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

Abstract 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 µWh cm −2 ), which exceed those of previously reported in‐plane MSCs.