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Sulfur-polyacrylonitrile (S-PAN) composite has been developed as a novel composite cathode material to address many issues with conventional Li-S batteries (LSBs). In this study, a freestanding S-PAN-CNT composite is first developed as the cathode material for LSBs, which is capable to deliver a high specific capacity of 1458 mAh g-1 at 0.2C and a desirable high-rate performance of 1097 mAh g-1 at 2.0 C. Furthermore, a Li2S-PAN-CNT cathode is obtained via in-situ direct pre-lithiation of S-PAN-CNT composite, which exhibits an even improved discharge capacity, cycling performance, and rate capability. Lastly, we develop Li-ion sulfur full batteries based on both S-PAN-CNT and Li2S-PAN-CNT cathode. The excellent electrochemical performance and corresponding theoretical estimation both demonstrate that the proposed system as a promising metal-free Li-ion battery with a high specific capacity, good cycle life, and low cost. 

Lithium-sulfur (Li-S) batteries suffer from poor utilization of active material and short cycle life due to the complicated multi-step reaction mechanisms. Herein, three conditional cycling methods, i.e. asymmetrical cycling, constant voltage (CV) discharge cycling, and partial cycling are designed in order to increase the cyclability of Li-S batteries. It is found that the solid deposition process that takes place during the lower plateau of discharge is the major limiting step for achieving high discharge capacity and cycle retention, and the cathode surface coverage can be deferred by applying an optimal discharge/charge rate and CV discharge cycling. The asymmetrical cycling renders a specific capacity of ca. 700 mAh g-1 after 200 cycles, 30% higher than that under symmetrical cycling, while applying a CV discharge cycling enables a full retention of target specific capacity of ca. 800 mAh g-1 over 50 cycles. The partial cycling with a low number of phase transformation steps and reduced surface coverage at the end of discharge/charge also enhances cyclability. This work paves the way for understanding and improving the cycling performance of Li-S batteries without increasing the cost of electrode design or changing the configuration of the cell.