*3.1.1 Manganese dioxide (MnO2)*

Manganese dioxide is well noticed because of high theoretical capacitance (1370 F/g), low cost, and natural abundance. The charge storage mechanism of MnO2 can be as follows [12]:

$$\text{MnO}\_2 + \text{M} + + \text{e}^- \leftrightarrow \text{MnOOM} \tag{1}$$

Where M corresponds to electrolyte cations such as K<sup>+</sup> , Li+ , Na +, etc. The multiple oxidation states exhibited by MnO2 result in transition from Mn2+ to Mn6+ within the potential window. The crystal structure variation by MnO2 with various polymorphs such as β, α, δ, γ, λ results in variation in electrochemical performance of the material. Other than various advantages, the low conductivity of MnO2 limits its application as SC electrode material [18].
