Editors’ Choice—Mechanistic Elucidation of Anion Intercalation into Graphite from Binary-Mixed Highly Concentrated Electrolytes via Complementary ¹⁹F 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 LiPF6, 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 19F magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements, GICs can be identified at various states-of-charge (SOCs). The 19F chemical shifts of intercalated anions indicate no significant charge transfer between anion and graphite. The observed narrow 19F linewidths of the GIC-signals are most likely caused by a high translational and/or rotational mobility of the intercalates. Furthermore, the 19F MAS NMR studies allow the identification of the molar ratios for PF6 − and TFSI− anions intercalated into graphite, suggesting a preferred intercalation of PF6 − anions, especially at lower SOCs.

Potentiometric MRI of a Superconcentrated Lithium Electrolyte: Testing the Irreversible Thermodynamics Approach

“…Below 1 Mthe concentration regime for standard lithium-ion-battery electrolytesour Raman analysis indicates that the majority of EMC moves freely, but the opposite holds above 3 M, where more than 90% of the EMC is coordinated with lithium. The monotonic increase in solvent coordination with concentration qualitatively agrees with prior studies of LiPF 6 in linear-carbonate solvents, 64 although infrared spectroscopy tends to show higher coordination at lower concentrations. 65 A relatively steady transition to a state wherein ions and solvent are highly coordinated occurs across the 1−3 M range, commensurate with behavior observed for other nonaqueous solutions.…”

Kinetic Behavior of the Anion Intercalation/De-intercalation into the Graphite Electrode in Organic Solution

“…6 This means the volumetric change of the graphite electrode is as large as 40-140 %, depending on the depth of charge. Even for the smaller PF6 − anion case, the galley height has been reported to be 7.7-7.8 Å, leading up to 130 % of volumetric change, 7 while that of the traditional oxide materials is generally less than10 %. 8 So far, some groups have studied on the anon intercalation behavior in organic solution or ionic liquid electrolytes, especially focusing on the structure of the graphite intercalation compound or the search for the electrolyte solutions with high anti-oxidation capability or excellent cycle performance.…”

Advanced Dual‐Ion Batteries with High‐Capacity Negative Electrodes Incorporating Black Phosphorus

“…[50] The main stage index n as well as the gallery height h C-A-C can be calculated from the 2𝜃 values of the most intense reflections ((00n+1) and (00n+2) (see Supporting Information for further information). [52] After the formation of a stage-3 GIC at 4.3 V, a stage-1 TFSI-based GIC is clearly developed with a gallery height of ≈8 Å at the highest SOC at 4.7 V (further information about the repeat distance of the unit cell (I Gallery ) and the gallery height (h C-A-C ) are shown in Table S3, Supporting Information). During discharge, the deintercalation mainly starts at the observed plateau at a cell voltage of 4.1 V and the staging mechanism progresses in reverse, ending up with the formation of a stage-5 TFSI-GIC at 2.0 V. The results show that the electrochemical data correlate well with the structural changes and a reversible staging mechanism expected for TFSI-based GICs.…”