Localized Mechanical Stress Induced Ionic Redistribution in a Layered LiCoO₂ Cathode

Controlling the transport of ions within electrodes is highly desirable for the operation of rechargeable ion batteries. Here, for the first time, we report the role of mechanical stress in controlling the redistribution of lithium ions in a layered LiCoO₂ electrode at a resolution of ∼100 nm. Under a higher stress field, more active redistribution of lithium ions was observed along the grain boundaries than the interiors of the layered LiCoO₂. The dynamic force ramping test proved the external stress field (<100 nN) is capable of inducing the resistive-switching effect of the layered LiCoO₂. The comparison test on the highly ordered pyrolytic graphite (HOPG) substrate further demonstrated the improved current responses from the layered LiCoO₂ resulted from the deficiency of lithium ions, rather than the increase of tip–sample contact area. Our findings will pave the road for a full understanding of how mechanical stimulus can affect the distribution of ions in the layered electrodes of rechargeable ion batteries

Preformed Seeds Modulate Native Insulin Aggregation Kinetics

Insulin aggregates under storage conditions via disulfide interchange reaction. It is also known to form aggregates at the site of repeated injections in diabetes patients, leading to injection amyloidosis. This has fueled research in pharmaceutical and biotechnology industry as well as in academia to understand factors that modulate insulin stability and aggregation. The main aim of this study is to understand the factors that modulate aggregation propensity of insulin under conditions close to physiological and measure effect of “<i>seeds</i>” on aggregation kinetics. We explored the aggregation kinetics of insulin at pH 7.2 and 37 °C in the presence of disulfide-reducing agent dithiothreitol (DTT), using spectroscopy (UV–visible, fluorescence, and Fourier transform infrared spectroscopy) and microscopy (scanning electron microscopy, atomic force microscopy) techniques. We prepared insulin “<i>seeds</i>” by incubating disulfide-reduced insulin at pH 7.2 and 37 °C for varying lengths of time (10 min to 12 h). These seeds were added to the native protein and nucleation-dependent aggregation kinetics was measured. Aggregation kinetics was fastest in the presence of 10 min seeds suggesting they were <i>nascent.</i> Interestingly, <i>intermediate</i> seeds (30 min to 4 h incubation) resulted in formation of transient fibrils in 4 h that converted to amorphous aggregates upon longer incubation of 24 h. Overall, the results show that insulin under disulfide reducing conditions at pH and temperature close to physiological favors amorphous aggregate formation and seed “maturity” plays an important role in nucleation dependent aggregation kinetics

Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor Deposition

Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and “puckering” mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles

Experimentally Validated Structures of Supported Metal Nanoclusters on MoS₂

In nanometer clusters (NCs), each atom counts. It is the specific arrangement of these atoms that determines the unique size-dependent functionalities of the NCs and hence their applications. Here, we employ a self-consistent, combined theoretical and experimental approach to determine atom-by-atom the structures of supported Pt NCs on MoS₂. The atomic structures are predicted using a genetic algorithm utilizing atomistic force fields and density functional theory, which are then validated using aberration-corrected scanning transmission electron microscopy. We find that relatively small clusters grow with (111) orientation such that Pt[11̅0] is parallel to MoS₂[100], which is different from predictions based on lattice-match for thin-film epitaxy. Other 4d and 5d transition metals show similar behavior. The underpinning of this growth mode is the tendency of the NCs to maximize the metal–sulfur interactions rather than to minimize lattice strain

Origin of the Phase Transition in Lithiated Molybdenum Disulfide

Phase transitions and phase engineering in two-dimensional MoS₂ are important for applications in electronics and energy storage. By <i>in situ</i> transmission electron microscopy, we find that H-MoS₂ transforms to T-LiMoS₂ at the early stages of lithiation followed by the formation of Mo and Li₂S phases. The transition from H-MoS₂ to T-LiMoS₂ is explained in terms of electron doping and electron–phonon coupling at the conduction band minima. Both are essential for the development of two-dimensional semiconductor-metal contacts based on MoS₂ and the usage of MoS₂ as anode material in Li ion batteries

