Donald R. Baer

Formation of the spinel phase in the layered composite cathode used in Li-Ion batteries

Pristine Li-rich layered cathodes, such as Li(1.2)Ni(0.2)Mn(0.6)O(2) and Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2), were identified to exist in two different structures: LiMO(2)R3[overline]m and Li(2)MO(3)C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO(2)R3[overline]m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li(2)MO(3)C2/m to spinel involves removal of Li(+) and O(2-), which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF(3)-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF(3)-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.

In Situ Transmission Electron Microscopy Observation of Microstructure and Phase Evolution in a SnO2 Nanowire during Lithium Intercalation

Recently we have reported structural transformation features of SnO(2) upon initial charging using a configuration that leads to the sequential lithiation of SnO(2) nanowire from one end to the other (Huang et al. Science2010, 330, 1515). A key question to be addressed is the lithiation behavior of the nanowire when it is fully soaked into the electrolyte (Chiang Science2010, 330, 1485). This Letter documents the structural characteristics of SnO(2) upon initial charging based on a battery assembled with a single nanowire anode, which is fully soaked (immersed) into an ionic liquid based electrolyte using in situ transmission electron microscopy. It has been observed that following the initial charging the nanowire retained a wire shape, although highly distorted. The originally straight wire is characterized by a zigzag structure following the phase transformation, indicating that during the phase transformation of SnO(2) + Li ↔ Li(x)Sn + Li(y)O, the nanowire was subjected to severe deformation, as similarly observed for the case when the SnO(2) was charged sequentially from one end to the other. Transmission electron microscopy imaging revealed that the Li(x)Sn phase possesses a spherical morphology and is embedded into the amorphous Li(y)O matrix, indicating a simultaneous partitioning and coarsening of Li(x)Sn through Sn and Li diffusion in the amorphous matrix accompanied the phase transformation. The presently observed composite configuration gives detailed information on the structural change and how this change takes place on nanometer scale.

Application of Surface Chemical Analysis Tools for Characterization of Nanoparticles

AbstractThe important role that surface chemical analysis methods can and should play in the characterization of nanoparticles is described. The types of information that can be obtained from analysis of nanoparticles using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), low-energy ion scattering (LEIS), and scanning-probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are briefly summarized. Examples describing the characterization of engineered nanoparticles are provided. Specific analysis considerations and issues associated with using surface-analysis methods for the characterization of nanoparticles are discussed and summarized, with the impact that shape instability, environmentally induced changes, deliberate and accidental coating, etc., have on nanoparticle properties. FigureAtomic force microscopy image of Cu2O nanodots formed on a SrTiO3 substrate.

In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li(x)Si phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li(15)Si(4) or Li(22)Si(5). Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li(15)Si(4) from amorphous Li(x)Si is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling.

Void formation during early stages of passivation: Initial oxidation of iron nanoparticles at room temperature

The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies in which sample preparation artifacts have been the cause of some uncertainties. Microstructure of iron nanoparticles passivated with iron oxide shell was studied using high-resolution transmission electron microscopy and high-angle annular dark-field imaging in aberration-corrected scanning transmission electron microscopy. Voids were readily observed on both small single-crystal α-Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticles has been engineered for Co at higher temperatures [Y. Yin, R. M. Riou, C. K. Erdonmez, S. Hughes, G. A. Somorjari, and A. P. Alivisatos, Science 304, 711 (2004)], they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. There exists a critical size of ∼8n...

The adsorption of liquid and vapor water on TiO2(110) surfaces: the role of defects

Abstract The adsorption of liquid and vapor water on defective and nearly defect-free TiO 2 (110) surfaces has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study focuses on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. Defective surfaces were prepared by electron-beam exposure and Ar + bombardment. With exposure up to 10 4 L low vapor pressure ( −5 Torr) water to defective surfaces, little change on Ti3d defect intensity was observed. However, defect intensities were greatly reduced after exposing defective surfaces to ∼ 10 8 L higher vapor pressure (0.2–0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar + bombardment were only partially removed. Surface defects created by Ar + bombardment were removed more readily than sub-surface defects. Water adsorption on the surface has been quantified using the OH signal from the O 1s photopeak. For a nearly defect-free surface, water coverage was ∼ 0.02 ML at 10 4 L exposure to low vapor pressure water, ∼ 0.07 ML at 10 8 L exposure to higher vapor pressure water, and ∼ 0.125 ML with liquid water exposure, respectively.

Morphology and electronic structure of the oxide shell on the surface of iron nanoparticles.

