Influence of structural defects on the insertion behavior of γ-MnO2: comparison of H+ and Li+

In an attempt to correlate the crystallographic disorder of γ-MnO2 compounds to their cationic uptake, the influence of their structural defects on their lithium and proton insertion behavior has been studied and compared. The rate of structural defects in the starting γ-MnO2 compounds strongly modifies both the voltage vs. cation composition profiles (in terms of shape, average voltage of each electrochemical process, polarization and cycled capacity) and the cycling behavior during the first cycles. This work illustrates that the relationships between structure and cation insertion behavior are different for Li and for proton. The study allows to select for better performing samples in terms of maximum cycled capacity: Li insertion is favored by a very small amount of initial disorder (i.e. a low rate of structural defects in starting compound), while proton insertion requires maximum initial disorder (i.e. an intermediate rate of structural defects). Extrapolation of the results demonstrates the interest of Ramsdellite γ-MnO2 compounds for cathode applications of rechargeable Li batteries.