Tunneling Effect-Induced Exfoliated NiCoLDH Quantum Sheets for High-Power and High-Energy Aqueous and Li-Ion Capacitors

The bulk nickel cobalt-layered double hydroxide (NiCoLDH) exhibits limited charge transfer and low conductivity due to clumping and blockage of the active sites in its layers. To tackle this, exfoliated nickel cobalt-layered double hydroxide quantum sheets (e-LDHQS) were developed by exfoliating bulk NiCoLDH via a simple sonication method. The spectroscopic and microscopic characterizations confirmed the formation of the e-LDHQS and exhibited a clear Tyndall effect, a major feature of a quantum material. The three-electrode cell setup was made to assess e-LDHQS electrochemical activity using cyclic voltammetry and galvanostatic charge–discharge studies in 1 M KOH. The e-LDHQS electrode was electrochemically active and delivered 655 C g^–1 specific capacity at 1 A g^–1 current rate. The Power law and Dunn’s approach were implemented to explore the charge storage kinetics. Subsequently, an aqueous hybrid supercapacitor and a nonaqueous lithium-ion capacitor were fabricated in the form of a CR-2032 coin cell. The fabricated aqueous hybrid supercapacitor device (e-LDHQS|1 M KOH|rGO) showed a specific power of 2500 W kg^–1 at a high specific energy of 133 Wh kg^–1 and an extremely high specific power of 27,000 W kg^–1 at a specific energy of 52 Wh kg^–1, whereas the nonaqueous lithium-ion capacitor (e-LDHQS|1 M LiPF₆|rGO) exhibited a specific power of 800 W kg^–1 at a high specific energy of 20 Wh kg^–1 and a high specific power of 7000 W kg^–1 at a specific energy of 2 Wh kg^–1. Such excellent performances were attributed to the quantum size effect (or quantum tunneling effect) associated with the exfoliated 2D LDH quantum sheets that resulted in efficient charge/ion transport.