| dc.description.abstract | Lithium ion battery is the dominant secondary storage technology for portable electronics, electric vehicles, medical devices and grid storage. While it has gained widespread acceptance, current generation battery materials are limited by the expensive and often toxic components, and safety concerns, and, still cannot meet projected storage requirements for future applications. A majority of studies rely on traditional synthesis techniques similar to those used for large scale manufacturing for development and investigation of new materials. Our work takes an alternate approach of harnessing thin film technology that has been extensively developed for semiconductor-related research. The thesis focuses on the investigation of the next generation environmentally friendly and low cost battery material using thin film- based studies. A better understanding and optimization using thin film techniques aids the development of traditional processes for bulk batteries and also paves a way for future all thin film-based battery for portable electronics. The work focuses on Mn-based materials because of its low cost and non-toxic nature.
A unique one step deposition processes have been developed and demonstrated using the growth and analysis of epitaxial and highly textured Li(NixMnyCo1-x-y)O2 (NMC) thin film on stainless steel with a thin gold buffer layer. The NMC thin film cathodes gave a high capacity of 167 mAh.g^-1 and 125 mAh.g^-1 at 0.1 C and 0.5 C respectively. Another promising next generation material Li2MnO3 has also been investigated. An unique process to extract Li2O during synthesis has been developed to activate this material directly during deposition. Li2MnO3 cathodes with capacity of 225 mAh.g^-1 were synthesized by tuning growth conditions appropriately. Another Mn based chemistry, commonly referred to as Li-rich oxide, has been investigated. Nano-domain and composite model structures have been developed to investigate unresolved questions about the microstructure of the films. It has been demonstrated that a nano-domain model best matches with the characteristic electrochemical properties of Li-rich cathodes with a demonstrated capacity of 293 mAh g^-1 at 0.05 C. Further study has been carried out to explore optimum composition and structure for Li-rich based thin film cathodes. Composition variation experiments indicate that the 50:50 deposition schemes gave the highest capacity. Moreover, the results indicate a strong correlation between Li2MnO3 domain size on the capacity of the Li rich electrode. A study to investigate correlation between oxygen stoichiometry and microstructure on behavior of Li2MnO3 showed strong inter-correlation. It shows that Li rich cathodes performed the best when deposited at low oxygen partial pressure while Li2MnO3 required a moderate partial pressure for optimum capacity.
Finally Li3PO4 and LiPON electrolytes have been investigated for all solid state battery development. An electrolyte coating on top of a NMC cathode film resulted in significant improvement to the observed capacity. Li3PO4 coated cathodes demonstrated a high capacity of ~ 80 µAh/µm.cm^2 at 0.75 µA.cm^-2, compared to 65 µAh/µm.cm^2 that was observed in bare NMC. This work contributes to the implementation of all solid state batteries using thin film approaches with great potentials for various applications such as integrated circuits, portable medical devices and space applications. | en |
