Exfoliated graphites

In fact, crystalline graphites usually cannot be operated in PC electrolytes, unless effective film forming electrolyte additives are used (see above) as propane gas evolution [35], creation of solvated graphite intercalation compounds (sGICs) [36], and graphite exfoliation take place. Recently [37, 38], it was found that propylene evolution is observed at graphite, while absent at lithium active metallic anodes, e.g., Sn and SnSb. 

In order to evaluate pore structure of exfoliated graphite, exfoliation volume, which is a volume occupied by a lump of exfohated graphite, or bulk density, which is an inverse of the exfohation volume, was used. ExfoHation volume includes overall information on the three kinds of pores mentioned above the exfohation volume is larger if pore structure is more developed. Information on each pore, however, could not be obtained separately. 

In this section, three case examples taken as representative for the development of different types of pores in carbon materials will be described. The first example to be considered constitutes a model for development of extrinsic nanopores by air oxidation. The second one concerns macropore development by graphite exfoliation. The third case example deals with intrinsic, two-dimensional slit pores formed between neighboring graphite layers by intercalation. 

The present results on exfoliation volume and pore structure inside the worm-like particles suggest that graphite exfoliation proceeds in three steps. Below 923 K, the exfoliation volume increases with temperature through exfoliation of each graphite flake to worm-like particles, in which ellipsoidal pores are developed. Above 923 K, the main process in the second step may be the introduction of a complicated entanglement of worm-hke particles and results in an increase in exfoliation volume, because growth of pores inside the... 

Figure 7.5 Color changes and exfoliation yield after 4 h of electrochemical graphite exfoliation at 7 V using 0.1 M IL/acetonitrile. Reprinted with permission from Ref. [26]. Copyright 2014, Elsevier Ltd.

When PC is used as a solvent, the electrolyte continues to decompose at about 0.9 V Vi. LP/Li, as shown in Fig. 4.4, and graphitic (crystalline) carbons cannot be charged due to their exfohation caused by the solvent co-intercalation. This is the reason why EC is dominantly used instead of PC, which was adopted for the first commercial lithium-ion cells with anongraphitic (amorphous) carbon anode. However, the addition of some compounds such as vinylene carbonate (VC) prevents the graphite exfoliation and enables the charge of graphitic carbons, as shown in Fig. 4.4. 

Since VC has a smaller lowest unoccupied molecular orbital (LUMO) energy due to the presence of a double bond in its structure, it is considered to be more susceptible to reduction than other carbonates such as EC and DMC. The reduction potential of VC is higher than those of other carbonate solvents, as given in Eig. 4.2, which were measured on a gold electrode in tetrahydrofuran (THE) solvent. " It is interpreted that the reductive decomposition of VC precedes the carbonate solvent decomposition, and the resultant good SEI film on the anode protects the further solvent decomposition and the graphite exfoliation by solvent co-intercalation. ... 

Besides VC, vinyl ethylene carbonate (VEC), phenylethylene carbonate (PhEC), ° phenylvinylene carbonate (PhVC), catechol carbonate (CC), - ally methyl carbonate (AMC), - allyl ethyl carbonate (AEC), vinyl acetate (VA), and other vinyl compounds, - - acryronitrile (AAN) - and 2-cyanofuran (CN-F), whose chemical structures are given in Fig. 4.10, showed the similar effect and no graphite exfoliation in PC solvent systems. 

The earUer study on graphite exfoliation by PC has led to a simple two-electron reduction process, producing Li2C03 and propylene as the major product, the same as PC reduction on lithium electrode. Scheme 5.1. As we mentioned above, this proposed mechanism was challenged by Aurbach and co-workers. They believed that the solvent preferentially formed a radical anion during the first cathodic process and then subsequently underwent intermolecular electron transfer between the two radical anions, as shown in Scheme 5.3 ... 

---