Highly stable cobalt-doped FeSe₂ anodes for unexpectedly fast sodium storage enabled by doping and structure engineering

Cobalt and iron selenides-based materials with high theoretical capacities, low toxicity and abundant sources have been identified as the promising anode materials for sodium-ion batteries (SIBs). However, they still face the challenges of high volume expansion and slow electrode kinetics, resulting in poor rate performance and fast capacity fading. In this paper, three-dimensional honeycomb-like Co-Fe based selenide composites with different molar ratios are successfully synthesized by one-pot solvothermal, annealing and selenization processes (expressed as Co-FeSe₂@C/CNs-fbs, Co-FeSe₂@C/CNs-irs and Co-FeSe₂@C/CNs-sbs). Benefitted from the design of three-dimensional porous compositing structure, the optimized Co-FeSe₂@C/CNs-fbs electrode material possesses more active sites and structural stability, resulting in stable cycling performance and fast electron/ion transport. As a result, Co-FeSe₂@C/CNs-fbs anode shows excellent rate capability (353.1 mAh g⁻¹ at 120 A g⁻¹) and long cycling performance (95.7% of capacity retention after 3700 cycles at 60 A g⁻¹), surpassing most previously reported anode materials for SIBs. Meanwhile, a full-cell made up with Na₃V₂(PO₄)₃/C cathode and Co-FeSe₂@C/CNs-fbs anode shows a high energy density (180.1 Wh kg⁻¹ at a power density 630.5 W kg⁻¹) and capacity retention rate. This study provides a feasible strategy to fabricate selenide-based composites as the anode materials for high-performance SIBs via doping and structure engineering.