(345e) Effects of Transition Metal Cation Additives on the Passivation of Lithium Metal Anode in Li-S Batteries

The effects of transition metal cation (Zn, Cu, Co, Ni, and Mn) additives on the passivation of lithium metal anode in Li-S batteries was investigated. The use of transition metal cations greatly improved the nature of SEI film, making it possible to suppressing further degradation of lithium metal. The specific discharge capacity with carbon nanofiber cathode after 100 cycles were found to be 873, 824, 767，751，718， and 694 mAh g^-1 for Zn, Cu, Co, Ni, and Mn and no salt, respectively. The effects of transition cation additives on improving rate capability and Coulombic efficiency were observed in the following order: Zn > Cu > Co > Ni > Mn > no salt. SEM images indicated with Zn additive, a smoother surface and less accumulation of sulfur was observed compared to the one without additive. Furthermore, EQCM shows less accumulated mass on the anode surface for Zn additive as compared to Cu and no additive. The findings can be attributed to transition metal sulfides deposited on lithium surface cooperate with lithium sulfide and electrolyte decomposition product resulting in a smoother and more robust SEI film. The enhanced homogeneity of passivation layer greatly hinders the parasitic reactions between lithium metal and polysulfides as well as organic electrolyte, reducing the loss of active material and improving Coulombic efficiency. On the other hand, the uniform and mechanically strong SEI film introduces less undesired lithium plating and further accumulation on active sites, which significantly suppresses the dendritic formation due to improved surface morphology and chemistry. The results of this study provide a facile method of in-situ chemical formation of passivation layer protecting the lithium metal in batteries.