Carbon shell-coated mackinawite FeS platelets as anode materials for high-performance sodium-ion batteries

Mackinawite iron sulfide (M-FeS), a rarely reported conversion-type anode material, undergoes repeated volume changes during the charge–discharge process, which eventually induces fast electrochemical degradation. However, commonly used strategies to mitigate volume changes using hybridization with carbon materials require high temperatures above 500 °C, which is limited for M-FeS due to its low thermal stability. Here, a novel and facile strategy to hybridize M-FeS with a carbon material using a one-step hydrothermal method under low-temperature conditions (125 °C) is reported. Carbon dots with functional groups were served as nucleation sites, and hybridized with M-FeS. The hybrid material (M-FeS@C) comprised micron-sized M-FeS particles wrapped with a carbon shell and exhibited improved structural stability during the charge–discharge process when tested for sodium-ion storage; stable cycle performance was achieved for 500 cycles delivering a capacity of 372 mAh g−1 at a current density of 1 A g−1. Moreover, when Ox-SWCNTs were also used as a conductive agent, a three-dimensional (3D) conductive network that provides electrical pathways was formed and preserved. The synergic effects of the carbon shell and CNT conductive agent (M-FeS@C+CNT) maximized the structural stability and the electrical conduction path, resulting in a capacity of 336 mAh g−1 at a high current density of 10 A g−1 and a capacity of 360 mAh g−1 for 860 cycles at 1 A g−1.