Nano-particle assembled porous core-shell ZnMn₂O₄ microspheres with superb performance for lithium batteries

Porous ZnMn₂O₄ microspheres were prepared via a facile co-precipitation method followed by calcination at various temperatures and evaluated as anode materials for lithium ion batteries. The sample prepared at 600 °C outperformed the other samples in terms of electrochemical performance with high reversible capacity, high-rate capability, and excellent cycling performance. The capacity of the sample remained as high as 999 mAh g^-1 at a current rate of 100 mA g^-1 after 50 cycles-one of the best ever reported for ZnMn₂O₄-based materials. A high reversible capacity of 400 mAh g^-1 was retainable at a current density of 2000 mA g^-1 after 2500 cycles. A novel electrochemical reaction mechanism of ZnMn₂O₄ anodes was established and investigated at length. The Mn₃O₄ observed during the charge process was largely responsible for the enhanced performance, as confirmed by x-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The relatively large surface area, abundant porosity, large ion exchange space, and strong mechanical stability of the porous connected 3D framework were responsible for the unique oxidation/reduction Mn²⁺ ↔ Mn³⁺ process we observed.