Volume Changes of Graphite Anodes Revisited: A Combined <i>Operando</i> X‑ray Diffraction and <i>In Situ</i> 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 <i>operando</i> 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 <i>in situ</i> 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

The Critical Role of Fluoroethylene Carbonate in the Gassing of Silicon Anodes for Lithium-Ion Batteries

The use of functionalized electrolytes is effective in mitigating the poor cycling stability of silicon (Si), which has long hindered the implementation of this promising high-capacity anode material in next-generation lithium-ion batteries. In this Letter, we present a comparative study of gaseous byproducts formed by decomposition of fluoroethylene carbonate (FEC)-containing and FEC-free electrolytes using differential electrochemical mass spectrometry and infrared spectroscopy, combined with long-term cycling data of half-cells (Si vs Li). The evolving gaseous species depend strongly on the type of electrolyte; the main products for the FEC-based electrolyte are H₂ and CO₂, while the FEC-free electrolyte shows predominantly H₂, C₂H₄, and CO. The characteristic shape of the evolution patterns suggests different reactivities of the various Li_<i>x</i>Si alloys, depending on the cell potential. The data acquired for long-term cycling confirm the benefit of using FEC as cosolvent in the electrolyte