Folding the Energy Storage: Beyond the Limit of Areal Energy Density of Micro-Supercapacitors

Despite the ever-growing interest in micro-supercapacitors (MSCs) as a promising power source for microelectronics, their low areal energy density has plagued practical applications. Herein, accordion foldable MSCs (af-MSCs) are presented as a cell architectural strategy in contrast to traditional material-driven approaches. The constituent unit cells of an in-plane MSC array are compactly stacked in a confined device footprint via accordion folding. Decoupling the energy storage (MSC cells) and folding section (electrical interconnection between the cells) in the MSC array, in combination with neutral plane-controlled flexible hydrophobic cellulose nanofiber (CNF) substrates, enables the realization of the af-MSCs. The af-MSCs achieve high areal integration density with a fill factor of 81.1% and on-demand (in-series/in-parallel) cell configurations owing to the microscale direct-ink-writing of rheology-tuned MSC cell components on the CNF substrates. The af-MSC with a miniaturized footprint (22.75 mm(2)) achieves exceptional areal electrochemical performances (areal energy density of 89.2 mu Wh cm(-2)), which exceed those of previously reported in-plane MSCs.