Triple-conducting oxides (TCOs) are an emerging class of mixed ionic and electronically conducting materials that show great promise for oxygen reduction/evolution (ORR/OER) electrocatalysis-primarily in high-temperature ceramic electrochemical cells- but also in aqueous alkaline environments. Their high activity is attributed, at least in part, to their ability to incorporate and transport three mobile charge carriers: protons, oxygen vacancies, and electron-holes. Despite their promise, fundamental studies of TCOs are challenging, as transport dynamics from three charge carriers cannot be fully disentangled via traditional electrical measurement techniques. Characterizing proton dynamics in TCOs is particularly difficult as protons are generally the minority carrier, and their conduction response is typically obscured by the oxygen vacancies and electron holes. Here, we demonstrate successful isolation of the proton behavior in an archetypal TCO, BaCo0.4Fe0.4Zr0.1Y0.1O3-delta (BCFZY4411), using a combination of non-electrical techniques. We determine proton uptake and oxygen non-stoichiometry (delta) using thermogravimetric analysis (TGA). X-ray absorption near edge structure (XANES) and neutron diffraction (ND) are used to validate the oxidation state of Co and the delta values obtained through TGA. We apply H-1 solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) to provide insights into local structure, dynamics, and proton kinetics. Finally, the proton transport properties are further quantified using tracer isotope exchange with time-of-flight secondary ion mass spectrometry (ToF-SIMS). Despite the very low proton concentrations in BCFZY4411 (<0.2 % under most conditions), our analysis suggests that the oxygen reduction and evolution reactions are nevertheless limited by the oxygen ion kinetics (e.g., oxygen surface exchange) rather than the proton kinetics at the reduced operating temperatures (<500 degrees C) that are targeted for electrochemical cell applications. These findings provide a comprehensive understanding of proton behavior in BCFZY4411 and pave the way for advancing the fundamental study of TCOs.

Ionogels, electrolytes containing an ionic liquid and a solid matrix, show potential for enhancing Li-ion battery performance and versatility. Such composite materials offer tunable mechanical properties with the advantages of ionic liquids, such as high ionic conductivity and nonflammability. However, ensuring proper interfacial contact between the gel and electrodes remains challenging. Here, the stability of epitaxial LiMn2O4 (111) electrode thin films was studied in an ionogel electrolyte containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and h-BN nanoplatelets. In operando synchrotron X-ray scattering was employed to examine this electrode's interfacial and structural evolution during operation. Despite the ionic liquid's capability to suppress Mn dissolution, loss of crystallinity and formation of irreversible Li2Mn2O4 phase were found, attributed to an inadequate contact between the gel and the cathode.

Multivalent batteries are often considered a natural successor to current lithium-ion technologies, but, in practice, often suffer from large overpotentials and limited capacity and cycle life. These issues largely stem from the morphology of electrodeposited metal anode, stability and mobility of ions in a multivalent anode, and bulk and interfacial solvation structure of ions within the electrolyte. Here we use synchrotron x-ray scattering and spectroscopy to better understand the evolving in situ structure of magnesium and zinc species during cycling , with an emphasis on interfacial speciation. Using operando surface scattering from model magnesium spinel thin film cathodes and electron microscopy, we highlight differences in Li and Mg mobility that drive Mg surface enrichment. Using density functional theory, we demonstrate that over-magnesiation results from strain with the underlying cathode and that different surface orientations have dramatically different rates of Mg intercalation. We then focus on aqueous and nonaqueous zinc electrolytes, using x-ray absorption and emission spectroscopy to capture the solvation structure of ions in bulk solutions in comparison to nanoconfined species or at a charged interface. We will also highlight the role of current collectors in templating the morphology of the final electrodeposited zinc species.

Successful deployment of a Mg-ion battery requires cathodes that can achieve reversible Mg intercalation and high energy density. Recent theoretical and experimental studies indicated that the overall transport is likely limited by sluggish Mg transport at the cathode-electrolyte interface and not Mg diffusion through bulk. In this work, we investigated the surface electrochemical activity of Mg ions by using a spinel-structured manganese oxide thin-film model system and in situ X-ray scattering. In combination with post-mortem microscopy analysis, we found that magnesium insertion was more favorable than subsequent extraction near the surface of the MgxMn2O4 film, resulting in overmagnesiation, and eventually amorphization of the surface. This structural irreversibility and high overpotential required for Mg extraction could explain significant voltage hysteresis and Mg surface enrichment previously observed in bulk cathodes. Density functional theory calculations suggested that the tendency for the Mg surface enrichment could be associated with Mg diffusion kinetics, which varies with the strain state evolved due to constrained film volume change during Mg insertion and extraction. Particularly, out-of-plane Mg migration was predicted to be favorable in the tensile strain rather than in the compressive case.

