Influence of the pyrolysis conditions on the nature of lithium inserted inhard carbons

The influence of different heating processes of a fibrous cellulose precurs or together with the application of anisotropic mechanical stretching has b een studied for the Li storage in the obtained hard carbons. A reversible c apacity up-to 470 mAh/g has been reached and its origin has been associated with a large plateau at low potential vs Li. The Li-7 NMR spectrum recorde d on the fully lithiated sample at room temperature presents a peak at 66 p pm. At lower temperature down to 230 K, this contribution is split into a t emperature-independent peak centere at 18 ppm and an ill-defined paramagnet ic shift. On the basis of the spin-lattice relaxation times, it is shown th at the temperature-independent Li sites exhibit a small deviation to the Ko rringa type behavior at low temperature, that is associated with a distance between Li sites of the same order as for Li-Li in the metallic state. The paramagnetic NMR contribution has been related to the lithium ions inserte d below 100 mV vs Li within the hard carbon. The reversibility of the elect rochemical process, and especially the polarization between insertion and r emoval of the Li ions, is found to be related to their chemical nature, dif ferent if they are located in the nanopores or between the graphitic type l ayers. The data for inserted lithium in these samples clearly shows that ex tended delocalization of conduction electrons is not occurring.