Structural and electronic characterization of fluorine-doped La0.5Sr0.5CoO3-δ using electron energy-loss spectroscopy

Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3-δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.

5D Analysis of Capacity Degradation in Battery Electrodes Enabled by Operando CT-XANES

For devices encountering long-term stability challenges, a precise evaluation of degradation is of paramount importance. However, methods for comprehensively elucidating the degradation mechanisms in devices, particularly those undergoing dynamic chemical and mechanical changes during operation, such as batteries, are limited. Here, a method is presented using operando computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES) that can directly track the evolution of the 3D distribution of the local capacity loss in battery electrodes during (dis)charge cycles, thereby enabling a five-dimensional (the 3D spatial coordinates, time, and chemical state) analysis of the degradation. This paper demonstrates that the method can quantify the spatiotemporal dynamics of the local capacity degradation within an electrode during cycling, which has been truncated by existing bulk techniques, and correlate it with the overall electrode performance degradation. Furthermore, the method demonstrates its capability to uncover the correlation among observed local capacity degradation within electrodes, reaction history during past (dis)charge cycles, and electrode microstructure. The method thus provides critical insights into the identification of degradation factors that are not available through existing methods, and therefore, will contribute to the development of batteries with long-term stability.

Fast fluoride ion conduction of NH4(Mg1-xLix)F3-x and (NH4)2(Mg1-xLix)F4-x assisted by molecular cations

Aiming development of the fast anion conductors, we proposed a new material design using flexible molecular cation as a host cation, and demonstrated it with fluoride ion conduction in NH₄MgF₃ and (NH₄)₂MgF₄ based materials. Dominant fluoride ion conduction with relatively high conductivities of 4.8 × 10^-5 S cm^-1 and 8.4 × 10^-6 S cm^-1 were achieved at 323 K in (NH₄)₂(Mg_0.85Li_0.15)F_3.85 and NH₄(Mg_0.9Li_0.1)F_2.9, respectively. It is implied that the molecular cation in the host lattice can assist the anion conduction. Our findings suggest molecular cation-containing compounds can be attractive material groups for fast anion conductors.

High-temperature ionic logic gates composed of an ionic rectifying solid–electrolyte interface

Direct data collection from extremely high temperature environments is vitally important for the progress of industrial technologies such as combustion-engines, turbines and furnaces for various purposes. However, present semiconductor-based information devices are not suitable for such high-temperature applications due to thermal excitation of electronic carriers. Herein, we demonstrate high-temperature ionic AND and OR logic gates composed of the oxide-ion-conducting yttria stabilized zirconia (YSZ) and the mixed oxide-ion and electron conducting La₂NiO_4+δ as an ultra-high temperature information device. The ionic AND and OR gates developed in this work exhibited proper and stable electrical responses at 1073 K. The ionic logic gates shown in this work are promising demonstrations for robust information devices in extreme environments.

In this work, high-temperature ionic logic gates composed of ion rectifying YSZ/La₂NiO_4+δ junctions are demonstrated.

Rate-Determining Process at Electrode/Electrolyte Interfaces for All-Solid-State Fluoride-Ion Batteries

Developing high-performance solid electrolytes that are operable at room temperature is one of the toughest challenges related to all-solid-state fluoride-ion batteries (FIBs). In this study, tetragonal β-Pb_0.78Sn_1.22F₄, a promising solid electrolyte material for mild-temperature applications, was modified through annealing under various atmospheres using thin-film models. The annealed samples exhibited preferential growth and enhanced ionic conductivities. The rate-determining factor for electrode/electrolyte interface reactions in all-solid-state FIBs was also investigated by comparing β-Pb_0.78Sn_1.22F₄ with representative fluoride-ion- and lithium-ion-conductive materials, namely, LaF₃, CeF₃, and Li₇La₃Zr₂O₁₂. The overall rate constant of the interfacial reaction, k⁰, which included both mass and charge transfers, was determined using chronoamperometric measurements and Allen-Hickling simulations. Arrhenius-type correlations between k⁰ and temperature indicated that activation energies calculated from k⁰ and ionic conductivities (σ_ion) were highly consistent. The results indicated that the mass transfer (electrolyte-side fluoride-ion conduction) should be the rate-determining process at the electrode/electrolyte interface. β-Pb_0.78Sn_1.22F₄, with a large σ_ion value, had a larger k⁰ value than Li₇La₃Zr₂O₁₂. Therefore, it is hoped that the development of high-conductivity solid electrolytes can lead to all-solid-state FIBs with superior rate capabilities similar to those of all-solid-state Li-ion batteries.

3D Operando Imaging and Quantification of Inhomogeneous Electrochemical Reactions in Composite Battery Electrodes

The performances of electrochemical systems such as solid-state batteries (SSBs) can be severely hindered by the three-dimensional (3D) and mesoscopically inhomogeneous electrochemical reactions that take place in the electrodes. However, the majority of existing methods for analyzing such inhomogeneous reactions are restricted to one- or two-dimensional observations. Herein, we performed 3D operando imaging of the mesoscopically inhomogeneous electrochemical reaction in a composite SSB electrode using hard X-ray computed-tomography with X-ray absorption near edge structure spectroscopy (CT-XANES). The 3D inhomogeneous reaction evolution during (dis)charge was successfully visualized for the first time. Furthermore, our 3D quantitative analysis unambiguously revealed the origin of the inhomogeneous reaction in the investigated electrode. Our results suggested that slow ion transport through active material particles can considerably restrict SSB performances. Our technique therefore provides new insights into the electrochemical reactions taking place in electrodes and enables us to maximize the performance of electrochemical systems.

