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Characterization Studies of Materials and Devices used for Electrochemical Energy Storage


There is a variety of energy storage systems but they all share a need for characterization. The principal motivation of this work is to understand the parameters that affect charge storage in three different systems. Studies were performed on carbon nanotube supercapacitors whose electrolyte was based on an encapsulated 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. This electrochemical double layer capacitor is an intrinsically high power device and the challenge was preserving that power with a quasi-solid electrolyte. The device was characterized by a suite of electrochemistry techniques and demonstrated both high power and high energy densities.

The second system investigated is the electrochemical pseudocapacitor orthorhombic Nb2O5, a transition metal oxide system with high rate capability. Pseudocapacitors are based on faradaic reactions that allow them high energy densities than traditional electrochemical capacitors. Nb2O5 has a high rate capability through its combination of electronic and ionic conductivities. This study quantified those values as a function of lithiation and shed light on a phase change when heating lithiated samples.

Batteries have greater energy densities than electrochemical capacitors but they have a lower power density due to slow ion kinetics through the electrodes. An approach to minimize the diffusion distance in the electrodes is to design the entire battery into an interpenetrating array. This structure needs a thin conformal solid electrolyte to prevent electrical shorts and maximize energy density. Due to the 3D nature of this structure, common techniques for imaging pinholes do not have the spatial resolution to characterize the film. A technique was adapted from the literature to characterize the pinholes using electron transfer from a metallocene molecule to the surface of the electrode. This technique offers a spatial sensitivity of a few angstroms in which to detect pinholes and can be used to characterize 3D conformal films.

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