Reversible Intercalation of Fluoride-Anion Receptor Complexes in Graphite

Abstract

We have demonstrated a route to reversibly intercalate fluoride-anion receptor complexes in graphite via a nonaqueous electrochemical process. This approach may find application for a rechargeable lithium-fluoride dual-ion intercalating battery with high specific energy. The cell chemistry presented here uses graphite cathodes with LiF dissolved in a nonaqueous solvent through the aid of anion receptors. Cells have been demonstrated with reversible cathode specific capacity of approximately 80 mAh/g at discharge plateaus of upward of 4.8 V, with graphite staging of the intercalant observed via in situ synchrotron X-ray diffraction during charging. Electrochemical impedance spectroscopy and 11 B nuclear magnetic resonance studies suggest that cointercalation of the anion receptor with the fluoride occurs during charging, which likely limits the cathode specific capacity. The anion receptor type dictates the extent of graphite fluorination, and must be further optimized to realize high theoretical fluorination levels. To find these optimal anion receptors, we have designed an ab initio calculations-based scheme aimed at identifying receptors with favorable fluoride binding and release properties. As cathodes in lithium batteries, carbon-fluoride ͑C-F͒ compounds offer very high theoretical specific capacity, on the scale of 1785 mAh/g for C 1.25 F, though to date, all C-F compounds regardless of preparation conditions are strictly non-rechargeable. A rechargeable C-F cathode would be very desirable in terms of specific energy relative to conventional lithium-ion transition metal oxide cathodes, though there are numerous difficulties associated with reversible fluorination of carbon which relate to a large extent to the nature of the C-F bond.The reaction of carbon and fluorine is well known to yield a wide range of useful compounds with varying properties, depending on preparation conditions and synthesis route. Many of these compounds, despite their relatively poor electronic conductivity, are useful as cathodes for primary Li batteries. Direct reaction of fluorine gas with graphite at temperatures greater than 350°C results in covalent bonding of C x -F with the transition of the planar sp 2 graphene sheets to buckled sp 3 carbon, and concomitant drop in electrical conductivity with decreasing x. Room temperature chemical fluorination of graphite in the presence of various fluorides such as HF, WF 6 , SbF 5 , and IF 5 has been demonstrated by others up to C 1.0 F 0.89 ͑I 0.02 H 0.06 ͒. 1 Under these preparation conditions, the sp 2 carbon hybridization is maintained, and the C-F bonding is ionic. Electrochemical fluorination of graphite in aqueous or anhydrous HF media is also possible, resulting in ionic or semi-covalent C x -F depending on the degree of fluorination, though this process is not significantly reversible and has poor Coulombic efficiency. 2-4 A candidate rechargeable C-F cathode must retain good electronic conductivity and thus the practical lower bound on C x F is likely near the C-F delocalizati...

Electrolytes and Interphases in Li-Ion Batteries and Beyond

“…780 More recently, West et al used a series of anion-receptors based on fluoroalkylborates or -boronates to form complex anions with fluoride (F − ) and showed that anions of such sizes could still electrochemically intercalate into graphite at ∼5.0 V in PC, accompanied by an expansion in interlayer distance between graphene sheets to ∼0.99 nm. 781 Although the process was still demonstrated in a cathode half cell, decent reversibility was obtained in this case as compared to previous studies. Realizing that most of the electrolyte solvents cannot satisfy interphasial chemistry requirements at graphitic anodes and graphitic cathodes simultaneously, Placke et al replaced the graphitic anode with another intercalation host, Li 4 Ti 5 O 12 , which operates at 1.5 V and does not require SEI formation.…”

Additives for Functional Electrolytes of Li-Ion Batteries

“…However, even in the early days of anion receptor research, researchers found other uses of these interesting molecules. To begin with, anion receptors can greatly increase the solubility of salts that are otherwise almost insoluble in organic electrolytes, such as LiF [70,119,138], Li 2 O, and Li 2 O 2 ( Fig. 10) [150].…”

“…Electrolytes based on these salts that are traditionally For example, the fluoride anion can be reversibly intercalated into graphite through a soluble fluoride-anion-receptor complex, making graphite as a cathode material. Based on this finding, a dual graphite intercalating cell with discharge voltage over 4.8 V has been realized [138]. Furthermore, anion receptors have been known to stabilize the metallic lithium anode in lithium metal batteries [31,173].…”

Boron based fluoride anion receptors: Electrochemical and sensory applications

“…However, the relative insolubility of the LiF in many nonaqueous electrolyte solvents limits its use as a viable electrolyte salt, and therefore the use of the fluoride specific anion receptors as electrolyte additives is a clear alternative towards achieving enhanced solubility of LiF and thereby the relatively increased lithium ion transference numbers. [5][6][7][8][9][10][11] The selective reversible binding of these boron-based anion receptors resulted in The use of these anion recepors allows the use of LiF as a low molecular weight electrolyte in the lithium ion batteries. Whereas LiF is essentially insoluble in the nonaqueous electrolyte solvents such as dimethyl ether (DME) and carbonate based solvents, in the presence of anion receptors LiF is relatively highly soluble, and typically 1 M solutions of LiF in dimethyl ether could be obtained so that high lithium ion conductivities could be achieved in lithium ion batteries.…”

“…Amine and coworkers' ab initio calculations, for example, show that the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 5 5 fluoride anion affinity is reduced by the introduction of the alkoxyl groups on the boron center and that the fluoride anion affinity increases with the substitution of the electron-withdrawing groups on the boron (e.g., fluoroalkyl or fluoroaryl aryl substitutents on the boron). 26 The anion receptor TPFPB was shown also to complex with PF 6 -anion (i.e., in addition to its complexation with the fluoride anion) so that the lithium ion transference number (and thereby the ionic conductivity) of the cells consisting of LiPF 6 electrolyte are significantly increased. 27 the NH bond of the urea deivative is successively, almost completely deprotonated upon addition of the tetrabutylammonium fluoride.…”