7Li NMR study of Li-doped polyacenic semiconductor (PAS) materials

The polyacenic semiconductor (PAS) material prepared from phenol resin at relatively low temperature (680 °C) can be doped by a much larger amount of lithium atoms up to the C2Li state compared with graphite (C6Li state). To understand the Li storage mechanism as well as the electronic structure, 7Li nuclear magnetic resonance (NMR) measurements have been performed for the Li-doped PAS materials. In the initial doping stage a broad signal was observed at 0 ppm, while the successive doping causes a Knight shift to 9 ppm. It is suggested from this Knight shift that the Li nucleus slightly undergoes the Fermi contact interaction with conduction electrons delocalizing through carbon π-atomic orbitals. From the spin-lattice relaxation rate ( 1/T1), it is found that the Li nucleus in the PAS material loses its mobility by the surrounding Li nuclei with the proceeding doping and that the Li nucleus is more loosely trapped than that in the C6Li state of graphite.