For the first time, in situ solid-state Li-7-nuclear magnetic resonance (Li-7-NMR) has been successfully realized on a supple ultrathin plastic carbon/lithium cell at various steps of galvanostatic cycling. Two lines related with lithium insertion are reversibly observed, the minor one in the 0-17 ppm region attributed to intercalated lithium (with a charge transfer to carbon), while the position of the most important is down-field shifted during lithium insertion to reach values of ca. 110 ppm, characteristic of quasi-metallic lithium at full reduction of carbon. Taking into account the electrochemical results, the Li-7-NMR data and the transmission electron microscopy observations on the host carbon, we propose a model which shows that lithium first intercalates in the small intervals between nanometer-size graphitic-type layers, and then penetrates the nanopores where growing quasi-metallic clusters are formed. (C) 2003 The Electrochemical Society.