Application of double-side modified separator with hollow carbon material in high-performance Li-S battery

To reduce the "shuttle effects" of lithium polysulfides (LIPs) and the lithium dendrites in Li-S batteries, the separator modified by hollow carbon material was prepared by the simple scraping method. It can be found from the contact angle tests that the layers formed by the porous carbon of uniform width exhibited both stronger attractions to LIPs and better permeability of electrolytes than the bare polypropylene (PP) separator. Permeation tests further showed an effective block over LIPs by the modification layers. Cathode symmetrical batteries with Celgard 3501 separator were assembled and the current response tests implied a conversion of LIPs to Li2S catalyzed by hollow carbon materials. Lithium symmetrical batteries with modified separators were assembled and the voltage-time profile of charge-discharge processes showed better stability owing to the prevention of lithium dendrites. The Li-S batteries were assembled with sulfur loading of 1.8-2.0 mg.cm(-2) and with the bare PP, single-side modified, and double-side modified separators. Calculations of the diffusion coefficient of lithium-ion from galvanostatic intermittent titration technique (GITT) tests and Nyquist plots both indicated the faster ion transportation for the modified separators. Smaller semicircles for impedance were also found in the plots. Nyquist plots after the 1st, 5th, 10th, 50th, and 100th cycles were analyzed to show a stable diffusion behavior of lithium ions, which should be caused by the multichannel from hollow carbon material to provide more paths for Li+ ion transportation. Li-S batteries with double-side modified separators presented a high specific capacity of 1 035 mAh.g(-1) in the first cycle and 500 mAh.g(-1) after 700 cycles at the current density of 0.2C, 630 mAh.g(-1) after 100 cycles at 1C, and 505 mAh.g(-1) after 100 cycles at 2C. The rate performance also behaved superior to the cells with bare PP as the separator. The cell assembled with higher sulfur content (3.2 mg.cm(-2)) also presented the reverse specific capacity of 500 mAh.g(-1) at 0.2C. These battery performances could be ascribed to the porous hollow carbon materials for their adsorption and conversion of LIPs and their prevention of dendrites. Thus, the physicochemical interaction between hollow carbon and LIPs effectively alleviates the shuttle effect and the bifunctional modification of the separator could prevent the growth of lithium dendrites to improve the safety of the Li-S batteries.