Ionic-Potential-Guided Fluoride Engineering of Reversible Mn-Based Cathodes for Sodium-Ion Batteries

Mn-based layered oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high capacity and cost-effectiveness. However, their practical application is often hindered by structural instability and Jahn-Teller distortion associated with Mn³⁺. Herein, an ionic-potential-guided metal-fluoride engineering strategy is proposed to address these challenges by coincorporating AlF₃ with transition-metal vacancies into a P2-type Mn-based layered oxide (Na_0.76Ni_0.225Al_0.0167Mn_0.75O_1.95F_0.05). This dual-site tuning elevates total ionic potential from 15.37 to 15.61 by simultaneously enhancing the cationic potential and reducing the anionic contribution, thereby promoting interlayer stability and suppressing Jahn-Teller effects. Multiscale characterization and density functional theory calculations reveal a reversible, solid-solution Na⁺ (de)intercalation mechanism with a negligible lattice strain (∼0.15%) and suppressed Mn³⁺ formation. The optimized cathode delivers an average voltage of ≈3.60 V within 2.0 to 4.3 V range, a reversible capacity of 134 mA h g^-1, and 83% capacity retention after 100 cycles (2.0-4.3 V, 1C). In contrast, the pristine counterpart shows a lower average voltage of 3.32 V and a rapid capacity drop to ∼50 mA h g^-1 by the second cycle. These findings establish the ionic-potential-guided AlF₃ incorporation as a robust and scalable strategy for designing highly reversible, high-voltage, and long-life cathodes for next-generation SIBs.