Geun-Chang Chung

Origin of Graphite Exfoliation An Investigation of the Important Role of Solvent Cointercalation

To elucidate the origin of graphite exfoliation, we have investigated the influence of various material parameters relevant to solvent co‐intercalation, such as the cation, the electrolytic solvents, and the structure of graphite, on the solvent decomposition behavior. By electrochemically probing changes in the electrode, we demonstrated that a large increase of surface area accompanies the decomposition of propylene carbonate (PC). Furthermore, such a change in surface area is dramatically amplified when is replaced by tetrabutylammonium ion. A slight structural modification of PC exerts a profound influence on the solvent decomposition behavior, as demonstrated with cis‐ and trans‐2,3‐butylene carbonate. These reaction behaviors are also altered significantly by the choice of graphite. Such an influence of graphite structure is particularly surprising for t‐BC electrolyte, in which SFG44 graphite undergoes extensive exfoliation, whereas SFG6 graphite and MCMB25 can be cycled reversibly. These results can be best explained by incorporating the co‐intercalation of cyclic carbonate as a critical process in the solid electrolyte interphase formation mechanism.

Effect of surface structure on the irreversible capacity of various graphitic carbon electrodes

In order to understand the structural effect on irreversible capacity, the electrochemical lithium intercalation of various graphitic carbons was studied in 1 M solution of ethylene carbonate/propylene carbonate/diethyl carbonate (EC/PC/DEC), as a function of PC content. Because the irreversible capacity was increased by PC only for the carbons with a considerable edge fraction, it was possible to selectively amplify and monitor the reaction on the edge surface. The important role of the edge surface was confirmed qualitatively by high resolution transmission electron microscope images of the surface structure and quantitatively by further analysis on the results, using a simple model that explicitly considered the different reactivity of EC and PC toward the basal and edge surfaces. Exfoliation behavior, emerging as the PC content increased above a certain structure‐dependent threshold, was examined also. The structural effect could be explained by assuming that the cointercalation of PC gave rise to exfoliation. Two controlling factors were suggested: the structural integrity affecting the expansion of layer spacing and the geometry of the edge surface affecting the cointercalation of PC.

New cyclic carbonate solvent for lithium ion batteries: trans-2,3-butylene carbonate

Abstract A new cyclic carbonate useful for lithium ion batteries with graphitic carbon anode is presented. Although it is structurally very similar to propylene carbonate, it is much less reactive toward graphite than propylene carbonate. The decrease in the reactivity can be explained in terms of its unique geometry that hinders its co-intercalation into the layer spacing of graphite structure. We demonstrate that electrolytes containing this solvent exhibit a satisfactory initial efficiency and discharge performance at low temperature.

Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Abstract Lithium ion batteries with graphitic carbon anodes and LiCoO 2 cathodes are cycled reversibly in electrolytes based on trans -2,3-butylene carbonate ( t -BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.