Vertically Aligned and Ordered Arrays of 2D MCo₂S₄@Metal with Ultrafast Ion/Electron Transport for Thickness-Independent Pseudocapacitive Energy Storage.

Abstract

Pseudocapacitance holds great promise for energy density improvement of supercapacitors, but electrode materials show practical capacity far below theoretical values due to limited ion diffusion accessibility and/or low electron transferability. Herein, inducing two kinds of straight ion-movement channels and fast charge storage/delivery for enhanced reaction kinetics is proposed. Very thick electrodes consisting of vertically aligned and ordered arrays of NiCo₂S₄-nanoflake-covered slender nickel columns (NCs) are achieved <i>via</i> a scalable route. The vertical standing ∼5 nm ultrathin NiCo₂S₄ flakes build a porous covering with straight ion channels without the "dead volume", leading to thickness-independent capacity. Benefiting from the architecture acting as a "superhighway" for ultrafast ion/electron transport and providing a large surface area, high electrical conductivity, and abundant availability of electrochemical active sites, the NiCo₂S₄@NC-array electrode achieves a specific capacity up to 486.9 mAh g^-1. The electrode even can work with a high specific capacity of 150 mAh g^-1 at a very high current density of 100 A g^-1. In particular, due to the advanced structure features, the electrode exhibits excellent flexibility with a unexpected improvement of capacity when being largely bent and excellent cycling stability with an obvious resistance decrease after the cycles. An asymmetric pseudocapacitor applying the NiCo₂S₄@NC-array as a positive electrode achieves an energy density of 66.5 Wh kg^-1 at a power density of 400 W kg^-1, superior to the most reported values for asymmetric devices with NiCo₂S₄ electrodes. This work provides a scalable approach with mold-replication-like simplicity toward achieving thickness-independent electrodes with ultrafast ion/electron transport for energy storage.