Lithium-sulfur batteries represent a promising alternative to the classical Li-ion technology, particularly due to the high specific energy density combined with the good environmental sustainability of sulfur. Compared to state-of-the-art cathode materials for Li-ion batteries (LiB), which usually contain cobalt and/or manganese, sulfur is less toxic and much more abundant [1]. Additionally, the theoretical capacity of 1.675 mAh g^-1 is more than eight times higher than typical cathode materials as Lithium-Nickel-Mangan-Cobalt-Oxide (NMC), Lithium-Cobalt-Oxide (LCO) or Lithium-Iron-Phosphate (LFP) [1,2]. One of the main challenges of sulfur is the so-called “shuttle effect” [1,3]. During lithiation soluble intermediates (polysulfides) migrate to the anode side, further react and precipitate, resulting in a continuous loss of active material and capacity. Next to that, the poor electrical conductivity (5∙10^-30 S cm^-1) does not allow the utilization of pure sulfur as cathode material [1].

To improve the low conductivity, carbon-containing composites have been proposed as cathode material for Li-S batteries for several decades. One possible carbon matrix material is reduced graphene oxide (rGO) [4]. The reduced form of partially oxidized graphite (graphite oxide; GO) is a graphene-like material with high potential for a scale-up to mass production.

In our group, we have developed a new synthesis method to rapidly reduce GO using a reactive spray drying technique, where most functional groups can be removed from GO within seconds. The rapid thermal processing leads to a reduced and exfoliated rGO, which still contains small amounts of epoxy groups within the carbon lattice. These remaining oxygen atoms influence the conjugated π-bonds of the graphene-like matrix and thus change the polarity of the surface. A thermal post-treatment for 2 h in Ar/H₂ can further reduce the remaining functional groups leading to a highly non-polar surface.

It is assumed that oxygen-containing groups (e.g. epoxy and hydroxy) result in a varying adsorption behavior of the polysulfides, affecting the unwanted “shuttle effect” and thus influencing the electrochemical performance [5]. Here, we show the impact of remaining functional groups in rGO on the electrochemical performance of S-rGO composites as cathode material vs. Li/Li⁺. We compare untreated GO to reactive spray dried and additionally (thermally) post-treated rGO. Furthermore, we analyze the composites in terms of asymmetric and symmetric bonding vibrations as well as crystallinity using FT-IR, RAMAN and XRD.

[1] Baikalov, N. et al., Frontiers Energy Research 2020, 8, 207

[2] Su, Y.-S. et al., Nature Communications 2013, 4, 2985

[3] Ren, W. et al., Energy Storage Materials 2019, 23, 707-732

[4] Shastri, M. et al., Ceramics International 2021, 47, 10, B, 14790-14797

[5] Ji, L. et al., Journal of the American Chemical Society 2011, 133, 18522-18525