Showing papers by "Mark S. Hybertsen published in 2018"

Energetics of Lithium Insertion into Magnetite, Defective Magnetite, and Maghemite

Abstract: At low concentrations of lithium insertion into inverse spinel magnetite Fe3O4, a phase change to rock-salt-like LixFe3O4 has been observed. We used density-functional-theory-based (DFT-based) calculations to study the structural origins of this phase change, the concentration at which it occurs, the role of iron vacancies, and the stability of the various motifs that form during the electrochemical reduction process in the Li–Fe–O ternary space up to x = 1.33. We compared our results to new experimental measurements of the open circuit voltage for 8–9 nm magnetite particles over a comparable range of lithium insertion. Of the vacant sites in magnetite (16c, 8b, and 48f) lithium insertion was found to be most stable on 16c. Coulomb interactions between the added lithium and iron at the 8a site in magnetite led to substantial displacement of the iron. As further lithium was added, the most energetically favored motif involved lithium clustering in 16c sites around the shifted 8a iron up to a total of three...

Atomic Scale Account of the Surface Effect on Ionic Transport in Silver Hollandite

Abstract: Nanosized electrodes for Li-ion batteries typically display improved electrochemical properties, which are generally attributed to the reduced dimensionality for lithiation. However, the intriguing roles of surface defects and disorder associated with the nanosized materials are often overlooked. Here, combining atomically resolved structural analysis with density functional theory calculations, we reveal that the formation of intrinsic oxygen vacancies near surface in silver hollandite nanorods modifies the local atomic structure and valence state. These surface reconstructions resulted from oxygen vacancies can significantly affect the diffusion pathways in what are otherwise one-dimensional (1D) tunneled structures. On the basis of energy barrier calculations, we demonstrate that the oxygen vacancies boost ionic transport through the edge sharing MnO6 polyhedra in the a–b plane. Thus, within a single rod different from the inherent 1D tunnel diffusion in the interior, the ionic transport at oxygen vaca...

Excitation and characterization of image potential state electrons on quasi-free-standing graphene

Abstract: We investigate the band structure of image potential states in quasi-free-standing graphene (QFG) monolayer islands using angle-resolved two-photon-photoemission spectroscopy. Direct probing by low-energy electron diffraction shows that QFG is formed following oxygen intercalation into the graphene-Ir(111) interface. Despite the apparent decoupling of the monolayer graphene from the Ir substrate, we find that the binding energy of the $\mathit{n}=1$ image potential state on these QFG islands increases by 0.17 eV, as compared to the original Gr/Ir(111) interface. We use calculations based on density-functional theory to construct an empirical, one-dimensional potential that quantitatively reproduces the image potential state binding energy and links the changes in the interface structure to the shift in energy. Specifically, two factors contribute comparably to this energy shift: a deeper potential well arising from the presence of intercalated oxygen adatoms and a wider potential well associated with the increase in the graphene-Ir distance. While image potential states have not been observed previously on QFG by photoemission, our paper now demonstrates that they may be strongly excited in a well-defined QFG system produced by oxygen intercalation. This opens an opportunity for studying the surface electron dynamics in QFG systems, beyond those found in typical nonintercalated graphene-on-substrate systems.

In-situ Probe of Lithium-ion Transport and Phase Evolution Within and Between Silver Hollandite Nanorods

Abstract: Porous manganese oxides such as Ag doped -MnO2 hollandites have gained significant attentions as electroactive materials as their tunnel-based crystallographic structure may provide sufficient structural rigidity to enable repeated ion exchange within their one-dimensional forms [1]. Specifically, hollandite type materials consist of edge-sharing MnO6 octahedra which interlink to form tunnels (Fig. 1n). Using an in situ TEM approach (Fig. 1a), lithiation of Ag1.6Mn8O16 nanorods was observed in ‘operando’, revealing Li diffusion in individual nanorods not only along their longitudinal (or tunnel) direction, but also in the lateral (a-b plane) direction between the nanorods. As shown in Fig. 1, upon applying the electrostatic potential, we observed changes in the interior contrast of nanorod I where many needle-like regions (region) became visible (Fig. 1c–e). After 3s, a distinct lithiation reaction front (RF) appeared and propagated longitudinally, Fig. 1f–k, while the region (-region) characterized by numerous tiny crystallites behind it underwent a 27.6% radial expansion. Comparing EELS measurement of the chemically lithiated reference samples, we estimated that the and regions correspond to ~0.9 Li and >6 Li equivalent, respectively (Fig. 1o). After ~24 s, the RF from nanorod I reached nanorod II where a second pathway for lithium transport was observed. At 34 s, the RF has crossed the boundary between the two rods and formed a reacted area centered at the top part of nanorod II (Fig. 1j). The RF reached the bottom edge of the nanorod II at 42 s (Fig. 1k). It also continued to propagate along the c axis of nanorod II. Our observations reveal the lithiation proceeded in both directions, away from the point of contact with nanorod I. Analysis of the RF progress in nanorod II indicated an estimated velocity of 1.9 nm/s across the diameter of the rod and a velocity of ~4.4 nm/s longitudinally, indicating an asymmetry for RF motion in the a-b direction and along the c axis. Starting at ~42 s, the lateral and longitudinal lithium transport pathways were observed simultaneously for nanorod III [2].