Characteristic Work Function Variations of Graphene Line Defects

Line defects, including grain boundaries and wrinkles, are commonly seen in graphene grown by chemical vapor deposition. These one-dimensional defects are believed to alter the electrical and mechanical properties of graphene. Unfortunately, it is very tedious to directly distinguish grain boundaries from wrinkles due to their similar morphologies. In this report, high-resolution Kelvin potential force microscopy (KPFM) is employed to measure the work function distribution of graphene line defects. The characteristic work function variations of grain boundaries, standing-collapsed wrinkles, and folded wrinkles could be clearly identified. Classical and quantum molecular dynamics simulations reveal that the unique work function distribution of each type of line defects is originated from the doping effect induced by the SiO₂ substrate. Our results suggest that KPFM can be an easy-to-use and accurate method to detect graphene line defects, and also propose the possibility to tune the graphene work function by defect engineering

Direct Evidence of Lithium-Induced Atomic Ordering in Amorphous TiO₂ Nanotubes

In this paper, we report the first direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO₂ nanomaterials and propose new reaction mechanisms that contradict the many works in the published literature on the lithiation behavior of these materials. The lithiation process was conducted in situ inside an atomic resolution transmission electron microscope. Our results indicate that the lithiation started with the valence reduction of Ti⁴⁺ to Ti³⁺ leading to a Li_<i>x</i>TiO₂ intercalation compound. The continued intercalation of Li ions in TiO₂ nanotubes triggered an amorphous to crystalline phase transformation. The crystals were formed as nano-islands and identified to be Li₂Ti₂O₄ with cubic structure (<i>a</i> = 8.375 Å). The tendency for the formation of these crystals was verified with density functional theory (DFT) simulations. The size of the crystalline islands provides a characteristic length scale (∼5 nm) at which the atomic bonding configuration has been changed within a short time period. This phase transformation is associated with local inhomogeneities in Li distribution. On the basis of these observations, a new reaction mechanism is proposed to explain the first cycle lithiation behavior in amorphous TiO₂ nanotubes

Directly Formed Alucone on Lithium Metal for High-Performance Li Batteries and Li–S Batteries with High Sulfur Mass Loading

Lithium metal is considered the “holy grail” of next-generation battery anodes. However, severe parasitic reactions at the lithium–electrolyte interface deplete the liquid electrolyte and the uncontrolled formation of high surface area and dendritic lithium during cycling causes rapid capacity fading and battery failure. Engineering a dendrite-free lithium metal anode is therefore critical for the development of long-life batteries using lithium anodes. In this study, we deposit a conformal, organic/inorganic hybrid coating, for the first time, directly on lithium metal using molecular layer deposition (MLD) to alleviate these problems. This hybrid organic/inorganic film with high cross-linking structure can stabilize lithium against dendrite growth and minimize side reactions, as indicated by scanning electron microscopy. We discovered that the alucone coating yielded several times longer cycle life at high current rates compared to the uncoated lithium and achieved a steady Coulombic efficiency of 99.5%, demonstrating that the highly cross-linking structured material with great mechanical properties and good flexibility can effectively suppress dendrite formation. The protected Li was further evaluated in lithium–sulfur (Li–S) batteries with a high sulfur mass loading of ∼5 mg/cm². After 140 cycles at a high current rate of ∼1 mA/cm², alucone-coated Li–S batteries delivered a capacity of 657.7 mAh/g, 39.5% better than that of a bare lithium–sulfur battery. These findings suggest that flexible coating with high cross-linking structure by MLD is effective to enable lithium protection and offers a very promising avenue for improved performance in the real applications of Li–S batteries

Atomistic Insights into the Oriented Attachment of Tunnel-Based Oxide Nanostructures

Controlled synthesis of nanomaterials is one of the grand challenges facing materials scientists. In particular, how tunnel-based nanomaterials aggregate during synthesis while maintaining their well-aligned tunneled structure is not fully understood. Here, we describe the atomistic mechanism of oriented attachment (OA) during solution synthesis of tunneled α-MnO₂ nanowires based on a combination of <i>in situ</i> liquid cell transmission electron microscopy (TEM), aberration-corrected scanning TEM with subangstrom spatial resolution, and first-principles calculations. It is found that primary tunnels (1 × 1 and 2 × 2) attach along their common {110} lateral surfaces to form interfaces corresponding to 2 × 3 tunnels that facilitate their short-range ordering. The OA growth of α-MnO₂ nanowires is driven by the stability gained from elimination of {110} surfaces and saturation of Mn atoms at {110}-edges. During this process, extra [MnO_<i>x</i>] radicals in solution link the two adjacent {110} surfaces and bond with the unsaturated Mn atoms from both surface edges to produce stable nanowire interfaces. Our results provide insights into the controlled synthesis and design of nanomaterials in which tunneled structures can be tailored for use in catalysis, ion exchange, and energy storage applications