A critical review of facets and defects in different MnO₂ crystalline phases and controlled synthesis – Its properties and applications in the energy field

Manganese dioxide (MnO₂) is an inexpensive and non-toxic compound that has eight typical crystal structures, namely α-, β-, γ-, ε-, δ-, λ-, OMS-6 and todorokite MnO₂. These crystal structures differ in their properties, such as charge states and vacancy character, and are used in a variety of applications. Although extensive research has been carried out on the use of different types of MnO₂ materials in energy-related applications, there are few papers dealing with MnO₂ with different facets, crystal phases and structural defects and the corresponding applications. Therefore, this article focuses on the commonly occurring crystal facets and defects of the different crystal phases of MnO₂ and their corresponding physiochemical properties and applications in energy-related fields. Furthermore, the controlled formation of MnO₂ with the desired crystal facets and performances is investigated in detail. Finally, research gaps and opportunities for future research on MnO₂ with different facets, crystalline phases and structural defects are discussed. Suggestions for the rational design and fabrication of MnO₂ materials for high efficiency energy applications are also discussed.