Li-7-NMR of well-graphitized vapor-grown carbon fibers and natural graphite negative electrodes of rechargeable lithium-ion batteries

Lithium intercalation of natural graphite and well-graphitized vapor-grown carbon fibers has been investigated by solid-state Li-7-NMR and by cyclic v oltammetry. Chemical shift of Li-7 in Li-graphite intercalation compounds ( Li-GICs) of natural graphite occurs in two regions, >40 and <10 ppm. The fo rmer shifts were assigned to GICs with the in-plane density of LIC6 and the latter shifts were assigned to those with the in-plane density of LiC9. Ph ase diagram of Li-GICs during electrochemical intercalation is discussed. I n comparison to natural graphite, Li-7 in the vapor-grown carbon fibers sho wed higher chemical shifts. The passivating film was also examined by Li-7- NMR and structural change of the film during the first charge-discharge cyc le is indicated. Scanning electron microscopy imaging of the passivation fi lm supports the Li-7 NMR data during charge-discharge. The swelling and shr inking of passivation is evident by scanning elecron microscope image and n uclear magnetic resonance data. The thickness of the solid electrolyte inte rface layer depends on the nature of salt, solvent, carbon origin, and curr ent density. (C) 1999 The Electrochemical Society. S0013-4651(98)06-053-4. All rights reserved.