From aqueous Zn-ion battery to Zn-MnO₂ flow battery: A brief story

Aqueous Zn-ion battery (AZIB) has become an attractive technology because of its unique features of low cost, high safety and the eco-friendliness. MnO₂ is the model cathode material for AZIB since the first report on reversible Zn-MnO₂ battery, but recent studies have unveiled different charge storage mechanisms. Due to revamping of the electrochemistry and redesigning of the electrolyte and interface, there is tremendous performance enhancement in AZIB. This mini Review will first give a brief introduction of ZIB, including fundamentals of materials and components, and the progress in recent years. Then, a general classification of working mechanisms related to MnO₂ in neutral and mildly acidic electrolyte is elaborated. Our focus is put on the recent blossoming Zn-MnO₂ electrolytic mechanism, which has given birth to the Zn-MnO₂ redox flow batteries that are highly promising for large-scale static energy storage.