Mn‐Rich P′2‐Na_0.67[Ni_0.1Fe_0.1Mn_0.8]O₂ as High‐Energy‐Density and Long‐Life Cathode Material for Sodium‐Ion Batteries

Abstract Herein, P′2‐type Na_0.67[Ni_0.1Fe_0.1Mn_0.8]O₂ is introduced as a promising new cathode material for sodium‐ion batteries (SIBs) that exhibits remarkable structural stability during repetitive Na⁺ de/intercalation. The ONiOMnOFeO bond in the octahedra of transition‐metal layers is used to suppress the elongation of the MnO bond and to improve the electrochemical activity, leading to the highly reversible Na storage mechanism. A high discharge capacity of ≈220 mAh g⁻¹ (≈605 Wh kg⁻¹) is delivered at 0.05 C (13 mAg⁻¹) with a high reversible capacity of ≈140 mAh g⁻¹ at 3 C and excellent capacity retention of 80% over 200 cycles. This performance is associated with the reversible P′2–OP4 phase transition and small volume change upon charge and discharge (≈3%). The nature of the sodium storage mechanism in a full cell paired with a hard carbon anode reveals an unexpectedly high energy density of ≈542 Wh kg⁻¹ at 0.2 C and good capacity retention of ≈81% for 500 cycles at 1 C (260 mAg⁻¹).