Rational design of a Fe1−xS/FeS2 homologous heterostructure with exceptional high-rate performance for advanced sodium-ion battery anodes

Abstract

A series of iron sulfides with high theoretical sodium storage capacities can be used as ideal anodes for sodium-ion batteries (SIBs). However, weak electronic conductivity and severe capacity degradation restrict their application. Based on density functional theory (DFT) calculation results, a built-in electric field (BIEF) can develop at the Fe_1−xS/FeS₂ heterogeneous interface, resulting in enhanced electronic conductivity and a lower Na⁺ diffusion barrier compared to Fe_1−xS or FeS₂. Using information from DFT calculations, a composite of Fe_1−xS/FeS₂ with a heterostructure was designed and prepared through a facile assembly-sulfurization strategy. In contrast to FeS₂ and Fe_1−xS, the optimized Fe_1−xS/FeS₂ electrode demonstrates superior sodium storage performance, excellent cycling stability (494.6 mA h g⁻¹ after 2500 cycles at 2 A g⁻¹) and rate-performance (444.1 mA h g⁻¹ at 5.0 A g⁻¹). Furthermore, at 1C, a full cell combining a NaFePO₄ cathode and an Fe_1−xS/FeS₂ anode exhibits exceptional capacity retention after 400 cycles (with only 0.036% decay per cycle). The Fe_1−xS/FeS₂ proposed in this work can improve the electron/Na⁺ transport rate and mitigate the volume expansion in the electrochemical process. This work provides new insights into designing and preparing transition metal sulfides with heterostructures for sustainable SIBs.