Mn‐Rich P′2‐Na _0.67 [Ni _0.1 Fe _0.1 Mn _0.8 ]O ₂ as High‐Energy‐Density and Long‐Life Cathode Material for Sodium‐Ion Batteries

Herein, P′2‐type Na 0.67 [Ni 0.1 Fe 0.1 Mn 0.8 ]O 2 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 ).