Heterojunction Vacancies‐Promoted High Sodium Storage Capacity and Fast Reaction Kinetics of the Anodes for Ultra‐High Performance Sodium‐Ion Batteries

Abstract Transition metal sulfides as anode materials for sodium‐ion batteries (SIBs) have the advantage of high capacity. However, their cycle‐life and rate performance at ultra‐high current density is still a thorny issue that limit the applicability of these materials. In this paper, the carbon‐embedded heterojunction with sulfur‐vacancies regulated by ultrafine bimetallic sulfides (vacancy‐CoS 2 /FeS 2 @C) with robust interfacial C‐S‐Co/Fe chemical bonds is successfully synthesized and explored as an anode material for sodium‐ion battery. By changing the ratio of two metal cations, the concentration of anion sulfur vacancies can be in‐situ adjusted without additional post‐treatment. The as‐prepared vacancy‐CoS 2 /FeS 2 @C anode material offers ultrahigh rate performance (285.1 mAh g −1 at 200 A g −1 ), and excellent long‐cycle stability (389.2 mAh g −1 at 40 A g −1 after 10000 cycles), outperforming all reported transition metal sulfides‐based anode materials for SIBs. Both in‐situ and ex‐situ characterizations provide strong evidence for the evolution mechanism of the phases and stable solid‐electrolyte interface (SEI) on the vacancy‐CoS 2 /FeS 2 @C surface. The density functional theory calculations show that constructing heterojunction with reasonable concentration of vacancies can significantly increase the anode electronic conductivity. Notably, the assembled vacancy‐CoS 2 /FeS 2 @C//Na 3 V 2 (PO 4 ) 3 /C full‐cell shows a capacity of 226.2 mAh g −1 after 400 cycles at 2.0 A g −1 , confirming this material's practicability.