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

Abstract

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.