19F MAS NMR study on anion intercalation into graphite positive electrodes from binary-mixed highly concentrated electrolytes

Dual-graphite batteries (DGBs), which are based on anion intercalation into graphite positive electrodes, exhibit great potential for stationary energy storage due to use of more sustainable and low-cost electrode materials and processing routes. Binary-mixed highly concentrated electrolytes (HCEs) appeal highly suitable for the high operating voltages of DGBs, although the lack of sufficient insights into the formation of graphite intercalation compounds (GICs) limits the cell performance in terms of specific capacity and lifetime so far. Herein, anion intercalation from single-salt HCEs (LiPF₆ and LiBF₄) and an equimolar binary mixture of LiPF₆/LiBF₄ are studied in graphite || Li metal cells, revealing an improved performance in terms of specific capacity and Coulombic efficiency in the order LiPF₆ > LiPF₆/LiBF₄ > LiBF₄. LiBF₄-based cells exhibit an increased onset potential for anion intercalation and higher area specific impedance, suggesting an ineffective interphase formation at graphite. X-ray diffraction reveals GIC formation, while a lower stage number is achieved for the LiBF₄-based HCE. ¹⁹F MAS NMR spectroscopy analysis at various states-of-charge confirms no significant charge transfer between the intercalated anions and the graphite host and suggest preferred intercalation of PF₆^- compared to BF₄^- as well as a high translational and/or rotational mobility of the intercalated anions.