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Department of Energy Engineering (Energy Engineering)With the demand to overcome the issues concerning environmental pollution of fossil fuel in large-scale system and various fields, numerous efforts have been devoted toward a design of rational energy storage system (ESS) in order to substitute present energy source. While lots of systems are suggested, lithium-ion batteries (LIBs) have been attracted as one of the promising ESS among various storage devices. Existing one which consists of a graphite anode unfortunately have the trouble to fulfilling required condition such as high power and energy density. Thus, new type of anode materials has been developed to achieve mentioned specifications. As possible candidates, silicon (Si) and germanium (Ge) have been emerged owing to their high gravimetric/volumetric capacity and low operating voltage. Nevertheless, those materials remain under challenge level because inferior electronic properties have the limit to catch high power density and unexpected volume expansion on a lithiation process into materials, resulting in the electrode failure and capacity decay where factors influence safety and stability issues in LIBs system. Here, we introduce approaches through dimensional manipulation to proceed. Overall synthetic processes are focused on versatile method, a possibility of mass production and evaluation methods obviously demonstrate intrinsic/extrinsic characteristic ways. With in situ microscopic/electrochemical techniques, specific properties and electrochemical reaction mechanism of synthesized materials are clearly unveiled to facilitate power density enhancement and volume change suppression. In Chapter II, we present zero-dimensional (0D) carbon wrapped-hollow Si microparticles which possess porous shell structure from various silica source regardless of their shape. Sequent top-down and bottom-up processes fabricate uniform 0D Si and a key factor for unique formation mechanism is verified through ex situ characterization and simulation results. In electrochemical view, creating cavities in a core and pores in the shell alleviate volume expansion and enable short ion-diffusion length. Surface carbon layer additionally provide fast electron movement to guarantee stable and considerable power density. Besides, in situ transmission electron microscopic (TEM) demonstrate the stability of morphological structure on charge/discharging cycle. In Chapter III, we design one-dimensional (1D) Ge/zinc (Zn)-based nanofibers. Homogeneous Ge/Zn nanofibers via electrospinning method and solid-gas reduction reaction own atomic-level distribution of each element. Well-dispersive metallic Zn in Ge nanofiber could effectively improve electronic conductivity/volumetric stability and nanosized structure also features facile ion transport and stress release by volume expansion on electrochemical cycles. In situ TEM/electrochemical impedance spectroscopy (EIS) deeply investigate the critical role of ionic bond of Zn element in Ge nanofibers. In Chapter IV, we introduce additional 1D Ge nanofibers, which feature numerous sizes of pores in whole morphological structure. Intrinsic metal oxide characters based on Ellingham diagram enable to carve heterogenous pore in and out of nanofibers. This structure shows stable electrochemical cyclability without a large volume expansion. Further, we confirm the unique behavior of Ge, called memory effect in LIBs. In situ TEM characterization supports that numerous pores work as volume buffer sites and keep spatial reversibility on charge/discharge cycles. In Chapter V, we finally suggest synthetic method of three-dimensional (3D) porous Ge clusters from zeotype-borogermanate microcubes, artificial Ge-rich zeolite. This starting material is prepared in a large quantity through a simple hydrothermal process as followed by sequential thermal and etching treatment to produce 3D porous Ge. As-fabricated product interestingly behaves like a pseudocapacitance exhibiting fast electrochemical kinetics. Further, the as-formed pores build up stable solid electrolyte interphase (SEI) layer on the surface for prolonged cycles, improving cycle stability. In Chapter VI, we briefly provide the insight for the correlation of the dimensions and electrochemical properties toward advanced lithium storage system. To handle unsettled issues in large-scale lithium batteries, it is essential to look around the overall circumstances to match the specific purpose.clos

Revealing salt-expedited reduction mechanism for hollow silicon microsphere formation in bi-functional halide melts

The thermochemical reduction of silica to silicon using chemical reductants requires high temperature and has a high activation energy, which depends on the melting temperature of the reductant. The addition of bi-functional molten salts with a low melting temperature may reduce the required energy, and several examples using molten salts have been demonstrated. Here we study the mechanism of reduction of silica in the presence of aluminum metal reductant and aluminum chloride as bi-functional molten salts. An aluminum-aluminum chloride complex plays a key role in the reduction mechanism, reacting with the oxygen of the silica surfaces to lower the heat of reaction and subsequently survives a recycling step in the reaction. This experimentally and theoretically validated reaction mechanism may open a new pathway using bi-functional molten salts. Furthermore, the as-synthesized hollow porous silicon microsphere anodes show structural durability on cycling in both half/full cell tests, attributed to the high volume-accommodating ability

