Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds

In this work, we report the presence of surface-densified phases (β-Ni₅O₈, γ-Ni₃O₄, and δ-Ni₇O₈) in LiNiO₂ (LNO)- and LiNi_0.8Al_0.2O₂ (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni²⁺ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi₂O₄ spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. These experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures.

Electrolyte-Induced Surface Transformation and Transition-Metal Dissolution of Fully Delithiated LiNi0.8Co0.15Al0.05O2

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[Ni_0.8Co_0.15Al_0.05]O₂ (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface Li₂CO₃ content on the magnitude and character of the dissolution reaction was studied.

Structure of a Near-Coincidence Σ9 Tilt Grain Boundary in Aluminum

A systematic study of the structure of tilt grain boundaries in aluminum has been initiated. High resolution transmission electron microscopy is being used to examine the interface structure of several bicrystals with <110> tilt axes. In this paper, we report the structure determination of a grain boundary close to the Σ9 (221) symmetric orientation. The grain boundary plane, which appears wavy at lower magnification, is actually composed of atomically flat microfacets. Two distinct, symmetric structures with (221) boundary planes have been identified within individual microfacets. These observations have been compared with structures calculated using the Embedded Atom Method. The semi-quantitative comparison between the observed and predicted grain boundary structures is accomplished using multislice image simulations based on the calculated structures. The results of these comparisons and the evaluation of the relative energies of the microfacets are discussed.

Hrtem Observations Of A Σ=3 {112} Bicrystal Boundary In Aluminum

We present here a study of the Σ=3 {112} incoherent twin boundary in aluminum. Atomistic studies of this boundary indicate that several high energy boundary structures may exist, with the lowest energy structure exhibiting a small rigid body shift parallel to the boundary. The observations presented here indicate that the rigid body shift does in fact occur and that its magnitude, as well as the local grain boundary structure, is well predicted by atomistic calculations using the Embedded Atom Method. The low energy boundary configuration is much narrower than the equivalent boundaries that have been observed in the lower stacking fault energy FCC metals.

Growth and Epitaxy of Au on TiO2 (110)

The growth of Au on TiO2 (110) has been examined by high resolution fielc emission scanning electron microscopy (HRSEM) in combination with electror backscattered diffraction (EBSD). The Au was evaporated under UHV conditions onto stoichiometric TiO2 (110) surfaces in the temperature range from 300 to 475 K. At 300 K and for low coverages (<1.5 nm), Au grows as discrete particles. For thicker coverages (>1.5 nm), the particles coalesce to form a network, but percolation is absent even aftel deposition of 5 nm Au. Upon annealing or deposition (≥ 5 nm) at 475 K, the particles, appear clearly faceted and are oriented along specific crystallographic directions. EBSE patterns taken from individual particles reveal two equivalent domain orientations rotated by 180° with epitaxial orientation relationships corresponding to (111)Au//(110)TiO and [110] // [001]TiO2 (Orientation I) and [110] // [001]TiO2.(Orientation II)

Next Generation Positive Electrode Materials Enabled by Nanocomposites: -Metal Fluorides-

Through the use of nanostructures and nanocomposites, the electrochemical activity of metal fluoride materials was opened as potential candidates as next generation high energy density positive electrodes for Li batteries. This class of materials, utilizing FeF3 as an example, is shown to exhibit good reversible behavior of approximately 200 mAh/g in the 3V region. The specific capacity is extended to 600 mAh/g when the discharge is extended to take into account the additional specific capacity associated with a 2V plateau. Through the use of XRD, SAED and high resolution TEM, the 2V reaction mechanism was associated to a reversible metal fluoride conversion mechanism. It is shown that LiF + Fe nanocomposite can be utilized as initial components in order to make the technology suitable for Li-ion applications. Although exhibiting relatively poor rate capabilities at this initial stage, reversible conversion metal fluorides enable for the first time the utilization of all the redox states of the constituent metal in a reversible manner in the positive electrode. This translates to 4X the specific capacity and double the energy density of today's state of the art LiCoO2.

Epitaxy and Atomic Structure Determination of Au/TiO2 Interfaces by Combined EBSD and HRTEM

We have studied the effects of deposition conditions on the epitaxial orientation of Au or TiO2 (110) and on the atomic structure of Au/TiO2 interfaces by combined EBSD and HRTEM. Two experimental conditions were explored consisting of deposition of a 12 nim Au film at 300K followed by annealing at 770K and direct deposition of a 12 nm Au film at 770K. Deposition at 300K followed by annealing at 770K give rise to a (111)Au//(110)TiO2 epitaxial orientation relationship, while direct deposition at 700K temperature give rise to an epitaxial orientation relationship given by (112)Au//(110)TiO2. For both orientations, two epitaxial variants are observed which are twin related. The (112)Au//(110)TiO2 orientation has been found to minimize the interfacial lattice misfit while maximizing the number of Au-Ti bonds across the interface.

EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries

A new type of positive electrode for Li-ion batteries has been developed recently based on FeF3/C and FeF2/C nanocomposites. The microstructural and redox evolution during discharge and recharge processes was followed by electron energy loss spectroscopy (EELS) to determine the valence state of Fe by measuring the Fe L3 line energy shift and from Fe L3/L2 line intensity ratios. In addition, transition metal fluorides were found to be electron beam sensitive, and the effect of beam exposure on EELS spectra was also investigated. The EELS results indicate that for both FeF3/C and FeF2/C nanocomposite systems, a complete reduction of iron to FeO is observed upon discharge to 1.5 V with the formation of a finer FeO/LiF subnanocomposite ( approximately 7 nm). Upon complete recharging to 4.5 V, EELS data reveal a reoxidation process to a Fe2+ state with the formation of a carbon metal fluoride nanocomposite related to the FeF2 structure.

Single-crystal-like materials by the self-assembly of cube-shaped lead zirconate titanate (PZT) microcrystals

We demonstrated the formation of single-crystal-like materials that contain preferentially oriented arrays of lead zirconate titanate (PZT) cube-shaped particles by self-assembly. Hydrothermally synthesized PZT particles with a bulk composition of Zr/Ti = 70/30 were used in making microcrystal arrays. Spreading a suspension containing PZT cube-shaped particles, 2-propanol, and mineral oil at the air-water interface produced a one-dimensional planar array of PZT particles on the water surface. The array so formed was subsequently transferred onto a flat or curved substrate. X-ray diffraction and electron backscattered diffraction analyses revealed that most of the cube-shaped particles in the array were oriented with their pseudocubic (001) direction aligned parallel to the normal direction of the substrate surface. Filling the arrays with matrixes produced monolayer or multilayer textured composites. The piezoelectric properties of oriented cube-shaped micron-sized particles in the self-assembled arrays were measured using a modified atomic force microscope to reveal the ferroelectric nature of the PZT arrays.