Correlation between the microstructures and the cycling performance of $RuO2$ electrodes for thin-film microsupercapacitors

We have fabricated all solid-state thin-film microsupercapacitors (TFSCs) using $RuO2$ electrodes and LiPON electrolytes. The $RuO2$ electrodes were grown at oxygen gas flow ratios $[O2/(O2+Ar)]$ of 10% and 30%. Room-temperature charge–discharge measurements show that specific capacitance is dependent on the oxygen gas flow ratio. Glancing angle x-ray diffraction (GXRD) and transmission electron microscopy (TEM) results show that the $RuO2$ electrodes grown at 10% contain nanocrystallites (0.7–10 nm across) embedded in the amorphous matrix, while the electrodes grown at 30% are polycrystalline (with grains of 0.7–15 nm in diameter). Based on the GXRD, TEM, Auger electron spectroscopy depth profile, and scanning electron microscopy results, the oxygen flow ratio dependence of the cycling performance of the $RuO2$-based TFSCs are discussed in terms of the combined effects of the microstructures of the electrodes, interfacial products, and the surface morphology of the electrodes.