Mass transport properties of manganese dioxide phases for use in electrochemical capacitors: Structural effects on solid state diffusion

The solid state mass transport characteristics of various manganese dioxide phases has been examined with a focus on their use in electrochemical capacitors. The phases examined included gamma-MnO2 (electrolytic manganese dioxide), beta-MnO2 (Pyrolusite), Ramsdellite, delta-MnO2 (Birnessite), alpha-MnO2 (Cryptomelane) and lambda-MnO2. Diffusion within each phase was examined using electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS). A root D (where A is surface area and D is the diffusion coefficient) decreases with depth of discharge, and is also affected by the phase of manganese dioxide studied, with gamma-MnO2 exhibiting the highest A root D value. Overall, values of A root D varied between 3 x 10(-8) - 2 x 10(-10) m(3)/s(1/2)/g, which is comparable with literature data. These results also provide information on the kinetics of lattice expansion and contraction which occurring during cycling. High surface area phases such as gamma-MnO2, Ramsdellite and Cryptomelane, showed significant hysteresis in lattice contraction which is attributed to the diffusion of protons through surface domains. Low surface area phases (Pyrolusite and lambda-MnO2) did not display this hysteresis, suggesting that proton diffusion occurs predominantly in the bulk of the material. No direct correlation between mass transport and specific capacitance is observed, suggesting that other material properties contribute to specific capacitance.