Revisiting the use of electrolyte additives in Li-S batteries: the role of porosity of sulfur host materials

In this work, we demonstrate the important role of porosity of the sulfur host material in the efficient functioning of the biphenyl-4,4 '-dithiol (BPD) electrolyte additive in Li-S batteries. We compare the electrochemical performance of Li-S cells fabricated using non-porous (CNFs), micro-porous (mi-CNFs), and micro-mesoporous carbon nanofibers (me-CNFs) as sulfur hosts. me-CNFs/S cathodes exhibit a stable specific capacity with 83% capacity retention at C/2 after 200 cycles and 90% retention at C/5 after 150 cycles, whereas mi-CNFs/S and CNFs/S cathodes retain close to only 30% capacity after 200 cycles. We investigate the role of porosity using two approaches - Li+ diffusion coefficient and shuttle-current measurements. The me-CNFs/S cathodes show a relatively higher Li+ ion diffusion coefficient during reduction and oxidation processes thus indicating a low concentration of BPD-S-x(2-) species in the electrolyte. Furthermore, the me-CNFs/S cathodes indicate a relatively lower shuttle effect compared to the other two cathodes, further validating the presence of a lower concentration of polysulfides in the electrolyte. Based on the size of the BPD-S-x(2-) complex, we infer that the use of mesoporous CNFs is essential for achieving long-term cycling. Our results show that the integration of suitable porous host materials with functional electrolyte additives presents a promising approach for developing high-performance Li-S batteries.