Two-dimensional nanosheet arrays with chemically bonded Cu₂S@NSC heterostructure for efficient sodium-ion storage

Metal sulfides have attracted significant interest as battery electrode materials due to their high theoretical capacity and redox reversibility. However, large volume changes that occur during cycling will result in structural degradation and rapid capacity loss. In this study, we reported on Cu2S nanosheet arrays with N, S co-doped carbon (NSC) encapsulation (Cu2S@NSC), with strong sulfur-bridged bond interactions (Cu-S-C). The synthesis involved a simple one-pot pyrolysis method using an environmentally friendly resin as the source of N, C, and S. As a sodium-ion battery anode material, Cu2S@NSC delivered outstanding cycling stability of 353.1 mAh g(-1) after 200 cycles at 0.1 A g(-1) (at 86.3 % capacity retention) and > 300 mAh g(-1) after 1000 cycles at 1 A g(-1) (at 94.1 % capacity retention). The cycling stability was attributed to the encapsulated nanoarray structure and strong binding interaction between Cu2S and NSC (bridge bond of Cu-S-C). The nanosheets provided a large contact area for electrolyte penetration, shorten Na+ diffusion distance and accelerated electron transfer. In situ TEM video analysis also confirmed that the carbon layer effectively buffered the volume expansion of Cu2S, while in situ XRD and Raman spectroscopy confirmed that the reaction mechanism occurred via a fully reversible conversion reaction between Cu2S, Cu metal and Na2S. The methods and findings of this work may be of interest for the improved electrode material design of conversion reaction anodes.