Ge Nanowires Anode sheathed with Amorphous Carbon for Rechargeable Lithium batteries

Interdisciplinary School of Green EnergyThe composite electrode composed of single crystalline Ge NWs sheathed with amorphous carbon showed excellent electrochemical properties of large reversible capacity, high coulombic efficiency, excellent rate capability and stable cycle performance. c-Ge NWs synthesized by using thermal decomposition of C2H2 gas at 700 °C under Ar atmosphere after SLS (solution-liquid-solid) growth were found to have good performance during cycling with Li. The rate capability for charging was shown reversible capacity of 963 mAh/g with a coulombic efficiency of 90% and 700 mAh/g at the rate of 6C (= 4800mA/g). Capacity retention after 100 cycles was 72% at the rate of 0.5C. The improved electrochemical performance of c-Ge-NWs fabricated in our experiment was attributed to the formation of amorphous Ge NWs during cycling and a homogenous carbon coating on Ge NWs. Thus, these results suggest that the use of nanowires structure can be promising for alloy anode materials in lithium ion batteries

Ultra-fast self-assembly and stabilization of reactive nanoparticles in reduced graphene oxide films.

Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material

Study on Nano-Engineering of High-Capacity Anode Materials for High-Power Energy Storage System

Department of Energy Engineering(Battery Science and Technology)Nano-engineering and nanotechnology issue in various industry fields such as semiconductor, chemistry, energy solution, material science, and medicine. A definition of nanotechnology includes quantum mechanics, molecular chemistry, biology, and atomic level behaviors. Also, nanostructured materials (e.g., nanoparticle, nanorod, nanotube, nanowire, hollow, and yolk-shell) improve properties of materials for performance enhancement of devices. These nanomaterials have been synthesized using bottom-up and top-down approaches. In the early 2000s, many researchers garnered information and experiences about the nanotechnology that led to innovation and progress in industry and academy of science. As a result, many electronic devices were developed for a convenience of our life. Especially, significant advances of devices lead to the development of another new device with more improved performances including faster processing ability, longer working time, light weight, and easy transportation. In this regard, gradual development of energy storage system must need to satisfy this demand for new electric device (e.g. electric vehicle (EV), energy storage system (ESS), even drone) As one of the powerful energy storage systems, lithium-ion batteries (LIBs) are critically important to operate portable electronic devices. However, they cannot meet requirements for more advanced applications, like electric vehicles and energy storage systems due to limitations of conventional cathode/anode materials in high power and high energy density. To overcome these limitations, several strategies have been developed, including nanostructured design of electrode materials, coating of active materials with electrically conductive layers, and control of electrode architectures. Herein, we study on a simple, cost effective and unique synthesis method of various shaped functional materials by nano-engineering process in an each chapter. Also, we conduct research about a mechanism of reaction, key for synthesizing good materials, change of chemical reaction in experiment. So, the developed materials appear outstanding properties such as structural stability, chemical stability in electrochemical test, and mainly used energy storage system like LIBs. In chapter III, we demonstrate a simple route for fabricating trench-type copper patterns by combining a photo-lithography with a wet etching process. Nanostructured CuO was grown on the patterned Cu current collectors via a simple solution immersion process. And silicon nanoparticles were filled into the patterned Cu current collectors. The strongly immobilized CuO on the patterned Cu exhibited high electrochemical performance, including