Enhancing the Sodium Storage of Zinc Oxide through Built-in Electric Field in N-Doped Carbon Coated ZnO/ZnS Heterostructures

Conversion-type anode zinc oxide (ZnO) has been considered one of the most promising anode materials for sodium ion batteries (SIB) due to its high theoretical capacities and appropriate working potential. Nevertheless, ZnO-based anodes suffer from huge volume change, poor electrical conductivity, and serious capacity decay during the sodiation/desodiation process. Constructing heterostructures is an efficient strategy to enhance sodium storage. The built-in electric field induced within heterostructures facilitates the diffusion of sodium ions and insertion progress for the batteries. Here, a hollow ZnO/ZnS heterostructure with a nitrogen-doped carbon coating layer (ZnO/ZnS@NC) was rationally designed through a facile multistep strategy of template-involved sulfidation, etching, polydopamine (PDA) coating, and a pyrolysis process. The heterostructure interface was demonstrated by HRTEM. XPS analysis indicated the existence of abundant covalent S–O bonds in ZnO/ZnS@NC, which might induce a built-in electric field in the heterostructure of the oxide and sulfide. The N-doped carbon coating not only suppresses zinc oxide aggregation but also buffers volume expansion during the sodiation/desodiation process. Consequently, the ZnO/ZnS@NC heterostructures exhibit improved Na+ storage capacity compared with that of ZnO nanorods. Combined with in situ XRD, the conversion-alloy mechanism of the ZnO/ZnS@NC heterostructure was disclosed. As anodes for SIBs, the hollow ZnO/ZnS@NC heterostructure exhibits better electrochemical performance than most reported zinc-oxide-based anodes for SIBs. The in situ disclosed reaction provides insights into the electrochemical sodiation mechanism of metal oxides and sulfides for sodium ion batteries.