MOF-derived zinc manganese oxide nanosheets with valence-controllable composition for high-performance Li storage

Zinc manganese oxide (ZMO) system represents a notable family of mixed transition metal oxides (MTMOs) because of their superiority of the high theoretical capacity, adequacy of natural content, and low cost. However, the methods to match both the reliable synthesis and the designable construction of large-sized two-dimensional (2D) ZMO nanosheets are still considered as grand challenges. Herein, we have successfully realized the preparation of 2D ZMO nanosheets with large lateral sizes up to ∼20 μm by simple pyrolysis of 2D metal-organic framework (MOF) nanosheets precursor. The growth mechanism of 2D MOF is proposed to be based on the lamellar micelles formed by polyvinyl pyrrolidone (PVP). The obtained 2D and porous ZMO nanosheets exhibit high specific capacity as well as good rate capability. More importantly, the as-prepared ZMO electrode shows a remarkable capacity increment upon cycling (from 832 mAh g^-1 at the 2nd cycle to 1418 mAh g^-1 at the 700th cycle, at 1 A g^-1). Through simple adjustment of the calcination temperature, the valence state of Mn species in the yielding ZMO samples can be fine-tuned. Through systematic investigation towards these ZMOs containing different Mn species, the extra specific capacity is revealed to be chiefly on account of the arising of the valence state of Mn upon the cycling process. Moreover, it is disclosed that the higher-valent Mn the pristine ZMO contains, the more additional capacity it gains upon cycling. We believe that this work will inspire more detailed analysis on the relationship between the valence state of Mn and extra capacity.