Properties of a novel hard-carbon optimized to large size Li ion secondary battery studied by ⁷Li NMR

The state of lithium in a novel hard-carbon optimized to the anode of large size Li ion secondary battery, which has been recently commercialized, was investigated and compared with other existing hard-carbon samples by ⁷Li NMR method. The new carbon material showed a peak at 85 ppm with a shoulder signal at 7 ppm at room temperature in static NMR spectrum, and the former shifted to 210 ppm at 180 K. The latter at room temperature was attributed to Li doped in small particles contained in the sample. The new carbon sample showed weaker intensity of cluster-lithium signal than the other hard-carbon samples in NMR, which corresponded to a tendency of less "constant voltage" (CV) capacity in charge-discharge curves of electrochemical evaluation. Smaller CV capacity and initial irreversible capacity, which are the features of the novel hard-carbon, are considered to correspond to a blockade of the diffusion of Li into pore of carbon.