Encapsulating Sulfur into Carbon Nanotube Framework for Robust Lithium-Sulfur Batteries
Authors: Cheng Xin-Bing , Peng Hong-Jie , Huang Jia-Qi , Zhang Qiang 
Published: February 2020

Lithium-sulphur (Li-S) battery, with the high theoretical capacity density (~1675 mAh g-1) and energy density (~2600 Wh kg-1) as well as the easy access and non-toxic of raw materials, demonstrates a promising prospect among the high energy storage systems for electric vehicles (EVs) and portable electronic devices. However, several issues must be handled before its commercial application.

Differing from traditional lithium-ion batteries, intermediate polysulfides generate and dissolve in the organic electrolyte during the discharge process of lithium-sulfur batteries. The dissolved polysulfides, shuttling between the cathode and anode, lower the utilization of sulfur. However, the dissolution of polysulfide in a lithium-sulfur battery is highly dependent on the sulfur/electrolyte loading. Thus an appropriate dosage of electrolyte will build a concentration gradient, which will limit the dissolution of polysfulfides. Herein, a lithium-sulfur cell with a high initial discharge capacity of 1053 mAh g-1 at a high rate of 1 C and an ultralow decay rate of 0.049 %/per cycle during 1000 cycles was obtained by using CNT-sulfur cathode and suppressing polysulfide shuttle to a shuttle factor of 0.02 by matching the sulfur/electrolyte loading[1].

Beyond the issue of polysulfide shuttle, Li-S battery is always with low practical energy density because sulfur has a low electrical conductivity (5×10-30 S cm-1 at 25 oC) and extra conductive network is highly required. Generally, the addition of conductive materials will neutralize the advantage in high energy density of Li-S battery. Herein, we employed CNTs to build the unblocked conductive skeleton and room-temperature, one-step ball-milling treatment of aligned CNTs and sulfur was applied to obtain high sulfur content of 90%. The 90% sulfur loading cathode exhibited a superior density of 1.98 g cm-3 (2.07 g cm-3 for sulfur), which greatly increases the volumetric energy density of lithium-sulfur battery[2]. To improve the areal density of sulfur in the cathode further, a 3D Al foam was employed as the long-range conductive matrix. The combination of 3D Al foam with CNT frameworks not only exhibits long-/short-range scaffolds as electron pathways through hierarchical point-line-plate contact, but also offers vast void space to accommodate huge amount of active materials and interconnected channels with short diffusion pathways and low resistance. A high sulfur loading of 7.0 mg cm-2 (based on the surface area of the electrode) was available with a discharge capacity of 6.02 mAh cm-2 (860 mAh g-1) at a current density of 1.17 mA cm-2 (167 mA g-1). Based on the total weight of the cell, a super high energy density of 367 Wh kg-1 was achieved, which is much higher than the cell with routine Al foil current collectors[3]. 

Reference

[1]Cheng X-B, Huang J-Q, Peng H-J et al., Journal of Power Sources. 2014, 253: 263-268.

[2]Cheng X-B, Huang J-Q, Zhang Q et al., Nano Energy, 2014, 4: 65–72.

[3]Cheng X-B, Peng H-J, Huang J-Q et al., Journal of Power Sources. 2014, 261: 264-270.