Factors controlling the stability of O3- and P2-type layered MnO2 structures and spinel transition tendency in Li secondary batteries

Abstract

Cathode properties of two layered manganese dioxides (AxMnO2+·yH2O, where A is the pillaring alkali cations) having different crystal structures were compared in 3 V Li secondary batteries. The materials were prepared from the mixture of KNO3, LiOH, and MnO at 800 and 1050°C, respectively. The 800°C-prepared MnO2 has a trigonal Rm space group with an O3-type oxide-packing pattern, whereas the 1050°C material has an orthorhombic Cmcm symmetry with a P2-type oxide-packing pattern. The gallery space where the pillaring cations and water molecules reside is wider in the case of the 800°C material. Due to the higher mobility of pillaring cations in the 800°C material and similarity in the oxide-packing pattern (O3-type) to the spinel phases, the pillaring cations are easily leached out during cell cycling, which ultimately leads to a lattice collapse and structural transition to the spinel-related phases. By contrast, as the 1050°C material has rather immobile pillaring cations and its oxide-packing pattern (P2-type) is far different from that of the spinel phases, this cathode shows better cycling performance, with its structural integrity being well maintained.