Li-S batteries, as noted many times, are attractive next-generation batteries, especially for electric vehicles, because of their advantageous of high theoretical capacity (1675 mAh/g), high specific energy density (2500 Wh/kg), low cost, and environmental friendliness.

However, the electrochemical inertness of bulk sulfur in the cathode of Li−S batteries is one of the main problems to be solved. This is due to that the inert bulk sulfur leads to a poor electronic conductivity, sluggish kinetics, and incomplete electrochemical conversion. This further results in short cycling life and low rate performance of Li−S batteries. The main strategy to solve such problems is impregnating sulfur into numerous carbon materials to build the cathode with nanostructures. For examples, porous carbon/sulfur composite with confined pores (3−5 nm) and 59 wt % S exhibited a capacity of 750 mAh/g after 100 cycles at 0.5 C.36 A graphene−sulfur−graphene sandwich cathode with 70 wt % S showed a capacity of 680 mAh/g after 300 cycles at 0.1 C.37 The impregnation of 60 wt % S into carbon nanotubes with the diameter of 200 nm displayed a capacity of 670 mAh/g after 100 cycles at 0.2 C.38 This means that cathodes with nanostructured carbon materials play important roles in enhancing the performance of Li−S batteries.

—Zhao et al.

However, these carbon-based cathodes also carry their own challenges, the authors note:

• The sulfur content in these carbon-based nanostructured composites is usually not more than 70 wt %; this leads to low capacity calculated by the whole mass of the cathode.

• The use of conductive additives such as carbon black (∼10 wt %) and binder such as polytetrafluoroethylene (PTFE, ∼ 10 wt %) further lowers the sulfur radio in the cathode. The binder will also result in the electrode polarization and sluggish kinetics particularly at high rate charge/discharge.

• The collapse of the electrode will result in fast decay of the cell; it is important therefore to prepare a cathode with high sulfur content (near 100%) while maintaining high integrity during the cycling for the elimination of the effect of electrode collapse. Studies of the reaction mechanism from S to Li₂S find two contradictory aspects: at low sulfur radios, sulfur signals rapidly attenuate because of the shielding effect of the carbon, while increasing the sulfur content will fade the reactivity and lead to incomplete reactions.

Therefore, it is worthwhile to prepare a highly reactive S cathode with carbon and binder-free for Li−S batteries. Because electrodeposition has been used not only in industry application but also in the preparation of nanomaterials, it would be a proper method to prepare a 100% sulfur cathode on a flexible substrate of Ni foam.

—Zhao et al.

The Nankai team prepared S nanodots on nickel foam through an electrodeposition method at room temperature in which nickel foam (thickness, 0.14 mm; area density, 0.028 g/cm2) was used as working and counter electrode. By using one-step electrodeposition process (and eliminating carbon and binders), the Nankai team shortened the traditional multistep preparation process for common sulfur cathodes.

Electrochemical performance. (a) Discharge and charge profiles at 0.1 C, (b) cycling performance of nano S cathodes with different S content and bulk S cathode (without the addition of Li2S8). (c) Rate performance, (d) cycling performance with Coulombic efficiency of selected nano S (0.45 mg/cm2) cathode and bulk S (0.50 mg/cm2) cathode (with the addition of Li2S8). Credit: ACS, Zhao et al.Click to enlarge.

The fundamental reversible reaction of S + 2Li = Li₂S on this cathode system was also shown with in situ Raman and TEM tests.

Resources

• Qing Zhao, Xiaofei Hu, Kai Zhang, Ning Zhang, Yuxiang Hu, and Jun Chen (2014) “Sulfur Nanodots Electrodeposited on Ni Foam as High-Performance Cathode for Li–S Batteries” Nano Letters doi: 10.1021/nl504263m