Zihua Zhu

Mitigating Voltage Fade in Cathode Materials by Improving the Atomic Level Uniformity of Elemental Distribution

Lithium- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, their voltage fading mechanism and its relationships with fundamental structural changes are far from being well understood. Here we report for the first time the mitigation of voltage and energy fade of LMR cathodes by improving the atomic level spatial uniformity of the chemical species. The results reveal that LMR cathodes (Li[Li0.2Ni0.2M0.6]O2) prepared by coprecipitation and sol-gel methods, which are dominated by a LiMO2 type R3̅m structure, show significant nonuniform Ni distribution at particle surfaces. In contrast, the LMR cathode prepared by a hydrothermal assisted method is dominated by a Li2MO3 type C2/m structure with minimal Ni-rich surfaces. The samples with uniform atomic level spatial distribution demonstrate much better capacity retention and much smaller voltage fade as compared to those with significant nonuniform Ni distribution. The fundamental findings on the direct correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials may also guide the design of other energy storage materials with enhanced stabilities.

Damage profile and ion distribution of slow heavy ions in compounds

Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range, and doping concentration are commonly estimated by the stopping and range of ions in matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au, and Pb ions, in other compound materials, such as GaN, AlN, and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stoppin...

Thermodynamic instability at the stoichiometric LaAlO3/SrTiO3(001) interface

Stoichiometric epitaxial LaAlO(3) grown on TiO(2)-terminated SrTiO(3)(001) by off-axis pulsed laser deposition is shown to exhibit strong cation intermixing. This result is corroborated by classical and quantum mechanical calculations of the relative stabilities of abrupt and intermixed interface configurations. The valence band offset was measured to be 0.16 ± 0.10 eV, and this value cannot be accounted for theoretically without including intermixing in the physical description of the interface.

Polyvinylpyrrolidone-induced anisotropic growth of gold nanoprisms in plasmon-driven synthesis.

After more than a decade, it is still unknown whether the plasmon-mediated growth of silver nanostructures can be extended to the synthesis of other noble metals, as the molecular mechanisms governing the growth process remain elusive. Herein, we demonstrate the plasmon-driven synthesis of gold nanoprisms and elucidate the details of the photochemical growth mechanism at the single-nanoparticle level. Our investigation reveals that the surfactant polyvinylpyrrolidone preferentially adsorbs along the nanoprism perimeter and serves as a photochemical relay to direct the anisotropic growth of gold nanoprisms. This discovery confers a unique function to polyvinylpyrrolidone that is fundamentally different from its widely accepted role as a crystal-face-blocking ligand. Additionally, we find that nanocrystal twinning exerts a profound influence on the kinetics of this photochemical process by controlling the transport of plasmon-generated hot electrons to polyvinylpyrrolidone. These insights establish a molecular-level description of the underlying mechanisms regulating the plasmon-driven synthesis of gold nanoprisms.

Making a hybrid microfluidic platform compatible for in situ imaging by vacuum-based techniques

A self-contained microfluidic-based device was designed and fabricated for in situ imaging of aqueous surfaces using vacuum techniques. The device is a hybrid between a microfluidic poly(dimethyl siloxane) block and external accessories, all portable on a small platform (10 × 8 cm2). The key feature is that a small aperture with a diameter of 2-3 μm is opened to the vacuum, which serves as a detection window for in situ imaging of aqueous surfaces. Vacuum compatibility and temperature drop due to water vaporization are the two most important challenges in this invention. Theoretical calculations and fabrication strategies are presented from multiple design aspects. In addition, results from the time-of-flight secondary ion mass spectrometry and scanning electron microscopy of aqueous surfaces are presented.

