Encapsulating Sulfides into Tridymite/Carbon Reactors Enables Stable Sodium Ion Conversion/Alloying Anode with High Initial Coulombic Efficiency Over 89%

Abstract Electrode dissolution/collapses and interfacial reactions pose challenges to batteries, leading to pronounced capacity loss particularly during the initial few cycles. As high‐capacity conversion/alloying anodes for sodium storage, metal sulfides generally show unsatisfactory performances like poor initial Coulombic efficiency (ICE; mostly <70% in the usual electrolyte) and inferior cyclic stability due to thick solid‐electrolyte interface (SEI) layer formation and ubiquitous volume/phase changes. Using SnS 2 as an example, here, sulfides are elaborately encapsulated into functionalized amorphous tridymite/carbon reactors to address the above issues. The outer tridymite/carbon manifests good ionic permeability and superb electrochemical/mechanical tolerance against destructive Na + insertion. Confining actives into tailored reactors endows SnS 2 full of nanoboundaries with an ultrahigh ICE of ≈89.13% and remarkable electrochemical attributes including large initial capacity (Max. 733.24 mAh g −1 ), prominent stability in subsequent cycles, and excellent rate capability. Detailed investigation unveils that thin and steady SEI condition on tridymite/carbon rather than SnS 2 is key to achieving outstanding ICE. Engineered reactors always keep intact and free of valence‐state changes, guaranteeing capacities running at a high level without an evident downtrend. Their peculiar functions on enlisting Na + diffusion/transport and inhibiting sulfides’ release are also discussed. Packed full‐cell Na‐ion batteries with less irreversibility may show great potential in practical utilizations.