Ultrahigh Energy Density Realized by a Single‐Layer β‐Co(OH)2 All‐Solid‐State Asymmetric Supercapacitor

A conceptually new all‐solid‐state asymmetric supercapacitor based on atomically thin sheets is presented which offers the opportunity to optimize supercapacitor properties on an atomic level. As a prototype, β‐Co(OH)2 single layers with five‐atoms layer thickness were synthesized through an oriented‐attachment strategy. The increased density‐of‐states and 100 % exposed hydrogen atoms endow the β‐Co(OH)2 single‐layers‐based electrode with a large capacitance of 2028 F g−1. The corresponding all‐solid‐state asymmetric supercapacitor achieves a high cell voltage of 1.8 V and an exceptional energy density of 98.9 Wh kg−1 at an ultrahigh power density of 17 981 W kg−1. Also, this integrated nanodevice exhibits excellent cyclability with 93.2 % capacitance retention after 10 000 cycles, holding great promise for constructing high‐energy storage nanodevices.