The recent rapid expansion of the Li-ion battery (LIB) market has been fueled by the rapid increase in demand for electric vehicles and energy storage systems (ESS) that are able to run and last longer. The Li-S battery (LSB), which has attracted much attention due to the very high theoretical reversible specific capacity of sulfur cathodes, 1675 mA h g⁻¹, leading to a very high theoretical energy density of LSBs, 2600 Wh kg⁻¹. In addition, sulfur is both inexpensive and abundant.¹

However, both sulfur and Li₂S (the final sulfur reduction product) have poor inherent electrical conductivity. Another inherent problem of any LSB containing a cathode with a high loading of sulfur, is the massive dissolution of the LiS_n moieties formed by sulfur reduction that leads to a subsequent loss of active material.² Once LiS_n products are dissolved in the electrolyte solution, they diffuse to the Li anode and undergo continuous reduction processes, causing a phenomenon known as the shuttle mechanism during cycling, which prevents full recharge of the sulfur reduction products in LSBs.

Much effort has been devoted in recent years to overcome the intrinsic problems of sulfur cathodes. We concentrate here on attempts to develop freestanding sulfur cathodes. A freestanding electrode is fabricated without binder and powdery conductive materials, thereby allowing higher percentages of active material in the cathode. In addition, because of their porous structure, freestanding electrodes can provide 3D interlinked pathways for electrons and Li⁺ ions, thereby improving the electrochemical response of sulfur in the cathode.³

In order to promote the practical development of LSBs, it is imperative to investigate the performance of scaled-up LSBs and solve technical problems which cannot be considered at the level of small-scale cells. For that purpose, we have developed an up-scaling methodology for Li-S batteries based on pouch cell assembly, using freestanding electrodes. The stacked pouch cells we prepared demonstrated high discharge capacity and rate capability at a high sulfur loading of 2.8 mg cm^-2. However, we experienced relatively poor cycling performance of pouch cells containing metallic lithium anodes, which can be attributed to the localized mossy growth of metallic lithium deposits in the charging process. In order to resolve this problem that arises from the poor behavior of Li metal anodes in these cells, we developed long-term lithium-ion sulfur pouch cells containing prelithiated C-coated Si/SiO_x anodes which do not suffer from dendritic growth of Li. In this work we did not intend to develop new materials or to develop sulfur electrodes with fully practical high loading. We concentrated in the challenge of up-scaling Li-S battery prototypes from small laboratory coin type cells to much larger pouch cells.

1. A. Rosenman, E. Markevich, G. Salitra, D. Aurbach, A. Garsuch, F. F. Chesneau, Adv. Energy Mater. 2015, 5, 1500212
2. H. A. Salem, G. Babu, C. V. Rao, L. M. R. Arava, J. Am. Chem. Soc. 2015,137, 11542
3. H.-S. Kang, Y.-K. Sun, Adv. Funct. Mater. 2016, 26, 1225