Todorokite as a Li insertion cathode -: Comparison of a large tunnel framework "MnO2" structure with its related layered structures

A detailed chemical/structural composition and electrochemical study of Li insertion in a manganese oxide phase with a tunnel (todorokite) framework and its 2D (birnessite/buserite) precursor is presented. The effect of ion substitution within the lattice and in the interstitial sites as a result of different methods of preparation was also examined for both structures. The results outline the effect of the arrangement of the MnO6 octahedra, the framework composition, and of thermal treatment of these inorganic host structures on the electrochemical response. At intermediate cycling rates. todorokite displays the highest discharge capacity of 0.6 Li/Mn (155 mAh/g) down to 2.0 V, and 0.52 Li/Mn reversible capacity in the voltage window 4.2-2.0 V. There is one primary site for Li insertion within the large tunnels, along with a secondary site of lesser capacity. Differences in the two preparation methods lead to pronounced differences in the ability to remove ions from within the structure, although the overall reversible capacities of the resultant materials are similar. These results are interpreted in the context of cation substitution within the framework structure of todorokite, and its relationship to the 2D frameworks that are precursors to its formation by hydrothermal methods.