Dealumination of small-pore zeolites through pore-opening migration process with the aid of pore-filler stabilization.

Small-pore zeolites are gaining increasing attention owing to their superior catalytic performance. Despite being critical for the catalytic activity and lifetime, postsynthetic tuning of bulk Si/Al ratios of small-pore zeolites has not been achieved with well-preserved crystallinity because of the limited mass transfer of aluminum species through narrow micropores. Here, we demonstrate a postsynthetic approach to tune the composition of small-pore zeolites using a previously unexplored strategy named pore-opening migration process (POMP).

Atomic-Level Changes during Electrochemical Cycling of Oriented LiMnO Cathodic Thin Films.

Spinel LiMnO is an attractive lithium-ion battery cathode material that undergoes a complex series of structural changes during electrochemical cycling that lead to rapid capacity fading, compromising its long-term performance. To gain insights into this behavior, in this report we analyze changes in epitaxial LiMnO thin films during the first few charge-discharge cycles with atomic resolution and correlate them with changes in the electrochemical properties. Impedance spectroscopy and scanning transmission electron microscopy are used to show that defect-rich LiMnO surfaces contribute greatly to the increased resistivity of the battery after only a single charge.

Oxide-ion diffusion in brownmillerite-type CaAlMnO from first-principles calculations.

Oxide-ion diffusion pathways in brownmillerite oxides CaAlMnO and CaAlMnO are systematically investigated using first-principles calculations. These structures reversibly transform into each other by oxidation and reduction. We examine oxide-ion migration in CaAlMnO and CaAlMnO using the nudged elastic band method.

Quantitative prediction of grain boundary thermal conductivities from local atomic environments.

Quantifying the dependence of thermal conductivity on grain boundary (GB) structure is critical for controlling nanoscale thermal transport in many technologically important materials. A major obstacle to determining such a relationship is the lack of a robust and physically intuitive structure descriptor capable of distinguishing between disparate GB structures. We demonstrate that a microscopic structure metric, the local distortion factor, correlates well with atomically decomposed thermal conductivities obtained from perturbed molecular dynamics for a wide variety of MgO GBs.

Characterisation of the temperature-dependent M to R phase transition in W-doped VO nanorod aggregates by Rietveld refinement and theoretical modelling.

Understanding the mechanism of the insulator-metal transition (IMT) in VO2 is a necessary step in optimising this material's properties for a range of functional applications. Here, Rietveld refinement of synchrotron X-ray powder diffraction patterns is performed on thermochromic V1-xWxO2 (0.0 ≤ x ≤ 0.

Systematic analysis of electron energy-loss near-edge structures in Li-ion battery materials.

Electrical conductivity, state of charge and chemical stability of Li-ion battery materials all depend on the electronic states of their component atoms, and tools for measuring these reliably are needed for advanced materials analysis and design. Here we report a systematic investigation of electron energy-loss near-edge structures (ELNES) of Li-K and O-K edges for ten representative Li-ion battery electrodes and solid-state electrolytes obtained by performing transmission electron microscopy with a Wien-filter monochromator-equipped microscope. While the peaks of Li-K edges are positioned at about 62 eV for most of the materials examined, the peak positions of O-K edges vary within a range of about 530 to 540 eV, and the peaks can be categorised into three groups based on their characteristic edge shapes: (i) double peaks, (ii) single sharp peaks, and (iii) single broad peaks.

Quantifying Anharmonic Vibrations in Thermoelectric Layered Cobaltites and Their Role in Suppressing Thermal Conductivity.

Optimizing multiple materials properties which are simultaneously in competition with each other is one of the chief challenges in thermoelectric materials research. Introducing greater anharmonicity to vibrational modes is one strategy for suppressing phonon thermal transport in crystalline oxides without detrimentally affecting electronic conductivity, so that the overall thermoelectric efficiency can be improved. Based on perturbed molecular dynamics and associated numerical analyses, we show that CoO layers in layered cobaltite thermoelectrics NaCoO and CaCoO are responsible for most of the in-plane heat transport in these materials, and that the non-conducting intermediate layers in the two materials exhibit different kinds of anharmonicity.

Microscopic mechanism of biphasic interface relaxation in lithium iron phosphate after delithiation.

Charge/discharge of lithium-ion battery cathode material LiFePO is mediated by the structure and properties of the interface between delithiated and lithiated phases. Direct observations of the interface in a partially delithiated single crystal as a function of time using scanning transmission electron microscopy and electron energy-loss spectroscopy help clarify these complex phenomena. At the nano-scale, the interface comprises a thin multiphase layer whose composition varies monotonically between those of the two end-member phases.

