MORPHOLOGY MODIFICATION AND DELITHIATION MECHANISMS OF LIMN2O4 AND LI2MNO3 BY ACID DIGESTION

The structural features of equilibrium phases, LiMn2O4 and Li2MnO3, an d their acid-delithiated products (MnO2) are compared using x-ray diff raction and electron microscopy. The observed morphological changes as sociated with acid-delithiated Li0.1Mn2O4 confirm the acid digestion m echanism proposed by Hunter. Three delithiated phases including alpha- , gamma-, and beta-MnO2-like phases can be derived from Li2MnO3 under different acid digestion conditions. An acid-delithiation mechanism co nverting Li2MnO3 to alpha- and gamma-MnO2 is proposed. This mechanism involves an acid-dissolution and precipitation process. A related sche me is also proposed for the formation of beta-MnO2-like materials from alpha- and gamma-MnO2 phases. The crystallography of the alpha-MnO2 s tructure is related to the grain/crystal geometry of alpha-MnO2 on the basis of convergent-beam electron diffraction and lattice imaging ana lysis. An in situ heating experiment in a transmission electron micros cope demonstrates that heat-treatment of hydrated alpha-MnO2 at simila r to 300 degrees C develops porosity within grains/crystals. It is pro posed that the microstructural changes induced by acid delithiation ma y contribute to the reported difference in the initial electrochemical behavior of acid delithiated Li0.1Mn2O4 and equilibrium LiMn2O4. The delithiated product derived from Li2MnO3 at similar to 115 degrees C f or 4-6 h, having gamma-MnO2 as the major phase, demonstrates superior electrochemical properties.