In situ quantification of the swelling of graphite composite electrodes by scanning electrochemical microscopy

The physical swelling of uncharged graphite composite electrodes due to electrolyte-binder interactions is investigated by scanning electrochemical microscopy (SECM) using 2,5-di-tert-butyl-1,4,-dimethoxybenzene as a redox mediator. A series of approach curves at the same location is conducted in order to quantify in situ and locally the physical swelling. The film thickness change dfilm amounted to 9.1 mm on average for a 80 mm thick uncharged graphite composite electrode in LP40 electrolyte between 1.1 and 5.9 h. Curves of dfilm vs. t usually reach a saturation within 12 h. The swelling ratio CH varies from 0.3% to 17.6% for uncharged graphite composite electrodes from the same batch in the same electrolyte. In contrast, the 8 mm thick polyvinylidene fluoride (PVDF) model sample swelled by CH = 99%. Approach curves demonstrate that swelling of the PVDF is the main cause for the physical swelling of uncharged graphite composite electrodes. Both PVDF model sample and uncharged graphite composite electrodes show locally different swelling ratios by SECM imaging. Based on these results a swelling model is proposed, where the uncharged graphite composite electrode swells physically on average by at least CH = 11% and the local topography is changing during swelling.