Reconstructing Solvation Chemistry via Cosolvent Regulation for Interfacial Engineering Toward Stable High‐Voltage O3‐Type Cathodes

ABSTRACT O3‐type layered oxides have emerged as promising cathode materials for sodium‐ion batteries (SIBs) due to their high reversible capacity. However, their fast‐charging and long‐term cyclability under high‐voltage operation are limited by interfacial and structural degradation, such as electrolyte decomposition, oxygen release, transition‐metal dissolution, and irreversible phase transitions. Herein, a multifunctional co‐solvent of dimethyl difluoromalonate (DMDF) is introduced into carbonate electrolytes to stabilize O3‐type Na 0.85 Mn 0.45 Ni 0.25 Li 0.05 Cu 0.1 Ti 0.15 O 2 (NMNLCTO) cathode within a voltage range of 1.5–4.5 V. In/ex situ characterizations and theoretical calculations reveal that the DMDF facilitates the formation of a robust NaF‐rich cathode‐electrolyte interface, suppresses electrolyte decomposition, and regulates solvation structure for fast Na + transport. The formulated NaClO 4 /carbonate‐DMDF electrolyte endows the NMNLCTO cathode with record‐breaking cycling stability, achieving 80% capacity retention over 600 cycles at 500 mA g − 1 . The assembled hard carbon//NMNLCTO full cells deliver high energy densities of 304.5 and 156.4 Wh kg − 1 (based on the mass of the cathode and anode) at 20 and 800 mA g − 1 , respectively. This work indicates that dimethyl difluoromalonate acts as an efficient co‐solvent to upgrade O3‐type SIB cathodes under high‐voltage operation.