Fluoride ion batteries are potential “next-generation” electrochemical storage devices that offer high energy density. At present, such batteries are limited to operation at high temperatures because suitable fluoride ion–conducting electrolytes are known only in the solid state. We report a liquid fluoride ion–conducting electrolyte with high ionic conductivity, wide operating voltage, and robust chemical stability based on dry tetraalkylammonium fluoride salts in ether solvents. Pairing this liquid electrolyte with a copper–lanthanum trifluoride (Cu@LaF3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in a fluoride ion electrochemical cell cycled at room temperature. Fluoride ion–mediated electrochemistry offers a pathway toward developing capacities beyond that of lithium ion technology.

Nonaqueous electrolyte solutions in current lithium-ion cells achieve stability toward the graphite anode (negative electrode) via the formation of passive surface films on the anode surface. These films are composed of reaction products resulting from electrolyte reduction and some reduced lithium. These films reportedly contain various lithium compounds, such as lithium carbonate (Li 2 CO 3 ), lithium oxide (Li 2 O), lithium hydroxide (LiOH), lithium alkoxides, lithium fluoride (LiF), as well as electrolyte salt reduction products that are still to be accurately characterized. 1 Relative amounts of these constituents are also equally uncertain. In addition to the expense of lithium for such surface films, termed solid electrolyte interphase (SEI), 2 a portion of lithium might be "trapped" in the anode material and is "kinetically inaccessible." Consequently, a differential exists between the intercalated lithium (charge capacity) and deintercalated lithium (discharge capacity), which is loosely termed irreversible capacity. This irreversible capacity depends not only on the rate of lithiation during the formation cycles and the temperature, but also on the extent of charge-discharge cycling, during which the surface film may grow. Irreversible capacity is typically estimated as the cumulative differential in the capacity after five cycles (one cycle in some reports), when the charge capacity/discharge capacity ratio approaches unity.It is difficult to separate the irreversible capacity into a component involving SEI formation and a component involving capacity loss due to kinetic effects, unless one of the components is estimated by a nonelectrochemical method. In this work, we attempted such a study on SEI formation on graphite in different electrolyte solutions using solid-state 7 Li nuclear magnetic resonance (NMR). Solid-state 7 Li NMR has been commonly used for qualitative detection and characterization of lithium intercalation in (and the SEI formation on) graphite and disordered carbons. 3-8 However, this technique has not been used extensively for quantitative determinations. In this work, the surface films were also examined ex situ by transmission electron microscopy (TEM) for elucidating the microstructure of film-covered graphite electrodes. These studies were further complemented by electrochemical impedance spectroscopy (EIS) measurements to understand the surface film characteristics of graphite anodes in different electrolytes.Improving low-temperature performance of lithium-ion cells remains a formidable technical challenge. The present investigation is an outgrowth of our previous studies 9 of novel electrolyte formulations for low-temperature performance. Similar efforts to improve the properties of electrolytes at low temperatures are being made elsewhere. 10-14 Our recent results have shown that a ternary mixture of alkyl carbonates, i.e., 1:1:1 (vol. %) of EC (ethylene carbonate):DEC (diethyl carbonate):DMC (dimethyl carbonate) with 1 M LiPF 6 exhibits favorable electrochemical characteris...

The electronic structure of chemically delithiated Li1 - x CoO2 (x = −0.02, 0.09, 0.12, 0.20, and 0.28) was investigated by electron energy-loss spectrometry (EELS). The O K edge and Co L2,3 edge were used to probe the density of unoccupied states around the O and Co ions at different states of lithiation. The O ions accommodate the incoming charge during Li intercalation. The net electron density surrounding the Co ions is less affected. This is in substantial agreement with prior electronic structure calculations of Van der Ven, et al., whose atomic structure data were used in the present calculations of EELS cross-sections. Calculations of the O 2p partial densities of state curves confirm the increase in unoccupied states that accompany Li extraction.

Transmission electron energy-loss spectrometry was used to investigate the electronic states of metallic Li and LiC 6 , which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges of metallic Li and LiC 6 were nearly identical, and the C K edges were only weakly affected by the presence of Li. These results suggest only a small charge transfer from Li to C in LiC 6 , contrary to prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation damage and sample oxidation in the transmission electron microscopy are also reported. © 2000 American Institute of Physics. ͓S0003-6951͑00͒04428-4͔Lithiated graphite is the standard anode material in Liion rechargeable batteries. 1 Highly crystallized graphite can intercalate Li atom to a maximum composition of LiC 6 . This is equivalent to a specific charge of 372 Ah kg Ϫ1 , 2 although in practice graphite anodes have specific energies of 320-360 Ah kg Ϫ1 . Graphite anodes have high voltages of 3-4 V versus the cathode, but the difference in electrochemical potential between metallic Li and lithiated graphite is small, of order 0.01 V. The intercalation of Li into highly crystallized graphite changes the stacking sequence of the hexagonal planes from an ABABAB to AAAAAA. 3 This change in stacking sequence and the high chemical potential of Li in graphite suggest that a better understanding of the interlayer states of LiC 6 may facilitate improvements to Liion electrochemical cells.The results of numerous studies on the band structure of Li intercalated graphite demonstrate the difficulty in determining the degree of hybridization between Li atomic orbitals and graphite interlayer states. Early theoretical calculations of the LiC 6 band structure began with the notion of complete charge transfer of Li valence electrons to the graphite p bands. 4,5 This evolved into an elegant theory of alkali-intercalated graphite interlayer states as interacting nonorthogonal hybrid states of Li 2s and graphite interlayer states. 6,7 This gives credence to x-ray photoelectron spectroscopy ͑XPS͒ results by Momose et al. 8 and others 9 claiming Li to be intercalated into graphite as ionic Li ϩ . Early experimental work by Grunes et al. 10 using electron energy-loss spectroscopy ͑EELS͒ demonstrated distortions of the graphite band structure upon intercalation of alkali metals. Hartwigsen et al. 11 used a density functional theory, local density approximation to determine the degree of charge transfer from Li to the intercalant host lattice to be 0.5e for LiC 6 and 0.4e for LiC 8 . Further experiments using inelastic x-ray scattering spectroscopy by Schülke 12 were able to correlate features of LiC 6 spectra to band structure calculations by Holzwarth et al. 13 The present letter reports transmission EELS measurements of the Li K edge in intercalated graphite and in metallic Li. After showing oxidation tendencies of the transmission electron microscopy ͑TEM͒ samples and how this was controlled, we show that the Li K edge for Li in LiC 6 ...