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 (Na0.76Ni0.225Al0.0167Mn0.75O1.95F0.05). 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.