Controllable Electrochemical Synthesis of Copper Sulfides as Sodium-Ion Battery Anodes with Superior Rate Capability and Ultralong Cycle Life

Sodium-ion batteries (SIBs) are prospective alternative to lithium-ion batteries for large-scale energy-storage applications, owing to the abundant resources of sodium. Metal sulfides are deemed to be promising anode materials for SIBs due to their low-cost and eco-friendliness. Herein, for the first time, series of copper sulfides (Cu₂S, Cu₇S₄, and Cu₇KS₄) are controllably synthesized via a facile electrochemical route in KCl-NaCl-Na₂S molten salts. The as-prepared Cu₂S with micron-sized flakes structure is first investigated as anode of SIBs, which delivers a capacity of 430 mAh g^-1 with a high initial Coulombic efficiency of 84.9% at a current density of 100 mA g^-1. Moreover, the Cu₂S anode demonstrates superior capability (337 mAh g^-1 at 20 A g^-1, corresponding to 50 C) and ultralong cycle performance (88.2% of capacity retention after 5000 cycles at 5 A g^-1, corresponding to 0.0024% of fade rate per cycle). Meanwhile, the pseudocapacitance contribution and robust porous structure in situ formed during cycling endow the Cu₂S anodes with outstanding rate capability and enhanced cyclic performance, which are revealed by kinetics analysis and ex situ characterization.