Editors’ Choice—Mechanistic Elucidation of Anion Intercalation into Graphite from Binary-Mixed Highly Concentrated Electrolytes via Complementary 19 F MAS NMR and XRD Studies

Abstract

Dual-graphite batteries have emerged as promising candidate for sustainable energy storage due to their potentially low costs and absence of toxic materials. However, the mechanism of anion intercalation and the structures of the resulting graphite intercalation compounds (GICs) are still not well understood. Here, we systematically evaluate the anion intercalation characteristics into graphite for three highly concentrated electrolytes containing LiPF 6 , LiTFSI and their equimolar binary mixture. The binary mixture exhibits a significantly enhanced capacity retention and improved intercalation kinetics compared to the single-salt electrolytes in graphite ∣∣ Li metal cells. In situ X-ray diffraction studies prove the formation of stage 1-GICs and a homogeneous distribution of anions within graphite. From ex situ solid-state 19 F magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements, GICs can be identified at various states-of-charge (SOCs). The 19 F chemical shifts of intercalated anions indicate no significant charge transfer between anion and graphite. The observed narrow 19 F linewidths of the GIC-signals are most likely caused by a high translational and/or rotational mobility of the intercalates. Furthermore, the 19 F MAS NMR studies allow the identification of the molar ratios for PF 6 − and TFSI − anions intercalated into graphite, suggesting a preferred intercalation of PF 6 − anions, especially at lower SOCs.