Selective Halide Incorporation in P2‐Na 0.66 Fe 1/3 Mn 2/3 O 2 Cathodes: Revealing a Compositional Regime for Enhanced Cycling

Mn‐ and Fe‐based layered oxides are candidates for sodium‐ion battery cathodes due to their high specific capacity and raw material abundance. However, rapid capacity degradation due to irreversible phase transitions during electrochemical cycling is a major challenge. In this study, we explore anionic substitution with A = F − , Cl − , and Br − of Na 0.66 Fe 1/3 Mn 2/3 O 2−x A x (with x < 0.2), both experimentally and through density functional theory calculations. We demonstrate that substitution with Cl − /Br − is challenged by their larger ionic radii compared to oxygen ions, following the classical Goldschmidt rule. Yet, we reveal the successful aliovalent F − for O 2− substitution with a solubility limit at x = 0.035, determined by quantitative phase analysis from powder X‐ray diffraction (P XRD), Rietveld refinement, and Raman spectroscopy. Moreover, galvanostatic electrochemical cycling tests on x = 0.02 mol F‐substituted cathodes showed 15% higher capacity retention after 75 cycles, when compared with the unsubstituted material. Using in situ XRD, we show that the incorporation of F − alters the kinetics of the P2‐P`2 transformation in the low voltage range in favor of the P2 phase. This highlights low‐level F‐substituted P2‐type layered oxides as promising candidates for resource‐efficient, high‐capacity cathodes in sodium‐ion batteries.