a high reversible capacity and a high rate capability. In chapter IV, we demonstrate multi-scale patterned electrodes that provide surface-area enhancement and strong adhesion between electrode materials and current collector. The combination of multi-scale structured current collector and active materials (cathode and anode) enables us to make high-performance Li-ion batteries (LIBs). When LiFePO4 (LFP) cathode and Li4Ti5O12 (LTO) anode materials are combined with patterned current collectors, their electrochemical performances are significantly improved, including a high rate capability (LFP : 100 mAhg-1, LTO : 60 mAhg-1 at 100 C rate) and highly stable cycling. Moreover, we successfully fabricate full cell system consisting of patterned LFP cathode and patterned LTO anode, exhibiting high-power battery performances. We extend this idea to Si anode that exhibits a large volume change during lithiation/delithiation process. The patterned Si electrodes show significantly enhanced electrochemical performances, including a high specific capacity (825 mAhg-1) at high rate of 5 C and a stable cycling retention. In chapter V, Chemical reduction of micro-assembled CNT@TiO2@SiO2 leads to the formation of titanium silicide-containing Si nanotubular structures. The Si-based nanotube anodes exhibit a high capacity (>1850 mAh g-1) and an excellent cycling performance (capacity retention of >99% after 80 cycles). In chapter VI, we revisit the metallothermic reduction process to synthesize shape-preserving macro-/nano-porous Si particles via aluminothermic and subsequent magnesiotheric reaction of porous silica particles. This process enables us to control the specific capacity and volume expansion of shape-preserving porous Si-based anodes. Two step metallothermic reactions have several advantages including a successful synthesis of shape-preserving Si particles, tunable specific capacity of as-synthesized Si anode, accommodation of a large volume change of Si by porous nature and alumina layers, and a scalable synthesis (hundreds of gram per batch). An optimized macroporous Si/Al2O3 composite anode exhibits a reversible capacity of ~1500 mAh g-1 after 100 cycles at 0.2 C and a volume expansion of ~34% even after 100 cycles. In chapter VII, we report a redox-transmetalation reaction-based route for the large-scale synthesis of mesoporous germanium particles from germanium oxide at temperatures of 420 ~ 600 oC. We could confirm that a unique redox-transmetalation reaction occurs between Zn0 and Ge4+ at approximately 420 oC using temperature-dependent in situ X-ray absorption fine structure analysis. This reaction has several advantages, which include (i) the successful synthesis of germanium particles at a low temperature (∼450 oC), (ii) the accommodation of large volume changes, owing to the mesoporous structure of the germanium particles, and (iii) the ability to synthesize the particles in a cost-effective and scalable manner, as inexpensive metal oxides are used as the starting materials. The optimized mesoporous germanium anode exhibits a reversible capacity of∼1400 mA h g-1 after 300 cycles at a rate of 0.5 C (corresponding to the capacity retention of 99.5%), as well as stable cycling in a full cell containing a LiCoO2 cathode with a high energy density. In chapter VIII, we report a unique synthesis of redox-responsive assembled carbon-sheathed germanium coaxial nanowire heterostructures without a need of metal catalyst. In our approach, germanium nanowires are grown by reduction of germanium oxide particles and subsequent self-catalytic growth mechanism during thermal decomposition of natural gas, and simultaneously, carbon sheath layers are uniformly coated on the germanium nanowire surface. This process is a simple (one-step process), reproducible, easy size-controllable and cost-effective (mass production) process which total mass of metal oxides can be transformed into nanowires. Furthermore, the germanium nanowires exhibit outstanding electrochemical performance including capacity retention of ~96% after 1000 cycles at 1C rate as lithium-ion battery anode.ope