An iron (Fe) nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell that is typically approximately 3 nm thick. The nature of this native oxide shell, in combination with the underlying Fe(0) core, determines the physical and chemical behavior of the core-shell nanoparticle. One of the challenges of characterizing core-shell nanoparticles is determining the structure of the oxide shell, that is, whether it is FeO, Fe(3)O(4), gamma-Fe(2)O(3), alpha-Fe(2)O(3), or something else. The results of prior characterization efforts, which have mostly used X-ray diffraction and spectroscopy, electron diffraction, and transmission electron microscopic imaging, have been framed in terms of one of the known Fe-oxide structures, although it is not necessarily true that the thin layer of Fe oxide is a known Fe oxide. In this Article, we probe the structure of the oxide shell on Fe nanoparticles using electron energy loss spectroscopy (EELS) at the oxygen (O) K-edge with a spatial resolution of several nanometers (i.e., less than that of an individual particle). We studied two types of representative particles: small particles that are fully oxidized (no Fe(0) core) and larger core-shell particles that possess an Fe core. We found that O K-edge spectra collected for the oxide shell in nanoparticles show distinct differences from those of known Fe oxides. Typically, the prepeak of the spectra collected on both the core-shell and the fully oxidized particles is weaker than that collected on standard Fe(3)O(4). Given the fact that the origin of this prepeak corresponds to the transition of the O 1s electron to the unoccupied state of O 2p hybridized with Fe 3d, a weak pre-edge peak indicates a combination of the following four factors: a higher degree of occupancy of the Fe 3d orbital; a longer Fe-O bond length; a decreased covalency of the Fe-O bond; and a measure of cation vacancies. These results suggest that the coordination configuration in the oxide shell on Fe nanoparticles is defective as compared to that of their bulk counterparts. Implications of these defective structural characteristics on the properties of core-shell structured iron nanoparticles are discussed.

DISSOLUTION KINETICS AT THE CALCITE-WATER INTERFACE

Abstract Although many geochemical processes, including mineral dissolution, are controlled by kinetic mechanisms, quantitative descriptions of the reaction kinetics are mostly lacking, principally due to an incomplete understanding of the molecular-scale processes controlling these reactions. In this paper, a combined experimental and theoretical approach involving atomic force microscopy, an analytical terrace-ledge-kink model, and kinetic Monte Carlo computer simulations was used to study the aqueous dissolution kinetics of the calcite (1014) surface. The study provides a determination of the elementary rates and activation energies associated with dissolution at specific kink sites on the calcite surface.

Creation of variable concentrations of defects on TiO2(110) using low-density electron beams

Low density (~μAcm2) 0.48 and 1.0 keV electron beams have been used to create surface defects on a TiO2(110) surface. These electron-beam induced defects were examined primarily by X-ray photoelectron spectroscopy (XPS) with supporting ultraviolet photoemission spectroscopy (UPS). Glancing and normal emission XPS spectra of nearly defect-free surfaces revealed that Ti atoms on the surface were similar to the bulk Ti, while some surface oxygen atoms were different from the bulk oxygen. XPS of Ti 2p32 was used to quantify the defect concentration and to examine the defect electronic structure. Based on our calculation of defect concentrations and the comparison of our results with results and models from the literature, we conclude that oxygen vacancies induced by electron beams in the current study are mostly from the bridging oxygen sites, in agreement with the previous work. A range of defect concentrations with similar electronic structure, mainly composed of Ti3+, have been induced by low-density electron beams. Beam energy and exposure were the experimental variables. The rates of defect formation at low beam exposure were beam-energy dependent, with a faster growth rate at 0.48 keV than at 1.0 keV. These defects were similar to those by thermal annealing in vacuum, but a higher concentration of defects could be obtained with longer beam exposure. However, the e-beam induced defects were different from those produced by Ar+ ion bombardment since both this and previous studies have found defects produced by Ar+ ion bombardment to be complex, with a variety of different local environments where oxygen and titanium surface atoms coexist.

The structure of Na2O–Al2O3–SiO2 glass: impact on sodium ion exchange in H2O and D2O

Abstract The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10–50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D 2 O compared to rates in H 2 O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the OH bond as the rate-limiting reaction in Na + –H + exchange whereas matrix dissolution is controlled by OH − or H 2 O catalyzed hydrolysis of SiOSi and SiOAl bonds. Changes in Na exchange rate with increasing Al 2 O 3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using NaO bond length and co-ordination as measured by Na K-edge X-ray absorption spectroscopy, and binding energy shifts for SiONa sites measured by X-ray photoelectron spectroscopy (XPS). The energy barrier was calculated to increase from 34 kJ mol −1 for Na 2 O·2SiO 2 glass to 49 kJ mol −1 for a glass containing 15 mol% Al 2 O 3 , consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion-exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.