Core-Shell Structure and Interaction Mechanism of γ-MnO2 Coated Sulfur for Improved Lithium-Sulfur Batteries.

Lithium-sulfur batteries have attracted worldwide interest due to their high theoretical capacity of 1672 mAh g-1 and low cost. However, the practical applications are hampered by capacity decay, mainly attributed to the polysulfide shuttle. Here, the authors have fabricated a solid core-shell γ-MnO2 -coated sulfur nanocomposite through the redox reaction between KMnO4 and MnSO4 . The multifunctional MnO2 shell facilitates electron and Li+ transport as well as efficiently prevents polysulfide dissolution via physical confinement and chemical interaction. Moreover, the γ-MnO2 crystallographic form also provides one-dimensional (1D) tunnels for the Li+ incorporation to alleviate insoluble Li2 S2 /Li2 S deposition at high discharge rate. More importantly, the MnO2 phase transformation to Mn3 O4 occurs during the redox reaction between polysulfides and γ-MnO2 is first thoroughly investigated. The S@γ-MnO2 composite exhibits a good capacity retention of 82% after 300 cycles (0.5 C) and a fade rate of 0.07% per cycle over 600 cycles (1 C). The degradation mechanism can probably be elucidated that the decomposition of the surface Mn3 O4 phase is the cause of polysulfide dissolution. The recent work thus sheds new light on the hitherto unknown surface interaction mechanism and the degradation mechanism of Li-S cells.