High-quality mesoporous graphene particles as high-energy and fast-charging anodes for lithium-ion batteries.

The application of graphene for electrochemical energy storage has received tremendous attention; however, challenges remain in synthesis and other aspects. Here we report the synthesis of high-quality, nitrogen-doped, mesoporous graphene particles through chemical vapor deposition with magnesium-oxide particles as the catalyst and template. Such particles possess excellent structural and electrochemical stability, electronic and ionic conductivity, enabling their use as high-performance anodes with high reversible capacity, outstanding rate performance (e.g., 1,138 mA h g-1 at 0.2 C or 440 mA h g-1 at 60 C with a mass loading of 1 mg cm-2), and excellent cycling stability (e.g., >99% capacity retention for 500 cycles at 2 C with a mass loading of 1 mg cm-2). Interestingly, thick electrodes could be fabricated with high areal capacity and current density (e.g., 6.1 mA h cm-2 at 0.9 mA cm-2), providing an intriguing class of materials for lithium-ion batteries with high energy and power performance

Controlled Synthesis and Characterization of Metal Oxide Nanowires by Chemical Vapor Deposition on Silicon and Carbon Substrates

Nanotechnology and nanomaterials have attracted considerable interest and are predicted to revolutionize many materials and technologies that we use in everyday life. In the past few years, significant research has focused on one dimensional metal oxide nanostructures due to their unique properties and potential applications in various fields from nanoelectronics to energy. However, controlled synthesis of these nanostructures is still a challenge. The objective of this thesis is to synthesize metal oxide nanowires by chemical vapour deposition directly on various substrates. The nanostructures include (i) silicon oxide nanostructures on silicon substrate, (ii) manganese oxide nanostructures on silicon substrate, and (iii) manganese oxide nanostructures on carbon paper substrate. Firstly, silicon oxide nanowires were synthesized on silicon substrate by a VO2 assisted chemical vapor deposition. Networked features of silicon oxide nanowires were found. Systematic study on the nanowire growth has indicated that morphology and composition of the final products are considerably sensitive to the catalyst components, reaction atmosphere and temperature. These results will help in better understanding the growth process of silicon oxide nanowires. Secondly, manganese oxide nanostructures were synthesized on silicon substrate by chemical vapor deposition method. It was found that MnO nanowires are high density and single crystalline with average diameter of 150 nm. These nanowires were characterized using FESEM, EDX, TEM and XRD. The synthesis process and effects of growth parameters such as temperature, heating rate and source/substrate distance on the morphology, composition and structure of the products were systematically studied. Finally, manganese oxide nanostructures were synthesized on carbon paper substrate by chemical vapor deposition method. It was revealed that manganese oxide nanowires and nanobelts can be selectively grown on carbon paper substrate by using a catalyst (gold) assisted or catalyst free thermal evaporation of manganese powder under an argon gas atmosphere. Various effects of growth parameters such as temperature, catalyst and buffered substrate on the growth product were also systematically investigated by using SEM, TEM and XPS

DECONVOLVING THE STEPS TO CONTROL MORPHOLOGY, COMPOSITION, AND STRUCTURE, IN THE SYNTHESIS OF HIGH-ASPECT-RATIO METAL OXIDE NANOMATERIALS

Metal oxides are of interest not only because of their huge abundance but also for their many applications such as for electrocatalysts, gas sensors, diodes, solar cells and lithium ion batteries (LIBs). Nano-sized metal oxides are especially desirable since they have larger surface-to-volume ratios advantageous for catalytic properties, and can display size and shape confinement properties such as magnetism. Thus, it is very important to explore the synthetic methods for these materials. It is essential, therefore, to understand the reaction mechanisms to create these materials, both on the nanoscale, and in real-time, to have design control of materials with desired morphologies and functions. This dissertation covers both the design of new syntheses for nanomaterials, as well as real-time methods to understand their synthetic reaction mechanisms. It will focus on two parts: first, the synthesis of 1-dimension (1-D) featured nanomaterials, including manganese-containing spinel nanowires, and tin dioxide and zinc oxide-based negative nanowire arrays; and second, a mechanistic study of the synthetic reactions of nanomaterials using in situ transmission electron microscopy (TEM). The work presented here demonstrates unique synthetic routes to single crystalline “positive” and “negative” metal oxide nanowires, and introduces a new mechanism for the formation of single-crystalline hollow nanorods

Characterization of vertically aligned carbon nanotube forests grown on stainless steel surfaces

Vertically aligned carbon nanotube (CNT) forests are a particularly interesting class of nanomaterials, because they combine multifunctional properties, such as high energy absorption, compressive strength, recoverability and super-hydrophobicity with light weight. These characteristics make them suitable for application as coating, protective layers and antifouling substrates for metallic pipelines and blades. Direct growth of CNT forests on metals offers the possibility to transfer the tunable CNT functionalities directly onto the desired substrates. Here, we focus on characterizing the structure and mechanical properties, as well as wettability and adhesion of CNT forests grown on different types of stainless steel. We investigate the correlations between composition and morphology of the steel substrates with the micro-structure of the CNTs, and reveal how the latter ultimately controls the mechanical and wetting properties of the CNT forest. Additionally, we study the influence of substrate morphology on the adhesion of CNTs to their substrate. We highlight that the same structure-property relationships govern the mechanical performance of CNT forests grown on steels and on Si

Controlled Synthesis and Characterization of Metal Oxide Nanowires by Chemical Vapor Deposition on Silicon and Carbon Substrates

