Interface Optimization of Cu₂S Nanoparticles by Loading N-Doped Carbon for Efficient Sodium-Ion Storage

Rapid capacity fading and sluggish diffusion kinetics resulting from crystal conversion/powder pulverization hinder practical energy storage application of conversion-type electrodes. To address this issue, we prepared a Cu2S/polyelectrolyte/graphene composite (denoted as Cu2S/PG) through interfacial optimization by incorporating a polyelectrolyte to enhance the connection between Cu2S powders and N-doped graphene. In comparison to CuS and Cu2S, the electrochemical performance of Cu2S/PG was significantly improved by nanocrystallization and carbon-coating, which delivers a capacity of 317 mAh g(-1) at 0.1 A g(-1) after 200 cycles. Moreover, we performed real-time analysis of the phase conversion and resistance evolution of the Cu2S/PG electrode during Na+ insertion/desertion using in situ X-ray diffraction (XRD) and in operando electrochemical impedance spectroscopy (EIS). Thus, the formation of the intermediate phase (Na2S2) was firstly discovered, which finally converts to Na2S by the end of the sodiation process. In sum, the N-doped carbon/graphene wrapping acts as a protective barrier against electrolyte side reactions, thereby effectively improving the cyclability of the conversion-type Cu2S electrodes.