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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.
Li et al. (Mon,) studied this question.