Unlocking Exceptionally Fast and Large‐Capacity Na ⁺ Storage of Fe ₂ SSe via Coupling Multicore‐in‐Multishell Design and Vacancy Engineering

Abstract Achieving simultaneous high‐rate capability and large reversible capacity remains a core challenge for sodium‐ion battery (SIB) anodes. Herein, the rational design and synthesis of a multicore Fe ₂ SSe architecture is reported, enriched with anion vacancies and confined within a multishell carbon matrix (V‐Fe ₂ SSe@MC), to address this challenge. This tailored structure effectively accommodates volume fluctuations, maintains robust electrode‐electrolyte interfaces, and significantly accelerates Na ⁺ and electron transport. The dense concentration of selenium/sulfur vacancies not only boosts intrinsic electrical conductivity but also generates abundant active sites while lowering the energy barrier for Na ⁺ diffusion. Most critically, the coupling of the multicore‐in‐multishell architecture with the induced anion vacancies produces a synergistic effect that alleviates the conventional trade‐off between storage capacity and rate capability. As a result, the V‐Fe ₂ SSe@MC anode unlocks exceptionally fast and large‐capacity Na ⁺ storage, delivering a high specific capacity of 505 mAh g ⁻¹ even at 40C and maintaining 97% capacity over 3000 cycles at 20C. Comprehensive in situ characterizations elucidate the reversible cleavage and regeneration of Fe─S/Se bonds and excellent structural integrity during cycling. This work provides compelling insight into the rational design of high‐performance SIB anodes via the integrated approach combining multicore‐in‐multishell structural design with vacancy engineering.