Graphite intercalation compounds GICs formation

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. 

If GO is used as a host lattice for Li+ in aprotic electrolytes, reversibility is improved [577]. The potential level is distinctly more positive than with donor GIC, at about —1 V vs. SHE. An all-solid-state Li/GO battery with PE0/LiC104 as solid electrolyte was reported by Mermoux and Touzain [578], but rechargeability is poor. Recently, the structure of graphite oxide was studied by its fluorination at 50-2()0 °C [579]. C-OH bonds were transformed into C-F bonds. The examples, in conjunction with Section 2, show that the formation or cleavage of covalent C-O (C-F) bonds makes the whole electrochemical process irreversible. Application was attempted in lithium primary batteries, which have a voltage of 2-2.5 V. Really reversible electrodes are only possible, however, with graphite intercalation compounds, which are characterized by weak polar bonds. 

The early studies have identified that the solvents could cointercalate into graphene layers before they decompose. On the basis of the knowledge about intercalation compounds and their reactions, a mechanism for SEI formation was proposed by Besenhard which involves the formation of ternary GIC [Li(solvent) cC5,] and its subsequent decomposition on the edge site of the graphite to form SEI [20], as shown in Fig. 5.4. 

As already mentioned the fundamental condition which must be fulfilled for intercalation to occur is electron transfer from the graphite macromolecule to intercalate or vice versa. This quantity determines directly many physiocochemical properties of GICs. For example, it is obvious that for an acceptor compound the quantity of electrons lost by the graphene layers (some other time understood as the hole concentration), must exactly be compensated by the amount of electrons accumulated in the intercalate layers to assure the electrical balance of the intercalation system. The formation of acceptor and donor-type compounds in the reactions of anodic oxidation and cathodic reduction may be represented by the following equations, respectively,... 

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