The structural features of a series of intergrown "layered-spinel" composite materials with initial formulation xLi[Mn1.5Ni0.5]O-4 center dot(1-x)(Li2MnO3 center dot Li[Mn0.5Ni0.5]O-2), previously shown to have attractive properties as positive electrodes in lithium batteries, were studied. The existence of a layered and a spinel component in samples with 0 < x < 1 was confirmed by both X-ray diffraction and Li-6 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The unit-cell volume of the layered component was shown to decrease as a function of x, while the unit-cell parameter of the cubic spinel phase did not vary significantly. The Li-6 MAS NMR spectra confirmed that the cation distribution in these layered-spinel composite structures is extremely complex; analysis of the data, particularly for intermediate values of x, suggested that Mn4+, Mn3+, Ni3+, and Ni2+ ions are present, rather than just Mn4+ and Ni2+ as implied by the ideal formula xLi[Mn1.5Ni0.5]O-4 center dot(1-x)(Li2MnO3 center dot Li[Mn0.5Ni0.5]O-2). The structural changes induced by the electrochemical removal and reinsertion of lithium in the composite electrode for x=0.5 were followed by Li-6 MAS NMR and related to the different signatures in the composition-voltage profiles. The activity of both the layered and the spinel component within a 5-2 V voltage window was confirmed.