Synthesis and characterization of MoS2-PANI composite materials for electrochemical applications

Over the past two hundred years, human history has been closely linked to energy sources development. Coal, oil and other fossil fuels have made possible the evolution of society, as we know it today. However, the unbridled use of these non-renewable and highly polluting resources has prompted the scientific community to seek other solutions. Thus renewable energies (in particular, wind and solar power) have been introduced on the market, supported by the batteries development: essential devices capable of storing and moving energy even in periods and places where such resources are not available. The following thesis work deepens the state of the art of one of the most recent and promising classes of batteries, the Lithium-Sulfur ones. Moreover, one of the most significant limitations that still limits the potential of this technology (the Shuttle phenomenon of polysulfides) has been studied and addressed. The Shuttle effect generates intermediate species, negatively charged (polysulfides), promotes their migration inside the cell and parasitic reactions with the electrodes. The suppression of this phenomenon is crucial due to the side effects affecting the cell performances: lower useful battery life, low coulombic efficiency, a significant drop in potential, moderate use of active materials and a high propensity to self-discharge. During the experimental campaign Double-Layers composites have been generated to be applied on the cathode to limit the Shuttle effect’s severity. The purpose of the protective coatings is to block polysulfides both from a chemical and physical point of view. Composite materials (so-called IN-SITU, EX-SITU, Hydrothermal) have been synthetized and applied. These materials, consisting of conductive polymer, PANI (polyaniline) and metallic dichalcogenide, Molybdenum Disulfide (MoS2) act in coherence with the target. The composite materials were then characterised from a chemical-physical, structural and morphological point of view through RAMAN, XRD, FESEM, EDS analysis, and finally electrochemically tested. Since the most encouraging results come from EX-SITU composites, these have been selected, optimised and applied as a double layer on the Sulfur-based cathode. The cells containing the composite materials have been tested with galvanostatic cycling, cyclic voltammetry, specific capacity test: the results, have been compared with the equivalents from cells containing only MoS2 and only PANI in the double layers. Finally a standard battery, without coating (so-called STD), has been assembled and used as a reference. These analyses show, that the two cells containing the synthesised materials present a lower polarisation and a higher specific capacity. The two selected composites material, in particular, show higher initial capacity values, a constant capacity trend and a rather limited decrease in its values compared to the other two cells. Only the standard cell (STD), without a protective coating, shows the Shuttle effect. This phenomenon confirms that the presence of a double layer (whether it consists only of PANI, only MoS2 or both composites) prevents the Shuttle phenomenon. Furthermore, the cells containing the composites materials show better electrochemical performances than the others, confirming the thesis’s target achievement.