Effects of transition metal cation additives on the passivation of lithium metal anode in Li-S batteries

The effect of additives of transition metal cations (Zn2+, Cu2+, Co2+, Ni2+, and Mn2+) on the passivation of lithium metal anode in Li-S batteries was investigated. The effects of transition cation additives on improving rate capability and Coulombic efficiency were observed in the following order: Zn2+ > Cu2+ > Co2+ > Ni2+ > Mn2+ > no salt. With Zn additives, less-accumulation of sulfur and smoother surfaces were observed using Scanning Electron Microscope (SEM) compared to that without additives. The result is consistent with Electrochemical Quartz Crystal Microbalance (EQCM) measurement, where less increase in mass changes of the anode 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 solid electrolyte interphase (SEI) films. 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 enhance the long-term stability 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. (C) 2019 Elsevier Ltd. All rights reserved.