Lithium and Deuterium NMR Studies of Acid-Leached Layered Lithium Manganese Oxides

6 Li and 2 H MAS NMR in conjunction with X-ray diffraction is used to study the acid leaching of the layered Mn(IV) compound Li 2 MnO 3 (i.e., Li[Li 0.33 Mn 0.67 ]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 H 1 - x Li x [Li 0.33 Mn 0.67 ]O 2, indicating that H + /Li + ion exchange has occurred. 2 H MAS NMR shows unequivocally that deuterons are present in these structures, following leaching in D 2 SO 4 /D 2 O. 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 HCrO 2 . 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 Li 2 O from the structure, which is more pronounced for Li 2 MnO 3 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 .