Lifen Wang

Fractal growth of platinum electrodeposits revealed by in situ electron microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth – either dense branching or ramified islands – are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Microstructural Evolution in Transition-metal-oxide Cathode Materials for Lithium-Ion Batteries

Layered transition metal oxides are promising materials for high-energy lithium-ion battery cathodes. These materials offer high capacity and rate capability, good safety, and relatively low cost compared to many alternative materials [1]. Mixed oxides such as NCA (LiNi0.8Co0.15Al0.05O2) exhibit high capacity but suffer from a significant capacity fade with cycling [2]. The composition of alternatives such as NMC (LiNi1-x-yMnxCoyO2) can be tuned to show less capacity fade, but generally at the cost of lower capacity. Consequently, there is great interest in cycling these materials to higher potentials for more energy, but then cycling performance decreases.

Dynamic Nanobubbles in Graphene Liquid Cell under Electron Beam Irradiation

The use of graphene windows in liquid cells for in situ electron microscopy has become popular because the single atom thickness, extraordinary mechanical strength and high conductivity of graphene allows the study of liquid in the confined environment with atomic resolution. Such liquid cells are commonly used for in situ observation of nanoparticle growth in liquid at the atomic scale using transmission electron microscopy (TEM). These studies improve our understanding of the initial growth mechanisms and future design of nanomaterials. However, the electron beam generates local heating or irradiation and can strongly influence the growth process [1]. At higher dose rates, electron beam irradiation can lead to liquid decomposition, ionization, vapor generation and even nanobubble formation that can influence the reliability of the in situ observation [2]. In addition, the hydrophic surface of graphene may influence these processes.

Modulating the Redox Equilibrium of Silver Using Electron Beams

1. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 2. School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Microand Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China 3. Science and Technology on High Strength Structural Materials Laboratory, Central South University, Changsha 410083, China

Atomic Surface Structures of Oxide Nanoparticles with Well-defined Shapes

Recent studies have shown that catalytic activities can be tuned by controlling the shape of nanoparticles such as SrTiO3, CeO2, and Co3O4 [1]. Therefore, determination of surface structure is very important to understand structure-property relationships for these oxide nanoparticles. The Argonne Chromatic-corrected TEM (ACAT) has an image corrector that corrects both spherical (Cs) and chromatic aberration (Cc). Cc correction allows the correction of Cs towards zero to improve resolution without compromising contrast. Using this unique feature, we correct both Cs and Cc to small values to achieve direct structure interpretable HREM images including oxygen atomic columns. In this study, atomic surface structures of SrTiO3, CeO2, Co3O4 nanocubes are observed by using aberration-corrected HREM.

Fractal Growth of Platinum Electrodeposits Revealed by in situ Electron Microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth - either dense branching or ramified islands - are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Exploring Lithium-ion Battery Performance through in situ Characterization

In this work, we focused on high capacity cathode materials that have a graded composition, with an Nirich core and an Mn-rich periphery. The concept for these materials with a general composition of LiNi1-x-yCoyMnxO2 is that the Ni-rich core provides high capacity while the Mn-rich periphery minimizes detrimental interaction with the electrolyte. [1,2] In addition to high capacity, these materials exhibit better long-term performance with less “fade” in capacity over many cycles compared to, for example, LiNi0.8Co0.15Al0.05O2 (“NCA”), which shows much more significant capacity fade. Our in situ single particle studies suggest one of the mechanisms for capacity fade in NCA is particle fracture that occurs during cycling. [3] In this work, we applied our approach to graded cathode materials to see if this was an important factor in their improved performance.

Dynamic Rate Mechanism of V2O5 Coated SnO 2 Nanowires for Lithium Ion Batteries Studied by in situ TEM

Coating strategies are commonly employed to avoid lithiation-induced fracture and improve electrochemical performance of lithium storage materials with higher capacity and longer cycle life [1, 2]. Correlation of structure with electrochemical performance of such materials is needed to guide further design. This work mainly focuses on realizing a working lithium battery inside the transmission electron microscope (TEM) to measure and understand electrochemical mechanisms of vanadium pentoxide loaded tin dioxide (V2O5/SnO2) nanowire. We achieve this by using a nanomanipulator to assemble an open electrochemical cell inside a TEM that consists of target materials, a Li-containing counter electrode (Li or Li cobalt oxide), and a solid-state electrolyte (Li2O) [3]. To directly capture the dynamic structural changes of electrode materials during the lithiation process, open lithium cells were characterized via real-time in situ high resolution transmission electron microscopy (HRTEM) imaging, electron diffraction (ED) and electron energy-loss spectroscopy (EELS).