Synthesis and lithium ion insertion/extraction properties of hollandite-type MnO2 prepared by acid digestion of Mn2O3

Single-phase specimens of α-MnO₂ (hollandite-type) and β-MnO₂ (rutile-type) were synthesized by the acid digestion of Mn₂O₃ under reflux conditions. The type of polymorph of MnO₂ products was strongly dependent on the reaction temperature, type of acid used, and its concentration. The pH titration curve of α-MnO₂ displayed a monobasic acid behavior toward Li⁺, but β-MnO₂ showed a poor ion-exchange property. In contrast, both α-MnO₂ and β-MnO₂ acted as a rechargeable active material in a liquid organic electrolyte lithium cell. The initial discharge capacities of both electrodes exceeded 200 mAh/g (cut-off voltage: 2 V). After discharge–charge repetition, the α-MnO₂ structure was still retained without structure collapse, although the β-MnO₂ structure was destroyed. These findings show that Li⁺ ions can be inserted only into the hollandite-type tunnels in α-MnO₂ and cannot diffuse the rutile-type linkages in α-MnO₂ as well as those in β-MnO₂ without structure collapse.