The current Li-ion battery based on lithiation chemistry has a theoretical specific capacity of 372 mAh g^-1. A battery chemistry beyond lithium ion vision is in urgent need to meet the ever-increasing energy storage demand. Lithium sulfur (Li-S) battery with a theoretical specific capacity of 1672 mAh g^-1, is a promising candidate for next generation rechargeable lithium battery due to its high energy density, low raw material cost and environmental friendliness¹. Despite these advantages, the current Li-S battery suffers from major problems including limited discharge capacity, poor cycle life and low coulombic efficiency, which severely hinder its commercialization.

In this work, freestanding, three-dimensional macroporous carbon nanotube (CNT) foams were used as sulfur hosts. To synthesize CNT foams, poly(methyl methacrylate) (PMMA) microspheres were used as a template and Polyac-rylonitrile (PAN) was used as a precursor to create crosslinks among CNTs^2,3. The sulfur was then infiltrated into the foam by melting diffusion method. The porosity and sulfur loading of the CNT/S composite can be easily tuned during synthesis. The as-obtained cathodes showed a stable electrochemical performance with an initial specific capacity varied from 1337 to 1016 mAh g^-1 at 0.2 C under sulfur loading of 1-3 mg cm^-1. The cell under sulfur loading of 2 mg cm^-1 retained a capacity of 812 mAh g^-1 after 100 cycles. The incomplete sulfur utilization and gradual capacity decay during cycles are mainly associated with the growth of insulating end product Li₂S_x (x≤2) and the dissolution of soluble intermediate product Li₂S_x (4≤x≤8). Several approaches to overcome the issues and further improve the electrochemical performance were studied, including surface modification of CNT for better polysulfide anchoring, optimizing cathode pore structure by mixing CNT with microporous carbon, applying an interlayer as an upper current collector and enforcing an all-solid-state reaction by partial recharging within the second discharge plateau. The results of these studies will be presented and discussed in the meeting.

1. A. Manthiram, Y. Fu, S. Chung, C. Zu, and Y. Su, Chem. Rev., 114, 11751–11787 (2014).

2. Y. Cui and M. Zhang, ACS Appl. Mater. Interfaces, 5, 8173–8178 (2013).

3. Y. Cui and M. Zhang, J. Mater. Chem. A, 1, 13984-13988 (2013).