Hollow cubic MnS-CoS₂-NC@NC designed by two kinds of nitrogen-doped carbon strategy for sodium ion batteries with ultraordinary rate and cycling performance

In order to obtain an advanced anode material that exhibits excellent electrochemical performance in sodium ion batteries, a hollow nanocube MnS-CoS₂-NC@NC (NC = nitrogen-doped carbon) with two kinds of nitrogen-doped carbon was synthesized by simple one-step calcination. One of the two kinds of nitrogen-doped carbon comes from the organic ligands in the precursor being mixed in the sulfide after calcination, and the other comes from the calcination of the coated polydopamine (PDA) to form a carbon shell wrapped outside the sulfide. The characteristic nanostructure with two kinds of nitrogen-doped carbon can not only improve the overall conductivity of the electrode material, which is obviously beneficial to the rapid transmission of sodium ions and thus outstanding rate performance, but also can alleviate volume expansion to maintain battery cycle stability. In the half-cell, the MnS-CoS₂-NC@NC electrode not only provides an ultra-high specific capacity of 608.5 mA·h·g⁻¹ at 0.2 A·g⁻¹ for 100 cycles, but also shows an outstanding rate performance of 560.5 mA·h·g⁻¹ at 5.0 A·g⁻¹ for 1,100 cycles. Even in a full-cell composed with Na₃V₂(PO₄)₃ as the positive material, it can still maintain a capacity of 436.7 mA·h·g⁻¹ after 900 cycles at 1.0 A·g⁻¹. In order to explore its sodium storage mechanism, in-situ and ex-situ X-ray diffraction (XRD) tests were carried out to prove that CoS₂ and MnS were reduced to produce metallic Co and metallic Mn during the discharging process, respectively, and reversibly returned during the charging process.