Regulating the zinc ion transport kinetics of Mn 3 O 4 through copper doping towards high-capacity aqueous Zn-ion battery.

High working voltage, large theoretical capacity and cheapness render Mn ₃ O ₄ promising cathode candidate for aqueous zinc ion batteries (AZIBs). Unfortunately, poor electrochemical activity and bad structural stability lead to low capacity and unsatisfactory cycling performance. Herein, Mn ₃ O ₄ material was fabricated through a facile precipitation reaction and divalent copper ions were introduced into the crystal framework, and ultra-small Cu-doped Mn ₃ O ₄ nanocrystalline cathode materials with mixed valence states of Mn ²⁺ , Mn ³⁺ and Mn ⁴⁺ were obtained via post-calcination. The presence of Cu acts as structural stabilizer by partial substitution of Mn, as well as enhance the conductivity and reactivity of Mn ₃ O ₄ . Significantly, based on electrochemical investigations and ex-situ XPS characterization, a synergistic effect between copper and manganese was revealed in the Cu-doped Mn ₃ O ₄ , in which divalent Cu ²⁺ can catalyze the transformation of Mn ³⁺ and Mn ⁴⁺ to divalent Mn ²⁺ , accompanied by the translation of Cu ²⁺ to Cu ⁰ and Cu ⁺ . Benefitting from the above advantages, the Mn ₃ O ₄ cathode doped with moderate copper (abbreviated as CMO-2) delivers large discharge capacity of 352.9 mAh g ^-1 at 100 mA g ^-1 , which is significantly better than Mn ₃ O ₄ (only 247.8 mAh g ^-1 ). In addition, CMO-2 holds 203.3 mAh g ^-1 discharge capacity after 1000 cycles at 1 A g ^-1 with 98.6 % retention, and after 1000 cycles at 5 A g ^-1 , it still performs decent discharge capacity of 104.2 mAh g ^-1 . This work provides new ideas and approaches for constructing manganese-based AZIBs with long lifespan and high capacity.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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