Volume Changes of Graphite Anodes Revisited: A Combined Operando X-ray Diffraction and In Situ Pressure Analysis Study

Lithium intercalation into graphite is one of the electrochemically best studied solid-state reactions, and its application in lithium-ion batteries was a pioneering step in the development of advanced electrochemical storage devices. Therefore, one might expect that virtually any aspect of this important reaction has been examined both qualitatively and quantitatively. All the more, it is surprising that there are only a few experimental studies on the volume expansion of graphite, especially under cycling conditions. To the best of our knowledge, there exists no comprehensive set of structural data as a function of lithium content. Here, we present this missing information using combined results from electrochemical testing and operando X-ray diffraction. The changes in lattice parameters and unit cell volume are examined and related to the different intercalation stages and phase transition regimes. A total volume expansion (from space-group-independent evaluation) of 13.2% is observed when C6 is fully lithiated to a composition of LiC6, of which approximately 5.9% occur in the early dilute stages. The remaining expansion of approximately 7.3% is due to transition from stage 2 to stage 1. These findings are corroborated by in situ pressure measurements on prelithiated Li4Ti5O12/graphite cells. Collectively, our data provide valuable information about one of the most important electrode materials for lithium-ion batteries and clearly demonstrate that even partially lithiated graphite experiences considerable crystallographic strain.