequencies), which have been previously used to predict Li-ion conductivity, are calculated using density functional theory. Band centers alone are found not to correlate with migration barriers ( R 2 = 0.02), perhaps due to poor alignment of low-frequency phonon eigenmodes with ion migration pathways in some materials. A new metric that incorporates both frequencies and alignments─the mean alignment-weighted frequency─is more strongly correlated to migration barriers ( R 2 = 0.25). Materials in this study with the lowest migration barriers consistently exhibit the lowest mean alignment-weighted frequencies, suggesting the utility of this metric for filtering out materials with high barriers in screening efforts. Comparisons to previous studies suggest that phonon band centers may be correlated to migration barriers only in compositionally similar materials and that adding alignment information may enable more reliable predictions among more diverse sets of materials. These results quantify the promise of using phonon frequencies and alignments, perhaps in combination with other properties, to efficiently screen for materials with high multivalent ionic conductivity.

Batteries based on the redox of multivalent cations (Mg2+, Ca2+, Zn2+, Al3+, etc.) offer potential advantages over today’s lithium-ion batteries, but their development is hindered by the sluggish migration of such ions in solid electrodes and electrolytes. Computational screening can accelerate the discovery of more conductive materials, provided that ionic conductivity can be estimated with sufficient accuracy and efficiency. The present study examines whether vibrational properties can be used to predict energetic barriers for cation migration in 24 prototypical multivalent solid electrolytes. Phonon band centers (i.e. mean frequencies), which have been previously used to predict Li-ion conductivity, are calculated using density functional theory. Band centers alone are found not to correlate with migration barriers (R^2 = 0.02), perhaps due to poor alignment of low-frequency phonon eigenmodes with ion migration pathways in some materials. A new metric that incorporates both frequencies and alignments—the mean alignment-weighted frequency—is more strongly correlated with migration barriers (R^2 = 0.25). Materials in this study with the lowest migration barriers consistently exhibit the lowest mean alignment-weighted frequencies, suggesting the utility of this metric for filtering out materials with high barriers in screening efforts. Comparisons to previous studies suggest that phonon band centers may be correlated with migration barriers only in compositionally similar materials, and that adding alignment information may enable more reliable predictions among more diverse sets of materials. These results quantify the promise of using phonon frequencies and alignments, perhaps in combination with other properties, to efficiently screen for materials with high multivalent ionic conductivity.

Batteries based on the redox of multivalent cations (Mg2+, Ca2+, Zn2+, Al3+, etc.) offer potential advantages over today’s lithium-ion batteries, but their development is hindered by the sluggish migration of such ions in solid electrodes and electrolytes. Computational screening can accelerate the discovery of more conductive materials, provided that ionic conductivity can be estimated with sufficient accuracy and efficiency. The present study examines whether vibrational properties can be used to predict energetic barriers for cation migration in 24 prototypical multivalent solid electrolytes. Phonon band centers (i.e. mean frequencies), which have been previously used to predict Li-ion conductivity, are calculated using density functional theory. Band centers alone are found not to correlate with migration barriers (R^2 = 0.02), perhaps due to poor alignment of low-frequency phonon eigenmodes with ion migration pathways in some materials. A new metric that incorporates both frequencies and alignments—the mean alignment-weighted frequency—is more strongly correlated with migration barriers (R^2 = 0.40). Comparing these findings to those of previous studies suggests that phonon band centers may be correlated with migration barriers only in compositionally similar materials, and that adding alignment information may improve correlations among more diverse materials. These results quantify the promise of using phonon frequencies and alignments, perhaps in combination with other properties, to efficiently screen for materials with high multivalent ionic conductivity.

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In “anode-free” configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive ( i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu 2 O, and Li 2 O are considered. The calculations predict that both Cu and Cu 2 O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu 2 O → Li 2 O + 2 Cu, that generates Li 2 O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li 2 O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li 2 O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

Evaluation of metal-organic frameworks (MOFs) for adsorbed natural gas (ANG) technology employs pure methane as a surrogate for natural gas (NG). This approximation is problematic, as it ignores the impact of other heavier hydrocarbons present in NG, such as ethane and propane, which generally have more favorable adsorption interactions with MOFs compared to methane. Herein, using quantitative Raman spectroscopic analysis and Monte Carlo calculations, we demonstrate the adsorption selectivity of high-performing MOFs, such as MOF-5, MOF-177, and SNU-70, for a methane and ethane mixture (95:5) that mimics the composition of NG. The impact of selectivity on the storage and deliverable capacities of these adsorbents during successive cycles of adsorption and desorption, simulating the filling and emptying of an ANG tank, is also demonstrated. The study reveals a gradual reduction in the storage performance of MOFs, particularly with smaller pore volumes, due to ethane accumulation over long-term cycling, until a steady state is reached with substantially degraded storage performance.

