In situ confined growth of Cu2-xSe nanoparticles in highly defective nitrogen-doped carbon for high-rate sodium-ion battery anodes

Transition metal selenides are considered as hopeful anode materials for sodium-ion battery (SIB) due to the high specific capacity and rich reserves in nature. However, the inherent low conductivity of these anode materials, as well as the inevitable volume expansion and structural collapse during cycling, result in their poor rate performance and cycling stability. Herein, a universal selenization-co-carbonization strategy is proposed to synthesize Cu2-xSe nanoparticles in-situ confined in highly defective nitrogen-doped carbon (Cu2-xSe@NC) with superior architectural features, which involves simultaneous selenization and carbonization procedure of octahedral metal–organic framework (Cu-BTC) precursor. The nitrogen-doped carbon matrix stabilizes/confines the Cu2-xSe nanoparticles to prevent their agglomeration and volume expansion during cycling while endowing high electrical conductivity to the hybrid materials. As anode for SIBs, the Cu2-xSe@NC exhibits a high reversible capacity of 340 mAh/g at 0.1 A/g with a high initial coulombic efficiency of 70 %, superior rate performance (capacity retention of 82.4 % from 0.02 to 5 A/g), and long-cycle durability. The ex-situ XRD and HRTEM techniques confirmed that Cu2-xSe undergoes complex intercalation and crystal evolution accompanied by reversible intermediate phases (NaCuSe and Na2Se) transformation during charge/discharge cycle. First-principles calculations and in-situ EIS confirmed that the intercalation of Na+ can greatly reducing the energy band gap and improving the electrical conductivity of Cu2-xSe@NC, thereby achieving excellent rate performance. Moreover, the Cu2-xSe@NC anode is also evaluated by matching with Na3V2(PO4)3 cathode to assemble a full SIB, which can provide high initial capacity and capacity retention, revealing its potential for practical application.