Nanotechnology and nanomaterials have attracted considerable interest and are predicted to revolutionize many materials and technologies that we use in everyday life. In the past few years, significant research has focused on one dimensional metal oxide nanostructures due to their unique properties and potential applications in various fields from nanoelectronics to energy. However, controlled synthesis of these nanostructures is still a challenge. The objective of this thesis is to synthesize metal oxide nanowires by chemical vapour deposition directly on various substrates. The nanostructures include (i) silicon oxide nanostructures on silicon substrate, (ii) manganese oxide nanostructures on silicon substrate, and (iii) manganese oxide nanostructures on carbon paper substrate. Firstly, silicon oxide nanowires were synthesized on silicon substrate by a VO2 assisted chemical vapor deposition. Networked features of silicon oxide nanowires were found. Systematic study on the nanowire growth has indicated that morphology and composition of the final products are considerably sensitive to the catalyst components, reaction atmosphere and temperature. These results will help in better understanding the growth process of silicon oxide nanowires. Secondly, manganese oxide nanostructures were synthesized on silicon substrate by chemical vapor deposition method. It was found that MnO nanowires are high density and single crystalline with average diameter of 150 nm. These nanowires were characterized using FESEM, EDX, TEM and XRD. The synthesis process and effects of growth parameters such as temperature, heating rate and source/substrate distance on the morphology, composition and structure of the products were systematically studied. Finally, manganese oxide nanostructures were synthesized on carbon paper substrate by chemical vapor deposition method. It was revealed that manganese oxide nanowires and nanobelts can be selectively grown on carbon paper substrate by using a catalyst (gold) assisted or catalyst free thermal evaporation of manganese powder under an argon gas atmosphere. Various effects of growth parameters such as temperature, catalyst and buffered substrate on the growth product were also systematically investigated by using SEM, TEM and XPS

Nanowire based materials and architectures as anodes for LI-ION batteries.

Energy independence requires that the nation reduce its dependence on foreign oil imports. This can be achieved through electrification of transportation vehicles if proper battery technology can be developed. In addition, the renewable energy sources such as solar and wind tend to be intermittent with time scales ranging from seconds to hours. So, a suitable energy storage technology is essential for integrating renewable sources for base load generation. Lithium ion battery technology is promising; however, the big challenge limiting its widespread implementation is with capacity, durability and safety. A dramatic advancement is needed in terms of materials used for both electrodes and electrolytes. Several materials such as tin, tin oxide (SnO2), cobalt oxide (CoO3), iron oxide (Fe2O3), intermetallic alloys and semiconductors like silicon (Si) and germanium (Ge) potentially provide much higher theoretical capacity compared to conventionally used carbon based materials for anodes. Although most of these materials have favorable characteristics, they come at the expense of enormous volume changes associated with lithium alloying as a result of which the material integrity is lost. One-dimensional nanowires are believed to have better charge transport and strain relaxation properties but mostly unproven. In this dissertation, a generic hybrid architecture concept involving one-dimensional nanowires covered with nanoclusters IS proposed for improving the durability of anodes with high capacity retention. Specifically, this concept is demonstrated with metal-nanocluster-covered metal oxide nanowires using Sn/SnO2 system. The results showed that Sn nanocluster covered SnO2 nanowires exhibited a capacity retention of ~800 mAhg-1 for up to 100 cycles, the highest reported until now. In this study, the presence of well-spaced nanoclusters provides adequate room for metal volume expansion on lithiation preventing cluster coalescence leading to stable material structure while the metal oxide base provides various channels for electron conductivity. Cyclic voltammetric studies are conducted to understand the fundamental behavior of mono layers of nanoscale and micron scale tin clusters supported on both metallic substrates and hybrid architectures. The results suggest that tin clusters with sizes less than 50nm undergo complete de-lithiation while larger clusters exhibit incomplete delithiation due to diffusion limitation. The hybrid architecture concept can also be extended to other high capacity materials systems using unique carbon structures and molybdenum oxide nanowire arrays as base materials. In this direction, carbon microtubes (CMTs) are synthesized in large quantities and tested for their lithiation and de-lithiation characteristics. CMTs are micron sized tubes with 50nm walls comprised of random nanographite domains. The results indicated that CMTs exhibited capacity retention of ~440 mAhg-l, higher than the theoretical capacity of graphite. More importantly, CMTs show excellent rate capability of ~135 mAhg-1 at rates as high as 5C which makes them ideal as base materials in hybrid architectures. Another material of interest is molybdenum oxide (MoO3) which has excellent theoretical capacity and stability. Nanowire arrays are grown on conducting substrates providing direct charge conduction pathways eliminating the use of conducting polymer, generally used in powder based electrodes. These arrays show good capacity retention of ~630 mAhg-1 along with rate capability. In addition, the capacity retention below 0.7 V is ~500 mAhg-1 , which is better than the performance of any other MoO3 based materials and hence, makes the material viable for practical application as electrodes. Technologically, the proposed concept of hybrid architectured materials involving I-D materials with nanoclusters should result in the development of new materials architectures for high capacity, high rate and durable anodes. Scientifically, for the first time, the study showed fundamental differences in the lithiationlde-lithiation behavior of tin clusters at nanoscale which could apply to several other material systems. In addition, the interesting aspects involved in high capacity retention and durability have been aptly studied and understood for further application in other material systems