Synthesis of hollandite-type LiyMn1−xCoxO2 (x = 0–0.15) by Li+ ion-exchange in molten salt and the electrochemical property for rechargeable lithium battery electrodes

The Li+ ion-exchange reaction of K+-type α-K0.14MnO1.93·0.18H2O and its Co-doped α-K0.14(Mn0.85Co0.15)O1.96·0.21H2O with a large (2 × 2) tunnel structure has been investigated in a LiNO3/LiCl molten salt at 300 °C. The Li+ ion-exchanged products were examined by chemical analysis, X-ray diffraction, and scanning and transmission electron microscopic measurements. Almost all the K+ ions and the hydrogens of water molecules in the (2 × 2) tunnel of α-MnO2 and its Co-doped one were exchanged by Li+ ions in the molten salt, resulting in Li+-type α-MnO2 and its Co-doped one containing Li+ ions as well as Li2O (lithium oxide) in the (2 × 2) tunnel with maintaining the original hollandite structure. The electrochemical properties including charge–discharge cycling of the Li+ ion-exchanged α-MnO2 and its Co-doped samples have been investigated as insertion compounds in the search for new cathode materials for rechargeable lithium batteries. The Li+ ion-exchanged α-MnO2 and its Co-doped samples provided higher capacities than the K+-type parent materials on initial discharge and charge–discharge cyclings, probably due to the structural stabilization with the existence of Li2O in the (2 × 2) tunnels.