Tailoring Surface Acidity of Metal Oxide for Better Polysulfide Entrapment in LiS Batteries

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com transport of PS into electrolyte under high concentration gradient and internal electric field. Introducing chemical interaction between functional groups/polar sites on the host with PS is emerging as a more effective approach for caging sulfur species.[5] Recent reports found that using functionalized carbon[6,7] and metal oxide[8] as sulfur host can significantly improve the cycling performance for Li-S cells. Density function theory calculation and surface chemistry analysis reveal that the polar–polar chemical interaction[9] between PS and the host are responsible for this performance improvement. In addition, the lone electron pairs of PS anion are prone to forming the strong Lewis acid–base bonds with the electron-accepting metal sites on host,[5] which have also been proven to capture PS and result in long-term cycling stability (89% after 100 cycles at 0.1 C) in latest report of metal-organic framework.[10] Nevertheless, the chemical nature of this interfacial phenomenon and the factors governing its strength are not well understood. Herein, we systematically tuned the surface chemistry of a sample metal oxide host (TiO2), investigated its interfacial interaction with PS, and demonstrated for the first time that the surface acidity of the host material plays a critical role in its capability to chemisorb PS. By tailoring the surface acidity of TiO2 via heteroatom doping, we were able to enhance the polysulfide-TiO2 interaction in the form of a strong Ti-S bond. Consequently, the doped TiO2/S composite cathode exhibits significantly better capacity retention (0.040% fading per cycle) than pure TiO2/sulfur composite (0.067% fading per cycle) and porous carbon/sulfur composite (0.114% fading per cycle). The discovery in our paper not only shed light on the mechanism of these unprecedented electrode-electrolyte interfacial phenomena, but also opens a new avenue for realizing practical Li-S battery with comparable capacity retention with intercalation cathode.