Fabrication and characterization of 3D thin-film batteries.

The technological fields of mobile devices, remote wireless sensors, implantable medical devices, smart cards and energy harvesters are rapidly evolving. A bottleneck in the development of such emerging systems is the energy storage component required for extended autonomous operation. Miniaturization of conventional batteries is limited by the use of particle materials and a liquid electrolyte. Commercially available planar (2D) thin-film solid-state batteries are intrinsically safe, can be charged quickly for thousands of cycles, and are available in the smallest form. Unfortunately, the thin-film electrodes in these batteries contain only a small amount of active electrode material, which results in a relatively low storage capacity. In order to increase the capacity of thin-film batteries, three-dimensional thin-film solid-state batteries are envisioned. By using high aspect ratio structures as the current-collector substrate, the amount of active material per footprint area can effectively be increased, while fast charging capabilities are retained. The main goal of this thesis was to fabricate and characterize titania-based thin-film electrodes for 3D thin-film batteries. Four main scientific goals were pursued:

1) Perform a fundamental study on the Li+-ion insertion/extraction properties of anatase versus amorphous TiO2 thin-films and on the influence of nanosizing.

2) Investigate doping of TiO2 thin-films to increase their rate-performance and Li-ion capacity.

3) Understand the impact of solid electrolytes on thin-film electrode performance.

4) Investigate conformal coating of TiO2 deposited by (spatial)-ALD on micropillar substrates.

TiO2 was chosen as thin-film Li+-ion insertion anode, as it offers a high volumetric capacity, shows a small volume expansion upon lithiation, and can be conformally coated using atomic layer deposition (ALD). Unfortunately, when using the common anatase structure, the rate-performance is extremely poor. Even for nanosized films (i.e. 35 nm), it was shown that anatase did not provide satisfactory rate-performance. For the first time, the electrochemical properties of amorphous TiO2 (am-TiO2) were systematically compared with anatase, using cyclic voltammetry, galvanostatic charge/discharge and potentiostatic intermittent titration technique experiments (PITT). The performance could be improved multifold by using the amorphous TiO2 structure. As a result of the promising Li+-ion insertion properties of am-TiO2, nanosize am-TiO2 thin-films were deposited on a 3D (aperiodic) carbon nanosheet substrate, highlighting the conformal nature of ALD, and the concept of surface area enhancement for increasing the (footprint) capacity.

Next, chlorine was investigated as a dopant for am-TiO2. By selecting TiCl4 as the Ti-precursor for the ALD process, Cl was incorporated during am-TiO2 deposition. The influence of the electrochemical properties as a function of Cl-content were investigated in detail. The rate-performance and storage capacity were found to improve with increasing Cl-contents. Compared to state-of-the art TiO2 from literature, Cl-doped amorphous TiO2 showed one of the highest storage capacity to date.

To unravel the properties and promises of 3D thin-film batteries, optimal micropillar array designs and their theoretical capacities were simulated. The use of a large-scale applicable deposition technique (i.e. Spatial ALD or S-ALD) was investigated. Specifically, the aspect of conformal coating across a high-surface area micropillar array using S-ALD were studied and compared to conventional ALD. As a demonstration, Cl-doped am-TiO2 films were deposited on Si micropillar arrays and their electrochemical properties were investigated. Finally, TiO2 was combined with a LiPON solid-state electrolyte thin-film and electrochemical properties of the TiO2/LiPON stacks were investigated.