A review on mechanistic understanding of MnO₂ in aqueous electrolyte for electrical energy storage systems

The demand for the large-scale storage system has gained much interest. Among all the criteria for the large-scale electrical energy storage systems (EESSs), low cost ($ k Wh⁻¹) is the focus where MnO₂-based electrochemistry can be a competitive candidate. It is notable that MnO₂ is one of the few materials that can be employed in various fields of EESSs: alkaline battery, supercapacitor, aqueous rechargeable lithium-ion battery, and metal-air battery. Yet, the technology still has bottlenecks and is short of commercialisation. Discovering key parameters impacting the energy storage and developing systematic characterisation methods for the MnO₂ systems can benefit a wide spectrum of energy requirements. In this review, history, mechanism, bottlenecks, and solutions for using MnO₂ in the four EESSs are summarised and future directions involving more in-depth mechanism studies are suggested.