Wentao Yao

The influence of large cations on the electrochemical properties of tunnel-structured metal oxides

Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.

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 in situ 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 [MnOx] 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.

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 M2 phase during the monoclinic-tetragonal (rutile) phase transition of W-doped VO2 nanowires

Identifying different phases of VO2 during the metal−insulator phase transition is critical for device application due to the difference of electrical, mechanical and magnetic properties of phases. However, most studies so far were carried out using microprobe analyses, which lack the spatial resolution needed to identify nanoscale phases and changes. Taking advantage of in situ low temperature aberration-corrected scanning transmission electron microscopy, we observed the existence of M2 phase alongside M1 and R phase in the W-doped nanowires close to transition temperature. The localized stress caused by adding W in the structure results in the stabilization of nanosize grains of M2 phase in structure along with M1 and R phases. The observation of the metastable M2 phase even for unclamped nanowires suggests the possibility of finely modulating the phase diagram of VO2 through a combination of finite size and doping.

Energy-driven surface evolution in beta-MnO2 structures

Exposed crystal facets directly affect the electrochemical/catalytic performance of MnO2 materials during their applications in supercapacitors, rechargeable batteries, and fuel cells. Currently, the facet-controlled synthesis of MnO2 is facing serious challenges due to the lack of an in-depth understanding of their surface evolution mechanisms. Here, combining aberration-corrected scanning transmission electron microscopy (STEM) and high-resolution TEM, we revealed a mutual energy-driven mechanism between beta-MnO2 nanowires and microstructures that dominated the evolution of the lateral facets in both structures. The evolution of the lateral surfaces followed the elimination of the {100} facets and increased the occupancy of {110} facets with the increase in hydrothermal retention time. Both self-growth and oriented attachment along their {100} facets were observed as two different ways to reduce the surface energies of the beta-MnO2 structures. High-density screw dislocations with the 1/2<100> Burgers vector were generated consequently. The observed surface evolution phenomenon offers guidance for the facet-controlled growth of beta-MnO2 materials with high performances for its application in metal-air batteries, fuel cells, supercapacitors, etc.

Elevated‐Temperature 3D Printing of Hybrid Solid‐State Electrolyte for Li‐Ion Batteries

While 3D printing of rechargeable batteries has received immense interest in advancing the next generation of 3D energy storage devices, challenges with the 3D printing of electrolytes still remain. Additional processing steps such as solvent evaporation were required for earlier studies of electrolyte fabrication, which hindered the simultaneous production of electrode and electrolyte in an all-3D-printed battery. Here, a novel method is demonstrated to fabricate hybrid solid-state electrolytes using an elevated-temperature direct ink writing technique without any additional processing steps. The hybrid solid-state electrolyte consists of solid poly(vinylidene fluoride-hexafluoropropylene) matrices and a Li+ -conducting ionic-liquid electrolyte. The ink is modified by adding nanosized ceramic fillers to achieve the desired rheological properties. The ionic conductivity of the inks is 0.78  × 10 -3 S cm-1 . Interestingly, a continuous, thin, and dense layer is discovered to form between the porous electrolyte layer and the electrode, which effectively reduces the interfacial resistance of the solid-state battery. Compared to the traditional methods of solid-state battery assembly, the directly printed electrolyte helps to achieve higher capacities and a better rate performance. The direct fabrication of electrolyte from printable inks at an elevated temperature will shed new light on the design of all-3D-printed batteries for next-generation electronic devices.

Simultaneous Structural and Electrical Analysis of Vanadium Dioxide Using In Situ TEM

Vanadium dioxide (VO2), a correlated electron material, has received significant attentions due to its metal-insulator transition (MIT) at ~ 67 °C [1]. This transition is associated with structural phase transition from the monoclinic (M1), an insulating phase, to rutile (R), a metallic phase. This metalinsulator transision is accompanied by a noticeable resistivity, optical transparency and magnetic changes. These distinctive properties have inspired many applications such as thermo/electrochromics, Mott transistors, memristors, thermal actuators, gas sensors, strain sensors and temperature sensors. Recent efforts focus on controlling of phase transition and domain structures in finite size VO2, which results in different material properties and play a critical role in device applications.

In situ cooling and heating study of VO 2 phase transition

Vanadium dioxide (VO2), one the correlated electron material has received many attentions through a metal-insulator transition (MIT) at ~ 340°K, close to room temperature. The transition in VO2 associated by structural phase transition from the monoclinic (M1), insolating phase, to rutile(R), metallic phase. This metal-insulator transmission is accompanied by a noticeable resistivity, optical transparency and magnetic changes. In addition, large hysteresis effects are reported at nanoscale size VO2, which opens much more application possibility such as gas sensors and optical data storage. However, there is still no clear explanation on hysteresis effect variations of different VO2 morphologies. In this work, we used aberration corrected scanning transition electron microscopy and in situ transition electron microscopy (TEM) cooling and heating techniques to study the different VO2 samples. The atomic resolution image of W-doped sample (x=0.8 atom %) Figure 1a. and corresponding diffraction patterns at 300 and 90 K are respectively shown in Figure 1b-c. This investigation correlates the atomic structural aspect of each sample to variations in hysteresis gap, which help controlling of the hysteresis properties for different needs. 816 doi:10.1017/S1431927616004931 Microsc. Microanal. 22 (Suppl 3), 2016

Atomistic Exploration of the Surface-Sensitive Oriented Attachment Growth of a-MnCh Nanowires and the Formation of Defective Interface with 2×3 and 2×4 Tunnel Intergrowth

1. Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan, USA 2. Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois, USA 3. Department of Chemistry, University of Bath, Bath, United Kingdom 4. Department of Materials Science and Engineering, Shandong University, 17923 Jingshi Road, Jinan, , China 5. Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan, USA 6. Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL, USA

Dynamic Study of Sodiation Process in Single Crystalline a-MnO2 Nanowires

α-MnO2 is widely applied as an energy storage electrode in rechargeable batteries due to its unique 2×2 tunneled structure that facilitates diffusion of charge carriers . By now, it is unclear how the intercalated charge carriers such as Li, Na and Mg interact with the tunnel-based host due to the lack of atomic scale understanding of the tunnel configuration and the complicated effect from generally existing tunnel stabilizers (like K).