Synthesis and lithium ion insertion/extraction properties of hollandite-type MnO 2 prepared by acid digestion of Mn 2O 3

Abstract

Single-phase specimens of α-MnO 2 (hollandite-type) and β-MnO 2 (rutile-type) were synthesized by the acid digestion of Mn 2O 3 under reflux conditions. The type of polymorph of MnO 2 products was strongly dependent on the reaction temperature, type of acid used, and its concentration. The pH titration curve of α-MnO 2 displayed a monobasic acid behavior toward Li +, but β-MnO 2 showed a poor ion-exchange property. In contrast, both α-MnO 2 and β-MnO 2 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-MnO 2 structure was still retained without structure collapse, although the β-MnO 2 structure was destroyed. These findings show that Li + ions can be inserted only into the hollandite-type tunnels in α-MnO 2 and cannot diffuse the rutile-type linkages in α-MnO 2 as well as those in β-MnO 2 without structure collapse.