As one of the most intensely investigated technologies in the electrochemical energy storage field, lithium−sulfur (Li−S) batteries have observed rapid improvements in their properties in recent years. Recent work experimentally realized the theoretical specific capacity of 1672 mAh g⁻¹ using elemental octaatomic S₈ as the cathode material. Highly stable Li−S cathodes with lifetimes of more than 1000 cycles were also reported. Li−S pouch batteries have achieved high energy densities in the range of 350−450 Wh kg⁻¹. Although the cycling stability of Li anodes is still an important issue to address, state-of-the-art performance should, in principle, enable niche applications that do not require particularly long cycling. However, such applications have not been possible with the exception of very selected cases because of the low rate performance. While 3C applications require relatively moderate discharging rates, new applications in electric vehicles, power backups, and portable power tools require much higher power densities (i.e., higher rates), which have been difficult to achieve with current Li−S technologies. Currently, the slow discharging rate inhibits the practical application of Li−S technology due to the limitations of the cathode.

In principle, the rate performance of Li−S batteries is significantly affected by polysulfide redox kinetics at the cathode as well as electron and ion transport in the electrodes and electrolyte. Previous works have shown that, due to the inherent insulating nature of elemental sulfur and the reduction product Li₂S as well as poor electrode kinetics, there exists a trade-off between rate performance and energy density. To improve both properties simultaneously, sufficient redox sites inside the cathode are necessary to promote high energy density, and integrated electron/ion pathways are essential to enhance electrode kinetics. Here, we report the rational design and implementation of a Li−S cathode structure that exhibits significantly improved rate performance while maintaining a high specific capacity and long cycle lifetime.

—Chen et al.

Freestanding cathode structures based on carbon nanomaterials that can be directly assembled into cells without using a metal current collector or carbon paste as a conducting agent have reported in a range of other work. The new cathode benefits from a holistic design approach, yielding a cathode with high energy densities, long cycle lifetimes, and excellent rate performances.

• The nanosized sulfur particles improve the degree of sulfur utilization and the initial specific capacity. The nanosized sulfur particles are also proven to promote high rate performance.

• The layered and porous rGO structure restricts polysulfide diffusion and also provides space to accommodate volume changes of the active material during cycling, both of which are beneficial for cycling stability.

• The layered rGO structure ensures the electronic conductivity of the electrode, while the interconnected pores provide reservoirs for the electrolyte and constitute connected open pathways for ion transport.

• PAQS is essential to the realization of the freestanding cathode. A PAQS polymer solution is mixed with nano-S on an rGO dispersion and vacuum filtered to assemble the freestanding film with a layered and porous structure.The PAQS additive may also restrict polysulfide diffusion and prolong the cycling lifetime. Furthermore, PAQS is also a good Li⁺ conductor, facilitating ion transport at high discharging rates.

• Because no conductive carbon, no binder, nor metal current collector is needed for the free-standing thin film cathode, the composite electrode has high effective sulfur loading, 48% sulfur content in the entire cathode structure. This effective loading is far higher than that in typical C/S composite active material based cathodes.

We believe the holistic design approach targeting not only specific capacity and cycle life but also rate performance is an essential step toward the practical application of Li−S technology.

—Chen et al.

Resources

• Hongwei Chen, Changhong Wang, Yafei Dai, Shengqiang Qiu, Jinlong Yang, Wei Lu, and Liwei Chen (2015) “Rational Design of Cathode Structure for High Rate Performance Lithium–Sulfur Batteries” Nano Letters doi: 10.1021/acs.nanolett.5b01837