Structural Evolution of Layered Li_xMn_yO₂: Combined Neutron, NMR, and Electrochemical Study

The conversion upon electrochemical cycling from a layered structure to a spinel in nonstoichiometric Li x Mn y O 2 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 Li x Mn y O 2 compounds begin to transform to a spinel. In the first 50 or so cycles, the various layered Li x Mn y O 2 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 LiMnO 2, with only 25% spinel observed after 100 cycles. Ultimately, upon extended cycling, the entire structure will form a well-ordered spinel phase.