The energy storage mechanisms of MnO2 in batteries

Manganese dioxide, MnO2, is one of the most promising electrode reactants in metal-ion batteries because of the high specific capacity and comparable voltage. The storage ability for various metal ions is thought to be modulated by the crystal structures of MnO2 and solvent metal ions. Hence, through combing the relationship of the performance (capacity and voltage) with the polymorphs of the MnO2 and metal ions in different solvents (organic and aqueous), three main energy storage mechanisms were found to be responsible for the different electrochemical processes. Furthermore, this review summarizes the main challenge and gives a direction for profound study in the future.