Hierarchical Porous Carbon Materials As Host Structure for Sulphur Encapsulation and Performance in Lithium-Sulphur Batteries

One of the key challenges of the 21^st century is the development of rechargeable energy-storage systems that can be coupled to renewable energy sources and for the use in consumer devices. Among them, rechargeable lithium batteries are very attractive candidates due to their high energy densities. One of the most promising next generation systems is the lithium-sulphur (Li-S) battery where sulphur as the cathode material accommodates a maximum of two lithium ions in a non-topotactical assimilation process and offers a theoretical capacity as high as 1672 mAh/g against Lithium significantly exceeding classical Li-ion devices based on intercalation. In addition, sulphur provides a broad operating temperature and an intrinsic protection mechanism against overcharging, which enhances the battery safety [1]. However, it is still associated with various drawbacks including the low electrical conductivity of elemental sulphur and the shuttling mechanism of polysulphides during charging hindering its practical use in these devices. In recent years, it was found that mesoporous carbon materials are very promising as conductive host structures for the active material [2]. Especially hierarchically structured carbon materials seem to be highly suitable due to their large specific surface areas that provide sufficient contact area/interface with the active material.

 In the present contribution, we report the synthesis and lithium-sulphur performance of two advanced mesoporous carbon structures. Carbide-derived carbon (CDC) mesofoams (designated as DUT-70) were prepared by nanocasting of mesocellular SiO₂ foams with polycarbosilanes followed by pyrolysis and template removal. The extraction of the semi-metal atoms from the resulting silicon carbide mesofoams by use of hot chlorine gas leads to the formation of DUT-70 with very high specific surface areas of 2700 m²/g and total pore volumes up to 2.6 cm³/g [3]. It can be infiltrated with sulphur and host the active material in lithium-sulphur battery cathodes. Reversible capacities of 790 mAh/g are achieved at a current rate of C/10 after 100 cycles rendering DUT-70 as an ideal support material for this electrochemical energy storage application due to the strong encapsulation of the active material in the hierarchical pore system.

Moreover, we report the synthesis of mesoporous carbon materials, so called Kroll-Carbons (KCs), which can be obtained by high-temperature chlorine treatment of TiO₂/Carbon nanocomposites. The reductive carbochlorination selectively removes the template according to the equation TiO₂ + 2 Cl₂ + (2 + x) C -> TiCl₄ + 2 CO + x C and meso- as well as micropores are inserted into the resulting porous carbon material. The mesopore size can be tailored in a wide range by using template particles of different size and high specific surface areas of 1980 m²/g coupled with total pore volumes up to 3.1 cm³/g are obtained. KCs prepared from a commercially available template material (Degussa P 25) show outstanding performance as sulphur host material in high capacity cathodes for lithium-sulphur batteries. Sulphur contents up to 72 wt% cause initial discharge capacities as high as 747 mAh/g and stable cycling with reversible capacities of more than 550 mAh/g (related to the mass of the cathode) [4].

[1] N. Jayaprakash, J. Shen, Surya S. Moganty, A. Corona, L. A. Archer: “Porous Hollow Carbon@Sulphur Composites for High-Power Lithium-Sulphur Batteries”, Angew. Chem. Int. Ed. 2011, 5904 .

[2] X. Ji, K. Lee, L. F. Nazar: “A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries”, Nat. Mater. 2009, 500 .

[3] M. Oschatz, L. Borchardt, K. Pinkert, S. Thieme, M. R. Lohe, C. Hoffmann, M. Benusch, F. M. Wisser, C. Ziegler, L. Giebeler, M. H. Rümmeli, J. Eckert, A. Eychmüller, S. Kaskel: „Hierarchical Carbide-Derived Carbon Foams with Advanced Mesostructure as a Versatile Electrochemical Energy-Storage Material“, Adv. Energy Mater. Adv. Energy Mater. 2014, 4, 1.

[4] M. Oschatz, S. Thieme, L. Borchardt, M. R. Lohe, T. Biemelt, J. Brückner, H. Althues, S. Kaskel: „A new route for the preparation of mesoporous carbon materials with high performance in lithium-sulphur battery cathodes”, Chem. Commun. 2013, 5832 .