Rechargeable energy storage relies mainly on lithium-ion battery technology, the same that supports most of the mobile world. This technology is under research by many groups around the world and is still considered the best way to store electrical energy from intermittent power sources. However, battery technology is limiting the evolution of many integrated electronics, especially in wearable applications; improvements in terms of energy density, higher number of life cycles, flexibility and safety are still needed. In thin-film batteries, the selection, the design structure, fabrication process and characterization of materials as well as film deposition techniques play an important role in the maximization of the battery performance, durability and reproducibility. This thesis contributes to battery technology in several ways. The use of a typical flexible substrate (Kapton®, by Dupont™) while fabricating all battery materials in the same chamber, including barrier and encapsulation materials, excluding the necessity for extra vacuum and glove-box chambers, was researched. Using only safe solid-state materials, on which no leakage or explosions can occur and replacing metallic lithium (Li) anode for a much more “friendly” material in terms of fabrication and battery cycling, battery energy density benefits. The thin-film lithium phosphorous oxynitride (LiPON) electrolyte is required to have high ionic conductivity, a negligible electrical conductivity and to be stable in contact with the anode and cathode electrodes. LiPON electrolyte was deposited by RF sputtering at different experimental conditions. The highest ionic conductivity of 1 x 10-6 S/cm was measured at ambient temperature of 35 ⁰C for a film deposited with power supply of 150 W, 20 sccm of nitrogen (N2) and a deposition pressure of 3 x 10-4 mbar. Samples with a silicon nitride (Si3N4) barrier layer, a titanium (Ti) adhesion layer and a platinum (Pt) cathode current collector layer (Kapton/Si3N4/Ti/Pt) were prepared for flexibility experiments of Kapton® substrate. Si3N4 Li barrier layer was deposited by RF sputtering deposition technique with an electric resistivity of 9.51 x 1011 Ωcm and a mean breakdown field of 1.67 MV/cm. Thin-films continued attached to the substrate after several bends. Lithium cobalt oxide (LiCoO2) cathode was deposited on top of Kapton/Si3N4/Ti/Pt structure. After LiCoO2 deposition, annealing at 400 ⁰C was performed during 1 hour at different atmospheres (vacuum and air). The films annealed in air atmosphere presented higher crystallinity, especially in the plane (101), the orientation required for batteries with improved performance and durability. Cathode LiCoO2 films were deposited by RF sputtering with a 120 W power supply, pressure of 6 x 10-3 mbar and 17/3 sccm of Ar/O2 gases, respectively. A thin-film flexible Li battery and a thin-film flexible Li-ion battery were successfully fabricated using only physical vapour deposition (PVD) techniques. The anodes of metallic Li (for Li-battery) and germanium (Ge) (for Li-ion battery) were deposited 3 μm thick by thermal evaporation and 300 nm thick by e-beam, respectively. A well-organized battery structure with smooth interfaces and good adhesion was observed by scanning electron microscope (SEM) analysis. A self-discharge was measured and related to a possible thinner electrolyte in some area between the cathode and the anode on both fabricated batteries. A low potential and retention fading along charge/discharge cycles were also measured and related to an amorphous LiCoO2. Despite the low capacity presented by the two batteries, an improvement when the Li anode was changed to Ge is evident (0.35 nAh/cm2 with Li anode, to 46 nAh/cm2 with Ge anode). A battery encapsulation with three sputtered layers: lithium phosphorous oxide (LiPO), LiPON and Si3N4, each 20 nm thick, was fabricated. After these depositions and at atmospheric conditions, an epoxy was applied on the PVD multilayer to complete the encapsulation for long term protection. Research indicates it is possible to fabricate flexible thin-film Li batteries on Kapton® substrate using only PVD deposition techniques, avoiding the necessity of extra vacuum and glove-box chambers.