Intercalation into HOPG

Figure 6. Early stages of intercalation into HOPG above, in EC-based electrolyte below, in PC-based electrolyte [4, 5. ...

Foelske-Schmitz, A., P. W. Ruch, and R. Kotz. 2010. Ion intercalation into HOPG in supercapacitor electrolyte—An X-ray photoelectron spectroscopy study. Journal of Electron Spectroscopy and Related Phenomena 182 57-62. 

Using dilatometry in parallel with cyclic voltammetry (CV) measurements in lmolL 1 LiC104 EC-l,2-dimethoxy-ethane (DME), Besenhard et al. [87] found that over the voltage range of about 0.8-0.3 V (vs. Li/Li+), the HOPG crystal expands by up to 150 percent. Some of this expansion seems to be reversible, as up to 50 percent contraction due to partial deintercalation of solvated lithium cations was observed on the return step of the CV. It was concluded [87] that film formation occurs via chemical reduction of a solvated graphite intercalation compound (GIC) and that the permselective film (SEI) in fact penetrates into the bulk of the HOPG. It is important to repeat the tests conducted by Besenhard et al. [87] in other EC-based electrolytes in order to determine the severity of this phenomenon. 

HOPG cylinders showed that intercalation starts close to the basal planes and proceeds thence to the central layers. Cylinders capped at both ends with glass caps and grease are not intercalated. The layer system is split into a number of discs at a rate that increases with the degree of alignment of layer planes. For non-heat-treated PG, the threshold pressure increases with the number of layer planes in the sample. Similar results confirming this mechanism have been obtained with metal halides (H13). A mechanism of intercalation based on these results has been discussed by Hooley (H22). 

Figure 49 Comparison between the AES spectra of HOPG(bp) obtained after the thermal treatment, immediately before the Li[C/R]/PE0(LiC104) assembly was introduced into the main chamber (curve A) and after electrochemical intercalation of Li+ (curve B, see text for details). (From Ref. 6.)... 

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