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_xSe_y) dissolution, and structural degradation. Herein, we uncover that the poor cycling reversibility of SnSe₂ 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 Na₂Se₆ into soluble Na₂Se₄, leading to the accumulation of shuttle-active intermediates and rapid capacity decay. Guided by theoretical calculations, a bimetallic selenide composite (Cu₂SnSe₄@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_xSe_y from Na₂Se₆ to the final Na₂Se, thereby substantially suppressing the solvation and shuttling of soluble intermediates. Consequently, the Cu₂SnSe₄@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.