Solid-state batteries (SSB) can potentially solve the main challenges facing today’s lithium-ion batteries (LIB) with liquid electrolytes, namely, safety issues due to the flammability of the liquid electrolyte and limited cycle life time due to unwanted side-reactions at the solid/liquid interface. The emerging Internet of Things will lead to an exponential growth in wireless sensor networks and autonomous microsystems, however, improvements in energy storage is a critical aspect in order to enable future applications. Thin-film solid-state batteries (TFB) are the candidate of choice for powering this wide variety of microsystems, such as smart cards, radio-frequency identification tags and medical implants, due to their intrinsic safety and great flexibility in device design and integration. In order to meet the future demands in terms of power and energy density the need for all-solid-state 3D Li-ion microbatteries arises. One of the key challenges for the realization of 3D SSB is the conformality of the layers on high surface area substrates. This is especially true for the solid electrolyte layer, as a single pinhole will short-circuit the entire cell. Atomic layer deposition (ALD) and molecular layer deposition (MLD) are two of the few techniques capable of depositing conformal and pin-hole free layers on complex substrates, as they both are gas-phase deposition techniques based on sequential self-limiting surface reactions. The main purpose of SUPER-Lion was to combine these two powerful techniques for development of a novel thin-film nano-composite electrolyte (NCE) which would provide both high ionic conductivity and good electrochemical stability. The NCE consists of a mesoporous oxide matrix, obtained through MLD, which provides both mechanical stability and a high effective internal surface area. The internal surface of the mesoporous matrix is coated by ALD with nanometer thin layers of a Li-compound that supply the necessary Li+ ions. The increased ionic transport at the interface between the surface of the oxide and the lithium compound is exploited to make an NCE with enhanced ionic conductivity. SUPER-Lion has provided detailed insight into the potential of composite electrolytes for use in SSB for the first time. Several NCE systems were fabricated to study the effect of different Li-compound/oxide combinations, oxide matrix composition and structure on the resulting ionic conductivity, battery performance and stability. In addition, the feasibility of integrating composite electrolytes in battery systems was also demonstrated by utilizing NCE systems made by MLD/ALD in TFB cells. The battery devices were based on TiO2 as cathode material, NCEs as electrolyte, and lithium metal as anode. The TFBs were characterized in order to determine their performance and dependence on the NCE properties. The direct application of this work will enable the fabrication and development of novel solid-state electrolytes, paving the way for next generation TFBs.