Structural evolution of layered LixMnyO2:: Combined neutron, NMR, and electrochemical study

The conversion upon electrochemical cycling from a layered structure to a spinel in nonstoichiometric LixMnyO2 has been studied by neutron diffraction and NMR. This process occurs in all layered lithium manganate materials, irrespective of the ion-exchange route. The high temperature of the ion exchange under reflux in n-hexanol is sufficient to promote formation of some spinel within the as-synthesized material. Upon cycling, all the different layered LixMnyO2 compounds begin to transform to a spinel. In the first 50 or so cycles, the various layered LixMnyO2 compounds behave rather differently, with a higher spinel fraction being observed for materials ion-exchanged under more extreme conditions. However, the rate of this transformation is significantly slower than was observed in stoichiometric layered LiMnO2, with only 25% spinel observed after 100 cycles. Ultimately, upon extended cycling, the entire structure will form a well-ordered spinel phase.