High-Energy Density Li-O2 Battery with Polymer-Electrolyte Coated CNT Electrode via Layer-by-Layer Method.

Li-air batteries have attracted considerable attention for several decades due to their high theoretical energy density (> 3400 Wh/kg). However, it has not been clearly demonstrated that their actual volumetric and gravimetric energy densities are higher than those of Li-ion batteries. In previous studies, considerable quantity of electrolyte was usually employed in preparing Li-air cells. In general, the electrolyte was considerably heavier than the carbon materials in the cathode, rendering the practical energy density of the Li-air battery lower than that of the Li-ion battery. Therefore, air cathodes with significantly lower electrolyte quantities need to be developed to achieve high specific energy density in Li-air batteries. In this study, we propose a core-shell-structured cathode material with a gel-polymer electrolyte layer covering the carbon nanotubes (CNTs). The CNTs are synthesized using the floating catalyst chemical vapor deposition (FCCVD) method. The polymeric layer corresponding to the shell is prepared by the layer-by-layer (LbL) coating method, utilizing Li-Nafion along with PDDA-Cl (poly(diallyldimethylammonium chloride)). Several bilayers of Li-Nafion and PDDA, on the CNT surface, are successfully prepared and characterized through XPS, FT-IR, and TGA. The porous structure of the CNTs is retained after the LbL process, as confirmed by the nitrogen adsorption/desorption profile and BJH pore-size distribution analysis. This porous structure can function as an oxygen channel for facilitating the transport of oxygen molecules for reacting with the Li ions on the cathode surface. These polymeric bilayers can provide an Li-ion pathway, after absorbing a small quantity of ionic liquid electrolyte, 0.5M LiTFSI EMI-TFSI (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide). Compared to a typical cathode, where only liquid electrolytes are employed, the total quantity of electrolyte in the cathode can be significantly reduced; thereby, the overall cell energy density can be increased. An Li-air battery with this core-shell-structured cathode exhibited a high energy density of approximately 390 Wh/kg, which was assessed by directly weighing all the cell components together, including the GDL (gas diffusion layer), interlayer (a separator containing a mixture of LiTFSI, 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR-14), and PDDA-TFSI), lithium anode, and LbL-CNT cathode. The cycle life of the LbL-CNT-based cathode was found to be 31 cycles at a limited capacity of 500 mAh/gcarbon. Although this is not an excellent performance, it is almost twice better than that of a CNT cathode without polymer coats.