Carbon nanostructures for enabling microstructured energy storage devices

Abstract

Commercial Li_ion batteries are based on liquid electrolytes which are considered unsafe as the organic solvents they contain are volatile and flammable. As an alternative, Li_ion batteries with solid electrolytes are proposed. Solid-state batteries need to be made in form of a thin-film stack to compensate for the low ion conductivity through the solid electrolyte. However, these batteries in their current planar format have low energy density. Solid-state batteries with higher energy density and similar power density are possible, if the architecture of the thin film battery is changed from planar to microstructured. This can be achieved by depositing the active materials as a thin-film stack with sub-micrometer thicknesses over these microstructured surfaces. To this date no commercial battery of this type is available. In this thesis, several challenges regarding the fabrication of this type of batteries were investigated. In particular, nanostructured graphitic carbon layers where explored. High aspect-ratio carbon nanosheets (CNS) were used as model material to study conformal deposition of pinhole free thin electrolyte films by electrodeposition. Thin planar graphitic carbon films with nano roughness were investigated as adhesion promotor for nanoporous electrolytic manganese dioxide films to be used as cathode in our 3D Li-ion thin-film batteries. Carbon nanosheets (CNS) layers were used as high surface area carbon nanostructures. The CNS consist of a maze of electrically interconnected thin graphitic carbon sheets which are oriented vertically with respect to the substrate. CNS form a self-supported, high aspect ratio network with a sheet thickness ranging from a few nanometers to tens of nanometer and with sheet heights up to 2 micrometer. The planar graphitic carbon layers were obtained with process of growing CNS, however, in this case, the process was interrupted right after the nucleation stage, providing graphitic carbon films with few tens of nanometers. In the first part of thesis an extensive electrochemical characterization of CNS layers with different morphology and surface area (height) in aqueous and non- aqueous electrolyte solutions was performed. During this investigation a direct correlation between electrochemical capacitance, wettablity and functional surface groups on CNS sheets was determined. Moreover, the electrochemical capacitance of the CNS layers was used to determine the area enhancement of these structures. An area enhancement 120x per micrometer of CNS was found. One of the biggest challenges in the fabrication of a microstructured battery is the ability to conformally coat high aspect ratio microstructures with active battery materials. This is especially difficult for the solid electrolyte. As the solid electrolyte is directly sandwiched between two active electrodes, it is required to be electronically insulating. Hence, the electrolyte film needs to be pinhole free. In this thesis we have used the high aspect ratio CNS layer as a template to investigate the conformal deposition of electrically insulating poly(phenylene oxide) or PPO films. So far, PPO films had only been electrodeposited on planar surfaces. According to literature, these films were electrically insulating but ionically conductive. In this thesis we were able to conformally coat about 10nm thin PPO films over the CNS layers. The as-deposited PPO films were pinhole-free, electrically insulating and showed some ionic conductivity. The planar graphitic carbon film was used as a conductive seed layer to investigate the growth and electrochemical properties of electrolytic manganese dioxide (EMD) battery electrode. The graphitic carbon coating provided an excellent adhesion between the TiN substrate and EMD which significantly improved the electrochemical performance compared to EMD layers for example grown on platinum seed. Moreover, with the help of these graphitic carbon films, up to 500 nm thick EMD films could be grown, which was not possible neither with Pt nor TiN current collectors. Finally, a working half-cell was composed based on the developed graphitic carbon adhesion layer, EMD thin-film cathode and thin PPO as solid Li-ion electrolyte. However, due to the ultrathin thickness of PPO, the attempts to build full battery were unsuccessful. It is suggested that the PPO can serve as ion conductive buffer or protective layer rather than the main solid electrolyte in the stack. Even though the half-cell materials and processes are fully transferable to a microstructured substrate, further research will be required to build a full 3D thin-film battery.status: publishe