Ion-selective and chemical-protective elastic block copolymer interphase for durable zinc metal anode

Aqueous rechargeable batteries based on zinc anodes are among the most promising systems to replace conventional lithium-ion batteries owing to their intrinsic safety, high ionic conductivity, and economic benefits. However, inferior reversibility of zinc anode resulting from zinc dendrites and surface side reactions limits the practical realization of zinc-ion batteries. Herein, we develop a thin but robust polymeric artificial interphase to enhance reversibility of zinc anode. The grafted maleic anhydride groups in the polymer structure restrain the detrimental reactions through selective zinc-ion penetration and homogenize ion distribution, leading to a smooth electrode surface after plating-stripping processes. Consequently, the coated zinc anode shows excellent stability with a long-term symmetric cell lifespan (>3,000 h at 3 mA??cm???2) and maintains capacity retention of 80% after 2,500 cycles, paired with a manganese oxide cathode. This study provides a facile fabrication process and accessible analysis methods to rationalize the development of high-performance zinc-ion batteries

Stretchable Aqueous Batteries: Progress and Prospects

The development of stretchable electronics is indispensable for realizing next-generation wearable devices, such as sensors, health care devices, and electronic skin. A key challenge for achieving complete and independent wearable devices is developing stretchable power sources. This issue should be addressed appropriately before the realization of wearable devices. Very recently, stretchable aqueous rechargeable batteries as power supplies have received much attention for wearable devices owing to their intrinsic safety and high power density. In this Perspective, we present the current status and the latest advances in research on stretchable aqueous batteries, especially aqueous Li-ion batteries and zinc-based batteries. Also, we briefly provide the design of stretchable materials and battery systems for stretchable aqueous batteries. Furthermore, an overview of general technical issues confronting their development is presented, and a brief discussion on the future outlook of this field is provided

BlockNet: A Deep Neural Network for Block-Based Motion Estimation Using Representative Matching

Owing to the limitations of practical realizations, block-based motion is widely used as an alternative for pixel-based motion in video applications such as global motion estimation and frame rate up-conversion. We hereby present BlockNet, a compact but effective deep neural architecture for block-based motion estimation. First, BlockNet extracts rich features for a pair of input images. Then, it estimates coarse-to-fine block motion using a pyramidal structure. In each level, block-based motion is estimated using the proposed representative matching with a simple average operator. The experimental results show that BlockNet achieved a similar average end-point error with and without representative matching, whereas the proposed matching incurred 18% lower computational cost than full matching.11Ysciescopu

Rational Structure Design of Fast-Charging NiSb Bimetal Nanosheet Anode for Lithium Ion Batteries

Although bimetallic materials with various structures have been used as anodes for advanced lithium ion batteries, structural degradation, caused during electrochemical reactions, leads to a shorter cycle lifespan. Herein, we propose a rational structural design, carbon-wrapped porous bimetallic nickel-antimony nanosheets (NiSbNS@C), with the help of dual-functional organic acid acting as a reducing agent and a carbon coating source upon the synthetic process. The structural evolution of NiSbNS@C is further confirmed as the NiSb crystal is transformed into a Ni-rich phase on fast charging. A well-constructed NiSbNS@C electrode exhibits outstanding high rate performance and structure stability owing to the fast electrochemical kinetics of the porous NiSb nanostructure and uniform carbon decoration in both half and full cells. This approach opens up an avenue to make a desirable structure for bimetallic anode materials toward high rate and stable lithium ion batteries

Recent Progress in Stretchable Batteries for Wearable Electronics

With the rapidly approaching implementation of wearable electronic devices such as implantable devices, stretchable sensors, and healthcare devices, stretchable power sources have aroused worldwide attention as a key component in this emerging field. Among stretchable power sources, batteries, which store electrical energy through redox reactions during charge/discharge processes, are an attractive candidate because of their high energy density, high output voltage, and long-term stability. In recent years, extensive efforts have been devoted to developing new materials and innovative structural designs for stretchable batteries. This review covers the latest advances in stretchable batteries, focusing on advanced stretchable materials and their design strategies. First, we provide a detailed overview of the materials aspects of components in a stretchable battery, including electrode materials, solid-state electrolytes, and stretchable separator membranes. Second, we provide an overview on various structural engineering strategies to impart stretchability to batteries (i.e., wavy/buckling structures, island-bridge structures, and origami/kirigami structures). Third, we summarize recently reported developments in stretchable batteries based on various chemistries, including Li-based batteries, multivalent-based batteries, and metal-air batteries. Finally, we discuss the future perspectives and remaining challenges toward the practical application of stretchable batteries with reliable mechanical robustness and stable electro-chemical performance under a physical strain

