Air‐Stable Copper‐Based P2‐Na7/9Cu2/9Fe1/9Mn2/3O2 as a New Positive Electrode Material for Sodium‐Ion Batteries

An air-stable copper-based P2-Na7/9Cu2/9Fe1/9Mn2/3O2 is designed and synthesized by a simple solid-state method and investigated as a positive electrode material for sodium-ion batteries. The attractive long cycling stability is demonstrated by the capacity retention of 85% after 150 cycles at 1 C rate without phase transformation. The reversible Cu2+/Cu3+ redox couple in P2 phase oxides is proved for the first time.