Spinel-type lithium manganese oxide (LiMn2O4) cathodes suffer from severe manganese dissolutionin the electrolyte,compromising the cyclic stability of LMO-based Li-ion batteries (LIBs).In addition to causing structural and morphological deteriorationto the cathode, dissolved Mn ions can migrate through the electrolyteto deposit on the anode, accelerating capacity fade. Here, we examinesingle-crystal epitaxial LiMn2O4 (111) thin-filmsusing synchrotron in situ X-ray diffraction and reflectivityto study the structural and interfacial evolution during cycling.Cyclic voltammetry is performed in a wide range (2.5-4.3 V vs Li/Li+) to promote Mn3+ formation,which enhances dissolution, for two different electrolyte systems:an imidazolium ionic liquid containing lithium bis-(trifluoromethylsulfonyl)imide(LiTFSI) and a conventional carbonate liquid electrolyte containinglithium hexafluorophosphate (LiPF6). We find exceptionalstability in this voltage range for the ionic liquid electrolyte comparedto the conventional electrolyte, which is attributed to the absenceof Mn dissolution in the ionic liquid. X-ray reflectivity shows anegligible loss of cathode material for the films cycled in the ionicliquid electrolyte, further confirmed by inductively coupled plasmamass spectrometry and transmission electron microscopy. Conversely,a substantial loss of Mn is found when the film is cycled in the conventionalelectrolyte. These findings show the significant advantages of ionicliquids in suppressing Mn dissolution in LiMn2O4 LIB cathodes.

Predictive understanding of the molecular interaction of electrolyte ions and solvent molecules and their chemical reactivity on electrodes has been a major challenge but is essential for addressing instabilities and surface passivation that occur at the electrode-electrolyte interface of multivalent magnesium batteries. In this work, the isolated intrinsic reactivities of prominent chemical species present in magnesium bis(trifluoromethanesulfonimide) (Mg(TFSI)2) in diglyme (G2) electrolytes, including ionic (TFSI-, [Mg(TFSI)]+, [Mg(TFSI):G2]+, and [Mg(TFSI):2G2]+) as well as neutral molecules (G2) on a well-defined magnesium vanadate cathode (MgV2O4) surface, have been studied using a combination of first-principles calculations and multimodal spectroscopy analysis. Our calculations show that nonsolvated [Mg(TFSI)]+ is the strongest adsorbing species on the MgV2O4 surface compared with all other ions while partially solvated [Mg(TFSI):G2]+ is the most reactive species. The cleavage of C-S bonds in TFSI- to form CF3- is predicted to be the most desired pathway for all ionic species, which is followed by the cleavage of C-O bonds of G2 to yield CH3+ or OCH3- species. The strong stabilization and electron transfer between ionic electrolyte species and MgV2O4 is found to significantly favor these decomposition reactions on the surface compared with intrinsic gas-phase dissociation. Experimentally, we used state-of-the-art ion soft landing to selectively deposit mass-selected TFSI-, [Mg(TFSI):G2]+, and [Mg(TFSI):2G2]+ on a MgV2O4 thin film to form a well-defined electrolyte-MgV2O4 interface. Analysis of the soft-landed interface using X-ray photoelectron, X-ray absorption near-edge structure, electron energy-loss spectroscopies, as well as transmission electron microscopy confirmed the presence of decomposition species (e.g., MgFx, carbonates) and the higher amount of MgFx with [Mg(TFSI):G2]+ formed in the interfacial region, which corroborates the theoretical observation. Overall, these results indicate that Mg2+ desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgV2O4 cathode surface. This work provides the first evidence of the primary mechanisms leading to electrolyte decomposition at high-voltage oxide surfaces in multivalent batteries and suggests that the design of new, anodically stable electrolytes must target systems that facilitate cation desolvation.

