Electrostatic Self‐Assembly of CoSe 2 HBs/Ti 3 C 2 T x Composites for Long‐cycle‐life Sodium Ion Batteries

Abstract Sodium ion batteries (SIBs) are considered as one of the most promising energy storage devices owing to lower cost and higher energy/power density. However, limited cycling life and poor rate capability of selectable materials restrict their worldwide application. To regulate the material's structure at the nanoscale, reducing volume expansion and increasing the intrinsic conductivity are two positive strategies to deal with these issues. Herein, CoSe 2 Hollow Boxes/Ti 3 C 2 T x flakes composites (CoSe 2 HBs/Ti 3 C 2 T x ) is designed and fabricated based on electrostatic self‐assembly. Benefitting from the structural and compositional advantages, synergistic effects between the high conductivity and large layer spacing structure of Ti 3 C 2 T x and the high capacity CoSe 2 is observed. The CoSe 2 HBs/Ti 3 C 2 T x composites exhibit excellent rate capability (Average specific capacities of CoSe 2 HBs/Ti 3 C 2 T x are 225.8, 210.5, 194.6, 169.9, 153.2, and 135.8 mAh g −1 at current densities of 50, 100, 200, 500, 1000 and 2000 mA g −1 , respectively, and it could rapidly assume a reversible capacity when the current density changed back to small value, 199.1 mAh g −1 at 100 mA g −1 ), exceptional cycling life (3000 cycles), and ∼100 % high coulombic efficiency. Such composites can be considered as another promising candidate for high performance energy storage devices.