Enabling Reversible Reaction by Uniform Distribution of Heterogeneous Intermediates on Defect‐Rich SnSSe/C Layered Heterostructure for Ultralong‐Cycling Sodium Storage

2D layered Sn-based materials have attracted enormous attention due to their remarkable performance in sodium-ion batteries. Nevertheless, this promising candidate involves a complex Na+ -storage process with multistep conversion-alloying reactions, which induces the uneven dispersion of heterogeneous intermediate accompanied by severe agglomeration of metallic Sn0 , inescapably resulting in poor reaction reversibility with sluggish rate capability and inferior cyclic lifespan. Herein, a delicately layered heterostructure SnSSe/C consisting of defect-rich SnSSe and graphene is designed and successfully achieved via a facile hydrothermal process. The equal anionic substitution of Se in SnSSe crystal can trigger numerous defects, which can not only facilitate Na+ diffusion but also accelerate the nucleation process by inducing quantum-dot-level uniform distribution of heterogeneous intermediates, Na2 Se/Na2 S and Sn0 . Concurrently, in situ formed uniform Na2 Se/Na2 S grain boundaries confined by this unique layered heterostructure may effectively suppress the agglomeration of metallic Sn0 nanograins and boost the reversibility of conversion-alloying reaction. As a result, the SnSSe/C displays significant improvement in Na-storage performance, in terms of remarkable rate capability and ultralong cycling lifespan. This work, focusing on controlling intermediate distribution, provides an effective strategy to boost reaction reversibility, which can be wildly employed in conversion-based electrodes for energy storage regions.