Frontier Orbital-Regulated Bifunctional Fluorinated Electrolyte for High-Voltage Sodium-Ion Batteries: Balancing Weak...

Developing electrolytes that can stably operate at high voltages is a pivotal challenge in enhancing the energy density of sodium-ion batteries (SIBs). This paper proposes a rational design strategy based on density functional theory calculations of the frontier molecular orbitals and electrostatic potential. Specifically, the highest occupied molecular orbital level of solvent molecules is regulated by introducing strongly electron-withdrawing fluorine atoms, producing a bifunctional electrolyte with weakened solvation and high oxidation stability. The weak coordination characteristics of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFETFE) are combined with the interface film-forming ability of fluorinated ethylene carbonate (FEC), synergistically optimizing the solvation structure of Na⁺ and the electrode/electrolyte interface. Theoretical calculations and spectroscopic analysis show that the weak coordination solvent allows more PF₆ ^- to enter the primary solvation sheath of Na⁺, forming a structure rich in ionic aggregates, thus accelerating Na⁺ transport. Meanwhile, the formation of a stable and inorganic-rich interface film on the cathode surface effectively inhibits oxidative decomposition at high voltages. Consequently, the cathode exhibits high long-cycle stability at a high cut-off voltage of 4.3 V (vs. Na⁺/ Na), with a capacity retention rate of 87.6% after 2000 cycles at room temperature. This work provides a novel paradigm for designing advanced electrolytes suitable for high-voltage/high-energy-density SIBs.