Lithium and deuterium NMR studies of acid-leached layered lithium manganese oxides

Li-6 and H-2 MAS NMR in conjunction with X-ray diffraction is used to study the acid leaching of the layered Mn(IV) compound Li2MnO3 (i.e., Li[Li0.33Mn0.67]O-2). Lithium NMR shows that the lithium ions are progressively removed from the lithium layers during leaching, the reduction in lithium content in the layers correlating directly with the concentration of a new local environment for lithium that contains both manganese ions and protons in the first cation coordination sphere. The composition of these samples may be written as H1-xLix[Li0.33Mn0.67]O-2, indicating that H+/Li+ ion exchange has occurred. H-2 MAS NMR shows unequivocally that deuterons are present in these structures, following leaching in D2SO4/D2O. X-ray diffraction confirms that leaching is associated with the shearing of the oxygen layers to form a structure with an AABBCC stacking sequence of the oxygen layers. This stacking sequence is also found in isostructural compound HCrO2. The hydrogen bonding between the oxygen layers is proposed to provide the driving force for oxygen shearing so as to minimize the O-O distances between the AA, BB, and CC layers to generate stronger hydrogen bonding. Li+/H+ ion exchange is also accompanied by the loss of Li2O from the structure, which is more pronounced for Li2MnO3 samples calcined for shorter times. These samples can be cycled as positive electrodes (versus Li), providing capacities of more than 150 mA h g(-1).