Plasma polymer electrolyte coatings for 3D microbatteries
As micron-scale electronic devices become ubiquitous, there is a growing need for similar-sized batteries for self-contained storage. Current thick film batteries fail in that they require an areal footprint of at least 1 cm2 (leaving the microscale) in order to have enough volume to satisfy energy storage requirements. The electrodes cannot simply be made thicker because that would slow ion diffusion. One solution is to increase the surface area available per unit area by replacing the standard thin film with an array of microscale posts. The challenge, then, is to develop an electrolyte system that can effectively coat tall posts in order to separate this structure from the opposing electrode, while still allowing for the efficient transport of ions. To solve this problem, a gas- (or “plasma”)-phase deposition process can be used, which allows for conformal contact of the depositing molecules with all surfaces of a 3-dimensional structure. I have designed and constructed the instrument for this deposition, have optimized both the system and the deposition parameters for optimal chemical composition, and am currently optimizing ion transport properties. A cross-linked, plasma polymer of polyethylene oxide was the target structure, and XPS shows that this composition have been achieved. Furthermore, impedance spectroscopy shows that the films are the first solid state electrolytes to be synthesized via a non-directional gas phase technique (having the potential to be conformal).