Intermixing of atomic species at the electrode-electrolyte boundaries can impact the properties of the interfaces in solid-state batteries. Herein, this work uses first-principles statistical mechanics along with experimental characterization to understand intermixing at the electrode-electrolyte interface. For the model presented in this work, lithium manganese oxide (LiMn2O4, LMO) and lithium lanthanum titanate (Li3xLa2/3-xTiO3, LLTO) are employed as the cathode and electrolyte, respectively. The results of the computational work show that Ti-Mn intermixing at the interface is significant at synthesis temperatures. The experimental results in this work find that, at some critical temperatures between 600 and 700 °C for material preparation, the interface of LLTO-LMO becomes blurred. Calculations predict that the interface is unstable with regard to Ti-Mn intermixing starting at 0 K, suggesting that the critical temperature found in the experiment is related to kinetics. The work overall suggests that, in designing a solid-state battery, the fundamental reactions such as intermixing need to be considered.

Lithium‐ion batteries (LIBs) have been the focus of research for decades owing to their superior energy storage potential and wide range of applications. However, long‐term stability issues still persist in LIB cathodes as the result of the formation of a solid electrolyte interface (SEI), structural transformations, or the loss of active cathode materials. In nanoscale cathodes, it is often impossible to isolate, let alone mitigate the causes of the observed capacity loss. Herein, a novel approach is presented to synthesizing LIB cathode model systems using thin‐film molecular beam epitaxy (MBE). Specifically, 100 nm thick LiMn 2 O 4 (LMO) thin‐film cathodes are grown on SrRuO 3 /SrTiO 3 (100) single‐crystal substrates and characterized in their pristine and cycled states. The pristine thin films exhibit the electro‐chemical cycling behavior and structural evolution previously seen in bulk LMO cathodes. The formation of surface‐layer rocksalt MnO and spinel Mn 3 O 4 along with the mass‐loss associated with the corrosion of active Mn ions is also reported. It is, therefore, demonstrated that these MBE‐grown model systems can be used for detailed studies of their surface structures and the electrode/electrolyte interfacial evolution in LIB cathodes, leading to the development of materials for rechargeable batteries with higher capacity and better stability.

Submitted for the MAR10 Meeting of The American Physical Society Structural signal of a dynamic glass transition1 SUDESHNA CHATTOPADHYAY (BANDYOPADHYAY), AHMET UYSAL, BENJAMIN STRIPE, GUENNADI EVMENENKO, PULAK DUTTA, Department of Physics and Astronomy, Northwestern University, STEVEN EHRLICH, Brookhaven National Laboratory, EVGUENIA A. KARAPETROVA, Argonne National Laboratory — Conventional wisdom states that there is no significant difference between the static structures of the glass and liquid states of a given material. Using x-ray reflectivity, we have studied pentaphenyl trimethyl trisiloxane, an isotropic liquid at room temperature with a dynamic glass transition at 224K. Surface density oscillations (surface layers) develop below 285K, similar to those seen in other metallic and dielectric liquids and in computer simulations [1]. Upon cooling further, there is a sharp increase in the penetration of the surface layers into the bulk material, i.e. an apparently discontinuous change in the static structure, exactly at the glass transition (224K) [2]. [1]. e.g. O. M. Magnussen et al., PRL 74, 4444 (1995); H. Mo et al. PRL 96, 096107 (2006); E. Chac’on et al., PRL 87, 166101 (2001) [2] S. Chattopadhyay et al, PRL 103, 175701 (2009) 1Supported by NSF grant no. DMR-0705137. Sudeshna Bandyopadhyay Department of Physics and Astronomy, Northwestern University Date submitted: 23 Nov 2009 Electronic form version 1.4

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Our results reveal that conversion reactions and structural changes in NiO thin film electrodes begin near the theoretical lithiation potential.

Oxide conversion reactions are known to have substantially higher specific capacities than intercalation materials used in Li-ion batteries, but universally suffer from large overpotentials associated with the formation of interfaces between the resulting nanoscale metal and Li2O products. Here we use the interfacial sensitivity of operando X-ray reflectivity to visualize the structural evolution of ultrathin NiO electrodes and their interfaces during conversion. We observe two additional reactions prior to the well-known bulk, three-dimensional conversion occurring at 0.6 V: an accumulation of lithium at the buried metal/oxide interface (at 2.2 V) followed by interfacial lithiation of the buried NiO/Ni interface at the theoretical potential for conversion (at 1.9 V). To understand the mechanisms for bulk and interfacial lithiation, we calculate interfacial energies using density functional theory to build a potential-dependent nucleation model for conversion. These calculations show that the additional space charge layer of lithium is a crucial component for reducing energy barriers for conversion in NiO.

