Combined Electrochemical, XPS, and STXM Study of Lithium Nitride as a Protective Coating for Lithium Metal and Lithium-Sulfur Batteries.

Li3N is an excellent protective coating material for lithium electrodes with very high lithium-ion conductivity and low electronic conductivity, but the formation of stable and homogeneous coatings is technically very difficult. Here, we show that protective Li3N coatings can be simply formed by the direct reaction of electrodeposited lithium electrodes with N2 gas, whereas using battery-grade lithium foil is problematic due to the presence of a native passivation layer that hampers that reaction. The protective Li3N coating is effective at preventing lithium dendrite formation, as found from unidirectional plating and plating-stripping measurements in Li-Li cells. The Li3N coating also efficiently suppresses the parasitic reactions of polysulfides and other electrolyte species with the lithium electrode, as demonstrated by scanning transmission X-ray microscopy, X-ray photoelectron spectroscopy, and optical microscopy. The protection of the lithium electrode against corrosion by polysulfides and other electrolyte species, as well as the promotion of smooth deposits without dendrites, makes the Li3N coating highly promising for applications in lithium metal batteries, such as lithium-sulfur batteries. The present findings show that the formation of Li3N can be achieved with lithium electrodes covered by a secondary electrolyte interface layer, which proves that the in situ formation of Li3N coatings inside the batteries is attainable.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3.

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH5 (PO4 )3 (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10-2  S cm-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Proton and Oxide Ion Conductivity in Palmierite Oxides.

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A3V2O8 (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO4 units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t H ∼ 0.6-0.8) and high protonic mobility. In particular, the proton conductivity of Sr3V2O8 is 1.0 × 10-4 S cm-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO4 units, which can stabilize oxygen defects via formation of V2O7 dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

Localized Spin Dimers and Structural Distortions in the Hexagonal Perovskite Ba3CaMo2O9.

Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba3CaMo2O9. Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo2O9 dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba3SrMo2O9 analogue, in which the Mo2O9 dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba3CaMo2O9 and Ba3SrMo2O9 have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo5+ hexagonal perovskites.

Variable Temperature Neutron Diffraction Study of the Oxide Ion Conductor Ba3VWO8.5.

Ba3VWO8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10-5 S cm-1 at 600 °C. Ba3VWO8.5 is anomalous to the other Ba3M'M″O8.5 (M' = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba3VWO8.5, which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba3M'M″O8.5 are dictated by water absorption. The significant water uptake in Ba3M'M″O8.5 (M' = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V5+ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.

Investigation of the Crystal Structure and Ionic Pathways of the Hexagonal Perovskite Derivative Ba3-xVMoO8.5-x.

The hexagonal perovskite derivatives Ba3NbMoO8.5, Ba3NbWO8.5, and Ba3VWO8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba3-xVMoO8.5-x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1Ox polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba3M'M″O8.5 family, Ba3-xVMoO8.5-x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba3-xVMoO8.5-x is unstable at lower temperatures, as illustrated by the formation of BaCO3 and BaMoO4 by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba3-xVMoO8.5-x.

High oxide ion and proton conductivity in a disordered hexagonal perovskite.

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

Hexagonal perovskite derivatives: a new direction in the design of oxide ion conducting materials.

Various structural families have been reported to support oxide ion conductivity; among these, perovskite conductors have received particular attention. The perovskite structure is generally composed of a framework of corner-sharing octahedral units. When the octahedral units share their faces, hexagonal perovskites are formed. Mixed combinations of corner-sharing and face-sharing octahedral units can give rise to a variety of hexagonal perovskite derivatives. However, the ionic conducting properties of these materials have not been well explored. In this feature article, we review the conducting properties of the most significant hexagonal perovskite derivatives, with special focus on Ba3MM'O8.5. Ba3MM'O8.5 is the first hexagonal perovskite derivative to exhibit substantial oxide ion conductivity, and here we outline the structural features that are key for the oxide ion conduction within this system. The results demonstrate that further investigation of hexagonal perovskite derivatives could open up new directions in the design of oxide ion conductors.

