MoS2-encapsulated CuFeS2/expanded graphite ternary sulfide with built-in electric field for high performance sodium-ion battery anodes

Transition metal sulfides (TMSs) represent a promising class of anode materials for sodium-ion batteries (SIBs) due to their high theoretical capacity. However, their practical implementation is hindered by the severe volume change, intrinsically poor electrical conductivity and polysulfide shuttle effect. Herein, a ternary sulfide composite, MoS 2 -encapsulated CuFeS 2 /expanded graphite (denoted as FeCuMo(1:1:1)S), is designed and synthesized via a facile molten-solid phase sulfidation strategy. In this architecture, MoS 2 nanosheets uniformly encapsulate CuFeS 2 particles, forming a heterostructure that generates a built-in electric field to accelerate electron transport kinetics. Furthermore, MoS 2 functions dually as a chemical anchor to immobilize polysulfide intermediates and as a structural buffer to mitigate volume fluctuations during cycling. The three-dimensional flocculent morphology of MoS 2 additionally exposes abundant active sites, facilitating rapid Na + adsorption and diffusion. Benefiting from these synergistic effects, the composite exhibits exceptional electrochemical performance, delivering a high initial discharge capacity of 511.6 mAh g −1 and retaining 490.7 mAh g −1 after 1800 cycles at a high current density of 5 A g −1 (95.9% retention). When coupled with a Na 3 V 2 (PO 4 ) 3 cathode, the full cell achieves 102.6 mAh g −1 at 50 mA g −1 . This study offers a viable and effective strategy for enhancing the stability and longevity of TMS-based anodes in SIBs.