Technologies for thermal energy storage (TES) are limited by the performance of the heat storage material. Therefore, it is desirable to develop materials with superior heat storage properties. The present study employs first-principles calculations to predict the properties of 7012 hypothetical hydrates based on chalcogenide and complex anion salts. Accounting for thermodynamic stability and energy densities, promising hydrates were identified for temperatures below 200 °C, including Li 2 S·9H 2 O, Ca(OH) 2 ·8H 2 O, and Li 2 CO 3 ·10H 2 O. System-level projections indicate that several of the proposed materials surpass the energy densities of known materials when incorporated into a solar-thermal storage system. Interpretable machine learning models were trained on the hydrate data set and used to identify features that control the enthalpy of dehydration. This analysis reveals similarities and differences in the thermodynamic behavior of hydrates based on chalcogenides, complex anions, and the previously studied halides. Hydrates based on chalcogenide anions exhibit a wide distribution of dehydration enthalpies; the low average enthalpies of these hydrates reflect the fact that relatively few are stable. In contrast, hydrates based on complex anions and halides exhibit enthalpies that are, on average, larger and more narrowly distributed. The enthalpies of the chalcogenide hydrates can be predicted by using only two machine-learned features, both of which implicate the electronegativity of the cation as a controlling property. This correlation agrees with a trend reported previously for halide-salt hydrates. In contrast, the behavior of the complex-anion hydrates requires twice as many features for machine-learning predictions, and some of these features are complex. Nevertheless, a combination of the molar volume and boiling point data is identified as a useful descriptor. In total, the hydrate compositions and design insights identified in this study are anticipated to catalyze the development of more efficient TES systems.

Maintaining interfacial contact between the Li metal anode and the solid electrolyte is a key challenge in developing Li metal-based solid-state batteries (LMSSB). At moderate discharge rates, relatively slower diffusion within the anode results in roughening and void formation in Li near this interface. The resulting reduction in interfacial contact focuses the Li-ion current during plating to a reduced number of contact points, generating high local current densities that nucleate dendrites. One approach to minimize void formation is to apply high stack pressure, which enhances plastic flow in the anode. Nevertheless, the use of pressure has drawbacks, as it facilitates fracture within the solid electrolyte. Here, an alternative strategy for minimizing void formation is described. Using a multi-scale model, it is shown that targets for capacity and current density in LMSSBs can be achieved by reducing the grain size of Li to exploit fast grain boundary (GB) diffusion. Diffusion rates along a diverse sampling of 55 tilt and twist GBs in Li were predicted using molecular dynamics, and found to be 3 to 6 orders of magnitude faster than in the bulk. Using these atomic-scale data as input, a meso-scale model of Li depletion in the anode during discharge was developed. The model predicts that smaller, columnar grains are desirable, with grain sizes of approximately 1 μm or less needed to meet performance targets. As micron-sized grains are two orders of magnitude smaller than those in common use, strategies for controlling grain size are discussed. In total, the model highlights the importance of the anode’s microstructure on the performance of LMSSBs.

