Unlocking Lewis‐Acid Catalysis and Crystalline Polyselenide Evolution for Ultra‐Stable Sodium‐Ion Batteries

Metal selenides (MSes) are promising anodes for sustainable sodium‐ion batteries (SIBs), but their practical application is fundamentally hindered by sluggish kinetics, severe sodium‐polyselenide (Na x Se y ) dissolution, and structural degradation. Herein, we uncover that the poor cycling reversibility of SnSe 2 originates from localized electron distribution and high energy barriers, which hinder complete conversion during cycling. We further demonstrate that the in‐situ generated Sn intermediates function as stage‐selective catalysts, preferentially promoting the conversion of Na 2 Se 6 into soluble Na 2 Se 4 , leading to the accumulation of shuttle‐active intermediates and rapid capacity decay. Guided by theoretical calculations, a bimetallic selenide composite (Cu 2 SnSe 4 @NC) was rationally designed, where copper incorporation delocalizes electrons and weakens Cu─Se bonding, thereby accelerating the initial conversion reaction. Crucially, the in situ generated Cu/Sn heterostructure enables Lewis‐acid‐regulated and stepwise crystalline evolution of Na x Se y from Na 2 Se 6 to the final Na 2 Se, thereby substantially suppressing the solvation and shuttling of soluble intermediates. Consequently, the Cu 2 SnSe 4 @NC electrode achieves excellent cycling stability, retaining 95% of its capacity after 7000 cycles at 5.0 A g −1 in half‐cells and sustaining over 5000 cycles at 1.0 A g −1 in full cells. This work establishes a new design paradigm for fabricating ultra‐long lifespan MSes anodes toward scalable SIBs.