Date on Master's Thesis/Doctoral Dissertation

12-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Sunkara, Mahendra K.

Author's Keywords

Nanowires; Nanowire arrays; Anodes; Lithium ion batteries; Hybrid architectures; Carbon microtubes; Tin oxide

Subject

Nanowires; Lithium ion batteries

Abstract

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.

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