Abstract: Transition metal sulfides have been regarded as promising anode materials for sodium-ion batteries (SIB). However, they face the challenges of poor electronic conductivity and large volume change, which result in capacity fade and low rate capability. In this work, a composite containing ultrasmall CoS (~7 nm) nanoparticles embedded in heteroatom (N, S, and O)-doped carbon was synthesized by an efficient one-step sulfidation process using a Co(Salen) precursor. The ultrasmall CoS nanoparticles are beneficial for mechanical stability and shortening Na — ions diffusion pathways. Furthermore, the N, S, and O — doped defect-rich carbon provides a robust and highly conductive framework enriched with active sites for sodium storage as well as mitigates volume expansion and polysulfide shuttle. As anode for SIB, CoS@HDC exhibits a high initial capacity of 906 mA h g^-1 at 100 mA g^-1 and a stable long-term cycling life with over 1000 cycles at 500 mA g^-1, showing a reversible capacity of 330 mA h g^-1. Meanwhile, the CoS@HDC anode is proven to maintain its structural integrity and compositional reversibility during cycling. Furthermore, Na — ion full batteries based on the CoS@HDC anode and Na₃V₂(PO₄)₃ cathode demonstrate a stable cycling behavior with a reversible specific capacity of ~ 200 mA h g^-1 at least for 100 cycles. Moreover, advanced synchrotron operando X-ray diffraction, ex-situ X-ray absorption spectroscopy, and comprehensive electrochemical tests reveal the structural transformation and the Co coordination chemistry evolution of the CoS@HDC during cycling, providing fundamental insights into the sodium storage mechanism.

Key words: Sodium-ion batteries, Cobalt sulfide nanoparticles, Heteroatom-doped porous carbon matrix, Synchrotron X-ray techniques, Reaction mechanisms