Nitrogen-doped graphene-wrapped Cu₂S as a superior anode in sodium-ion batteries

Sodium-ion batteries (SIBs) have been attracting an ever-growing research interest, mainly ascribed to their cost-effectiveness. However, SIBs have been significantly hindered by lack of a suitable anode. Herein, an exceptional Cu₂S-based composite anode is developed via a facile ball-milling method, in which Cu₂S particles are wrapped by nitrogen-doped graphene sheets (Cu₂S@NG). This Cu₂S@NG composite anode enables extremely long cycling life, ultra-stable cyclability with high capacity, and excellent rate capability. The superior performance of the Cu₂S@NG composite is owing to its intriguing core-shell structure and the exceptional properties of both the Cu₂S and NG. In this study, it is found that the NG shell yields multiple merits in improving the performance of Cu₂S: (i) mitigating the loss of active materials, (ii) constituting a stable interface, (iii) providing improved electrical conductivity and good ionic transfer, and (iv) enhancing mechanical integrity. Additionally, the vital effects of different voltage windows and a surface coating via atomic layer deposition on further enhancing performance are clarified. Significantly, the electrochemical mechanism of Cu₂S during sodiation/desodiation is unveiled using advanced synchrotron-based in-situ X-ray diffraction and X-ray adsorption spectroscopy. This work represents a great advance in seeking high-performance anodes in SIBs.