Suppressing Solvent Co‐Intercalation for Fast and Durable Sodium Storage in Layered Transition Metal Sulfide Cathodes

ABSTRACT Titanium disulfide (TiS 2 ) is a promising layered cathode for sodium‐ion batteries (SIBs), but suffers from rapid capacity fading, which was often ascribed to multi‐step phase transitions. Herein, we demonstrated that solvent co‑intercalation that typically occurs at low Na + content plays a more decisive role in driving structural degradation by promoting lattice distortion and triggering layer delamination. By replacing the conventional solvent propylene carbonate with ethyl methyl carbonate (EMC), which is a linear carbonate with a lower dielectric constant and larger kinetic diameter, solvent uptake is significantly suppressed. Further improvement is achieved by introducing fluoroethylene carbonate (FEC) and increasing NaFSI concentration to 3 m in an EMC/FEC (9:1 v/v) blend. This formulation enables the formation of a uniform, thin cathode‐electrolyte interphase, which is robust and rich in NaF that facilitates desolvation of Na + and further limits solvent co‐intercalation. Consequently, the TiS 2 cathode delivers exceptional long‐term cycling stability (88.9% capacity retention after 1200 cycles at 2000 mA g −1 ) and superior rate capability (75% capacity retention from 20 to 2000 mA g −1 ). Since solvent co‐intercalation is a common issue in layered hosts during Na + intercalation, the insights and electrolyte design strategy presented here provide a generalizable approach to developing high‐performance, durable cathodes for SIBs.