Modulation of the structure and morphology of NiCo2S4 by varying the anion types of nickel and cobalt salts to achieve high-rate supercapacitive performance

Abstract

In this work, the structure and morphology of NiCo₂S₄ (NCS) were modulated by varying the anion types (Cl⁻, CH₃COO⁻, and NO₃⁻) of nickel and cobalt salts. Extensive material characterization and analyses revealed that the grain size of the obtained NCSs was determined by different solvation free energies, capping effects, and steric hindrance during the crystal growth process. Among these three anions, Cl⁻, with the smallest ionic size, exhibited the lowest capping effect, steric hindrance, and solvation free energy, leading to the largest average grain size of 15.34 nm for Cl⁻-based NiCo₂S₄ (NCS-C). Moreover, the sea urchin-like morphology of NCS-C provided a high reaction interface for electrochemical energy storage. As a result, the specific capacitance of NCS-C could reach 1112.4 F/g at a current density of 6 A/g, retaining 692 F/g even at a high current density of 16 A/g. The assembled asymmetric supercapacitor could also deliver a high energy density of 23.4 Wh kg⁻¹. This work highlights the significant influence of anion type on the structural and morphological evolution of NCS materials, providing new insights for the development of high-rate NCS-based electrode materials.