Abstract Thin intermetallic Li 2 Te–LiTe 3 bilayer (0.75 µm) derived from 2D tellurene stabilizes the solid electrolyte interphase (SEI) of lithium metal and argyrodite (LPSCl, Li 6 PS 5 Cl) solid‐state electrolyte (SSE). Tellurene is loaded onto a standard battery separator and reacted with lithium through single‐pass mechanical rolling, or transferred directly to SSE surface by pressing. State‐of‐the‐art electrochemical performance is achieved, e.g., symmetric cell stable for 300 cycles (1800 h) at 1 mA cm −2 and 3 mAh cm −2 (25% DOD, 60 µm foil). Cryo‐stage focused ion beam (Cryo‐FIB) sectioning and Raman mapping demonstrate that the Li 2 Te–LiTe 3 bilayer impedes SSE decomposition. The unmodified Li–LPSCl interphase is electrochemically unstable with a geometrically heterogeneous reduction decomposition reaction front that extends deep into the SSE. Decomposition drives voiding in Li metal due to its high flux to the reaction front, as well as voiding in the SSE due to the associated volume changes. Analysis of cycled SSE found no evidence for pristine (unreacted) lithium metal filaments/dendrites, implying failure driven by decomposition phases with sufficient electrical conductivity that span electrolyte thickness. DFT calculations clarify thermodynamic stability, interfacial adhesion, and electronic transport properties of interphases, while mesoscale modeling examines interrelations between reaction front heterogeneity (SEI heterogeneity), current distribution, and localized chemo‐mechanical stresses.

Maintaining interfacial contact between the Li metal anode and the solid electrolyte is a key challenge in developing Li metal-based solid-state batteries (LMSSB). At moderate discharge rates, relatively slower diffusion within the anode results in roughening and void formation in Li near this interface. The resulting reduction in interfacial contact focuses the Li-ion current during plating to a reduced number of contact points, generating high local current densities that nucleate dendrites. One approach to minimize void formation is to apply high stack pressure, which enhances plastic flow in the anode. Nevertheless, the use of pressure has drawbacks, as it facilitates fracture within the solid electrolyte. Here, an alternative strategy for minimizing void formation is described. Using a multiscale model, it is shown that targets for capacity and current density in LMSSBs can be achieved by reducing the grain size of Li, thereby exploiting fast grain boundary (GB) diffusion. Diffusion rates along a diverse sampling of 55 tilt and twist GBs in Li was predicted using molecular dynamics, and found to be 3 to 6 orders of magnitude faster than in the bulk. Using these atomic-scale data, a meso-scale model of Li depletion in the anode during discharge was developed. The model predicts that grain sizes of approximately 1 𝜇m are needed to meet performance targets for LMSSBs. As these grain sizes are two orders of magnitude smaller than those in common use, strategies for controlling grain size are discussed. In total, the model highlights the importance of the anode’s microstructure on the performance of LMSSBs.

Abstract Diffusion of transition metals across the cathode–electrolyte interface is identified as a key challenge for the practical realization of solid‐state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials. Herein, the hypothesis that formation of interphases is associated with the incorporation of Co into the Li 7 La 3 Zr 2 O 12 lattice representing the starting point of a cascade of degradation processes is investigated. It is shown that Co incorporates into the garnet structure preferably four‐fold coordinated as Co 2+ or Co 3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 per formula unit, whereby concentrations beyond this limit causes a cubic‐to‐tetragonal phase transition. Moreover, the temperature‐dependent Co diffusion coefficient is determined, for example, D 700 °C = 9.46 × 10 −14 cm 2 s −1 and an activation energy E a = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al 2 O 3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid‐state batteries.

Interdiffusion of transition metals across the cathode-electrolyte interface is identified as a key challenge for the practical realization of solid-state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials thus limiting the battery performance. Herein, we investigate the hypothesis that formation of interphases is associated with the incorporation of Co into the LLZO lattice representing the starting point of a cascade of degradation processes. It is shown that Co incorporates into the garnet structure preferably four-fold coordinated as Co2+ or Co3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 pfu, whereby concentrations beyond this limit causes a cubic-to-tetragonal phase transition. Moreover, the temperature-dependent Co diffusion coefficient is determined, e.g., D700 °C = 9.46 × 10-14 cm2/s and an activation energy Ea = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al2O3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid-state batteries.

Solid State Electrolytes Solid-state conductors utilizing the rotation of atomic clusters to facilitate fast-ion transport provide an alternative mechanism to the classical picture of ion hopping through a fixed lattice. The antiperovskite compound Na2NH2BH4, described in article number 2203284 by Yet-Ming Chiang and co-workers, is an example where two such clusters work in concert to increase Na+ conduction.

A multi-scale model reveals that the microstructure of the Li metal anode can impact the performance of solid-state batteries. Micron-sized, columnar grains are preferred for minimizing void formation at the solid electrolyte interface.