Graphite Electrode - an overview

3.1 Graphite Electrode

[6]

The results reported here refer to a graphite electrode prepared by mixing 95  wt.% of natural graphite with 5   wt.% of poly(vinylidene fluoride) (PVdF)-binder. Graphite electrodes without PVdF-binder were also fabricated. The electrolyte was 1   M LiPF6/ethylene carbonate (EC)   +   dimethyl carbonate (DMC) (1:1 v/v) and the counter electrode a Li metal sheet.

The cells were cycled between 0.01 and 1.5   V with a relaxation period of 60   min at the end of charge, at a constant current of 0.2   mA/cm2. After two cycles in this condition, the cells were charged to 0   V with the time limit of 372   mAh/g to obtain a fully charged negative electrode.

Figure 20.2 shows DSC curves for fully lithiated or delithiated graphite (a–d) and the electrolyte (e). Sample (a) shows a mild heat generation starting at 130   °C with a small peak at 140   °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. From our experiments, the small peak at 140   °C is caused by SEI formation. There is already SEI on the sample (lithiated graphite), which is formed during cycling for sample preparation. This original SEI protects the reaction of the electrolyte and Li in graphite at a lower temperature, and there is no heat generation. However, at ∼140   °C, the protection effect of the original SEI is not sufficient, and a new, thicker SEI is formed. When this becomes thick enough, its formation speed decreases, and a small exothermic heat peak is observed at 140   °C. The mild heat generation continued until a sharp exothermic peak appeared at 280   °C, because the SEI formation continues with increase in temperature even if there is a protection effect of the SEI. If the original SEI formed during cycling is thick enough, the small peak at 140   °C does not appear because the protection effect of the original SEI is enough even at this temperature. Sample (b) is charged at a very low current density because PVdF-binder is not used to make the electrode. Therefore, SEI of sample (b) is very thick, and no peak appeared at 140   °C. Samples (c) and (d) did not show the small peak at 140   °C. Therefore, the lithiated graphite and the electrolyte are necessary to show the small peak at 140   °C. This fact also supports that the small peak at 140   °C is the formation of SEI. This peak is sometimes large and the peak temperature is different from 140   °C, because the thickness of original SEI is different (the charge current density is different).

There is evidence [7] that the peak at 280   °C in Figure 20.2 is caused by decomposition of SEI with reaction of Li in graphite. DSC curves of charged electrode powder (without electrolyte) obtained after the 2nd charge are shown in Figure 20.3, together with that of discharged electrode powder (without electrolyte) after the 2nd discharge. No exothermic peak was seen at around 100–130   °C. The heat values, which were evaluated by integrating DSC curves, were proportional to the amount of charged electrode powder. These results suggest that SEI formed on graphite during charging would react with charged graphite at ∼280   °C accompanied by exothermic heat.

As shown in Figure 20.3 no exothermic peak was visible at 100–160   °C for charged graphite only, thus the electrolyte should be directly involved in the exothermic reaction at this temperature. To identify the effect of solvent and LiPF6 in the electrolyte separately, the thermal behavior of the charged graphite in solvent was studied firstly [8]. Figure 20.4(a) shows DSC curves for 4   mg of Li0.92C6 mixed with a given amount of the EC   +   DMC solvent (from 0.25 to 4   μl). When the amount of the solvent was 0.25   μl, an exothermic peak was observed at ∼160   °C. When the amount of solvent increased from 0.25 to 2   μl, the heat values of the peak increased significantly. However, the heat value was almost constant when the amount of the coexisting solvent increased to 3 and 4   μl. Therefore, the exothermic peak at ∼160   °C is caused by the reaction between solvent and intercalated Li. The protection effect of original SEI, which was formed during sample preparation, has to be considered. The heat value of the peak increased with the increase of the solvent until the amount of solvent became to 3   μl. All the coexisting solvent was used for the reaction, and some of intercalated Li remained, because the reaction was limited by the amount of the solvent. With 4   μl, the heat value did not increase too much from that of 3   μl solvent, because the reaction was limited by the amount of intercalated Li. All the intercalated Li was consumed, and excess solvent remained after the reaction.

To confirm the above supposition of exothermic peak at around 160   °C, half-charged graphite (Li0.48C6) with solvent was also quantitatively studied by DSC. Figure 20.4(b) shows DSC curves for 4   mg of Li0.48C6 mixed with a given amount of EC   +   DMC solvent (from 0.25 to 4   μl). Compared with the DSC curves of the mixtures of Li0.92C6 and solvent (Figure 20.4(a)), it was easy to find that the dominant peak was quite similar to that obtained for Li0.92C6 in the solvent, including peak position and peak shape. At the same time, similar tendency of the heat value was visible. The heat values in both cases increased with the increasing of the amount of solvent, and then remained almost constant when all the intercalated Li was consumed with excess solvent. The heat value became almost constant when the solvent was about 3   μl with 4   mg of Li0.92C6, and the solvent was from 1 to 2   μl with 4   mg of Li0.48C6. Furthermore, the largest heat value of Li0.48C6 was almost half the value of Li0.92C6. Based on these results, it was clear that the amount of solvent limits the reaction when its amount is small, and the amount of intercalated Li limits the reaction when the amount of solvent is large. LiPF6 in the electrolyte is needed to form SEI (with protection effect) on the charged graphite.