Structure and lithiation mechanism of the nongraphitizable carbon (hard carbon) fiber were determined by X-ray and neutron diffraction-pattern simulation, neutron small-angle scattering, and neutron total scattering techniques. The structure of the hard carbon is formed by two-dimensional graphene sheets, which are spaced by narrow interlayer space and slightly larger interlayer space, corresponding to the periodically layered region and the interlayer voids, respectively. The shape of these voids on the nanosize was characterized by a combination of ellipsoid and random branching form. The lithiation proceeded by two steps. At a shallow lithiation region, lithium intercalated into the periodically layered region and the interlayer spacing of this region expanded. At the deep lithiation stage, lithium was inserted into the interlayer nanovoid and the void size increased. The coordination numbers of the constituent ions indicated a two-dimensional graphene sheet in the whole structure; this supports the "wave" model for the hard carbon structure. Based on our X-ray and neutron scattering experiments, a new structure model and a lithiation mechanism are proposed. (c) 2006 The Electrochemical Society.