Investigating the Phase Transitions of Graphite By In Situ Neutron Diffraction

Abstract

Graphite is one of the most common negative electrode materials in today’s lithium-ion batteries [1]. Intensive research has revealed most of its thermodynamic properties [2-5] and modeling of its free energy was able to determine some of these properties like the phase diagram and the staging phenomena to some extent [6, 7]. However, these tools were partially insufficient to describe the open-circuit potential and phase transitions in the low state-of-charge regime of the Li-graphite system.The phase transitions during the intercalation of lithium in graphite were therefore investigated using in situ neutron diffraction. In situ neutron diffraction data have been collected to determine the transitions of the lithium-graphite intercalation compound in the range of 7-30 % of the state-of-charge (100 % = LiC6). Only this limited SOC range has been investigated in detail since previous findings by Billaud and Dahn already describe the range from 0-7 % and 30-100 % very precisely [3,5]. This new set of data suggests a continuous lithium intercalation in a certain solid-solution regime for each pure phase (stage) at room temperature. This result is found to be consistent with the phase diagram of Woo et al. [4] and is able to extract the in-plane density of lithium in each stage.