Density functional study of the phase stability and Raman spectra of YbO, YbSiO and YbSiO under pressure.

The phase stability and Raman spectra of Yb2O3, Yb2SiO5 and Yb2Si2O7 under hydrostatic pressure are investigated using density functional theory calculations. The calculated energies of polymorphs of each compound show that the stable phases at zero pressure, viz., C-type Yb2O3, X2-Yb2SiO5 and β-Yb2Si2O7, exhibit a pressure-induced phase transition as compressive pressure increases, which is consistent with available experimental data.

Quantitative analysis of Li distributions in battery material Li1-xFePO4 using Fe M2,3-edge and valence electron energy loss spectra.

The spatial distribution of Li ions in a lithium iron phosphate (Li1-xFePO4) single crystal after chemical delithiation is quantitatively investigated using Fe M2,3-edge and valence electron energy loss (EEL) spectroscopy techniques. Li contents between those of end-member compositions LiFePO4 and FePO4 are found to correspond to reproducible changes in Fe M2,3-edge and valence EEL spectra across an interface between LiFePO4 and FePO4 regions. Quantitative analysis of these changes is used to estimate the local valence states of Fe ions, from which the Li concentration in the intermediate phase can be deduced.

Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors.

Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder.

Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors.

Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder.

In situ electron holography of electric potentials inside a solid-state electrolyte: Effect of electric-field leakage.

In situ electron holography is used to observe changes of electric-potential distributions in an amorphous lithium phosphorus oxynitride (LiPON) solid-state electrolyte when different voltages are applied. 2D phase images are simulated by integrating the 3D potential distribution along the electron trajectory through a thin Cu/LiPON/Cu region. Good agreement between experimental and simulated phase distributions is obtained when the influence of the external electric field is taken into account using the 3D boundary-charge method.

Atomic-Scale Observations of (010) LiFePO4 Surfaces Before and After Chemical Delithiation.

The ability to view directly the surface structures of battery materials with atomic resolution promises to dramatically improve our understanding of lithium (de)intercalation and related processes. Here we report the use of state-of-the-art scanning transmission electron microscopy techniques to probe the (010) surface of commercially important material LiFePO4 and compare the results with theoretical models. The surface structure is noticeably different depending on whether Li ions are present in the topmost surface layer or not.

Particle shapes and surface structures of olivine NaFePO₄ in comparison to LiFePO₄.

The expansion of batteries into electric vehicle and grid storage applications has driven the development of new battery materials and chemistries, such as olivine phosphate cathodes and sodium-ion batteries. Here we present atomistic simulations of the surfaces of olivine-structured NaFePO4 as a sodium-ion battery cathode, and discuss differences in its morphology compared to the lithium analogue LiFePO4. The calculated equilibrium morphology is mostly isometric in appearance, with (010), (201) and (011) faces dominant.

Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties.

Energy storage technologies are critical in addressing the global challenge of clean sustainable energy. Major advances in rechargeable batteries for portable electronics, electric vehicles and large-scale grid storage will depend on the discovery and exploitation of new high performance materials, which requires a greater fundamental understanding of their properties on the atomic and nanoscopic scales. This review describes some of the exciting progress being made in this area through use of computer simulation techniques, focusing primarily on positive electrode (cathode) materials for lithium-ion batteries, but also including a timely overview of the growing area of new cathode materials for sodium-ion batteries.

First-principles calculations of lithium-ion migration at a coherent grain boundary in a cathode material, LiCoO(2).

Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO(2) . This study suggests that internal interfaces in LiCoO(2) strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all-solid-state Li-ion batteries.

Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features.

This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented.

Disproportionation, dopant incorporation, and defect clustering in Perovskite-structured NdCoO3.

Atomistic simulation techniques are used to examine the defect chemistry of perovskite-structured NdCoO(3), a material whose electrochemical properties make it attractive for use in heterogeneous oxidation catalysis, as well as in gas sensors and mixed ionic/electronic conductors. In practice, dopants are added to NdCoO(3) to obtain the desired properties, such as high electrical conductivity and rapid gas adsorption/desorption; thus, a wide range of dopants substituted on both Nd and Co sites are examined. Charge compensation for aliovalent dopants is predicted to occur via formation of oxide ion vacancies; these are understood to be key sites with respect to catalytic and sensor activity.