This dissertation describes the development of a high-power, plastic-based, thick-film lithium-ion microbattery for use in a hybrid micropower system for autonomous microsensors. A composite porous electrode structure and a liquid state electrolyte were implemented in the microbatteries to achieve the high power capability and energy density. The use of single-walled carbon nanotubes (SWNTs) was found to significantly reduce the measured resistance of the cathodes that use LiAl0.14Mn1.86O4 as active materials, increase active material accessibility, and improve the cycling and power performance without the need of compression. Optimized uncompressed macro cathodes were capable of delivering power densities greater than 50 mW/cm2, adequate to meet the peak power needs of the targeted microsystems. The anodes used mesocarbon microbeads (MCMB) with multi-walled carbon nanotubes (MWNTs) and had significantly better power performance than the cathodes. The thick-film microbattery was successfully fabricated using techniques compatible with microelectronic fabrication processes. A Cyclic Olefin Copolymer (COC)-film was used as both the substrate and primary sealing materials, and patterned metal foils were used as the current collectors. A liquid-state electrolyte and Celgard separator films were used in the microbatteries. These microbatteries had electrode areas of c.a. 2 mm x 2 mm, and nominal capacities of 0.025-0.04 mAh/cell (0.63-1.0 mAh/cm2, corresponding to an energy density of ~6.3-10.1 J/cm2). These COC-based batteries were able to deliver constant currents up to 20 mA/cm2 (100% depth of discharge, corresponding to a power density of 56 mW/cm2 at 2.8 V) and pulse currents up to 40 mA/cm2 (corresponding to a power density of 110 mW/cm2). The high power capability, small size, and high energy density of these batteries should make them suitable for the hybrid micropower systems; and the flexible plastic substrate is also likely to afford some unique integration possibilities for autonomous microsystems. The mechanism by which the SWNTs improved the rate performance of composite cathodes was studied both experimentally and theoretically. It was concluded that the use of SWNT improved cathode performance by improving the electronic contacts to active material particles, which consequently improved the accessibility of these particles and improved the rate capability of the composite cathodes.