Built-in ElectricField Formed In Situ and SulfurVacancy Synergistically Promoting Copper Sulfide Anode Efficient SodiumStorage

Copper sulfides with abundant redox active sites are hailed as a great promising anode material for sodium-ion batteries. However, sluggish reaction kinetics, poor conductivity, and structural stability affect their electrochemical performance. Here, in situ formation of built-in electric field and vacancy strategies were adapted to synergistically promote copper sulfide anode material efficient sodium storage. First, S/CuS_1–<i>x</i>@PANI with S vacancies was synthesized, and then Cu₂S/CuS_1–<i>x</i> heterogeneous interface was formed in situ during the discharge–charge process. The experimental and theoretical calculation results indicate that sulfur vacancies provide additional reaction sites for storage of Na⁺ ions, and accelerate the transfer of Na⁺ ions. The built-in electric field from the Cu₂S/CuS_1–<i>x</i> heterogeneous interface can effectively improve its stability and electronic conductivity. Polyaniline (PANI) serves as a strong protective layer to effectively ease the volume effects and boost conductivity during the cycle. The anode material shows excellent rate performance (493 mA h g^–1 at 15 A g^–1) and long cycle stability (620.5 mA h g^–1 with a high-capacity preservation ratio of 98.4% after 3000 cycles at 5 A g^–1), which exceeds the theoretical specific capacity of CuS and Cu₂S. Our study proposes a promising idea for developing sustainable and high-performance sodium-ion battery anode materials.