Carbon-Loaded Flexible Electrode Films for Li-O2 Cells: Preparation, Porosity, Homogeneity and Electrochemical Characterization

Porous (up to 70 vol. %) self-standing flexible carbon/polymer (Ketjen Black / PVdF-HFP) film electrodes are produced by leaching out a plasticizer-porogen agent (DBP = DiButyl Phtalate) from precursor films (80% ³ DBP w% ³ 40%). Textural analysis reveals that i) these films exhibit copious initial macroporosity, ii) extra open macro- and then meso-porosity are created along the leaching, iii) this leaching process impacts the internal films texture resulting in a partial closing of the pre-existing porosity, iv) only films with the lowest initial DBP contents (≤ 60 w%) are homogeneous in composition/porosity/texture. In Li/O2 cells, the first discharge capacities of these films are compared to those calculated assuming a total filling of this porosity by electrochemically formed solid Li2O2 (2700 mAh.cm-3 of pores). Up to 80 % of the maximum capacity can be reached (i.e. 80 % of the porosity filled by Li2O2), confirming the positive attributes of mixed interconnected macro/meso porosity. This is further emphasized by the very low capacities obtained with electrodes having similar porosity vol. % but totally different internal texture (GDL). These conclusions can be made despite a large discrepancy in the data, even for homogeneous films, due to reproducibility issues intrinsic to the system.