Guidelines for All-Solid-State Battery Design and Electrode Buffer Layers Based on Chemical Potential Profile Calculation

Protective coatings on cathode active materials have become paramount for the implementation of solid-state batteries; however, the development of coatings lacks the understanding of the necessary coating properties. In this study, guidelines for the design of solid electrolytes and electrode coatings in all-solid-state batteries are proposed from the viewpoint of the steady-state Li chemical potential profile across the battery cell. The model calculation of the (electro)chemical potential profile in all-solid-state batteries is established by considering the steady-state mixed ionic and electronic conduction in the solid electrolyte under the assumption of local equilibrium. For quantitative discussion, the potential profiles within oxygen ion conductors are calculated instead of Li/Na ion conductors as their partial electronic conductivities have not been reported so far in sufficient detail. Based on the calculated chemical potential profile, two main conclusions are obtained: (1) the decisive factor for the formation of the chemical potential profile of the neutral mobile component (e.g., oxygen or lithium) in the solid electrolyte is its electronic conductivity (and the activity dependence), and (2) a particularly large potential drop is formed in a region where the electronic conductivity becomes small. While these conclusions are valid and general for any solid electrolyte device, they are particularly important for the design of protective coatings and the understanding of the functionality of self-assembled solid electrolyte interphases in all-solid-state batteries. To protect the solid electrolyte from decomposition by reduction/oxidation at the anode/cathode interfaces, a sufficient chemical potential drop is necessary within the coating layer or directly at the interphase layer. To achieve this situation, the coating/interphase materials need to have a lower electronic conductivity than the solid electrolyte.

High-valence-state manganate(v) Ba3Mn2O8 as an efficient anode of a proton-conducting solid oxide steam electrolyzer

High-valence-state Mn(v) oxide Ba₃Mn₂O₈ exhibits good performances for the anodic OER on H⁺-SOEC.

Effect of Cation Ordering on the Performance and Chemical Stability of Layered Double Perovskite Cathodes

The effect of A-site cation ordering on the cathode performance and chemical stability of A-site cation ordered LaBaCo₂O_5+δ and disordered La_0.5Ba_0.5CoO_3−δ materials are reported. Symmetric half-cells with a proton-conducting BaZr_0.9Y_0.1O_3−δ electrolyte were prepared by ceramic processing, and good chemical compatibility of the materials was demonstrated. Both A-site ordered LaBaCo₂O_5+δ and A-site disordered La_0.5Ba_0.5CoO_3−δ yield excellent cathode performance with Area Specific Resistances as low as 7.4 and 11.5 Ω·cm² at 400 °C and 0.16 and 0.32 Ω·cm² at 600 °C in 3% humidified synthetic air respectively. The oxygen vacancy concentration, electrical conductivity, basicity of cations and crystal structure were evaluated to rationalize the electrochemical performance of the two materials. The combination of high-basicity elements and high electrical conductivity as well as sufficient oxygen vacancy concentration explains the excellent performance of both LaBaCo₂O_5+δ and La_0.5Ba_0.5CoO_3−δ materials at high temperatures. At lower temperatures, oxygen-deficiency in both materials is greatly reduced, leading to decreased performance despite the high basicity and electrical conductivity. A-site cation ordering leads to a higher oxygen vacancy concentration, which explains the better performance of LaBaCo₂O_5+δ. Finally, the more pronounced oxygen deficiency of the cation ordered polymorph and the lower chemical stability at reducing conditions were confirmed by coulometric titration.

Operando Soft X-ray Absorption Spectroscopic Study on a Solid Oxide Fuel Cell Cathode during Electrochemical Oxygen Reduction

An operando soft X-ray absorption spectroscopic technique, which enabled the analysis of the electronic structures of the electrode materials at elevated temperature in a controlled atmosphere and electrochemical polarization, was established and its availability was demonstrated by investigating the electronic structural changes of an La₂ NiO_4+δ dense-film electrode during an electrochemical oxygen reduction reaction. Clear O K-edge and Ni L-edge X-ray absorption spectra could be obtained below 773 K under an atmospheric pressure of 100 ppm O₂ /He, 0.1 % O₂ /He, and 1 % O₂ /He gas mixtures. Considerable spectral changes were observed in the O K-edge X-ray absorption spectra upon changing the PO2 and application of electrical potential, whereas only small spectral changes were observed in Ni L-edge X-ray absorption spectra. A pre-edge peak of the O K-edge X-ray absorption spectra, which reflects the unoccupied partial density of states of Ni 3d-O 2p hybridization, increased or decreased with cathodic or anodic polarization, respectively. The electronic structural changes of the outermost orbital of the electrode material due to electrochemical polarization were successfully confirmed by the operando X-ray absorption spectroscopic technique developed in this study.

The determining factor for interstitial oxygen formation in Ruddlesden–Popper type La2NiO4-based oxides

In this paper, we report the electronic structure of the layered perovskite oxides and suggest the determining factor for interstitial oxygen formation.

Oxygen nonstoichiometry, the defect equilibrium model and thermodynamic quantities of the Ruddlesden–Popper oxide Sr3Fe2O7−δ

Oxygen nonstoichiometry of the Ruddlesden–Popper oxide Sr₃Fe₂O_7−δwas measured at intermediate temperatures (773–1073 K) by coulometric titration and high temperature gravimetry.