Electrical and Structural Characterization of Ba3Mo1-xNb1+xO8.5-x/2: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba3MoNbO8.5.

The electrical and structural properties of the series Ba3Mo1-xNb1+xO8.5-x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba3Mo1-xNb1+xO8.5-x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O4 and (Mo/Nb)O6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10-6 S cm-1 for x = 0.0 to 1.1 × 10-7 S cm-1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba3MoNbO8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MoNbO8.5 system.

Oxide Ion Conductivity in the Hexagonal Perovskite Derivative Ba3MoNbO8.5.

Oxide ion conductors are important materials with a range of technological applications and are currently used as electrolytes for solid oxide fuel cells and solid oxide electrolyzer cells. Here we report the crystal structure and electrical properties of the hexagonal perovskite derivative Ba3MoNbO8.5. Ba3MoNbO8.5 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures. This is a new and so far unique crystal structure that contains a disordered distribution of (Mo/Nb)O6 octahedra and (Mo/Nb)O4 tetrahedra. Ba3MoNbO8.5 shows a wide stability range and exhibits predominantly oxide ion conduction over a pO2 range from 10-20 to 1 atm with a bulk conductivity of 2.2 × 10-3 S cm-1 at 600 °C. The high level of conductivity in a new structure family suggests that further study of hexagonal perovskite derivatives containing mixed tetrahedral and octahedral geometry could open up new horizons in the design of oxygen conducting electrolytes.

Small is beautiful: the unusual transformation of nanocrystalline layered α-zirconium phosphate into a new 3D structure.

Nanosized α-zirconium phosphate, α-ZrP, undergoes a phase transition at 120 °C, which is not observed with microcrystalline α-ZrP in the same conditions, and which leads to a new 3D phase. The new compound, with formula Zr(HPO4)2 (τ'-ZrP), consists of cubelike nanoparticles and has a tetragonal unit cell (space group P43212, a = 7.955 Å, c = 10.744 Å). The structure of τ'-ZrP is in close relationship with that of the already known τ-ZrP. Both structures are made of packed chains of eight-membered rings, composed of Zr atoms connected to bridging HPO4 groups. The main difference between the two structures concerns the different orientation of the uncoordinated P-OH groups, pointing into the channels. The in situ XRPD analysis on nanosized α-ZrP, performed at 120 °C as a function of time, provided information about the kinetics of the formation of τ'-ZrP, showing that the α-ZrP phase is directly transformed into τ'-ZrP. Moreover, τ'-ZrP is converted into α-ZrP at room temperature in the presence of water vapor. It was proved that the free phosphoric acid, which is originally present in small amounts in nanosized α-ZrP and τ'-ZrP, is necessary for the interconversion between the two phases. As a matter of fact, the removal of phosphoric acid, by washing α-ZrP and τ'-ZrP with anhydrous ethanol, inhibits the above conversion.

Aminoalcohol functionalized zirconium phosphate as versatile filler for starch-based composite membranes.

Microcrystalline zirconium phosphate was exfoliated by treatment with aqueous solutions of α,ω-alkylaminoalcohols and employed for the fabrication of potato starch composite membranes. Glycerol-based and glycerol-free composite membranes, containing 5 wt% of filler, were prepared from gelatinized starch and characterized for their physico-chemical properties. Despite of a partial filler reaggregation, as revealed by XRD and SEM analysis, all the composites exhibited a significant increase in the Young's modulus with respect to the glycerol-starch membrane, up to 80% and 190% for the glycerol-based and the glycerol-free composites, respectively. For both kinds of membranes the filler delays to a large extent the starch decomposition above about 300°C. A significant reduction in the water uptake of the composites was also observed with respect to the neat glycerol-based membrane, up to about 70% for the glycerol-free composites.