Electron energy-loss spectrometry on lithiated graphite

Abstract

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 ...

Determination of Lithium Insertion Sites in LixTiP4 (x = 2−11) by Electron Energy-Loss Spectroscopy

“…This latter result implies a decrease of the effective charge of the lithium atom as a function of the lithium content (edge at a lower energy). The lithium atoms in Li 11 TiP 4 have a local electronic structure closer to Li metal than Li + , which justifies the strong correlation between our Li 11 TiP 4 spectrum and the spectrum measured by Hightower et al 9 on pure amorphous lithium. Let it be noted that for this composition, the reduction of Li x TiP 4 occurs at a very low potential versus lithium metal, 20 in perfect agreement with a decrease of the effective charge of the lithium atom.…”

Local field effects at Li K edges in electron energy-loss spectra of Li,Li2Oand LiF

“…12 This probably explains why EELS is becoming increasingly popular in this area. [9][10][11][13][14][15][16] Most EELS studies focus on the host matrix of the electrode material and their transition metal, carbon or oxygen edges. 9,[13][14][15] Very few concentrate on the sole common atom between all these batteries: the lithium atom.…”

The Science of Electrode Materials for Lithium Batteries

“…The name "Li-ion" battery is therefore a misnomer. Although published in the year 2000 [33], this result produces strong reactions whenever it is presented at any meeting on lithium batteries. …”

“…One of our first efforts under this DOE BES program was the comparison of the Li K-edges in Li metal and in graphite near the composition LiC 6 [33]. The experimental challenge was working with reactive samples in a TEM, especially the evaporated films of Li metal.…”