MOFs Derived Hierarchically Porous TiO2 as Effective Chemical and Physical Immobilizer for Sulfur Species as Cathodes for High-Performance Lithium-Sulfur Batteries

Despite recent progress in lithium-sulfur batteries (LSB), the chemically immobilizing mechanism for sulfur species hasn't been understood totally. For the first time, we report on insight into the immobilizing nature for soluble lithium polysulfides (LPS) based on metal-organic frameworks (MOFs) derived porous TiO2-S cathodes for LSB. The LSB based on hierarchically porous TiO2-S cathode displays greatly enhanced rate capability with superior specific capacity and coulombic efficiency. Electrochemical impedance spectroscopy for fresh and cycled cells indicates that the charge transfer kinetics of the TiO2-S cathode can be obviously enhanced, and the diffusion coefficient of Li-ions greatly increases due to the Ti-S chemical bonding during charge/discharge process, almost 17 times higher than that of fresh TiO2-S hybrid cathode. The charge resistance and diffusion coefficient can be comparable to that of previously reported carbon/sulfur cathode. It is revealed that dissolution of lithium ploysulfides, deposition of Li2S/Li2S2 and shuttle effect can be greatly mitigated due to the chemical immobilization effect from Ti-S chemical bonding. Furthermore, the porous structures of MOFs-derived TiO2 effectively alleviate the volume change, shortens the diffusion path of Li+. The present work provides a hint to design rational cathodes for high performance LSB. (C) 2016 Published by Elsevier Ltd.