Hollow sphere of heterojunction (NiCu)S/NC as advanced anode for sodium-ion battery

Metal sulfide is considered as a potential anode for sodium-ion batteries (SIBs), due to the high theoretical capacity, strong thermodynamic stability and low-cost. However, their cycle capacity and rate performance are limited by the excessive expansion rate and low intrinsic conductivity. Herein, heterogeneous hollow sphere NiS-Cu9S5/NC (labeled as (NiCu)S/NC) based on Oswald ripening mechanism was prepared through a simple and feasible methodology. From a structural perspective, the hollow structure provides an expansion buffer and raises the electrochemical active area. In terms of electron/ion during the cycles, Na+ storage mechanism is optimized by NiS/Cu9S5 heterogeneous interface, which increases the storage sites and shortens the migration path of Na+. The formation of built-in electric field strengthens the electron/ion mobility. Based on the first principle calculations, it is further proved the formation of heterogeneous interfaces and the direction of electron flow. As the anode for SIBs, the synthesized (NiCu)S/NC delivers high reverse capacity (559.2 mA h g−1 at 0.5 A g−1), outstanding rate performance (185.3 mA h g−1 at 15 A g−1), long-durable stability (342.6 mA h g−1 at 4 A g−1 after 1500 cycles, 150.0 mA h g−1 at 10 A g−1 after 20,000 cycles with 0.0025% average attenuation rate). The matching cathode electrode Na3V2(PO4)3/C is assembled with (NiCu)S/NC for the full-battery that achieves high energy density (253.7 W h kg−1) and reverse capacity (288.7 mA h g−1). The present work provides a distinctive strategy for constructing electrodes with excellent capacity and stability for SIBs.