Abstract
We investigate sulfur infiltration and formation of lower order allotropes in heated porous hosts during fabrication of lithium-sulfur (Li-S) battery cathodes. Sulfur existence in cathode ultramicropores has been an important question for Li-S batteries, as ultramicropores reduce the polysulfides " shuttle effect " but also delay sulfur dissolution and Li + ion diffusion in the trapped solid sulfur. A novel continuum-level model is presented including heat transfer and sulfur infiltration, either from the top of a porous host or from the porous host particle surface, and taking into account the pore size distribution. A novel decay factor in modeling sulfur infiltration incorporates the pore wall repulsion energy and allotrope formation energy (predicted by density functional theory [DFT] simulations). Simulations are performed for a microporous carbon fabric host and an activated carbon powder host with bimodal micropore and macropore size distribution , with Raman and X-ray photoemission spectroscopy (XPS) spectroscopy confirming the predicted existence of linear S 6 and S 4 in ultramicropores. K E Y W O R D S lithium-sulfur batteries, melt and vapor infiltration, pore size distribution in cathode, sulfur allotropes 1 | INTRODUCTION Lithium-sulfur (Li-S) batteries have been in the research focus in the last decade because of their high theoretical energy density, 2510 Wh kg À1 , abundance and low toxicity of sulfur. 1 Intensive effort has been devoted on the cathode, where sulfur is incorporated in a porous conductive host with the aim for the host to not only enhance electronic conductivity but also allow for cathode expansion on the insertion of Li + ions and formation of polysulfides Li 2 S x during discharge. 2 Additionally, effective cathode hosts offer means to trap the sulfur and polysulfides to limit polysulfide shuttling between cathode and anode, which would cause reduction in capacity and coulombic efficiency. 3,4 Suitable microstructural architectures, including small pores, pores with necks and hollow particles, 5,6 and hosts functionalized with chemical groups with high adsorption energy to sulfur and sulfides 7 can provide such good traps. Additional specifications for host porosity and pore size distribution (PSD) target a sulfur mass greater than 5 mg cm À2 , making up at least 70% of the cathode weight, to realize the high theoretical energy density of Li-S batteries. 8 There is a vast range of porous host material candidates for Li-S battery cathodes: porous carbons, 5,6,9 activated carbon (AC) fabrics or fibers, 10,11 graphene and graphene oxide, 12–15 and