Unveiling the Mystery of Lithium Bis(fluorosulfonyl)imide as a Single Salt in Low-to-Moderate Concentration Electrolytes of Lithium Metal and Lithium-Ion Batteries

This work aims to unveil the mystery about the suitability of using lithium bis(fluorosulfonyl)imide (LiFSI) as a single salt in low-to-moderate concentration electrolytes of Li metal and Li-ion batteries by studying the corrosion behavior of Al and stainless steel (SS) in 3:7 (wt) ethylene carbonate/ethyl methyl carbonate electrolytes. It is found that regardless of LiFSI concentrations (0.8–1.8 m), there is a ∼4.3 V vs Li/Li + of “threshold potential,” above which Al suffers uncontrollable corrosion. This finding is confirmed in graphite/LiNi 0.80 Co 0.15 Al 0.05 O 2 pouch cells, however, none of Li/LiNi 0.80 Co 0.10 Mn 0.10 O 2 coin cells can be charged to 4.2 V in the first charge. Post-mortem analysis of the failed coin cells reveals that the failure is due to anodic corrosion of the SS spacer. Further study on two types of spacers, made by SS304 and SS316L, indicates that the SS316L spacer is more corrosion resistant, however, it still cannot support batteries for long-life cycling. The suitability for using LiFSI as a single electrolyte solute in Li-ion batteries can be guaranteed as long as, at the cathode, the charge voltage is strictly limited to the “threshold potential” and the SS components are avoided. This conclusion is further verified by using four different sources of LiFSI.