Lithium Intercalation into Graphitic Carbons Revisited: Experimental Evidence for Twisted Bilayer Behavior

The crystal structure of the 18650-type Li-ion cell constituents has been studied at ambient temperature by high-resolution neutron powder diffraction at different states of charge. The structure evolution occurring on the anode (graphite) site as a function of the cell state has been accurately monitored by simultaneous electrochemical measurements and powder diffraction. A set of 128 neutron powder diffraction patterns each over a 20 angular range of 0.95 degrees-160 degrees has been collected during slow cell discharge/charge in a high-resolution mode. Severe deviations from the previously reported details of Li-intercalation into graphite have been observed, such as: structural evolution of the intercalated carbons depending either on charge or discharge; pronounced dependence of the interatomic spacings for the higher ordered LixC6 (incl. previously unknown ones). Instead of higher stages of Li-intercalation in the region of low Li-contents, which would have been observed by Bragg reflections at low 20 angles, a modulation within the ab-planes is proposed, which is correlated between neighboured graphene layers and changes in dependence of the Li-content. On the basis of these results the phase diagram for lithium-interealated carbons has been reconsidered and the alternative ordering model for the LixC6 (x < 0.5) has been proposed. (C) 2013 The Electrochemical Society. All rights reserved.