One major issue is related to the insulating nature of S and its fully discharged product (Li2S), which often leads to low utilization of the active material and poor rate capability. The poor electronic conductivity of these species can be overcome by utilizing conductive hosts, though they are dilutive and decrease the energy density, meaning that their mass ratio to the active material should be as low as possible [1]. Another crucial issue relates to the undesired solubility of certain sulfur discharge products, so-called long-chain Li polysulfides (LiPSs), in the conventional ether-based liquid electrolyte. The solubility of long-chain LiPSs promotes their free back-and-forth transport between the positive and negative electrodes, which results in poor cyclability and capacity decay [2, 3]. Despite the efforts to engineer and control the undesired LiPSs shuttling effect, advances have been mostly limited to a small number of cycles (100-200), or the need for complex and often expensive synthesis that has limited the rational development of new sulfur cathodes.
At present, a large majority of the sulfur cathode research has focused on nano-architectured electrodes using 2D and 3D host materials for sulfur, such as carbon nanotubes, graphene, conductive scaffolds, yolk-shell structures, and the like, to increase the conductivity and alleviate the LiPSs shuttling [4]. Although these approaches have helped to increase the achievable capacity, and sometimes the cyclability, their synthesis methods have been highly complex, meaning that their manufacturing cost will be high. Also, in operating cells, it is highly unlikely that these complex structures can be effectively reproduced upon many charge-discharge cycles – meaning that capacity loss is essentially inevitable. Thus, developing novel, yet affordable and scalable, cathode architectures that can enhance the rapid transport of Li-ions to active sites for electrode reactions, accommodate discharge-induced volume expansion, and minimize the shuttling mechanism by sulfur encapsulation are still in great need.
In this work, we present a low-cost and scalable processing method for highly durable sulfur cathodes containing commercial sulfur, carbon black, and polyvinylidene fluoride (PVDF). The sulfur cathode slurry was prepared through a simple and scalable recipe where the degree of binder dissolution into the solvent was controlled before electrode deposition. Variables such as the solvent:binder ratio, dissolution time, and agitation will be discussed. The microstructure of the sulfur cathodes was characterized using scanning electron microscopy. Through controlled dissolution of binder, a porous, swollen network of binder was achieved that adhered the sulfur and carbon particles while providing a highly porous structure that can accommodate the sulfur volume expansion during discharge and impede dissolution of the discharge products into the electrolyte by physically trapping them. The cycling performance of the sulfur cathodes prepared through the present novel processing was tested at C/10 and compared with those prepared through the conventional production techniques. The sulfur cathodes prepared with this novel electrode processing offered impressive capacity retention of 80% after 1000 cycles suggesting a considerable improvement in the shuttling effect and active material preservation. These results are expected to help move the production and manufacturing of Li-S batteries forward.
References
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