High energy and power density rechargeable micro-batteries are a necessity for powering the next generation of flexible electronics, internet of things and MedTech devices. In theory, significant improvements in the capacity, current and power densities of micro-batteries would result if 3-dimensional architectures were used, as they have both enhanced interdigitated component interface areas and shortened ion diffusion path lengths. Vertically aligned nanocomposite (VAN) films, an example of a thin film 3D architecture, have shown promise in solid oxide fuel cells devices displaying enhanced ionic conductivity, reduced areal surface resistances and improved cell performance by enhancing the interfacial surface area. These VAN attributes may be transferable to solid-state micro-batteries, enabling improvements in the aforementioned battery properties, while also compensating for intrinsic low diffusivity due to nanoscale path lengths. VANs can be grown with high control of the crystallographic and interface orientation and act as a scaffold to stabilize challenging phases; hence, systems can be optimized to maximize capacity and performance, particularly when working with materials with anisotropic properties. Thus, VANs may allow a wider selection of materials to be utilised in miniaturised batteries.

In the first part of this thesis, two battery based vertically aligned nanocomposite thin films are presented. The first system, a LL(Nb,Ti)O-(Ti,Nb)O₂ VAN, showcases the essential physical properties of a VAN film that could be incorporated into an all solid-state battery. Namely, high Li⁺ ionic conductivity and distinct regions of electron conducting nanocolumns embedded in an insulating matrix. The second system, a LiMn₂O₄-SrRuO₃ VAN, is comprised of LiMn₂O₄ cathode nanocolumns embedded in a SrRuO₃ current collecting matrix. This system exhibits high discharge capacities and excellent rate performance. It is demonstrated that the cycling performance is dependent on both the LiMn₂O₄ columnar orientation and dimensions. In the third part of this thesis, a new potential current collector is explored, NiCo₂O₄. Preliminary work on epitaxial LiMn₂O₄ films grown on NiCo₂O₄ current collector is presented, demonstrating that it is possible to grow epitaxial LiMn₂O₄ at 360 °C, ∼ 200 °C lower than previously reported. This cathode system also displays a high discharge capacity, > 100 mAh g⁻¹ for 6000 cycles. The final chapter deviates away from batteries and explores the stabilisation of oxide ion conducting fluorite δ-Bi₂O₃. A series of systems are discussed, cumulating in the development of a dysprosium stabilised δ-Bi₂O₃-DyMnO₃ VAN exhibiting an ionic conductivity of 10⁻³ S cm⁻¹ at 500 °C.

This thesis communicates important advancements for the micro-battery community. First, two new battery oxide VAN systems are introduced, expanding the total systems reported to three (at the time of writing). With the first VAN system discussed, LL(Nb,Ti)O-(Ti,Nb)O₂, promising physical properties are reported alongside key criteria that should be met in order to develop a battery VAN that exhibits clear redox behaviour. These criteria are successfully implemented in the second VAN presented, LiMn₂O₄-SrRuO₃. This VAN system exhibits clear LiMn₂O₄ redox, the first VAN to achieve this feat, and demonstrates remarkable capacity retention under high-rate regimes. It is also shown that the electrochemical performance is dependent on both the LMO pillar crystallographic orientation and dimensions. The latter dependence has not been shown before and it has very important ramifications for the development and future of 3D architectured thin film VAN batteries. Beyond VAN batteries, the LiMn₂O₄/NiCo₂O₄ system reduces the epitaxial growth temperature of LiMn₂O₄ to within the complementary metal-oxide-semiconductor stability window (< 450 °C). This shows a route towards and may enable the implementation of epitaxial cathodes in next-generation micro-batteries.