A facile synthesis of CuSe nanosheets for high-performance sodium-ion hybrid capacitors.

Due to the low price and abundant reserves of sodium resources, sodium-ion batteries have become the main candidate for the next generation of energy storage equipment, particularly for large-scale grid storage and low-speed electric vehicles. Transition metal selenides have attracted considerable attention because of their high reversible capacity, superior electrical conductivity and versatile structures. In this study, two-dimensional CuSe nanosheets are synthesized via a simple hydrothermal reaction. When acting as an electrode material for sodium-ion batteries, the CuSe electrode exhibits an initial coulombic efficiency of 96.7% at a current density of 0.1 A g -1 and a specific capacity of 330 mA h g -1 after 100 operation cycles, as well as retains a specific capacity of 211 mA h g -1 even at a high current density of 10 A g -1 . Moreover, the anode delivers a specific capacity of 236 mA h g -1 after 3300 cycles at 5 A g -1 with a capacity retention of 91.2%. In sodium-ion hybrid capacitors (SHICs) with the two-dimensional CuSe nanosheets and Ti 3 C 2 T x MXene as the negative and positive materials, respectively, the nanosheets without any pre-sodiation present a lifespan of up to 2000 cycles at 2 A g -1 and a capacity retention of about 77.7%.