Uncoordinated chemistry enables highly conductive and stable electrolyte/filler interfaces for solid-state lithium-sulfur batteries

Composite-polymer-electrolytes (CPEs) embedded with advanced filler materials offer great promise for fast and preferential Li+ conduction. The filler surface chemistry determines the interaction with electrolyte molecules and thus critically regulates the Li+ behaviors at the interfaces. Herein, we probe into the role of electrolyte/filler interfaces (EFI) in CPEs and promote Li+ conduction by introducing an unsaturated coordination Prussian blue analog (UCPBA) filler. Combining scanning transmission X-ray microscope stack imaging studies and first-principle calculations, fast Li+ conduction is revealed only achievable at a chemically stable EFI, which can be established by the unsaturated Co-O coordination in UCPBA to circumvent the side reactions. Moreover, the as-exposed Lewis-acid metal centers in UCPBA efficiently attract the Lewis-base anions of Li salts, which facilitates the Li+ disassociation and enhances its transference number (tLi+). Attributed to these superiorities, the obtained CPEs realize high room-temperature ionic conductivity up to 0.36 mS cm-1 and tLi+ of 0.6, enabling an excellent cyclability of lithium metal electrodes over 4,000 h as well as remarkable capacity retention of 97.6% over 180 cycles at 0.5 C for solid-state lithium-sulfur batteries. This work highlights the crucial role of EFI chemistry in developing highly conductive CPEs and high-performance solid-state batteries.

Breakdown of the Small-Polaron Hopping Model in Higher-Order Spinels

The small-polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its accuracy has not been rigorously tested for higher-order oxides. Here, the small-polaron transport model is tested by using a spinel system with mixed cation oxidation states (Mn_x Fe_{3- x} O₄ ). Using molecular-beam epitaxy (MBE), a series of single crystal Mn_x Fe_{3- x} O₄ thin films with controlled stoichiometry, 0 ≤ x ≤ 2.3, and lattice strain are grown, and the cation site-occupation is determined through X-ray emission spectroscopy (XES). Density functional theory + U analysis shows that charge transport occurs only between like-cations (Fe/Fe or Mn/Mn). The site-occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the Mn O h 2 + polaron is energetically preferred to the Fe O h 2 + polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small-polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearest-neighbor hopping equation are introduced to account for percolation, cross-hopping, and polaron-distribution, and it is found that a near-perfect correlation can be made between experiment and theory for the electronic conductivity.

Towards the synthesis of mixed oxides with controlled stoichiometry from Prussian blue analogues

The calcination of (nano) Prussian blue analogues is now a fully controlled and understood route to synthesize Co–Fe spinel oxides.

X-ray emission spectroscopy: an effective route to extract site occupation of cations

Cation site occupation is an important determinant of materials properties, especially in a complex system with multiple cations such as in ternary spinels. In this work, we show that XES provides not only the site occupation information as EXAFS, but also additional information on the oxidation states of the cations at each site. Additionally, we show that XES is a superior and a far more accurate method than EXAFS.

High-Energy Resolution Fluorescence Detected X-Ray Absorption Spectroscopy: A Powerful New Structural Tool in Environmental Biogeochemistry Sciences

The study of the speciation of highly diluted elements by X-ray absorption spectroscopy (XAS) is extremely challenging, especially in environmental biogeochemistry sciences. Here we present an innovative synchrotron spectroscopy technique: high-energy resolution fluorescence detected XAS (HERFD-XAS). With this approach, measurement of the XAS signal in fluorescence mode using a crystal analyzer spectrometer with a ∼1-eV energy resolution helps to overcome restrictions on sample concentrations that can be typically measured with a solid-state detector. We briefly describe the method, from both an instrumental and spectroscopic point of view, and emphasize the effects of energy resolution on the XAS measurements. We then illustrate the positive impact of this technique in terms of detection limit with two examples dealing with Ce in ecologically relevant organisms and with Hg species in natural environments. The sharp and well-marked features of the HERFD-X-ray absorption near-edge structure spectra obtained enable us to determine unambiguously and with greater precision the speciation of the probed elements. This is a major technological advance, with strong benefits for the study of highly diluted elements using XAS. It also opens new possibilities to explore the speciation of a target chemical element at natural concentration levels, which is critical in the fields of environmental and biogeochemistry sciences.

Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship

The structure of cobalt oxide (CoO_x) nanoparticles dispersed on rutile TiO₂ (R-TiO₂) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and X-ray photoelectron spectroscopy. The CoO_x nanoparticles were loaded onto R-TiO₂ by an impregnation method from an aqueous solution containing Co(NO₃)₂·6H₂O followed by heating in air. Modification of the R-TiO₂ with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co₃O₄ nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO₂ surface, resulting in visible light absorption and active sites for water oxidation.

Temperature dependence of X-ray absorption and nuclear magnetic resonance spectra: probing quantum vibrations of light elements in oxides

Probing the quantum thermal fluctuations of nuclei in light-element oxides using XANES and NMR spectroscopies.