Regulating the zinc ion transport kinetics of Mn3O4 through copper doping towards high-capacity aqueous Zn-ion battery.

ABSTRACT

High working voltage, large theoretical capacity and cheapness render Mn3O4 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, Mn3O4 material was fabricated through a facile precipitation reaction and divalent copper ions were introduced into the crystal framework, and ultra-small Cu-doped Mn3O4 nanocrystalline cathode materials with mixed valence states of Mn2+, Mn3+ and Mn4+ 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 Mn3O4. Significantly, based on electrochemical investigations and ex-situ XPS characterization, a synergistic effect between copper and manganese was revealed in the Cu-doped Mn3O4, in which divalent Cu2+ can catalyze the transformation of Mn3+ and Mn4+ to divalent Mn2+, accompanied by the translation of Cu2+ to Cu0 and Cu+. Benefitting from the above advantages, the Mn3O4 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 Mn3O4 (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.