Fluorine-engineered high-entropy layered oxyfluorides enable ultrafast and stable sodium storage: Synergistic stabilization and kinetics enhancement

Abstract

The development of high-performance cathodes remains pivotal for advancing sodium-ion batteries (SIBs), yet conventional O3-type layered oxides suffer from irreversible phase transitions, sluggish Na⁺ kinetics, and lattice oxygen loss. While high-entropy oxides (HEOs) mitigate structural degradation through cationic disorder, restricted interlayer spacing and unaddressed anion sublattice instability persist. Herein, we propose a fluorine-mediated multi-sublattice engineering strategy to design an O3-type high-entropy oxyfluoride (HEOF) cathode, Na_0.83Li_0.1Ni_0.25Co_0.2Mn_0.3Ti_0.15O_2-x/2F_x, synergizing cationic entropy stabilization with fluorine substitution to establish multi-sublattice engineering. Fluorine's elevated anionic potential (Φ_anion) expands interlayer spacing (d_O-Na-O) through intensified electrostatic repulsion while strengthening transition metal-oxygen (TM-O) bonds, as confirmed by structural and spectroscopic analyses. The optimized HEOF-002 achieves a reversible capacity of 168.9 mAh g⁻¹ at 0.1C, 91.2 % capacity retention over 50 cycles, and exceptional rate capability (58 % capacity retention at 5C). Mechanistic studies reveal fluorine's triple roles: (1) enabling rapid Na⁺ diffusion via widened channels, (2) suppressing oxygen redox degradation through reinforced TM-O interactions, and (3) redistributing lattice strain via entropy-fluorine synergy. In situ impedance analysis further demonstrates fast interfacial kinetics with minimal charge transfer and Na⁺ diffusion resistance. This work establishes a universal paradigm for anion-regulated high-entropy design, advancing the development of high-energy and durable energy storage systems.