Synthesis of dual porous structured germanium anodes with exceptional lithium-ion storage performance

Dual-porous Ge nanostructures are synthesized via two straightforward steps. Compared with conventional approaches related to porous Ge materials, different types of pores can be readily generated by adjusting the relative ratio of the precursor amounts for GeO2 and SiO2. Unlike using hard templates with different sizes for introducing secondary pores, this system makes a uniformly blended structure of porogen and active sites in the nanoscale range. When GeO2 is subjected to zincothermic reduction, it is selectively converted to pure Ge still connected to unreacted SiO2. During the reduction process, primary pores (larger than 50 nm) are formed by eliminating zinc oxide by-products, while inactive SiO2 with respect to zinc metal could contribute to retaining the overall structure. Finally, the HF treatment completely leaches remaining SiO2 and formed secondary pores (micro/mesopores) to complete the dual-porous Ge structure. The resulting Ge structure is tested as an anode material for lithium-ion batteries. The Ge electrode exhibits an outstanding reversibility and an exceptional cycling stability corresponding to a capacity retention of 100% after 100 cycles at C/5 and of 94.4% after 300 cycles at C/2. Furthermore, multi-scale pores facilitate a facile Li-ion accessibility, resulting in an excellent rate capability delivering similar to 740 mAh g(-1) at 5C

A Three-Dimensional Nano-web Scaffold of Ferroelectric Beta-PVDF Fibers for Lithium Metal Plating and Stripping

Lithium metal has been considered as an anode material to improve energy densities of lithium chemistry-based rechargeable batteries (that is to say, lithium metal batteries or LMBs). Higher capacities and cell voltages are ensured by replacing practically used anode materials such as graphite with lithium metal. However, lithium metal as the LMB anode material has been challenged by its dendritic growth, electrolyte decomposition on its fresh surface, and its serious volumetric change. To address the problems of lithium metal anodes, herein, we guided and facilitated lithium ion transport along a spontaneously polarized and highly dielectric material. A three-dimensional web of nanodiameter fibers of ferroelectric beta-phase polyvinylidene fluoride (beta-PVDF) was loaded on a copper foil by electrospinning (PVDF#Cu). The electric field applied between the nozzle and target copper foil forced the dipoles of PVDF to be oriented centro-asymmetrically and then the beta structure induced ferroelectric polarization. Three-fold benefits of the ferroelectric nano-web architecture guaranteed the plating/stripping reversibility especially at high rates: (1) three-dimensional scaffold to accommodate the volume change of lithium metal during plating and stripping, (2) electrolyte channels between fibers to allow lithium ions to move, and (3) ferroelectrically polarized or negatively charged surface of beta-PVDF fibers to encourage lithium ion hopping along the surface. Resultantly, the beta-PVDF web architecture drove dense and integrated growth of lithium metal within its structure. The kinetic benefit expected from the ferroelectric lithium ion transport of beta-PVDF as well as the porous architecture of PVDF#Cu was realized in a cell of LFP as a cathode and lithium-plated PVDF#Cu as an anode. Excellent plating/stripping reversibility along repeated cycles was successfully demonstrated in the cell even at a high current such as 2.3 mA cm(-2), which was not obtained by the nonferroelectric polymer layer

Hierarchically Structured Multidimensional Carbon Composite Anchored to a Polymer Mat for a Superflexible Supercapacitor

A carbon electrode was designed to guarantee flexibility of symmetric electric double layer capacitors (EDLCs) based on its architecture. Three different dimensional carbon materials were combined to achieve the flexibility without sacrificing high performances: highly capacitive but poorly conductive three-dimensional graphene (3D-Gn*) as a platform for electric double layer formation, one-dimensional carbon nanotube (1D-CNT) as an electrically conductive highway, and two-dimensional graphene (2D-Gn) for facilitating electron communications between 3D-Gn* and 1D-CNT. From a mechanical standpoint, the 1D-CNT provided an intertangled framework to integrate the main active material 3D-Gn* and anchored the active layer to a flexible polymer matrix. Resultantly, the symmetric EDLC based on the hierarchically structured multidimensional carbon electrodes anchored to flexible substrates was operated successfully at 3 V, ensuring high energy densities even under repetitive mechanical stress conditions