Damage and microstructure evolution in GaN under Au ion irradiation

Damage and microstructure evolution in gallium nitride (GaN) under Au+ ion irradiation has been investigated using complementary electron microscopy, secondary ion mass spectrometry and ion-beam analysis techniques. Epitaxially-grown GaN layers (2??m thick) have been irradiated by 2.0?MeV Au ions to 1.0 ? 1015 and 1.4 ? 1015?cm?2 at 155?K and to 7.3 ? 1015?cm?2 at 200?K. The irradiation-induced damage has been analysed by Rutherford backscattering spectroscopy in a channelling direction (RBS/C). For a better determination of the ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) are combined to analyse the ion channelling spectra. With increasing irradiation dose, separated amorphous layers develop from the sample surface and near the damage peak region. Formation of large nitrogen bubbles with sizes up to 70?nm is observed in the buried amorphous layer, while the surface layer contains small bubbles with a diameter of a few nanometres due to significant nitrogen loss from the surface. Volume expansion from 3% to 25% in the irradiated region is suggested by cross-sectional transmission electron microscope and RBS/C measurement. The anomalous shape of the Au distributions under three irradiations indicates out-diffusion of Au towards the sample surface. The results from the complementary techniques suggest that nitrogen is retained in the damaged GaN where the crystallinity is preserved. Once the amorphous state is reached in the surface region, GaN starts to decompose and nitrogen escapes from the surface. Furthermore, experimental results show considerable errors in both the disorder profile and the ion range predicted by the Stopping and Range of Ions in Matter code, indicating a significant overestimation of electronic stopping powers of Au ions in GaN.

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells: III. Role of Volatile Boron Species on LSM/YSZ and LSCF

Boron oxide is a key component to tailor the softening temperature and viscosity of the sealing glass for solid oxide fuel cells (SOFCs). The primary concern regarding the use of boron-containing sealing glasses is the volatility of boron species, which possibly results in cathode degradation. In this paper, we report the role of volatile boron species on the electrochemical performance of LSM/yttria-stabilized zirconia (YSZ) and LSCF cathodes at various SOFC operation temperatures. The transport rate of boron, ~3.24 × 10 -12 g/cm 2 sec was measured at 750°C with air saturated with ~3% moisture. A reduction in power density was observed in the cells with the LSM/YSZ cathodes after the introduction of boron source to the cathode air stream. A partial recovery of the power density was observed after the boron source was removed. Results from post-test secondary-ion mass spectroscopy (SIMS) analysis showed that the partial recovery in the power density correlated with the partial removal of the deposited boron by the clean air stream. The presence of boron was also observed in the LSCF cathodes by SIMS analysis; however, the effect of boron on the electrochemical performance of the LSCF cathode was negligible. The coverage of triple phase boundaries in LSM/YSZ was postulated as the cause for the observed reduction in the electrochemical performance.

In Situ Mass Spectrometric Determination of Molecular Structural Evolution at the Solid Electrolyte Interphase in Lithium-Ion Batteries

Dynamic structural and chemical evolution at solid-liquid electrolyte interface is always a mystery for a rechargeable battery due to the challenge to directly probe a solid-liquid interface under reaction conditions. We describe the creation and usage of in situ liquid secondary ion mass spectroscopy (SIMS) for the first time to directly observe the molecular structural evolution at the solid-liquid electrolyte interface for a lithium (Li)-ion battery under dynamic operating conditions. We have discovered that the deposition of Li metal on copper electrode leads to the condensation of solvent molecules around the electrode. Chemically, this layer of solvent condensate tends to be depleted of the salt anions and with reduced concentration of Li(+) ions, essentially leading to the formation of a lean electrolyte layer adjacent to the electrode and therefore contributing to the overpotential of the cell. This observation provides unprecedented molecular level dynamic information on the initial formation of the solid electrolyte interphase (SEI) layer. The present work also ultimately opens new avenues for implanting the in situ liquid SIMS concept to probe the chemical reaction process that intimately involves solid-liquid interface, such as electrocatalysis, electrodeposition, biofuel conversion, biofilm, and biomineralization.

Electronic properties of H and D doped ZnO epitaxial films

ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2 or bulk single crystals annealed in H2. These include temperature-independent resistivities of ∼0.1Ωcm, carrier (electron) concentrations in the 1018cm−3 range, mobilities of 20–40cm2∕Vs, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultrashallow donor or conduction band states not achievable by postgrowth annealing in H2.

Electronic stopping powers for heavy ions in SiC and SiO2

Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to me...