Two-dimensional medium-entropy SeSI composite cathodes with cascaded redox reactions for advanced sodium batteries

Na−S/Se/I2 batteries represent a promising rechargeable Na battery system because of high theoretical energy density and low raw material cost. However, the inherent electrical insulation and poor structural stability of S/Se/I2 cathodes result in the unsatisfactory battery performance. Here we develop a new medium-entropy Se-S-I composite cathode via a straight chemical reaction of 2xSe+yI2+CS2=2SexSIy+C, simultaneously consolidating a ternary Se-S-I compound within carbon nanocage, forming a SeSI@C core-shell structure. In the composite, SeSI has a distinctive single crystal structure with homogeneous elemental composition, while carbon shell shows a uniform annular nanoscale that can completely envelop SeSI active mass. The cathode structural design bears high electrical conductivity, fast synergistic activation and excellent structural integrity, accordingly enabling a high capacity with an ultralong cycle life in a representative Na−Se-S-I battery (425.4 mAh g−1 at 10.0 A g−1, 91.6% retention for 2000 cycles). Notably, a tertiary cascade of Se, S, and I redox has been demonstrated for providing high capacity, and an in-situ grown carbon shell ensures electrical conductivity and structural stability through the entire cathode. This SeSI@C cathode architecture significantly alleviates issues of traditional S/Se/I2-based cathodes, and makes the Na−Se-S-I battery more promising for application.