Metal–organic framework derived copper chalcogenides-carbon composites as high-rate and stable storage materials for Na ions

Transition metal chalcogenides have been regarded as promising storage materials for sodium ions owing to their high theoretical capacity. Herein, copper-based metal–organic frameworks (Cu-BTC) are reported as precursors to fabrica copper chalcogenides-carbon composites, namely Cu1.8S@C and Cu2-xSe@C. The materials exhibit excellent electrochemical performance with high specific capacities (504 mAh g–1 for Cu1.8S@C and 317 mAh g–1 for Cu2-xSe@C at 0.1 A g–1) and long-term cycling stability when used as anode materials in cells employing carbon-coated Na3V2(PO4)3 (NVP/C) positive electrodes. The Cu2-xSe@C||NVP/C cell delivers a specific capacity of 73 mAh g–1 at 1.2 A g–1 (based on cathode mass) and excellent cycling stability (capacity retention of 85% after 500 cycles at 0.12 A g–1) with Coulombic efficiency of ≈99.9%. Moreover, the Cu2-xSe@C composite performs well as positive electrode storage material in a sodium-metal cell, offering a high reversible capacity of 216 mAh (per gram of Cu2-xSe@C) after 1800 cycles at 2 A g–1 and enabling high specific energy and power.