Design, Fabrication and Electrochemical Performance of Nanostructured Carbon Based Materials for High-Energy Lithium–Sulfur Batteries

Despite the great promise of Li–S batteries, many challenges need to be addressed before they can find practical applications. For example, the low electrical conductivities of sulfur, polysulfide products and the final Li₂S product affect the utilization of the active sulfur material and the rate capability of the battery. The highly soluble polysulfides in the electrolyte, which can shuttle between the anode and cathode and form a deposit of solid Li₂S₂/Li₂S on the cathode and anode, cause an irreversible loss of sulfur, which leads to low Coulombic efficiency, low cyclic capacity and an increase in impedance. A volume change (80%) between sulfur and Li₂S during charge/discharge induces stress in the electrode and destroys its structural stability, which leads to rapid capacity decay. In this book, firstly I will introduce the background of rechargeable Li–S batteries, then I will demonstrate three strategies to improve the performance of Li–S batteries from physical confinement, chemical binding and battery configuration design. Finally, conclusions and perspective were included.

A microporous-mesoporous carbon with graphitic structure was developed as a matrix for the sulfur cathode of a Li–S cell using a mixed carbonate electrolyte. Sulfur was selectively introduced into the carbon micropores by a melt adsorption-solvent extraction strategy. The micropores act as solvent-restricted reactors for sulfur lithiation that promise long cycle stability. The mesopores remain unfilled and provide an ion migration pathway, while the graphitic structure contributes significantly to low-resistance electron transfer. The cathode is able to operate reversibly over 800 cycles with a 1.8 C discharge–recharge rate. This integration of a micropore reactor, a mesopore ion reservoir, and a graphitic electron conductor represents a generalized strategy to be adopted in research on advanced sulfur cathodes.

A template-directed synthesis of sulfur-carbon nanotubes and their use to form a membrane that is binder-free, highly-conductive and flexible was reported. This nanostructured membrane is used as a self-supporting cathode without metal current-collectors for Li–S batteries. The membrane cathode has a high electrical conductivity and renders a long life of sulfur over 100 charge/discharge cycles. High discharge capacity of sulfur was attained at 712 mAh g _sulfur ⁻¹ (23 wt% S) and 520 mAh g _sulfur ⁻¹ (50 wt% S) at a high current density (6 A g _sulfur ⁻¹ ). The overall capacity of the flexible cathode correspondingly reaches 163 mAh g⁻¹ (23 wt% S) and 260 mAh g⁻¹ (50 wt% S). These results demonstrate the great potential of this nanostructured flexible membrane as a cathode for Li–S batteries with fast charge/discharge performance and long life.

Graphene–sulfur (G–S) hybrid materials with sulfur nanocrystals anchored on interconnected fibrous graphene are obtained by a facile one pot strategy using a sulfur/carbon disulfide/alcohol mixed solution. The reduction of graphene oxide and the formation/binding of sulfur nanocrystals were integrated. The G–S hybrids exhibit a highly porous network structure constructed by fibrous graphene, many electrically conducting pathways and easily tunable sulfur content, which can be cut and pressed to pellets to be directly used as lithium–sulfur battery cathodes without using metal current-collector, binder and conductive additive. The porous network and sulfur nanocrystals enable rapid ion transport and short Li⁺ diffuse distance, the interconnected fibrous graphene provides highly conductive electron transport pathways, and the oxygen-containing (mainly hydroxyl/epoxide) groups show strong binding with polysulfides preventing their dissolution into electrolyte based on first-principles calculations. As a result, the G–S hybrids show a high capacity, an excellent high-rate performance and a long life over 100 cycles. These results demonstrate the great potential of this unique hybrid structure as cathodes for high performance lithium-sulfur batteries.

Lithium-sulfur (Li–S) batteries have high specific capacities and are considered as next-generation batteries for large-scale energy storage and electric vehicles. However, rapid capacity fade and low sulfur utilisation inhibit their use. We designed a unique sandwich structure with pure sulfur between two graphene membranes, which are continuously produced over a large area, as a very simple but effective approach for the fabrication of Li–S batteries with ultrafast charge/discharge rates and long-life. One membrane was used as a graphene current collector (GCC) to replace the conventional aluminium foil current collector, and sulfur was coated onto this membrane as the active material. The other membrane was coated onto a conventional polymer separator (G-separator). This electrode showed a high specific capacity of 1340 mA h g⁻¹ at 300 mA g⁻¹, a Coulombic efficiency approaching 100%, excellent high-rate performance and long cyclic stability. The GCC and G-separator not only effectively reduce the internal resistance of the sulfur cathode but also function as buffer layers to trap/immobilise and reuse the dissolved lithium polysulfides. Furthermore, for the first time, three-dimensional X-ray microtomography was used to investigate sulfur diffusion during electrochemical charge/discharge.

Flexible energy storage devices are becoming increasingly important for future applications but are limited by the lack of suitable lightweight electrode materials with robust electrochemical performance under cyclic mechanical strain. Here, we proposed an effective strategy to obtain flexible Li–S battery electrodes with high energy density, high power density and long cyclic life by adopting graphene foam-based electrodes. Graphene foam can provide a highly electrically conductive network, robust mechanical support and sufficient space for a high sulfur loading. The sulfur loading in graphene foam-based electrodes can be tuned from 3.3 to 10.1 mg cm⁻². The electrode with 10.1 mg cm⁻² sulfur loading could deliver an extremely high areal capacity of 13.4 mAh cm⁻², much higher than the commonly reported Li–S electrodes and commercially used lithium cobalt oxide cathode with a value of ~3–4 mAh cm⁻². Meanwhile, the high sulfur-loaded electrodes retain a high rate performance with reversible capacities higher than 450 mAh g⁻² under a large current density of 6 A g⁻² and preserve stable cycling performance with ~0.07% capacity decay per cycle over 1000 cycles. These impressive results indicate that such electrodes could enable high performance, fast-charging and flexible Li–S batteries that show stable performance over extended charge/discharge cycling.

In this chapter, main conclusion and innovations, as well as perspective for future works are included.