Ultrathin Carbon Nanosheets for Highly-efficient Capacitive K-ion and Zn-ion Storage

Porous carbon has attracted extensive attentions as the electrode material for various energy storage devices considering its advantages like high theoretical capacitance/capacity, high conductivity, low cost and earth abundant inherence. However, there still exists some disadvantages limiting its further applications, such as the tedious fabrication process, limited metal-ion transport kinetics and undesired structure deformation at harsh electrochemical conditions. Herein, we report a facile strategy, with calcium gluconate firstly reported as the carbon source, to fabricate ultrathin porous carbon nanosheets. The as-prepared Ca-900 electrode delivers excellent K-ion storage performance including high reversible capacity (430.7 mAh g^-1), superior rate capability (154.8 mAh g^-1 at an ultrahigh current density of 5.0 A g^-1) and ultra-stable long-term cycling stability (a high capacity retention ratio of ~81.2% after 4000 cycles at 1.0 A g^-1). Similarly, when being applied in Zn-ion capacitors, the Ca-900 electrode also exhibits an ultra-stable cycling performance with ~90.9% capacity retention after 4000 cycles at 1.0 A g^-1, illuminating the applicable potentials. Moreover, the origin of the fast and smooth metal-ion storage is also revealed by carefully designed consecutive CV measurements. Overall, considering the facile preparation strategy, unique structure, application flexibility and in-depth mechanism investigations, this work will deepen the fundamental understandings and boost the commercialization of high-efficient energy storage devices like potassium-ion/sodium-ion batteries, zinc-ion batteries/capacitors and aluminum-ion batteries.