Utilization of MEMS Techniques for the Fabrication of Scalable Energy Storage Devices

The research reported herein attempts to tackle the challenges of designing new fabrication technologies for the next generation of energy storage devices for MEMS and IoT systems. In order to satisfy the needs of the advanced sensors and actuators that are flooding the marketplace, new energy storage devices must 1) have excellent specific power/energy performance, 2) have scalable devices geometries in all three directions, and 3) have tunable output currents or voltages to match the specifications of the circuitry of the systems they are powering. It was found that utilizing MEMS fabrication techniques, specifically electrodeposition, lithography, and multilayer assembly, were crucial in the development of fabrication schemes that can achieve all three of these goals. These techniques were incorporated into lithium ion battery, zinc air battery, and inductor core devices within this study. For the lithium ion system, the approach centered on a vertically interdigitated electrode architecture, where the total height and volume of the cells were decoupled from the individual active layer thicknesses, thus enabling discharge rates of up to 100 C. The substrate free electrode layer stacking facilitated tunable device volumes from 5 to 100 mm3 without sacrificing specific performance, which measured up to 115 Wh/L and 1900 W/L. For the zinc air devices, a similar layer stacking approach was incorporated, but in this case it was to take advantage of integrated series connection of individual cells to create battery stacks with tunable output voltages. All layers in the batteries were electrodeposited, and a porous gold cathode layer was developed to provide channels for air flow and oxygen intrusion during discharge. Utilization of all solid-state materials, including a gel electrolyte, enhanced the ability of these devices to be integrated with future piezoelectric or electrostatic based MEMS devices, while simultaneously maintaining high energy densities of up to 1720 Wh/L and voltages from 1.2–5.1 V. Lastly, these multilayer electrodeposition processes were applied to the fabrication of laminated structures for the use in micro-inductor cores for potential on-chip power systems. Studying the electropolymerization of conductive polymers, in particular polypyrrole, led to the discovery of a five-bath plating technology that enables scalable, uniform deposition of both magnetic and polymeric layers through a photoresist mold that is required to shape the cores for eventual alignment on a chip. Due to the suppression of lateral polymer growth during polymerization, the laminated structures can be scaled to any number of layers that can fit inside an appropriate plating mold, without the need for any etching or subtractive fabrication steps. Thus, this technology could lead to commercialization of these magnetic and polymer layered constructs