Copper-substituted NaxMO2 (M = Fe, Mn) cathodes for sodium ion batteries: Enhanced cycling stability through suppression of Mn(III) formation

Adding Cu2+ has substantially boosted the practical potentiality of Fe/Mn-based layered cathodes for sodium ion batteries (SIBs) owing to the enhanced stabilities, which were previously ascribed to the raised valence of Mn. Herein, the roles of Cu2+ are verified by investigating Cu2+-substituted materials with the stoichiometry of Na0.5+xCuxFe0.5-xMn(IV)0.5O2. Surprisingly, it is found that Mn valence can hardly reach the expected value (IV) even by adjusting Cu2+ content. For the first time, the separation of CuO, which has been previously detected but rarely explained, is ascribed to the restrained chemical states of Mn. Detailed analyses show that, Mn(II) is generated while Mn(III) is decreased in pace of Cu2+ substitution, actually lowering down the oxidation states of Mn. Moreover, Mn4+/3+ redox can be efficiently restricted by importing Cu2+. Albeit the loss of capacity, the cycling stability is greatly enhanced, achieving a high capacity retention of 92.3% after 200 cycles within 4.2–2.5 V. Therefore, the suppression of Jahn-Teller Mn(III) should be intrinsically responsible for the superior cycling stability after Cu2+ substitution. These findings may present a new sight to probe the roles of Cu in layered NaxMO2 system for the design of advanced cathodes for SIBs.