Elemental sulfur (S) is an attractive cathode material for next-generation, high-capacity rechargeable Li batteries because of its lightweight and capability of multi-electron reactions. (Earlier post.)

Li/S batteries have a theoretical specific capacity of 1,672 mAh/g and energy density of ∼2600 Wh/kg—much higher than those of Li-ion batteries. Additionally, sulfur is naturally abundant and with low environmental impact, as opposed to transition metal oxides or phosphates.

However, Li-S batteries face a number of issues that need to be overcome, the authors noted, including:

• The electrically insulating nature of S at room temperature (5 × 10−30 S/cm) inevitably causes poor electrochemical accessibility and low active material use.

• The dissolution of long-chain lithium polysulfide intermediates Li₂Sn (4 ≤ n ≤ 8) and their notorious shuttle mechanism in organic electrolytes lead to rapid capacity degradation and low Coulombic efficiency of Li/S cells.

• Volume variation of the sulfur cathode during cycling and aggregation of insulating discharge products, Li₂S₂ and Li₂S, on the cathode surface dramatically change the electrode morphology and shorten the cycle life by hindering electron transfer and ion diffusion pathways.

One solution that has been explored by numerous teams is to encapsulate elemental S in a multifunctional carbon matrix, thereby enhancing the electronic conductivity of the S cathode, trapping soluble Li₂S_n intermediates, and accommodating volume variation of the cathode during cycling, the team observed.

A number of approaches have been tried along these lines, including the use of various nanostructured carbon materials as scaffolds; the multi-walled carbon nanotube (MWCNT) is also an attractive carbon matrix due to its excellent electrical and mechanical properties.

However, earlier efforts have found that with the MWCNTs, the large surface area of the S cathode is in direct contact with the electrolyte, leading to large mass loss of active material. Too, the one-dimensional conductive network of MWCNTs inherently limits the ion and electron transfer pathway along the long axis of the CNTs, which results in low rate performance of the cell.

While graphene sheets—two-dimensional and one-atom-thick carbon layers—have been tailored to host S with good capacity results, the aggregation of individual GSs results in poor cyclability. Embedding sulfur within the conductive polymers, amphiphilic surface modification of hollow carbon nanofibers or formation of S-TiO₂ yolk-shell nanoarchitecture with internal void space have also been reported to improve the cycle life of the Li−S batteries, the authors write.

Despite these advances, the performance of the studied Li/S cells is, however, still not satisfying in terms of cycling stability, solid- electrolyte interface protection, and rate capability.

To overcome these shortcomings, we propose a conceptually new approach to design and fabricate MWCNT@S nanostructures embedded in graphene galleries using a facile, environmentally friendly assembly process.

By taking advantage of the excellent electrical and mechanical properties of both MWCNTs and GSs, the proposed sulfur−carbon composite with unique hierarchical sandwich-type architecture aims to maximize the usage of S and improve the cycling stability and rate capability of Li/S batteries.

—Chen et al.

In their study, the team compared the electrochemical performance of the sandwich-type sulfur−carbon composite with MWCNT@S and GS@S composite cathodes.

The synergistic effects of GS and MWCNTs provide a 3D conductive network for electron transfer, open channels for ion diffusion, strong confinement of polysulfide (Li₂S_n), and an effective buffer for volume variation of the S cathode...The much improved electrochemical performance of this electrode suggests that the hierarchical, sandwich-type GS-MWCNT@S composite could be a promising cathode material for rechargeable Li/S batteries.

—Chen et al.

Resources

• Renjie Chen, Teng Zhao, Jun Lu, Feng Wu, Li Li, Junzheng Chen, Guoqiang Tan, Yusheng Ye, and Khalil Amine (2013) Graphene-Based Three-Dimensional Hierarchical Sandwich-type Architecture for High-Performance Li/S Batteries. Nano Letters doi: 10.1021/nl4016683