Enhanced Anionic Redox Stability for Sodium Ion Battery Cathodes via Mg‐Modified P2/O3 Biphasic Architecture

Abstract Layered cathode materials for sodium ion batteries (SIBs) have garnered significant attention due to their high theoretical capacity and tunable crystal structure. However, they still confront severe structural attenuation at high voltages, especially those undergoing anionic redox reactions. To mitigate these limitations and elucidate the underlying mechanism, herein, a biphasic cathode material, Na 0.67 Fe 0.3 Mn 0.5 Li 0.1 Mg 0.1 O 2 , comprising 76.6% P2 and 23.4% O3 phases is proposed. The “Na─O─Mg” structure with robust Mg─O bonds enhances oxygen redox activity but also suppresses lattice oxygen evolution at elevated voltages, enabling a high initial discharge capacity of 187.8 mA h g⁻¹ at 0.1C. Moreover, the biphasic structure suppresses O/P structural rearrangement and mitigates harmful phase changes via forming OP2 phase at high‐voltage, resulting in excellent cycling stability with a capacity retention of 96.6% after 100 cycles at 1C within an extended voltage window of 1.8–4.3 V. These findings deepen the understanding in the interlocking effect of biphasic materials and offer valuable insights for designing high‐performance cathode materials.