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Although lithium-ion batteries that run on the conversion reaction have high capacity, their cyclability remains problematic due to large volume changes and material pulverization. Dimensional confinement, such as 2D thin film or nanodots in a conductive matrix, is proposed as a way of improving the cyclic stability, but the lithiation mechanism of such dimensionally controlled materials remains largely unknown. Here, by in situ transmission electron microscopy, lithiation of thin RuO2 films with different thicknesses and directions of lithium-ion diffusion are observed at atomic resolution to monitor the reactions. From the side-wall diffusion in approximate to 4 nm RuO2 film, the ion-diffusion and reaction are fast, called "interface-dominant" mode. In contrast, in approximate to 12 nm film, the ion diffusion-reaction only occurs at the interface where there is a high density of defects due to misfits between the film and substrate, called the "interface-to-film" mode. Compared to the side-wall diffusion, the reaction along the normal direction of the thin film are found to be sluggish ("layer-to-layer" mode). Once lithiation speed is higher, the volume expansion is larger and the intercalation stage becomes shorter. Such observation of preferential lithiation direction in 2D-like RuO2 thin film provides useful insights to develop dimensionally confined electrodes for lithium-ion batteries.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Oxide conversion reactions may yield Li-ion battery electrodes with exceptionally high capacities in comparison to intercalation materials, with Li2O-encompassed transition metal nanoparticles as the ideal discharge product. Nanoscale confinement has been demonstrated to facilitate the reversibility of these conversion reactions1–3. The NiO conversion reaction, in particular, has a high theoretical capacity (718 mAh g-1), but there is significant hysteresis between the experimental and theoretical voltage (0.6 V and 1.9 V, respectively), and the molecular origins of this hysteresis are poorly understood. In this work, we will present an integrated computational modeling approach, using classical molecular dynamics and first principles density functional theory calculations, with classical nucleation theory, to directly probe the role of interfaces during NiO conversion. These efforts are informed, in part, by recent work suggesting a strong role of interfaces during conversion, as lithiated domains propagate from the surface to the bulk of NiO nanomaterials4. Different nucleation schemes are evaluated, considering potential-dependent bulk and interfacial free energies to determine critical nucleation radii and energy barriers, as well as possible mechanisms for exceeding theoretical capacity limits. Theoretical predictions are directly compared to experimentally observed Ni/NiO multilayer electrode morphologies determined from in-situ X-ray reflectivity measurements. This research was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

X-ray reflectivity and transmission electron microscopy (TEM) were used to characterize the morphological changes in thin film electrodes with alternating Ni and NiO layers during lithiation as a function of the Ni buffer layer thickness. Complete lithiation of the active NiO layers occurs only when the thickness of the Ni/NiO bilayers are less than 75 Å - a threshold value that is determined by the sum of the Ni quantity in the Ni/NiO bilayer of the multilayer stack. Thicker Ni/NiO bilayers present a kinetic barrier for lithium ion diffusion inside the stack resulting in partial lithiation of the multilayer electrodes in which only the top NiO layer lithiates. Lithiation of NiO layers in a multilayer stack also leads to an interface-specific reaction that is observed to increase the thicknesses of adjacent Ni layers by 3-4 Å and is associated with the formation of a low-density Li2O layer, corresponding to an interfacially-driven phase separation of the NiO. Rate dependent cyclic voltammetry studies reveal a linear relation between the peak current and scan rate suggesting that the lithiation kinetics are controlled by charge transfer resistance at the liquid-solid interface.

In this paper, we demonstrate how photochemically enhancing the permeability of metal-organic assemblies results in a significant enhancement of the electrochemical activity of metal complexes located within the assembly. The molecular assemblies consist of different layers of redox-active metal complexes ([M(mbpy-py)3][PF6]2; M = Ru or Os) that are separated by redox-inactive spacers consisting of 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPEB) and PdCl2 of variable thicknesses (0-13.4 nm). UV-irradiation (λ = 254 nm) of our assemblies induces a photochemical reaction in the redox-inactive spacer increasing the permeability of the assembly. The observed increase was evident by trapping organic (nBu4NBF4) and inorganic (NiCl2) salts inside the assemblies, and by evaluating the electrochemical response of quinones absorbed inside the molecular assemblies before and after UV irradiation. The increase in permeability is reflected by higher currents and a change in the directionality of electron transfer, i.e., from mono- to bidirectional, between the redox-active metal complexes and the electrode surface. The supramolecular structure of the assemblies dominates the overall electron transfer properties and overrules possible electron transfer mediated by the extensive π-conjugation of its individual organic components.