Morphology modification and delithiation mechanisms of LiMn2O4 and Li2MnO3 by acid digestion

The structural features of equilibrium phases, LiMn{sub 2}O{sub 4} and Li{sub 2}MnO{sub 3}, and their acid-delithiated products (MnO{sub 2}) are compared using x-ray diffraction and electron microscopy. The observed morphological changes associated with acid-delithiated Li{sub 0.1}Mn{sub 2}O{sub 4} confirm the acid digestion mechanism proposed by Hunter. Three delithiated phases including {alpha}-, {gamma}-, {beta}-MnO{sub 2}-like phases can be derived from Li{sub 2}MnO{sub 3} under different acid digestion conditions. An acid-delithiation mechanism converting Li{sub 2}MnO{sub 3} to {alpha}- and {gamma}-MnO{sub 2} is proposed. This mechanism involves an acid-dissolution and precipitation process. A related scheme is also proposed for the formation of {beta}-MnO{sub 2}-like materials from {alpha}- and {gamma}-MnO{sub 2} phases. The crystallography of the {alpha}-MnO{sub 2} structure is related to the grain/crystal geometry of {alpha}-MnO{sub 2} on the basis of convergent-beam electron diffraction and lattice imaging analysis. An in situ heating experiment in a transmission electron microscope demonstrates that heat-treatment of hydrated {alpha}-MnO{sub 2} at {approximately} 300 C develops porosity within grains/crystals. It is proposed that the microstructural changes induced by acid delithiation may contribute to the reported difference in the initial electrochemical behavior of acid delithiated Li{sub 0.1}Mn{sub 2}O{sub 4} and equilibrium LiMn{sub 2}O{sub 4}. The delithiated product derived from Li{submore » 2}MnO{sub 3} at {approximately} 115 C for 4--6 h, having {gamma}-MnO{sub 2} as the major phase, demonstrates superior electrochemical properties.« less