Salt Template‐Mediating Synthesis of In Situ Carbon‐Encapsulated FeSe ₂ /Fe ₂ VO ₄ Heterostructure Nanosheets for Durable and Ultrafast Sodium Storage

Abstract Currently, heterostructure engineering is considered an optimization strategy to enhance anode performance for sodium‐ion batteries. However, ferric vanadate‐based heterostructures are rarely reported due to their rapid nucleation. Furthermore, it is a huge challenge to synthesize carbon‐coated heterostructure materials through an efficient strategy. Herein, a strategy is proposed for constructing in situ carbon‐encapsulated metal selenide/ferric vanadate heterostructure (FeSe ₂ /Fe ₂ VO ₄ @C) nanosheets by a selenization‐assisted salt template method for the first time. Density functional theory calculations reveal that a built‐in electric field at the heterointerface accelerates electron transfer and induces d‐electron delocalization in V atoms, increasing the electronic conductivity. Additionally, the confinement effect of the salt template mediates in situ carbon encapsulation and 2D nanostructure formation, alleviating the volume expansion and ensuring structural stability. The introduction of FeSe ₂ generates more active sites and accelerates reaction kinetics. Notably, the as‐prepared FeSe ₂ /Fe ₂ VO ₄ @C‐based anode demonstrates outstanding rate performance, delivering capacities of 423 mAh g ⁻¹ at 0.5 A g ⁻¹ , 228.1 mAh g ⁻¹ at 100 A g ⁻¹ , and excellent long‐term stability with a capacity of 219.9 mAh g ⁻¹ at 40 A g ⁻¹ after 5000 cycles. This universal strategy for constructing ferric vanadate‐based heterostructures and in situ carbon coating offers a promising pathway for secondary battery systems.