Cut-off-voltage-controlled phase transformations in hollow Cu_2-xSe cube enables high-rate and long-life sodium-ion batteries

Cu_2-xSe is a promising sodium-ion battery anode material, but its sodium storage performance is severely limited by significant volume expansion and irreversible phase transition during the charge-discharge process. Herein, a novel flower-like Cu_1.8Se/Cu₃Se₂ cube with hollow structure was fabricated by a template selenization method. The hollow architecture offers sufficient internal space to buffer volume variation of Cu_1.8Se/Cu₃Se₂ during cycling, thereby effectively suppressing structural pulverization and electrode delamination. Furthermore, the sheet-like structure significantly increases the specific surface area of the electrode, facilitating electrode-electrolyte interaction and thereby enhancing the rate capability. Additionally, optimizing the discharge cut-off voltage effectively suppresses the irreversible phase transition of Cu_1.8Se/Cu₃Se₂. Specifically, when the discharge cut-off voltage was maintained at 0.2 V, Cu_1.8Se/Cu₃Se₂ delivered a high specific capacity of 228.2 mAh g⁻¹ after 2000 cycles at a high current density of 4 A g⁻¹, corresponding to an ultralow capacity fading rate of only 0.05 % per cycle. Overall, this work presents a promising strategy for enhancing the structural stability and phase reversibility of Cu_1.8Se/Cu₃Se₂ electrodes, thereby achieving high-performance Na⁺ storage.