Facile synthesis and electrochemical sodium storage of CoS₂ micro/nano-structures

We report the synthesis and electrochemical sodium storage of cobalt disulfide (CoS₂) with various micro/nano-structures. CoS₂ with microscale sizes are either assembled by nanoparticles (P-CoS₂) via a facile solvothermal route or nanooctahedrons constructed solid (O-CoS₂) and hollow microstructures (H-CoS₂) fabricated by hydrothermal methods. Among three morphologies, H-CoS₂ exhibits the largest discharge capacities and best rate performance as anode of sodium-ion batteries (SIBs). Furthermore, H-CoS₂ delivers a capacity of 690 mA·h·g^-1 at 1 A·g^-1 after 100 cycles in a potential range of 0.1–3.0 V, and ~240 mA·h·g^-1 over 800 cycles in the potential window of 1.0–3.0 V. This cycling difference mainly lies in the two discharge plateaus observed in 0.1–3.0 V and one discharge plateau in 1.0–3.0 V. To interpret the reactions, X-ray diffraction (XRD) and transmission electron microscopy (TEM) are applied. The results show that at the first plateau around 1.4 V, the insertion reaction (CoS₂ + xNa⁺ + xe^- → Na_xCoS₂) occurs; while at the second plateau around 0.6 V, the conversion reaction (Na_xCoS₂ + (4 - x) Na⁺ + (4 - x)e^- → Co + 2Na₂S) takes place. This provides insights for electrochemical sodium storage of CoS₂ as the anode of SIBs.